Implantable Hearing Aids: Where are we in 2020?

Mechanical stimulation of cochlea for treatment of hearing loss: Comparison between forward stimulation and reverse stimulation with an active cochlear model

BACKGROUND AND OBJECTIVE

Reverse stimulation is a stimulation mode of the active middle-ear implants (AMEIs), targeted at moderate conductive hearing loss and mixed hearing loss. However, previous studies investigated reverse stimulation through passive cochlear models that simulate profound sensorineural hearing loss, which is beyond the AMEI's indications. Therefore, we investigated the cochlear responses to reverse stimulation under different hearing loss and compared them with those to forward stimulation.

METHODS

The human ear model consists of a human ear macro dynamic model, a cochlear micro dynamic model, and a cochlear circuit model. The human ear macro dynamic model and cochlear micro dynamic model were developed by simplifying the human ear tissues into stiffness, damping, and mass. The cochlear active amplification was realized by coupling the cochlear circuit model. Based on the model, the cochlear responses to forward and reverse stimulation were calculated.

RESULTS

The results show that the cochlear responses to reverse stimulation are higher than those to forward stimulation, and the difference in cochlear responses decreases and then increases with increasing stimulus magnitude. Conductive hearing loss significantly reduces cochlear response to forward stimulation but has less effect on reverse stimulation. Outer hair cell hearing loss significantly reduces cochlear response to both forward and reverse stimulation, but the effect diminishes to nothing as the stimulation amplitude increases.

CONCLUSIONS

This study compared the cochlear responses differences in normal hearing and hearing loss to forward and reverse stimulation, contributing to the optimization of the round window stimulating AMEIs.

Rehabilitation of human hearing with a totally implantable cochlear implant: a feasibility study

BACKGROUND

Cochlear implants (CIs) are neuroprosthetic devices which restore hearing in severe-to-profound hearing loss through electrical stimulation of the auditory nerve. Current CIs use an externally worn audio processor. A long-term goal in the field has been to develop a device in which all components are contained within a single implant. Here, we present initial clinical results with the totally implantable cochlear implant (TICI). The primary objective of this study was to assess the safety of the device in adults who suffer from bilateral severe-to-profound sensorineural hearing loss.

METHODS

This study used a design with non-randomized single group assignment (trial registration: NCT04571333). Six implantations took place beginning in September 2020. Data collection took place at the two participating CI centers. Adverse events (the primary outcome), speech perception, patient reported outcomes, and device usage statistics were collected over the subsequent 52 weeks. A within-subjects comparison was used in which each participant was evaluated both with the TICI and with an external SONNET audio processor.

RESULTS

One anticipated serious adverse device effect (ASADE) occurred. After treatment the event resolved without sequelae. No unanticipated serious adverse device effects (USADE) occurred. Speech perception in quiet and in noise scores were comparable between the TICI and the SONNET audio processor. Scores on the validated patient reported outcome instruments HUI3, SSQ-12, and HISQUI-19 all increased over the duration of the study. User satisfaction scores as reported in their daily diary also increased over the duration of the study. Based on device usage metrics, all but one user used the TICI without an external processor the majority of the time.

CONCLUSIONS

The primary outcome of assessing the safety of the device was achieved. The TICI provides high levels of hearing performance, comparable to those of a conventional CI. The development of the TICI expands the range of options for treatment of hearing loss.

Optimal Position and Orientation of an Ossicular Accelerometer for Human Auditory Prostheses

In this study, a method for determining the optimal location and orientation of an implantable piezoelectric accelerometer on the short process of the incus is presented. The accelerometer is intended to be used as a replacement for an external microphone to enable totally implantable auditory prostheses. The optimal orientation of the sensor and the best attachment point are determined based on two criteria-maximum pressure sensitivity sum and minimum loudness level sum. The best location is determined to be near the incudomalleolar joint. We find that the angular orientation of the sensor is critical and provide guidelines on that orientation. The method described in this paper can be used to further optimize the design and performance of the accelerometer.

Esteem® Active Middle Ear Implant versus Conventional Hearing Aids: Long-Term Performance

INTRODUCTION

The totally implantable active middle ear implant Esteem® may be considered an effective alternative to conventional hearing aids (cHAs) to manage moderate-to-severe forms of sensorineural hearing loss. This study aimed to provide long-term comparative data of Esteem performances with those achieved by cHA.

METHODS

From a total of 46 subjects who received unilateral application of Esteem®, and were followed up over the years, ten underwent an audiological assessment that compared the outcomes with those achieved in the contralateral ear by a cHA, considering the initially symmetric auditory thresholds in both ears. Other than pure tone audiometry and speech audiometry in quiet, the assessment was performed by using the adaptive speech in noise, i.e., Matrix test.

