1. Microsystems, Department of Mechanical Engineering, Eindhoven University of Technology, 5600 MB Eindhoven, The Netherlands
2. Institute for Complex Molecular Systems (ICMS), Eindhoven University of Technology, 5600 MB Eindhoven, The Netherlands
3. Philips Research, Royal Philips, High Tech Campus, 5656 AE Eindhoven, The Netherlands
4. Novel Devices Laboratory, Biomedical Engineering Dept., Univ. of Cincinnati, Cincinnati, Ohio 45221, USA