Imagine your favorite food. Now, imagine it tattooed with incredibly thin, edible sensors that could give you useful information. Maybe they could tell you the food’s nutritional content. Or show where and when the food was made — even how it got to your plate. That’s what scientists are working toward. With a special laser, a research team has learned to etch an ultrathin layer of carbon onto everyday materials such as foods and fabric. It’s an early, but important, step toward making a new type of electronic sensor. They could become wearable or even edible.
Yieu Chyan is a chemist at Rice University in Houston, Texas. His team uses lasers to convert carbon in the very topmost layer of different items into an incredibly useful material called graphene (GRAA-feen). It’s a single-atom-thick layer of carbon. The researchers are perfecting their tattooing technique for use on common carbon-containing materials. These include paper, cork, wood, cardboard, cloth — even foods such as potatoes and toast!
The way that carbon atoms are arranged makes graphene appear black. So tattoos created with the laser can look like drawings. But the researchers don’t add ink or anything else to the surface. The graphene forms from carbon atoms that were already part of the material.
Graphene has a honeycomb-shaped structure and is arranged in sheets just one atom thick. As a laser passes over some material’s surface, its intense energy locks together those carbon atoms on the surface, turning them into graphene. Sometimes, the laser instead jumbles up the carbon atoms. This creates a foam, which has no structure. When this happens, the researchers pass the laser over the surface again. Its second pass converts the carbon foam into graphene. Scientists call the carbon treated this way laser-induced graphene, or LIG.
Scientists have worked with LIG for a long time. “We knew it to be very useful in many applications — including sensing, water-splitting, making special coatings and batteries,” Chyan says. But previous work, he adds, only made LIG on a material known as polyimide. That’s a type of plastic. The Rice team didn’t want to be limited to that. Explains Chyan: “We wanted to be able to put LIG on a whole range of different materials.”
To do this, they changed their approach. They found a way to use multiple laser treatments to make LIG. Along the way, they discovered that the laser worked especially well on materials high in lignin. This carbon-rich substance is found in many plants. Cork, coconuts and potato skins, for instance, tattooed very well. Chyan’s team described its findings February 13 in ACS Nano.
Graphene in electronics
Graphene is not only very strong but also conducts heat and electricity well. That’s why scientists are particularly interested in using LIG for electronics. “For example,” said Chyan, “you could put LIG on cloth and make wearable electronics. Imagine a jacket that had its own power source, sensors and wiring.”
James Tour is a chemist at Rice and a co-author of the new study. He would like to see electronic sensors on foods. Manufacturers already put small computer chips on many items. One type of chip, called a radio frequency identification (RFID) tag, can store many types of information. People can retrieve that stored information by scanning the tag with a small device. Tour now envisions a day when every food “will have a tiny RFID tag that gives you information about where it's been, how long it's been stored, its country and city of origin and the path it took to get to your table."
LIG tags might even help detect the presence of dangerous germs. How? Perhaps, Tour says, “they could light up and give you a signal that you don't want to eat this."
Jian Lin is a mechanical engineer at the University of Missouri in Columbia, who once worked with Tour. Lin is optimistic about LIG’s potential. He says the new study describes “very exciting work.” The Rice team’s improvements basically show “that anything with the proper carbon content can be turned into graphene.”
Lin’s own team is exploring how to use LIG in robotics. They are testing it for energy storage, biosensors and soft robotics, he says. That last application requires soft and bendable materials.
Chyan, too, is looking at LIG applications for robotics. He would like to give robots “touch” sensors that help them respond to how things feel. In robotic gloves, for example, they might signal how firmly to grip something so that it won’t fall — but also won’t get crushed.
application A particular use or function of something.
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.
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.
computer chip (also integrated circuit) The computer component that processes and stores information.
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.
germ Any one-celled microorganism, such as a bacterium or fungal species, or a virus particle. Some germs cause disease. Others can promote the health of more complex organisms, including birds and mammals. The health effects of most germs, however, remain unknown.
graphene A superthin, superstrong material made from a single layer of carbon atoms connected together.
laser A device that generates an intense beam of coherent light of a single color. Lasers are used in drilling and cutting, alignment and guidance, in data storage and in surgery.
lignin A natural substance that helps strengthen the cell walls of plants. Although lignin is made from a large number of sugar molecules, which should provide energy, livestock can’t digest this material because of the way its sugars are chemically bonded together.
plastic Any of a series of materials that are easily deformable; or synthetic materials that have been made from polymers (long strings of some building-block molecule) that tend to be lightweight, inexpensive and resistant to degradation.
polyimide A class of synthetic resins made from some polymer (chain-like molecule with repeating imide subunits). These fairly rugged resins can withstand corrosive environments and high temperatures.
radio frequency identification, or RFID It’s a technology that uses tiny computer chips implanted in products, packaging or animals. A device can scan those tags to retrieve information, using radio frequencies, that has been stored on them.
range The full extent or distribution of something. For instance, a plant or animal’s range is the area over which it naturally exists. (in math or for measurements) The extent to which variation in values is possible. Also, the distance within which something can be reached or perceived.
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.