Functionalisation of implants by use of shape memory materials for temperature and force sensitive applications

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Shape memory materials, above all Nitinol, are of particular interest for minimally invasive applications in medicine. Their unique material properties allow a high degree of miniaturization and the realization of actuator functions. However, the design process of such shape memory actuators is complex and insufficiently controlled. Therefore, the use of Nitinol in medical engineering is up to now restricted to applications which do not require an accurate temperature-dependent behavior or for which the actuation movement can be generously and coarsely designed. This is the case when the transformation temperatures can be chosen considerably below or above body temperature or when the surrounding tissues are sufficiently robust (e.g. bone around compression staples).The situation is different when functionalizing the electrode carrier of a cochlear implant (CI) by use of Nitinol. The desired change of its shape from a straight configuration required for implantation towards a spiral shape (adapted to the cochlea) suggests the use of shape memory materials. However, an unsolved challenge is the concurrent preservation of the functionally important, membranous structures inside the inner ear. Consequently, the temperature-dependent actuation movement and the corresponding forces on the surrounding tissue have to be controllable and adjusted very accurately. In addition, the narrow surgical access causes a high heat input from the surrounding tissues, so that a precise timing of the temperature-dependent actuator is crucial. Thus, the cochlear implant serves as a prime example of a clinical application that requires both a very accurate adjustment of the thermo-mechanical behavior of the actuator to the clinical conditions as well as sufficient strategies to control the heat flow.The proposed research project will address the special challenges of using thin Nitinol wires as actuator elements applied to the concrete example of the CI. The aim is to develop a sound knowledge for the dimensioning of wiry Nitinol shape memory actuators. Therefore, the influence of external forces on the thermal hysteresis will be quantified by a systematic thermo-mechanical characterization of different wire samples. This will be complemented by investigating the interdependence of the thermo-mechanical behaviour and the clinical preconditions (thermal: physiological threshold values, geometrical: anatomical boundaries, mechanical: maximum permissible contact forces). In addition, cooling methods suitable for the OR will be developed and active heating (externally controlled activation of the shape memory effect) will be experimentally investigated. Resulting surface temperatures will be quantified by use of thermography. This application-oriented basic research should support a broader utilization of miniaturized Nitinol shape memory actuators in medical technology.