Soft Optical Fibers Made of Agar

Biodegradable optical waveguides are emerging technologies for light delivery and sensing in various in-vivo applications, such as imaging systems, optogenetics, and the surveillance of intra-body parameters. In this context, agar extracted from red algae is a promising low-cost, renewable, and edible material for developing optical devices with remarkable characteristics, such as transparency, flexibility, moldability, and thermal reversibility.<br/>This work introduces biocompatible optical fibers fabricated by pouring melted agar solutions into cylindrical molds. The solidified waveguides assume the mold geometry, allowing the manufacture of assorted waveguides, including no-core and structured holey fibers. Furthermore, the refractive index and mechanical strength are tailorable by choosing the agar concentration or adding glycerol to the precursor. Cutback tests with a 2 wt% agar, 60 wt% glycerol fiber revealed optical losses of ~0.8 dB/cm at 633 nm, comparable to waveguides made of typical biopolymers.<br/>Subsequently, we investigate the ability of agar optical fibers to perceive physical and chemical stimuli. A coherent light excites multiple fiber modes due to the core dimensions, creating an output speckle field whose spatiotemporal distribution transcribes the fiber state. Besides traditional bending and strain measurements, one may explore the agar’s swelling and syneresis behaviors to evaluate humidity, temperature, and its degradation over time. The electrical conductivity of agar also establishes current-driven modulation of the optical response. Moreover, the holey structure of agar waveguides can transport fluids and provide refractive index detection through interaction with the core modes.<br/>The developed agar-based optical fibers are customizable, easy to fabricate, and exhibit versatile sensing capabilities, which inspires several applications as a biocompatible and degradable probe for medical, biochemical, and environmental measurements.