Since the beginning of the polymer age, silicates – like clays and other finely particulated solids such as silica, calcium carbonate and carbon black – have been incorporated as fillers at the micrometer dimension into plastics and elastomers with the aim of improving the mechanical and rheological properties of these polymers. Kaolinite was the clay mineral initially most widely used as a silicate filler of diverse polymeric matrices.¹  More recently, swelling clays such as smectites have received great interest due to their ability to exfoliate, giving rise to elemental silicate platelets of 1 nm thickness, which represent a way to develop fillers at the nanometer dimension (nanofillers). In this context, the concept of clay delamination encompassed by its high dispersion in polymers, introduced by Fukushima and other researchers at the Toyota Central Laboratory almost three decades ago,^2,3  represents a revolutionary idea, not only for the use of clays as reinforcing charges but also to introduce functionality in the resulting materials.

Very quickly, polymer–clay nanocomposites became a popular topic with a rapid increase of publications and registered patents and, in the last 20 years, about 13 000 articles and 200 patents have been published or registered, according to the ISI Web of Science data. Practically all these works make reference to polymer–clay nanocomposites belonging to the smectite layered clays family, i.e. 2 : 1 charged phyllosilicates, such as montmorillonite, hectorite and saponite natural clay minerals as well as analogous synthetic hectorites and fluoro-hectorites (e.g. LAPONITE® and the so-called “synthetic mica”).⁴  The smectite layers exhibit a high aspect ratio as each one is approximately 1 nm thick, while the diameter may rise to several microns or larger, as is the case with vermiculites. So, polymer–clay nanocomposites based on smectites have been extensively studied from basic aspects to applications, and a significant number of reviews have been published on this topic (see, for instance, ref. 5–11). Compared to smectites, other types of clay minerals, such as kaolinite, halloysite, imogolite, sepiolite and palygorskite, have barely been studied as nanofillers of polymers. However, as we report below in this chapter, the structural and textural characteristics of the fibrous clays could be of great benefit for the properties and applications of the novel polymer–clay nanocomposites derived from them.

The sepiolite and palygorskite fibrous clays also appear to be attractive nanofillers to reinforce polymer matrices.^12–19  They do not exhibit intercalation capacity, but these two natural silicates offer interesting characteristics, such as microporosity and large specific surface area. Interestingly, the presence of hydroxyls (silanol groups, Si–OH) at their external surface allows easy functionalization by controlled modification based on chemical reactions, e.g. with coupling agents or by anchoring of nanoparticles (NPs).^16,20  This behavior allows the preparation of a wide variety of polymer–clay nanocomposites provided with diverse functionality.

Sepiolite is a natural hydrated magnesium silicate with fibrous morphology, displaying a crystal structure consisting of talc-like ribbons arranged parallel to the fiber direction (c-axis) with Si₁₂O₃₀(OH)₄(H₂O)₂₄·8H₂O as the ideal formula^21,22  (Figure 1.1A). To a variable extent, isomorphous substitution of magnesium at the octahedral layer by trivalent metals, mainly Al(iii), provokes a charge deficiency in the structure that is compensated by extra-framework cations. This is the origin of the cation-exchange capacity (CEC) of sepiolite samples, which has been generally established in the 10–20 mEq 100 g⁻¹ range, depending on the origin of the clay and always being about 4–5 times inferior to typical CEC values found in smectite clays. As often occurs in clays formed in lacustrine sediments, certain sepiolite samples show that hydroxyls located at the octahedral layers (mainly Mg–OH) can be partially substituted by fluorine.²³  One of the most interesting features of this silicate is the existence of microporosity ascribed, as occurring in zeolites, to their backbone structural arrangement – in this case with discontinuity of the phyllosilicate layers. This characteristic results in an alternating distribution of structural blocks, each one composed of two tetrahedral silica sheets and a central sheet of magnesium hydroxide, which determine the presence of structural cavities (tunnels) that grow along the c-axis direction (Figure 1.1A). The cross-section of the sepiolite tunnels is 1.06 × 0.37 nm², which determines a structural microporosity typically superior to 0.3 cm³ g⁻¹ as determined from nitrogen adsorption isotherms.²⁴  Palygorskite (Figure 1.1B) is a related silicate with a higher content of aluminum and shorter dimensions of the structural blocks than sepiolite, with tunnel dimensions of 0.64 × 0.37 nm². The irregular arrangement of blocks and tunnels gives rise to fibers, as represented for instance in Figure 1.1C, and determines an elevated surface area in both silicates. Sepiolite and palygorskite exhibit BET (Brunauer–Emmett–Teller theory) specific surface area values in the order of 300 and 200 m² g⁻¹, respectively.²⁴ 

The roughness of fibers at the nanometric scale in both silicates favors the interaction with their surroundings, which makes these materials excellent adsorbents for many applications in diverse sectors such as industry, agronomy and environmental remediation. The peculiar morphology and textural characteristics of sepiolite and palygorskite make these silicates also appropriate as fillers in polymer matrices. The fiber length of sepiolite and palygorskite clays depends on their origin: for instance, sepiolite from Taxus basin deposits in Spain are in the micrometer dimensions, whereas fibers from Finland and China can be much longer. The high aspect ratio^18,25  and the mechanical properties of sepiolite fibers²⁶  make these silicates useful as fillers for the reinforcement of polymers. The elastic properties of individual sepiolite fibers have been measured by atomic force microscopy (AFM) (Figure 1.2), obtaining values of the modulus in the order of 10 GPa in bending mode.²⁶  The abovementioned characteristics indicate that sepiolite and palygorskite could act as efficient fillers of polymers to improve their structural properties.

Fibrous clays are present in Nature as aggregates of the fibers, often forming interwoven bundles as observed by electronic microscopy techniques.²⁷  Disaggregation can be conducted under mechanical or sonomechanical treatment of the fiber aggregates in water dispersion, which after drying, results in xerogels that in turn produce highly viscous aqueous dispersions useful for many applications.²⁸  These processes only produce a small proportion of individualized fibers, and an entire defibrillation of sepiolite and palygorskite appears to be extremely difficult. However, it is evident that high disaggregation degrees can facilitate the fibrous clay dispersions into polymer matrices, leading to the corresponding clay nanocomposites.

The aim of this chapter is to show how sepiolite and palygorskite fibrous clays can be incorporated as efficient fillers into polymer matrices, not only to improve their mechanical properties but also to provide functionality to the resulting polymer-clay nanocomposites. To achieve this objective, diverse approaches may be needed as preliminary procedures, including the disaggregation of the bundles of fibers by physical methods. In some cases, as occurs in other types of clays used as nanofillers, it could also be necessary to modify the nature of the silicate surface in view of making it compatible with the polymer.

As indicated above, raw fibrous clays are present as aggregates of fibers or laths that form interwoven bundles, leading to rigid particles. Their disaggregation, based, for instance, on micronization processes conducted at the industrial level, represents an essential route for applications as fillers and rheological additives for water-based systems.²⁷  For many applications, such as those based on rheological properties, it is crucial that sepiolite or palygorskite are separated into individual fibers. This is the case for important uses such as drilling fluids, construction, asphalt and bitumen, paints and coatings, liquid animal feeds, mortars, fluid fertilizers or grease thickener.²⁷  The micronized fibrous clays, e.g. Pangel® commercialized by TOLSA S.A., are also useful as filler for rubber and provide reinforcing characteristics with polar polymers. This type of material is produced by a patented milling process of the fibrous clays using humidified samples, leading to defibrillation and producing the commercially called sepiolite of rheological grade, capable of producing very viscous suspensions at relatively low concentrations compared with other clays.³⁰  More recently, a new patented invention claims the replacement of the grinding step and wet disaggregation by treatments based on ultrasound irradiation.²⁸ 

The high viscosity developed by sepiolite samples conveniently disaggregated in water can be exploited as a so-called suspension capacity enhancer²⁰  that can be applied in diverse systems. As an illustrative example, multiwalled carbon nanotubes (MWCNTs) can be maintained in aqueous suspension by assembly with sepiolite under ultrasound irradiation.^31,32  These carbon nanotube (CNT) dispersions are extraordinarily stable, avoiding settling over very long periods of time, greater than a year (Figure 1.3). The resulting sepiolite–MWCNT materials can be easily isolated from the dispersions giving rise to the so-called hybrid buckypapers – similar to self-supported films of MWCNTs alone.³¹  These materials exhibit electrical conductivity as well as reinforcing ability when incorporated as fillers in polymers such as polyvinyl alcohol (PVA).³¹  In this way, functionalization of sepiolite introduced by means of physical processes represents an option for the further production of functional polymer–clay nanocomposites, in this case related to conducting nanocomposites.

As the external surface of fibrous clays is covered by silanol groups (≡Si–OH), a useful approach to introduce chemical modifications is based on their reaction with coupling agents such as organosilanes and other reagents such as epoxides and isocyanates.³³  Organosilane coupling agents containing Si–X groups (X = OR, Cl) can react with the silanol groups; the organic part remains grafted to the mineral surface through very stable siloxane bridges (≡Si–O–Si≡).³⁴  In agreement with thermogravimetry (TG) and Differential Thermal Analysis (DTA) data, the grafted groups are very stable, being eliminated by heating at temperatures above 400 °C. In this way, the characteristic hydrophilic surface becomes organophilic and the fibrous clays can then be easily dispersed in low-polar polymers.¹⁷  Sepiolite has been functionalized by treatment with silanes containing unsaturated or thiol groups, such as vinyl and methacryloxy or 3-propylmercapto, respectively, giving organic derivatives of sepiolite capable of further copolymerization reactions.¹⁷  Surface modifications by treatment of sepiolite with alkyl and functional silanes in the form of aqueous gels have been recently reported.³⁵  Other reagents used to functionalize sepiolite by grafting reactions contain epoxide and isocyanate groups, which react with silanol groups, remaining covalently attached to the silicate through ≡Si–O–C≡ bonds and showing lower stability than siloxane bridges.³³  However, the most usual approach to modify the external surface of fibrous clays to reduce their elevated polarity is based on treatments with alkylammonium salts, leading to fillers showing good compatibility with low-polar polymer matrices.

