Functional photonic structures for external interaction with flexible/wearable devices

In addition to vital functions, more subsidiary functions are being expected from wearable devices. The wearable technology thus far has achieved the ability to maintain homeostasis by continuously monitoring physiological signals. The quality of life improves if, through further developments of wearable devices to detect, announce, and even control unperceptive or noxious signals from the environment. Soft materials based on photonic engineering can fulfil the abovementioned functions. Due to the flexibility and zero-power operation of such materials, they can be applied to conventional wearables without affecting existing functions. The achievements to freely tailoring a broad range of electromagnetic waves have encouraged the development of wearable systems for independent recognition/manipulation of light, pollution, chemicals, viruses and heat. Herein, the role that photonic engineering on a flexible platform plays in detecting or reacting to environmental changes is reviewed in terms of material selection, structural design, and regulation mechanisms from the ultraviolet to infrared spectral regions. Moreover, issues emerging with the evolution of the wearable technology, such as Joule heating, battery durability, and user privacy, and the potential solution strategies are discussed. This article provides a systematic review of current progress in wearable devices based on photonic structures as well as an overview of possible ubiquitous advances and their applications, providing diachronic perspectives and future outlook on the rapidly growing research field of wearable technology.