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International Journal of Audiology Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/iija20
Technical design of a new bone conduction implant (BCI) system a
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Hamidreza Taghavi , Bo Håkansson , Sabine Reinfeldt , Måns Eeg-Olofsson , Karl-Johan a
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Fredén Jansson , Emil Håkansson & Bayan Nasri a
*Department of Signals and Systems, Chalmers University of Technology, Gothenburg, Sweden b
ENT Department, Sahlgrenska University Hospital, Department of Otorhinolaryngology, Head and Neck Surgery, The Sahlgrenska Academy, University of Gothenburg, Gothenburg, Sweden c
Department of Computer Science and Engineering, Chalmers University of Technology, Gothenburg, Sweden Published online: 15 Jun 2015.
To cite this article: Hamidreza Taghavi, Bo Håkansson, Sabine Reinfeldt, Måns Eeg-Olofsson, Karl-Johan Fredén Jansson, Emil Håkansson & Bayan Nasri (2015): Technical design of a new bone conduction implant (BCI) system, International Journal of Audiology To link to this article: http://dx.doi.org/10.3109/14992027.2015.1051665
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International Journal of Audiology 2015; Early Online: 1–9
Technical Report
Technical design of a new bone conduction implant (BCI) system Hamidreza Taghavi*, Bo Håkansson*, Sabine Reinfeldt*, Måns Eeg-Olofsson†, Karl-Johan Fredén Jansson*, Emil Håkansson* & Bayan Nasri‡
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*Department of Signals and Systems, Chalmers University of Technology, Gothenburg, Sweden, †ENT Department, Sahlgrenska University Hospital, Department of Otorhinolaryngology, Head and Neck Surgery, The Sahlgrenska Academy, University of Gothenburg, Gothenburg, Sweden ‡Department of Computer Science and Engineering, Chalmers University of Technology, Gothenburg, Sweden
Abstract Objective: The objective of this study is to describe the technical design and verify the technical performance of a new bone conduction implant (BCI) system. Design: The BCI consists of an external audio processor and an implanted unit called the bridging bone conductor. These two units use an inductive link to communicate with each other through the intact skin in order to drive an implanted transducer. Study sample: In this study, the design of the full BCI system has been described and verified on a skull simulator and on real patients. Results: It was found that the maximum output force (peak 107 dB re 1 mN) of the BCI is robust for skin thickness range of 2–8 mm and that the total harmonic distortion is below 8% in the speech frequency range for 70 dB input sound pressure level. The current consumption is 7.5 mA, which corresponds to 5–7 days use with a single battery. Conclusions: This study shows that the BCI is a robust design that gives a sufficiently high output and an excellent sound quality for the hearing rehabilitation of indicated patients.
Key Words: Bone conduction implant; bone anchored hearing aid; implanted transducer; inductive links; long-term implantation earing-impaired patients with sensorineural hearing loss are generH ally rehabilitated by using air conduction (AC) hearing aids. However, in some patients with middle ear and ear canal disorders, AC devices are not sufficient or cannot be used. In these conditions, a bone conduction device (BCD) is an alternative to the AC device. In BCDs, the sound is converted to vibrations that are transmitted through the skull bone directly to the cochlea, bypassing the outer and middle ear (Håkansson et al, 1985; Stenfelt & Goode, 2005a). In conventional skin-drive BCDs (Reinfeldt et al, 2015a), the plastic casing of the transducer is pressed against the skin by a static force, using a steel spring headband, a softband, or frames of a pair of glasses. These solutions have drawbacks such as discomfort related to the static force and reduction of the blood circulation in the compressed skin. Moreover, the high frequency sensitivity is attenuated by the skin and soft tissues, and feedback problems that limit the gain are caused by sound radiation from the transducer via the casing or the skull bone to the microphone (Snik et al, 1995). In the early 1980s, the bone anchored hearing aid (BAHA) was introduced and is currently the most widely used BCD. The principal design of a generic BAHA is described in Håkansson et al (1985). The main indications for BAHA are pure conductive or mixed hearing losses (Håkansson et al, 1985; Tjellström et al, 2001; Snik et al, 2005). It can also be fitted in single-sided deaf patients in order to
reduce the head shadow effect (Wazen et al, 2010; Hol et al, 2010). The surgery for the BAHA is uncomplicated and safe. Today more than 150 000 patients have been rehabilitated with the BAHA (Cochlear, 2013; Oticon Medical, 2014) and two companies are commercially providing such devices; Cochlear Bone Anchored Solutions, Mölnlycke, Sweden, and Oticon Medical, Askim, Sweden. The most frequently described BAHA complications are skin and soft tissue irritations, infections around the skin penetrating implant, and complications related to the implant anchorage to the bone leading to implant losses (Tjellström & Granström, 1994; Tjellström & Stalfors, 2012; Wazen et al, 2011; Dun et al, 2012; de Wolf et al, 2008, 2009). Some patients do not accept the screw sticking out through the skin due to aesthetic reasons. Furthermore, in patients with need of high gain, the BAHA may cause oscillations due to feedback, mostly in higher frequencies (Taghavi et al, 2012a). To reduce some of these complications, several improvements have been made (Hultcrantz & Lanis, 2014; Håkansson, 2011; Dun et al, 2011).
The bone conduction implant A new bone conduction implant (BCI) system has been developed in a collaboration project between Chalmers University of Technology and the Sahlgrenska Academy, University of Gothenburg, both
Correspondence: Hamidreza Taghavi, Chalmers University of Technology, Department of Signals and Systems, 412 96, Gothenburg, Sweden. E-mail: [email protected] (Received 16 May 2014; accepted 8 May 2015) ISSN 1499-2027 print/ISSN 1708-8186 online © 2015 British Society of Audiology, International Society of Audiology, and Nordic Audiological Society DOI: 10.3109/14992027.2015.1051665
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Abbreviations AC AM AP ASIC BAHA BBC BC BCD BCI BEST DSP MPO MRI OFL RF SAR SPL THD USD
Air conduction Amplitude modulation Audio processor Application specific integrated circuit Bone anchored hearing aid Bridging bone conductor Bone conduction Bone conduction device Bone conduction implant Balanced electromagnetic separation transducer Digital signal processor Maximum power output Magnetic resonance imaging Output force level Radio frequency Specific absorption rate Sound pressure level Total harmonic distortion dollar
in Gothenburg, Sweden. The BCI has been introduced as an alternative to the percutaneous BAHA (Håkansson et al, 2008, 2010; Håkansson, 2011; Taghavi et al, 2012b; 2012c, Eeg-Olofsson et al; 2014; Reinfeldt, 2015b), and the principal design of the BCI system is shown in Figure 1. For the BCI, the skin and subcutaneous tissues are kept intact and no screw attachment to the skull bone is needed. Moreover, the BCI transducer is implanted closer to the cochlea than a BAHA, which increases the ipsilateral sensitivity to bone-conducted sound (Håkansson et al, 2008, 2010; Stenfelt et al, 2000; Stenfelt & Goode, 2005b; Eeg-Olofsson et al, 2008; Reinfeldt et al, 2014). In more technical details, the BCI consists of an externally worn audio processor and an implanted unit called the bridging bone conductor (BBC), as shown in Figure 2. The audio processor includes directional microphones, a single 675 hearing-aid battery, a digital signal processor (DSP), and an application specific integrated circuit
(ASIC) that drives the tuned transmitter coil. The BBC comprises a tuned receiver coil, a demodulator unit, and the capsuled transducer. To transmit the sound to the transducer through the intact skin, an inductive link (tuned transmitter and receiver coils) is used. The sound is amplitude modulated (AM) and transmitted wirelessly to the BBC. A retention magnet system, similar to those used in cochlear implants, is used to keep the audio processor in place and align the transmitter and receiver coils to optimize the signal transmission. The BCI transducer is less than half the size of the BAHA transducer and is permanently implanted in the temporal bone. The BCI transducer uses the balanced electromagnetic separation transducer (BEST) technology (Håkansson, 2003) and is sealed in a laserwelded titanium casing. A first inherent resonance is located in the low speech frequency range around 0.8 kHz. An additional spring suspension is added between the transducer and the titanium casing, creating a second resonance in the frequency response around 4.5 kHz that boosts the high frequency content of the speech.