Organoclays derived from sepiolite and palygorskite can be easily prepared by treatment with neutral or cationic surfactants in aqueous solution. In this last case, treatment with long-chain alkylammonium salts^36,37  following an ion-exchange process is carried out in a similar way to that in montmorillonite and other smectite clay minerals. These organically modified fibrous clays are attaining great importance because they are commercialized not only for applications as nanofillers of polymers, like in epoxy systems and plastisols, but also in solvent-based paints, greases, solvent asphalt coatings, inks, foundry washes and adhesives.²⁰ 

New possibilities to modify sepiolite and palygorskite in view of their use as fillers for diverse polymer matrices are still open and novel developments in this sense may find applications in the future. One of them consists of the encapsulation of molecular species into the structural cavities of nanometric dimensions grown along the silicate fibers (tunnels), and the other one is based on the assembly of diverse NPs to the external surface of those clays. The presence of tunnels allows the access of small molecular species into the interior of the silicate by replacing the zeolitic water that is usually filling these structural cavities and is reversibly adsorbed and desorbed by heating or by vacuum exposure.²⁴  Organic dyes such as methylene blue can penetrate at least partially into the tunnels of sepiolite and palygorskite.^24,38  In fact, several centuries ago Maya people in Mesoamerica developed a technique to encapsulate indigo into palygorskite clay, giving rise to a pigment known as Maya Blue.³⁹  It was used during the pre-Spanish period and later-on; its use was continued in Spain till the 17th century and until the 19th century in Cuba, where it was known as Havana Blue.^40,41  This clay-dye hybrid material shows a remarkable stability against weathering and microbiological activity, also being resistant to thermal treatments and solvent extractions, attributed to the encapsulation of the dye molecules inside the silicate tunnels.⁴²  Due to this quality, the existence of a strong interaction between the dye and the host silicate could be admitted, giving rise to the high stability of the Maya Blue pigment.^41–44  Maya Blue analogs prepared by adsorption of molecular dyes such as methylene blue (MB) and methyl red (MR) by both sepiolite and palygorskite have been used for coloring polymer matrices, opening the way to introduce stable colors in polymer–clay nanocomposites.⁴⁵  More recently, Ouellet-Plamondon et al.⁴⁶  reported the functionalization of inorganic polymers (geopolymers) using sepiolite-dye hybrids inspired by Maya Blue. In this case, it is also confirmed that the encapsulation of MB and MR dyes into sepiolite leads to the stability of the pigment in the geopolymer matrix despite the chemical aggressivity of that system in showing strong alkalinity. The protection of the colors towards light and external reagents such as hydrogen peroxide and acids has been clearly shown in the resulting materials.⁴⁶ 

As indicated above, another possibility for introducing functionality in fibrous clays used as polymer fillers is based on the immobilization of diverse types of NPs that, in some cases, can be anchored on the silicate surface by interaction with the external silanol groups.¹⁶  This method could represent an excellent opportunity for the introduction of specific characteristics in fibrous clays-based nanocomposites such as antimicrobial activity, for instance by incorporation of Ag NPs,¹⁶  superparamagnetic properties, for instance by introduction of magnetite–maghemite NPs,^47,48  or photoactivity by assembly of TiO₂ NPs.^49,50 

Research on polymer–clay nanocomposites is a discipline with a continuous increase in the number of publications since they were reported for the first time around 25 years ago.²  For a long time, the majority of the publications were related to smectite-based nanocomposites, however in recent years, the use of fibrous clays, sepiolite and palygorskite, has attracted increasing interest. Since 2014 until September 2015 more than 150 publications have been reported in the Web of Science, searched using “(sepiolite* or palygorskite* or attapulgite*) and nanocomposite*” for a total number of around 670 entries.⁵¹  The increasing interest in this type of nanocomposite may be related to the potentiality that the incorporation of fibrous clays offers in comparison to layered clays from the point of view of functionality. Actually, the most part of the interest in polymer–smectite-based nanocomposites has been directed to the improvement of mechanical properties of polymers. Additionally, the presence of delaminated clay nanosheets results of interest in the improvement of other properties such as the reduction of gas diffusion, as they introduce more tortuous paths which work as a barrier for gases. Hence, for a long time, fibrous clays have scarcely been employed for those purposes, but they may prove interesting for introducing other types of properties and functionalities.

In this section, diverse examples will be introduced in order to illustrate different synthetic polymer–fibrous clay nanocomposites with special emphasis on their implications for developing functional materials. In this way, the use of fibrous clays as nanofillers of thermoplastics and other conventional polymers has been explored as their high aspect ratio and specific surface area may prove favorable for the improvement of certain properties when used instead of layered clays. In this case, the application of specific strategies to overcome the high hydrophilicity of these clay minerals must be considered to promote adequate interfaces with conventional polymers such as polyethylene (PE) or polypropylene (PP).^16–18  In some cases the clay is modified just to render the interface organophilic and hence, compatible, but others preferred to use approaches involving the assembly of monomeric species that could be further used in copolymerization reactions. As mentioned above, more recent advances try to incorporate additional functionalities, for instance by combining two or more types of polymers, addition of CNTs or other NPs, for instance magnetite, that may lead to new applications. Anyway, from a practical point of view, advances in this field may consider that the synthesis of nanocomposites based on those polymers should be integrated in processes that are already well established for large scale production.

As in the case of nanocomposites based on layered silicates and highly hydrophobic thermoplastics such as PE, PP, polybutylene (PB), polystyrene (PS), polyethylene terephthalate (PET), polyvinyl chloride (PVC), polymethylmethacrylate (PMMA) or polyamides (e.g. Nylon®, polyamide 6 (PA6),…), the use of fibrous clays also requires their modification to procure compatibility between the components. As mentioned above, the specific reactivity of the external surface of palygorskite and sepiolite fibrous clays facilitates the incorporation of organophilicity, as well as other particular functionalities, to improve the interactions at the polymer–clay interface region, which in general drives to enhanced thermal and mechanical properties.^16,17  Moreover, such reactivity is currently also exploited to achieve the integration of other NPs or functionalities, making the development of new functional plastics possible.

Polyamides such as PA6 have been frequently reinforced with sepiolite or palygorskite directly⁵²  or after modification with organosilanes.⁵³  The fibrous shape of these nanofillers may contribute to enhancing mechanical properties during their processing. For instance, by controlling experimental conditions during melt spinning of fibers of PA6 reinforced with palygorskite modified with N-b-aminoethyl-caminopropyltrimethoxy silane it is possible to favor the orientation of the nanofillers in the drawing direction, resulting in improved tensile properties.⁵⁴  The presence of the fibrous clay enhanced the effect of fire retardant agents, e.g. the aluminum salt of diethylphosphinic acid (AlPi), as well as the thermal stability and char yield of PA66 nanocomposites incorporating sepiolite.⁵⁵  In this case, the presence of sepiolite shows an effective role favoring the formation of a compact char layer during burning (Figure 1.4). PA6–fibrous clay nanocomposites have also been prepared by in situ polymerization in the presence of the clay modified with appropriate organosilanes.⁵⁶  This methodology can be used to prepare, in two steps, nanocomposites containing two types of polymers, for instance PA6 and poly(ethylene glycol) (PEG) reinforced with palygorskite. Thus, PA6–palygorskite was firstly in situ polymerized and then combined with PEG to produce nanocomposites of PA6–PEG block-copolymers with enhanced heat-resistant and flame retardant properties by conveniently adjusting the ratio of the components.⁵⁷ 

In the case of polyolefins, such as PE, the presence of sepiolite or palygorskite contributes to reducing thermo-oxidation processes and thus improves the thermostability and mechanical properties.⁵⁸  In general, the processing methodology may affect the final characteristics of the resulting nanocomposites. Dynamic packing injection has been proposed over conventional injection molding for favoring the dispersion of the nanofiller in the preparation of PE–palygorskite nanocomposites.⁵⁹  The way in which the clay is modified also affects the final properties of the nanocomposites. Thus, for instance, modification of sepiolite with vinyl triethoxy silane before (ex situ) or during (in situ) heat mixing produces, in the latter case, nanocomposites with improved thermal stability and tensile strength and modulus but reduced elongation at break.⁶⁰  Specific catalysts have also been supported on the clay to act during the polymerization in the presence of the nanofiller.⁶¹  When the co-catalyst methylaluminoxane (MAO) is directly grafted on sepiolite, the process maximizes the interaction between the matrix and the nanofiller, which leads to PE–sepiolite nanocomposites that show better performance than the ones obtained by melt intercalation.⁶²  In the same way, it is possible to graft other reactive species to the fibrous clays, such as octadecyl acrylate, which can react, for instance, with atactic PP producing nanocomposites with improved mechanical properties.⁶³  Palygorskite modified by grafting reaction with aminopropyltrimethoxysilane (APTMS) and treated with ethylene octene elastomer-grafted maleic anhydride (POE-g-MAH) was used to prepare PP nanocomposites in the presence of metallocene linear low density PE (m-LLDPE). Interestingly, the resulting PP–m-LLDPE nanocomposites present enhanced ultraviolet (UV) photoresistance with little change in their mechanical properties.⁶⁴  The incorporation of flame retardant additives (e.g. magnesium hydroxide) and vinyltriethoxysilane as a crosslinking agent in PE–sepiolite nanocomposites results not only in improved flame retardancy but also thermal stability and mechanical properties of the resulting systems.^65,66  Thus, PP–sepiolite nanocomposites prepared from organosepiolites containing cethyltrimethylammonium ions and zinc borate⁶⁷  or from sepiolite and MWCNTs,⁶⁸  present improved fire retardancy properties ascribed to the superior char formation. The filler may introduce other functionalities to the nanocomposites, thus PE–sepiolite and PS–sepiolite nanocomposites showing plasmonic properties have been prepared by previous assembly of Au and Ag NPs to sepiolite.⁶⁹  The presence of FeCo NPs associated to a sepiolite that is treated with methyltrimethoxysilane results in a functional nanofiller for PS-based nanocomposites that show magneto-optical Faraday activity.⁷⁰  Similar properties can be achieved when using sepiolite to which γ-Fe₂O₃ NPs of around 10–30 nm were previously associated.⁷¹  Magnetic PS–palygorskite nanocomposites have been prepared by in situ polymerization of styrene in the presence of a palygorskite, to which magnetite NPs were previously assembled to avoid their aggregation.⁷² 

Sepiolite has also been employed as nanofiller in the production of ethylene-vinyl acetate (EVA) with improved mechanical and thermal properties, including flame retardancy, due to the presence of the clay.⁷³  The use of sepiolite modified by APTMS in EVA allows the preparation of reinforced plastics that show interesting properties as membranes for gas separation – with an increase in the permeability of O₂, CO₂, CH₄, and N₂ gases with just the addition of 3 wt% of sepiolite and the CO₂/CH₄ and CO₂/N₂ selectivity also being improved.⁷⁴  Sepiolite modified with 3-aminopropyltriethoxysilane (APTES) has recently been explored as a nanofiller of polynorbornene matrices, resulting in hybrid nanocomposite films with enhanced mechanical and oxygen barrier properties, which may be of interest in packaging applications.⁷⁵ 

Fibrous clay has also been employed to produce thermoplastic PMMA-based nanocomposites. In general, a previous modification of the clay surface is necessary, for instance with cationic surfactants⁷⁶  or isocyanates,⁷⁷  to improve their compatibility. In some cases, the modifier may act as a crosslinking agent during the melt-compounding process.⁷⁶  Most frequently PMMA is prepared by in situ polymerization using different approaches.^78,79  The clay modification by grafting specific groups, for instance by treatment with mercaptopropyltrimethoxysilane (MPTMS), favors their ulterior coupling with N,N-dimethylaminoethyl methacrylate (DMAEMA) inducing copolymerization via Ce(iv) induced redox polymerization with reactive thiol groups.⁸⁰  In the same way, Cu(0)-mediated radical polymerization of DMAEMA has been coupled to a palygorskite containing 2-bromoisobutyryl groups.⁸¹  Interestingly, the resulting poly(2-(diethylamino)ethyl methacrylate) (PDEAEMA)–palygorskite nanocomposite particles can be used as recyclable Pickering emulsifiers responsive to pH (Figure 1.5). Moreover, the possibility of using methacrylate monomers provided with two sites of polymerization (e.g. ethylene glycol dimethacrylate) allows for the incorporation of additional functionalities to the final nanocomposite. For instance, their combination with organoclays produces nanocomposites with interesting properties for oil adsorption.⁸²  In other cases, copolymerization of functional monomers such as 2-(2-methoxyethoxy) ethyl methacrylate, oligo (ethylene glycol) methacrylate and acrylic acid in the presence of palygorskite results in the formation of stimuli-responsive nanocomposite hydrogels.⁸³  Further functionalization can be reached when magnetite NPs are previously assembled to palygorskite, in this case rendering magnetic hydrogels with improved mechanical properties and sensitivity to pH and temperature changes.^84,85 

