Page 66 - IJB-9-6

International Journal of Bioprinting 3D Aerosol Jet® printing for microstructuring

1. Introduction from 70,000 to a half million of euros, and the printing
on objects with 3 or 5 axes over an usual area of 20 × 30 ×
Microfabrication processes for high aspect ratio (AR) three- 20 mm . Side backs of this technology are intrinsic in
[20]
dimensional (3D) structures have been widely investigated the complex aerosol dynamics, which occur across the
since the beginning of the 21st century. In particular, the printing process, often resulting in a poor process control
microforming process aims to produce semi-planar or 3D and repeatability. However, process optimizations have
[1]
structures from nanometers to sub-millimeters scales . As been investigated both from an industrial and academic
[2]
reported by Vaezi et al. , many traditional and state-of- perception, especially encouraging its use in the last
the-art processes are currently accessible from subtractive decade. Two-dimensional (2D) applications have been
lithography-based, additive manufacturing (AM)-based, extensively reviewed mainly for PE, such as conformal
and hybrid-based approaches. The range of applications printing, antennas, batteries, smart packaging, wearable
is in continuous expansion, ranging from electronics and devices, and (stretchable) sensors [22-25] . Despite an
life science to aerospace and automotive. In particular, 3D increasing interest in research and several emerging
periodic microstructures, such as arrays of pillars or lattice applications in bioelectronics , surface structuring, and
[26]
units, with well-defined geometrical characteristics (aspect biological interfaces , the exploitation of this technology
[27]
ratio [AR], diameter, height, and interspacing), have as a 3D printing technique is still very limited, and only a
been efficiently used for microelectromechanical systems couple of examples have been reported in the literature.
(MEMS) , energy harvesting 3D microbatteries [4,5] , This is mainly related to the fact that this technology was
[3]
[6]
bioinspired architectures , microsensors (electrochemical developed for planar-printed electronics and only a few
sensing , microactuators ), micro-optical devices , studies have investigated the actual possibility to use it for
[8]
[9]
[7]
circuit packaging , among others . In life science, 3D 3D printing as well.
[10]
[11]
periodic microstructures have also been embedded in
[28]
bacterial sensors , scaffold-based cell culture systems Saleh et al. were the first to AJ®-print silver
[12]
for guiding and cell growth [13-17] , electrophysiological nanoparticles (AgNPs)-based fully dense truss elements,
recording sensors , and microbial electrolytic cell . like lattices and micropillars arrays at high ARs ~ 20.
[18]
[19]
Zips et al. also AJ®-printed a composite of poly(3,4-
[29]
In the case of AM, 3D structures are produced ethylenedioxythiophene) polystyrene sulfonate
by direct printing through a functional (multi)- (PEDOT:PSS) and multiwalled carbon nanotube ink with
material deposition process. AM methods to fabricate an AR ~ 3.3. Finally, Hohnholz et al., Di Novo et al., and
3D nano/microstructures have been extensively Vlnieska et al. explored the use of photo-reactive polymers,
reported in the literature, including two-photon such as polydimethylsiloxane (PDMS) and ultraviolet
polymerization, (superfine) inkjet printing (IJP), (UV)-curable adhesives or epoxies to obtain 3D-printed
stereolithography, syringe extrusion printing, etc. [20,21] . structure with AJP [30-32] . In each case, the ink formulation
Among AM techniques, Aerosol Jet® printing (AJ®P) was and print parameters were controlled and adapted for the
introduced in the early 1990s by Optomec© Inc. (USA), specific application. However, these works are dictated
mainly for printed electronics (PE) applications on free- by trial-and-error explorations and they lack a thorough
form (e.g., flat/curved, rigid/flexible) substrates. AJ®P investigation on 3D AJ®P capabilities and limitations,
is a noncontact, maskless, direct writing (DW) material associated with different inks formulations.
jet-based technology which, differently from the others
previously mentioned, enables the deposition of (multi-) This paper aims at fulfilling this research gap with
functional materials in the form of an aerosol through a the purpose of identifying guidelines for AJ®P of 3D
nozzle at a variable stand-off distance, z [1–5] mm, from microstructures of increasing complexity, with respect
the substrate. The result is a well-defined printed pattern, to materials and print strategies. Three inks, among
with a minimum feature size starting from 15 µm up to a which Newtonian and non-Newtonian fluids, that
few cm in width and 0.1 µm in thickness. A high variety drastically differentiate in ink composition and application
of ink viscosities in the range of 1–1000 mPas can be functionality (electronics, bioelectronics, and biological
AJ®-printed, from low (ultrasonic configuration) to high interfaces) and three print strategies were selected and
(pneumatic configuration) viscous solutions, without the combined in this study to realize 3D-printed structures of
use of photo-sensitive materials. Although limited AJ®P different complexity and aspect ratios.
ink formulations are available in the market, most of the This work is the first to provide a systematic discussion
IJP inks can be AJ®-printed if aerosolized, showing less on 3D AJ®P capabilities for different ink compositions and
clogging occurrences. Moreover, compared to laser-based print strategies, and it highlights the ability of AJ®P as a 3D
AM techniques, AJ®P offers a relatively high process speed microfabrication technology, emphasizing its future use
up to 12 m/min, flexible solutions with start-up costs for 3D micromanufacturing, electronics, and life science.

Volume 9 Issue 6 (2023) 58 https://doi.org/10.36922/ijb.0257