3D force sensors for laparoscopic surgery tool

3D force sensors were developed to further integration in laparoscopic heads of surgery robots. The Si sensors operate with piezoresistive transduction principle by measuring the stress induced signals of the symmetrically arranged four piezoresistors in a deforming membrane. As the chip size has to be reduced to a few mm2, the conventional anisotropic alkaline etching technique was replaced by deep reactive ion etching (DRIE) for membrane formation. Moreover, DRIE enables to form any geometry of the membrane and offers the formation of monolith force transfer rod protruding over the chip surface. This rod increases shear sensitivity of the structure, thereby plays crucial role in tactile sensing. The technology applies SOI (silicon on insulator) wafers of appropriate device layer thickness, which provide highly uniform membranes and reproducibility of the process. According to the medical and functional requirements the sensors must be covered by biocompatible and sterilisable elastic polymers. As the elastomer drastically effect on the performance of the device, the proposed sensor structures were modelled by coupled finite element simulation to determine the appropriate geometric parameters meet the functional requirements. Sensors were covered with spherically shaped PDMS (polydimethylsiloxane) polymer and the effect of the elastic coating was also studied in terms of sensitivity and response time. Finally, the design of the laparoscopic head with the integrated 3D MEMS force sensors is also demonstrated.