Page 162 - IJB-9-1

International Journal of Bioprinting Bio-inks for 3D printing cell microenvironment

Figure 3. Stereolithography bio-inks for engineering stiffness gradient microenvironment. (A) A schematic diagram of ink mixing controlled by a micro-
fluidic chip. (B) Mixing three colored poly(ethylene glycol) diacrylate (PEGDA) inks to obtain continuous gradient colors for printing two-dimensional
(2D) and three-dimensional (3D) gradient structures .
[76]

recent study, composable gradients of stereolithography achieve viscoelasticity independent of the initial stiffness.
were achieved by using a microfluidic chip to control the This feature is particularly important in studying stem cell
mixing of bio-inks of different components (Figure 3) . differentiation and the mechanical microenvironment of
[76]
The multiple printing of different bio-inks can increase the cancer cells. According to preliminary studies on stress
scale of complex structures. For example, cells are loaded relaxation of bio-inks, cross-linked covalent bonds store
into bio-inks to make gel microfibers, and after mixing, pure elasticity, whereas weaker non-covalent bonds
the microfibers are aligned by secondary extrusion, allow for some modulus dissipation . Alginate is one of
[80]
thereby obtaining the directional arrangement of cells. the commonly used materials to tune viscoelasticity due
The utilization of these bio-inks can partially resolve the to its ionic cross-linking properties. In a study, alginate
anisotropy of the mechanical microenvironment, but and PEG were covalently grafted together to achieve
further studies are still required for more precise control, different stress relaxations by changing the molecular
which may be solved by dynamic regulation . weight of PEG and the ionic cross-linking concentration
[77]
of alginate . The increase in PEG concentration and
[81]
5.2. Dynamic mechanical microenvironment molecular weight resulted in faster stress relaxation, higher
When structures and cells are co-cultured, the initial loss modulus, increased creep, and a significant impact on
stiffness gradually changes over time, resulting in a fibroblast proliferation and osteoblast differentiation .
[81]
temporal stiffness gradient. As a general rule, bioactive In addition, this system can attenuate the interference of
bio-inks are degraded by cells in a gradual manner, and biodegradation on stiffness changes. Stress relaxation can
the loss of mass due to degradation inevitably reduces also be achieved with interpenetrating network hydrogels
stiffness. On the other hand, cells proliferate while based on HA-hydrazine and collagen. This combination
secreting ECM to deposit on the scaffold and changing the has biocompatibility closer to native ECM than alginate .
[80]
matrix’s mechanical properties. This process is called ECM The dynamic mechanical stimulation of engineered
remodeling . The rate of degradation can be controlled cell microenvironment imposes requirements on bio-ink
[78]
by material design, while remodeling depends on the structures in addition to stiffness. It is necessary to ensure
state of cells and tissues . Remodeling and degradation structural integrity throughout the dynamic mechanical
[79]
usually occur simultaneously, and their synergistical stimulation loading cycle. Cyclic load testing, for example,
effects determine the temporal change of the mechanical requires a material with a suitable fatigue limit that can
microenvironment. This process is both inevitable and withstand a certain number of stretches or compressions
difficult to control, and remains as one of the relatively without breaking or chipping. The non-hydrogel scaffold
uncharted territories of bio-inks.
material has greater plasticity and is more prone to
Stress relaxation is another dynamic mechanical cue that plastic deformation when bent. The fracture properties
can be simulated, and by tuning the material, it is possible to of hydrogels are affected by the cross-linking bond and

Volume 9 Issue 1 (2023) 154 https://doi.org/10.18063/ijb.v9i1.632