Page 65 - IJB-8-1

Seiti, et al.
repairing and/or regenerating neural cells and tissues. In and substrate). AJ P 3D micro-structuring has also been
®
particular, NTE in vitro concepts have been established as proven recently [15-17] , in which different inks composed of
an efficient and ethical alternative to in vivo studies. In vitro silver nanoparticles (AgNPs), polymers (PEDOT: PSS) or
smart bio-architectures, namely (neural) scaffolds, can be biological (collagen) dispersions, were exploited for the
indeed developed to recreate, for instance, a biomimetic fabrication of 3D micropillars and lattice structures.
brain extra cellular matrix in laboratory to investigate In this study, poly(3,4-ethylenedioxythiophene):
mechanisms, such as cellular differentiation, degeneration poly(styrenesulfonate) (PEDOT: PSS) electrodes and
or regeneration, offering new possibilities for disease interconnects, up to 30 mm thick, are AJ printed on a
®
modelling and/or personalized medicine . Neural microstructured silicon (Si)-based wafer with proven
[1]
differentiation, as from induced pluripotent stem cells cellular adhesion, to develop a customized bioelectrical
(iPSCs), can be modulated or accelerated using various NTE in vitro device with combined topographical and
(combined) stimuli, such as topographical, electrical, electrical axon guidance cues. To the author’s knowledge,
mechanical, or biochemical stimuli, which are known this combination of material, substrate and application
as axon guidance cues . As an example, micro-/nano- is unique in the literature. PEDOT: PSS was selected
[2]
structured scaffolds, such as channelled substrates, have for this study because of its known good conductivity,
demonstrated to enhance directional guidance for neural biocompatibility, and electrochemical stability, when
networks growth. Electrical stimulation (ES) of iPSCs in it is mixed with specific co-solvents and additives .
[18]
culture medium is also known to influence direction and Several examples of PEDOT: PSS inks have also
triggering of neurite outgrowths . Microelectrode arrays been already reported in the literature for bioelectrical
[3]
(MEAs) are currently the most widely adopted devices neural applications [3,19-21] . Despite the guidelines and
for combined digital neural electrophysiological activity experimental framework about the optimization of AJ P
®
stimulation and also recording. However, current MEAs printing parameters [22-24] , the literature focuses on the
solutions are not integrated into TE oriented substrates, typically empirical approaches specifically oriented to a
and the majority of them are fabricated with flat electrodes given combination of ink solution and substrate, such as
on stiff substrates, which do not reassemble the complex silver ink combined with flat/flexible supports. Moreover,
three-dimensional (3D) neural architecture. Hence, these they lack in generality and applicability, for example, when
devices usually lack a 3D structure and do not allow the dealing with micro-structured substrates. Furthermore,
mimicking of axonal guidance. they usually refer to thin printed patterns for the purpose
In this context, Aerosol Jet Printing (AJ P) can of PEs, instead of thick designs for multifunctional
®
®
provide new opportunities. AJ P, which is an additive applications, such as bioelectronics interfaces. In this
®
manufacturing technology commercialized by Optomec , paper, the use of a generalized print transfer methodology
®
is able to fabricate printed electronics (PE) applications is proposed instead. It is based on the concept of wettability
on various substrates (interconnects, electrodes, antennas, as indication of the substrate-ink interaction. Hence, the
resistive and capacitive sensors, RFID, electrochemical printability of the selected ink was firstly investigated
sensors, etc.) [4-7] . It is a nozzle-based direct writing on a substrate of reference (glass slides) and optimized
technology, and it makes use of a functional ink, which for the desired purpose (thick printed interconnects and
is atomized into an aerosol mist (gas suspension of electrodes). The print strategy was then transferred to
microsized material droplets) through piezoelectric the substrate of interest, previously treated to match the
ultrasonic or pneumatic methods. The mist is then wettability of the reference support, hence reducing the
transferred to the deposition head through a (nitrogen, N ) time and cost of investigation and process optimization.
2
carrier gas and focused in the nozzle into a high dense In addition, a manual and automated detection protocol,
aerosol beam by means of an annular (N ) sheath gas. As based on quantitative and qualitative data, was proposed.
2
such, microscale resolutions down to 10 μm in width and The printed lines characterized with respect to accuracy
~100 nm in thickness can be attained via AJ P. Moreover, and geometrical profiles, electrical conductivity and
®
functional nanoinks with a wide range of viscosity biocompatibility, and interesting new insights concerning
(1 – 100 mPas) can be printed, including conductive, the cytotoxicity of the selected PEDOT: PSS ink were
dielectrics and biological solutions, such as metal loaded reported. The electrical impedance of the final device was
inks (particle size ≤ 0.5 μm) [8,9] , polymer or carbon- eventually tested in saline solution.
[10]
based solutions [11,12] , and hydrogels (as for collagen [13,14] ).
Furthermore, it can print theoretically on any substrate 2. Materials and methods
displaying a certain surface energy, such as smooth/rough 2.1. Inks and substrates
supports, flexible foils, textile, or papers, including free
form parts, because of the variable stand-off distance A PEDOT: PSS inkjet ink (ORGACON™ Transparent
(i.e., the distance 1 – 5 mm, between the tip of the nozzle Conductive Inkjet Ink IJ-1005 AGFA NV, BE) was used

International Journal of Bioprinting (2022)–Volume 8, Issue 1 51