Tactile sensors for determination of elastic properties of brain tissue mimicking phantoms

In spite of the outstanding importance of neuronavigational techniques the neurosurgeon has still highly to rely on his visual as well as tactile sense and his experience in distinguishing tumor tissue from intact brain parenchyma. All the more - as neuronavigation is based on preoperative imaging scans - considerable brain shift may occur during surgery and that will lead to erroneous results. Furthermore the surgeon loses his tactile sense during endoscopic surgery. Therefore control of tissue manipulation is limited today to the fiberoptic image techniques, possibly ensured by neuronavigational supervision or haptic feedback systems. In robotic guided surgery the missing tactile sensing will be particularly crucial. The human sense of touch is a highly complex sensory perception comprising epicritical and propiozeptive information processing. The development of a tool for intraoperative tactile perception will be a highly challenging task to improve the safety during surgery. Different sensor concepts (elasticity probe and resonant piezoelectric sensor) have been set up and adapted to distinguish elasticity differences in soft tissue mimicking gel phantoms. The elasticity probe is based on two leaf springs of different bending stiffness. The two tips of the beams act both on the material to be tested. The two deflections measured by strain gauges are processed in a mathematical model calculating the phantoms' stiffness. In contrast to this differential sensing concept the piezoelectric sensor, which is driven at its resonance, measures stiffness and damping of the phantoms by evaluating frequency or phase shift and amplitude change. Gelatine gels of different concentrations (5{\%}, 10{\%}, 20{\%} of gelatine) have been used for a characterization of sensor elements and their sensitivity to elasticity distinctions. Using the hand-guided elasticity probe the determination of gel elasticity is performed with high reliability. By constant mechanical load the piezoelectric sensor also presents a proportional dependency of tissue elasticity.