Mechanical Testing Device for Viscoelastic Biomaterials

Nearly all biologic tissues exhibit viscoelastic behavior. This behavior is characterized by hysteresis in the response of the material to load or strain. This information can be utilized in extrapolation of life expectancy of vascular implant materials including native tissues and synthetic materials. This behavior is exhibited in many engineering materials as well such as the polymers PTFE, polyamide, polyethylene, etc. While procedures have been developed for evaluating the engineering polymers the techniques for biologic tissues are not as mature. There are multiple reasons for this. A major one is a cultural divide between the medical and engineering communities. Biomedical engineers are beginning to fill that void. A digitally controlled drivetrain designed to evaluate both elastic and viscoelastic characteristics of biologic tissues has been developed. The initial impetus for the development of this device was to evaluate the potential for human umbilical tissue to serve as a vascular graft material. The consequence is that the load frame is configured for membrane type specimens with rectangular dimensions of no more than 25mm per side. The designed load capacity of the drivetrain is to impose an axial load of 40N on the specimen. This drivetrain is capable of assessing the viscoelastic response of the specimens by four different test modes: stress relaxation, creep, harmonic induced oscillations, and controlled strain rate tests. The fluorocarbon PTFE has mechanical properties commensurate with vascular tissue. In fact, it has been used for vascular grafts in patients who have been victims of various traumas. Hardware and software validation of the device was accomplished by testing PTFE and comparing the results to properties that have been published by both researchers and manufacturers. Parallel loop structure for data collection and GUI display along with sub-sampling logic structure to enhance iteration rate of the control loop. These structures are highlighted by yellow shaded boxes outlined by red dashed lines.. ACKNOWLEDGMENTS The author wishes to express sincere appreciation to Professors Nelson, Barton, and Gu for their assistance in the preparation of this manuscript. In addition, special thanks to Dr. James Hammel whose vision served as the impetus to fabricate this device. Further thanks to all persons listed below whom contributed to this research as well. INDEPENDENT VARIABLES t Time = seconds ω Angular frequency = Rad/sec τ Dummy integration variable in convolution integral = seconds i Complex # = 1 − DEPENDENT VARIABLES σ Tensile stress = Force/Area σ shear Shear …