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Recrystallization of Poly(L-lactic acid) into Submicrometer Particles in Supercritical Carbon Dioxide Mi Yeong Kim,† Youn Woo Lee,‡ Hun-Soo Byun,⊥ and Jong Sung Lim*,§ Clean Technology Research Center, Korea Institute of Science and Technology, Seoul 136-791, Korea, Department of Chemical Engineering, Seoul National UniVersity, Seoul 151-742, Korea, Department of Chemical Engineering, Yosu National UniVersity, Yosu, Chonnam 550-749, South Korea, and Department of Chemical and Biomolecular Engineering, Sogang UniVersity, Seoul 121-742, Korea
The ASES (aerosol solvent extraction system) process, which is one of the SAS (supercritical antisolvent) processes, was selected to recrystallize PLLA into submicrometer particles. In the ASES process, there are two key factors. One is atomization for fine droplets, and the other is mass transfer of droplets during precipitation in the vessel, which causes nucleation and growth of particles. They are affected by several elements such as temperature, pressure, density, concentration, injection rate of solution, and feed rate of CO2. In this work, we studied the effect of CO2 density, solution injection rate, and CO2 feed rate on atomization and mass transfer. In addition, to investigate the influence of the solvent on particle production, DCM (dichloromethane), THF (tetrahydrofuran), and 1,4-dioxane were used as solvents for PLLA. In the variation of CO2 densities, there was not a consistent tendency. From variation of solution injection rate and feed rate of CO2 we found out that the relative velocity difference between CO2 and the PLLA/DCM solution was an important factor for fine PLLA particles. The particles increased in size with relatively high temperature and pressure for fixed CO2 density. Particles in which DCM was used as the solvent for PLLA were finer than the others. 1. Introduction The supercritical antisolvent (SAS) process is used in the case where a solute produced into fine particles is not soluble in the supercritical fluid. The fine particles are prepared by sudden reduction of the solubility of the solvent when the solution containing the issued materials and appropriate solvent is dissolved in the supercritical solvent. It is effective in the preparation of submicrometer particles of thermally sensitive medicine or protein without residual solvent and needs no washing and drying process. Also, it is generally known for minimizing the process parameters of each batch operation. In the past few years, several supercritical fluid-based techniques have been proposed for the production of micrometric and nanometric particles for potential applications in many fields such as pharmaceutical compounds, superconductors, catalyst precursors, coloring matters, explosives, polymers, and biopolymers because of such advantages.1 It is classified into GAS, ASES, and SEDS according to the operation method.2 Among these three methods, we used the ASES process in this study. The ASES (aerosol solvent extraction system) process takes a way to spray solution containing the material of concern into the precipitator in which supercritical fluid flows continuously. By spraying the solution into the precipitator filled with supercritical antisolvent, fine liquid droplets are obtained. The droplets are collected on the filter as the fine particles at the bottom of the precipitator after going through the two-way mass transfer with supercritical antisolvent.3 Semicontinuous, this process can obtain a higher supersaturation than that of the GAS process in an instant. As a result, the particles produced by this process have a very small size and narrow particle distribution. * To whom correspondence should be addressed. Tel.: (822) 7058918. Fax: (822) 705-7899. E-mail:
[email protected]. † Korea Institute of Science and Technology. ‡ Seoul National University. ⊥ Yosu National University. § Sogang University.
PLLA (poly(L-lactic acid)) has attracted the attention of many researchers because of the many advantages such as good biocompatibility, biodegradability, mainly by simple hydrolysis, very good processability, and a wide range of degradation rates because it is a thermoplastic polyester with high strength and high modulus that can be produced from renewable resources.4 Also, it is able to achieve other properties by various molecular weights and its copolymers.5 PLLA can be used in the industrial packaging field as well as in the biocompatible market for dental, orthopedic, and drug delivery devices.4 As an example, it was already used as a synthetic biocompatible suture approved for clinical use at the first time. In this study, we produced submicrometer particles of PLLA to apply to the DDS (drug delivery system) in the form of microparticles/microspheres by using the ASES process. Supercritical carbon dioxide was used as an antisolvent, and DCM (CH2Cl2), THF (tetrahydrofuran), and 1,4-dioxane were selected as solvents for PLLA. We investigated the effect of different solvents for PLLA as well as of process parameters such as CO2 density, injection rate of liquid solution, and CO2 feed rate on the particle size and morphology of PLLA. Also, we examined the case of different sets of temperature and pressure with the same density of CO2. 2. Experimental Section 2.1. Materials. PLLA of molecular mass Mw ) 85 000 to ∼160 000 was purchased from Sigma Aldrich (Korea). It was a long rod shape having an average length of 350 µm and width of 100 µm. Carbon dioxide (purity 99.9%), used as an antisolvent for PLLA, was obtained from Sinyang Oxygen. DCM (99.9%, Biotech grade solvent), THF (99.9%, biotech grade solvent), and 1,4-dioxane (98%) were used as solvents for PLLA. Methanol (98%, J. T. Baker) was selected for dispersion of the PLLA particles. They were used without further purification. 2.2. Apparatus. The schematic diagram of the ASES apparatus is shown in Figure 1. It is composed of an antisolvent
10.1021/ie050711b CCC: $33.50 © 2006 American Chemical Society Published on Web 09/13/2005
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Figure 1. Apparatus for supercritical antisolvent process.
