Keywords inhalation drug delivery - flashing jet - inhaler prototype
Introduction
Inhalation drug delivery is a method of administration whereby medications in aerosolized
form are inhaled into the respiratory tract and subsequently the pulmonary system
for topical or systemic applications.[1 ]
[2 ] The efficiency of inhalation drug delivery is determined by the atomization performance
of the aerosol device, the physicochemical characteristics of the formulation, and
the inhalation skill of the patient.[3 ]
[4 ]
[5 ]
[6 ] Flashing is an instantaneous boiling process that occurs when the external pressure
of a liquid drops below its saturated vapor pressure.[7 ] When overheated liquid is forced through an orifice, flashing happens after the
orifice, resulting in a flashing jet.[8 ]
[9 ] The expansion and rupture of flashing bubbles break the flashing jet into droplets.[8 ]
[10 ]
[11 ]
[12 ]
[13 ] Our previous study established a flashing jet inhaler prototype (FJ prototype) and
recovered the potential effects of flashing and jetting parameters on the atomization
performance of the device.[14 ] In terms of the FJ prototype, when normal saline is used as the atomized liquid,
fine droplets (<5 μm) can be obtained. However, when drugs and surfactants are introduced,
there will be significant changes in the aerodynamic particle size distribution (APSD).
Therefore, further research is necessary to investigate the potential effects of formulation
ingredients on the atomization performance of the FJ prototype.
Evidence suggests a variety of factors that affect the atomization performance of
the flashing jet, and those factors may be surface tension, ion concentration, and
viscosity.[10 ]
[15 ]
[16 ]
[17 ]
[18 ] In addition, several theoretical formulas have been proposed to elucidate the relationship
between these factors and the droplet size.[7 ]
[15 ]
[19 ]
[20 ] However, these theoretical formulas mainly consist of empirical equations. The mechanism
through which these factors affect the flashing jet remains unclear.
The primary objective of this study is to investigate the impact of surfactant, electrolyte,
and drug concentration on the atomization performance of the FJ prototype. In this
work, Tween 80 was employed as a surfactant, sodium chloride (NaCl) was used as an
electrolyte, and salbutamol sulfate (SBS) was chosen as a model drug used in pulmonary
drug delivery. The mass median aerodynamic diameter (MMAD) and APSD of the aerosol
produced were measured to compare the atomization performances. Additionally, the
drug distribution was evaluated to assess the drug delivery capacity at different
SBS concentrations. The study is helpful to explore the formulation of the FJ prototype
with improved atomization performance.
Material and Methods
Instruments and Materials
The following instruments were used in this study: high-performance liquid chromatography
(HPLC; Shimadzu, LC-20AT), aerodynamic particle sizer (APS; TSI, Model 3321), impactor
inlet for pharmaceutical research (IIPR; TSI, Model 3306), vacuum pump (SH-Deying
Vacuum & Lighting Equipment Co., Ltd., 2XZ(s)-2), air flow meter (QF-meter, LZB-10),
glass microfiber filter (Whatman GF/F), analytical balance (Mettler Toledo, XS205DU),
Pari nebulizer, and Flashing Jet prototype.
The following reagents were used in this study: Tween 80 (Sigma-Aldrich, BCBR8595V),
sodium chloride (Sinopharm Chemical Reagent Co., Ltd., 20161207), SBS (SPH Sine Pharmaceutical
Laboratories Co., Ltd., cpc-007–1712005), methanol (CNW Technologies GmbH, 16891140),
phosphoric acid (Sinopharm Chemical Reagent Co., Ltd., 20120920), and sodium dihydrogen
phosphate dihydrate (Sinopharm Chemical Reagent Co., Ltd., 20170209). Methanol was
of chromatographically pure grade, while the other reagents were analytical grade.
The water used in the experiments was purified water.
FJ Prototype
The FJ prototype is illustrated in [Fig. 1A ], and the atomization process of the FJ prototype ([Fig. 1B ]) was conducted according to a reported study.[14 ]
Fig. 1 Diagrammatic view of FJ prototype: (A ) prototype composition, and (B ) actuation process.[14 ]
The FJ prototype was operated using an overheat temperature of 150°C, an initial chamber
pressure of 150 kPa, and a jetting rate of 25 μL/s. The jetting volume was set at
150 μL, which approximates the atomized volume at the Pari nebulizer in 60 seconds.
