Drug delivery to the lungs: challenges and opportunities

Traditional Chinese medicine combined with pulmonary drug delivery system and idiopathic pulmonary fibrosis: Rationale and therapeutic potential

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Pathogenesis and characteristics of idiopathic pulmonary fibrosis (IPF) are presented.

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The history and current situation of traditional Chinese medicine (TCM) in treating lung diseases are introduced.

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Therapeutic mechanisms of different TCM to treat IPF are summarized.

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Advantages and types of pulmonary drug delivery systems (PDDS) are emphasized.

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Combining TCM with PDDS is a potential strategy to treat IPF.

Idiopathic pulmonary fibrosis (IPF) is a progressive pulmonary interstitial inflammatory disease of unknown etiology, and is also a sequela in severe patients with the Coronavirus Disease 2019 (COVID-19). Nintedanib and pirfenidone are the only two known drugs which are conditionally recommended for the treatment of IPF by the FDA. However, these drugs pose some adverse side effects such as nausea and diarrhoea during clinical applications. Therefore, it is of great value and significance to identify effective and safe therapeutic drugs to solve the clinical problems associated with intake of western medicine. As a unique medical treatment, Traditional Chinese Medicine (TCM) has gradually exerted its advantages in the treatment of IPF worldwide through a multi-level and multi-target approach. Further, to overcome the current clinical problems of oral and injectable intakes of TCM, pulmonary drug delivery system (PDDS) could be designed to reduce the systemic metabolism and adverse reactions of the drug and to improve the bioavailability of drugs. Through PubMed, Google Scholar, Web of Science, and CNKI, we retrieved articles published in related fields in recent years, and this paper has summarized twenty-seven Chinese compound prescriptions, ten single TCM, and ten active ingredients for effective prevention and treatment of IPF. We also introduce three kinds of inhaling PDDS, which supports further research of TCM combined with PDDS to treat IPF.

Recent applications and strategies in nanotechnology for lung diseases

Lung diseases, including COVID-19 and lung cancers, is a huge threat to human health. However, for the treatment and diagnosis of various lung diseases, such as pneumonia, asthma, cancer, and pulmonary tuberculosis, are becoming increasingly challenging. Currently, several types of treatments and/or diagnostic methods are used to treat lung diseases; however, the occurrence of adverse reactions to chemotherapy, drug-resistant bacteria, side effects that can be significantly toxic, and poor drug delivery necessitates the development of more promising treatments. Nanotechnology, as an emerging technology, has been extensively studied in medicine. Several studies have shown that nano-delivery systems can significantly enhance the targeting of drug delivery. When compared to traditional delivery methods, several nanoparticle delivery strategies are used to improve the detection methods and drug treatment efficacy. Transporting nanoparticles to the lungs, loading appropriate therapeutic drugs, and the incorporation of intelligent functions to overcome various lung barriers have broad prospects as they can aid in locating target tissues and can enhance the therapeutic effect while minimizing systemic side effects. In addition, as a new and highly contagious respiratory infection disease, COVID-19 is spreading worldwide. However, there is no specific drug for COVID-19. Clinical trials are being conducted in several countries to develop antiviral drugs or vaccines. In recent years, nanotechnology has provided a feasible platform for improving the diagnosis and treatment of diseases, nanotechnology-based strategies may have broad prospects in the diagnosis and treatment of COVID-19. This article reviews the latest developments in nanotechnology drug delivery strategies in the lungs in recent years and studies the clinical application value of nanomedicine in the drug delivery strategy pertaining to the lung.

Volatile molecules for COVID-19: A possible pharmacological strategy?

COVID-19 is a novel coronavirus disease with a higher incidence of bilateral pneumonia and pleural effusion. The high pulmonary tropism and contagiousness of the virus, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), have stimulated new approaches to combat its widespread diffusion. In developing new pharmacological strategies, the chemical characteristic of volatility can add therapeutic value to the hypothetical drug candidate. Volatile molecules are characterized by a high vapor pressure and are consequently easily exhaled by the lungs after ingestion. This feature could be exploited from a pharmacological point of view, reaching the site of action in an uncommon way but allowing for drug delivery. In this way, a hypothetical molecule for COVID-19 should have a balance between its lung exhalation characteristics and both antiviral and anti-inflammatory pharmacological action. Here, the feasibility, advantages, and disadvantages of a therapy based on oral administration of possible volatile drugs for COVID-19 will be discussed. Both aerosolized antiviral therapy and oral intake of volatile molecules are briefly reviewed, and an evaluation of 1,8-cineole is provided in view of a possible clinical use and also for asymptomatic COVID-19.

