Control of Robotic Micro- and Nanoswimmers at Low Reynolds Number using Magnetic Fields

Abstract

Microrobotics is a field of engineering with the goal to find suitable and simplistic ways to navigation and perform tasks in small spaces that is not easily accessible in a non-invasive manner using conventional means. The microrobots/microswimmers studied in this work have the capability to swim in microfluidic spaces at low Reynolds number and have promising potential for various biomedical applications; including, and not limited to, drug delivery which is the envisioned application in this work. In drug delivery, the main contribution of microswimmers is the use of precise control to navigate drug carriers to target sites and mechanical forces to overcome physical barriers en route. Here, the hydrodynamics of swimming in low Reynolds number environments and control strategies for manipulation of magnetic microswimmers were studies. The first and foremost principle is the low Reynolds number condition which implies that microswimmers in motion, due to their small size and velocity, ignore inertial forces. This is problematic due to the ineffectiveness of conventional swimming motions, such as that of fishes, and hinders the development of artificial microswimmers. Most existing artificial microswimmers, inspired by various microorganisms, focused on using chiral or flexible structures to generate non-reciprocal swimming motions in low Reynolds numbers; this inevitably brings complicity to the swimmers' structures and fabrication process. Here, it was shown that magnetically actuated simple microswimmers can be fabricated and controlled using self-assembly of magnetic beads; hence called bead-based microswimmers. This approach offer simplicity in both geometrical design and fabrication, at the same time, offer easy scalability in both physical size and deployment number. Notably, the simplest bead-based microswimmer is the three-bead achiral microswimmer which can produce propulsion through rotation with neither flexibility nor chirality. An achiral microswimmer is composed of three firmly connected magnetic beads, at the length scale of 10 µm, and relatively easy to fabricate with low cost and manufacturing time. A magnetic control system with approximate Helmholtz coils was used to control the microswimmers. Both directional and velocity control were successfully implemented to navigate the microswimmers at low Reynolds number. Multi-robot manipulation, modular robot control, and PIV characterization had been explored. Moreover, nanoscale swimmers had been fabricated and controlled in a similar manner, but imaging limitations confined most analysis within the microscale regime. The implication of the swimming phenomenon and robotic control demonstrated the potential to revision future developments of microrobots for biomedical applications by advancing the existing paradigms of low Reynolds number propulsion; possibly enabling simpler fabrication and design of micro- and nanoswimmers; and to take an important step towards addressing realistic concerns that had hindered the development for practical…