[1] ETH, Inst Robot & Intelligent Syst, Multiscale Robot Lab, CH-8092 Zurich, Switzerland
[2] Univ Autonoma Barcelona, Dept Fis, Fac Ciencias, E-08193 Barcelona, Spain
[3] ICN2 Inst Catala Nanociencia & Nanotecnol, Barcelona 08193, Spain
[4] ICREA, Barcelona, Spain