How graphene deters mosquitoes

Materials World magazine
,
27 Sep 2019

Graphene proves to protect against mosquito bites and also neutralises their awareness of a human presence. Ceri Jones reports.

An ultra-thin layer of graphene oxide (GO) appears to prevent mosquito bites by acting as a physical and chemical inhibitor, making the mosquitoes unable to register a human presence. With the potential to be implemented into protective coverings and clothing, the material could protect people against mosquito bites, therefore also related illnesses, without the use of expensive, toxic chemical deterrents.

Gauging interest

A team from Brown University, USA, set out to develop a puncture barrier using light and strong GO. To test its effectiveness, lab-hatched, disease-free mosquitoes were placed in a box with a small open area for a person to press their arm against, exposing it to the mosquitoes.

‘We wanted to give the mosquitoes every chance to bite, so wanted nothing between them and the skin, and then the graphene and mosquito. But we found that if you just lay graphene on skin, you get air gas below it, which we didn’t want to be a barrier. So we used cheesecloth on the outside which was pressed down against the graphene element and that pushed against the skin,’ the university’s Professor of Engineering, Robert Hurt, told Materials World.

According to Hurt, when using a thin layer of GO, the mosquitoes did not even land on the arm and appeared uninterested, which hinted at blocked chemo-sensing. ‘We did behavioural analyses and came up with the conclusion that the mosquitoes were not sensing the human. We also put sweat on the outside of the material [facing inside the box] and then they did come, which confirmed that earlier on they did not detect a chemical signal,’ he said.

This is unsurprising as graphene generally prevents molecules from passing through it and as Hurt said, there is a whole variety of chemicals that mosquitoes are believed to sense and ‘any one of them or all could be blocked by graphene, if it is between the mosquito and the source’.

Wet-dry vulnerability

Blocking sweat molecules could be problematic in practice, as any wearable protective clothing would need to be breathable, but GO has been found to allow water moisture passage.

‘We developed breathable films for chemical protection that you can sweat through and regulate your body temperature because the back-permeation of various toxicants was mostly inhibited, although a few could leak back in. The graphene oxide films pass water selectively, so we imagine they would not pass these chemo-attracted, larger organic molecules at a significant rate,’ Hurt said.

‘Under normal conditions, the films are dry enough to block everything. But in conditions where they get wet externally, like if you waded through a river or if you were sweating profusely, then you could saturate it with water and it might become permeable at that point to some molecules. Either way, if the water alone or the sweat attracts mosquitoes and then the graphene oxide becomes soft and they are able to bite through it.’

The team will continue developing the GO due to its breathability, and is focused on solving the protection plus breathability conundrum. According to Hurt, several paths are being explored. ‘One can cross-link it, add a polymer, and add an external film that stabilises it. In all cases, we are looking to make it mechanically stable so it would still offer puncture resistance, but while retaining its ability to pass water vapour. It is going to take some work, but we are optimistic,’ he said.

‘That’s the big obstacle – how to modify the film to be stable and still pass water and not become brittle.’

Building up protection

Mosquitoes have a measured bite pressure of around 10µN, so although a GO monolayer could be an effective chemical barrier, this would be easier to bite through, necessitating multiple layers.

Hurt’s team made several films in thicknesses of a quarter, a half and one micrometre, consisting of around 250, 500 and 1,000 layers respectively. Tests indicated that the GO could be reduced to ultra-thin levels without compromising performance, but that this was not necessary in general, and would present more complicated and expensive methods in any future textile manufacturing.

‘We imagine you might have a sandwich architecture with two layers, with the graphene fully embedded,’ he said. ‘I think any additional layer could only increase barrier properties. Whether it is uniforms or recreational textiles or chemicals, there is a desire to make these better in a way that has less risk for human health and the environment, or even to replace the chemicals with some other technology.’