Page 264 - IJB-10-6

International Journal of Bioprinting 3D-printed contractive pennate muscle

laden hydrogel has a good molding accuracy. This could location. The combined effect between printing speed and
be due to the use of molds as external support, which flow rate, particularly when there is a mismatch between
enhances the forming quality macroscopically, especially them, would affect layer thickness during the printing
in the horizontal direction. During the fabricating process, process. This, in turn, affects the accuracy of microchannel
the support had dimensional limitations on both horizontal formation and ultimately the orientation effect.
ends of the print-outs, while in the height direction, it only The contraction performance was also compared
restricted the bottom of the fabricated tissues. Therefore, between our tissues and previously engineered muscle
the actual height of the print-outs is significantly higher tissues cultured from C2C12 cells by traditional fabrication
than the theoretical value.
approaches. Some researchers developed skeletal muscle
Regarding the microscale formation, we observed that strips and rings with the aim of actuation in biohybrid
the surface of the microchannels was relatively rough. Due robots, 20,36–40 which were fabricated by molding in
to temperature changes during the printing process, there 3D-printed strip/ring molds. The latter involves two
is solidification of the bioink from liquid to solid. During muscle rings tightly integrated with the mechanical body
this process, the bioink is extruded from the extruder to during fabrication, thus generating a greater force during
the print piece. At this time, the hydrogel retains a certain contraction. On one hand, the 3D printing process may
liquid fluidity, and the surface tension may lead to the cause damage to cell viability during extrusion, which can
boundary of microchannels being rough, thus affecting negatively affect cell differentiation and functionalization.
the microscale forming quality. The microchannels were On the other hand, decreased cell density observed in
designed in a diamond shape with a self-supporting ability our study should also be considered a factor influencing
to ensure its shape integrity during manufacturing and functional performance. Although our study has room
cultivation processes. However, we also noticed that in for improvement, its strength lies in the successful 3D
subsequent cultivation, some microchannels fused under bioprinting of large-sized in vitro tissues with a designed
gravity. Other remaining microchannels could support the organization of pennate fibers, achieving relatively high
diffusion of nutrients for the long-term viability of artificial viability and functionality.
muscles. In contrast, GelMA, as a porous material, can also
facilitate the permeation of culture medium into the in vitro Currently, there has been significant progress in the
muscle for nutrient transport. Considering the change in construction of 3D-bioprinted muscle tissue structure
cell viability in the engineered tissues after biofabrication, in vitro, with limited consideration of the functional
we observe a trend consistent with previous results. properties, especially contraction performance. 31,33,41
However, overall cell viability is slightly lower compared In this paper, we developed a novel frog gastrocnemius
to traditional fabrication techniques, 31–34 especially on the muscle-inspired 3D-bioprinted muscle mimetic for the
first day after printing. To ensure the printability of GelMA, regeneration of damaged skeletal muscles and actuation
the cell-laden bioink was preserved in the cooling system of biohybrid robotics. Using 3D bioprinting, we created
for about 30 min before printing. This step initially reduces skeletal muscle tissues with a fusiform shape and pennate
the cell activity to some extent after fabrication. Thereafter, angle microchannels, displaying good cytocompatibility
cell viability gradually increased with culture time and and fiber alignment. The engineered muscle tissues with
reached approximately 80% after 7 days, providing a basis biomimetic design exhibited much higher contraction
for contraction performance evaluation. forces compared to those of common parallel-printed
muscle, suggesting the excellent potential of the developed
We also investigated the influence of printing speed muscle models. Thus, this paper presents a developed
and air pressure on myotube alignment in the engineered model aimed at achieving higher contraction performance,
tissues. The cell orientation rate would increase with higher contributing to future advancements in SMTE applications.
printing speed, while there is no evident relationship
between air pressure and cell orientation rate. As the 5. Conclusion
printing speed increases, the nozzle moves faster during
the extrusion printing process, causing the diameter of the In our study, we designed and 3D printed modular
formed hydrogel filaments to shrink and resulting in smaller skeletal muscle tissues with customized shuttle micro
microchannels. A previous study has demonstrated that shapes, pennate fiber arrangement, and microchannels
reducing the size of cell culture substrate can improve the for nutrient transport during culture. The engineered
directional differentiation effect of cells. Thus, increasing tissues could achieve a maximum contractile force of
35
the printing speed can improve the directional effect of cell over 400 μN when subjected to electrical stimulation after
alignment. In contrast, an increase in air pressure would functionalization. Our study proved that 3D printing
increase the accumulation of hydrogels at the same spatial could be used to effectively achieve uniform distribution of

Volume 10 Issue 6 (2024) 256 doi: 10.36922/ijb.4371