3D BIOPRINTING TECHNIQUES - 3D Bioprinting and Nanotechnology in Tissue Engineering and Regenerative Medicine - page 66

FIGURE 3.5
Solenoid valve-based bioprinting capable of depositing 20 pl or higher droplets of living cells and biological
molecules.
an inert gas source. Multiple print head assemblies can be fitted together to improve the throughput
of the system. Figure 3.5 shows the schematic of a solenoid-based jetting nozzle. The droplet volume
of the printed materials can be controlled by the applied air pressure and the frequency of the sole-
noid valve open time. Electrical pulse signals sent from the computer can engage or disengage the
solenoid leading to droplet ejection from the nozzle. Different nozzle diameters can be attached to the
print head to deliver precise quantities of the fluid. The solenoid valve system does not involve heat
and is capable of accepting viscous polymers such as collagen and 1-2% sodium alginate. Multiple
nozzles can be fitted to the robotic stage to print multiple materials to form a complex heterogeneous
construct.
Yoo and coworkers ( Lee et al . , 2010 ) reported using this technique for the on-demand fabrication of
cellular constructs containing a neural cell line, a fibrin matrix containing a vascular endothelial growth
factor (VEGF), and a collagen hydrogel. Since fibrin gel cannot be preloaded into the cartridge, its con-
stituents—fibrinogen, thrombin, and heparin—were separated into two different material cartridges.
A third cartridge contained the neural cells to be printed and a fourth cartridge contained a sodium
bicarbonate to help in the cross-linking of the collagen gel. With an initial cell density of 1 × 10 6 cells/
ml, each printed droplet of volume 11 ± 0.6nl contained about 56 ± 9 cells. Reported viability of cells
within the 500 m m thick collagen construct was greater than 93% soon after printing. This work dem-
onstrates the feasibility of precisely placing desired concentrations of VEGF within spatial locations in-
side the construct to affect cellular behavior, namely proliferation and differentiation, by controlling the
time release behavior of these growth factors. In another study using a similar technique, Karande and
his team demonstrated the solenoid-based printing technique to engineer human skin. Fibroblasts and
keratinocytes representing the epidermis and dermis respectively, along with collagen were bioprinted
to showcase the capability of fabricating a complex living system. The printed skin tissue provides ap-
plications in topical drug formulation discovery and screening along with designing autologous grafts
for wound healing ( Lee et al., 2014b ).