[1] ETH, Inst Robot & Intelligent Syst, CH-8092 Zurich, Switzerland
[2] ETH, Micro & Nanosyst Grp, CH-8092 Zurich, Switzerland
[3] ETH, Particle Technol Lab, CH-8092 Zurich, Switzerland
[4] Univ Autonoma Barcelona, Dept Fis, E-08193 Bellaterra, Spain
[5] Univ Autonoma Barcelona, ICREA, E-08193 Bellaterra, Spain
[6] Univ Manchester, Fac Life Sci, Manchester M13 9PT, Lancs, England