Preparation, optimization, and in vivo evaluation of an inhaled solution of total saponins of Panax notoginseng and its protective effect against idiopathic pulmonary fibrosis

Idiopathic pulmonary fibrosis (IPF) is a chronic and progressive pulmonary disease that can cause fibrotic remodeling of the surrounding lung, thus leading to respiratory failure. Although IPF is the most common form of idiopathic interstitial pneumonia, the precise mechanisms underlying this condition remain unknown. In this study, we used total saponins of Panax notoginseng inhalation solution (TIS) to induce idiopathic bleomycin-induced pulmonary fibrosis in rats. The uniformity of delivery dose was investigated by analyzing the aerodynamic particle size distribution and drug stability. The potential of hydrogen potential of hydrogen (pH) of the inhalation solution was 7.0 and the solvent 0.9% NaCl solution, thus meeting physiological requirements for pulmonary drug administration. The delivery rate was 1.94 ± 0.16 mg·min-1 and the total dose was 17.40 ± 0.04 mg. TIS was composed of five key components: notoginsenoside R1, ginsenosides Rg1, ginsenosides Re, ginsenosides Rb1, and ginsenosides Rd. The mass median aerodynamic diameter (MMAD) for these five components were 3.62 ± 0.05 µm, 3.62 ± 0.06 µm, 3.65 ± 0.10 µm, 3.62 ± 0.06 µm, and 3.61 ± 0.05 µm, respectively. Fine particle fraction (FPF) was 66.24 ± 0.73%, 66.20 ± 0.89%, 66.07 ± 1.42%, 66.18 ± 0.79%, and 66.29 ± 0.70%, respectively. The MMAD for inhalation solutions needs to be 1-5 µm, which indicates that the components of TIS are suitable for inhalation. It is important to control the particle size of targeted drugs to ensure that the drug is delivered to the appropriate target tissue. In vitro experiments indicated that TIS exhibited high rates of deposition in lung tissue, thus indicating that pulmonary delivery systems may represent a good therapeutic option for patients.

Inhalation of sustained release microparticles for the targeted treatment of respiratory diseases

Delivering drugs through inhalation for systemic and local applications has been in practice since several decades to treat various diseases. In recent times, inhalation drug delivery is becoming one of the highly focused areas of research in the pharmaceutical industry. It is being considered as one of the major portals for delivering drugs because of its wide range of advantages like requirement of low concentrations of drug to reach therapeutic efficacy, surpassing first pass metabolism and a very low incidence of side effects as compared to conventional delivery of drugs. Owing to these favorable characteristics of pulmonary drug delivery, diverse pharmaceutical formulations like liposomes, nanoparticles, and microparticles are developed through consistent efforts for delivery drugs to lungs in suitable form. However, drug-loaded microparticles have displayed various advantages over the other pharmaceutical dosage forms which give a cutting edge over other inhalational drug delivery systems. Assuring results with respect to sustained release through inhalational delivery of drug-loaded microparticles from pre-clinical studies are anticipative of similar benefits in the clinical settings. This review centralizes partly on the advantages of inhalational microparticles over other inhalational dosage forms and largely on the therapeutic applications and future perspectives of inhalable microparticle drug delivery systems.

Two-Year Outcomes for the Double-Blind, Randomized, Sham-Controlled Study of Targeted Lung Denervation in Patients with Moderate to Severe COPD: AIRFLOW-2

Purpose

COPD exacerbations are associated with worsening clinical outcomes and increased healthcare costs, despite use of optimal medical therapy. A novel bronchoscopic therapy, targeted lung denervation (TLD), which disrupts parasympathetic pulmonary innervation of the lung, has been developed to reduce clinical consequences of cholinergic hyperactivity and its impact on COPD exacerbations. The AIRFLOW-2 study assessed the durability of safety and efficacy of TLD additive to optimal drug therapy compared to sham bronchoscopy and optimal drug therapy alone in subjects with moderate-to-severe, symptomatic COPD two years post randomization.

