Encapsulation of a Low Aqueous Solubility Substance in a

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Encapsulation of a Low Aqueous Solubility Substance in a Biodegradable Polymer using Supercritical Fluid Extraction of Emulsion Candy S Lin, Jane J. Xu, Ka Ming Ng, Christianto Wibowo, and Kathy Qian Luo Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/ie300612r • Publication Date (Web): 01 Jun 2012 Downloaded from http://pubs.acs.org on June 7, 2012

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Industrial & Engineering Chemistry Research

Encapsulation of a Low Aqueous Solubility Substance in a Biodegradable Polymer using Supercritical Fluid Extraction of Emulsion Candy S. Lin, Jane J. Xu and Ka M. Ng* Department of Chemical and Biomolecular Engineering The Hong Kong University of Science and Technology Clear Water Bay, Hong Kong Christianto Wibowo ClearWaterBay Technology, Inc. 4000 Valley Blvd., Suite 100 Pomona, CA 91789-1532, U.S.A. Kathy Q. Luo School of Chemical and Biomedical Engineering Division of Bioengineering Nanyang Technological University, 70 Nanyang Drive, Singapore 637457 A manuscript submitted to Ind. Eng. Chem. Res. Original Submission: March 7, 2012; Revision Submission: May 31, 2012 Special issue honoring Professor L.T. Fan Key words: Supercritical fluid, Extraction of emulsion, Controlled drug delivery, Polymer encapsulation, Process design. Abstract The use of the supercritical fluid extraction of emulsion (SFEE) process for the polymer encapsulation of a low aqueous solubility drug substance was investigated, with focus placed on the carbon dioxide extraction step. The effects of process parameters – copolymer ratio, extraction time, pressure, and temperature – on the drug content, encapsulation efficiency and drug release profile of the polymer-drug composite particle were studied. Ibuprofen and polylactic-glycolic acid were chosen as the model system. Encapsulated particles of 100-300 nm were successfully obtained after freeze-drying. XRD and SEM analysis confirmed the entrapment of ibuprofen in an amorphous polymer matrix and the absence of ibuprofen external to the encapsulated particles. In addition to the solubility of ibuprofen, the particle surface area and, to a small extent, the glass transition temperature were found to influence the drug content and drug release profiles. These understandings are expected to facilitate the rational design of the extraction step of the SFEE process, especially for drugs that are relatively soluble in supercritical carbon dioxide. * To whom correspondence concerning this article should be addressed. Tel.: +8522358-8228. Fax: +852-2358-0054. Email: [email protected]

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Introduction The considerable interest in the use of biodegradable polymers for the encapsulation of drug substance is mostly aimed for developing controlled release drug formulations. Conventional drug administration, such as capsules and tablet dosage forms, provides a single burst of drug concentration in the blood followed by decreasing drug concentration. With a polymer encapsulated drug particle, it is possible to release the drug at a relatively constant rate, thus extending the time of drug presence in the blood from a single dosage. The removal of the non-toxic degraded polymer from the body after drug release is unnecessary. There are two types of polymer-drug composite particles; the first is a core (drug) and shell (polymer) particle typically formed by coating an existing drug particle. The second form is a homogeneous particle with the drug dispersed in it, produced by extraction, coprecipitation or impregnation of the core material in the polymeric material. A wide variety of techniques for fabricating such particles including supercritical fluid processing,1,2 spray drying,3 and co-crystallization4 are available. The most interesting is supercritical carbon dioxide (CO2) processing due to its relatively low cost, environmentally benign nature, and excellent transport properties at mild supercritical conditions (TC = 31.1°C and PC = 73.8 bar). The rapid expansion of supercritical solutions is one of the supercritical fluid techniques that use supercritical fluid as a solvent for both the drug and polymer. The resulting solution is then depressurized through an orifice, leading to the precipitation of the drug polymer composite particle. This method has limited applications because it is not easy to achieve reasonable solubility for both compounds.5 The supercritical anti-solvent process is a more popular method, where the polymer and the drug substance are first dissolved in an organic solvent. Then, by contacting the solution with a supercritical fluid, saturation of the liquid solution occurs, resulting in the precipitation of the composite solute.6,7 However, this method may experience difficulty in achieving the specified particle size and particle size distribution. In some cases the addition of the supercritical CO2 may trigger a liquid-liquid phase split at moderate temperatures, and failure of the encapsulation operation.8


