This is one in a series presenting news on technology and innovation, made possible with generous support from the Lemelson Foundation.
Engineers have built a new type of gadget that doesn't look like most others. It's stretchy, thin and so light that a human hair can lift it. It's made from cheap, nontoxic materials. It can also vanish. After a month in acid — even just kitchen vinegar — the device is gone.
Chemical engineer Ting Lei helped design the innovation. He works in the lab of engineer Zhenan Bao at Stanford University in California. “We're focusing on low-cost sensors for flexible electronics,” Lei says. For about five years, Bao's group has been studying materials that can break down naturally.
The researchers see many uses for such a technology. For example, scientists could build sensors to measure heat or humidity in remote forests. Many soils are acidic. So when the scientists are done taking measurements, their sensors would fall apart and degrade naturally in the soil. Or doctors could implant medical devices that use this technology into patients. The patients wouldn’t need surgery to remove the devices. Instead, their bodies would simply absorb them.
Huanyu Cheng says this new work shows that it’s possible to make electronics that are less toxic to the environment. He is a materials scientist at Pennsylvania State University in State College. Although he did not work on the new research, he too designs dissolvable devices. Electronic sensors and other devices made this way would be “greener.," Cheng says .By that, he means they would be more friendly to the environment.
Building a dissolvable circuit
To demonstrate the new technology, Lei and his team built a stretchy transistor. A transistor controls the flow of current in electronics. The scientists used iron to build the conducting part of the device. Iron occurs naturally, and it won't cause harmful pollution when the device breaks down. Iron also is more environmentally friendly than gold, which has been used in other stretchy electronics.
However, building the circuit wasn't the hard part. The researchers had to find some material that could both hold the circuit together and later fall apart in acid. Many flexible electronic devices rely on polymers. A polymer is a material made from long chains of identical molecules. Because polymers can bend and stretch, they're perfect for electronics that need to flex.
But Lei and his team didn't want any old polymer. They wanted one that would dissolve in something as simple as vinegar.
Two years ago, they came up with a recipe. They began making a polymer out of imines (IH-meens). These are chemical compounds held together by double bonds between carbon atoms and nitrogen atoms. The bonds stay strong in water, but fall apart in acid. An imine polymer was perfect for Lei and his team.
The last piece of the puzzle was a material to hold the whole device together. The researchers used cellulose. This is the fibrous stuff in plants and trees (and used to make paper).
The team put all of those pieces together to make the first electronic device that can break down into environmentally friendly components. They introduced their transistor May 16 in Proceedings of the National Academy of Sciences. The researchers also showed that the device could conduct electricity.
Electronics that break down can help solve a growing problem. Worldwide, people add around 41 million metric tons (45 million short tons) of electronic waste to landfills every year. And that amount is growing. But if a device dissolves, it won’t add to the problem.
Lei says you probably won’t see his team's new technology in phones and laptops. It doesn't perform as well as the transistors in those devices now. But the technology could work for tools that don't need much power — like forest sensors or implanted medical devices.
“Our target is focusing on small, low-cost, flexible sensors,” he says.
atom The basic unit of a chemical element. Atoms are made up of a dense nucleus that contains positively charged protons and uncharged neutrons. The nucleus is orbited by a cloud of negatively charged electrons.
bond (in chemistry) A semi-permanent attachment between atoms — or groups of atoms — in a molecule. It’s formed by an attractive force between the participating atoms. Once bonded, the atoms will work as a unit. To separate the component atoms, energy must be supplied to the molecule as heat or some other type of radiation.
carbon The chemical element having the atomic number 6. It is the physical basis of all life on Earth. Carbon exists freely as graphite and diamond. It is an important part of coal, limestone and petroleum, and is capable of self-bonding, chemically, to form an enormous number of chemically, biologically and commercially important molecules.
cellulose A type of fiber found in plant cell walls. It is formed by chains of glucose molecules.
chemical A substance formed from two or more atoms that unite (bond) in a fixed proportion and structure. For example, water is a chemical made when two hydrogen atoms bond to one oxygen atom. Its chemical formula is H2O. Chemical also can be an adjective to describe properties of materials that are the result of various reactions between different compounds.
chemical engineer A researcher who uses chemistry to solve problems related to the production of food, fuel, medicines and many other products.
circuit A network that transmits electrical signals. In the body, nerve cells create circuits that relay electrical signals to the brain. In electronics, wires typically route those signals to activate some mechanical, computational or other function.
component Something that is part of something else (such as pieces that go on an electronic circuit board or ingredients that go into a cookie recipe).