Pari Nebulizer
The Pari nebulizer is globally recognìzed as one of the most commonly used jet nebulizers.
Jet nebulizers employ compressed air or oxygen to transform liquid into aqueous aerosol.
During the atomization process, the compressed air generates a localized area of negative
pressure around the nozzle, causing the liquid from the reservoir to be drawn and
fragmented into droplets ([Fig. 2A ]).[21 ]
Fig. 2 Diagram view of atomization mechanism of (A ) Pari nebulizer and (B ) FJ prototype.
The Pari nebulizer was used as a reference device in the study due to its fundamental
atomization mechanism, which involves using an air jet to break the liquid jet. In
contrast, the atomization mechanism of the FJ prototype relies on using flashing bubbles
to disrupt the liquid jet ([Fig. 2B ]). By observing the distinct patterns of changes in atomization performance at different
solvent concentrations, it is possible to discern whether these variations are related
to the flashing process.
The APS/IIPR system was used to measure the APSD and MMAD of the aerosol output. It
should be noted that the measurement results obtained using this system may deviate
from the original droplet size due to potential shrinkage caused by the sampling flow.[22 ]
[23 ] Therefore, the Pari nebulizer was chosen as a reference device since it also produces
aqueous aerosol and is subjected to similar evaporation effects within the APS/IIPR
system. The APSD and MMAD of the aerosol output from the Pari nebulizer were used
as benchmarks to assess the atomization quality of the FJ prototype.
APSD and MMAD Measurement
The particle size of the delivered aerosol was measured by the APS based on the time
of flight method. The IIPR, which has a United States Pharmacopeia/European Pharmacopeia
(USP/Ph Eur) inlet and an APS sampling probe, was integrated with the APS. APSD and
MMAD of the aerosol were measured according to a reported study.[14 ]
Atomized Volume Measurement
Atomized volume was defined as a 60-second continuous output for the Pari nebulizer
and a single spray output for the FJ prototype. The residual gravimetric method was
used to measure the atomized volume.[24 ]
[25 ] For the Pari nebulizer, a 2 mL liquid was filled into the nebulizer cup, and the
initial weight of the cup before atomization was recorded. After 60 seconds of continuous
atomization, the end weight of the cup was recorded. The difference between the initial
and end weights, divided by the liquid density, represents the atomized volume of
the Pari nebulizer.
Regarding the FJ prototype, a 2 mL liquid was filled into the liquid pool during preparation.
The weight of the pool before and after actuation was recorded. The difference between
these two weights, divided by the liquid density, provides the atomized volume of
the FJ prototype. For simplification, an approximate density of 1 g/cm3 was used for all solutions in this study during the calculation.
Dosage Measurement
The drug dosage of the delivered aerosol was measured using the USP throat, a 4.7
μm single-stage impactor, and an outlet filter within the APS/IIPR system.[14 ] Following the atomization process, the atomization block of the FJ prototype, connecter,
USP throat, and impact plates were washed by the mobile phase. The lotion of the atomization
block was collected as the device residual dosage sample, while the lotions from the
connecter and USP throat were combined as a throat residual dosage sample. The lotions
from the impact plates were merged to form a large particle dosage (LPD) sample. The
glass microfiber filter, situated on the outlet holder in the IIPR, was immersed in
the mobile and subjected to ultrasonic cleaning for 30 minutes. The resulting lotion
was then filtered through a 0.22 μm membrane and the subsequent filtrate was used
as fine particle dose (FPD) samples. All samples were analyzed by HPLC.
The sum of the device residual dosage, throat residual dosage, LPD, and FPD represents
the total delivered dosage (TD). The dosage calculated based on the atomized volume
and SBS concentration is referred to as the normal dosage.
Chromatographic Conditions
The chromatographic analysis was conducted using a Kromasil C18 column (4.6 × 250 mm,
5 μm particle size). The mobile phase consisted of a phosphate buffer solution prepared
by dissolving 12.48 g of sodium dihydrogen phosphate dihydrate in 1,000 mL of purified
water and adjusting the pH to 3.1 using phosphoric acid. The mobile phase was mixed
with methanol in a ratio of 85:15 (v/v). The detection wavelength was set at 276 nm.
