Salinity gradients exhibit a great potential for production of renewable energy. Several techniques such as pressure-retarded osmosis and reverse electrodialysis have been employed to extract this energy. Unfortunately, these techniques are restricted by the high costs of membranes and problems with membrane fouling. However, the expansion and contraction of hydrogels can be a new and cheaper way to harvest energy from salinity gradients since the hydrogels swell in freshwater and shrink in saltwater. We have examined the effect of cross-linker concentration and different external loads on the energy recovered for this type of energy-producing systems. Poly(allylamine hydrochloride) hydrogels were cross-linked with glutaraldehyde to produce hydrogels with excellent expansion and contraction properties. Increasing the cross-linker concentration markedly improved the energy that could be recovered from the hydrogels, especially at high external loads. A swollen hydrogel of 60 g could recover more than 1800 mJ when utilizing a high cross-linker concentration, and the maximum amount of energy produced per gram of polymer was 3.4 J/g. Although more energy is recovered at high cross-linking densities, the maximum amount of energy produced per gram of polymer is highest at an intermediate cross-linking concentration. Energy recovery was reduced when the salt concentration was increased for the low-concentration saline solution. The results illustrate that hydrogels are promising for salinity gradient energy recovery, and that optimizing the systems significantly increases the amount of energy that can be recovered.

The effects of microencapsulated phase-change materials (MPCM) on the rheological properties of a geopolymer paste (GPP) were investigated. In order to quantify the time-dependent viscosity increase of the geopolymer paste containing MPCM (GPP-MPCM), a new rheological model was successfully developed. Three different MPCMs were compared in order to examine the effect of the hygroscopic nature of the microcapsule shells and the size distribution of the microcapsules. In addition, the effect of microcapsule concentration was investigated. It was found that microcapsules with polar functional groups on the shells affect the viscosity and the geopolymerization reaction of the geopolymer paste much more than microcapsules with hydrophobic shells. In addition, aggregated microcapsules influence the viscosities less than unaggregated microcapsules.

Thermal management is important for medical ultrasound probes to maintain optimal performance, reliability, and lifetime of the devices, as well as to avoid heat-induced damage to the patients' tissue. This paper presents heat transfer simulations of the scan head of a trans-esophageal echocardiography (TEE) ultrasound probe, which operates temporarily inside the human esophagus for cardiac imaging. The current encapsulation design of the scan head requires manual assembly of several prefabricated parts to ensure functionalities such as heat spreading, electromagnetic interference (EMI) shielding, electrical isolation and biocompatibility. New encapsulation concepts that provide a more efficient manufacturing process while maintaining the multifunctional performance are desirable. The objective of this study is to screen encapsulation designs and materials which can simplify the encapsulation of the scan head. The main output to consider from the simulations is the maximum surface temperature of the scan head, which must be below 43 ºC to ensure thermal safety for patients. Two encapsulation concepts are analyzed: single-layer encapsulation and double-layer encapsulation. The simulation results show that a double-layer encapsulation, such as a polymer-coated metal encapsulation or a metallized polymer encapsulation, can fulfill the requirements regarding heat transfer, EMI shielding, electrical isolation and biocompatibility.

This paper presents a thermal study of a double-layer encapsulation for an interventional medical device, which operates temporarily inside the human esophagus for cardiac imaging. The surface temperature of test samples, representing the device, was assessed by experiments and numerical simulations. The test samples consisted of a heat source, a heat sink and a doublelayer encapsulation consisting of a 3D printed biocompatible polymer (thickness 0.9 mm), with an electroplated Cu inner layer (0, 10, 80 or 150 µm thick). The surface temperature of test samples was measured in a tissue-mimicking thermal phantom at 37°C, with different heat source power levels. Experimental results showed that the maximum steady-state surface temperature could be reduced significantly by a 10 µm thick Cu layer (compared to no Cu layer). Increasing the Cu layer thickness further had a rather small effect, at least for low power levels. The maximum steady-state surface temperature was an exponential function of the Cu layer thickness. Test samples with a Cu electroplated polymer encapsulation and a heat source power of 0.5 W satisfied the maximum temperature limit for thermal safety (43°C) when the Cu layer was thicker than about 80 µm. Simulated surface temperatures were in good agreement with experimental values, for a model using two different thermal contact conductance coefficients (for different materials) for the sample-phantom boundary condition. The simulation model was also used to suggest alternative materials for the outer layer of an encapsulation with a metal inner layer, for reducing the surface temperature.

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