Table 1.

Different additives in hydrogels with different mechanism to methodology.

Function	Additive		Mechanism	Methodology	References
Vascularization	Growth factors	VEGF	VEGF can induce endothelial cell proliferation, promotes cell migration, and inhibits apoptosis. VEGF-induced angiogenesis with increased vascular permeability plays a critical regulatory role in the generation of blood vessels.	Wang et al. made BMP-2 and VEGF adsorbed onto silk fiber (SF) microspheres (diameter of 1.5 ± 0.3 μm) that were prepared using a co-flow capillary device. These microspheres were subsequently doped into the SF/nHAp scaffolds to provide regulated and controlled release.	[16]
		FGF-2	FGF-2 can stimulate VEGF expression, granulation tissue formation, and vascular maturation.	Xiong et al. used 3D printing to fabricate a gelatin-sulfonated silk composite scaffold (3DG-SF-SO3-FGF). The basic FGF-2 was incorporated in this scaffold by binding with a sulfonic acid group (SO3).	[17]
		PDGF	PDGF can recruit SMC/pericytes to immature vessels to stabilize and remodel them.	/	/
	Heparin and its derivatives	Heparin	Heparin can accelerate neovascularization by binding angiogenic factors, such as VEGF, and improve their stability.	An et al. coated poly-L-lysine (PLL) and heparin on the surface of 3D printed hydrogel scaffolds, which were made from cross-linking of GelMA and HAMA via electrostatic interactions between PLL and heparin.	[18]
		Heparan sulfate	Heparan sulfate can not only protect growth factors from degradation by proteases, but it also promote the binding of growth factors to their receptors, thus promoting growth factor activity.	Jiang et al. fabricated a scaffold with collagen and heparan sulfate	[19]
	Other materials	Angiogenic peptides	Angiogenic peptides are functionally equivalent to VEGF and can bind to the same receptors to initiate angiogenesis.	Wang et al. made a dual-delivery bone tissue engineering scaffold by low-temperature 3D printing of β-tricalcium phosphate and—osteogenic peptide (OP) containing water/PLGA/DCM emulsion—and coating AP on the scaffold surface.	[20]
		Desferrioxamine	Desferrioxamine can promote HIF1-α synthesis by simulating hypoxia, which was demonstrated to be essential for the regulation of genes related to angiogenesis. Desferrioxamine can lead to the activation of a cascade of pro-angiogenic genes, such as VEGF when it was activated.	Yan et al. designed a bionic degradable polycaprolactone (PCL) scaffold using 3D printing technology. It can control the release of DFO by surface degradation and layer-by-layer assembly techniques.	[21]
		Catalase	Catalase can break down hydrogen peroxide and produce oxygen, which not only reduces the damage caused by H2O2, but the oxygen that it produces also helps to induce angiogenesis	Rija et al. intercalated catalase in hydrogels to form functional decellularized adipose tissue-alginate (DAT-A) hydrogels using 3D printing technology.	[22]
		Decellularized extracellular matrix	The dECM plays a huge role in pro-vascularization with the removal of immunogenicity while retaining a large amount of nutrients such as large amounts of growth factors. It can facilitate the induced differentiation of pluripotent stem cells on dECM scaffolds into tissue-specific cell types	Kim et al. used S-dECM bioink to print and fabricate 3D pre-vascularized skin patches for wound healing with infusion of adipose-derived stem cells (ASCs) and endothelial progenitor cells (EPCs).	[23,24]
				Bejleri et al. fabricated a 3D bioprinted patch containing cECM for the delivery of pediatric cardiac progenitor cells hCPCs which is printed with bioinks consisting of cECM, hCPCs, and GelMA.
