The calibration of continuous Digital Light Processing (cDLP) for the highly accurate additive manufacturing of tissue engineered bone scaffolds

It is helpful if the rendering of both the external and internal geometry of bone tissue engineering scaffolds is highly accurate. The external geometry must accurately fit the defect site if the scaffold is to be incorporated by the host tissue. It may also be useful to load internal pore spaces with cells and growth factors prior to implantation. Optimal pore space size has been judged to be between 200 and 1600 microns. Continuous Digital Light processing (cDLP) is one of the most promising methods for the highly accurate rendering of tissue engineering scaffolds using biocompatible dye-initiator packages and resorbable polymers. The high accuracy of cDLP scaffold rendering results in part from two technical developments. The first technical achievement reported here is the integration of DLP® (Texas Instruments, Dallas, TX) technology using a Digital Micromirror Device (DMD) chip into an additive manufacturing device, such as the Perfactory® SXGA+ Standard UV device (envisionTEC, Ferndale, MI). The Perfactory UV device is capable of continuously polymerizing 35.5 × 35.5 × 50μm voxels. The second technical achievement reported here is the development of a biocompatible dye-initiator package for the rendering of resorbable polymer tissue engineering scaffolds. A dye is used to block light, thereby limiting the depth of polymerization. In this study we demonstrate the fabrication of scaffolds from the well-studied resorbable polymer, poly(propylene fumarate) (PPF). We have used a Perfactory UV device to render porous cylindrical PPF scaffolds with a diameter of 6mm and a length of either 1.2mm (N=10) or 12.4mm (N=8) with either 2 or 4 minute exposure using a "plate and post" geometry. Our Computer Aided Design for this scaffold is rendered on the Perfactory UV via 50μm thick layers. We used a 120μm curing depth to ensure sufficient overcuring (inter-layer binding). A yellow azo chromium or titanium dioxide (TiO ₂) dye, Irgacure® 819 (BASF [Ciba], Florham Park, NJ) initiator, and diethyl fumarate solvent were added to the primary material, PPF, and used for scaffold production. A 500-195-20 Mitutoyo (Aurora, IL) caliper was used to measure scaffold features. The 12.4mm long azo chromium scaffolds were micro-CT (μCT) scanned. The 1.2mmlong scaffolds were imaged via scanning electron microscope (SEM).We found that qualitative analysis of these μCT images presented anisotropic but predictable shrinkage. Qualitative analysis of SEM images presented thinning at layer margins. The 1.2mm azo chromium scaffolds presented an average observed post diameter (expected 0.4 mm) of 0.43mm (0.02 std dev) and an average observed plate diameter (expected 0.6 mm) of 0.63mm (0.01 std dev). The 12.4mm azo chromium, 4 minute exposure, scaffold group presented an average diameter (expected 6 mm) of 6.03mm (0.03 std dev). The 12.4mmTiO ₂ average diameter was 5.92mm (0.07 std dev). Accurate calibration of overcuring ensures interlayer binding and full formation of the smallest, 400μm in this study, scaffold features.