Study of the hygroscopic properties of selected pharmaceutical aerosols using single particle levitation

A database for deliquescence and efflorescence relative humidities of compounds with atmospheric relevance

Deliquescence relative humidity (DRH) and efflorescence relative humidity (ERH), the two parameters that regulate phase state and hygroscopicity of substances, play important roles in atmospheric science and many other fields. A large number of experimental studies have measured the DRH and ERH values of compounds with atmospheric relevance, but these values have not yet been summarized in a comprehensive manner. In this work, we develop for the first-of-its-kind a comprehensive database which compiles the DRH and ERH values of 110 compounds (68 inorganics and 42 organics) measured in previous studies, provide the preferred DRH and ERH values at 298 K for these compounds, and discuss the effects of a few key factors (e.g., temperature and particle size) on the measured DRH and ERH values. In addition, we outline future work that will broaden the scope of this database and enhance its accessibility.

Count- and mass-based dosimetry of MDI spray droplets with polydisperse and monodisperse size distributions

Most previous numerical studies of inhalation drug delivery used monodisperse aerosols or quantified deposition as the ratio of deposited particle number over the total number of released particles (i.e., count-based). These practices are reasonable when the aerosols have a sufficiently narrow size range. However, spray droplets from metered-dose inhalers (MDIs) are often polydisperse with a wide size range, so using monodisperse aerosols and/or count-based deposition quantification may lead to significant errors. The objective of this study was to develop a mass-based dosimetry method and evaluate its performance in lung delivery in a mouth-lung (G9) geometry with an albuterol-CFC inhaler. The conventional practices (monodisperse and polydisperse-count-based) were also simulated for comparison purposes. The MDI actuation in the open space was studied using both high-speed imaging and LES-Lagrangian simulations. Experimentally measured spray velocities and size distribution were implemented in the computational model as boundary conditions. Good agreement was achieved between recorded and simulated spray plume evolution spatially and temporally. The polydisperse-mass-based predictions of MDI doses compared favorably with the measurements in all three regions considered (device, mouth-throat, and lung). Significant errors in MDI regional deposition were predicted using the monodisperse and count-based methods. The new polydisperse-mass-based method also predicted local deposition hot spots that were one order of magnitude higher in intensity than the two conventional methods. The results of this study highlighted that a presentative polydisperse size distribution and appropriate deposition quantification method should be applied to reliably predict the MDI drug delivery in the human respiratory tract.

Pulmonary aerosol delivery and the importance of growth dynamics

Aerosols are dynamic systems, responding to variations in the surrounding environmental conditions by changing in size, composition and phase. Although, widely used in inhalation therapies, details of the processes occurring on aerosol generation and during inhalation have received little attention. Instead, research has focused on improvements to the formulation of the drug prior to aerosolization and the resulting clinical efficacy of the treatment. Here, we highlight the processes that occur during aerosol generation and inhalation, affecting aerosol disposition when deposited and, potentially, impacting total and regional doses. In particular, we examine the response of aerosol particles to the humid environment of the respiratory tract, considering both the capacity of particles to grow by absorbing moisture and the timescale for condensation to occur. [Formula: see text].

Nanoscale interfacial gradients formed by the reactive uptake of OH radicals onto viscous aerosol surfaces

A key but poorly understood chemical process is how gas phase uptake is governed by the relative mobility of molecules at an interface of an atmospheric aerosol. Citric acid (CA), a model system for oxygenated organic aerosol, is used to examine how changes in viscosity, due to changing water content, govern the reactive uptake of gas phase hydroxyl radicals (OH). By comparing the reaction kinetics measured when probing the outer aerosol surface layers with measurements of the bulk particle composition, the effective OH reaction probability is observed to be a complex and non-linear function of the relative humidity (RH). At RH < 50%, the reactive decay of CA is controlled by the viscosity of the particle, where the depletion of CA and the formation of reaction products occurs over a narrow region near the aerosol interface, on the order of 8 nm at 20% RH. At RH = 50% the reaction zone increases to the particle dimensions (i.e. ∼50 nm) and at RH > 50%, the aerosol becomes aqueous and well-mixed on the timescale of the heterogeneous reaction. These results imply that in the atmosphere, the formation and dissipation of interfacial chemical gradients could be significant in viscous and semisolid aerosol and play important roles altering gas-particle partitioning and aging mechanisms (i.e. bulk vs. interface).

