Table 2.

Conductive additives used for fabricating conductive biomimetic materials and their biomedical applications and performance.

Type of additive	Conductive additive	Hydrogel	Concentration	Method	Advanced property	Application	Performance	Ref.
Conductive polymers	PANi	PEGDA	1 and 3 wt%	In situ precipitation of PANi in PEGDA solution	Improved conductivity of 1.1 × 10−3 mS/cm, enhanced water retention and proton conductivity	Neural tissue engineering	Promote cell attachment and neural differentiation of PC12 and hMSC cells with improved extension of neuritis and expression of membrane neurite marker growth associated proteins	944
		Cellulose	/	Oxidizing aniline inside of cellulose hydrogel using Ammonium persulfate	Improved conductivity to 0.68 S/cm, reduced strength, elongation at break and moisture content	Neural tissue engineering	Excellent biocompatibility, promote cell adhesion of RSC96 cells, and guide extension of neurons in neural repairing in adult SD rats	945
		Quaternized chitosan	1–4 wt%	Adding aniline and reacting	Improved conductivity of 4.3–15.7 × 10−4 S/cm, electrostatic adherence ability	Tissue engineering and antibiosis	Good antibacterial activity for both Gram-negative and Gram-positive bacteria in vitro and vivo, improved biocompatibility, promote the proliferation of C2C12 myoblast cells	946-947
		Polyaniline/myo-inositol hexakisphosphate	/	Gelating PANi using myo-inositol hexakisphosphate	Conductive and biocompatible	Tissue engineering	Promote cell adhesion and proliferation of rat endothelial progenitor cells, and induce milder inflammatory responses after implantation	948
		GelMA	0.16 M aniline monomers	In situ polymerization of aniline monomers within GelMA matrix	Improved conductivity with low resistance and impedance	Tissue engineering	Biocompatible, promote cell adhesion and spreading of C3H/10T1/2 murine mesenchymal progenitor cells, and can be fabricated to microarchitecture with defined pattern to guide cell growth	913
	PPy	Chitosan	3 and 30%	Grafting pyrrole to chitosan using chemical oxidative polymerization	Improved conductivity of 2.4 × 10−4 S/cm, semiconductive properties	Cardiac tissue engineering	No influence on cell attachment, metabolism or proliferation in vitro, enhanced Ca2+ signal conduction and improved electric coupling in vitro, decreased QRS interval and increased transverse activation velocity and improved heart function when injected in vivo	897
		Oligo(polyethylene glycol) fumarate	0.4 M pyrrole solution	Immersing oligo(polyethylene glycol) fumarate network in pyrrole solution	Improved conductivity and reinforced mechanical properties	Neural tissue engineering	Biocompatible, promote cell attachment and neural differentiation, increase neurite lengths	949
		Alginate	0.001-0.02 M pyrrole solution	Chemically polymerizing pyrrole within alginate hydrogels using FeCl3	Improved conductivity of 1.1 × 10−4 S/cm, increased stiffness	Neural tissue engineering	Promote cell adhesion, growth and expression of neural differentiation markers of hBMSCs, and induce mild inflammatory reactions after implantation	950
		HA	0.01-0.1 M pyrrole solution	Mixing pyrrole and PyHA solution and chemically polymerizing pyrrole using ammonium persulfate	Improved conductivity of 7.27 mS/cm, increased stiffness	Tissue engineering	Promote attachment and growth of fibroblasts	951
		Collagen/alginate	19-30 wt% of Alg-graſt-PPy	Incorporating Alg-graſt-PPy with collagen/alginate solution	Improved conductivity of ~25 mS/cm	Injectable hydrogel for tissue engineering	Rheological capacity and syringeability, support cell viability of hBMSCs	952
		Poly(hydoxyethyl methacrylate)	/	Potentiostatic electropolymerization of pyrrole	Charge storage capacity of 10 mC/cm2	Enzyme-based biosensor	Increased apparent Michaelis constant and biotransducer sensitivity, improved stabilization	953
		Poly(acrylic acid)	/	Dispersing of PPy powder in poly(acrylic acid) solution	Improved conductivity, electro-induced gel swelling capacity	Controlled drug delivery	Controlled drug diffusion from conductive hydrogels under electric field	954
