3D PRINTING AND NANOMANUFACTURING - 3D Bioprinting and Nanotechnology in Tissue Engineering and Regenerative Medicine - page 43

3 m m. Woodpile scaffolds could be used for cell transmigration study, where spacing between lines
was chosen similar to the typical size of cancer cells. The distance among each line could be changed
according to the size of cells. 10T1/2 cells were seeded on the web structures with various shapes for
tuning Poisson's ratio, which was hypothesized to change cell response ( Zhang et al. , 2013 ).
GelMod was utilized to fabricate a 3D scaffold used for adipose-derived stem cell (ASC) adhesion,
proliferation, and differentiation into adipogenic lineage ( Ovsianikov et al. , 2011a ). A cavity-dumped
oscillator was employed for femtosecond laser. A layer of scaffolds was produced at a scanning speed
of 10 mm/s with a constant average laser power of 3.5 mW. Each layer had a distance of 15 m m. The
pore size was a square cross-section of 250 m m by 250 m m. Cell adhesion and proliferation were evalu-
ated using cell staining and fluorescence image analysis. The differentiation into adipogenic lineage
was assessed by using Oil Red O staining. Ovsianikov et al. conducted a similar study on fabricated 3D
scaffold using 2PP with GelMod ( Ovsianikov et al. , 2011b ). Instead of analysis of cell differentiation,
degradation of hydrogels was investigated by measuring mass loss of fabricated scaffolds in the pres-
ence of collagenase (Type I, collagenase digestion unit (CDU)). Degradation of scaffolds depended on
incubation time. Incubation time of 3-4 h was needed to degrade half of the scaffolds.
Stereolithography using DMD was used to fabricate scaffold for tissue engineering with GMHA. The
DMD is composed of an array of micromirrors (1024 by 768) ( Suri et al. , 2011 ). For polymerization, the
power of UV light was determined to be ∼ 8 mW/cm 2 and each layer was exposed under UV light for
30 s. A platform having polymerized layer moved downward 0.5 mm for each layer. The pore sizes were
100-200 m m in one side and diameter with hexagonal and circular geometries, respectively. After the
protein grafting process, Schwann cells were seeded and cultured for 24 h. Scaffold degradation was per-
formed in 500 U/ml of hyaluronidase. The longer the UV exposure time the slower the degradation of the
scaffold. This is because longer UV exposure could lead to a higher cross-linking density. Cell adhesion
was analyzed and the scaffolds represented cell adhesion and retention of cell viability for at least 36 h.
Human umbilical vein endothelial cells (HUVECs) were seeded on 3D scaffolds fabricated with
GelMA via DMD-SL ( Figure 2.12 ) ( Gauvin et al. , 2012 ). The created scaffold had a dimension of
∼ 2 mm with a micrometer-scale resolution. Unconfined compression test was conducted for scaffolds
with different designs, the results of which are biphasic stress-strain curves consisting of low-strain
(20-40%) and high-strain (70-90%) compressive moduli ( Figure 2.13 ). At the higher strain stage, the
stress-strain relationship showed a linear behavior similar to that of elastic materials. Cell viability,
proliferation, and functionality were evaluated by using a confocal microscope after fluorescence stain-
ing, which demonstrated that the scaffolds can support cell adhesion and proliferation without damag-
ing the biological function and phenotype of the cells ( Figure 2.14 ).
Elastic modulus and Poisson's ratio are two fundamental mechanical properties that reflect the
tissue-engineered scaffolds' ability to handle various loading conditions ( Soman et al. , 2012 ). While
elastic modulus can be easily tuned by the compositions and fabrication conditions, it is substantially
more challenging to tune the Poisson's ratio of the scaffolds ( Soman et al. , 2012 ). In the work of
Soman and his colleagues, scaffolds with various Poisson's ratio (negative, positive, and zero) were
constructed using PEGDA via DMD-SL, as shown in Figures 2.15 and 2.16 ( Soman et al. , 2012 ).
Human mesenchymal stem cells (hMSCs) were seeded and cultured on the scaffolds to demonstrate
the feasibility of these scaffolds for tissue engineering and other biological applications. As long as the
scaffolds are deformed in elastic region, Poisson's ratio is solely dependent on the geometry of scaf-
folds. Thus, the tunable Poisson's ratio property can be imparted to any photocurable materials, which
shows great promise for a variety of bioengineering applications ( Soman et al. , 2012 ).