Liangbing Hu makes wood stronger than steel

Regular old wood becomes a wonder material in Liangbing Hu’s laboratory. The University of Maryland, College Park, materials scientist and his colleagues have made the material transparent, as strong as steel, squishy and bouncy like rubber, and most recently, moldable like plastic.

Using simple chemical processes, Hu’s lab partially washes away lignin—the polymer that holds cellulose fibers in wood together—and exploits the natural complexity of wood’s nanostructure. Hu licensed the technology to InventWood, a University of Maryland spin-off company that looks for commercial applications, such as greener materials that could replace the glass, metal, and plastics in buildings and vehicles.

Hu studied carbon nanotubes for his PhD thesis. He was drawn to wood when he found that the structure and ion-transport capabilities of cellulose nanofibers are similar to those of carbon nanotubes while being sustainable and low cost.

Prachi Patel spoke with Hu about the often-overlooked properties of wood and the applications for biobased materials that his lab envisions for the future. This interview was edited for length and clarity.

Why is wood a good material to engineer?

If I had read a textbook about wood structure and chemistry, I would never have worked with this material because it is so complicated. I dared to work with it because I really liked the nanofibers.

At the nanoscale, wood has mechanical properties very similar to carbon nanotubes produced in a lab. A tree is an anisotropic structure—it has a growth direction that’s well defined. When you look at wood under a microscope, you see these tubular structures, these hollow fibers pointing in the same direction, glued together by lignin. And inside the cell wall of each fiber you see these amazing nanofibers that are four orders of magnitude smaller than the big fibers. The typical strength of a piece of wood is about 20–50 MPa, but the nanofibers are about 100 times as strong. So the microscopic building blocks of wood have a unique structure and amazing properties that we could take advantage of by removing lignin chemically.

Trees also pump ions and water efficiently 24/7. So we want to study ion transport in these tiny fibers and see if we can do that for batteries.

What applications for nanoengineered wood are we most likely to see soon?

That’s the question InventWood, which licensed the technologies from my lab, has been asking almost every day. Structural applications of advanced wood—for example, the superstrong wood and moldable wood we’ve demonstrated—can happen in 2–3 years.

Using our lab’s super wood—which really takes advantage of the nanofibers’ mechanical properties and has material strength similar to some metals—we’re looking to replace steel and aluminum to save carbon dioxide emissions. Producing those metals uses a lot of heat and electricity and emits a lot of carbon dioxide. But growing wood removes carbon dioxide, and our method of processing wood at room temperature using water, sodium sulfite, and sodium hydroxide is more energy efficient.

And now for the first time, we are able to shape wood like you can mold plastic and metal. When you think about plastics, you can melt them and reshape them, but when you try to bend wood, you can break it. Our material is environmentally friendly compared with plastic composites because this material, in the end, is biodegradable but still retains strength for structural applications.

On the other hand, in a battery you have many components; everything needs to work together, and you have to compete with many other technologies. So I think that using wood to make batteries will require more development.

Why are you interested in the ion-transport properties of wood for batteries?

Wood is used mainly as a structural material or to make paper. But its interesting physics and chemistry come up at the nanoscale. For example, in the tiny nanofibers, ions move much faster than they do in the big fibers because, in the tiny space, ions can only go one at a time and have to get rid of all their counterions, which would normally be attracted to them and hold them back. This speed is very important for a battery, where you move lithium ions when you charge. One reason your battery doesn’t charge quickly enough is because lithium ions come with copper ions and are not very mobile. With this kind of nanofiber structure, you could tailor how the ions move.

More generally, people have tried to make new nanomaterials. But wood nanofibers are the most abundant material around us, with about 400 trees per person on a global average. We need to really explore wood nanotechnology more.

What are the limitations of making new materials out of wood? How could you overcome those?

Everything comes with a downside. We don’t add anything to the wood. That’s why it’s sustainable and biodegradable. But at the end of the day, it’s a piece of wood. If you put it in a flame long enough, it will catch fire. But the wood-based materials we made ignite significantly slower, at one-hundredth the speed of natural wood.

Wood also comes with size limitations, and the shape is not always straight. So we have to be smart about how to deal with this. For example, we’re trying to get around the size of the wood by rotary cutting it. The diameter of wood is limited, but if you can peel it spirally and unroll it, you can make a much larger piece.

What do you do with the lignin you take out of the wood?

There’s a saying in the paper industry that you can make anything out of lignin but money. There’s never been a good way of using lignin. We’re trying to turn that around and do something that can benefit from lignin instead of trying to remove it. We haven’t fully focused on engineering or recycling lignin yet, but we have something very interesting going on here that I hope to talk to you about in the future.

Prachi Patel is a Pittsburgh-based freelance journalist who writes about energy, materials science, nanotechnology, biotechnology, and computing. A version of this story first appeared in ACS Central Science: cenm.ag/superwood.