Capacitive Sensors to Prevent Suture Breakage in Robotic Surgery

Omeed Paydar, PhD, Ahmad Abiri, MS, Bradley Genovese, MD, Robert Candler, PhD, Warren Grundfest, MD, Eric Dutson, MD. University of California, Los Angeles

Objective. This work is focused on the development and characterization of surgical sensors for the prevention of tissue crush injuries and suture failure during surgical procedures. Medical robots, specifically the Da Vinci surgical system, have gained significant popularity for gastrointestinal operations. Lack of haptic feedback in robotic surgical systems, however, has contributed to intraoperative suture breakage. We have previously reported tensile strength data for the most commonly used suture types and gauges under constant strain rates. Instrument mounted normal and shear force sensors with high dynamic range and sensitivity can detect the magnitude of tension applied to the suture and aid in the development of warning systems that predict and prevent suture failure.

Description & Methods. Single-sided floating capacitive tactile sensors are fabricated with microelectromechanical systems (MEMS) processes using biocompatible materials (e.g., gold, silicone, and glass). These sensors are integrated onto custom printed circuit boards for the capacitive measurements. Force application (Figure 1) results in micro-changes: compressive forces reduce separation between electrode layers (single-ended measurement) and shear forces alter the overlap area (differential measurement). Sensors are mounted on robotic forceps and used to measure normal and shear forces. This real-time data will aid in the development of warning systems that detect suture failure. Sensors were tested under various load conditions using an Instron universal testing machine. Experiments were repeated in triplicate.

Preliminary Results. The baseline (no force) capacitance of the compression sensor elements was 514.9 femtoFarads (fF), within 3% of the theoretical value. The measured shear baseline capacitance was 26 fF (theoretically 0 fF with perfect fabrication alignment). The sub-Newton (0.1N) ultimate sensor resolution was determined from the noise performance of the sensor (215 attoFarad (aF) RMS noise). The sensor response to shear results in a positive differential measurement to pulling forces and a negative differential output for pushing forces. The noise performance of the shear component (117 aF) and sensitivity (1 fF/N) provide sub-Newton resolution for gripping tasks common during suturing.

Conclusions. Combined with our previous results on suture tensile strength (SAGES 2014), implementation of the sensor during surgical tasks can provide advanced warning prior to suture breakage. The successfully fabricated capacitive sensors can continuously inform the surgeon, through a tactile display, of the applied load and trigger a warning system to prevent complications.

Figure 1. The capacitor in a non-deflected state (a&b, left). Application of a compressive force (a, right) causes the top and bottom electrodes to come closer together. A shear force (b, right) causes a change in the overlap area between the top and bottom electrodes.

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