REVIEW

Preparation of Uniform-Sized Muive

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absorb drugs or biomolecules onto their exterior size and size distribution. DDSs with small size canlsrp-eere)a-gsurfaces. They are designed for targeting delivery andcontrolled release of drug into systemic circulationmaintaining consistent efficacy and reducing dose ofthe drug and its related side effects. DDSs have

easily pass through the fine capillary blood vesseand the lymphatic endothelium.9 They have longecirculation times in the blood, higher binding caability and accumulation at the target sites, and givless inflammatory and immune response from thtissues and cells of the body than those with biggesize.10,11 The narrow size distribution (monodispersgives better control over the dose and release behvior of the encapsulated drug, yields higher dru

Correspondence to: W.S. Winston Ho (Telephone: 1-614-292-9970; Fax: 1-614-292-3769; E-mail: ho@chbmeng.ohio-state.edu)

Journal of Pharmaceutical Sciences, Vol. 100, 7593 (2011)

2010 Wiley-Liss, Inc. and the American Pharmacists AssociationINTRODUCTION

Drug delivery systems (DDSs) represent one ofthe most rapidly advancing areas of pharmaceuticalscience and technology.1,2 DDSs, such as multipleemulsions, micro/nano solid lipid and polymer parti-cles (spheres or capsules) can entrap drugs orbiomolecules into their interior structures and/or

already been applied with great success today andstill have greater potential for many applications,including anti-tumor therapy, gene therapy, AIDStherapy, and radiotherapy in the delivery of proteins,antibiotics, virostatics, and vaccines as drug carriersto pass the blood-brain barrier.38

The targeting drug delivery and controlled drugrelease properties of DDSs are closely related to theirsystems, such as multiple emulsions, micro/nano solid lipid and polymer particles (spheres orcapsules). Precise control of particle size and size distribution is especially important in such fineapplications. Membrane emulsification can be used to prepare uniform-sizedmultiple emulsionsand micro/nano particulates for drug delivery. It is a promising technique because of the bettercontrol of size and size distribution, the mildness of the process, the low energy consumption,easy operation and simple equipment, and amendable for large scale production. This reviewdescribes the state of the art of membrane emulsification in the preparation of monodispersemultiple emulsions and micro/nano particulates for drug delivery in recent years. The princi-ples, influence of process parameters, advantages and disadvantages, and applications inpreparing different types of drug delivery systems are reviewed. It can be concluded that themembrane emulsification technique in preparing emulsion/particulate products for drug deliv-ery will further expand in the near future in conjunction with more basic investigations on thistechnique. 2010 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 100:7593,2011

Keywords: membrane emulsification; drug delivery systems; multiple emulsion; micro/nanoparticulate; uniform size; controlled releaseABSTRACT: Much attention has in recent years been paid to fine applications of drug deliveryPublished online 29 June 2010 in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/jps.22272Nano Particulates for Drug DelEmulsification

WEI LIU,1,2 XIANG-LIANG YANG,1 W.S. WINSTON HO2,3

1College of Life Science and Technology, Huazhong University

2William G. Lowrie Department of Chemical and BiomoleculaColumbus, Ohio 43210-1180

3Department of Materials Science and Engineering, The Ohio S

Received 24 March 2010; revised 20 May 2010; accepted 21JOURNALltiple Emulsions and Micro/ry by Membrane

cience and Technology, Wuhan 430074, China

ineering, The Ohio State University, 140 West 19th Avenue,

University, Columbus, Ohio 43210-1178

2010OF PHARMACEUTICAL SCIENCES, VOL. 100, NO. 1, JANUARY 2011 75

developed, such as cross-flow membrane emulsifica-tion, premix membrane emulsification, microchannelemulsification, and other membrane-based methods.

Cross-Flow Membrane Emulsification

The process and typical apparatus of cross-flow mem-brane emulsification are shown schematically inFigures 1 and 2, respectively.In cross-flow membrane emulsification, the disper-

sed phase is pressed through a microporous mem-brane (micropore diameter is dp) while the continuousphase flows along the membrane surface. Dropletsgrow at micropores and detach at a certain size (dd),which is determined by the balance between theforces acting on the droplet.2729 Emulsifiers in thecontinuous phase stabilize the newly formed inter-face, to prevent droplet coalescence immediately afterformation.26,30 Some fundamental process parame-ters, such as membrane parameters, process para-meters and phase properties, affect the emulsiondroplet size/size distribution and the dispersed phaseflux during cross-flowmembrane emulsification. Thesemembrane parameters include the type of membranematerial, average micropore diameter, porosity/micropore spacing, micropore geometry, membrane

76 LIU, YANG, AND WINSTON HOlsion droplet size and size distribution, the mildnessof the process, the low energy consumption, and easyto industrial-scale preparation.1719 This reviewdescribes the state of the art of the membrane emul-sification technique in manufacturing uniform-sizedmultiple emulsions and micro/nano particulates fordrug delivery in recent years. The principles, effect ofprocess parameters, and especially applications inpreparing different kinds of drug delivery systems arereviewed.

PRINCIPLES OF MEMBRANE EMULSIFICATION

Membrane emulsification is a technique that involvesusing an applied pressure to force a dispersed phaseto go through membrane pores into a continuousphase.17,20 Small droplets are formed at the poreopenings on the membrane surface and dispatched bythe relative shear motion between the membraneand continuous phase. The resulting droplet size iscontrolled primarily by the choice of the porousmembrane. The membrane emulsification techniquewas first proposed by Nakashima et al. to preparemonodisperse emulsion using a particular glassmembrane called Shirasu Porous Glass (SPG) mem-brane (SPG Technology, Miyazaki, Japan),2123 andlater developed to create a variety of particulateproducts with novel structures and functional-ities.13,15,24,25 To date, in addition to the SPG mem-branes, a broad range of other types of microporousmembranes, such as polymeric, ceramic, metallicand microengineered devices, have been used.And several types of membrane emulsification areencapsulation efficiency and better biocompatibilitywith cells and tissues of the body than that ofpolydisperse DDSs.9,11,12

However, the preparation of these DDSs with con-trolled size and size distribution is still a challenge.The conventional techniques for manufacturingmultiple emulsions and micro/nano particulatesinclude high pressure homogenization, ultrasonica-tion method, rotor/stator systems (such as stirredvessels, colloid mills, and toothed disc dispersingmachines), solvent evaporation, solvent diffusion,coacervation, spray drying and direct polymeriza-tion.1315 None of these techniques can give a goodcontrol over the size and size distribution of above-mentioned emulsion and particulate products. Also,the encapsulated biomolecules such as proteins, geneand vaccines are liable to lose their bioactivities understrong mechanical processing and macro organicsolvents.15,16

The membrane emulsification technique receivedbroad attention as a novel tool for manufacturingmonodisperse emulsion over the last 20 years. Thetechnique is attractive for the better control of emu-JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 100, NO. 1, JANUARY 2011thickness, and wetting property (wall contact angle).SPG membranes are the earliest and most com-

monly used membranes for the excellent char-acterization of uniform cylindrical interconnectedmicropores, a wild range of available mean micro-pore diameter (0.0530mm), high membrane surface

Figure 1. Schematic drawing of the cross-flowmembraneemulsification process.26DOI 10.1002/jps

PREPARATION OF DRUG DELIVERY SYSTEMS BY MEMBRANE EMULSIFICATION 77Figure 2. Schematic of a typical cross-flow membraneemulsification apparatus.17 (a) membrane module, (b) pres-surizing source, (c) reservoir of dispersed phase, (d) emul-sion/continuous phase storage vessel, (e) recirculationpump, (f) needle valve, and (g) pressure gauge.porosity (5060%), and the possibility of surfacemodification.13,17,19 In addition to the SPG membra-nes, many other types of membranes include ceramicaluminum oxide (a-Al2O3),

31,32 zirconium oxide,33

microporous silica glass34 and stainless steel mem-branes,35 and polymer membranes such as polytetra-fluoroethylene (PTFE)36,37 and polycarbonate38

membranes as well as polypropylene hollow fibersmembrane,39 have been successfully used to preparemicro/nano emulsions and particulate products.Some studies show that there is a linear relation-

ship between the droplet size (dd) and the averagemicropore diameter (dp)

15,26,40

dd mdp (1)The values reported form are from 3 to 50, dependingon the ingredients and type of surfactants used, andthe properties of the membrane. For SPG mem-branes, the range of m is between 2 and 10. Formembranes other than SPG, the values of m arehigher.Timgren et al.41 investigated the effects of mem-

brane porosity/micropore spacing in the direction ofthe cross-flow continuous phase on the size of thedroplets using computational fluid dynamics (CFD).Themaximum porosity can be estimated by assuminga square array of membrane micropores, while allmicropores are active and steric hindrance and

DOI 10.1002/jps JOcoalescence are prevented. With the assumption thatthe distance between the pores may be equal to thedroplet diameter, the porosity e can then be calculatedwith the following equation:

" 0:25pd2p

d2d 0:25p 1

x

2(2)

Abrahamse et al.42 calculated the maximum mem-brane porosity to prevent coalescence of dropletsgrowing on neighboring micropores of 5mm diameterto be 1.5%. However, a low porosity has the negativeeffect on obtaining a dispersed phase flux that makesindustrial application feasible.4345

Microporous geometry has an important effect onthe uniform-sized droplet obtained.46 So far, random,round, square and rectangle pores have been appliedin membrane emulsification. When random-shaped/round pore membranes are used in cross-flow mem-brane emulsification, a significant external shearforce is required to obtain the best control over thedroplet size and size distribution.47 In cross-flowprocess with a tubular ceramic membrane was emp-loyed, the tube Reynolds numberwas required to be inthe range from transient (2300 4000).48 Recently, more studiestowards the effects of the microporous shape werecarried out with microengineered membranes.49

The membrane thickness (micropore length) playssome important roles in the control of droplet uniformand productivity.20,48 According to Darcys law, thedispersed phase flux ( fd) has the following relation-ship with the transmembrane pressure (DP) and themembrane thickness (L):

fd KDPmL

(3)

where K is the membrane permeability, and m is thedispersed phase viscosity. So at a given transmem-brane pressure, the membrane thickness is one of thefactors to determine the dispersed phase flux.Some studies indicated that the wetting property

(wall contact angle) is one of the most importantcharacteristics affecting the droplet size/size distri-bution.2729,38,42,50,51 For both detachment mechan-isms, the membrane should be wetted with thecontinuous phase and should not be wetted withthe dispersed phase for proper droplet formation anddroplet detachment. The wall contact angle measuredin the continuous phase should be smaller than 908,that means a hydrophilic membrane for O/W emul-sions is wetted with the aqueous phase and a hydro-phobic membranes for W/O emulsions is wettedwith the oil phase.19 The wall contact angle of themembrane depends on the dynamics of the emulsifierand may be calculated with CFD.42URNAL OF PHARMACEUTICAL SCIENCES, VOL. 100, NO. 1, JANUARY 2011

