RESEARCH SUMMARY

March 2003

Chemistry and Rheology of Otoplastic Materials
Research by Robert J. Oliveira, Ph.D., President of Hearing Components, and Vasant V. Kolpe, Ph.D., Company Scientist, Hearing Components

The knowledge of earmold materials and ear impression compounds is important in providing solutions to hearing impaired people who use behind the ear (BTE) hearing aids. Viscoelastic foam provides a solution.

Ear canal activity and non-compliant materials are the key problems to achieving trouble-free physical fit of earmolds.

• To research a solution, the sound attenuation properties of silicones, acrylates and polyurethane foams have been measured using a specially designed acoustic test fixture (ATF). Also, the moduli (softness) of earmold and shell materials have been assessed.
• For the most part, shells that enclose the electronics of ITE, CIC and other miniature hearing instruments use hard, polymerized acrylates and earmolds use polymerized acrylates, flexible polyvinylchloride, or flexible silicone rubbers prepared from replicas of ear impressions.
• Traditionally, a rigid, or at most flexible, earmold is placed into a soft ear canal. The earmold expands the soft tissue of the ear canal and makes a seal. This leads to discomfort when a tighter seal is required with high gain BTEs.

Making of ear impressions:

• When an ear impression is taken, it is made with a polymer that will shrink. No matter how careful, the material naturally shrinks, so it’s never 100 percent perfect.
• We now have state of the art silicone ear impression by modifying similar compounds used in dental science. Rarely, for an ear impression compound, we use a polymer solution of low molecular weight acrylate in an acrylate monomer (powder and liquid).
• Modern silicone ear impression putties undergo almost negligible polymeric shrinkage (.5 % when the impression compounds are set and removed from the ear). This material is not very useful as an earmold since it is not very durable.

Poor fit of earmolds are caused by:

• Poor impression taking technique or material.
• Poor laboratory replication of impressions.
• Changing ear canal geometry with jaw motion.
• Use of non-compliant earmold materials.

Earmold solution for the hearing impaired:
Compliant viscoelastic polyurethane foam provides the best current earmold solution for the hearing impaired.

• It overcomes the limitations of material shrinkage and mechanical compatibility to the dynamic ear canal.
• It is compliant. It expands and contracts in the ear canal, changes shape as the ear canal enlarges or reduces in volume due to jaw motion.
• Compliant foam provides an ultrasoft modulus at body temperature and superior energy dissipation in the audible acoustic range of frequencies.
• It retains the fidelity of DSP amplified sound.
• It prevents leakage of amplified sound by an active seal.
• It is comfortable and has a good physical fit in a warm, humid and mobile ear canal.
• The clogging of earmold sound tubes by ear is reduced if there is a cerumen filter built in to the compliant foam.
• Hearing Components’ viscoelastic foam is not a plasticized PVC foam. Plasticized PVC foams have a relatively poor shelf life due to the migration of the plasticizer which results in the PVC foam becoming stiff and scratchy.

Other results of the research show:

• Hearing Components’ viscoelastic foam is superior. For 100dB SPL input the HFA (Pure Tone High Frequency Attenuation) of the Hearing Components’ viscoelastic foam averages at 52dB over a thickness range 4 to 12 mm at 98° Fahrenheit temperature. It easily exceeds attenuation for silicones (46dB) and hard acrylic (35dB) both held at the same temperature of 98° F.
• The superiority of Hearing Components’ viscoelastic foam is maintained in the composite mode test where attenuation is determined for a wide band speech weighted composite signal.
• Testing of the materials also determined that the Hearing Components’ viscoelastic foam is 30 times softer than the softest current earmold material and even softer than skin.

Conclusion: Compliant viscoelastic polyurethane foam has provided a solution to leak-free sound seals in ear canals. It provides an ideal fit to volume changes generated by the active ear canal.

Viscoelastic polyurethane foams’ compliance, superior attenuation and softness, in comparison to common acrylates, plasticized PVC, and silicone rubber, makes it the preferred new earmold material.

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