LAdvanced Drug Delivery Reviews 45 (2000) 47–56www.elsevier.com/ locate /drugdeliv

Particle control of emulsion by membrane emulsification and itsapplications

*Tadao Nakashima , Masataka Shimizu, Masato Kukizaki

SPG Research Laboratory, Miyazaki Prefectural Industrial Technology Center, 16500-2 Higashi-kaminaka, Sadowara,Miyazaki 880-0303, Japan

Abstract

Particle-size control of emulsion is very important for maintaining stability and giving emulsions new functional roles.Porous glass membrane, prepared by phase separation of a glass composition, is available as an emulsifying element, fromwhich, one can obtain monodispersed emulsion with different particle sizes, and useful water /oil /water (W/O/W) emulsionin very high yield. The authors have called this new technology ‘membrane emulsification’. Applications of membraneemulsification technology to drug delivery systems were carried out under cooperative research with Miyazaki MedicalCollege. It was found that the clinical administration of a W/O/W drug emulsion that encapsulated an anticancer drug in itsinner droplets was surprisingly effective for both terminal and multiple nodules of hepatocellular carcinoma when the drugwas injected to damaged liver through a catheter inserted in the hepatic artery. Other applications have been tried anddeveloped elsewhere. 2000 Elsevier Science B.V. All rights reserved.

Keywords: Particle-size control; Membrane emulsification; Porous glass membrane; Phase separation; Drug delivery systems; W/O/Wemulsion as carrier; Hepatic artery injection method

Contents

1. Introduction ............................................................................................................................................................................ 472. Membrane emulsification ......................................................................................................................................................... 483. Fundamentals.......................................................................................................................................................................... 504. Applications ........................................................................................................................................................................... 51

4.1. Food emulsions................................................................................................................................................................ 514.2. Synthesis of monodispersed microspheres .......................................................................................................................... 524.3. Drug delivery systems ...................................................................................................................................................... 53

5. Conclusion ............................................................................................................................................................................. 55References .................................................................................................................................................................................. 55

1. Introduction

The primary significance of particle design is of*Corresponding author. Tel.: 181-985-74-4311; fax 181-985-course related to emulsion stabilization [1]. The74-4488.

E-mail address: nakasima@iri.pref.miyazaki.jp (T. Nakashima). general tendency of emulsions to break down is due

0169-409X/00/$ – see front matter 2000 Elsevier Science B.V. All rights reserved.PI I : S0169-409X( 00 )00099-5

48 T. Nakashima et al. / Advanced Drug Delivery Reviews 45 (2000) 47 –56

to the strong dependence of emulsion stability on thesurface energy of the dispersed particles. From thispoint of view, micelles and microemulsions are verydifferent from emulsions in that these stand onthermodynamic stability. Secondly, depending on theparticle design, we can easily prepare, not only liquidmonodispersed particles, but also solid monodis-persed microsphere or multiple emulsions, such aswater /oil /water (W/O/W) emulsions. The tertiarysignificance is in contributions to scientific research.Although a number of studies have been carried outon mass transportation and rheology of emulsions,the phenomena were not very easy to explainprecisely because the emulsions used in most studieswere not monodispersed but, rather, were polydis-persed. If it is possible to develop complete particledesign technology, then the desired particle systems

Fig. 2. Schematic of the membrane emulsification apparatus. (a)of particular emulsions can be prepared at any time.Membrane module; (b) pressurizing source; (c) reservoir of oil tobe dispersed; (d) medium water tank; (e) circulation pump; (f)needle valve and (g) pressure gauge.

2. Membrane emulsification

In order to answer this practical requirement, the as uniform a pore size as possible and of highauthors proposed membrane emulsification [2–4] at mechanical strength, is attached to a membranethe annual meeting of the Society of Chemical module (a).Engineers, Japan, in 1988. This method differs from The aqueous liquid that is to be the continuousconventional emulsification methods, of both the phase containing a surfactant is placed into a tank (d)mixing and ultrasonic type, in that it uses a specially and is circulated through that module. Another oilydeveloped microporous membrane as an emulsifying liquid, which is to act as the dispersed phase, iselement placed in the reservoir (c); this is pressurized using

Figs. 1 and 2 show the principles of membrane nitrogen and is directed towards the membraneemulsification and a schematic diagram of the module. When the pressure reaches a specific value,emulsification apparatus, respectively. Initially, a which depends on the pore size of the membrane andmicroporous membrane that has countless pores, of the 0/W interface tension, emulsion particles are

formed on the surface of the membrane and emulsifi-cation truly begins. As a result, the concentration ofthe emulsion in tank (d) may increase with time,although it is zero at the beginning.

