Enhancing detection accuracy via controlled release of 3D-printed microlattice nasopharyngeal swabs

Nasopharyngeal swabs are widely used for specimen collection in diagnostics (e.g., SARS-CoV-2 testing), but conventional swabs typically require elution into buffer (diluted release, DR), which dilutes analytes and can reduce test sensitivity, leading to slow positives and false negatives. Recovery efficiency is often poor, with studies reporting more than 50% of DNA remaining in various swab types after collection. Enhancing assay sensitivity by modifying detection chemistries is laborious and costly. The authors hypothesize that improving the sample release step—by preventing dilution and maximizing recovery—can significantly increase analyte concentration at the point of measurement, thereby improving detection sensitivity and accuracy. They propose 3D-printed microlattice NP swabs featuring an open-cell architecture enabling controlled, undiluted sample release (CR) via centrifugal force, addressing dilution limits while offering mechanical advantages and customizable performance.

3D printing alleviated NP swab shortages during COVID-19; early 3D-printed solid swabs with micro-convex arrays showed comparable performance to commercial swabs and enabled individualized designs. Subsequent incorporation of microlattice structures offered large surface area, open unit cells enhancing capture/retention, flexibility for better nasal conformity, capillary-driven rapid uptake, energy absorption, and tunable mechanical/sampling properties. Nevertheless, these advantages were not uniquely superior once commercial supplies resumed. Literature also documents limitations of DR methods and variable POCT sensitivity; nylon flocked swabs can release better due to open-fiber structures, but dilution and incomplete recovery persist. This context motivates microlattice-based swabs with controlled undiluted release to surpass concentration and recovery limitations inherent to DR.

Design and manufacturing: Three microlattice swab heads—Auxetic (A), Dodecahedron (D), and Body-Centered Cubic (BCC, X)—were modeled in Rhinoceros. Printing used a high-resolution LCD 3D printer (Whale 2, Shenzhen Nova Intelligent Technology) with transparent tough ABS-like resin (G217, RESIONE). Layer thickness 50 µm; exposure 75 s for first 8 layers and 6 s for subsequent layers. Post-processing: ethanol and DI water wash, then vacuum drying with silica gel desiccant. Multiple swabs were printed simultaneously to enhance print success for slender parts. SEM confirmed print quality and open-cell architecture. Mechanical characterization: Bending (3-point), tensile, and compression tests were performed on a micro testing system (Gatan Microtest). A custom 3-point bending fixture was 3D-printed and attached for bending tests. Swab tips were tested; tensile specimens length ~10 mm; compression specimens ~5 mm. Each swab design was tested >3 times for reproducibility. Force–elongation curves and in-situ deformation were recorded. Sample release characterization (food dye model): Yellow food dye and DI water (1:9 v/v) prepared as initial specimen. Swabs were immersed for 10 s to load ~1 mL aliquots. DR: swabs stirred 20 times in 3 mL DI water. CR: microlattice swabs placed in empty tubes and centrifuged (manually or using a centrifuge) to expel liquid without dilution; released sample collected at tube bottom. UV–Vis spectrophotometry (BioDrop µLITE) measured absorbance of transfer buffers. A standard curve related dye concentration (vol%) to absorbance for quantification of release concentration and volume. Variants with different BCC strut diameters (e.g., 0.10 mm vs 0.16 mm) assessed customization of storage/release volume. Viscous liquid (mucus) models: Honey:water mixtures (10:1 for cervical mucus, 1:4 for nasal mucus) were prepared. Rheometry (KINEXUS Pro+, Malvern) confirmed viscosity–shear rate profiles within physiological ranges. DR and CR procedures were applied as above; CR volumes were tuned by centrifuge rotation speed (e.g., 300–500 rpm) and duration (2–10 s). Centrifugal force F = mω²r = m(2πn)²r; manual centrifuging ω ~35 rad/s; 300/400/500 rpm give ω ~31.4/41.9/52.4 rad/s. Experiments were run in triplicate; for each group, three absorbance readings were averaged. Rapid test kit antibody detection demonstration: Anti-SARS-CoV-2 Spike RBD neutralizing IgG (SAD-S35, Acro Biosystems) stock 2000 µg/mL was prepared; 10,000 ng/mL intermediate in PBST; working 300 ng/mL IgG prepared in PBST. Swabs were loaded with 300 ng/mL IgG and released: microlattice A-swab via CR into empty tube; commercial flocked swab via DR into 3 mL PBST. Transfer buffers were applied to COVID-19 IgM/IgG rapid test kits (BIOSYNEX) and results recorded. ELISA quantification: SARS-CoV-2 IgG ELISA (Abcam ab275300) quantified IgG concentrations in swab transfer buffers. Samples (10× diluted) were processed per kit protocol (antigen-coated wells, detection antibody, TMB substrate, stop solution) and OD measured at 450 and 570 nm. A standard curve related IgG concentration (ng/mL) to absorbance. Each ELISA was performed in duplicate. Statistics and reproducibility: For release experiments, each condition was repeated three times; each replicate measured in triplicate and averaged.

