Auxetic material girl

Daphne Attard talks to Tech Sunday about how the University of Malta Auxetics Group has gained a leading edge in the field of auxetics.

Daphne Attard, 27, has always nursed a passion for science. “As a young girl I attended government schools – Ġużeppi Aġius Primary School, Paola, and Margaret Mortimer Girls’ Junior Lyceum.

“In my secondary school years, I developed a passion for science subjects – the dedication my teachers had for both the subjects and the students left a good influence on me.”

After obtaining her secondary education certificate, Dr Attard continued her studies at the Ġan Franġisk Abela Junior College and at the University of Malta.

She graduated in 2007 with a BSc (Hons) degree in chemistry and physics and was awarded a scholarship grant through the Malta Government Scholarship Scheme to read for a PhD in chemistry, which she obtained in 2011.

“My main area of research is in systems with negative thermo-mechanical properties,” Dr Attard says.

“This refers to materials which expand laterally when stretched (auxetic materials), expand in one or more directions when compressed from all sides (negative compressibility materials) or shrink in one or more directions when heated (negative thermal expansion materials).

This type of counter-intuitive behaviour is not what one would usually expect from common materials, which very often show the opposite behaviour.”

Dr Attard has a fascination for auxetics.

“It all started when, during my undergraduate course, I attended the National Chemistry Symposium organised by the Chemistry Department. One of the talks which caught my attention was about auxetic materials – it was delivered by a student of Prof. Joseph Grima, who is the founder of the Malta Auxetics Research Group.

“The subject was very interesting primarily because these auxetic materials behaved against common expectations and also because the area seemed to be interdisciplinary.

“Besides, the area seemed to be very promising in terms of the practical applications these materials can have – so I made it a point to choose this area for my dissertation the following year.”

It turned out that after a year of work in this area, Dr Attard fell in love with the subject and decided to continue her studies on a doctoral level. After finishing here studies, she was given an academic post at the University of Malta, allowing her to continue her research on these materials.

Auxetics isn’t exactly a new area of research. However, it is only recently that auxetic materials received renewed attention.

“The fact that materials may theoretically exhibit this counter-intuitive behaviour has been known for a little over 100 years,” says Dr Attard.

In the early 1900s, German physicist Woldemar Voigt was the first to report that he experimentally found this property in a mineral called iron pyrite. His work suggested that the crystals somehow grew thicker when stretched.

Voigt did not explain this peculiar behaviour, and since no one had ever looked into the possible applications of this property at that time, it was ignored for decades.

Then in the late 1980s, researchers managed to intentionally make an auxetic foam from a commercially available one. This was a major breakthrough because it showed that it is possible to induce this property in materials and structures through their design. With this discovery, researchers started to realise the myriad applications these materials had. In view of this, a significant effort has been directed towards designing these materials and finding new applications.

The potential applications of auxetic materials are various, ranging from biomedical to protective and structural applications.

“For example, it is known that auxetic materials have a higher resistance to indentation so that when a force is applied, an auxetic material densifies at the point of indentation, providing extra protection where needed,” says Dr Attard.

“This property can be highly desirable in materials that are used in safety equipment like elbow and knee pads, bullet-proof vests, linings in crash helmet, automotive seats, cushioning and packaging.

“Besides, when compared to common materials, auxetic materials also tend to behave differently in response to bending – they form a dome-shaped surface as opposed to common materials which form a saddle-shaped surface.

“This means jackets and other protective equipments made of auxetic foams should also be more comfortable to wear due to the auxetics’ natural ability to wrap around and adapt to body curvature in a much better way than common materials.

“Obviously, soft foam-like auxetics may also be used in mattresses and ergonomic cushions. These may alleviate discomfort associated with prolonged times spent in a bed or a chair as very often experienced by bed-ridden and wheelchair-bound people, the elderly and passengers taking long flights who spend a significant amount of time sitting down.”

Auxetic materials also have a number of applications in the biomedical field.

“Smart textiles are being developed by a number of researchers – these may be employed in smart dressings which are impregnated with medication that is trapped in the textile in its unstretched state. When the wound swells, the dressing stretches and the pores open up to release the medication in proportion to the degree of swelling.

“When the swelling subsides, the pores close up again and prevent further release of medication. This is an efficient way to minimise the overexposure of patients to unnecessary medication.”

Auxetic systems are also being incorporated in heart stents. Stents keep blood vessels open and are aimed to treat blockages. The stents currently used are sometimes inflexible and may not behave in the same way that a blood vessel inside the body would.

“This can result in stresses within the walls of the blood vessels, resulting in adverse affects on the patient. Additionally, when the stents are deployed, they tend to get shorter and this may cause some difficulties in the correct placement of the stent within the vessel. Together with clinicians, experts in cardiac surgery and design and manufacturing companies, we are working on new auxetic geometries that can help minimise the negative effects of current stent designs.”

Of course, the development of new materials requires a lot of investment both financially and in terms of human resources.

“Much of the applications concerning auxetic materials are still at a research stage,” says Dr Attard.

However, there have been a number of improvements in this area, especially in auxetic foams, polymers and textiles which currently seem to be the most promising in terms of their applications. Efforts are being made to upscale and optimise production techniques to be able to manufacture them on a larger and ultimately industrial scale.

“Obviously, the end products would need to be further tested, especially if they relate to biomedical applications and where possible further refined to ensure the well-being of the end user. Thus we probably would have to wait for a few more years until we can see auxetic materials readily available on the market.”

The University of Malta Auxetics Group has a leading edge in the field of auxetics.

“The group is very active and publishes a significant number of articles yearly. Overall, the number of international publications by the Malta Group accounts for around 15 per cent of the total number of international publications in this area. This shows that the research we do is of a very high standard, despite the limitations that a small country like Malta may have.”