Fig. 3a shows a photomicrograph of an 0/Wemulsion obtained by this method, with kerosene asthe dispersed phase and water as the continuousphase.

The water phase contains the surfactant, sodiumdodecyl sulfate (SDS). The particle size of theemulsion was remarkably uniform, compared with

Fig. 1. Principle of membrane emulsification in the case ofthat (b) obtained by a conventional homogenizer.preparing an O/W emulsion. The pressurized oil to be dispersed is

Now microporous membranes must be discussedpassed through controlled pores of membrane and forms countlessoil droplets on the inner surface of the membrane. briefly. Although many porous membranes are exten-

T. Nakashima et al. / Advanced Drug Delivery Reviews 45 (2000) 47 –56 49

separation of calcium aluminoborosilicate glass syn-thesized from a volcanic ash called Shirasu, whichoccurs abundantly in the southern area of KyushuIsland. Its preparation is described briefly in Fig. 4.

Calcium carbonate and boric acid are added torefined Shirasu in a fixed ratio and mixed. Themixture is heated to above 13508C and, after reach-ing glass fusion, this is followed by the formation ofa tube or sheet that is about 1 mm in thickness. Inthe next process, annealing above the glass transitiontemperature, primary glass decomposes into cal-cium–borate-rich glass and aluminosilicate-richglass, according to spinodal decomposition. Finally,if the phase-separated glass is treated with acid,calcium borate is leached out, as it is acid soluble,and a porous glass membrane is obtained. Since thepore size of the membrane directly depends on thesize of the separated microphase, various membraneswith different average pore sizes are obtained bymodifying the annealing temperature and time. Formembrane emulsification, various membranes withpore sizes ranging from below 1 mm to tens of mmare required, thus, in the manufacturing process, theannealing temperature and time are fixed over arange of 700 to 7508C and from several hours to tensof hours, respectively. According to observation of

Fig. 3. Comparison of an O/W emulsion prepared from a such porous glass using a scanning electron micro-kerosene–water sodium dodecyl sulfate (SDS) system by mem-

scope, we can see a characteristic interconnectedbrane emulsification with a corresponding emulsion preparedusing a conventional homogenizer. The scale bar shows 10 mm.(a) Emulsion manufactured by membrane emulsification; (b)emulsion prepared with a conventional homogenizer.

sively applied to advanced separative membranetechnology and, thus, various types of membranesare available, porous glass membranes, using phaseseparation of glass, are the best for providing uni-formity of pore size. The authors have studied suchporous glass and its use since 1978.

With regard to conventional porous glass, porousvycor glass, which Nordberg and his coworkers ofCorning Glass Works America developed in 1930s,is well known around the world [5]. However, thisglass has not been used as a membrane because ofthe limited pore size and its lack of both mechanicalstrength and chemical durability. In 1980, the authors

Fig. 4. Flow chart of the preparation of Shirasu porous glassdeveloped a new porous glass membrane [6–8] that (SPG) membrane. Phase separation occurs during the heat treat-is truly practicable as an emulsifying element as well ment step and the pore size of the membrane is determined by thisas a separate membrane, and is made by the phase process, although real pores are formed by acid leaching.

50 T. Nakashima et al. / Advanced Drug Delivery Reviews 45 (2000) 47 –56

structure comprising very uniformly controlled teristics are indispensable for an emulsifying ele-pores, as shown in Fig. 5. ment.

In addition, the homogeneity of the pore structureof the glass membrane can be proved using amercury penetration porosimeter. In addition, the 3. Fundamentalsglass membrane has a high mechanical strength thatis superior to that of conventional membranes and its There is a close connection between the pore sizesurface can be easily modified with surface chemical of membranes and the particle size of the emulsionreagents. Thermal sterilization and recycling of used when using the membrane emulsification technique.membranes by firing are completed without problem Plotting the average particle size of emulsions as abecause of its thermal stability. All of these charac- function of the average pore size of the membrane

gives a linear relation, as shown in Fig. 6.Particle-size distributions of emulsions correspond

to pore-size distributions of the membrane used and,when a larger pore size of membrane is used, alarger particle size of emulsion is prepared. It isinteresting that such a relation is satisfied not only inO/W but also in W/O emulsions. The surfactant isone of the most important factors that determinessuccess or failure in this technology. With increasingconcentration of the surfactant, below its criticalmicelle concentration in a continuous water phase,during preparation of an O/W emulsion, the permea-tion flux of kerosene increased as a function ofpressure. Because of decreasing surface tension, g,the minimum permeation pressure, P is expected toc

decrease, based on the following equation, andexperimental data support this expectation.