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The supercritical fluid extraction of emulsion (SFEE) technique is used in this study with the aim to produce small, encapsulated particles of narrow size distribution. It comprises of two steps; the first is the formation of an oil-in-water (O/W) emulsion, and the second step utilizes supercritical CO2 to remove the oil phase from within the emulsion droplets to yield high purity, uniform encapsulated particles suspended in the aqueous phase. Recent applications of the SFEE process include β-carotene in polyethylene glycol with dichloromethane as the organic solvent used in the oil phase,9 and piroxicam in polylactic-glycolic acid (PLGA) with ethyl acetate as the solvent.10 Since the starting emulsion provides a template for the final particles, relatively narrow particle size distribution can be obtained in SFEE. Much has been done on the emulsion formation step where the critical parameters to obtain an emulsion with desirable particle attributes were investigated. Della Porta and Reverchon carried out extensive studies of the effects of the amount of PLGA in the oil phase on the SFEE process.10 The droplet size increased with an increase in PLGA amount in the emulsion. Similar observations were made in other studies.11-13 Obviously, smaller emulsion droplets are preferred as they yield smaller encapsulated particles. In general, the second step of the SFEE process in which the organic solvent is extracted from the prepared emulsion to form the final product has received less attention. One of the exceptions was the study by Della Porta and Reverchon in which the extraction conditions were optimized to minimize the residual solvent in the product suspension, which is an important aspect of product quality. The encapsulation efficiency was very high because the model drug, piroxicam, had very low solubility in supercritical carbon dioxide. However, the effects of the conditions of the extraction step on the release characteristics of the polymer-drug particle were not emphasized. In this article, we investigate in the extraction step the effects of operating temperature, pressure, copolymer ratio, and extraction time on the drug content, encapsulation efficiency, and drug release profile of the final product particle. Ibuprofen (IBU), a non-steroidal anti-inflammatory drug, and PLGA were selected as our model system. Since ibuprofen is relatively soluble in supercritical carbon dioxide, it is expected to be more significantly affected by the operating conditions of the extraction step than piroxicam. PLGA is an FDA approved biodegradable polymer that undergoes hydrolysis


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in the body to form non-toxic byproducts. A wide range of PLGA products with different specifications is available to cater for different release requirements. The same emulsion was used throughout this study in order to credit the changes in final product performance to the effect of altered operating conditions of the CO2 extraction step.

Experimental Section Materials IBU with minimum purity of 98.0% was obtained from TCI Japan. PLGAs with lactic-glycolic copolymer ratio of 50:50, 75:25 and 85:15 (inherent viscosity = 0.64 dL/g) were obtained from Jinan Diagang Biomaterials Co. Polyvinyl alcohol (PVA) with MW 31,000-50,000 (98-99% hydrolyzed) were obtained from Sigma-Aldrich and ethyl acetate with purity not less than 99.5% from Merck. Distilled and deionized (using Millipore Milli-Q purification system) water (DDI) was used throughout the study. The carbon dioxide was supplied by Hong Kong Specialty Gases with a purity of 99.99%. Dimethyl sulfoxide (DMSO) for determination of drug content was supplied by Sigma-Aldrich (99.9%). Phosphate buffer solution (pH 7.4, 0.1 M) required for the drug release experiment was made up of dipotassium phosphate (K2HPO4, 98%) and monopotassium phosphate (KH2PO4, 98%). Dialysis bags (8000-14000 kDa) were used in dissolution tests. For HPLC analysis, acetonitrile (ACN) of 99.9% minimum purity and acetic acid of 99.99% minimum purity were obtained from Tedia and BDH, respectively. 12 mm bronze specimen mount stages and double-sided carbon 12 mm adhesive tabs were used in SEM sample preparation. Copper grids were used for sample preparation for TEM viewing. All chemicals were used directly as received.

Apparatus and Procedure Emulsion preparation An oil phase and a water phase at a mass ratio of 20:80 w/w were mixed to form an emulsion. For the oil phase, 1 g of PLGA and 0.05 g of IBU were dissolved in 13.33 g of ethyl acetate to form an organic solution. This was added into the aqueous phase, consisting of 0.5 wt% of PVA in water. A mechanical homogenizer (IKA, T25 UltraTurrax) at 24,000 rpm was used to process the oil and water phase into an emulsion for 8


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minutes. Due to the heat produced during the homogenization process, an ice bath was used exterior to the emulsion flask to keep the emulsion from overheating. The same recipe and conditions were used throughout the study to make emulsion batches, 70 g per batch for each extraction experiment. A particle sizing apparatus (Brookhaven Instruments Corp., 90Plus/BI-MAS) was used to monitor the droplet size for each batch to ensure consistency. The emulsion droplets produced using this procedure were found to be stable within a 2-hour time frame, which was longer than the CO2 extraction run time.

