Our interest is in biosensors where micron-sized magnetic beads, clad with antibodies, are used to scavenge proteins from a fluid. This fluid has been extracted from a patient, and the sensor is used to detect extremely small concentrations of marker proteins which are indicators for diseases. These proteines must make it to the surface of the spheres, so that they can be detected.
It appears that diffusion is much too slow, and we must move around the beads actively. The problem is that at these small scales, fluids cannot be mixed easily, so that also moving the spheres does not help, unless we do something smart.
The tiny magnetic beads of the biosensor can be manipulated by external magnetic fields. For example, they be rotated in complicated patterns while sitting still in the fluid. The question is how to rotate them to enhance radial mixing and to enhance the capture rate of proteines on the bio-activated surface of the beads. It turns out that very specfic ways of rotation can create "worm holes" that can funnel proteins in a radial direction to the surface. In these simulations ordinary molecular diffusion is turned off, and we see the position of marker molecules at stroboscopic instants at each period of the motion protocol.
Computer simulations are one thing, but experiments must be done to test the robustnes of magnetic bead motion protocols. After all chaos is chacterized by a sensitive dependence on the variation of parameters, and it is chaos which will be exploited to enhance the working of the biosensor. It turns out that the scavenging efficiency of a micro bead can be modelled by the heat transfer of the sphere in the experiment. In this way, a macro-scale experiment can be used to optimize the micro-scale biosensor.
Neehar Moharana, Ruben Trieling, Herman Clercx