	PEDOT	PVA	1-3 wt%	Electropolymerization	Charge storage capacity of 52-72 mC/cm2, low electrical impedance	Neural tissue engineering	Improved proliferation of olfactory ensheathing cells, significant increasd proportion of flatter cells with extended cytoplasm	955
		Agarose	0.01 M EDOT solution	Electrodepositing of EDOT inside of hydrogel	Improved conductivity	Neural tissue engineering	Promote directional and controlled axonal regeneration in nerve gap in rats	914
		Poly(3,4 ethylenedioxythiophene)/para-toluenesulfonate	0.1 M EDOT solution	Electrodepositing of EDOT	Improved conductivity with low impedance	Neural electrodes for cochlear	Improve electrical properties without affecting the mechanical properties of the electrode array, remain conductive under 2 billion electrical pulses	639,956
		Methacrylated PVA	/	Electrodepositing of EDOT	Improved conductivity	Neural electrodes and neural tissue engineering	Promote proliferation of olfactory ensheathing cell and neural differentiation of OECs co-cultured with PC12 cells under electrical stimulation	957
		PVA	0.1 M EDOT solution	Galvanostatic electrodeposition of ethylenedioxythiophene	Enhanced conductivity of 1.1±0.2 S/cm, increased charge storage capacity and stiffness	Medical electrodes with drug delivery capability	Promote cell proliferation of PC12 cells and neurite extension, and improve attachment and differentiation of neural like cells with the delivery of nerve growth factor	958-959
		Bacterial cellulose	/	Immersing BC microfiber in EDOT solution	Core/shell BC/PEDOT hydrogel microfiber, improved conductivity	Controlled rug delivery and tissue engineering	Controlled delivery of diclofenac sodium under electrical stimulation, excellent biocompatibility and electroactivity of the hybrid microfibers for PC12 cell culture	960
	Tetraaniline	PEG	3 wt% of chitosan-graft-aniline tetramer	Grafting tetraaniline on chitosan and then blending with PEGDA solution	Improved conductivity of ~10−3 S/cm, reinforced mechanical properties	Cardiac tissue engineering	Adhesiveness to host tissue and antibacterial property, support cell viability of C2C12 cells before and after injection, tunable release C2C12 myoblasts and H9c2 cardiac cells after cell delivery	961
		Oxidized alginate	7.1-21.3 wt%	Incorporating tetraaniline-graft-OA with oxidized alginate	Improved conductivity of 7.52 × 10−6 S/cm and reinforced mechanical properties	Injectable hydrogel for tissue engineering	Support cell growth of MSCs, induce mild inflammation response when implanted into chick chorioallantoic membrane for 1 week	962
Carbon-based materials	CNT	Collagen	0.5-2.0 mg/mL	Dispersing in chitosan solution and mixing with collagen solution	Improved conductivity and reinforced mechanical properties	Cardiac tissue engineering	Promote cell viability, adhesion and beating function of HL-1 cardiomyocytes	963
		GelMA	1-5 mg/ml	Blending and sonicating	Low impedance, increased compression modulus	Cardiac tissue engineering	Improved cardiac cell adhesion, organization and cell-cell coupling, and the tissues resist damage by a model cardiac inhibitor	898
		Gelatin	0.5-2.5 mg/mL	Mixing SWCNTs in gelatin solution	Improved conductivity of ~4× 10−5 S/cm, reinforced mechanical properties	Cardiac tissue engineering	Promote cardiac contraction and the expression of electrochemical associated proteins in vitro, and the conductivity hydrogels structurally integrate with the host myocardium and improve the heart function after implantation in vivo.	964
		Collagen I/Matrigel	20-100 μg/mL	Mixing carbon nanotube dispersion with hydrogel solution	Improved conductivity of 2.4 S/m	Neural tissue engineering	Promote neurite outgrowth in isolated dorsal root ganglia, and further promote the neurite outgrowth and neurite length under electrical stimulation	965
		Collagen	10-50 μg/mL	Mixing carbon nanotube dispersion with collagen solution	Improved conductivity	Neural tissue engineering	Support cell viability but inhibit cell proliferation of Schwann cells in 2D culture, while in 3D culture the cell proliferation, viability, or morphology are not influenced	966