Extraction of emulsion: setup and procedure The SFEE extraction setup includes vessels and piping supplied by Thar Design Technologies (Figure 1). The CO2 supply was connected to a cooler, which kept the CO2 in liquid phase before being pumped to the heater. A flow meter was installed to constantly monitor the flow rate of CO2 through the system. CO2 was pumped into the 500 mL extraction vessel at the bottom. There was a metal filter with 5 µm pores to keep the particles in the vessel. The pressure of the vessel was controlled by a back pressure regulator (BPR) preset to an experimental value. The CO2 and the organic solvent were separated in a cyclone separator downstream of the extraction vessel. Temperature control was monitored by a computerized system governing the whole extraction system. In a typical CO2 extraction experiment, the emulsion was placed in the extraction vessel. With the passage of CO2 at a fixed flow rate of 8g/min, ethyl acetate was extracted out of the droplets into the surrounding dense CO2 (Figure 2). This resulted in a sharp rise in supersaturation within each droplet, causing the polymer and drug inside the droplet to precipitate out to form composite particles. These fine particles remained suspended in the aqueous phase of the original emulsion, while the dense fluid of CO2 and ethyl acetate exited to the downstream cyclone where the temperature and pressure were set to allow gas-liquid separation. The flow of CO2 was stopped when a designated amount of CO2, or equivalently extraction time, was reached. Procedure for solid particle recovery


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The particles collected from the aqueous suspension in the extraction vessel required washing to remove the PVA still present in the collected sample. This aqueous suspension was centrifuged at 20,000 rpm for 15 min. The supernatant was removed and the sediment particles were re-suspended in fresh water and washed again in the same manner. After washing, the solids were re-suspended in water again to be freeze-dried to powder form and subjected to a series of analysis to determine their characteristics.

Product analysis and characterization Particle size distribution The same particle sizer (Brookhaven Instruments) for determining the emulsion droplet size distribution was used for the solid particles suspended in water after the extraction step. Liquid samples were loaded into a quartz cell for direct measurement.

Solid –state analysis A powder X-Ray diffractometer (XRD Panalytical X’pertpro) was used to determine the crystalline structure of all the composite particles obtained from the experiments as well as the unprocessed drug and polymer prior to the experiments. Powder used for this characterization was obtained directly from the freeze-dried samples. The measuring conditions were as follows: Ni-filtered CuKα radiation, λ = 1.54 Å, 2θ angle ranging from 3° to 35° with a step size of 0.033°. Differential scanning calorimetry (DSC) was used to determine the glass transition temperature (Tg) of the composite particles by using sealed aluminum pans heated at a rate of 20°C/min.

Morphology A high resolution scanning electron microscope JSM-6700F model from Jeol was used for obtaining SEM images of the composite particles. A gold sputter coater, supplied by Emitech (K575X Turbo Sputter Coater) was used to prepare conductive samples. A high resolution transmission electron microscope JEM-2010F was used to obtain TEM images which were compared with those obtained by SEM.

Particle surface area


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A BET surface area analyzer (Beckman Coulter SA3100) was used to determine the surface area of the particles obtained under different operating conditions. All samples were out-gassed overnight and measurements were carried out at 40°C. Each measurement was made twice to ensure reproducibility.

Drug content, encapsulation efficiency and release profile Drug content was expressed as the weight percentage of the drug to product particles obtained from each experiment. The amount of drug in the composite particles was determined by dissolving 10 mg of the product particles in 5 mL of DMSO, which is a solvent for both the drug and polymer. The concentration of drug in the solution, hence the weight of drug in the particles, was determined by HPLC analysis. This experiment was carried out three times to ensure reproducibility. Since not all the drug placed in the original emulsion ended up in the product particles, the encapsulation efficiency was calculated as follows:

Wt. of drug content in final particle Encapsulation (%) =   × 100 Efficiency  Wt. of drug placed in initially 

(1)

