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Rapid Protyping System Transforms Hearing Aid Development

Mon, 12/06/2004 - 6:45am
The Project: To design, optimize, and produce better fitting hearing aids in less time.
The Solution: Use CAD/CAM and 3D computer imaging to create custom devices in audiologist offices.

These images show various angles of two hearing aid shells. One shell is on top of the other. Raw data is illustrated in yellow. Finished models are depicted in green.
Todd Grimm is the founder and president of T. A. Grimm & Associates Inc., 3028 Beth Ct., Edgewood, KY 41017, a consulting services firm specializing in rapid prototyping and related technologies. He has worked in the product development industry for the last 17 years, has a mechanical engineering degree from Purdue University, and has written "User's Guide to Rapid Prototyping," a book published this year by SME. He also contributed to McGraw-Hill's "Manufacturing Engineering Handbook." Grimm can be reached at 859-331-5340 and grimm@tagrimm.com. More information is available from Roland at 800-542-2307, PolyMesh at 401-413-2954, and 3D Systems at 661-295-5600.

By Todd Grimm
Advances in CAD/CAM technology and 3D computer imaging are making it possible for audiologists to participate in the creation of customized hearing aids. The Ocean State Hearing Aid Center in Providence, RI, is among the first to begin scanning patient ear canals to produce custom hearing aids.

Its CAD/CAM workflow consists of three steps: scan, design, and produce. Ocean State uses a system from PolyMesh, a product development firm in Providence, to accomplish all three steps. The system relies on the LPX-250 3D laser scanner from Roland DGA Corp. in Irvine, CA, for the scanning, PolyMesh's EarFORM 2004 software for the designing, and the Viper laser-based prototype/production machine from 3D Systems in Valencia, CA, for production.

Compared to conventionally manufactured hearing aid shells, the shells developed with the PolyMesh system provide a more comfortable fit, improved acoustics, higher reliability, and a fully concealed "in-the-ear-canal" design that is cosmetically pleasing.

During the scanning phase, the audiologist makes a silicone impression of the patient's ear canal. The impression is removed and digitized in a matter of minutes. Using plane scanning, the LPX-250 3D laser scanner captures complex contours of the ear canal through triangulation. The data points are connected by the scanning computer, and a wire frame of the impression is generated. The computer automatically interpolates data from the wire frame and creates an exact digital image of the impression.

In the design phase, EarFORM 2004 software lets the audiologist create custom designs, making it possible to decimate, edit, and heal scanned data. The audiologist can try different modifications or receiving and venting layouts before deciding on the final design. In addition, all changes—including the exact amount of material added or removed from the virtual impression—are recorded in the computer for later retrieval and analysis.

Final production is handled with the Viper stereolithography system. It traces a laser beam on the surface of a vat of liquid photopolymer to build the hearing aid shell one layer at a time. The material quickly solidifies when the laser touches the surface. This advanced technology automatically creates the shell in a matter of hours.

In comparison, the conventional method of designing and developing hearing aid shells requires the following nine steps.

1. A cast of the impression is made.

2. The ear impression is trimmed to the model size.

3. The impression is dipped in wax.

4. A hydrocolloid mold of the impression is made.

5. Acrylic resin is poured into the hydrocolloid cast.

6. Excess acrylic resin is drained from the hydrocolloid cast.

7. The faceplate end of the shell is trimmed.

8. The vent is laid into the shell.

9. The finished shell is readied for electronics.

With the conventional method, the skill of the technician is extremely important in ensuring that there is an even coating of wax on the impression. Despite all precautions, wax will adhere in different thicknesses at different points. As a result, the shell can be too loose or tight, yielding pressure points that cause discomfort. With the CAD/CAM method, waxing is done in the modeling stage. A consistent 0.3 mm increase (or any other magnitude) in thickness is specified over the entire surface of the impression. Thus, no buffing is necessary on these shells after they are produced. Without the need for buffing and trimming, the transition between the first and second bends and the area medial to the second bend is improved. The result is a more accurate and comfortable fit.

In addition, the CAD/CAD method encourages experimentation in the design phase. Virtual components—IC chips, receivers, the microphone, the faceplate, vents, receiver tubings, and wax guards—can be placed in different parts of the virtual shell. In this way, it can be guaranteed that all parts fit properly into the shell and that the hearing aid is cosmetically acceptable. This virtual modeling process should also result in better acoustics because the placement of different components can be optimized. For example, it should minimize the risk of the receiver pointing at the ear-canal wall or being misdirected, which can result in feedback.

The CAD/CAM process also allows the thickness of the shell wall to be specified during the modeling stage. Typically, a 0.7 mm thickness is specified to approximate the average thickness found in conventional shells. A hearing aid shell that has a uniform thickness has better mechanical resistance to impact and potential damage. It is interesting to note that these shells break less frequently than conventional shells.

ONLINE
For additional information on the technologies discussed in this article, see Medical Design Technology online at www.mdtmag.com and the following websites:
• www.roland3d.com
• www.polymesh.net
• www.3dsystems.com

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