Biomedical Engineering Reference
In-Depth Information
and spatial patterns within a Z -range of several hundreds of microns. Another
research group even produced PEG-based scaffolds in the millimetre scale [ 132,
149- 151 ]. Several other research groups also selected PEGDA as starting material
[ 31, 152 ] . Yasar et al. [ 31 ] successfully plotted 100 mm-sized complex scaffold
architectures. The swelling effects of the PEGDA, however, prevented the fabrica-
tion of highly reproducible samples below 100 mm. Higher resolutions could be
obtained in the presence of UV absorbers since they prevent the internal reflection
of the UV light within the polymer solution. More recently, the feasibility of plot-
ting cell-encapsulated hydrogels has been evaluated. Several research groups have
already reported on the cell encapsulation in photocrosslinkable poly(ethylene gly-
col) (PEG) microgels [ 30, 153 ] .
In addition to synthetic polymers, photocrosslinkable biopolymers including
hyaluronic acid (HA) [ 154 ] and gelatin [ 70 ] derivatives have already been printed
with or without cells using SLA.
In order to enhance the cell-interactive properties of a material, different surface
functionalization strategies can be elaborated [ 31 ] . Luo et al. [ 155 ] grafted RGD-
containing peptide sequences on the surface of scaffolds composed of cysteine-
modi fi ed agarose. Han et al. [ 141 ] applied a fibronectin coating on the surface of a
PEGDA scaffold to improve the attachment of murine marrow-derived progenitor
cells. However, important limitations of laser-based systems include both the need
for photocrosslinkable materials as well as the effect of the applied UV light on the
encapsulated cells [ 156 ] .
Since shrinkage occurs after post-processing of scaffolds developed using SLA,
a major drawback is its limited resolution [ 138 ]. In addition, due to scattering phe-
nomena of the applied laser beam, a significant deformation occurs when relatively
small objects are developed. The produced hydrogel is often weak upon removal
and post-curing is often essential.
Therefore, m-SLA was introduced to counter the limitations of SLA from a reso-
lution point of view. For example, Lee et al. [ 135, 136, 157 ] developed a hybrid
scaffold consisting of an acrylated trimethylene carbonate/trimethylolpropane
(TMC/TMP) framework and an alginate hydrogel for chondrocyte encapsulation.
The encapsulated cells retained their phenotypic expression within the structure and
the scaffold remained mechanically stable up to 4 weeks after implantation in mice.
Barry et al. [ 158 ] have combined direct ink writing (DIW) with in situ photopoly-
merization to create hydrogel scaffolds possessing micrometre-sized features. Using
this approach, another research group even realized submicron range structures
based on PEGDA [ 159 ] .
In addition to SLA techniques, SGC also shows potential to be applied in the
development of porous hydrogel-based scaffolds for tissue engineering, as already
indicated before [ 138 ]. However, up to now, no literature data regarding this appli-
cation and hydrogel processing can be found.
In order to achieve 3D subcellular resolution during scaffold development, 2PP
can offer a suitable alternative for SLA. Since this technique was only properly
introduced recently, only few reports can be found in literature regarding the appli-
cation of 2PP to produce porous hydrogels.
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