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Moisture Resistance

Overview

Exposure to moisture, wax and other foreign materials will degrade hearing aid performance over time. This issue may be exacerbated in patients with more active lifestyles (such as baby boomers and children).

Common causes of device failure that occur as a result of ingress of moisture, sweat, and other foreign materials are:

  • blockage of acoustic paths, such as acoustic ports;
  • damage to transducers and mechanical components, such as switches;
  • leakage and corrosion of the zinc air battery;
  • circuit malfunctions due to damage to the protective layer and solder joints.

Traditionally, it has been difficult to protect batteries and electronic circuits from moisture ingress. Typical forms of moisture ingress management include: barriers, gaskets o-rings, water-repellent fabrics, and water-repellent foams. These approaches are most often implemented within battery compartments, case seams, microphones and receivers. However, these techniques may not be practical in some cases due to size constraints. Additionally, damage, such as a cut or tear, to these physical seals may negate any protection offered. Attempts to prevent damage from moisture using acoustically transparent, but water repellent fabrics and foams have been more positive as they reduce the amount of unwanted substances that can reach the transducers; however, the effectiveness of these methods has been found to be insufficient when used alone.

The limitations to moisture barriers include:

  • degradation in performance of the barrier over time, requiring frequent replacement;
  • difficulty in attaching acoustic fabric reliably, due to the miniature nature of the protective mechanism;
  • degradation in acoustic transparency over time due to the condensation of water droplets on the fabric surface.

Hydrophobic and Superhydrophobic Nano-Coating

One solution for improving moisture resistance lies in the application of moisture repellent nano-coatings to hearing devices as an enhancement to the traditional approaches. Water repellent phenomena found in plants, such as the lotus, offer unique insight into the behavior of a hydrophobic coating. The lotus leaf has non-wetting properties that act as a self-cleaning mechanism; water droplets completely roll off the leaves carrying dirt and mud with them. This self-cleaning, or Lotus Effect, is caused by the roughness of the leaf surface (composed of micrometer sized papillae, or hairs), and the intrinsic hydrophobicity of the waxy surface layer covering these papillae. The roughness enhances the natural non-wetting nature of the surface, leading to even greater repellency to a liquid drop on the surface.

How well a solid surface repels a liquid depends upon two factors: surface energy and surface morphology. Surface energy affects the interface between the liquid and the solid surface by influencing the attractive forces at the molecular level. When liquid contacts a surface, the liquid molecules may have a stronger attraction to the molecules of the solid surface than to each other. This affects the degree of water repellency of the surface.

Alteration of surface morphology, on the other hand, at the micro- or nano-scale can allow a layer of air to form in the spaces on the contact surface. This layer of air is shown in Figure 1 as gaps in the black surface. This surface roughness/texture is crucial in that it is the source of the hydrophobic properties. Panel A of Figure 1, shows a water droplet that completely wets a textured surface. This is known as the “Wenzel state”. In this state, the water droplet will leave a water trail as it slides and spreads, which may negatively affect a hearing aid. Conversely, a water droplet that rests on the layer of air within the textured surface is in the “Cassie state”. Shown in Panel B of Figure 1, the water droplet sits on top of the surface and is less likely to adhere to the surface. A water droplet in the Cassie state will easily roll off of a surface without leaving residual moisture.

Wenzel State Cassie State
(a) Wenzel State (b) Cassie State
Figure 1. Comparison of Behaviors for a Liquid Droplet on a Textured Surface


The Lotus Effect, or Cassie State, can be achieved artificially by introducing textures on a surface of interest at the nano scale (such as a nano tube forest, nano particles, or etching). This surface coating can be applied through a harmless photochemical or plasma treatment. Starkey Laboratories' research team has completed extensive development that resulted in Advanced Hydrashield, a nano-coating technology that produces a non-wettable surface achieving water resistance for hearing devices.

Evaluating Moisture Resistance

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