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Materials Science in Additive Manufacturing Materials for 3D-printed electrodes

3. Carbon-based materials electrodes with excellent stability and reusability, which
has comparable sensitivity to commercial wet Ag/AgCl
In addition to their ductility and stable cycling performance, electrodes . In another study, Qian et al. demonstrated
[84]
carbon-based materials are more widely available and less that reduced graphene oxide (rGO) and elastomeric resins
expensive than metal materials, making them attractive formed a composite material that could be prepared as a
topic of research in the domain of flexible electronic flexible strain sensor using DLP 3D printing (Figure 4B).
devices in recent years [71,72] . Some examples, such as carbon This sensor has high mechanical stability of more than
nanotubes (CNTs), graphene, and other polymer-derived 10,000 stretching–relaxing cycles and a sensitivity of 6.723
carbon materials, are widely used as functional materials over a linear strain detection range from 0.01% to 40% .
[85]
for flexible medical electrodes.
In addition, there are studies on graphene materials to
CNTs are one-dimensional quantum materials prepare biological scaffolds with conductive properties.
with a special structure consisting of several layers of For instance, Fang et al. mixed rGO with polycaprolactone
hexagonally arranged carbon atoms in a coaxial circular (PCL) and printed it into an orderly arranged microfiber
tube . The unique structure of CNTs gives them strong layer by melt electro-writing technology (Figure 4C) .
[73]
[86]
physical properties with single-walled CNTs reaching Incorporating rGO enhances the mechanical properties
tensile strengths of about 800 GPa and elastic moduli of of the PCL scaffolds while conferring better electrical
up to 1 TPa [74,75] . Thus, CNTs not only have a hardness conductivity on the scaffolds, which is essential for the
close to that of diamond but also have good flexibility, functional recovery of peripheral nerves. The experimental
making them a preferred material for preparing flexible results showed that the nerve guidance conduits with
medical electrodes. There are two main strategies for conductive carbon-based materials could promote nerve
prepare CNTs-based conductive elastomers. The first regeneration, myelin sheath formation, and functional
strategy is to use CNTs as fillers uniformly dispersed in regeneration of nerve tissues .
[86]
the polymer matrix. For example, Sun et al. composited MXene is a novel 2D carbon nanofiber material, mainly
CNTs with polydimethylsiloxane (PDMS) to prepare composed of carbon (C), nitrogen (N), titanium (Ti),
flexible piezoresistive tactile sensors . However, this niobium (Nb), and tantalum (Ta) . MXene was first
[76]
[87]
approach tends to affect the mechanical properties of the used as a contact layer for electrochemical biosensors
polymers and limits the sensitivity of the sensor devices . in 2014 . The unique layered structure gives MXene a
[77]
[88]
The second strategy is to coat the CNTs on the surface of conductivity similar to that of metals, which exhibit good
the polymer matrix. Kim et al. deposited CNT film onto sensitivity in sensors [88-90] . In addition, MXene has other
polystyrene (PS) substrates and transferred CNT film excellent properties, including controllability in elemental
[78]
to PDMS to form flexible thin-film pressure sensors , composition and structure, as well as favorable optical and
which are highly sensitive. Nevertheless, the CNTs may be mechanical properties (Young’s modulus ~0.4TPa, fracture
detached from the surface of the polymer matrix during strength ~26 GPa) [91-93] . These properties give MXene
the deformation of the sensor, which is unfavorable for the materials a significant advantage in sensing elements for
long-term monitoring of physiological electrical signals flexible sensors. Cui et al. prepared a wearable MXene-
in vitro . To overcome these problems, Yu et al. adopted polyurethane mesh (MPM) electronic skin (e-skin) by
[79]
a strategy to embed CNTs into the surface of FDM-printed embedding or wrapping MXene nanosheets into porous
thermoplastic elastomer (TPE), as shown in Figure 4A . polyurethane (PU) nanogrid scaffolds generated by
[80]
The CNTs are encapsulated by the melted TPE on the electrospinning (Figure 4D). With ultra-low electrode-
surface through a high-temperature heating process. This skin contact impedance (4.68 kΩ at 1 kHz) and high
flexible electronic sensor has a sensitivity of up to 136.8 kPa signal-to-noise ratio (16.5 dB), MPM e-skin demonstrated
-1
at an applied pressure of <200 Pa while compressing and good stability and signal acquisition accuracy in long-
recognizes human facial activity at a thickness of only term electrocardiographic testing. The fiber film produced
2 mm. Its potential application extends to monitoring and by electrostatic spinning allows the MPM e-skin to be
recognizing various human physiological signals. breathable, minimizing the risk of adverse effects as a
First discovered in 2004 by Andre Geim and Konstantin result of blocked sweat evaporation . It has also been
[94]
Novoselov, graphene has a high carrier mobility studied to make composite gels of MXene with conductive
(∼10 000 cm /V·s) and a high Young’s modulus (∼1 TPa) at polymers, which can be used for DIW 3D printing.
2
room temperature [81-83] . Some studies have been conducted The incorporation of MXene nanosheets enables the
to apply graphene to prepare conductive devices, such as connection of disconnected structural domains between
sensors and supercapacitors. Yang et al. integrated patterned neighboring conductive polymer molecules, facilitating
graphene on PDMS film to prepare graphene/PDMS ion/electron transport in the gel. The composite gel exhibits

Volume 2 Issue 4 (2023) 6 https://doi.org/10.36922/msam.2084