To obtain the drug release profile of the particles, 80 mg of the product powder was suspended in 1 mL of phosphate buffer (pH 7.4, 0.1 M), placed in a dialysis bag, sealed and submerged in a bottle with 39 mL fresh phosphate buffer medium. These bottles were placed in a temperature controlled water bath set at 37°C with magnetic stirring. Each of these bottles had septum lids for drawing samples of 0.5 mL at timed intervals from 10 min to 1 week. Samples were filtered through with syringe filters (0.2 µm) before collection and 0.5 mL of fresh buffer was replaced after each sampling step. Collected portions were analyzed by HPLC. This experiment was carried out three times to ensure reproducibility. The following equations were used to fit the release profiles in order to quantitatively compare them. First order kinetics is assumed.


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dW = k (W Max − W ) dt W −W W F = W Max − Max kt 0 e SE =

W − WF N −1

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(2)

(3)

2

(4)

where WF and W0 are the cumulative weight of drug released at any time t and at time zero, respectively, WMax is the maximum cumulative weight of drug released, k is rate constant for drug release, and SE is standard error obtained between the actual data and the fitting value with the number of entries (N). Note that the surface area of the particles has been incorporated in the rate constant. The HPLC conditions for the determination of drug concentration were as follows: mobile phase used for the chromatography was 0.1% acetic acid in DDI water (60%) and ACN (40%) at flow rate 0.4 mL/min on column Xterra RP18 3.5 µm 2.1 x 50 mm, with UV signal measured at 265 nm. All samples for analysis were injected twice to ensure correct measurement.

Results and Discussion As mentioned, the same emulsion formulation was used for all the experimental runs for the CO2 extraction of emulsion. Figure 3 shows the effective particle size of the emulsion droplets as measured by the particle sizer in the SFEE process and in the subsequent washing steps. The particle size (23.1nm) of the suspension after extraction (denoted as ‘After SFEE’) was smaller than the droplet size (31.1nm) of the original emulsion (denoted as ‘Before SFEE’) as expected, but increased with each wash step in the downstream processing. For example, the size was 7.5nm and 40.8nm after the first and second wash, respectively. Due to the removal of PVA from each wash, particles tended to aggregate to form larger particles. This effect was very pronounced as the number of washes increased. Particles after the first wash were still heavily covered with emulsifier but were mostly free of the emulsifier after the second wash, as viewed under


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the SEM. Also, despite the low solubility of ibuprofen in water, it was possible that some of the ibuprofen would be lost with excessive washing. Thus, particles were all washed twice before subsequent experiments. The temperature, pressure, copolymer ratio (also referred to as LG ratio) and amount of CO2 for extraction covered in this study are summarized in Table 1. The temperature and pressure were selected based on the solubility of ibuprofen in CO2 at supercritical conditions. Figure 4 shows the solubility data of ibuprofen in supercritical CO2 at three different temperatures as measured by Charoenchaitrakool et al.14 The general trend is that the solubility of ibuprofen increases with pressure since denser CO2 has higher solvation power.15 At pressures lower than 110 bar, a small increase in pressure can markedly increase the solubility of ibuprofen. Also at low pressure, as temperature is increased, the solubility decreases quite significantly. For example, at 90 bar, the solubility at 45°C seems to be over 10 times lower than that at 35°C, although there are not enough data to make definitive conclusions The opposite trend, however, is observed at pressures higher than 110 bar. As temperature increase, the solubility at the same pressure increases. The effects of temperatures at 35, 45, 55 and 65°C with a fixed pressure of 80 bar and a pressure of 80, 150, and 200 bar at a fixed temperature of 45°C were selected for investigating the effects on final product attribute (Expt. 1-6). Two other parameters under investigation included the LG ratio and extraction time. The LG ratio is the molar ratio of polylactic acid to polyglycolic acid in the copolymer. It is expected to affect the drug release properties of the encapsulated product. The glass transition temperature of PLGA tends to decrease with decreasing LG ratio, thus affecting drug diffusion out of the polymer matrix.16 Expt. 2, 9, and 10 varied the LG ratios from 50:50, to 75:25 and to 85:15. The amount of CO2 used for extraction is expected to impact the final product characteristics. This is quantified in this study by the CO2:EA ratio; that is, the mass of CO2 passed through the extraction vessel to the mass of ethyl acetate present in the original emulsion. Expt. 7, 2, and 8 considered the CO2:EA ratio of 20, 40 and 80, respectively, at fixed temperature and pressure. Encapsulated particles of particle size of 100-300 nm were achieved after freezedrying in all the experimental conditions. Figure 5 shows a typical SEM and TEM image obtained for Expt. 1 operated at 35°C. Similar SEM images were obtained for all


