Wearable watch and fitness tracker. Image by © Tim Sandle.
Imagine a material that is mouldable, biocompatible and glitters like gold. This is a new form of plastic (a conjugated polymer) that can conduct an electric charge. This material can be used for almost everything which is connected – from sensors that can monitor our health to selfcooling clothing or electronic adhesive plasters that can be applied to the skin and send data directly to a mobile phone.
Conductive plastics can also be used for various kinds of implants or they can be 3Dprinted to create electronic adhesive plasters that can detect an infection.
This innovation comes from researchers at Chalmers University of Technology in Swede – a conductive plastic made without the use of harmful chemicals, and in a much more costeffective way compared with other approaches.
Currently, the market price for just 100 grams of this type of conductive plastic would be around USD 100,000 – about ten times as much as actual gold. But for the human body, it is in fact the absence of metals that makes this material so valuable.
“While some metals can corrode in humid environments, conductive plastic is an organic material that our bodies are comfortable with. The material is compatible with the body’s own tissue, while also being a semiconductor. There’s also an environmental advantage in that you don’t need to use the rare earth elements required for today’s electronics,” says Joost Kimpel, the led researcher.
Electronic adhesive plasters with connectivity
The key to the new manufacturing method was discovered serendipitously – during a routine experiment in a lab. When a chemical reaction was happening too fast and the resulting plastic was reaching its final state too quickly, the idea came up of reducing the heat in the process. This was what led to the discovery that you can produce this material at room temperature – involving significantly fewer steps, with lower energy consumption, and without toxic chemicals.
- The basic ingredients originate from the aromatic compounds thienothiophene and bithiophene, which are the basic building blocks of many organic semiconductors. These substances are mixed in the benign solvent Nbutyl2pyrrolidone in the presence of a palladium catalyst.
- Almost immediately, the transparent solution begins to change colour as the building blocks start to assemble into polymer chains. These larger and longer molecules form the basis of the conductive plastic.
- Once the colour has changed from yellow to deep red and then to deep purple, the reaction is complete. The mixture is then washed using several different solvents to remove impurities. Finally, the solvents are removed using rotary evaporation – a method similar to distillation.
- After the separation process, a glittering goldcoloured substance remains, the colour being an indication that the material is electrically conductive. The production of the conductive plastic is now complete.
Organic electronics
The next step in this research will be to continue working on a method that makes it possible to produce even larger volumes – continuously and with exactly the same results every time.
One important ingredient for plastics that can conduct electricity are conjugated polymers. Conjugated polymers are a type of semiconductor and have properties that make it possible to produce a new type of technology – organic electronics – that can be used in many different applications such as energy conversion and storage, wearable electronics, electronic textiles, and biotechnology attached to or close to the body.
Unlike inorganic materials such as metals, conjugated polymers can be made flexible and soft. They can be applied to surfaces and used to manufacture solar cells, and are compatible with liquids such as sweat and blood, which is important for bioelectronic applications. Research to make conjugated polymers stable and improve their conductive properties has been conducted for decades.
The research article “Openflask, ambient temperature direct arylation synthesis of mixed ionicelectronic conductors” appears in the journal Science Advances.