78 LIU, YANG, AND WINSTON HOThe process parameters include transmembranepressure, cross-flow velocity/wall shear stress andtemperature. The transmembrane pressure (DP) isdefined as the difference between the pressure ofthe dispersed phase and the mean pressure of thecontinuous phase. In membrane emulsification, thetransmembrane pressure is a critical operating para-meter because it has a great influence on theemulsification results, including the dispersed phaseflux through the membrane.52 According to Eq. (3),the dispersed phase flux ( fd) increases as the trans-membrane pressure increases. The capillary pressure(Pc) is the critical pressure to make the discontinuousphase flow and can be predicted from the Laplaceequation, assuming that the micropores are idealcylinders:30

Pc 4g cos udp

(4)

where g is the O/W interfacial tension, u is thewall contact angle, and dp the is average microporediameter. The actual transmembrane pressure requi-red to make the discontinuous phase flow may begreater than the capillary pressure due to tortuositiesin the micropores, irregular micropore openings atthe membrane surface, and the significant effects ofsurface wettability.29,48,53

Cross-flow velocity is a fundamental process para-meter to determine membrane emulsification char-acteristics because wall shear stress caused by thecontinuous phase is a major force to drive the dropletsthat are departing from the membrane micropores.Studies show that the droplet size becomes smaller asthe cross-flow velocity increases, but the droplet sizedistribution may quickly changed to be broad withfurther increases in the cross-flow velocity.30,38,48,50,54

The effect of the cross-flow velocity/wall shear stresson reducing droplet size is dependent on the mem-brane micropore size, more effective for smallermicropore size.The phase properties that influence the cross-flow

membrane emulsification process include the dis-persed phase viscosity, continuous phase viscosity,and type of emulsifier/surfactant. The viscosities ofdispersed and continuous phase have an importanteffect on the membrane emulsification results.52,54,55

According to the Darcys law (Eq. 3), an increase inthe dispersed phase viscosity can result in a decreasein the dispersed phase flux through a porous mem-brane, which leads to a larger droplet size and broadersize distribution. The continuous phase viscosityinfluences the diffusion of the surfactant moleculesand thus reduces the rate of the oilwater interfacialtension.56

During the droplet formation process, the surfac-tant molecules adsorb to the newly formed oilwaterJOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 100, NO. 1, JANUARY 2011interface to reduce the interfacial tension and con-sequently to facilitate droplet formation. A great dealof research work indicates that the interfacial tensionis one of the key factors to control the droplet for-mation during the membrane emulsification proc-ess.38,5154,56 Emulsifiers/surfactants reduce theinterfacial tension between oil and water phases,facilitate droplet distribution, and decrease thetransmembrane pressure (see Eq. 4). The type andconcentration of emulsifier/surfactant greatly influ-ences the adsorption kinetics and thus the dynamicinterfacial tension.52 The other important role ofemulsifier/surfactant is to stabilize the droplet agai-nst coalescence and/or aggregation, which greatlyinfluence the droplet size/size distribution.54,57

Premix Membrane Emulsification

The conventional cross-flow membrane emulsifica-tion process is known as direct membrane emulsifica-tion (DME). There are some potential disadvantageswith this technique:19,25 (i) the relatively low max-imum dispersed phase flux (typically 0.010.1m3/(m2h)) that leads to low productivity; (ii) it is difficultto prepare uniform emulsion droplets when thedispersed phase has high viscosity; and (iii) uniformemulsion can only be prepared using a microporousmembrane with very uniform pores. Because of theserestricted conditions, there have been some limita-tions in choosing the dispersed phase, the continuousphase, and the membrane to obtain the desiredemulsification products. The DME technique is moresuitable for preparing relatively diluted emulsionswith disperse phase contents up to 30 vol% for its lowproductivity and long production time.Recently, an alternative technique of membrane

emulsification based on DME has been developed,which is called premix membrane emulsification(PME). Figure 3 shows the processes of DME andPME.In the process of PME, a preliminarily emulsified

coarse emulsion (rather than a single dispersed phasein DME) is passed through a microporous membrane.The coarse emulsion can be achieved by mixing thetwo immiscible phases (oil and aqueous phases) to-gether using a conventional stirrer mixer.58,59 FromFigure 3, we can see there are two cases in the processof PME, PME without phase inversion and PMEwith phase inversion. If the membrane is wetted bythe dispersed phase of coarse emulsion, for example,hydrophobic membrane wetted by oil phase, andsuitable surfactants are dissolved in both phases. ThePME process may result in a phase inversion, that is,a coarse O/W emulsion may be inverted into a fine W/O emulsion.Studies indicated that PME provides several ad-

vantages over DME:6064 (i) the optimal flux withregard to droplet uniformity is much higher thanDOI 10.1002/jps

1.0m3/(m2h); (ii) the average droplet size is smallerwith the same membrane and phase compositions;(iii) the experimental set-up in PME is generally

microchannels were manufactured using micromachin-ing technology.32,6871 A schematic representation of thedroplet formation process in microchannel emulsifi-

Figure 3. Schematic diagrams of DME and PME processes.25

PREPARATION OF DRUG DELIVERY SYSTEMS BY MEMBRANE EMULSIFICATION 79simpler than that in DME; and (iv) the PME processparameters in PME are easier to control than those inDME. The driving pressure and emulsifier propertiesare not critical in the PME operation as in the DMEprocess.40

However, there are some disadvantages of PMEsuch as a higher polydispersity of emulsion dropletscompared with that prepared by DME. In order tocombine the advantages of the both techniques, amulti-stage PME or repeated membrane homoge-nization method was developed.56,65 In the novelmembrane process, the coarse emulsion is repeatedlyforced through the same microporous membrane anumber of times to achieve fine and uniform-sizedemulsion droplets.66,67

Microchannel Emulsification

Microchannel emulsification is the new developmentin the field of membrane emulsification with the app-lication of micromachining technology. Some newstructure of membranes (microfluidic devices) withprecisely designed geometry of pores and noncylindricalFigure 4. A Schematic representation of thenel emulsification.68

DOI 10.1002/jps JOcation is shown in Figure 4.In the process of terrace-based microchannel

emulsification, emulsion droplets are produced byforcing the dispersed phase through the microchan-nels. The interfacial tension (sow) is the driving forcein the droplet formation process that is divided intoinflation and detachment processes.72,73 The disper-sed phase is forced into a disk-like shape on theterrace in the membrane. This elongated shape hasa higher interfacial area with at least one radius ofcurvature smaller than a spherical shape in thewell with radius Rd, resulting in different Laplacepressures, that is DP1 on the terrace and DP2 inthe well.19,68DP1 andDP2 can be calculated as follows:

DP1 sow 1R1 1R2

sow

R1(5)

DP2 2 sowRd

(6)

From Eqs. (5) and (6), when DP1>DP2, R1 is lessthan Rd/2. Droplet can spontaneously transform intodroplet formation process in microchan-

URNAL OF PHARMACEUTICAL SCIENCES, VOL. 100, NO. 1, JANUARY 2011

80 LIU, YANG, AND WINSTON HObrane emulsification techniques based on the con-ventional methods have been developed, such asvibrating membrane emulsification,79,80 rotatingmembrane emulsification,47,49,8183 and stirred cellmembrane emulsification.84,85 The optimized processconditions and the effects of various parameters ofthese new techniques need to be investigated in moredetail.

PREPARATION OF DRUG DELIVERY SYSTEMUSING MEMBRANE EMULSIFICATION

Membrane emulsification has shown promisingapplications in various fields, such as food industry,pharmaceutical industry, cosmetic industry, chem-ical industry, and other fields like agriculture andenvironment protection. Among these applications,preparation of DDSs is one of the most attractivesubjects.17 The membrane emulsification techniquecan be directly utilized to prepare monodisperse mul-tiple emulsions for drug delivery. Also, this techniquecan afford the preparation of a variety of structure ofuniform-sized particulate products by means ofsequential secondary processes/reactions after pri-mary emulsification, such as solidification, crystal-lization, freeze-drying, evaporation, droplet swelling,gelation, polymerization, etc.13,15,24,25

Multiple Emulsions

Single and multiple emulsion productions by a directmethod are the most investigated systems for mem-brane emulsification applications. However, singlea spherical shape with a more favorable thermo-dynamic property and detachment occurs.From the process of microchannel emulsification,

we can see that the emulsion droplet size is onlydetermined by the geometry of microchannel whensow is regarded as a constant, and no flow of thecontinuous phase is needed during the emulsiondroplets are formed. These make the microchannelemulsification technique attractive for producingmonodisperse emulsions.The main disadvantage of microchannel emulsifi-

cation for practical applications is its inherent lowproductivity. The dispersed phase flux is less than0.01m3/(m2h) for a microchannel plate of 1 cm2.Recently, some novel microchannel emulsificationtechniques such as larger microchannel plates,74,75

straight-through microchannels,7678 and multiplemicrochannel plates have been investigated to inc-rease the production rate.To date, considerable amounts of work about the

mechanisms of membrane emulsification have beendone, both experimentally and computationally. Inorder to better control size distribution and obtainuniform droplets at higher productivity, novel mem-JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 100, NO. 1, JANUARY 2011emulsion is not a very suitable system for drugdelivery since W/O emulsion with high viscosity isdifficult to be perfused into arteries or capillariesvia catheters, and O/W emulsion cannot encapsulatemost anticancer drugs that are water-soluble.86

Water-in-Oil-in-Water (W/O/W) Emulsions

Higashi et al.8789 prepared W/O/W emulsions bymembrane emulsification as a new drug carrier forthe treatment of liver cancer by arterial injectionchemotherapy. The W/O/W emulsion was prepared intwo steps. First, the submicron-sized W/O emulsionwas prepared using a conventional rotor-stator emu-lsification device. The aqueous phase contained ananticancer drug (epirubicin or carboplation), and theoil phase was made up of an iodized poppy-seed oil(Lipiodol) with polyoxyethylene (40) hydrogenatedcaster oil being used as the hydrophobic surfactant.Second, the W/O emulsion was passed through ahydrophilic SPG membrane into a glucose solutionto obtain the multiple drug emulsion. The W/O/Wemulsions with droplets sizes in a range of 170mmcan be prepared by using different SPGmembranes ofan appropriate micropore size. The stability experi-ment showed that separation or breakdown of themultiple drug emulsions did not occur for at least40 days. Clinical studies showed that the multipledrug emulsions were effective in contracting the livercancer texturewhen injected directly into the liver viathe hepatic artery.90 The studies also indicated thatthe emulsion droplet size influenced the anti-tumoreffect of the therapy: the emulsions with proper sizecould accumulate in the small vessels in the tumor,while very small droplets passed through the tumorand very large droplets did not reach the tumor. Theresults concluded that the membrane emulsificationtechnique makes it possible to prepare emulsions forcontrolled drug release with the precise design ofdroplet size.Vladisavljevic et al.91 produced multiple W/O/W

emulsions for drug delivery by extruding a coarse W/O/W emulsion five times under pressures of 70150kPa through the SPG membranes with a meanpore size of 5.4, 7.6, 10.7, 14.8, and 20.3mm. Thestudies showed that the ratio of the mean droplet sizeto the mean pore size after five extrusions decreasedfrom 1.25 to 0.68 as the pore size increased from 5.4 to20.3mm at the wall shear stress of continuous phasein the pores of 200Pa. And at low continuous phaseviscosity, uniform droplets with the coefficient ofvariation values of 0.280.34 were produced at veryhigh transmembrane fluxes exceeding 200m3/(m2h).The encapsulation efficiency of a model drug (CaNa2-EDTA) determined was 8385% in the emulsionproducts containing 30 vol% of inner droplets and3050 vol% of outer drops. The results confirmed arepeated SPG membrane homogenization for con-DOI 10.1002/jps

