represent inferior compatibility with biological systems (especially soft brain tissues) due to their high stiffness and long-time stability, which lead to deleterious effects like tissue damage, inflammation, and rejection. [4] Flexible and stretchable polymeric fibers with designed microstructures are explored for neurological studies, enabling integrated electrical sensing, optical stimulation, and controlled microfluidic delivery. [5] In medical practice, biodegradable organic and inorganic materials including metals, semiconductors, and polymers have been exploited to form biological structures (drugs, stents, scaffolds, etc.) [6] and functional devices (electrical, optical, mechanical, and thermal sensors). [7] In comparison to devices and systems made of inert materials, these biodegradable implants that can physically disappear in biological tissues to eliminate the risk associated with further retraction or removal, providing enormous potential to biomedical applications and particularly clinical uses. Recently, waveguides and photonic structures based on biodegradable materials like ceramics (e.g., calcium phosphates), [8] synthetic polymers, [9] hydrogels, [10] and even bioderived materials (e.g., silks) [11] are investigated in both in vitro and in vivo studies. So far, these biodegradable photonic materials and structures have not been utilized as implantable fibers to study deep-brain neural activities. Here, we present poly(l-lactic acid) (PLLA)-based optical fibers as a biodegradable Advanced optical fibers and photonic structures play important roles in neuroscience research, along with recent progresses of genetically encoded optical actuators and indicators. Most techniques for optical neural implants rely on fused silica or long-lasting polymeric fiber structures. In this paper, implantable and biodegradable optical fibers based on poly(l-lactic acid) (PLLA) are presented. PLLA fibers with dimensions similar to standard silica fibers are constructed using a simple thermal drawing process at around 220 °C. The formed PLLA fibers exhibit high mechanical flexibility and optical transparency, and their structural evolution and optical property changes are systematically studied during in vitro degradation. In addition, their biocompatibility with brain tissues is evaluated in living mice, and full in vivo degradation is demonstrated. Finally, PLLA fibers are implemented as a tool for intracranial light delivery and detection, realizing deep brain fluorescence sensing and optogenetic interrogation in vivo. The presented materials and device platform offer paths to fully biocompatible and bioresorbable photonic systems for biomedical uses. Biodegradable Fibers

Optical fibers and waveguides in general effectively control and modulate light propagation, and these tools have been extensively used in communication, lighting and sensing. Recently, they have received increasing attention in biomedical applications. By delivering light into deep tissue via these devices, novel applications including biological sensing, stimulation and therapy can be realized. Therefore, implantable fibers and waveguides in biocompatible formats with versatile functionalities are highly desirable. In this review, we provide an overview of recent progress in the exploration of advanced optical fibers and waveguides for biomedical applications. Specifically, we highlight novel materials design and fabrication strategies to form implantable fibers and waveguides. Furthermore, their applications in various biomedical fields such as light therapy, optogenetics, fluorescence sensing and imaging are discussed. We believe that these newly developed fiber and waveguide based devices play a crucial role in advanced optical biointerfaces.

Photobiomodulation (PBM) has recently started to gain popularity in clinical therapeutics. Visible light, in particular, plays a critical role in osteogenesis modulation. However, the limited penetration depth of visible light in biological tissues has constrained the application of this technology in vivo. Herein a green light‐based PBM technique with implantable and biodegradable poly(L‐lactic acid) & poly(L‐actic‐co‐glycolic acid) optical fibers to achieve accelerated bone regeneration is explored. Facilitated with experimental characterizations as well as numerical simulations, optical and thermal behaviors of fibers operated in the biological environment are understood. The optical regulation of bone regeneration is systematically studied both in vitro and in vivo. Under green light irradiation, biochemical activities of bone marrow‐derived mesenchymal stem cells and their expression of osteogenic‐related factors are significantly elevated. By introducing green light into defective bone structures via fibers in a rodent model, the process of bone regeneration and repair is accelerated. Furthermore, fibers exhibit ideal biocompatibility with both cultured cells and living tissues and undergo complete degradation in vivo after ≈1 month. Assisted with degradable optical materials and devices, such as photobiomodulation technique provides a promising solution to tissue regeneration in various biomedical applications.

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