Modeling, design, and testing of a resonator device for the measurement of regional tissue viscoelasticity

This paper introduces a device able to quantitatively characterise tissue viscoelastic properties. One of the key indicators of tissue health is its physical properties. The proposed device is a dual spring-mass resonator system driven by a piezoelectric actuator, designed for use in an invivo setting. This device aims to measure tissue viscoelastic properties by assessing the change in natural frequency and the bandwidth of the system as a result of tissue contact. Initially the device was modelled using the lumped mass method of analysis, assuming friction and damping negligible. Using this model, the natural frequencies were predicted as 44Hz and 151Hz. Once manufactured, the system was tested. By obtaining the frequency response plot of the system without the casing, the lower and upper longitudinal natural frequencies were found to be 7.4% and 6.3% lower than predicted, respectively. With the casing in place, friction dominated the system, considerably reducing the response displacement amplitude. The upper and lower longitudinal natural frequencies became 114.75Hz and 269.5Hz, 161% and 78% higher than predicted by the model, respectively. Whilst the model was able to accurately predict the system response without the casing, the model broke down when this was in place. To confirm the principles of operation, the system was tested on an ox kidney without the casing. As a result of tissue contact, the lower natural frequency increased by 9.5%, and bandwidth by 0.068. Furthermore, the upper natural frequency increased by 0.4%, and damping ratio increased by 0.022. The fully assembled device was then tested on the same ox kidney. As a result of tissue contact, the lower natural frequency increased by 0.9%, and damping ratio increased by 0.015. Whilst the magnitude of this change was much less with, than without the casing, the modified frequency response as a result of tissue contact confirms the ability of the system to assess tissue viscoelastic properties.