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Electrically conducting hydrogels for health care: Concept, fabrication methods, and applications
amount of silica nanoparticles. In another report, a which allows users to define desired toolpaths.
hybrid hydrogel of silica/alumina was modified by Lack of such a capability does not allow the users
adding poly(N-isopropylacrylamide) (PNIPAm) to write customized codes or load their model files.
microparticles, resulting in a printable material that
responded to both heat and electrical stimuli . 3.3 Fabrication enabling spatial distribution
[76]
The water molecules trapped in PNIPAm are Conducting hydrogels can now be processed
released to the silica/alumina matrix on heating. into complex shapes and structures with the help
PNIPm particles are dehydrated and act as light of 3D printing. However, one critical challenge
scattering centers leading to an optical change still plagues them. When conducting material is
from opaque to transparent. The response of the added to the hydrogel, they may be distributed
material to electrical stimulation was exploited for homogeneously but they cannot be confined
fabricating an optical switch. Resin based SLA spatially in desired spots. There has been growing
was also used to print all polymer-based composite needs to confine conducting material in space
based on the polyethylene glycol diacrylate within hydrogels and not distribute evenly.
(PEGDA). Poly(3,4-ethylenedioxythiophene) This can lead to building electrical circuits
(PEDOT):polystyrene sulfonate (PSS) aqueous encompassed within hydrogels. Some emerging
solution was freeze-dried and mixed with applications may require the conducting material
PEGDA to make it conducting . Increasing not distributed in the entire hydrogel matrix but
[77]
the PEDOT:PSS concentration, enhanced the be restricted in specific spots to enable electrical
electrochemical characteristics of the printed
platform, which was used to electrically stimulate stimulation. This section of the paper deals
cells for neuronal differentiation. 3D printing with latest research efforts directed to print soft
enables fabricating complex construct shapes with electrical circuits on hydrogels. With the assistance
customization as anatomy of humans are very of transfer techniques and 3D printing, it is now
different from each-other. possible to lay circuits on hydrogel surfaces. Such
Commercialization of 3D printing whether for platforms find application in wound management,
conducting hydrogels or other biomaterials is still drug delivery, on-skin patches, implantable,
far away. This is due to many issues that need to and bioelectronic devices. In one of the early
be resolved before industries can take it up for reports, Sekine et al. used micropatterning to
mass-production. Hydrogels are considered soft lay down PEDOT on agarose hydrogel in a
[78]
materials and their lack of mechanical strength predefined fashion . The fabricated platform
poses limitation in printing sturdy 3D constructs was organic, moist and served as flexible electrode
and shapes. These materials are unable to follow to cultivate contractile myotubes. A similar peel
the original design models, as the printed construct and transfer process were applied to fabricate
does not retain the original shape. Achieving silver nanowire (AgNW)-based microelectrode
functional gradients and hierarchical properties on polydimethylsiloxane (PDMS) substrate [79]
have also been challenging and new design (Figure 4A). The desired microelectrode pattern
approaches are being developed to tackle them. was first fabricated on glass substrate using
3D printing is heralded as a unique solution to photolithography and later directly transferred to
print human organs off-the-shelf to counter their PDMS based hydrogel. Electrical conductivity
storage. However, precision printing in layer- could be control by varying the density of silver
by-layer fashion mimicking human tissues is a nanowires. Peel and transfer methods are neither
mammoth task. A prominent weakness is the lack of reproducible nor up-scalable for commercial use.
precise control on the cellular microenvironment, Shay et al. and their group created microfluidic
which governs cell attachment and proliferation. channels through soft lithography in PDMS .
[80]
Various professional and off-the-shelve hobbyist Combination of acrylamide (AAm) and acrylic acid
3D printers are available, but few are open-source (AA) was used as the hydrogel material to fill the

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