ASSETm is a soft, stretchable, biodegradable material for use in wearable sensors. Photo: Xiaohong Lan, University of Groningen.
ASSETm is a soft, stretchable, biodegradable material for use in wearable sensors. Photo: Xiaohong Lan, University of Groningen.

Despite their obvious utility, one problem with soft, wearable sensors is that these flexible and stretchable electronic devices are nearly impossible to recycle. As a result, this electronic waste usually ends up in landfills or polluting the environment.

Now, however, polymer scientists at the University of Groningen in the Netherlands have developed a starch-based polymer that can be used to create a fully biodegradable soft material for sensors. The scientists report their work in a paper in ACS Applied Materials & Interfaces.

Soft, stretchable polymers are used to make various kinds of electronic devices. These include smart watches that make contact with the skin, sensors in shoes or clothing, and smartphone screens.

“These soft materials are often made from mixtures of polymers, which are difficult to purify,” said Xiaohong Lan, a postdoc in the University of Groningen’s Polymer Science group and first author of the paper. “As a result, they are dumped in landfills, often with the toxic metal components of the sensor still present. This kind of electronic waste is becoming a serious problem.”

Lan and his colleagues in the Polymer Science group have developed an alternative to these complex polymers: a biodegradable material that decomposes in a matter of weeks to a few months.

“There are easy ways to remove the metal and polymers from the e-waste,” says Lan. “Of course, you could try to recycle the soft polymers, but that is often too complex, and therefore too expensive.” Instead, the researchers' new polymer decomposes, leaving only water and carbon dioxide behind.

“To create a biodegradable polymer, we started out with a backbone of starch-derived dextrin carbohydrates,” explains Lan. “Most polymer backbones contain chemical bonds, which are very strong. The dextrin backbone can be degraded by natural enzymes that are present in soil.”

By adding long fatty acid tails to the dextrin in the backbone, the researchers were able to regulate the hydrophobicity of the polymer. “The enzymes that degrade the polymer require water, so if a material is too hydrophobic, they cannot do it,” says Lan. “If the polymer is too hydrophilic, on the other hand, the material will not have the right properties.”

The material needs to be soft and stretchable, but also dielectric, so that the sensors can charge themselves with the electricity created by rubbing against fabric. In addition to the fatty acid tails, the researchers also grafted lactone monomers onto the modified dextrin polymers in a brush-like pattern. These brushes give the material its stretchability.

The resulting ‘Advanced Scalable Supersoft Elastic Transparent material’ (ASSETm) has just the right properties, with experiments confirming that it is suited for sealing in electrodes to produce sensors. “We compared our sensors with state-of-the-art commercial sensors, and found that ours worked at least as well,” says Lan.

The production process is scalable, so there is no reason why this biodegradable ASSETm should not replace traditional soft polymers in smart electronics. “However, we do have to change our attitude towards starch, which is usually seen as a food product,” says Lan. Currently, approximately 60% of all starch is used in animal feed, 30% for human consumption and 10% in medical applications. “However, starch consumption is decreasing, and there is a downward trend in cattle numbers.”

Group leader Katja Loos is also enthusiastic about the new material: “We hope that our paper will launch a discussion on further curbing e-waste. This degradable polymer can really help reduce the amount of e-waste.”

This story is adapted from material from the University of Groningen, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.