18.3 Applications of Hydrogels in Bioelectronics

18.3.1 Coating of Hydrogel on the Neural Electrode

Neuron acts as an interface for the communication between the central nervous system

and bioelectronics devices. But chemical and the mechanical disparity between neuron

and bioelectronics devices cause aggravated inflammation response, an unreliable signal

collection due to nonconforming contact between the devices and the surface of skin or

tissue. The coating of a hydrogel on neural electrodes improves the functionality of

bioelectronics devices by providing intimate cellular integration and mechanical buffer

between hard electrodes and soft tissues. But as the thickness of a hydrogel coated on

neural electrodes increases, it may hamper the optimal performance of neural electrodes

due to lack of neurons near the electrode. The possible solution is to coat a conducting

polymer on the electrode to restore loss of functionality by an increase of the thickness of

the hydrogel on the electrode. To visualize the effect of hydrogel thickness on the re­

cording quality of neural electrodes, ionically cross-linked alginate hydrogel (AH) having

different thicknesses were prepared on the neural electrode by dip coating. It was ob­

served that as the AH thickness increased, the number of clearly detectable units gra­

dually decreased, which could be due to a lack of neurons immediately around the

electrode sites. Furthermore, the conducting polymer PEDOT was also deposited on the

neural electrode along with AH. This improved the recording functionality of the AH-

coated electrodes. The biocompatible hydrogel was also applied for the differentiation of

human neural stem cells to enhance neuritogenesis via the electrical stimulation process.

Flexible PEDOT-based sodium alginate hydrogel-coated neural electrodes for the sensitive

neural recordings in guinea pig auditory were reported. PEDOT-CNT encapsulated fibrin

hydrogel-coated electrodes were designed to record somatosensory induced potentials into

a rat cortex through the deflection of multi-whisker. While agarose hydrogels doped with

surface-modified cellulose nanocrystals were fabricated to produce a diode [23]. A bionic

ear via 3D printing of a cell-seeded hydrogel matrix in the geometry of a human ear, with an

intertwined conducting polymer embedded with silver nanoparticles, was fabricated [24].

In-vitro culturing of cartilage tissue around an inductive coil antenna in the ear was per­

formed, which enabled the readout of inductively coupled signals from cochlea-shaped

electrodes. Table 18.1 presents an overview of conductive hydrogels with their specific

features and applications.

18.3.2 Artificial Skin

The physiological environment has a huge impact on the performance of hydrogel

bioelectronic devices. Some hydrogel bioelectronic devices become unstable and fragile

on exposure to aqueous solutions or harsh physiological environments, significantly

impeding their desired applications. Biostable hydrogel bioelectronic devices that can

maintain their super mechanical and conductive properties, even when exposed to bio­

fluids are highly desirable. By utilizing biocompatible cellulose and conducting reduced

graphene oxide (rGO), a biostable conducting hydrogel was prepared. A 2D planar cel­

lulose crystal structure using the polydopamine-reduced graphene oxide was prepared.

This 2D planar cellulose crystal after physical and chemical cross-linking self-assembled

into a conducting hydrogel. This hydrogel showed high biostability and could withstand

long-term immersion in aqueous environments and implantation for over 30 days [45].

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Bioelectronics