Micro ônibus Proton Bus Download
Fajar Roux
The piezo-resistive pressure sensor is based on the fact that the resistance of the material changes with the applied pressure. The resistance change of the material can be expressed by the equation ΔR/R = (1 + 2ν)ε + (Δρ/ρ), where (1 + 2ν), ε, and (Δρ/ρ) are geometric effect term, strain by pressure, and resistivity effect term, respectively. That is, the change in resistance originates from a change in dimension by pressure. The pressure sensor using this mechanism is very simple to manufacture, and it has been studied extensively because it can detect a wide range of pressures. Usually, conductive fillers (reduced graphene oxides (rGO), carbon nanotubes (CNTs), or metal particles) are added to the elastomer to produce materials with piezoresistive properties, but these sensors have low sensitivity. To solve this problem, studies have been conducted to increase the sensitivity using a microstructure or a porous structure. Park et al. introduced stretchable pressure sensors using the unique geometry of interlocked microdome structures [45,73]. These pressure sensors can be attached to the skin and distinguish a variety of mechanical stimuli. However, the fabrication method requires the use of an Si mold, which is complicated, expensive, and has poor reproducibility. Therefore, Jung et al. developed a wearable piezoresistive pressure sensor by creating pores in a conventional pressure-sensitive rubber (PSR) [39]. Sensors based on these methods have greater sensitivity than conventional PSR-based sensors. This wearable porous PSR pressure sensor successfully conforms to the skin for various applications, such as human-machine interfaces, healthcare monitoring, and radio control of robots (Figure 2c). In addition to the aforementioned pressure sensors based on the microstructure and porous structures, piezoresistive pressure sensors based on other materials also have been developed [36,74]. Gong et al. reported a flexible pressure sensor with high sensitivity by interposing the gold nanowires (AuNWs) between two PDMS films [36]. The AuNW-based pressure sensors provide real-time monitoring of blood pressure with high sensitivity and flexibility, as shown in Figure 2d,e.
Wearable strain sensors. (a) Schematic illustration of the cross-section of the strain sensor consisting of the three-layer stacked nano hybrid structure of polyurethane-poly(3,4-ethylenedioxythiophene) polystyrenesulfonate (PU-PEDOT:PSS)/single-wall carbon nanotube (SWCNT)/PU-PEDOT:PSS on a polydimethylsiloxane (PDMS) substrate. (b) Time-dependent ΔR/R0 responses of the sensor attached to the forehead when the subject was crying. Reprinted with permission from Ref. [34]. Copyright 2015, American Chemical Society. (c) Optical micrograph of a graphene woven fabrics (GWFs)-PDMS-tape composite film (scale bar 0.1 mm). (d) Relative change of resistance between 0% and 0.2% strain. Reprinted with permission from Ref. [86]. Copyright 2014, John Wiley and Sons. (e) Schematic illustration of stretchable capacitor with transparent electrode (top) and photograph of the same device reversibly adhered to a backlit liquid-crystal display (bottom) (scale bar 1 cm). (f) Change in capacitance ΔC/C0 versus strain ε (top) and ΔC/C0 versus time t over four cycles of stretching (bottom). Reprinted with permission from Ref. [69]. Copyright 2011, Nature Publishing Group. (g) Schematic image of multicore-shell printing process for fiber-type capacitive strain sensor. (h) Normalized decay time output of the sensor for different walking speeds up to 4 mph. Reprinted with permission from Ref. [92]. Copyright 2015, John Wiley and Sons.
An electrochemical gas sensor operates by reacting with gas molecules and creating an electrical signal. This sensor consists of a sensing electrode and a counter electrode that are separated by an electrolyte. When the target gas diffuses into the sensors, through the porous membrane to the working electrodes where it is oxidized or reduced. This electrochemical reaction generates an electric current flow between the sensing and counter electrodes. Electrochemical gas sensors are among the most promising approaches for wearable devices due to their low power consumption, low cost, high sensitivity, and high selectivity. Also, they can be operated easily by using a single microelectronics chip [116]. However, conventional electrochemical sensors have limited lifetimes because they use volatile electrolytes. To overcome this limitation, many researchers have replaced the volatile electrolytes with room temperature ionic liquids (RTILs). Unlike conventional electrolytes, RTILs are non-volatile, and they have negligible vapor pressure and high thermal stability. Mu et al. demonstrated a flexible electrochemical oxygen (O2) gas sensor using an RTIL as the electrolyte with a porous polytetrafluoroethylene (PTFE) substrate [117]. The porous structure of the PTFE substrate is suitable for an electrochemical gas sensor membrane due to its outstanding chemical resistance, flexibility, and gas permeability. Figure 4e shows a micro-fabricated, flexible, RTIL-based gas sensor. An O2 gas sensor can respond linearly to O2 gas concentrations ranging from 0 to 21%, and Figure 4f shows that the sensor recovered fully from these responses when the O2 gas was removed.
Wearable biosensors (a) The images of the temporary transfer-tattoo biosensor attached on deltoid. (b) Amperometric response as a function of lactate concentration for the sensor at 37 C. Inset: profiles of current at different lactate concentrations. Reprinted with permission from [136]. Copyright 2013, American Chemical Society. (c) Conceptual images of conformally contacted devices on an artificial eye for glucose sensing in tears are shown. Thin-film sensors remained in contact with skin even during tension and relaxation (scale bars 10 mm). (d) Scanning electron microscope image of a representative device (thickness of 1.7 μm) on an artificial PDMS skin replica indicating conformal contact between the device and the substrate (scale bar 500 μm). (e) Representative responses of In2O3 sensors to physiologically relevant d-glucose concentrations found in human diabetic tears (lower range) and blood (upper range). Inset: data from five devices. Error bars represent standard deviations of the means. Reprinted with permission from Ref. [40]. Copyright 2013, American Chemical Society. (f) The preparation route of the 4-boronobenzaldehyde (4-BBA)-modified poly(vinyl alcohol) (PVA) gelated colloidal crystal array (GCCA)-lens. (g) The diffraction response at low glucose concentration. Insert: the photograph of the GCCA-lens sample. Reprinted with permission from Ref. [144]. Copyright 2017, MDPI AG.
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