Another in-vivo skin-mounted skeletal muscle tissue stimulation was shown by ionic

hydrogel system localized in-vitro cultured cells electrical stimulation. Yuk et al. reported

ionically conductive tough hydrogel for constructing electronic devices under large de­

formation using soft, flexible, and stretchable conductive material [12]. For the sake of

brevity, details of such applications are presented in Table 18.1.

18.3.3 Flexible and Implantable Bioelectronics

Regular and continued monitoring of vital signs of the body such as body temperature,

blood pressure, and estimation of analytes in body fluids is necessary for maintaining a

healthy life. Currently, electrocardiography (ECG), electroencephalography (EEG), and

electromyography (EMG) are used to monitor these vital signs of the body. The acquisition

of their signals is typically achieved by metal electrodes which can cause skin damage/

irritation. Moreover, these electrodes are rigid and cannot withstand stretching and

bending. Therefore, soft and flexible epidermal patches are highly desirable. Conductive

hydrogels are tissue-friendly and their tunable electrical properties, flexibility, and bio­

compatibility make them useful for epidermal patches. A conducting hydrogel was pre­

pared by mixing polyvinyl alcohol (PVA), borax, and PEDOT:PSS screen-printing paste to

use in epidermal patches. Prepared hydrogels exhibited high skin adhesion, high plastic

stretchability, moderate conductivity, and self-healing properties. The hydrogel was ap­

plied for the recording of ECG and EMG signals, which showed high-quality recording.

An ion-conducting (Ch-CMC-PDA) hydrogel using chitosan (Ch), cellulose (CMC), and

dopamine (DA) [29]. Ch-CMC-PDA the hydrogel was applied for ECG signal detection; the

result showed that the ECG pattern obtained using Ch-CMC-PDA was identical to com­

mercial gel Cardijelly.

PEDOT:PSS-PAAm organogels possess a better transport of electrical signals and was

highly stretchable. PEDOT:PSS added to CS/PVA scaffolds was introduced for the sake

of better mechanical and electrical properties for cardiovascular engineering. The con­

ductive PDA–pGO–PAM hydrogels with high stretchability, self-healing ability, and self-

adhesiveness potential pave the way as cell stimulators and implantable bioelectronics for

the human body (Figure 18.2) [18]. Soft and conductive r(GO/PAAm) hydrogels were

also found to be useful material for skeletal muscle tissue engineering scaffolds [19]. The

incorporation of small amounts of CNTs into gelatin-chitosan-based hydrogel supports

cardiomyocyte function and helped to attain the electrical conductivity of the beating rate

of the hearts [22]. Tissue-engineered scaffolds with the combined properties of CNTs

improved the cardiovascular defect repairs. The conductive biopolymer-based hydrogel

can behave as an artificial nerve in a 3D-printed robotic hand. This may allow tunable

electrical signals to pass and full recovery with robotic hand movements. This natural

highly elastic (up to 900 kPa) ionic conductive hydrogel is visualized to contribute to

artificial flexible electronics. The conducting 3D-printable scaffolds showed good cell

adhesion, are biodegradable, and have cytocompatible properties to be used in tissue

engineering. C2C12 myoblasts grown on the hybrid GelMA-vertically aligned CNT hy­

drogels yielded functional myofibers [46], after applying electrical stimulation in the

direction of the aligned CNTs, than cells that were cultured on the GelMA hydrogels with

randomly distributed and horizontally aligned CNTs.

The development and application of printed MEA arrays on soft substrates including

PDMS and hydrogels were conducted [47]. To this end, a straightforward printing pro­

cess was introduced that exploits controlled wetting properties of carbon and polyimide

inks on PDMS, curtailing major problems that are often faced in printing structures. The

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Bioelectronics