Chapter 1: Polysaccharide and Protein Based Aerogels: An Introductory Outlook

Aerogels have been in use for almost nine decades now. Synthetic aerogels are being used as super capacitors, electrodes, insulators, and so forth. Increased aerospace applications and rising energy costs have increased the prominence of insulating nanoporous materials (Figure 1.1). Nanocrystals based on a wide range of biomaterials have found their way into the production of nanoporous materials by replacing synthetic counterparts. In addition, awareness of using non benign, environmental friendly materials is driving researchers to develop novel green materials. Aerogels based on several polysaccharides and proteins have been developed recently, involving comprehensive research such as theory modelling and life cycle analysis of such materials.

The aim of this book is to bring together the results of research studies in the area of polysaccharide and protein based aerogels. Aerogels based on these biomaterials, their preparation from various sources, characterisation methods and applications in various domains of science and day to day life will be discussed in detail.

It is indeed commendable that all the contributors in this book have done their best to profoundly review recent advances in this growing area of strong international interest, providing a comprehensive idea on the preparation, properties and applications of polysaccharide and protein based aerogels. In-depth studies and reviews on the technological developments concerning processing strategies and structural analyses have also been included, providing a general idea of the individual types. The chapters are designed in such a way as to edify readers from various realms: academics, students, researchers and industrialists in particular.

Aerogels are porous ultralight materials derived from gels, in which the liquid component of the gel has been replaced with a gas.²  These advanced materials are highly porous solids that hold gas (usually air) within the pores or networks of solid substances. Due to their light weight, low density, large surface area and high mechanical strength, aerogels are useful for many applications, such as heat insulators, particle filters, particle trappers and catalyst supports.³  Aerogels are composed of a network of clustered nanoparticles. The materials usually have unique properties including high strength to density, and high surface area to volume ratios. They are manufactured by subjecting a wet gel precursor to critical point drying in order to remove the liquid through supercritical drying, without disturbing the network.

Well known aerogels are those of silica and metal oxides such as TiO₂ and Fe₂O₃. However, these inorganic aerogels usually lack mechanical strength and tend to collapse easily when subjected to small stresses. In contrast, aerogels made of organic polymers are stronger and can also be used as carbon precursors for pyrolysis, the resorcinol–formaldehyde aerogel being a prominent example. Aerogels fabricated from synthetic polymers are fragile. As a result, such aerogels have limitations to be used in situations that need robustness. As aerogels combine the properties of highly divided solids and metastable characteristics, they can develop very attractive physical and chemical properties that are not achievable by other means of low temperature soft chemical synthesis. In other words, they form a new class of solids showing a sophisticated potential for a range of applications.⁴ 

Aerogels can be classified according to their appearance (as monoliths, powders and films), their different microstructural characteristics (as microporous, mesoporous and mixed porous) or by defining their composition (Figure 1.2).

Polysaccharide-based aerogels are commonly obtained in the form of cylindrical monoliths, although many other shapes (Figure 1.3) can be found in the literature (such as beads, microspheres, etc.).^6,7 

The size and morphology of aerogels can be customized by means of shaping the gel by moulding, extrusion or any other suitable physical techniques. In general, gels take the shape of the mould in which gelation takes place and this shape is preserved in the monolithic aerogels after supercritical drying (Figure 1.4).

The development of innovative materials from renewable and abundant bio-resources is becoming an important area of research as such materials exhibit high physical properties with a low impact on the environment. Increasing demand for products made from sustainable and non-petroleum based resources is also a major driving force for the development of new bio-based products. Polymers derived from non-petrochemical feedstocks are gaining a great deal of momentum from both commercial and scientific points of view. Biopolymers can be derived from natural sources such as plants, exoskeletons of arthropods, skin, silkworm cocoons, spider webbing, hair, and so forth. These materials are carbon neutral, sustainable, renewable, recyclable, nontoxic and environmental friendly, and can replace petroleum based products.^8,9 

Fundamental research in the production, modification, property enhancement and new applications of these materials is important. The new materials, concepts and utilizations that result from these efforts will shape the future of polymers from renewable resources.^10,11  Various biopolymers, predominantly polysaccharides and proteins, are used in aerogel production because they are ecological materials that can transform numerous industrial processes from being petroleum-dependent into biomaterial-dependent. In particular, they have numerous applications in food and non-food industries.

