Development of a Sperm‐Flagella Driven Micro‐Bio‐Robot

Abstract

A new biohybrid micro-robot is developed by capturing bovine sperm cells inside magnetic microtubes that use the motile cells as driving force. These micro-bio-robots can be remotely controlled by an external magnetic field. The performance of micro-robots is described in dependence on tube radius, cell penetration, and temperature. The combination of a biological power source and a microdevice is a compelling approach to the development of new microrobotic devices with fascinating future applications. The design of nanomotors and micro-bio-robots is of rising interest in nanomedicine and nanotechnology. There have been many successful approaches to develop artificial micromotors of various architectures which are driven by chemical fuels,1-6 surface tension gradients,7, 8 magnetic9-11 or electric fields.12, 13 The use of micromotors towards biologically related applications has led to interesting advances such as the drilling of tissues14, 15 and fixing cancer cells.2, 16 However, the use of toxic fuels is currently a challenge for any potential biomedical application of such motors and hence, a next generation of self-propelled microdevices is desperately sought after.2 The fascinating biomolecular motors found in nature offer a great source of inspiration for the design of artificial motors. As Steven Boxer mentions, “since we can't beat them (biomolecular systems) we should join them.”17 Motors found in nature are small but powerful and thus a reason to take them as archetypes. The first demonstration of integrating a biomolecular motor in a nanodevice was a combination of ATPase with a metal propeller.18 It has been demonstrated that there is potential in using motile microorganisms for the development of micro-bio-robots when they are integrated into microsystems.19-26 There are several mechanisms towards the directional control of micro-bio-robots such as taxis-based motion (magnetotaxis,22 chemotaxis,22, 23 thermotaxis etc.), electrokinetic control24 or geometrical asymmetry.25, 26 Bacteria-powered microrobots, magnetotactic bacteria, artificial bacterial flagella, flexible magnetic filaments, and our sperm-flagella driven micro-robots have all in common that they use flagella as driving force in low Reynolds conditions. Magnetotactic bacteria require generally a small magnetic field strength (0.4 mT)22 and the speed of a MC-1 cell (marine coccus strain) decreases only 15% when steered by an external magnetic field. Artificial bacterial flagella that are driven by a rotating magnetic field tend to require a slightly higher magnetic field strength for actuation. Magnetotactic, bacteria-powered and sperm-driven microrobots have in common, that they do not need an external power source for actuation, can be controlled by an external signal and their activity range is restricted to physiological conditions (temperature and pH tolerance of bacteria species and spermatozoa, respectively). Artificial bacterial flagella10 and flexible magnetic swimmers27 offer more flexibility in the design of the device and can tolerate a large temperature range. However, the artificial flagella are only actuated and steered if a continuously rotating magnetic field is applied. Flexible magnetic microswimmers require an oscillating transverse field in order to be actuated. Both kinds of artificial swimmers do not have an on-board power source. The artificial flagella presented by Tottori et al.10 moves at 100 μm s−1 at a frequency of magnetic rotation of 50 Hz and a field strength of 4 mT. However, none of these approaches offer the selective encapsulation of a single motile cell and directed control over its motility. Our sperm-tube hybrid system provides more flexibility regarding the type of organism that can be integrated: it can be adjusted to any flagellated organism if needed. One advantage of using sperm cells for the micro-bio-robot actuation is that these cells are readily available, easy to handle, do not need to be cultivated, and