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Journal of Controlled Release 47 (1997) 233–245

Microencapsulation techniques using ethyl acetate as a dispersedsolvent: effects of its extraction rate on the characteristics of PLGA

microspheres

*Hongkee SahDepartment of Pharmaceutical Sciences, Temple University School of Pharmacy, 3307 N. Broad Street, Philadelphia, PA 19140, USA

Received 20 August 1996; revised 24 January 1997; accepted 29 January 1997

Abstract

Ethyl acetate solvent evaporation and extraction processes were developed to prepare poly(d,l-lactide-co-glycolide)microspheres. The microencapsulation processes first emulsified a polymer-containing ethyl acetate solution with a 1%aqueous polyvinyl alcohol solution (W ) to make an oil-in-water (O/W ) emulsion. The O:W phase ratio was carefully1 1 1

chosen so as to saturate the W by a small proportion of the dispersed solvent and to form successfully embryonic1

microspheres without generating polymer precipitates. The effects of the O:W phase ratio on the morphology and size of1

microspheres were interpreted in terms of the solvent miscibility with water, as well as the influence of the W volume on1

breakup of the dispersed phase. The extraction rate of ethyl acetate from nascent microspheres was then adjusted by makinguse of both its miscibility with water and its volatility at atmospheric pressure.Variation of these parameters made it possibleto fabricate hollow- or matrix-type microspheres with different size distributions. It was also found that the tendency ofmicrospheres to aggregate on drying was related to the extent of microsphere hydration and the residual ethyl acetate in wetmicrospheres. 1997 Elsevier Science Ireland Ltd.

Keywords: Ethyl acetate; Poly(d,l-lactide-co-glycolide); Microspheres; Solvent extraction rate

1. Introduction A variety of methods that rely on the two phases arevery well documented in a number of publications.

Recent investigations have invested considerable The use of halogenated alkanes, such as methyleneefforts in the microencapsulation of peptides, pro- chloride and chloroform, however, is not desirableteins and antigens into poly(d,l-lactide-co-glycolide) from the viewpoints of environmental and human(PLGA) microspheres for their controlled release. safety, so they are not recommended for routineThe most commonly used microencapsulation tech- manufacturing process. Furthermore, the use ofnique is based on the concept of solvent evaporation / methylene chloride may also impose a problem inextraction and employs methylene chloride and water obtaining product approval by regulatory agencies.

as dispersed and continuous phases, respectively [1]. As evidenced by Lupron Depot , a small amount ofmethylene chloride remaining in a microsphere

* product is acceptable by the FDA, but only if theCorresponding author. Tel: 1215 7075895; fax: 12157073678; e-mail: [email protected]. product’s therapeutic benefits clearly outweigh a

0168-3659/97/$17.00 1997 Elsevier Science Ireland Ltd. All rights reservedPII S0168-3659( 97 )01647-7

234 H. Sah / Journal of Controlled Release 47 (1997) 233 –245

Table 1Examples of non-chlorinated solvents used for preparing PLGA particles

Solvent Preparation method

Acetone A salting-out procedure utilizing an electrolyte-saturated continuous phase [2]Ethyl acetate Rapid freezing of a polymeric phase in a liquefied gas, followed by solvent

extraction [3]Water-in-oil-in-water emulsion techniques [4,5]

Ethyl formate Spray drying of the dispersed phase to generate spherical particles [6]N-methyl-2-pyrrolidone dimethyl sulfoxide In situ solidificantion of polymeric materials due to dissipation of solvents to

external aqueous media [7,8]Methylethyl ketone A solvent extraction technique making use of partial water-miscibility of the

solvent [9]Phthalic acid diethyl ether Freeze-drying of the polymeric solution followed by ball milling to produce

randomly shaped particles [10]

safety concern over the residual solvent. However, ences observed with the microspheres prepared fromfacing a microsphere product to be marketed as ethyl acetate and methylene chloride.preventive medicaments such as vaccine, the regula-tory agency may raise a concern over the possiblerisk that the residual solvent triggers: methylene 2. Materials and methodschloride is a suspected carcinogen and mutagen.

