In situ pulmonary mucus hydration assay using rotational and translational diffusion of gold nanorods with polarization-sensitive optical coherence tomography

Abstract

Significance: Assessing the nanostructure of polymer solutions and biofluids is broadly useful for understanding drug delivery and disease progression and for monitoring therapy.

Aim: Our objective is to quantify bronchial mucus solids concentration (wt. %) during hypertonic saline (HTS) treatment in vitro via nanostructurally constrained diffusion of gold nanorods (GNRs) monitored by polarization-sensitive optical coherence tomography (PS-OCT).

Approach: Using PS-OCT, we quantified GNR translational ($D_T$) and rotational ($D_R$) diffusion coefficients within polyethylene oxide solutions (0 to 3 wt. %) and human bronchial epithelial cell (hBEC) mucus (0 to 6.4 wt. %). Interpolation of $D_T$ and $D_R$ data is used to develop an assay to quantify mucus concentration. The assay is demonstrated on the mucus layer of an air-liquid interface hBEC culture during HTS treatment.

Results: In polymer solutions and mucus, $D_T$ and $D_R$ monotonically decrease with increasing concentration. $D_R$ is more sensitive than $D_T$ to changes above 1.5 wt. % of mucus and exhibits less intrasample variability. Mucus on HTS-treated hBEC cultures exhibits dynamic mixing from cilia. A region of hard-packed mucus is revealed by $D_R$ measurements.

Conclusions: The extended dynamic range afforded by simultaneous measurement of $D_T$ and $D_R$ of GNRs using PS-OCT enables resolving concentration of the bronchial mucus layer over a range from healthy to disease in depth and time during HTS treatment in vitro.

Keywords: gold nanorods; mucus; optical coherence tomography; particle diffusion; polyethylene oxide.

Fig. 1

PS-OCT system schematic.

Fig. 2

GNR diffusion in aqueous 4 MDa PEO solutions. In the top panels of (a), $D_T$ and $D_R$ are plotted as a function of PEO solids concentration. In many cases, error bars are not visible due to small standard deviation size. Zero concentration represents GNR diffusion in solvent (distilled water). In the bottom panel, intrasample variability is presented as the percent ratio of the standard deviation to the mean for both $D_T$ and $D_R$. In panels (b) and (c), co-polarized (HH) and cross-polarized (HV) B-mode images of the 0.25 and 2.5 wt. % PEO solutions are displayed along with corresponding $D_T$ and $D_R$ values within each ROI derived from B + M-mode images; excluded ROIs based on thresholding criteria described in the methods are displayed as the background color.

Fig. 3

GNR diffusion in hBE mucus sample. In the top panels of (a), $D_T$ and $D_R$ are plotted as a function of mucus solids concentration. In many cases, error bars are not visible due to small standard deviation size. Zero concentration represents GNR diffusion in solvent (DPBS). In the bottom panel, intrasample variability is presented as the percent ratio of the standard deviation to the mean for both $D_T$ and $D_R$. (b), (c) Co-polarized (HH) and cross-polarized (HV) B-mode images of the 0.5 and 6.4 wt. % hBE samples are displayed along with corresponding $D_T$ and $D_R$ values within each ROI derived from B + M-mode images; excluded ROIs based on thresholding criteria described in the methods are displayed as the background color. (c) Multiple ROIs are rejected due to air bubbles (indicated by blue arrows) in the 6.4 wt. % sample.

Fig. 4

(a), (b) $D_T$ and $D_R$ in ALI cultures with HTS introduced are color mapped. (c) The diffusion coefficients are mapped to mucus concentration using trendlines produced from stationary mucus experiments. If an ROI had a valid measurement of both $D_T$ and $D_R$, the calculated concentrations were averaged for a final reported value.