The overarching goal of this research work is to fabricate dimensionally-accurate porous bone scaffolds for the treatment of large osseous fractures. In pursuit of this goal, the objective is to investigate the dimensional accuracy and morphological properties of triply periodic minimal surface (TPMS) bone scaffolds, composed of Polycaprolactone (PCL) and fabricated using pneumatic micro-extrusion (PME) additive manufacturing process. PME is a high-resolution direct-write method, widely utilized for the fabrication of biological tissues, structures, and organs. It has a layer resolution and positioning precision of 100 μm and 10 µm, respectively. In addition, the PME process allows for non-contact, multi-material deposition of a wide range of functional bioinks for tissue engineering applications. However, the PME process is inherently complex (highly nonlinear), governed by complex multi-physics phenomena (e.g., phase-change, coalescence, receding-relaxation, and wetting equilibrium). The PME complexity, in addition, stems from the presence of a broad spectrum of process parameters – e.g., photo-polymerization light intensity, material viscosity, flow pressure, material deposition temperature, and solidification rate – as well as material-process interactions. Consequently, investigation of the effects of significant process parameters and their interactions on scaffold dimensional accuracy, morphological properties, and functional properties would be inevitable. Although the PME process has been utilized in various applications, a review of literature reveals that the effects of PME process parameters on the dimensional accuracy as well as the morphological properties of PCL-based TPMS scaffolds have not been rigorously explored. This gap is addressed in this work.&nbsp;In the PME process, a high-pressure flow (typically, compressed air) is injected into a barrel (also known as cartridge), which contains a polymer material (such as PCL), leading to pressure-driven material deposition on a surface with the aid of a converging micro-capillary nozzle. In this study, PCL powder was loaded into the cartridge, maintained at 180 °C. To ensure steady-state and uniform material deposition, the loaded PCL was kept in the heated barrel for 15 minutes prior to deposition. Supplied by an oilless, rust-free air compressor, the flow pressure was set to 300 kPa. Laminar molten PCL flow was deposited on a glass surface (uniformly kept at 10 °C) with a print speed of 2 mm/s, using a 200 μm nozzle. Having a porosity of 60%, the following TPMS scaffolds were characterized in this study: (i) Schwarz-Gyroid, (ii) Schwarz-Diamond, (iii) Schwarz-Primitive, (iv) Neovius, and (v) Schoen I-WP. Furthermore, the scaffolds were fabricated using a layer height and line width of 200 μm. The dimensional accuracy as well as morphological properties of the fabricated bone scaffolds were measured based on monochromatic images, acquired in situ using a high-resolution charge-coupled device (CCD) camera. Subsequently, the acquired images were analyzed using a digital image processing algorithm developed, in house, in the MATLAB environment. The veracity of the image-based measurements was corroborated using physical measurements. In addition, several morphology features (e.g., scaffold dimensions, pore size, entropy, uniformity, and energy) were extracted from the acquired images with the aim to quantify the morphological properties of the scaffolds. The results of this study pave the way for accurate fabrication of complex porous scaffolds for the treatment of bone fractures and defects.

A Novel Image-Based Method for In Situ Characterization of the Pore Size Distribution and Dimensional Accuracy of Bone Tissue Scaffolds

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

ASME 2021 International Mechanical Engineering Congress and Exposition (IMECE2021)

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