Patients and Methods

TLD was performed in COPD patients (FEV1 30-60% predicted, CAT≥10 or mMRC≥2) in a 1:1 randomized, sham-controlled, double-blinded multicenter study (AIRFLOW-2) using a novel lung denervation system (Nuvaira, Inc., USA). Subjects remained blinded until their 12.5-month follow-up visit when control subjects were offered the opportunity to undergo TLD. A time-to-first-event analysis on moderate and severe and severe exacerbations of COPD was performed.

Results

Eighty-two subjects (FEV1 41.6±7.4% predicted, 50.0% male, age 63.7±6.8 yrs, 24% with prior year respiratory hospitalization) were randomized. Time-to-first severe COPD exacerbation was significantly lengthened in the TLD arm (p=0.04, HR=0.38) at 2 years post-TLD therapy and trended towards similar attenuation for moderate and severe COPD exacerbations (p=0.18, HR=0.71). No significant changes in lung function or SGRQ-C were found 2 years post randomization between groups.

Conclusion

In a randomized trial, TLD demonstrated a durable effect of significantly lower risk of severe AECOPD over 2 years. Further, lung function and quality of life remained stable following TLD.

Clinical Trial Registration

NCT02058459.

An Overview on Spray-Drying of Protein-Loaded Polymeric Nanoparticles for Dry Powder Inhalation

The delivery of therapeutic proteins remains a challenge, despite recent technological advances. While the delivery of proteins to the lungs is the gold standard for topical and systemic therapy through the lungs, the issue still exists. While pulmonary delivery is highly attractive due to its non-invasive nature, large surface area, possibility of topical and systemic administration, and rapid absorption circumventing the first-pass effect, the absorption of therapeutic proteins is still ineffective, largely due to the immunological and physicochemical barriers of the lungs. Most studies using spray-drying for the nanoencapsulation of drugs focus on the delivery of conventional drugs, which are less susceptible to bioactivity loss, compared to proteins. Herein, the development of polymeric nanoparticles by spray-drying for the delivery of therapeutic proteins is reviewed with an emphasis on its advantages and challenges, and the techniques to evaluate their in vitro and in vivo performance. The protein stability within the carrier and the features of the carrier are properly addressed.

Exogenous Surfactant as a Pulmonary Delivery Vehicle for Budesonide In Vivo

Background

Lung inflammation is associated with many respiratory conditions. Consequently, anti-inflammatory medications, like glucocorticoids, have become mainstay intrapulmonary therapeutics. However, their effectiveness for treating inflammation occurring in the alveolar regions of the lung is limited by suboptimal delivery. To improve the pulmonary distribution of glucocorticoids, such as budesonide to distal regions of the lung, exogenous surfactant has been proposed as an ideal delivery vehicle for such therapies. It was therefore hypothesized that fortifying an exogenous surfactant (BLES) with budesonide would enhance efficacy for treating pulmonary inflammation in vivo.

Methods

An intratracheal instillation of heat-killed bacteria was used to elicit an inflammatory response in the lungs of male and female rats. Thirty minutes after this initial instillation, either budesonide or BLES combined with budesonide was administered intratracheally. To evaluate the efficacy of surfactant delivery, various markers of inflammation were measured in the bronchoalveolar lavage and lung tissue.

Results

Although budesonide exhibited anti-inflammatory effects when administered alone, delivery with BLES enhanced those effects by lowering the lavage neutrophil counts and myeloperoxidase activity in lung tissue. Combining budesonide with BLES was also shown to reduce several other pro-inflammatory mediators. These results were shown across both sexes, with no observed sex differences.

Conclusion

Based on these findings, it was concluded that exogenous surfactant can enhance the delivery and efficacy of budesonide in vivo.

Inhaled cytotoxic chemotherapy: clinical challenges, recent developments, and future prospects

INTRODUCTION

Since 1968, inhaled chemotherapy has been evaluated and has shown promising results up to phase II but has not yet reached the market. This is due to technological and clinical challenges that require to be overcome with the aim of optimizing the efficacy and the tolerance of drug to re-open new developments in this field. Moreover, recent changes in the therapeutic standard of care for treating the patient with lung cancer also open new opportunities to combine inhaled chemotherapy with standard treatments.