Summary of ongoing clinical study Eight patients have been operated on in a clinical study under an approval from the Swedish Medical Products Agency and the regional ethical review board in Gothenburg. It has been concluded that the surgical procedure is regarded as both safe and uncomplicated with minor engagement of the mastoid portion of the temporal bone (EegOlofsson et al, 2014; Reinfeldt et al, 2015b). The BCI transducer is placed in a 4–5 mm deep recess, drilled in the mastoid part of the temporal bone (see Figure 1), and is secured using a thin titanium wire over the casing. Unlike the BAHA that has a screw attachment in the skull bone, the BCI transducer uses a flat surface contact to the skull bone. If there is a need for replacing the transducer, the surgical intervention is anticipated to be easier with this method of attachment compared to if the transducer is anchored with screws in the bone. The surgical method and audiological results of the first patient have been presented by Eeg-Olofsson et al (2014). Audiological and quality of life outcomes with the BCI on the first six patients have been addressed in a clinical study by Reinfeldt et al (2015b). In this study it was shown that the BCI improvement over the unaided condition was statistically significant in all audiometric measures. The improvement in puretone average hearing thresholds was 31.0 dB; the speech recognition threshold improvement in quiet was 27.0 dB, and the improvement of the speech recognition score in noise at normal speech level was 51.2 %. The signal-to-noise ratio threshold with the BCI was 5.5 dB at speech levels using Swedish “Hagerman sentences” (Hagerman, 1982) and the APHAB as well as GBI scores were improved compared to the unaided condition. Furthermore, preoperatively and according to the study protocol, the patients had been using a conventional BCD (Ponto Pro Power) on a softband for a period of one month. The BCI audiometric results compared to the conventional BCD results showed to be similar or better.
Technical design
Figure 1. The full-scale BCI system with an implanted and capsuled BEST transducer using a flat surface contact to the temporal bone. The vibrations are transmitted to the cochlea by means of bone conduction.
Several methods have been used for the design and development of the BCI system. In a previous design the robustness of the output force to skin flap thickness variations was optimized and the current consumption was reduced according to Taghavi et al (2012c). Thanks to Class-E design (Sokal et al, 1975), using a tuned switching power amplifier, an efficient power and signal transmission was achieved. This design was sufficiently insensitive to skin flap thickness variations in the power transmission to the implant, but still had a high current consumption. In order to further reduce
Technical design of a new bone conduction implant 3
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Figure 2. Technical details of the full-scale BCI design. The sound signal is picked up by directional microphones and is fed to the DSP. The processed and filtered signal is then transferred to the ASIC and thereafter transmitted through the intact skin using AM of the radiofrequency carrier (120 kHz) wave. On the implant side, the sound is extracted by the tuned demodulator unit and then fed to the BEST. The BCI audio processor is powered by a single 675 battery. current consumption, a switching power amplifier was designed and the results are presented in this paper. It uses a low power ASIC that includes gate-driver circuitry as well as a half-bridge Class-D switching topology (El-Hamamsy, 1994). This ASIC has so far been used in all patient implantations. A comparison of important features between the percutaneous BAHA and the transcutaneous BCI are summarized in Table 1.
Aim of study The aim of this report is (1) to describe the technical design and development of the BCI system that is used in the first implantations in patients, and (2) to verify its objective performance data.
Design aspects In what follows, the BCI system will be presented in more details with focus on the electronics and mechanical design.
Audio processor
implanted components are all passive. The On/Off switch is currently triggered by opening the battery door. A smart battery On/Off switch is now being tested, turning the device on when it is attached to the head, and turning the device off when it is removed from the head. This is a more user-friendly approach and it allows the battery to last longer, as no power is wasted when the device is inactivated (sometimes patients forget to switch off, and for short-term removal some patients do not consider switching off to be necessary). Figure 3 shows the AP and the BBC with their components.
Digital signal processor The digital signal processor offers a multitude of adjustments in 16 frequency bands and eight compression channels, and gives the patients up to six programmable programs to choose from (RHYTHM R3910, On Semiconductor, Phoenix, USA). It provides several automatic features such as environmental classification, adaptive noise reduction, feedback cancellation, and fully automated adaptive microphone directionality. For specific frequency response
The audio processor (AP) comprises a DSP with two microphones for either omni or directional configuration to improve the hearing by means of adaptive directionality, feedback cancellation, and noise reduction algorithms. The audio processor uses a single 675 hearingaid battery, which supplies the entire electronic unit in the device. In contrast, the implanted unit (BBC) does not contain any battery and the Table 1. Summary of the main features of the BAHA and the BCI systems. BAHA Permanent skin penetration. Stimulation posterior to the mastoid bone. Vibrations passing through a screw implant. Speaker in same housing as the audio processor. Conventional transducer. At least 15-mm protrusion of the audio processor from skin level. Coupling procedure requires a certain degree of manual dexterity.
BCI Intact skin; magnetic inductive link. Stimulation in the mastoid bone closer to the cochlea. Vibrations passing through a flat surface contact to the bone. Speaker implanted and distant from the audio processor. BEST technology with high-frequency boost. Approximately 9-mm protrusion of the audio processor from skin level, allowing use of soft headwear. Simple management of coupling procedure; magnet attraction when audio processor is nearby.
Figure 3. The BCI device with the audio processor comprising the directional microphones, battery, program selector, and volume control (opposite side of program selector, not visible here). The DSP, ASIC, transmitter coil, and external retention magnet are located inside the housing. The implanted unit (BBC) includes receiver coil, internal retention magnet, demodulator, and the BEST transducer. The BBC is sealed and molded in medical implant grade silicon except for the sound transmission part (facing inwards) where pure titanium and bone tissue are in direct contact to promote osseointegration and transmission efficiency.
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Figure 4. Basic structure of an inductive power and data link. The implanted unit is completely passive and includes an envelope detector (demodulator unit), which extracts the sound signal from the carrier wave and then feeds it to the transducer. adjustments, multiple second order biquad filters are available. The accuracy in the sound processing is based on a 20-bit audio processing capability. In a specific software provided by the DSP manufacturer, all parameters are accessible for fitting of the DSP to the particular hearing loss. However, no specific fitting algorithms have yet been developed as the presently used DSP platform does not support that feature. The fitting procedure has so far been made mainly by a linear gain approach and the audibility was optimized in an interaction between the operator and the patient, and also from the knowledge of the patient’s BC in-situ threshold measured by Ponto Pro Power (Oticon Medical, Askim, Sweden) on softband. To start with, the patient has been given about three or four different programs to choose between, where program A has the widest frequency response for best speech understanding, program B has a softer sound with high frequency cutting for more noisy sound environments, program C makes use of directional microphones for cocktail party situations, and finally, program D has some optional features for the patient’s own wishes, e.g. in one case a tele-coil option. For some patients the available programs were reduced to only A and B for ease of handling.
Application specific integrated circuit As the sound is transmitted wirelessly through the intact skin by use of an inductive link, the sound is modulated on a radio frequency (RF) carrier wave of 120 kHz. In the BCI design, amplitude modulation (AM) is used for signal and power transmission through the inductive link. The inductive link is driven by a low power ASIC. More details about the ASIC design can be found in Taghavi (2014) and Nasri (2011).
Radio frequency power and data link The BCI includes an inductive RF tuned power and data link system as shown in Figure 4. The AM power amplifier embedded in the
ASIC drives the tuned transmitter coil and sends the sound to the implanted unit by magnetic induction across the intact skin of the patient. One very important design issue is that the inductive link power transmission should be relatively insensitive to variations of skin flap thickness of patients that causes variations in the coupling coefficient (k) between the two coils. The BCI uses the techniques for tuning the transmitter and receiver coils that make the link less sensitive to skin flap thickness, as suggested by Ko et al (1977) and Galbraith et al (1987). Important design parameters to consider are the physical sizes of the receiver and transmitter coils. For the BCI system, two approaches are used; (1) the geometric method (Ko et al, 1977) introducing a transmitter coil that is larger in diameter than the implanted coil and, (2) the stagger tuning method (Galbraith et al, 1987), which is based on pole placement where one RF link resonance frequency is designed to be above the carrier frequency and one below. Using these methods, the gain of the link is desensitized from the skin flap thickness variations. It was shown by Raine et al (2007), that the skin flap thickness of the cochlear implant patients is reduced to 5 mm after six months, which has therefore been assumed to be the optimum skin thickness for the BCI link design. With a skin thickness of 5 mm, the total distance between adjacent faces of the coils is 7 mm taking into account that the thickness of the casing is 2 mm. Figure 5a shows the frequency response of the voltage to the transducer versus frequency, and in Figure 5b the frequency response of the voltage to the transducer versus coupling coefficient (k) for the designed inductive link is shown.
