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3D Bioprinting Photo-crosslinkable Hydrogels for Bone and Cartilage Repair
substituting groups, such as azido-functionalized CS, density and matrix stiffness . This can be attributed to
[12]
vinylated CS, and MA-CS . However, this mechanical energy-dependent regulation of photoinitiator activation
[38]
strength of azido or vinylated-functionalized hydrogel is and crosslinker reactivity . While the presence of
[44]
relatively low, and the insufficient biocompatibility limits photoinitiators can greatly lower the energy barrier for
further applications. At present, the most common method initiating the often-simple click-reactions of crosslinking
for preparing photo-crosslinkable CS is by grafting MA groups such as acrylates, under appropriate photo-
groups to provide good mechanical properties as well as crosslinking conditions, sufficient photonic energy is
suitable biocompatibility . required to complete the photo-crosslinking process,
[39]
Polyethylene glycol (PEG) has high hydrophilicity whereas by intentionally limiting light exposure intensity
with abundant functional groups for chemical modification and duration, partially crosslinked matrixes are formed
and adjustable properties, which is widely used in various with reduced stiffness and strength to mimic softer tissues.
biomedical applications. PEG hydrogel formed by photo- This creates a two-way array to regulate the mechanical
click reaction is one of the common methods to achieve features of photo-crosslinkable hydrogels entirely from
photo-polymerization. The biophysical or biochemical tuning the light source alone. To achieve complete
properties in the thiol-ene PEG hydrogel can be tuned crosslinking, light intensity and exposure time are
with the degree of substitution . Meanwhile, PEGDA, interdependent as the amount to the total energy supplied
[40]
PEG dimethacrylate (PEGDMA), and ethylene linkage to the reaction between polymer chains and crosslinkers.
multi-arm PEG (n-PEG) are the most commonly used Herein, a high intensity enables low exposure time, and
macromolecules in photo-polymerization and of which, vice versa. This creates an interesting dynamic between
PEGDA is very popular in terms of bioprinting. Under these two parameters when applied in photo-bioprinting
ultraviolet (UV) or visible light exposure, the double- as vigorous limitations exist in terms of biocompatibility
bond acrylate groups in PEGDA can initiate rapid photo- in additive manufacturing . In addition, cells are highly
[45]
polymerization to form a 3D polymer network and in sensitive to external changes in the microenvironment
the presence of a photoinitiator , and the mechanical including shear, heat, and radiation. As radioactive
[41]
strength can be controlled by changing the molecular damage and heat generated by light sources such as
weight or the concentration of the hydrogel . lasers are unavoidable when crosslinking cell-embedded
[42]
hydrogels, a balance of intensity and exposure is required
2.2. Control of photo-crosslinkable structures to preserve cell viability of printed structures. For example,
with various parameters alginate/GelMA bio-inks can be crosslinked at as low as

−2
The integrity of photo-crosslinkable scaffolds is 4 mW cm with UV light source to ensure 80% survival
determined by both the strength of the individual polymer of cells in printed scaffolds, spanning over 20 – 60 kPa
[46]
chain as well as the density of crosslinking networks modulus under various UV exposure . Yet the same
within the hydrogel matrix. Naturally, high molecular approach faces difficulties in SLA, where light intensity
weight hydrogels possess higher matrix stiffness due to decays as it goes through the medium in which it is
the larger proportion of less bendable polymer backbones absorbed in accordance with the Beer–Lambert equation.
compared with the more flexible connective ends between This creates dilemma where light intensity cannot be
polymer chains. The increase in polymer concentration maintained in deeper areas of the hydrogel matrix to
naturally enhances matrix strength as the density of inter- surmount the energy barrier required to achieve photo-
linkage between polymer chains increase. Yet increase crosslinking. In addition, a decrease in intensity impedes
in hydrogel concentration in aqueous solutions often resolution as it increases diffraction of light, which greatly
exhibits high viscosity, which impedes the extrusion limits the fidelity of printed structures in reservoir-based
process due to the shear stress. Alternatively, the density printing. Hence, to elucidate the effects on light intensity,
of crosslinks can be chemically modified through the exposure time, and cell density, the previous studies have
degree of substitution, which is the ratio of substituent created GelMA-printed model phase diagram between 7 –
group to unmodified group in a photopolymer. A high 16 mW/cm and 15 – 45 s, where areas for underexposure
2
[47]
degree of substitution greatly increases the chance of and overexposure were plotted . With the advancement
successful crosslinks throughout the hydrogel matrix in neural network technology, machine learning has also
without actual addition of polymer chains, and contributes been used to predict cell viability in SLA bioprinting,
toward increased complexity of the polymer network as with exceptional accuracies in predictions at as low as
well as improved stiffness and lowered porosity of printed 10% of total data supplied . The learning algorithm
[48]
structures . concluded that exposure time had the greatest effect
[43]
By modulating the light intensity and exposure time, on cell viability, followed by layer thickness, GelMA
photo-polymerization controls the formation process of concentration, and light intensity. Thus, the adjustment of
the hydrogel and its properties, including the crosslinking light source alone in terms of intensity and exposure time

40 International Journal of Bioprinting (2021)–Volume 7, Issue 3