The second generation BAHA HC-200: 1987-1991

In 1984, after the thesis2 was defended, the mother company Nobel Industries was interested to expand the dental business to also cover extra oral applications. They formed a sister company to Nobelpharma AB with the name Nobel Biotech AB that signed an exclusive license in January 1985 with P&B to use know-how and immaterial rights for the BAHA. At that time the second generation of the BAHA, that was called the HC-200, was already in the pipeline including patents and patent applications for a damped transducer, the Skull simulator and the bayonet coupling.

The plan was to collaborate with an internationally recognized hearing aid company. An agreement with the Starkey Inc in Minneapolis was negotiated and ready for signing in the spring 1985. However, three month after the license was signed Nobel industries made a strategic change and closed down all activities with the BAHA and told us at Chalmers to do something else. We had at that time from our research seen that there was a real need for a BAHA in patients with draining ears who actually did not have any other functioning alternative. We had also convinced the Swedish board for technical development (STU) to further support our work and therefore we continued the development program for the HC-200 at Chalmers. After some time we also convinced the Nobelpharma (who took over from the license from Nobel Biotech) to finance a prototype series of the HC-200. The project was then restarted more officially in 1987 and key persons (David Eriksson and Einar Jörgensen) were engaged at Nobelpharma and they played an important role in this sensitive phase of the project. A manufacturing site was established at Chalmers facilities as the project was not yet accepted to a level that it could be fully integrated in the Nobelpharma business. The BAHA project was at that time marketed under the name Nobelpharma Auditory System (NAS).

The need for damped transducer was as mentioned before a first priority. This need became apparent when we had one patient, a deaconess, who came back with the HC-100 and told us that the she absolutely didn’t want to keep her device because it was extremely unpleasant. When she said “the problem occurs only when visitors come to my home” we thought it was something religious. Later she explained: “I feel like my head will explode when visitors ring the doorbell”. Then we understood what it was all about. As you can see in Figure 14 (graph to the left below) the Oticon transducer was undamped which causes the sharp peak in the frequency response when it is connected to the skull via the implant. The doorbell was sounding on that frequency and thus producing an extremely high output sound. So even if the device is stable it might be very annoying in some cases and when the amplification is increased to near instability the device can respond with a ringing or singing echo of a sound. It should be emphasized that the Oticon transducer was designed to be used in conventional bone conduction applications and there the soft tissues act like a damper and the peak is smoothed out and these feedback problems disappear (other feedback problems from sound radiation will appear). Another problem with the undamped peak in the BAHA application is that it limits the maximum gain because of feedback problems at the resonance frequency. As can be seen in Figure 14 (right) the Oticon transducer used in the HC-100 have a “S” shaped metallic spring without damping whereas the HC-200 transducer has a blade spring with a layer of damping material on top.

Figure 14: The HC-100 (undamped) vs HC-200 (damped) transducer.

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