Sensor for Meso-Scale Tissue Stiffness Characterization

Force sensing is fundamental for developing smart surgical devices capable of in-vivo tissue stiffness measurement. The sensing technique necessitates integration of micromachined force sensors at the end-effectors of tactile probes. We discuss fabrication and characterization of a small footprint silicon diaphragm-based force sensor with boron-doped piezoresistors connected in Wheatstone bridge configuration. The sensor performance characteristics for axial indentation experiments yield sensitivity 0.34 mV/mN, linearity > 0.99, and hysteresis 1.18% for force range 0–250 mN. To validate the utility of the force sensor for mechanical characterization of soft tissues, we perform indentation experiments on sheep tracheal tissues. The study involved two important mechanical characterization techniques: compression and stress relaxation. Tissue samples comprised of cartilage tissue from the cranial, middle, and caudal portions of the trachea and smooth muscle tissue. We record the hysteresis patterns of the tissue samples at
$10 ~\mu \text{m}$
/s strain rate. The stiffness of cranial, middle, and caudal cartilage at 20% strain are 23±1.39 N/m,22.36±2.05 N/m, and 22.41±2.04 N/m. The cranial cartilage is innately stiffer than other tracheal segments; however, the results show no statistical variation (p ≥ 0.1488) in cartilage stiffness based on anatomical region. The stiffness of smooth muscle at 30% strain measures to be 14.02±0.76 N/m. The stress relaxation experiments reveal cartilage samples have faster relaxation time, and the smooth muscle tissues respond less rapidly to compression loads and have slower relaxation time. The feasibility of characterizing soft tissues at low contact force and small sensing area extends the utility of the sensor in airway diagnostic tools for localized stiffness measurement.