Table 1.

Overview of popular natural and synthetic components used in bioinks for extrusion-based bioprinting sorted by the main component. The table summarizes strategies for enabling the printability of common biomaterials using various functionalities and crosslinking approaches.

Natural	Characteristics/chemical structure	Functionalities/derivates	Crosslinking/fabrication approaches	Remarks/biological activity
Polypeptides
Gelatin	Denaturated collagen Mainly collagen I Gly-Pro-X (most abundant) Type A (derived from acidic-treated gelatin) → positive-charge at neutral pH Type B (derived from alkali-treated gelatin) → negative-charge at neutral pH Thermoreversible gelation	Without functionalization[35–37]	Blending with stabilizing component (e.g., alginate, fibrin[37]) Enzymatic crosslinking: transglutaminase[38] tyrosinase[39] Radical (photo-) polymerization	Biodegradable Cell adhesion motifs are present Moderate biological activity if used alone Common sources: Bovine skin Porcine skin Fish skin
		Methacrylated[23,40-44] Allylated[45] Norbornenet[46] Thiolatedt[47,48] Tyraminet[49] Furfury[50]	Radical (photo-) polymerization
Collagen type I	Most abundant collagen in human body Gly-Pro-X pH-dependent fibrillogenesis and gelation	Without functionalization	pH-dependent crosslinking[51–53] photo-crosslinking (e.g., with ribofavin[54]) crosslinked with genipin[55]	Biodegradable Cell adhesion motifs are present Common sources: Rat tail Bovine skin Rabbit skin Calf skin
Fibrinogen/fibrin	Fibrous and nonglobular glycoprotein Thrombin (factor IIa) and factor XIIIa can be used to covalently crosslink fibrinogen	Without functionalization[18,35–37,43,51]	Enzymatic crosslinking (factor IIa, XIIIa and IV) Additionally blended with stabilizing component (e.g., gelatin, hyaluronic acid, and Pluronic F127[37])	Biodegradable Cell adhesion motifs are present Limited long-term stability (can be prolonged by addition of aprotinin to culture medium) Common sources: Human plasma Bovine plasma Rat plasma
Silk/fibroin	Silkworm cocoons Gly-Ser-Gly-Ala-Gly-Ala units[56] Recombinant spider protein GSSAAAAAAAASGPGGYGPE NQGPSGPGGYGPGGP Physical crosslinking by β-sheet crystal formation	Without functionalization	Physical crosslinking[57] Blended with stabilizing component (e.g., gelatin[57]) Enzymatic crosslinking by tyrosinase[39]	Biodegradable Poor cell adhesion due to hydrophobic character[58] Common sources: B. mori silkworm Recombinant silk protein eADF4(C16) mimicking Araneus diadematus silk protein sequences[59]
Poly-saccharides
Agarose	d-Galactose and 3,6-anhydro-l-galactopyranose Thermoreversible gelation	Without functionalization[60,61]	Physical crosslinking	Biologically inert Biodegradable Often used as sacrifcial material[12] Can be used to modulate the viscosity[60] Common sources: Red algae
Alginate	Varying sequences and blocks of β-d-mannuronate (M) and α-l-guluronate (G) Ionic gelation by divalent cations (e.g., Ca[2+])	Without functionalization	Physical crosslinking via divalent ions Often blended with, e.g., GelMA, nanocellulose or agarose[40,44,60,62] to improve properties	Biologically inert Sulfate can bind growth factors, such as FGF, TGF, and HFG Common sources: Brown algae
		Sulfated[63] Methacrylated[64]	Radical (photo-) polymerization
	Characteristics/chemical structure	Functionalities/derivates	Crosslinking/fabrication approaches	Remarks/biological activity
Hyaluronic acid	Glycosaminoglycan (GAG) Units of d-glucuronic acid and N-acetyl-d-glucosamine Forms weak entangled molecular network[65]	Without functionalization[37,66]	Needs modifcation or blending with stabilizing component	Biodegradable Cell adhesion motifs are present Can be used to modulate the viscosity Common sources: Bacteria Bovine vitreous humor Rooster comb
		Methacrylated[41,67–69] Phenolic hydroxyl[49] Thiolated[47,48]	Radical (photo-) polymerization
		Poly(N-isopropylacrylamide)[67] Adamantane (guest)[68] β-Cyclodextrin (host) Cucurbituril (host)[70] 1,6-diaminohexane (guest)	Lower critical solution temperature behavior (LCST) Guest–host supramolecular assembly
