The ability to control the motion of soft robots, coupled with their flexibility, gives them potential for use in applications ranging from biomedical technologies to manufacturing processes. Researchers are interested in using magnetic fields to control the movement of these soft robots because it can be done remotely and because magnetic fields are easily obtained from permanent magnets and electromagnets.
The NC State researchers have found a way of embedding long chains of nanoscale magnetite particles in sheets of elastic polymer to form a magnetic polymer nanocomposite. By applying a magnetic field, the researchers can control the way the nanocomposite bends.
The process begins by dispersing nanoparticles of magnetite into a solvent. A polymer is then dissolved into the mixture, which is poured into a mould to form the desired shape. A magnetic field is then applied, causing the magnetite nanoparticles to arrange themselves into parallel chains. The solution is dried, locking the chains into place, and the finished nanocomposite can be cut, to further refine its shape.
Sumeet Mishra, a Ph.D. student at NC State, said: "The nanoparticle chains give us an enhanced response, and by controlling the strength and direction of the magnetic field, you can control the extent and direction of the movements of soft robots."
The mechanism stems from the structure of the chains. The researchers have also constructed a simple model to explain how the chained nanoparticles affect the mechanical response in magnetic fields.
"The key here is that the nanoparticles in the chains and their magnetic dipoles are arranged head-to-tail, with the positive end of one magnetic nanoparticle lined up with the negative end of the next, all the way down the line," said Joe Tracy, an associate professor of materials science and engineering at NC State. "At issue is something called magnetic anisotropy, which is caused by assembling the nanoparticles into chains. When a magnetic field is applied in any direction, the chain re-orients itself to become as parallel as possible to the magnetic field, limited only by the constraints of gravity and the elasticity of the polymer."
The researchers believe this technique may be especially attractive for some biomedical applications. "Electrical control can raise safety issues for some medical applications," says Mishra. "And both electrical and light signals pose challenges in terms of communicating those signals to devices embedded in the body. Magnetic fields, on the other hand, pass through easily - and pose fewer safety challenges."
This technique is said to be relatively simple and easy to execute and uses inexpensive and widely available materials.
