From Patch-Clamps to SPAD Arrays – Evolution and Future Perspectives of Invasive Neural Interfacing Toward Integrated Photonic Implants

The investigation of neural circuit dynamics faces a fundamental challenge: existing tools cannot simultaneously achieve cellular resolution, millimeter-depth penetration, and compatibility with freely behaving subjects. Electrophysiology offers temporal precision at depth, while advanced microscopy provides superb resolution, but is physically constrained to superficial layers or head-fixed preparations. In this review, we propose that implantable photonic devices are emerging as the critical solution to address this challenge. We first critically examine the evolution of electrophysiology and microscopic imaging, establishing their inherent trade-offs. We then detail how integrated photonic probes, leveraging semiconductor innovations like single-photon avalanche diode (SPAD) arrays and µLEDs, enable optical sensing and manipulation deep within the brain of behaving animals. By establishing frameworks to compare important performance and synthesizing the latest research, we provide analysis of this transformative shift. Finally, we outline the multidisciplinary challenges in scaling, thermal management, data processing, and biocompatibility, which must be overcome to realize the full potential of implantable photonics as a new paradigm for closed-loop neuroscience and clinical translation.