Strain-sensitive alginate/polyvinyl alcohol composite hydrogels with Janus hierarchy and conductivity mediated by tannic acid.

To construct conductive hydrogels with a conductive and a non- or weakly conductive layer for comfortable and safe electronic application, marine biobased anionic polysaccharide sodium alginate (SA) and neutral polyvinyl alcohol (PVA) were employed as the hydrogel matrixes. Tannic acid (TA) was exploited to mediate the demixing of the miscible aqueous solution of SA and PVA in view of the much larger interaction strength of TA with PVA than both of TA with SA and PVA with SA calculated from the density functional theory (-40.21, -29.77 and -21.00 kcal·mol-1 respectively). The finally-fabricated alginate/PVA composite hydrogels not only possess a "Janus" hierarchy but manifest asymmetrical conductivity, i.e., one layer strongly conductive and another weakly conductive. The strongly conductive layer achieves a conductivity of more than 2.95 S·m-1, facilitating their application in soft electronic areas like human-machine interfaces, smart wearable devices and soft robots. The weakly conductive layer with the conductivity less than 0.60 S·m-1 and the thickness adjustable, constitutes a protective screen for another layer. The Janus hydrogels exhibit good mechanical performance, excellent strain-sensing performance and fatigue-resistant mechanics, conductivity and sensitivity.