Hygroscopic aerosol deposition in the human upper respiratory tract under various thermo-humidity conditions

The deposition of hygroscopic aerosols is highly complex in nature, which results from a cumulative effect of dynamic particle growth and the real-time size-specific deposition mechanisms. The objective of this study is to evaluate hygroscopic effects on the particle growth, transport, and deposition of nasally inhaled aerosols across a range of 0.2-2.5 μm in an adult image-based nose-throat model. Temperature and relative humidity fields were simulated using the LRN k-ω turbulence model and species transport model under a spectrum of thermo-humidity conditions. Particle growth and transport were simulated using a well validated Lagrangian tracking model coupled with a user-defined hygroscopic growth module. Results of this study indicate that the saturation level and initial particle size are the two major factors that determine the particle growth rate (d/d0), while the effect of inhalation flow rate is found to be not significant. An empirical correlation of condensation growth of nasally inhaled hygroscopic aerosols in adults has been developed based on a variety of thermo-humidity inhalation conditions. Significant elevated nasal depositions of hygroscopic aerosols could be induced by condensation growth for both sub-micrometer and small micrometer particulates. In particular, the deposition of initially 2.5 μm hygroscopic aerosols was observed to be 5-8 times that of inert particles under warm to hot saturated conditions. Results of this study have important implications in exposure assessment in hot humid environments, where much higher risks may be expected compared to normal conditions.

Dynamic growth and deposition of hygroscopic aerosols in the nasal airway of a 5-year-old child

Hygroscopic growth within the human respiratory tract can be significant, which may notably alter the behavior and fate of the inhaled aerosols. The objective of this study is to evaluate the hygroscopic effects upon the transport and deposition of nasally inhaled fine-regime aerosols in children. A physiologically realistic nasal-laryngeal airway model was developed based on magnetic resonance imaging of a 5-year-old boy. Temperature and relative humidity field were simulated using the low Reynolds number k - ε turbulence model and chemical specie transport model under a spectrum of four thermo-humidity conditions. Particle growth and transport were simulated using a well validated Lagrangian tracking model coupled with a user-defined hygroscopic growth module. The subsequent aerosol depositions for the four inhalation scenarios were evaluated on a multiscale basis such as total, subregional, and cellular-level depositions. Results of this study show that a supersaturated humid environment is possible in the nasal turbinate region and can lead to significant condensation growth (d / d(0)  > 10) of nasally inhaled aerosols. Depositions in the nasal airway can also be greatly enhanced by condensation growth with appropriate inhalation temperature and humidity. For subsaturated and mild inhalation conditions, the hygroscopic effects were found to be nonsignificant for total depositions, while exerting a large impact upon localized depositions.

Bulk, surface, and gas-phase limited water transport in aerosol

The influence of solute species on mass transfer to and from aqueous aerosol droplets is investigated using an electrodynamic balance coupled with light scattering techniques. In particular, we explore the limitations imposed on water evaporation by slow bulk phase diffusion and by the formation of surface organic films. Measurements of evaporation from ionic salt solutions, specifically sodium chloride and ammonium sulfate, are compared with predictions from an analytical model framework, highlighting the uncertainties associated with quantifying gas diffusional transport. The influence of low solubility organic acids on mass transfer is reported and compared to both model predictions and previous work. The limiting value of the evaporation coefficient that can be resolved by this approach, when uncertainties in key thermophysical quantities are accounted for, is estimated. The limitation of slow bulk phase diffusion on the evaporation rate is investigated for gel and glass states formed during the evaporation of magnesium sulfate and sucrose droplets, respectively. Finally, the effect of surfactants on evaporation has been probed, with soluble surfactants (such as sodium dodecyl sulfate) leading to little or no retardation of evaporation through slowing of surface layer kinetics.

CFD simulations of enhanced condensational growth (ECG) applied to respiratory drug delivery with comparisons to in vitro data

