Flexible polyurethane foam that could be used as a sensor material in industrial and sports applications was science fiction… not any more with nanoparticles, as Sarah Houlton reports.
Foam mattresses that can use a person’s motion to generate a voltage, allowing the movement of bed-ridden patients to be monitored, could be a step closer with technology from XOnano. The technology has already been successfully applied to denser foams used in sporting equipment.
The research that led to XOnano Smartfoam dates back to 2010. At that time Jake Merrell, now the company’s president, enrolled as an engineering undergraduate at Brigham Young University in Utah. Merrell spent some time in a nanocomposite lab that was making strain gauges out of silicone rubber. Merrell was adding nanoparticles to silicone materials that generated a piezoresistive response. This turns the silicone into a strain gauge that allowed the amount of stretch to be measured.
While the gauges allowed stretch to be measured, compression was a different matter. A project to create a foam-based sensor that might be able to measure compression was initiated. At the heart of the technology is a piezoresistive foam that undergoes a change in electrical resistance when it is compressed.
The real breakthrough came when Merrell accidentally attached the wrong monitor to the foam – one that measured voltage, not resistance. ‘We inadvertently discovered that the foam was creating a voltage when it was compressed,’ he said. ‘We’d discovered a piezoelectric foam.’
The foam’s piezoelectric response is caused by the triboelectric effect. This occurs when the polymer matrix and the nanoparticles embedded within it rub against each other.
‘This is a transfer of electrons from a material with one triboelectric affinity to one with a different affinity,’ Merrell said. It’s the same effect that you see if you rub a balloon on your sweater, and then put it near your hair, causing the hair to be attracted to the balloon.
‘When the two materials rub against each other, the electrons transfer,’ he said. ‘That same jump of electrons occurs when you impact the foam. This results in an internally generated charge.’
They found that this effect occurs with different foams, but polyurethane gives a good response, and it is easy to incorporate nanoparticles into the material, Merrell said.
‘We have played with silicone and latex foams and they do work, as long as there is that triboelectric disparity,’ he said. ‘Polyurethane foam has a very high affinity, and the affinity of the nanoparticles is low, so there is a very good response. The effect is less strong with latex and silicone.’
XOnano Smartfoam has been working with Chandler, Arizona-based Rogers to develop the foams. ‘Rogers has been an excellent source of polyurethane knowledge and they helped us develop our technology in each of their incredible resilience and high-tech foams,’ Merrell said.
‘The polyurethane foams we market under the Poron brand name are interpenetrating polymer networks of polyester and polyether urethanes,’ said Dave Sherman, new business development manager at Rogers. ‘There is a good match between the technology and our foams because they have such a high compression set resistance and low compression set, so the same electrical effect is seen every time.’ The nanoparticles are easy to incorporate, Sherman said, by simply mixing particles with the liquids before the foam is made.
Because it was easy to incorporate the nanoparticles into the formulation, a lot of development work was done with polyurethanes. Prototyping and iteration were straightforward, he said.
‘The Rogers foam has a highly micro-cellular structure, which enables a lot of interaction to occur between the particles and the foam, giving a high response,’ Merrell said. ‘Rogers can cast our foam in a wide range of thicknesses and densities, and the foams are already in use in many of the markets that we are trying to penetrate with the sensing capabilities.’
Rogers’ XRD material is designed to give extreme impact protection, and is already in widespread use. This protection can be achieved with thin layers of material because the polyurethane foam is rate-dependent. It behaves like a hard material if challenged with a sharp force, but is more rubbery and flexible if it is more gradually compressed.
The XRD material is widely used in protective sporting products such as American football helmets and mountainbike kneepads. Incorporating smart technology into the foam would allow impacts generated in football games, for example to be measured and monitored.
A taekwondo vest incorporating XOnano Smartfoam technology in Rogers’ XRD foam was the first product to hit the market. This was developed by start-up company 2020Armor in conjunction with XOnano and Rogers. 2020Armor is now working with the International Taekwondo Federation to get the vest used in competitive matches, including the Olympics.
The vest measures the number of kicks that hit it, and their magnitude. ‘We are ramping up production now, and it already has a commercial presence,’ Merrell said. ‘It makes the sport non-subjective. It gives more information about what is actually occurring to the person wearing the vest.’ It can also be used for training by putting it on a dummy. It could even ‘gamify’ the sport by allowing players to battle it out until health points run out, a little like computer games such as Mortal Kombat.
The foam is also being used in other sporting products, including shoes and insoles. ‘Using the Rogers foam, we put a layer in an insoles, and add electronics to measure the voltage the foam is generating,’ Merrell said. ‘That voltage allows us to perform gait analysis. This includes many aspects that could previously only be measured in a laboratory with wires attached to the shoes.’
XOnano is working with the US military to monitor the gait of soldiers in training. They believe the technology may make it possible to predict via gait analysis whether the soldiers are likely to develop an injury.
It even has potential industrial applications. The Smartfoam could be used as a bushing in a vibrating system. The change in resistance to strain could be used to monitor the magnitude and frequency of vibrations. Another application could be in gaskets to measure the pressure between two different surfaces. XOnano and Rogers are currently developing pressure sensing systems, and initial results are promising. ‘We hope to have products available to early adopters in the coming months,’ Merrell said.
XOnano is now working independently on a lower density foam for the bedding industry that can measure pressure. ‘While we were developing our bedding foam, we discovered that adding metal particles to the foam also increases its thermal conductivity,’ he said.
The company is working with a large US bed companies. It hopes that in 2019 it will be possible to release a thermal foam for mattresses that wicks heat away from the body. ‘We hope it will be in upwards of a million beds come 2019,’ Merrell said.
