Highly stretchable and fatigue resistant hydrogels with low Young's modulus as transparent and flexible strain sensors

Abstract

It can be envisaged that in the future, wearable electronics will be skin-like and “unfeelable”. High stretchability (strain > 100%), skin-like Young's modulus (<25 kPa), and high conductivity under a highly stretchable state are desired for advancing electronics. In this study, a novel waterborne hexamethylene diisocyanate trimer (WHDT) and N,N′-methylene bisacrylamide (MBAA) were used as synergic crosslinkers to prepare conductive hydrogels. These hydrogels were cut resistant and highly stretchable (strain > 900%). The hydrogels could withstand over 15 000 cycles of 200% stretching–releasing at high speed. The Young's modulus was about 8 kPa. The strain sensitivity of these “skin-like” hydrogels confirmed that the relative change in electric resistance increased to 700% with 400% stretching, showing good exponential fitting with 0.9997 of R-square. Furthermore, the strain sensor could maintain high transparency with transmittance of 87%. These combined important features make these hydrogels potentially useful for transparent strain sensors, skin-mountable electrics, and flexible conductors in the fields of artificial skins and stretchable electronics.