Water-induced polymer swelling and its application in soft electronics

Research highlights• Micro- and nanoscale wrinkled structures were enabled by water-induced swelling. • Swelling is governed by osmotic pressure and constraining stress of polymer chains. • Water transport to elastomer involves Fickion diffusion and non-Fickion interaction. • Conductive materials of gold and graphene were deposited on the swollen elastomers. • The wrinkled conductors exhibited good electrical performance in strain sensing. Polymer blend system has been commonly applied in a wide variety of applications. Herein, we propose to introduce sugar particles to polymer matrix, which results in a controllable polymer swelling under the action of osmotic pressure upon soaking in water. Taking advantage of this economic and environment-friendly, water-induced polymer swelling process, we have fabricated wrinkled conductive films and 3D structures by depositing conductive materials on the swollen polymer substrates for stretchable strain sensing devices. Several commercial silicone elastomers were utilized in the study. Key processing factors affecting the polymer swelling were investigated, including film thickness, sugar concentration, and temperature of soaking medium. The swelling ratio can be regulated by changing these parameters and the soaking duration. By sputtering gold (Au) on the swollen films (through vapor deposition) or depositing graphene on 3D complex structures companied by polymer swelling (in aqueous dispersion), wrinkled conductive elastomer films and 3D structures have been fabricated. These conductors not only demonstrated high sensitivity in electrical response when subjected to external deformations from 5% up to 100%, but also possessed good stability after long-term stretching/releasing cycles. The excellent electrical properties of these elastomer conductors implied a great potential in applications of flexible and stretchable sensors.