Preparation of Uniform-Sized Multiple Emulsions and Micro/ Nano Particulates for Drug Delivery by Membrane Emulsification

Transcription

1 REVIEW Preparation of Uniform-Sized Multiple Emulsions and Micro/ Nano Particulates for Drug Delivery by Membrane Emulsification WEI LIU, 1,2 XIANG-LIANG YANG, 1 W.S. WINSTON HO 2,3 1 College of Life Science and Technology, Huazhong University of Science and Technology, Wuhan , China 2 William G. Lowrie Department of Chemical and Biomolecular Engineering, The Ohio State University, 140 West 19th Avenue, Columbus, Ohio Department of Materials Science and Engineering, The Ohio State University, Columbus, Ohio Received 24 March 2010; revised 20 May 2010; accepted 21 May 2010 Published online 29 June 2010 in Wiley Online Library (wileyonlinelibrary.com). DOI /jps ABSTRACT: Much attention has in recent years been paid to fine applications of drug delivery systems, such as multiple emulsions, micro/nano solid lipid and polymer particles (spheres or capsules). Precise control of particle size and size distribution is especially important in such fine applications. Membrane emulsification can be used to prepare uniform-sized multiple emulsions and micro/nano particulates for drug delivery. It is a promising technique because of the better control 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 review describes the state of the art of membrane emulsification in the preparation of monodisperse multiple emulsions and micro/nano particulates for drug delivery in recent years. The principles, influence of process parameters, advantages and disadvantages, and applications in preparing different types of drug delivery systems are reviewed. It can be concluded that the membrane emulsification technique in preparing emulsion/particulate products for drug delivery will further expand in the near future in conjunction with more basic investigations on this technique. ß 2010 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 100:75 93, 2011 Keywords: membrane emulsification; drug delivery systems; multiple emulsion; micro/nano particulate; uniform size; controlled release INTRODUCTION Drug delivery systems (DDSs) represent one of the most rapidly advancing areas of pharmaceutical science and technology. 1,2 DDSs, such as multiple emulsions, micro/nano solid lipid and polymer particles (spheres or capsules) can entrap drugs or biomolecules into their interior structures and/or absorb drugs or biomolecules onto their exterior surfaces. They are designed for targeting delivery and controlled release of drug into systemic circulation maintaining consistent efficacy and reducing dose of the drug and its related side effects. DDSs have Correspondence to: W.S. Winston Ho (Telephone: ; Fax: ; ho@chbmeng.ohio-state.edu) Journal of Pharmaceutical Sciences, Vol. 100, (2011) ß 2010 Wiley-Liss, Inc. and the American Pharmacists Association already been applied with great success today and still have greater potential for many applications, including anti-tumor therapy, gene therapy, AIDS therapy, and radiotherapy in the delivery of proteins, antibiotics, virostatics, and vaccines as drug carriers to pass the blood-brain barrier. 3 8 The targeting drug delivery and controlled drug release properties of DDSs are closely related to their size and size distribution. DDSs with small size can easily pass through the fine capillary blood vessels and the lymphatic endothelium. 9 They have longer circulation times in the blood, higher binding capability and accumulation at the target sites, and give less inflammatory and immune response from the tissues and cells of the body than those with bigger size. 10,11 The narrow size distribution (monodisperse) gives better control over the dose and release behavior of the encapsulated drug, yields higher drug JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 100, NO. 1, JANUARY

2 76 LIU, YANG, AND WINSTON HO encapsulation efficiency and better biocompatibility with cells and tissues of the body than that of polydisperse DDSs. 9,11,12 However, the preparation of these DDSs with controlled size and size distribution is still a challenge. The conventional techniques for manufacturing multiple emulsions and micro/nano particulates include high pressure homogenization, ultrasonication method, rotor/stator systems (such as stirred vessels, colloid mills, and toothed disc dispersing machines), solvent evaporation, solvent diffusion, coacervation, spray drying and direct polymerization None of these techniques can give a good control over the size and size distribution of abovementioned emulsion and particulate products. Also, the encapsulated biomolecules such as proteins, gene and vaccines are liable to lose their bioactivities under strong mechanical processing and macro organic solvents. 15,16 The membrane emulsification technique received broad attention as a novel tool for manufacturing monodisperse emulsion over the last 20 years. The technique is attractive for the better control of emulsion droplet size and size distribution, the mildness of the process, the low energy consumption, and easy to industrial-scale preparation This review describes the state of the art of the membrane emulsification technique in manufacturing uniform-sized multiple emulsions and micro/nano particulates for drug delivery in recent years. The principles, effect of process parameters, and especially applications in preparing different kinds of drug delivery systems are reviewed. developed, such as cross-flow membrane emulsification, premix membrane emulsification, microchannel emulsification, and other membrane-based methods. Cross-Flow Membrane Emulsification The process and typical apparatus of cross-flow membrane emulsification are shown schematically in Figures 1 and 2, respectively. In cross-flow membrane emulsification, the dispersed phase is pressed through a microporous membrane (micropore diameter is d p ) while the continuous phase flows along the membrane surface. Droplets grow at micropores and detach at a certain size (d d ), which is determined by the balance between the forces acting on the droplet Emulsifiers in the continuous phase stabilize the newly formed interface, to prevent droplet coalescence immediately after formation. 26,30 Some fundamental process parameters, such as membrane parameters, process parameters and phase properties, affect the emulsion droplet size/size distribution and the dispersed phase flux during cross-flow membrane emulsification. These membrane parameters include the type of membrane material, average micropore diameter, porosity/ micropore spacing, micropore geometry, membrane thickness, and wetting property (wall contact angle). SPG membranes are the earliest and most commonly used membranes for the excellent characterization of uniform cylindrical interconnected micropores, a wild range of available mean micropore diameter ( mm), high membrane surface PRINCIPLES OF MEMBRANE EMULSIFICATION Membrane emulsification is a technique that involves using an applied pressure to force a dispersed phase to go through membrane pores into a continuous phase. 17,20 Small droplets are formed at the pore openings on the membrane surface and dispatched by the relative shear motion between the membrane and continuous phase. The resulting droplet size is controlled primarily by the choice of the porous membrane. The membrane emulsification technique was first proposed by Nakashima et al. to prepare monodisperse emulsion using a particular glass membrane called Shirasu Porous Glass (SPG) membrane (SPG Technology, Miyazaki, Japan), and later developed to create a variety of particulate products with novel structures and functionalities. 13,15,24,25 To date, in addition to the SPG membranes, a broad range of other types of microporous membranes, such as polymeric, ceramic, metallic and microengineered devices, have been used. And several types of membrane emulsification are JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 100, NO. 1, JANUARY 2011 Figure 1. Schematic drawing of the cross-flow membrane emulsification process. 26 DOI /jps

