Improved hearing through mass-customized hearing aids

Additive manufacturing is applied by Sonova AG for mass-customizing hearing aids. For a worldwide supply Sonova AG make use of a distributed production network consisting of several manufacturing facilities in different parts of the world.

In the hearing instrument industry, additive manufacturing has been used since the early 2000`s for the fabrication of customized hearing aids. A hearing aid helps a person with impaired hearing to compensate this hearing loss by selective amplification of sound. Additive manufacturing allows to individualize and adapt the shape of these devices to the specific geometry of a customer`s hearing channel.

Over time, this use case for additive manufacturing has become mature and today, customized hearing aids are sold worldwide in very large quantities. In this respect, there are a number of drivers that have a significant influence on the development and manufacturing of hearing aids. First, hearing devices should be as small as possible and provide a good wearing comfort while integrating all necessary electronic components. Second, customers from different parts of the world demand a short delivery time. Third, customers as well as hearing healthcare professionals expect a selection of different features, optical appearance, and other customizable options.

Sonova AG, a Swiss company specialized in hearing care solutions, addresses these market requirements with its brands Phonak, Unitron and Hansaton by employing a decentralized network of production facilities and applying a digital fabrication process., The overall process for in-the ear hearing aids starts by capturing a customer`s hearing channel through a silicone impression. Besides this impression, a hearing aid professional defines the required functional components regarding acoustics e.g. loudspeaker size. In addition, the customer adds additional specifications e.g. type of feature (battery, tele-coil), color, and lacquering. Together with the requirement list, the silicon impression is sent to a local production facility of Sonova AG.

Laser scanning the silicon impression digitizes the geometry of a customer`s ear channel geometry. In this regards, it has to be noted that in general between individuals the anatomic differences can be quite large. Therefore, customizing the shape based on the 3D scan allows achieving an improved wearing comfort and better fit. The generated point cloud serves as basis for the modeling of the customized hearing instrument. The task of the modeler is to design the shape of the hearing aid and place the electronic components in it. For this process step, Sonova AG developed its own Rapid Shell Modelling (RSM) software together with the company Materialise. The design automation software guides the modeler step-by-step through the shell design process. It provides ear-anatomy recognition capabilities thereby accelerating the design process and enabling initial placement of electronic components automatically. Furthermore, the software allows the company to offer its customers benefits not possible with traditional manufacturing methods such as Acoustically Optimized Vents tailor-made for each customer based on individual acoustic requirements

After designing the shell of the hearing aid, it is manufactured using Direct Light Processing (DLP) based on vat polymerization. The technology uses a light source to cure and harden acryl resin. In comparison to Stereolithography (SLA), the resin is not cured by a single beam but by projecting a single image at once for a given layer.. DLP printer have the advantage that they can produce parts in a short build time and at reduced costs with a high level of precision, process stability, and good material properties. Typical wall thicknesses of the shell range from 0.5 to 0.7 mm. Moreover, DLP allows for a wide range of colors. As a result, Sonova AG offers over ten different materials for hearing aids. For a given material, multiple shells can be printed in a few hours. After the printing process, the shells are separated from the build plate and support structures removed. A post-curing step causes the final curing of the resin. A quality check ensures the integrity of the printed shells.

For the final assembly of the hearing aid, an operator places the specified electronic components in the shell. Prefabricated electronic kits and layout instructions accelerate the manual positioning. After all components are integrated in the shell, it is closed and sealed. Applying a transparent varnishing improves the optical appearance and makes it possible to fine-tune the retention of the hearing aid. The marking with a serial number allows the traceability of the medical device. A final quality check tests the acoustic and visual characteristics of the device. In this regard, it is worth mentioning that is not only the geometric personalization of a hearing aid but also the acoustic settings, which are tailored to a customer`s degree of hearing loss and the assessment of a hearing care professional. The final product is delivered to a customer within one to five days.

To achieve such a fast delivery, Sonova AG make use of a distributed production network consisting of several manufacturing facilities in different parts of the world. Each local facility possesses multiple DLP printers to deliver hearing aids with the described process chain to regional customers. To attain a more continuous production flow and increase, production flexibility orders are printed in small batch jobs using multiple small-sized DLP printers. In general, the printer size and number needs to match the required local capacity. Overall, a worldwide, decentralized network of local production facilities offers a high degree of flexibility, simplifies logistics, and enables a short turnaround time for customized hearing aids.

More recently, Sonova AG announced the release of new solutions for hearing aids with a metal shell. These shells are made of titanium through laser melting. The material’s excellent biocompatibility allows its use for customers who are allergic to typical acrylic otoplastic materials. Fabricating the shell out of metal enables a decreased wall thickness to only 0.2 mm, thereby allowing the integration of larger components such as loudspeakers or batteries at constant size or making hearing aids even smaller and discrete. In the future, it will be interesting to see how advancements in additive manufacturing will affect and further improve hearing aid applications.

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