Biomedical applications

Key personnel: Dr Feodor Ogrin, Dr Peter Petrov, Prof Peter Winlove

Magnetic nanobots: construction and study of microscopic swimming machines.

The demand for miniaturisation in many areas of technology and medicine faces a number of challenges. How to deliver a precise amount of active agents to a localised target in the human body? How to design and implement complex spatio-temporal cascades of processes in e.g. microfluidic devices, such as timely delivery and homogenisation of reagents and transporting out of undesirable products in liquid volumes of the order of several nanoliteres? These are just a few questions which so far have proved difficult to solve. It is not surprising, therefore, that considerable efforts have and are being made to understand the physics behind the swimming of autonomous microscopic devices able to self-propel in viscous environments.

Diagrammatic representation of the dipole-pair system. The magnetic forces experienced by the particles are shown for two orientations of the external field.

In this study we aim to address the question of dynamics of swimming at low Re (Reynolds number), by suggesting a novel actuation mechanism generating the necessary shape sequence and providing the requisite energy for the motion. The main objective is to construct a microscopic magnetically driven device, which will generate linear motion by using non-reciprocal displacement of its components. The proposed system employs pairs of particles, one with “hard” and the other with “soft” magnetic properties. The alternating uniform external field can “switch” the magnetisation of the soft particle while leaving the state of the hard particle the same. The interaction of the particles with two different moments (parallel and anti-parallel) provides the required driving mechanism to activate linear motion of the system.

Trajectories of the hard bead (1), the middle particle (2), the centre of mass (3), and the soft particle (4), after application of an elliptically rotating external field. Different colours demonstrate the change in trajectories as a function of stiffness in the bending of two springs. In the case of minimal bending (theta ~ 180 degrees, green trajectory) the swimming is inhibited.

Recently we have developed a numerical model demonstrating swimming of such a device in glycerine. The simulations show that a device based on two ~1 micron diameter bids can generate a directional displacement with velocities exceeding 100 microns per second. This performance is well exceeding the natural microscopic swimmers such E-coli bacterium or sperm cells. The model also predicts that the motion of the magnetic swimmers can be controlled. In particular, the orientation of swimming depends on a number of parameters, including the frequency and the amplitude of the magnetic actuation. By changing either of these the swimmer can change direction within a broad range of angles, thus providing a simple mechanism to manipulate the motion.

Following the theoretical work we have constructed the first experimental prototype (see the attached film) of a magnetic swimmer based on the principles proposed. The device consists of two particles, a ‘hard’ neodymium boron iron particle, typically 0.03 – 0.2 mm³ in volume and a soft ‘iron’ particle 0.01- 0.1 mm³ in volume connected by a latex ring 1 to 3 mm in diameter. To observe the motion of the device it was floated on a glycerol/air interface and its trajectories determined from analysis of video images acquired through a low power microscope. The device was activated by an alternating uniform magnetic field produced by two parallel Helmholtz coils. The field amplitude and the frequency were varied between 0-100 G and 20 to 2000 Hz respectively. The motion consisted of two components, rapid oscillation of each particle with an associated deformation of the elastic ring, and translation of the centre of mass (‘swimming’). The details of the translational motion depended on the dimensions of the device and the applied field. In particular, as predicted by the model, it was possible to control its direction and speed by varying the amplitude and frequency of the applied field. Translational velocities of up to 500 microns/second were produced in a field of 100 G, thus demonstrating swimming in the low-Re domain (Re ~ 0.0005).

Magnetic stimulators atatched on to an elastic membrane

The demonstrated mechanism of actuation can be utilised in a number of applications. This includes a range of microfluidic devices: such as microscopic pumps, stirrers and mixers, and other applications where locally induced forces are required. Our group is currently looking at the possibility to utilise these principles in tactile devices, where the dipole-pairs would be playing a role of touch sensors/stimulators.

Theoretical aspects of this work were published in Physical Review Letters, vol 100 Article number 218102 (May 2008), which can be downloaded here.
This download is made available for personal use only. Any other use requires prior permission of the author and the copyright holder. Copyright (2008) American Physical Society. The article above may also be found on the journal's website.