Designing bio-compatible gel electrolyte for implantable Zn-O ₂ battery

Metal-bio-oxygen batteries establish a paradigm-shifting energy architecture for biomedical implants, endowing these devices with extended service life in continuous physiological surveillance and precision theranostic operations. However, the conventional electrolyte in these semi-opened batteries fail to meet the requirements in bio-compatibility and bio-safety for <em>in vivo</em> applications. Herein, we report a bio-compatible composite gel electrolyte for implanted Zn-O₂ battery (ZOB), while also sustainably powering a mechanical sensor <em>in vivo</em>. This electrolyte composes a poly(L-lactide-co-epsilon-caprolactone) (PLCL) framework with a Gelatin methacryloyl (GelMA) modification layer, the salt in body fluid serves as ion transport carriers in electrolyte. It displays an O₂ impermeable property and lower polarization potentials as electrolyte in Zn||Zn asymmetric cell. <em>In vitro</em> assay results demonstrate that the battery components illustrate excellent biocompatibility with negligible cytotoxicity. <em>In vivo</em> histopathological and hematological analyses further verified the biosafety of the ZOB during operation, while capillary regeneration around the cathode ensured adequate oxygen supply for sustained performance. The assembled ZOB delivers a power density of 1.96 μW/cm² at 0.98 V <em>in vivo</em>, which also successfully powers an integrated hydrogel mechanical sensor and monitors cardiac signals in rats. The unique two-electron transfer pathway of oxygen reduction in blood has also been elucidated. This work offers a new insight into bio-compatible electrolyte design for next-generation implantable power sources, enabling robust implantable devices for healthcare technologies.