Retention magnet system The BCI system uses one permanent magnet positioned in the center of the transmitter coil in the AP and one in the center of the receiver coil in the BBC. Their mutual attraction is used for retention of the AP and to align the two coils to each other. The alignment is necessary to obtain an optimal signal transmission. Because this static axial force is for retention and alignment, and not for allowing efficient transmission of vibrations through the skin as in conventional skin-drive BCDs, this force can be considerably lower. The retention force of all BCI patients is below 1 N and the target force is 0.7 N, which is far below the force that is deemed to cause skin and blood circulation problems, i.e. below the capillary blood pressure (Raicevich, 2008). Furthermore, this retention force is significantly smaller than the attachment force used in conventional BCDs, which is typically at least 2 N (von Békésy, 1960). When the retention force goes below 0.4 N a stronger magnet can be considered, and when it
Figure 5. (a) Frequency response of the voltage to the transducer as the coil spacing is varied at Vsound 0.65 VDC. Maximum voltage to transducer occurs at d 5 mm at the carrier frequency. (b) Voltage transfer function of the voltage to the transducer relative to coupling coefficient k. The peak voltage to the transducer occurs approximately at coupling coefficient k 0.17.
Figure 6. Frequency response function of the BEST measured on a skull simulator when driven by a voltage source. The high frequency boost has been designed to occur around 4.5 kHz. exceeds 1 N a weaker magnet should be considered. All patients in the BCI clinical study still use the same standard strength magnet.
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Bridging bone conductor The implanted unit in the BCI system is called the bridging bone conductor (BBC) and comprises: the tuned receiver coil; the retention magnet in the center of the coil; the demodulator circuitry; and the BEST transducer. Both the magnet and the transducer are hermetically sealed in laser welded titanium casings. The hermeticity of the casings is verified by a helium method, where the welding takes place in helium atmosphere and helium leakage can be detected afterwards. Another important component is the “feed thru” of the electrical wires through the titanium casing. This feed thru is made of a titanium supported ceramically isolated platinum-iridium pin design. All the demodulation electronics are embedded in the transducer casing. The demodulator unit is designed to optimize the power and signal transfer to the transducer that converts the electrical sound signal to vibrations. In the final assembly process all components are sealed and supported by implant grade silicon except the bone contact surface of the transducer casing. It was realized early that a traditional BAHA transducer was too big to be used in the temporal bone. Therefore, a new transducer principle called balanced electromagnetic separation transducer (BEST) was developed by Håkansson (2003). This transducer principle does not only offer smaller size, but also lower distortion and higher efficiency than the transducers used in the BAHA. Figure 6 illustrates the characteristics of the BEST frequency response function measured on the skull simulator. It can be seen that the high frequency boost of the transducer is designed to be at approximately 4.5 kHz, which is located in the upper part of the audible frequency range for increased speech understanding (Håkansson et al, 2010). Above this frequency range, there will be a gradual decrease in sensitivity. The titanium transducer casing is designed to have a flat and firm surface contact to the bone in order to promote osseointegration and to achieve an efficient vibration transmission. It is assumed that if a constant attachment force is applied by, e.g. a titanium wire or a
Technical design of a new bone conduction implant 5 plate, and overlaying soft tissues, a sufficient contact between the titanium casing and the bone tissue will be achieved. It has been shown by Taghavi et al (2013) in an animal model study that the mechanical point impedance of such a flat contact to the bone is increased with time, which indicates that more bone contact, i.e. osseointegration, is developed during the healing period. It was also shown that the transcranial vibration transmission was stable, efficient and linear over time. The flat surface attachment method allows for the implant to be smaller in size, and it is anticipated that it will be easy to replace if needed. In a study by Reinfeldt et al (2015c) it was shown that the size of the present transducer casing is sufficiently small to be fitted in the mastoid portion of the temporal bone in a majority of indicated patients with normal anatomy. With a small protrusion of the titanium casing from the bone surface, this casing will fit more than 95% of the patients with normal temporal bones if it is placed with its center approximately 20 mm behind the ear canal. Figure 7a shows the BBC dimensions while Figure 7b shows the implant position on a skull bone where the center of the transducer casing is placed approximately 20 mm behind the center of the ear canal. Since the distance between the center of the magnet and the center of the transducer is 35 mm, the center of the magnet is approximately located where the BAHA implant is typically placed (55 mm behind the ear canal).
Materials and Methods Audio processor programming The BCI audio processor fitting was conducted using a DSP programmer (GENNUM, version 1.0) with appropriate programming software. In all measurements, the microphone was switched to omnidirectional mode. Furthermore, all adaptive and digital sound enhancement features were disabled and no compression was used in the gain settings of the device in these laboratory measurements for performance testing.
Frequency response measurement The frequency response measurement set up of the BCI is shown in Figure 8, in which the output force level (OFL) in dB re 1 mN was measured at different sound pressure levels (SPL) for one BBC transducer attached to a skull simulator with a special adapter. The skull simulator, which is generating a voltage proportional to the force applied at the connection point (Håkansson & Carlsson, 1989), was used to simulate the mechanical impedance of the skull bone. During the tests, the BCI audio processor, located in an anechoic chamber type 4222 (Brüel & Kjær A/S, Denmark), was driving the receiver inductive link with full-on gain i.e. the volume control of the AP was set on maximum given that the force
Figure 7. (a) Dimensions of the implanted unit (BBC), and (b) position of the implant on the skull bone.
6 H. Taghavi et al. 90 dB SPL using a Brüel & Kjær reference microphone system type 2804. Total harmonic distortion (THD) of the BCI was measured at 70 dB SPL at 5 mm skin thickness.
Current consumption The BCI current consumption was measured based on American National Standard for specification of hearing-aid characteristics (ANSI/ASA S3.22-2009) over the whole frequency range and also with respect to skin thickness.
Results
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Frequency response
Figure 8. Acoustical measurement set up. The BCI is placed in the anechoic chamber and the transducer is attached to the skull simulator. A Brüel & Kjær PULSE system generates the sound signal (stepped sine 0.1–10 kHz) via an amplifier with the sound pressure equalized. The PULSE system measures the linear spectrum of the output force from the skull simulator. A DSP programmer (GENNUM, ver 1.0) is used to conduct BCI fitting. output should not saturate until the input sound pressure level reaches 80 dB SPL at 1 kHz. A range of 2–8 mm thick stacked silicon sheets (1 mm each) between the transmitter and receiver coils was used to represent the patient’s skin flap thickness. A Brüel & Kjær PULSE analyser type 3560C was used to generate and analyse signals. A stepped sine signal in the frequency range of 0.1 to 10 kHz was used in all measurements. Linear spectra of the OFLs were measured while the input sound pressure level was equalized at 60, 70, and
The acoustical performance of the BCI measured on a skull simulator is shown in Figure 9. OFLs of the BCI were measured at 60, 70, and 90 dB SPL, where the 60 and 70 dB SPL curves reveal that the device is linear, as a 10-dB increase in the input SPL results in a 10-dB increase of the OFL. The most important curve is the maximum power output (MPO) because it determines the power capacity of the system. The MPO was measured at 90 dB SPL when the device is saturated and limited by the battery voltage capacity at the output. The MPO was also measured for different skin flap thicknesses. It can be seen in Figure 9 that the MPOs will change only 1.5 dB over the skin flap thickness variations of 2 to 8 mm and it has a maximum value of 107 dB re 1 mN at 5 mm skin thickness. It is also shown by the OFL graphs that the BEST design has the main resonance peak at around 0.8 kHz and also an additional high frequency resonance at around 4.5 kHz. Furthermore, the gray area under OFL curves shows that the THD of the BCI device is below 8% for frequencies from 0.6 to 6 kHz when the input was 70 dB SPL with a skin thickness of 5 mm. The input sound pressure levels to the microphone (60, 70, and 90 dB SPL) were equalized within 2 dB at each level in the entire frequency range.
