Morphing out-of-surface channels enable strain-based control over fluid flow in skin-mountable patches

The volume of natural materials increases under tension, thus conventionally biomechanical actuation of fluidic pumps relies on compression for pressure generation. Here, we report on out-of-surface microchannels (OSMiCs) that exhibit negative volumetric strain (i.e., pressure generation) under skin-induced tensile strain. Monolithic polydimethylsiloxane (PDMS) patches were fabricated and characterized. The complex relations between the wrinkling and buckling of the OSMiC shell and the fluid flow patterns were investigated. OSMiCs were shown to snap-back (-through) between two stable states that lead to (ir)reversible fluid flow depending on their architecture. Unlike standard microchannels that only generate pressure symmetrically upon application and release of tensile strain, OSMiCs are shown to be tunable for providing an asymmetrical pressure owing to their shape-change property (i.e., morphing). The maximum forward (backward) flow pressure of 10 (-14) kPa was measured upon 20% uniaxial strain application (release). The versatile fabrication technique allowed the integration of OSMiCs with different Q values, leading to a discrete strain-actuated flow control element. Numerical simulations were conducted and shown to support the experimentally observed wrinkling and buckling behavior. Finally, the operation of the power-free OSMiC skin patch for strain-based liquid administration on skin was demonstrated.