AREAS COVERED

Clinical and technological concerns are highlighted from the reported clinical trials made with inhaled cytotoxic chemotherapies. This work then focuses on new pharmaceutical developments using dry powder inhalers as inhalation devices and on formulation strategies based on controlled drug release and with sustained lung retention or based on nanomedicine. Finally, new clinical strategies are described in regard to the impact of the immunotherapy on the patient's standard of care.

EXPERT OPINION

The choice of the drug, inhalation device, and formulation strategy as well as the position of inhaled chemotherapy in the patient's clinical care are crucial factors in optimizing local tolerance and efficacy as well as in its scalability and applicability in clinical practice.

Ivermectin: an award-winning drug with expected antiviral activity against COVID-19

Ivermectin is an FDA-approved broad-spectrum antiparasitic agent with demonstrated antiviral activity against a number of DNA and RNA viruses, including severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Despite this promise, the antiviral activity of ivermectin has not been consistently proven in vivo. While ivermectin's activity against SARS-CoV-2 is currently under investigation in patients, insufficient emphasis has been placed on formulation challenges. Here, we discuss challenges surrounding the use of ivermectin in the context of coronavirus disease-19 (COVID-19) and how novel formulations employing micro- and nanotechnologies may address these concerns.

A dry powder inhalable formulation of salvianolic acids for the treatment of pulmonary fibrosis: safety, lung deposition, and pharmacokinetic study

Salvianolic acids (SAL), the main bioactive component extracted from Salvia miltiorrhiza, is a natural product with a reported anti-pulmonary fibrosis (PF) effect. SAL is commonly administrated orally; however, it has a low oral bioavailability (less than 5%). The objective of this work was to develop a new dry powder inhalable formulation intended to facilitate the access of SAL to the target place. We prepared the new SAL powder formulation containing L-arginine and 2% of lecithin using the ball milling technique. L-arginine was used to regulate the strong acidity of the SAL solution, and lecithin was added to disperse the powder and improve the flowability. The resulting powder had a content in salvianolic acid B (SALB, the main active principle of SAL) of 66.67%, a particle size of less than 5 μm and a good flowability. In vivo fluorescence imaging showed that the powder could be successfully aerosolized and delivered to the lung. The acute lung irritation study proved that the presence of L-arginine improved the biocompatibility of the powder. Finally, according to the pharmacokinetic study, the new SAL powder formulation was found to significantly increase drug concentration in the lung and the bioavailability. In conclusion, the new dry powder inhalable formulation of SAL developed in this study could be a strategy to enhance the performance of SAL at the lung level. Graphical abstract.

Engineering large porous microparticles with tailored porosity and sustained drug release behavior for inhalation

Sustained drug delivery is considered as an effective strategy to improve the treatment of local lung diseases. In this context, inhalation administration of large porous microparticles (LPPs) represents promising prospects. However, one major challenge with said delivery technology is to control the drug release pattern (especially to decrease the burst release) while maintaining a low mass density/high porosity, which is of high significance for the aerodynamic behavior of LPP systems. Here, we show how to engineer drug-loaded, biodegradable LPPs with varying microstructure by means of a premix membrane emulsification-solvent evaporation (PME-SE) method using poly(vinyl pyrrolidone) (PVP) as the pore former. The influence of PVP concentration on the physicochemical properties, in-vitro drug release behavior and in-vitro aerodynamic performance of the drug-loaded microparticles was tested. We demonstrated that the PME-SE technique led to LPPs with favorable pore distribution characteristics (i.e., low external but high internal porosity) as a function of the PVP concentration. In general, more PVP conditioned a larger discrepancy of the internal vs. external porosity. When the external porosity of the LPP formulation (15% of PVP during the manufacturing process) was less than 3%, the burst release of the embedded drug was significantly reduced compared to LPPs prepared by a "conventional" emulsification solvent evaporation method. All the formulations prepared by the PME-SE method had aerodynamic properties suitable for inhalation. This is the first report indicating that the microstructure of LPPs can be tailored using the PME-SE technology with PVP as a suitable pore former. Doing so, we designed LPP formulations having full control over the drug release kinetics and aerodynamic behavior.

