Crisscross-designed piezoresistive strain sensors with a cracked microtectonic architecture for direction-selective tensile perception

Flexible strain sensors are a key component of electronic skin (e-skin), a technology that is currently receiving considerable research attention with a view to future applications ranging from human healthcare monitoring to robotic skins and environmental risk detection. Here, we developed a highly sensitive, simple, and low-cost piezoresistive strain sensor, which acted as a flexible reactive resistor with a cracked microtectonic architecture that could be fabricated over a large area. In particular, our strain sensor recognizes the direction of tensile stimulation through its rational crisscross electrode design, allowing it to overcome some of the shortcomings of traditional flexible strain sensors. Under a given stress, the strain sensor developed here showed a variation in the relative resistance (ΔR/R0) of up to 24-fold depending on the direction of the applied stress. For example, application of a 1% strain changed ΔR/R0 by 0.11 when the strain was applied parallel to the direction of current flow, but by only 0.012 when the strain was applied perpendicular to that direction. Similarly, a 5% strain changed ΔR/R0 by 0.85 and 0.062, and a 20% strain changed ΔR/R0 by 2.37 and 0.098, depending on whether the strain was applied parallel or perpendicular to the current flow, respectively. In addition, ΔR/R0 varied approximately linearly as a function of the strain over the operational range. The results thus show that the proposed sensor is sensitive to the direction in which an external stress is applied. Finally, we demonstrated that our sensor could be used to detect the bending of a human finger.