Functional Hydrogels for Wearable and Implantable Neural Interfaces: Materials, Properties, and Applications

Functional hydrogels are increasingly used in neural interfaces. As hydrophilic polymer networks that combine tissue-like softness with tunable conductivity and adhesion, they relieve two fundamental limitations of rigid conventional electrodes within a single material: the mechanical mismatch that drives the chronic foreign body response, and the electrical mismatch that raises interfacial impedance. This review discusses functional hydrogels through their core interfacial properties of biocompatibility, adhesion, and interfacial impedance/conductivity, and highlights how these requirements differ between skin-mounted wearable and implantable devices. Their applications in neural recording, electrical and ultrasound neuromodulation, and external and robotic device control are then covered in detail. Finally, the key challenges and future opportunities are summarized, including gelation control, long-term in vivo stability, integration with robotic hardware, and clinical translation. As the field advances, functional hydrogels are poised to become a core platform for next-generation neural interfaces.