RESULTS

The mean speech intelligibility in quiet shows in the unaided situation a recognition of 50.7% at 71 dBHL, 71% at 69 dBHL with only contralateral cHA, 92% at 66 dBHL with only Esteem device and 94% at 61 dBHL with Esteem® device and contralateral cHA. The mean speech intelligibility in noise shows in the unaided situation a recognition of 36% at 71 dBHL, 56% at 69 dBHL with only contralateral cHA, 79% at 66 dBHL with only Esteem® device and 84% at 61 dBHL with Esteem® device and contralateral cHA. At Matrix test in the unaided condition, 4 patients reached 50% of intelligibility and the 50% threshold was obtained with a mean sound/noise ratio of +10 dBHL. In the contralaterally aided condition, 10 patients reached a 50% threshold in a condition of mean S/N ratio of +10.6 dBHL. In the Esteem® only and Esteem® plus cHA condition, all patients reached the 50% threshold with a mean S/N ratio of +3.4 dBHL with the Esteem® device and +0.92 dBHL with Esteem® plus a contralateral cHA, with a statistically nonsignificant difference. The mean deviation from the reference value (7.1 dB in the normal hearing population) was 17.1 dBHL, in unaided situation; this condition did not change with only the contralateral cHA (17.6 dBHL), whilst a significant improvement could be identified with only Esteem® device, where the mean deviation was 10.5 dBHL, and mostly with Esteem® device associated with the contralateral cHA, with a value of 8.02 dBHL.

CONCLUSIONS

The adaptive speech audiometry in noise (Matrix Test) showed that binaural stimulation provides greater benefits in the speech recognition in noise test in comparison to monaural stimulation, especially when this is carried out only by the cHA. However, the Esteem® device allowed to obtain audiological benefits that are significantly superior to those offered by cHAs, especially in cases where the hearing loss is severe and, in some cases, profound, achieving performances almost comparable to those of a cochlear implant.

Effect of electromagnetic middle-ear implant simulating sites on the stapes spatial motion: A finite element analysis

The electromagnetic middle-ear implant (MEI) is a new type of hearing device for addressing sensorineural and mixed hearing loss. The hearing compensation effect of the MEI varies depending on the transducer stimulation sites. This paper investigates the impact of transducer stimulation sites on MEI performance by analyzing stapes spatial motion. Firstly, we constructed a human-ear finite element model based on micro-CT scanning and inverse molding techniques. This model was validated by comparing its predictions of stapes spatial motion and cochlear response with experimental data. Then, stimulation force was applied at four common sites: umbo, incus body, incus long process and stapes to simulate the electromagnetic transducer. Results show that at low and middle frequencies, stapes-stimulating and incus-long-process-stimulating produce similar spatial motion to normal hearing; at high frequencies, incus-body-stimulating produces similar results to normal hearing. The equivalent sound pressure level generated by the stapes piston motion is less sensitive to the stimulation direction than that deduced by the stapes rocking motion.

Design of Piezoelectric Dual-Bandwidth Accelerometers for Completely Implantable Auditory Prostheses

For the last 20 years, researchers have developed accelerometers to function as ossicular vibration sensors in order to eliminate the external components of hearing aid and cochlear implant systems. To date, no accelerometer has met all of the stringent performance requirements necessary to function in this capacity. In this work, we present an accelerometer design with an equivalent noise floor less than 20 phon equal-loudness-level over a 0.1-8 kHz bandwidth in a package small enough to be implanted in the middle ear. Our approach uses a dual-bandwidth (two sensing elements) microelectromechanical systems piezoelectric accelerometer, sized using an area-minimization process based on an experimentally-validated analytical model of the sensor. The resulting bandwidth of the low-frequency sensing element is 0.1-1.25 kHz and that of the high-frequency sensing element is 1.25-8 kHz. These sensing elements fit within a silicon frame that is 795 μm × 778 μm, which can reasonably be housed along with a required integrated circuit in a 2.2 mm × 2.7 mm × 1 mm package. The estimated total mass of the packaged system is approximately 14 mg. This dual-bandwidth MEMS sensor fills a technological gap in current completely implantable auditory prosthesis research and development by enabling a device capable of meeting physical and performance specifications needed for use in the middle ear.

On the battery life of a totally implantable active middle ear device: a retrospective study in a single implanting center

BACKGROUND

Totally-implantable active middle ear devices (AMED) rely on a non-rechargeable battery encased with the implantable sound processor that needs to be replaced with a minor surgical procedure after its depletion.

OBJECTIVES

This study aimed to investigate the most significant factors affecting the implant's battery life.

MATERIALS AND METHODS

The implanted subjects (29 patients) were divided into three groups; group A with 17 patients who underwent one battery change surgery; group B with ten patients who underwent two battery changes; and group C with two patients and three surgeries. The battery life was put in correlation with several variables, including daily use and the auditory threshold.

