Design, Prototyping, and Characterization of a Micro-Force Sensor Intended for Tissue Assessment in Confined Spaces

The quantitative characterization of soft tissue viscoelastic properties can aid in disease prognosis and diagnosis. Existing technologies present challenges to measuring localized in vivo tissue relaxation data while meeting load and geometric constraints. This research presents the design, prototype, and characterization of a micro-force sensor that could enable better access to confined spaces of the human body. The novel design of the uniaxial micro-force sensor has an external diameter of less than or equal to 3.5 mm and 1 N load capacity for transurethral palpation of the bladder interior wall. The conceptual design of the micro-force sensor and a finite element-based discrete optimization procedure to determine the optimum values of the identified design parameters of bend radius, bend angle, and thickness while meeting defined operational and geometric constraints are presented. These optimum values guided the prototyping of an aluminum sensing element with 2.18 mm bend radius, 104.9° bend angle, and 0.3 mm thickness. A miniature metal foil strain gauge was attached at defined location on the sensing element for measurement purposes. An experimental testbed was developed, calibrated, and used for characterization experiments. The performance matrix of the prototyped micro-force sensor was experimentally evaluated. The sensitivity, resolution, accuracy, precision, and repeatability band of the sensor were evaluated to be
$859.73~\mu \epsilon /$
N, 2.6 mN, 28.6 mN, 87.22%, and ±2.87%, respectively, with a hysteresis of 118 mN. These experimental results provide confidence to further employ the sensor for in vivo experiments toward the identification of viscoelastic properties of soft tissue.