Table 1.

Structural engineering of hydrogel wound dressing.

	Method of fabrication	Material	Crosslinking method	Additives	Study	Structural features	Biological results	Ref
1	Prefabricated casting	GelMA	Photocrosslinking	------	In vitro	Compressive modulus up to 200 kPa is dependent on polymer concentration and UV exposure time. Prolonged degradation up to 8 weeks using 20% GelMA.	Over 90% viability of human epidermal keratinocytes (HaCaTs) cultured on the hydrogels. Cellular attachment increases on stiffer hydrogels. Differentiation and stratification of HaCaTs into a multilayered epidermis.	[34]
2	Prefabricated casting	Heparin and PEG-DA	Photocrosslinking	hEGF	In vitro/in vivo	Swelling capacity was higher in heparin-containing hydrogels. Heparin-containing hydrogels have a G′ of 14 kPa and a WVTR of 1010 g/m2/day. Heparin extended the release of hEGF up to 21 days.	hEGF loaded heparin-based hydrogel enhanced wound closure, granulation tissue formation, epithelialization and capillary formation compared to control groups.	[36]
3	Prefabricated casting	CS, PEG, and PVP	Ionic crosslinking	Tetracycline hydrochloride	In vitro/in vivo	In the presence of PVP, a spherical porous structure was observed, and the swelling rate increased from ⁓400%–1210%. Tensile strength of 0.8 MPa for 60% humidity and WVTR of ⁓2230 g/m2/day were detected for the CS/PEG/PVP sample.	80% of drug was released after 48 h antimicrobial effect was observed in vitro against both gram-positive and gram-negative bacteria. Antibiotic containing samples accelerated wound healing with minimum scar formation.	[40]
4	Prefabricated casting	HA	Dynamic coordinate bond between EDTA−Fe3+ complexes cross-linked with hyaluronic acid (HA)	Platelet-derived growth factor BB (PDGFBB)	In vitro/in vivo	Higher G′ values than G″ for different ratios of Fe3+−EDTA. G′ ≥ 200 Pa for Fe3+−EDTA complexes and HA units equal to 1:2.	In vitro antibacterial effect against E. coli and S. aureus due to release of Fe3+. Promoted in vivo skin regeneration with no inflammation 10 days post implantation.	[42]
5	Prefabricated casting	Pectin & PAA	Interpenetrating network with free-radical polymerization	Lignin-coated Ag NPs & epidermal growth factor (EGF) EGF	In vitro/in vivo	High resilience and stretchability up to 26 times the primary length. NPs-P-PAA hydrogel showed tensile strain of 2660%, ductility of 300 MPa%, fracture energy of 5500 Jm−2 and adhesion strengths to porcine skin of 25 kPa for the NPs-P-PAA hydrogel.	High antibacterial activity in vivo with increased wound healing and skin tissue regeneration compared to control groups.	[43]
6	In situ forming hydrogel	Collagen type I, Poly-d-Lysine (PDL) and chondroitin sulfate	Covalent and ionic bonding	------	In vitro/in vivo	Chondroitin sulfate extended the gelation time to 1 h. A pore size of 10 μm was observed for the collagen/PDL sample. Adding air microbubbles increased the pore size to ≈30–40 μm.	Support for migration of MSTC-derived cells. Bacterial growth inhibition in vivo and promoted wound healing.	[47]
7	In situ forming hydrogel	Silk fibroin (SF) extracted from two different silkworm	Physical entanglements and crosslinking	------	In vitro/in vivo	Pore size of ≈155 μm, Water retention of 75% for SF blend hydrogel after 12 h compared to 20% for the collagen control group. G′ of 336.42 Pa, compressive modulus of ≈7.4 kPa and 85% of the weight preservation after 14 days was observed for SF hydrogel.	Proliferation of human dermal fibroblasts and migration keratinocytes in vitro. Improved wound healing and more collagen deposition were observed for SF hydrogel compared to the collagen control group.	[51]
