Page 10 - IJB-6-2

Electrically conducting hydrogels for health care: Concept, fabrication methods, and applications
Gold nanoparticles were added into the solution of One of the most common issues encountered is
monomer of N-isopropylacrylamide/acrylamide agglomeration and poor dispersion of the additives
(NIPAAm/AAm) followed by addition of the in the hydrogel matrix. As discussed above, this
gelation initiator ammonium persulfate (APS) issue can be overcome to some extent by selecting
and accelerator tetramethylethylenediamine suitable solvents for dispersion. Due to the use
(TMEDA) by Sershen et al. to form gold- of metal nanoparticles, namely, silver and gold,
hydrogel composite . Gold nanoparticles tend the overall cost of the final material may be high.
[51]
to agglomerate under electric field and hence This prevents their use in commercial application.
cannot be electropolymerized. To this end, There are also some reports wherein the response
gold nanoparticles were incorporated in PAAM time of devices fabricated using conducting
hydrogel after the gel has been formed. The hydrogels is slower. Much work needs to be done
hydrogel was formed through a “breathing in” to improve the overall device performance in
process which consists of (i) collapsing the gel conducting hydrogels.
by placing it in acetone and causing the water to
be expelled; and (ii) placing the dehydrated gel in 3.2 3D printing
aqueous solution of gold nanoparticles. This step Additive manufacturing, also called 3D printing,
caused the hydrogel to swell up again, termed as is defined as “process of joining materials to
breathing in; and (iii) washing the hydrogel with make parts from 3D model data, usually layer on
water to remove any weakly surface-adsorbed layer, as opposed to subtractive manufacturing
nanoparticles. The above steps were repeated
many times to obtain the desired nanoparticle and formative manufacturing methodologies”
density in the final hydrogel composite. There are according to the International Organization for
many reports available, which involve loading the Standardization (ISO)/American Society for
nanoparticle precursors into the hydrogel matrix Testing and Materials (ASTM) 52900:2015
rather than adding the preformed nanoparticles. standard. It has been a disruptive technology for
Although Langer’s group initiated the work fabricating complex shapes in various materials,
in this direction , Saravanan et al. improved thus overcoming the limitations of traditional
[52]
the methodology to form silver laden NIPAAm manufacturing techniques [57,58] . 3D printing of
hydrogel . The group carried out free-radical metals is a mature technology and polymer
[55]
cross-linking polymerization of acrylamide printing is making its way to industries. This
monomer in an aqueous medium containing silver technology has forayed into electronic and
ions. Different reducing agent for nanoparticle biomaterials and opened doors for design and
ions and precursors has been reported. The benefit device innovation [59-61] . Conducting hydrogels
of this method is the formation of hydrogels with have seen a surge in being processed using 3D
enhanced mechanical strength. In an alternate printing technique due to ease of constructing
approach, the surface of nanoparticles was complex shapes, customized constructs, and time
functionalized by adding cross-linking groups to efficient processing [61-64] . 3D printing is an umbrella
bind with the hydrogel. Zhang et al. extensively term and mostly three different techniques have
studied several combination of semiconductor been applied to print conducting hydrogels
nanoparticle-hydrogel composites through self- namely 3D bioplotting, inkjet, and light-based
initiated polymerization under light irradiation . technique [65-67] . In 3D bioplotting, the conducting
[56]
Silicon nanoparticles were added to a hydrogel hydrogel material is extruded from an orifice on to
framework by in situ polymerization to synthesize a substrate of choice (Figure 3A). The technique
a well-connected 3D structure. generally relies on the shear thinning behavior of
Fabricating electrically conducting hydrogels the hydrogels, thus making them flow on applying
mostly employs traditional approaches. They may pressure and allowing their deposition [65,68] . Inkjet
be well-established, but there are some limitations. technique uses a piezoelectric head for the orifice.

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