We present the development of a flexible bimodal sensor using a paper platform and inkjet printing method, which are suited for low-cost fabrication processes and realization of flexible devices. In this study, we employed a vertically stacked bimodal device architecture in which a temperature sensor is stacked on top of a pressure sensor and operated on different principles, allowing the minimization of interference effects. For the temperature sensor placed in the top layer, we used the thermoelectric effect and formed a closed-loop thermocouple composed of two different printable inks (conductive PEDOT:PSS and silver nanoparticles on a flexible paper platform) and obtained temperature-sensing capability over a wide range (150 °C). For the pressure sensor positioned in the bottom layer, we used microdimensional pyramid-structured poly(dimethylsiloxane) coated with multiwall carbon nanotube conducting ink. Our pressure sensor exhibits a high-pressure sensitivity over a wide range (100 Pa to 5 kPa) and high-endurance characteristics of 10. Our 5 × 5 bimodal sensor array demonstrates negligible interference, high-speed responsivity, and robust sensing characteristics. We believe that the material, process, two-terminal device, and integration scheme developed in this study have a great value that can be widely applied to electronic skin.
The printing and modular fabrication of a paper-based active microfluidic lab on a chip implemented with electrochemical sensors (ECSs) is developed and integrated on a portable electrical control system. The electrodes of a chip plate for active electrowetting actuation of digital drops and an ECS for multiple analysis assays are fabricated by affordable printing techniques. For enhanced sensitivity of the sensor, the working electrode is modified through the electrochemical method, namely by reducing graphene with voltammetry and coating gold nanoparticles by amperometry. Detachable sensor and absorber modules are assembled modularly on an open chip plate, forming various novel hybridized open-closed chip formats. By varying the coupled or decoupled sensor modules, excellent detection of three diagnostic biological molecules is demonstrated (glucose, dopamine, and uric acid in human serum). With a newly designed portable power supply and wireless control system, the active paper-based chip platform can be utilized as an advanced point-of-care device for multiple assays in digital microfluidics.
A simple programmable contact printing method using ballpoint pens with silver nanoparticle (AgNP) and carbon nanotube (CNT) ink and a digital plotter were developed for quick patterning of electrodes on paper. This printing method enables sequential and programmable printing with two different inks and with ink consisting of high viscosity materials and is amenable to reproducibility of printed electrodes and customized designs. With this printing method, AgNP and CNT patterns with low electrical resistance and high density of the material can be printed. Using these AgNP and CNT inks, we fabricated disposable electrochemical sensors (ECSs) on paper. The ECSs were successfully used to detect glucose at various concentrations from 0 to 15 mM. The characteristics of the printed AgNP and CNT patterns, such as the printing resolution, surface morphology, and electrical properties, were also studied. The proposed contact printing method opens an avenue for printing paper-based electronics and devices.
The effects of an immiscible, lubricating polydimethylsiloxane fluid, referred to as silicone oil, on the static deformation and on the dynamic motion of a water drop on paper induced by electrowetting were investigated. The deformation of a drop on a hydrophobic film of amorphous fluoropolymers top-coated with less hydrophobic silicone oil was much more predictable, reversible and reproducible than on the uncoated surface. In the dynamic tribological experiment for a sliding drop along an inclined surface, a significant decrease in the friction coefficient, with an unexpected dependency of the contact area, was observed. Based on the curve fitting analysis, the shear stress and the net friction force were estimated quantitatively. Because of the tribological effect and the reduced shear friction force of the oil film, the static and the dynamic electrowetting states of the water drop were enhanced.
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