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 ("De novo rational design of a freestanding, supercharged polypeptide, proton-conducting membrane"): a new class of polymers based on protein-like materials that work as proton conductors and might be useful in future bio-electronic devices.
What
do spiders have in common with batteries? Nothing so far, but material
developed by G. Portale and co-workers may change this in the future. The inset
shows a robust membrane created using a spider silk inspired polyelectrolyte
that is capable of efficiently transport protons.
Courtesy: Giuseppe Portale.
'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.'
Scheme
of the structure for the spider silk inspired proton conducting membrane
(left). Spider silk-like beta-sheet domains assembled together (centre). Their
surface is decorated by carboxylic acid groups able to release a proton at high
relative humidly (right).
Courtesy: Giuseppe Portale.
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.'