3 PREPARATION OF DRUG DELIVERY SYSTEMS BY MEMBRANE EMULSIFICATION 77 coalescence are prevented. With the assumption that the distance between the pores may be equal to the droplet diameter, the porosity e can then be calculated with the following equation: " ¼ 0:25pd2 p d 2 d ¼ 0:25p 1 x 2 (2) Figure 2. Schematic of a typical cross-flow membrane emulsification apparatus. 17 (a) membrane module, (b) pressurizing source, (c) reservoir of dispersed phase, (d) emulsion/continuous phase storage vessel, (e) recirculation pump, (f) needle valve, and (g) pressure gauge. porosity (50 60%), and the possibility of surface modification. 13,17,19 In addition to the SPG membranes, many other types of membranes include ceramic aluminum oxide (a-al 2 O 3 ), 31,32 zirconium oxide, 33 microporous silica glass 34 and stainless steel membranes, 35 and polymer membranes such as polytetrafluoroethylene (PTFE) 36,37 and polycarbonate 38 membranes as well as polypropylene hollow fibers membrane, 39 have been successfully used to prepare micro/nano emulsions and particulate products. Some studies show that there is a linear relationship between the droplet size (d d ) and the average micropore diameter (d p ) 15,26,40 d d ¼ md p (1) The values reported for m are from 3 to 50, depending on the ingredients and type of surfactants used, and the properties of the membrane. For SPG membranes, the range of m is between 2 and 10. For membranes other than SPG, the values of m are higher. Timgren et al. 41 investigated the effects of membrane porosity/micropore spacing in the direction of the cross-flow continuous phase on the size of the droplets using computational fluid dynamics (CFD). The maximum porosity can be estimated by assuming a square array of membrane micropores, while all micropores are active and steric hindrance and Abrahamse et al. 42 calculated the maximum membrane porosity to prevent coalescence of droplets growing on neighboring micropores of 5 mm diameter to be 1.5%. However, a low porosity has the negative effect on obtaining a dispersed phase flux that makes industrial application feasible Microporous geometry has an important effect on the uniform-sized droplet obtained. 46 So far, random, round, square and rectangle pores have been applied in membrane emulsification. When random-shaped/ round pore membranes are used in cross-flow membrane emulsification, a significant external shear force is required to obtain the best control over the droplet size and size distribution. 47 In cross-flow process with a tubular ceramic membrane was employed, the tube Reynolds number was required to be in the range from transient (2300 <Re < 4000) to turbulent region (Re > 4000). 48 Recently, more studies towards the effects of the microporous shape were carried out with microengineered membranes. 49 The membrane thickness (micropore length) plays some important roles in the control of droplet uniform and productivity. 20,48 According to Darcy s law, the dispersed phase flux ( f d ) has the following relationship with the transmembrane pressure (DP) and the membrane thickness (L): f d ¼ KDP ml where K is the membrane permeability, and m is the dispersed phase viscosity. So at a given transmembrane pressure, the membrane thickness is one of the factors to determine the dispersed phase flux. Some studies indicated that the wetting property (wall contact angle) is one of the most important characteristics affecting the droplet size/size distribution ,38,42,50,51 For both detachment mechanisms, the membrane should be wetted with the continuous phase and should not be wetted with the dispersed phase for proper droplet formation and droplet detachment. The wall contact angle measured in the continuous phase should be smaller than 908, that means a hydrophilic membrane for O/W emulsions is wetted with the aqueous phase and a hydrophobic membranes for W/O emulsions is wetted with the oil phase. 19 The wall contact angle of the membrane depends on the dynamics of the emulsifier and may be calculated with CFD. 42 (3) DOI /jps JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 100, NO. 1, JANUARY 2011

4 78 LIU, YANG, AND WINSTON HO The process parameters include transmembrane pressure, cross-flow velocity/wall shear stress and temperature. The transmembrane pressure (DP) is defined as the difference between the pressure of the dispersed phase and the mean pressure of the continuous phase. In membrane emulsification, the transmembrane pressure is a critical operating parameter because it has a great influence on the emulsification results, including the dispersed phase flux through the membrane. 52 According to Eq. (3), the dispersed phase flux ( f d ) increases as the transmembrane pressure increases. The capillary pressure (P c ) is the critical pressure to make the discontinuous phase flow and can be predicted from the Laplace equation, assuming that the micropores are ideal cylinders: 30 P c ¼ 4g cos u d p (4) where g is the O/W interfacial tension, u is the wall contact angle, and d p the is average micropore diameter. The actual transmembrane pressure required to make the discontinuous phase flow may be greater than the capillary pressure due to tortuosities in the micropores, irregular micropore openings at the membrane surface, and the significant effects of surface wettability. 29,48,53 Cross-flow velocity is a fundamental process parameter to determine membrane emulsification characteristics because wall shear stress caused by the continuous phase is a major force to drive the droplets that are departing from the membrane micropores. Studies show that the droplet size becomes smaller as the cross-flow velocity increases, but the droplet size distribution may quickly changed to be broad with further increases in the cross-flow velocity. 30,38,48,50,54 The effect of the cross-flow velocity/wall shear stress on reducing droplet size is dependent on the membrane micropore size, more effective for smaller micropore size. The phase properties that influence the cross-flow membrane emulsification process include the dispersed phase viscosity, continuous phase viscosity, and type of emulsifier/surfactant. The viscosities of dispersed and continuous phase have an important effect on the membrane emulsification results. 52,54,55 According to the Darcy s law (Eq. 3), an increase in the dispersed phase viscosity can result in a decrease in the dispersed phase flux through a porous membrane, which leads to a larger droplet size and broader size distribution. The continuous phase viscosity influences the diffusion of the surfactant molecules and thus reduces the rate of the oil water interfacial tension. 56 During the droplet formation process, the surfactant molecules adsorb to the newly formed oil water JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 100, NO. 1, JANUARY 2011 interface to reduce the interfacial tension and consequently to facilitate droplet formation. A great deal of research work indicates that the interfacial tension is one of the key factors to control the droplet formation during the membrane emulsification process. 38,51 54,56 Emulsifiers/surfactants reduce the interfacial tension between oil and water phases, facilitate droplet distribution, and decrease the transmembrane pressure (see Eq. 4). The type and concentration of emulsifier/surfactant greatly influences the adsorption kinetics and thus the dynamic interfacial tension. 52 The other important role of emulsifier/surfactant is to stabilize the droplet against coalescence and/or aggregation, which greatly influence the droplet size/size distribution. 54,57 Premix Membrane Emulsification The conventional cross-flow membrane emulsification process is known as direct membrane emulsification (DME). There are some potential disadvantages with this technique: 19,25 (i) the relatively low maximum dispersed phase flux (typically m 3 / (m 2 h)) that leads to low productivity; (ii) it is difficult to prepare uniform emulsion droplets when the dispersed phase has high viscosity; and (iii) uniform emulsion can only be prepared using a microporous membrane with very uniform pores. Because of these restricted conditions, there have been some limitations in choosing the dispersed phase, the continuous phase, and the membrane to obtain the desired emulsification products. The DME technique is more suitable for preparing relatively diluted emulsions with disperse phase contents up to 30 vol% for its low productivity 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 and PME. In the process of PME, a preliminarily emulsified coarse emulsion (rather than a single dispersed phase in DME) is passed through a microporous membrane. The coarse emulsion can be achieved by mixing the two immiscible phases (oil and aqueous phases) together using a conventional stirrer mixer. 58,59 From Figure 3, we can see there are two cases in the process of PME, PME without phase inversion and PME with phase inversion. If the membrane is wetted by the dispersed phase of coarse emulsion, for example, hydrophobic membrane wetted by oil phase, and suitable surfactants are dissolved in both phases. The PME 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 advantages over DME: (i) the optimal flux with regard to droplet uniformity is much higher than DOI /jps