Fig. 5. SEM images of the surface and inner structures of porousglass membrane obtained by a phase separation method. Both Fig. 6. Relation between particle size of the emulsion and thephotographs show the typical interconnected structure of the phase pore size of the membrane used. Blank and solid keys showseparation. The scale bar indicates 5 mm. (a) Surface structure and average particle sizes in preparing a kerosene-in-water emulsion(b) inner structure. and a water-in-kerosene emulsion, respectively.

T. Nakashima et al. / Advanced Drug Delivery Reviews 45 (2000) 47 –56 51

P 5 4g/D cos u positive polar groups of cationic surfactant moleculesc m

onto the negatively charged membrane surface,where D and u are the pore radius of the membrane resulting from silanol group dissociation. Such ad-m

and the contact angle of oil to membrane wall, sorption of CTMABr apparently makes the mem-respectively. When different types of surfactants are brane hydrophobic and highly wettable with disper-used for membrane emulsification, both anionic sion oil. This explanation is strongly supported bysurfactants, such as SDS and sodium dodecylbenzene results obtained in the emulsification process usingsulfate (SDBS) and nonionic surfactant, such as another charge-modified membrane and only SDS asTween 20, lead to successful emulsification, produc- the surfactant. That is to say, when the emulsificationing excellent monodispersions. However, the use of is carried out with a porous glass membrane intro-cationic surfactants, such as cetyltrimethyl ammo- duced a negatively charged sulfonic group or posi-nium bromide (CTMABr) does not lead to the tively charged ammonium group to the surface,formation of monodispersed emulsions, as shown in monodispersed emulsion is successfully obtained inFig. 7 [9]. the former. However in the latter, size-controlled

This results from the electrostatic adsorption of the monodispersions cannot be prepared since SDS isadsorbed on the positively charged membrane.Therefore, special attention should be paid to theselection of the type of surfactant when using themembrane emulsification technique, because themembrane may become highly wettable with thedispersion phase, depending on the type of surfactantthat is used.

The applications of membrane emulsification ex-tend to various fields, such as the cosmetics, plastics,pharmacy and food industries, and some applicationsof this technology are now being commerciallyrealized. Their scale varies from large plants in thefood industry through medium-scale use in thepolymer industry, to the bench scale in laboratories,or to microkits that produce tens of ml of emulsions.

Fig. 8 shows practical membrane modules andmembrane emulsification apparatus.

4. Applications

4.1. Food emulsions

In preparing W/O emulsion in which the continu-ous phase is soybean oil, polyglycerin condensedricinolate is used as the surfactant in the oil phase

Fig. 7. Effect of surfactants in water on the control of oil particle and a small amount of sodium chlorite is added tosize in preparing an O/W emulsion from a kerosene /water (SDS) the water phase. One of the most characteristicsystem. Canon type surfactant is adsorbed on the surface of the factors in preparing a W/O emulsion is the use of anegatively charged glass membrane and results in wetting by oil.

hydrophobic porous glass membrane that has beenThis type of wetting makes size control of the emulsion unsuit-treated previously. Through this treatment, the emul-able. A porous glass membrane with a pore size of 0.52 mm was

used. sion may be kept from being polydispersed by

52 T. Nakashima et al. / Advanced Drug Delivery Reviews 45 (2000) 47 –56

wetting the membrane with water. In order to givethe membrane hydrophobicity, chemical modificationusing a silane coupling agent, such as octadecyltrichlorosilane (ODS) or a surface treatment methodwith a specific resin has been employed.

The ODS method gives the membrane stronghydrophobicity due to chemical bonding, but it canbe a little troublesome. The surface treatment methodis much simpler, but the resulting membrane seemsto be somewhat lacking in durability. Margarine is atypical example of a W/O emulsion. Recently,decreasing the calories in margarine is a strategicdirection among food industries because of marketneeds for healthy food. This may result in a shift tolow-fat spread.

