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Polymer in smartphones: sensors to meld with human body, screen coating with energy-harvesting material

Polymer in smartphones: sensors to meld with human body, screen coating with energy-harvesting material

Microscopic sensors and motors in smart phones detect movement, and could one day help their cameras focus. Scientists have devised components for these machines that are compatible with the human body, potentially making them ideal for use in medical devices such as bionic limbs and other artificial body parts. The technology is called microelectromechanical systems (MEMS), and involves parts less than 100 microns wide, the average diameter of a human hair. For example, the accelerometer that tells a smart phone if its screen is being held vertically or horizontally is a MEMS sensor; it convert signals from the phone's environment, such as its movement, into electrical impulses. MEMS actuators work in the opposite way, by converting electrical signals into movement. MEMS are typically produced from silicon. But now researchers have devised a way to print highly flexible parts for these micro-machines from a rubbery, organic polymer more suitable for implantation in the human body than is silicon. The new polymer is attractive for MEMS because of its high mechanical strength and how it responds to electricity. It is also nontoxic, making it biocompatible, suitable for use in the human body. The method the scientists used to create MEMS components from this polymer is called nanoimprint lithography. The process works much like a miniaturized rubber stamp, pressing a mold into the soft polymer to create detailed patterns, with features down to nanometers in size. The scientists printed components just 2 microns thick, 2 microns wide and about 2 centimeters long. "The printing actually worked, that is to say that we were able to get the recipe right," researcher Leeya Engel, a materials scientist at Tel Aviv University in Israel, told LiveScience. The fact that nanoimprint lithography does not rely on expensive or cumbersome electronics makes the new process simple and cheap. Introducing polymer MEMS to industry can only be realized with the development of printing technologies that allow for low-cost mass production. Scientists have previously created biocompatible MEMS parts, but the team's method offers an advantage: it can manufacture these biocompatible parts quickly and inexpensively. As a bonus, MEMS parts made from this organic polymer are highly flexible; they may be hundreds of times more flexible than such components made from conventional materials. This flexibility could make, for example, MEMS sensors more sensitive to vibrations and MEMS motors more energy efficient, leading to better cameras and smartphones with longer battery lives.

UCLA researchers have developed a 2 layer, see-through solar film that could be placed on windows, sunroofs, smart phone displays and other surfaces to harvest energy from the sun. The new device is composed of two thin polymer solar cells that collect sunlight and convert it to power. It is more efficient than previous devices, because its two cells absorb more light than single-layer solar devices, because it uses light from a wider portion of the solar spectrum, and because it incorporates a layer of novel materials between the two cells to reduce energy loss. While a tandem-structure transparent organic photovoltaic (TOPV) device developed at UCLA in 2012 converts about 4% of the energy it receives from the sun into electric power (conversion rate), the new tandem device-which uses a combination of transparent and semi-transparent cells, achieves a conversion rate of 7.3%. Researchers led by Yang Yang, the Carol and Lawrence E. Tannas, Jr., Professor of Engineering at the UCLA Henry Samueli School of Engineering and Applied Science, said the new cells could serve as a power-generating layer on windows and smartphone displays without compromising users' ability to see through the surface. The cells can be produced so that they appear light gray, green or brown, and so can blend with the color and design features of buildings and surfaces. The research was published by Energy & Environmental Science. This device could offer new directions for solar cells, including the creation of solar windows on homes and office buildings. The tandem polymer solar cells are made of a photoactive plastic. A single-cell device absorbs only about 40% of the infrared light that passes through. The tandem device, which includes a cell composed of a new infrared-sensitive polymer developed by UCLA researchers, absorbs up to 80% of infrared light plus a small amount of visible light. Using transparent and semi-transparent cells together increases the device's efficiency, and that the materials were processed at low temperatures, making them relatively easy to manufacture.

Batteries of smart phones drain much quicker than the previous generation of cell phones that required to be recharged just twice a week. Researchers have created the first ever “self-healing” battery electrodes. The secret is a stretchy polymer that coats the electrode, binds it together and spontaneously heals tiny cracks that develop during battery operation, said the team from Stanford University and the Department of Energy’s SLAC National Accelerator Laboratory. Chao Wang, a postdoctoral researcher at Stanford and one of two principal authors of the paper, said “We want to incorporate the feature of self healing into lithium ion batteries so they will have a long lifetime as well.” Chao developed the self-healing polymer in the lab of Zhenan Bao, a professor of chemical engineering at Stanford, whose group has been working on flexible electronic skin for use in robots, sensors, prosthetic limbs and other applications. For the battery project, Chao added tiny nanoparticles of carbon to the polymer so it would conduct electricity. It was found that the silicon electrodes lasted 10 times longer when coated with the self-healing polymer, which repaired any cracks within just a few hours. The electrodes worked for about 100 charge-discharge cycles without significantly losing their energy storage capacity. The team is trying to reach a goal of about 500 cycles for cell phones and 3,000 cycles for an electric vehicle. To make the self-healing coating, scientists deliberately weakened some of the chemical bonds within polymers long, chain-like molecules with many identical units. The resulting material breaks easily, but the broken ends are chemically drawn to each other and quickly link up again, mimicking the process that allows biological molecules such as DNA to assemble, rearrange and break down.

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