Introduction to peptide soft materials

Abstract

Peptides are versatile biomolecules which can be used to design innovative functional and structural soft materials. This themed issue is devoted to a selection of highlights of research from leading teams around the world, working in this rapidly developing field. Peptides can now be created to produce soft materials with a huge range of properties and applications using recently established molecular design rules based on sequence, residue properties and defined secondary structure. Peptides can be synthesized at high yield and purity with advanced and automated methods, and new methods are being developed to create new kinds of peptide-based and peptide-inspired constructs and conjugates. The design of peptide sequences is no longer limited to the use of the 20 natural amino acids, because non-proteinogenic and unusual amino acid residues can be incorporated within peptides using a variety of recently-developed chemical and chemical biology approaches. Peptide self-assembly can be driven by a range of non-covalent interactions including hydrogen bonding, electrostatic interactions, π–π and hydrophobic interactions. This, often coupled with peptide secondary structure formation, leads to a range of soft materials under suitable conditions. Self-assembling peptides can form different soft nano-structures including nanospheres (micelles), vesicles, nanotubes, nanofibrils, nanosheets, ribbons, and nanofilms.1–7 Peptide-based hydrogels are important peptide-based soft materials that exhibit fascinating properties such as excellent biocompatibility, tunable mechanical stiffness, thermal stability and stimulus-responsiveness. The mechanical strength of such gels can be easily regulated by varying the peptide structure, solvent, pH, ionic strength of the medium, the presence of additives, etc. Similarly, the thermal stability of these gels can be varied by altering any of the above mentioned parameters. Regulation of the thermal and mechanical properties of these gels can tune their functions. Moreover, the responsiveness of peptide hydrogels to chemical, physical and biological stimuli8,9 makes them very useful for many biological and other applications. Apart from having tuneable structural properties, peptide gels have various applications in health care,10 environmental remediation11 and as renewable non-conventional energy sources.12 These gels form a three-dimensional network structure from self-assembling peptide-based nanofibers that entrap solvents, often at low peptide content. Peptide hydrogels provide porous structures that aid cell migration and growth. They also have the ability to offer an environment that mimics the extra-cellular matrix, and this opens up opportunities for different biomedical applications including drug delivery, 3D cell culture, tissue engineering, wound healing, antimicrobial agents and 3D bio-printing.13,14 Due to their high water content and extra-cellular matrix like properties, these hydrogels offer an excellent platform even for three-dimensional neural tissue culture.15 Peptide-based hydrogels can be used to regulate the differentiation of stem cells, and in cancer therapy.