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Electrically conducting hydrogels for health care: Concept, fabrication methods, and applications
new generation of smart materials that allow tissue. This study demonstrated positive effects on
the direct delivery of electrical signals to cardiac differentiation efficiency . A conducting
[42]
control the delivery of the drug. PAAM and PPy hydrogel composed of PPy/graphene/chitosan
were used to make a cylindrical drug delivery composite good adhesion, proliferation, and
device . Controlled release of compounds was viability toward fibroblast cells . A conducting
[28]
[85]
demonstrated for the treatment of schizophrenia tissue engineering scaffold for muscle and nerve
and bipolar disorder. Some treatments may require tissues was fabricated using PEDOT:PSS and PEG
delayed release of the medicinal compounds. To hydrogel mix . In another variation, PEDOT
[44]
this end, poly(p-phenylenevinylene) (PPV) was and PU were mixed to fabricate a hydrogel-based
used to create a hydrogel with PAAM, where device. The PEDOT/PU hydrogel exhibited high
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the release profile was optimized using cathodic electrical conductivity of up to 120 S. cm at
[45]
potential . The release of salicylic acid was 100% elongation .
[86]
delayed from 3 h to 15 h after application of
appropriate potential. Some other conducting 5 Conclusion and Outlook
hydrogels investigated for drug delivery are based We have reviewed conducting hydrogel
on gum ghatti, vinyl monomers, and aniline [87-89] . composites as the state-of-the-art and versatile
The electro-stimulated devices made from these class of materials that are gaining attention
materials benefit from high loading capacity and due to their suitability for various applications.
low voltage actuation. There are, however, many The review summarizes synthesis methods and
limitations to the current system of drug delivery strategies to achieve electrical conduction in
using conducting hydrogels. First, there are active otherwise insulating hydrogels. Incorporation
and passive losses of the loaded drug through of nanoparticles, carbon-based materials, and
exchange with the environment and by diffusion, polymers has been discussed, and this provides
respectively. Second, many of these materials better understanding to the readers to design novel
suffer from low diffusion coefficients of the drug combination of materials for desired applications.
resulting in poor release kinetics. Finally, many of Working with conducting hydrogels has its own
these materials are affected from low drug loading limitations, especially at the hydrogel device
capacity. Incorporating nanoparticles is not just interface. Adhesion of hydrogels layers and their
impart electrical conductivity but also improves dehydration over longer time span pose stability
drug loading capacity through increase in surface issues. In many reports, the biocompatibility tests
area . Graphene oxide (GO) has been widely are limited to in vitro screening, and further animal
[90]
researched to make conducting hydrogels, where studies may be required. Conducting scaffolds
the resulting material has shown no passive drug can provide ideal platform for regenerative
diffusion [91,92] . GO was combined with PPy and tissue engineering. However, role of electrical
PEDOT to generate electrically active composite. stimulation for cell growth is poorly understood.

4.3 Tissue engineering We have also delved into the area of 3D printing
for conducting hydrogels. Much work is needed
Hydrogels have been a success in tissue engineering to determine most promising printing technique
and tissue regeneration for various human organs. and functionalization approaches for 3D printed
Conducting hydrogels have been attractive conducting hydrogels. We have also discussed on
for cell growth, adhesion, and proliferation of how 3D printing can lay down materials in desired
muscles, cardiovascular nerves, and bone tissue locations within a hydrogel, thus creating flexible
cells [78,93-95] . PTAA and methacrylate aminated circuits instead of homogeneous conducting
gelatin were used in cardiac tissue engineering material. The area of conducting hydrogels is still
demonstrating a conductivity up to 10 m.S.cm , full of unresolved technological challenges, and
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which is similar to the conductivity of myocardial thus provides researchers with opportunity for
10 International Journal of Bioprinting (2020)–Volume 6, Issue 2