Figure 9. Output force levels (OFL) measured at equalized 60, 70, and 90 dB SPL, and the total harmonic distortion (THD) measured at 70 dB SPL at 5 mm skin thickness. Maximum power output (MPO) was varied for skin flap thickness between 2 and 8 mm. The highest MPO occurs at 5 mm skin thickness, which was the design target.
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Technical design of a new bone conduction implant 7
Figure 10. (a) The BCI battery current consumption with the input signal of 1 kHz at 65 dB SPL (asterisk) and MPO (diamond) when the skin thickness varied from 1 to 10 mm. For comparison, the Class-E design (circle) at 65 dB SPL from Taghavi et al (2012c) is also shown. (b) The BCI current consumption in the optimal skin thickness of 5 mm in frequency range of 0.1 to 10 kHz with 65 and 90 dB SPLs compared with Taghavi et al (2012c) measured at 65 dB SPL.
Current consumption The BCI battery current consumption was measured with a pure tone of 1 kHz at 65 dB SPL when the skin flap thickness was changed from 1 to 10 mm, see Figure 10a. It can be seen that the current consumption increases slightly with the skin thickness increment and the maximum current consumption occurs at 10 mm skin thickness (10 mA). Figure 10b shows that the battery current is fairly frequency independent in the frequency range from 0.1 to 10 kHz when measured at 65 dB SPL with the skin flap thickness of 5 mm. The highest current consumption is 10 mA at 65 dB SPL and 13 mA at 90 dB SPL and occurs at 10 mm skin thickness. The current consumption at 65 dB SPL is reduced by up to 50% in comparison with the Class-E design (Taghavi et al, 2012c).
Discussion Frequency analysis A more close-up image of the MPO curves, illustrated in Figure 9 as the OFLs at 90 dB SPL, shows that the maximum output of the BCI occurs at 5 mm skin flap thickness, which has been chosen as the optimal skin thickness for the inductive link design. Comparing with the Class-E design, described by Taghavi et al (2012c), in which the maximum output occurred at 4 mm skin thickness due to reduced quality factor of the transmitter circuit, it can be seen that the Class-D power amplifier operates as a voltage source with low output impedance and is essentially independent of variations in loading by the transmitter and receiver circuits. As a consequence, the MPOs using the Class-D design are fairly insensitive to variations in the skin flap thickness. The peak MPO at the transducer resonance frequency of 107 dB re 1 mN and a THD less than 8% in the frequency range of 0.6 to 6 kHz is considered to be acceptable for indicated patients.
Current consumption The present BCI using the Class-D design has a current consumption, which is approximately 7.5 mA, see Table 2. This current consumption is 4–8 mA less than using a tuned Class-E power amplifier as proposed by Taghavi et al (2012c) and is meeting the design target
that the device should operate for up to seven days. With this current consumption, a typical 675 battery will last for approximately 80 hours of effective use, which equates to 12 hours effective use per day for seven days. Battery cost is approximately 1 USD/week, which corresponds to 50 USD/year, which is considered as acceptable. Table 2 also shows the current consumption of each audio processor block of the BCI. Not surprisingly, most of the current is consumed by the Class-D stage driving the inductive link. It should be noted that the current consumption in the Class-D stage is, unlike in direct drive systems like the BAHA, mainly independent of the sound level. The reason for this is that the carrier wave must be on its full amplitude all the time. In fact, the current consumption even increases by 54% when the audio processor is not in place over the BBC because of mistuning of the inductive link. This is one of the reasons for us developing a smart on/off switch that automatically turns the audio processor off when it is detached from the head, to ensure that the BCI only consumes current when used.
Safety aspects An initial study on the magnetic resonance imaging (MRI) compatibility and safety of the implanted retention magnet of the BCI has been done by Fredén Jansson et al (2014). It was found that the BCI system might be MR conditional up to 1.5 Tesla if a compression band is used to counteract the magnetically induced torque of the retention magnet. The magnet materials for the retention magnet and the transducer magnets are chosen properly to avoid demagnetization. Regarding the Table 2. Current consumption of each part of the BCI. BCI audio processor blocks DSP and microphones amplifier Clock circuit ASIC Class-D output stage (5 mm skin thickness at 65 dB SPL) Total
Current consumption 500 10 50 7
mA mA mA mA
Approximately 7.5 mA
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8 H. Taghavi et al. transducer, the BEST has four magnets that are oriented in a way so that force and torque from the static magnetic field are cancelled. The BCI will undergo substantial testing in order to be approved as MR conditional up to at least 1.5 Tesla. In terms of human exposure to electric and magnetic fields, and based on basic restrictions for time varying electric and magnetic fields for frequencies between 100 kHz to 10 MHz (ICNIRP, 1998, table 4), the localized specific absorption rate (SAR) for head and trunk should not exceed 2 W/kg. This value has been calculated for the BCI and found to be 1 W/kg and is thus below hazardous levels. Also, based on the risk for nervous system responses and nerve stimulation, there are basic restrictions for human exposure of time-varying electric and magnetic field (ICNIRP, 2010, table 2) in the frequency range from 3 kHz to 10 MHz. For the transmitter coil geometry used in the BCI system, the internal electric field has been calculated and found to be 4 V/m. The critical value that should not be exceeded in all tissues of head and body is 16.2 V/m and undoubtedly BCI is below that value as well.
Conclusions The bone conduction implant (BCI) system has been designed and developed for long-term use and is now in clinical trial on patients. The major advantage over percutaneous bone anchored hearing aids is that the BCI leaves the skin and subcutaneous tissue intact. In this study it was found that the BCI: •• can generate high enough output force levels for indicated patients with a sufficiently low total harmonic distortion, •• has an output power that is robust to skin flap thickness variations, and •• has a battery drain that is reasonably low and where the battery will last for five to seven days depending on the patient use.
Acknowledgements This study was supported by VINNOVA “Swedish Governmental Agency for Innovation Systems”, Swedish Research Council, Hörselforskningsfonden research fund, and Promobilia. Declaration of interest: All authors report no conflict of interest except the co-author Bo Håkansson who holds several patents related to the BCI device and has started a partnership with Oticon Medical, to make the BCI commercially available.
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Reinfeldt S., Östli P., Håkansson B., Taghavi H., Eeg-Olofsson M. et al. 2015c. Study of the feasible size of a bone conduction implant (BCI) transducer in the temporal bone. Otol Neurotol, 36, 631–637. Snik A.F.M., Myllanus E.M. & Cremers C. 1995. The bone-anchored hearing aid compared with conventional hearing aids. Otolaryngol Clin North Am, 28, 73–78. Snik A.F.M., Mylanus E.M., Proops D.W. et al. 2005. Consensus statements on the BAHA system: Where do we stand at present? Ann Otol Rhinol Laryngol Suppl., 195, 2–12. Sokal N. & Sokal A. 1975. Class E, a new class of high efficiency tuned single ended switching power amplifiers. IEEE J. Solid State Circuits, 3, 168–176. Stenfelt S., Håkansson B. & Tjellström A. 2000. Vibration characteristics of bone conducted sound in vitro. J Acoust Soc Am, 107, 422–31. Stenfelt S. & Goode R.L. 2005a. Bone-conducted sound: Physiological and clinical aspects. Otol Neurotol, 26(6), 1245–61. Stenfelt S. & Goode R.L. 2005b. Transmission properties of bone conducted sound: Measurements in cadaver heads. J Acoust Soc Am, 118, 2373–91. Taghavi H., Håkansson B., Reinfeldt S., Eeg-Olofsson M. & Akhshijan S. 2012a. Feedback analysis in percutaneous bone-conduction device and bone-conduction implant on a dry cranium. Otol Neurotol, 33 413–420. Taghavi H., Håkansson B. & Reinfeldt S. 2012b. A novel bone conduction implant system: Analog radio frequency data and power link design. In: Proceeding of the 9th IASTED international conference on biomedical engineering, Innsbruck, Austria, 327–335.
