Besides the classical laser-based SLA process, alternative processes using digital mask generators, e.g. liquid crystal displays or digital mirror devices (DMDs), have been used successfully to build structures out of polymers and ceramics. In the RP literature this process is also termed solid ground curing (SGC). In contrast to traditional UV laser based SLA machines, DLP systems are significantly cheaper and therefore more versatile with respect to material modifications. At the same time, DLP machines can expose a whole layer at once, whereas laser-based systems have to scan the contour of the object sequentially. DLP systems are based on a digital micromirror device (as used in consumer electronics). By projecting a bitmap onto the photosensitive resin, the liquid resin can be solidified selectively. Theoretically, DLP systems can be used to fabricate scaffolds with high resolution and geometric complexity. However, a prerequisite is the availability of a light-curable, biocompatible and bioresorbable polymer material.
The wider application of SGC in designing scaffolds is mainly driven by developments in photochemically driven gelation technology of biomacromolecules chemically modified with photodimerizable groups. Recent reviews summarize the chemistry and rationale for using these polymers in scaffold-based tissue engineering (Nguyen and West, 2002). Photopolymerizable and biodegradable poly(ethylene glycol)-based macromers, acrylated poly(ethylene glycol) derivatives including poly(ethylene glycol)-co-poly(a-hydroxy acid) diacrylate and poly(ethylene glycol)-poly(lysine) diacrylate, both of which are end capped with acryloyl groups, have been studied in detail by Matsuda's group (Mizutani, Arnold and Matsuda, 2002).
Using SCG technology, tubular photoconstructs were prepared by photocopolymerization of vinylated polysaccharide and vinylated gelatin (Mizutani, Arnold and Matsuda, 2002). The mixing of diacrylated poly(ethylene glycol) with vinylated polysaccharide improved the burst strength of photogels against the gradual infusion of water. These photocurable polysaccharides may be used as photocured scaffolds in tissue-engineered devices.
Liu and Bhatia (2002) describe the development of a photopatterning technique that allows localized photoencapsulation of live mammalian cells to control the tissue architecture. Cell viability was characterized using HepG2 cells, a human hepatoma cell line. The utility of this method was demonstrated by photopatterning hydrogels containing live cells in various single-layer structures, patterns of multiple cellular domains in a single 'hybrid' hydrogel layer and patterns of multiple cell types in multiple layers. The authors observed that UV exposure itself did not cause cell death over the doses and timescale studied, while the photoinitiator 2,2-dimethoxy-2-phenylacetophenone was itself cytotoxic in a dose-dependent manner. Furthermore, the combination of UV and photoinitiator was the least biocompatible condition, presumably owing to formation of toxic free radicals.
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