Optical Fiber Senses Human Motion

A silicone strain sensor glued to a rubber glove bends easily with the wearer’s finger. The amount of light transmitted by the fiber changes with the bending. (Source: C. Yang, Tsinghua U.)

The exciting appli­cations of wearable sensors have sparked a tremen­dous amount of research and business invest­ment in recent years. Sensors attached to the body or inte­grated into clothing could allow athletes and physical therapists to monitor their progress, provide a more detailed level of motion capture for computer games or animation, help engineers build robots with a lighter touch or form the basis for new types of real-time health monitors. A team led by Changxi Yang of the State Key Laboratory of Precision Measure­ment Tech­nology and Instruments at Tsinghua Uni­versity in Beijing now offers the first demon­stration of optical fibers sturdy enough to sense a wide range of human motion.

The new fiber is sensitive and flexible enough that it can detect joint movements, unlike currently used fiber sensors. “This new technique provides a fiber-optic approach for measuring extremely large defor­mations,” said Yang. “It’s wearable, mountable and also possesses intrinsic advan­tages of optical fibers such as inherent elec­trical safety and immunity to electro­magnetic inter­ference.” Optical fibers have been used for strain sensing on bridges and buildings for years; stretch or bend the fiber a little and light going through it is shifted in a way that can be easily picked up by a monitor. Tradi­tionally optical fibers haven’t been the best choice for strain sensing on the human body because they are typi­cally made of plastic or glass, which are stiff and don’t bend well. A silica glass fiber, for example, can handle a maximum strain of less than 1 percent, while a bending finger joint would strain it by more than 30 percent.

This barrier has meant that most wearable sensor deve­lopments so far have been based on elec­tronic sensors. These sensors detect movement by measuring changes in elec­trical pro­perties such as resis­tance as the sensor bends. However, these systems are difficult to miniaturize, can lose their electrical charge and are sensitive to electro­magnetic inter­ference from devices such as cars and cell phones. A bendable optical fiber could avoid these problems and poten­tially create wearable devices that are more stable and sustainable than those based on elec­tronics.

When the researchers started looking for a fiber that could stand up to the amount of bending and stretching involved in human movements, they first tried fibers made of hydrogel, a soft, jelly-like substance that can hold strains of up to 700 percent. But hydrogel consists mostly of water, and therefore only worked in wet environments. When exposed to air, the fibers quickly dried out and shrank. In a second attempt, Yang and his students, Jingjing Guo and Mengxuan Niu, developed a fiber made of silicone – specifically a soft polymer called poly­dimethyl­siloxane (PDMS). They created the fiber by putting the liquid sili­cone into a tube-shaped mold and heating it to 80° C (176° F) for 40 minutes to get it to thicken, then used water pressure to push a thin fiber out of one end of the mold. They put the resul­ting fibers through an elaborate series of tests, such as repea­tedly stretching them out to double their length. Even after 500 stretches, a fiber still returned to its original length.

“The fabri­cated PDMS fibers exhibited excellent mecha­nical flexi­bility, and could easily be tied and twisted,” said Yang. What’s more, when the team reduced the diameter of the fibers they produced, from 2 milli­meters to 0.5 milli­meters, the mechanical strength of the fibers actually increased. To aid in sensing, the resear­chers mixed the fluores­cent dye Rhoda­mine B into the silicone. When light shines through the fiber, some of the light is absorbed by the dye. The more the fiber stretches, the more light the dye absorbs. So simply measuring the trans­mitted light with a spectro­scope provides a measure­ment of how much the fiber is being stretched or bent, which tells an observer about the movement of any body part it is attached to.

The researchers tested that idea by gluing their fiber to a rubber glove with epoxy, and then monitoring it as a wearer flexed and extended his fingers. During that movement, they measured a strain in the fiber of 36 percent, in line with what others had measured using elec­tronic sensors. “The remarkable flexi­bility and stretcha­bility of the PDMS fiber makes it especially attractive for sensing of large strains,” said Yang, adding that this is the first time researchers have used an optical sensor to capture human motion.

The sensor also performed well in situations involving more subtle strains, such as the minute movements of neck muscles as a person breathes or speaks. “All the results show that the optical strain sensor can be used for monitoring of various human motions and may provide a new approach for explo­ration of human-machine inter­faces,” said Yang. The team tested how well their fibers sensed strain over longer periods of time and in different environ­ments, such as in water, glycerol and air. They learned that the fibers held up well, although the sensing accuracy did change in different environ­ments, suggesting devices using the optical fiber-based sensors would need to be cali­brated for the specific environ­ment they’d be used in.

The team illu­minated the fiber by attaching it to a halogen lamp, and measured the light passing through it with a spectrometer. To adapt the tech­nology to create a wearable device, Yang said it should be possible to develop a compact light source and spectro­meter that can be easily worn on the body. (Source: OSA)

Reference: J. Guo et al.: Highly flexible and stretchable optical strain sensing for human motion detection, Optica 4, 1285 (2017); DOI: 10.1364/OPTICA.4.001285

Link: State Key Laboratory of Precision Measurement Technology and Instruments, Dept. of Precision Instruments, Tsinghua University, Beijing, China


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