In recognition of this issue, a number of in- 2.1. Materialsvestigations have sought safer solvents (Table 1).Among them, ethyl acetate is considered one of the Birmingham Polymers Inc. (Birmingham, AL) wasmost preferable solvents. Considering the issues of the supplier of poly(d,l-lactide-co-glycolide) with aboth environmental /human safety and product ap- lactide /glycolide ratio 85:15 (inherent viscosity5

proval, ethyl acetate is regarded as a better solvent 0.28 dl /g in chloroform at 308C). This polymer wasthan dichloromethane. The major physical properties noted as PLGA85:15 in the text. A 88% hydrolyzedof ethyl acetate and dichloromethane are compared polyvinyl alcohol (M 525 000) was obtained fromw

in Table 2. Investigations relating to the effects of Polysciences Inc. (Warrington, PA). HPLC gradeethyl acetate on microsphere quality have not been acetone, ethyl acetate, methylene chloride, and N,N-fully reported in current scientific or patent publi- dimethyl formamide were from Fisher Scientificcations. Therefore, this study focused on the de- (Malvern, PA).velopment of a microencapsulation process utilizingethyl acetate as a dispersed solvent. It also investi- 2.2. Preparation of microspheresgated key process parameters that affected thecharacteristics of PLGA microspheres, such as their 2.2.1. Solvent extraction process with use ofmorphology, size distribution, and aggregation on different O:W phase ratios1

drying. Finally, assessment was made of the differ- PLGA85:15 (700 mg) was dissolved in 8 ml of

Table 2aSome physical properties of dichloromethane and ethyl acetate

Solvent Bp (8C) Density at 208C Solubility (wt%) at 20–258C

Solvent in water Water in solvent

Dichloromethane 39.8 1.3255 1.32 0.2Ethyl acetate 76.7 0.9018 8.70 3.3aFrom Chapter 12. Properties of individual solvents. In: A.K. Doolittle (Ed.), The Technology of Solvents and Plasticizers, Wiley, NewYork, 1954, pp. 492–742 [21].

H. Sah / Journal of Controlled Release 47 (1997) 233 –245 235

ethyl acetate. A 1% aqueous polyvinyl alcohol during various microencapsulation processes.solution was selected as a continuous phase, and its Aliquots (5 ml) of the emulsions were collected byvolume was 20, 50, 80, or 110 ml. The dispersed centrifugation after they were stirred for 15, 40, 150,phase was then poured into the continuous phase. and 300 min. The samples were then observed underDuring the addition, the aqueous phase (W ) was a Zeiss light microscope.1

stirred at 400 rpm with a magnetic stirrer (400HPS/VWR Scientific Co.) to produce an oil-in-water 2.4. Measurement of microsphere hydration(O/W ) emulsion. After 2 min, the emulsion was1

transferred quickly to additional distilled water (W ); At the end of each microencapsulation process,2

the combined total volume of W and W was always microspheres were collected by filtration, weighed1 2

maintained at 200 ml. After 60 min, microspheres immediately (M ) and after drying to a constant1were collected by filtration and redispersed in 100 ml weight (M ). The percentage of microsphere hydra-2of a 0.1% aqueous polyvinyl alcohol solution (W ). tion was then calculated by:3

The microsphere suspension was stirred for 2 h andMicrosphere hydration % 5 (M /M ) 3 100 (1)1 2was wet sieved (mesh size 355 mm) to remove big

microspheres and polymer precipitates. The remain-2.5. Determination of the residual ethyl acetate ining microspheres were then collected by filtrationmicrospheresand dried overnight under vacuum at room tempera-

