Recent innovations in the field of wearable electronics have increased research into flexible and stretchable electronic systems.
While decades’ worth of work in the field of traditional CMOS-based components has led to dramatic miniaturization, these devices are still brittle and inflexible. Although they can be placed on flexible PCB substrates to achieve some degree of flexibility, this solution will not enable truly body-conformed devices any time soon.
As such, researchers and design engineers worldwide have been looking at different ways of creating completely flexible electronic components at the device level, particularly wearable sensors that have several useful applications in a range of consumer, industrial, defense, and medical technologies.
Stretched Out Sensors
There is an inherent problem in stretchable physical sensors, though—elasticity. When stretchable sensors are too elastic and stretch too far, unwanted interactions can lead to measurements in one axis producing errors in another. This could hold back progress in the crucial development of advanced electrical systems such as wearable devices and soft robotics.
For instance, a completely normal and regular movement such as the bend of an elbow or knee can be enough to push a sensor beyond its structural integrity. This produces a significant error on the pressure movement measurement and stops the sensor from being able to measure pressure and strain simultaneously.
In this demonstration, pressure and strain sensing are controlled independently by each motion. Image used courtesy of Scientific Reports
Pressure sensors (transducers) work by utilizing a sensing element of constant area and respond to force applied to it by fluid pressure. The force applied deflects the transducer’s diaphragm, which is then measured and converted into an electrical output.
If one of the transducer’s axes is off by a large enough factor (e.g. because it has been stretched too far), this will lead to an inaccurate reading because pressure (P) is calculated by dividing force (F) by area (A)—P = F/A.
In consumer wearables, these inaccuracies would represent an annoyance for the user. In medical or safety-critical applications, they could be dangerous.
Making Sensors Bounce Back
Researchers at Yokohama National University (YNU) in Japan claim to have found a way to combat this problem, proposing a “monolithic array of pressure and strain sensors” capable of simultaneously and independently detecting force and bending deformation of motion.
In the published paper, researchers describe using two different materials—one soft and one hard—to protect the sensor’s ability to both stretch and accurately measure movement. A hard silicone (PDMS) was placed along electrodes over the array, and at the core of each placement, they placed soft porous silicone that senses pressure.
The silicone substrate, which is made from two different types of silicone—one hard and one soft. The harder silicone (PDMS) can suppress the deformation of the pressure sensing elements under strain. Image used courtesy of Hiroki Ota, Yokahama National University
“The PDMS around the pressure-sensing elements prevents the development of large deformations of the elements during the developed device tension,” said Hiroki Ota, paper author and associate professor in the Faculty of Engineering at YNU.
The soft porous silicone pressure center at the core of the PDMS is protected by the PDMS’ hard shell. This allows it to measure the force of pressure without being overextended beyond reliable error margins. It also allows the sensors to measure both pressure and strain as independent contributors to movement.
Understanding Human Motion Through Bendable Sensors
In addition, the resistance of the column and row electrodes in the matrix of the mapped array is lower than the pressure sensors’ electrodes. “This substrate and control of electrode resistances can prevent stretch deformation of the device from affecting the sensing of pressure,” Ota added.
The electrodes in the stretchable array can measure strain at a lower rate than is required to detect pressure, which enables the independent sensing of pressure and strain.
The researchers plan to apply their sensor to a physical keyboard and mount it to a human body. This keyboard, they say, will be able to bend with the body and still detect fingertip pressure. They also hope to use the sensor to further understanding of the touch and motion of the human hand.
Learn More About Stretchable Electronics
Recent Research (Literally) Stretches the Meaning of “Sensor” and “Battery”
Designing Stretchable Devices and Displays with Transparent Electrodes
Stretchable Supercapacitors for the Next Stage of Wearables
Flexible and Stretchable Crystals as an Alternative Material for Electronic Applications
Silicon Nanowires Could Stretch Up to 23% Farther than Previously Thought