Sensing with agar-based optical waveguides

The growing demand for biocompatible and biodegradable sensing devices emerges to fulfill applications in medicine, tissue engineering, and environmental monitoring. Nowadays, optical waveguides conceived with unconventional materials (like silk, cellulose, green polymers, and hydrogels) have replaced silica and polymer-based devices in optogenetics, phototherapy, and intra-body assessment. However, most biodegradable optical materials rely on expensive resources and intricate processing. Agar is an edible, soft, and renewable alternative presenting singular features: gelation at low temperatures, thermal reversibility, moldability, and transparency. Furthermore, one may enhance the optical and mechanical properties of the agar samples by choosing the chemical composition. This work proposes the design, characterization, and application of agar-based devices for optical sensing. Firstly, melt solutions comprising food-grade agar and water undergo solidification inside molds to create standard and structured optical fibers and waveguides. Besides, adding glycerol improves mechanical strength and stability, reduces optical losses, and provides reliable refractive index control. Subsequently, experiments evaluate the optic response of agar devices to mechanical, thermal, electrical, and chemical stimuli. Illumination with a visible laser creates speckle fields susceptible to mode coupling and phase deviation effects. Therefore, one may analyze these speckle patterns with image processing techniques to detect subtle changes in the output light and retrieve the measured parameters with high sensitivity through a straightforward camera-based setup. The agar optical waveguides provide new perspectives for physical and biochemical sensing based on an edible and biodegradable material for intra-body applications and environmental monitoring.