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Water constitutes approximately 60% of the human body and up to 80% of its soft tissues. It is thus not surprising that hydrogels, themselves highly hydrated materials, are often employed as preferred biomaterials. Their engineerability and resultant range of mechanical and (bio)chemical properties have resulted in their use in biomedical applications from sensing and drug/gene delivery to tissue engineering and regeneration. This versatility is especially applicable to multicomponent hydrogels, in which two or more different constituents are combined to provide additive or synergistic properties for improved functionality and outcomes.

Natural hydrogels comprising polysaccharides and/or polypeptides have the advantage that they are generally biocompatible, contain the motifs and bio-active signals for cellular interaction, can be reconstituted to resemble the extracellular matrix (from which they are often derived), and are recognized, degraded and metabolized by the body according to established pathways. Disadvantages associated with natural gels are, however, that their properties are not fully controllable/reproducible due to the variation and complexity of their inherent and crosslinking chemistries.

Although the term “hydrogel” was first used in the late 19th century to describe colloidal gels of inorganic salts, the use of the term to describe hydrated polymers for biomedical use started in the 1960s with the development of poly(hydroxyethylmethacrylate) (PHEMA) contact lenses. Since then, the types and varieties of synthetic gels based on chemistry, crosslinking techniques, etc. have increased rapidly, with much work driven by the need for more sophisticated biomaterials for biomedical applications. Synthetic hydrogels are more readily engineerable than natural hydrogels in terms of physicochemical properties and reproducibility, but may lack mechanical strength and the appropriate signals for positive tissue interaction with cells and tissue.

The physical properties in terms of improving mechanical strength may be addressed by using multicomponent systems comprising copolymers, admixtures and (semi-)interpenetrating networks (IPNs), dendritic materials and enhanced crosslinking techniques. In order to introduce activity and other functionalities to these gels, the inclusion of additional components, whether they be drugs for sustained delivery, bioactive molecules for cell adhesion or growth, or analytes for sensing, is often used. In addition, the controlled degradation of hydrogels is achieved by including hydrolytically and/or enzymatically degradable monomers or crosslinks. Hybrid hydrogels, aimed at improving both mechanical properties and tissue interaction, may, of course, be derived from combinations of synthetic and natural materials.

After an introduction to definitions, classification, characterization and applications, the design architectures, reactions and crosslinking strategies of hydrogels are described in Chapter 2. The remainder of the book can be classified into four sections. The first describes different types/classes, such as low molecular weight, dendritic, click, and self-healing and stimuli responsive gels, in detail. The second elaborates on characterization, modelling and simulation of gels, and the third on the processing of multicomponent hydrogels into three-dimensional structures and scaffolds using techniques such as electrospinning and 3D printing.

The remaining part of the volume starts with an analysis of the challenges and opportunities that exist for multicomponent hydrogel applications, followed by in-depth chapters on applications in tissue engineering, drug delivery, wound healing, clinical and pharmaceutical applications, biosensing and imaging, and cancer diagnosis and therapy. As the ultimate goal of research in this field is to provide useful clinical therapies and devices, a chapter on regulatory and commercial aspects related to the use of hydrogels as biomaterials concludes the work.

It is hoped that this book provides a useful and comprehensive reference to teachers and students in biomaterials science and related fields who aim to use multicomponent hydrogels in biomedical applications intended for diagnosis and pharmacological treatment, and/or for the normalization, restoration, replacement or regeneration of bodily tissues.

Jagan Mohan Dodda

Kalim Deshmukh

Deon Bezuidenhout

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