5 PREPARATION OF DRUG DELIVERY SYSTEMS BY MEMBRANE EMULSIFICATION 79 Figure 3. Schematic diagrams of DME and PME processes m 3 /(m 2 h); (ii) the average droplet size is smaller with the same membrane and phase compositions; (iii) the experimental set-up in PME is generally simpler than that in DME; and (iv) the PME process parameters in PME are easier to control than those in DME. The driving pressure and emulsifier properties are not critical in the PME operation as in the DME process. 40 However, there are some disadvantages of PME such as a higher polydispersity of emulsion droplets compared with that prepared by DME. In order to combine the advantages of the both techniques, a multi-stage PME or repeated membrane homogenization method was developed. 56,65 In the novel membrane process, the coarse emulsion is repeatedly forced through the same microporous membrane a number of times to achieve fine and uniform-sized emulsion droplets. 66,67 Microchannel Emulsification Microchannel emulsification is the new development in the field of membrane emulsification with the application of micromachining technology. Some new structure of membranes (microfluidic devices) with precisely designed geometry of pores and noncylindrical microchannels were manufactured using micromachining technology. 32,68 71 A schematic representation of the droplet formation process in microchannel emulsification is shown in Figure 4. In the process of terrace-based microchannel emulsification, emulsion droplets are produced by forcing the dispersed phase through the microchannels. The interfacial tension (s ow ) is the driving force in the droplet formation process that is divided into inflation and detachment processes. 72,73 The dispersed phase is forced into a disk-like shape on the terrace in the membrane. This elongated shape has a higher interfacial area with at least one radius of curvature smaller than a spherical shape in the well with radius R d, resulting in different Laplace pressures, that is DP 1 on the terrace and DP 2 in the well. 19,68 DP 1 and DP 2 can be calculated as follows: 1 DP 1 ¼ s ow þ 1 R 1 R 2 s ow R 1 (5) DP 2 ¼ 2 s ow (6) R d From Eqs. (5) and (6), when DP 1 > DP 2, R 1 is less than R d /2. Droplet can spontaneously transform into Figure 4. A Schematic representation of the droplet formation process in microchannel emulsification. 68 DOI /jps JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 100, NO. 1, JANUARY 2011

6 80 LIU, YANG, AND WINSTON HO a spherical shape with a more favorable thermodynamic property and detachment occurs. From the process of microchannel emulsification, we can see that the emulsion droplet size is only determined by the geometry of microchannel when s ow is regarded as a constant, and no flow of the continuous phase is needed during the emulsion droplets are formed. These make the microchannel emulsification technique attractive for producing monodisperse emulsions. The main disadvantage of microchannel emulsification for practical applications is its inherent low productivity. The dispersed phase flux is less than 0.01 m 3 /(m 2 h) for a microchannel plate of 1 cm 2. Recently, some novel microchannel emulsification techniques such as larger microchannel plates, 74,75 straight-through microchannels, and multiple microchannel plates have been investigated to increase the production rate. To date, considerable amounts of work about the mechanisms of membrane emulsification have been done, both experimentally and computationally. In order to better control size distribution and obtain uniform droplets at higher productivity, novel membrane emulsification techniques based on the conventional methods have been developed, such as vibrating membrane emulsification, 79,80 rotating membrane emulsification, 47,49,81 83 and stirred cell membrane emulsification. 84,85 The optimized process conditions and the effects of various parameters of these new techniques need to be investigated in more detail. PREPARATION OF DRUG DELIVERY SYSTEM USING MEMBRANE EMULSIFICATION Membrane emulsification has shown promising applications in various fields, such as food industry, pharmaceutical industry, cosmetic industry, chemical industry, and other fields like agriculture and environment protection. Among these applications, preparation of DDSs is one of the most attractive subjects. 17 The membrane emulsification technique can be directly utilized to prepare monodisperse multiple emulsions for drug delivery. Also, this technique can afford the preparation of a variety of structure of uniform-sized particulate products by means of sequential secondary processes/reactions after primary emulsification, such as solidification, crystallization, freeze-drying, evaporation, droplet swelling, gelation, polymerization, etc. 13,15,24,25 Multiple Emulsions Single and multiple emulsion productions by a direct method are the most investigated systems for membrane emulsification applications. However, single JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 100, NO. 1, JANUARY 2011 emulsion is not a very suitable system for drug delivery since W/O emulsion with high viscosity is difficult to be perfused into arteries or capillaries via catheters, and O/W emulsion cannot encapsulate most anticancer drugs that are water-soluble. 86 Water-in-Oil-in-Water (W/O/W) Emulsions Higashi et al prepared W/O/W emulsions by membrane emulsification as a new drug carrier for the treatment of liver cancer by arterial injection chemotherapy. The W/O/W emulsion was prepared in two steps. First, the submicron-sized W/O emulsion was prepared using a conventional rotor-stator emulsification device. The aqueous phase contained an anticancer drug (epirubicin or carboplation), and the oil phase was made up of an iodized poppy-seed oil (Lipiodol) with polyoxyethylene (40) hydrogenated caster oil being used as the hydrophobic surfactant. Second, the W/O emulsion was passed through a hydrophilic SPG membrane into a glucose solution to obtain the multiple drug emulsion. The W/O/W emulsions with droplets sizes in a range of 1 70 mm can be prepared by using different SPG membranes of an appropriate micropore size. The stability experiment showed that separation or breakdown of the multiple drug emulsions did not occur for at least 40 days. Clinical studies showed that the multiple drug emulsions were effective in contracting the liver cancer texture when injected directly into the liver via the hepatic artery. 90 The studies also indicated that the emulsion droplet size influenced the anti-tumor effect of the therapy: the emulsions with proper size could accumulate in the small vessels in the tumor, while very small droplets passed through the tumor and very large droplets did not reach the tumor. The results concluded that the membrane emulsification technique makes it possible to prepare emulsions for controlled drug release with the precise design of droplet 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 kpa through the SPG membranes with a mean pore size of 5.4, 7.6, 10.7, 14.8, and 20.3 mm. The studies showed that the ratio of the mean droplet size to the mean pore size after five extrusions decreased from 1.25 to 0.68 as the pore size increased from 5.4 to 20.3 mm at the wall shear stress of continuous phase in the pores of 200 Pa. And at low continuous phase viscosity, uniform droplets with the coefficient of variation values of were produced at very high transmembrane fluxes exceeding 200 m 3 /(m 2 h). The encapsulation efficiency of a model drug (CaNa 2 - EDTA) determined was 83 85% in the emulsion products containing 30 vol% of inner droplets and vol% of outer drops. The results confirmed a repeated SPG membrane homogenization for con- DOI /jps