A flow rate of 1.0 mL/min was maintained during the analysis. The injection volume
was 20 μL, and the column temperature was set to 35°C.
System Cleaning
The Pari nebulizer cup was cleaned after each test. The interior and exterior surfaces
of the cup were flushed with a combination of 95% ethanol and pure water. Subsequently,
a nonwoven fabric was used to wipe and dry the cup in a shaded area. The net weight
difference of the cup was kept within 50 mg. Similarly, the atomization block of the
FJ prototype was cleaned after each test. The interior and exterior surfaces of the
block were flushed with a mixture of 95% ethanol and pure water. The block was then
dried in a shaded area. After each sample group, the pressure chamber, metering pump,
and liquid pool were sequentially flushed with 95% ethanol and pure water. For the
IIPR system, the inlet and impactor plates were cleaned after each sample group except
for the SBS group. The inlet, upper impact plate, lower impact plate, and outlet holder
of the IIPR were successively flushed with 95% ethanol and pure water. Additionally,
the exterior surface of the APS probe was cleaned using 75% ethanol wipes, and a nonwoven
fabric was used to wipe and dry the probe. In the SBS group, the IIPR system was cleaned
after each test.
Statistical Analysis
Data analysis was performed using the SPSSAU data analytics platform. The results
were expressed as means ± standard deviation. One-way ANOVA was employed to evaluate
data dependency. A statistically significant difference was defined as a p -value less than 0.05.
Results
Reference Output
When pure water was atomized by the Pari nebulizer, the MMAD of output aerosol was
7.12 ± 0.27 μm (n = 5), and the atomized volume in 60 seconds was 166.9 ± 9.8 μL. In comparison, when
the FJ prototype was used, the MMAD of output aerosol was 8.30 ± 0.94 μm (n = 5), and the single spray volume was 165.7 ± 1.03 μL. Typical APSD of output aerosol
is shown in [Fig. 3 ].
Fig. 3 APSD of pure water at (A ) Pari nebulizer and (B ) FJ prototype. APSD, aerodynamic particle size distribution.
MMAD was significant difference between the Pari nebulizer and the FJ prototype (p = 0.0284). To facilitate the follow-up work in this study, the single spray volume
of the prototype was adjusted to an approximate volume (150 μL) as the average 60-second
output volume of the Pari nebulizer. Therefore, there is no significant difference
in the atomized volume between the two devices.
Effect of Surfactant Concentration
Tween 80 was chosen for this study due to its ability to induce significant changes
in surface tension while causing minimal alterations in viscosity. Tween 80 concentrations
ranging from 0.0001% to 1.0% were used to compare the atomization performances of
both the Pari nebulizer and the FJ prototype.
The aerosol MMAD decreased as Tween 80 concentrations increased and no abnormal changes
were found around the critical micelle concentration (CMC) point at the FJ prototype
([Fig. 4A ]). At the Pari nebulizer, the aerosol MMAD first increased, but then decreased when
Tween 80's concentration increased above the CMC ([Fig. 4A ]).
Fig. 4 Effect of different concentrations of Tween 80 on (A ) aerosol MMAD and (B ) atomized volume. MMAD, mass median aerodynamic diameter.
The addition of Tween 80 to pure water resulted in a significant increase in the atomized
volume. However, as the concentrations of Tween 80 increased, no significant changes
in atomized volume were observed in the two devices ([Fig. 4B ]). In addition, the influence of different Tween 80 concentrations on the APSD of
aerosol is shown in [Fig. 5 ].
Fig. 5 Effect of different concentrations of Tween 80 on APSD. APSD, aerodynamic particle
size distribution.
Effect of Electrolyte Concentration
Normal saline is generally adopted as a solvent in an inhaled liquid prescription
according to the requirement of osmotic pressure equilibrium. Hence, NaCl was used
to study the potential effect of electrolyte concentration on atomization performance.
Upon adding NaCl to pure water, the MMAD of aerosol significantly decreased for both
the FJ prototype and Pari nebulizer ([Fig. 6 ]). When NaCl concentration was above 0.1%, the MMAD of aerosol at the FJ prototype
was increased dose-dependently, while the MMAD of aerosol at the Pari nebulizer showed
almost no change. Moreover, no significant changes in the atomized volume were observed
at the Pari nebulizer or FJ prototype. The aerosol APSD at different NaCl concentrations
is presented in [Fig. 7 ].