Bearing the need for a reduction in calories inmind, the Moringa Milk Industry, one of the biggestmilk industries in Japan, developed and commercial-ized a very low fat spread using this technology. InFig. 9a, a micrograph of the product is shown. It hassurprising stability, which is guaranteed for at least 6months without the use of preservative, while thevolume fraction of dispersion water phase reaches upto 75%. It is reported that it has a creamy, soft, tastein addition to the low number of calories and istherefore a popular addition to the market.

4.2. Synthesis of monodispersed microspheres[10,11]

Fig. 9b shows an electron scanning micrograph ofpolydivinylbenzene microspheres that were de-veloped as spacer material for liquid crystal displays.The spacers must be made as uniform as possible forhomogeneous and clear display and this aim hasbeen achieved by the use of the membrane emulsifi-cation process. The microspheres are produced bysuspension polymerization of a monodispersed O/Wemulsion that is made of divinylbenzene, with ben-zoyl peroxide and water, dissolving specific surfac-tant through the membrane emulsification at the firststep. Furthermore, monodispersed silica powder, asshown in Fig. 9c, which is used as an HPLC packingmaterial and as a cosmetic foundation, was preparedfrom sodium silicate dissolved in monodispersedFig. 8. SPG membrane modules and emulsification apparatuswater drops in a joint project by Miyazaki Industrialmade by Kiyomoto Iron Works Co., Ltd. (a) Small scale module;

(b) large scale module; (c) bench plant and (d) production plant. Technology Center and Suzuki Yushi Industries.

T. Nakashima et al. / Advanced Drug Delivery Reviews 45 (2000) 47 –56 53

4.3. Drug delivery systems [12,13]

Among the applications of membrane emulsifica-tion, a drug delivery system called DDS is one of themost attractive subjects. The authors have carried outsome projects related to DDS through joint researchwith Dr. Higashi, of Miyazaki Medical College.DDS is used largely for arterial injection chemo-therapy of liver cancer with an emulsion that hasbeen designed and prepared using the membraneemulsification process, to carry the anticancer drug.

The important thing in the design of the emulsionwas to prepare a very stable W/O/W emulsionparticle like Fig. 10 in which countless water drop-lets were encapsulated with an iodized poppy seedoil, called Lipiodol, which has already been used inthe medical field as an oily contrast medium. In thisway, it is possible not only to suppress the strongside effects of the anticancer drug, but also toconcentrate the dosage selectively to focus on thecancer, when a suitably designed emulsion at op-timum conditions is injected directly into the liver.Furthermore, the use of a contrast medium gives theadditional advantage that the disposition of theadministered drug emulsion can easily be observed.

Preparation of this emulsion begins with theproduction of a submicron W/O emulsion compris-ing water as the dispersion phase, and containing theanticancer drug, epirubicin and Lipiodol as thecontinuous phase, with polyoxyethylene(40) hydro-

Fig. 9. Industrial applications of membrane emulsification. (a)Super low fat spread made by Morinaga Milk Industry Co. Ltd.; Fig. 10. Ideal structure of W/O/W drug emulsion particle. (a)(b) polydivinylbenzene microsphere made by Sekisui Chemical Inner water droplet encapsulated anticancer drug, epirubicin; (b)Industries Co. Ltd. for liquid display and (c) monodispersed silica iodized poppy seed oil as a contrast medium with the hydrophobicpowder made by Suzuki Yushi Industries Co. Ltd. for liquid surfactant polyoxyethylene(40) hydrogenated caster oil (HCO-40)chromatography. and (c) outer phase consisting of physiological saline.

54 T. Nakashima et al. / Advanced Drug Delivery Reviews 45 (2000) 47 –56

genated caster oil, as the hydrophobic surfactant. clearly and look like a conglomeration of blackThen, a W/O/W emulsion is made using the W/O points, but it is the result of the W/O particles beingemulsion as the dispersion phase. The attractive uniformly controlled. This fact also correspondscharacteristics of this method of processing can be closely to the result of particle-size distributionsummarized as follows. Such double membrane measurement, as predicted.emulsifications are schematically shown in Fig. 11. Finally, I would like to present recent results ofFirst, a medically safe emulsion can be prepared clinical trials in Miyazaki Medical College Hospitalsince the precise design of the emulsion particle is in relation to the medical treatment for liver cancerpossible. Second, the production yield of W/O/W using a W/O/W emulsion. Fig. 13 shows administra-emulsion is kept at an extremely high level in tion of emulsion drug by hepatic arterial injectioncomparison with that obtained using conventional method in Miyazaki Medical College Hospital. W/O/methods. The photomicrograph shown in Fig. 12 is W drug emulsion encapsulating anticancer drug isof such a W/O/W emulsion. directly administered into the liver using a catheter

Inner water droplets are too small to be observed inserted into the hepatic artery.

