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JUNE 2019 I DESIGN007 MAGAZINE 63 of humidity to enable the resin to cure. High ambient temperatures can reduce the shelf life of certain resins, and where the resin has a high percentage of fillers, these conditions can also cause filler sedimentation. Low temperatures bring their own set of problems. When the temperature drops below 10°C for a long period, certain resins and hard- eners begin to crystallise, and the crystals drop out of solution. Many materials can be heated so that the crystals are dissolved back into so- lution, but some cannot be treated in this way. Again, it is important to consult the material's technical data sheet to ascertain the resin's op- timum storage conditions. Always consider what rules and regulations are valid in various regions of the world and the preferences of the different industries that are to be served in these regions. The choice of resin chemistry is very likely to vary, such as for health and safety or other environmental regulatory requirements. In some areas, epox- ies will be preferred over polyurethanes, while in others, silicone resins would simply be a non-starter. Resins play an integral part in ensuring that an electronics product can survive for at least its design life and often well beyond. Be it chemical, environmental, physical, or thermal, whatever the conditions, there is a resin sys- tem that can be found or developed to provide the protection required. DESIGN007 Alistair Little is global business/ technical director—resins—at Electrolube. To read past col- umns from Electrolube, click here. Also, visit to download your free copy of Elec- trolube's book, The Printed Circuit Assembler's Guide to… Conformal Coatings for Harsh Environments, as well as other free, educational titles. Wearable devices that harvest energy from movement are not a new idea, but a material created at Rice Univer- sity may make them more practical. The Rice lab of chemist James Tour has adapted laser- induced graphene (LIG) into small, metal-free devices that generate electricity. Like rubbing a balloon on hair, putting LIG composites in contact with other surfaces produces static electricity that can be used to power devices. In experiments, the researchers connected a folded strip of LIG to a string of light-emitting diodes and found that tapping the strip produced enough energy to make them flash. A larger piece of LIG embedded within a flip- flop let a wearer generate energy with every step, as the graphene composite's repeated contact with skin pro- duced a current to charge a small capacitor. The lab turned polyimide, cork and other materials into LIG electrodes to see how well they produced energy and stood up to wear and tear. They got the best results from materials on the opposite ends of the triboelectric series, which quantifies their ability to generate static charge by contact electrification. In the folding configuration, LIG from the tribo-negative polyimide was sprayed with a protective coating of poly- urethane, which also served as a tribo-positive material. When the electrodes were brought together, electrons transferred to the polyimide from the polyurethane. The best configuration, with electrodes of the poly- imide-LIG composite and aluminum, produced voltag- es above 3.5 kilovolts with a peak power of more than 8 milliwatts. (Source: Rice University) Flexible Generators Turn Movement Into Energy

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