64

PREPARATION OF DRUG DELIVERY SYSTEMS BY MEMBRANE EMULSIFICATION 81Kukizaki prepared a hydrophilic drug, vitamin B12(VB12) multiple S/O/W emulsion by premix membraneemulsification. S/O dispersion was obtained by waterremoval of the water droplets containing 1.1wt% VB12in aW/O emulsion, followed bymixingwith an externalwater phase at 608C to form a coarse S/O/W dis-persion. By forcing the resultant S/O/W dispersionthrough a SPGmembrane with a mean pore diameterof 14.8mm under a transmembrane pressure of25 kPa, uniformly sized S/O/W droplets were formedat a very high transmembrane flux of 11.8m3/(m2h).Eventually, solid lipid microcapsules for drug deliv-ery with a narrow particle size distribution andhigh hydrophilic drug entrapment efficiency can beachieved by subsequent solidification of the S/O/Wdroplets.trolled production of multiple emulsions with highencapsulation efficiency at high production rates.W/O/W multiple emulsions can also be manufac-

tured with the microchannel emulsification tech-nique.69 The W/O emulsions were prepared byhomogenizing a mixture of water and oil phasesusing a conventional homogenizer, then penetratedthrough the microchannel device into a secondaqueous phase containing a suitable emulsifier foroil phase stabilization. Tetraglycerin polyricinolateat 5wt% dissolved in medium chain triglycerides(decane or ethyl oleate) was selected as the oil phase.A high entrapment efficiency (91%) was achievedunder the low shear stress conditions of microchannelemulsification.

Solid-in-Oil-in-Water (S/O/W) Emulsions

Multiple S/O/W emulsion for oral administration ofinsulin was produced by Toorisaka et al.92 with SPGmembrane emulsification. First, surfactant-coatedinsulin was achieved bymixing an aqueous solution ofinsulin with a hexane solution containing a lipophilicsurfactant (sugar ester ER-290), followed by freeze-drying. Then, surfactant-coated insulin was dis-persed into soybean oil using an ultrasonicationmethod to obtain S/O emulsion. The S/O emulsionwasmixed with aqueous solution containing a hydrophilicsurfactant (sugar ester L-1695), sodium cholate andD-glucose to prepare a coarse S/O/W dispersion usinga rotor-stator homogenizer. Finally, the preliminaryemulsified S/O/W dispersion was forced through anSPG membrane with a mean pore size of 1.1mmseveral times (three or more). The monodisperse ins-ulin multiple emulsions showed hypoglycemic activ-ity for a long period after oral administration to rats.The authors indicated that the S/O/W emulsions withtransforming insulin into a lipophilic complex anduniform droplets had good potential in the treatmentof diabetes.DOI 10.1002/jps JOEthanol-in-Oil-in-Water (E/O/W) Emulsions

Nakajima et al.93 presented novel E/O/W multipleemulsions that were prepared with membraneemulsification. The E/O/W emulsions are suitable toencapsulate some ingredients, such as polyphenols,taxol, androstenedione, and validamycin, which havelow solubility with respect to water and oil but aresoluble in ethanol. The ingredient is first dissolved inethanol at the concentration of 2030% and then theethanol solution is dispersed into an oil phase using aconventional homogenizer. The E/O emulsion can befurther dispersed into an aqueous phase to produce E/O/W multiple emulsions with membrane or micro-channel emulsification. These kinds of multiple emu-lsions have a wide range of applications as emulsiondelivery systems in drugs and functional food andcosmetics.

Solid Lipid Microspheres and Microcapsules

Solid lipid microspheres (SLMSs) and microcapsules(SLMCs) are interesting particulate carriers forcontrolled drug delivery. They have several advan-tages of lower toxicity, better biocompatibility, higherbioavailability, higher drug encapsulation, and long-time storage than emulsions and traditional colloidalcarriers.9496 The drugs encapsulated in SLMSs andSLMCs are released mainly due to the gradualdegradation of the solid lipid by lipase for oral admi-nistration.97 The drug release properties of SLMSsand SLMCs for oral drug delivery are closely relatedto their size and size distribution. So it is very imp-ortant to control the size and size distribution ofSLMSs and SLMCs.

Solid Lipid Microspheres

Sugiura et al.98 prepared SLMSs using temperature-controlled microchannel emulsification and subse-quent solidification of the dispersed oil phase (a highmelting point edible oil). In the microchannel emu-lsification process, triglyceride (tripalmitin) or hyd-rogenated fish oil with the melting point of 588Cpenetrated through the microchannel device into anaqueous solution of emulsifier at 708C. MonodisperseO/W emulsionwith amean droplet size of 21.7mmanda coefficient of variation value of smaller than 4.6%was obtained. Uniform SLMSs with approximatelythe same size as emulsion droplets were obtained bycooling and freeze-drying of the O/W emulsion.

Solid Lipid Macrocapsules

Kukizaki and Goto95 produced uniform-sized SLMCsby a two-step membrane emulsification techniqueusing tubular SPG membranes with a mean porediameter of 0.39.9mm. Triglyceride was used asthe high-melting point solid lipid material andURNAL OF PHARMACEUTICAL SCIENCES, VOL. 100, NO. 1, JANUARY 2011

resultant W/O/W emulsions were immediately cooledto solidify the oil phase, and were then filtered.Uniformly sized SLMCs with mean size 3.132.8mmand high encapsulation yields of vitamin B12 of 93.297.7% (w/w) were obtained. The stability experimentshowed that no leakage of vitamin B12 from theSLMCs was observed at body temperature (310K)over a period of 10 days when the SLMCs were re-dispersed into normal saline.Kukizaki64 presented a novel method for prepara-

tion of hydrophilic drug-encapsulated SLMCs with anarrow particle size distribution via S/O/W dispersionby premix membrane emulsification, which can pro-duce SLMCs at higher production rates and reducethe amount of water contained in the SLMCs. Themicrocosmic structures of SLMCs prepared from W/

82 LIU, YANG, AND WINSTON HOvitamin B12 as a model drug in the study. The flowchart of the preparation of SLMCs using the two-step membrane emulsification process is shown inFigure 5.W/O emulsions were prepared in the first mem-

brane emulsification process using a hydrophobicSPG membrane. At the second membrane emulsifica-

Figure 5. Flow chart of the preparation of SLMCs usingtwo-step SPGmembrane emulsification.95 The temperaturewas kept at (a) 343K and (b) 293K.tion step, W/O emulsion was dispersed through ahydrophilic SPG membrane into the outer aqueousphase. The temperature of the whole membraneemulsification process was kept at a higher tempera-ture (343K) than the melting point of the oil phase(triglyceride). W/O/W emulsions with a narrow dropletsize distribution were achieved through the tempe-rature-controlled membrane emulsification step. The

Figure 6. Schematic illustrations of the druW/O/W emulsion and (b) S/O/W dispersion.64

JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 100, NO. 1, JANUARY 2011O/W emulsion and S/O/W dispersion are shown inFigure 6.As shown in Figure 6a, SLMCs prepared from W/O/

W emulsion contain small aqueous droplets withinlarger solid lipid particles and hydrophilic drugs aredissolved in the aqueous droplets. Figure 6a illustratesthat SLMCs prepared from S/O/W dispersion have amatrix type structure with nano-order particles ofhydrophilic drugs embedded in the capsule. Theproperties of the S/O/W dispersion and hence SLMCswere affected by the homogenization process of S/O/Wdispersions by premix membrane emulsification. Theparticle size of SLMCs and the transmembrane flux ofthe S/O/W dispersion were controlled by adjusting themembrane pore size and transmembrane pressure.The author concluded that micro-encapsulation ofhydrophilic drugs into solid lipids by this novel met-hod may have great potential as drug carriers.

Polymer Microspheres and Microcapsules

Biodegradable polymer microspheres and micro-capsules with uniform size have good potential ascarriers in drug delivery because of better reprodu-cibility, more repeatable controlled release behavior,

g-encapsulated SLMCs prepared from (a)DOI 10.1002/jps

PREPARATION OF DRUG DELIVERY SYSTEMS BY MEMBRANE EMULSIFICATION 83higher bioavailability, targeted delivery, and func-tionality.99101 In recent years, uniform-sized poly-mer microspheres and microcapsules for drugdelivery with different characteristics, hydrophilicand hydrophobic, smooth and rough, solid and hollow,porous and uniform, and with different surface cha-rge, morphologies and diameters raging from 1 to100mm, were successfully produced by combining themembrane emulsification technique and subsequentpolymerization process.

Membrane EmulsificationDirect Polymerization

In this method, the dispersed phase containing mono-mer, solvent, crosslinker and initiator, as well aswater-insoluble oligomer, were permeated throughmembrane micropores into the aqueous solution ofemulsifier and stabilizing agent to form uniformmonomer emulsion droplets (O/W or W/O/W emul-sion). The suspension polymerization is then carriedout by transferring the emulsion into a reactor andheating it to above the decomposition temperatureof the initiator under mild agitation and nitrogenatmosphere. The initial narrow size distribution ofmonomer emulsion droplets can be retained through-out the polymerization process.The studies showed that the uniform-sized chitosan

microspheres have potential applications in oral andother mucosal administration of protein and peptidedrug because they show repeatable release behaviorand excellent mucoadhesive and permeation enhan-cing effect across the biological surfaces.101103 Thechitosan microspheres were prepared by the mem-brane emulsification-direct polymerization technique.Chitosan was dissolved in 1.0wt% aqueous acetic acidcontaining 0.9wt% sodium chloride, which was usedas an aqueous phase. A mixture of liquid paraffinand petroleum ether 7:5 (v/v) containing emulsifierwas used as an oil phase.101 The aqueous phase waspermeated through the uniform pores of a SPGmembrane into the oil phase by the pressure of nitro-gen gas to form W/O emulsion. Uniform chitosanmicrospheres were achieved by direct polymerizationof the chitosan droplets with glutaraldehyde satu-rated toluene as the crosslinking agent.Wei et al.104,105 prepared monodisperse chitosan

microspheres with different structural and auto-fluo-rescent properties for biological tracing and proteindrug delivery by SPG membrane emulsification com-bined with different polymerization systems. Fourdifferent types of uniform chitosanmicrosphereswereprepared using different crosslinkers and monomersderived from chitosan. In addition to uniformity, theSPG membrane technique also enables the prepara-tion of chitosan microspheres with a specific particlesize by the appropriate choice of the membranemicropore size. The tunability of chitosan micro-DOI 10.1002/jps JOsphere structural properties such as surface charge,cavity size, and wall porosity enables the modificationof these systems to cater to specific requirements foruse as protein drug carriers.105

