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Agarwala
for reinforcement material in hydrogel matrix. cobalt nanomagnets into the hydrogel provided a
The final composite is synergistic, unique and has safe way to reduce magnetic particle migration
desired properties that are not found in individual or loss. Iron oxide magnetic nanoparticles have
components. The final properties depend on the been added in a hydrogel to provide remedy for
type of nanoparticles incorporated, which in methylene blue contaminated water .
[17]
turn determines the proposed application. Silver Apart from 3D nanoparticles, 1D (one-
(Ag) and gold (Au) are the most commonly used dimensional) nanostructures have been used to
conducting materials for hydrogels. Silver has an enhance the electrical conductivity of hydrogels.
added antimicrobial property and for this reason, This is motivated by the fact that 1D nanostructures
it is blended with hydrogels to make functional have better electrical transport . Stretchability
[18]
coatings . A study done by Endo et al. revealed the is an additional property that can be achieved
[6]
relationship between silver ion concentration and in the final hydrogel composite through 1D
swelling rate of hydrogel . A higher concentration nanostructures, as they act as electrical connectors
[7]
of silver ions results in better conductivity, but during bending, stretching, or flexing of the
reduced the swelling ratio and vice versa. So far, material . A recent report by Lim et al. confirmed
[19]
silver nanoparticles have been added to many the theory where silver nanowires were chosen as
hydrogels, namely polyacrylamide (PAAM), fillers for alginate hydrogel . A wearable antenna
[20]
polyacrylic acid (PAA), N-isopropylacrylamide patch was fabricated using the silver nanowire
(NIPAAm), methyl methacrylate, and polyvinyl alginate composite with excellent conductivity,
alcohol (PVA) [8-12] . Silver nanoparticles added to tough structural integrity, and stretchability. The
PAA through in situ reduction and polymerization fabrication procedure did not involve formation of
showed swelling dependent electrical covalent bonds by polymerization, thus making it
conductivity, which could be varied from 13.6 compatible to incorporate different inorganic and
to 572 mS.cm -1[13] . The use of gold for making metallic nanomaterials.
conducting hydrogel remains limited due to its
high cost. Gold nanoparticles were utilized to make 2.2 Conducting hydrogels with carbon material
conducting poly(3,4-ethylenedioxythiophene)/
poly(acrylic acid) (PEDOT/PAA) hydrogel, Carbon based materials such as carbon nanotubes
which exhibited excellent catalytical activity (CNT), carbon black (CB), and graphene have
for p-nitophenol . It has been proven that by played an important role in wide range of
[14]
varying gold concentrations, it is possible to research fields due to their electrical, thermal, and
change the properties of the final composite. mechanical properties. Their enhanced electrical
There are sporadic reports on employing metal properties are highly attractive, and this shows
nanoparticles for making conducting hydrogels. their potential to be used as reinforcement and
Platinum (Pt) nanoparticles have also been additive material for composites. Hence, metal
deployed in polyaniline (PANI) hydrogel to make nanoparticles are being replaced by carbon-based
heterostructures for a glucose sensor . The materials such as carbon nanotubes, graphene, and
[15]
porous structure of hydrogels favors trapping carbon black to add more functionalities, especially
the nanoparticles and immobilizing enzymes for because they have shown higher compatibility for
glucose sensing. In one of the early reports, Fuhrer biological species compared to metal counterparts.
et al. incorporated magnetic cobalt nanoparticles Better response to biological species comes
to hydrogel backbone . The cobalt nanoparticles from the fact that carbon-based materials can be
[16]
were encapsulated in stable carbon shells, which trapped in synthetic and natural hydrogels. This
were covalently modified to have vinyl-termination immobilization leads to better biocompatibility.
groups. Copolymerization of carbon coated cobalt An early attempt was made by Cho et al. to
and 2-hydroxyethyl methacrylate was carried out to encapsulate CNTs into collagen to electrically
form a magnetically active hydrogel. Crosslinking stimulate PC12 cells . A loading of <1% CNTs
[21]
International Journal of Bioprinting (2020)–Volume 6, Issue 2 3