Fluid-powered soft actuators have shown potential in applications such as prosthetics and orthotics due to their natural motion. Robots using these actuators require stretchable sensors that can be embedded in their bodies for sophisticated functions; however, these sensors usually rely on the electrical properties of materials and composites to measure a signal and many suffer from hysteresis, complex fabrication, environmental instability, and material incompatibility with soft actuators.

Researchers have solved many of these problems through the use of stretchable optical waveguides for strain sensing in the prosthetic hand. The optoelectronic strain sensors are easy to fabricate, are chemically inert, and have low hysteresis and high precision in their output signals.

The waveguide is fabricated to be intentionally lossy — as light propagates through it, some radiates to the environment and the more it is deformed, the more light is lost. The light power loss of the waveguide was measured using a photodetector to indicate its deformation. The sensory waveguide is a step-index multimode optical fiber composed of a core with a high refractive index that has a cross-sectional area of about 1 × 1 mm and is clad with an elastomer that has a lower refractive index with a cross-sectional area of about 3 × 3 mm.

A four-step soft lithography process was used to fabricate the stretchable waveguide: 1) 3D-print a mold to make the cladding, 2) pour a pre-elastomer for the cladding into the mold and demold after curing, 3) fill the cladding with the pre-elastomer of the core material, and 4) pour the pre-elastomers of the cladding to enclose the core. Two holes were cast at each end of the waveguide to house the LED and the photodetector.

The waveguides operate as bending, elongation, and pressure sensors for most situations a hand would typically encounter (e.g., pressing, touching, and grasping). The core material has a relatively large propagation loss compared with those used for fiber-optic communication. Because the waveguides are applied to prosthetic hands for sensation, this relatively large propagation loss improves sensitivity during elongation, while still allowing a detectable amount of light over the length scales typical of a human hand using a low-cost photodiode and a simple current amplifying circuit.

After the waveguides were fabricated, three of them were cast into a finger actuator using overmolding. The 3D integration of the sensors and actuators means that the waveguides are parts of the body and will deform when the actuator does, serving as proprioceptive sensors. The more the prosthetic hand deforms, the more light is lost through the core. That variable loss of light, as detected by the photodiode, is what allows the prosthesis to “sense” its surroundings.

The prosthesis was used to perform a variety of tasks including grasping and probing for both shape and texture. Most notably, the hand was able to scan three tomatoes and determine, by softness, which was the ripest.

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This article first appeared in the July, 2020 issue of Tech Briefs Magazine.

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