A new kind of wood chip: collaboration could lead to biodegradable computer chips Portable electronics � typically made of non-renewable, non-biodegradable and potentially toxic materials � are discarded at an alarming rate in consumers� pursuit of the next best electronic gadget.

In an effort to alleviate the environmental burden of electronic devices, a team of University of Wisconsin-Madison researchers has collaborated with researchers in the Madison-based U.S. Department of Agriculture Forest Products Laboratory (FPL) to develop a surprising solution: a semiconductor chip made almost entirely of wood.

A cellulose nanofibril (CNF) computer chip rests on a leaf. Image credit: Yei Hwan Jung, Wisconsin Nano Engineering Device Laboratory
A cellulose nanofibril (CNF) computer chip rests on a leaf. Image credit: Yei Hwan Jung, Wisconsin Nano Engineering Device Laboratory

The research team, led by UW-Madison electrical and computer engineering professor Zhenqiang �Jack� Ma, described the new device in a paper published on May 26, 2015 by the journal Nature Communications. The paper demonstrates the feasibility of replacing the substrate, or support layer, of a computer chip, with cellulose nanofibril (CNF), a flexible, biodegradable material made from wood.

�The majority of material in a chip is support. We only use less than a couple of micrometers for everything else,� Ma says. �Now the chips are so safe you can put them in the forest and fungus will degrade it. They become as safe as fertilizer.�

Zhiyong Cai, project leader for an engineering composite science research group at FPL, has been developing sustainable nanomaterials since 2009.

�If you take a big tree and cut it down to the individual fiber, the most common product is paper. The dimension of the fiber is in the micron stage,� Cai says. �But what if we could break it down further to the nano scale? At that scale you can make this material, very strong and transparent CNF paper.�

Working with Shaoqin �Sarah� Gong, a UW-Madison professor of biomedical engineering, Cai group addressed two key barriers to using wood-derived materials in an electronics setting: surface smoothness and thermal expansion.

�You don�t want it to expand or shrink too much. Wood is a natural hydroscopic material and could attract moisture from the air and expand,� Cai says. �With an epoxy coating on the surface of the CNF, we solved both the surface smoothness and the moisture barrier.�

Gong and her students also have been studying bio-based polymers for more than a decade. CNF offers many benefits over current chip substrates, she says.

�The advantage of CNF over other polymers is that it a bio-based material and most other polymers are petroleum-based polymers. Bio-based materials are sustainable, bio-compatible and biodegradable,� Gong says. �And, compared to other polymers, CNF actually has a relatively low thermal expansion coefficient.�

The group work also demonstrates a more environmentally friendly process that showed performance similar to existing chips. The majority of today wireless devices use gallium arsenide-based microwave chips due to their superior high-frequency operation and power handling capabilities. However, gallium arsenide can be environmentally toxic, particularly in the massive quantities of discarded wireless electronics.

Yei Hwan Jung, a graduate student in electrical and computer engineering and a co-author of the paper, says the new process greatly reduces the use of such expensive and potentially toxic material.

�I�ve made 1,500 gallium arsenide transistors in a 5-by-6 millimeter chip. Typically for a microwave chip that size, there are only eight to 40 transistors. The rest of the area is just wasted,� he says. �We take our design and put it on CNF using deterministic assembly technique, then we can put it wherever we want and make a completely functional circuit with performance comparable to existing chips.�

While the biodegradability of these materials will have a positive impact on the environment, Ma says the flexibility of the technology can lead to widespread adoption of these electronic chips.

�Mass-producing current semiconductor chips is so cheap, and it may take time for the industry to adapt to our design,� he says. �But flexible electronics are the future, and we think we�re going to be well ahead of the curve.