Taking the Strain

In order for the devices and machine learning algorithms that make our lives easier and more efficient to operate as intended, they need some way of gathering information about the environment around them. A seemingly endless variety of sensors exist to fill this role, from cameras and microphones to motion and temperature sensors. Another class of sensors — stretchable strain sensors — are able to detect and measure strain or deformation in a material. This gives them capabilities that cannot easily be replicated by other sensing methods, and makes them a natural choice for a variety of applications, including structural health monitoring, robotics, and wearable technologies.

One of the most common technologies employed to create stretchable strain sensors is the use of piezoresistive materials. These materials, such as carbon nanotubes or graphene, change their electrical resistance in response to applied strain. Using this approach, it is possible to detect even very small strains with a high degree of sensitivity. However, these materials do not have a very large range of motion, and tend to break when subjected to higher levels of strain. While other methods can create sensors that are able to take a lot of strain, they are not nearly as sensitive.

And therein lies the problem — strain sensors can either be sensitive, yet minimally stretchable, or very stretchy, yet not very sensitive. To accommodate more applications, a best of both worlds approach is needed. A team of researchers at North Carolina State University may have developed just such a strain sensor that hits the sweet spot between durability and sensitivity. They have recently reported on a soft, stretchable resistive strain sensor constructed with a novel fabrication method that gives it these unique properties.

The sensor consists of a network of silver nanowires that have been embedded in an elastic polymer made of polydimethylsiloxane. A series of parallel cuts are made on the surface of the polymer in a zigzag pattern. This patterning allows the polymer to stretch further, adding to the range of motion of the sensor. Additionally, as the zigzag pattern stretches, portions of the sensor lose electrical contact with one another, which in turn changes the electrical resistance of the material. This change in resistance is measurable, and allows for very accurate strain measurements to be captured.

After conducting a study to better understand the effect of differing slit depths, slit lengths, and pitch between the slits, the researchers were able to optimize the performance of their strain sensor. They then created a number of devices to demonstrate the unique capabilities of the sensor. In one example, to show how sensitive their sensor is, they created a wearable blood pressure monitor. To demonstrate that this sensitivity is also paired with durability, they developed a wearable device for monitoring motion in a person’s back, which may be useful in physical therapy applications. Finally, they showed off all of these qualities at once with their three-dimensional touch controller for a video game.

Looking ahead, the team believes that it will be easy to incorporate their sensor into existing wearable materials, like clothing and athletic tapes. And that is not the end of it either — they are continuing to refine their work and see many more possible applications ahead in the future.