Page 545 - IJB-10-4

International Journal of Bioprinting 3D-bioprinting of osteochondral plugs

reinforcing lattice and a cell-laden bioink. The chondral Methacrylated gelatin (GelMA) was selected for the
section is intended to generate neocartilage and integrate bone bioink due to its printability, biocompatibility, and
with adjacent healthy cartilage after implantation. resorbability. Gelatin is derived from collagen, the most
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Similarly, the bone portion is intended to serve as the abundant protein in the human body and a large constituent
subchondral bone and integrate with surrounding of ECM in many tissues. Similar to the HAMA–HMWHA
tissue while providing mechanical support and access bioink, the 5 wt% GelMA bioink composition exhibits
to nutrients. A diameter of 12 mm was selected, which significant shear-thinning characteristics, suggesting good
is substantially larger than most biofabricated OC plugs printability (Figure 3A).
reported in the literature. This size closely approximates Photorheology of the chondral and bone bioinks was
the size of plugs needed in clinical settings for treating conducted to further understand their flow behavior,
lesions after traumatic injury in humans. The 2 mm curing characteristics, and stiffness after crosslinking.
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thickness of the chondral section is similar to the thickness Viscous flow behavior is characterized by the storage
of human articular cartilage. We included a subchondral modulus (G’) and the loss modulus (G”). For printability
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bone section in the biphasic plug design due to its with an extrusion bioprinter, a bioink needs a G’ that is
importance in cartilage maintenance. Articular cartilage greater than its G”. Photorheology was performed with an
receives much of its nutrient supply from the subchondral oscillatory shear test at 0.1% strain and 1 Hz using a 500 µm
bone, and a stable subchondral bone environment is gap. Both bioinks exhibited gel-like behavior both prior to
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essential to maintaining healthy cartilage. 6,50 We selected and after photocuring (G’ > G”) and had similar storage
materials and bioinks for the 3D bioprinting of this OC moduli between 8 and 11 kPa (Figure 3B). The bioinks
plug based on their biocompatibility, ability to withstand respectively reached their maximum G’ approximately 60 s
physiological loads, suitable rheological properties that after the onset of illumination.
allow extrusion printing, and ability to maintain their
shape and dimensional fidelity during and after printing. 3.3. 3D printing
The evaluation of the biological and mechanical properties Polycaprolactone (PCL) was selected for 3D printing the
of each individual component within the multi-material reinforcing lattice of the bone portion of the OC plug
OC plug is challenging. Therefore, we investigated the due to its bioresorbability, biocompatibility, and ability
chondral bioink, bone bioink, and bone lattice separately. to be processed at relatively low temperatures so it can be
co-printed alongside cell-laden bioinks. The addition of
3.2. Bioink composition and TCP and HAp, ceramics found in native bone, to the PCL
rheological characterization increases its stiffness and may improve osteoinduction
Hyaluronic acid (HA) is a non-sulfated GAG found in and osteoconduction in the surrounding gel. The
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articular cartilage and synovial fluid, making it a logical 3D-printed PCL lattices demonstrated high fidelity and
choice for use in a chondral bioink. The chondral bioink resolution with a scaffold diameter of 12.47 mm, layer
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was formulated using methacrylate-modified hyaluronic thickness of 308 µm, and line width of 574 µm (Figure 4).
acid (HAMA) to enable UV crosslinking for enhanced The 3D-printed chondral and bone bioinks retained their
stiffness. The shear thinning properties of the bioink were cylindrical shape after extrusion. The co-printing of the
characterized using a shear rate sweep from 0.01 to 100 three different materials demonstrated the ability to 3D
s at 23°C. Shear-thinning bioinks, which exhibit lower bioprint a complete biphasic OC plug.
−1
viscosities at higher strain rates, reduce the shear stress
that encapsulated cells experience while also allowing 3.4. Compression testing
the bioink to retain its shape after extrusion. However, a It is important that the materials selected for the OC
formulation of 2.5 wt% HAMA has only moderate shear plug exhibit sufficient strength to function properly in
thinning and relatively low zero-shear viscosity, indicating the bone/cartilage environment. Therefore, we evaluated
that it will not be able to retain its shape after extrusion the stiffness of the individual OC plug components with
(Figure 3A). The incorporation of unmodified but high- compression testing. The 3D-printed PCL and ceramic
molecular weight hyaluronic acid (HMWHA) as a viscosity composite lattices were compressed to 40% strain. The
modifier enhanced the shear-thinning characteristics linear portion of the stress–strain curve indicated a
of the HAMA-based bioink (Figure 3A). The greater compressive modulus of 68.1 ± 10.4 MPa (Figure 5A).
viscosity at low shear rates allows the extruded material Despite the high ceramic content (40 wt%) and strains
to retain its shape during bioprinting until crosslinking by experienced, the lattices did not fracture but were
photocuring, and the decrease in viscosity with increasing plastically deformed (Figure 5A, inset). The 3D-printed
shear rates demonstrates the shear-thinning properties of lattices did have a considerably lower compressive
the chondral bioink. modulus compared to trabecular and subchondral bone,

Volume 10 Issue 4 (2024) 537 doi: 10.36922/ijb.4053