7 PREPARATION OF DRUG DELIVERY SYSTEMS BY MEMBRANE EMULSIFICATION 81 trolled production of multiple emulsions with high encapsulation efficiency at high production rates. W/O/W multiple emulsions can also be manufactured with the microchannel emulsification technique. 69 The W/O emulsions were prepared by homogenizing a mixture of water and oil phases using a conventional homogenizer, then penetrated through the microchannel device into a second aqueous phase containing a suitable emulsifier for oil phase stabilization. Tetraglycerin polyricinolate at 5 wt% dissolved in medium chain triglycerides (decane or ethyl oleate) was selected as the oil phase. A high entrapment efficiency (91%) was achieved under the low shear stress conditions of microchannel emulsification. Solid-in-Oil-in-Water (S/O/W) Emulsions Multiple S/O/W emulsion for oral administration of insulin was produced by Toorisaka et al. 92 with SPG membrane emulsification. First, surfactant-coated insulin was achieved by mixing an aqueous solution of insulin with a hexane solution containing a lipophilic surfactant (sugar ester ER-290), followed by freezedrying. Then, surfactant-coated insulin was dispersed into soybean oil using an ultrasonication method to obtain S/O emulsion. The S/O emulsion was mixed with aqueous solution containing a hydrophilic surfactant (sugar ester L-1695), sodium cholate and D-glucose to prepare a coarse S/O/W dispersion using a rotor-stator homogenizer. Finally, the preliminary emulsified S/O/W dispersion was forced through an SPG membrane with a mean pore size of 1.1 mm several times (three or more). The monodisperse insulin multiple emulsions showed hypoglycemic activity for a long period after oral administration to rats. The authors indicated that the S/O/W emulsions with transforming insulin into a lipophilic complex and uniform droplets had good potential in the treatment of diabetes. Kukizaki 64 prepared a hydrophilic drug, vitamin B 12 (VB 12 ) multiple S/O/W emulsion by premix membrane emulsification. S/O dispersion was obtained by water removal of the water droplets containing 1.1 wt% VB 12 in a W/O emulsion, followed by mixing with an external water phase at 608C to form a coarse S/O/W dispersion. By forcing the resultant S/O/W dispersion through a SPG membrane with a mean pore diameter of 14.8 mm under a transmembrane pressure of 25 kpa, uniformly sized S/O/W droplets were formed at a very high transmembrane flux of 11.8 m 3 /(m 2 h). Eventually, solid lipid microcapsules for drug delivery with a narrow particle size distribution and high hydrophilic drug entrapment efficiency can be achieved by subsequent solidification of the S/O/W droplets. Ethanol-in-Oil-in-Water (E/O/W) Emulsions Nakajima et al. 93 presented novel E/O/W multiple emulsions that were prepared with membrane emulsification. The E/O/W emulsions are suitable to encapsulate some ingredients, such as polyphenols, taxol, androstenedione, and validamycin, which have low solubility with respect to water and oil but are soluble in ethanol. The ingredient is first dissolved in ethanol at the concentration of 20 30% and then the ethanol solution is dispersed into an oil phase using a conventional homogenizer. The E/O emulsion can be further dispersed into an aqueous phase to produce E/ O/W multiple emulsions with membrane or microchannel emulsification. These kinds of multiple emulsions have a wide range of applications as emulsion delivery systems in drugs and functional food and cosmetics. Solid Lipid Microspheres and Microcapsules Solid lipid microspheres (SLMSs) and microcapsules (SLMCs) are interesting particulate carriers for controlled drug delivery. They have several advantages of lower toxicity, better biocompatibility, higher bioavailability, higher drug encapsulation, and longtime storage than emulsions and traditional colloidal carriers The drugs encapsulated in SLMSs and SLMCs are released mainly due to the gradual degradation of the solid lipid by lipase for oral administration. 97 The drug release properties of SLMSs and SLMCs for oral drug delivery are closely related to their size and size distribution. So it is very important to control the size and size distribution of SLMSs and SLMCs. Solid Lipid Microspheres Sugiura et al. 98 prepared SLMSs using temperaturecontrolled microchannel emulsification and subsequent solidification of the dispersed oil phase (a high melting point edible oil). In the microchannel emulsification process, triglyceride (tripalmitin) or hydrogenated fish oil with the melting point of 588C penetrated through the microchannel device into an aqueous solution of emulsifier at 708C. Monodisperse O/W emulsion with a mean droplet size of 21.7 mm and a coefficient of variation value of smaller than 4.6% was obtained. Uniform SLMSs with approximately the same size as emulsion droplets were obtained by cooling and freeze-drying of the O/W emulsion. Solid Lipid Macrocapsules Kukizaki and Goto 95 produced uniform-sized SLMCs by a two-step membrane emulsification technique using tubular SPG membranes with a mean pore diameter of mm. Triglyceride was used as the high-melting point solid lipid material and DOI /jps JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 100, NO. 1, JANUARY 2011

