Exploring the role of electrostatic deposition on inhaled aerosols in alveolated microchannels

Large amounts of net electrical charge are known to accumulate on inhaled aerosols during their generation using commonly-available inhalers. This effect often leads to superfluous deposition in the extra-thoracic airways at the cost of more efficient inhalation therapy. Since the electrostatic force is inversely proportional to the square of the distance between an aerosol and the airway wall, its role has long been recognized as potentially significant in the deep lungs. Yet, with the complexity of exploring such phenomenon directly at the acinar scales, in vitro experiments have been largely limited to upper airways models. Here, we devise a microfluidic alveolated airway channel coated with conductive material to quantify in vitro the significance of electrostatic effects on inhaled aerosol deposition. Specifically, our aerosol exposure assays showcase inhaled spherical particles of 0.2, 0.5, and 1.1 μm that are recognized to reach the acinar regions, whereby deposition is typically attributed to the leading roles of diffusion and sedimentation. In our experiments, electrostatic effects are observed to largely prevent aerosols from depositing inside alveolar cavities. Rather, deposition is overwhelmingly biased along the inter-alveolar septal spaces, even when aerosols are charged with only a few elementary charges. Our observations give new insight into the role of electrostatics at the acinar scales and emphasize how charged particles under 2 µm may rapidly overshadow the traditionally accepted dominance of diffusion or sedimentation when considering aerosol deposition phenomena in the deep lungs.

Exploring pulmonary distribution of intratracheally instilled liquid foams in excised porcine lungs

The applicability of inhalation therapy to some severe pulmonary conditions is often compromised by limited delivery rates (i.e. total dose) and low deposition efficiencies in the respiratory tract, most notably in the deep pulmonary acinar airways. To circumvent such limitations, alternative therapeutic techniques have relied for instance on intratracheal liquid instillations for the delivery of high-dose therapies. Yet, a longstanding mechanistic challenge with such latter methods lies in delivering solutions homogeneously across the whole lungs, despite an inherent tendency of non-uniform spreading driven mainly by gravitational effects. Here, we hypothesize that the pulmonary distribution of instilled liquid solutions can be meaningfully improved by foaming the solution prior to its instillation, owing to the increased volume and the reduced gravitational bias of foams. As a proof-of-concept, we show in excised adult porcine lungs that liquid foams can lead to significant improvement in homogenous pulmonary distributions compared with traditional liquid instillations. Our ex-vivo results suggest that liquid foams can potentially offer an attractive novel pulmonary delivery modality with applications for high-dose regimens of respiratory therapeutics.

Biomimetics of the pulmonary environment in vitro: A microfluidics perspective

The entire luminal surface of the lungs is populated with a complex yet confluent, uninterrupted airway epithelium in conjunction with an extracellular liquid lining layer that creates the air-liquid interface (ALI), a critical feature of healthy lungs. Motivated by lung disease modelling, cytotoxicity studies, and drug delivery assessments amongst other, in vitro setups have been traditionally conducted using macroscopic cultures of isolated airway cells under submerged conditions or instead using transwell inserts with permeable membranes to model the ALI architecture. Yet, such strategies continue to fall short of delivering a sufficiently realistic physiological in vitro airway environment that cohesively integrates at true-scale three essential pillars: morphological constraints (i.e., airway anatomy), physiological conditions (e.g., respiratory airflows), and biological functionality (e.g., cellular makeup). With the advent of microfluidic lung-on-chips, there have been tremendous efforts towards designing biomimetic airway models of the epithelial barrier, including the ALI, and leveraging such in vitro scaffolds as a gateway for pulmonary disease modelling and drug screening assays. Here, we review in vitro platforms mimicking the pulmonary environment and identify ongoing challenges in reconstituting accurate biological airway barriers that still widely prevent microfluidic systems from delivering mainstream assays for the end-user, as compared to macroscale in vitro cell cultures. We further discuss existing hurdles in scaling up current lung-on-chip designs, from single airway models to more physiologically realistic airway environments that are anticipated to deliver increasingly meaningful whole-organ functions, with an outlook on translational and precision medicine.

Particle sizing of pharmaceutical aerosols via direct imaging of particle settling velocities

We present a novel method for characterizing in near real-time the aerodynamic particle size distributions from pharmaceutical inhalers. The proposed method is based on direct imaging of airborne particles followed by a particle-by-particle measurement of settling velocities using image analysis and particle tracking algorithms. Due to the simplicity of the principle of operation, this method has the potential of circumventing potential biases of current real-time particle analyzers (e.g. Time of Flight analysis), while offering a cost effective solution. The simple device can also be constructed in laboratory settings from off-the-shelf materials for research purposes. To demonstrate the feasibility and robustness of the measurement technique, we have conducted benchmark experiments whereby aerodynamic particle size distributions are obtained from several commercially-available dry powder inhalers (DPIs). Our measurements yield size distributions (i.e. MMAD and GSD) that are closely in line with those obtained from Time of Flight analysis and cascade impactors suggesting that our imaging-based method may embody an attractive methodology for rapid inhaler testing and characterization. In a final step, we discuss some of the ongoing limitations of the current prototype and conceivable routes for improving the technique.

A novel aerodynamic sizing method for pharmaceutical aerosols using image-based analysis of settling velocities

This article discusses a novel method to estimate aerodynamic particle size distributions (APSDs) of pharmaceutical aerosols through direct measurement of particle settling velocities using image-based analysis and particle tracking techniques. This simple, optical method provides accurate and fast measurements (approximately 1 minute) with few sources of bias due to specific device design choices or operation conditions. A proof-of-concept for the method is demonstrated by measuring APSDs for widely available commercial dry powder inhalers (DPIs), then comparing the results with previously published data from cascade impactors (CIs) and the Aerodynamic Particle Sizer (APS).