Another relevant group of polymer–clay nanocomposites involves epoxy, polyurethane (PU) and other thermosetting resins of importance in construction and aviation, amongst other areas of application. In those nanocomposites, the clay is incorporated mainly to procure mechanical properties.⁸⁶  The use of fibrous clays as nanofillers plays a double role as, besides improvement of mechanical and thermal properties, they may cause crosslinking effects that affect the mobility of the polymer chains.¹⁶  Thus, for instance, the incorporation of commercial organosepiolite in glassy bisphenol A-based epoxy resin has only a slight effect on the thermal stability of the polymer but enhances its storage modulus in flexion.⁸⁷  In the same way, epoxy resins filled with palygorskite, either pristine or modified with cetyltrimethylammonium ions and APTES, present a better distribution of the nanofiller as well as stronger interfacial adhesion when using the modified clay.⁸⁸  The presence of the clay improves the thermal and mechanical properties of the resulting nanocomposites, although in the case of the nanofiller that incorporates grafted amino groups, the degradation of these properties is strongly affected by the loading, where a maximum effect is reached at around 2 wt%. When the nanofiller is modified with more specific agents, for instance glycidyl silane, it is possible to get better toughness although the concentration of the nanofiller has to be kept below certain concentrations to avoid clay agglomeration that may compromise the final mechanical behavior.⁸⁹  Clearly the rheology of sepiolite–epoxy systems is affected by the type of modification introduced to the clay to achieve a better compatibility between the two components, and in general, only physical gels are formed when adding modified clays.⁹⁰  The use of modifiers containing epoxy functionalities, for instance prepared from surfactants of natural origin, procures sites for reaction with the epoxy monomers favoring compatibility between the two phases, which results in enhanced properties in the final nanocomposites.⁹¹  Alternative approaches imply the modification of palygorskite with acrylic resins and its further compatibilization with the epoxy resin to reach more efficient interactions that result in remarkable enhancement in the impact strength, Young’s modulus and the tensile strength relative to the neat epoxy resin.⁹²  In some cases, organosepiolites have been modified with SiO₂ NPs revealing a more effective adhesion between the two components, probably due to the existence of stronger interactions between the nanofiller and the polymer in the presence of the interfacial agent.⁹³  Moreover, the use of a magnetic sepiolite, prepared by chemical coupling of aminosilane-functionalized sepiolite with epoxysilane-functionalized magnetite, allows for the preparation of epoxy resin systems with a reinforcing effect ascribed to the alignment of the magnetic nanofiller in the presence of a magnetic field.⁹⁴  Most sophisticated systems involve the preparation of epoxy-based nanocomposites incorporating palygorskite and graphite oxide NPs under ultrasound irradiation, in which the presence of both types of NPs results in synergies with improved properties compared to nanocomposites containing only one type of NP.⁹⁵ 

In the same way, most research related to PU–fibrous clay nanocomposites is focused on the improvement of thermal properties due to the necessity of fulfilling the regulations for applications in the construction and building sector.^96–100  There is recent interest in exploring the use of the so-called water-borne polyurethane (WPU); a non-toxic, nonflammable and non-pollutant polymer. However, it is insoluble and shows lower thermal stability and mechanical properties than other PU. In this way, the incorporation of nanofillers such as palygorskite to this system has been explored with the view of producing nanocomposites with improved mechanical properties.¹⁰¹  The incorporation of other types of NPs, such as carbon black, results in nanocomposites that may also show electrical conductivity.¹⁰²  WPU combined with palygorskite modified with silanes providing organophilic groups (e.g. octadecyltrichlorosilane, OTCS) has been used to modify stainless steel meshes (Figure 1.6) and provide them with superhydrophobic and superoleophilic properties.¹⁰³  Interestingly, they can be used to separate diverse oil–water mixtures (e.g. kerosene, chloroform and petroleum) with efficiencies as high as for the kerosene–water system. The application of such coatings to Cu-based meshes results in superoleophobic systems with efficiencies over 99.0% in the removal of kerosene and showing stable recyclability after 50 separation cycles.¹⁰⁴  The coating of the mesh may be prepared from other palygorskite-based nanocomposites, for instance epoxy resins, allowing for the separation of diverse oil–water mixtures, containing for instance hexadecane, lubrication oil or paroline, that can be filtered using the coated mesh with a separation efficiency above 98%.¹⁰⁵ 

Another relevant group of functional fibrous clay nanocomposites based on conventional polymers are based on certain hydrophilic polymers such as PVA, polyacrylic acid (PAA) or polyacrylamide (PAAm) related polymers. Since these polymers, as well as their related monomers, are hydrophilic, they can be easily associated with sepiolite and palygorskite by interaction with the silanol groups located at the external surface of the clay. In this way, the simple mixture of the sepiolite with PVA results in the formation of the nanocomposite.¹⁰⁶  Aqueous suspensions of them alone or incorporating other biocompatible polymers, such as PAA, can be directly submitted to freeze-drying to produce macroporous nanocomposites that can be explored as scaffolds in tissue regeneration applications.¹⁰⁷  However, the majority of the research related to nanocomposites generated by polymerization (or copolymerization) of acrylic acid and/or acrylamide and other related monomers incorporating sepiolite or palygorskite intends to develop superadsorbent materials. The strong hydrophilic character of fibrous clays favors their dispersion in hydrogels and imparts greater consistency to the generated systems, which results in improved water uptake properties, relevant in agriculture and horticulture, sanitary goods, drug–delivery systems or water remediation. Besides applications as water superabsorbents PAAm-based nanocomposites can be used in the removal of pollutants in water such as Pb(ii) and Cu(ii)^108,109  or dyes (e.g. methylene blue).¹¹⁰  The preparation of these nanocomposites usually employs crosslinking agents (e.g. N,N′-methylenebisacrylamide) that in the presence of an initiator (e.g. ammonium persulfate) produces the growth of PAA, PAAm or PAA–PAAm copolymers that become bonded to the silanol groups at the external surface of sepiolite and palygorskite.^111,112  The final water absorbency properties of the developed nanocomposites are strongly dependent on the amount of clay, crosslinker content, initiator dosage, nature of monomer or ratio of monomers in AA–AAm copolymer systems amongst other parameters of synthesis.^112–114  The use of modified clays has also been tested, for instance palygorskites treated with APTMS and methacryloxypropyltrimethoxysilane (MAPTMS) were simultaneously incorporated to AA, where the first clay-hybrid (APTMS–palygorskite) acted as initiator and the latter MAPTMS–palygorskite hybrid acted as crosslinker in the polymerization reaction.¹¹⁵  Microgels are obtained when the polymerization of organopalygorskites, e.g. modified with APTMS, occurs in the presence of an emulsifier and paraffin.¹⁰⁸  The presence of vinyl groups in certain organopalygorskites can be exploited to copolymerize with 2-acrylamido-2-methylpropane-sulfonic acid, giving rise to the formation of a nanocomposite network in which acrylamide can be further polymerized to produce a double network hydrogel with improved mechanical properties.¹¹⁶ 

The development of polymer–clay nanocomposites equipped with electrical and ionic conductivity was probably one of the first research fields regarding the incorporation of functionality with implications in the development of electrochemical and electroanalytical applications.¹¹⁷  Although smectites have mainly been used in the preparation of such nanocomposites, diverse examples of systems based on sepiolite and palygorskite fibrous clays have emerged lately. In the case of smectites, the presence of conducting polymers between the clay layers improves their stability and often determines anisotropy in the observed electrical behavior.^117,118  Thus, this section will focus on the description of the main features and the latest advances related to polymer–fibrous clay nanocomposites exhibiting ionic and/or electrical conductivity.

Polyaniline (PANI), polypyrrole (PPy) or poly(3,4-ethylenedioxythiophene) (PEDOT) are typical electron conducting polymers exhibiting characteristic semiconductor behavior. Although the majority of them are practically insoluble in most common solvents, they can be easily prepared from their corresponding unsaturated monomers (e.g. aniline, pyrrole, thiophene, isoprene…), which has been exploited to produce diverse examples of conducting polymer–fibrous clay nanocomposites.^119,120  The small size of these monomers may result in their incorporation into the structural tunnels of the clay, which may then act as templating pores for the growth of single chains of the corresponding conducting polymer. Thus, the formation of PPy–sepiolite nanocomposites by in situ polymerization of pyrrole has been reported, which after doping with I₂ vapors, introduces conducting properties to the nanocomposites. This conductivity has been explained by considering polaron and bipolaron mechanisms on the basis of ultraviolet-visible (UV-vis) and electron paramagnetic resonance (EPR) evidence.¹²¹  In the case of palygorskite, PPy polymerization in the interior of the tunnels does not seem as evident but PPy–palygorskite nanocomposites containing rhodamine B (RhB) have been reported.¹²²  The resulting nanocomposites show higher conductivity values and better electrochemical behavior when the RhB dye is incorporated, which has been interpreted considering the existence of π–π interactions and stacking between RhB molecules and the pyrrole rings.¹²²  This effect should be exploited and so the incorporation of chromophores in other PPy–fibrous clay nanocomposites could lead to the development of, for instance, novel photoconductor materials. PPy can also be in situ polymerized at the external surface of sepiolite fibers modified with alkylammonium-based surfactants (e.g. CTAB), giving rise to core–shell structures in which PPy encapsulates the clay fibers.¹²³  Interestingly, these nanocomposites exhibit sufficient conductivity, as well as good electrical double-layer capacitance and specific capacitance, to be explored as an alternative to pure PPy electrodes for supercapacitor devices.¹²³  Organopalygorskites modified with hexadecylpyridinium ions have also been used to produce PPy–palygorskite nanocomposites in which the conducting polymer encapsulates the clay fibers (Figure 1.7). These nanocomposites exhibit excellent electrical conductivity values (around 85 S cm⁻¹) as well as a higher thermal degradation temperature (>240 °C) after doping with HCl, which could be of interest in view of enlarging the field of applications of this type of nanocomposite to anti-corrosion coatings, shielding of electromagnetic interferences and other areas.¹²⁴ 

PPy–fibrous clay nanocomposites have also been evaluated as adsorbents in the removal of anionic pollutants, such as chromate ions, from industrial wastewater.^125,126  This property seems to be ascribed to the ion-exchange ability of the formed polymer, which after doping, results in the presence of PPy⁺X⁻ sites at the external surface of the clay. Alternative approaches intend to profit from the possibility of different associated active species on the fibrous clay that are later assembled to in situ electropolymerized PPy to produce drug delivery systems. In this way, PPy nanocomposites incorporating aspirin-loaded palygorskite were tested to produce a controlled aspirin release by using an external electrical stimulus.¹²⁷ 

