Cell-Stress-Free Percutaneous Bioelectrodes.

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.