Agar-based optical sensors for electric current measurements

Abstract

Biodegradable optical waveguides are breakthrough technologies to light delivery and sensing in biomedical and environmental applications. Agar emerges as an edible, soft, low-cost, and renewable alternative to traditional biopolymers, presenting remarkable optical and mechanical characteristics. Previous works introduced agar-made optical fibers for chemical measurements based on their inherent response to humidity and surrounding concentration. Therefore, we propose, for the first time, an all-optical, biodegradable electric current sensor. As flowing charges heat the agar matrix and modulate its refractive index, we connect the optical device to a DC voltage source using pin headers and excite the agar sample with coherent light to project spatiotemporally deviating speckle fields. Experiments proceeded with spheres and no-core fibers comprising 2 wt% agar/water. Once the increasing current stimulates the speckles’ motion, we acquire such images with a camera and evaluate their correlation coefficients, yielding exponential decay-like functions whose time constants provide the input amperage. Furthermore, the light granules follow the polarization of the applied voltage drop, providing visual information about the current direction. The results indicate a maximum resolution of $$\sim $$ ∼ 0.4 $$\upmu $$ μ A for electrical stimuli $$\le $$ ≤ 100 $$\upmu $$ μ A, which fulfills the requirements for bioelectrical signal assessment.

Recent developments in agar-based optical devices

“…Albeit the gel conductivity subtly rises with the agarose concentration, such an approach may be unfeasible for electro-optic modulation due to the turbidity constraints [40]. Alternatively, one may dissolve salts in the agar solution to improve ions availability in the dilute phase.…”

“…The electrical conductivity linearly increases with the NaCl or KCl contents, quadrupling the value for concentrations of ∼ 1 g/mL [39,41,42]. Furthermore, the salt's contribution also prevails over the conductivity of glycerol solutions, assisting the creation of electrically-tunable yet strengthened and transparent gels [39,40,43]. Regarding agar, scientists developed a plano-convex lens molded with a glass lens template, as illustrated in Fig.…”

“…Fig. 4 Fabrication of agar-based optical fibers: (a) silicone tubing filled with melted agar produces no-core fibers; (b) glass tube mold with stacked rods produces structured fibers with air holes surrounding the solid core (adapted from [31,40]). and cleaves the end faces according to Fig.…”

“…Mineral impurities in the gel composition establish the current flow, promoting localized heating and modulating the agar's refractive index by thermooptic effect. Such an aspect culminates in specklegram fluctuations that become more vigorous as the electric current magnitude increases [40]. Based on the correlation curve's decrease rate, this sensor attained a practical resolution of 0.4 µA for stimuli ≤ 100 µA, matching bioelectrical signals standards [61].…”

Mechanically Robust Biopolymer Optical Fibers with Enhanced Performance in the Near-Infrared Region

“…29 In a recent study, they developed agar-based fiber and sphere as optical sensors for electric current measurements. 30 In another approach, a combination of 60% glycerol and 2% agar was utilized to fabricate cylindrical optical fiber with an optical loss of 0.81 dB cm −1 at 633 nm. 31 Chen et al utilized chitosan film to coat the tip of single-mode GOF to develop a humidity sensor.…”