PANI–fibrous clay nanocomposites have been prepared using diverse experimental approaches with the polymer assembled only on the external surface of palygorskite and sepiolite.^128,129  Here again, the role of the clay interface influences the growth and characteristics of the formed polymer. Thus, palygorskite submitted to an acid treatment to generate fresh silanol groups before modification with APTES notably improves the assembly of PANI, favored by the presence of the aminopropyl functions.¹²⁸  The electrical conductivity of these palygorskite–PANI nanocomposites may reach values of around 2.2 S cm⁻¹, which is higher than that of the pure PANI prepared under analogous experimental conditions. This result points to a favorable effect of the clay interface that favors the growth of the polymer in a more favorable conformation leading to PANI chains with a greater conjugation length, which results in enhanced conductivity of the nanocomposite.¹²⁸  Polymerization of PANI in the presence of sepiolite slurry results in nanocomposites in which PANI is adhered to the sepiolite surface, giving rise to interesting electrorheological properties in systems containing up to 50% of conducting polymer.¹²⁹  This type of conducting PANI–sepiolite nanocomposite has been explored due to its potential interest as an electrorheological system.¹³⁰  The electrorheological behavior of pure polyaniline compared to that of sepiolite coated with polyaniline has been ascribed to a higher aspect ratio of sepiolite fibers,¹³¹  which can be used as fillers to reinforce, for instance, as an alternative to the incorporation of magnetite-clay into epoxy resins with the aim of producing a specific orientation of the fibers before polymerization.⁹⁴  Poly(2,3-dimethylaniline) has been grown in the presence of palygorskite previously modified with APTES to produce nanocomposites that have been postulated as self-healing coatings with improved corrosion resistance.¹³²  In the same way, palygorskite modified with diverse types of cationic surfactants has been explored as a nanofiller of PANI–PVA based nanocomposite films. It was observed that the type of clay modifier strongly influences the conducting properties of the resulting materials.¹³³  Multifunctional nanocomposites have been prepared in a one-pot process using Fe(iii) salts that act as a precursor to Fe₃O₄ NP formation and oxidant for aniline polymerization in the presence of attapulgite.¹³⁴  The morphology (Figure 1.8) and magnetic properties of the nanocomposites can be tuned by controlling the molar ratio of aniline to Fe(iii). These superparamagnetic PANI–Fe₃O₄–palygorskite nanocomposites can be tested as adsorbents for the removal of dyes and the enrichment of Au(iii) from the solution, with the possibility of being easily removed from the solution with the help of a magnet (Figure 1.8). Interestingly, these nanocomposites also exhibit catalytic activity toward the catalytic reduction of 4-nitrophenol in the presence of NaBH₄.¹³⁴ 

Other polymers of great interest in electrochemical applications are those related to polyelectrolytes such as poly(ethylene oxide) (PEO) and nanocomposites based on layered clays that have been studied for a long time in view of developing ion-conductors for application as solid electrolytes of solid state batteries.^{117,118,135–138}  In those cases, the 2D structure of the inorganic host acted as the counter-ion, where the interlayer cations associated to the intercalated polymer are the only mobile species, i.e., the transport number of anions is practically zero (t− ≈ 0) in the resulting nanocomposites.^117,139  PEO has also been assembled to sepiolite, either from solution in acetonitrile or from melt by MW (microwave) irradiation, giving rise to nanocomposites in which a partial penetration of the polymer chains takes place.^24,117  More recently, diverse studies have been reported focusing on aspects such as the control and modulation of the silicate interphase in order to understand diverse physical characteristics of PEO–sepiolite nanocomposites.¹⁴⁰  Sepiolite modified with various types of PEG and other additives such as vitamin E, tocopherol or polyethylene glycol succinate (TPGS) has been tested as a mechanical reinforcer of PEO in the preparation of ethylene carbonate (EC) containing PEG and PEO blends, together with lithium triflate (LiTf) salt, in the preparation of liquid and solid nanocomposite polymer electrolytes.¹⁴¹  From this study, it is established that TPGS–sepiolite disperses better in EC–PEG mixtures, being completely stable for weeks or even longer periods of time, in contrast to neat sepiolite or PEG–sepiolite which sediment progressively under the same conditions. Sepiolite-based nanocomposites prepared with an EC : PEO weight ratio of around 1 present ionic conductivity close to 10⁻³ S cm⁻¹ at 30 °C, which is in the order of that reported for liquid electrolytes.¹⁴²  Moreover, ionic conductivity on these nanocomposites varies in the range of 10⁻⁶ to 10⁻⁴ S cm⁻¹ at 30 °C with increasing concentration of EC and LiTf, being close to that of liquid polyelectrolytes, while they show solid-like mechanical properties above the melting point of PEO.¹⁴³  This combination of properties appears very promising in view of developing new clay-based solid polymer electrolytes.

Nanocomposites based on fibrous clay showing proton conductivity have also been explored as conducting membranes for fuel-cell applications. For this purpose, Nafion® membranes containing neat sepiolite or sepiolite modified with sulfonic groups have been tested and showed enhanced properties when compared to the pure polymer, especially when working at low relative humidity and high temperature.¹⁴⁴  Conducting membranes based on sulfonated poly(ether sulfone) containing palygorskite and phosphotungstic acid have also been evaluated for application in direct methanol fuel cells (DMFCs).¹⁴⁵  Membranes containing 10% of palygorskite show very attractive properties, reducing methanol permeability by up to 25% that of Nafion® membranes, with proton conductivity values of around 3 × 10⁻² S cm⁻¹, although a drop of the tensile strength has been observed. Anyway, the study confirms that mechanical properties still remain adequate for palygorskite loadings up to 15%.¹⁴⁵  Palygorskite modified with quaternary ammonium has been assembled to bromide polyphenyl ether (PPO) to produce nanocomposite membranes for application in alkaline fuel cells. These membranes exhibit anion-exchange properties (1.07 meq g⁻¹), good thermal and chemical stability as well as excellent mechanical properties in the hydrated state, with a conductivity ranging from 1.6 × 10⁻² S cm⁻¹ at 20 °C to 2.1 × 10⁻² S cm⁻¹ at 80 °C.¹⁴⁶ 

Certain polymer–fibrous clay nanocomposites have been explored as a precursor of conducting materials by controlled thermal transformation of the involved polymer. For instance, polyacrylonitrile (PAN) produced inside the sepiolite tunnels by in situ polymerization of acrylonitrile monomers can be further transformed into a conducting carbon by treatment at temperatures of around 700 °C in an inert atmosphere.^147,148  The produced carbon–sepiolite nanocomposites show electrical conductivity that has been attributed to the presence of graphene-like species that remain strongly associated with the silicate substrate, similar to that occurring to PAN intercalated into montmorillonite.¹⁴⁹  The formed carbonaceous material can be removed from the nanocomposite by its successive treatment with HCl and HF acids, although the carbon–sepiolite nanocomposites can be directly employed as an electrode in secondary battery electrodes and supercapacitor electrochemical devices.^117,150  Interestingly, the carbon recovered from sepiolite-based nanocomposites, compared to that templated on smectites, shows better properties for being used as an electrode in rechargeable Li-batteries.¹⁵¹ 

Fillers are introduced in a continuous matrix with the aim of improving a given property of a material.¹⁵²  Relevant examples of this behavior are the improvement of mechanical properties, the increase of thermal stability or the flame retardancy, and the enhancement of the gas–vapor barrier properties of biopolymers. Such properties, together with the biodegradable and biocompatible character of biopolymer compounds, make these bionanocomposites based on sepiolite and palygorskite susceptible materials for diverse applications, e.g. drug delivery, environmental remediation, bioplastics, packaging and other food contact surfaces.

Due to their biodegradable character when composted industrially, biopolyesters such as poly(lactic acid) (PLA) or polycaprolactone (PCL) are ecological alternatives to conventional polymers in the production of bioplastic materials. Nevertheless, the use of these compounds as main raw materials for bioplastic is still limited, mainly due to their brittleness, slow crystallization rate and high gas permeability.¹⁵³  In order to overcome these problems, clay minerals, including sepiolite and palygorskite clays, have been employed as a nanofiller in PLA and PCL matrices. Within this perspective, PLA and PCL films loaded with sepiolite clay showed a thermal–mechanical improvement, which is related to the good compatibility with biodegradable polyester matrices.¹⁵⁴  The control of the biodegradation rate of both PLA and PCL films reinforced with sepiolite was also investigated, and the results obtained indicated a significant level of degradation in compost at 40 °C, together with evidence of erosion after 35 days.^14,154  On the other hand, hydrolytic degradation studies suggest that the presence of sepiolite favors a delay in degradation of the PLA matrix at 37 °C, likely due to the increase of the PLA crystallization effect induced by the clay mineral.¹⁵  Based on this fact, Russo and co-authors¹⁵⁵  have studied the influence of sepiolite on the crystallinity of the PLA, and according to the authors, the presence of sepiolite not only could modify the crystallinity of the PLA, but also influence both gas barrier and mechanical properties of the resulting PLA–sepiolite films.

Bionanocomposites based on PLA and palygorskite prepared via melt blending were studied by Jiang and co-authors for application in agricultural packaging.¹⁵⁶  In addition to significant enhancements in tensile properties, which are likely occasioned by the good integration of palygoskite fibrils in the PLA matrix as revealed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images, spectroscopy measurements such as Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) indicated interfacial interactions through hydrogen bonds between palygorskite and PLA. Alternatively, previous surface modification of fibrous clays by silane compounds was the strategy employed by Moazeni et al.¹⁵⁷  Although the addition of modified sepiolite into PLA resulted in a reduction in the elongation at break, noteworthy increases of tensile modulus and oxygen and carbon dioxide barrier properties were evidenced, showing in both cases the best results for the PLA film that incorporates 1.5 wt% silane treated sepiolite. According to the authors, this behavior is due to better dispersion and interaction of the sepiolite nanofiller within the PLA matrix, which are evidenced by SEM and spectroscopy analyses.

The capacity of sepiolite and palygorskite to act as a nucleating agent in PLA or in other biodegradable polyesters was recently explored by several authors.^158–160  In the case of PLA, the crystallinity, nucleation density and the G constant for a specific crystallization temperature, were significantly enhanced in the presence of 1.0 wt% sepiolite, whilst a reduction of crystallization time from 17.7 to 2.1 min at the optimum crystallization temperature (110 °C) was evidenced. On the other hand, FTIR and WAXD results revealed that the crystal structure of PLA–sepiolite samples adopted a common α-form, which indicates that the introduction of sepiolite in the PLA matrix did not induce the formation of a new crystal structure.¹⁶⁰  Alternatively, biodegradable materials prepared from a combination of organo-palygorskite and poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) can also be of great interest in the plastics industry. Recent studies¹⁵⁸  demonstrated that PHBV films loaded with organophilic palygorskite showed a decrease of the thermal stability due to the presence of quaternary ammonium compounds used as modifier agents in the clay. Importantly, however, an improvement of the crystallization ability of PHBV was evidenced, which can be attributed to a nucleation effect of the clay in the polymer matrix during the non-isothermal crystallization process. Palygorskite clay was also used as a nucleating agent and filler in poly(butylene succinate) (PBS) using 1,2-octanediol as comonomer to synthesize branched PBS copolymers.¹⁵⁹  Besides the improved thermal stability and the crystallization temperature of the PBS matrix, which was promoted by the presence of palygorskite fibrils, the role of the filler agent was also evidenced, where clay-reinforced branched PBS containing 3 wt% of silicate in PBS matrix resulted in break elongation values of 550%. All these salient features are due to homogeneous dispersion of palygorskite fibrils in the PBS matrix, which results from strong hydrogen bonding interactions established between the components.

Polysaccharides are composed of sugar units joined by glycosidic linkages. These polymeric carbohydrates, the most abundant polymers in Nature, are emerging as a very attractive alternative to replace synthetic polymers in the development of nanocomposite materials. Thus, the resulting bionanocomposites show biocompatibility and biodegradability, together with low cost and positive ecological impact.