Technical design of a new bone conduction implant 9 Taghavi H., Håkansson B. & Reinfeldt S. 2012c. Analysis and design of RF power and data link using amplitude modulation of Class-E for a novel bone conduction implant. IEEE Trans Biomed Eng, 59, 3050–59. Taghavi H., Håkansson B., Eeg-Olofsson M., Johansson C., Tjellström A. et al. 2013. A vibration investigation of a flat surface contact to skull bone for direct bone conduction transmission in sheep skulls in vivo. Otol Neurotol, 34, 690–698. Taghavi H. 2014. The bone conduction implant (BCI): Preclinical studies, technical design and a clinical evaluation. PhD thesis. Chalmers University of Technology. ISBN: 978–91–7385–970–7. Tjellström A., Håkansson B. & Granström G. 2001. The bone-anchored hearing aids: Current status in adults and children. Otolaryngol Clin North Am, 34, 337–64. Tjellström A. & Stalfors J. 2012. Bone-anchored hearing device surgery: A 3- to 6-year follow-up with life table and worst-case scenario calculation. Otol Neurotol, 33, 891–894. Tjellström A. & Granström G. 1994. Long-term follow-up with the bone anchored hearing aid: A review of the first 100 patients between 1977 and 1985. ENT J, 73, 21–3. von Békésy G. 1960. Experiments in Hearing. New York: McGraw-Hill. Wazen J., Van Ess M.J., Alameda J., Ortega C., Modisett M. et al. 2010. The Baha system in patients with single-sided deafness and contralateral hearing loss. Otolaryngol Head Neck Surg, 142, 554–9. Wazen J., Wycherly B. & Daugherty J. 2011. Complications of boneanchored hearing devices. In: M. Kompis & M.D. Caversaccio, Implantable Bone Conduction Hearing Aids. Basel: Karger, pp. 63–72.
International Journal of Audiology Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/iija20
Technical design of a new bone conduction implant (BCI) system a
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a
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Hamidreza Taghavi , Bo Håkansson , Sabine Reinfeldt , Måns Eeg-Olofsson , Karl-Johan a
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Fredén Jansson , Emil Håkansson & Bayan Nasri a
*Department of Signals and Systems, Chalmers University of Technology, Gothenburg, Sweden b
ENT Department, Sahlgrenska University Hospital, Department of Otorhinolaryngology, Head and Neck Surgery, The Sahlgrenska Academy, University of Gothenburg, Gothenburg, Sweden c
Department of Computer Science and Engineering, Chalmers University of Technology, Gothenburg, Sweden Published online: 15 Jun 2015.
To cite this article: Hamidreza Taghavi, Bo Håkansson, Sabine Reinfeldt, Måns Eeg-Olofsson, Karl-Johan Fredén Jansson, Emil Håkansson & Bayan Nasri (2015): Technical design of a new bone conduction implant (BCI) system, International Journal of Audiology To link to this article: http://dx.doi.org/10.3109/14992027.2015.1051665
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International Journal of Audiology 2015; Early Online: 1–9
Technical Report
Technical design of a new bone conduction implant (BCI) system Hamidreza Taghavi*, Bo Håkansson*, Sabine Reinfeldt*, Måns Eeg-Olofsson†, Karl-Johan Fredén Jansson*, Emil Håkansson* & Bayan Nasri‡
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*Department of Signals and Systems, Chalmers University of Technology, Gothenburg, Sweden, †ENT Department, Sahlgrenska University Hospital, Department of Otorhinolaryngology, Head and Neck Surgery, The Sahlgrenska Academy, University of Gothenburg, Gothenburg, Sweden ‡Department of Computer Science and Engineering, Chalmers University of Technology, Gothenburg, Sweden
Abstract Objective: The objective of this study is to describe the technical design and verify the technical performance of a new bone conduction implant (BCI) system. Design: The BCI consists of an external audio processor and an implanted unit called the bridging bone conductor. These two units use an inductive link to communicate with each other through the intact skin in order to drive an implanted transducer. Study sample: In this study, the design of the full BCI system has been described and verified on a skull simulator and on real patients. Results: It was found that the maximum output force (peak 107 dB re 1 mN) of the BCI is robust for skin thickness range of 2–8 mm and that the total harmonic distortion is below 8% in the speech frequency range for 70 dB input sound pressure level. The current consumption is 7.5 mA, which corresponds to 5–7 days use with a single battery. Conclusions: This study shows that the BCI is a robust design that gives a sufficiently high output and an excellent sound quality for the hearing rehabilitation of indicated patients.
Key Words: Bone conduction implant; bone anchored hearing aid; implanted transducer; inductive links; long-term implantation earing-impaired patients with sensorineural hearing loss are generH ally rehabilitated by using air conduction (AC) hearing aids. However, in some patients with middle ear and ear canal disorders, AC devices are not sufficient or cannot be used. In these conditions, a bone conduction device (BCD) is an alternative to the AC device. In BCDs, the sound is converted to vibrations that are transmitted through the skull bone directly to the cochlea, bypassing the outer and middle ear (Håkansson et al, 1985; Stenfelt & Goode, 2005a). In conventional skin-drive BCDs (Reinfeldt et al, 2015a), the plastic casing of the transducer is pressed against the skin by a static force, using a steel spring headband, a softband, or frames of a pair of glasses. These solutions have drawbacks such as discomfort related to the static force and reduction of the blood circulation in the compressed skin. Moreover, the high frequency sensitivity is attenuated by the skin and soft tissues, and feedback problems that limit the gain are caused by sound radiation from the transducer via the casing or the skull bone to the microphone (Snik et al, 1995). In the early 1980s, the bone anchored hearing aid (BAHA) was introduced and is currently the most widely used BCD. The principal design of a generic BAHA is described in Håkansson et al (1985). The main indications for BAHA are pure conductive or mixed hearing losses (Håkansson et al, 1985; Tjellström et al, 2001; Snik et al, 2005). It can also be fitted in single-sided deaf patients in order to
reduce the head shadow effect (Wazen et al, 2010; Hol et al, 2010). The surgery for the BAHA is uncomplicated and safe. Today more than 150 000 patients have been rehabilitated with the BAHA (Cochlear, 2013; Oticon Medical, 2014) and two companies are commercially providing such devices; Cochlear Bone Anchored Solutions, Mölnlycke, Sweden, and Oticon Medical, Askim, Sweden. The most frequently described BAHA complications are skin and soft tissue irritations, infections around the skin penetrating implant, and complications related to the implant anchorage to the bone leading to implant losses (Tjellström & Granström, 1994; Tjellström & Stalfors, 2012; Wazen et al, 2011; Dun et al, 2012; de Wolf et al, 2008, 2009). Some patients do not accept the screw sticking out through the skin due to aesthetic reasons. Furthermore, in patients with need of high gain, the BAHA may cause oscillations due to feedback, mostly in higher frequencies (Taghavi et al, 2012a). To reduce some of these complications, several improvements have been made (Hultcrantz & Lanis, 2014; Håkansson, 2011; Dun et al, 2011).
