Small-Scale Robotics. From Nano-to-Millimeter-Sized Robotic Systems and Applications

We designed microrobots in the form of autonomous and remotely guided microtubes. One of the challenges at small scales is the effective conversion of energy into mechanical force to overcome the high viscosity of the fluid at low Reynolds numbers. This can be achieved by integration of catalytic nano-materials and processes to decompose chemical fuels. However, up to now, mostly hydrogen peroxide has been employed as a fuel which renders the potential applications in biomedicine and in vivo experiments. Therefore, other sources of energy to achieve motion at the micro- nanoscale are highly sought-after. Here, we present different types of tubular micro- and nanorobots, alternative approaches to toxic fuels and also, steps towards the use of tubular microrobots as micro- and nanotools.

Building, powering, and operating structures that can navigate complex fluidic environments at the sub-mm scale are challenging. We discuss some of the limitations encountered when translating actuation mechanisms and design-concepts from the macro- to the micro-scale. The helical screw-propeller or drill is a particularly useful geometry at small scales and Reynolds numbers, and is one of the mechanisms employed by microorganisms to swim. The shape necessarily requires three-dimensional fabrication capabilities which become progressively more challenging for smaller sizes. Here, we report our work in building and operating these screw-propellers at different sizes. We cover the length scales from the sub 100 nm to drills that are a few hundred microns in length. We use a known physical deposition method to grow micron-sized magnetic propellers that we can transfer to solutions. We have recently succeeded in extending the fabrication scheme to grow nanohelices, and here we briefly review the technical advances that are needed to grow complex shaped nanoparticles. The microstructures can be actuated by a magnetic field and possible applications of the micro- and nanohelices are briefly discussed. We also present a system of polymeric micro-screws that can be produced by micro-injection molding and that can be wirelessly driven by an external rotating magnetic field through biological phantoms, such as agarose gels with speeds of ~200 μm/s. The molding technique faithfully reproduces features down to a few microns. These microdrills can serve as a model system to study minimally invasive surgical procedures, and they serve as an efficient propeller for wireless microrobots in complex fluids. The fabrication scheme may readily be extended to include medically approved polymers and polymeric drug carriers.