A new polymer film that can generate electricity, or even serve as a motor itself, with nothing more than water as fuel has been developed by an engineering research team at the Massachusetts Institute of Technology. Where larger-scale power plants need an entire river or ocean of water, the MIT polymer can manage to capture energy from nothing more than a hint of water vapor. The material consists of two separate polymers that are integrated into one another; polypyrrole-a sturdy but bendable material that maintains the compound’s structure, and polyol-borate, a soft gel that expands and contracts when water vapor is nearby. The way the material works is as follows: First, when water is introduced, the polyol-borate forces the bottom layer to curl away from the exposed surface. Once the bottom layer is exposed to air again, the moisture is released, the material somersaults forward, and the cycle repeats itself again and again until the water is removed. Mechanical energy is produced by the repetition
This concept as well is not entirely original, as researchers have attempted to capture the movement of water vapor in the past using a "hard but flexible" polymer known as polypyrrole. In previous attempts, however, the polypyrrole has been used by itself. A team led by Mingming Ma, a post-doctoral researcher at the MIT David H. Koch Institute for Integrative Cancer Research, took a different approach, combining that primary polymer, formed into a matrix, with a gel-like polymer called polyol-borate. This gel readily absorbs water, expanding substantially when it does. This combination material can then be used to effectively harvest the energy from water vapor by causing the film to curl as water is absorbed. Incorporating the two different kinds of polymers can generate a much bigger displacement, as well as a stronger force. As the material curls toward the drier air above the film, the water is released and reverts to vapor, causing the material to snap back to its original orientation. This creates a cycle that allows the material to generate constant power through a differential in moisture levels between the air above and below it. The most interesting aspect of this new approach to energy is that the motion produced by the film can be harnessed in two distinct ways: as a small, but surprisingly power motor or as a source of electricity for micro- and nanoelectronics. As an engine, despite being no more than 20 micrometers thick and 25 milligrams, one test film was able to lift glass slides weighing around 9.5 grams. Because water vapor is extremely common and the demands of the material are so limited, this new polymer film could be used to power devices such as temperature and humidity sensors, which often need to be left in large numbers over a wide area, making replacing batteries a hassle. There is even the possibility that this material could be used to power biosensors based on nothing more than human sweat, or even large-scale power plants located over rivers and lakes. MIT researchers believe the material can not only harvest energy from water vapor, but it can also generate electricity without any waste product like carbon dioxide. When coupling the polymer film with a device to convert mechanical energy to electricity (known as a piezoelectric material) it can generate an average charge of 5.6 nanowatts. Although this may not seem like much, it can be used on a small scale for many purposes like temperature and humidity sensors, or even monitoring sensors on clothing with evaporating sweat as the ‘water source’.
"What's really impressive about this work is that they were able to figure out a scheme where a gradient in humidity would cause the polymer to cyclically roll up, flip over and roll in the other direction, and were able to harness that energy to do work," noted Ryan Hayward, an associate professor of polymer science and engineering at the University of Massachusetts at Amherst. This material could ultimately be scaled up to the point of being able to control the movement of robotic limbs with only a small amount of water. Though perhaps less efficient, probably the more broadly applicable use is actually this technology's potential when combined with piezoelectric materials. These types of crystals can create an electric charge from physical stress, and are seeing increasing use in small-scale electronics as a source of readily available energy for devices with minimal power needs. One of the more impressive features of this new creation is the amount of physical energy it exudes. Being referred to as ‘artificial muscle,’ a 25-milligram film is able to either lift up nearly 400 times its own weight or carry metals ten times its weight across a flat surface. Mingming Ma, lead author for the paper being featured in Science on the material, sees value in harnessing its continuous motion and durability to drive robotic limbs in the future, “with a sensor powered by a battery, you have to replace it periodically. If you have this device, you can harvest energy from the environment so you don’t have to replace it very often.”