Computer assisted optimization of an electromagnetic transducer design for implantable hearing aids
Introduction
Implantable hearing aids are a dynamic area of research. Several different types of both, totally and partially implantable hearing aids, are currently under development [1], [2]. When compared to conventional hearing aids, such implantable aids hold the promise of substantial improvements regarding reduced sound distorsion and, consequently, better sound quality and speech recognition, reduced feed-back, better cosmetic appearance and less discomfort due to the occlusion of the ear canal [2].
The single most important component of an implantable hearing aid is the output transducer. It is the equivalent of the loudspeaker in conventional hearing aids but provides a direct mechanical interface in the human middle ear, usually at the ossicular chain. Different types of output transducers have been proposed. Electromagnetic [3] or piezoelectric [4] transducers driving the ossicular chain by means of a driving rod have been shown to be able to provide high output levels of up to sound pressure level (SPL) [3]. However, the surgical procedure is usually complex, and additional conductive hearing losses due to the additional mechanic load of the driving rod cannot be ruled out. Electromagentic floating mass transducers [5] provide only limited output at low frequencies and currently cannot be implanted through the external auditory canal, necessitating a mastoidectomy. Simple electromagnetic transducers consisting of a coil and a permanent magnet have been proposed by several authors [6], [7]. This simple contactless design promises several advantages over piezoelectric and more sophisticated electromechanic transducers [8] including
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reduced risk of malfunctions due to wear and time
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apart from the weight of the part of the transducer which is attached to the ossicular chain, an absence of bias-forces which could cause tissue erosion and promote mechanical device failure
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both parts of the transducer can be implanted or, if necessary, exchanged independently
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minimally invasive implantation through the ear canal, similar to the transcanal approach in middle ear surgery, is probably possible.
The force generated by such contactless electromagnetic transducers depends strongly on geometry and on the relative placing of the components. To our knowledge, so far the effect of changes in geometry and relative placement have not been investigated systematically. For hearing aid applications, this knowledge is essential in order to insure sufficient acoustic output at acceptable power consumption by optimizing the transducer design. This investigation aims to close this gap for a specific contactless electromagnetic transducer designed to be implanted using a minimally invasive transcanal approach.
The paper is organized as follows. Section 2 introduces a simple contactless electromagnetic transducer. In Section 3, the factors limiting the range of realistic design parameters are discussed. Section 4 describes the materials and methods used in the computer simulations and in the experimental verification. Results are presented and discussed in 5 Results, 6 Discussion, respectively.
Section snippets
A simple electromagnetic transducer
Several configurations of electromagnetic transducers for implantable middle ear hearing aids are conceivable and have already been described [6], [7]. All of them are based on the principle of a controlled variable force between a coil and a permanent magnet.
Fig. 1 shows a schematic representation of the configuration of an electromagnetic transducer considered in this research. The transducer consists of an axially polarized permanent magnet PM mounted at the manubrium mallei MM, and a coil
Factors limiting the design parameters
A number of factors, including geometry, weight, biocompatibility and tissue warming due to power dissipation of the coil, limit the range of design parameters of electromagnetic transducers.
Reference coil
A simulation procedure using Matlab (The MathWorks Inc, Massachusetts, US) was developed to calculate the static force of a coil–magnet configuration with finite dimensions. Calculations were based on the laws of Biot Savart and force calculation on a magnetic dipole in an inhomogenous magnetic field [17]. This procedure has been found to be more efficient than finite element models which are optimized for systems where most of the magnetic flux is guided in metallic materials. In our
Reference coil
The left-hand diagram in Fig. 4 shows the axial force as a function of the width of the air gap z and the radial displacement ρ of the permanent magnet. The right-hand side of Fig. 4 shows the vector force field, i.e. the axial and the radial component, as a function of the radial displacement of the magnet at a fixed air gap of .