Antibacterial	Photothermal materials		MXenes can kill microorganisms such as Gram-positive and negative bacteria by disrupting bacterial membranes directly through physical contact, especially when supplemented with NIR irradiation for higher antimicrobial efficiency	Nie et al. fabricated a personalized MXene composite hydrogel scaffold GelMA/β-TCP/sodium alginate (Sr2+)/MXene (Ti3C2) (GTAM) with both photothermal antibacterial and osteogenic capabilities using 3D printing technology.	[25]
	Antibiotic	Vancomycin + Ceftazidime	Vancomycin can exert a strong bactericidal ability by inhibiting cell wall biosynthesis through specific binding to the dipeptide d-Ala-d-Ala (AA) at the end of bacterial cell wall precursors. Ceftazidime can inhibit bacterial peptidoglycan synthesis by inhibiting penicillin-binding proteins, leading to cell wall instability and inhibition of synthesis and cell death.	Yu et al. fabricated a mesh-like PCL scaffold by using 3D printing technology and poly(lactic-co-glycolic-acid) (PLGA) nanofibers with a hybrid sheath core structure by using co-axial electrospinning technique. Subsequently, they fabricated this nanofiber membrane in two layers (electrospun PLGA/vancomycin/ceftazidime layer and a coaxially spun PLGA/BMP-2 layer).	[26]
		Levofloxacin	Levofloxacin can inhibit bacterial helicase activity, which leads to the inability of DNA to replicate and synthesize properly, ultimately causing bacterial death.	Sadaba et al. produced poly (lactic acid) (PLA) scaffolds with 3D printing technology and subsequently added polydopamine-coated BaSO4 particles within the scaffolds and adsorbed levofloxacin in it.	[27]
		Minocycline	Minocycline can inhibit protein synthesis in bacteria.	Martin et al. utilized 3D printing technology to prepare a PLA scaffold and combined the scaffold with collagen, minocycline and bio-inspired citrate hydroxyapatite nanoparticles (cHA).	[28]
	NO		NO can play a bactericidal role by damaging DNA, proteins and lipids of microorganisms.	Kabirian et al. designed a 3D printed small-diameter vascular graft (SDVG) which was printed from polylactic acid and coated with blending of 10 wt% S-nitroso-N-acetyl-D-penicillamine mixed in a polymer matrix consisting of poly (ethylene glycol) and polycaprolactone to achieve controlled release of NO.	[29]
	Satureja cuneifolia		Satureja cuneifolia (SC) is a natural aromatic plant which is rich in phenolic compounds with antimicrobial activity, which are strongly oxidizing and can cause protein coagulation and destroy bacterial proteins thus exerting antimicrobial effects.	Ilhan et al. used 3D printing technique to make SA/PEG composite scaffold and loaded methanol extract of SC into it.	[30]
	Nanodiamonds		The active oxygen-containing groups on the surface of NDS promote their interaction with cellular components to quickly kill Gram-positive and negative bacteria and prevent bacterial adhesion.	Rifai et al. fabricated selective laser melting titanium (SLM-Ti) scaffolds by using 3D printing and selective laser melting (SLM), and applied nanodiamond (ND) coating on the scaffolds for functionalization modification.	[31]
	Chitosan		Its antimicrobial activity results from the interaction of the positive charge it carries with ions on the surface of negatively charged cells. Such activity disrupts bacterial cell membranes, prevents the transport of cellular material, increase the internal osmotic pressure, and cause the rupture of microbial cells, among other things. Chitosan can interact with bacterial DNA and prevents DNA transcription, thus inhibiting microbial ribonucleic acid (RNA) synthesis.	Intini et al. used 3D printing technology to manufacture porous chitosan scaffolds with 200 μm inter-filament opening.	[32]
Immunomodulation	Adrenocorticos-teroids	Dexamethasone	It can reduce the expression of cyclooxygenase 2 (COX-2). It inhibits prostaglandin production, inflammatory signaling, and neutrophil and macrophage exudation and aggregation to the site of inflammation.	Lee et al. designed a robust and biodegradable 3D tubular scaffold by the combination of electrostatic spinning technique (ELSP) and 3D printing technique, and subsequently loaded DEX onto this scaffold using a mild surface modification reaction of PDA, polyethyleneimine (PEI), and carboxymethyl β-cyclodextrin (βCD).	[33]
		Prednisolone	It can inhibit the accumulation of inflammatory cells (including macrophages and leukocytes) at sites of inflammation, and their phagocytosis. It can prevent the release of lysosomal enzymes, and the synthesis and release of chemical mediators of inflammation.	Farto-Vaamonde et al. utilized two different ways to load prednisolone or dexamethasone into a 3D printed PLA scaffold. The first one is immersing the pre-printed 3D PLA scaffold in a prednisolone solution, which covers its surface with prednisolone and allows for rapid release to exert its antimicrobial properties. The second one is immersing the polylactic acid filament in dexamethasone solution to make the polylactic acid swells reversibly.	[34]
	Animal sources	Interleukin-4	IL-4 can activate the production of Th2 cytokine and thus promotes the polarization of M2-type macrophages. IL-4 has the ability to antagonize the Th1-driven pro-inflammatory immune response, as evidenced by the downregulation of the synthesis of pro-inflammatory cytokines such as IL-10 and TNF-α and the inhibition of pro-inflammatory chemokines.	Wang et al. used GelMA-Dextran (PGelDex) as bioink and incorporated both IL-4 loaded silver-coated gold nanorods (AgGNRs) and hMSCs.	[35]
