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Munjal, Tina MD; Grosskopf, Abigail K. MS; Stingel, Jon P. MS; Chellappa, Nitika S. MTM; Dever, David MBA; Webel, Aaron David MD, MBA; Fitzgerald, Matthew B. PhD, CCC-A
*The first four authors share first authorship; corresponding author can be reached at [email protected] From left: Dr. Munjal is a resident physician and research fellow in Otolaryngology—Head & Neck at Stanford University. Her primary research is on tinnitus and personalized therapeutics for auditory disorders. Ms. Grosskopf is a chemical engineering PhD candidate at Stanford University. Her research focuses on the development of biomaterials for controlled drug delivery and cell transplantation. Mr. Stingel, MS and PhD candidate (expected 2023) in mechanical engineering at Stanford University, is most interested in developing both mechanical and data-driven technologies that improve human health and wellness using a needs-driven approach. Ms. Chellappa is a bioengineer developing clinically relevant product solutions focused on patient and clinician needs. Mr. Dever works as a consultant and is a graduate of the Stanford Byers Center for Biodesign. Dr. Webel is an ophthalmologist and glaucoma specialist at the University of Missouri-Columbia. His research focuses on novel glaucoma drainage and diagnostic devices to address the problem of ocular fibrosis after glaucoma surgery. Dr. Fitzgerald is the chief of Audiology at Stanford University. His primary research interests are developing new tools to assess auditory function and translating those tools into audiologic practice.
The most common device used to manage pediatric sensorineural hearing loss (SNHL) is the behind-the-ear (BTE) hearing aid (HA), which typically requires the use of a custom earmold in pediatric patients to maximize their access to sound. 1 As children grow, these earmolds must be replaced to deliver sound optimally. Otherwise, the amount of feedback is likely to increase and the hearing aid will become less comfortable and useful. 1 Maintaining access to sound is crucial given that duration of HA use is associated with better outcomes. 2,3 These difficulties are likely to be further exacerbated in children in developing countries, who lack consistent access to audiologic care. 4
In conventional audiologic settings, at least two visits are required per set of earmolds: one to generate the impression and one to fit the mold (Figure 1A). The manufacturing turnaround for molds is typically 8 to 12 days, with additional time required for the fitting process. 5 This inefficient cycle is repeated many times during childhood due to rapid growth. Furthermore, current earmolds are static in that they lose contact with key points of the canal after growth, leading to poor acoustic seal, loss of retention, and discomfort (Figure 1B).
One solution to reduce the amount of time with suboptimal or no sound input would be to have an earmold that could be adjusted, rather than replaced, by either the caregiver or audiologist. Then, children could maintain access to sound as they grow despite changes in the size and shape of the ear. Here, we describe a novel, low-cost manufacturing technique that allows for adjustment of the size and fit of the earmold using standard materials. The earmold, inspired by novel soft robotic engineering techniques, allows for selective inflation of the canalicular portion of the earmold, thus maintaining optimal contact with the anatomic points most important for comfort, retention, and acoustic fidelity (Figure 1C). 6,7 The objective of such a device would be to reduce time and sound input lost due to poorly fitting earmolds.
All molds consist of two materials: silicone for the body of the mold and temperature-responsive Pluronic F127 for the sacrificial ink. 6,8,9
Pluronic F127 (Sigma-Aldrich) was dissolved in milliQ water (Millipore Sigma) at 20 wt% and dissolved over 2 days at 4°C. The solution was kept in 10 mL aliquots and stored at room temperature. Two drops of aqueous-based red food dye were added to each aliquot.
Figure 2A displays the overall fabrication process to create the adjustable earmold using silicone and Pluronic F127 with figure numbers aligning with steps detailed subsequently. First, a set of casting molds were designed and 3D printed for an ear impression that was scaled 1.5x original size for prototyping. The casting mold was altered to have the same conchal eminence dimensions as the scaled impression, a canal with the same length, and a diameter no more than 50% of the scaled impression diameter to allow for ease of subsequent manufacturing steps. 10 Figure 2A 1.1 displays the core casting mold creation.
20 g of SORTAClear 40 silicone (Smooth-On) was mixed according to manufacturer’s instructions, degassed, and loaded into the first small casting mold by filling one half of the mold, covering with the other half and injecting silicone through the injection port until the mold was full. This became the earmold core once cured overnight and removed from the casting mold (Figure 2A 1.2).
The same 1.5x scaled digital ear impression was used to create a second casting mold that would result in creation of an earmold with the initial target size.
A spatula was used to paint the entire ear canal portion of the earmold core with a 1 mm-thick layer of 20 wt% Pluronic F127 (Figure 2A 2.1).
