TABLE 2.

Common natural biopolymers used for tissue engineering and clinical applications.

Biopolymer	Structure	Biological interactions	Applications and references
Alginate	1 → 4 linked β-d-mannuronic acid (M) and α-l-guluronic acid (G) residues G residues function with divalent cations to form ionic crosslinks	Variations in local mechanical properties controlled by concentration of calcium ions Form soft gels, composite materials, interpenetrating networks Electrospun fibers with improved mechanical integrity	Wound healing, drug delivery, soft tissue engineering, cell delivery, in vitro stem cell maintenance48,78,79,112,126,135,141,142,200,201,216,223,230,240,243,247,248,252,264,265,270,287,295
Chitosan	Linear polysaccharide from deacetylation of chitin (main component in the exoskeletons of crustaceans) Randomly distributed β-(1–4)-linked d-glucosamine and N-acetyl-d-glucosamine	Soluble, cationic in acidic conditions and insoluble in neutral and basic conditions	Wound healing, orthopedics, cardiac repair, neural tissue engineering, cornea repair, drug and gene delivery34,56,94,146,159,189,196,205,217,288,293
Collagen	monomeric and crosslinked 3D triple helical structures Many types, type I consists of two identical α1 chains and one α2 chain Utilized in vitro in fibrous and hydrogel formats Content measured by quantifying hydroxyproline content Gelatin is a commonly used animal-based collagen product for gels	Easily remodeled and degraded by cells Chemical crosslinking decreases degradation and improves long-term Mechanical properties Many types available, which are specific to different tissue types Collagen IV primarily found in the basal lamina, secreted by epithelial cells, collagen I is the main component in scar tissue, cartilage, bone, tendon, and other connective tissues and muscles Improper expression or mutation leads to disease	Wound healing, skin grafts, muscle repair, nerve regeneration, anti-aging1,33,74,82–85,152–154,190
Elastin	Elastic component in soft tissues that allows tissue to return to normal shape following stretch or pinch Crosslinking smaller tropoelastin polymers using lysyl oxidase to form mesh-like structures	Small tropoelastin polymers can be used to form composite materials Can form films, gels, scaffolds, and electrospun fibers Easily remodeled by cells	Cardiac stent coatings, soft tissue reconstruction, orthopedics5,10,14,86,110,172,273
Fibrin	Formed when thrombin cleaves fibrinogen, generating soft gels Gelation kinetics controlled by ratio of thrombin to fibrinogen, calcium concentration, temperature	Fibrinolytic inhibitors, like aprotinin or aminocaproic acid, reduce in vitro degradation rates	Wound healing, cardiac repair, in vivo cell delivery25,38,178,222,244,260,285,292
Fibronectin	Glycoprotein secreted by fibroblasts Binds integrins, transmits mechanical cues from the environment to the cell binds other ECM proteins such as collagen and fibrin	Improper regulation of expression leads to diseases such as cancer, fibrosis Fibronectin can be grafted onto 2D and 3D surfaces to improve biocompatibility Many sites for cell-based protein degradation	Wound healing, stem cell differentiation, cardiac repair, bone regeneration158,193,227,275
Keratin	Bundles form intermediate filaments that make up hair (α-keratins) and nails (β-keratins) Tough, insoluble polymer found in non-mineralized tissues Monomers form stable left-handed superhelical structures to form filaments	Structure attributed to disulfide bridges; more bridges yields lower elasticity Classified as neutral-basic or acidic, dictating in vivo occurrence	Cornea tissue engineering, wound healing, skin regeneration, cardiac repair, drug delivery, nerve repair23,210,213,238
Silk	Extracted from the cocoons of Bombyx mon silkworm Foams, sponges, films and hydrogels are formed from silk solution	In vivo degradation rate controlled by crystallinity (β-sheet content) Mechanics tailored by modifying concentration, crystallization, molecular weight, and scaffold format	Muscle repair and regeneration, bone tissue engineering, cornea repair, drug delivery6,8,17,67,86,110,165,168,192,197,211,212,280