supplying system, a solution feeding system, a precipitation vessel (KISTEC), two filters (0.5 µm, Swagelok), a back pressure regulator (Tescom Corp.), and a gas vent system. A precipitator is placed on the center of a thermostated air chamber. Its pressure is controlled by a back pressure regulator (Tescom Corp.) located between the filters and the depressurizing tank. Pure CO2 is cooled by a refrigeration bath circulator (Jeio tech) and continuously fed into a precipitator from the top of the vessel by using a high-pressure pump (Oriental Motors). It is continuously vented through passing a back pressure regulator and a dry gas meter (Taekwang Energy S/T3). The solution containing the PLLA is sprayed into the precipitator (KISTEC) by using a high-pressure liquid pump (Oriental Motors) through a fine nozzle (0.63 mm i.d.) located on the top of the vessel. It remains in the depressurizing tank (SUS 316) or is vented with CO2 as passing a dry gas meter. 2.3. Experimental Procedure. The precipitation vessel was filled with CO2 until the desired pressure was reached and was heated to the desired operating temperature. After the flow rate of CO2 was maintained constantly at a designated value, liquid solution containing PLLA was injected into a precipitator, which was broken up into droplets by a fine nozzle. As the droplets passed the precipitator charged with CO2, they went through supersaturation by expansion of the DCM and CO2. At this time, the solution became cloudy and nucleation followed. Consequently, they crystallized to fine particles and collected on a metal filter by a continuous flow of CO2. This continued until the liquid solution in the solution feed tank was injected completely into the precipitator while CO2 continued to pump into the precipitator to remove the residual liquid content, which may remain on the particles for several minutes and make particles staying in the vessel leave for a filter. To obtain the collected particle, the valve between the vessel and filter was closed and then the filter was gradually depressurized by using a back pressure regulator. Solvent separated with the PLLA by CO2 during the spraying process remains in the depressurizing tank or is vented with CO2 passing through dry gas meter. 2.4. Particle Characterization. The morphology of the collected particles was analyzed by FE-SEM (field emission scanning electron microscopy; HITACHI). The samples for SEM were attached by using double-coated adhesive tape and coated with gold by a sputter coater. PSA (laser diffraction
Figure 2. Effect of solution injection rate on PLLA particles at 35 °C, 100 bar, 1 wt % concentration, and 0.53 kg/h CO2 feed.
particle size analyzer; LS 230, Coulter Electronics) was used to measure the mean particle size and particle size distribution. It uses an interaction between a laser and the particles, such as diffraction, refraction, reflection, and absorbance. Before analyzing particles, the particles were dispersed in methanol for 1 min by using a sonicator (Power Sonic 510, Hwa Shin Technology Co.). 3. Results and Disscusion The core of the ASES process is to spray a solution in the antisolvent environment through a fine nozzle. Through this action, fine droplets are generated in the precipitator. This is called atomization and is expressed by the dimensionless Weber number defined as the ratio of the deformation force and the reformation force:
Nwe )
FAV2d σ
(1)
where FA is the density of the antisolvent, V is the relative velocity, d is the diameter of the droplets by spraying the solution, which depends on the relative antisolvent-drop velocity and the antisolvent density. The term σ is the surface tension.7 The numerator of eq 1 means the deformation force, and the denominator means the reformation force. Generally, the higher Nwe it has, the smaller the droplets are as the result of atomization. As shown in eq 1, atomization was influenced by CO2 density, viscosity, and the energy for solution pumping. The droplets broken up by passing a fine nozzle go through evaporation (mass transfer) by diffusion of the surrounding CO2. The energy for solvent evaporation from the droplet surface was supplied from the surrounding antisolvent by conduction and convection. The evaporated solvent is incorporated into the flow of the antisolvent by convection and back diffusion. The overall rate of evaporation is affected by pressure, temperature, droplet diameters, and the velocity difference between the droplet and the surrounding gas.8 Because enhanced atomization and mass transfer between CO2 and droplets leads to fine particles, it is important to maximize their effect. The experimental conditions and results are indicated in Table 1. 3.1. Influence of Solution Injection Rate on PLLA Particles. Figure 2a-d shows the SEM photographs representing
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Figure 3. Effect of solution injection rate on PLLA particle size distribution, which was measured by PSA.
