Pressure sensors have been widely utilized to detect physical contacts or physiological signals in human-machine interfaces of many stretchable systems, such as wearable devices or electronic skin. Among the various types of pressure sensors, piezoresistive type pressure sensors have many advantages of a simple fabrication method and a read-out circuit appropriate for large-area, scalable manufacture. However, when the tensile stress is applied to the sensing materials of piezoresistive sensors, they are vertically contracted by the Poisson effect, and the resistance characteristics change immediately. Although the sensor properties affected by strain are a significant problem in this deformable system, there have been few studies on the strain characteristics of pressure sensors. Some studies adopted micro-pyramid structures that are less affected by strain but have issues in the complex fabrication process. To get strain insensitivity and take advantage of the existing piezoresistive pressure sensor, we found a way to control the tensile stress through a simple additional structure to the existing pressure sensor.
Here, we present a strain-insensitive pressure sensor array based on a rigid island array designed for strain engineering. Our pressure sensor is composed of micro-sized nickel (Ni) particles embedded in a polydimethylsiloxane (PDMS) composite. By applying a strong magnetic field (>0.1 T) in the z-direction, Ni, the ferromagnetically conductive particles, are vertically aligned, and conduction paths are formed. Using the magnetic field modulator in the form of an iron pillar array, we can pattern the sensor array in the shape of a magnetic field modulator. The sensing layer shows stable pressure characteristics during 0-373 kPa, covering the entire pressure range of human activities. Then we add inkjet-printed rigid islands on the pixels. Because Young's modulus ratio of the rigid island and PDMS is large enough (>1000), most of the stress applied to the sensor is concentrated on the soft part outside the islands. Therefore, the tensile strain does not affect the sensing pixels, resulting in the strain-insensitivity of the whole sensor array. We simulated and experimented under various conditions of strain-engineering structures, optimizing the sensor to effectively protect the pixels without overloading the soft part.
