Why we write about this topic:
Soft and stretchable electronics and robotics promise new applications. Healthcare, for example, could benefit from wearables or implantable electronics. Examples include disposable sensors to measure health data or smart implants that release drugs in the body. The research topic is complex and keeps scientists around the world busy. One of its latest achievements is the first stretchable and biodegradable battery. It is only a few centimeters in size – and has the potential to supply the intelligent health products of the future with energy. It was invented by Professor Martin Kaltenbrunner and his doctoral students at Johannes Kepler University (JKU) in Linz, Austria.
Growing electronic waste
Professor Kaltenbrunner heads the soft matter physics group at JKU and conducts basic research into new materials for soft robotics and elastomers. One of his special areas of expertise is new energy carriers. His predecessor, Siegfried Bauer, presented the world’s first stretchable battery in 2010. He has now succeeded in developing the first stretchable AND biodegradable battery. “Applications in the medical field, such as sensors on the skin, are only worn for a short period of time, and for disposable products, environmentally compatible and biodegradable materials are particularly important,” says Kaltenbrunner. He continues: “As the amount of tools grows, so inevitably does the electronic waste mountain. Already in 2019, 140,000 tons of e-waste were created every day – and the amount is growing exponentially. That’s why we need to start thinking now about how to make electronics and even robotics sustainable – and how to do that for new research fields, like soft forms of electronics and robotics.”
As the amount of tools inevitably grows, so does the electronic waste mountain. Already in 2019, 140,000 tons of e-waste were created every day – and the amount is growing exponentially. That’s why we need to start thinking now about how we can make electronics and also robotics sustainable – and how we can do that for new research fields, like soft forms of electronics and robotics.”
Professor Martin Kaltenbrunner, JKU Linz
The stretchable AND biodegradable battery
Research already has approaches for stretchable batteries and for biodegradable batteries, but a there was still no combination of these crucial properties. This is because there are several aspects that separate conventional batteries from sustainable flexible electronics: They are rigid, usually contain toxic metals and cannot yet be easily recycled. That was the starting point for the project, Kaltenbrunner explain
Batteries usually consist of a skeleton and active materials. By skeleton, we mean the casing and a separator. The separator is a layer that separates the two electrodes (anode and cathode). Ions are exchanged between the anode and cathode and the flow of ions is made possible by the electrolyte, which can be, for example, potassium hydroxide (KOH). The separator located between the anode and cathode prevents a short circuit from occurring.
A question of materials
In ordinary batteries, the case is made of steel and the electrodes of manganese oxide and zinc. The separator is made of a paper-like fabric and the electrolyte is potassium hydroxide (KOH). None of these materials is particularly stretchy or biologically compatible – especially the concentrated potassium hydroxide solution. Therefore, the researchers had to first identify and/or create each individual part for their stretchable and biodegradable battery.
The materials had to make a battery, meaning they had to have the electrochemical requirements – while also being harmless to health, biodegradable and mechanically stretchable. The choice of materials that meet all these requirements is not very large, Kaltenbrunner points out.
For the housing, the researchers synthesized an elastomer – polyglycerol sebacate (PGS) – that is stretchable and biodegradable. There are enzymes, among others in compost, that even need the material as food. For the active materials – the anode and the cathode – a film of magnesium and molybdenum oxide was used. Both substances are harmless and also occur in the body as trace elements. The electrolyte and separator were replaced by a biodegradable gel made of calcium alginate
Both substances are harmless and also occur in the body as trace elements. The electrolyte and separator have been replaced by a biodegradable gel made of calcium alginate. Since it is a hydrogel and contains dissolved salt (calcium chloride), it can both establish the mechanical separation of the anode and cathode and conduct the ions.
Finding a material for the anode was particularly challenging. Since this had to be made of metal and metal is generally not flexible. The magnesium foil used also faced the problem of lack of ductility. To get around this, the team used kirigami, a precisely defined Japanese cutting technique that turns rigid materials such as paper or metal foil geometrically and mechanically stretchable ones. Nevertheless, the stretchability of the magnesium foil remains limited, explains Kaltenbrunner. Even when stretched, as much electrode area as possible must still be usable. This is the only way the foils can conduct electrons and thus provide electricity, the researcher says.
The stretchable and biodegradable battery that Kaltenbrunner and his team developed in this way has an energy density of 1.72 milliwatt-hours per square centimeter. That’s not much compared to a lithium polymer battery, which has 60 times the power. But this energy density is already enough to supply a sensor with energy for several hours. Athletes, for example, could use this battery to analyze their training progress. A sensor could measure the sodium content on the skin and transmit the data to a smartphone.
Water-soluble and degradable in the body
After use, the entire energy storage system can be easily composted. The materials are water-soluble and decompose as soon as they come into contact with water. In the test, the battery was exposed to a water temperature of 37 degrees Celsius and had decomposed by more than 70 percent after eleven weeks. What is important with regard to future applications in the health sector is that the battery is completely harmless. In principle, it could even be swallowed – and completely degraded in the stomach.
For Kaltenbrunner and his doctoral students, the research goal has been achieved. All the materials are stretchable, environmentally compatible and biodegradable. But they want to continue research to achieve higher energy density. “These are great materials, but maybe they’re not the best possible materials yet, and maybe they’re not the best possible design yet,” Kaltenbrunner says.
