Multilayered Microcoils for Microactuators and Characterization of Their Operational Limits in Body-Like Environments

Multilayered microcoils are of great importance for the development of advanced electromagnetic microactuators and especially important to develop high-sensitivity microsensors and magnetic field neural stimulators for medical applications. A clean room-free procedure for manufacturing multilayered micrometric coils is presented in this article. The production of miniaturized multilayered coils from tens to hundreds of micrometres long is demonstrated. The microcoils have outer diameters ranging between 150 and 300 μm, arrangements of up to five consecutive layers, and an average fill factor of 85%. This means three times smaller diameters than the smallest diameter ever achieved by winding techniques while keeping a high fill factor and a large number of layers. Such small and highly performant microcoils have never been demonstrated neither by winding processes nor epitaxial growth techniques. These microcoils were tested inside different human body-like environments. Maximum current density versus temperature was measured in air, fat tissue, muscle tissue, and simulated body fluid at 36 °C. A maximum current density of 3600 A/mm² has been measured before coil failure. Experiments demonstrated that current densities of up to 610 A/mm² can safely be supplied to coils without risk of harm to internal tissues. These counterintuitive values are orders of magnitude larger than typical current densities used in macroscale actuators windings.