the effect of different injection rates on the PLLA particles. The injection rate of the solution ranged from 0.2 to 1.0 mL/min at 35 °C, 100 bar, 1 wt %, and 5.0 L/min CO2 feed rate. DCM was used as the solvent for PLLA. As shown in Figure 2, parts a and b, and Table 1, the PLLA particle size was slightly reduced from 707.85 to 684.66 nm while the solution injection rate was increased from 0.2 to 0.3 mL/min. But, as shown in Figure 2b-d and Table 1, the particle size was significantly increased up to 2.3 µm by increasing the injection rate of the solution from 0.3 to 1.0 mL/min. In particular, very large particles were observed at 0.5 and 1.0 mL/min as 1.995 and 2.3 µm, respectively. Besides, somewhat rough particles were observed at 0.5 and 1.0 mL/min. Figure 3 shows the particle size distribution by PSA (laser diffraction particle size analyzer). The horizontal axis indicates particle size (nm), and the vertical axis means particle number (%) equivalent to the horizontal axis. The particle size distribution became large by increasing the solution injection rate from 0.3 to 1.0 mL/min. At 0.3 mL/min, the particle size distribution was the narrowest, while at 1.0 mL/ min the largest particle size distribution was observed. When solution was injected into the precipitator at an increased rate, the droplets became small by advanced atomization, like a piped water system. This was the reason that the particle size and distribution were smaller and narrower at 0.2 mL/min than those of 0.3 mL/min. But this is possible only when the volume of the precipitator is so large that it allows all the spraying droplets to interact with the surrounding CO2. Because our system did not satisfy this, as the injection rate was increased, droplets could not achieve enough mass transfer
Figure 4. Effect of various CO2 feed rates on PLLA particles at 40 °C, 80 bar, 1.0 wt % concentration, and 0.2 mL/min solution injection rate.
to get to fine particles. In addition, due to insufficient CO2, drying droplets by surrounding CO2 collided with another one in the precipitator or melted already recrystallized particles. Owing to the lengthened solution jet, the flying time of the droplets did not sufficiently allow the droplets to dry before they arrived at the particle collection device. As a result, large particles which have a large particle size distribution were produced with increasing solution injection rate. 3.2. Influence of Feed Rate of CO2 on PLLA Particles. To investigate the effect of feed rate of CO2, the other conditions were fixed at 35 °C, 100 bar, 1 wt %, and 0.2 L/min solution injection rate. Figure 4 shows the changes of the PLLA particles according to increasing the CO2 feed rate from 2.96 to 8.41 L/min. In Figure 4, the SEM photographs were not clear but could find out that particle size was heavily influenced by the CO2 feed rate. The PLLA particle size was decreased from 871.86 to 380.06 nm during increasing CO2 feed rate. As shown in Figure 5, the particle size distribution became narrow with increasing CO2 feed rate. This was attributed to the difference between the CO2 flow velocity and that of the droplet increased by increasing the CO2 flow rate and CO2/solution ratio in the vessel. Increased CO2 reduced the droplet diameter by enhancing the aerodynamic force. The diameter of the droplet was also reduced owing to enhanced atomization by increased surface tension. In addition, CO2 was diffused into the droplet rapidly, and the solvent in the droplet was simultaneously evaporated for CO2 circumstance
Table 1. Experimental Conditions to Obtain Fine PLLA Particles solvent DCM
DCM THF 1,4-dioxane
temp [°C]
pressure [bar]
CO2 density [g/cm3]
concn [wt %]
injection rate of solution [mL/min]
35 50 60 35 45 55 35
76.3 100 113.6 100 137 174 100
0.397
1.0
0.2
5.0
0.719
0.2
5.0
0.719
5.0
40
80
0.293
0.2 0.3 0.5 1.0 0.2
60
100
0.297
0.3
0.2
feed rate of CO2 [L/min]
2.96 4.65 8.41 6.0 ( 0.5
mean particle size [nm] 630.22 793.45 1408.45 707.85 743.04 764.98 707.85 684.66 1995.25 2301.17 871.86 525.02 380.06 657.85 909.18 1293.45
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Figure 5. Effect of CO2 feed rate on PLLA particle size distribution, which was measured by PSA.
