Spider silk inspires new class of functional synthetic polymers

Date:
August 12, 2020
Source:
University of Groningen
Summary:
Synthetic polymers have changed the world around us. However,
It is hard to finely tune some of their properties, such as the
ability to transport ions. To overcome this problem, researchers
decided to take inspiration from nature and created a new class
of polymers based on protein-like materials that work as proton
conductors and might be useful in future bio-electronic devices.



FULL STORY ========================================================================== Synthetic polymers have changed the world around us, and it would be hard
to imagine a world without them. However, they do have their problems. It
is for instance hard from a synthetic point of view to precisely control
their molecular structure. This makes it harder to finely tune some of
their properties, such as the ability to transport ions. To overcome this problem, University of Groningen assistant professor Giuseppe Portale
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 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 a proton so I worked a lot on 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. He came to this idea together with professor
Andreas Hermann, a former colleague at the University of Groningen,
now working at the DWI -- Leibniz Institute for Interactive Materials in Germany. 'We could immediately see that proton- conducting bio-polymers
could be very useful for applications like bio- electronics or sensors', Portale says.

More active groups, more conductivity But first, they had to see if the
idea would work. Portale: 'Our first goal was to prove that we could
precisely tune the proton conductivity of the protein- based polymers by
tuning the number of ionisable groups per polymer chain'. To do this,
the researchers prepared a number of unstructured biopolymers that
had different numbers of ionisable groups, in this case, carboxylic
acid groups.

Their proton conductivity scaled linearly with the number of charged
carboxylic acid groups per chain. 'It was not groundbreaking, everybody
knows this concept. But we were thrilled that we were able to make
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,
which dramatically boosts proton conductivity.' Since the first batch
of biopolymers was completely amorphous, the researchers had to switch
to a different material. They decided to use a known protein that had
the shape of a barrel. 'We engineered this barrel-like protein and
added strands containing carbocyclic acid to its surface', Portale
explains. 'This increased conductivity greatly.' Novel Spider silk
polymer Unfortunately, the barrel-polymer was not very practical. It
had no mechanical strength and it was difficult to process, so Portale
and his colleagues had to look for an 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. 'I knew spider silk has a fascinating nanostructure, so we engineered a protein-like polymer that
has the main structure of spider silk but was modified to host strands
of carbocyclic acid.' 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.' The measured proton conductivity was higher than any previously known biomaterials, but they
are not there yet according to Portale: 'This was mainly fundamental
work. In order to apply this material, we really have to improve it
and make it processable.' Dreams But even though the work is not yet
done, Portale and his co-workers can already dream about applying their polymer: 'We think this material could be useful as a membrane in fuel
cells. Maybe not for the large scale fuel cells 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 factors in the environment,
like humidity or temperature. So if you want to store something at a
certain humidity you can place this polymer between two electrodes and
just measure if anything changes.' 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 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.'

========================================================================== Story Source: Materials provided by University_of_Groningen. Note:
Content may be edited for style and length.


========================================================================== Journal Reference:
1. Chao Ma, Jingjin Dong, Marco Viviani, Isotta Tulini, Nicola
Pontillo,
Sourav Maity, Yu Zhou, Wouter H. Roos, Kai Liu, Andreas Herrmann,
Giuseppe Portale. De novo rational design of a freestanding,
supercharged polypeptide, proton-conducting membrane. Science
Advances, 2020; 6 (29): eabc0810 DOI: 10.1126/sciadv.abc0810 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/08/200812144025.htm

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