Photocrosslinkable Materials for the Fabrication of Tissue-Engineered Constructs by Stereolithography - Tissue Engineering: Computer Modeling, Biofabrication and Cell Behavior - page 153

Fig. 5 Scanning electron microscopy (SEM) images of microbanks ( a ), microwells ( b ) and mi-
croneedle-array ( c ) built by stereolithography using the CL and TMC oligomers [ 66 ]
3.1.1 Synthetic Polymers and Hydrogels
The interest on synthetic polymers concerns good and adjustable physical, chemical
and mechanical properties, easy processing into a wide range of shapes and large-
scale production. On the other hand, their main drawback regard its limited biocom-
patibility, poor interaction with living cells, and in some cases the toxicity of the
degradation products [ 87 , 96 ]. The surface of synthetic polymers can be modified
through different approaches, such as plasma treatment, polymer coating, chemical
modification, peptide immobilization and photochemical modifications, to improve
cell interactions or provide specific interactions with different cell types [ 40 , 102 ].
In stereolithography, the most used biodegradable macromers are based on func-
tionalized oligomers containing hydrolyzable ester- or carbonate linkages in the
main chain [ 71 ]. Examples include PPF [ 56 ], PCL [ 35 ], poly(D,L-lactide) (PDLLA)
[ 70 ] and poly(trimethylene carbonate) (PTMC) [ 89 ]. Usually, these polymers are
mixed with reactive and non-reactive diluents, such as diethyl fumarate (DEF), to
control the viscosity and the degree of cross-linking, allowing the fabrication of
constructs with adequate mechanical properties [ 35 , 50 , 58 , 71 ].
Matsuda et al. [ 67 ] reported the preparation of photocurable liquid biodegradable
copolymers through the ring-opening copolymerization of ε -caprolactone (CL) and
trimethylene carbonate (TMC), using polyol as initiator and tin(II) 2-ethylhexanoate
as a catalyst. The copolymers were subsequently derivated at the hydroxyl end with
a photodimerizable coumarin group. Results showed that higher coumarin function-
ality and UV light intensity, and a reduced layer thickness of the liquid film precur-
sor increases the photocuring reaction. The same oligomers were used by Matsuda
and Mizutani [ 66 ] to produce photocurable copolymers, using trimethylene glycol
or PEG as initiator and an acrylate group (acryloyl chloride) to end-functionalize the
oligomers. Different structures were produced using the photocurable copolymers
through stereolithography, as shown in Fig. 5 .
PPF, an unsaturated linear polyester, has been used in stereolithography due to
its ability to be cross-linked through the carbon-carbon double bonds, presenting
excellent mechanical properties [ 54 , 87 ]. The subunits of PPF can be crosslinked by
using different agents, such as DEF, methyl methacrylate and N-vinylpyrrolidone
[ 54 ]. In addition, PPF also undergoes degradation into biocompatible and non-toxic