Photocrosslinkable Materials for the Fabrication of Tissue-Engineered Constructs by Stereolithography - Tissue Engineering: Computer Modeling, Biofabrication and Cell Behavior - page 152

enable an easy modification at the ester functionality [ 13 , 44 , 71 , 91 ]. However,
the obtained structures are scarcely applied on tissue engineering as they are pre-
dominantly glassy, rigid and brittle, preventing the fabrication of flexible and elas-
tomeric structures to resemble the properties of human tissues [ 13 , 58 ]. In addition,
these polymeric systems are usually not biocompatible or biodegradable, thereby
preventing their application for the fabrication of scaffolds [ 35 , 44 , 91 ]. To address
the need for photocurable biomaterials, both biocompatible and biodegradable, sig-
nificant technological advances are used to develop novel polymeric systems. The
fabrication of scaffolds through stereolithography involves the use of natural and
synthetic polymers, and polymer/ceramic blends.
3.1 Polymers
Polymeric materials are the most commonly used materials for biomedical applica-
tions, due to the wide range of its properties, easy processing and versatility. Both
natural and synthetic polymers can be modified to allow the processing through
stereolithography, without affecting or even improving the interaction with living
cells.
Biocompatible hydrogels, based on either natural or synthetic polymers, repre-
sent a relevant group of materials widely employed for several biomedical applica-
tions, including tissue engineering [ 97 ], wound dressings [ 84 ], controlled drug de-
livery [ 113 ] and cell encapsulation [ 103 ]. Hydrogels are tridimensional hydrophilic
networks with the ability to absorb and retain large amounts of water without dis-
solution [ 84 ], due to the establishment of physical (reversible) or chemical (irre-
versible) bonds between the polymeric chains [ 43 , 69 , 98 , 100 ]. These are attractive
materials for tissue engineering applications due to its excellent biocompatibility,
biodegradability, elasticity, smoothness and compositional similarities regarding the
ECM of the human body [ 43 , 98 , 101 ]. In addition, some hydrogels can be pho-
topolymerized using in vitro and in vivo conditions in the presence of cells and
photoinitiators [ 13 , 63 , 100 , 111 ], which can potentially improve its use for the in
situ regeneration of damaged tissues. The high water content of the hydrogels makes
them useful carriers for the transport and delivery of fragile molecules (e.g. proteins,
drugs or cells), providing a 3D environment similar to the one in human tissues, pro-
tecting these molecules from denaturation or degradation [ 69 , 98 , 101 ]. It is possible
to manipulate the diffusion and transport of biological materials by changing both
the density of cross-links and the pore size of the gel network. The control over the
cross-links density also enables to tailor different properties, such as its swelling be-
havior, degradation rate, mechanical properties, pore size and permeability [ 50 , 69 ].
Hydrogels used in stereolithography comprise natural polymers (e.g. alginate,
chitosan, hyaluronic acid, gelatin), synthetic polymers (e.g. poly(ethyleneglycol)
(PEG), propylene fumarate (PPF), poly( ε -caprolactone) (PCL)) and a combination
of both [ 13 , 59 , 100 , 101 ]. Synthetic and natural hydrogels are usually modified
using photoreactive and crosslinkable groups, such as acrylates and methacrylates,
to enable their processing by stereolithographic processes [ 4 , 69 , 71 , 111 ].