Download Experimental Study of Nano Particles Formation through Rapid

Experimental Study of Nano Particles Formation through
Rapid Expansion of Supercritical CO2
Mahya Sameia, Alireza Vatanarab, Shohreh Fatemia,*
a
School of Chemical Engineering, University College of Engineering, Tehran
University, Tehran, 11365–4563, Iran
b
Department of Pharmaceutics, School of Pharmacy, Tehran University of Medical
Sciences, Tehran, Iran
* School of Chemical Engineering, University College of Engineering, Tehran
University, Tehran, 11365–4563, Iran. Email: [email protected]
Reducing the size of pharmaceutical particles enhances their solubility and
bioavailability and therefore inhibits their side effects. An advantageous way to produce
fine particles is using supercritical fluids technologies among which the rapid expansion
of supercritical solutions (RESS) is of a significant importance in producing pure
pharmaceuticals since no organic solvent is used. This method is limited to active
substances highly soluble in supercritical fluid. Because of solubility limitations of large
molecules and polar substances in supercritical CO2, rapid expansion of supercritical
solutions with solid co-solvent (RESS-SC) can be used to overcome this restriction.
Megestrol Acetate (MA) is a poorly water-soluble drug, used as an appetite enhancer to
treat weight loss in pediatric patients with malignancies, cystic fibrosis and HIV/AIDS.
In this work, submicron particles of MA were produced by RESS-SC method, using
CO2 as supercritical fluid in the presence of menthol as solid co-solvent. The effect of
operating conditions including extraction temperature, extraction pressure and percent
of co-solvent on the average particle size of produced particles was investigated. The
experiments were carried out at three temperature levels from 40 to 60 ºC, three
pressure levels from 15 to 25 MPa, and with 2, to 6 percents of co-solvent. The particle
size could be reduced from about 10 µm to less than 110 nm. The results showed that an
increase in temperature or pressure results in smaller particles while by increasing the
amount of co-solvent, larger particles were produced.
1. Introduction
Controlling the size of pharmaceutical particles is an important goal in pharmaceutical
industry since it increases the bioavailability of drug by improving the drug dissolution
rate. Compared with conventional techniques which require numerous manufacturing
steps, cause thermal and chemical degradation of products and need further purification
steps, SCF-based particle formation methods produce very narrow size distribution of
particles with controlled morphology in a single-step operation with mild operating
temperatures. Due to its distinguishing features, CO2 is widely used as SCF. It is
nontoxic, inexpensive and gaseous at ambient conditions which simplifies the problem
of solvent residues. It also has mild critical conditions which minimizes thermal
degradation of drugs. Among methods using SCF, rapid expansion of supercritical
solutions (RESS) is the first choice since it is simpler and less expensive. This process
relies on solvent properties of CO2. The active substance is dissolved in supercritical
CO2 (SC-CO2) and the mixture formed is expanded in a very short time (less than 10-5
sec), causing high supersaturation ratios and formation of small particles. The main
problem with RESS process is that it is not practical for substances with low solubility
in SC-CO2 which is true about most polar drugs. In a modified RESS process, a cosolvent is added to provide a higher solubility at lower pressures due to specific
molecular interactions (RESS-SC). Menthol is a common co-solvent which is widely
used in food and pharmaceutical industry. It shows high solubility in SC-CO2, is solid at
ambient conditions, does not react with SC-CO2 or solute, and is easily removed by
sublimation (Kayrak et al., 2003; Fages et al., 2004; Thakur and Gupta, 2006; Pasquali
and Bettini, 2008). The precipitation phenomenon in RESS process is quite complicated
and many factors simultaneously affect the nucleation and growth of particles and result
in particles with different sizes and shapes. Many researchers have tried to find out how
operating conditions alter the final product. However, the obtained results cannot be
extended to all cases and are sometimes contradictory. The main factors that should be
considered in controlling the size of particles formed by RESS-SC method are pressure
and temperature of extraction and expansion units, concentration of drug, amount of cosolvent, nuzzle length and diameter, spray distance and collision angle (Huang et al.,
2005; Yildiz et al., 2007).
The aim of this work is to study the effect of main parameters on nano particle
fabrication of Megestrol Acetate (MA) which was rapidly expanded from supercritical
CO2 to atmosphere. In this research the way that extraction pressure and temperature
and co-solvent content affect the average particle size of outlet material is investigated.
2. Experimental
CO2 was used as solvent, megestrol acetate (MA) as the model substance and menthol
as solid co-solvent. A schematic of apparatus used in RESS-SC process is illustrated in
figure 1.
Figure 1: Schematic diagram of RESS process
CO2 reached the desired temperature and pressure by passing the pump and heater, and
in supercritical state, entered the extraction cell previously filled with MA and menthol
in specific ratios and glass beads as packing. The mixture was left for hours to provide
the time needed for MA to dissolve in SC-CO2. The exit valve was then opened to let
the solution expand into atmospheric conditions through a heated nozzle, and particles
precipitated on a surface perpendicular to and 10 cm from the nozzle outlet.
The extraction pressure, extraction temperature and amount of co-solvent were the
varying parameters with low and high levels of 15 and 25 MPa, 40 and 60 ºC, and 2 and
6% respectively. A two-level factorial design with three factors was used and the results
were assessed by Design Expert software. The morphology of formed particles was
characterized by scanning electron microscopy (SEM) and average particle sizes were
obtained using a nanosizer device (Malvern, NANO-ZS).
3. Results and discussions
The results of all 11 experiments are summarized in table 1. The average particle sizes
are in range of 102.8 to 516.3 nm.
