Page 602 - IJB-10-3

International Journal of Bioprinting Five-axis printer for hybrid 3D scaffolds

bioactivity of co-printed synthetic photopolymers by Table 1. Printing parameters for hybrid printing combining
promoting colonization (e.g., hydrogels resembling inkjet and extrusion printing on a planar surface
ECM in this study).
Printing parameter Value
(iii) The support structure of hydrogels can be removed Inkjet printing speed, ν (mm/s) 41.67
IJP
after printing to fabricate overhanging structures Jetting frequency, ƒ (kHz) 13
with interconnected porosity, avoiding the use of Resolution, ϕ (dpi) 8100
cytotoxic support materials for inkjet printing as there
are no biocompatible support materials available. Extrusion printing speed, ν EXT 1.17
(mm/s)
The use case is first defined based on extrusion printing Extrusion rate, C (mm/s) 0.2
of a hydrogel and inkjet printing of a photocurable resin
on a planar surface (κ = 0; κ = 0) (Figure 3a-1i). This is lithographic ceramic manufacturing. This process resulted
1
2
followed by the preparation of printing files, including in a multi-material and multiscale tri-phasic hybrid
G-code generation and bitmap preparation for both
inkjet and extrusion printing (Figure 3a-2ii). The printing structure (Figure 4a–c). Notably, the microporosity of the
process starts with inkjet printing of the synthetic 3D-printed ceramic facilitated adequate bonding between
photopolymer, followed by a curing step (Figure 3a-3iii) the photopolymer and the ceramic hard phase. The design
and the deposition of the peptide hydrogel using extrusion featured square struts with w of 2 mm and w of 2 mm.
s
p
printing (Figure 3a-3iv). This sequence is repeated to The accuracy of the printed model perpendicular to
fabricate multilayer scaffolds. the printing direction was characterized with respect to
In Figure 3b, the multilayer channel structure of the the pore width (w ) and strut width (w ) (Figure 4d and
s
p
inkjet-printed photopolymer on a glass slide substrate is f). Struts exhibited a discrepancy of 7.73 ± 6.74%, whereas
presented before the deposition of the hydrogel. A total of pores displayed a mismatch of 6.73 ± 8.04% (n = 3 struts
240 layers of photopolymer struts were printed, followed and pores).
by the extrusion of the hydrogel. The process is then In the channels perpendicular to the printing
complemented by 60 additional photopolymer layers for direction, a notable printing artifact was evident, i.e., the
the topmost layer, effectively closing the channels filled presence of angled struts (approximately 22°) (Figure 4e).
with the hydrogel. The multiscale and multi-material bi- This observation implies a directional influence on the
phasic structure is illustrated in Figure 3c, highlighting structural characteristics of the printed object. This artifact
a nanofiber network in the millimeter-sized peptide may be linked to the optimal printing speed of 41.67 mm/s,
hydrogel-filled pores of the photopolymer. Figure 3d and e which was determined through an iterative process to be
presents a hybrid-printed sample after mechanically the most suitable. However, the optimized speed was still
removing the hydrogel from the channels. relatively slow, further increasing the time for the UV lamp
The printed model was characterized with respect to to cure the deposited ink and resulting in ink spreading
the pore width (w ), pore height (h ), strut width (w ), and the occurrence of angled struts. Hence, accurate
s
p
p
and strut height (h ) (Figure 3f and g). The analysis (n = measurements of the strut and pore sizes were challenging
s
3 pores) revealed an accuracy mismatch for the intended due to an angled wall, resulting in a gradient in pore size
h = w = 1.8 mm, revealing a deviation of 5.79 ± 2.1% in (Figure 4e).
p
p
pore width and -6.06 ± 8.64% in pore height. Regarding 3.2. Multi-material 3D inkjet printing on single-
the intended square strut size of h = w = 3.6 mm, the curved surfaces for osteochondral defects
s
s
mismatch was 19.64 ± 2.09% for strut height and -2.5 ± Subsequently, we utilized 3D inkjet printing on a single-
0.94% for strut width of the printed model (Figure 3h). The curved surface to fabricate implants for osteochondral
more pronounced mismatch in height may be attributed defects. Osteochondral defect refers to damage of both
to two primary factors: (i) the use of soft materials leads articular cartilage and subchondral bone in a joint and is
to noticeable irregularities in the form of a wavy top layer one of the most common orthopedic lesions, leading to
pattern, and (ii) a mismatch in layer thickness between pain, inflammation, and potential functional impairment.
inkjet-printed and extrusion-printed structures was The articular cartilage tissue has a thickness of 2–4 mm
observed. The printing parameters are presented in Table 1. and lacks blood vessels, and its ECM exhibits a network
Thereafter, we printed an interconnected network in of fibers with diverse orientations. Additionally, the
the soft phase on top of a ceramic hard phase. The hard adaptation of implants to the curvature of the femoral
phase was composed of hydroxyapatite, fabricated through condyle can be beneficial due to the anatomical irregular

Volume 10 Issue 3 (2024) 594 doi: 10.36922/ijb.3189