Monodisperse poly(glycidyl methacrylate-divinylben-zene) (P(GMA-DVB)) and poly(glycidyl methacrylate-ethyleneglycol dimethacrylate) (P(GMA-EGDMA)) por-ous microspheres were also prepared by the membraneemulsification-direct polymerization technique.106

This kind of PGMA microspheres covered with manyreactive epoxy groups is easily derived to multi-functional materials and has good potential in theprotein separation and enzyme immobilization.Nagashima et al.107,108 produced poly(acrylamide-

co-acrylic acid) hydrogel microspheres by directpolymerization of monomer droplets in a W/O emul-sion after membrane emulsification. An aqueousmixture of monomers was dispersed into the oilphase to form monodisperse W/O emulsion using ahydrophobic SPG membrane treated with octadecyl-trichlorosilane and trimethylchlorosilane. Uniform-sized hydrogel microspheres were obtained by thepolymerization of W/O emulsion at 708C. The hydro-gel microspheres suitable for use in swelling-con-trolled drug delivery systems can absorb water orother body fluids and swell when placed in the body.The swelling increases the aqueous content withinthe formulation as well as the polymer mesh size,enabling the drug to diffuse through the swollennetwork into the external biological environment.

Membrane EmulsificationSolvent Evaporation

The solvent evaporation method is one of the mostgeneral and simple preparation techniques of polymermicrospheres for drug delivery.109 However, polydis-perse emulsion droplets are obtained and polymermicrosphereswith a relativelywide size distribution areprepared due to the fusion or coagulation of emulsiondroplets under mechanical stirring of emulsion dro-plets. Recently, themembrane emulsification techniquehas been combinedwith the solvent evaporationmethodfor the preparation of polymer microspheres.110114

After the uniform droplets were prepared with mem-brane emulsification, the volatile solvents such asdichloromethane, chloroform, acetonitrile, toluene, etc.were evaporated, and the polymer solidified to formmicrospheres containing the drug in its matrix.Ma et al.110 prepared uniform biodegradable poly

(lactide) microspheres, using a mixture composed ofpoly(lactide) (PLA) polymer, dichloromethane solvent,and lauryl alcohol cosurfactant as the oil phase andan aqueous phase containing a dispersion stabilizerpoly(vinyl alcohol) (PVA) as a continuous phase. AfterSPG membrane emulsification, dichloromethane wasevaporated at room temperature for 24h to obtainPLA microspheres.URNAL OF PHARMACEUTICAL SCIENCES, VOL. 100, NO. 1, JANUARY 2011

Ito et al.112114 prepared monodisperse rifampicin-loaded poly(lactide-co-glycolide) (RFP-PLGA) micro-spheres by the membrane emulsification-solventevaporation method. RFP-PLGA microspheres withdifferent sizes were prepared by changing themicropore sizes of the SPG membranes and the drugrelease kinetics were measured in pH 7.4 PBS at378C. Effect of polyethylene glycol (PEG) added toPVA solution (continuous phase) as a stabilizer uponthe monodispersity of microspheres was studied.113

SEM photographs of RFP-PLGA microspheres mod-ified with PEG20000 are shown in Figure 7.The results indicated that RFP-PLGA micro-

spheres modified with PEG20000 were apparentlymore uniform than those prepared without PEG. Theyield of RFP-PLGA microspheres was 100%. Theinitial burst observed in the release of RFP from RFP-PLGA microspheres was suppressed by the additionof PEG. The effects of the types and the ratios ofvarious organic solvents to dissolve PLGA were alsostudied.114 The authors reported that the particle size

drugs loaded polymer microspheres due to hydro-phobic drugs are easily dissolved together withpolymeric material in the organic solvent. However,hydrophilic drugs would be poorly encapsulated withthis method. So the membrane emulsification-multi-ple emulsion-solvent evaporation method was devel-oped. In this method, a hydrophilic drug is dissolvedin an aqueous phase and dispersed in polymer andorganic solvent to form W/O emulsions. The W/Oemulsions is then dispersed again into a secondaqueous phase to obtain W/O/W multiple emulsions.With this method, both hydrophobic and hydrophilicdrugs can be successfully encapsulated.100,115

Uniform-sized biodegradable PLA microcapsuleswith lysozyme and PLA/PLGA microcapsules con-taining recombinant human insulin (rhI) weresuccessfully prepared by combining the SPG mem-brane emulsification technique and the multipleemulsion-solvent evaporation method.100,116 For pre-paring lysozyme-loaded PLA microcapsules, an aqu-eous phase containing lysozyme was used as the

GAmm, (

84 LIU, YANG, AND WINSTON HOand drug loading efficiency of drug-loaded PLGAmicrospheres dependent on the types of solvents useddue to the interfacial tension between the organicsolvent and aqueous phase. The organic solvents withhigh interfacial tension used for the preparation ofPLGA microspheres were found to be suitable interms of improvement on the properties of drugdelivery formulations.

Membrane EmulsificationMultiple Emulsion-SolventEvaporation

The membrane emulsification-solvent evaporationmethod is suitable for preparation of hydrophobic

Figure 7. SEM photographs of RFP-PLMembranes with a pore size of (a) 1.00m(e) 3.63mm were used.JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 100, NO. 1, JANUARY 2011internal aqueous phase, and PLA and Arlacel 83 weredissolved in amixture solvent of dichloromethane andtoluene which was used as the oil phase.100 Thesetwo solutions were emulsified by a homogenizer toform aW/O primary emulsion. TheW/O emulsionwaspushed through the uniform micropores (5.25mm) ofSPG membrane into the external water phase by thepressure of nitrogen gas to form the uniform W/O/Wdroplets. The lysozyme loaded PLA microcapsuleswere obtained by simply evaporating the solvent. Forpreparing rhI-loaded PLA/PLGA microcapsules, anaqueous phase containing rhI was used as the inneraqueous phase, and PLA/PLGA and Arlacel 83 weredissolved in amixture solvent of dichloromethane and

icrospheres modified with PEG20000.113

b) 1.50mm, (c) 1.95mm, (d) 2.60mm, andDOI 10.1002/jps

drug delivery by an external magnetic field and bio-separation. Uniformly sized polymer microcapsulescontaining inorganic magnetite can be prepared bycombining the SPG membrane emulsification andevaporation method in the multiple emulsions. Omiet al.122 encapsulated Fe3O4 magnetite in poly(styr-ene-co-acrylic acid) (PS-AA) copolymers. A solution ofPS-AA copolymers in toluene was used as the oilphase, and the 2025wt% magnetic fluid was dis-persed in the oil phase. An aqueous phase containingdissolved PVA and sodium lauryl sulfate (SLS) wasused as a continuous phase. After membrane emul-sification, toluene solvent was evaporated underreduced pressure at 50608C, and PS-AA microcap-sules entrapping 3040wt% Fe3O4 were obtained.

Membrane EmulsificationDroplet Swelling

For hydrophilic monomers (such as acrylate mono-mer), it is different to obtain polymer macrosphereswith a narrow droplet size distribution using themembrane emulsification-direct polymerization tech-

PREPARATION OF DRUG DELIVERY SYSTEMS BY MEMBRANE EMULSIFICATION 85toluene, which was used as the oil phase.116 The twosolutions were emulsified by a homogenizer to form aW/O primary emulsion, and then the emulsion wasforced through a SPG membrane into an outeraqueous phase to form the uniform W/O/W droplets.The drug-loaded microcapsules were then achievedby evaporating solvent from droplets.Ito et al.117119 synthesized monodisperse PLGA

microspheres containing the hydrophilic modeldrug, blue dextran (BLD), by the solvent evaporationmethod and the SPG membrane emulsificationtechnique. In order to prepare PLGA microsphereswith a higher drug loading efficiency, the testsof stability and productivity of the primary W/Oemulsion were preliminarily examined by changingspecies or concentration of the oil-soluble surfactantand the ratio of water and organic solvent. BLD-PLGA microspheres with various sizes between 2.0and 10.0mm were prepared by variation of averagemicropore diameters of the SPG membranes. Thestudy indicated that the yield, monodispersity, drugloading efficiency, and drug release rate of the BLD-PLGA microspheres prepared by addition of PEG ascodispersant into the outer aqueous phasewere betterthan those of microspheres prepared without anadditive.For subcutaneous drug delivery, biocompatible

microparticles with diameters in a range of 20100mm are required. These are of sufficient size tocontain a reasonable amounts of active ingredients,but not too big as to cause discomfort in administra-tion. Gasparini et al.115 reported a novel membraneemulsification apparatus, a stirred dispersion celland micropore array membrane, combined with themultiple emulsion-solvent evaporation method, toprepare uniformly sized PLGA microspheres forsubcutaneous controlled drug release. In the disper-sion cell, the discontinuous phase is injected atthe base of the cell, where it passes through themembrane, and the droplets emerge into the conti-nuous phase. The continuous phase is agitated by asimple two-bladed paddle controlled by a DC motor.The membrane is a thin flat metal disc with mono-sized circular pores distributed in a highly regulararray and is chemically treated in order to make thesurface hydrophilic. The dispersion cell provides theability to control the droplet size and size distributionby changing operating conditions and the chemicalproperties of the phases.120,121 The authors think thatthe membrane type used in this study is ideal for theproduction of water-soluble drug loaded PLGAmicro-spheres for subcutaneous drug delivery as it ispossible to produce microparticles in the size rangerequired and with encapsulation efficiencies as highas 100%.Polymer microcapsules containing inorganic sub-

stances have good potential applications in targetedDOI 10.1002/jps JOnique due to the strong hydrophilic property of theSPG membrane surface. So SPG membrane emulsi-fication was combined with the droplet swellingmethod to produce uniform polymer macrospheresfrom hydrophilic monomers. Figure 8 illustrates aschematic diagram of the droplet swelling method forthe manufacture of uniform polymer microspheres.The seed emulsion containing uniform droplets was

prepared by SPG membrane emulsification while thesecondary small emulsion composed of hydrophilicmonomers was obtained by conventional homogeni-zation. Then, the two emulsions were mixed. The

Figure 8. Schematic diagram of droplet swellingmethod.15URNAL OF PHARMACEUTICAL SCIENCES, VOL. 100, NO. 1, JANUARY 2011

86 LIU, YANG, AND WINSTON HOadsorption of the hydrophilic monomers from thesecondary emulsion on the seed emulsion dropletsoccurred due to the degradative diffusion mechan-ism.25 The uniform size of the seed droplets led to theuniformity of adsorbing the hydrophilic monomers,resulting in monodispersity of the swollen dropletseven though their volume was increased by manytimes due to swelling. Uniform-sized polymer macro-spheres were obtained after a subsequent polymer-ization process.The hydrophilic poly(styrene-co-divinylbenzene),123

polystyrenepolyimide,124 poly(methylmethacrylate),125

composite poly(styrene-co-methylmethacrylate) with ahigh content of 2-hydroxyethyl methacrylate,126 poly(styrene-co-N-dimethylaminoethyl methacrylate),127

poly (styrene-co-PEGMMA) and PEGPMMA mono-disperse microspheres,128 and poly(methylmethacry-late-co-2-hydroxyethyl methacrylate) monodispersehollow microspheres129 were produced with the mem-brane emulsification-droplet swelling technique.

