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Liang, et al. PEG has been shown to facilitate chondrogenic ECM

regeneration back in 2002 . It is capable for maintaining
[86]
the viability of non-adhesive cells such as chondrocytes,
cells including osteoblast and fibroblast . Studies have
[87]
Full name Extrusion-based printing Digital light processing Interpenetrating network KLF3 Antisense RNA 1: KLF3-AS1 G-protein-coupled receptor kinase interacting protein 1: GIT-1 while discouraging the adhesion and spreading of adhesive
found that this biologically inert property of PEG could
be improved by the inclusion of hydroxyapatite and
. As for its
Laponite, a kind of synthetic smectite clay
[88,89]
mechanical properties, the compressive modulus of PEG
Acronyms EBP DLP IPN hydrogel is about 0.75 MPa, which is relatively stronger
than other hydrogels but still low compared with that of
native human articular cartilage .
[90]
Scientists have been working on the development of
PEG hydrogel in 3D printing for the past decade. In 2014,
Lithium acylphosphinate salt Bone marrow-derived mesenchymal stem cells Human mesenchymal stem Transforming growth factor β of tricalcium phosphate used as bone graft substitute .
Zhang et al. developed a 3D-printed PEG scaffold with
β-tricalcium phosphate ceramic, which is a special form
[91]
The scaffold was of 50%-65% porosity and was fully
interconnected. Formation of new tissue with smooth but
raised surface was observed 24 weeks after implantation
Full name Platelet-rich plasma Glycosaminoglycan Hydroxyproline using rabbit model with trochlea defects . Gao and
[90]
other scientists fabricated a PEG-GelMA scaffold
cells
encapsulating hMSCs by inkjet printing in 2015 . The
[92]
contained cells were shown to maintain in their initially
deposited position during printing and the cell viability
Acronyms LAP BMSC hMSC PRP GAG TGFβ HYP was over 80%. 63% of embedded hMSCs underwent
chondrogenic differentiation after 21 days of in vitro
chondrogenesis, but cell hypertrophy was also observed.
The compressive modulus of either cell-laden or non-
cell-laden PEG-GelMA scaffold was also lower than that
of their corresponding PEG scaffold . In 2018, Wang et
[92]
and PEG diacrylate (PEGDA) (Table 4) . Their results
[78]
showed that the compressive stress of hydrogels with
Full name Polyethylene glycol Polymer poly (N-isopropylacrylamide) grafted hyaluronan Methacrylated hyaluronic acid Polycaprolactone Methacrylated SF PEG diacrylate Tannic acid al. developed a UV-crosslinkable ink containing GelMA
both GelMA and PEGDA was significantly higher than
that of GelMA alone, but the novel ink became fragile as
Table 4. Acronyms applied in this review and their full name
the concentration of GelMA increased due to enhanced
Acronyms PEG pNIPAAM MeHA PCL Sil-MA PEGDA crosslinking density. For its cytocompatibility, the
viability of MC3T3-E1 cells during a 7-day culture was
maintained above 99%. Recently, Qiao et al. combined
triblock polymer networks of PCL-b-PEG-b-PCL with
TA
GelMA, BMSCs and growth factors to construct a native-
like tri-layered 3D scaffold via melt electrowriting, a
high-resolution additive manufacturing process
. The
[93,94]
and subchondral bone (B), were fabricated with PCL-b-
Full name Osteoarthritis Extracellular matrix Metalloproteinase A disintegrin and a metalloproteinase with thrombospondin motifs Hyaluronic acid Collagen Silk fibroin Gelatin methacryloyl three layers, superficial cartilage (S), deep cartilage (D),
PEG-b-PCL filaments of different diameters, spacing, and
orientations based on the native osteochondral COL fiber
architecture. The compressive moduli of the structure
were benign (S: 283.6 ± 22.3 kPa, D: 964.2 ± 56.8 kPa,
B: 55.8 ± 5.4 MPa), but the mechanical strength of
Acronyms OA ECM MMP ADAMTS HA COL SF GelMA cartilage parts was still inferior to that of natural tissue.
Significant accumulation of osteochondral tissue-related
zonal marker proteins and the presence of spatially

International Journal of Bioprinting (2022)–Volume 8, Issue 3 23