RESULTS

The battery life ranged from 26 to 67 months, with a mean of 48.93 ± 13.47. Pearson's correlation coefficient revealed that the battery life was statistically correlated only with the mean post-implantation bone conduction thresholds (p-value <.0001).

CONCLUSIONS

Although the non-rechargeable battery system of the AMED under study overcomes the drawbacks of daily charging, it needs to be surgically changed after its depletion. The different rates of battery life were shown to mainly depend on the post-implantation BC thresholds, which in some cases showed a deterioration concerning the pre-implanting values.

The remaining obstacles for a totally implantable cochlear implant

PURPOSE OF THE REVIEW

For years, the development of a totally implantable cochlear implant (TICI) has faced several technical challenges hindering any prototypes from reaching full commercialization. This article aims to review the necessary specifications for a viable TICI. An overview of the remaining challenges when designing TICIs will be provided, focusing on energy supply and implantable microphones.

RECENT FINDINGS

The literature review highlights how research efforts to generate sufficient power to supply a fully implantable CI could take advantage of microelectromechanical systems (MEMS)-based energy harvesters incorporating piezoelectric materials. Using one of the various energy sources in the vicinity of the temporal bone would allow the development of a self-sufficient implant, overcoming the limitations of electrochemical batteries. Middle ear implantable microphones could also use similar fabrication techniques and transduction mechanisms to meet the sensor requirements for a TICI.

SUMMARY

Recent breakthroughs in power supply using MEMS-based energy harvesting technologies and piezoelectric implantable microphones may make TICIs become a more practical reality in the foreseeable future. Once available, TICIs will have major impact on our patients' quality of life and may help to make hearing rehabilitation a more appealing option to a greater proportion of those who fulfill our candidacy criteria.

Long-Term Follow-Up of the Auditory Threshold After a Fully Implantable Middle Ear Implant

A fully implantable active middle ear device has been proposed and indicated for the rehabilitation of bilateral moderate or moderate-to-severe sensorineural hearing loss, assuming it would overcome the disadvantages of a conventional hearing aid. The indications have further been extended to severe or severe-to-profound forms of hearing loss in the case of an expected limited or null efficacy of hearing aids. While the literature has highlighted several positive aspects of the device, including a better quality of life related to its invisibility, the improvement of auditory and perceptual functions has not been controlled for throughout a long period of follow-up. The present study aimed to verify the behavior of the auditory threshold, especially the bone conduction (BC) component, in the implanted ear in a group of implantees affected by initial bilateral symmetric hearing loss of different severity grades. The BC threshold was assessed preoperatively at activation and at the last follow-up (ranging from 4 to 12 years) in the implanted ear, and preoperatively and at the last follow-up in the contralateral ear, to monitor eventual deteriorated values in both ears over time. The pure tone average (PTA; 250–4,000 Hz), speech reception threshold (SRT) and the maximum word recognition score as a percentage (% WRS) and in dB HL were measured in the implanted ear to verify the efficacy of the device after the first fitting at device activation. A significant worsening of the BC threshold with respect to the baseline threshold was noticed during further follow-up. When comparing the implanted ear with the contralateral ear, a significant worsening of the bone PTA was assessed in the former with respect to the contralateral ear. Despite the worsened hearing found in the implanted ears, the beneficial gains in PTA and speech audiometry observed at the first activation remained constant at the follow-up, thus showing an extension of the efficacy of this device in aiding those with up to the most severe forms of sensorineural hearing loss.

Round Window Stimulation of the Cochlea

Mixed hearing loss associated with a sensorineural component and an impaired conductive mechanism for sound from the external ear canal to the cochlea represents a challenge for rehabilitation using either surgery or traditional hearing amplification. Direct stimulations of the ossicular chain and the round window (RW) membrane have allowed an improved hearing in this population. The authors review the developments in basic and clinical research that have allowed the exploration of new routes for inner ear stimulation. Similar changes occur in the electrophysiological measures in response to auditory stimulation through the traditional route and direct mechanical stimulation of the RW. The latter has proven to be very effective as a means of hearing rehabilitation in a group of patients with significant difficulties with hearing and communication.

Systematic and audiological indication criteria for bone conduction devices and active middle ear implants

Certain patients with conductive or mixed hearing loss can benefit from bone-conduction hearing devices or active middle ear implants. Available devices differ in coupling site, energy transfer from the sound processor to the implant, and the active or passive actuator technology. The audiological benefit of those devices depends on the maximum stable power output and the noise floor of the device, the degree and expected stability of the sensorineural hearing loss and the coupling efficiency with the aim on achieving a minumum of 30-35 dB effective dynamic range. The choice of the device is often a trade-off between the optimal audiological solution with respect to the hearing loss, technical device-related parameters and the expected coupling efficiency, the optimal surgical solution with respect to patho-anatomical aspects, device dimensions and the coupling site, invasiveness or surgical risks, and other patient factors with respect to the patients' wish and expectations, social aspects, device usability and connectivity. This review article lists all currently available implantable and conventional bone-conduction hearing devices and active middle ear implants with respect to technical features like maximum power output, market availability, and the expected effective output dynamic range.