Figure 7. Particle size distribution at 0.397 g/cm3 CO2 density (a) and at 0.719 g/cm3 CO2 density (b). Table 2. Property of Solvents (From NIST Chemistry Web-Book)
Figure 6. Effect of temperature and pressure on PLLA particles at 0.397 g/cm3 CO2 density (a-c) and at 0.719 g/cm3 CO2 density (d-f).
by sufficient CO2. This caused accelerated supersaturation and nucleation. In other words, increased CO2 feed rate led to small particles which had a narrow particle size distribution by enhancing atomization and mass transfer, the core of the ASES process. 3.3. Particles Produced at Fixed Density. In Figure 6a-f, the SEM photographs show the effect of different temperatures and pressures at fixed CO2 density as well as the density on the PLLA particles. As shown in Figure 6a-c, the particle size was increased from 630.22 nm to 1.41 µm by increasing the temperature and pressure at 0.397 g/cm3 CO2 density. But, as shown in Figure 6d-f, there was not a significant change in the aspect of the particle size. Comparing part a with part d, the particle size became larger, from 630.22 to 707.85 nm, due to increased CO2 density from 0.397 to 0.719 g/cm3. As shown in Figure 6, parts b and e, their particle size was similar despite the changed CO2 density. At relatively high temperature,
solvent
boiling point [°C]
polarity index
dielectric constant
density at 25 °C [g/cm3]
viscosity at 20 °C [cP]
DCM THF 1,4-dioxane
39.8 66 101.32
3.1 4.0 4.8
8.93 7.58 2.25
1.325 0.881 1.028
0.44 0.55 1.37
particles prepared at high density were smaller than those of low density, comparing part c with part f. In other words, there was a different tendency according to the temperature and pressure of the system despite the same density. Figure 7, parts a and b, shows the particle size distribution. At 0.397 g/cm3 CO2 density, the particle size distribution became wide with increasing temperature and pressure despite the same density. In the case of 0.719 g/cm3 density, the particle size distribution slightly became wide by changing the temperature and pressure in contrast with the case of 397 g/cm3 CO2 density. Comparing parts a and b in Figure 7, high CO2 density led to a relatively wide particle size distribution. From these results, we found out that it could be a mistake to consider only the density, ignoring the temperature and pressure when the effect of density is investigated. 3.4. When Various Solvents Were Used for Recrystallization. DCM, THF, and 1,4-dioxane were used as solvents for PLLA as shown in Figure 8. Figure 9 shows their particle size distribution. Their physical properties are indicated in Table 2. The particle size in Figure 8a was the smallest, at 657 nm, and
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viscosity leads to increased droplet size.7 From this, we found out the viscosity of the solvent was also a critical factor for recrystallization into particles. 4. Conclusions
Figure 8. Effect of various solvents on PLLA particles at 60 °C, 100 bar, 0.3 wt % concentration, 0.2 mL/min solution injection rate, and 0.53 kg/h CO2 feed.
PLLA submicrometer particles were successfully recrystallized by using the ASES process, one of the SAS processes. Atomization and mass transfer (evaporation), the core of ASES process, were easily adjusted by controlling the solution injection rate, feed rate of CO2, and solvent. Also, particle size was easily controlled from 830 nm to 2.3 µm by adjusting them. Despite enhanced atomization, particle size was increased with increasing solution injection rate because of the lack of mass transfer with the surrounding CO2 and drying time of the droplet. Increased CO2 velocity results in a favorable effect on the PLLA particles because it maximizes atomization and mass transfer. Also, from the result related with CO2 density, it was very dangerous to consider only the density effect, ignoring the temperature and pressure when PLLA was recrystallized into submicrometer particles by using the ASES process. It was found that DCM, a relatively low-viscosity solvent, was the most appropriate for the preparation of fine PLLA particles. Acknowledgment The authors gratefully acknowledge financial support from the Korea Ministry of Commerce, Industry and Energy and the Korea Energy Management Corporation. Literature Cited
Figure 9. Effect of solvent for PLLA on PLLA particle size distribution.
its distribution was the narrowest. THF was also a relatively good solvent to recrystallize the PLLA into submicrometer particles as shown in Figure 8b. In Figure 8, parts a and b, particles had a spherical shape without aggregation, while in Figure 8c, several agglomerates were observed. The particle size was the biggest and its distribution was also the widest in Figure 8c. As shown in Table 2, particle size increased with the viscosity of the solvent. Weber, in 1931, explained that increased
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ReceiVed for reView June 15, 2005 ReVised manuscript receiVed August 8, 2005 Accepted August 11, 2005 IE050711B
