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

Table 1. AJ®P inks selected for the print investigation of 3D microstructures and related features a
AJ P inks and features
®
Ink Metal-based Polymer-based (synthetic) Polymer-based (natural)
Supplier Novacentrix Metalon® JS-A221AE Sigma Aldrich (BE) and Enzyme extracted Bovine Collagen
(USA) own-developed formulation Type I [3 mg mL ] supplemented with
−1
hydroxyapatite nanoparticles
Abbreviation AgNP-based ink PEDOT:PSS-based ink Collagen-based ink
Type Nano-dispersion Solution
Density, ρ [g/cm ] - ~1 ~1
3
Viscosity, η [mPas] 5.9 ≤ 10 100–150
Surface tension, σ[mN/m] 35.2 75 76
Main solvent DI water DI water 0.01 M HCl
Co-solvents Diethylene glycol (≥ 3 – Own-formulation: polyethylene Glycerol (1 M)
≤ 10 v/v%), isopropyl alcohol glycol and ethylene glycol
(≥ 2 – ≤ 10 v/v%)
Loading particle 35nm Ø (50 wt.%) PEDOT:PSS (1.3 wt.%) 200 nm Ø Hap (3–6 mg/mL)
avg
avg
Additives - Carboxymethyl cellulose 1 M glycerol
Notes (resistivity, ρ [Ω·cm]) Conductive, ρ = 4.2 × 10 to 9.1 × Conductive, ρ = 1.66 × 10 Ω·cm Bioactive and osteoconductive ink
−4
−3
10 Ω·cm
−6
a (i) an AgNPs-based ink for electronics, (ii) a PEDOT:PSS-based ink for bioelectronics, and (iii) a collagen-based ink for biological interfaces.
analyzed in its standard commercial solution and as an printing strategies, i.e., a continuous jet deposition (CJD),
own-formulated composition. In addition, a collagen and a traditional layer-by-layer (LBL), and a point-wise (PW)
collagen composite ink were formulated by dissolving approach, were exploited. CJD is a continuous deposition of
freeze-dried collagen type I in 0.01 M hydrochloric acid the aerosol jet on a given spot, while LBL is the traditional
(HCl) at a concentration of 3 mg mL . The collagen was layer-by-layer printing in a pattern. Finally, PW is a spot-
−1
extracted according to a previously published protocol . by-spot of the aerosol jet in a pattern. Unlikely, LBL and PW,
[36]
The composite ink was prepared for use in bone tissue CJD does not require the use of a shutter and a print speed,
applications. Hence, HAp was added to the collagen s [mm/sec]. For each strategy, the nozzle was settled at a
solution as particle loading by adding 6 v/v% of a 10 wt.% stand-off distance, z (mm), of 3 mm (increase of + 0.5 mm
aqueous HAp suspension (< 200 nm; Sigma Aldrich, BE), every + 0.5 mm height of the printed microstructures).
as this 1:2 collagen to HAp ratio mimics the composition of For CJD, a continuous deposition of aerosolized material
human bone tissue. Glass slides (Superfrost or Micro cover on a single spot was performed with the aim to realize 3D
glasses, VWR, BE) were selected as reference substrate high ARs pillars. In the LBL approach, the targeted samples
and ultrasonically cleaned at T = 25°C (EMMI - 20 HC, were arrays of 6 × 4 pillars, each one detected as a circle
Emag) for 10 min prior to use, with a mixture of DI water of 50 µm in diameter. The geometry was designed using
and 2-propanol (IPA, Sigma Aldrich, BE). Before printing, AutoCAD software (Autodesk, USA) and then converted
the conductive inks were ultrasonically sonicated for in a .prg toolpath code compatible with the printer using
10 min at 25°C. On the other hand, for the collagen inks, VM Tools (VMware, USA). The PW approach was instead
glass cover slips (Ø12 mm, VWR, Belgium) were used as used to realize lattice units of pyramidic shape. A MatLab
the reference substrate and cleaned in an ultrasonic bath script was created, which controlled the shutter opening
using MilliQ water and ethanol, before being stored in a time (in milliseconds, ms) as well as the position of the
1× phosphate-buffered saline (PBS; Lonza, BE) in order print table. Accordingly, the print head moves in a square-
to remove any surface charges. The collagen inks could shaped sequence and stops at the corners where the shutter
not be ultrasonically homogenized as this would result opens for a short period (50–100 ms) before moving to the
in degradation of the collagen molecule, as shown by next point. By decreasing the square dimensions by each
Gibney et al. . layer, the corner-points ultimately coincide in the center,
[36]
generating a pyramid. By using a slow printing speed
2.3. AJ P tools and experimental methodology (s = 0.01 mm/s), the deposited material is allowed to dry,
®
An Optomec AJ®P 300s system equipped with an ultrasonic providing a supporting surface for the consecutive printed
atomizer (U-AJ®P) was used to print samples. Three 3D AJ®P layer.

Volume 9 Issue 6 (2023) 60 https://doi.org/10.36922/ijb.0257