8	In situ forming hydrogel	Benzaldehyde-terminated PEG and dodecyl-modified chitosan (DCS)	Reversible Schiff base	VEGF	In vitro/in vivo	Highest G′ was observed ≈600 Pa. Rheological recovery analysis demonstrated self-healing capability.	Strong blood cell coagulation and hemostasis and anti-infective properties. An increase in wound closure, deposition of collagen, angiogenesis, and granulation tissue formation by using a VEGF-loaded hydrogel.	[56]
9	In situ forming hydrogel	Oxidative hyaluronic acid (OHA), Poly-ε-l-lysine (EPL) and Pluronic F127 (F127)	Reversible Schiff's base	Adipose mesenchymal stem cells (AMSCs)-derived exosomes	In vitro/in vivo	G′ in the range of 103 Pa was observed for all samples at 37 °C, indicating formation of hydrogel. Self-healing capability was shown as G′ of hydrogel was decreased from ∼10 kPa to several pascals by an increase of strain up to 1000% and recovered at a strain of 1%.	Exosome-containing hydrogel enhanced proliferation and tube formation ability of HUVECs in vitro. In vivo analysis showed enhanced wound healing rate, angiogenesis, deposition of collagen and re-epithelialization.	[57]
10	In situ forming hydrogel	HA	Dynamic coordination crosslinking	Ag+ ions	In vitro/in vivo	G′ of ≈400 Pa and G″ of ≈100 Pa. Non-fibrous architecture	Antimicrobial effect against both Gram-positive and Gram-negative bacteria. In vivo analysis showed higher wound closure and more complete epithelium layer compared to control group.	[50]
11	In situ forming hydrogel	PDA nanoparticles and glycol chitosan (GC)	Schiff's base reaction/Michael addition	Ciprofloxacin	In vitro/in vivo	G′ ≈500 Pa. Photothermal behavior of samples was demonstrated and when exposed to NIR with a power of 0.5 W/cm2, hydrogels reached 46.8 °C to release antibiotics.	On demand antibacterial effect In vitro against S. aureus. Antibacterial efficacy and enhanced wound closure in vivo without inflammatory responses.	[58]
12	In situ forming hydrogel	Quaternized hydroxyethyl cellulose (HEC)/mesocellular silica foam (MCF)	Radical graft copolymerization	------	In vitro/in vivo	Sol-gel transition characteristics and the highest G′ and viscosity for HEC were up to 103 and ≈70 Pa, respectively. By increasing MCF, G′ and viscosity were enhanced to the range of 104 and ≈450 Pa, respectively. Swollen composite hydrogel under 37 °C showed G′ in the range of 104 Pa.	9.82 w/w% of MCF could trigger the coagulation factors. QHM hydrogel could decrease plasma clotting time to 59% in vitro compared to commercially available hemostatic. Great antibacterial and biocompatibility and enhanced wound healing.	[239]
13	In situ Injectable/Microfluidics	Alginate (microcapsules)	Ionic crosslinking	copper-/zinc-niacin	In vitro/in vivo	Size of ∼300 μm for alginate capsules containing copper-/zinc-niacin Smart release of calcium, copper, and zinc ions.	Decreased inflammation and improved angiogenesis and collagen deposition	[60]
14	In situ Injectable/Microfluidics	Poly (hydroxypropyl acrylate-co-acrylic acid)-magnesium ions (poly-(HPA-co-AA) and carboxymethyl chitosan (CMCS) (micro-gel ensemble)	Hydrogen bonding/amide bonding	Mg2+	In vitro/in vivo	Microfluidics internal/external flow rate determined microbead size (1–2 mm)	Persistent wound pH modulation Collagen deposition, macrophage polarization, and blood vessel development	[61]
15	Prefabricated 3D printing (Extrusion-based)	Cellulose nanofibril (CNF)	Ionic and chemical (by BDDE) crosslinking	------	In vitro	YM of 3.45 kPa for 1 wt% CNF hydrogel and reached 7.44 kPa after second (chemical crosslinking). No failure at 50% strain, indicating excellent elasticity.	Viability of fibroblast cells. Promoted cell proliferation compared to the 2D structure. Cell proliferation is enhanced with an increase in rigidity of the hydrogel.	[66]
