Design of a Load Frame for Nonlinear Viscoelastic Biological Materials

The purpose of this study was to design and fabricate a load frame that could be used to help researchers determine the mechanical properties of intervertebral discs. Intervertebral discs display viscoelastic behavior, their reactions to deformations are rate dependent. Traditional materials testing systems could not be used for this characterization due to the intervertebral disc’s rate dependent nature. A load frame would have to be custom built to accomplish the task. In accordance with the required motion profile for characterization of 0.5 seconds per cycle, 1 mm of displacement, and up to 250 N of reactionary force, actuation and motion control components were identified, and structural components were designed. Fixtures to hold the specimen in place during testing were also designed. An electromagnetic voice coil stage was the chosen actuator due to its high precision positioning and force capabilities. Programmable motion control instrumentation was chosen due to its open source nature, allowing various motion profiles to be tested. A data acquisition system was chosen due to its high data collection rate of 1000 Hz. A load cells was incorporated into the design that could withstand testing conditions. Finally, a translation stages were included into the design for specimen positioning purposes. All structural components were validated using finite element methods to ensure structural components would maintain integrity and not fail or displace during testing. After fabrication, the load frame was assembled; motion control and data acquisition instrumentation were integrated. The result of the design effort yielded a fully functional load frame capable of mechanically characterizing intervertebral discs, and other viscoelastic materials alike.