Due to abundance, biodegradable and biocompatible properties, hierarchical structure and good film-forming ability, the polysaccharides are attractive matrices for the development of “green” nanocomposites. Although the layered clays such as montmorillonite are inorganic solids traditionally employed as filler in polymeric matrices, the use of fibrous clays (sepiolite and palygorskite) as promising reinforcing agents of biopolymers in the development of bionanocomposite materials has emerged in recent years. In this sense, it is worth mentioning the pioneering work reported by Lynch and co-authors,¹⁶¹  who studied the effect of palygorskite fibrils in the degradation of a cellulose dextrin matrix in the presence of soil suspensions containing microorganisms. Thenceforward, studies on the properties and applications resulting from the assembling of both types of natural components, microfibrous clays and biopolymers, began to explore the development of new ecological nanocomposites provided with structural and/or functional properties.

Although it is well known that cellulose is the most abundant renewable and biodegradable natural polymer on Earth, the investigation focusing on cellulose films reinforced with fibrous clays is still recent. In contrast to the major polysaccharides, cellulose shows low solubility in water or other common solvents as a result of its strong inter- and intra-molecular hydrogen bonds, which causes a certain limitation in the preparation of cellulose bionanocomposites based on fibrous clay minerals. To overcome this problem, new synthetic strategies were employed, such as for example, the use of ionic liquids as solvent.^162,163  Ionic liquids are considered “green” solvents and have attracted a great deal of scientific attention, mainly due to their unique physicochemical properties such as chemical and thermal stability, non-flammability, immeasurably low vapor pressure and recyclability.¹⁶³  Thereby, using a simple, cost effective and “green” method, novel regenerated cellulose–sepiolite bionanocomposite films have been prepared using the environmentally friendly ionic liquid 1-butyl-3-methylimidazolium chloride (BMIMCl) as main solvent.¹⁶⁴  The results obtained from physicochemical studies showed that interactions between sepiolite and regenerated cellulose were established, where images from field-emission scanning electron microscopy (FE-SEM) and TEM revealed that the sepiolite fibers are uniformly dispersed in the cellulose matrix with contact between them. Mechanical properties and thermal stability of the bionanocomposite films were significantly improved when compared to pure cellulose films. In the case of the mechanical properties, the Young’s modulus of bionanocomposite films with 8 wt% sepiolite increased by 166% compared to pristine cellulose films. Interestingly, in contrast to other biopolymer films that incorporate fibrous clays as filler, cellulose–sepiolite films showed an improvement of the ductility when sepiolite was added, where the incorporation of 6 wt% sepiolite resulted in an increase of 35% elongation at break, maintained nearly constant for higher sepiolite loadings. The incorporation of sepiolite in the regenerated cellulose matrix was also reflected in the water resistance and gas barrier properties. Thus, O₂ permeability values decreased gradually with the increasing sepiolite content, reaching values of about 56% of the initial value by addition of 8 wt% sepiolite. All the properties shown by cellulose–sepiolite bionanocomposite films make these materials promising for application in the areas of biomaterials, membranes and food packaging.

The affinity of sepiolite and palygorskite towards cellulose derivatives, such as carboxymethylcellulose (CMC) and hydroxypropylmethylcellulose (HPMC), in the development of bionanocomposite films for packaging applications was also evaluated. On this occasion, self-standing films loaded with different amounts of fibrous clays not only exhibited enhanced mechanical properties and water resistance, but also significant barrier properties against UV light (Figure 1.9) and reduced water absorption capacity, which make these bionanocomposites very attractive as bioplastics in the food packaging sector.¹⁹  These properties are a consequence of the strong interaction by hydrogen bonding between the hydroxyl groups of the biopolymers and the silanol groups located on the surface of the fibrous silicates. Xylan-type hemicelluloses, such as arabinoxylan, are also interesting candidates in the preparation of bionanocomposites based on sepiolite.¹⁶⁵  In addition to a significant increase in the Young’s modulus and tensile strength exhibited by arabinoxylan–sepiolite films compared to those previously reported for xylan–layered clay systems,¹⁶⁶  the drawback of the high water uptake typical of hemicellulose matrices was overcome by the introduction of sepiolite clay in the biopolymer film. Thus, materials become more resistant to water and are promising for use in the food sector.

Besides cellulose, interesting structural and functional properties in bionanocomposites based on fibrous clays were also observed with other abundant polysaccharides like chitosan. In this case, Darder and co-authors observed that the assembly of positively charged chitosan to sepiolite produced a clear reinforcing effect in the polymer film, which derived mainly from the good distribution of sepiolite fibrils in the chitosan matrix, originating from electrostatic interactions between the charged amine groups of the biopolymer and silanol groups of the silicate as well as from hydrogen bonding interactions.¹⁶⁷  In addition to excellent mechanical properties, these chitosan–sepiolite bionanocomposite films showed interesting results as membranes for N₂ separation, being an attractive material in membrane processes.¹⁶⁸ 

Since the main goal in the use of fibrous silicates as nanofiller materials in polymer matrices is related to providing improved mechanical properties, this reinforcement strategy was employed by different authors in order to enhance the elastic modulus of polymer blends. Thus, a series of reinforced nanocomposite films prepared by a solution casting method using sepiolite¹⁶⁹  and palygorskite¹⁷⁰  as filler in chitosan–PVA blends was prepared. In these studies, the authors indicated that, due to good compatibility occasioned by the interactions through hydrogen bonds between hydroxyl groups of the polymer matrix and the silicate, the incorporation of fibrous clays can enhance the mechanical and barrier properties and water resistance of bionanocomposites compared to pristine blend films, favoring their application as a bioplastic in the food sector. Although the incorporation of sepiolite or palygorskite in chitosan–PVA matrices seems to be well studied, assorted synthesis strategies have recently been employed in order to modulate the crystallization and mechanical behaviors of the resulting bionanocomposite film. Within this context, once the chitosan–PVA films loaded with palygorskite were prepared, Lu et al.¹⁷¹  adopted a post treatment by immersion of these films in a solution of NaOH and ethanol to control the crystallization degree of the polymer chains. In this work, it was evidenced that the hybrid film treated with NaOH and ethanol solutions exhibited good mechanical performance, likely due to higher crystallinity of the NaOH–ethanol treated films. On the one hand, this effect can be attributed to a decrease of the solubility of chitosan by a consequent neutralization effect of the acetic acid used in the solubilization of chitosan by NaOH solution. Otherwise, the films treated by ethanol showed different crystallinities, suggesting that the mechanical properties of these bionanocomposite films could be controlled by altering the dehydration process occasioned by the treatment with ethanol.

In recent years, Avérous and co-authors^12,172  developed different synthetic approaches in order to obtain bionanocomposite films based on thermoplastic starch and fibrous clays. One of the strategies employed in this case was the surface modification of sepiolite by a cationic starch, achieving bio-organosepiolites which were used as filler in starch matrices. The compatibility provided by the assembly of cationic starch to sepiolite produced a considerable improvement of the mechanical properties in the starch bionanocomposite films, increasing the Young’s modulus by a factor of up to 2.5 as compared to those analogous materials based on montmorillonite.¹²  Surface modification of sepiolite was also the strategy employed by Samper-Madrigal et al.¹⁷³  in studies addressing the combination of hydrophilic thermoplastic starch and hydrophobic “green” PE obtained from bioethanol derived from sugarcane. In order to compatibilize the system, pristine sepiolite was modified with hydrophobic groups from the silane (PTMS) resulting in a filler with dual functionality, where modified sepiolite is able to act as a bridge between hydrophobic PE chains and hydrophilic plasticized starch chains. The usefulness of silane-treated sepiolite as a filler in starch–PE blends was corroborated by a significant increase in the elongation at break of films from 90% (uncompatibilized starch–PE blend) up to values of 260% in the case of starch–PE films reinforced with 3 and 5 wt% of modified sepiolite.

Recently, a study focused on the incorporation of nano-sized sepiolite clay into thermoplastic starch–poly(butyrate adipate terephthalate) (PBAT) blends. According to the authors,¹⁷²  the addition of only 3 wt% sepiolite resulted in an important enhancement of the mechanical properties of starch–PBAT films, reaching up to 300% and 150% increase in the Young’s modulus and tensile strength, respectively. These results confirm that fibrous clays can act as a filler, representing an interesting route to produce more resistant polysaccharide-based films.

This type of bionanocomposite involving polysaccharides and fibrous clays can also be processed as cellular solids¹⁷⁴  aimed at applications in insulation and cushioning. The available techniques for preparation of porous materials include phase separation/emulsion freeze-drying, solvent casting and particle leaching, gas foaming or fiber knitting, freeze-drying and supercritical drying. Several bionanocomposites based on smectites were prepared by supercritical drying with CO₂ ¹⁷⁵  or by foaming in a heated press in the presence of a blowing agent,¹⁷⁶  and these technologies could be analogously extended to the development of fibrous clay-based foams. However, the most commonly employed technique for processing polysaccharide-based materials as cellular solids is freeze-drying, due to the use of water as solvent in the preparation of these bionanocomposites.¹⁷⁷  This technique involves an ice-templating process in which the solid fraction in the aqueous suspension is pushed away towards the voids between adjacent crystals as the ice crystals grow. Finally, sublimation of the ice under vacuum leads to the generation of the porous structure. As a result of this process, low density materials exhibiting macroporosity are produced, and diverse examples of polysaccharide–fibrous clay-based materials have been reported in recent years.¹⁷⁸  For instance, cellular materials involving sepiolite or palygorskite and a wide variety of polysaccharides (alginate, starch, carrageenan, locust bean gum or xanthan gum) showed macropores with an average diameter around 100 µm and apparent density values below 0.20 g cm⁻³. However, they offered good mechanical resistance and those samples with a sepiolite content around 50–75 wt% reached values of compression modulus of about 40 MPa.¹⁷⁹  The presence of the fibrous clays can help to increase the fire retardancy of the resulting bionanocomposite foams (Figure 1.10), and it was reported that some polysaccharide–sepiolite materials with a clay content higher than 25% avoid the propagation of flames, thus behaving as auto-extinguishable foams that could replace polyurethane and analogue foams as building materials.¹⁸⁰  Unidirectional freezing leading to well-aligned microchannels in the freezing direction was applied in a recent work in order to obtain anisotropic foams with thermal insulation and fire retardancy properties from a mixture of nanocellulose fibers, graphene oxide and sepiolite.¹⁸¹  The resulting ultra-lightweight materials showed a lower thermal conductivity in the radial direction than conventionally used insulation materials like polyurethane, good mechanical properties and moisture resistance, as well as fire retardant behavior, avoiding the self-propagation of the flame in a vertical burning test.

Another application of polysaccharide-based bionanocomposites is their use as low cost biosorbents in environmental remediation.¹⁸²  Several examples of polysaccharide–fibrous clay materials applied for the removal of pollutants can be found in the literature. For this purpose, a wide variety of charged and neutral polysaccharides have been evaluated. The recently discovered polysaccharide sacran, an anionic megamolecule extracted from the cyanobacterium Aphanothece sacrum, shows a great affinity towards heavy metal and rare earth ions.¹⁸³  Such a property was exploited for the development of biosorbent materials with increased stability in water and the ability to retain neodymium ions selectively.¹⁸⁴  For this purpose, the assembly of sacran with sepiolite fibers was carried out in the optimal conditions that lead to the organization of sacran chains as liquid crystals within the bionanocomposite, which seems to favor the adsorption of neodymium in comparison to other lanthanide ions (Figure 1.11).

