Digitally Controlled Self-Assembly of Magnetic Materials by Micro Atmospheric Pressure Plasma Jet

Introducing a magnetic response to soft materials is of growing interest to a number of fields, with the ability to fabricate soft structures featuring spatially varied anisotropic magnetic properties being key to enabling many applications. Magnetically actuated soft robots are an example of this, demonstrating remarkable feats throughout healthcare and micro-robotic applications owing to their untethered operation and inherent dimensional scalability. In particular for medical applications, it is desirable to be able to tune the design of the soft robot to the specificities of the patient. Digitally driven fabrication approaches, such as 3D printing, provide the flexibility to do this, but come with limitations linked to processing speed, resolution, and compatible materials. Here we present a novel approach to the patterning of magnetic materials whereby they self-assemble into a programmable shape, overcoming these limitations. This is accomplished by exposing a silicone elastomer film to a highly localised source of plasma, produced by a bespoke computer-controlled micro atmospheric pressure plasma jet(μAPPJ), to selectively create patterns of plasma treatment across its surface. The plasma induces a chemical change on the surface of the film, which corresponds to a large increase in the polar component of free surface energy. Consequently, polar fluid dispersions containing customised magnetic nanoparticles, which have undergone chemical functionalisation to enable stable suspension in the fluid, align with the treated regions autonomously. Subsequent evaporation of the polar fluid then leaves solely the magnetic material. We show that through variation of the voltage and frequency used to generate plasma in the μAPPJ the resolution of the treatment can be varied between 50μm and 1200μm. Furthermore, a maximum processing rate of 3.6mm²s^-1 is orders of magnitude quicker than comparable 3D printing techniques and due to the dynamically controllable resolution, large patterns with fine details can be processed efficiently. We demonstrate the process through the fabrication of stretchable, soft and magnetically responsive thin films.