Page 142 - IJB-9-4

International Journal of Bioprinting 3D-printed scaffolds for osteochondral defects

form a thrombus. As a result, mobilized MSCs and blood parameters, such as temperature, extrusion pressure, print
cells form hematomas to repair the cartilage defects . In speed, speed of hydrogel crosslinking or gelation [48,49] .
[43]
this case, only fibrocartilaginous scar tissue is generated, ME can be used to fabricate scaffolds with relatively high
which has more type I collagen and less type II collagen porosity, which facilitates seed cell adhesion, proliferation,
in the ECM. Additionally, its mechanical properties are far chondrogenesis and osteogenic differentiation. Porous
inferior to those of hyaline cartilage, and it does not form structures similar to SB can be printed with thermopolymer-
an effective and long-lasting bond with the surrounding based ME technology to promote bone growth. The pore
tissue. Bone marrow stimulation techniques, including size for the SB section of the multiphase osteochondral
microfracture and deep drilling, use similar principles to scaffold is usually 0.3–1.0 mm, with a porosity of 70%–
penetrate the subchondral bone to the bone marrow cavity 80% [50,51] . ME printing technology based on bioceramics is
and mobilize cells to the cartilage defect site to achieve mainly used in the CCZ and SB sections of osteochondral
regenerative repair. However, the regenerative repair by scaffolds . In general, the printed bioceramic scaffolds
[52]
these techniques ultimately only generates fibrocartilage achieve low porosity (20%–60%) and small pore sizes
tissue in the defect and does not completely regenerate the (0.1–0.4 mm). Many scaffolds failed to produce pore sizes
articular cartilage to restore its original functional state . >0.3 mm to promote bone growth in the SB section [53,54] .
[44]
Although hydrogel ME printing is commonly used for the
[55]
3. 3D-printed osteochondral repair AC section, Gao et al. fabricated a biphasic osteochondral
materials scaffold using this technique. The addition of β-tricalcium
phosphate (β-TCP) to the SB section of the hydrogel
The articular osteochondral tissue units are an ordered increases the mechanical stiffness and osteoinductive
and integrated whole. In normal tissue, articular cartilage properties of the hydrogel, while transforming growth
polysaccharide chains have a pore size of approximately factor-beta 1 (TGF-β1) is incorporated into the AC section
6 nm between them and the collagen fibril network to enhance cartilage formation.
has a pore size of 60–200 nm and extends vertically to MEW and ES technologies allow long filaments to
[45]
the CCZ . Unlike hyaline cartilage layer, the internal be deposited layer-by-layer through a nozzle . Its fiber
[56]
structure of CCZ and SB is much denser. This structural diameters range from microns to nanometers. In addition,
difference poses a major challenge for the bionic fabrication ES is a solvent-based printing technique that deposits
of CCZ-containing osteochondral scaffolds, particularly material fibers randomly on a collector bed, whereas
in the selection of the scaffold raw material and its design MEW is a solvent-free method that regulates where and
strategies.
how the fibers are deposited, thus controlling the final
3.1. 3D printing techniques in osteochondral tissue pattern. Polycaprolactone (PCL) is the most used material
engineering in MEW, as well as gels, chitosan, polyvinyl alcohol (PVA),
[57]
Currently, the most common 3D printing techniques hyaluronic acid, and collagen . Despite the increased
for articular osteochondral scaffolds include electro- availability of suitable materials in ES, the solvents used
[58]
spinning (ES), material extrusion (ME), stereolithography are often biotoxic and require significant attention .
(SLA), digital light processing (DLP), and melt electro- When applying MEW and ES to the construction of
writing (MEW). However, every technique has their own articular cartilage scaffolds, the main challenge is the
advantages and limitations, as well as their appropriate limited total thickness of the structure printed in the Z-axis
[59]
printing materials. In terms of material selection, there is direction . The current solution is to print the material
no evidence to date that one material is definitely better onto various collectors and body beds in order to increase
[60]
than another. In general, hydrogels are mostly used for the the structure height in the Z-axis direction . On the other
printing of hyaline cartilage layer; bioceramics, hyaluronic hand, given the limited height and strength of the micro/
acid, tricalcium phosphate (TCP), and metallic materials nanofibers, MEW and ES often produce soft scaffolds that
are more suitable for the printing of SB . In addition, the are well suited for the AC section manufacture of articular
[46]
development of new materials with better biocompatibility, osteochondral scaffolds.
plasticity, and modifiability is one of the most important SLA and DLP technologies are used to achieve
issues in the future. 3D-printed shapes by depositing material layer-by-layer.
ME technology involves depositing material via nozzles However, these technologies are not based on a nozzle
on a print bed in the X-Y plane and then stacking it layer- approach, but rather on a liquid material in a resin bath.
by-layer in the Z-axis plane . It is suitable for a wide range The difference between SLA and DLP technology is the
[47]
of materials, including thermopolymers, bioceramics, and light source used; SLA uses a laser while the light source
[61]
hydrogels. Each material requires fine-tuning of printing of DLP comes from projection . The accuracy of SLA/
Volume 9 Issue 4 (2023) 134 https://doi.org/10.18063/ijb.724