ture.Accurately weighed microsphere samples (20.3–

44.6 mg) were completely dissolved in 4 ml of2.2.2. Solvent evaporation processN,N-dimethyl formamide, and the solutions wereWithout being transferred to the W and W , a2 3then spiked with an internal standard (acetone).primary O/W emulsion was stirred overnight at1Samples were prepared in triplicate for each micro-room temperature. Microspheres were then collectedsphere formulation. A Hewlett Packard 5890 gasby filtration and vacuum dried. During the microen-chromatograph with a flame ionization detector wascapsulation process, the O:W phase ratio was fixed1used in this experiment. The initial oven temperatureat 8:20.was set at 358C for 2 min and then increased to afinal temperature of 1808C at the rate of 108C/min.2.2.3. Solvent change from ethyl acetate toThe injector and detector temperature was main-methylene chloridetained at 180 and 2608C, respectively. The HP-1PLGA85:15 (700 mg) was dissolved in 8 ml ofcross-linked methyl silicone gum capillary columnmethylene chloride. To be consistent with the ethylwas used as a stationary phase with helium as aacetate extraction process described earlier, the poly-carrier gas. Calculation of the concentration of ethylmeric dispersed phase was poured into 20 ml of aacetate in microsphere samples was based on the1% aqueous polyvinyl alcohol solution. After 2 min,standard calibration curve constructed with 0.0001 tothe emulsion was transferred to 180 ml of distilled0.0005 ml of ethyl acetate /ml of N,N-dimethylwater. The O/W emulsion was stirred at 400 rpm forformamide.300 min in order to let methylene chloride partition

from polymeric microdroplets and evaporate throughthe air / liquid interface. Microspheres were then 2.6. Determination of the size distribution ofcollected by filtration and dried overnight under microspheresvacuum at room temperature.

The size distribution of microspheres was mea-2.3. Observation of various O /W and O /( W 1 sured using a Microtrac SRA 150 particle size1 1

W ) emulsions analyzer (Leeds and Northrup Co., FL). To do so,2

aliquots (200 mg) of final dried microspheres wereThe morphology of O/W or O/(W 1W ) emul- dispersed in Isopar G solution (70 ml) by a gentle1 1 2

sions with different formulations was monitored stirring. In one case, the suspension was loaded


directly into the particle size analyzer. In the other crease in the W to 50 and 80 ml, without the change1

case, the Isopar G suspension was homogenized for in the volume of ethyl acetate (8 ml), yielded bigger30 s to deagglomerate microspheres prior to sample microspheres. Especially, some microspheres pre-loading into the analyzer. The patterns of micro- pared with use of the phase ratio of 8:80 displayedsphere size distribution determined by the two surface defects and irregularity. It was previouslymethods were compared to study the effect of reported that during a methylene chloride O/Wprocess variables on microsphere aggregation. emulsion process an increase in microparticle size

was observed if the volume of an aqueous phase was2.7. Scanning electron microscopy (SEM) increased [11]. However, assessment was not offered

to which mechanism could account for the results. InThe morphology of microspheres was investigated our ethyl acetate extraction microencapsulation pro-

using an Amray 1400 scanning electron microscope cess, the effects of the O:W phase ratio on the1

(Amray Inc., MA). Their internal structures were morphology and size of microspheres were inter-revealed as follows: microspheres were mixed with preted in terms of the miscibility of ethyl acetate

Cole-Parmer epoxy kit comprising epoxy resin and with water and the influence of the W phase volume1

hardener, allowed to set for 2 h at room temperature, upon breakup of the dispersed phase. Firstly, 1.93 mland sliced with a blade (cat. [71960/Electron of ethyl acetate is miscible with 20 ml of water sinceMicroscopy Sciences). Samples were then mounted the solvent solubility in water is 8.7 wt% (Table 2).on aluminum holders and sputter-coated in an argon Therefore, upon emulsification of the two phases atatmosphere. the ratio of 8:20, the W can be saturated by only1

24.1% of the dispersed solvent (Fig. 2). As a result,the majority of ethyl acetate resided in polymeric

3. Results and discussion microdroplets. The subsequent dilution of the O/W1

emulsion with additional water (W ) extracted most2

When 8 ml of PLGA85:15-containing ethyl ace- of the residual solvent, thereby inducing microspheretate solution was poured into 20 or 50 ml of the W hardening. In contrast, as the O:W phase ratio1 1

phase, the polymeric solution was well disintegrated decreased, the fraction of ethyl acetate diffusing intointo microdroplets. The subsequent dilution of the the W increased. For example, 96.5% of ethyl1

contents to 200 ml with additional distilled water acetate could leach to the W when its volume was1

(W ) converted the microdroplets into solid micro- 80 ml. The fast leaching of most solvent resulted in2

spheres. In contrast, when the W volume was 80 ml, the immediate microsphere solidification at the first1

microspheres became hardened instantly before O/W emulsification step. In this case, some micro-1

transfer to the W . At the same time, some of spheres tended to be irregularly shaped so that2