For any constant radial displacement ρ, force decreases for larger air gaps z. For small air gaps , there are two separate maximums as a function of the
Discussion
For the reference coil, the force generated by the transducer has been both simulated and verified experimentally. The results of our computer simulations and of the experiments are in good agreement, confirming the correctness of the computer simulation approach used. The actual time to perform a complete set of simulations (i.e. the data for one coil, as shown in Fig. 4) is approximately on a Sun Ultrasparc 10 Workstation. This is substantially longer than the actual measuring time of
Summary
A contactless electromagnetic transducer design for implantable hearing aids, consisting of a coil and a permanent magnet, is investigated. The transducer is intended to be implanted in the middle ear using a minimally invasive surgical procedure through the external auditory canal. The transducer is investigated and optimized using computer simulations for different coil designs and for a range of radial displacements and air gaps. A subset of the simulation results are verified experimentally
Acknowledgements
We would like to thank Ms. E. Clamann for her help in preparing this text. This work has been supported by the Gebert Ruef Foundation, Basel, Switzerland and the Commission for Technology and Innovation (CTI), Berne, Switzerland.
Christof Stieger received his Diploma in Electrical Engineering and his master diploma in Medical Physics from the Swiss Federal Institute of Technology (ETH), Zurich in 1998 and 2002, respectively. He works currently as an academic assistant at the Audiology Department at the University-ENT clinic of Berne, Switzerland. His research interests include biomedical transducers and implantable devices.
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Evaluation of implantable actuators by means of a middle ear simulation model
2010, Hearing ResearchAnatomical study of the human middle ear for the design of implantable hearing aids
2006, Auris Nasus LarynxThe influence of implantable actuators on residual hearing: A computational study
2014, Applied Mechanics and MaterialsHuman temporal bones versus mechanical model to evaluate three middle ear transducers
2007, Journal of Rehabilitation Research and Development
Christof Stieger received his Diploma in Electrical Engineering and his master diploma in Medical Physics from the Swiss Federal Institute of Technology (ETH), Zurich in 1998 and 2002, respectively. He works currently as an academic assistant at the Audiology Department at the University-ENT clinic of Berne, Switzerland. His research interests include biomedical transducers and implantable devices.
Daniela Wäckerlin received her Diploma in Micro Engineering from the University of Applied Sciences, Biel in 2001. She works currently as research assistant at the Department of Micro Engineering of the School of Engineering and Architecture in Biel, Switzerland. Her research interests include development and simulation of biomedical transducers and implantable devices.
Hans Bernhard received his diploma in Mikroengineering and his master diploma in Boimedical Engineering from the Swiss Federal Institute of Technology (ETH), Lausanne both in 1999. He works currently in the micro and medical engineering team at Helbling Technik AG Berne, Switzerland. His field of activity is focused on miniaturized transducers and implantable devices.
Martin Kompis received his Diploma and doctoral degree in Electrical Engineering from the Swiss Federal Institute of Technology (ETH), Zurich in 1989 and 1993, and his diploma in Medicine and MD degree from The University of Zurich in 1994 and 1995, respectively. He is currently Assistant Professor and Head of Audiology Department at the University-ENT.clinic of Berne, Switzerland. His research interests include biomedical acoustics and signal processing.
Andreas Stahel received his diploma (1983) and doctoral degree (1987) in Mathematics from the University of Zurich. He is currently teaching Mathematics at the HTA Biel. His interests include partial differential equations and numerical analysis.
Jürgen Burger received his Diploma and doctoral degree in Physics from the University Erlangen-Nürnberg, Germany, in 1987 and 1993. He is currently Professor for Microtechnology at the Berne University of Applied Sciences, Switzerland. His research interests include biomedical Microsensors and -actuators and implantable devices.
Rudolf Häusler received his diploma in Medicine and MD degree from the University of Bern in 1970. He was awarded the title of a Privat Docent at the University of Geneva in 1980 and became Professor of otolaryngology at the University of Berne in 1991. He is currently Head of the University-ENT clinic of Berne, Switzerland.