Designing bio-compatible gel electrolyte for implantable Zn-O₂ 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 electrolytes in these semi-opened batteries fail to meet the requirements in bio-compatibility and bio-safety for in vivo applications. Herein, we report a bio-compatible composite gel electrolyte for implanted Zn-O₂ battery (ZOB), while also sustainably powering a mechanical sensor in vivo. This electrolyte composes a poly(L-lactide-co-epsilon-caprolactone) (PLCL) framework with a gelatin methacryloyl (GelMA) modification layer, and the salt in body fluid serves as ion transport carriers in the electrolyte. It displays an O₂ impermeable property and lower polarization potentials as electrolyte in Zn||Zn symmetric cell. In vitro assay results demonstrate that the battery components illustrate excellent biocompatibility with negligible cytotoxicity. In vivo histopathological and hematological analyses further verified the biosafety of 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 in vivo, 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.