LEDs that can be utilized as implantable light sources have been playing increasingly important roles in neuroscience research, along with the development of genetically encoded actuators and indicators. [3,4] In particular, gallium nitride (GaN)/indium gallium nitride (InGaN) based blue LEDs are utilized as implantable light sources for optogenetic stimulation and/or exciting fluorophores for neural signal sensing. [5,6] High performance GaN blue LEDs are typically grown on rigid, single crystalline substrates including sapphire, [7,8] silicon (Si) [1] and silicon carbide (SiC), [9] and novel strategies on growing and releasing GaN devices on unusual substrates like zinc dioxide coated graphene, [10] boron nitride (BN), [11] amorphous glasses, [12] nanovoid-mediated substrates, [13] etc. are also actively explored. [14] Although diced bare LED chips have found their uses in wearable and implantable systems by flip-chip bonding, [15,16] thinfilm LEDs (with thicknesses less than 10 µm) with various emission wavelengths that are released from original growth substrates and integrated onto flexible and stretchable substrates are more desirable for biomedical applications. [17] Recent results have successfully demonstrated that released, thinfilm LEDs are integrated with flexible, stretchable, and even biodegradable substrates with better biocompatibilities like improved skin conformance and reduced lesion during implantation. [5,18,19] Thin-film, freestanding red and infrared (IR) LEDs based on gallium arsenide can be easily formed by selective sacrificial etching; [20] however, conventional techniques for thinfilm GaN based purple/blue/green LED release and integration typically rely on sophisticated process steps including laser liftoff (LLO) (for GaN on sapphire), [21,22] chemical etching (for GaN on Si), [1,23,24] wafer bonding, [25][26][27] layer transfer, [11] device pick and place, [28,29] etc., [30,31] thus limiting their use. While GaN LED epitaxial liftoff and integration with flexible substrates have been extensively exploited for various planar device architectures like GaN LEDs on graphene, [7,10] BN, [11] glasses with nanovoids, [12,13] Si, [32] SiC-on-insulator, [9] the performance of these devices is still inferior to their counterparts on conventional sapphire substrates, in terms of their current-voltage characteristics, quantum efficiencies, etc. Therefore, it is highly desirable to develop simple and reliable technologies to implement the mainstream, state-of-the-art GaN LEDs (for example,

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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.

In this paper, the roles of mechanical stretch and electric poling on phase transition and piezoelectric response of Poly (vinylidene fluoride) (PVDF) IntroductionAs a promising piezoelectric polymer, poly (vinylidene fluoride) (PVDF) has its unique features such as high piezoelectric constant, low acoustic impedance, wide frequency response, chemical stability, flexibility and light weight. It attracts interests from scientific to industrial areas and has been therefore investigated for applications such as pressure sensor [1], energy harvester [2], battery separation [3], etc. Generally, there are five crystal phases for PVDF, α-, β-, γ-, σ-and ξ-phase, among which the β-phase is responsible for the piezoelectricity of PVDF, since it has the strongest spontaneous polarity. PVDF membranes prepared with traditional methods such as spin-coat contains are predominated with α-and γ-phase [4], and post treatments of mechanical stretching and electric poling are needed to make it piezoelectric, which is time consuming [5].Compared with traditional processes, far field electrospinning combines mechanical stretching and in-situ electric poling simultaneously. And various methods have been applied to collect patterned PVDF fibers from nano to micro scale, such as parallel electrodes, rotating disk. However, this technique is not competent when it comes to the application in miniaturized devices, since it is hard to deposit the fiber precisely using far field electrospinning. The Near Field Electrospinning (NFES) shows its advantage to deposit fibers of nano or micro scale into patterns with high precision, apart from the features that far field electrospinning owns [6]. Chieh Chang, et al, utilized this method to prepare piezoelectric nanogenerator with high energy conversion efficiency [7]. Juan Pu, et al, studied the piezoelectric actuation of PVDF fibers prepared by NFES and found their average piezoelectric coefficient is about twice larger than that of reported PVDF films [8]. ZH Liu, et al, investigated the piezoelectric properties of PVDF/MWCNT nanofiber prepared with different NFES parameters [9].Although it is known that near field electrospinning combines mechanical stretch and electric poling in one step, how these coupled processes influencing the final piezoelectric performance of PVDF nanofibers is still to be understood. With this regard, this study investigates the phase transition and piezoelectric response of PVDF nanofibers prepared under different mechanical stretch and electric poling by adjusting the speed of motion stage and applied voltage.

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