**Ultrasensitive Wearable Strain Sensors of 3D Printing Tough and Conductive Hydrogels**

**Jilong Wang 1, Yan Liu 1, Siheng Su 2, Junhua Wei 3, Syed Ehsanur Rahman 3, Fuda Ning 4, Gordon Christopher 3, Weilong Cong 5 and Jingjing Qiu 3,\***


Received: 15 September 2019; Accepted: 11 November 2019; Published: 13 November 2019

**Abstract:** In this study, tough and conductive hydrogels were printed by 3D printing method. The combination of thermo-responsive agar and ionic-responsive alginate can highly improve the shape fidelity. With addition of agar, ink viscosity was enhanced, further improving its rheological characteristics for a precise printing. After printing, the printed construct was cured via free radical polymerization, and alginate was crosslinked by calcium ions. Most importantly, with calcium crosslinking of alginate, mechanical properties of 3D printed hydrogels are greatly improved. Furthermore, these 3D printed hydrogels can serve as ionic conductors, because hydrogels contain large amounts of water that dissolve excess calcium ions. A wearable resistive strain sensor that can quickly and precisely detect human motions like finger bending was fabricated by a 3D printed hydrogel film. These results demonstrate that the conductive, transparent, and stretchable hydrogels are promising candidates as soft wearable electronics for healthcare, robotics and entertainment.

**Keywords:** hydrogels; 3D printing; tough; sensor