PLGA85:15 became irregular precipitates, rather indentation was observed in their surface. Thisthan microspheres. This caused a reduction in micro- observation was in good agreement with the earliersphere yield. The further increase of the W to 110 reports substantiating that a fast removal of solvents1

ml provided worse results: during emulsification, from polymeric droplets affected the surface mor-about 30% of PLGA85:15 was transformed into phology of microspheres [4,12].precipitates. Moreover, the microspheres tended to Secondly, attention should be paid to the im-aggregate during the filtration of the O/(W 1W ) portant aspect that the volume of a continuous phase1 2

suspension, so they could not be well transferred to affects the breakup of a dispersed phase. For stirredthe W phase. tanks, Eq. (2) was proposed to correlate droplet size3

After various O/(W 1W ) emulsions were stirred to mixing conditions [13,14];1 2

for 15 min, the morphology of embryonic micro- 1 / 3 3 / 5dp dp23 / 5] ]F S D G5 0.054We 1.0 1 4.42Ca (2)spheres was observed under a light microscope L L(LM). As illustrated in Fig. 1, the use of the O:W1

phase ratio of 8:20 resulted in the fabrication of If Weber number (We) is written in terms ofrelatively small microspheres. Interestingly, the in- power per unit mass,


Fig. 1. LM photographs of O/(W 1W ) emulsions taken during the ethyl acetate extraction process. The volume of the W phase was (A)1 2 1

20, (B) 50, or (C) 80 ml. Magnification: 1003 and 4003.

agitated tank; Ca, Calabrese number; r , density of a3 / 5 c3 / 5s s V] ] ]S Dd ~0.054 5 0.054 (3) continuous phase; s, interfacial tension; e, energyF GS Dp r e r Pc c dissipation; P, power; and V, volume. At the phase

where d is droplet size; L, impeller diameter for ratio 8:20, the interfacial tension between the dis-p


the fact that polymer precipitation took place over ashort time scale at the O:W phase ratio of 8:80 and1

8:110.One of novel methods to remove an organic

solvent from embryonic microspheres is to quenchan initial microsphere suspension with a largeamount of water. Such a procedure, utilizing ethylacetate as a dispersed solvent, was briefly mentionedin US patent 5 288 496 [15]. Prior to emulsification,an aqueous phase was mixed with 4.9–11.3 wt% ofethyl acetate, and the resultant phase was thenemulsified with a polymer-containing ethyl acetatesolution. After embryonic microspheres were pro-duced by this technique, a large amount of water wasadded to quickly extract the ethyl acetate remainingin the microspheres. The organic:aqueous phase ratiodisclosed in the patent ranged from 1:155 to 1:413.In our study, after O/(W 1W ) emulsions were1 2

Fig. 2. The effect of W volume on the percentage of the dispersed stirred for 1 h, microspheres were collected by1

solvent (ethyl acetate) leaching into the W phase.1 filtration and redispersed in the W to shorten the3

time required to harvest microspheres. In an addi-persed and continuous phases was reduced by a tional effort to develop a microencapsulation processsmall portion of the dispersed solvent ethyl acetate that could avoid using a large volume of extractionthat leached to the continuous phase (This conclu- water and a big reactor to accommodate such asion was backed up by the following experiment: quenching step, an ethyl acetate evaporation processboth the ethyl acetate-free and ethyl acetate-saturated was also exploited in this study. To do so, an O/W1

W droplets were spread over a glass surface, and the (8 /20) emulsion was stirred at room temperature1

contact angle between each liquid droplet and the without being transferred to the W and W . After2 3

surface was compared. The presence of ethyl acetate evaporation was carried out overnight, discrete andin the W significantly lowered the contact angle). As free-flowing microspheres were obtained. This satis-1

described in Eq. (3), the decrease in the interfacial factory result suggested that the continuous phasetension gives rise to a reduction in microsphere size. does not have to be doped with additional ethylHowever, the increase in the W to 80 ml, while acetate, prior to emulsification with the dispersed1

maintaining a constant volume of ethyl acetate, phase. In addition, a small amount of water isdiminished such effect. In this case, PLGA85:15 sufficient to fabricate microspheres successfully. Theprecipitation concurred with the diffusion of ethyl organic:aqueous phase ratio (8:20) used in this ethylacetate into the W that was not saturated by ethyl acetate evaporation process was only 1:2.5. The1

