Feeling the Squeeze

The human skin is a remarkable organ that possesses an astonishing level of pressure sensitivity. This ability serves as one of our body's most important ways of sensing the external world, allowing us to perceive and interact with our surroundings. The intricate network of mechanoreceptors embedded within the skin enables us to detect even the slightest variations in pressure, making our sense of touch incredibly sophisticated.

This exquisite pressure sensitivity does not come without a large degree of complexity, however. Specialized nerve endings, such as Merkel cells, Meissner's corpuscles, Pacinian corpuscles, and Ruffini endings, are distributed throughout the skin, each responsible for detecting different types of pressure stimuli. This intricate network allows us to perceive a wide range of sensations, from gentle caresses to firm grips.

Complexities such as these have rendered the reproduction of this level of pressure sensitivity artificially in a sensor impossible to date. To be certain, we have many pressure sensors available to us that can perform with impressive accuracy. But designing a sensor that can simultaneously detect a broad pressure range, measure with high sensitivity, and detect small changes in pressure, after already being loaded with a large amount of pressure, has remained an elusive task.

A team of researchers from Penn State University and Hebei University of Technology have recently reported on their work in which they build a novel type of pressure sensor that can do all of these things. With the rise of intelligent machines, sensors such as these are likely to become more important, giving robots the ability to interact with the world in a more human-like way. This technology also offers the promise of allowing for the development of new types of human-computer interaction devices.

The sensors are composed of gradient pyramidal microstructures fabricated using a CO2 laser with a Gaussian beam profile. Producing them in this way is more cost-effective and less complex than other competing technologies, like photolithography. The shapes and heights of the microstructures were adjusted until they were capable of providing even deformation as pressure increases. The layer of microstructures was then combined with an ultrathin layer of iontronic liquid to enhance the electric field, which gives the device both high sensitivity and a wide linear sensing range, which traditional sensors are not capable of.

The present design allows the sensor to detect an ultralow pressure of 0.36 Pa. And with a large pressure of 2 MPa already loaded on the sensor, it can recognize tiny pressure variations of 145 Pa. One of the researchers described this capability as being akin to detecting a fly that lands on the back of an elephant.

To demonstrate some of the unique characteristics of this device, a number of experiments were conducted. In one, the sensor was leveraged to detect the subtle movements of the fingertip to measure a person’s pulse. In another demonstration showing the wide range of detectable pressures, the device was used in building a smart weight scale chair that can measure the weight of a person. An interactive robot hand was also shown off that can detect objects, and also aid in grasping those objects with an appropriate amount of force.

At present, the team is exploring ways to further enhance the performance of their pressure sensor. They believe that they can improve the sensitivity and sensing range by increasing the concentration of iontronic liquids and using larger pyramids, respectively. Further, they believe that they may even be able to detect more subtle pressures by introducing an air gap between the electrode and the dielectric layer.

This novel sensor's unique properties, in conjunction with the scalable manufacturing method employed and the low production cost could mean that we will be seeing iontronic pressure sensors put to work in all sorts of applications in the near future.