Biopolymers from various sources such as alginate, cellulose, lignin, pectin, chitosan, proteins and others have been tested as precursors. The resulting aerogels exhibit both the specific inheritable functions of the starting polymer and the distinctive features of aerogels (open porous structure with high specific surface and pore volume). This synergy of properties has prompted researchers to view biopolymer aerogels as promising candidates for a wide range of applications.¹²  More recent reports on biopolymer aerogels, as chronicled in this book, describe their use for thermal insulation, tissue engineering, regenerative medicine, drug delivery systems, functional foods, and as catalysts and sensors.

Some of the most studied uses of aerogels include different aerospace-related applications, thermal super insulations, acoustic devices, and so forth. Recently, a growing interest in aerogels has been observed in the fields of pharmaceutical science, food related technology, biosensors, and diagnostics and biotechnology, to name a few (Figure 1.5).⁵ 

Polysaccharide based aerogels have proved to be useful in scientific domains where biocompatibility and biodegradability are needed, such as for medicinal, cosmetic and pharmaceutical applications, opening up the field of aerogel research even further.^13,14  Polysaccharides that could be used to prepare aerogels include cellulose, marine polysaccharides and starch, all of which have in common the ability to form gels either by themselves in the presence of water or with di-cations, other cross-linking agents, and/or other blended or mixed polysaccharides.¹⁵  Ultraporous nanocellulose aerogels have been used as a separation medium for liquid mixtures of oil/water. Polysaccharide-based aerogels are highly porous (ε=90–99%), lightweight (ρ=0.07–0.46 g cm⁻³) drug carriers with a high surface area (S_a=70–680 m² g⁻¹), enabling them to provide enhanced drug bioavailability and drug loading capacity.¹⁶ 

Several outstanding properties of polysaccharide aerogels make them promising materials for catalysis, especially their high surface area and high density of functional groups (up to 5.8 mmol g⁻¹ amino groups for chitosan, 5.6 mmol g⁻¹ carboxylic groups for alginic acid, and 2.8 mmol g⁻¹ sulphate groups for k-carrageenan). With surface areas as high as 500 m² g⁻¹, polysaccharides can compete with inorganic solids as supports for organometallic or metal catalysts.¹³ 

Protein based aerogels, though not as advanced as their polysaccharide counterparts, have gone through numerous stages of research in recent years. The foremost aerogels include those created from proteins of plant and animal origin, such as those derived from soy, wheat, silk fibroin, egg white (ovalbumin), milk (caseins and whey proteins), and so forth. They have proved to exhibit superior gel forming properties and ease of availability as they can be readily obtained from the food processing industry as by-products.

Other than the single-component aerogels, those composed of one of the precursors with specific additives have often conferred additional functionalities such as mechanical strength, hydrophobicity and catalytic features to the pristine materials, and this has improved the usefulness for some high-performance applications.¹⁷  The prospect of hybrid aerogels widens the horizon for development and innovations in this field. A broad realm of such aerogels has been established with diverse combinations of organic, inorganic, synthetic and bio-based raw materials.^12,18–22  Aerogels thus developed have found an extensive range of applications in the fields of drug delivery, nutraceuticals and oil absorption, thanks to their superior mechanical properties, thermo-responsiveness, biocompatibility, non-cytotoxicity, porous structure and reduced brittleness.^12,23,24 

Continuing on this flow of ongoing studies, this book documents the history, chemistry and recent developments in the field of bio-based aerogels, which will undoubtedly be an inevitable reference text for both academic and research communities alike. Even though one can find several research publications in the domain of aerogels, to the best of our knowledge, no systematic scientific reference book has been exclusively written on bio-based aerogels.

This multi-author book provides a useful summary of current knowledge in the realm of polysaccharide and protein based aerogels. The book commences with an introduction to different types of bio-based aerogels, their processing techniques and morphological analyses (Chapters 2–7). A detailed description is given in Chapter 8 on theory modelling and simulations of such aerogels. Chapters 9–12 deal with various properties possessed by these aerogels, while highlighting the procedures used for the tuning and tailoring of certain parameters that have influential effects on their properties. Chapters 13–16 shed light on the significance and applicability of bio-based aerogels in various arenas, with special reference to life science, food engineering, insulation, aerospace, packaging and biomedical applications.

The entire book has been brought together with the anticipation of providing a better understanding, and assisting further development, in the field of bio-based aerogels.