Table 2.

Details of natural polymers commonly used for growth factor and cytokine delivery

Polymer	Class	Properties	Forms	Crosslinking Mechanisms
Chitosan	Polysaccharide	Polycationic polymer with antimicrobial activity but weak mechanical strength and stability	Hydrogels, films	Covalent: GA, dextran aldehyde, ionic: oxalic acid90
Alginate	Polysaccharide	Negatively charged polymer, adjustable mechanical and biological properties by varying the content of two monomers	Beads, hydrogel, sponge	Physical: Calcium chloride or other divalent metal chlorides94,95
Hyaluronate	Polysaccharide	Negatively charged GAG, main connective tissue ECM component, good biocompatibility and mechanical properties	Hydrogel, film	Covalent: amine-modified HA reacted with oxidized GAG to form imine bonds99
Heparin	Polysaccharide	Negatively charged sulphated GAG	Hydrogel	Covalent: disulphide bond between cysteine residues101
Collagen	Protein	Slightly anionic, most abundant protein in the body, provides cell-matrix and matrix–matrix interactions, good biocompatibility, low immunogenicity, poor mechanical properties	Scaffold, sponges, membranes, hydrogel	Covalent: GA, TG, three polypeptide chains that form pH and temperature dependent hydrogels103
Gelatin	Protein	Anionic, denaturalized form of collagen, similar properties as collagen	Microspheres, sheets, sponge	Covalent: genipin, GA112,114
Fibrin/fibrinogen	Protein	Anionic, component of tissue architecture, cell-matrix and matrix–matrix interactions, good biocompatibility	Matrix, hydrogel	Initiated by thrombin115

ECM, extracellular matrix; GA, glutaraldehyde; GAG, glycosaminoglycan; TG, transglutiminase.