		Ac2-26 peptide	It can inhibit tumor necrosis factor-α (TNF-α) production in monocytes, inhibit NF-κB signaling of the proinflammatory pathway, and promote phagocytosis of neutrophils.	Xu et al. fabricated a polylactic acid/4-arm polyethylene glycol hydrogel (PCL@tetra-PEG) composite scaffold with the encapsulation of Ac2-26 peptide.	[36]
	Plant sources	Curcumin	Its inhibitory effect on cytokine production and expression. Curcumin can exert anti-inflammatory activity in LPS/interferon-γ (IFN-γ) treated macrophages through several mechanisms, for example, by blocking nuclear factor-κB (NF-κB) and signal transducer and activator of transcription 1 (STAT1) signaling pathways, thereby inhibiting LPS-induced IL-6 expression in RAW264.7 cells. Flavin derivatives can also inhibit NO, TNF-α and IL-1β expression by suppressing the mitogen-activated protein kinase (MAPK)/NF-κB pathway in IFN-γ/LPS-stimulated macrophages.	Chen et al. made mesoporous CS (MesoCS/curcumin) scaffolds with curcumin. MesoCS nanoparticles were first prepared using a template followed by dissolving turmeric as a stock solution in 0.5 M NaOH to various concentration and finally mixing the scaffolds at the same 0.4 mL/g liquid/powder ratio to make the scaffolds.	[37]
		Isoflavones	It can eliminate free radicals produced by macrophages during inflammation, thereby reducing the release of various inflammatory mediators induced by ROS. It have ability to inhibit the phosphorylation of IκB, an inhibitor of NF-κB, thereby reducing the degradation of IκB, inhibiting NF-κB activation, and subsequently reducing the pro-inflammatory cytokine production among others.	Sarkar et al. loaded three soy isoflavones (Genistein, Daidzein, and Glycitein) in a 5:4:1 ratio mimicking their original ratios in soy on 3D printed TCP scaffolds with pores.	[38]
		Quercetin	It can induce phosphorylation of protein kinase B and promote signal transduction and nuclear translocation of transcriptional activator protein 6 (STAT6), it can increase the expression of the M2 phenotype upon binding to anti-inflammatory cytokine-related genes. Quercetin can block the NF-κB signaling pathway, thus promoting M2 polarization for anti-inflammatory repair effects.	Wang et al. fabricated layered micro/nano surfaces on the surface of Ti6Al4V implants using 3D printing technique, alkali heat treatment and hydrothermal treatment after deposition of titanium dioxide (TiO2) on this surface. Because of the excellent ability to chelate metal cations of quercetin, it can be adsorbed as a monomer on TiO2, effectively creating a quercetin coating on the surface of the 3D printed Ti6Al4V implant.	[39]
Regulation of collagen deposition	Collagen		Different types and ratios of collagen deposition play different roles in different periods of wound repair.	Martin et al. utilized 3D printing technology to prepare a PLA scaffold and combined the scaffold with collagen, minocycline and bio-inspired citrate hydroxyapatite nanoparticles (cHA).	[28]
	Extracellular Matrix		Extracellular matrix is mainly composed of various components such as collagen, non-collagen, and elastin, and collagen as the main component can not only give the extracellular matrix the function of directly regulating collagen deposition, but also avoid the degradation and shrinkage process caused by a single collagen.	Kim et al. used S-dECM bioink to print and fabricate 3D pre-vascularized skin patches for wound healing with infusion of ASCs and EPCs.	[23]
	Silk fiber		As one of the filamentous proteins, Antheraea assama SF can improve cell fate and promote ECM secretion owing to its Arg-Gly-Asp (RGD) content.	Bandyopadhyay et al. mixed a blend of SF from mulberry (Bombyx mori) and non-mulberry (Antheraea assama) silk with gelatin as inks for 3D bioprinting to simulate the extracellular microenvironment of the meniscus. Subsequently, EDC and NHS were chemically cross-linked to enhance their stability and mechanical strength.	[40]
	Curcumin		Curcumin has the function of inhibiting the NF-κβ signaling pathway, thus promoting osteoblast differentiation and the secretion of ECM.	Bose et al. fabricated a PCL-PEG+curcumin-coated TCP scaffold using 3D printing technology.	[41]
	Granulocyte colony-stimulating factor		As an anti-inflammatory protein, endometrial stem cell-derived G-CSF can reduce scar formation by reducing Gli2 protein expression levels to regulate collagen deposition and reduce endometrial fibrosis.	Wen et al. made G-CSF-loaded slow-release microsphere (G-CSF-SRM) hydrogel scaffolds with 3D printing technology.	[42]
	Polyvinylidene fluoride		Polyvinylidene fluoride (PVDF) has excellent piezoelectric properties, biocompatibility, thermal stability and resistance to chemical irritation. Electricity generated by piezoelectric materials can reverse the effects of the gradual weakening of bioelectric stimulation with wound healing which may cause disruption of gene regulation, leading to downregulation of the wound healing cascade and eventually disorganized deposition of collagen fibers and abnormal remodeling of the ECM.	Liang et al. fabricated a novel ZnO nanoparticle-modified PVDF/SA piezoelectric hydrogel 3D scaffold (ZPFSA). The incorporation of SA precisely reduces the hydrophobicity of PVDF and creates the possibility of preparing hydrogels from its 3D printing.	[43]