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

and human clinical devices. For example, Utah electrodes stability and processability by modification [60-62] . Thus,
and Michigan electrodes use metal as conductors metal nanomaterials become the preferred materials for 3D
and are utilized as invasive electrodes for intracranial printing of flexible electrodes for medical applications. In
applications [46,47] . Nevertheless, conventional hard metals another study, Im et al. added multifunctional thiols to AuNP
are often subjected to mechanical mismatches at biological ink to modulate the cohesion of AuNP by inducing strong
tissue interfaces, resulting in irreversible damage and interactions between the thiol groups and the gold surface
inflammatory reactions [48-51] . These issues greatly limit so that microcracks and pores are not easily generated in
the use of metal electrodes in animal research and human the AuNP film after heat exposure (Figure 3C) . The
[11]
clinical neurological recording devices. modified AuNP ink is more structurally stable during
Recent years have seen the emergence of several processing and can form stable, flexible conductive devices
strategies for combining flexible polymer materials by IJP onto flexible substrates. These flexible electrodes
with metallic materials, effectively mitigating the are stable in high humidity and salt-rich liquids and can
abovementioned problems. Hui et al prepared an Ag-based maintain stable conductivity after more than 1,000 bending
[11]
ink for extruded 3D printing by mixing silver with hydrogel cycle tests . The improvements of the ink properties are
and showed that the ink can be printed in a hydrogel- made possible only after the tremendous advancements in
support matrix to form a 3D conductive structure that materials science, which further spawn the development of
can be stretched and compressed . Electronic devices various flexible and elastomeric materials with mechanical
[52]
prepared in this way can be used as biomedical electrodes. properties equivalent to those of human tissues and organs,
Ma et al. reported the optimization of a drop-on-demand, and lay solid foundation for preparing flexible medical
high-resolution electrohydrodynamic-based jet-printing electrodes based on metal materials.
method for generating 3D gold (Au) micropillar electrode In addition to the traditional solid metal,
arrays on flexible substrates (Figure 3A) . The electrode room-temperature liquid metals represented by gallium
[53]
array showed mechanical flexibility under planar, concave, (Ga)-based alloys have good room-temperature mobility
and convex conditions, and its sensing capability remained and low toxicity. The liquid metals can reach a conductivity
virtually unchanged . Another study conducted by a of 3.8 × 10 S/m, making them ideal materials for
6
[53]
research team from Carnegie Mellon University reported manufacturing flexible electrodes [63,64] . However, it is still
the formation of shanks with a diameter of only a few challenging for liquid metals to form a continuous and stable
tens of micrometers by stacking atomized metal ink as conductive structure due to their highly oxidizable surface,
an aerosol on a two-dimensional (2D) substrate. A high- large surface tension, and low viscosity [65-67] . Wu et al.
density microarray electrode with 2,600 shanks per square printed liquid metal in an acrylamide/nanoclay support
centimeter was obtained. This microarray electrode caused bath with oxidizing properties, forming a viscoelastic
very minimal gross tissue damage and yielded an excellent oxide skin instantaneously on the extruded liquid metal
signal-to-noise ratio given the low impedance of the surface . It allows the formation of continuous liquid
[67]
metal ink . On the other hand, Morgan et al. attempted metal filaments (150 μm) with tensile strains up to 1400%,
[54]
to integrate 3D microarrays on flexible polyimide or as shown in Figure 3D . This flexible electrode can be
[67]
parylene C films through two-photon lithography and used as a strain sensor and a passive resonant sensor, which
modify the metallic platinum on arrays to obtain flexible has potential applications in wearable biomonitoring
microarray electrodes . The microarray electrode with and untethered robotics. In addition, a common method
[55]
flexible substrates significantly reduces tissue damage for fabricating flexible electrodes based on liquid metals
caused by hard metals. This electrode can be used for signal involves the preparation of liquid metals into nano-sized
acquisition inside tissues to enhance the spatial resolution particles, followed by printing and molding through
of physiological electrical signals and hold immense annealing or other operations [68-70] .
promise for use as implantable brain-computer interfaces. This section summarizes the research on metallic
Compared to common metals, metal nanoparticles materials used in the preparation of 3D-printed flexible
(NPs)/nanowires/nanosheets have relatively small sizes medical electrodes. These studies have facilitated flexible
and more interactions between grains [56-58] . A recent metal-based electrodes in implantable neural interfaces,
study used CuO NP inks for electrospinning, as shown in sensors, and wearable devices by improving 3D printing
Figure 3B. After post-processing, a Cu wire with a width techniques or modifying the materials. The involved
of 50 μm, a thickness of 1.48 μm and a resistivity of only methods not only allow better machinability while
5.46 μΩ·cm was obtained . Moreover, the high surface maintaining the good properties of the metallic material
[59]
area to volume ratio of metal nanomaterials allows them but also increase the mechanical properties of the flexible
to adsorb small molecules to achieve better chemical electrodes, such as tensile and flexibility.

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