Flexible Organic Crystals for Light Delivery in Biological Tissues

Optically transmissive materials are indispensable for the transmission of light or light-encoded signals in telecommunications and optobiomedical techniques. Here, we propose that slender crystals of small organic molecules can be used as optically transparent, flexible, lightweight, and emissive media to deliver photons into or through biological tissues as an alternative to silica- or polymer-based light waveguides. We demonstrate that organic crystals remain transmissive in various tissues, and their efficiency in light transduction depends on the intrinsic optical properties of the crystal, optical path, geometry of excitation, and the type of tissue. Moreover, elastically or plastically deformable organic crystals remain mechanically compliant and can be bent after they have been embedded in the tissue, opening prospects for designing a new class of biocompatible light waveguiding elements based on crystalline organic materials. In vivo implantation and toxicity assays capitalize on mechanical flexibility and biocompatibility in animal models. Within a broader context, the high transparency, anisotropy, and biocompatibility of some organic crystals turn this emerging class of materials into a prospective platform for delivering photons for specific interaction with target cells in tissues for applications such as photodynamic therapy and optogenetics.