Hybrid Flexible–Soft Neural Interface as a Practical Solution for Long-Term Implants

Implantable neural interfaces face persistent challenges in long-term functionality due to chronic immune responses, mechanical mismatch with tissue, and material corrosion in vivo. The commonly used flexible materials, such as polyimide and parylene-C (poly[monochloro-p-xylylene]), have advantages in low water vapor permeability and high processing resolution, but their GPa-level modulus and limited stretchability may lead to fibrotic encapsulation and signal loss. In contrast, soft materials, such as silicones and hydrogels, provide tissue-like compliance and intrinsic stretchability with a kPa–MPa modulus but have poor barrier properties and fabrication precision. Fortunately, by integrating a flexible thin-film encapsulation with a compliant soft substrate, hybrid flexible–soft neural interfaces provide a promising approach to overcoming the limitations of both material systems. This review first systematically summarizes flexible and soft neural interfaces, respectively, from materials and properties to lifespan. Then, the latest advances are highlighted in hybrid flexible–soft interface performance across different applications, including superior mechanical stability, chronic functionality, and improved signal quality. Existing challenges and future perspectives for the next-generation implantable neural interfaces are also discussed. By examining diverse implantable applications of hybrid flexible–soft devices, this review further draws attention to practical issues that are critical for long-term implantation, including failure criteria, lifespan estimation, interfacial mechanisms, and material innovation.