MECHANICS OF EXTRUSION
3D Printing techniques have been flourished in the fabrication of advanced scaffolds for tissue engineering and regenerative medicine. In addition to design and development of bioprinting platforms, bioink formulation should be optimized for the application of interest. A proper optimization of bioink formulation requires an inclusive understanding of bioprinting process. In this project, we will itemize the governing parameters in extrusion based bioprinting process: printing resolution and instability. Assuming known geometry and constant flow rate, we try to describe the behavior of a general viscoelastic fluid. This would lead to derivation of the relations between fluid properties and printing resolution as well as threshold of instability. We then develop a printability map based on fluid viscosity, input flow and nozzle size. This project will help researchers in understanding current challenges of bioprinting and selecting the proper approach.
Cartilage tissue engineering is of significant interest in regenerative medicine because damaged articular cartilage lacks the ability to self-repair. Recent studies focus on induced pluripotent stem cells (iPSCs) that possess an extraordinary capacity to differentiate into different cell types. Mechanical stimulations have been shown to enhance iPSC differentiation into chondrocytes; however, designing microenvironments that control iPSC differentiation and plasticity is still challenging. ECM-derived hydrogels provide an instructive microenvironment for chondrogenesis. I hypothesize that mathematically defined periodic mesostructures will provide biomechanical tuning in cell-laden constructs under cyclic loading. This project will initially involve designing negative-stiffness honeycombs, introduced by Correa et al. (Rapid Proto J 2015), which exhibit relatively large positive stiffness followed by a region of plateau stress as the building blocks buckle, with full recovery after compression. This can be elicited by a curved beam structure in regular honeycomb structures. The mechanism underlying energy-absorbing behavior is elastic buckling rather than plastic deformation (i.e., full recovery). The proposed platform will then be used to fabricate 3D methacrylated gelatin-based constructs, loaded with pluripotent stem cells. Next, the construct will be stimulated by cyclic loading, at physiological deformation and frequency, to accelerate the kinetics of the stem cell differentiation. Finally, computational simulations will be used to correlate the relations between differentiated stem cell markers and structural pore size ranging from microns to millimeters.