Bionanocomposites based on the assembly of sepiolite or palygorskite with alginate, chitosan and starch were tested in the uptake of copper and lead from aqueous solution.¹⁸⁵  All these materials showed better mechanical properties and enhanced stability in water than the pristine polymers, and the best results were obtained for alginate-based systems in the case of lead uptake and for chitosan-based materials in the removal of copper ions. Chitosan–palygorskite bionanocomposites were also applied in the removal of Fe(iii) and Cr(iii) cations by a complexation mechanism, showing a higher sorption capacity than the individual components.¹⁸⁶  The removal of uranium from aqueous solutions is also of great interest. The same chitosan–palygorskite biosorbent¹⁸⁷  retained uranium ions based on the interaction of U(vi) cations with the –OH and –NH₂ groups of chitosan. The optimal adsorption takes place at pH 5.5, when the competing effects with other charged species at extreme pH values are minimized. In other examples, the ion-imprinting concept was used in the production of biosorbents for selective removal of heavy metal ions. Thus, chitosan, previously mixed with a solution of a Co(ii) salt, was combined with γ-glycidoxypropyltrimethoxysilane and palygorskite in order to produce an ion-imprinted material with selectivity towards Co(ii) ions.¹⁸⁸  In a recent work, a similar Cu(ii)-imprinted material was reported, based on chitosan, palygorskite and ethylene glycol dimethacrylate (EGDMA),¹⁸⁹  which showed a selective adsorption of copper ions.

The association of chitosan to fibrous clays takes place mainly by hydrogen bonding interaction between the hydroxyl groups on the biopolymer and the silanol groups located at the external surface of the silicate fibers, and in a minor extent through electrostatic interaction due to the protonated amino groups of chitosan.¹⁶⁷  Due to the low cation-exchange capacity of sepiolite and palygorskite (<25 mEq 100 g⁻¹)²⁰  to compensate the protonated amino groups of chitosan, the resulting bionanocomposites usually show anionic-exchange ability due to the excess of surface positive charges, thus being useful for the removal of diverse anionic pollutants. For instance, a chitosan–palygorskite biosorbent was successful in the removal of tannic acid from drinking water by electrostatic interaction, together with hydrogen bonding and van der Waals interactions established with the uncharged parts of the polysaccharide.¹⁹⁰  A similar system was prepared for the same purpose, but in this case an emulsified chitosan–palygorskite mixture was processed as microspheres using formaldehyde and glutaraldehyde as crosslinking agents.¹⁹¹  Other chitosan-based bionanocomposites were applied in the removal of anionic dyes like reactive yellow 3RS, such as for instance, the material prepared with sepiolite fibers modified with APTES and chitosan, which were crosslinked with glutaraldehyde.¹⁹² 

In a different type of application, the affinity of chitosan–sepiolite and chitosan–palygorskite materials towards anionic dyes like Acid Yellow 25 or chrysophenine G, among others, was exploited to create a photoprotective matrix for Beauveria bassiana conidia, which was used as a microbial biocontrol agent.¹⁹³  The biocompatibility of polysaccharide–fibrous clay materials allows their application as a support for living species, such as for instance, microalgal cells that were able to colonize chitosan–sepiolite bionanocomposites processed as foams.¹⁹⁴  Similarly, a recent work described the immobilization of Pseudomonas sp. DG17 in sodium alginate–palygorskite materials,¹⁹⁵  and the resulting system was successfully applied in petroleum hydrocarbon biodegradation.

Polysaccharide–fibrous clay bionanocomposites can also be applied to treatment of wastewater effluents by means of coagulation–flocculation processes. A recent example of such applications is the use of chitosan–sepiolite materials for clarification of winery wastewater by neutralization of the colloids with the highly charged bionanocomposites, as well as of olive mill wastewater by van der Waals or OH interactions between specific organic colloids and the glucosamine units of the chitosan in low charge nanocomposites.¹⁹⁶  Similarly, chitosan–sepiolite bionanocomposites were used as flocculants to remove harmful microalgal cells from the Taihu Lake, showing a good removal efficiency with a reduced competing effect from humic acids.¹⁹⁷ 

Several multifunctional materials involving polysaccharides and previously modified fibrous clays have been reported. For instance, palygorskite fibers can be modified with superparamagnetic NPs and then coated by chitosan and cysteine modified β-cyclodextrin deposited by means of a layer-by-layer process.¹⁹⁸  The resulting multifunctional bionanocomposite was applied to the recovery of precious metal ions, showing a preferential adsorption of Ag⁺ and Pd²⁺ over Pt⁴⁺. Wang et al.¹⁹⁹  reported the preparation of a bionanocomposite material involving Au NPs assembled by electrostatic interaction to a chitosan layer that covers Fe₃O₄-modified palygorskite. The catalytic activity of this material was exploited in the decoloration of Congo red solutions. In both examples, the superparamagnetic properties of the materials are helpful to facilitate the recovery of the bionanocomposites by means of a magnet.

Another type of multifunctional bionanocomposite involves the combination of the polysaccharides and the fibrous clays with PAA or PAAm. This additional polymer confers superabsorbent properties to the final system that can be of benefit for a wide variety of applications. Amongst them, some of these bionanocomposite hydrogels are devoted to agriculture and horticulture, for instance for the controlled release of nutrients. Such fertilizer granules were prepared by coating a mixture of alginate, palygorskite, urea and KH₂PO₄ with an alginate-g-PAA-co-PAAm–humic acid hydrogel.²⁰⁰  In another recent example, the carboxymethyl chitosan-g-PAA–attapulgite composite was used as an outer coating of a complex system entrapping co-granulated ammonium zinc phosphate and urea as fertilizer core.²⁰¹  In these examples, the goal is the reduction of environmental pollution due to the excessive use of fertilizers, in a similar way to other bionanocomposites based on alginate and palygorskite and processed as microbeads that were used for the controlled release of the organophosphate insecticide chlorpyrifos in order to minimize the adverse environmental impact.²⁰² 

Another interesting application of this type of hydrogel system is related to their use as sorbents in environmental remediation, with special interest in the removal of heavy metal ions. For instance, CMC-g-PAA–palygorskite systems were applied as adsorbents of Pb(ii),²⁰³  chitosan-g-PAA–palygorskite was useful in the removal of Hg(ii)²⁰⁴  and, more recently, alginate-g-poly(acrylic acid-co-2-acrylamido-2-methyl-1-propane sulfonic acid)–attapulgite was successfully applied in the uptake of Pb(ii), Cd(ii), Cu(ii) and Zn(ii),²⁰⁵  while the hydroxypropyl cellulose-g-PAA–palygorskite hydrogel was an effective sorbent for the recovery of rare earth elements like La(iii) and Ce(iii).²⁰⁶  There is also great interest in the removal of ammonium nitrogen (NH₄⁺–N), which is a primary indicator of water quality²⁰⁷  and also of dyes like crystal violet, which was effectively sorbed on a κ-carrageenan-g-PAAm–sepiolite,²⁰⁸  as well as methylene blue²⁰⁹  or malachite green,²¹⁰  which were removed making use of materials involving chitosan-g-PAA and palygorskite fibers processed as beads (Figure 1.12a). The presence of palygorskite can help to enhance the adsorption kinetics, as shown in the removal of malachite green (Figure 1.12b).

In some applications, polysaccharide–fibrous clay bionanocomposites have been used as intermediates in the preparation of adsorbent carbonaceous materials. Several recent examples report the use of chitosan^211,212  and cellulose^213,214  as low cost raw materials in combination with palygorskite. For instance, chitosan was transformed into carbon by calcination at temperatures above 280 °C,²¹¹  which gave rise to larger pores and increased the adsorption sites, being successfully applied in the bleaching of palm oil. Alternatively, chitosan²¹²  or cellulose^213,214  can be submitted to a hydrothermal carbonization process at lower temperatures, below 250 °C, resulting in carbon–silicate materials that were effective in the removal of methylene blue or phenol from aqueous solution. Similarly, caramel–sepiolite materials derived from the disaccharide sucrose were transformed at around 800 °C into conducting carbon–sepiolite nanocomposites, useful as electrode materials after functionalization with appropriate organoalkoxysilanes.²¹⁵  The covalent grafting of thiol groups results in electrochemical sensors that could be applied in the amperometric determination of heavy metal ions in aqueous solution.²¹⁶ 

Nanoparticulated materials have been receiving enormous interest for biomedical applications such as drug delivery due to the high loading capacities of pharmacological molecules related with their large specific surface area and the possibility to direct these particles to the required site.^217–219  The combination of nanomaterials such as clays with biopolymers (e.g. polysaccharides) offers several advantages: (i) control over the drug release profile, (ii) control over carrier assembly and its microstructure and (iii) improved uptake.^220,221 

Recently, Wu et al.²²²  prepared emulsion crosslinked microbeads of chitosan and organo-palygorskite. The clay was modified with the amphoteric surfactant hexadecyl betaine (BS-16) to improve the compatibility and affinity with the chitosan matrix. Furthermore, the incorporated clay increased the encapsulation of diclofenac sodium (DS) and slowed down the drug release kinetics. A similar delivery system was reported by Wang et al.,²²³  who prepared hydrogel microbeads composed of chitosan-g-poly(acrylic acid)–attapulgite–sodium alginate (CTS-g-PAA–APT–SA) suitable for pH-sensitive delivery of DS. The model drug was loaded into CTS-g-PAA–APT microparticles and, subsequently, the microparticles were confined in Ca²⁺-crosslinked sodium alginate hydrogel beads. This configuration prolongs the drug diffusion paths and, hence, retards the drug release. Importantly, the drug can be released by changing the pH of the medium. This ensures that the release of DS in gastric fluid (acidic pH) is avoided, but allows for its complete and controlled delivery in intestinal fluid (neutral pH). In acidic conditions, hydrogen bonding between the polyelectrolytes and the clay results in non-swollen microbeads, which greatly limits the migration of DS. Contrarily, at a pH above 5.5 both the clay and sodium alginate are negatively charged, which results in electrostatic repulsion, in extensive swelling and, consequently, in pronounced release of DS. In another work, pH-sensitive microbeads were prepared by spray-drying of glutaraldehyde crosslinked chitosan–attapulgite gels.²²⁴  The microbeads allowed the controlled release of DS in the simulated intestinal fluid. The presence of attapulgite has also been observed to favor the production of small and uniform microbeads of narrow size distribution around 5 µm. The same authors have also proposed composite hydrogel beads composed of carboxymethyl cellulose-g-poly(acrylic acid)–attapulgite–sodium alginate (CMC-g-PAA/APT/SA) as a pH-sensitive release system for DS.²²⁵  The bionanocomposite was mixed with the model drug and dropped into Ca²⁺ crosslinking solution to obtain the desired nanocomposite beads. A clay content of up to 20 wt% ensured reduced swellability, and thus, prevented undesired burst release of the drug. The cumulative release ratio of DS from the hydrogel beads in simulated gastric fluid was only 3.7% within 3 h, but in simulated intestinal fluid was about 50% for 3 h and up to 90% after 24 h.