16	Prefabricated 3D printing (Extrusion-based)	Chitosan	Ionic crosslinking	------	In vitro/in vivo	Total thickness of 2.1 mm. Distance between filaments ≈200 μm and a Feret diameter of ≈3.5 μm and 5 μm for the surface and within the structure, respectively. The YM of the samples ≈105 kPa.	In vitro proliferation of human fibroblasts and keratinocyte. Formation of a skin-like layer. Improved wound closure compared to non-treated groups in vivo.	[68]
17	Prefabricated Bioprinting (Extrusion-based	Pectin methacrylate (PECMA)	Ionic crosslinking & Photocrosslinking	Arginylglycylaspartic acid (RGD)	In vitro	Range of G′ between ≈79.6 Pa and 2600 Pa based on concentration and DM. After swelling, the elastic modulus of hydrogels (1.5 wt%) decreased significantly from ⁓1000 Pa to ⁓400 Pa. Polymer solutions supplemented with 5 mM and 7 mM of CaCl2 were able to form stable, neat filaments.	RGD-PECMA high hydrogels supported metabolic activity of fibroblasts when photocrosslinked for 160 s (G′ of 229.8 ± 44.0 Pa). Although with secondary ionic crosslinking, higher concentration and UV exposure time followed to stiffer hydrogels (160 s: 842.0 ± 97.8 Pa; 300 s: 1210.0 ± 148.5 Pa), spreading of the cells was significantly reduced.	[72]
18	Prefabricated Bioprinting (Extrusion-based)	Calcium silicate (Calsil) nanowires, sodium alginate (SA), Pluronic F127, and l(+)-Glutamic acid (CS + SA hydrogel)	Photothermal & Ionic crosslinking	OPC	In vitro/in vivo	Hydrogels containing OPC exhibited compacted porous structure with a small pore size.Photothermal conversion efficiency was observed by changing NIR power. G′ in the range of 102 and mechanical stiffness of 8 kPa for Calsil + SA + 6%OPC after 30 min of NIR laser exposure.	Supported proliferation, and migration of fibroblasts and human umbilical vein endothelial cells (HUVECs). Improved angiogenesis and skin regeneration in vivo.	[73]
19	Prefabricated Bioprinting (Extrusion-based)	Fibrinogen and decellularized human skin-derived extracellular matrix (dsECM).	Thrombin-Fibrinogen (Enzymatic) crosslinking	------	In vitro	Addition of dsECM to fibrinogen matrix increased G′ reached 452.6 Pa vs. 207.7 Pa for control fibrinogen and after thrombin crosslinking reached ≈800 Pa. 15 days after culture of fibroblasts, G′ was increased compared to cell-free hydrogels, 400 kPa vs 200 kPa. Fibrinogen + dsECM exhibited better shear thinning properties compared to control dsECM and fibrinogen.	Viability of fibroblast was supported till only day 8 for fibrinogen and significantly improved till day 15 in the fibrinogen + dsECM	[76]
20	Prefabricated Bioprinting (DLP 3D printing)	GelMA & HA-NB	Photocrosslinking	------	In vitro/in vivo	Tunable mechanical properties. The compressive modulus was 30–100 kPa based on polymer concentration. The pore size ranged from 200 μm to 400 μm.	Promoted fibroblast and HUVEC migration and proliferation, and Tissue in growth and improved skin regeneration in vivo.	[77]
21	Prefabricated 3D printing (FDM) 3D printer	Polyacrylamide (PAM)/hydroxypropyl methylcellulose (HPMC)	Silver−ethylene interaction	AgNPs	In vitro/in vivo	14 times the dead weight of water uptake capacity was observed for hydrogel. An average pore diameter of 100 μm observed for all samples. The addition of AgNPs and HPMC made pores interconnected. A porosity of 91% and a YM of less than 0.4 MPa were observed for super porous hydrogels.	Enhanced infected wound regeneration and inhibited scar tissue formation in vivo.	[79]
