Wood instead of plastic? The dream of sustainable products

Dr. Geiger, what exactly do you want to achieve in the "Hypelignum" project?

Dr. Thomas Geiger: In the project, we are working with wood substrates that we can subsequently use for circuit boards. We are experimenting with wood wool on the one hand, and with wood fibers and cellulose nanofibrils on the other. The wood wool consists of thin wood threads planed from a tree trunk. For the cellulose nanofibrils, cellulose is pulped again using special processes. So we start with three materials and try different processes to obtain sheets on which the required electronics can finally be printed. However, the application of the electronics does not fall within our scope of work, but is taken over by our colleagues in the EU project.

How should I imagine the production? Do you get a solid mass that you can press, similar to paper production?

Geiger: Once we have produced the cellulose nanofibrils, they are then dispersed in water. The result is a solid paste containing around 12 to 14 percent cellulose, with the rest being water. We put this paste into a large mold. The mold is built in such a way that the water can escape when it is pressed together. This is how we get a thin sheet.

If you squeeze cellulose tighter and tighter, then it starts to keratinize. That means the hydrogen bonds between the cellulose macromolecules find each other again, and the whole material becomes very stiff. So it becomes a very strong sheet. And you can use that to print electronics on. This also works with the larger cellulose fibers, but here we need a binder. This is because the self-bonding properties of the cellulose are not as pronounced as with the cellulose nanofibrils. The same applies to the wood wool product. The wood threads have to be glued together to obtain a board.

How do the sheets behave when the electronics are printed on them?

Geiger: Cellulose loves water. But you don't want water near electronics. This topic, among others, is driving the project. We are currently looking at what materials from nature can help us protect the plates from absorbing water vapor or liquid water. A raw panel, whether made of pure cellulose nanofibrils, cellulose fibers or wood wool, will absorb water over time if exposed to high humidity.

And depending on what electronics are applied, that can be disruptive. That's why you need an insulation layer. In addition, we want to prevent the cellulose sheet from swelling and causing it to warp, so that it's no longer a flat sheet, but the sheet curves. Water vapor over time can really penetrate very deep into the structure and then the mechanical stability of the board suffers.

As a first step with the material, you have completed a cover for computer mice. Now you test the printed circuit boards. If you succeed in designing the material according to your wishes, would you have a sustainable alternative to plastic?

Geiger: Yes, but with a 'but'. The industry always has plastics, they always have the opportunity to produce large volumes very quickly. And since cellulose is not a thermoplastic, but we have to go through dewatering system or have to work with binders and adhesives, production takes a certain amount of time. In addition, we are not unlimited in terms of shaping. If you look at the inside of the cover of a computer mouse, you'll see a lot of webs and add-ons. These can not be shaped with cellulose at the moment. You would first have to redesign the cover. That's why the industry is looking more to thermoplastic bio-polymers that can be processed directly in existing machines. It is difficult to convince the industry to try out our process. Even though we can offer a sustainable closed-loop systems and make everything CO2-neutral.

Certain plastics such as PFAS are soon to be banned. Would this perhaps be the moment to say: Here, we have something that is not harmful to the environment?

Geiger: Absolutely. Of course, it would take someone with the courage to take the first step. I could well imagine, for example, that our process is very well suited for products that are exposed to little humidity. Office products such as our computer mouse or similar.

COMPAMED deals with medical technology and the products of suppliers for medical technology. What applications do you see for the material here?

Geiger: First of all, I think the packaging sector is interesting for our process. There are many uses for paper and cardboard here.

What about 3D printed products, for example? Could the cellulose nanofabrics also be used here?

Geiger: That's possible, but you have to switch to an aqueous suspension. You can concentrate it so highly that it becomes paste-like. You can 3D print with it. We do that here in our department, too. You could also consider implementing living cells or additives like enzymes. Then you would 3D print a body, dry it and then use it. In addition, you could 3D print hydrogels for medical technology. We have already tried that here. The process is interesting for wound care and bone replacement and the like. A research team at ETH Zurich, for example, has printed a complete tube from cellulose. If you animate the cells to grow along the tube so that they take on the same shape, you might be able to grow organ substitutes. However, we are still at the beginning here.

That sounds very exciting. Where are you in the project at the moment?

Geiger: The project was properly started in October 2022 and we have already practically produced the first samples. Our project partners from Hypelignum have already been able to print on these plates. So there are already the first demonstrators with which we can show the principle. Now the next step is to protect the board against humidity. And we still have to do a lot of testing to ensure that the compatibility of the inks that are printed on it works. In the project it is formulated that we use technologies to apply multilayer boards with 3D printing.

Another topic of the project is "compostability". Can the circuit boards simply go on the compost heap after use?

Geiger: A student dealt with this topic in her Matura thesis (a technical paper written by the school in Switzerland). How did the board decompose? Do the conductive tracks, if copper is included, for example, have an influence on the result? What happens if electronic components are mounted? If I put them in the compost pile, does it still degrade completely? Whole samples were made and then placed in a compost pile. After five months they were taken out and you could see that all the cellulose had degraded. You could separate the copper conductors and also the electronic components. If you could separate these components industrially to recycle or reuse them, you would have developed an environmentally friendly process. But there are still many open questions. For example, what about the copper input into the compost. That would have to be investigated in more detail.

What is the service life of the wooden panels? A computer mouse would probably be subjected to a lot of wear and tear. Is the service life comparable to that of plastic?

Geiger: We will never have the raw surface of the cellulose in our finished product; a coating must always be applied. Only then do we get a certain gloss to the product. It is also always a question of appearance. That's why you can extend the service life with a biological coating system. From a mechanical point of view, I can well imagine that we can come close to plastics here.