Multifunctional Hydrogel Sensor with Curved Macro Cracks: A Strategy for High Sensitivity and Wide Detection Range

Multi-sensing in simple devices, but with a high sensitivity and a large detection range, is desirable for soft machines. Stretchable sensors based on the resistance changes of bulk ionic hydrogels are inherently limited by the single function and low sensitivity at small deformations. Here, a design enabled by a highly cracked hydrogel (HCHG) that is hypersensitive to tensile strain, bending, and tactile force in a wide range is proposed. The mechanism relies on the continuous sharp changes of the cross-sectional area flowing ionic current when pre-cut curved cracks are closed/opened by external load. The high fracture toughness of the hydrogel inhibits the crack propagation, making the sensing robust. By designing the crack patterns, sensitivities of 80 for 0-20% tensile strain and 0.45 kPa(-1) for tactile force are achieved. Compared to the sensor made from bulk hydrogel, the sensitivities are enhanced by two and three orders of magnitude, respectively, meanwhile the detectable strain range is maintained (up to 215%). A sandwich design is also developed to distinguish elongation, compression, and bending. Applications of HCHG sensors in manipulating a robotic arm and nondestructive grasping an even softer object by a soft gripper are demonstrated.