soft MEAs were applied for localized recordings of action potentials from HL-1 cells,

testifying to the suitability of the printed devices for electrophysiological measurements.

This work represents a far-reaching step toward the design of soft hydrogel–based

bioelectronic devices using inkjet printing.

An ultra stretchable hydrogel device with custom-designed microchannel patterns per­

fused with ionic liquids was formed. The hydrophobic ionic liquids were sufficiently con­

ductive and remain stably separated with aqueous surroundings in the air as well as

underwater. A hydrogel matrix was prepared using highly water-soluble elastin peptide

cryogel to achieve ultra-flexible scaffold, and further reinforced with gelatine, yielding an

excellent and biocompatible gelation material [48]. This conductive hydrogel with a fixed

shape showed excellent flexibility and injectable property, suggesting its potential appli­

cation as a syringe-injectable biosensor or bioelectronics. A biocompatible ionic hydrogel

made of polyvinyl alcohol, silk fibroin, and borax was prepared, which showed ultrahigh

stretchability, water retention, self-healing, tunable conductivity, and adhesion. This hy­

drogel could be used as a sensing platform to monitor surrounding body motion for

applications in healthcare monitoring, soft robotics, and human-machine interfaces. A

gelatin/ferric-ion-cross-linked polyacrylic acid (GEL/PAA) dual dynamic supramolecular

network was formed, which, on soaking into a NaCl glycerol/water solution to further

toughen the gelatin network via solvent displacement, yielded a high toughness and high

ionic conductivity [49]. Highly stretchable and multifunctional ionic microdevices are then

fabricated based on the organohydrogel electrolytes by simple transfer printing of carbon-

based microelectrodes onto the prestretched gel surface. Proof-of-concept microdevices

including resistive strain sensors and micro-supercapacitors are demonstrated, which dis­

played outstanding stretchability to 300% strain, resistance to dehydration for >6 months,

autonomous self-healing, and rapid room-temperature degradation within hours.

Smart and robust nanofibrillar poly(vinyl alcohol) (PVA) organohydrogels were fab­

ricated via one-step physical cross-linking. The nanofibrillar network cross-linked by

numerous PVA nanocrystallites enables the formation of organohydrogels with high

transparency, drying resistance, high toughness, and good tensile strength. For strain

sensor application, the PVA ionic organohydrogel, after soaking in a NaCl solution,

shows excellent linear sensitivity (GF = 1.56, R2 > 0.998) owing to the homogeneous

nanofibrillar PVA network. The potential application of the nanofibrillar PVA-based

organohydrogel in smart contact lenses and emotion recognition was demonstrated. Such

strategy paved an effective way to fabricate strong, tough, biocompatible, and ionically

conductive organohydrogels, shedding light on multifunctional sensing applications in

next-generation flexible bioelectronics.

A phenylboronic acid-based, hydrogel-interlayer radio-frequency (RF) resonator is de­

monstrated as a highly responsive, passive, and wireless sensor for glucose monitoring [50].

Constructs are composed of unanchored, capacitively coupled split rings interceded by

glucose-responsive hydrogels. These sensors exhibited no signal drift or hysteresis over

the period. This non-degradative, long-term nature of both RF read-out and phe­

nylboronic acid-based hydrogels will enable biosensors capable of long-term, remote

read-out of glucose. A conducting hydrogel immobilized enzyme-based amperometric

biosensor was devised for glucose determination on to platinum electrode as a viable

biotransducer. Dong et al. designed injectable self-healing conductive hydrogels as cell

delivery vehicles for cardiac cell therapy in case of myocardial infarction [6]. The de­

veloped CS-AT and PEG-DA hydrogel exhibited excellent self-healing, tissue adhesive,

cell proliferation, antibacterial activity, and cell delivery ability in chosen H9c2 and

C2C12 myoblasts for cardiac repair.

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