Needle-free, nasal vaccine administration²²⁶  is another field where clay–polysaccharide materials can make an important contribution as adjuvants. An interesting aspect of using microfibrous clays such as sepiolite as vaccine adjuvants is connected with their needle-like morphology that is thought to enhance mucosal immune responses provoked by irritation of the nasal mucous through the fibers.²²⁷  This can help to improve the efficacy of vaccination and thus reduce the dose necessary for immunization. However, the bare surface of inorganic carriers can cause adsorption-induced alterations of the protein structure of antigens provoked by strong electrostatic interactions. This can result in diminished immunogenicity of the antigen, as observed for pristine sepiolite²²⁷  and sometimes even in the case of standard aluminum containing adjuvants.^228,229  Therefore, sepiolite was modified with the biopolymer xanthan gum to mimic the surface of nasal mucous, the natural adsorption site of the influenza virus during entry into an organism.²²⁷  Mimicking this environment in terms of surface charge, water retention and functional sugar groups is considered favorable for the interaction between carrier and virion and thus determining the preservation of immunogenic activity of the antigen. Influenza virus particles of the strain PR/8(H1N1) were immobilized on sepiolite–xanthan and elicited high levels of seroprotection in a balb/c mouse model.²²⁷ 

Proteins are biopolymers composed of amino acid units that are linked by peptide bonds to form polypeptide chains. They can be obtained from plants, animals or bacteria and can be classified as fibrous, globular or membrane proteins depending on their shape and solubility. Proteins have been used in the preparation of nanocomposite materials, although in a lesser extent than polysaccharides. Common non-food applications of proteins include adhesives, glues, paper coatings, paints, textile fibers, as well as molded plastic items.

Among the diverse bionanocomposites, maybe the most interesting are those derived from proteins. These biomacromolecules have become very attractive due the variety of functional groups present on their peptide chain, which may result in interaction points with inorganic solids and thus provide the resulting bionanocomposite materials with diversified structural and functional properties.

Analogously to carbohydrates, various routes and strategies of synthesis were explored using fibrous clays as filler in order to improve the water resistance and mechanical properties of protein-based films. In this sense, films based on gelatin, a structural protein, were reinforced with sepiolite clay, varying the sepiolite content between 0.05 and 0.50 mass fraction.¹³  The most salient feature of these gelatin–sepiolite films was the substantial improvement of the elastic modulus, which showed an increase of up to 250% at 50 wt% clay loading compared to unmodified gelatin films. This is a consequence of interactions established between the sepiolite and the available functional groups of gelatin.^13,230  More recently, Fernandes et al.²³⁰  evaluated the mechanical properties of gelatin films, as function of the crystallinity of sepiolite. Interestingly, the changes in the nature of the silicate induced by thermal treatments resulted in different values of the Young’s modulus, where elastic modulus increased up to 220% at 20% clay loading using raw sepiolite, while no improvement was found at 20% clay loading using the protoenstatite form. Bionanocomposite films resulting from the assembly of type I collagen, another structural protein, to palygorskite were evaluated by Su and co-authors.²³¹  In addition to enhanced mechanical properties in comparison with neat collagen films, it was evidenced that the increase of palygorskite content in collagen–palygorskite films contributes to better thermal behavior. In this case, although spectroscopic studies point out that the presence of palygorskite in a collagen matrix causes a contraction and aggregation of the collagen chains, the triple-helix backbone of the protein is preserved throughout the synthesis process.

The role of sepiolite fibers in the release of bioactive compounds from gelatin–egg white films containing clove essential oil was investigated by Giménez et al.²³²  Although an improvement of the Young's modulus from the introduction of the clay mineral was evidenced, the presence of clove essential oil in the nanocomposite films promoted a slight decrease in the water vapor permeability (WVP), this latter compound acting as a plasticizer of the system. Furthermore, the release of protein components and eugenol from the matrix was promoted by the presence of sepiolite in gelatin–egg white films. This contributed to the antioxidant and antimicrobial activity of the films, which makes them very attractive as bioactive packaging for the food sector.

Another interesting protein for the production of reinforced films is zein, a storage protein extracted from corn. Due to non-polar amino acid residues, zein shows a relatively strong hydrophobic character, which makes it a very attractive biopolymer in the preparation of moisture resistant materials. In this context, Alcântara et al.^19,233–235  have reported the assembly of zein to sepiolite and palygorskite giving rise to new bio-organohybrids that can be used as a “green” alternative for the development of clay hybrids for food packaging. The incorporation of zein–fibrous clays as a biofiller in alginate films produced a reinforcing effect, helping to overcome the drawback related with the low water resistance of the alginate matrix. Moreover, these reinforced alginate films showed improved water vapor barrier properties and gas permeation toward CO₂, N₂, He and O₂, even at high humidity conditions.¹⁹  It was observed that oxygen permeability and water uptake are significantly influenced by the total zein content in the alginate matrix, reducing the values of both properties, which is an advantageous feature that would allow the application of these materials in the food packaging sector (Figure 1.13).

Pristine palygorskite clay was also used as a nanofiller in wheat gluten, another storage protein. In this study, carried out by Yuan et al.,²³⁶  the structure and biodegradable properties of transparent thermos-pressed films were studied. Thanks to the good dispersion of palygorskite particles into the wheat gluten matrix, a significant enhancement of the mechanical and viscoelastic properties was observed, where the best results were found for those bionanocomposite films loaded with 7 wt% of clay. Furthermore, an increase in the biodegradability due the presence of palygorskite was evidenced, leading to the disaggregation and dwindling of wheat gluten–palygorskite films after 15 days of burial.

Recently, several works reporting the use of soybean protein as organic counterpart in the preparation of hybrid films have emerged.^237–239  In most cases, this class of protein has been associated with unmodified and silylated palygorskite clay resulting in homogeneous nanocomposite films. For instance, Wang and co-authors^238,239  investigated the structure and loading effects of raw and silylated palygorskite on the properties of soy polyol-derived polyurethane nanocomposites. In addition to an increase of the glass transition temperature, the mechanical properties were strongly influenced by the presence of palygorskite fibrils, where at 12 wt% pristine palygorskite and silylated palygorskite loadings, a 223% and 100% increase in tensile strength and 455% and 182% increase in Young’s modulus were obtained, respectively. These new nanocomposites represent a promising alternative in preparing environmentally friendly materials with excellent structural properties for practical applications.

Together with smectites, kaolinite, talc and other phyllosilicates, fibrous clays are also widely used as excipients²⁴⁰  and even as active ingredients²⁴¹  in both the pharmaceutical and the cosmetic industries. As excipients, sepiolite and palygorskite are used for diverse purposes as thickening and anticaking agent, flavor corrector or carrier–releaser of active principles, while for their use as active ingredients they can be orally administered as antacid, gastrointestinal protector or anti-diarrheal drugs. Another application of palygorskite in the health field is related to its topical use as wound dressings due to its anti-inflammatory action and its contribution to providing better rehydration of the damaged area.²⁴²  Recently, a detailed study on the anti-inflammatory properties of both sepiolite and palygorskite has been carried out by Cervini-Silva et al.²⁴³  These authors have determined that clay hydration due to structural water in these fibrous clays together with the presence of external silanol groups contribute to edema inhibition and migration of neutrophils. They have also confirmed the non-cytotoxic effect of these fibrous clays when the cells are exposed to low concentrations of the silicates.

Bionanocomposites involving fibrous clays and proteins have been reported for biomedical applications, with special emphasis on regenerative medicine. The first works reporting the biocompatibility of collagen–sepiolite complexes and their potential use in bone tissue repair appeared about three decades ago.^244,245  Initially, in vitro experiments confirmed the suitability of these bionanocomposites to favor the adhesion and proliferation of fibroblasts, and some years later in vivo experiments were carried out with the implantation of the collagen–sepiolite materials in rat calvaria defects.²⁴⁶  These studies confirmed the non-cytotoxicity of these bionanocomposites and their ability to promote bone regeneration in the damaged area, which was accompanied by a complete resorption of the implanted material. The rate of resorption of the collagen–sepiolite implants was tuned by means of crosslinking with 1% glutaraldehyde, which contributed to extend the resorption for several months after subcutaneous implantation.²⁴⁷  Recent studies have been focused on the assembly between collagen or its derivative gelatin and fibrous clays,^230,231  as well as on the development of macroporous bionanocomposite scaffolds²⁴⁸  that could be applied in reparative medicine. In this last example, gelatin–sepiolite bionanocomposites were freeze-dried in order to produce macroporous foams, showing a porosity around 98% and a maximum Young’s modulus of 6 MPa.²⁴⁸ 

Another interesting application of bionanocomposites related to health is drug delivery, benefiting from the swelling properties, bioadhesion or possibility of cell uptake of these systems, which allow the controlled release of a constant dose of the entrapped drug.²²⁰  Similarly to those reported systems involving layered silicates, bionanocomposites based on proteins and fibrous clay minerals can be used for drug delivery. A recent work reports the use of silk fibroin–palygorskite materials for controlled release of diclofenac.²⁴⁹  In this case, a core–shell structure was formed by coating the drug-loaded palygorskite with silk fibroin produced by spiders, the larvae of Bombyx mori and other insects, and the resulting system showed a reduction in the release rate of the entrapped diclofenac by about 50% with respect to the pristine clay. Other examples are related to the use of bionanocomposites involving antibiotics for wound-dressing applications. For instance, the controlled release of neomycin from sepiolite-based bionanocomposites prepared with the corn protein zein was recently studied.²⁵⁰  In this case, zein was combined with CMC in order to increase the water resistance of the polysaccharide and to reduce the swelling degree of the resulting biopolymer blend. All the prepared systems involving neomycin showed antibacterial properties, but the release rate of this antibiotic was reduced when it was previously loaded on sepiolite, in comparison to systems with neomycin directly added to the biopolymer blend.

Enzymes are globular proteins that act as catalysts in complex chemical reactions, and for many applications in biosensors and bioreactors they are previously immobilized on a solid support, including fibrous clays. These silicates can provide a protective support for the biological molecule, preventing its denaturation and loss of catalytic activity, and also enhancing the thermal and storage stabilities of the enzymes, as determined in catalase supported on sepiolite²⁵¹  or alcohol dehydrogenase on palygorskite.²⁵²  In addition, the solid support allows the recovery of the enzyme, which can be reused in consecutive cycles. Similarly, invertase immobilized on sepiolite showed higher thermal stability and increased resistance to leaching, which was exploited for its use in a packed bed reactor.²⁵³ 

In many cases, the enzyme is directly adsorbed on the fibrous clays. For instance, Rhizomucor miehei and Candida cylindracea lipases were immobilized on sepiolite and palygorskite by ionic adsorption through the positively charged groups of the proteins and the silanol groups of the silicate surface,²⁵⁴  and the resulting material yielded higher rates of hydrolysis of ethyl carboxylates than analogous systems involving layered silicates. In contrast, sometimes the enzymatic activity can be partially lost due to the immobilization process. This was the case for a pig pancreatic lipase adsorbed on sepiolite, which showed a decrease in the enzymatic activity up to 42% in comparison to the free enzyme.²⁵⁵  This fact is most likely due to a partial deactivation of the active sites as a consequence of adsorption or to a steric effect of the supported lipase in the reaction. In spite of this reduction, the supported lipase was applied in the production of biofuel from sunflower oil, showing high stability and easy recyclability.²⁵⁵  A previous modification of the surface of the fibrous clays can be carried out before enzyme immobilization in order to avoid the decrease in the catalytic activity. For instance, sepiolite was modified with phospholipids forming a biomimetic supported membrane for immobilization of urease and cholesterol oxidase,²⁵⁶  as will be detailed in the next section. The fibrous clays can also be functionalized in order to facilitate the covalent immobilization of the enzymes and enlarge their stability during reuse, as shown in a recent work by Luna et al.²⁵⁷  In this example, sepiolite was coated with an external AlPO₄ layer to which Rhizopus oryzae lipase was covalently grafted through phosphamide bonds. The immobilized lipase was applied in the production of biofuel by ethanolysis of sunflower oil, minimizing the glycerol waste production and showing a significant stability together with the possibility of reusing it more than five times. Similarly, acid activated palygorskite was modified with APTES in order to facilitate the covalent immobilization of lipase from Burkholderia cepacia, using glutaraldehyde as crosslinker.²⁵⁸  This system was used to obtain biodiesel from jatropha oil, showing a higher stability of the immobilized lipase even after 15 cycles of reuse.

