# Development and Validation of a Wearable Softness Sensor Based on Fingernail Deformation and an Inertial Measurement Unit for Quantifying Palpation

## TL;DR

This paper introduces a wearable sensor that measures softness during palpation without blocking touch, aiming to improve medical training by making diagnosis more objective.

## Contribution

A novel wearable sensor system that quantifies softness using fingernail deformation and an IMU, preserving tactile sensation.

## Key findings

- The sensor achieved high accuracy in estimating fingertip force (RMSE: 0.17 N) and displacement (RMSE: 0.75 mm).
- The system successfully discriminated between three sponge types with varying softness levels.
- Estimated stiffness values aligned well with reference measurements from a force gauge.

## Abstract

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.