Researchers in China have become the first to develop optical fiber that can sense a wide range of motion, paving the way for a strain-sensing fabric for sensing that can replace individual sensors in wearable, robotic, and other types of technology.
A team from the State Key Laboratory of Precision Measurement Technology and Instruments at Tsinghua University in Beijing demonstrated the fiber, which is sensitive and flexible enough that it can detect joint movements--unlike currently used fiber sensors, said Changxi Yang, a professor in the Department of Precision Instruments at the university.
“For human-motion detection, strain sensors with high flexibility and stretchability are demanded,” he explained to Design News. “For such applications, fiber-optic strain sensors offer attractive advantages such as inherent electrical safety, immunity to electromagnetic interferences, and small size. However, the low stretchability and stiffness of the conventional optical fibers, typically made of glass or plastics, are fundamental limits for measurements of large deformations.”
For example, Yang said, the bending motion of a finger joint can reach a strain more than 30 percent, which is far beyond the stretchability of a silica fiber, which has a maximum strain of less than 1 percent.
To stretch the fundamental limits of optical fiber—literally—Yang and the team fabricated a highly flexible and stretchable polymer optical fiber using PDMS, a soft and stretchable material commonly used for stretchable electronics. PDMS has a number of advantages for this application, in that it is thermally stable, chemically inert, and most importantly highly transparent in a wide spectral range, he said.
“In our previous work, we also demonstrated a highly stretchable and implantable hydrogel optical fiber that can hold strains up to 700 percent,” Yang explained. “However, as hydrogels are polymer networks infiltrated with water, the fiber can only be used in wet environments. When exposed to air, the drying of the fiber suffered from volume shrinkage and structural damage.”
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. Researchers at Tsinghua University in China developed the strain-sensing fiber using PSMS and dye doping. (Source: Changxi Yang, Tsinghua University)
For the new material, researchers achieved the sensing of the fiber by doping it with dye—specifically, fluorescent dye called Rhodamine B—in high-absorption molecules that caused light attenuation of the fiber, he said. Upon stretching, the fiber length increased along with the amount of light absorption, allowing for the applied strain to be measured from the changes of the transmitted absorption spectra, Yang said.
“The sensor can detect extremely large strains up to 100 percent, which thus allows human motion detection with a fiber-optic setup,” he said.
The team published a paper on their work in the journal Optica from The Optical Society.
Researchers tested the technology 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 sensor also performed well in situations involving more subtle strains, such as the minute movements of neck muscles as a person breathed or spoke, researchers said.
The technology has the potential to be used in a wide array of applications, including wearable smart devices for medical, entertainment, or sporting and fitness uses, Yang said.
“For example, the flexible and stretchable strain sensors can be mounted in different parts of the body and function for the sport-performance monitoring,” he said. “The data can be used for the body-movement analysis during sport activities, which are beneficial for the continuous health and wellness monitoring, and evaluation of athletes' sport performances.”
Robotics is another area of potential use, with the strain sensors being used to actuate smart robots to remotely control a gripper robot by measurements of the finger-joint motion, Yang added.
“Flexion or extension of fingers can be used to control the robot to perform surgical procedures or delicate and dangerous tasks that are out of reach for the human body,” he said.
The team plans to continue its research to further improve the sensor performance in areas of sensitivity and stability, as well as to minimize the current physical set-up of the sensing mechanism to be more wearer-friendly, Yang said.
Elizabeth Montalbano is a freelance writer who has written about technology and culture for more than 15 years.