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3D Bioprinting Photo-crosslinkable Hydrogels for Bone and Cartilage Repair
cartilage repair . 3D bioprinting can be considered an and chondrogenic phenotype in hydrogels with medium
[6]
additive manufacturing technique where biomaterials, stiffness (~17 kPa) . Cell proliferation could also be
[14]
cells and growth factors, often referred to as “bio- influenced by cross-linking density. In one study, Bryant
ink,” are printed to create tissue-like structures that et al. found that the increase in crosslinking density of
imitate natural tissues . It has been applied in bone and poly(ethylene glycol) diacrylate (PEGDA) hydrogel
[7]
cartilage tissue engineering as it can precisely fabricate resulted in a decrease of chondrocytes proliferation and
the 3D scaffolds by controlling the pore size, porosity, protein expression; in another study, Marklein et al.
and interconnectivity [8,9] . To date, there are four leading discovered that human mesenchymal stem cells (hMSCs)
3D printing technologies: Inkjet-based bioprinting, exhibited increased cell proliferation on a stiff hyaluronic
extrusion-based bioprinting, laser-assisted bioprinting, acid methacrylate (HAMA) hydrogel compared to a softer
and stereolithography (SLA) bioprinting. In extrusion- one in 2D condition [15,16] . In terms of cell differentiation,
based bioprinting, bio-inks are extruded as filaments Bian et al. investigated the effect of crosslinking density
and undergo fast crosslinking to maintain the desired of photo-crosslinkable hydrogels on encapsulated
shape and structure. Laser-assisted bioprinters use a laser mesenchymal stem cells (MSCs) . They found that
[17]
pulse to produce a cell suspended bio-ink and deposit it the degree of photo-crosslinking can regulate the
into the substrate in an orderly manner. In inkjet-based differentiation of MSCs: Enhanced chondrogenesis in a
bioprinting, a certain volume of bio-inks was injected onto soft environment and osteogenesis in a stiff environment,
a substrate to form a precise pattern with either thermal which provides important clues for the design of photo-
or piezoelectric energy. In SLA bioprinting, a digital crosslinking hydrogels for cartilage and bone repair. Due
projector is used to selectively crosslink bio-ink plane-by- to these unique features, photo-crosslinkable hydrogels
plane into desired shapes. The four primary bioprinting have shown a wide range of applications in biomedical
techniques each have specific strengths, weaknesses, and fields including organ printing, tissue engineering,
limitations. Although no single bioprinting technology disease modeling, and high-throughput drug screening .
[18]
can achieve the complete replication of complexities In this review, we summarize the classification,
of various tissues, extrusion and SLA bioprinting are crosslinking mechanism and application of photo-
commonly used for preparing bone and cartilage scaffolds crosslinkable hydrogels for 3D bone and cartilage
due to their good biocompatibility and easy combination bioprinting (Figure 1). The cell types and additives
of multiple crosslinking mechanisms [4,10] . encapsulated in hydrogels to promote bone and cartilage
One of the key elements for 3D bioprinting is reconstruction are additionally discussed. Finally, the
the bio-ink. Unlike conventional 3D printing process future prospects of bone and cartilage 3D bioprinting are
in which inks can be printed in melt form at high outlined.
temperature (ceramics and alloys) or as a polymer
solution dissolved in organic solvents, elevated 2. Photo-crosslinkable hydrogel bio-ink
temperature or organic solvents are unfortunately not systems
cytocompatible with depositing living cells and growth
factors in a 3D bioprinting process. Hydrogels, which In this section, we focus on photo-crosslinkable hydrogels,
can provide nutritious environments suitable for cell and introduce additives (such as nanomaterials, functional
survival, proliferation, and differentiation, have unique cells, and drugs or cytokines) which can improve the
advantages in 3D printing living cells and/or growth physical and biological properties of hydrogels.
factors. In general, click chemistry, enzymatic reactions, 2.1. Photo-crosslinkable hydrogels
Schiff’s base reaction, and photo-polymerization can be
used to crosslink hydrogels. In this review, we would For photo-crosslinkable materials, when they are
like to focus on photo-crosslinking due to its rapid in situ exposed to a suitable light, the liquid state solution can be
gelling, good cytocompatibility and low toxicity [11,12] . solidified. In general, photo-polymerization includes free-
Besides, the crosslinking density and physicochemical radical initiated chain polymerization and bio-orthogonal
properties of photo-crosslinkable hydrogel could be click reaction. In free radical photo-polymerization, the
precisely controlled through adjusting the intensity of functionalization of hydrogel prepolymers with active
light and exposure time to promote cell proliferation groups (such as methacrylates, vinyl esters, and acrylates)
and differentiation [12,13] . For example, by controlling is an essential step. Under light irradiation, the absorbed
crosslinking density of the photo-crosslinkable hydrogel, photons of photoinitiator promote its cleavage, thereby
Li et al. regulated the morphology of chondrocytes in encouraging the generation of free-radical molecules.
3D gelatin methacrylate (GelMA) environment. Cells Then, these molecules will react with the vinyl bonds,
exhibited round shape in hard hydrogel with stiffness leading to the formation of chemical crosslinks between
at ~30 kPa; elongated shape in soft hydrogel (~4 kPa) prepolymers . In photo-click reaction, the thiol-ene
[19]
38 International Journal of Bioprinting (2021)–Volume 7, Issue 3