compound (often used as a synonym for chemical) A compound is a substance formed when two or more chemical elements unite (bond) in fixed proportions. For example, water is a compound made of two hydrogen atoms bonded to one oxygen atom. Its chemical symbol is H2O.
conductor (in physics and engineering) A material through which an electrical current can flow.
current A fluid — such as of water or air — that moves in a recognizable direction. (in electricity) The flow of electricity or the amount of electricity moving through some point over a particular period of time.
dissolve To turn a solid into a liquid and disperse it into that starting liquid. (For instance, sugar or salt crystals, which are solids, will dissolve into water. Now the crystals are gone and the solution is a fully dispersed mix of the liquid form of the sugar or salt in water.)
double bond A type of bond between two atoms within a molecule. In a single bond, atoms share two electrons. In a double bond, they share four. This bond is slightly less stable than a single bond.
electricity A flow of charge, usually from the movement of negatively charged particles, called electrons.
electronics Devices that are powered by electricity but whose properties are controlled by the semiconductors or other circuitry that channel or gate the movement of electric charges.
engineer A person who uses science to solve problems. As a verb, to engineer means to design a device, material or process that will solve some problem or unmet need.
environment The sum of all of the things that exist around some organism or the process and the condition those things create. Environment may refer to the weather and ecosystem in which some animal lives, or, perhaps, the temperature and humidity (or even the placement of components in some electronics system or product).
green (in chemistry and environmental science) An adjective to describe products and processes that will pose little or no harm to living things or the environment.
humidity A measure of the amount of water vapor in the atmosphere. (Air with a lot of water vapor in it is known as humid.)
imine A type of organic (carbon-containing) chemical that contains a “functional” group made of a nitrogen atom that is bound to a hydrogen atom and to either of two other types of functional groups (known as alkyl or aryl groups).
innovation (v. to innovate; adj. innovative) An adaptation or improvement to an existing idea, process or product that is new, clever, more effective and/or more practical.
iron A metallic element that is common within minerals in Earth’s crust and in its hot core. This metal also is found in cosmic dust and in many meteorites.
landfill A site where trash is dumped and then covered with dirt to reduce smells. If they are not lined with impermeable materials, rains washing through these waste sites can leach out toxic materials and carry them downstream or into groundwater. Because trash in these facilities is covered by dirt, the wastes do not get ready access to sunlight and microbes to aid in their breakdown. As a result, even newspaper sent to a landfill may resist breakdown for many decades.
materials science The study of how the atomic and molecular structure of a material is related to its overall properties. Materials scientists can design new materials or analyze existing ones. Their analyses of a material’s overall properties (such as density, strength and melting point) can help engineers and other researchers select materials that are best suited to a new application.
molecule An electrically neutral group of atoms that represents the smallest possible amount of a chemical compound. Molecules can be made of single types of atoms or of different types. For example, the oxygen in the air is made of two oxygen atoms (O2), but water is made of two hydrogen atoms and one oxygen atom (H2O).
nitrogen A colorless, odorless and nonreactive gaseous element that forms about 78 percent of Earth's atmosphere. Its scientific symbol is N. Nitrogen is released in the form of nitrogen oxides as fossil fuels burn.
polymer A substance made from long chains of repeating groups of atoms. Manufactured polymers include nylon, polyvinyl chloride (better known as PVC) and many types of plastics. Natural polymers include rubber, silk and cellulose (found in plants and used to make paper, for example).
Proceedings of the National Academy of Sciences A prestigious journal publishing original scientific research, begun in 1914. The journal's content spans the biological, physical, and social sciences. Each of the more than 3,000 papers it publishes each year, now, are not only peer reviewed but also approved by a member of the U.S. National Academy of Sciences.
sensor A device that picks up information on physical or chemical conditions — such as temperature, barometric pressure, salinity, humidity, pH, light intensity or radiation — and stores or broadcasts that information. Scientists and engineers often rely on sensors to inform them of conditions that may change over time or that exist far from where a researcher can measure them directly.
toxic Poisonous or able to harm or kill cells, tissues or whole organisms. The measure of risk posed by such a poison is its toxicity.
transistor A device that can act like a switch for electrical signals.
waste Any materials that are left over from biological or other systems that have no value, so they can be disposed of as trash or recycled for some new use.
Journal: T. Lei et al. Biocompatible and totally disintegrable semiconducting polymer for ultrathin and ultralightweight transient electronics. Proceedings of the National Academy of Sciences. Vol. 114, May 16, 2017, p. 5107. doi: 10.1073/pnas.1701478114.