Fig. 6 Effect of different concentrations of NaCl on MMAD of aerosol; *p < 0.05. MMAD, mass median aerodynamic diameter.
Fig. 7 Effect of different concentrations of NaCl on APSD. APSD, aerodynamic particle size
distribution.
Effect of Drug Concentration
The effect of different concentrations of SBS on the atomization performance was assessed.
Upon adding SBS to the pure water, a significant decrease in the MMAD while an increase
in the atomized volume were observed ([Fig. 8 ]). At the FJ prototype, the aerosol MMAD continued to decrease with increasing SBS
concentration, whereas, at the Pari nebulizer, no significant changes were observed.
In addition, the atomized volume fluctuated at the Pari nebulizer, but no significant
difference was observed.
Fig. 8 Effect of different concentrations of SBS on (A ) aerosol MMAD and (B ) atomized volume; *p < 0.05. MMAD, mass median aerodynamic diameter; SBS, salbutamol sulfate.
As SBS concentration increased, the fine particle fraction (FPF) at the FJ prototype
increased, and FPF at the Pari nebulizer substantially fluctuated ([Fig. 9 ]). Also, the APSD of aerosol at different SBS concentrations is presented in [Fig. 10 ].
Fig. 9 Effect of different concentrations of SBS on FPF; *p < 0.05. FPF, fine particle fraction; SBS, salbutamol sulfate.
Fig. 10 Effect of different concentrations of SBS on APSD. APSD, aerodynamic particle size
distribution; SBS, salbutamol sulfate.
Discussion
Surfactant and MMAD
In the Pari nebulizer, changing the surface tension of the liquid may either influence
the aerosol size or the atomized volume. Tween 80 has a CMC value of 0.0014% (w/v)
at 25°C.[26 ] Tween 80A increased MMAD to a peak value followed by a decreased value, which was
observed in the other three types of jet nebulizers in a previous study.[27 ] This pattern change is likely caused by the volume change of the primary and satellite
droplets.[28 ]
[29 ]
Reducing the surface tension first leads to a larger primary droplet due to decreased
flow resistance of the liquid. As a result, the nebulization rate increases,[30 ] and larger droplets will be generated. This explains the presence of large droplets
of Pari nebulizer at low Tween 80 concentrations ([Fig. 5 ]). Beyond the CMC point, reducing surface tension theoretically reduces the volume
of the primary droplet, which inadvertently increases the volume of the satellite
droplet.[31 ] This is consistent with the displacement of the super-fine droplet (<1 μm) distribution
at high Tween 80 concentrations ([Fig. 5 ]). This explanation is also suitable for the FJ prototype. Below CMC, the aerosol
MMAD did not increase with increasing Tween 80 concentrations at the FJ prototype,
because the atomized volume was controlled by a pusher.
Dynamic surface tension may be another explanation for the MMAD changes in the FJ
prototype. The dynamic surface tension indicates the equilibrium process of surface
tension in the liquid surface.[32 ] The initial surface tension of a newly generated liquid surface is the same as pure
water. Soon after, this value is decreased with the diffusion of the surfactants until
an equilibrium value is reached. The time required for this equilibrium is the dynamic
surface tension depending on the diffusion rate and concentration of the surfactant.[33 ]
[34 ] Same as the surface tension, the dynamic surface tension improves the bubble bursting
strength in a flashing jet. The growth of a bubble in a liquid involves balancing
three forces: the pressure of the external liquid, the vapor pressure inside the bubble,
and the interfacial tension.[7 ]
[35 ] In order for a bubble to expand, its internal pressure must exceed the external
pressure and interfacial tension. Therefore, a lower surface tension not only results
in a faster expansion process but also leads to more intense bubble formation in the
flashing jet. However, since the bubble expansion happens in microseconds, the surface
tension on the newly generated bubble interface may not reach the equilibrium value
measured in the static conditions. In such cases, a lower dynamic surface tension
can effectively decrease the surface tension throughout the entire bubble expansion
process, ultimately resulting in a more drastic flashing process and smaller aerosol
droplet sizes.