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conditions with the exception of Expt. 7 where agglomeration of the particles was observed. These particles obtained after freeze drying were on the order of 100nm. This size was larger than those in Figure 3 based on the particles after the second wash. This is in agreement with the observation that similar particles are known to increase in size during freeze drying.17

XRD analysis XRD was used to compare the crystalline pattern of the unprocessed ibuprofen, PLGA, a physical mixture of these two, the SFEE processed final product obtained from Expt. 2, alongside the characteristic peaks for ibuprofen (Figure 6). It can be seen that the diffraction pattern for ibuprofen matched the characteristic peaks perfectly, revealing its crystalline nature, while PLGA is shown to be amorphous. The physical mixture of these two chemicals shows the combined signals of their original chemicals although the characteristic peaks for ibuprofen are relatively small (shown by the arrows on the figure). The diffraction pattern for the final product shows an amorphous material without the presence of crystalline drug particle on the sample. This indicates that the drug content is indeed encapsulated inside the amorphous polymer matrix. All other experimental conditions are shown to have a similar diffraction pattern (Figure 7). The diffraction pattern of SFEE processed IBU without polymer and that of raw IBU are also presented here, showing similar crystalline structures.

Impact of LG Ratio, CO2:EA ratio, pressure and temperature on product quality The quality of the composite particles is quantified by the rate constant (k), WMax (as fitted from the release profile data to be reported below), drug content, encapsulation efficiency, BET and glass transition temperature (Tg). Table 2 shows the data for each of the 10 experiments performed. It can be observed that the drug content, WMax and encapsulation efficiency closely track one another from experiment to experiment, suggesting consistency of results obtained from the different methods. In addition, if one calculates Wmax based on drug content, with the exception of Expt. 3, the calculated Wmax is consistently higher than the experimental Wmax. Since the drug content was measured by completely dissolving the drug and the polymer in DMSO, the discrepancy indicated


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that some of the ibuprofen was not released from the composite particle even at long time.

Effect of LG ratio in PLGA Figure 8 shows the release profiles for particles obtained from the extraction step where only the LG ratio was changed (Expt. 2, 9 and 10). It should be noted that the release data for time less than 30 min or so were captured by the trend line. The LG ratio in copolymer had significant effect on the drug content and drug release. The rate constant of 0.918 for LG ratio of 85:15 (Expt. 10) was more than 3 times slower than that (k = 3.463) of 75:25 (Expt. 9). The Tg decreased from 52.26°C, to 49.94°C and to 46.48°C as the LG ratio decreased. Higher drug content was observed in particles with a higher LG ratio (Expt. 9 and 10 as compared with Expt. 2).

Effect of CO2:EA ratio Expt. 7 had a CO2:EA ratio of 20. The SEM image for this run showed that the particles merged together on the surface (Figure 9) and the particles have a relatively low BET value. This might be caused by the insufficient removal of the organic solvent, and subsequent fusion of particles. The low rate constant of drug release profile of 0.944 correlated well with the low surface area obtained (5.319 m2/g). The drug content value, on the other hand, was high (1.97 % w/w) due to the short contact time between the emulsion and CO2, which reduced the drug loss to the surrounding supercritical CO2 during extraction. The observation of decreasing WMax with increasing amount of CO2 used in extraction is clearly seen in Figure 10, where the drug profiles of Expt. 2, 7 and 8 are plotted. Expt. 8 with a CO2:EA ratio of 80 gave the lowest drug content value (0.49) of all the experimental batches. Due to the solubility of ibuprofen in the surrounding supercritical CO2, the continuous flow of fresh CO2 through the emulsion drew the drug out of the particles already depleted of organic solvent, causing low drug content, WMax value and encapsulation efficiency of the composite particle.

Effect of pressure


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Figure 11 shows the release profiles of Expt. 2, 5 and 6, for which the extraction of CO2 was performed under different operating pressures. The WMax and drug content decreased with increasing pressure because the solubility of ibuprofen increased with increasing pressure (Figure 4). Indeed, the encapsulation efficiency is the lowest among all 10 experiments. Relatively high BET surface areas were achieved for these conditions (Table 2) with small, round particles, with a typical image shown in Figure 12. High rate constants agreed with the high particle surface area obtained in this group of experiments.