Table 1: Experimental conditions and results
Run
1
2
3
4
5
6
7
8
9
10
11
Temperature
(ºC)
40
60
40
60
40
60
40
60
50
50
50
Pressure
(MPa)
15
15
25
25
15
15
25
25
20
20
20
Co-solvent
(wt%)
2
2
2
2
6
6
6
6
4
4
4
Average Size
(nm)
403.5
302.6
138.1
119.2
516.3
403.2
118.2
102.8
191.9
268.2
232.6
3.1 Effect of pressure
It is clear from figure 2 that in both 40 and 60 ºC smaller particles were formed at
higher pressures. The same results have been reported for taxol (Yildiz et al., 2007) and
sulindac (Hezave and Esmaeilzadeh, 2010), but about salicylic acid and aspirin there
was an optimum pressure and increasing the pressure more than that, formed larger
particles (Huang et al., 2005; Yildiz et al., 2007).The decrease in particle size with
increasing pressure can be explained as follows: At constant temperature, the solubility
of MA in SC-CO2 increases with pressure and so does the supersaturation, therefore
nucleation rate rises and particle size is reduced.
Figure 2: Average particle size of MA vs. Extraction pressure
3.2 Effect of temperature
Figure 3 shows that increasing the temperature results in smaller particles. This increase
is more considerable at lower pressures. An increase in temperature causes the density
of SCF solvent to fall and decreases the solubility. On the other hand, the volatility of
the substance increases as the temperature rises and leads to a higher solubility
(Mukhopadhyay, 2000). The existing data (dean et al.,1995) indicate that in pressures
from 80 to 22.5 MPa, MA becomes more soluble in CO2 at higher temperatures. A
greater solubility means larger supersaturation and smaller particles as was observed in
our experiments. This is similar to what is reported for lidocaine (Kim et al., 2010). It
has been reported that for salicylic acid (Yildiz et al., 2007) and aspirin (Huang et al.,
2005), up to a point increasing the temperature led to smaller particles but further
increase of temperature showed the opposite effect and resulted in larger particles.
Figure 3: Average particle size of MA vs. Extraction temperature
3.3 Effect of co-solvent
As indicated in figure 4, co-solvent has different effects at different pressures. At 25
MPa adding co-solvent has a positive effect on particle size while at 15 MPa adding
more co-solvent leads to larger particles. The amount and nature of co-solvent affects
the degree to which the polarity of supercritical fluid phase is modified. Here at 15
MPa, more MA is dissolved in SC-CO2 by adding more co-solvent. Particle collision
rate is directly proportional to the square of particle concentration, so this higher
concentration of MA caused by menthol means more coagulation and larger particles.
Figure 4: Average particle size of MA vs. Percent of co-solvent
Another effect of adding co-solvent is hindering particle growth in expansion zone by
surrounding the drug and preventing surface to surface interaction between drug
particles. This effect causes getting smaller particles by more amount of co-solvent at 25
MPa.
3.4 SEM results
Among three factors discussed above, Pressure has the greatest effect. All experiments
done at the highest pressure (25 MPa) resulted in smallest particles and their size
differences are not considerable in nano scale. SEM images of run 4 are presented in
figure 5.
Figure 5: SEM images of run 4
Figure 5 shows that very fine particles formed primarily, are agglomerated. It can be
referred to menthol as solid co-solvent, which is left inside droplets in the very short
time CO2 changes to gas. Some spherical larger particles are also formed as can be seen
in figure 5. SEM images of particles in other experiments have almost the same shape,
but for example in run 6, more large spherical particles were formed.
4. Conclusion
In this work, MA particles were formed in nano scale through rapid expansion of
supercritical CO2 and with menthol as solid co-solvent. Effect of temperature, pressure
and weight percent of co-solvent on average particle size was investigated performing a
2-level full factorial design. It was found that all three parameters influence the particle
size of outlet material, although it was observed that the average size of precipitated
particles was most sensitive to pressure and at higher pressures, finer particles were
produced. At the best conditions, the average size of MA particles could reach from
nearly 10 µm to less than 120 nm.
References
Dean J. R., Kane M., Khundker S., Dowle C., Tranter R.L. and Jones P., 1995, Analyst
120, 2153-2157.
Fages J., Lochard H., Letourneau J.J., Sauceau M. and Rodier E., 2004, Particle
generation for pharmaceutical applications using supercritical fluid technology,
Powder Technology 141, 219-226.
Hezave A.Z. and Esmaeilzadeh F., 2010, Crystallization of micro particles of sulindac
using rapid expansion of supercritical solution, Journal of Crystal Growth 312,
3373-3383.
Huang Z., Sun G.B., Chiew Y.C. and Kawi S., 2005, Formation of ultrafine aspirin
particles through rapid expansion of supercritical solutions (RESS), Powder
Technology 160, 127-134.
Kayrak D., Akman U., Hortacsu O., 2003, Micronization of Ibuprofen by RESS,
Journal of Supercritical Fluids 26, 17-31.
Kim J.T., Kim H.L. and Ju C.S., 2010, Micronization and characterization of drug
substances by RESS with supercritical CO2, Korean Journal of Chemical
Engineering 27, 1139-1144.
Mukhopadhyay M., 2000, Natural Extracts Using Supercritical CO2, CRC Press, Boca
Raton, Florida, 21-23.
Pasquali I. and Bettini R., 2008, Are pharmaceutics really going supercritical?,
International Journal of Pharmaceutics 364, 176-187.
Thakur R. and Gupta R.B., 2006, Formation of phenytoin nanoparticles using rapid
expansion of supercritical solution with solid cosolvent (RESS-SC) process,
Internatonal Journal of Pharmaceutics 308, 190-199.
Yildiz N., Tuna S., Doker O. and Calimli A., 2007, Micronization of salicylic acid and
taxol (paclitaxel) by rapid expansion of supercritical fluids (RESS), Journal of
Supercritical Fluids 41, 440-451.