acetate. Therefore, breakup of the dispersed phase O:W phase ratio was manipulated in a way that only1

was not facilitated by the reduction in the interfacial a small portion of the dispersed solvent was utilizedtension between the two phases as noted for the case to saturate the continuous phase in order to formbased on the phase ratio of 8:20. Consequently, embryonic microspheres successfully. This simplelarger microspheres were formed. Finally, it should procedure may be easily tailored for the scale-up of abe noted that a magnetic stirrer was used to make microencapsulation process for many drugs. TheO/W emulsions throughout this experiment. The evaporation process likely could be shortened by1

mixing technique might provide poor initial dis- several methods such as blowing a gas into thetribution of the dispersed phase into the continuous emulsion, adjusting temperature, or reducing thephase, thereby contributing to the formation of pressure of the system. Further investigation of thesepolymer precipitates. This elaboration was based on process parameters is warranted because microsphere


quality can be drastically influenced by these vari-ables.

In the present study, LM photographs taken at theend of the solvent evaporation process revealed thatmicrospheres with different quality were fabricated.They were completely transparent and became muchsmaller than those produced following the solventextraction process (Fig. 3). Further analysis ofmicrosphere size distribution confirmed that the twodifferent microencapsulation processes greatly affect-ed microsphere size. The average mean volume size(M ) of the microspheres prepared following thev

solvent extraction process was 93 mm, whereas thatprepared following the solvent evaporation processwas 44 mm (Fig. 4). It is believed that the effect ofthe microencapsulation processes upon microspheresize originates from the different onset of micro-

Fig. 4. Size distribution of microspheres prepared following theethyl acetate evaporation (filled bars) and extraction processes(hatched bars) with the O:W phase ratio of 8:20.1

sphere solidification. Both microencapsulation pro-cesses emulsified the organic phase (8 ml) with theW (20 ml) and produced temporarily stabilized1

microdroplets. When the O/W emulsion was diluted1

with the W during the solvent extraction process,2

the ensuing PLGA85:15 precipitation led to theimmediate hardening of microspheres. Owing to therapidity of the microsphere hardening, the sub-sequent in-liquid drying did not affect its particlesize. By contrast, the size of polymeric microdropletsdispersed in the W decreased with stirring time1

during the solvent evaporation process. This hap-pened due to the slow leaching of ethyl acetate fromthe polymeric microdroplets and subsequent inwardpolymer shrinkage. Under this condition, the solventdiffusion from embryonic microspheres to the con-tinuous phase was preceded by solvent evaporationat the air / liquid interface. In the process of thesolvent diffusion, a viscous surface layer aroundpolymeric microdroplets could also impede ethylacetate transfer to the continuous phase, as previous-ly suggested [16,17].

A similar explanation is proposed for the methyl-Fig. 3. LM photographs of microspheres fabricated following (A)

ene chloride evaporation process based on the phasethe ethyl acetate evaporation and (B) the ethyl acetate extractionratio of 8:200. Despite a 10-fold increase in theprocesses with the O:W phase ratio of 8:20. Magnification:1

4003. volume of the continuous phase, compared to the


ethyl acetate evaporation, the majority (about 75%) which microspheres were well dispersed (Figs. 1 andof the solvent still resided in initial polymeric 3). Surprisingly, the microspheres displayed differentmicrodroplets (the solubility of methylene chloride behaviors in response to vacuum drying. The micro-in water is 1.32 wt%, as shown in Table 2). As a spheres prepared following the solvent extractionresult, embryonic microspheres were more likely process with the phase ratios of 8:80 had a seriousviscous liquid microdroplets at the initial microen- drawback in that they were labile to aggregation oncapsulation stage and coalesced together to form drying (Fig. 6A). In contrast, better microspheresaggregates without mechanical stirring (Fig. 5A). from an aggregation point of view were preparedThe evaporation of methylene chloride through the with the phase ratio 8:20 for both the solventair / liquid interface, followed by the solvent diffusion extraction and evaporation processes (Fig. 6B).out of the microdroplets, led to microsphere shrink- To provide more quantitative comparisons, micro-age (Fig. 5B–D), as well as hardening [18]. Com- sphere aggregation on drying was assessed by thepared to the ethyl acetate evaporation, evaporation of patterns of microsphere size distribution determinedmethylene chloride took place more rapidly due to its by two different methods (After drying, microsphereslower boiling point (39.88C). were gently dispersed in Isopar G solution and their

