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Electrically conducting hydrogels for health care: Concept, fabrication methods, and applications
in collagen led to enhanced conductivity and The synthesized composite demonstrated high
flexibility. It is argued that adding CNTs in collagen mechanical performance with low percolation
has many advantages compared to pure CNT or threshold of electrical conductivity at 0.4% with
only collagen scaffolds. First, CNTs enhances added feature of pH sensitivity [30] . Graphene was
the structural integrity of the fabricated scaffold. also added to PANI and PPy [31,32] . Most of the
Second, such composites reduce the mismatch other work on graphene-based hydrogels was
between Young’s modulus of human tissues and targeted toward electro-chemical and electrical
rigid electrodes. Third, collagen matrix is effective applications [33,34] .
in confining CNTs and thus reducing harmful effect
due to migration. Khademhosseini’s research 2.3 Conducting hydrogels with polymers
group developed conducting cardiac patches by Concerns on biocompatibility of metal nanoparticles
mixing CNT in gelatin methacrylate (GelMA) and carbon-based materials have led to increased
hydrogel . The composite material not only had interest in replacing them with polymeric
[22]
excellent mechanical integrity but also showed materials. Polymeric materials are used in various
advanced electrophysiological functions. In a forms namely particles, core-shells, micelles,
similar work, to construct heart patches, Pok et al. and dendrimers [35-38] . There are two common
added CNTs to chitosan . The resulting scaffold routes through which polymers are added into the
[23]
supported cardiomyocyte functionalization and hydrogels, (i) electro- or chemical polymerization
speeded up conduction velocity to achieve beating of conducting monomer in prefabricated hydrogel
heart functionality of a rat. CNTs have been added and (ii) mixing the precursor monomer followed
repeatedly to various hydrogels such as chitosan, by polymerization. The main idea is to entrap the
poly(3,4-ethylenedioxythiophene) doped with conducting polymer chains in the hydrogel matrix.
poly(4-styrenesulfonate) (PEDOT:PSS), and Polyaniline (PANI) and cellulose composite
poly (N-Isopropylacrylamide) (PNIPAM) for hydrogel exhibited a continuous and linear
electrocatalysis, battery electrode, and fuel crawling motion under a low applied electric
cell [24-26] . A different morphology of core-shell of field, apart from high compressive strength .
[39]
hydrogel composite was synthesized using PANI Cellulose was also mixed with polypyrrole (PPy)
and CNT . in an ionic liquid, giving rise to high electrical
[27]
CNTs were replaced with graphene in conductivity of 7.83 × 10 S.cm −1[40] . Gilmore
−3
chitosan for tissue engineering application. It et al. demonstrated the fabrication of hybrid
showed similar swelling mechanism and yielded composite composed of PPy and PAAM. PPy was
higher mechanical strength of the final composite directly electropolymerized on the hydrogel .
[41]
material [28] . Biological scaffold made of graphene The work has since led to plethora of polymer-
oxide and PAAM composite hydrogel displayed hydrogel combinations for various applications.
muscle such as stiffness apart (Young’s modulus Most common tissue engineering application is
of approximately 50 kPa) from conductivity [29] . for cardiac tissues. Poly(triaryl amine) (PTAA)
Enhanced proliferation and myogenic was homogeneously combined with methacrylate
differentiation were observed in the scaffold. hydrogel achieving conductivity similar to
Graphene oxide shows good suspension in water myocardial tissues . High-quality conductive
[42]
but aggregates in acidic medium. In a modified composite hydrogels composed of single-walled
approach, Alam et al. tailored the oxidation carbon nanotubes (SWNTs), polypyrrole (PPy)
degree of graphene sheets by controlling the and poly(ethylene glycol) diacrylate (PEGDA)
sonication time and acid concentration [30] . hydrogel were successfully fabricated through
Graphene/polyacrylic acid hydrogel was formed interfacial polymerization (IP) . PEDOT:PSS is
[43]
using in situ polymerization process, where a high conductivity polymer and heavily used in
graphene sheets are mixed with acrylic acid in organic electronic applications. PEDOT:PSS was
the presence of a cross-linker and an initiator. mixed with polyethylene glycol (PEG) to make

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