8 82 LIU, YANG, AND WINSTON HO Figure 5. Flow chart of the preparation of SLMCs using two-step SPG membrane emulsification. 95 The temperature was kept at (a) 343 K and (b) 293 K. vitamin B 12 as a model drug in the study. The flow chart of the preparation of SLMCs using the twostep membrane emulsification process is shown in Figure 5. W/O emulsions were prepared in the first membrane emulsification process using a hydrophobic SPG membrane. At the second membrane emulsification step, W/O emulsion was dispersed through a hydrophilic SPG membrane into the outer aqueous phase. The temperature of the whole membrane emulsification process was kept at a higher temperature (343 K) than the melting point of the oil phase (triglyceride). W/O/W emulsions with a narrow droplet size distribution were achieved through the temperature-controlled membrane emulsification step. The resultant W/O/W emulsions were immediately cooled to solidify the oil phase, and were then filtered. Uniformly sized SLMCs with mean size mm and high encapsulation yields of vitamin B 12 of % (w/w) were obtained. The stability experiment showed that no leakage of vitamin B 12 from the SLMCs was observed at body temperature (310 K) over a period of 10 days when the SLMCs were redispersed into normal saline. Kukizaki 64 presented a novel method for preparation of hydrophilic drug-encapsulated SLMCs with a narrow particle size distribution via S/O/W dispersion by premix membrane emulsification, which can produce SLMCs at higher production rates and reduce the amount of water contained in the SLMCs. The microcosmic structures of SLMCs prepared from W/ O/W emulsion and S/O/W dispersion are shown in Figure 6. As shown in Figure 6a, SLMCs prepared from W/O/ W emulsion contain small aqueous droplets within larger solid lipid particles and hydrophilic drugs are dissolved in the aqueous droplets. Figure 6a illustrates that SLMCs prepared from S/O/W dispersion have a matrix type structure with nano-order particles of hydrophilic drugs embedded in the capsule. The properties of the S/O/W dispersion and hence SLMCs were affected by the homogenization process of S/O/W dispersions by premix membrane emulsification. The particle size of SLMCs and the transmembrane flux of the S/O/W dispersion were controlled by adjusting the membrane pore size and transmembrane pressure. The author concluded that micro-encapsulation of hydrophilic drugs into solid lipids by this novel method may have great potential as drug carriers. Polymer Microspheres and Microcapsules Biodegradable polymer microspheres and microcapsules with uniform size have good potential as carriers in drug delivery because of better reproducibility, more repeatable controlled release behavior, Figure 6. Schematic illustrations of the drug-encapsulated SLMCs prepared from (a) W/O/W emulsion and (b) S/O/W dispersion. 64 JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 100, NO. 1, JANUARY 2011 DOI /jps

9 PREPARATION OF DRUG DELIVERY SYSTEMS BY MEMBRANE EMULSIFICATION 83 higher bioavailability, targeted delivery, and functionality In recent years, uniform-sized polymer microspheres and microcapsules for drug delivery with different characteristics, hydrophilic and hydrophobic, smooth and rough, solid and hollow, porous and uniform, and with different surface charge, morphologies and diameters raging from 1 to 100 mm, were successfully produced by combining the membrane emulsification technique and subsequent polymerization process. Membrane Emulsification Direct Polymerization In this method, the dispersed phase containing monomer, solvent, crosslinker and initiator, as well as water-insoluble oligomer, were permeated through membrane micropores into the aqueous solution of emulsifier and stabilizing agent to form uniform monomer emulsion droplets (O/W or W/O/W emulsion). The suspension polymerization is then carried out by transferring the emulsion into a reactor and heating it to above the decomposition temperature of the initiator under mild agitation and nitrogen atmosphere. The initial narrow size distribution of monomer emulsion droplets can be retained throughout the polymerization process. The studies showed that the uniform-sized chitosan microspheres have potential applications in oral and other mucosal administration of protein and peptide drug because they show repeatable release behavior and excellent mucoadhesive and permeation enhancing effect across the biological surfaces The chitosan microspheres were prepared by the membrane emulsification-direct polymerization technique. Chitosan was dissolved in 1.0 wt% aqueous acetic acid containing 0.9 wt% sodium chloride, which was used as an aqueous phase. A mixture of liquid paraffin and petroleum ether 7:5 (v/v) containing emulsifier was used as an oil phase. 101 The aqueous phase was permeated through the uniform pores of a SPG membrane into the oil phase by the pressure of nitrogen gas to form W/O emulsion. Uniform chitosan microspheres were achieved by direct polymerization of the chitosan droplets with glutaraldehyde saturated toluene as the crosslinking agent. Wei et al. 104,105 prepared monodisperse chitosan microspheres with different structural and auto-fluorescent properties for biological tracing and protein drug delivery by SPG membrane emulsification combined with different polymerization systems. Four different types of uniform chitosan microspheres were prepared using different crosslinkers and monomers derived from chitosan. In addition to uniformity, the SPG membrane technique also enables the preparation of chitosan microspheres with a specific particle size by the appropriate choice of the membrane micropore size. The tunability of chitosan microsphere structural properties such as surface charge, cavity size, and wall porosity enables the modification of these systems to cater to specific requirements for use as protein drug carriers. 105 Monodisperse poly(glycidyl methacrylate-divinylbenzene) (P(GMA-DVB)) and poly(glycidyl methacrylateethyleneglycol dimethacrylate) (P(GMA-EGDMA)) porous microspheres were also prepared by the membrane emulsification-direct polymerization technique. 106 This kind of PGMA microspheres covered with many reactive epoxy groups is easily derived to multifunctional materials and has good potential in the protein separation and enzyme immobilization. Nagashima et al. 107,108 produced poly(acrylamideco-acrylic acid) hydrogel microspheres by direct polymerization of monomer droplets in a W/O emulsion after membrane emulsification. An aqueous mixture of monomers was dispersed into the oil phase to form monodisperse W/O emulsion using a hydrophobic SPG membrane treated with octadecyltrichlorosilane and trimethylchlorosilane. Uniformsized hydrogel microspheres were obtained by the polymerization of W/O emulsion at 708C. The hydrogel microspheres suitable for use in swelling-controlled drug delivery systems can absorb water or other body fluids and swell when placed in the body. The swelling increases the aqueous content within the formulation as well as the polymer mesh size, enabling the drug to diffuse through the swollen network into the external biological environment. Membrane Emulsification Solvent Evaporation The solvent evaporation method is one of the most general and simple preparation techniques of polymer microspheres for drug delivery. 109 However, polydisperse emulsion droplets are obtained and polymer microspheres with a relatively wide size distribution are prepared due to the fusion or coagulation of emulsion droplets under mechanical stirring of emulsion droplets. Recently, the membrane emulsification technique has been combined with the solvent evaporation method for the preparation of polymer microspheres After the uniform droplets were prepared with membrane emulsification, the volatile solvents such as dichloromethane, chloroform, acetonitrile, toluene, etc. were evaporated, and the polymer solidified to form microspheres containing the drug in its matrix. Ma et al. 110 prepared uniform biodegradable poly (lactide) microspheres, using a mixture composed of poly(lactide) (PLA) polymer, dichloromethane solvent, and lauryl alcohol cosurfactant as the oil phase and an aqueous phase containing a dispersion stabilizer poly(vinyl alcohol) (PVA) as a continuous phase. After SPG membrane emulsification, dichloromethane was evaporated at room temperature for 24 h to obtain PLA microspheres. DOI /jps JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 100, NO. 1, JANUARY 2011

