A Promising Optical Bio‐Interface: Graded‐Index Hydrogel Fiber

Abstract Light in human‐machine interactions requires a bio‐friendly, efficient, and stable optical bio‐interface. Optical hydrogel fibers (HFs) with excellent biocompatibility will advance next‐generation implantable optical bio‐interfaces. However, the efficiency of HFs is limited by high optical attenuation. Moreover, the stability of implanted HFs is degraded by refraction leakage due to contact with high‐refractive‐index (RI) tissue and bending losses due to in vivo motion or organ micro‐stress. Here, a graded‐index hydrogel fiber (GIHF) is created for high‐performance, anti‐interference optical bio‐interface. The gradient RI of GIHF provides a “self‐focusing” ability to confine the light field, achieving a high transmittance and resistance to stress bending and interference of high‐RI tissue. Additionally, GIHF with optimized RI distribution is fabricated by a projection‐suspended photocuring (PSP) 3D printing method. The optical loss (0.25 dB cm −1 ), tissue‐contact transmittance (98%), and bending loss (0.24 dB per 90°) of the developed GIHF indicate that it outperforms other HFs. The GIHF optical bio‐interface is used to guide sufficient light to excite fluorescence in phantom brain, and the fluorescence signal is stable under multi‐frequency vibration. It paves the way for advances in optogenetics and brain‐machine interfaces (BMI).