Measuring Forces on Immobilized Yeast Cells

Abstract

The interaction of cells with biomedical sensors or sampling systems of bioreactors is one of the most crucial problems affecting the long-term stability of such devices. Cell adhesion on sensors can be influenced by the use of different sensor materials or material coatings [1]. The cell-materialinteraction can be characterized, e.g., by the adhesion force. Additionally, filter blocking on sampling devices for large scale bioreactors can be avoided under different hydrodynamic conditions. A new approach for small volume bioreactors consists in the application of high frequency electrical fields in connection with the utilization of microsytem components in sampling systems [2]. The physical background is the so called dielectrophoretic effect, which describes forces on non-conducting but polarizable particles caused by high frequency alternating fields in the range of a few kHz to some hundreds of MHz. This dynamic effect is also detectable on microorganisms. Forces are repulsive (negative dielectrophoresis) or attractive (positive dielectrophoresis) depending on frequency, conductivity, geometric conditions of the electrode structure, but also on the complex dielectric properties of the cells (or microorganisms) and the surrounding medium [3]. The action of the dielectrophoretic force can be visualised easily by light microscopy [2], but a direct measurement of the force itself is impossible by that method. Therefore, we used the Atomic Force Microscope (AFM) as a force sensor. A necessary step in such experiments is the immobilization of cells on an AFM cantilever [4]. In our study yeast cells were immobilized on the cantilever. In one application the adhesion force of the cells was measured on different materials. Another goal of this work was the characterization of the dielectrophoretic force on immobilized cells. But an immediate measurement of the dielectrophoretic force [5] is complicated by a superimposition with electrostatic forces [6]. Nevertheless, the measured force can be splitted because of different dependencies of both forces on the AC voltage and a DC offset. As a result, a qualitative detection of the dielectrophoretic force is shown for immobilized yeast cells. A suitable method was found to immobilize yeast cells on an AFM cantilever (fig. 1). Force-distance-curves had shown a very small adhesion force of the cantilever with cells to an optical glass of type Borofloat B33 and virtually no adhesion of a blank cantilever on B33. In comparison a gold coated glas was used as a test system. We found small interactions of a blank cantilever to the gold surface, but considerable interactions in the case of the cantilever with immobilized cells. Finally, there was a larger adhesion force on a phosphorylcholine coated glass (PC 1036) in comparison to B33. On this occasion nearly equal forces appeared for blank and treated cantilevers with slight differences depending on the used cantilever type. First results were obtained to characterize the dielectrophoretic force acting on immobilized yeast cells.