Effects of Electrolyte
The addition of electrolytes, specifically NaCl, has two effects of electrolyte on
the atomization performance. First, it leads to a significant decrease in the aerosol
MMAD at both the FJ prototype and Pari nebulizer. Second, the aerosol MMAD significantly
increased with increased NaCl concentration at the FJ prototype while no significant
change was observed at the Pari nebulizer.
The first effect is not caused by the physical properties of the liquid, since the
NaCl has not significantly changed the surface tension, viscosity, or density at low
concentrations (<1.0%). For example, the surface tension, kinematic viscosity, dynamic
viscosity, and density of 0.9% normal saline at 20°C are 72.90 mN/m, 10.042 mSt, 1.009
mPa.s, and 1.004 g/cm3 , respectively, which are almost the same as those of pure water (72.88 mN/m, 10.07
mSt, 1.005 mPa.s, and 0.998 g/cm3 , respectively).
A similar effect was observed with mesh nebulizers, where the aerosol MMAD decreased
with increased ion concentration.[36 ] One possible cause of this effect is the streaming potential caused by the electrolyte
solution flow. The streaming potential refers to the potential difference produced
by the convective flow of charge due to a pressure gradient through a nozzle.[37 ] When the streaming potential is present, an additional reverse pulling force acts
on the jet, and the stability of the liquid interface is significantly disturbed.
As a result, the liquid jet can break into finer droplets under the same jetting or
flashing strength ([Fig. 7 ]). Furthermore, the transfer of electrons makes the jetting liquid positively charged.
Electrostatic repulsion in the flashing jet may also help the generation of fine droplets.[38 ]
The mechanism of the second effect is less clear, as an increased NaCl concentration
did not increase the MMAD of aerosol at the Pari nebulizer. The second effect of the
FJ prototype is likely associated with the flashing or bubbling process. A bubble-bursting
experiment of seawater spray indicated that as salinity increases, the aerosol speed
and geometric mean diameter will also increase.[39 ] However, the underlying mechanism remains unclear.
Effect of Drug Concentration
Evidence suggests that SBS does not greatly influence the surface tension of the solution.[30 ] In this study, SBS concentration had no effect on the MMAD of aerosol in a Pari
nebulizer, however, decreased the MMAD of aerosol in the FJ prototype in a dose-depended
manner, which is not caused by changes in the physical properties of the liquid.
A similar effect of drug concentration can be observed in mesh nebulizer and thermal
inkjet.[40 ]
[41 ] Possible causes for this effect include the streaming potential and the changes
in dynamic viscosity. When liquid viscosity is relatively low, an inter-droplet filament
appears in the initial stage of droplet generation.[42 ] As the droplet separation progresses, the filament breaks into a string of fine
satellite droplets. However, when the viscosity becomes higher, the liquid in the
filament is pulled back into the main droplet. As a result, fewer fine satellite droplets
are produced, and the volume of the main droplet increases. This can be supported
by the disappearance of large droplets and the shift in the fine droplet distribution
in the APSD induced by a high SBS concentration ([Fig. 10 ]).
Besides, the molecular behavior of SBS in solution is another possible cause of the
MMAD change. Several thermodynamic parameters of the solution are influenced by the
SBS concentration, such as apparent molar volume, partial molar volume, molar expansivity,
isobaric thermal expansion coefficient, isentropic compressibility, apparent compressibility,
partial molar compressibility, and viscosity coefficient.[43 ] Apart from the viscosity coefficient, all other thermodynamic parameters are related
to the flashing process. Considering the reduction of large droplet proportion in
APSD, the flashing intensity may be enhanced by the SBS concentration. However, the
mechanism by which these parameters exert their impact remains unknown. The measurement
of boiling point, specific heat capacity, and latent heat of vaporization in different
SBS concentrations would be beneficial in further understanding the impact of these
changes in thermodynamic parameters.