10 84 LIU, YANG, AND WINSTON HO Ito et al prepared monodisperse rifampicinloaded poly(lactide-co-glycolide) (RFP-PLGA) microspheres by the membrane emulsification-solvent evaporation method. RFP-PLGA microspheres with different sizes were prepared by changing the micropore sizes of the SPG membranes and the drug release kinetics were measured in ph 7.4 PBS at 378C. Effect of polyethylene glycol (PEG) added to PVA solution (continuous phase) as a stabilizer upon the monodispersity of microspheres was studied. 113 SEM photographs of RFP-PLGA microspheres modified with PEG20000 are shown in Figure 7. The results indicated that RFP-PLGA microspheres modified with PEG20000 were apparently more uniform than those prepared without PEG. The yield of RFP-PLGA microspheres was 100%. The initial burst observed in the release of RFP from RFP- PLGA microspheres was suppressed by the addition of PEG. The effects of the types and the ratios of various organic solvents to dissolve PLGA were also studied. 114 The authors reported that the particle size and drug loading efficiency of drug-loaded PLGA microspheres dependent on the types of solvents used due to the interfacial tension between the organic solvent and aqueous phase. The organic solvents with high interfacial tension used for the preparation of PLGA microspheres were found to be suitable in terms of improvement on the properties of drug delivery formulations. Membrane Emulsification Multiple Emulsion-Solvent Evaporation The membrane emulsification-solvent evaporation method is suitable for preparation of hydrophobic drugs loaded polymer microspheres due to hydrophobic drugs are easily dissolved together with polymeric material in the organic solvent. However, hydrophilic drugs would be poorly encapsulated with this method. So the membrane emulsification-multiple emulsion-solvent evaporation method was developed. In this method, a hydrophilic drug is dissolved in an aqueous phase and dispersed in polymer and organic solvent to form W/O emulsions. The W/O emulsions is then dispersed again into a second aqueous phase to obtain W/O/W multiple emulsions. With this method, both hydrophobic and hydrophilic drugs can be successfully encapsulated. 100,115 Uniform-sized biodegradable PLA microcapsules with lysozyme and PLA/PLGA microcapsules containing recombinant human insulin (rhi) were successfully prepared by combining the SPG membrane emulsification technique and the multiple emulsion-solvent evaporation method. 100,116 For preparing lysozyme-loaded PLA microcapsules, an aqueous phase containing lysozyme was used as the internal aqueous phase, and PLA and Arlacel 83 were dissolved in a mixture solvent of dichloromethane and toluene which was used as the oil phase. 100 These two solutions were emulsified by a homogenizer to form a W/O primary emulsion. The W/O emulsion was pushed through the uniform micropores (5.25 mm) of SPG membrane into the external water phase by the pressure of nitrogen gas to form the uniform W/O/W droplets. The lysozyme loaded PLA microcapsules were obtained by simply evaporating the solvent. For preparing rhi-loaded PLA/PLGA microcapsules, an aqueous phase containing rhi was used as the inner aqueous phase, and PLA/PLGA and Arlacel 83 were dissolved in a mixture solvent of dichloromethane and Figure 7. SEM photographs of RFP-PLGA microspheres modified with PEG Membranes with a pore size of (a) 1.00 mm, (b) 1.50 mm, (c) 1.95 mm, (d) 2.60 mm, and (e) 3.63 mm were used. JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 100, NO. 1, JANUARY 2011 DOI /jps

11 PREPARATION OF DRUG DELIVERY SYSTEMS BY MEMBRANE EMULSIFICATION 85 toluene, which was used as the oil phase. 116 The two solutions were emulsified by a homogenizer to form a W/O primary emulsion, and then the emulsion was forced through a SPG membrane into an outer aqueous phase to form the uniform W/O/W droplets. The drug-loaded microcapsules were then achieved by evaporating solvent from droplets. Ito et al synthesized monodisperse PLGA microspheres containing the hydrophilic model drug, blue dextran (BLD), by the solvent evaporation method and the SPG membrane emulsification technique. In order to prepare PLGA microspheres with a higher drug loading efficiency, the tests of stability and productivity of the primary W/O emulsion were preliminarily examined by changing species or concentration of the oil-soluble surfactant and the ratio of water and organic solvent. BLD- PLGA microspheres with various sizes between 2.0 and 10.0 mm were prepared by variation of average micropore diameters of the SPG membranes. The study indicated that the yield, monodispersity, drug loading efficiency, and drug release rate of the BLD- PLGA microspheres prepared by addition of PEG as codispersant into the outer aqueous phase were better than those of microspheres prepared without an additive. For subcutaneous drug delivery, biocompatible microparticles with diameters in a range of mm are required. These are of sufficient size to contain a reasonable amounts of active ingredients, but not too big as to cause discomfort in administration. Gasparini et al. 115 reported a novel membrane emulsification apparatus, a stirred dispersion cell and micropore array membrane, combined with the multiple emulsion-solvent evaporation method, to prepare uniformly sized PLGA microspheres for subcutaneous controlled drug release. In the dispersion cell, the discontinuous phase is injected at the base of the cell, where it passes through the membrane, and the droplets emerge into the continuous phase. The continuous phase is agitated by a simple two-bladed paddle controlled by a DC motor. The membrane is a thin flat metal disc with monosized circular pores distributed in a highly regular array and is chemically treated in order to make the surface hydrophilic. The dispersion cell provides the ability to control the droplet size and size distribution by changing operating conditions and the chemical properties of the phases. 120,121 The authors think that the membrane type used in this study is ideal for the production of water-soluble drug loaded PLGA microspheres for subcutaneous drug delivery as it is possible to produce microparticles in the size range required and with encapsulation efficiencies as high as 100%. Polymer microcapsules containing inorganic substances have good potential applications in targeted drug delivery by an external magnetic field and bioseparation. Uniformly sized polymer microcapsules containing inorganic magnetite can be prepared by combining the SPG membrane emulsification and evaporation method in the multiple emulsions. Omi et al. 122 encapsulated Fe 3 O 4 magnetite in poly(styrene-co-acrylic acid) (PS-AA) copolymers. A solution of PS-AA copolymers in toluene was used as the oil phase, and the wt% magnetic fluid was dispersed in the oil phase. An aqueous phase containing dissolved PVA and sodium lauryl sulfate (SLS) was used as a continuous phase. After membrane emulsification, toluene solvent was evaporated under reduced pressure at C, and PS-AA microcapsules entrapping wt% Fe 3 O 4 were obtained. Membrane Emulsification Droplet Swelling For hydrophilic monomers (such as acrylate monomer), it is different to obtain polymer macrospheres with a narrow droplet size distribution using the membrane emulsification-direct polymerization technique due to the strong hydrophilic property of the SPG membrane surface. So SPG membrane emulsification was combined with the droplet swelling method to produce uniform polymer macrospheres from hydrophilic monomers. Figure 8 illustrates a schematic diagram of the droplet swelling method for the manufacture of uniform polymer microspheres. The seed emulsion containing uniform droplets was prepared by SPG membrane emulsification while the secondary small emulsion composed of hydrophilic monomers was obtained by conventional homogenization. Then, the two emulsions were mixed. The Figure 8. Schematic diagram of droplet swelling method. 15 DOI /jps JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 100, NO. 1, JANUARY 2011

