. 2023 Mar 24;11:1150147. doi: 10.3389/fbioe.2023.1150147

TABLE 2.

Selection of hydrogel-based bioelectronic devices, classified according to the fabrication technique. Devices are qualitatively scored based on a level of maturity (LM) scale: 1- Proof of concept (mechanical, electrical and electrochemical testing); 2- In vitro testing (e.g., cytotoxicity); 3- In vivo acute testing on small animals (rodents); 4- In vivo acute testing on large animals (pigs, monkeys, cats, dogs); 5- In vivo chronic testing on rodents; 6- In vivo chronic testing on large animals; 7- Clinical trials.

Fabrication process	Hydrogel material	Function of the hydrogel in the device	Application	LM	Key metrics	Resolution	Ref
Photo-lithography	PEDOT: PSS hydrogel	Conductive hydrogel encapsulated in elastomer PFPE-DMA	Stimulation of the sciatic nerve	5	• σ = 47.4 ± 1.2 S/cm	2 µm	Liu et al. (2019)
					• t_H = 200 nm
					• E = 32 ± 5.1 kPa (compression)
					• Z > 100 MΩ
	PEG	Coating of the electrode, hydrogel loaded with anti-inflammatory drugs and PEDOT:PSS	Stimulation of the sciatic nerve	1	• Z = 580.2 ± 40.1 Ω (1 kHz)	200 µm	Heo et al. (2016)
					• t_H = 208 ± 11 μm
					• CDC = 2.67 ± 0.37 μC/mm²
Soft lithography	HA–Collagen I–laminin	Encapsulation of polyimide microelectrodes	TE electronic nerve interface	1	• Diameter = 1mm	1 mm	Spearman et al. (2020)
	HA–Collagen I–laminin	Encapsulation of polyimide microelectrodes	TE electronic nerve interface	1	• E_H = 2.55 ± 0.05 kPa	1 mm	Spearman et al. (2020)
	PU	PU encapsulation and PEDOT/PU–hydrogel hybrid (working electrode)	Advanced tissue engineering including electronics	2	• σ = 120 S/cm	40 µm	Sasaki et al. (2014)
					• t_H = 200µm
					• Elongation ratio = 100%
Direct ink writing and Inkjet printing	PEDOT:PSS hydrogel	PEDOT: PSS electrical circuit encapsulated in PDMS	Neural probe	5	• E = 1.1 MPa	30 μm	Yuk et al. (2020)
					• σ = 28 S/cm
					• Z = 50–150 Ω (1 kHz)
	PANI hydrogel	Conductor track	N.A.	1	• Capacitance = 480 F/g	18 μm	Pan et al. (2012)
					• R = 3.2 Ω
					• σ = 0.23 S/cm
Transfer printing	Fe³⁺ -[PEG-Dopa]₄	Encapsulation	Microelectrode array for recording the sciatic nerve	6	• E = 17.9 ± 0.3 kPa	25 μm	Huang et al. (2018)
					• t_H = 1 mm
					• Z = 32.2 ± 8.3 kΩ (f = 1 kHz)
	Gelatin and GelMa	All hydrogel-based device GelMa doped with Ag NWs, Pt NWs, and PEDOT:PSS	Microelectrode array for neural interface	2	• Z_GelMa = 38.3–52.4 Ω (f = 1 kHz)	30 µm	Lin et al. (2021)
					• E = 180 kPa
					• R_s = 592 ± 22.7 Ω
Laser patterning	PEDOT: PSS	PEDOT: PSS hydrogel encapsulated in PDMS	Stimulation and recording of the sciatic neve	5	• E = 57 MPa	6 µm	Won et al. (2022)
					• σ = 670 S/cm
					• Z = 6 kΩ (f = 1 kHz)