Electrical conductivities and sensing mechanisms of low-temperature 3D printing conductive hydrogels with good sensitivity

Flexible strain sensors are core components of wearable strain sensors, and they have attracted much attention because of their applications for electronic skin and monitors of human activity. However, hydrogel-based strain sensors are greatly limited by the trade-offs between mechanical performance, electrical sensing properties and diverse structures. In this paper, we present a facile approach for preparing conductive hydrogels with good mechanical properties, outstanding electrical sensing properties and complex structures. First, a low-temperature 3D printer was designed and fabricated in this laboratory to prepare lignosulfonate/poly(vinyl alcohol) hydrogels with complex structures. The hydrogels with a 30° angle between adjacent layers showed the best optimal tensile properties. Then, extremely thin and dense silver layers were prepared by reducing silver on their surfaces using a two-step immersion process to obtain electrical sensing functionality. The process of preparing layers of reduced silver was carefully analyzed according to the Lamer model, and the resulting silver layers showed relatively high conductivity of 8571 S/cm. By changing the print path angles of the upper surface, hydrogels were prepared with different sensitivities and strain ranges. The maximum sensitivity and widest strain range were 177.65 and 0–55%, respectively. The corresponding hydrogels showed durable sensing performances in various tensile strain and long-term cyclic loading and relaxation tests. The sensing mechanisms of the hydrogels were the opening and closing of microcracks in the silver layers located in the print paths and the gaps between adjacent print paths. The sensors composed of conductive hydrogels successfully detected subtle vibrations in strains and various human activities, indicating their great potential for applications in monitors for human health and soft robotics.