Stretchable Optical Memristor Design, Fabrication and Characterization

Wearable electronics/photonics are developing rapidly in the era of artificial intelligence (AI) and the Internet of Things (IoT). However, wearable computing devices developed so far are mainly limited to "electronic hardware" implementations, which are incompatible with wearable photonic components. A highly attractive vision is to develop wearable photonic computing devices that can be attachable to the human skin for direct interface with wearable photonic components, such as optical sensors. In this work, we present a novel material combination for a buried waveguide-type, hydrogel-based stretchable optical memristor that can be used for next-generation wearable photonic computing.<br/>To fabricate a stretchable optical memristor, we first fabricate a PDMS substrate with a straight waveguide channel. Then we synthesize polyoxometalates (POMs) doped polyacrylamide (PAAm) hydrogel with a refractive index of 1.43, which is higher than that of PDMS (1.41) at visible light wavelengths thus allowing total internal reflection in the hydrogel, inside the waveguide channel. The POMs within the hydrogel can be switched between a reduced, colored state and an oxidized, uncolored state under UV and oxygen environments, respectively, thereby modulating the transmittance across the visible light spectrum. We use UV light for programming the waveguide transmittance and a 650 nm light for optical readout due to its large photochromic response. By controlling the UV and oxygen environments, we achieve a maximum attenuation of 12 dB, 16 levels of memory, retention over 10000 s, and reversibility under an oxygen environment. Moreover, the stretchable optical memristor exhibits intrinsic stretchability owing to the hydrogel matrix, enabling it to withstand up to 50% strain while maintaining performance after repeated stretching. The combination of POM-doped hydrogel and PDMS substrate enables a buried-type waveguide that functions as an effective optical memristor with inherent stretchability. Such a stretchable optical memristor holds promise for applications in wearable photonic computing, serving as a key component of photonic skin for smart wearable devices, personal healthcare devices, and human-machine interaction interfaces.