Measurements of physiological parameters such as pulse rate, voice, and motion for precise health care monitoring requires highly sensitive sensors. Flexible strain gauges are useful sensors that can be used in human health care devices. In this study, we propose a crack-based strain gauge fabricated by fused deposition modeling (FDM)-based three-dimensional (3D)-printing. The strain gauge combined a 3D-printed thermoplastic polyurethane layer and a platinum layer as the flexible substrate and conductive layer, respectively. Through a layer-by-layer deposition process, self-aligned crack arrays were easily formed along the groove patterns resulting from stress concentration during stretching motions. Strain gauges with a 200-µm printing thickness exhibited the most sensitive performance (~442% increase in gauge factor compared with that of a flat sensor) and the fastest recovery time (~99% decrease in recovery time compared with that of a flat sensor). In addition, 500 cycling tests were conducted to demonstrate the reliability of the sensor. Finally, various applications of the strain gauge as wearable devices used to monitor human health and motion were demonstrated. These results support the facile fabrication of sensitive strain gauges for the development of smart devices by additive manufacturing.
Micro-drilling devices with different blade shapes were fabricated with a rapid and facile manufacturing process using three-dimensional (3D) printing technology. The 3D-printed casting mold was utilized to customize the continuous shape of the blades without the need for expensive manufacturing tools. A computational fluid dynamics simulation was performed to estimate the pressure differences (fluidic resistance) around each rotating device in a flowing stream. Three types of blades (i.e., 45°, 0°, and helical type) were manufactured and compared to a device without blades (i.e., plain type). As a result, the device with the 45° blades exhibited the best drilling performance. At a rotational speed of 1000 rpm, the average drilling depth of the device with the 45° blades to penetrate artificial thrombus for 90 s was 3.64 mm, which was ~ 2.4 times longer than that of helical blades (1.51 mm). This study demonstrates the feasibility of using 3D printing to fabricate microscale drilling devices with sharp blades for various applications, such as in vivo microsurgery and clogged water supply tube maintenance.
In this study, an artificial compound eye lens (ACEL) was fabricated using a laser cutting machine and polyvinyl alcohol (PVA) solution. A laser cutter was used to punch micro-sized holes (500 μm diameter—the smallest possible diameter) into an acrylic plate; this punched plate was then placed on the aqueous PVA solution, and the water was evaporated. The plate was used as the mold to obtain a polydimethylsiloxane (PDMS) micro lens array film, which was fixed to a dome-shaped three-dimensional-printed mold for further PDMS curing, and a hemispherical compound eye lens was obtained. Using a gallium nitride (GaN) photodetector, a light detection experiment was performed with the ACEL, bare lens, and no lens by irradiating light at various angles under low temperatures. The photodetector with the ACEL generated a high photocurrent under several conditions. In particular, when the light was irradiated at 0° and below -20 °C, the photocurrent of the GaN sensor with the ACEL increased by 61% and 81% compared with the photocurrent of GaN sensor with the bare lens and without a lens, respectively. In this study, a sensor for detecting light with ACEL was demonstrated in low-temperature environments, such as indoor refrigerated storages and external conditions in Antarctica and Arctic.
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