The mechanism behind the effect of SBS concentration was investigated, and it can
be argued that the fine particle dose (FPD) values of the FJ prototype were better
at higher SBS concentrations ([Table 1 ]). When the SBS concentration was 5 and 10 mg/mL, the FJ prototype demonstrated comparable
FPF and FPD (%TDD) in a single actuation ([Fig. 9 ]). With increased SBS concentration, the proportion of large droplets (>10 μm) in
APSD decreased and nearly disappeared after reaching 5 mg/mL ([Fig. 10 ]). No significant change was observed in the device residual dosage level, while
the residual ratio in the USP throat decreased with increased SBS concentration. Therefore,
the reduced proportion of the large droplet is not caused by more interception in
the USP throat or device. The enhanced FPD can be attributed to the further breakup
of large droplets.
Table 1
Drug dosage distribution proportion in different SBS concentrations (n = 3)
FJ prototype
Pari nebulizer
0.5
1
2
5
10
0.5
1
2
5
10
Device (%TD ± SD)
15.51 ± 2.14
13.78 ± 4.21
16.40 ± 2.59
12.80 ± 3.13
16.78 ± 4.12
9.01 ± 3.53
8.58 ± 0.56
9.40 ± 1.22
15.56 ± 2.32
14.09 ± 2.40
Throat (%TD ± SD)
34.57 ± 12.00
31.17 ± 4.75
25.19 ± 4.26
17.67 ± 4.63
22.07 ± 10.26
24.98 ± 2.30
15.80 ± 5.76
16.80 ± 2.59
23.52 ± 2.89
27.28 ± 2.67
LPD (%TD ± SD)
15.73 ± 7.29
13.95 ± 0.67
13.76 ± 6.02
18.55 ± 2.68
12.05 ± 6.50
11.41 ± 8.13
8.52 ± 2.08
8.96 ± 1.65
7.41 ± 0.61
7.90 ± 2.67
FPD (%TD ± SD)
34.18 ± 5.28
41.11 ± 6.01
44.66 ± 4.20
50.99 ± 5.88
49.11 ± 7.95
54.58 ± 4.58
67.0 ± 5.55
64.83 ± 5.40
53.51 ± 4.58
50.73 ± 3.07
TD (%ND ± SD)
111.2 ± 12.1
103.4 ± 3.50
98.8 ± 8.70
100.8 ± 4.6
92.6 ± 4.90
87.6 ± 9.75
96.86 ± 13.06
95.05 ± 6.50
81.93 ± 2.10
87.31 ± 7.05
Abbreviations: ND, normal dosage; SBS, salbutamol sulfate; TD, total delivered dosage.
Research Limits
In this study, the MMAD and FPD were not measured using Next Generation Impactor (NGI)
or Andersen Cascade Impactor (ACI). This is because the solution containing Tween
80 and NaCl used in the study is not suitable for the measurement of impactor-based
MMAD. Instead, the APS/IIPR system was preferred because its droplet size measurement
does not rely on HPLC. However, it should be noted that the MMAD values obtained from
the APS/IIPR system represent reference values and are only suitable for studying
the trend of influence in different solutions, due to the shrinking effect observed
in droplet size. When the design of FJ prototype and liquid prescription is further
refined in the future, the NGI or ACI will still be necessary for MMAD measurement.
The Pari nebulizer was chosen as the reference device in this study due to its ability
to deliver the same aqueous aerosol as the FJ prototype, and its support for a wide
range of atomization liquids. However, the nebulizer is a continuous atomization device,
which is different from the FJ prototype. Second, the jetting volume of the FJ prototype
was increased to 150 μL to obtain the approximate TD with the nebulizer resulting
in a 6-second spray duration, which is too long for single actuation. In future studies,
a better comparison option would be the soft mist inhaler Respimat. Not only does
it deliver fine aqueous aerosols, but also it is known for its limitation in low delivery
doses, whereas the FJ prototype has the potential advantage of high delivery doses.
Conclusion
The atomized volume MMAD decreased at the FJ prototype with increasing concentrations
of Tween 80 and SBS. When NaCl was added to pure water, the aerosol MMAD decreased
initially and then increased with increasing concentrations. These findings indicate
that surfactant, electrolyte, and drug concentration all have significant impacts
on atomization performance at the FJ prototype. Higher concentrations of surfactant
and drug result in improved performance, while the lowest MMAD is achieved between
0.05% and 0.1% NaCl concentration. However, the mechanisms underlying these effects
are not fully understood, thus, it is important to consider the effects of these ingredients
on the atomization performance of the device in future studies.