Recent Advances in Polymeric Nanoparticle-Encapsulated Drugs against Intracellular Infections

Polymeric nanocarriers (PNs) have demonstrated to be a promising alternative to treat intracellular infections. They have outstanding performance in delivering antimicrobials intracellularly to reach an adequate dose level and improve their therapeutic efficacy. PNs offer opportunities for preventing unwanted drug interactions and degradation before reaching the target cell of tissue and thus decreasing the development of resistance in microorganisms. The use of PNs has the potential to reduce the dose and adverse side effects, providing better efficiency and effectiveness of therapeutic regimens, especially in drugs having high toxicity, low solubility in the physiological environment and low bioavailability. This review provides an overview of nanoparticles made of different polymeric precursors and the main methodologies to nanofabricate platforms of tuned physicochemical and morphological properties and surface chemistry for controlled release of antimicrobials in the target. It highlights the versatility of these nanosystems and their challenges and opportunities to deliver antimicrobial drugs to treat intracellular infections and mentions nanotoxicology aspects and future outlooks.

Recent advances in the development of microparticles for pulmonary administration

Pulmonary drug delivery offers several benefits for the management of various conditions over other conventional routes. Inhalation of drugs can also be useful for targeting alveolar macrophages and for maintaining a higher drug concentration in the lung tissues to improve the efficacy of drugs and shorten the duration of treatment, thereby reducing drug toxicities. Thus, such an approach is useful in the treatment of various pulmonary and nonpulmonary diseases. Newer techniques and delivery devices have been used for the formulation of inhalable microparticles. Here. we not only focus on advances in inhalation therapy and in the preparation of microparticles, but also address the clinical development and regulatory aspects of such therapies.

Pulmonary Delivery of Fenretinide: A Possible Adjuvant Treatment In COVID-19

At present, there is no vaccine or effective standard treatment for severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) infection (or coronavirus disease-19 (COVID-19)), which frequently leads to lethal pulmonary inflammatory responses. COVID-19 pathology is characterized by extreme inflammation and amplified immune response with activation of a cytokine storm. A subsequent progression to acute lung injury (ALI) or acute respiratory distress syndrome (ARDS) can take place, which is often followed by death. The causes of these strong inflammatory responses in SARS-CoV-2 infection are still unknown. As uncontrolled pulmonary inflammation is likely the main cause of death in SARS-CoV-2 infection, anti-inflammatory therapeutic interventions are particularly important. Fenretinide N-(4-hydroxyphenyl) retinamide is a bioactive molecule characterized by poly-pharmacological properties and a low toxicity profile. Fenretinide is endowed with antitumor, anti-inflammatory, antiviral, and immunomodulating properties other than efficacy in obesity/diabetic pathologies. Its anti-inflammatory and antiviral activities, in particular, could likely have utility in multimodal therapies for the treatment of ALI/ARDS in COVID-19 patients. Moreover, fenretinide administration by pulmonary delivery systems could further increase its therapeutic value by carrying high drug concentrations to the lungs and triggering a rapid onset of activity. This is particularly important in SARS-CoV-2 infection, where only a narrow time window exists for therapeutic intervention.

Multifunctional Nanocarriers for Lung Drug Delivery

Nanocarriers have been increasingly proposed for lung drug delivery applications. The strategy of combining the intrinsic and more general advantages of the nanostructures with specificities that improve the therapeutic outcomes of particular clinical situations is frequent. These include the surface engineering of the carriers by means of altering the material structure (i.e., chemical modifications), the addition of specific ligands so that predefined targets are reached, or even the tuning of the carrier properties to respond to specific stimuli. The devised strategies are mainly directed at three distinct areas of lung drug delivery, encompassing the delivery of proteins and protein-based materials, either for local or systemic application, the delivery of antibiotics, and the delivery of anticancer drugs-the latter two comprising local delivery approaches. This review addresses the applications of nanocarriers aimed at lung drug delivery of active biological and pharmaceutical ingredients, focusing with particular interest on nanocarriers that exhibit multifunctional properties. A final section addresses the expectations regarding the future use of nanocarriers in the area.