22	Prefabricated 3D printing (Extrusion-based) + Electrospinning	Alginate and PLGA	Ionic crosslinking	------	In vitro/in vivo	Bilayer membrane scaffold. Thickness of ⁓20 μm for PLGA and ⁓100 μm for alginate hydrogel. PLGA layer enhanced water retention, mechanical properties (YM ≈ 24 kPa and 531 kPa for alginate hydrogel and bilayer scaffold), and decreased degradation rate.	Bilayer membrane improved cell proliferation in vitro and promoted, wound closure, collagen deposition, and vascularization in vivo.	[78]
23	In situ bioprinting (Pressure-driven)	Heparin-conjugated hyaluronic acid (HA-HP)	Photocrosslinking	FGF and VEGF	In vitro/in vivo	Hydrogels crosslinked with four-arm crosslinkers showed a higher release rate compared to eight-arm HA hydrogels. Average pore size of ≈100 μm, 50 μm and 25 μm and G′ of ≈200 Pa, 2000 Pa, and 5700 Pa for linear, 4-armed, and 8-armed, respectively.	Prolonged release of heparin-binding growth factors. Promoted wound closure, ECM production, re epithelialization, and vascularization in vivo.	[86]
24	In situ bioprinting (handheld Skin Printer with microfluidic cartridge)	Alginate and fibrinogen	Ionic crosslinking and enzymatic crosslinking	------	In vitro/in vivo	Thickness of 100 μm–600 μm via a one-step process. Faster gelation and higher YM for alginate-based compared to fibrin-based hydrogels (≈0.25 MPa vs ≈ 0.1 MPa)	Supported interaction and viability of keratinocytes and fibroblasts. Normal re-epithelialization and wound contraction.	[82]
25	In situ bioprinting (handheld)	A two-phase aqueous emulsion bioink made of GelMA and polyethylene oxide (PEO)	Photocrosslinking	------	In vitro	The pore size ranged between 20 μm −100 μm based on the concentration and ratio of GelMA and PEO and mixing time.	Improved survival and proliferation of fibroblasts compared to standard GelMA hydrogel.	[85]
26	In situ bioprinting (handheld)	GelMA	Photocrosslinking	VEGF	In vitro/in vivo	The fiber diameter ranged from 500 μm to 2500 μm (reduction by increasing the printing speed). Compression modulus of ≈10.6 kPa and adhesion strength to porcine skin of ≈10.3 kPa were reported for samples with GelMA 12% (w/v). The average pore size was 9.57 μm.	Improved migration of endothelial cells in vitro due to the sustained release of VEGF. Improved wound closure, neoepidermis formation, angiogenesis, and decreased scar formation compared to control treatments in vivo.	[81]
27	3D printing/Microfluidics	Alginate and GelMA (hollow fibers containing microalgae)	Ionic and photocrosslinking	Chlorella pyrenoidosa	In vitro/in vivo	Depending on CaCl2 concentration, hollow fiber channel diameter could be 150–350 μm.	Promoted cell proliferation, migration, angiogenesis, and wound closure in vivo	[90]
28	Electrospinning	GelMA	Photocrosslinking	------	In vitro/in vivo	Average diameters of ≈1.52 μm and 2.18 μm for GelMA for electrospun fibers before and after incubation in PBS. Decrease in permeability was by increase of MD. Tensile strength up to ≈260 kPa.	Supported adhesion, migration, and proliferation of endothelial and fibroblast cells. Improved vascularization in vivo.	[94]
29	Electrospinning	GelMA	Photocrosslinking	------	In vitro/in vivo	Average diameter of 1200 nm after 24 h in water. Water retention capacity and YM (350 kPa) improved by an increase in UV exposure time.	Supported adhesion, migration, and proliferation of fibroblast cells. Improved wound closure and collagen deposition in vivo.	[95]
30	In situ electrospinning	Chitosan/poly (vinyl alcohol) (PVA)	Chemical crosslinking mediated by glyoxal	Halloysite nanotubes (HNT)	In vitro	HNT's hydrophilic nature led increase swelling ratio. nanofibers. Nanofibers with 3 and 5% HNT had 2.4 and 3.5 times the tensile strength of chitosan/PVA nanofibers.	HNT increased nanofibers' biocompatibility. Glyoxal does not harm fibroblast cells and can be used to crosslink chitosan/PVA nanofibers.	[100]