A tri-coil bellows-type round window transducer with improved frequency characteristics for middle-ear implants

Abstract

A number of methods to drive the round window (RW) using a floating mass transducer (FMT) have been reported. This method has attracted attention because the FMT is relatively easy to implant in the RW niche. However, the use of an FMT to drive the RW has been proven to produce low outputs at frequencies below approximately 1 kHz. In this study, a new tri-coil bellows-type transducer (TCBT), which has excellent low frequency output and is easy to implant, is proposed. To design the frequency characteristics of the TCBT, mechanical and electrical simulations were performed, and then a comparative analysis was conducted between a floating mass type transducer (like the FMT) and a fixed type transducer (like the TCBT). The features of the proposed TCBT are as follows. First, the TCBT's housing is fixed to the RW niche so that it does not vibrate. Second, the internal end of a tiny bellows is connected to a vibrating three-pole permanent magnet located within three field coils. Finally, the rim of the bellows bottom is attached to the end of the housing that hermetically encloses the three field coils. In this design, the only vibrating element is the bellows itself, which efficiently drives the RW membrane. To evaluate the characteristics of this newly developed TCBT, the transducer was installed in the RW niche of temporal bones and the velocity of the stapes was measured using a laser Doppler vibrometer. The experimental results indicate that the TCBT can produce 100, 111, and 129 dB SPL equivalent pressure outputs at below 1 kHz, 1-3 kHz, and above 3 kHz, respectively. Thus, the TCBT with one side coupled to the RW via a bellows will be easy to implant and offer better performance than an FMT.

Keywords: Middle-ear implants; Round window (RW); Tri-coil bellows-type transducer (TCBT).

Fig. 1

Conceptual illustration of a middle-ear implant that drives the cochlear round window (RW).

Fig. 2

(a) Floating mass type driving method, and (b) fixed type driving method.

Fig. 3

(a) Mechanical model of floating mass type transducer, (b) electrical analog model of a floating mass type transducer, (c) mechanical model of floating mass type transducer installed in the RW, (d) electrical analog model of a floating mass type transducer installed in the RW, and (e) comparison of frequency characteristics between unloaded (solid line: I₁) and loaded (dashed line: I₃) floating mass type transducers.

Fig. 4

(a) Mechanical model of fixed type transducer, (b) electrical analog model of the fixed type transducer, (c) mechanical model of the fixed type transducer installed in the RW, (d) electrical analog model of the fixed type transducer installed in the RW, (e) comparison of frequency characteristics between unloaded (solid line: I′₁) and loaded (dashed line: I′₂) of the fixed type transducer, and (f) comparison of loading-effect characteristics between the floating mass type transducer (black dashed line: I₃) and the fixed type transducer (red dashed line: I′₂). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 5

(a) The structure of the proposed bellows, and (b) the implemented bellows.

Fig. 6

Designed structure of the TCBT (left) and a photograph of a fabricated TCBT (right).

Fig. 7

(a) The unloaded vibrational characteristics of a circuit model response of the TCBT and a manufactured TCBT, (b) distortion characteristics of the TCBT under the unloaded condition, for three different input powers and (c) distortion characteristics of the TCBT with a 30 mg load on the bellows surface, for three different input powers.

Fig. 8

(a) Temporal-bone stapes velocity measurements at 1 Pa (94 dB SPL), compared with the ASTM-F2504 standard, (b) photographs of the implantation process (left) and after TCBT implantation in the RW niche of a human cadaveric temporal bone (right), and (c) the measured stapes velocity of a TCBT for a 2 mW input (pink solid line), compared with the sound-driven results at 94 dB SPL (black solid line) (N = 5). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 9

(a) Comparison of equivalent SPL of TCBT (black) and FMT (red: from Shimizu et al., 2011) driven by 2 mW (Shimizu et al.), and (b) comparison of response tendency according to RW stimulation by TCBT (black) and FMT (red: from Nakajima et al., 2010). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)