Our global obsession with ephemeral consumer electronics is fast resulting in a massive global garbage problem. As many as 50 million metric tons of our old smartphones, PCs, TVs and other devices were discarded last year in favor of the next new thing.
But researchers at the University of Wisconsin-Madison have developed a surprising way to make tossing out future smartphones and tablets easier on the environment and the conscience. They’re replacing the bulk of toxic and non-biodegradable materials in modern microprocessors with wood.
The research was done in collaboration with the U.S. Department of Agriculture Forest Products Laboratory and is detailed in a paper published recently in Nature Communications.
Specifically, the researchers’ method replaces the rigid base or substrate material in smartphone and tablet chips, often comprised of the arsenic-containing compound gallium arsenide, with cellulose nanofibril (CNF). CNF is a flexible, transparent material made by breaking down the cell walls of wood to the nano scale and forming it into sheets, much like paper.
The tiny transistors and other components on the team’s chips are still made of metals and other potentially toxic materials. But the amount of those materials used is so small that lead researcher and UW-Madison electrical and computer engineering professor Zhenqiang "Jack" Ma says the chips can be consumed by fungus and become “as safe as fertilizer.”
Of course, wood-based CNF doesn’t have the same characteristics as petroleum or metal-based materials more typically used as substrates in mobile chips. Like any wood-based material, CNF has a tendency to attract moisture and expand and contract with temperature changes—both major problems for tightly packed, moisture-averse microchips. To make the material more suitable for use in electronics, Zhiyong Cai at the U.S. Department of Agriculture and Shaoqin "Sarah" Gong at UW-Madison worked together to create a biodegradable epoxy coating, which keeps the material from attracting water and expanding. It also makes the material smoother, an important property for a material used to build tiny chips. Ma says the amount of epoxy used depends on how long the chip needs to last. Using less epoxy also means that fungus can break down the chip more quickly, but Ma says fungus will always eventually make its way through the epoxy.
Like gallium arsenide, CNF also needs to have a low radio frequency energy loss, so wireless signals being transmitted and received by the chip won't be degraded or blocked. “Our group did the radio frequency energy loss testing,” Ma says, “and we found, oh cool, everything looks good.”
Once the researchers were sure the material was a viable substitute, the next step was figuring out how to remove as much gallium arsenide from a chip as possible and replace it with CNF. For that, Ma borrowed a technique from some of his other work designing flexible electronics.
“When we do flexible electronics, we peel off a very thin layer of silicon or gallium arsenide, and the substrate [material underneath] can be saved,” says Ma. “So why don’t we just do the same thing and peel off a single layer of the original substrate and put it on CNF, this wood-based substrate.”
Gallium arsenide is used in phones as a substrate, rather than the silicon that’s common in computer processors, because it has much better properties for transmitting signals over long distances—like to cell phone towers. But Ma says despite the environmental and scarcity issues with gallium arsenide (it's a rare material), no one had created a thin-film-type transistor or circuit out of the material, and the existing techniques were using more of the potentially toxic substance than necessary.
As few as 10 transistors are needed for some types of chips, and the technique they’ve developed allows for many more than that to be created in a 4-millimeter-by-5-millimeter area. “Actually, we can build thousands of transistors out of that area, and just move those transistors to the wood substrate,” says Ma. “This CNF material is surprisingly good and no one ever tried high-frequency applications with it.”
Of course, there are other potentially toxic materials in portable electronics, including in batteries, and the glass, metal and plastic shells of the devices make up the bulk of e-waste. But advances in eco-friendly plastics and recent work using wood fibers to create three-dimensional batteries offer hope that we might one day feel better about replacing our aging devices.
The real challenge, however, will likely be getting massive chip-fabrication plants, and the companies that employ or own them, to shift to newer, more eco-friendly methods when current techniques are so inexpensive. When scaled up however, the costs for creating CNF from renewable wood should be inexpensive as well, helping entice device makers to switch from more traditional substrates. After all, wood is abundant, and doesn’t need to be mined from the ground like gallium. The nearly two-millennia history of wood-based paper should also help keep the cost of making CNF low. “The wood breakdown process is established very well,” says Ma.
The pliable nature of CNF will make it a good fit for the emerging field of flexible electronic devices. But Ma warns that the emergence of flexible, wearable, low-cost devices will also likely substantially increase the amount of e-waste in the not-too-distant future.
“We are on the horizon of the arrival of flexible electronics,” says Ma. “The number of flexible electronic gadgets will be much more than just one phone and one tablet or laptop. We are probably going to have ten PCs.”
Ma hopes the amount of potential e-waste generated by all these devices combined with the amount of rare materials—gallium arsenide and others—that can be saved by using wood-based materials in electronics will eventually make both financial and environmental sense.