Membrane Emulsification Combined WithOther Methods

Chu et al.130 prepared monodisperse environmentalstimulus-responsive controlled-release core-shellmic-rocapsules using SPG membrane emulsification com-bined with interfacial polymerization from O/Wemulsions. The dispersion phase, organic solventcontaining a certain amount of monomer (terephtha-loyl dichloride, TDC), was stored in a pressure-tightvessel and allowed to pass through the SPG mem-brane into the continuous phase under a certainpressure. The continuous phase, an aqueous solutioncontaining an emulsifier and a stabilizer, was forcedto pass through the SPG membrane surface by mag-netic stirring. After emulsification, another monomer(ethylene diamine, EDA), was added to the O/Wemulsion dispersion in a stirred reactor, to start theinterfacial polycondensation reaction between thetwo monomers at the O/W interface. The core-shellmicrocapsules with porous membranes and linear-grafted functional polymeric chains in the pores,acting as stimulus-responsive gates, were obtainedafter isolation by gravity precipitation and washingwith deionized water.Monodisperse poly(N-isopropylacrylamide) (PNI-

PAM) microspheres and hollow microcapsules wereproduced by employing monomer-containing W/Oemulsion droplets obtained from membrane emulsi-fication as the polymerization templates and subse-quent UV-induced free radical polymerization.131

When aqueous-soluble initiator ammonium persul-fate (APS) was added in the dispersed aqueous phase,monodisperse PNIPAM microspheres were obtainedafter UV irradiation polymerization. On the otherhand, monodisperse hollow PNIPAM microcapsulesJOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 100, NO. 1, JANUARY 2011were synthesized while the oil-soluble initiator 2,2-dimethoxy-2-phenylacetophenone (BDK) was intro-duced into the continuous oil phase. The authorsindicated that the PNIPAM microspheres and hollowPNIPAM microcapsules with good monodispersitypresented good potential applications for tempera-ture stimuli-responsive controlled drug release.Uniform-sized pH-sensitive quaternized chitosan

microspheres were prepared by combining the SPGmembrane emulsification technique and a novel ther-mal-gelation method.132 The mixture of quaternizedchitosan solution and glycerophosphate was used asaqueous phase and dispersed in an oil phase to formuniform W/O emulsion with membrane emulsifica-tion. The droplets solidified into microspheres at 378Cby the thermal-gelation method. The microspheresobtained had porous structure and showed apparentpH-sensitivity. Bovine serum albumin (BSA) as amodel drug was encapsulated in the microspheres,and it was released rapidly in an acid solution andslowly in a neutral medium. The novel quaternizedchitosan microspheres with pH-sensitivity can beutilized as a drug carrier (such as tumor-targeteddrug carrier) targeted to organs with different pHvalues.From the discussion presented above, premix mem-

brane emulsification would be a promising techni-que due to its high transmembrane flux and easilycontrollable process parameters. More recently, pre-mix membrane emulsification combined with othertechniques, such as emulsion-solvent extraction andnonsolvent-freeze drying methods, was developed.Uniform-sized amphiphilic poly(ethylene glycol-co-lactide) (PELA) microspheres with high encapsula-tion efficiency of antigen were prepared with a novelmethod combining emulsion-solvent extraction andpremix membrane emulsification.61 In this method,the preparation of coarse double emulsions wasfollowed by additional premix membrane emulsifica-tion with proper pressure, and protein-loaded micro-spheres were obtained by further solidification. Also,narrowly-dispersed polylactide hollowmicrocapsules,with sizes 0.355.0mm that can be used as ultrasoundcontrast agents, were successfully prepared bypremix membrane emulsification of a polylactide/dichloromethane/dodecane solution in alcoholwatermixtures.63

Albumin microspheres have found many applica-tions in the drug delivery and medical diagnosis inrecent years. Monodisperse albumin microsphereswere prepared bymembrane emulsification combinedwith a heat-denaturing method.133,134 A monodis-perse W/O emulsion was first prepared by passingalbumin solution through a hydrophobic SPG mem-brane into an oil phase. Then, albumin microsphereswere prepared after heating the W/O emulsion. Theresult from the experiment showed that the shapeDOI 10.1002/jps

and size of the albumin microspheres strongly de-pended on the concentration of the albumin solutionand the heat-denaturing temperature.

Nanoparticles

Over the recent years, nanoparticles (nanospheresand nanocapsules) have provided huge advantagesregarding drug delivery and release and emergedwith their additional potential to combine diagnosisand therapy as one of the major tools in nanomedi-cine.135,136 Many studies have shown that nanopar-ticles with small size and narrow size distributionare of great importance in pharmaceutical appli-cations.137139 Presently, the membrane emulsifica-

contactor allowed the preparation of SLNswith a lipidphase flux between 0.15 and 0.35m3/(m2h)), and amean size between 70 and 215nm.147 While hydro-philic SPG membranes with the micropore sizes of0.2 and 0.4mm, and hydrophobic SPG membraneswith 0.4 and 1.0mm were used, the membraneemulsification process produced SLNs with a meansize between 50 and 750nm, and a lipid phase fluxbetween 0.008 and 0.84m3/(m2h)).150 The high dis-persed phase fluxes obtained with the SPG mem-branes suggest that the scale-up could be possible forindustrial applications.Zhang et al.151 presented a novel method for

generating SLNs in a microchannel system with a

em

PREPARATION OF DRUG DELIVERY SYSTEMS BY MEMBRANE EMULSIFICATION 87tion technique with a moderate condition has beenapplied successfully to prepare nanoparticles for drugdelivery and controlled release.Solid lipid nanoparticles (SLNs) composed of phy-

siological solid lipids represent a second generationof colloidal drug carriers due to the advantages ofcontrolled release, long-term stability, well biocom-patibility and prevention of loaded drugs from deg-radation, and they have been investigated for variouspharmaceutical applications, for example, parente-ral, peroral, dermal, ocular, and pulmonary admin-istration.140146 Charcosset et al.147150 investigateda cross-flow membrane emulsification process forthe preparation of SLNs using amembrane contactor.A schematic drawing of the process is shown inFigure 9.In this process, the aqueous phase is stirred conti-

nuously and circulates tangentially to the membranesurface. The lipid phase is pressed, at a temperatureabove themelting point of the lipid, through themem-brane micropores, allowing the formation of nano-sized lipid droplets. The lipid droplets are thendetached from the membrane surface by the aqueousphase flowing tangentially inside the membranemodule. Finally, SLNs are obtained by the coolingof the nano-sized lipid droplets to room temperature.When Kerasep ceramic membranes with microporesize of 0.1, 0.2, and 0.45mmwere used, the membrane

Figure 9. Schematic drawing of the mSLNs.147DOI 10.1002/jps JOcross-shaped junction formed by amainmicrochanneland two branches. The SLN formation process thatoccurs in the microchannel is depicted schematicallyin Figure 10.As shown in Figure 10, a lipid solution by dissolving

the lipid in a water-miscible organic solvent is passedthrough the main channel while an aqueous surfac-tant solution is introduced into the branches sim-ultaneously. These two liquids meet together at thecross-shaped junction and pass along the mainchannel. The solvent diffuses from the lipid solutionstream into the aqueous phase, which results in thelocal supersaturation of lipid and thus leads to theformation of SLNs. The mean size, size distribution,and morphology of SLNs prepared with the micro-channel system have been examined. Figure 11 showsthe transmission electron microscopy (TEM) micro-photograph of the obtained SLNs.From the TEM photograph, it can be seen that the

SLNs obtained are nearly spherical in shape and thesize distribution is narrow. The statistical measure-ment result of TEM has indicated that the particlesize has distributed in the range of 70160 nm and themean diameter has been 115nm.Uniform-sized biodegradable PLA nanoparticles

were prepared by combining emulsion-solvent removaland premix membrane emulsification.60 The standardformulation conditions were in the following: the

brane contactor for the preparation ofURNAL OF PHARMACEUTICAL SCIENCES, VOL. 100, NO. 1, JANUARY 2011

micropore size of membrane was 1.4mm, the trans-membrane pressure was 1000kPa, and the volume

Figure 10. Schematic drawing of SLN format

88 LIU, YANG, AND WINSTON HOratio of oil phase and external water phase was 1:5.The study indicated that high transmembrane pre-ssure was a key factor to achieve uniform-sized PLAnanoparticles. The result of dispersed phase fluxwas not given in this study. However, for scaling upthis process, it seems that the transmembranepressure and dispersed phase flux should be furtherconsidered.According to the literature, the nanocapsules have

some advantages over nanospheres, such as a lowerpolymer content and a higher loading capacity forlipophilic drugs. In addition, burst effect may beavoided due to the drug within a central cavity, andthe drug may be better protected from degradationboth during storage and after administration.152

Spironolactone-loaded polycaprolactone (PCL) nano-capsules were prepared by using a membrane cont-actor in laboratory and pilot scales.153 The membraneused was a Kerasep ceramic membrane, which hadan active ZrO2 layer on an Al2O3TiO2 support and amean macropore size of 0.1mm. The optimizedformulations in laboratory and pilot scales led tothe preparation of spironolactone-loaded PCL nano-capsules with the mean sizes of 320 and 400nm asFigure 11. TEM image of SLNs prepared with the micro-channel system.151

JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 100, NO. 1, JANUARY 2011well as the high encapsulation efficiencies of 96.21%and 90.56%, respectively, and both were stable for6 months. The researchers concluded that the pilot-scale production of spironolactone-loaded nanocap-sules prepared using the membrane contactor waspossible in an easy and reproducible way.In the field of medicine, nanobubbles/microbubbles

are expected to be applicable to areas such as thedevelopment of ultrasound contrast agents and tar-geted drug delivery.57,154 Kukizaki and Goto155

preparedmonodisperse nanobubbles using SPGmem-branes with uniform micropores in a system com-posed of dispersed gaseous and continuous aqueousphases containing a surfactant as stabilizer. Theeffects of the surfactant type on the monodispersity ofnanobubbles formed, bubble/pore diameter ratio, andgaseous-phase fluxwere examined. The study showedthat monodisperse nanobubbles with a mean dia-meter of 360720 nm were stably produced frommembranes with mean pore size of 4385nm. Themean diameter of nanobubbles was shown to be 8.6times larger than the mean pore size of the mem-brane. Therefore, the nanobubbles diameter could becontrolled by the membrane pore size.More basic investigations are needed on the

membrane emulsification technique used in prepar-ing nanoparticles for drug delivery. One such investi-gation is the use and preparation of nanoporousmembranes with uniform-sized pores. They may be

ion process in themicrochannel system.151prepared by the self-assembly of block copolymers,resulting in a highly-ordered nanoporous struc-ture.156,157 But the self-assembly technique typicallyrequires either an inorganic support or a minimumthickness to obtain sufficient mechanical strength.However, a new approach by combining the self-assembly technique with the phase inversion methodmay give asymmetric membranes with uniformlynanoporous structures.158 The membrane preparedby involving the phase inversion can give a highporosity, very thin top layer and excellent mechanicalstrength. In the preparation of nanoporous mem-branes, the effects of the molecular weights of thecopolymer blocks and the ratios of these blocks on

DOI 10.1002/jps

time, and casting solution composition.

various average diameters and narrow size-distribu-

Ohio State University for their helpful discussion.

