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Agarwala
A C

D

B
E

Figure 4. Schematic representation of (A) peel and transfer process (Reproduced with permission ),
[79]
and (B) 3D printing microvalve process for spatially resolved microelectrode circuits on hydrogel
substrates (Reproduced with permission ). Photograph of (C) bended AgNW-based microelectrodes
[81]
on hydrogel (Reproduced with permission ), (D) liquid metal injected PDMS layers for ECG electrode
[79]
(Reproduced with permission ), and (E) interdigitated electrode sandwiched between two hydrogel
[80]
layers (Reproduced with permission ).
[81]
holes, while liquid metal eutectic gallium indium reduce the performance of the electrodes by
(EGaIn) was injected into the channels to form adsorption of biological materials such as proteins,
flexible electrodes. Agarwala et al. demonstrated cells, and oligonucleotides . Implantable devices
[83]
the usability of 3D printing by printing silver and electrodes have been found to give rise to
nanoparticle ink over gelatin methacryloyl inflammatory response due to exposure to body
(GelMA) hydrogel . They printed interdigitated fluids. This leads to tissue damage and can be
[81]
electrode and micro-heater design with the silver tackled through material – tissue interactions that
nanoparticle ink sandwiched between printed include the electro-stimulated release of bioactive
layers of hydrogel loaded with cells (C2C12 mouse agents at the site of implantation. Non-conducting
cells and human fibroblast cells) (Figure 4B). hydrogel coatings have shown high interfacial
Authors studied cell proliferation under electrical impedance and thus, conducting hydrogels are
stimulation. finding much use in such scenarios. Composites
of poly(HEMA-co-PEGMA) and a cross-linker,
4 Biomedical Applications for Conducting tetraethylene glycol diacrylate (TEGDA), have
Hydrogels been formulated for biosensing applications .
[84]
4.1 Biosensors 4.2 Drug delivery vehicles
Biosensors have been at the forefront of the Drug delivery vehicles are scaffolds and platforms
biomedical research in helping to collect clinically that allow a pharmaceutical compound to be
relevant data for medicinal activity, surgery, and released on or within human body for therapeutic
intensive care . Electrodes used for sensing effects. Hydrogels are natural candidate for such
[82]
biological functions or stimulating tissues usually application due to their porous network, which
require non-fouling surface, which are resistant can be controlled by managing the density of
to protein adsorption. Fouling can significantly cross-linkers. Electroactive hydrogels are the

International Journal of Bioprinting (2020)–Volume 6, Issue 2 9