Enhanced condensational growth (ECG) is a newly proposed concept for respiratory drug delivery in which a submicrometer aerosol is inhaled in combination with saturated or supersaturated water vapor. The initially small aerosol size provides for very low extrathoracic deposition, whereas condensation onto droplets in vivo results in size increase and improved lung retention. The objective of this study was to develop and evaluate a CFD model of ECG in a simple tubular geometry with direct comparisons to in vitro results. The length (29 cm) and diameter (2 cm) of the tubular geometry were representative of respiratory airways of an adult from the mouth to the first tracheobronchial bifurcation. At the model inlet, separate streams of humidified air (25, 30, and 39 °C) and submicrometer aerosol droplets with mass median aerodynamic diameters (MMADs) of 150, 560, and 900 nm were combined. The effects of condensation and droplet growth on water vapor concentrations and temperatures in the continuous phase (i.e., two-way coupling) were also considered. For an inlet saturated air temperature of 39 °C, the two-way coupled numerical (and in vitro) final aerosol MMADs for initial sizes of 150, 560, and 900 nm were 1.75 μm (vs. 1.23 μm), 2.58 μm (vs. 2.66 μm), and 2.65 μm (vs. 2.63 μm), respectively. By including the effects of two-way coupling in the model, agreements with the in vitro results were significantly improved compared with a one-way coupled assumption. Results indicated that both mass and thermal two-way coupling effects were important in the ECG process. Considering the initial aerosol sizes of 560 and 900 nm, the final sizes were most influenced by inlet saturated air temperature and aerosol number concentration and were not largely influenced by initial size. Considering the growth of submicrometer aerosols to above 2 μm at realistic number concentrations, ECG may be an effective respiratory drug delivery approach for minimizing mouth-throat deposition and maximizing aerosol retention in a safe and simple manner. However, future studies are needed to explore effects of in vivo boundary conditions, more realistic respiratory geometries, and transient breathing.

Minimizing variability of cascade impaction measurements in inhalers and nebulizers

The purpose of this article is to catalogue in a systematic way the available information about factors that may influence the outcome and variability of cascade impactor (CI) measurements of pharmaceutical aerosols for inhalation, such as those obtained from metered dose inhalers (MDIs), dry powder inhalers (DPIs) or products for nebulization; and to suggest ways to minimize the influence of such factors. To accomplish this task, the authors constructed a cause-and-effect Ishikawa diagram for a CI measurement and considered the influence of each root cause based on industry experience and thorough literature review. The results illustrate the intricate network of underlying causes of CI variability, with the potential for several multi-way statistical interactions. It was also found that significantly more quantitative information exists about impactor-related causes than about operator-derived influences, the contribution of drug assay methodology and product-related causes, suggesting a need for further research in those areas. The understanding and awareness of all these factors should aid in the development of optimized CI methods and appropriate quality control measures for aerodynamic particle size distribution (APSD) of pharmaceutical aerosols, in line with the current regulatory initiatives involving quality-by-design (QbD).

Physical characterization of component particles included in dry powder inhalers. I. Strategy review and static characteristics

The performance of dry powder aerosols for the delivery of drugs to the lungs has been studied extensively in the last decade. The focus for different research groups has been on aspects of the powder formulation, which relate to solid state, surface and interfacial chemistry, bulk properties (static and dynamic) and measures of performance. The nature of studies in this field, tend to be complex and correlations between specific properties and performance seem to be rare. Consequently, the adoption of formulation approaches that on a predictive basis lead to desirable performance has been an elusive goal but one that many agree is worth striving towards. The purpose of this paper is to initiate a discussion of the use of a variety of techniques to elucidate dry particle behavior that might guide the data collection process. If the many researchers in this field can agree on this, or an alternative, guide then a database can be constructed that would allow predictive models to be developed. This is the first of two papers that discuss static and dynamic methods of characterizing dry powder inhaler formulations.

Influence of Humidity on the Electrostatic Charge and Aerosol Performance of Dry Powder Inhaler Carrier based Systems

To investigate the influence of storage relative humidity (RH) on the aerosolisation efficiency and tribo-electrification of carrier based dry powder inhaler (DPI) formulations using the next generation impactor (NGI) in vitro methodology and the electrostatic low pressure impactor (ELPI). Micronised salbutamol (d (0.5) 1.48 +/- 0.03 microm) was blended with 63-90 microm sieve fractioned alpha-lactose monohydrate carrier and stored at a range of humidities (0-84% RH). The aerosolisation efficiency after storage for 24 h periods was investigated using the NGI. The same experiment was conducted using the ELPI, with corona charger switched off, to measure the net charge vs. mass deposition profile. Significant variations in the aerosolisation efficiency of the formulation were observed with respect to storage RH. In general, the fine particle fraction aerosol performance measured by NGI and ELPI (fraction with mass median aerodynamic diameter <4.46 and 4.04 microm, respectively) followed a positive parabola with aerosol performance increasing over the range 0-60% RH before decreasing >60% RH. Analysis of the ELPI charge data suggested that the micronised salbutamol sulphate had an electronegative charge when aerosolised from lactose based carriers, which was most electronegative at low RH. Increased storage RH resulted in a reduction in net charge to mass ratio with the greatest reduction at RH >60%. The aerosol performance of this binary system is dependent on both electrostatic and capillary forces. The use of the ELPI allows a degree of insight into how these forces affect formulation performances after storage at different RH.