PREPARATION OF DRUG DELIVERY SYSTEMS BY MEMBRANE EMULSIFICATION 89REFERENCES

1. Allen TM, Cullis PR. 2004. Drug delivery systems: Enteringthe mainstream. Science 303:18181822.tion have been successfully prepared by employingdifferent pore sizes and types of membranes, andcombining membrane emulsification with sequentialsecondary processes/reactions, such as solidification,evaporation, droplet swelling, crystallization, gela-tion, and polymerization, etc. In comparison withconventional emulsification methods, the membraneemulsification technique not only has superior size-controllability for drug delivery systems, but alsooffers other advantageous: (1) higher drug encapsula-tion efficiency, (2) excellent reproducibility, (3) mildemulsification for preserving the bioactivities ofprotein and peptide drugs, (4) low energy consump-tion, and (5) easy scale-up. From the discussionpresented in this review, premix membrane emulsi-fication would be a promising technique due to itshigh transmembrane flux and easily controllableprocess parameters. Therefore, we can anticipate thatthe utilization of the membrane emulsification tech-nique in manufacturing emulsion/particulate pro-ducts for drug delivery will further expand in the nearfuture in conjunction with more basic investigationson this technique.

ACKNOWLEDGMENTS

Theworkwas supported financially byNational BasicResearch Program of China (No.2007CB935800) andKey Technology R&D Program of Hubei Province ofChina (No.2006AA304A07). We wish to thankDr. Fawn Liu of Chemical Abstracts Service andDr.Michael Vilt of theWilliamG. Lowrie Departmentof Chemical and Biomolecular Engineering at TheCONCLUSIONS

All the studies carried out so far have shown thatmembrane emulsification offers great potential inmanufacturing uniform-sizedmultiple emulsions andmicro/nano particulate products for drug delivery.Multiple emulsions (W/O/W, S/O/W, E/O/W), solid lipidmicrospheres/microcapsules, biodegradable polymermicrospheres/microcapsules, and nanoparticles withself-assembled structures should be investigatedthoroughly. Several parameters involved in the phaseinversion process should also be investigated, forexample, solvent, nonsolvent, additive, evaporationDOI 10.1002/jps JO2. Kaparissides C, Alexandridou S, Kotti K, Chaitidou S. 2006.Recent advances in novel drug delivery systems. J Nanotech2:111.

3. Brannon-Peppas L, Blanchette JO. 2004. Nanoparticle andtargeted systems for cancer therapy. Adv Drug Del Rev56:16491659.

4. Couvreur P, Puisieux F. 1993. Nano- and microparticles forthe delivery of polypeptides and proteins. Adv Drug Del Rev10:141162.

5. Ghosh PK, Murthy RS. 2006. Microemulsions: A potentialdrug delivery system. Curr Drug Del 3:167180.

6. Tseng YC, Mozumdar S, Huang L. 2009. Lipid-based systemicdelivery of siRNA. Adv Drug Del Rev 61:721731.

7. Peek LJ, Middaugh CR, Berkland C. 2008. Nanotechnology invaccine delivery. Adv Drug Del Rev 60:915928.

8. Kreuter J. 2001. Nanoparticulate systems for brain delivery ofdrugs. Adv Drug Del Rev 47:6581.

9. Win KY, Feng SS. 2005. Effects of particle size and surfacecoating on cellular uptake of polymeric nanoparticles fororal delivery of anticancer drugs. Biomaterials 26:27132722.

10. Gref R,Minamitake Y, PeracchiaMT, TrubetskoyV, TorchilinV, Langer R. 1994. Biodegradable long-circulating polymericnanospheres. Science 263:16001602.

11. Gaumet M, Vargas A, Gurny R, Delie F. 2008. Nanoparticlesfor drug delivery: The need for precision in reporting particlesize parameters. Eur J Pharm Biopharm 69:19.

12. Lince F, Marchisio DL, Barresi AA. 2008. Strategies to controlthe particle size distribution of poly-e-caprolactone nanopar-ticles for pharmaceutical applications. J Colloid Interf Sci322:505515.

13. Charcosset C, Limayem I, Fessi H. 2004. The membraneemulsification processA review. J Chem Tech Biotech79:209218.

14. Ito F, Fujimori H, Honnami H, Kawakami H, Kanamura K,Makino K. 2009. Study of types and mixture ratio oforganic solvent used to dissolve polymers for preparationof drug-containing PLGA microspheres. Eur Polym J45:658667.

15. Ma GH. 2003. Control of polymer particle size using porousglass membrane emulsification. China Particuol 1:105114.

16. Liu R, Huang SS, Wan YH, Ma GH, Su ZG. 2006. Preparationof insulin-loaded PLA/PLGA microcapsules by a novel mem-brane emulsification method and its release in vitro. 2006.Colloids Surf B: Biointerf 51:3038.

17. Nakashima T, Shimizu M, Kukizaki M. 2000. Particle controlof emulsion by membrane emulsification and its applications.Adv Drug Del Rev 45:4756.

18. Joscelyne SM, Tragardh G. 2000. Membrane emulsificationA literature review. J Membr Sci 169:107117.

19. Van der Graaf S, Schroen CGPH, Boom RM. 2005. Prepara-tion of double emulsions by membrane emulsificationAreview. J Membr Sci 251:715.

20. Peng SJ, Williams RA. 1998. Controlled production of emul-sions using a crossflow membrane: Part I: Droplet formationfrom a single pore. Trans Inst Chem Eng 76:894901.

21. Nakashima T, Shimizu M, Kukizaki M. 1991. Membraneemulsification by microporous glass. Key Eng Mater 61/62:513516.

22. Kandori K, Kishi K, Ishikawa T. 1991. Preparation of mono-dispersedW/O emulsions by Shirasu-porous-glass filter emul-sification technique. Colloids Surf 55:7378.

23. Kandori K, Kishi K, IshikawaT. 1991. Formationmechanismsof monodispersed W/O emulsions by SPG filter emulsificationmethod. Colloids Surf 61:269279.

24. Gijsbertsen-Abrahamse AJ, van der Padt A, Boom RM. 2004.Status of cross-flow membrane emulsification and outlook forindustrial application. J Membr Sci 230:149159.URNAL OF PHARMACEUTICAL SCIENCES, VOL. 100, NO. 1, JANUARY 2011

90 LIU, YANG, AND WINSTON HO25. Vladisavljevic GT, Williams RA. 2005. Recent developmentsin manufacturing emulsions and particulate products usingmembranes. Adv Colloid Interf Sci 113:120.

26. Yuan QC, Williams RA, Biggs S. 2009. Surfactant selectionfor accurate size control of microcapsules using membraneemulsification. Coll Surf A: Physicochem Eng Aspects347:97103.

27. Abrahamse AJ, van Lierop R, van der Sman RGM, van derPadt A, Boom RM. 2002. Analysis of droplet formation andinteractions during cross-flow membrane emulsification.J Membr Sci 204:125137.

28. Yasuno M, Nakajima M, Iwamoto S, Maruyama T, Sugiura S,Kobayashi I, Shono A, Satoh K. 2002. Visualization andcharacterization of SPG membrane emulsification. J MembrSci 210:2937.

29. Luca GD, Maio FPD, Renzo AD, Drioli E. 2008. Dropletdetachment in cross-flow membrane emulsification: Compar-ison among torque- and force-based models. Chem Eng Proce47:11501158.

30. Vladisavljevic GT, Schubert H. 2003. Influence of processparameters on droplet size distribution in SPG membraneemulsification and stability of prepared emulsion droplets.J Membr Sci 225:1523.

31. Vladisavljevic GT, Schubert H. 2003. Preparation of emul-sions with a narrow particle size distribution using micro-porous alumina membranes. J Dispers Sci Technol 24:811819.

32. Vladisavljevic GT, Lambrich U, Nakajima M, Schubert H.2004. Production of O/W emulsions using SPG membranes,ceramic a-aluminum oxide membranes, microfluidizer and asiliconmicrochannel plate: A comparative study. Colloids Surf232:199207.

33. Joscelyne SM, Tragardh G. 1999. Food emulsions using mem-brane emulsification: Conditions for producing small droplets.J Food Eng 39:5964.

34. Fuchigami T, Toki M, Nakanishi K. 2000. Membrane emulsi-fication using sol-gel derived macroporous silica glass. J Sol-Gel Sci Technol 19:337341.

35. Dowding PJ, Goodwin JW, Vincent B. 2001. Production ofporous suspension polymer beads with a narrow size distribu-tion using a cross-flow membrane and a continuous tubularreactor. Coll Surf A: Physicochem Eng Aspects 180:301309.

36. Yamazaki N, Yuyama H, Nagai M, Ma GH, Omi S. 2002.A comparison ofmembrane emulsification obtained using SPG(Shirasu porous glass) and PTFE [poly(tetrafluoroethylene)]membranes. J Dispers Sci Technol 23:279292.

37. Yamazaki N, Naganuma K, Nagai M, Ma GH, Omi S. 2003.Preparation of W/O (water-in-oil) emulsions using a PTFE(polytetrafluoroethylene) membrane-a new emulsificationdevice. J Dispers Sci Technol 24:249257.

38. Kobayashi I, Yasuno M, Iwamoto S, Shono A, Satoh K, Naka-jima M. 2002. Microscopic observation of emulsion dropletformation from a polycarbonate membrane. Coll Surf A: Phy-sicochem Eng 207:185191.

39. Vladisavljevic GT, Tesch S, Schubert H. 2002. Preparation ofwater-in-oil emulsions using microporous polypropylene hol-low fibers: Influence of some operating parameters on dropletsize distribution. Chem Eng Process 41:231238.

40. Zhou QZ, Ma GH, Su ZG. 2009. Effect of membrane para-meters on the size and uniformity in preparing agarose beadsby premix membrane emulsification. J Membr Sci 326:694700.

41. Timgren A, Tragardh G, Tragardh C. 2009. Effects of porespacing on drop size during cross-flow membrane emulsifica-tionA numerical study. J Membr Sci 337:232239.

