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By Renee Moezelaar
Synthetic polymers have changed the world around us, and it would be hard to imagine a world without them. They have pervaded our life, making their way from simple plastic goods to sophisticated devices such as batteries and fuel cells. However, they do have their problems. It is for instance hard to precisely control their molecular structure from a synthetic point of view. This makes it harder to finely tune some of their properties, such as their ability to transport ions. To overcome this problem, a group of researchers headed by Dr. Giuseppe Portale from the University of Groningen (the Netherlands) decided to take inspiration from nature. The result was published in Science Advances on July 17: a new class of polymers based on protein-like materials and inspired to spider silk that work as proton conductors and might be useful in future bio-electronic devices.
‘I have been working on proton conducting materials on and off since my PhD’, says Portale. ‘I find it fascinating to know what makes a material transport ions so I work a lot on understanding and optimizing structures at the nanoscale level to get greater conductivity.’ But it was only a few years ago that he considered the possibility of making them from biological, protein-like structures. That was something the assistant professor came up together with Prof. Andreas Hermann, a former RUG-colleague now working at the DWI - Leibniz Institute for Interactive Materials in Germany and Prof. Kai Liu from Changchun Institute of Applied Chemistry at the Chinese Academy of Sciences: ‘We could immediately see that proton conducting bioinspired polymers could be very useful for applications like bio-electronics or sensors.’
But first they had to see if the idea would work. "Our first goal was to prove that we could precisely tune the proton conductivity of the protein-based polymers by tuning the number of ionic groups per polymer chain". A number of unstructured biopolymers that had different numbers of ionisable carboxylic acid (-COOH) groups was prepared. Their proton conductivity scaled linearly with the number of charged carboxylic acid groups per chain. ‘It was not ground breaking, everybody knows this concept. But we were thrilled by the possibility to design something that worked as expected’ Portale says.
For the next step, Portale relied on his expertise in the field of synthetic polymers: ‘I have learned over the years that the nanostructure of a polymer can greatly influence the conductivity. If you have the right nanostructure, it allows the charges to bundle together and increase the local concentration of these ionic groups and that gives a dramatic boost to the proton conductivity.’ Since the first batch of biopolymers was completely disordered, the researchers had to switch to a different material. They decided to use a known protein having the shape of a nanosized barrel. ‘We engineered a protein already existing in nature that has the structure of a barrel of few nanometers and added strands containing carbocyclic acid to its surface’, Portale explains. ‘This increased the conductivity greatly.’
Unfortunately, the barrel-polymer was not very useful. It had no mechanical strength and it was difficult to process, so Portale and his colleagues had to look for another alternative. They landed on a well-known natural polymer: spider silk. ‘This is one of the most fascinating materials in nature, because it is very strong but can also be used in many different ways’, says Portale. ‘During my time at the European Synchrotron laboratory, I have learned about the fascinating nanostructure of the spider silk. We have thus engineered a protein-like polymer that has the main structure of spider silk but was modified to host the strands of carbocyclic acid that we prepared before’
The novel material worked like a charm. ‘We found that it self-assembles at the nanoscale similarly to spider silk while creating dense clusters of charged groups, which are very beneficial for the proton conductivity’, Portale explains. ‘And we were able to turn it into a robust centimetre-sized membrane.’ A nice achievement if we consider that the measured proton conductivity is one order of magnitude higher than those of any previously known biomaterials.
But they are not there yet according to Portale: ‘This was mainly fundamental work. In order to really apply this material, we really have to improve it and make it processable.’
But even though the work is not yet done, Dr. Giuseppe Portale and his co-workers can already dream about applying their polymer: ‘We think this material could be useful as a membrane in future energy devices. Maybe not for the large scale systems that you see in cars and factories, but more on a small scale. There is a growing field of implantable bio-electronic devices, for instance glucose-powered pacemakers. In the coming years we hope to find out if our polymer can make a difference there, since it is already bio-compatible.’
For the short term, Portale mainly thinks about sensors. ‘The conductivity we measure in our material is influenced by the environment, like humidity, volatile chemical species or temperature. So changes in all these quantities can be measured using our material.’ However, before all these dreams come true, there are a lot of questions to be answered. ‘I am very proud that we were able to design, make and control these new materials on a molecular scale, and build them from scratch. But we still have to learn a lot about their capabilities and see if we can improve them even further.’
De novo rational design of a freestanding, supercharged polypeptide, proton-conducting membrane. Chao Ma, Jingjin Dong, Marco Viviani, Isotta Tulini, Nicola Pontillo, Sourav Maity, Yu Zhou, Wouter H. Roos, Kai Liu, Andreas Herrmann and Giuseppe Portale
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