Design of 3D-Printed Nasopharyngeal Swabs for Children is Enabled by Radiologic Imaging

MATERIALS AND METHODS

The institutional review board approved all clinical data use. Examination of maxillofacial CT scans of patients 1–3 years of age suggested that primary deflection of the swab in the sagittal plane would be sufficient to navigate from the external nares to the posterior nasopharynx. We, therefore, designed elliptical cross-section swabs (Fig 1) and verified that sagittal shaft deflection was adequate. We then made 3D prints of the nasopharyngeal passage itself, and physically tested the ability of individual swab prototypes to navigate the passage. We noted that less flexible designs would get trapped in the crenulations of the nasopharyngeal passage and might require multiple attempts to reach the posterior nasopharynx, thus lengthening the time required to reach the sampling location and possibly injuring the delicate pediatric anatomy. On the assumption that increased navigation time and resistance correlated with patient discomfort, we chose the navigation time and a qualitative assessment of resistance as measures of patient discomfort, ultimately selecting the design with the shortest navigation time and the lowest resistance score for clinical use.

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FIG 1.

A, Sagittal section of a maxillofacial CT scan of a 22-month-old patient. Shown in red is the trajectory of a virtually inserted swab reaching the posterior nasopharynx. The white box shows the portion of the anatomy chosen to print the nasopharyngeal passage. B, A commercial flocked pediatric swab (COPAN Flock Technologies swab) reaching the posterior nasopharynx within the 3D-printed nasopharyngeal passage. C, Design of the elliptical-section 3D-printed pediatric swab (Design ES).

Maxillofacial CT datasets of 5 patients, 11 to 34 months of age and unremarkable for pathologies involving the maxillofacial region, were randomly selected from our data base. Images were processed in 3D Slicer (Version 4.10; http://www.slicer.org) to create Standard Triangle Language files for printing. Nasopharyngeal passages were printed on a Form2 printer using Elastic Resin (Formlabs). Swab designs were made in Fusion 360 (AUTODESK) and printed on a Form2 printer using Surgical Guide Resin (Formlabs).

We tested 5 swabs designs: 2 commercial mini-swabs, Flocked 1, PURITAN Diagnostics and Flocked 2, COPAN Flock Technologies; and 3 in-house-printed, all of which had a flexible shaft length of approximately 50 mm, a handle of 78 mm, and a transition zone of 14 mm, as shown in Fig 1, and varied in the flexible shaft cross-section: Design E, 1.2 mm × 0.8 mmø; Design I, 1.2 mm; and Design ES, 1.2 mm × 0.8 mmø, with an additional slanted posterior edge of the brush. The Table shows dimensions (in millimeters) for each of the swab types tested. Navigation of each swab from the external nares to the posterior nasopharynx was done by 1 individual, the order of testing of the individual swabs and anatomic models was randomized, and each test was repeated 3 times. The time to complete navigation was timed by an observer using a hand-held stopwatch, while the resistance on insertion was classified using a 3-level resistance score (1, easy insertion: no resistance; 2, medium: mild resistance; and 3, hard: resistance requiring extra force). All statistical calculations were performed in Matlab R19a (MathWorks).

The quality of specimen recovery from Design ES nasopharyngeal swabs was determined by performing a crossover collection study with the commercial swabs (Puritan Diagnostics) and prototype NB swabs in 2 healthy adult volunteers, a 26-year-old man and a 28-year-old woman. Both nostrils of the volunteers were sampled by each swab in successive order. The swabs were placed into a MicroTest M4RT viral transport containing 3 mL of media (Remel). Nucleic acid was extracted using 200 uL from each viral transport on the eMAG system (bioMérieux) and eluted into 50 uL of buffer. Real-time polymerase chain reaction (RT-PCR) was performed on each eluate using the Centers for Disease Control’s 2019-Novel Coronavirus (2019-nCoV) Emergency Use Authorization assay,² which includes 2 targets in the Severe Acute Respiratory Syndrome coronavirus 2 nucleocapsid region and 1 target in the human RNase P gene to assess the cellular quality of the sample.