# Cell‐Stress‐Free Percutaneous Bioelectrodes

## TL;DR

A new type of soft, painless microneedle electrode is developed that can monitor body signals more stably and comfortably than existing wearable devices.

## Contribution

The novel electrode design uses an effervescent core to achieve cell-stress-free, long-term biocompatible sensing in the dermis.

## Key findings

- The ultrathin electrode dynamically softens after insertion, minimizing tissue damage and immune activation.
- It provides stable electrophysiological signals under sweat and dehydration in both rats and humans.
- The design enables environment-independent signal acquisition with high fidelity and mechanical compatibility.

## Abstract

Wearable bioelectronics have advanced dramatically over the past decade, yet remain constrained by their superficial placement on the skin, which renders them vulnerable to environmental fluctuations and mechanical instability. Existing microneedle (MN) electrodes offer minimally invasive access to dermal tissue, but their rigid, bulky design—often 100 times larger and 10,000 times stiffer than dermal fibroblasts—induces pain, tissue damage, and chronic inflammation, limiting their long‐term applicability. Here, a cell‐stress‐free percutaneous bioelectrode is presented, comprising an ultrathin (<2 µm), soft MN (sMN) that dynamically softens via an effervescent structural transformation after insertion. The sMN exhibits near‐zero Poisson's ratio deformation, preserving cellular morphology and minimizing immune activation over multiple days of use in rats and humans. Synchrotron imaging and histological analysis reveal reduced tissue disruption, while electrophysiological measurements demonstrate stable signal‐to‐noise ratios under sweat, dehydration, and extended use. This architecture shifts the biosensing interface from the epidermis to the dermis, establishing a mechanically and electrically stable platform for environment‐independent signal acquisition. The findings establish dermal electronics as a next‐generation paradigm for long‐term, biocompatible wearable sensing.

A structurally adaptive soft microneedle bioelectrode is developed with an effervescent sacrificial core that dissolves after insertion, leaving an ultrathin and highly compliant electrode integrated with skin. This design enables ultra‐flexible, cell‐stress‐free, stable, and high‐fidelity electrophysiological monitoring under dynamic conditions such as sweat and dehydration, surpassing conventional film and gel electrodes in both long‐term signal stability and mechanical compatibility.