12 86 LIU, YANG, AND WINSTON HO adsorption of the hydrophilic monomers from the secondary emulsion on the seed emulsion droplets occurred due to the degradative diffusion mechanism. 25 The uniform size of the seed droplets led to the uniformity of adsorbing the hydrophilic monomers, resulting in monodispersity of the swollen droplets even though their volume was increased by many times due to swelling. Uniform-sized polymer macrospheres were obtained after a subsequent polymerization process. The hydrophilic poly(styrene-co-divinylbenzene), 123 polystyrene polyimide, 124 poly(methylmethacrylate), 125 composite poly(styrene-co-methylmethacrylate) with a high content of 2-hydroxyethyl methacrylate, 126 poly (styrene-co-n-dimethylaminoethyl methacrylate), 127 poly (styrene-co-pegmma) and PEGPMMA monodisperse microspheres, 128 and poly(methylmethacrylate-co-2-hydroxyethyl methacrylate) monodisperse hollow microspheres 129 were produced with the membrane emulsification-droplet swelling technique. Membrane Emulsification Combined With Other Methods Chu et al. 130 prepared monodisperse environmental stimulus-responsive controlled-release core-shell microcapsules using SPG membrane emulsification combined with interfacial polymerization from O/W emulsions. The dispersion phase, organic solvent containing a certain amount of monomer (terephthaloyl dichloride, TDC), was stored in a pressure-tight vessel and allowed to pass through the SPG membrane into the continuous phase under a certain pressure. The continuous phase, an aqueous solution containing an emulsifier and a stabilizer, was forced to pass through the SPG membrane surface by magnetic stirring. After emulsification, another monomer (ethylene diamine, EDA), was added to the O/W emulsion dispersion in a stirred reactor, to start the interfacial polycondensation reaction between the two monomers at the O/W interface. The core-shell microcapsules with porous membranes and lineargrafted functional polymeric chains in the pores, acting as stimulus-responsive gates, were obtained after isolation by gravity precipitation and washing with deionized water. Monodisperse poly(n-isopropylacrylamide) (PNI- PAM) microspheres and hollow microcapsules were produced by employing monomer-containing W/O emulsion droplets obtained from membrane emulsification as the polymerization templates and subsequent UV-induced free radical polymerization. 131 When aqueous-soluble initiator ammonium persulfate (APS) was added in the dispersed aqueous phase, monodisperse PNIPAM microspheres were obtained after UV irradiation polymerization. On the other hand, monodisperse hollow PNIPAM microcapsules JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 100, NO. 1, JANUARY 2011 were synthesized while the oil-soluble initiator 2,2- dimethoxy-2-phenylacetophenone (BDK) was introduced into the continuous oil phase. The authors indicated that the PNIPAM microspheres and hollow PNIPAM microcapsules with good monodispersity presented good potential applications for temperature stimuli-responsive controlled drug release. Uniform-sized ph-sensitive quaternized chitosan microspheres were prepared by combining the SPG membrane emulsification technique and a novel thermal-gelation method. 132 The mixture of quaternized chitosan solution and glycerophosphate was used as aqueous phase and dispersed in an oil phase to form uniform W/O emulsion with membrane emulsification. The droplets solidified into microspheres at 378C by the thermal-gelation method. The microspheres obtained had porous structure and showed apparent ph-sensitivity. Bovine serum albumin (BSA) as a model drug was encapsulated in the microspheres, and it was released rapidly in an acid solution and slowly in a neutral medium. The novel quaternized chitosan microspheres with ph-sensitivity can be utilized as a drug carrier (such as tumor-targeted drug carrier) targeted to organs with different ph values. From the discussion presented above, premix membrane emulsification would be a promising technique due to its high transmembrane flux and easily controllable process parameters. More recently, premix membrane emulsification combined with other techniques, such as emulsion-solvent extraction and nonsolvent-freeze drying methods, was developed. Uniform-sized amphiphilic poly(ethylene glycol-colactide) (PELA) microspheres with high encapsulation efficiency of antigen were prepared with a novel method combining emulsion-solvent extraction and premix membrane emulsification. 61 In this method, the preparation of coarse double emulsions was followed by additional premix membrane emulsification with proper pressure, and protein-loaded microspheres were obtained by further solidification. Also, narrowly-dispersed polylactide hollow microcapsules, with sizes mm that can be used as ultrasound contrast agents, were successfully prepared by premix membrane emulsification of a polylactide/ dichloromethane/dodecane solution in alcohol water mixtures. 63 Albumin microspheres have found many applications in the drug delivery and medical diagnosis in recent years. Monodisperse albumin microspheres were prepared by membrane emulsification combined with a heat-denaturing method. 133,134 A monodisperse W/O emulsion was first prepared by passing albumin solution through a hydrophobic SPG membrane into an oil phase. Then, albumin microspheres were prepared after heating the W/O emulsion. The result from the experiment showed that the shape DOI /jps

