Optical strain sensor can measure finger bending

Researchers from Tsinghua University, China, have devised a highly flexible optical-fiber sensor for wearable technology that can measure the large strains common in human motion—such as the bending of a finger, which can impose extensions as large as 30 percent. [Image: Changxi Yang, Tsinghua University]

As the world increasingly ramps up to a future of wearable technology, sensors that can measure strain, and thus bodily motions, in real time have become a hot commodity. But figuring out an optical sensor that can stand up to large strains, such as those across a bent elbow or a clenched fist, has proved a tough problem to crack.

A team of researchers at Tsinghua University, China, led by OSA member Changxi Yang, now believes it’s come up with an answer: a fiber optic sensor made of a silicone polymer that can stand up to, and detect, elongations as great as 100 percent—and effortlessly snap back to an unstrained state for repeated use (Optica, doi: 10.1364/OPTICA.4.001285).

A problem of materials

At present, the strain sensors envisioned in wearable-tech designs tend to be electronic: graphene, carbon-nanotube, or nanofiber-based gadgets that measure strain based on changes in reistance or capacitance. While cost-effective and reasonably sensitive, those sensors have had trouble finding practical use, as they’ve been difficult to miniaturize and suffer from current leakage, sensitivity to ambient electromagnetic noise, and other problems.

An obvious alternative lies in optical strain sensors—particularly fiber-based sensors, which have a long pedigree as strain sensors in bridges, oilfields and other infrastructure. In such facilities, expanses of embedded fiber can measure minute temperature and strain changes integrated over long distances. But stiff conventional glass and plastic fibers do not have anything close to the flexibility required to operate as strain sensors in the more intimate realm of wearable technology, where the bending of a finger joint, for example, can cause strains greater than 30 percent.

Almost exactly a year ago, Yang, working with colleagues at the Masschusetts Institute of Technology and Harvard University, USA, demonstrated a stretchable fiber made of a water-infused “hydrogel” polymer network to get around some of these problems. The hydrogel fiber proved resilient to massive strains of up to 700 percent, and, as a biocompatible material, looked promising for applications such as implantable sensors and diagnostics. But the water-logged hydrogel isn’t a natural fit for applications in wearable tech, since it’s apt to dry out and deteriorate when exposed to air.

The silicone route

Now, the Tsinghua researchers have adopted another route toward a wearable optical strain sensor: fibers made of polydimethylsiloxane (PDMS), a soft, stretchable silicone elastomer that’s become a common substrate in stretchable electronics. The team developed the fiber by cooking up a liquid silicone solution in tube-shaped molds at 80 °C, and doping the fiber mix with Rhodamine B dye molecules, whose light absorption is wavelength dependent. Because stretching of the fiber will shrink its diameter, leaving the total volume invariant, a fiber extension has the effect of increasing the optical length for light passing through the dye-doped fiber. That increase, in turn, can be read in the attenuation of the fiber’s transmission spectra, and tied to the amount of strain in the fiber.

In tests Yang’s team found that a 4-cm length of the fiber, with a diameter of 0.5 mm, repeatedly held up to strains on the order of 100 percent, snapping back to its original length with no change even after 500 cycles. To test its ability to measure actual strains in the field, the researchers tied the short fiber length with epoxy to two silica multimode fibers and measured the attenuation spectrum of a white-light signal as the flexible portion was subjected to various strains. The team found that the sensor measured strains with an error of less than 0.6 percent over repeated measurements, and that bending of the fiber didn’t materially reduce its effectiveness as a strain sensor.

From finger motions to deep breathing

To underscore the material’s potential in the wearable-tech realm, the researchers even epoxied the sensor to a rubber glove, and demonstrated its sensitivity to finger motions—which, the authors maintain, is the first report “to the best of our knowledge … of human motion detection using optical sensors.” And they mounted it on the neck of a lab volunteer and managed to tease out a strong signal of the subtler strain associated with breathing, which suggests another application for the material.

The team concludes that the material “may find widespread applications in wearable smart devices, especially for human motion detection.”