Design, processing, and characterization of nickel titanium micro-actuators for medical implants

Cochlear implants (CI) are complex medical implants used as a common therapeutic measure for deaf people who suffer from damage to the inner ear. The success of CI insertion, a manual surgery procedure, is highly dependent on the surgeon's experience. Additionally, more precise positioning of the electrode close to the membrane structures could increase the effectiveness of frequency selectivity and stimulus conduction. To overcome these limitations, the degree of deformation of the electrode during its insertion has to be controllable. This ability can be achieved by integrating micro-actuator elements of a nickel titanium (NiTi) shape memory alloy (SMA) inside the electrode. These elements are manufactured using selective laser micromelting (SLμM). Initially, different concepts of activation mechanisms for SMA actuators for CI electrodes are discussed. Following the rules of additive manufacturing on a microscale, the corresponding actuator design and manufacturing strategies are presented. Suitable SLμM process parameters to achieve high spatial resolution are identified. Due to the high process temperatures, material chemical properties, respectively, its functional behavior, may be affected using SLμM. Therefore, analyses of SLμM NiTi parts manufactured using carrier gas hot extraction as well as differential scanning calorimetry (DSC) are carried out. Force measurements verify the available recovery forces of the produced micro-actuators activated thermally by one way effect. A suitable additive manufacturing strategy that allows the repeatable production of micro-actuators at a resolution of less than 100 μm could be evaluated. Different anatomical geometries could be transferred from clinical data model to the manufacturing process. The processed NiTi parts meet the requirements of the ASTM F2063 concerning oxygen inclusion, which is an important condition to preserve shape memory functionality. DSC analyses reflect stable functional properties of the processed NiTi alloy independent of the adjusted laser parameters. Phase transformation of actuators could be actively proved using electrical current and passively using an external heat source.