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

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 degrees) 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).