Modeling and Simulation of Medical Devices Undergoing Complex Thermo-Mechanical Loadings

The Dynamic and Smart Systems Laboratory at the University of Toledo is the home for Nitinol Commercialization Accelerator (NCA). NCA has a portfolio of medical devices ranging from intervertebral fusion devices for minimally invasive surgery to adaptive ankle foot orthoses to address various neuromuscular deficiencies. Numerical tools at NCA have developed into to distinct categories. Each group of tools is applied for simulation and design of one class of SMA-based device. For isothermal and quasi-static analysis of SMA components we sued finite element analysis. A UMAT subroutine, based on the Boyd and Lagoudas 3-D phenomenological constitutive model, is implemented in ABAQUS to simulate and analyze a number of SMA applications. An example of such devices is a smart tissue clamp. The clamp while providing a firm contact can be remotely activated to release the tissue. This cycle can be repeated as needed to complete the treatment in a minimally invasive approach. We used the FEA to find the optimal geometry of the shape memory and superelastic components of the clamp. The superelastic element provides the required clamping force while the shape memory element releases the clamp. A series of prototype devices have been successfully fabricated for in vitro testing.

When a medical device requires fast actuation or if the device undergoes complex thermo-mechanical loading we use another modeling and simulation approach. The second modeling approach is based on an exact solution form of the coupled SMA 3-D constitutive equations. The method is particularly appropriate for devices with dynamic behavior in which the high-speed motion creates a significant temperature change in the SMA component due to the latent heat effect.  Coupled equations along with appropriate thermal boundary conditions are solved using a semi-analytical method coded. This code is developed in MATLAB and has been extensively verified.