A team of international researchers led by engineers at Pennsylvania State University has developed a new type of ferroelectric polymer that’s exceptionally good at converting electrical energy into mechanical strain. According to the researchers, the polymer could be used in high-performance motion controllers or ‘actuators’ in medical devices, advanced robotics and precision positioning systems.
Mechanical strain – how a material changes shape when force is applied – is an important property for an actuator. Traditional actuator materials are rigid, but soft actuators such as ferroelectric polymers display higher flexibility and environmental adaptability.
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The research demonstrated the potential of ferroelectric polymer nanocomposites to overcome the limitations of traditional piezoelectric polymer composites, offering a promising avenue for the development of soft actuators with enhanced strain performance and mechanical energy density. Soft actuators are of particular interest to robotics researchers due to their combination of strength, power and flexibility.
‘Potentially, we can now have a type of soft robotics that we refer to as artificial muscle,’ said Qing Wang, Penn State professor of materials science and engineering. ‘This would enable us to have soft matter that can carry a high load in addition to a large strain. So that material would then be more of a mimic of human muscle – one that is close to human muscle.’
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However, there are a few obstacles to overcome before these materials can meet their promise, and potential solutions to these obstacles were proposed in the present study. Ferroelectrics are a class of materials that demonstrate a spontaneous electric polarisation when an external electric charge is applied – the positive and negative charges in the materials heading to different poles. Strain in these materials during the phase transition, in this case conversion of electrical energy to mechanical energy, can completely change properties such as shape, making them useful as actuators.
A common application of a ferroelectric actuator is an inkjet printer, where electrical charge changes the shape of the actuator to precisely control the tiny nozzles that deposit ink on the paper to form text and images.
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While many ferroelectric materials are ceramics, they also can be polymers. Ferroelectric polymers exhibit a tremendous amount of the electric-field-induced strain needed for actuation. This strain is much higher than that generated by ferroelectric ceramics.
This property of ferroelectric polymers, along with their high level of flexibility, reduced cost compared to other ferroelectric materials, and low weight, holds great interest for researchers in the growing field of soft robotics.
‘In this study we proposed solutions to two major challenges in the soft material actuation field,’ said Wang. ‘One is how to improve the force of soft materials. We know soft actuation materials that are polymers have the largest strain, but they generate much less force compared to piezoelectric ceramics.’
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The second challenge is that a ferroelectric polymer actuator typically needs a very high driving field – a force that imposes a change in the system, such as the shape change in an actuator. In this case, the high driving field is necessary to generate the shape change in the polymer required for the ferroelectric reaction needed to become an actuator.
The researchers proposed to improve the performance of ferroelectric polymers by developing a percolative ferroelectric polymer nanocomposite – a kind of microscopic sticker attached to the polymer. By incorporating nanoparticles into polyvinylidene fluoride, the researchers created an interconnected network of poles within the polymer.
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This network enabled a ferroelectric phase transition to be induced at much lower electric fields than would normally be required. This was achieved via an electro-thermal method using Joule heating, which occurs when electric current passing through a conductor produces heat. Using the Joule heating to induce the phase transition in the nanocomposite polymer resulted in only requiring less than ten per cent of the strength of an electric field typically needed for ferroelectric phase change.
‘Typically, this strain and force in ferroelectric materials are correlated with each other in an inverse relationship,’ Wang said. ‘Now we can integrate them together into one material, and we developed a new approach to drive it using the Joule heating. Since the driving field is going to be much lower, less than ten per cent, this new material can be used for many applications that require a low driving field to be effective, such as medical devices, optical devices and soft robotics.’
The research has been published in Nature Materials.