Micro-Actuated Tunable Hierarchical Silver Nanostructures to Measure Tensile Force for Biomedical Wearable Sensing Applications.

Commercially available biomedical wearable sensors to measure tensile force/strain still struggle with miniaturization in terms of weight, size, and conformability. Flexible and epidermal electronic devices have been utilized in these applications to overcome these issues. However, current sensors still require a power supply and some form of powered data transfer, which present challenges to miniaturization and to applications. Here, we report on the development of flexible, passive (thus zero power consumption), and biocompatible nanostructured photonic devices that can measure tensile strain in real time by providing an optical readout instead of an electronic readout. Hierarchical silver (Ag) nanostructures in various thicknesses of 20-60 nm were fabricated and embedded on a stretchable substrate using e-beam lithography and a low-temperature dewetting process. The hierarchical Ag nanostructures offer more design flexibility through a two-level design approach. A tensional force applied in one lateral ( or ) direction of the stretchable substrate causes a Poisson contraction in the other, and as a result, a shift in the reflected light of the nanostructures. A clear blue shift of more than 100 nm in peak reflectance in the visible spectrum was observed in the reflected color, making the devices applicable in a variety of biomedical photonic sensing applications.