Gecko-Inspired Slant Hierarchical Microstructure-Based Ultrasensitive Iontronic Pressure Sensor for Intelligent Interaction

Abstract

Highly sensitive flexible pressure sensors play an important role to ensure the safety and friendliness during the human-robot interaction process. Microengineering the active layer has been shown to improve performance of pressure sensors. However, the current structural strategy almost relying on axial compression deformation suffers structural stiffening, and together with the limited area growth efficiency of conformal interface, essentially limiting the maximum sensitivity. Here, inspired by the interface contact behavior of gecko's feet, we design a slant hierarchical microstructure to act as an electrode contacting with an ionic gel layer, fundamentally eliminating the pressure resistance and maximizing functional interface expansion to achieving ultrasensitive sensitivity. Such a structuring strategy dramatically improves the relative capacitance change both in the low- and high-pressure region, thereby boosting the sensitivity up to 36000 kPa^-1 and effective measurement range up to 300 kPa. To verify the advantages of high sensitivity, the sensor is integrated with a soft magnetic robot to demonstrate a biomimetic Venus flytrap. The ability to perceive weak stimuli allows the sensor to be used as a sensory and feedback window, realizing the capture of small live insects and the transportation of fragile objects.

Figure 1

Design of the iontronic capacitive pressure sensor. (a) Gecko's feet consisting of slant scales (scale bar, 500 μm) and setae (scale bar, 100 μm) to increase the adhesive surface. (b) Schematic illustration of the iontronic sensor with slant hierarchically structured electrode and ionic gel layer. (c) Equivalent electrical circuit of the sensor. (d) Manufacturing process of the slant hierarchical electrode. (e) SEM images of the structured electrode, the scale bar is 25 μm. (f) Bending enhancement of slant scales and compression enhancement of beads on contact area change at the interface. (g) Comparison of capacitance response of three different microstructures, including slant hierarchical scale, upright bead, and slant flat scale.

Figure 2

Mechanism of contact area growth. (a) The proportional relationship between capacitance brought by EDLs and the contact interface area. (b) Contact process of different structures with increasing pressure, including upright beads, slant scales, and slant hierarchical scales, the top two scale bars are 5 μm and the bottom two scale bars are 10 μm. (c) Numerical simulation results of the contact area change and the displacement of the upper electrode as functions of pressure. (d) SEM images showing the deformation process of the slant hierarchical microstructures under different pressures, the scale bars are 25 μm. (e) Elemental analysis of different domains on the hierarchical scales to determine the contact state with ionic gel.

Figure 3

Typical characteristics of the pressure sensor. (a) Capacitance response in pressure region from 0.1 Pa to 53 kPa. (b) Stepped increase associated with the accumulation process of five dandelion seeds on the sensor. (c) Comparison of the maximum sensitivity and LOD between the proposed sensor and capacitive pressure sensors reported in the literature. (d) Repeatable response under periodical pressures of 1.5 kPa, 2 kPa, 8 kPa, 14.5 kPa, and 30 kPa. (e) Durability test results after 5000 loading and unloading cycles at a pressure of 10 kPa, showing no degradation in the relative capacitance change. (f) The stable capacitance response under pressure of 10 kPa in the environment with the humidity of 30%, 45%, 60%, and 75%. (g) Capacitance response of the sensor to sound waves of 80 dB generated form a conventional speaker. (h) Human pulse signal curves tested by the sensor attached to the wrist. (i) Detection of micropressure under low and high pressures. (j) Capacitance response to a fallen water droplet of 10 μL, with the insets depicting the water droplet falling process.

Figure 4

Intelligent autonomous system using the sensor as the sensory and feedback window. (a) Images of the Venus flytrap and the leaf closure triggered by mechanical stimuli. (b) Schematic diagram of the biomimetic Venus flytrap sensing the stimuli and closing the trap. (c) Self-built experimental flying object automatic capture device. (d) Capacitance response used as the feedback signal for automatically capturing objects. (e) Biomimetic Venus flytrap passively capturing a dandelion and a bee, respectively, that fall on its surface. (f) Schematic diagram of the active intelligent grabbing system. (g) Capacitance and force change during the process of the gripper successfully grabbing a quail egg without any damage, inset showing the nondestructive grabbing of quail egg under ultra-sensitive feedback compared with that without sensitive feedback. (h) Showcase of grabbing process.