Borrowing a page from wireless phone communications, hearing aids have become firmly entrenched in the digital realm during the past decade. At GN ReSound, the manufacturing process for hearing aid shells has entered the digital age as well, promising faster production, greater customization, and fewer remakes.

Digital shape sampling and processing (DSSP) from Geomagic (Research Triangle Park, NC) is at the center of GN ReSound's new process, enabling the company to automatically transform 3D scan data into an airtight model for rapid manufacturing.

The Path to Better Customization

Once the 3D scan of the ear impression has been created, the Geomagic software rapidly surfaces the data in order to make it usable as a 3D model.
In the United States alone, 24 million people suffer from significant hearing loss; a number that is increasing as the baby-boomer generation ages. Custom-fit hearing instruments are popular in the U.S., making up 80 percent of the market. Some of these instruments are so advanced that they could be called hearing computers — the earliest wearable computers.

Custom hearing instruments need highly individualized shells. For decades, making custom shells for hearing instruments was done manually by sculptors. The process was labor-intensive and could not be easily scaled to increase production efficiency. Sculptors had different levels of skill, making it difficult to control quality.

The scenario is quickly changing, however, due to the combination of DSSP and rapid-manufacturing systems put to innovative use by hearing instrument manufacturers.

GN ReSound (Bloomington, MN) is one of the world's largest hearing instrument manufacturers, known in the U.S. for its ReSound and Beltone hearing aid brands. The company began testing digital methods in the early 1990s, but technologies and materials were not ready for production use. As new digital technologies became available, GN ReSound was among the first to adapt them to its production process.

The 3D shell of the hearing aid itself is created and tested to ensure that it will fit comfortably into the ear, as well as handle electronic parts once the product is manufactured.
For GN ReSound, the new digital process not only means increased manufacturing efficiency, it holds the promise of reaching the holy grail for hearing instrument companies: designing products attractive enough for the 80 percent of people who need a hearing instrument, but refuse to buy one for aesthetic, comfort, or cost reasons.

The process on the front end of creating a customized hearing aid is the same as always: A customer visits his or her doctor, acrylic or silicone is injected into the ear, and an impression of the ear canal is created after the product hardens. The finished mold is shipped to GN ReSound, where similarities to old processes end.

The sculptor's knife and other carving tools used in the past to replicate the impression have been replaced with a 3D scanner, computers running specialized software, and rapid-manufacturing systems.

Once the impression arrives, it is digitally captured with a 3D scanner from 3Shape ApS (www.3shape.com ) that is designed for scanning ear canal impressions. A complete 360-degree surfaced scan in STL format is output from the scanner in 150 seconds.

An operator imports the STL file into Geomagic eShell, customized software that creates digital models from 3D scanner data. The 3D model is then digitally sculpted — the ear canal section is trimmed to the appropriate length, and the end rounded for easier insertion into the ear.

Most people need hearing aids for both ears. In these cases, Geomagic eShell creates models from the 3D scans of the right and left ear impressions. As the operator sculpts one model, the software simultaneously applies the same sculpting features to the model for the other ear. At the end of the process, the operator reviews the results and accepts or modifies the model as needed.

“The ability to sculpt two shells at the same time provides a major productivity increase,” said Russ Schreiner, GN ReSound's project director. “Working with digital models enables us to further customize the shells as we see fit, creating an entire product line with exclusive features, no matter how small the run of final parts.”

Once the digital model is complete, electronic components are projected into it to test for fit. The operator is warned if there are areas of collision where the components might interfere with the manufacturing process.

Once the shell designs are completed, they are manufactured using liquid acrylic in a Perfactory rapid-prototyping machine. The visible light from the projector cures the acrylic, creating a hardened shell that is then cleaned and cured. Electronics can then be inserted into the manufactured shell.
The sculpted digital model is then made into a shell with an inner and outer surface. The inner surface varies in thickness throughout the finished shell in order to better support areas subject to fracture and to aid in assembly. Items such as counter-bore holes and air vents are added to the shell before exporting the model as a watertight STL file to a rapid-prototyping machine.

The entire process — from sculpting to outputting the final STL file to the rapid prototyping machine — takes only a few minutes.

The individualized shell designs for the hearing aids are manufactured with Envisiontec GmbH's (www.envisiontec.de ) Perfactory Rapid Prototyping System.

STL models from several orders are imported into the Perfactory software and automatically positioned for maximum space utilization. Once the shell placement is approved, the models are broken into bitmap image slices that are projected into liquid acrylic in the rapid-prototyping machine. The visible light from the projector cures the acrylic, creating a hardened shell that is then cleaned and cured.

The shells then move on to the manufacturing line for electronics insertion. There the electronics are inserted into the shell, a faceplate is added, and the entire unit is trimmed and buffed for fit and appearance. The finished products are assembled and shipped to the distributor, often a doctor, in as few as two days.

Automation Key to Improvements

Major improvements are expected to come from greater automation of the sculpting process, more efficient finishing and machining, and implementing new designs that are not possible in the traditional process.

Further automation of the sculpting process is an immediate priority. Although sculpting is performed on a computer, it is still “human-driven” according to Schreiner. GN ReSound is working on embedding the logic behind sculpting — considered more of an art form than a process — into Geomagic eShell software. These changes can easily be implemented in each of its manufacturing plants throughout the U.S. and Europe.

“Once that is accomplished,” said Schreiner, “software can be written to automatically perform most, if not all, tasks.” He also expects digital processes to add new functionality to hearing instruments.

“Rapid manufacturing machines allow you to produce features that are not possible with conventional molding or casting,” he said. “We expect to design features in the shell that enhance product performance and lead to more efficient manufacturing.”

GN ReSound’s digital processes have helped improve efficiency in producing remakes, where a new shell is required due to a fit-related problem. The need for remakes is a perennial problem in the hearing instrument industry. GN ReSound now has all the digital information on file and the customer does not have to go through the process of making a new impression.

An additional benefit of an all-digital process, said Schreiner, is that it will be less expensive to make shells in multiple styles and sizes for the customer to try out before manufacturing. This will help ensure the best fit and feel, and reduce returns.

“The new process has proven that it can be just as good if not better compared to the traditional process in overall fit and performance of the product,” said Schreiner. “The real breakthrough will come when we make new products that are attractive to those who need a hearing instrument but would not consider one in the past.”

This article was written by Jean-Michel Koch, Marketing Manager EMEA for Geomagic in Research Triangle Park, NC. Contact Jean-Michel at This email address is being protected from spambots. You need JavaScript enabled to view it., or visit http://info.hotims.com/34452-189.

Medical Design Briefs Magazine

This article first appeared in the March, 2011 issue of Medical Design Briefs Magazine.

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