13 PREPARATION OF DRUG DELIVERY SYSTEMS BY MEMBRANE EMULSIFICATION 87 and size of the albumin microspheres strongly depended on the concentration of the albumin solution and the heat-denaturing temperature. Nanoparticles Over the recent years, nanoparticles (nanospheres and nanocapsules) have provided huge advantages regarding drug delivery and release and emerged with their additional potential to combine diagnosis and therapy as one of the major tools in nanomedicine. 135,136 Many studies have shown that nanoparticles with small size and narrow size distribution are of great importance in pharmaceutical applications Presently, the membrane emulsification technique with a moderate condition has been applied successfully to prepare nanoparticles for drug delivery and controlled release. Solid lipid nanoparticles (SLNs) composed of physiological solid lipids represent a second generation of colloidal drug carriers due to the advantages of controlled release, long-term stability, well biocompatibility and prevention of loaded drugs from degradation, and they have been investigated for various pharmaceutical applications, for example, parenteral, peroral, dermal, ocular, and pulmonary administration Charcosset et al investigated a cross-flow membrane emulsification process for the preparation of SLNs using a membrane contactor. A schematic drawing of the process is shown in Figure 9. In this process, the aqueous phase is stirred continuously and circulates tangentially to the membrane surface. The lipid phase is pressed, at a temperature above the melting point of the lipid, through the membrane micropores, allowing the formation of nanosized lipid droplets. The lipid droplets are then detached from the membrane surface by the aqueous phase flowing tangentially inside the membrane module. Finally, SLNs are obtained by the cooling of the nano-sized lipid droplets to room temperature. When Kerasep ceramic membranes with micropore size of 0.1, 0.2, and 0.45 mm were used, the membrane contactor allowed the preparation of SLNs with a lipid phase flux between 0.15 and 0.35 m 3 /(m 2 h)), and a mean size between 70 and 215 nm. 147 While hydrophilic SPG membranes with the micropore sizes of 0.2 and 0.4 mm, and hydrophobic SPG membranes with 0.4 and 1.0 mm were used, the membrane emulsification process produced SLNs with a mean size between 50 and 750 nm, and a lipid phase flux between and 0.84 m 3 /(m 2 h)). 150 The high dispersed phase fluxes obtained with the SPG membranes suggest that the scale-up could be possible for industrial applications. Zhang et al. 151 presented a novel method for generating SLNs in a microchannel system with a cross-shaped junction formed by a main microchannel and two branches. The SLN formation process that occurs in the microchannel is depicted schematically in Figure 10. As shown in Figure 10, a lipid solution by dissolving the lipid in a water-miscible organic solvent is passed through the main channel while an aqueous surfactant solution is introduced into the branches simultaneously. These two liquids meet together at the cross-shaped junction and pass along the main channel. The solvent diffuses from the lipid solution stream into the aqueous phase, which results in the local supersaturation of lipid and thus leads to the formation of SLNs. The mean size, size distribution, and morphology of SLNs prepared with the microchannel system have been examined. Figure 11 shows the transmission electron microscopy (TEM) microphotograph of the obtained SLNs. From the TEM photograph, it can be seen that the SLNs obtained are nearly spherical in shape and the size distribution is narrow. The statistical measurement result of TEM has indicated that the particle size has distributed in the range of nm and the mean diameter has been 115 nm. Uniform-sized biodegradable PLA nanoparticles were prepared by combining emulsion-solvent removal and premix membrane emulsification. 60 The standard formulation conditions were in the following: the Figure 9. SLNs. 147 Schematic drawing of the membrane contactor for the preparation of DOI /jps JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 100, NO. 1, JANUARY 2011

14 88 LIU, YANG, AND WINSTON HO Figure 10. Schematic drawing of SLN formation process in the microchannel system. 151 micropore size of membrane was 1.4 mm, the transmembrane pressure was 1000 kpa, and the volume ratio of oil phase and external water phase was 1:5. The study indicated that high transmembrane pressure was a key factor to achieve uniform-sized PLA nanoparticles. The result of dispersed phase flux was not given in this study. However, for scaling up this process, it seems that the transmembrane pressure and dispersed phase flux should be further considered. According to the literature, the nanocapsules have some advantages over nanospheres, such as a lower polymer content and a higher loading capacity for lipophilic drugs. In addition, burst effect may be avoided due to the drug within a central cavity, and the drug may be better protected from degradation both during storage and after administration. 152 Spironolactone-loaded polycaprolactone (PCL) nanocapsules were prepared by using a membrane contactor in laboratory and pilot scales. 153 The membrane used was a Kerasep ceramic membrane, which had an active ZrO 2 layer on an Al 2 O 3 TiO 2 support and a mean macropore size of 0.1 mm. The optimized formulations in laboratory and pilot scales led to the preparation of spironolactone-loaded PCL nanocapsules with the mean sizes of 320 and 400 nm as Figure 11. TEM image of SLNs prepared with the microchannel system. 151 JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 100, NO. 1, JANUARY 2011 well as the high encapsulation efficiencies of 96.21% and 90.56%, respectively, and both were stable for 6 months. The researchers concluded that the pilotscale production of spironolactone-loaded nanocapsules prepared using the membrane contactor was possible in an easy and reproducible way. In the field of medicine, nanobubbles/microbubbles are expected to be applicable to areas such as the development of ultrasound contrast agents and targeted drug delivery. 57,154 Kukizaki and Goto 155 prepared monodisperse nanobubbles using SPG membranes with uniform micropores in a system composed of dispersed gaseous and continuous aqueous phases containing a surfactant as stabilizer. The effects of the surfactant type on the monodispersity of nanobubbles formed, bubble/pore diameter ratio, and gaseous-phase flux were examined. The study showed that monodisperse nanobubbles with a mean diameter of nm were stably produced from membranes with mean pore size of nm. The mean diameter of nanobubbles was shown to be 8.6 times larger than the mean pore size of the membrane. Therefore, the nanobubbles diameter could be controlled by the membrane pore size. More basic investigations are needed on the membrane emulsification technique used in preparing nanoparticles for drug delivery. One such investigation is the use and preparation of nanoporous membranes with uniform-sized pores. They may be prepared by the self-assembly of block copolymers, resulting in a highly-ordered nanoporous structure. 156,157 But the self-assembly technique typically requires either an inorganic support or a minimum thickness to obtain sufficient mechanical strength. However, a new approach by combining the selfassembly technique with the phase inversion method may give asymmetric membranes with uniformly nanoporous structures. 158 The membrane prepared by involving the phase inversion can give a high porosity, very thin top layer and excellent mechanical strength. In the preparation of nanoporous membranes, the effects of the molecular weights of the copolymer blocks and the ratios of these blocks on DOI /jps