Auxetic structures are a fascinating class of engineered materials that exhibit an extraordinary and counterintuitive property: it expands when stretched, in stark contrast to most common materials that get thinner under tension. This unique behaviour has captured the attention of academics and industry across various fields due to its potential for groundbreaking applications in biomedicine, aerospace, smart sensing and more.

One the largest fields of interest due to its market potential has been auxetic foams. The properties of auxetic foams have led to their exploration in diverse applications such as protective armour, sports equipment, biomedical tools, and even furniture and bedding. Its ability to enhance impact resistance and improve comfort in various applications makes it a promising avenue for advanced material design. While in small scale the properties of auxetic foam has undoubtedly been shown to outperform conventional foam, it has not seen widespread market use due to the previous complex methods of production. 

Principles of Auxetic Behaviour

The fascinating principles underlying auxetic behaviour in foam represents a significant departure from the typical mechanical response observed in conventional materials. At the heart of this phenomenon is the concept of negative Poisson’s ratio, which quantifies the material’s deformation when subjected to external forces. If we take a couple steps back and look at what this means, Poisson’s ratio helps us understand how materials respond to being pulled or squashed. 

It’s a way to describe whether they change shape a lot or just a little when you apply force to them. A material with a positive Poisson’s ratio, like conventional foam, becomes fatter when squeezed. Whereas a material with a negative Poisson’s ratio, just like Smart Materials’ foam, contracts when compressed. 

These contradictory responses to stress do not originate from the chemical composition of the individual foams, instead the structure of the cells within the foam dictates the behaviour. By taking a thin slice of each and placing them under a microscope you will realise that conventional foam has the cell structure similar to that of a honeycomb, whereas an auxetic foam has a re-entrant honeycomb structure. A honeycomb structure (Figure a) consists of hexagonal cells interconnected in a repeating pattern. In contrast, a re-entrant honeycomb (Figure b) features cells that fold inward, forming a pattern where each cell’s sides dip inward rather than extending straight out, resulting in a negative Poisson’s ratio and highly desirable properties such as enhanced support factor, toughness, breathability, energy absorption and even acoustic dampening.

How it’s Made

Manufacturing auxetic foam was never a simple process, it involved first synthesising conventional foam, which would then undergo complex treatment to be converted to auxetic foam. In essence the fundamentals behind all conversion methods are the same. An initial softening of the cell, followed by compression to buckle the cell, forming a convoluted structure. This creates the required cellular structure which exhibits auxetic behaviour under stress. While effective for research and laboratory-scale experiments, the conversion method faces several challenges that make it less viable for large-scale industrial applications.

The answer to this problem is Smart Materials. Our proprietary manufacturing technique enables us to tailor the cell structure, allowing for versatile customization of auxetic foam to suit a diverse array of potential applications. This positions Smart Materials as the ideal partner for anyone seeking to integrate auxetic foams into their product. We are able to achieve and deliver auxetic foam to the masses by foregoing the conversion method and making auxetic foam in-situ. This one pot process is cost-efficient, simple and environmentally friendly by avoiding the extra step of compression and post processing.

Enhanced Properties and Applications

With the one-pot process, we are making auxetic foam available to all. This technology boasts a remarkable set of characteristics that render it indispensable in various commercial applications. A quick literature search on the properties of auxetic foam will highlight the superior support factor, fatigue resistance and energy absorption compared to current foams on the market. 

Entirely unprecedented, the support factor of our auxetic foams have an initial soft and luxurious feel, yet as greater force is applied, a symmetrically increasing response of support follows as seen in the video below, making the foam contract under the point of force. This also means weight is distributed over a given area more evenly, thus improving pressure distribution and making it an ideal material to be used for comfort and support in mattresses and seats.

Fatigue resistance refers to the ability to withstand repeated cycles of compression without sinking in. Making it ideal for applications requiring long-term durability, such as in cushioning materials, sportswear, and automotive components, ensuring reliability and performance over extended periods of use. So even after a lifetime of use, the foam still feels brand new.

Auxetic foams possess exceptional energy absorption properties, efficiently dissipating impact and shock energy. Their unique cellular structure allows them to absorb and distribute forces evenly, making them invaluable for applications in safety gear, sports equipment, and automotive components, enhancing safety and reducing the risk of injury during impacts.

Conclusion

Auxetic foams represent a fascinating class of materials with remarkable mechanical properties, including their distinctive negative Poisson’s ratio, which sets them apart from conventional materials. These foams have potential across a wide range of industries, from healthcare and sports to upholstery and acoustics, thanks to their versatility, durability, and ability to enhance performance and safety. As our research and development continue, the potential applications for auxetic foams are likely to expand further, opening up new avenues for innovation and improved product design in the future.