Isotonic and hypertonic saline droplet deposition in a human upper airway model

The evaporative and hygroscopic effects and deposition of isotonic and hypertonic saline droplets have been simulated from the mouth to the first four generations of the tracheobronchial tree under laminar-transitional-turbulent inspiratory flow conditions. Specifically, the local water vapor transport, droplet evaporation rate, and deposition fractions are analyzed. The effects of inhalation flow rates, thermodynamic air properties and NaCl-droplet concentrations of interest are discussed as well. The validated computer simulation results indicate that the increase of NaCl-solute concentration, increase of inlet relative humidity, or decrease of inlet air temperature may reduce water evaporation and increase water condensation at saline droplet surfaces, resulting in higher droplet depositions due to the increasing particle diameter and density. However, solute concentrations below 10% may not have a very pronounced effect on droplet deposition in the human upper airways.

Changes in lung deposition of aerosols due to hygroscopic growth: a fast SPECT study

Our objective was to use rapid dynamic SPECT to study the effect of hygroscopic growth on aerosol deposition in the lung. Six healthy volunteers inhaled radiolabeled ((99m)Tc-DTPA) saline aerosols of two different droplet sizes (MMAD Small 3.2 +/- 0.2 and Large 6.5 +/- 0.2 microm, span 1.8 and 1.7, respectively) and tonicities (normal saline, NS, 0.9% and hypertonic saline 'HS' 7.0% NaCl) reproducibly on 4 occasions. Simultaneous emission-transmission scanning with a triple detector gamma camera was used to collect the lung images. Regional lung deposition was quantified by the penetration index or PI. The results revealed significant differences of aerosol deposition in the lung, which follows precisely the expectation from particle size and hygroscopicity. The difference in the PI values between the aerosols increases in the order of: Large NS-Large HS < Small NS-Small HS < Small HS-Large NS < Small HS-Large HS < Small NS-Large NS < Small NS-Large HS.

The hygroscopic properties of dicarboxylic and multifunctional acids: measurements and UNIFAC predictions

The role of water-soluble organic compounds on the hygroscopic properties of atmospheric aerosols has recently been the subject of many studies. In particular, low molecular weight dicarboxylic acids and some multifunctional organic acids have been found or are expected to exist in atmospheric aerosols in urban, semiurban, rural, and remote sites. Unlike for their inorganic counterparts, the hygroscopic properties of organic acids have not been well characterized. In this study, the hygroscopic properties of selected water-soluble dicarboxylic acids (oxalic acid, malonic acid, succinic acid, and glutaric acid) and multifunctional acids (citric acid, DL-malic acid, and L-(+)-tartaric acid) were studied using single droplets levitated in an electrodynamic balance at 25 degrees C. The water activities of bulk samples of dilute solutions were also measured. Solute evaporation was observed in the dicarboxylic acids but not in the multifunctional acids. Oxalic acid, succinic acid, and glutaric acid droplets crystallize upon evaporation of water, but, except for glutaric acid droplets, do not deliquesce even at 90% relative humidity (RH). Mass transfer limitation of the deliquescence process was observed in glutaric acid. Neither crystallization nor deliquescence was observed in malonic acid, citric acid, DL-malic acid, or L-(+)-tartaric acid. Malonic acid and these three hydroxy-carboxylic acids absorb water even at RH much lower than their respective deliquescence RH. The growth factor (Gf), defined as the ratio of the particle diameter at RH = 10% to that at RH = 90%, of oxalic acid and succinic acid was close to unity, indicating no hygroscopicity in this range. The remaining acids (malonic acid, glutaric acid, citric acid, malic acid, and tartaric acid) showed roughly similar hygroscopicity of a Gf of 1.30-1.53, which is similar to that of "more hygroscopic" aerosols in field measurements reported in the literature. A generalized equation for these four acids, Gf = (1-aw)-0.163, was developed to represent the hygroscopicity of these acids. Water activity predictions from calculations using the UNIFAC model were found to agree with the measured water activity data to within 40% for most of the acids but the deviations were as large as about 100% for malic acid and tartaric acid. We modified the functional group interaction parameters of the COOH(-H20, OH-H20, and OH-COOH pairs by fitting the UNIFAC model with the measured data. The modified UNIFAC model improves the agreement of predictions and measurements to within 38% for all the acids studied.