Flexure-based multi-degrees-of-freedom in-vivo force sensors for medical instruments

This paper presents novel multi-degrees-of-freedom force sensors based on flexures used as mecano-optical transducers (named flexure body) and white light interferometers used as opto-electrical transducers. Together, these transducers make up a load cell exploiting the nanometric accuracy of Fabry-Pérot interferometric measurement to reach milli-Newton force accuracy. The design focuses on the flexure body composed of three sections: a base (attached to the measuring device), a compliant section which deforms under applied forces and a pointed rigid section whose tip touches tissues during surgery. The fiber interferometer measures the distal displacement with respect to the base using one 125 μm diameter optical fiber for each load cell DOF. The key advantages of this design are: compact design (1 to 4 mm diameter shaft), simple optical alignment during assembly, scalability from Newton down to milli-Newton force levels, insensitivity to electrical charge and compatibility with sterilization procedure. These properties satisfy the requirements of in-vivo force measurements during surgery. The paper presents analytical stiffness estimation of 1 DOF flexure bodies and finite element stiffness analysis of multiple-DOF structures followed by the design, manufacturing and assembly process. The realized sensors are then characterized experimentally on a specifically designed motorized test-bench, which allows application of calibrated forces from various directions onto the senor tip. A specific calibration strategy was developed improving measurement accuracy of the sensor.