tissue softness. Due to greater electronic conductivity, ionically conductive hydrogels

found wide applicability in bioelectronic applications. Ionic electrolytes have been in­

troduced within a hydrogel as a promising material to improve the conductivity of edible

electrodes [11]. They fabricated highly swollen, robust, and conductive hydrogel mate­

rials made from food material for the enhancement in the edible device. This hydrogel

electrode was developed by soaking the alginate-gelatin hydrogels in the electrolytic

solution i.e., saturated CaCl2 and NaCl solution. The conductivity of hydrogel was

highly enhanced with the addition of ionic species. The conductivity for alginate-

gelatin hydrogels from edible supermarket foods was 190 ± 20 mS/cm while that of

gelatin/gellan gum soaked in a solution of NaCl or CsCl as ionic species was found to

be 200 ± 20 mS/cm and 380 ± 20 mS/cm, respectively. Ionically conductive, robust

hydrogel was fabricated by crosslinking polyacrylamide (PAAm) and alginate with

calcium sulfate. The PAAm-alginate hydrogel via UV irradiation was bonded with

Ecoflex elastomer by gelation process which looks to be a valuable candidate for

electronic devices under large deformation using soft, flexible, and stretchable con­

ductive material [12] (Figure 18.1).

The electrical conduction within the electronically conducting polymers infused with

ionic electrolytes is also monitored using electrical impedance spectroscopy (EIS).

Conductance of alginate-based hydrogels as a function of different ionic species using

EIS indicated that the lower concentration of electrolyte showed minimal frequency

dependence, whereas the higher concentration of electrolyte displays a larger con­

duction charge between the lower stimulation frequency to higher stimulation fre­

quency [13]. Ionically conductive hydrogel-based circuits using salt-soaked poly

(ethylene glycol) diacrylate were designed to generate programmed ionic circuits [14].

High conductivity salt solutions were incubated within a PEG hydrogel to give rise to

patterned ionic current to enable localized in-vivo muscle electrical stimulation. This

strategy offered integrated electronic platforms to distribute ionic electrical signals

between tailored and biological systems. The ability of the ionic hydrogel system

is displayed for light-emitting diode (LED) activation, localized in-vitro cultured cells

electrical stimulation, and in-vivo skin-mounted skeletal muscle tissue stimulation. A

biocompatible, elastic rubber-like ionic conductive hydrogel consisting of polyvinyl

alcohol (PVA) and hydroxypropyl cellulose (HPC) biopolymer fibers enhanced the

ionic conductivity up to 3.4 S/m, at 1 MHz frequency on ions migration within the

hydrogel. It can behave as an artificial nerve in a 3D-printed robotic hand allowing

tunable electrical signals.

18.2.5 Conductive Filler–Based Hydrogels

Conductive fillers including graphene, carbon nanotubes, and metal nanoparticles

within the hydrogel network are used to augment the conductivity, toughness, and

stretchability of hydrogels. These include metallic nanoparticles, graphene-based ma­

terials, nanofibers, nanotubes, or conducting polymers. Metallic nanoparticles have

been added to attain the desired electrical conductivity of hydrogels. Gold nano­

particles (AuNPs) embedded in thiol 2-hydroxyethyl methacrylate nanocomposite-

based conductive hydrogel were designed with tunable electrical and mechanical

properties. The neonatal rat cardiomyocytes were grown the conductive scaffold en­

hanced the expression of connexin-43 with or without any electrical stimulation. Silver

nanoparticles (AgNPs) incorporated polyacrylic acid-based hydrogel using methylol

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Bioelectronics