BLOOD VESSEL REGENERATION - 3D Bioprinting and Nanotechnology in Tissue Engineering and Regenerative Medicine - page 152

FIGURE 7.3 Stereolithography.
Two different stereolithography techniques are shown above. Each technique controls the Z -resolution by the use of
a stepper motor that moves the build plate. The X - and Y -axis resolutions are controlled through the use of mirrors
or direct illumination of each volumetric pixel (voxel). Both stereolithography techniques can utilize the same light
source for photo-initiation and either print layer-by-layer or one voxel at a time; however, the direction of the build
plate in relation to the printing resin is different. In the first case, (A) the resin reservoir remains stationary and
the build plate moves upward, away from the resin, as each layer is polymerized. In the second case, (B) the resin
reservoir moves up in height as each layer is built up from the stationary build plate. Both of these techniques can
be used in the fabrication of scaffolds for vascular regeneration applications; however, there can be size limitations
based upon the ability of the printed resin to adhere to the build plate. This can be problematic for printing method
(A) where the adhered first layer must support the weight of the entire printed scaffold.
This printing method is of interest for vascular engineering due to the variety of materials that are
available for the printing process and the fact that many of these materials can be biodegradable, thus,
as they degrade, the native tissue can invade and resorb the printed scaffold. Since either a UV or visible
light source can be used in this process, photo-initiators that are commonly used, and potentially ap-
proved by the FDA, for the polymerization of biomaterials can be utilized. Scaffolds generated using
this method of fabrication can also have their mechanical properties modulated based upon the curing
time for each layer of the scaffold. This allows for the generation of scaffolds that have mechanical
properties very similar to the native vasculature, which is critical for the prevention of intimal hy-
perplasia. However, there are some drawbacks to the use of this sort of rapid fabrication process to
manufacture vascular grafts. The most important drawback is the potential of the solvent to remain on
or in the scaffold after the manufacturing process. This results in the need to wash the scaffold during
the postprocessing steps to ensure that all of the uncross-linked monomers are removed, along with
any remaining solvent. Another drawback is that support scaffolds must be generated to help hold
the print in place, and as a result, these supports must be removed, which can prove to be a tedious
endeavor for complex geometries and requires some additional postprocessing in order to obtain the
desired surface finish on the printed vascular graft. However, as mentioned previously, one important
and desirable trait of this printing process is that multiple grafts can be printed simultaneously and the
printing speed is independent of the complexity of the implant design. For vascular grafts or implants,
this feature can be seen as advantageous in the generation of patient-specific implants (i.e. every patient
will have their own requirements that will vary in complexity based upon the native tissue architecture
being replaced). Much like the other grafts/scaffolds generated for vascular regeneration, cell-free scaf-
folds generated in this manner can be modified postprinting to contain various bioactive molecules to
encourage proper cell attachment and discourage an immune response.