New Considerations for a Totally Implantable Active Middle Ear Implant

Totally implantable active middle ear implants (AMEI) provide full-time hearing amplification to those with moderate to severe sensorineural hearing loss. While technology in conventional hearing aids (CHA) has advanced greatly, limitations remain for people with active lifestyles, limited vision or dexterity, and hearing aid fit issues. Furthermore, direct-drive properties of AMEI are thought to provide those with inefficient middle ear transfer functions a distinct advantage in delivering prescribed sound to the cochlea, ultimately improving speech understanding with less distortion. AMEI safety, stability, and efficacy outcomes are well documented and fitting strategies continue to improve. Recent studies show how simple aided speech testing can help predict whether a patient struggling with CHA may instead benefit from an AMEI. Totally implantable AMEI continue to be a viable option for patients who cannot or will not utilize traditional hearing aids.

Polyharmonic Vibrations of Human Middle Ear Implanted by Means of Nonlinear Coupler

This paper presents a possibility of quasi-periodic and chaotic vibrations in the human middle ear stimulated by an implant, which is fixed to the incus by means of a nonlinear coupler. The coupler represents a classical element made of titanium and shape memory alloy. A five-degrees-of-freedom model of lumped masses is used to represent the implanted middle ear for both normal and pathological ears. The model is engaged to numerically find the influence of the nonlinear coupler on stapes and implant dynamics. As a result, regions of parameters regarding the quasi-periodic, polyharmonic and irregular motion are identified as new contributions in ear bio-mechanics. The nonlinear coupler causes irregular motion, which is undesired for the middle ear. However, the use of the stiff coupler also ensures regular vibrations of the stapes for higher frequencies. As a consequence, the utility of the nonlinear coupler is proven.

Mechanical Energy Sensing and Harvesting in Micromachined Polymer-Based Piezoelectric Transducers for Fully Implanted Hearing Systems: A Review

The paper presents a comprehensive review of mechanical energy harvesters and microphone sensors for totally implanted hearing systems. The studies on hearing mechanisms, hearing losses and hearing solutions are first introduced to bring to light the necessity of creating and integrating the in vivo energy harvester and implantable microphone into a single chip. The in vivo energy harvester can continuously harness energy from the biomechanical motion of the internal organs. The implantable microphone executes mechanoelectrical transduction, and an array of such structures can filter sound frequency directly without an analogue-to-digital converter. The revision of the available transduction mechanisms, device configuration structures and piezoelectric material characteristics reveals the advantage of adopting the polymer-based piezoelectric transducers. A dual function of sensing the sound signal and simultaneously harvesting vibration energy to power up its system can be attained from a single transducer. Advanced process technology incorporates polymers into piezoelectric materials, initiating the invention of a self-powered and flexible transducer that is compatible with the human body, magnetic resonance imaging system (MRI) and the standard complementary metal-oxide-semiconductor (CMOS) processes. The polymer-based piezoelectric is a promising material that satisfies many of the requirements for obtaining high performance implantable microphones and in vivo piezoelectric energy harvesters.

A New Type of Wireless Transmission Based on Digital Direct Modulation for Use in Partially Implantable Hearing Aids

In this study, we developed a new type of wireless transmission system for use in partially implantable hearing aids. This system was designed for miniaturization and low distortion, and features direct digital modulation. The sigma-delta output, which has a high SNR due to oversampling and noise shaping technology, is used as the data signal and is transmitted using a wireless transmission system to the implant unit through OOK without restoration as an audio signal, thus eliminating the need for additional circuits (i.e., LPF and a reference voltage supply circuit) and improving the ease of implantation and reliability of the circuit. We selected a carrier frequency of 27 MHz after analysis of carrier attenuation by human tissue, and designed the communication coil with reference to both the geometry and required communication distance. Circuit design and simulation for wireless transmission were performed using Multisim 13.0. The system was fabricated based on the circuit design; the size of the device board was 13 mm × 13 mm, the size of the implanted part was 9 mm × 9 mm, the diameter of the transmitting/receiving coil was 26 mm, and the thicknesses of these coils were 0.5 and 0.3 mm, respectively. The difference (error) between the detected and simulation waveforms was about 5%, and was thought to be due to the tolerances of the fabricated communication coil and elements (resistors, capacitors, etc.) used in the circuit configuration of the system. The number of windings was reduced more than 9-fold compared to the communication coil described by Taghavi et al. The measured THD was <1% in the frequency band from 100 Hz to 10 kHz, thus easily meeting the standard specification for hearing aids.