The bone conduction implant A new bone conduction implant (BCI) system has been developed in a collaboration project between Chalmers University of Technology and the Sahlgrenska Academy, University of Gothenburg, both
Correspondence: Hamidreza Taghavi, Chalmers University of Technology, Department of Signals and Systems, 412 96, Gothenburg, Sweden. E-mail: [email protected] (Received 16 May 2014; accepted 8 May 2015) ISSN 1499-2027 print/ISSN 1708-8186 online © 2015 British Society of Audiology, International Society of Audiology, and Nordic Audiological Society DOI: 10.3109/14992027.2015.1051665
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Abbreviations AC AM AP ASIC BAHA BBC BC BCD BCI BEST DSP MPO MRI OFL RF SAR SPL THD USD
Air conduction Amplitude modulation Audio processor Application specific integrated circuit Bone anchored hearing aid Bridging bone conductor Bone conduction Bone conduction device Bone conduction implant Balanced electromagnetic separation transducer Digital signal processor Maximum power output Magnetic resonance imaging Output force level Radio frequency Specific absorption rate Sound pressure level Total harmonic distortion dollar
in Gothenburg, Sweden. The BCI has been introduced as an alternative to the percutaneous BAHA (Håkansson et al, 2008, 2010; Håkansson, 2011; Taghavi et al, 2012b; 2012c, Eeg-Olofsson et al; 2014; Reinfeldt, 2015b), and the principal design of the BCI system is shown in Figure 1. For the BCI, the skin and subcutaneous tissues are kept intact and no screw attachment to the skull bone is needed. Moreover, the BCI transducer is implanted closer to the cochlea than a BAHA, which increases the ipsilateral sensitivity to bone-conducted sound (Håkansson et al, 2008, 2010; Stenfelt et al, 2000; Stenfelt & Goode, 2005b; Eeg-Olofsson et al, 2008; Reinfeldt et al, 2014). In more technical details, the BCI consists of an externally worn audio processor and an implanted unit called the bridging bone conductor (BBC), as shown in Figure 2. The audio processor includes directional microphones, a single 675 hearing-aid battery, a digital signal processor (DSP), and an application specific integrated circuit
(ASIC) that drives the tuned transmitter coil. The BBC comprises a tuned receiver coil, a demodulator unit, and the capsuled transducer. To transmit the sound to the transducer through the intact skin, an inductive link (tuned transmitter and receiver coils) is used. The sound is amplitude modulated (AM) and transmitted wirelessly to the BBC. A retention magnet system, similar to those used in cochlear implants, is used to keep the audio processor in place and align the transmitter and receiver coils to optimize the signal transmission. The BCI transducer is less than half the size of the BAHA transducer and is permanently implanted in the temporal bone. The BCI transducer uses the balanced electromagnetic separation transducer (BEST) technology (Håkansson, 2003) and is sealed in a laserwelded titanium casing. A first inherent resonance is located in the low speech frequency range around 0.8 kHz. An additional spring suspension is added between the transducer and the titanium casing, creating a second resonance in the frequency response around 4.5 kHz that boosts the high frequency content of the speech.
Summary of ongoing clinical study Eight patients have been operated on in a clinical study under an approval from the Swedish Medical Products Agency and the regional ethical review board in Gothenburg. It has been concluded that the surgical procedure is regarded as both safe and uncomplicated with minor engagement of the mastoid portion of the temporal bone (EegOlofsson et al, 2014; Reinfeldt et al, 2015b). The BCI transducer is placed in a 4–5 mm deep recess, drilled in the mastoid part of the temporal bone (see Figure 1), and is secured using a thin titanium wire over the casing. Unlike the BAHA that has a screw attachment in the skull bone, the BCI transducer uses a flat surface contact to the skull bone. If there is a need for replacing the transducer, the surgical intervention is anticipated to be easier with this method of attachment compared to if the transducer is anchored with screws in the bone. The surgical method and audiological results of the first patient have been presented by Eeg-Olofsson et al (2014). Audiological and quality of life outcomes with the BCI on the first six patients have been addressed in a clinical study by Reinfeldt et al (2015b). In this study it was shown that the BCI improvement over the unaided condition was statistically significant in all audiometric measures. The improvement in puretone average hearing thresholds was 31.0 dB; the speech recognition threshold improvement in quiet was 27.0 dB, and the improvement of the speech recognition score in noise at normal speech level was 51.2 %. The signal-to-noise ratio threshold with the BCI was 5.5 dB at speech levels using Swedish “Hagerman sentences” (Hagerman, 1982) and the APHAB as well as GBI scores were improved compared to the unaided condition. Furthermore, preoperatively and according to the study protocol, the patients had been using a conventional BCD (Ponto Pro Power) on a softband for a period of one month. The BCI audiometric results compared to the conventional BCD results showed to be similar or better.
Technical design
Figure 1. The full-scale BCI system with an implanted and capsuled BEST transducer using a flat surface contact to the temporal bone. The vibrations are transmitted to the cochlea by means of bone conduction.
Several methods have been used for the design and development of the BCI system. In a previous design the robustness of the output force to skin flap thickness variations was optimized and the current consumption was reduced according to Taghavi et al (2012c). Thanks to Class-E design (Sokal et al, 1975), using a tuned switching power amplifier, an efficient power and signal transmission was achieved. This design was sufficiently insensitive to skin flap thickness variations in the power transmission to the implant, but still had a high current consumption. In order to further reduce
Technical design of a new bone conduction implant 3
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Figure 2. Technical details of the full-scale BCI design. The sound signal is picked up by directional microphones and is fed to the DSP. The processed and filtered signal is then transferred to the ASIC and thereafter transmitted through the intact skin using AM of the radiofrequency carrier (120 kHz) wave. On the implant side, the sound is extracted by the tuned demodulator unit and then fed to the BEST. The BCI audio processor is powered by a single 675 battery. current consumption, a switching power amplifier was designed and the results are presented in this paper. It uses a low power ASIC that includes gate-driver circuitry as well as a half-bridge Class-D switching topology (El-Hamamsy, 1994). This ASIC has so far been used in all patient implantations. A comparison of important features between the percutaneous BAHA and the transcutaneous BCI are summarized in Table 1.
Aim of study The aim of this report is (1) to describe the technical design and development of the BCI system that is used in the first implantations in patients, and (2) to verify its objective performance data.
Design aspects In what follows, the BCI system will be presented in more details with focus on the electronics and mechanical design.
Audio processor
implanted components are all passive. The On/Off switch is currently triggered by opening the battery door. A smart battery On/Off switch is now being tested, turning the device on when it is attached to the head, and turning the device off when it is removed from the head. This is a more user-friendly approach and it allows the battery to last longer, as no power is wasted when the device is inactivated (sometimes patients forget to switch off, and for short-term removal some patients do not consider switching off to be necessary). Figure 3 shows the AP and the BBC with their components.
Digital signal processor The digital signal processor offers a multitude of adjustments in 16 frequency bands and eight compression channels, and gives the patients up to six programmable programs to choose from (RHYTHM R3910, On Semiconductor, Phoenix, USA). It provides several automatic features such as environmental classification, adaptive noise reduction, feedback cancellation, and fully automated adaptive microphone directionality. For specific frequency response
The audio processor (AP) comprises a DSP with two microphones for either omni or directional configuration to improve the hearing by means of adaptive directionality, feedback cancellation, and noise reduction algorithms. The audio processor uses a single 675 hearingaid battery, which supplies the entire electronic unit in the device. In contrast, the implanted unit (BBC) does not contain any battery and the Table 1. Summary of the main features of the BAHA and the BCI systems. BAHA Permanent skin penetration. Stimulation posterior to the mastoid bone. Vibrations passing through a screw implant. Speaker in same housing as the audio processor. Conventional transducer. At least 15-mm protrusion of the audio processor from skin level. Coupling procedure requires a certain degree of manual dexterity.
BCI Intact skin; magnetic inductive link. Stimulation in the mastoid bone closer to the cochlea. Vibrations passing through a flat surface contact to the bone. Speaker implanted and distant from the audio processor. BEST technology with high-frequency boost. Approximately 9-mm protrusion of the audio processor from skin level, allowing use of soft headwear. Simple management of coupling procedure; magnet attraction when audio processor is nearby.
Figure 3. The BCI device with the audio processor comprising the directional microphones, battery, program selector, and volume control (opposite side of program selector, not visible here). The DSP, ASIC, transmitter coil, and external retention magnet are located inside the housing. The implanted unit (BBC) includes receiver coil, internal retention magnet, demodulator, and the BEST transducer. The BBC is sealed and molded in medical implant grade silicon except for the sound transmission part (facing inwards) where pure titanium and bone tissue are in direct contact to promote osseointegration and transmission efficiency.
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Figure 4. Basic structure of an inductive power and data link. The implanted unit is completely passive and includes an envelope detector (demodulator unit), which extracts the sound signal from the carrier wave and then feeds it to the transducer. adjustments, multiple second order biquad filters are available. The accuracy in the sound processing is based on a 20-bit audio processing capability. In a specific software provided by the DSP manufacturer, all parameters are accessible for fitting of the DSP to the particular hearing loss. However, no specific fitting algorithms have yet been developed as the presently used DSP platform does not support that feature. The fitting procedure has so far been made mainly by a linear gain approach and the audibility was optimized in an interaction between the operator and the patient, and also from the knowledge of the patient’s BC in-situ threshold measured by Ponto Pro Power (Oticon Medical, Askim, Sweden) on softband. To start with, the patient has been given about three or four different programs to choose between, where program A has the widest frequency response for best speech understanding, program B has a softer sound with high frequency cutting for more noisy sound environments, program C makes use of directional microphones for cocktail party situations, and finally, program D has some optional features for the patient’s own wishes, e.g. in one case a tele-coil option. For some patients the available programs were reduced to only A and B for ease of handling.
