A force study of on-chip magnetic particle transport based on tapered conductors

Recently, magnetic biosensors have shown to be promising alternatives for classical fluorescence-based detection schemes. While on-chip detection of magnetic particles is well established, research groups now start to explore the unique possibility to manipulate these particles by applying controlled magnetic forces. By immobilizing biomolecules onto the particle\´s surface, this results in a number of additional functionalities, turning the label-which was previously solely a means of detection-into a "smart" label. In this paper, we give an overview of the dominant forces acting on a magnetic particle when manipulated by transporting devices such as the ones that are currently developed at our lab. First, we describe a computational approach to predict the motion of the magnetic particle. It is based on a force balance of perpendicular forces to determine the particle/substrate separation distance. After this, we calculate the in-plane forces that determine the mobility of the particle and which are dependent on this separation distance. Next, we validate this model for the movement of particles in water. We then show how the surface forces influence the particle/substrate separation distance-and therefore the mobility of the particle-for various pH values and ionic strengths of the liquid.