Justin J Baker and Dr. William Pitt, Chemical Engineering

Hydrogels are polymers that absorb water and thus swell in the presence of water and/or body fluids. Hydrogels have been used in many drug-delivery and medical applications.1 One such application is to apply a thin coating of a hydrogel to the outer surface of a hearing aid to eliminate acoustic feedback. Acoustic feedback is the high-pitched squeal caused by an incomplete seal between the hearing aid and the surface of the ear canal. This incomplete seal allows amplified sound to escape and be re-amplified by the hearing aid. By coating a hearing aid with a hydrogel, the humidity and oils from the ear will cause the hydrogel to swell and create a seal between the hearing aid and the ear canal. The compliant nature of swollen hydrogels also allows the hydrogel to deform as the ear canal changes shape when hearing aid users talk or eat.

However, current hydrogels do not swell sufficiently or rapidly enough for this application. Research supports the theory that the cross-linking branches formed during polymerization give the hydrogel better strength but at the same time inhibit the hydrogel from further swelling.2 Our hypothesis was that the presence of water in the hydrogel monomer solution prior to polymerization would separate cross-linking branches during polymerization allowing the hydrogel to swell more and faster than would hydrogels without water in the monomer solution prior to polymerization. We also hypothesized that the presence of the water would create more pores in the hydrogel, thus facilitating faster swelling.

The monomer solution used in this study consisted of 1 weight percent (wt%) of the cross-linker polyethylene glycol dimethacrylate (PEGDMA), 0.5 wt% of the photo-initiator 2,2-dimethoxy-2-phenyl-acetophenone (DPAP), as well as a one-to-one mole ratio of hydroxyethylmethacrylate (HEMA, 53.13 wt%) and N-vinyl-pyrrolidone (NVP, 45.37 wt%).3 Two different types of tests, swelling tests and a tensile test, were performed to determine the effect that the prior water concentration had on the hydrogels swelling and mechanical properties. The swelling tests added 0 wt%, 5 wt%, and 10 wt% water to different samples and then photo-polymerized the hydrogels for equal lengths of time. The different samples were then dried and placed in water and the percent swelling was measured gravimetrically (by weight) at intervals of 10 minutes for the first hour and then at intervals of an hour or more up to approximately 18 hours. The formula used for determining the swelling was (% swelling)=((Ws-Wd)/Wd)*100%, where Ws is the weight of the swollen hydrogel and Wd is the weight of the dry hydrogel. The tensile tests were used to determine the mechanical properties and durability of the hydrogels. The tensile tests pulled in tension dog-bone-shaped samples until the hydrogels broke, thus giving the hydrogels ultimate tensile strength and modulus of elasticity.

Throughout the swelling tests several challenges emerged that had to be overcome. Some of the hydrogels were burnt or overpolymerized during drying prior to the swelling tests, possibly causing these hydrogels to crumble apart during the swelling test. This reduced the number of samples available to measure the swelling and the swelling standard deviation. Another difficulty encountered was making and preserving the molds in which the hydrogels were polymerized. Removing the hydrogels from the molds often ripped the mold material, rendering the mold useless. Ultimately, inexpensive, disposable test tube caps were used as molds.

Because of the hydrogel’s brittle nature, prior to tensile testing, both ends of the dog-bone-shaped hydrogels were mounted in epoxy. The epoxy mounts were to help distribute and absorb the force from the grips of the tensile test machine. Despite the epoxy mounts, the hydrogels remained extremely brittle, often breaking as the grips were tightened even before a tensile test could be performed. In an effort to keep the hydrogels from breaking, many samples slipped in the tensile machine’s grips. Despite these setbacks, several samples yielded data suggesting that the presence of water prior to polymerization was not a detriment to the hydrogel’s mechanical properties.

The results of the swelling tests showed that the hydrogels with a higher wt% of water swelled more overall and also swelled faster than did the control hydrogels without water in the monomer solution. The hydrogels with the prior concentration of 5 and 10 wt% water swelled 15% more in an hour (p≈0.022) and 20-30% more overall (p=0.025) than did the hydrogels without water prior to polymerization. These results supported our hypothesis that the presence of water in the monomer solution prior to polymerization allows the hydrogel to swell faster and more overall without harming the mechanical properties of the hydrogel.

We recommend that further research on the swelling of hydrogels with prior water concentration be performed on hearing aid molds with thin (approximately 1 mm thick) coatings of hydrogel to determine if the small thickness facilitates swelling. We also recommend that further tensile testing be performed because many of the samples broke, slipped, or yielded inconclusive data. A three-point flexural test may be a better method of determining mechanical properties while avoiding some of the problems with the tensile grips due to the hydrogel’s brittleness. In conclusion, the project was a success and we believe that the prior water concentration in hydrogels may present an effective method of eliminating acoustic feedback in hearing aids.4

References

1. Nikolaos A. Peppas, ed. Hydrogels in Medicine and Pharmacy. Boca Raton, Florida. CRC Press 1986.
2. Zhiqiang Yang, Yuehua Zhang, Peter Markland, Victor C. Yang. Poly(glutamic acid) poly(ethylene glycol) hydrogels prepared by photoinduced polymerization: Synthesis, characterization, and preliminary release studies of protein drugs. Journal of Biomedical Materials Research Vol. 62 Issue 1. pp. 14-21. 28 Jun 2002.
3. This solution was deemed an appropriate balance between swelling and mechanical properties for a hydrogel by research performed by Sungil Choi. For more information see: Sung-il Choi. Hydrogel-Coated Hearing Aid Earmold to Obtain an Acoustic Seal. Thesis, Brigham Young University. August 2003.
4. For further information on this project, please see my Brigham Young University Honors Thesis Effect of Water Concentration During Polymerization of a Hydrogel on Swelling and Mechanical Properties. 2006.