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Biomaterials

Biomaterials - Poster Session D

Poster C1 - PDMS Compositions for Pediatric Conductive Hearing Aids

Friday, October 25, 2024
3:30 PM - 4:30 PM EST
Location: Exhibit Hall E, F & G

Introduction:  Over 400 million people globally have disabling hearing loss, with at least 34 million of those being children(WHO, 2021). Many children are diagnosed with conductive hearing loss(CHL) due to microtia, aural atresia, and other complications. Implantable bone conduction hearing airs require surgery and are invasive for pediatric patients. In addition, non-surgical solutions such as wearable and behind-the-ear hearing aids present a set of problems such as stigmatization and discomfort(van Zyl et al., 2022).

Thin, flexible, conductive hearing aids produce vibrations using piezoelectric(PZT) actuators, which generate vibrations that are transmitted through soft tissue to the skull and cochlea(Moghimi et al., 2023). The components of the flexible aid are implemented on a polydimethylsiloxane(PDMS) substrate. The thin PDMS is interfaced between skin and PZT actuator; therefore, vibrations from PZT transmit through PDMS to skin and bone. When vibrational waves travel through different mediums, a portion of the waves are reflected and damped, so the signal to the subsequent layer is attenuated. If the acoustic impedances of both layers are similar, maximum transmission of vibration occurs(Kinsler et al., 2000). Young’s modulus of PDMS also influences transmitted vibrations through soft tissue(Moebs et al., 2016). Changing the proportion of base:curing agent changes the Young’s modulus of PDMS(Khanafer et. al., 2009). Therefore, the substrate’s composition can be optimized to achieve similar impedance to the skin layer, increasing the transmission of vibrations to the cochlea without increasing the device power. We investigated the effects of various PDMS base:curing agent ratios on the transmission of vibrations across soft material.

Materials and

Methods:  Figure 1 A shows the experimental setup to measure vibrations from the PZT actuator on PDMS substrates. A PZT actuator was attached to a PDMS sample (Fig. 1 B), which was subsequently attached to a 1-mm thick aluminum plate. The structure of the system is shown in Fig. 1 C. A sinusoidal voltage of varying frequencies was applied to the actuator, and a laser doppler vibrometer (LDV) was used to measure the resulting vibrations. Different PDMS compositions, with base-to-curing agent ratios of 10:1 (baseline), 15:1, and 20:1, were used for this experiment. Subsequently, a 2-mm thick Ecoflex layer was added between the PDMS and metal plate to simulate soft tissue.

Results, Conclusions, and Discussions: Changing the base-to-curing agent ratio of the PDMS had a significant effect on the resulting intensity of vibration. Fig. 2 A shows the velocity of vibrations from the PZT and PDMS layer that was directly attached to the aluminum plate. All three ratios showed a similar average velocity of 125 ± 0.8 dB. By increasing the ratio of base to curing agent from 10:1 to 20:1, we observed that the resonance frequency shifted upward from 5 to 9 kHz. With the Ecoflex layer substituting as a skin layer (Fig. 2 B), the baseline 10:1 ratio is attenuated by 5 dB. The peaks at 7 and 9 kHz were repeated for all three samples, and both the 15:1 and 20:1 ratios improved the response by 2.2 dB and 1.4 dB, respectively. At 9 kHz, the 15:1 showed the highest vibrational magnitude at 130.4 dB, compared to the 10:1 at 123.5 dB and 20:1 at 126 dB. Our results demonstrate that a PDMS composition of 15:1 transmitted vibration more efficiently through a flexible substrate compared to the original 10:1 and the 20:1 composition.

The flexible hearing aid is a noninvasive solution to address CHL. However, the attenuation from the flexible substrate can reduce the vibration level and affect the perceived sounds. This can be managed by changing the mechanical properties of the flexible substrate (e.g., PDMS layer). Our results show that changing the ratio of the base and curing agent of the PDMS affects the vibration transmission and changes the frequency of maximum vibration. We found that the optimum ratio of the base to curing agent is 15:1. This can allow the vibrations produced by the PZT actuator of the flexible hearing aid to be transmitted to the skull with minimal attenuation, allowing users to hear louder sounds without increasing the power of the device.

Acknowledgements (Optional):  This project has been funded by Rowan University and the National Institutes of Health, National Institute of Deafness and other Communication Disorders under Award R21DC01889.