Detailed Studies for Sensors
1. Strain Sensors
Flexible strain sensors are a key component of electronic skin (e-skin), a technology that is currently receiving considerable research attention with a view to future applications ranging from human healthcare monitoring to robotic skins and environmental risk detection. Here, we developed a highly sensitive, simple, and low-cost piezoresistive strain sensor, which acted as a flexible reactive resistor with a cracked microtectonic architecture that could be fabricated over a large area. In particular, our strain sensor recognizes the direction of tensile stimulation through its rational crisscross electrode design, allowing it to overcome some of the shortcomings of traditional flexible strain sensors. Under a given stress, the strain sensor developed here showed a variation in the relative resistance (ΔR/R0) of up to 24-fold depending on the direction of the applied stress. For example, application of a 1% strain changed ΔR/R0 by 0.11 when the strain was applied parallel to the direction of current flow, but by only 0.012 when the strain was applied perpendicular to that direction. Similarly, a 5% strain changed ΔR/R0 by 0.85 and 0.062, and a 20% strain changed ΔR/R0 by 2.37 and 0.098, depending on whether the strain was applied parallel or perpendicular to the current flow, respectively. In addition, ΔR/R0 varied approximately linearly as a function of the strain over the operational range. The results thus show that the proposed sensor is sensitive to the direction in which an external stress is applied. Finally, we demonstrated that our sensor could be used to detect the bending of a human finger.
Flexible pressure sensors are a key component of electronic skin (e-skin) for use in future applications ranging from human healthcare monitoring to robotic skins and environmental risk detection. Here, we demonstrated the development of a highly sensitive, simple, and low-cost capacitive pressure sensor, which acted as a flexible capacitive dielectric, based on a microstructured elastomeric template that could be fabricated over a large area. To achieve this goal, the dielectric template was prepared simply by stretching and releasing a flexible Ecoflex film to produce wrinkled surface microstructures with a feature size on the order of tens of micrometers. The effects of the wrinkled surface microstructure on the sensing performance were systematically investigated by comparing the nonwrinkled film, one-side wrinkled film, and double-side wrinkled film. The response and release times of the double-side wrinkled pressure sensor were improved by 42% and 25% in comparison with the values obtained from the unwrinkled case, respectively. These results showed that the introduction of wrinkled surface microstructures to the elastomeric template efficiently enhanced the pressure sensor performance. We also demonstrated that our sensor could be used to detect a variety of changes in the surroundings, such as variations in the angle of a stimulus, object loading/unloading, or an exhaled breath.
2. Pressure Sensors
3. Chemical Sensors
A person's sweat contains various ingredients, and the composition of sweat changes according to a person's health condition. Lactic acid, one of the components of sweat, is a chemical component that is an indicator of fatigue. Lactic acid is produced in cells during anaerobic exercise, which is accumulated in the muscles when fatigued, and causes fatigue in the body. In addition, lactic acid can be a good indicator of pathological disorders. Lack of oxygen in the blood, especially when the sweat gland cells produce lactic acid, the concentration of lactic acid in the sweat increases. This means that a disease that causes hypoxia can be diagnosed by detecting the concentration of lactic acid. Other studies have shown that lactic acid is a good indicator of reduced oxygen delivery in tissues from patients with peripheral arterial occlusive disease. Therefore, sensors that detect lactic acid may be required not only for personal health care, but also for specialized medical applications.
In the present study, we use transistors with a special structure called an interference gate. The interference gate is an electrode located on the dielectric. The sensing principle of this particular device acts as the sensing area of transistor-based sensors and is related to the threshold voltage shift by affecting the transfer characteristics of the transistor depending on the charge applied to the surface. This structure has the advantage of requiring a small amount of sample for detection without requiring a separate reference electrode. Carbon nanotube (CNT) was used as a sensing material in the interference gate sensing area. CNTs have functional groups that can chemically react with other materials such as carboxyl groups. Lactate oxidase (LOD) and peroxidase from horseradish were chemically combined with CNT-COOH to detect lactic acid. LOD oxidizes lactic acid to produce hydrogen peroxide (H2O2), and Peroxidase from horseradish (HRP) causes oxidation-reduction reaction while decomposing H2O2 to enable sensor detection.