The fibrous clays used as a support for enzymes can also be modified with magnetic particles in order to facilitate the recovery of the enzymatic system after the catalytic cycle. In this way, palygorskite was modified with γ-Fe₂O₃ assembled to its external surface, and then Candida sp. 99–125 lipase was adsorbed on the functionalized silicate.²⁵⁹  The immobilized lipase showed a good performance in the hydrolysis of olive oil and higher stability to changes in pH and temperature compared with free lipase. This high stability allowed its repeated use in at least eight consecutive cycles, profiting from easy recovery from the reaction medium by means of a magnet.

Immobilization of enzymes on solid supports is also of great interest for biosensing applications. In certain cases, the fibrous clay is used to develop surface-modified biosensors, in which the silicate is deposited as a film that entraps the enzyme, preserving its catalytic activity. This is the case for palygorskite used to immobilize tyrosinase,²⁶⁰  leading to the construction of a biosensor that was used in the electrochemical determination of phenol. Similarly, glucose oxidase was immobilized with palygorksite on the electrode surface, and the resulting biosensor was used to determine glucose in blood and urine samples.²⁶¹  In other cases, bulk-modified electrodes are developed. A recent work describes the immobilization of peroxidase extracted from a fruit native to South America on sepiolite,²⁶²  and this system is subsequently mixed with graphite powder, MWCNTs, mineral oil and nafion perfluorinated resin (Figure 1.14). The resulting composite can act simultaneously as the sensing and conducting phase of the biosensor, which was applied in the determination of tert-butylhydroquinone, used as an antioxidant in foods.

Naturally occurring surfactants such as fatty acids, phospholipids, lipopeptides, etc.²⁶³  are versatile and environmentally benign alternatives to synthetic surfactants like quaternary alkyl ammonium salts^8,264  for the organophilization of clay minerals. Organoclays find important application as thickeners or nanofillers in polymers and resins.^8,265  However, an important drawback of organoclays is connected with the toxicity of many quaternary alkylammonium components and hence makes their extension into biological fields difficult.²⁶⁶  Therefore, increased attention has been directed toward biosurfactants as biocompatible and eco-friendly alternatives.^263,267  Moreover, these molecules can form solid-supported bilayers with certain biomimetic features, which can be prepared via self-assembly routes²⁶⁸  and find potential applications in biomedical or other biotechnological fields.^269–271 

The first example of a fibrous clay–lipid hybrid was reported by Wicklein et al. who adsorbed phosphatidylcholine (PC) on sepiolite.^272,273  Lipid deposition was carried out either by liposome adsorption from aqueous media or by molecular adsorption from organic media.^229,273  In both cases, the adsorption isotherms show the characteristic shape of a mono- and bilayer formation as a function of the equilibrium PC concentration. This study has also shown that liposomal deposition is more effective for the formation of a lipid membrane in this specific system. The reason is that in aqueous media PC adsorbs as aggregates (i.e. bilayered liposomes) while in ethanol single PC molecules adsorb on the clay. The molecular interaction between the lipid headgroup and the clay surface was investigated by infrared (IR) and nuclear magnetic resonance (NMR) spectroscopy techniques. These investigations strongly indicated hydrogen bonding between the ester and phosphatidyl groups of the lipid and the silanol groups located on the sepiolite surface.²⁷²  The adsorption of phospholipids on sepiolite also has a significant influence on the surface wettability of the resultant material, as evidenced by contact angle measurements on cast sepiolite–PC films. The initially hydrophilic sepiolite becomes more hydrophobic as the first monolayer is completed, which corroborates the presence of hydrocarbon chains at the solid–liquid interface. With subsequent deposition of lipid molecules the contact angle gradually decreases again as hydrophilic lipid headgroups assemble at the external surface of the lipid bilayer.

Self-assembly of biosurfactants can be used to prepare mixed lipid membranes that offer diverse functionalities, which can be of interest in different technological applications.²⁷⁴  These hybrid layers are composed of individual membrane leaflets of different molecules.^275–277  Such a hybrid interface was assembled on sepiolite, where the inner leaflet was comprised of PC and the outer leaflet of n-octyl-β-d-galactoside (OGal). The mixed membrane resulted from subsequent monolayer deposition from the corresponding aqueous biosurfactant solutions.²⁵⁶  It has been assumed that the PC monolayer serves as nucleation site for the growth of the OGal outer membrane leaflet by hydrophobic interaction between the hydrocarbon chains.²⁷⁸  This assumption is also supported by Persson and co-workers who investigated adsorption of n-dodecyl-β-d-maltopyranoside on hydrophobized surfaces based on silane and concluded that insertion of alkyl chains of the sugar-based surfactant into the hydrophobic silane layer acts as an anchoring mechanism.²⁷⁹  The adsorption of OGal molecules causes an inversion of the surface hydrophilicity of the resultant material, as evidenced from water adsorption isotherms.²⁵⁶  Nonionic sugar-based surfactants (SBSs) are a type of biosurfactant which are particularly interesting for incorporation into these hybrid layers, e.g. for their antifouling properties^280,281  or protein stabilization ability.^282–284 

Thanks to their biocompatibility, clay–lipid hybrids are interesting candidates for in vivo applications like enterosorbents for mycotoxins. Infestation of crops and foodstuffs by fungi-produced toxic compounds such as mycotoxins has been a severe threat for millennia, with occasional catastrophic consequences for the lives and health of both animals and humans.²⁸⁵  A convenient and efficient method for elimination of these compounds from animal alimentation is their in vivo adsorption and sequestration (i.e. enterosorption) by food additives like clay and soil minerals.²⁸⁶  As many of these mycotoxins are rather hydrophobic (e.g. alflatoxins) the use of organophilic sorbents would increase their retention. In fact, bio-organoclays like sepiolite–lipid hybrids demonstrated higher in vitro sequestration efficiencies for aflatoxin B1 compared to the neat clay analogue²⁷²  and could thus be a promising candidate material for in vivo applications.

Another in vivo application of these hybrids is their employment as vaccine adjuvants where they can act as an antigen carrier. Recently, the bilayer lipid membrane supported on sepiolite fibers was reported to serve as a promising immobilization site for influenza A antigens (i.e. whole virus particles, hemagglutinin protein).²²⁹  The rationale was to limit the strong support–antigen interactions that are frequently observed for inorganic vaccine adjuvants and which are prone to compromise the immunogenicity of the vaccine.²²⁸  Functional studies in mice revealed that, in contrast to a standard aluminum hydroxide adjuvant, influenza vaccines based on sepiolite–lipid induced high titers of specific antibodies with a Th1 profile that is often associated with efficient clearance of viral infections.²⁸⁷ 

The biomimetic character of the clay-supported lipid interface is also beneficial for the immobilization of certain proteins, especially membrane-bound enzymes. The catalytic activity of such enzymes is very sensitive to the immobilization site and can be easily compromised by protein structure deformations. It could be shown in electrochemical assays that, for instance, cholesterol oxidase (COx) maintained its catalytic activity when supported on sepiolite, offering a bilayer lipid membrane.²⁵⁶  Contrarily, after immobilization on sepiolite hybrids displaying cetyltrimethylammonium or hybrid lipid–octyl-galactoside layers, the activity of COx was greatly reduced, underlining the importance of biomimetic interfaces like the bilayer lipid membrane for good stabilization of such enzymes. This offers possibilities for preparing, for instance, selective biocatalysts²⁸⁸  and sensitive biosensors²⁵⁶  employing immobilized enzymes on sepiolite–lipid hybrids as active species.

Fibrous silicates have been proposed as non-viral vectors for gene transfection. The spontaneous assembly of short chain DNA molecules on sepiolite was recently reported. In these bionanocomposites, the silicate can provide a protective environment limiting the DNA degradation by DNase,²⁸⁹  as observed for other clays of the smectite family.²⁹⁰  The assembly of the negatively charged nucleic acids on fibrous silicates is most likely due to hydrogen bonding interactions with the silanol groups in their structure. In order to evaluate the possible application of DNA–sepiolite materials for gene transfection, they were mixed with bacteria and deposited on agar plates. According to the reported Yoshida effect,²⁹¹  nano-sized acicular materials, stimulated by sliding friction on the surface of the hydrogel, are able to form penetration intermediates that can incorporate exogenous DNA in the recipient bacteria, and therefore, the nucleic acid can be expressed therein.²⁹²  Similarly, sepiolite fibers with a diameter at the nanometer scale, obtained from Spanish deposits, can be used as a carrier for the transfer of exogenous DNA into Escherichia coli and other bacterial cells.²⁹³  The use of this type of sepiolite with a diameter around 50 nm is due to its apparent lack of carcinogenic potential and lower biological activity compared to sepiolite from other origins.²⁹⁴  Further studies were focused on the optimization of the Yoshida effect, making use of sepiolite fibers to transfect Escherichia coli, Yersinia enterocolitica and Acinetobacter baumannii into bacterial cells.²⁹⁵  Later on, it was demonstrated that the use of sonicated sepiolite increases the bacterial transformation efficiency from 20 to 30-fold compared to methods based on the Yoshida effect.²⁹⁶  Interestingly, the use of sepiolite–DNA systems has been recently extended to gene transfection to eukaryotic cells.²⁹⁶ 

Conventional polymer–clay nanocomposites are based on the delamination ability of phyllosilicates like smectites, giving rise to highly dispersed elemental silicate layers in the polymer matrix that introduce significant improvement in key properties for advanced applications. In the case of sepiolite and palygorskite fibrous clays, which are not exfoliable, it is necessary to use fibrillated silicates produced by physical processes that may help to enable homogeneous distribution in the polymer matrix. This is especially useful when hydrophilic polymers are used, as for instance, in the case of bionanocomposites based on polysaccharides, proteins and nucleic acids. The functionality of the resulting polymer–clay nanocomposites can be provided not only by the nature of the polymer matrix but also by the functions deliberately introduced in the starting silicates by chemical reactions or by NP assembly, always based on interactions with silanol groups located at the external surface of the clays. In this way, the versatility of functions that can be introduced in the nanocomposites derived from sepiolite and palygorskite can be useful for a wide variety of applications. So, it can be expected that not only typical applications, such as reinforcing nanofillers for rubbers and thermoplastics, but also novel uses emerge based on the development of functional composites with magnetic, optical and conducting properties, as well as the development of active phases for sensor devices, membranes and bioplastics for food packaging, drug delivery systems, adjuvant of vaccines and gene vectors in non-viral transfection processes. A great future can therefore be estimated for this type of advanced material, which still require important progress in the basic and applied research on this topic.

This work was partially supported by the CICYT, Spain (project MAT2012-31759) and the EU COST Action MP1202.