Effect of temperature The effect of temperature is the most complex of the four parameters investigated in this study (Figure 13). First of all, consider the results obtained for Expt. 4 (65°C). The drug content and WMax are much higher than those operated at other extraction temperatures (Expt. 1, 2 and 3 in Table 2). Because of the significant effect of temperature, these experiments had been repeated to ensure that Expt. 4 was not an outlier. The observation can be explained by the extremely low solubility of ibuprofen according to the data trend at 80 bar in Figure 4. At this high temperature, ibuprofen is almost insoluble in supercritical CO2, leading to the very high drug content of the composite particles. Figure 4 also shows that WMax at 35°C is the second highest, not the lowest among the four experiments. One plausible reason is the possibility that 35°C might be below the Tg of the particle, which led to slower internal diffusion and higher WMax for Expt. 1. This argument, however, is rather tenuous because of the well-known “Tg lowering effect” due to carbon dioxide sorption.18 Anyway, the fact that WMax for Expt. 1, 2 and 3 is of nearly the same magnitude requires further investigation.

Conclusions This article has delineated the influence of the important operating parameters during the CO2 extraction step of the SFEE process on the final product quality. Drug solubility in supercritical CO2 was found to play an important role on drug content in the composite particles. Its value had the same trends as those obtained for WMax and encapsulation efficiency. Our data showed that the encapsulation efficiency ranged from 9.72 to 58.47% depending on the choice of pressure and temperature. Also, it was


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observed that not all ibuprofen was released from the composite particles under the given conditions even at long time. The results of this study offer a better understanding of the different operating parameters involved in the SFEE process. They are also expected to contribute to the rational design of the emulsion extraction step of the SFEE process to provide the desired product qualities such as drug content and release characteristics, particularly for drugs that are relatively soluble in supercritical carbon dioxide. This study can be expanded in a number of directions. The residual organic solvent in the composite particles was not considered in this study. Additional data could be collected for the solubility of ibuprofen in supercritical carbon dioxide. The mechanism of the increase in particle size in freeze-drying should be investigated

Acknowledgments We thank Dr. Johannes Kluge and Prof. Marco Mazzotti of ETH Zurich for their helpful information and advice on developing the SFEE process.


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14. Charoenchaitrakool, M.; Deghani, F.; Foster, N. R.; Chan, H. K. Micronization by Rapid Expansion of Supercritical Solutions to Enhance the Dissolution Rates of Poorly Water-soluble Pharmaceuticals. Ind. Eng. Chem. Res. 2000, 39, 4794. 15. Gurdial, G. S.; Foster, N. R. Solubility of o-Hydroxybenzoic Acid in Supercritical Carbon Dioxide. Ind. Eng. Chem. Res. 1991, 30, 575. 16. George, S. C.; Thomas, S. Transport Phenomena Through Polymeric Systems. Prog. Polym. Sci. 2001, 26, 985. 17. Saez, A.; Guzmán, M.; Molpeceres, J.; Aberturas, M. R. Freeze-drying of Polycaprolactone and Poly(D,L-lactic-glycolic) Nanoparticles Induce Minor Particle Size Changes Affecting the Oral Pharmacokinetics of Loaded Drugs. European J. Pharmaceutics and Biopharmaceutics 2000, 50, 379. 18. Pini, R.; Storti, G.; Mazzotti, M.; Tai, H.; Shakesheff, K. M.; Howdle, S. M. Sorption and Swelling of Poly(DL-Lactic acid) and Poly(lactic-co-glycolic acid) in Supercritcal CO2: An Experimental and Modeling Study. J. Poly. Sci.: Part B: Poly. Phy. 2008, 46, 483.


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List of Tables

Table 1. Experimental conditions investigated for the SFEE process. Table 2. Key results for the experiments performed.

