Shape memory polymers for Electrode Array insertion

Abstract

This thesis investigates whether shape memory polymers (SMPs) can serve as the basis of a removable smart stylet for cochlear implant (CI) electrode array (EA) insertion. The concept targets the advanced-off-stylet (AOS) technique, which uses a passive metallic stylet to straighten a pre-curved EA during perimodiolar placement. Despite its clinical adoption, this technique carries risks of tip fold-over and friction-induced force spikes that can cause intracochlear trauma. The proposed smart stylet addresses these limitations by replacing the passive stylet with an actively controllable SMP-based alternative. Rather than a single passive release, the smart stylet supports insertion, aims to reduce uncontrolled force spikes during withdrawal and can be reactivated in vivo to correct tip fold-over without revision surgery. To evaluate this concept, the research combined theoretical design criteria, COMSOL Multiphysics finite element modelling, polymer fabrication, and experimental validation. Two finite element models were developed to establish the quantitative design boundaries within which a functional SMP stylet must operate. A transient heat transfer model was developed based on a validated uncoiled cochlear geometry [1]. Using the CEM43 thermal dose criterion [2], this model revealed a clear trade-off between activation temperature and cochlear tissue safety. Activation temperatures equal to and above 44◦C reduced the safe heating window too strongly for practical surgical use. This established a target glass transition temperature range of 38− 44◦C for any candidate SMP material. Furthermore, a beam-based mechanical model quantified the minimum counter-moment required to maintain the pre-curved electrode array in a fully straightened configuration at 2.21· 10−5Nm [3]. The basal segment contributed approximately 67% of the total resistance. Together, these models determined the safe temperature window and mechanical boundary conditions under which the smart stylet concept can operate. Additionally, they established the quantitative constraints that guide minimal viable product creation later in this research.
In practice, a poly(ethylene glycol) diacrylate (PEGDA-400) / N-hydroxyethyl acrylamide (HEAA) / Irgacure 2959 formulation was selected as the only fabrication route achievable within the practical constraints of the Else Kooi Laboratory (EKL) at TU Delft. An initial wafer-based spin-coating approach proved unsuitable for the low-viscosity resin. Therefore, fabrication was successfully transitioned to a mould-based UV-curing method. The resulting specimens demonstrated an intrinsic SME under dry conditions, with a mean shape fixity ratio of 98.7 ± 1.0% and a mean shape recovery ratio of 97.7 ± 2.2% [4]. However, the formulation showed two fundamental limitations. The estimated dry glass transition temperature of the 55:45 PEGDA-400:HEAA specimens was−9.6 ± 1.1◦C, which lies far below the clinically required activation window [5, 6]. Furthermore, the transition temperature proved highly unstable under ambient humidity due to moisture plasticization [7]. Even more so, all specimen geometries fractured or dissolved under aqueous conditions due to swelling-induced stress and hydrolytic degradation of the acrylate ester network [8]. These failures were fundamental to the material chemistry and could not be resolved by extended UV post-curing. Overall, this thesis should be seen as a foundational feasibility study. The COMSOL models provide a clear quantitative design framework for future material selection. Additionally, the laboratory work confirms both the fabrication route and the basic shape memory mechanism of PEGDA-based covalent networks within the EKL infrastructure. However, the PEGDA-400/HEAA formulation is not suitable as a final clinical material for the smart stylet design. Future progress will depend primarily on selecting a more suitable SMP chemistry. One that achieves a wet Tg in the desired 38− 44◦C range, maintains structural integrity under aqueous conditions, and provides reliable actuation behaviour within the mechanical boundaries defined by this work.