		PHEMA	1-6 wt% to HEMA	Mixing	Improved conductivity of 8.0 × 10−2 S/cm, reinforced mechanical properties	Neural tissue engineering	Improved biocompatibility, protect SHSY5Y neuroblastoma cells from electrical potential application with no loss of cell activity	967
		Chitosan	0.5 mg/mL	Blending, sonicating and electrodepositing	Excellent electrochemical activity and conductivity, high content of oxygen functional groups	Microbial electrocatalysis	Increased current generation and the maximum power density	968
	Carbon nanobrush	Poloxamer	0.1-5 vol%	Blending	Improved conductivity	Tissue engineering	Cardiac fibroblasts and myocytes are survived and proliferated in hydrogel containing 0-1vol% carbon nanobrush	647
	GO	Methacryloyl-substituted tropoelastin	1 and 2 mg/mL	Mixing and sonicating	Improved conductivity and resilience	Cardiac tissue engineering	Biocompatible, support growth and function, and enhance activity and maturation of cardiomyocytes, induce mild inflammatory response after implantation in vivo	899
		PEGDA700-Melamine/HA	0.5 mg/mL	Mixing	Improved conductivity of 2.84 × 10−4 S/cm, soft and anti-fatigue mechanical property	Cardiac tissue engineering	Promote the expression of cardiac specific proteins of adipose tissue-derived stromal cells after injection of cell-laden hydrogel into MI area of rats, and improve the transmission of mechanical and electrical signals and heart functions	969
	rGO	PA	0.3 wt%	Blending GO with PA and then reducing	Improved conductivity of 1.3× 10−4 S/cm, reinforced mechanical properties	Muscle tissue engineering	Enhance proliferation and myogenic differentiation of C2C12 cells, and combining electrical stimulation further enhanced the myogenic gene expression	937,970
		Acrylamide	2-4 wt% to acrylamide	Reducing GO by polydopamine and then polymerizing acrylamide	Improved conductivity of 0.18 S/cm, high stretchability and toughness, self-healable	Medical electronics	Self-adhesiveness to skin, self-healability resembling to natural tissue both mechanically and electrically, excellent biocompatibility without causing any inflammation after implantation in vivo	971
	Carbon nanofiber/rosette nanotube	PHEMA	5-10 mg/mL CNF, 0.01-0.05 mg/mL RNT	Mixing	Improved conductivity and hydrophilicity, increased surface roughness	Cardiac tissue engineering	Promote cell viability and adhesion of a transformed human cardiomyocyte cell line, injectable	972
	rGO/CNT	Oligo(poly(ethylene glycol) fumarate)	~5% rGO, ~0.5% CNT	Covalently embedding by chemical cross-linking and followed by reducing	Improved conductivity of 5.75 × 10−3 S/m, reinforced mechanical properties	Neural tissue engineering	Biocompatible, promote cell proliferation and spreading of PC12 cells, and improve the neural differentiation and robust neurite formation under the application of nerve growth factor	973
Gold based materials	AuNP	Chitosan	0.5-1.5 wt%	Blending	Improved conductivity of 0.13 S/m,	Cardiac tissue engineering	Support viability, metabolism, migration and proliferation of MSCs, enhanced cardiomyogenic differentiation under electrical stimulation	974
		Thiol-2-hydroxyethyl methacrylate/hydroxyethyl methacrylate	/	Reducing colloidal Au and Au3+ ions inside of hydrogel in sodium borohydride solution	Improved conductivity of 15.3 S/m, reduced stiffness	Cardiac tissue engineering	Tunable conductivity and elasticity suitable for engineering cardiac tissues, excellent biocompatibility, support cell adhesion of neonatal rat cardiomyocytes, promote Cx-43 expression and cardiac function under electrical stimulation	920
	AuNR	GelMa	0.5-1.5 mg/mL	Mixing AuNR with GelMA solution	Improved conductivity with low impedance, and reinforced mechanical properties	Cardiac tissue engineering	Promote cell retention, viability, metabolic activity, tissue formation, cardiac specific protein expression and synchronous beating function of isolated cardiomyocytes	975