20 g of Ecoflex 20 (Smooth-On) was mixed according to manufacturer’s instructions and degassed. 0.1 g of colored silicone dye, SilcPig (Smooth-On) was added to the Ecoflex 20 to improve visualization. 7 g of Ecoflex 20 was poured into one half of the outer casting mold and the earmold core coated with Pluronic F127 was submerged into this half. Then, the second half of the casting mold was placed on top to envelop the earmold core within and more Ecoflex 20 was injected via the injection port into the mold until completely filled (Figure 2A 2.2) and allowed to cure overnight.
The complete mold—composed of the outer mold, Pluronic F127 coating within, and earmold core within—was placed at 4°C to liquify the Pluronic F127 using the temperature-dependent behavior of Pluronic F127 seen in Figure 2B. The temperature-dependent viscosity of Pluronic F127 measured with shear rheology is also shown in Figure 2C, where 20 wt% Pluronic F127 in water has a lower critical solution temperature of 23°C. Therefore, when the solution is at low temperatures, it behaves as a liquid and when above room temperature, it behaves as a solid-like gel. The mold was punctured through the conchal eminence with a 21 G needle and 10 mL syringe to access the Pluronic F127 pocket. A syringe, aided with the use of cold water, was used to remove the liquid Pluronic F127, leaving an empty void (Figure 2A 3.1).
Both a 21 G needle and a male 1/16” Luer lock barb were inserted into the same puncture site used for draining Pluronic. The barb was coated in a silicone-bonding epoxy to seal the opening to the earmold. This fitting was secured to a syringe, as shown in Figure 3A, using a 1/16” inner diameter silicone tubing and a Luer-activated valve (Qosina Corp.). These components create a simple inflation action via the syringe, while the Luer-activated valve seals the pressure inside the earmold when the syringe is disconnected (Figure 2A 3.2).
This prototype consisted of a 1.5x scaled earmold with a 1 mm-thick void around the external auditory canal region. The canal region was selected for the inflation to optimize acoustic seal, retention, and comfort. 7
When pressure was applied through the syringe during inflation, we achieved a continuous range of anisotropic inflation of the earmold’s canal portion. Figure 3B displays the final adjustable earmold prototype with labeled designations; pink shading represents the earmold canal portion before inflation, while the white silicone represents the inflated earmold canal. Measurements of canal diameter before and after inflation were taken at three locations along the canal portion of the earmold. The apex of the first bend of the ear canal experienced an expansion of 24.9%. A point in the canal 5 mm medial to the apex of the first bend expanded by 13.3%, and a point 5 mm lateral to the apex of the first bend expanded by 7.0%.
This prototype demonstrates the capabilities of an adjustable, yet still personalized, earmold. This earmold could allow children with BTE hearing aids to maintain optimal access to sound during critical periods of development. Moreover, this approach has the potential to minimize the number of patient visits and reduce the need for “remakes” of earmolds due to poor impressions or manufacturing of the device. The adjustable earmold concept could minimize barriers to access in the pediatric audiology workflow and may be particularly useful for children in developing countries or rural areas who have limited access to specialized audiologic care. Finally, the adjustable earmold approach may also be useful for adults who need compressible molds to accommodate dynamic canal changes with jaw movement. 11
Future optimization could take many forms, including validation across a range of anatomical shapes and sizes, as well as testing adjustment in the conchal eminence. Additionally, acoustic integration and testing using real-ear measurements is needed to assess transmission capabilities across a wide range of hearing losses. Finally, an algorithm must be developed to help determine when the earmold needs to be adjusted. Algorithm inputs may include information from the feedback manager, the output from the HA measured as gain, and also predicted growth patterns of external ear anatomy over time. This final input, the expected pediatric growth pattern, might involve an adjustment schedule. Figure 3C illustrates a proposed method to arrive at a possible inflation schedule using a calibration study in which mold diameter is correlated with input pressure, and canal diameter is correlated with age. This would potentially enable the prediction of input pressure required for a given age and subsequently allow for the creation of a typical inflation schedule.
Once these steps to validate the prototype have been achieved, the manufacturing process would need to be streamlined, perhaps by utilizing another casting mold for consistent Pluronic F127 layering, replacing the manual coating method seen in Figure 2A 2.1. Various areas of Pluronic F127 placement may also be tested, including the key anatomical points depicted in Figure 1B and 1C.
To our knowledge, this is the first report of a prototype for an earmold that can be adjusted outside of the clinic. This prototype uses biocompatible materials and allows for selective inflation of different portions of the mold. This approach has the potential to ensure that the earmold can maintain comfort and acoustic retention as the child grows. Thus, an adjustable earmold could reduce the amount of time a child has suboptimal access to sound and may be particularly beneficial for patients who do not have consistent access to audiologic services. Further work is required to validate this approach, but we are optimistic that an adjustable earmold could provide audiologists with additional tools to maximize a child’s access to sound.
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