Release date: 2017-10-17
With the proliferation of various wearable devices, the demand for various wearable sensors is increasing. These sensors can be attached to the body or integrated into the garment, allowing athletes to better monitor their movements, while physiotherapists can be used to help patients recover, as well as help in computer games and animations to make detailed motion captures. Help engineers build robots that can be soft touched or form a new real-time health monitoring device.
In fact, it is very complicated to accurately capture the movement of the human body. Our hands, feet, various parts of the body, facial expressions, etc., often have complex bending and deformation. Currently, there are also very good electronic strain sensors, but they are often affected and limited by external electromagnetic fields.
Fiber optic sensors are good at avoiding the above effects. Fiber optic sensors have long been available and have been used to measure buildings and bridges in many cases. When the fiber is bent, it will be affected by the passage of light. Fiber optic sensors measure such changes and then convert the data through a computer into forces that affect bridges and buildings. However, the current fiber-optic sensor materials are usually glass or brittle plastic, and several degrees of deformation will cause damage to them, which makes the original fiber-optic sensor difficult to apply to wearable devices.
The Changxi Yang team from the State Key Laboratory of Precision Measurement Technology and Instruments at Tsinghua University published a study in Optica magazine that demonstrated for the first time a flexible fiber that senses a wide range of motions.
Image source: medgadet
This new fiber is both sensitive and flexible, and can detect joint motion, which is different from the fiber optic sensors currently in use. Yang said: "This new technology provides a fiber-optic method for measuring extremely large deformations. It is wearable, mountable, and has the inherent advantages of optical fibers such as inherent electrical safety and immunity to electromagnetic interference. â€
Stretching puzzle
Traditional optical fibers are not the best choice for human sensing because they are usually made of hard plastic or glass. They are hard and brittle and are not easily bent. For example, silicon fiberglass can withstand up to 1 % strain, while curved finger joints deform more than 30%. Obviously, such an optical fiber is difficult to apply to flexible and complex human deformation.
This obstacle means that the development of most wearable sensors to date is based on electronic sensors. These sensors detect motion by measuring changes in electrical properties such as resistance when the sensor is bent. However, these systems are difficult to miniaturize, lose charge, and are sensitive to electromagnetic interference from devices such as automobiles and mobile phones.
A flexible fiber can be used to avoid these problems and potentially create a more stable and sustainable wearable device than electronics.
Material screening
Researchers began looking for a fiber that could withstand the bending and stretching of human motion. They first tried fibers made from hydrogels, which are known to be a soft, gelatinous substance that can withstand strains of up to 700%. But hydrogels are mainly composed of water and therefore work only in wet environments. When exposed to air, the fibers quickly dry and shrink, a feature that limits the hydrogel as a material for the fiber.
In the second attempt, Yang and his students Jingjing Guo and Mengxuan Niu jointly developed a fiber made of silicon, a soft polymer called polydimethylsiloxane (PDMS). They heated the liquid silicone into a tubular mold to 80 ° C for 40 minutes, and then used water pressure to push one end of the fiber mold to make the fiber. They doubled the fiber length through a series of well-designed tests, such as repeated stretching. Even after 500 stretching, the PDMS fibers recovered to their original length.
“The PDMS fibers we make are very mechanically flexible and can easily be bundled and twisted,†says Yang. More importantly, when the team reduced the diameter of the fiber from 2 mm to 0.5 mm, the mechanical strength of the fiber actually increased.
In order to make the silica fiber have sensing function, the researchers mixed a fluorescent dye called rhodamine B into silica gel. As the light passes through the fiber, some of the light is absorbed by the dye - the more the fiber is stretched, the more light the dye absorbs. So simply measuring the transmitted light with a beam splitter measures the extent to which the fiber is stretched or bent so that it reflects the movement of any part of the body to which it is attached.
Glove test
To test whether the silicone fiber was as effective as expected, the researchers glued the fiber to the rubber glove with epoxy, and then the wearer bent and stretched the finger to monitor it. In the process, they measured the strain on the fiber to be 36%, consistent with other measurements using electronic sensors.
This type of sensor also performs well in the face of more subtle pressures, such as the tiny movements of the neck muscles when a person breathes or speaks. "All the results show that optical strain sensors can be used to monitor a variety of human motion and may provide a new approach to the exploration of human-machine interfaces," Yang said.
Reference material
[1]In a first for wearable optics,researchers develop stretchy fiber to capture body motion
[2]Highly Stretchable and FlexibleFiber Measures Tiny Changes in Body Movements
Source: Health New Vision (Micro Signal HealthHorizon)
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