Both the ethyl acetate evaporation and the ex- size distribution was analyzed before and aftertraction processes produced the O/W suspensions in homogenization). The microspheres prepared follow-

Fig. 5. LM photographs of O/W emulsions sampled at (A) 15, (B) 40, (C) 150, and (D) 300 min during the methylene chloride evaporationprocess. Magnification: 1003.


Fig. 6. SEM micrographs of dried microspheres fabricated following (A) the ethyl acetate extraction with the O:W phase ratio of 8:80 and1

(B) the ethyl acetate evaporation with the phase ratio of 8:20.

ing the ethyl acetate evaporation process were rela- with the hydration of 145.5(65.5)%. Increasing thetively well dispersed in the Isopar G solution without W volume to 80 ml enhanced microsphere hydration1

the aid of an homogenizer (Fig. 7A). The methylene to 173.9 (63.5)% (Table 3). If microspheres werechloride evaporation permitted by far the easier prepared following the ethyl acetate or methylenedispersion of final dried microspheres, and the M chloride evaporation processes, a significant reduc-v

measured before and after homogenization of the tion in microsphere hydration was observed and themicrosphere suspension was 110 and 99 mm, respec- resultant microspheres were less prone to aggrega-tively (Fig. 7B). tion on drying.

As a means to elucidate the aggregation of some GC analysis provided the quantitative data on themicrospheres on drying, the degree of microsphere residual ethyl acetate in microspheres throughouthydration before vacuum drying was determined various microencapsulation stages (Fig. 8). Duringusing Eq. (1). The ethyl acetate extraction process the solvent extraction process, the initial O:W phase1

with the phase ratio of 8:20 provided microspheres ratio had an effect on the level of ethyl acetate


Table 3Effects of process variables on the yield and hydration ofmicrospheres

Process Variables Microsphere wt (g) Microsphere

Wet Dried Yield (%) Hydration (%)aO:W (8:20) 0.901 0.618 88.3 145.71

0.871 0.577 82.4 150.90.872 0.617 88.1 139.9

bO:W (8:80) 1.029 0.589 84.1 174.71

0.893 0.525 75.0 170.11.240 0.579 82.7 176.4

cO:W (8:20) 0.792 0.651 93.0 121.70.801 0.643 91.9 124.60.809 0.636 90.9 127.2

dO:W (8:200) 0.694 0.629 89.9 110.30.664 0.602 86.0 110.30.665 0.642 91.7 106.1

a,b Microspheres were prepared following the ethyl acetate ex-traction process with the initial phase ratio specified.c dEthyl acetate or methylene chloride evaporation process wasused to prepare microspheres.

persed in the W (the aggregated microspheres,3

without being transferred to the W , were further3

subjected to vacuum drying and the amount of theresidual ethyl acetate was then determined). This

Fig. 7. Size distribution of microspheres prepared following (A)the ethyl acetate evaporation and (B) the methylene chlorideevaporation processes. Microsphere size was analyzed before(hatched bar) and after (filled bar) homogenization of Isopar Gmicrosphere suspensions.

(6S.D., weight%) in wet microspheres collectedfrom O/(W 1W ) suspensions by filtration. The1 2

microspheres prepared with use of the O:W phase1

ratio of 8:20, 8:80 and 8:110 contained 5.9860.78,7.0761.20 and 8.1260.55 wt% of ethyl acetate,respectively (Fig. 8A). Hardening the correspondingmicrospheres in the W for 2 h, as well as drying3

Fig. 8. The residual ethyl acetate in microspheres. (A) During theunder our experimental condition, did not decreaseethyl acetate extraction process, microsphere samples were pre-the residual ethyl acetate to a great extent. It shouldpared after collection from (a) O/(W 1W ), (b) O/W suspen-1 2 3be mentioned that the microspheres prepared with sions, and (c) after drying. (B) During the ethyl acetate evapora-

the phase ratio of 8:110 became aggregated during tion process, samples were prepared (a) before and (b) afterthe first filtration step and could not be well redis- drying.


