1. Laboratory of Physical Chemistry of Polymers and Membranes, Ecole Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland, Microsystems Laboratory, EPFL, CH-1015 Lausanne, Switzerland, and Centre Suisse d'Electronique et de Microtechnique (CSEM), CH-2007 Neuchâtel, Switzerland