Investigation of the extrinsically activated shape memory effect for the functionalization of implants in temperature and force sensitive applications

• Applicants: Dr.-Ing. Sebastian Herbst; Dr. Thomas Stephan Rau
• Subject Area: Biomaterials ; Otolaryngology, Phoniatrics and Audiology ; Synthesis and Properties of Functional Materials
• Term: since 2016
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 316068314

Objective of the research project is the functionalization of the electrode array (EA) of a cochlear implant (CI) by use of a shape memory effect (SME). The aim is to change the shape of the EA from the straight configuration required for the insertion process into a spiral or curved shape, which has been described to be advantageously for the lifetime functioning of the implant. The challenge is that delicate and fundamental anatomical structures in the inner ear need to be preserved, hence the temperature driven phase transition and the temporal and spatial coordination of the resulting shape change need to be carefully controlled. In a first funding phase, an EA with SME effect activated by body temperature (TB) was realized. However, this variant requires artificial cooling of the cochlea (local hypothermia) to prevent unwanted premature shape change due to body heat. Therefore, an extrinsic activation of the shape change by resistance heating will be developed, which is independent of TB and allows a shape change that is freely controllable in time and geometry. Previous work has shown that the widespread Nitinol alloy SE508 is not suitable for that purpose. The focus of the project is therefore on alternative NiTi alloys, such as NiTi with approx. 50.1 to 50.3 at.% nickel as well as ternary NiTiCu alloys. Fundamental investigations of the thermo-mechanical behavior in combination with numerical simulations of the heating process will clarify whether these alloys can be used to meet the requirements of force and temperature-sensitive applications. Therefore, it has to be investigated whether actual transformation temperatures above TB can be achieved for a stable straight configuration while maintaining a preferably low energy input for thermal activation. By modeling and controlling the energy input into the implant, the heating of the surrounding cochlea is to be limited to a physiologically acceptable level. Additionally, methods for monitoring the electrical resistance will be developed that will allow for conclusions about the heating process and progress of the phase transformation. For the first time the speed of the resulting shape change will be included in the research as rapid movement inside the organ may cause damage. Pulsed or “cascaded” power supply strategies will be applied to slow down the shape change in a targeted manner. Furthermore, the question of which influence anatomical variations of the cochlea have on the perimodiolar forces will be addressed. This should allow an initial assessment of whether an average-sized SM inlay can be used across different patients, or whether different (“convection sizes”) or even individualized inlay geometries will be necessary in the future.

• DFG Programme: Research Grants