Applications utilizing solar energy are being actively developed due to increasing demand for environmentally friendly and sustainable energy solutions. The sun is a source of tremendous energy spanning from ultraviolet to infrared wavelengths, with the maximum energy in the visible range, corresponding to a blackbody temperature of 5780 K. [ 1 ] It is estimated that the amount of solar energy reaching Earth's surface is enough to Effi cient absorption of solar radiation is desired for the renewable energy sector, such as solar thermophotovoltaics and solar thermal applications. In order to minimize thermal re-radiation, wavelength-selective devices are required. Absorbers with structured surfaces are attractive because they derive their electromagnetic properties to a greater extent from their geometry and to a lesser extent from the intrinsic properties of the constituent materials. Thus, they offer greater fl exibility in the design and control of absorber features and can be tailored to suit requirements. This article reviews various classes of patterned structures: photonic crystals, metal-dielectric-metal slab arrays, metamaterials, and nanostructures operating in the visible and infrared wavelength ranges. Operation requirements, design principles and underlying physical phenomena, material and temperature considerations, as well as fabrication methods are discussed. Recent progress in achieving various desirable absorber features, such as broadband and multiband operation, polarization and angle independence, fl exibility, and tunability is presented. Suggestions are also given regarding future research directions.
The recent use of graduated compression therapy for treatment of chronic venous disorders such as leg ulcers and oedema has led to considerable research interest in flexible and low-cost force sensors. Properly applied low pressure during compression therapy can substantially improve the treatment of chronic venous disorders. However, achievement of the recommended low pressure levels and its accurate determination in real-life conditions is still a challenge. Several thin and flexible force sensors, which can also function as pressure sensors, are commercially available, but their real-life sensing performance has not been evaluated. Moreover, no researchers have reported information on sensor performance during static and dynamic loading within the realistic test conditions required for compression therapy. This research investigated the sensing performance of five low-cost commercial pressure sensors on a human-leg-like test apparatus and presents quantitative results on the accuracy and drift behaviour of these sensors in both static and dynamic conditions required for compression therapy. Extensive experimental work on this new human-leg-like test setup demonstrated its utility for evaluating the sensors. Results showed variation in static and dynamic sensing performance, including accuracy and drift characteristics. Only one commercially available pressure sensor was found to reliably deliver accuracy of 95% and above for all three test pressure points of 30, 50 and 70 mmHg.
In this work, we characterize the electromagnetic properties of polydimethylsiloxane (PDMS) and use this as a free-standing substrate for the realization of flexible fishnet metamaterials at terahertz (THz) frequencies. Across the 0.2 to 2.5 THz band, the refractive index and absorption coefficient of PDMS are estimated as 1.55 and 0-22 cm -1 , respectively. Electromagnetic modeling, multi-layer flexible electronics microfabrication, and terahertz time-domain spectroscopy are used in the design, fabrication, and characterization of the metamaterials, respectively. The properties of PDMS adds a degree of freedom to terahertz metamaterials, with the potential for tuning by elastic deformation or integrated microfluidics.
Flexible pressure sensors are increasingly being used in medical and non-medical applications, and particularly in innovative health monitoring. Their efficacy in medical applications such as compression therapy depends on the accuracy and repeatability of their output, which in turn depend on factors such as sensor type, shape, pressure range, and conformability of the sensor to the body surface. Numerous researchers have examined the effects of sensor type and shape, but little information is available on the effect of human body parameters such as support surfaces’ curvature and the stiffness of soft tissues on pressure sensing performance. We investigated the effects of body parameters on the performance of pressure sensors using a custom-made human-leg-like test setup. Pressure sensing parameters such as accuracy, drift and repeatability were determined in both static (eight hours continuous pressure) and dynamic (10 cycles of pressure application of 30 s duration) testing conditions. The testing was performed with a focus on compression therapy application for venous leg ulcer treatments, and was conducted in a low-pressure range of 20–70 mmHg. Commercially available sensors manufactured by Peratech and Sensitronics were used under various loading conditions to determine the influence of stiffness and curvature. Flat rigid, flat soft silicone and three cylindrical silicone surfaces of radii of curvature of 3.5 cm, 5.5 cm and 6.5 cm were used as substrates under the sensors. The Peratech sensor averaged 94% accuracy for both static and dynamic measurements on all substrates; the Sensitronics sensor averaged 88% accuracy. The Peratech sensor displayed moderate variations and the Sensitronics sensor large variations in output pressure readings depending on the underlying test surface, both of which were reduced markedly by individual pressure calibration for surface type. Sensor choice and need for calibration to surface type are important considerations for their application in healthcare monitoring.
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