Highly Stretchable, Strain Sensing Hydrogel Optical Fibers

Abstract

A core-clad fiber made of elastic, tough hydrogels is highly stretchable while guiding light. Fluorescent dyes are easily doped into the hydrogel fiber by diffusion. When stretched, the transmission spectrum of the fiber is altered, enabling the strain to be measured and also its location.

Keywords: absorbance spectroscopy; fiber optics; hydrogels; strain sensing.

Figure 1

Optical properties of step-index, core/clad alginate-polyacrylamide hydrogel fibers. a, The dependence of optical attenuation in hydrogels on the acrylamide (AAm) concentration, averaged over the visible spectrum (400–700 nm). b, The refractive index of alginate-polyacrylamide precursors (solid diamonds) and crosslinked hydrogels (open squares) as a function of the concentration of acrylamide. c, Linear expansion ratio, determined from the cube root (red squares) of the measured weight ratio between a fully-swollen state and immediately after fabrication (pre-swelling) or the cube root (cyan squares) of the swelling ratio between fully-swollen samples and dry mass. Dotted lines: curve fits based on a linear line (magenta) or y = A/x, where A is constant (cyan). d, Photos of fabricated hydrogel fibers. Scale bar, 1 cm. e, Phase-contrast images of three hydrogel fibers in deionized water. Arrows indicate the interface between the core and clad; the numbers indicate the core and cladding diameters (420/920 means 420-μm-diameter core and 920-μm clad). Scale bar, 500 μm. f, Photos showing 532-nm laser light guided through and scattered from a hydrogel fiber (top) and a hydrogel fiber connected to a multimode silica fiber (core diameter: 100 μm; NA=0.37) (bottom). g, Propagation loss of pre-swelling hydrogel fibers in air with/without cladding, measured by a cutback technique. h, Propagation loss of pre-swelling hydrogel fibers as a function of external refractive index. Symbols, experimental data. Lines, curve fits based on the Fresnel Equation. Error bars, s.d. (n=3) in g and h.

Figure 2

Mechanical properties of alginate-polyacrylamide hydrogel fibers. a, Photos demonstrating 3X stretching of a hydrogel fiber. b, Elasticity of fibers (750/1100 μm) against repeated 3X stretching for core/clad, showing no change in fiber length after 100 cycles. Photos show a 5-cm-long fiber in the relaxed state, top, and stretched state, bottom. c, Optical microscope images of light transmission through a hydrogel fiber in the relaxed (left) and stretched (right) conditions. d, A typical stress-strain curve of a fully-swollen hydrogel fiber. The slope in the low-strain region gives a Young’s modulus of ~ 80 kPa. e, Illustration of the optical loss in a sensing region containing absorption and scattering elements. As the fiber is stretched, the distribution of the loss elements is altered, resulting in loss increasing exponentially with the length of the sensing region. This relation offers a simple technique for strain sensing.

Figure 3

Dye-loaded hydrogel fibers for strain sensing. a, Setup for stretching a section of fiber doped with fluorescein (green region). b, The measured attenuation spectra as the sensing region was stretched. c, The attenuation spectra when the strain was applied to an un-doped section. d, Dye absorption spectra, a₁(l₁)R₁(λ), extracted from the attenuation spectra. e, The magnitude of dye absorption at the peak wavelength (480 nm) as a function of strain applied to the sensing (red) and un-doped (blue) regions. Data for three repeated measurements are shown. f, Time responses of the fiber sensor with respect to step-tuned strain up to ε₁ of 100%. The strain readout of the sensor is in good agreement with the applied strain values.

Figure 4

Multiplexed strain sensing. a, Schematic of a fiber with three sensor regions doped with different dyes, respectively: RB: rose Bengal, MB: methylene blue, FL: fluorescein. b, Photos showing a dye-doped fiber on a glass slide, without (top) and with (bottom) excitation broadband light. c, Extracted absorption spectra of the three sensors when local strain was applied to each sensor at a time. d, The dye absorption as a function of strain, showing linear readout in terms of dB/strain and negligible crosstalk between sensors.