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aids are constructed using a hard, external shell that fits within the ear canal and contains the electroacous- tic components: microphone(s), signal processing and amplification electronics, sound amplification trans- ducer, and the battery. The electroacoustic components are the same within any given product line, and the sig- nal-processing and amplification characteristics can be tailored to the individual user according to the specific hearing impairment using a simple programming step after audiological evaluation.
In contrast, the shape of the shell itself is highly unique due to the need to correctly position the device in the ear canal and maintain a physical seal between the device and ear canal. This shape cannot be easily reconfigured and thus each individual shell must be custom constructed. Further- more, traditional fabrication methods, such as UV-cured polymers molded to fit each user, require a significant effort by individual technicians with years of experience. This approach to mass customization of components resulted in low device-to-device repeatability, even when starting with the same biometric customer information.
For all the reasons given, these types of hearing devices are particularly well suited for the strengths of LPBF and SLA technology. Namely, AM fabrication of custom hear- ing devices exploits the fact that (1) each part has a highly irregular and custom shape in order to fit within the ear canal of the individual user; (2) the components are phys- ically small, and thus parts for different customers may be fabricated in a single build and each build can be differ- ent without changing the fabrication settings; (3) the final product is subjected to mild environmental mechanical loading over the entire life cycle; and (4) 3D optical scan- ning can be used to gather the ear canal geometry, and thus final parts can be created with virtually no contact with the customer. Given all of these benefits, there has been a massive transition to rapid production of hearing devices since the early 2000s, including nonacoustic or mechanical improvements such as incorporating antib- iofilm properties into the printed material to reduce ear infection (Vivero-Lopez et al., 2021).
As an example, Figure 5 shows photos of 3D-printed hearing aids printed with a digital light processing polymer, a liquid resin-type process, which has been implanted with an antibiofilm drug in varying quanti- ties. In this case, the drugs are mixed into the liquid resin
before printing. The study also used different materials, both a hard and a flexible resin, to assess the resulting variations in the printed final product.
Conclusion
This article provided a multidisciplinary review of current uses for AM in acoustics. AM can be applied to a wide swath of acoustic applications, including musical instru- ments, scale models, acoustic metamaterials, ultrasonic NDE, transducers, and hearing prostheses. These examples highlight how AM is pushing the frontiers of acoustics, and conversely, how acoustics can be used to advance dif- ferent AM technologies. Advances in both acoustic design and AM technology mean that the range and depth of these applications will certainly expand in the future. For example, advances in AM that push the limitations of a smallest printable feature size would expand the frequency range of acoustics applications, and advances that increase the maximum size of AM parts could enable larger scale acoustics applications such as noise control of large struc- tures. From another perspective, advances in acoustics and ultrasonics could enable better quality control of AM parts that would allow safety-critical industries to adopt AM technologies. Additionally, reduced costs for basic print- ing approaches mean that many of these applications will become more and more accessible to acoustic researchers and hobbyists alike.
Figure 5. AM hearing aids made from different resins. a: Sample is a resin. b: Sample is the result of printing the same hearing aid shell from a flexible resin. The samples were manufactured with differing levels of an antibiotic in the material to investigate the ability of the material to reduce ear infection from long-duration hearing aid use. Reproduced from Vivero-Lopez et al. 2021, Figure 1), with permission from Elsevier.
Spring 2022 • Acoustics Today 55