Introduction . Small-scale robots hold enormous potential for minimally-invasive medicine, for example, they may allow targeted drug delivery, in vivo sensing, and new minimally-invasive surgical procedures1 , 2. Recently, significant progress in the field has been made to wirelessly power and actuate nano- to millimeter robots in biological environments3 , 4. Controlled propulsion has been shown in in vivo or ex vivo experiments, such as microrobots swimming in mouse int...