# FOCUS: A Four‐In‐One Consolidated Unison Strain Sensor with Enhanced Sensitivity

## TL;DR

FOCUS is a new strain sensor that improves sensitivity and accuracy for detecting tiny movements in biomechanical applications.

## Contribution

FOCUS introduces a folded design with four liquid metal sensors in a Wheatstone bridge to enhance sensitivity fivefold.

## Key findings

- FOCUS achieves fivefold sensitivity improvement over single liquid metal sensors.
- The sensor maintains stability across wide temperature ranges and detects strains down to 25 micrometers.
- FOCUS successfully tracks periocular micro-motions and joint deformations with high precision.

## Abstract

Liquid metal strain sensors possess outstanding stretchability, low modulus, and excellent repeatability, making them ideal for large‐deformation measurements. However, traditional liquid metal sensors inherently suffer from low sensitivity, which severely limits their capability in detecting subtle deformations. To achieve enhanced sensitivity, we have proposed a new paradigm: FOCUS, a Four‐in‐One Consolidated Unison Strain Sensor specifically created for microscale biomechanical deformation. In this work, four liquid metal strain sensors were printed on an elastic planar substrate and subsequently folded to overlap at the same location. Upon stretching, two of the sensors exhibited an increase in resistance, whereas the other two displayed a greater decrease. By leveraging this complementary response within a Wheatstone bridge configuration, FOCUS exhibits a fivefold enhancement in sensitivity compared to a single liquid metal sensor, while simultaneously maintaining signal stability against wide temperature range. FOCUS can accurately capture strains ranging from centimeters to micrometers, resolution reaches 25 micrometers. To demonstrate the powerful performance of FOCUS, we used it to perform periocular micro‐motion tracking and potential deformation monitoring of other human joints.

Through a folding transformation that stacks four printed liquid metal sensors into a unified 3D orthogonal construct, FOCUS harnesses complementary bidirectional resistance responses within a Wheatstone bridge. This architecture yields fivefold sensitivity enhancement and 25 µm resolution, overcoming LM sensors’ micro‐strain limitations and enabling stable, high‐precision monitoring of multidirectional physiological micro‐motions.