Development and Validation of a Wearable Softness Sensor Based on Fingernail Deformation and an Inertial Measurement...

Introduction Palpation plays a crucial role in diagnosis; however, the technique relies heavily on subjectivity, presenting challenges for quantitative instruction in medical education. Many existing palpation sensors cover the fingertip, thereby obstructing the tactile sensation essential for palpation. Therefore, in this study, we aimed to develop and evaluate a wearable softness sensor that does not obstruct tactile sensation, with a view toward its application in medical education. Methods The proposed sensor system consists of fingertip force estimation using fingernail strain and indentation depth estimation using an inertial measurement unit (IMU). Accuracy verification experiments were conducted for each estimation method, followed by a softness discrimination experiment using three types of sponges with varying levels of softness. Results The results demonstrated that the proposed sensor achieved high estimation accuracy for both fingertip force (root mean square error (RMSE): 0.17 N) and displacement (RMSE: 0.75 mm). Furthermore, the system successfully and significantly discriminated the stiffness values of the three sponge types, with the estimated values showing good agreement with reference values measured using a force gauge. Conclusion The proposed sensor enables the quantification of object softness without interfering with natural palpation movements. These findings demonstrate the system's potential as a tool for the objective evaluation of palpation skills and as an effective training device.