A miniature piezoelectric spiral tactile sensor for tissue hardness palpation with catheter robot in minimally invasive surgery

The sense of touch plays a critical role in traditional open surgeries since it could provide tactile feedback to surgeons and is also used to acquire intrinsic properties of tissues through palpation. However, it is partially or completely lost in most existing robot-assisted minimally invasive surgeries. To solve this problem, a miniature tactile sensor with diameter less than 8 mm suitable for catheter robot-based tissue hardness palpation is presented in this paper. The stringent size constraint of minimally invasive surgery (MIS) is met by a unique spiral shape as well as a vertically configured piezoelectric transducer. The spiral shape also helps it achieve a low operating frequency suitable for testing biological tissues. The relationship between electrical impedance of the sensor and mechanical impedance of a load is derived based on the transduction matrix model, which forms the basis of the unique simultaneous actuation and sensing (SAS) technique. As a result, hardness of the load could be sensed from the sensor's electrical impedance by extracting the resonant frequency, with simple instrumentation. The proposed sensor and SAS technique are verified numerically on a finite element model and experimentally on a prototype. After properly choosing the vibration mode and operating frequency range, the sensor is able to perform hardness sensing in a wide range of 0-1.7 MPa. In addition, both simulation and experiment results indicate that the sensor has high sensitivity and low variance in the low-hardness region, and relatively lower sensitivity and higher variance in the high-hardness region, suggesting that the sensor can be used in two different sensing modes (quantitative measurement and qualitative classification) in the two regions, respectively. An ex vivo experiment confirms that the sensor could detect the presence, shape and location of an embedded lump from spatial distribution of tissue hardness acquired through grid-based palpation, followed by an improved k-means clustering algorithm. Compared with traditional hardness sensors, this tactile sensor is developed with a unique spiral shape which reduces the operating frequency for enhancing the interaction with biological tissues while keeps the overall size of the sensor as small as possible. And the proposed unique simultaneous actuation and electrical impedance sensing mode helps simplify the instrumentation, making it easier to integrate the sensor into MIS equipment.