New Material Could Give Robots Artificial Muscles

An international research team led by scientists at Penn State University has developed a new type of polymer that they say could be used as artificial muscle in medical devices and advanced robots. A research paper describing the synthesis and testing of this new material was recently published in the journal Nature Materials.

Building a high-performance actuator

To make a robot move, you need an actuator. Actuators can be any kind of device or material that converts energy into physical motion. Since mechanical strain – how much a material deforms when force is applied to it – is an important characteristic for actuators, these parts are traditionally made out of very rigid materials. But in a bid for more flexibility and environmental adaptability, researchers have also begun to investigate whether softer materials could also be used.

“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 and a co-corresponding author of the study. “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.”

Ferroelectric materials are one of the most promising materials that are currently being studied for new actuator designs. They have a unique property whereby they spontaneously polarize, making one end of the material positively charged and the other negatively charged, in a way that can be reversed when an electric field is applied. The strain that occurs in these materials during that transition can cause the material to change shape – converting that electrical energy into mechanical energy – making them potentially useful for actuator applications.

Hard ceramic ferroelectric actuators have already been reported. But ferroelectric polymers are cheaper, lighter and more flexible – all important characteristics if a material is looking to be used in robotics. They also exert a significantly higher amount of electric-field-induced strain compared to their ceramic counterparts. However, they also generate comparatively less force, and this has severely limited their practical applications.

Nanoparticles make ferroelectric muscles a reality

In this new study, the Penn State University researchers set out to design a new class of composite ferroelectric polymers that can overcome these current limitations.

"In this study we proposed solutions to two major challenges in the soft material actuation field,” Wang said. "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.”

The second challenge is that ferroelectric polymer actuators typically require a very high driving field – that initial force that triggers the strain and shape change – in order to function.

The solution developed by the researchers was the creation of percolative ferroelectric polymer nanocomposites – a kind of microscopic sticker attached to the polymer.

Distributing functional nanoparticles into the polymer effectively creates a network of poles within the polymer. In testing, the researchers found that this network enabled the ferroelectric phase transitions to be induced by much weaker electric fields than a ferroelectric polymer would normally require. Indeed, using Joule heating – a process where passing electric current through a conductor produces heat – the researchers could induce the phase transition using less than 10% of the electric field strength typically required.

“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 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 robotics.”

Reference: Liu Y, Zhou Y, Qin H, et al. Electro-thermal actuation in percolative ferroelectric polymer nanocomposites. Nat Mater. 2023;22(7):873-879. doi: 10.1038/s41563-023-01564-7

This article is a rework of a press release issued by Penn State University. Material has been edited for length and content.