Application specific integrated circuit As the sound is transmitted wirelessly through the intact skin by use of an inductive link, the sound is modulated on a radio frequency (RF) carrier wave of 120 kHz. In the BCI design, amplitude modulation (AM) is used for signal and power transmission through the inductive link. The inductive link is driven by a low power ASIC. More details about the ASIC design can be found in Taghavi (2014) and Nasri (2011).
Radio frequency power and data link The BCI includes an inductive RF tuned power and data link system as shown in Figure 4. The AM power amplifier embedded in the
ASIC drives the tuned transmitter coil and sends the sound to the implanted unit by magnetic induction across the intact skin of the patient. One very important design issue is that the inductive link power transmission should be relatively insensitive to variations of skin flap thickness of patients that causes variations in the coupling coefficient (k) between the two coils. The BCI uses the techniques for tuning the transmitter and receiver coils that make the link less sensitive to skin flap thickness, as suggested by Ko et al (1977) and Galbraith et al (1987). Important design parameters to consider are the physical sizes of the receiver and transmitter coils. For the BCI system, two approaches are used; (1) the geometric method (Ko et al, 1977) introducing a transmitter coil that is larger in diameter than the implanted coil and, (2) the stagger tuning method (Galbraith et al, 1987), which is based on pole placement where one RF link resonance frequency is designed to be above the carrier frequency and one below. Using these methods, the gain of the link is desensitized from the skin flap thickness variations. It was shown by Raine et al (2007), that the skin flap thickness of the cochlear implant patients is reduced to 5 mm after six months, which has therefore been assumed to be the optimum skin thickness for the BCI link design. With a skin thickness of 5 mm, the total distance between adjacent faces of the coils is 7 mm taking into account that the thickness of the casing is 2 mm. Figure 5a shows the frequency response of the voltage to the transducer versus frequency, and in Figure 5b the frequency response of the voltage to the transducer versus coupling coefficient (k) for the designed inductive link is shown.
Retention magnet system The BCI system uses one permanent magnet positioned in the center of the transmitter coil in the AP and one in the center of the receiver coil in the BBC. Their mutual attraction is used for retention of the AP and to align the two coils to each other. The alignment is necessary to obtain an optimal signal transmission. Because this static axial force is for retention and alignment, and not for allowing efficient transmission of vibrations through the skin as in conventional skin-drive BCDs, this force can be considerably lower. The retention force of all BCI patients is below 1 N and the target force is 0.7 N, which is far below the force that is deemed to cause skin and blood circulation problems, i.e. below the capillary blood pressure (Raicevich, 2008). Furthermore, this retention force is significantly smaller than the attachment force used in conventional BCDs, which is typically at least 2 N (von Békésy, 1960). When the retention force goes below 0.4 N a stronger magnet can be considered, and when it
Figure 5. (a) Frequency response of the voltage to the transducer as the coil spacing is varied at Vsound 0.65 VDC. Maximum voltage to transducer occurs at d 5 mm at the carrier frequency. (b) Voltage transfer function of the voltage to the transducer relative to coupling coefficient k. The peak voltage to the transducer occurs approximately at coupling coefficient k 0.17.
Figure 6. Frequency response function of the BEST measured on a skull simulator when driven by a voltage source. The high frequency boost has been designed to occur around 4.5 kHz. exceeds 1 N a weaker magnet should be considered. All patients in the BCI clinical study still use the same standard strength magnet.
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Bridging bone conductor The implanted unit in the BCI system is called the bridging bone conductor (BBC) and comprises: the tuned receiver coil; the retention magnet in the center of the coil; the demodulator circuitry; and the BEST transducer. Both the magnet and the transducer are hermetically sealed in laser welded titanium casings. The hermeticity of the casings is verified by a helium method, where the welding takes place in helium atmosphere and helium leakage can be detected afterwards. Another important component is the “feed thru” of the electrical wires through the titanium casing. This feed thru is made of a titanium supported ceramically isolated platinum-iridium pin design. All the demodulation electronics are embedded in the transducer casing. The demodulator unit is designed to optimize the power and signal transfer to the transducer that converts the electrical sound signal to vibrations. In the final assembly process all components are sealed and supported by implant grade silicon except the bone contact surface of the transducer casing. It was realized early that a traditional BAHA transducer was too big to be used in the temporal bone. Therefore, a new transducer principle called balanced electromagnetic separation transducer (BEST) was developed by Håkansson (2003). This transducer principle does not only offer smaller size, but also lower distortion and higher efficiency than the transducers used in the BAHA. Figure 6 illustrates the characteristics of the BEST frequency response function measured on the skull simulator. It can be seen that the high frequency boost of the transducer is designed to be at approximately 4.5 kHz, which is located in the upper part of the audible frequency range for increased speech understanding (Håkansson et al, 2010). Above this frequency range, there will be a gradual decrease in sensitivity. The titanium transducer casing is designed to have a flat and firm surface contact to the bone in order to promote osseointegration and to achieve an efficient vibration transmission. It is assumed that if a constant attachment force is applied by, e.g. a titanium wire or a
Technical design of a new bone conduction implant 5 plate, and overlaying soft tissues, a sufficient contact between the titanium casing and the bone tissue will be achieved. It has been shown by Taghavi et al (2013) in an animal model study that the mechanical point impedance of such a flat contact to the bone is increased with time, which indicates that more bone contact, i.e. osseointegration, is developed during the healing period. It was also shown that the transcranial vibration transmission was stable, efficient and linear over time. The flat surface attachment method allows for the implant to be smaller in size, and it is anticipated that it will be easy to replace if needed. In a study by Reinfeldt et al (2015c) it was shown that the size of the present transducer casing is sufficiently small to be fitted in the mastoid portion of the temporal bone in a majority of indicated patients with normal anatomy. With a small protrusion of the titanium casing from the bone surface, this casing will fit more than 95% of the patients with normal temporal bones if it is placed with its center approximately 20 mm behind the ear canal. Figure 7a shows the BBC dimensions while Figure 7b shows the implant position on a skull bone where the center of the transducer casing is placed approximately 20 mm behind the center of the ear canal. Since the distance between the center of the magnet and the center of the transducer is 35 mm, the center of the magnet is approximately located where the BAHA implant is typically placed (55 mm behind the ear canal).
Materials and Methods Audio processor programming The BCI audio processor fitting was conducted using a DSP programmer (GENNUM, version 1.0) with appropriate programming software. In all measurements, the microphone was switched to omnidirectional mode. Furthermore, all adaptive and digital sound enhancement features were disabled and no compression was used in the gain settings of the device in these laboratory measurements for performance testing.
Frequency response measurement The frequency response measurement set up of the BCI is shown in Figure 8, in which the output force level (OFL) in dB re 1 mN was measured at different sound pressure levels (SPL) for one BBC transducer attached to a skull simulator with a special adapter. The skull simulator, which is generating a voltage proportional to the force applied at the connection point (Håkansson & Carlsson, 1989), was used to simulate the mechanical impedance of the skull bone. During the tests, the BCI audio processor, located in an anechoic chamber type 4222 (Brüel & Kjær A/S, Denmark), was driving the receiver inductive link with full-on gain i.e. the volume control of the AP was set on maximum given that the force
Figure 7. (a) Dimensions of the implanted unit (BBC), and (b) position of the implant on the skull bone.
6 H. Taghavi et al. 90 dB SPL using a Brüel & Kjær reference microphone system type 2804. Total harmonic distortion (THD) of the BCI was measured at 70 dB SPL at 5 mm skin thickness.
Current consumption The BCI current consumption was measured based on American National Standard for specification of hearing-aid characteristics (ANSI/ASA S3.22-2009) over the whole frequency range and also with respect to skin thickness.