Fig. 11. Preparation of a W/O/W emulsion by double membrane emulsification. (a) Preparation of a W/O emulsion as the first stage, usinga hydrophobic membrane and (b) completion of the W/O/W emulsion as the second stage, using a hydrophilic membrane.

T. Nakashima et al. / Advanced Drug Delivery Reviews 45 (2000) 47 –56 55

Fig. 13. Administration of drug emulsion by the hepatic arterialinjection method in Miyazaki Medical College Hospital. W/O/Wdrug emulsion encapsulating anticancer drug is directly adminis-tered into the liver using a catheter inserted into the hepatic artery.

tion of the emulsion, the concentration of a-fetopro-tein was very high, in the hundreds of ng/ml region.

Fig. 12. Photomicrograph and particle-size distributions of W/O/ 5. ConclusionW drug emulsions. (a) Photomicrograph: inner water dropletscontaining drug are observed as countless small dots in each oil

The authors believe that the membrane emulsifica-particle; (b) particle size distributions: left and right distributioncurves show those of inner water droplets, containing drug, and tion process will become a very useful means ofapparent oil particles, respectively. obtaining stable and high quality emulsions, that it

will in the design and development of functionalmicrospheres, as well as contribute to scientificresearch on emulsion engineering. Membrane

From a computed tomogram that was taken soon emulsification has been the subject of worldwideafter injection of a W/O/W emulsion, it is obvious attention among colleagues interested in emulsionthat the administered drug emulsion is selectively processes since it was proposed at the 2nd Interna-deposited only in the cancerous tumor. For 20 days tional Conference on Inorganic Membranes, held inafter the injection, the texture of the cancer rapidly Montpellier, France, in 1991.contracted and its volume decreased to a quarter ofits initial size. On the other hand, the medical effectof the emulsion can be seen by following the

Referencesconcentration of a-fetoprotein in the blood, whichcancer cells specifically produce. After the adminis-

[1] D.J. Shaw, in: Introduction to Colloid and Surface Chemis-tration of the drug emulsion, the concentration of try, Butterworths, London, 1980, p. 234.a-fetoprotein in blood is rapidly lowered and reaches [2] T. Nakashima, M. Shimizu, M. Kukizaki, Key Engineeringnormal minimal levels, whereas before administra- Materials 61/62 (1991) 513–516.

56 T. Nakashima et al. / Advanced Drug Delivery Reviews 45 (2000) 47 –56

[3] T. Nakashima, M. Shimizu, M. Kukizaki, US Patent [10] S. Omi, K. Katami, A. Yamamoto, M. Iso, J. Appl. Polymer5326484 (1994). Sci. 51 (1994) 1–11.

[4] T. Nakashima, M. Shimizu, Kagaku Kogaku Ronbunshu 19 [11] T. Nakashima, M. Kukizaki, M. Shimizu, Y. Nakahara, H.(1993) 984–990. Kageyama, F. Nakahara, M. Mizuuchi, US Patent 5278106

[5] H.D. Hood, M.E. Nordberg, US Patent 2106744 (1934), US (1994).Patent 2215039 (1940). [12] S. Higashi, M. Shimizu, T. Nakashima, K. Iwata, F. Uchi-

[6] T. Nakashima, M. Shimizu, M. Kawano, US Patent 4657875 yama, S. Tamura, T. Setoguchi, Cancer 75 (1995) 1245–(1987). 1254.

[7] T. Nakashima, M. Shimizu, M. Kukizaki, J. Ceram. Soc. [13] S. Higashi, N. Tabata, K. Kondo, Y. Maeda, M. Shimizu, T.Japan 100 (1992) 1411–1415. Nakashima, T. Setoguchi, J. Pharmacol. Exp. Therapeutics

[8] T. Nakashima, M. Shimizu, J. Ceram. Soc. Japan 101 (1993) 289 (1999) 816–819.528–532.

[9] T. Nakashima, M. Shimizu, M. Kukizaki, Kagaku KogakuRonbunshu 19 (1993) 991–997.