Improvements in Wearable Optics Lead to Stretchy Optical Fiber Sensors

By: Nelson Monterrosa

Wearable sensor research has increased in the last years. The expansion of wearable technologies encourages the development of more efficient sensors for collecting and processing data from the real world. Small wearable sensors allow researchers to track body motion more efficiently. Wearable sensors applications range from allowing physicians to monitor life-threatening conditions to improving the quality of animations in video games. Efficient sensors are small, durable and sample large amounts of data in real time. An improvement in any of these three areas can increase the price of a sensor drastically.

Optica, the journal for high impact research at The Optical Society, offers a first look at a new type of sensor that could improve sensor efficiency while reducing the price per unit. Stretchy Optical Fibers sturdy enough to sense human motion were developed by a team led by Changxi Yang of the State Key Laboratory of Precision Measurement Technology and Instruments at Tsinghua University in Beijing.

The Stretchy Optical Fibers are as thin as 0.5 mm in diameter and are both durable and stretchy. This last feature allows them to provide detailed data. The Stretchy Optical Fiber sensors can be mapped to both a location and a tensile strength in the surface attached. The new optical fiber is different from currently used fiber sensors in that it is sensitive and flexible enough that it can detect joint movements.

“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.”

Previous Applications

Common optical fiber sensors have been used in bridges and buildings for years. The fiber refracts light when it stretches and bends allowing monitors to detect it more easily. However, fiber optics can usually handle a maximum strain of less than 1 percent, making them unsuitable candidates to sense body motion given that bending just one finger would cause a restrain of at least 30 percent. This is why most of the motion sensing research has gone to electronic sensors. Such sensors usually measure movement in the human body by measuring changes in electrical properties, such as resistance as the sensor bends. These systems are difficult to scale to the point of tracking body motion while helping the user feel comfortable. They are also susceptible to interference from many of the electromagnetic signals we encounter daily (i.e., cell phones, credit cards or even other sensors). A bendable optical fiber could avoid these problems and potentially create wearable devices that are more stable and sustainable than those based on electronics.

The New Research

At Tsinghua University, after several attempts, Yang and his students developed a fiber made of silicone-specifically a soft polymer called polydimethylsiloxane (PDMS). 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 to a quarter of the original size, the mechanical strength of the fibers actually increased.

To aid in sensing, the researchers mixed a fluorescent dye called 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 to which it is attached.

The research team tested the idea by attaching the optical fiber to a rubber glove and then monitored when the user bent and flexed his fingers. The results during movement were clear. The measured strain of the fiber was over 36 percent from less than 1 percent in regular silicone optical fiber. This quality of strain is in line with the bending of our fingers are capable of and definitely in line with the strain measured using electronic sensors.

“The remarkable flexibility and stretchability of the PDMS fiber make 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 Stretchy Optical Fibers were also tested to perceive more subtle strain, such as the movement of neck muscles when a person breaths 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 also tested the Stretchy Optical Fiber in different environments, such as in water, glycerol, and air. The tests were a success although the accuracy changed slightly in different environments, this behavior suggests that the new sensors would need to be calibrated for the specific environment they would 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.