- 3D-printed open-cell microlattice NP swabs (A, D, X) exhibit markedly improved mechanics: reactive bending force up to ~7× lower than commercial flocked swabs, translating to up to ~11× higher flexibility; all swabs showed elastic recovery after unloading. - Food dye model (initial 10% v/v dye): After DR in 3 mL water, released dye concentrations (vol%) were: commercial 0.23%, A 0.52%, D 0.40%, X 0.35% (microlattice 1.5–2.5× higher than commercial). After CR, microlattice swabs released undiluted samples at ~10% (≈50× commercial DR). Quantified release volumes (initial solution) via DR: commercial 70.6 µL, A 164.6 µL, D 125.0 µL, X 108.8 µL (microlattice up to ~2.3× commercial). Released dye amounts (µL dye): commercial 7.1, A 16.5, D 12.5, X 10.9. Open-cell geometry enabled high recovery with minimal residual dye after CR. - Customizable release volume: Altering microlattice cell/strut size (e.g., BCC 0.10 mm vs 0.16 mm strut) changed storage and release volumes. Smaller cells are recommended for dilute or low-volume specimens to enhance storage and concentration. - Viscous mucus models: For cervical model (10:1 honey:water), post-DR concentrations (vol%) were: commercial 0.06%, A 0.33%, D 0.17%, X 0.08% (microlattice up to ~6× commercial). For nasal model (1:4), post-DR: commercial 0.013%, A 0.17%, D 0.14%, X 0.03% (up to ~13×). Post-CR concentrations matched initial solutions (~90% for cervical, ~20% for nasal), representing ~1500× vs commercial DR. Under identical centrifugal force, higher-viscosity samples released less volume (more retained), demonstrating viscosity-dependent quantitative control. CR volumes were tunable by centrifuge speed (300–500 rpm) and time (2–10 s). - Antibody detection: Rapid test kits yielded positive results for microlattice CR-derived IgG transfer buffers (300 ng/mL input), but negative for commercial DR-derived buffers due to dilution. ELISA quantified IgG ~300 ng/mL (microlattice CR) vs ~5 ng/mL (commercial DR), a ~60× increase in concentration with CR. - Overall, microlattice swabs provide: higher flexibility, customizable and larger release volumes, undiluted high-concentration recovery (dozens to thousands-fold higher than DR), and near-complete recovery efficiency (~100%), enabling quantification of analyte levels.

The study addresses the central challenge that conventional swab DR significantly dilutes analytes and suffers from incomplete recovery, degrading diagnostic sensitivity and accuracy. By leveraging open-cell microlattice architectures and applying controlled release via centrifugal force, the proposed swabs prevent dilution and expel retained liquid efficiently, preserving original analyte concentration and maximizing recovery. Enhanced flexibility allows better nasal conformity, potentially increasing sampling area/volume and reducing patient discomfort. Quantitative control over release volume is achieved both structurally (cell size, strut diameter, lattice topology) and operationally (centrifuge speed/time), enabling tailoring to sample viscosity and diagnostic requirements. Empirical results across food dye and physiologically relevant mucus analogs show substantial gains in released concentration (up to ~1500× vs commercial DR for viscous models) and increased release volumes (up to ~2.3×), while antibody testing demonstrates practical benefits with improved rapid test positivity and ELISA-verified ~60× higher IgG concentration. Together, these findings validate that undiluted CR from microlattice swabs can break concentration limits imposed by DR, improving the sensitivity and accuracy of POCT and laboratory diagnostics, especially for low-titer or low-volume specimens.

This work introduces 3D-printed open-cell microlattice NP swabs with a simple, equipment-light controlled release (CR) approach that undilutedly expels collected specimens via centrifugal force. The swabs exhibit up to ~11× higher flexibility than commercial counterparts, larger and customizable release volumes (up to ~2.3×), and substantially higher released concentrations (dozens to thousands-fold above DR) with high recovery efficiency (~100%). Demonstrations with food dye, mucus analogs, and anti-SARS-CoV-2 IgG highlight improved diagnostic sensitivity and accuracy, including positive rapid test outcomes and ELISA-confirmed concentration gains (~60×). The microlattice design also enables quantification of released analyte amounts. Future work should: (i) optimize and customize microlattice parameters (cell topology, strut diameter, cell size) using comprehensive mechanical and fluidic data; (ii) validate performance with diverse real clinical specimens and pathogens/biomarkers; (iii) integrate with standardized collection tubes/centrifugation protocols for quantitative release; and (iv) explore micro/nano-lattice architectures to enhance capture for low-volume or dilute analytes and broaden applications across biomedical devices.

Experimental validations used model systems (food dye, honey-based mucus analogs) and purified IgG rather than diverse clinical specimens, which may exhibit more complex rheology, biofouling, and matrix effects. In practical sampling, insufficient specimen volume may not fully fill swab heads, causing released volumes lower than laboratory estimates and potentially larger concentration differences between CR and DR. The study focuses on benchtop performance; clinical usability, comfort, safety, and sterilization workflows, as well as broad regulatory validation, remain to be established.