Page 78 - IJB-9-6

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

that the addition of glycerol resulted in thicker walls for two main gas flows, that is CGF and SGF, have the most
the hollow pillars, closing the central channel. Hence, significant impact on the output quality. For 3D AJ®P, the
adding glycerol improved the 3D printing capabilities, authors detected narrowed gas flow working windows, that
but reduced the printing resolution and accuracy. It is is, 10 ≤ CGF ≤ 19 and 30 ≤ SGF ≤ 40 sccm, with a R in the
f
suggested that adding a solvent with low volatility, such range of 1.66–4. Specifically, at an equal CGF = (18–19)
as glycerol, aids in the build-up of material by keeping the sccm, a relative higher SGF was necessary in Col-based inks
ink hydrated. Moreover, as glycerol has a higher viscosity (SGF = 40 sccm) and in the PEDOT:PSS-based ink (SGF =
compared to the aqueous collagen solution, it ensures 35 sccm) than in the AgNPs-based ink (SGF = 30 sccm).
cohesion of the structure upon deposition, as the viscosity The cause may the detected in the volatility of the ink co-
of the deposited droplets increases while drying out in the solvent systems, which require higher SGFs in the presence
in-flight jet, resulting in a gel-like liquid being deposited. of low volatile co-solvents, such as glycerol (normal boiling
This combined effect of wetting the ink and increasing the point, NBP = 290°C) or PEG (NBP = 250°C), while lower
viscosity while drying eventually leads to the printing of SGFs for high volatile ones or mixtures, such as DEG (NBP
3D structures. = 244°C) and IPA (NBP = 82.4°C) in the AgNPs-based
Preliminary insights on the process window 3D AJ® ink. This hypothesis is based on the assumption that the
printability have been also investigated for each ink, aerosolized microdroplets are in the same size range of
comparing good- and poor-quality results. For all cases, it 1–5 µm and that the higher is the microdroplets weight
is clear that a strict control over specific print parameters and loading content, the lower is the SGF value. Further
combinations is crucial to obtain the desired output, in analyses to validate this statement are ongoing by means of
terms of 3D AJ®P (no 2D deposition), reproducibility, phase Doppler anemometry (PDA) systems. With respect
and desired quality. Based on the authors’ observations, to the building-up of the microstructures. It is suggested
the print nozzle is recommended to have a Ø ≤ 150 µm in to avoid values of SGF > 40 sccm for a stand-off distance
order to obtain high-resolution 3D microstructures. Larger z = 3 mm, in order to reduce any bending and diversion of
nozzle diameters indeed favor a 2D AJ®P (increased mass the in-flight aerosol jet, which may result in a remarkable
flow rate) or less control over the in-flight jet and overspray. overspray. The maximum ARs ~ 20 were obtained with
If necessary, it is suggested to use R ≥ 4 and low CGFs. the AgNPs-based ink, which are 8 and 4.5 times higher

f
Moreover, any type of in situ presintering, -annealing, or than Col- and PEDOT:PSS-based inks, respectively. The
-crosslinking while printing is suggested to initiate and AgNPs-based ink indeed contains a high particle loading
have control over the building up of the structure. In this content and volatile co-solvents, hence it is able to resist the
study, AgNPs- and PEDOT:PSS-based inks were thermally SGF effect even at high ARs (no bending). This statement
presintered and preannealed, respectively, with the thermal is strictly correlated to the R , CAD, and print speed used.
f
treatment chosen as postprinting process due to the CAD patterns and gas flows indeed directly influenced the
possibility to change the platen T from room temperature s (mm/s). After screening investigations, the speed was
to 200°C. Other postprocessing methods include plasma, chosen at s = 0.4 mm/s for 3D-LBL and at s = 0.1 mm/s
laser, photonic, or microwave sintering . Their selection for 3D-PW. Higher speeds induced the printing of porous
[54]
and relative parameters are believed to affect the quality, structures with wide depositions of overspray, while lower
dimension, and shape of the 3D microstructures. In this ones provoked excessive deposition of materials over each
case, specifically in the 3D-LBL approach, AgNPs-based step, with restraints on the building-up of uniform layers.
ink requires an ideal T = 100°C, while the PEDOT:PSS- 5. Conclusion and future perspectives
based ink a T = 80°C. Since the two inks have water as main
solvent, the in situ presintering process applied mainly This paper focuses on the feasibility and validation of 3D
affected the overall dimension and shape deformation of the printability with the AJP process. To summarize, the AJ®P
printed structures. Hence, no further significant difference technique is considered an enabling 3D printing technology
was detected after the postprinting thermal process. The to realize high AR 3D bio(electrical) microstructures.
specific value of temperature depends on the properties of Three inks categories, that is, metal NPs-based (AgNPs),
the ink, especially particle mass loading and (co-)solvents conductive polymers (PEDOT:PSS), and natural polymers
boiling points. Trial tests are recommended to detect the (collagen), were printed and investigated with at least two
most suitable T for the designated ink. Instead, in the of the three main print AJ®P strategies, i.e., CJD, LBL, and
case of Col-based inks (or similar), since the temperature PW. The results indicate that an ideal 3D U-AJ®P ink should
must be kept at T ≤ 37°C to avoid any degradation, it may have: (i) standard U-AJ®P ink requirements, (ii) a solid
be interesting to explore other methods, such as UV- (metal, polymeric, ceramic) loading content in the range
crosslinking, to improve the printing quality and time. The of 10–25 wt.% and additional binders, (iii) a co-solvent

Volume 9 Issue 6 (2023) 70 https://doi.org/10.36922/ijb.0257