Gellan gum	Tetrasaccharide of repeating units of β-d-glucose, one β-d-glucuronic acid and one α-l-rhamnose[71] Ionic gelation by mainly divalent cations Thermoreversible gelation	Without functionalization	Physically crosslinked by cations Blended with stabilizing component (e.g., GelMA[42,72])	Biodegradable Can be used to modulate the viscosity Common sources: Bacteria
Nano-cellulose	Linear linked d-glucose units Cellulose nanocrystals (CNC) Cellulose nanofibers (CNF) Bacterial nanocellulose (BNC)	Without functionalization	Blended with stabilizing and/or bioactive component (e.g., algi-nate[63,73,74] or hyaluronic acid[62,75])	Biodegradable Can be used to modulate the viscosity Common sources: Plants Bacteria
Dextran	Branched or linear poly-α-d-glucose	Methacrylatedt[66]	Radical (photo-) polymerization	Biologically inert
Chitosan	Progressively deacetylated chitins Linear and random dispersed β-(1-4)-linked d-glucosamine and N-acetyl-d-glucosamine Long gelation time and low mechanical properties[76]	Without functionalization	Blended with stabilizing components (e.g., agarose and alginate[60])	Biodegradable[77] improves cell survival[60] cell adhesion properties controlled by N-acetylation groups[78] antibacterial ability[79] Common sources: Chitin shells of seafood (e.g., crabs, shrimps, and prawns)
		Carboxymethylated[60] (water soluble)
Chondroitin sulfate	Sulfated glycosaminoglycan (GAG) units of N-acetylgalactosamine and glucuronic acid	Methacrylatedt[67]	Radical (photo-) polymerization	Biodegradable Common sources: bovine trachea Shark cartilage
Carrageenan	Kappa-carrageenan Ionic gelation mainly by potassium ions iota-carrageenan ionic gelation mainly by calcium ions Thermoreversible gelation	Without functionalization[80]	Physically crosslinked by cations Blended with a secondary component for covalent polymer network[80] or nanosilicates[81]	Kappa-carrageenan mostly used due to its resemblance to natural glycosaminoglycans (GAGs)[82] Common sources: Red algae
		Methacrylatedt[83]	Radical (photo-) polymerization
	Characteristics/chemical structure	Functionalities/derivates	Crosslinking/fabrication approaches	Remarks/biological activity
ECM mixtures
Matrigel	Solubilized basement membrane preparation Mainly laminin (L-111) and collagen IV, but in total over 1851 unique proteins and over 14 060 unique peptides[84] Temperature-dependent gelation at around RT-37 °C	Without functionalization	Temperature-triggered gelation	Biodegradable Cell adhesion motifs are present Highly bioactive due to growth factors and vast amount of proteins and peptides No defned composition Batch-to-batch variations Common sources: Engelbreth–Holm–Swarm mouse sarcoma
dECM	Decellularized extracellular matrix of a specifc tissue like heart,[85–87] cartilage,[85] adipose,[85,88,89] aorta,[90] skeletal muscle,[91] and liver[92] Temperature-dependent gelation	Without functionalization	Temperature-triggered gelation Blended with stabilizing component (e.g., thiolated gelatin, hyaluronic acid, and PEGDA[29]) Vitamin B2-induced UV-A crosslinking[86]	Biodegradable Cell adhesion motifs are present Biodegradable Highly bioactive No defned composition Batch-to-batch variations Common sources: mammalian tissue
Synthetic
PEG	Poly(ethylene glycol) H(OCH2CH2)nOH Linear or branched	Diacrylatedt[18] Tetra-acrylatedt[44,48]	Radical (photo-) polymerization	Biologically inert
		Methacrylated[61] Fibrinogen and diacrylate functionalization[18]
		Succinimidyl valeratet[43]	Amin-carboxylic acid coupling (NHS ester reaction) with, e.g., proteins
Pluronic	Triblock copolymer of poly(ethylene oxide) Nonionic tenside	Without functionalization[37]	Temperature-triggered gelation	Biologically inert Often used as sacrifcial material[37] Not suitable for long-term cell culture
		Diacrylated[93]	Radical (photo-) polymerization
Additives
Nanosilicates	Laponite Na[+] 07[(Mg5.5Li0.3)Si8O20(OH)4]-0.7)	Without functionalization[83,94]	Physically crosslinked Blended with other bioink components (e.g., kappa-carrageenan and GelMA[95]	Used to modulate the viscosity of an ink to improve printability Can improve cell adhesion and response[96–98]
Glycerol	C3H8O3 Viscous and hygroscopic liquid Backbone of many lipids	Without functionalization	Blended with other bioink components	Can help to induce crosslinking of silk[57] Can prevent nozzle clogging and used as rheology modifer[37]