Introduction: Film-type biosensors that can be applied to the skin and organs of living organisms in a stress-free manner are a key technology for accelerating personalized and tailor-made medicine. Herein, we have developed a metal-based stretchable conductor with a two-dimensional array of micrometer-scale fine sinusoidal structures. The conductor is stretchable and conductive, has a simple and low-cost fabrication, and enables desired wiring patterns. In this study, the electrical properties of the fabricated conductor were evaluated under stretching and bending loads. The conductor was then applied to detect pulse waves at the fingertip.
Materials and
Methods: Three-dimensional sinusoidal waveforms with various wavelengths (500–2000 μm) and amplitudes (50–200 μm) were microfabricated on an aluminum substrate using a horizontal machining center (Figure a). The structures were transferred to a dimethylpolysiloxane (PDMS) sheet and sputtered with Au through a conductive pattern mask (Figure b). The conductors were stretched, and their sheet resistances were measured with a milliohm meter using the four-probe method. Similarly, the conductor was then mounted on aluminum cylinders with curvatures of 0–200 m^−1 and the sheet resistances were measured. Microcracks caused in the conductor under stretching stress were visualized using a field-emission scanning electron microscope. Light-emitting diodes (LEDs) and photodiodes were connected to the conductor on a PDMS film for fingertip-pulse wave detection, as shown in Figure (c). The LED light transmitted through the fingertip was photoelectrically converted by a photodiode. The resulting signal was amplified through an operational amplifier and acquired using an oscilloscope.
Results, Conclusions, and Discussions: The sheet resistances of the sinusoidal conductors were 1.6 Ω/sq at a wavelength of 500 µm and an amplitude of 200 µm under 10% stretch loading. For the microfabricated conductors, conductivity was maintained at over 30% stretch loading, while the flat conductors broke under 1% stretch load. Owing to its uniquely structured two-dimensional array of sinusoidal waves, the conductor maintained its current flow even when stretched in any direction or in multiple directions simultaneously. Electron microscopy observations revealed small cracks in the valleys of the sinusoidal structure under stretch loading, which decreased conductivity. Therefore, crack control would further improve the conductivity of the proposed conductor. The LED–photodiode pair connected to the sinusoidal conductor detected pulse waves at the fingertip with a high signal-to-noise ratio (Figure d). This highly conductive and stretchable sinusoidal conductor is composed of biocompatible materials and can withstand 2000 cycles of mechanical stretching. When applied to sensors, the conductor is expected to realize the stress-free physiological monitoring of body surfaces and organs such as the heart.
Acknowledgements (Optional): This work was partially supported by AMED under Grant Number JP24ym0126813 (H500TR) and LIP Yokohama Trial Grant 2023 from Kihara Memorial Yokohama Foundation for the Advancement of Life Sciences.
