(Nanowerk News) A new type of ferroelectric polymer that is excellent at converting electrical energy into mechanical voltage holds promise as a high-performance motion controller or “actuator” with great potential for applications in medical devices, advanced robotics and precision positioning systems, according to an international research team led by Penn state.
Mechanical strain, the way a material deforms when a force is applied, is an important property for an actuator, which is any material that will deform or deform when an external force such as electrical energy is applied. Traditionally, these actuator materials are rigid, but soft actuators such as ferroelectric polymers exhibit greater flexibility and environmental adaptability.
The research demonstrates 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 improved strain performance and mechanical energy density. Soft actuators are of great interest to robotics researchers because of their strength, power and flexibility.
“Potentially we could now have the kind of soft robotics we call artificial muscles,” said Qing Wang, Penn State professor of materials science and engineering and co-author of a study recently published in Natural Ingredients (“Electro-thermal actuation in percolative ferroelectric polymer nanocomposites”). “This will allow us to have soft materials that can carry high loads in addition to large loads. So that material will then be more like human muscle, which is close to human muscle.”
However, there are several obstacles that must be overcome before this material can fulfill its promise, and potential solutions to these obstacles are proposed in this study. Ferroelectrics are a class of materials that exhibit spontaneous electrical polarization when an external electric charge is applied and the positive and negative charges on the material go to different poles. Strain in these materials during a phase transition, in this case the conversion of electrical energy to mechanical energy, can completely change their shape-like properties, making them useful as actuators.
A common application of ferroelectric actuators is inkjet printers, where an electric charge changes the shape of the actuator to precisely control tiny nozzles that deposit ink on paper to form text and graphics.
While many ferroelectric materials are ceramics, they can also be polymers, a class of natural and synthetic materials made of many similar units bonded together. For example, DNA is a polymer, like nylon. The advantage of ferroelectric polymers is that they exhibit a large amount of electric field-induced strain required for actuation. This strain is much higher than that produced by other ferroelectric materials used for actuators, such as ceramics.
These properties of ferroelectric materials, together with their high degree of flexibility, lower cost compared to other ferroelectric materials, and low weight, are of great interest to researchers in the growing field of soft robotics, robot design with flexible parts and electronics.
“In this study we propose solutions to two major challenges in the field of soft material actuation,” said Wang. “One is how to increase the strength of soft materials. We know soft actuation materials which are polymers have the greatest strain, but generate much less force compared to piezoelectric ceramics.”
The second challenge is that ferroelectric polymer actuators usually require a very high driving field, which is a force that forces changes to the system, such as deformation in the actuator. In this case a high driving field is required to produce the deformation in the polymer required for the ferroelectric reaction required to become the actuator.
A proposed solution to improve the performance of ferroelectric polymers is to develop percolative ferroelectric polymer nanocomposites – a type of microscopic sticker that adheres to the polymer. By incorporating the nanoparticles into a type of polymer, polyvinylidene fluoride, the researchers created a network of interconnected poles within the polymer.
This network allows a ferroelectric phase transition to be induced at a much lower electric field than would normally be required. This is achieved through the electro-thermal method using Joule heating, which occurs when an electric current passing through a conductor generates heat. Using Joule heating to induce phase transitions in polymer nanocomposites results in less than 10% of the electric field strength normally required for a ferroelectric phase change.
“Usually, strain and force in ferroelectric materials are correlated with each other, in an inverse relationship,” said Wang. “Now we can integrate them together into one material, and we are developing a new approach to power them using Joule heating. Because the driving field will be much lower, by less than 10%, this is why 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 robots.”