Aerosol Characteristics and Physico-Chemical Compatibility of Combivent® (Containing Salbutamol and Ipratropium Bromide) Mixed with Three Other Inhalants: Budesonide, Beclomethasone or N-Acetylcysteine

Inhalation therapy with a nebulizer is widely used in chronic respiratory disease. Mixing inhalation solutions/suspensions for simultaneous inhalation is more convenient and might simplify the administration procedure. However, there are no data available to address the in vitro aerosol characteristics and physico-chemical compatibility of Combivent^® (containing Salbutamol and Ipratropium bromide) with other inhalation solutions/suspensions. In order to investigate the in vitro aerosol characteristics and physico-chemical compatibility of Combivent^® with Budesonide, Beclomethasone, and N-acetylcysteine, the appearance, pH, osmotic pressure, chemical stability, mass median aerodynamic diameter (MMAD), fine particles fraction (FPF), particle size corresponding to X50 (particle size, which accounts for 50% of the total cumulative percentage of volume of all particles), delivery rate, and total delivery of the mixed inhalation solution/suspension were tested. There was no change in the appearance such as a change in color or precipitation formation at room temperature. The pH, osmolality, and chemicals of the mixtures were stable for 24 h after mixing. There were no significant differences between Combivent^®, Budesonide, Beclomethasone, N-acetylcysteine, and the mixtures in MMAD, FPF, X50, the delivery rate, and the total delivery. This indicates that the mixtures were physically and chemically compatible. The mixing did not influence the particle size, distribution, or delivery compatibility of the mixtures.

Pharmacokinetic profile analyses for inhaled drugs in humans using the lung delivery and disposition model

The kinetic clarification of lung disposition for inhaled drugs in humans via pharmacokinetic (PK) modeling aids in their development and regulation for systemic and local delivery, but remains challenging due to its multiplex nature. This study exercised our lung delivery and disposition kinetic model to derive the kinetic descriptors for the lung disposition of four drugs [calcitonin, tobramycin, ciprofloxacin and fluticasone propionate (FP)] inhaled via different inhalers from the published PK profile data. With the drug dose delivered to the lung (DTL) estimated from the corresponding γ-scintigraphy or in vivo predictive cascade impactor data, the model-based curve-fitting and statistical moment analyses derived the rate constants of lung absorption (k_a ) and non-absorptive disposition (k_nad ). The k_a values differed substantially between the drugs (0.05-1.00 h^-1 ), but conformed to the lung partition-based membrane diffusion except for FP, and were inhaler/delivery/deposition-independent. The k_nad values also varied widely (0.03-2.32 h^-1 ), yet appeared to be explained by the presence or absence of non-absorptive disposition in the lung via mucociliary clearance, local tissue degradation, binding/sequestration and/or phagocytosis, and to be sensitive to differences in lung deposition. For FP, its k_a value of 0.2 h^-1 was unusually low, suggesting solubility/dissolution-limited slow lung absorption, but was comparable between two inhaler products. Thus, the difference in the PK profile was attributed to differences in the DTL and the k_nad value, the latter likely originating from different aerosol sizes and regional deposition in the lung. Overall, this empirical, rather simpler model-based analysis provided a quantitative kinetic understanding of lung absorption and non-absorptive disposition for four inhaled drugs from PK profiles in humans.

Stability of Polymer Coatings on Nebulizer Membranes During Aerosol Generation

The dimensions of orifices found in aperture plates used for nebulization can be modified by thin polymer coatings with the aim to control the size distribution of the generated aerosol droplets. However, the stability of such polymer coatings on the surface of nebulizer membranes during aerosol generation has not been elucidated. Nebulizer membranes made of stainless steel were covered with a thin film of poly(chloro-p-xylylene) (~1 μm) in the presence or absence of a silane-based adhesion promoter. Thereby, the orifice cross-sections of the nebulizer membrane were reduced by ~50%, accompanied by a remarkable decline in droplet size. Upon continuous nebulization of aqueous test liquids, the droplet size generated by the nonconditioned (no silane), poly(chloro-p-xylylene)-coated membranes reverted to that of the uncoated nebulizer membrane within ~5 min. By contrast, no such rapid return of droplet size to "baseline" values was noticed for the silane-conditioned, poly(chloro-p-xylylene)-coated counterparts. Scanning electron microscopy exhibited significant polymer detachment from the orifices of the nonconditioned (no silane) membranes and thus confirmed the findings from laser diffraction. Overall, silane-based adhesion promoters can increase the persistence of poly(chloro-p-xylylene) coatings on nebulizer membranes during aerosol generation.