42. Abrahamse AJ, van der Padt A, Boom RM, Heij WBC. 2001.Process fundamentals of membrane emulsification: Simula-tion with CFD. AIChE J 47:12851291.JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 100, NO. 1, JANUARY 201143. Schadler V, Windhab EJ. 2006. Continuous membrane emul-sification by using a membrane system with controlled poredistance. Desalination 189:130135.

44. Kosvintseva SR, Gasparinib G, Holdich RG. 2008. Membraneemulsification: Droplet size and uniformity in the absence ofsurface shear. J Membr Sci 313:182189.

45. Egidia E, Gasparinia G, Holdicha RG, Vladisavljevica GT,Kosvintsevb SR. 2008. Membrane emulsification using mem-branes of regular pore spacing: Droplet size and uniformity inthe presence of surface shear. J Membr Sci 323:414420.

46. Kobayashi I, NakajimaM, ChunK, Kikuchi Y, Fujita H. 2002.Silicon array of elongated through-holes for monodisperseemulsion droplets. AIChE J 48:16391644.

47. Yuan QC, Hou RZ, Aryanti N, Williams RA, Biggs S, LawsonS. 2008. Manufacture of controlled emulsions and particu-lates usingmembrane emulsification. Desalination 224:215220.

48. Williams RA, Peng SJ, Wheeler DA, Morley NC, Taylor D,Whalley M, Houldsworth DW. 1998. Controlled production ofemulsions using a cross-flow membrane, Part II: Industrialscale manufacture. Chem Eng Res Des 76:902910.

49. Yuan QC, Aryanti N, Hou RZ, Williams RA. 2009. Perfor-mance of slotted pores in particle manufacture using rotatingmembrane emulsification. Particuology 7:114120.

50. Luca GD, Renzo AD, Maio FPD, Drioli E. 2006. Modelingdroplet formation in cross-flow membrane emulsification.Desalination 199:177179.

51. Luca GD, Drioli E. 2006. Force balance conditions for dropletformation in cross-flow membrane emulsifications. J ColloidInterf Sci 294:436448.

52. Kukizaki M. 2009. Shirasu porous glass (SPG) membraneemulsification in the absence of shear flow at the membranesurface: Influence of surfactant type and concentration, visc-osities of dispersed and continuous phases, and transmem-brane pressure. J Membr Sci 327:234243.

53. Christov NC, Danov KD, Danova DK, Kralchevsky PA. 2008.The drop size in membrane emulsification determined fromthe balance of capillary and hydrodynamic forces. Langmuir24:13971410.

54. Hao DX, Gong FL, Hu GH, Zhao YJ, Lian GP, Ma GH, Su ZG.2008. Controlling factors on droplets uniformity in membraneemulsification: Experiment and modeling analysis. Ind EngChem Res 47:64186425.

55. Vankova N, Tcholakova S, Denkov ND, Ivanov I, Vulchev VD,Danner T. 2007. Emulsification in turbulent flow: 1.Mean andmaximum drop diameters in inertial and viscous regimes.J Colloid Interf Sci 312:363380.

56. Vladisavljevic GT, Shimizu M, Nakashima T. 2006. Produc-tion of multiple emulsions for drug delivery systems byrepeated SPG membrane homogenization: Influence of meanpore size, interfacial tension and continuous phase viscosity.J Membr Sci 284:373383.

57. Kukizaki M, Baba Y. 2008. Effect of surfactant type on micro-bubble formation behavior using Shirasu porous glass (SPG)membranes. Coll Surf A: Physicochem Eng Aspects 326:129137.

58. Shima M, Kobayashi Y, Fujii T, Tanaka M, Kimura Y, AdachiS, Matsuno R. 2004. Preparation of fine W/O/W emulsionthrough membrane filtration of coarse W/O/W emulsion anddisappearance of the inclusion of outer phase solution. FoodHydrocolloid 18:6170.

59. Van der Zwan E, Schroen K, van Dijke K, Boom R. 2006.Visualization of droplet breakup in pre-mix membrane emul-sification using microfluidic devices. Colloid Surf A 277:223229.

60. Wei Q, Wei W, Lai B, Wang LY, Wang YX, Su ZG, Ma GH.2008. Uniform-sized PLA nanoparticles: Preparation by pre-mix membrane emulsification. Int J Pharm 359:294297.DOI 10.1002/jps

PREPARATION OF DRUG DELIVERY SYSTEMS BY MEMBRANE EMULSIFICATION 9161. Wei Q, Wei W, Tian R, Wang LY, Su ZG, Ma GH. 2008.Preparation of uniform-sized PELA microspheres with highencapsulation efficiency of antigen by premix membraneemulsification. J Colloid Interf Sci 323:267273.

62. Sawalha H, Purwanti N, Rinzema A, Schroen K, Boom R. 2008.Polylactide microspheres prepared by premix membrane emul-sification: Effects of solvent removal rate. JMembr Sci 310:484493.

63. Sawalha H, Fan YX, Schroen K, Boom R. 2008. Preparation ofhollow polylactide microcapsules through premix membraneemulsification: Effects of nonsolvent properties. J Membr Sci325:665671.

64. Kukizaki M. 2009. Preparation of solid lipid microcapsules viasolid-in-oil-in-water dispersions by premix membrane emul-sification. Chem Eng J 151:387396.

65. Vladisavljevic GT, Shimizu M, Nakashima T. 2004. Prepara-tion of monodisperse multiple emulsions at high productionrates by multi-stage premix membrane emulsification.J Membr Sci 244:97106.

66. Zhou QZ, Wang LY, Ma GH, Su ZG. 2008. Multi-stage premixmembrane emulsification for preparation of agarose microbeads with uniform size. J Membr Sci 322:98104.

67. Surh J, Jeong YG, Vladisavljevic GT. 2008. On the prepara-tion of lecithin-stabilized oil-in-water emulsions by multi-stage premix membrane emulsification. J Food Eng 89:164170.

68. Sugiura S, Nakajima M, Tong J, Nabetani H, Seki M. 2000.Preparation of monodispersed solid lipidmicrospheres using amicrochannel emulsification technique. J Colloid Interface Sci227:95103.

69. Sugiura S, Nakajima M, Yamamoto K, Iwamoto S, Oda T,Satake M, Seki M. 2004. Preparation characteristics of water-in-oil-in-water multiple emulsions using microchannel emul-sification. J Colloid Interf Sci 270:221227.

70. Van der Graaf S, Steegmans MLJ, van der Sman RGM,Schroen CGPH, Boom RM. 2005. Droplet formation in a T-shaped microchannel junction: A model system for membraneemulsification. Coll Surf A: Physicochem Eng Aspects266:106116.

71. Husny J, Cooper-White J. 2006. The effect of elasticity on dropcreation in T-shaped microchannels. J Non-Newtonian FluidMechanics 137:121136.

72. Sugiura S, Nakajima M, Iwamoto S, Seki M. 2001. Interfacialtension driven monodispersed droplet formation from micro-fabricated channel array. Langmuir 17:55625567.

73. Van Dijke KC, Boom RM, Schroen Karin CPGH. 2008. Micro-channel emulsification: From computational fluid dynamics topredictive analytical model. Langmuir 24:1010710115.

74. Kobayashi I, Mukataka S, Nakajima M. 2005. Production ofmonodisperse oil-in-water emulsions using a large siliconstraight-through microchannel plate. Ind Eng Chem Res44:58525856.

75. Vladisavljevic GT, Kobayashi I, Nakajima M. 2008. Genera-tion of highly uniform droplets using asymmetric microchan-nels fabricated on a single crystalsilicon plate: Effect ofemulsifier and oil types. Powder Tech 183:3745.

76. Kobayashi I, Mukataka S, Nakajima M. 2004. Effect of slotaspect ratio on droplet formation from silicon straight-through microchannels. J Colloid Interf Sci 279:277280.

77. Kobayashi I, Takano T, Maeda R, Wada Y, Uemura K, Naka-jima M. 2008. Straight-through microchannel devices forgenerating monodisperse emulsion droplets several micronsin size. Microfluid Nanofluid 4:167177.

78. Kobayashi I, Murayama Y, Kuroiwa T, Uemura K, NakajimaM. 2009. Production of monodisperse water-in-oil emulsionsconsisting of highly uniform droplets using asymmetricstraight-through microchannel arrays. Microfluid Nanofluid7:107119.DOI 10.1002/jps JO79. Zhu J, Barrow D. 2005. Analysis of droplet size during cross-flow membrane emulsification using stationary and vibratingmicromachined silicon nitride membranes. J Membr Sci261:136143.

80. Kelder JDH, Janssen JJM, Boom RM. 2007. Membrane emul-sification with vibrating membranes: A numerical study.J Membr Sci 304:5057.

81. Aryanti N, Williams RA, Hou RZ, Vladisavljevic GT. 2006.Performance of rotating membrane emulsification for o/wproduction. Desalination 200:572574.

82. Vladisavljevic GT, Williams RA. 2006. Manufacture of largeuniform droplets using rotating membrane emulsification.J Colloid Interf Sci 299:396402.

83. Aryanti N, Hou RZ, Williams RA. 2009. Performance of arotating membrane emulsifier for production of coarse dro-plets. J Membr Sci 326:918.

84. Stillwell MT, Holdich RG, Kosvintsev SR, Gasparini G, Cum-ming IW. 2007. Stirred cell membrane emulsification andfactors influencing dispersion drop size and Uniformity. IndEng Chem Res 46:965972.

85. Dragosavac MM, Sovilj MN, Kosvintsev SR, Holdich RG,Vladisavljevic GT. 2008. Controlled production of oil-in-wateremulsions containing unrefined pumpkin seed oil using stir-red cell membrane emulsification. J Membr Sci 322:178188.

86. Hino T, Kawashima Y, Shimabayashi S. 2000. Basic study forstabilization of W/O/W emulsion and its application to trans-catheter arterial embolization therapy. Adv Drug Del Rev45:2745.

87. Higashi S, Shimizu M, Nakashima T, Iwata K, Uchiyama F,Tateno S, Tamura S, Setogushi T. 1995. Arterial-injectionchemotherapy for hepatocellular carcinoma using monodis-persed poppy-seed oil microdroplets containing fine aqueousvesicles of epirubicin, initial medical application of a mem-brane emulsification technique. Cancer 75:12451254.

88. Higashi S, Maeda Y, Kai M, Kitamura T, Tsubouchi H,Tamura S, Setoguchi T. 1996. A case of hepatocellular carci-noma effectively treated with epirubicin aqueous vesicles inmonodispersed iodized poppy-seed oil microdroplets. Hepato-Gastroenterology 43:14271430.

89. Higashi S, Tabata N, Kondo KH, Maeda Y, Shimizu M,Nakashima T, Setoguchi T. 1999. Size of lipid microdropletseffects results of hepatic arterial chemotherapy with an antic-ancer agent in water-in-oil-in-water emulsion to hepatocellu-lar carcinoma. J Pharmacol Exp Ther 289:816819.