List of Figures Figure 1. Schematic diagram of the SFEE setup. Figure 2. Illustration of the extraction of emulsion step of the SFEE process operated at conditions above critical point. Figure 3. Particle size distribution of the emulsion droplets before SFEE, particles after SFEE, and particles after a number of washes that followed. Figure 4. Solubility of ibuprofen in supercritical CO2. Figure 5. (a) SEM and (b) TEM image of Expt. 1 (35°C) which is a typical image of all the experiments. Figure 6. Comparison of the XRD diffraction pattern of the original chemicals and that of the SFEE processed final product. Figure 7. Comparison of XRD pattern for all the experiments (Expt. 1-10) against rawand SFEE processed ibuprofen. Figure 8. Drug release profiles for particles obtained at different copolymer LG ratios, Expt. 2 (50:50), 9 (75:25), 10 (85:15). Figure 9. SEM image obtained from SFEE process for Expt. 7 (CO2:EA ratio of 20) with magnification (a) 5,000× and (b) 35,000×. Figure 10. Drug release profiles for particles obtained at different CO2:EA ratios, Expt. 2 (40:1), 7 (20:1), 8 (80:1). Figure 11. Drug release profiles for particles obtained at different pressures, Expt. 2 (80 bar), 5 (150 bar), 6 (200 bar). Figure 12. SEM image obtained from SFEE process for Expt. 6 (200 bar) with magnification (a) 7,500× and (b) 35,000×. Figure 13. Drug release profiles for particles obtained at different temperatures, Expt. 1 (35°C), 2 (45°C), 3 (55°C), 4 (65°C).


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Table 1. Experimental conditions investigated for the SFEE process. Expt. 1 2 3 4 5 6 7 8 9 10

Temperature (°C) 35 45 55 65 45 45 45 45 45 45

Pressure (bar) 80 80 80 80 150 200 80 80 80 80

LG Ratio

CO2:EA Ratio

50:50 50:50 50:50 50:50 50:50 50:50 50:50 50:50 75:25 85:15

40 40 40 40 40 40 20 80 40 40


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Table 2. Key results for the experiments performed. Expt. 1 2 3 4 5 6 7 8 9 10

k

WMax

Drug Content (% w/w)

1.363 4.054 4.161 3.099 4.074 4.123 0.944 3.757 3.463 0.918

0.877 0.741 0.622 1.866 0.451 0.171 1.348 0.332 1.182 1.219

1.67 1.18 0.73 2.92 0.73 0.24 1.97 0.49 2.01 1.98

Drug Release Profile

Encapsulation Efficiency (%)

BET Surface Area (m2/g)

(°C)

33.43 23.50 14.68 58.47 14.66 4.86 39.41 9.72 40.18 39.56

4.993 12.270 12.354 9.505 15.500 13.360 5.319 14.390 13.620 12.080

45.63 46.48 48.50 47.69 49.65 53.06 45.39 49.68 49.94 52.26


Tg

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Figure 1. Schematic diagram of the SFEE setup.


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Figure 2. Illustration of the extraction of emulsion step of the SFEE process operated at conditions above critical point.


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Figure 3. Effective particle size of the emulsion droplets before SFEE, particles after SFEE, and particles after a number of washes that followed.


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Figure 4. Solubility of ibuprofen in supercritical CO2 (After Charoenchaitrakool et al.14)


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(a)

(b)

Figure 5. (a) SEM and (b) TEM image of Expt. 1 (35°C) which is a typical image of all the experiments.


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Figure 6. Comparison of the XRD diffraction pattern of the original chemicals and that of the SFEE processed final product.


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Figure 7. Comparison of XRD pattern for all the experiments (Expt. 1-10) against raw and SFEE processed ibuprofen.


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Figure 8. Drug release profiles for particles obtained at different copolymer LG ratios, Expt. 2 (50:50), 9 (75:25), 10 (85:15).


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(a)

(b)

Figure 9. SEM image obtained from SFEE process for Expt. 7 (CO2:EA ratio of 20) with magnification (a) 5,000× and (b) 35,000×.


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Figure 10. Drug release profiles for particles obtained at different CO2:EA ratios, Expt. 2 (40:1), 7 (20:1), 8 (80:1).


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Figure 11. Drug release profiles for particles obtained at different pressures, Expt. 2 (80 bar), 5 (150 bar), 6 (200 bar).


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(a)

(b)

Figure 12. SEM image obtained from SFEE process for Expt. 6 (200 bar) with magnifications (a) 7,500× and (b) 35,000×.


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Figure 13. Drug release profiles for particles obtained at different temperatures, Expt. 1 (35°C), 2 (45°C), 3 (55°C), 4 (65°C).


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Encapsulation of a Low Aqueous Solubility Substance in a

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