Comprehensive review of hydrogel-based brain–machine interface materials, design, and integration strategies emphasizing their mechanical, electrical, and biocompatible properties and discussing invasive/noninvasive systems, clinical translation, and applications including Parkinson’s disease.

Hydrogel-enabled BMIs promise more compliant, long-lasting neural recording and stimulation interfaces that could reduce inflammation and enable reliable chronic sensing and closed-loop neuromodulation important for advancing Parkinson’s diagnostics and therapies.

Brain-machine interfaces (BMIs), which serve as revolutionary tools for neural recording, modulation, and rehabilitation, are highly dependent on the biocompatibility and mechanical suitability of their electrode materials. Although traditional metal electrodes possess excellent conductivity, their inherent rigidity causes a substantial mechanical mismatch with soft neural tissue, leading to chronic inflammatory responses and poor long-term stability. The emergence of hydrogel electrodes has provided a breakthrough solution to this fundamental limitation. Hydrogels, characterized by their softness, high ionic conductivity, and tissue-like compliance, offer a viable solution to mitigate these issues. This review systematically explores the material properties of hydrogel-integrated BMIs, providing an in-depth investigation of key hydrogel characteristics, including toughness, adhesion, conductivity, and biocompatibility. Additionally, hydrogel-based BMIs are categorized into non-invasive and invasive systems, each defined by its characteristic operational principles and signal-acquisition mechanisms. The study further reviews critical issues, including surgical implantation strategies, multimodal data fusion, integration of artificial intelligence, as well as system integration and clinical translation. From a therapeutic perspective, this work highlights the application of BMIs in treating neurological disorders such as Alzheimer's disease, Parkinson's disease, epilepsy, stroke, neuropathic pain, and depression. Furthermore, this review critically examines the persistent challenges faced by hydrogel-based BMIs and proposes innovative strategies for future development. Ultimately, it outlines a developmental roadmap for next-generation hydrogel-based biotherapeutic technologies aimed at achieving high-fidelity, stable and clinically translatable BMI systems.