Results
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Frequency response
Figure 8. Acoustical measurement set up. The BCI is placed in the anechoic chamber and the transducer is attached to the skull simulator. A Brüel & Kjær PULSE system generates the sound signal (stepped sine 0.1–10 kHz) via an amplifier with the sound pressure equalized. The PULSE system measures the linear spectrum of the output force from the skull simulator. A DSP programmer (GENNUM, ver 1.0) is used to conduct BCI fitting. output should not saturate until the input sound pressure level reaches 80 dB SPL at 1 kHz. A range of 2–8 mm thick stacked silicon sheets (1 mm each) between the transmitter and receiver coils was used to represent the patient’s skin flap thickness. A Brüel & Kjær PULSE analyser type 3560C was used to generate and analyse signals. A stepped sine signal in the frequency range of 0.1 to 10 kHz was used in all measurements. Linear spectra of the OFLs were measured while the input sound pressure level was equalized at 60, 70, and
The acoustical performance of the BCI measured on a skull simulator is shown in Figure 9. OFLs of the BCI were measured at 60, 70, and 90 dB SPL, where the 60 and 70 dB SPL curves reveal that the device is linear, as a 10-dB increase in the input SPL results in a 10-dB increase of the OFL. The most important curve is the maximum power output (MPO) because it determines the power capacity of the system. The MPO was measured at 90 dB SPL when the device is saturated and limited by the battery voltage capacity at the output. The MPO was also measured for different skin flap thicknesses. It can be seen in Figure 9 that the MPOs will change only 1.5 dB over the skin flap thickness variations of 2 to 8 mm and it has a maximum value of 107 dB re 1 mN at 5 mm skin thickness. It is also shown by the OFL graphs that the BEST design has the main resonance peak at around 0.8 kHz and also an additional high frequency resonance at around 4.5 kHz. Furthermore, the gray area under OFL curves shows that the THD of the BCI device is below 8% for frequencies from 0.6 to 6 kHz when the input was 70 dB SPL with a skin thickness of 5 mm. The input sound pressure levels to the microphone (60, 70, and 90 dB SPL) were equalized within 2 dB at each level in the entire frequency range.
Figure 9. Output force levels (OFL) measured at equalized 60, 70, and 90 dB SPL, and the total harmonic distortion (THD) measured at 70 dB SPL at 5 mm skin thickness. Maximum power output (MPO) was varied for skin flap thickness between 2 and 8 mm. The highest MPO occurs at 5 mm skin thickness, which was the design target.
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Technical design of a new bone conduction implant 7
Figure 10. (a) The BCI battery current consumption with the input signal of 1 kHz at 65 dB SPL (asterisk) and MPO (diamond) when the skin thickness varied from 1 to 10 mm. For comparison, the Class-E design (circle) at 65 dB SPL from Taghavi et al (2012c) is also shown. (b) The BCI current consumption in the optimal skin thickness of 5 mm in frequency range of 0.1 to 10 kHz with 65 and 90 dB SPLs compared with Taghavi et al (2012c) measured at 65 dB SPL.
Current consumption The BCI battery current consumption was measured with a pure tone of 1 kHz at 65 dB SPL when the skin flap thickness was changed from 1 to 10 mm, see Figure 10a. It can be seen that the current consumption increases slightly with the skin thickness increment and the maximum current consumption occurs at 10 mm skin thickness (10 mA). Figure 10b shows that the battery current is fairly frequency independent in the frequency range from 0.1 to 10 kHz when measured at 65 dB SPL with the skin flap thickness of 5 mm. The highest current consumption is 10 mA at 65 dB SPL and 13 mA at 90 dB SPL and occurs at 10 mm skin thickness. The current consumption at 65 dB SPL is reduced by up to 50% in comparison with the Class-E design (Taghavi et al, 2012c).
Discussion Frequency analysis A more close-up image of the MPO curves, illustrated in Figure 9 as the OFLs at 90 dB SPL, shows that the maximum output of the BCI occurs at 5 mm skin flap thickness, which has been chosen as the optimal skin thickness for the inductive link design. Comparing with the Class-E design, described by Taghavi et al (2012c), in which the maximum output occurred at 4 mm skin thickness due to reduced quality factor of the transmitter circuit, it can be seen that the Class-D power amplifier operates as a voltage source with low output impedance and is essentially independent of variations in loading by the transmitter and receiver circuits. As a consequence, the MPOs using the Class-D design are fairly insensitive to variations in the skin flap thickness. The peak MPO at the transducer resonance frequency of 107 dB re 1 mN and a THD less than 8% in the frequency range of 0.6 to 6 kHz is considered to be acceptable for indicated patients.
Current consumption The present BCI using the Class-D design has a current consumption, which is approximately 7.5 mA, see Table 2. This current consumption is 4–8 mA less than using a tuned Class-E power amplifier as proposed by Taghavi et al (2012c) and is meeting the design target
that the device should operate for up to seven days. With this current consumption, a typical 675 battery will last for approximately 80 hours of effective use, which equates to 12 hours effective use per day for seven days. Battery cost is approximately 1 USD/week, which corresponds to 50 USD/year, which is considered as acceptable. Table 2 also shows the current consumption of each audio processor block of the BCI. Not surprisingly, most of the current is consumed by the Class-D stage driving the inductive link. It should be noted that the current consumption in the Class-D stage is, unlike in direct drive systems like the BAHA, mainly independent of the sound level. The reason for this is that the carrier wave must be on its full amplitude all the time. In fact, the current consumption even increases by 54% when the audio processor is not in place over the BBC because of mistuning of the inductive link. This is one of the reasons for us developing a smart on/off switch that automatically turns the audio processor off when it is detached from the head, to ensure that the BCI only consumes current when used.
Safety aspects An initial study on the magnetic resonance imaging (MRI) compatibility and safety of the implanted retention magnet of the BCI has been done by Fredén Jansson et al (2014). It was found that the BCI system might be MR conditional up to 1.5 Tesla if a compression band is used to counteract the magnetically induced torque of the retention magnet. The magnet materials for the retention magnet and the transducer magnets are chosen properly to avoid demagnetization. Regarding the Table 2. Current consumption of each part of the BCI. BCI audio processor blocks DSP and microphones amplifier Clock circuit ASIC Class-D output stage (5 mm skin thickness at 65 dB SPL) Total
Current consumption 500 10 50 7
mA mA mA mA
Approximately 7.5 mA
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8 H. Taghavi et al. transducer, the BEST has four magnets that are oriented in a way so that force and torque from the static magnetic field are cancelled. The BCI will undergo substantial testing in order to be approved as MR conditional up to at least 1.5 Tesla. In terms of human exposure to electric and magnetic fields, and based on basic restrictions for time varying electric and magnetic fields for frequencies between 100 kHz to 10 MHz (ICNIRP, 1998, table 4), the localized specific absorption rate (SAR) for head and trunk should not exceed 2 W/kg. This value has been calculated for the BCI and found to be 1 W/kg and is thus below hazardous levels. Also, based on the risk for nervous system responses and nerve stimulation, there are basic restrictions for human exposure of time-varying electric and magnetic field (ICNIRP, 2010, table 2) in the frequency range from 3 kHz to 10 MHz. For the transmitter coil geometry used in the BCI system, the internal electric field has been calculated and found to be 4 V/m. The critical value that should not be exceeded in all tissues of head and body is 16.2 V/m and undoubtedly BCI is below that value as well.
Conclusions The bone conduction implant (BCI) system has been designed and developed for long-term use and is now in clinical trial on patients. The major advantage over percutaneous bone anchored hearing aids is that the BCI leaves the skin and subcutaneous tissue intact. In this study it was found that the BCI: •• can generate high enough output force levels for indicated patients with a sufficiently low total harmonic distortion, •• has an output power that is robust to skin flap thickness variations, and •• has a battery drain that is reasonably low and where the battery will last for five to seven days depending on the patient use.
Acknowledgements This study was supported by VINNOVA “Swedish Governmental Agency for Innovation Systems”, Swedish Research Council, Hörselforskningsfonden research fund, and Promobilia. Declaration of interest: All authors report no conflict of interest except the co-author Bo Håkansson who holds several patents related to the BCI device and has started a partnership with Oticon Medical, to make the BCI commercially available.
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