Improved Evaluation of Dynamic Mechanical Properties of Soft Materials With Applications to Minimally Invasive Surgery

Minimally invasive surgery minimizes trauma to the patient, however, the loss of tactile feedback impedes the surgeon´s ability to locate tumors among healthy tissue. In this paper, a resonance-based instrument to measure the stiffness, damping, and effective mass of a soft material such as biological tissue is presented. It was designed to be small yet has two natural frequencies below 100 Hz so that the effective mass of the tissue would not impact the determination of the stiffness. A state-space model was used to develop a fast and accurate method of extracting the tissue parameters by measuring the natural frequencies and the bandwidth at the first natural frequency. A fast and robust phased-locked-loop-based feedback system is described, which was used to measure the required frequencies. Simulations showed that the system was robust, while subjected to disturbances including hand tremor, tissue parameter variation, and preload. A prototype system showed that the instrument could accurately predict the stiffness, damping, and mass with an average error of 6%, 6%, and 7%, respectively. Experiments on a simulated tissue phantom showed the ability of the instrument to detect a tumour while it was stationary and in motion.