Fig. 9. SEM micrographs featuring the internal morphology of microspheres prepared following either the ethyl acetate extraction with theO:W ratio of (A) 8:20 and (B) 8:80, or (C) the ethyl acetate evaporation with the phase ratio of 8:20.1


suggested that the wet microspheres were so elastic A methylene chloride solvent evaporation processthat they could not offer resistance against micro- usually results in the fabrication of compact, mono-sphere fusion. Compared to the solvent extraction lithic matrix type microspheres. During the microen-process, the lowest concentration of ethyl acetate capsulation process, water poorly penetrates into thewas obtained with the microspheres prepared follow- dispersed phase due to its negligible solubility ining the solvent evaporation process. The levels of the methylene chloride (0.2 wt%). Therefore, the re-residual ethyl acetate before and drying were sultant microspheres are hardly hydrated at the end3.8360.46 and 2.6260.41 wt%, respectively (Fig. of the microencapsulation process and are less prone8B). The issue of solvent selection for conventional to aggregation on drying. By contrast, the ethylevaporation /extraction processes has been addressed acetate evaporation and extraction processes reportedin some publications from standpoints of the mecha- in this study produce microspheres with a matrix ornism of microsphere formation, as well as environ- hollow structure, depending on the organic /aqueousmental and human safety [17,19]. It was proposed phase ratio and the rate of the solvent removal fromthat rapid polymer precipitation at the nascent poly- embryonic microdroplets. In addition, since watermeric microdroplet surface was of primary impor- solubility in ethyl acetate is 3.3 wt%, the polymerictance for the successful microencapsulation. In addi- microdroplets can absorb a considerable amount oftion, the rate of polymer precipitation was reported water during the microencapsulation processes.to depend heavily on solvent properties such as Therefore, during a drying process, microspheresinteractions with polymers and water-miscibility. may become soften due to the residual ethyl acetateOur results reported in this study provide additional and water migrating into the surface of microspheres.information that, if ethyl acetate was used as a As this event leads to the aggregation of adjacentdispersed solvent, polymer precipitation was affected microspheres, a drying method should be taken intoby the organic:aqueous phase ratio. This process deliberation.parameter influenced to a great extent the onset ofPLGA85:15 precipitation and microsphere harden-ing, which in turn affected many microspherecharacteristics as discussed so far. 4. Conclusion

Fig. 9 shows the internal morphology of micro-spheres prepared under different process conditions. The disintegration of PLGA85:15-containing ethylWhen microspheres were produced following the acetate solution into microdroplets was significantlyethyl acetate extraction process with the initial phase affected by the O:W phase ratio. Depending on the1

ratio of 8:80, the resultant microspheres tended to phase ratio, emulsification resulted in the generationpossess a hollow core encased by a nonporous shell of either discrete microdroplets or polymer precipi-layer. The quick leaching of ethyl acetate into the W tation. Therefore, in conjunction with a proper1

phase seemed to be responsible for inducing an mixing technique, the phase ratio should be takeninterfacial polymer deposition immediately and lead- into consideration in order to form embryonic micro-ing to the formation of hollow microspheres. This spheres successfully. Both of the microencapsulationinterpretation is supported by earlier reports that the processes reported in this study worked well andrate of solvent removal from embryonic polymeric featured two essential strategies: (a) adjustment ofmicrodroplets determines the morphology of micro- the O:W phase ratio so as to saturate the W phase1 1

spheres [9,20]. Moreover, as illustrated by the other with only a minor proportion of the dispersedSEM micrographs, the morphology of microspheres solvent; and (b) manipulation of the rate of solventcould also be manipulated to possess a monolithic removal from embryonic microspheres. These pro-matrix type by adjusting microencapsulation con- cess variables were found to be critical with regardditions. to polymer precipitation, the morphology and size of

Finally, it is of interest to suggest that the degree microspheres, microsphere hydration, and the re-of microsphere hydration was also affected by sidual ethyl acetate in microspheres, as well assolubility properties of solvents and water (Table 2). microsphere aggregation on drying.


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