90. Higashi S, Setoguchi T. 2000. Hepatic arterial injection che-motherapy for hepatocellular carcinoma with epirubicin aqu-eous solution as numerous vesicles in iodinated poppy-seed oilmicrodroplets: Clinical application of water-in-oil-in-wateremulsion prepared using a membrane emulsification techni-que. Adv Drug Deliv Rev 45:5764.

91. Vladisavljevic GT, Shimizu M, Nakashim T. 2006. Productionof multiple emulsions for drug delivery systems by repeatedSPG membrane homogenization: Influence of mean pore size,interfacial tension and continuous phase viscosity. J MembrSci 284:373383.

92. Toorisaka E, Ono H, Arimori K, Kamiya N, Goto M. 2003.Hypoglycemic effect of surfactant-coated insulin solubilized ina novel solid-in-oil-in-water (S/O/W) emulsion. Int J Pharm252:271274.

93. Nakajima M, Nabetani H, Ichikawa S, Xu QY. 2003. Func-tional emulsions, US Patent No. 6538019.

94. Long CX, Zhang LJ, Qian Y. 2006. Preparation and crystalmodification of ibuprofenloaded solid lipid microparticles.Chin J Chem Eng 14:518525.

95. Kukizaki M, Goto M. 2007. Preparation and evaluation ofuniformly sized solid lipid microcapsules using membraneemulsification. Coll Surfa A: Physicochem Eng Aspects 293:8794.URNAL OF PHARMACEUTICAL SCIENCES, VOL. 100, NO. 1, JANUARY 2011

92 LIU, YANG, AND WINSTON HO96. Jaspart S, Bertholet P, Piel G, Dogne JM, Delattre L, EvrardB. 2007. Solid lipid microparticles as a sustained releasesystem for pulmonary drug delivery. Eur J Pharm Biopharm65:4756.

97. Long K, Zubir I, Hussin AB, Idris N, Ghazali HM, Lai OM.2003. Effect of enzymatic transesterification with flaxseed oilon the high-melting glycerides of palm stearin and palm olein.J Am Oil Chem Soc 80:133137.

98. Sugiura S, NakajimaM, Tong JH, Nabetani H, SekiyM. 2000.Preparation of monodispersed solid lipidmicrospheres using amicrochannel emulsification technique. J Colloid Interf Sci227:95103.

99. Couvreur P, Puisieux F. 1993. Nano- and microparticles forthe delivery of polypeptides and proteins. Adv Drug Del Rev10:141162.

100. Liu R,MaGH,MengFT, Su ZG. 2005. Preparation of uniform-sized PLA microcapsules by combining Shirasu Porous Glassmembrane emulsification technique and multiple emulsion-solvent evaporation method. J Control Rel 103:3143.

101. Wang LY, Ma GH, Su ZG. 2005. Preparation of uniform sizedchitosanmicrospheres bymembrane emulsification techniqueand application as a carrier of protein drug. J Control Rel106:6275.

102. Wang LY, Gu YH, Su ZG, Ma GH. 2006. Preparation andimprovement of release behavior of chitosan microspherescontaining insulin. Int J Pharm 311:187195.

103. Wang LY, Gu YH, Zhou QZ, Ma GH, Wan YH, Su ZG. 2006.Preparation and characterization of uniform-sized chitosanmicrospheres containing insulin by membrane emulsificationand a two-step solidification process. Coll Surf B: Biointerf50:126135.

104. Wei W, Wang LY, Yuan L, Wei Q, Yang XD, Su ZG, Ma GH.2007. Preparation and application of novel microspheres pos-sessing autofluorescent properties. Adv FunctMater 17:31533158.

105. Wei W, Yuan L, Hu G, Wang LY, Wu J, Hu X, Su ZG, Ma GH.2008. Monodisperse chitosan microspheres with interestingstructures for protein drug delivery. AdvMater 20:22922296.

106. Wang RW, Zhang Y, Ma GH, Su ZG. 2006. Preparation ofuniform poly(glycidyl methacrylate) porous microspheres bymembrane emulsification polymerization technology. J ApplPolym Sci 102:50185027.

107. Nagashima S, Ando S, Tsukamoto T, Ohshima H, Makino K.1998. Preparation of monodisperse poly(acrylamide-co-acrylicacid) hydrogel microspheres by a membrane emulsificationtechnique and their size-dependent surface properties. CollSurf B: Biointerf 11:4756.

108. Nagashima S, Koide M, Ando S, Makino K, Tsukamoto T,Ohshima H. 1999. Surface properties of monodisperse poly-acrylamide-co-acrylic acid hydrogel microspheres prepared bya membrane emulsification technique. Coll Surf A: Physio-chem Eng Aspects 153:221227.

109. ODonnell PB, McGinity JW. 1997. Preparation of micro-spheres by the solvent evaporation technique. Adv DrugDel Rev 28:2542.

110. Ma GH, Nagai M, Omi S. 1999. Preparation of uniformpoly(lactide) microspheres by employing the Shirasu PorousGlass emulsification technique. Coll Surf A: Physicochem EngAspects 153:383394.

111. Sawalha H, Purwanti N, Rinzema A, Schroen K, Boom R.2008. Polylactide microspheres prepared by premix mem-brane emulsificationEffects of solvent removal rate.J Membr Sci 310:484493.

112. Ito F, Makino K. 2004. Preparation and properties of mono-dispersed rifampicin-loaded poly(lactide-co-glycolide) micro-spheres. Coll Surf B: Biointerf 39:1721.

113. Ito F, Fujimori H, Honnami H, Kawakami H, Kanamura K,Makino K. 2008. Effect of polyethylene glycol on preparationJOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 100, NO. 1, JANUARY 2011of rifampicin-loaded PLGA microspheres with membraneemulsification technique. Coll Surf B: Biointerf 66:6570.

114. Ito F, Fujimori H, Honnami H, Kawakami H, KanamuraK, Makino K. 2009. Study of types and mixture ratio oforganic solvent used to dissolve polymers for preparationof drug-containing PLGA microspheres. Eur Polym J45:658667.

115. Gasparini G, Kosvintsev SR, Stiliwell MT, Holdich RG. 2008.Preparation and characterization of PLGA particles for sub-cutaneous controlled drug release by membrane emulsifica-tion. Coll Surf B: Biointerf 61:199207.

116. Liu R, Huang SS, Wan YH, Ma GH, Su ZG. 2006. Preparationof insulin-loaded PLA/PLGA microcapsules by a novel mem-brane emulsification method and its release in vitro. Coll SurfB: Biointerf 51:3038.

117. Ito F, Fujimori H, Makino K. 2007. Incorporation of water-soluble drugs in PLGA microspheres. Coll Surf B: Biointerf54:173178.

118. Ito F, Fujimori H, Makino K. 2008. Factors affecting theloading efficiency of water-soluble drugs in PLGA micro-spheres. Coll Surf B: Biointerf 61:2529.

119. Ito F, Honnami H, Kawakami H, Kanamura K, Makino K.2008. Preparation and properties of PLGA microspheres con-taining hydrophilic drugs by the SPG (Shirasu porous glass)membrane emulsification technique. Coll Surf B: Biointerf67:2025.

120. Kosvintsev SR, Gasparini G, Holdich RG, Cumming IW,Stillwell MT. 2005. Liquidliquid membrane dispersion in astirred cell with and without controlled shear. Ind Eng ChemRes 44:93239330.

121. Stillwell MT, Holdich RG, Kosvintsev SR, Gasparini G, Cum-ming IW. 2007. Stirred cell membrane emulsification andfactors influencing dispersion drop size and uniformity. IndEng Chem Res 46:965972.

122. Omi S, Kanetaka A, Shimamori Y, Supsakulchai A, Nagai M,Ma GH. 2001. Magnetite (Fe3O4) microcapsules prepared usinga glass membrane and solvent removal. J Macroencapsul18:749765.

123. Omi S, Taguchi T, Nagai M, Ma GH. 1997. Synthesis of100mm uniform porous spheres by SPG emulsification withsubsequent swelling of the droplets. J Appl Polym Sci 63:931942.

124. Omi S, Matsuda A, Imamura K, Nagai M, Ma GH. 1999.Synthesis of monodisperse polymeric microspheres includingpolyimide prepolymer by using SPG emulsification technique.Colloids Surf A 153:373381.

125. Omi S, Katami K, Taguchi T, Kaneko K, Iso M. 1995. Synth-esis of uniform PMMAmicrospheres employing modified SPG(Shirasu Porous Glass) emulsification technique. J ApplPolym Sci 57:10131024.

126. Ma GH, Nagai M, Omi S. 1997. Synthesis of uniform micro-spheres with higher content of 2-hydroxyethyl methacrylateby employing SPG emulsification technique followed by swel-ling process of droplets. J Appl Polym Sci 66:13251341.

127. Ma GH, Nagai M, Omi S. 2001. Study on preparation of mono-dispersed poly(styrene-co-N-dimethylaminoethyl methacry-late) composite microspheres by SPG (Shirasu Porous Glass)emulsification technique. J Appl Polym Sci 79:24082424.

128. Yoshizawa H, Maruta M, Ikeda S, Hatate Y, Kitamura Y.2004. Preparation and pore-size control of hydrophilic mono-dispersed polymer microspheres for size-exclusive separationof biomolecules by the SPG membrane emulsification techni-que. Colloid Polym Sci 282:965971.

129. Ma GH, Omi S, Dimonie VL, Sudol ED, El-Aasser MS. 2002.Study of the preparation and mechanism of formation ofhollow monodisperse polystyrene microspheres by SPG (Shir-asu Porous Glass) emulsification technique. J Appl Polym Sci85:15301543.DOI 10.1002/jps

130. Chu LY, Xie R, Zhu JH, Chen WM, Yamaguchi T, Nakao S.2003. Study of SPG membrane emulsification processes forthe preparation of monodisperse coreshell microcapsules.J Colloid Interf Sci 265:187196.

131. ChengCJ, ChuLY, Zhang J, ZhouMY,XieR. 2008. Preparationofmonodisperse poly(N-isopropylacrylamide) microspheres andmicrocapsules via Shirasu-porous-glass membrane emulsifica-tion. Desalination 234:184194.

132. Wu J, Wei W, Wang LY, Su ZG, Ma GH. 2008. Preparation ofuniform-sized pH-sensitive quaternized chitosanmicrosphereby combining membrane emulsification technique and ther-mal-gelation method. Coll Surf B: Biointerfaces 63:164175.

133. El-Mahdy M, Ibrahim ES, Safwat S, El-Sayed A, Ohshima H,Makino K, Muramatsu N, Kondo T. 1998. Effects of prepara-tion conditions on the monodispersity of albumin micro-spheres. J Microencapsul 15:661673.

134. Muramatsu N, Nakauchi K. 1998. A novel method to preparemonodisperse microparticles. J Microencapsul 15:715723.

135. Morr