Streamline crossing: An essential mechanism for aerosol dispersion in the pulmonary acinus

The dispersion of inhaled microparticles in the pulmonary acinus of the lungs is often attributed to the complex interplay between convective mixing, due to irreversible flows, and intrinsic particle motion (i.e. gravity and diffusion). However, the role of each mechanism, the exact nature of such interplay between them and their relative importance still remain unclear. To gain insight into these dispersive mechanisms, we track liquid-suspended microparticles and extract their effective diffusivities inside an anatomically-inspired microfluidic acinar model. Such results are then compared to experiments and numerical simulations in a straight channel. While alveoli of the proximal acinar generations exhibit convective mixing characteristics that lead to irreversible particle trajectories, this local effect is overshadowed by a more dominant dispersion mechanism across the ductal branching network that arises from small but significant streamline crossing due to intrinsic diffusional motion in the presence of high velocity gradients. We anticipate that for true airborne particles, which exhibit much higher intrinsic motion, streamline crossing would be even more significant.

A Microfluidic Model of Biomimetically Breathing Pulmonary Acinar Airways

Quantifying respiratory flow characteristics in the pulmonary acinar depths and how they influence inhaled aerosol transport is critical towards optimizing drug inhalation techniques as well as predicting deposition patterns of potentially toxic airborne particles in the pulmonary alveoli. Here, soft-lithography techniques are used to fabricate complex acinar-like airway structures at the truthful anatomical length-scales that reproduce physiological acinar flow phenomena in an optically accessible system. The microfluidic device features 5 generations of bifurcating alveolated ducts with periodically expanding and contracting walls. Wall actuation is achieved by altering the pressure inside water-filled chambers surrounding the thin PDMS acinar channel walls both from the sides and the top of the device. In contrast to common multilayer microfluidic devices, where the stacking of several PDMS molds is required, a simple method is presented to fabricate the top chamber by embedding the barrel section of a syringe into the PDMS mold. This novel microfluidic setup delivers physiological breathing motions which in turn give rise to characteristic acinar air-flows. In the current study, micro particle image velocimetry (µPIV) with liquid suspended particles was used to quantify such air flows based on hydrodynamic similarity matching. The good agreement between µPIV results and expected acinar flow phenomena suggest that the microfluidic platform may serve in the near future as an attractive in vitro tool to investigate directly airborne representative particle transport and deposition in the acinar regions of the lungs.

Biomimetics of fetal alveolar flow phenomena using microfluidics

At the onset of life in utero, the respiratory system begins as a liquid-filled tubular organ and undergoes significant morphological changes during fetal development towards establishing a respiratory organ optimized for gas exchange. As airspace morphology evolves, respiratory alveolar flows have been hypothesized to exhibit evolving flow patterns. In the present study, we have investigated flow topologies during increasing phases of embryonic life within an anatomically inspired microfluidic device, reproducing real-scale features of fetal airways representative of three distinct phases of in utero gestation. Micro-particle image velocimetry measurements, supported by computational fluid dynamics simulations, reveal distinct respiratory alveolar flow patterns throughout different stages of fetal life. While attached, streamlined flows characterize the shallow structures of premature alveoli indicative of the onset of saccular stage, separated recirculating vortex flows become the signature of developed and extruded alveoli characteristic of the advanced stages of fetal development. To further mimic physiological aspects of the cellular environment of developing airways, our biomimetic devices integrate an alveolar epithelium using the A549 cell line, recreating a confluent monolayer that produces pulmonary surfactant. Overall, our in vitro biomimetic fetal airways model delivers a robust and reliable platform combining key features of alveolar morphology, flow patterns, and physiological aspects of fetal lungs developing in utero.

Mixed alkanethiol monolayers on submicrometric gold patterns: a controlled platform for studying cell-ligand interactions

Nanoscale organization of surface ligands often has a critical effect on cell-surface interactions. We have developed an experimental system that allows a high degree of control over the 2-D spatial distribution of ligands. As a proof of concept, we used the developed system to study how T-cell activation is independently affected by antigen density and antigen amount per cell. Arrays of submicrometer gold islands at varying surface coverage were defined on silicon by electron beam lithography (EBL). The gold islands were functionalized with alkanethiol self-assembled monolayers (SAMs) containing a small antigen, 2,4,6-trinotrophenyl (TNP), at various densities. Genetically engineered T-cell hybridomas expressing TNP-specific chimeric T-cell antigen receptor (CAR) were cultured on the SAMs, and their activation was assessed by IL-2 secretion and CD69 expression. It was found that, at constant antigen density, activation increased monotonically with the amount of antigen, while at constant antigen amount activation was maximal at an intermediate antigen density, whose value was independent of the amount of antigen.

Electrically controlled molecular recognition harnessed to activate a cellular response

Seamless embedment of electronic devices in biological systems is expected to add the outstanding computing power, memory, and speed of electronics to the biochemical toolbox of nature. Such amalgamation requires transduction of electronic signals into biochemical cues that affect cells. Inspired by biology, where pathways are directed by molecular recognition, we propose and demonstrate a generic electrical-to-biological transducer comprising a two-state electronic antigen and a chimeric cell receptor engineered to bind the antigen exclusively in its "on" state. T-cells expressing these receptors remain inactivated with the antigen in its "off" state. Switching the antigen to its "on" state by an electrical signal leads to its recognition by the T-cells and correspondingly to cell activation.