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Water (containing hydrophilic molecule to be entrapped)

Emulsification organic solvent

Emulsification organic solvent

Primary W/O emulsion

Add primary W/O

emulsion to an aqueous phase containing a stabilizer

Solvent evaporation

'oo w/o/w emulsion

FIG. 1 Emulsification-solvent evaporation, (a) Simple emulsion process; (b) double-emulsion process.

W/O/W emulsion

triblock PLA-b-POE-PLA [6] was used in the elaboration of POE-covered particles that would display low interactions with plasma proteins.

The nature of the stabilizer in the water phase can be a surfactant such as sodium dodecyl sulfate, potassium oleate, or a macromolecule such as poly(vi-nyl alcohol) (PVA), POE-PPO-POE triblocks (PPO being the polypropylene oxide block that constitutes the hydrophobic segment of the macromolecular stabilizer), a protein like albumin [2] or a hydrophobized dextran [7]. On the nature of the stabilizer depend the size and the colloidal stability of the particles, as well as the nature of the resulting solid-liquid interface. It has been shown that PVA could bind in an irreversible manner to poly(DL-lactide-co-glycolide) (PLGA), and that the influence of the surface PVA layer would be larger in smaller particles but be independent of the PVA concentration of the continuous phase during the manufactoring process [8].

The solvent evaporation method, based on the formation of an o/w emulsion, allows entrapment within the core of the particles of a lipophilic substance by dissolution of the latter in the organic phase. To encapsulate hydrophilic compounds such as proteic antigenes, for example, a double-emulsion w/o/w is required (Fig. 1b). First the protein solution is emulsified in dichloromethane containing PLGA by homogenization at 5000 rpm. Thereafter, this first emulsion is poured into a PVA-containing aqueous solution and homogenized at 10,000 rpm. Then, the solvent is removed under reduced pressure. Using bovine serum albumin (BSA) as a model protein, 64% efficiency was attained with PLGA and only 49% with PEO-PLGA diblock copolymer [9]. The use of the diblock copolymer allowed an increase of the half-life of BSA in rats from 14 min to 4.5 h. The use of a PLGA-POE-PLGA triblock copolymer improved the release profile of BSA as compared to PLGA alone [10].

Lipoparticles composed of triglycerides can be obtained by either the o/w or the w/o/w methods [11]. In this approach, copolymers such as PVA, PEO derivatives, or polysaccharides are used as stabilizers or surface modifiers [11].

2. Salting Out

The classical emulsification-solvent evaporation method described above relies on the use of water-immiscible solvents, most of them being incompatible for further in vivo applications. To allow the separation from an aqueous phase of water-miscible solvents, more prone to be accepted for in vivo use, a method was developed using a salting-out effect. Acetone is often used as a water-miscible solvent because it is easily separated from aqueous solutions by salting-out with electrolyte [2]. A polymer solution in acetone, eventually containing a drug to be entrapped, is emulsified under mechanical stirring in an aqueous solution containing the salting-out agent and a colloidal stabilizer. The resulting emulsion is then diluted with a large amount of water to allow the diffusion of acetone into the aqueous phase, which induces the formation of the particles. Solvents and electrolytes can be removed by cross-flow filtration [2].

3. Emulsification-Diffusion

Recently, a new method for manufacturing nanoparticles from preformed polymers was developed to reduce the level of energy of the emulsification step, to allow the use of pharmaceutically acceptable organic solvents, and to produce particles in high yields with high reproducibility [12]. Named emulsification-diffusion, the process involves the formation of an o/w emulsion of a water-saturated organic solvent containing the polymer in a solvent-saturated aqueous phase containing a colloidal stabilizer. The subsequent dilution of the system in water causes diffusion of the solvent into the aqueous phase responsible for particle formation (Fig. 2).

In this process, besides increasing the PLA concentration in the organic phase, the critical step in the control of the particle size of the resulting colloidal dispersion is the size of the droplets formed during the emulsification step. A decrease in particle size (from 1.66 pm to 170 nm) and of polydispersity was obtained with an increase of the steering rate [13] for the emulsion step but not for the dilution step. Having shown as well that there was no particle size dependency on the solvent diffusion rate, the authors suggested that the particle formation arose from a chemical instability due to the presence of polymer-supersaturated regions rather than mechanical instability during the diffusion step [12]. Nevertheless, the nature of the stabilizer also has to be taken into consideration. Using propylene carbonate (PC) as an organic solvent, large aggregates were formed independently of the polymer concentration and process conditions, with polysorbate 80 or polyvinylpyrrolidone and dextran. But in the presence of po-loxamer 188, stable nanoparticles were obtained [13].

Nanocapsules containing an oily core surrounded by a hard polymeric shell have also been obtained by the emulsification-diffusion method. Using ethyl acetate as a solvent, capsules containing medium-chain triglycerides have been obtained. Density gradient centrifugation allowed confirmation of the formation of nanocapsules whose densities were intermediate between those of nanoemul-sions and nanospheres [14].

B. Solvent Displacement Method

This technique does not require the prior formation of an emulsion to obtain a dispersion of colloidal particles in an aqueous phase. It relies on the spontaneous emulsification of a polymer solution in a semipolar solvent such as acetone or ethanol. This solution is poured or injected, under moderate magnetic stirring, into an aqueous medium containing a stabilizer, and particles are formed instantaneously by rapid diffusion of the solvent in water. Then the organic solvent is

Polymer in water-saturated solvent

Stabilizer in solvent-saturated water

Emulsification -►

To induce solvent diffusion

O/W emulsion

Solvent evaporation

Diluted particle dispersion

Particle dispersion

FIG. 2 Emulsification-diffusion process.

removed under reduced pressure (Fig. 3) [15]. Lipophilic drugs and, eventually, phospholipids, acting as stabilizers, can be added to the organic phase. Poly(DL-lactic acid) or poly(e-caprolactone) can be used to obtain particles and the mostly used stabilizers in the water phase are pluronics or PVA. In this system, superficial instabilities of interfaces are sufficient to allow emulsification.

Nanocapsules can be obtained by the solvent displacement method. An ace-tonic solution of benzyl benzoate and poly(e-caprolactone) was added to an aqueous solution of pluronic F68. Nanocapsules composed of a benzyl benzoate core and poly(e-caprolactone) hard shell were obtained. The incorporation of benzyl benzoate was evaluated by a gas chromatography-mass spectrometry procedure. Particles of mean diameter 200 nm were obtained with an incorporation higher than 86% [16].

Finally, nanoemulsions can be obtained with this procedure. Elbaz et al. have described a positively charged submicrometer emulsion based on lecithin, medium-chain triglycerides, poloxamer 188, and stearylamine using the solvent displacement method [17]. With a slightly modified approach, Trimaille et al. showed that it was possible to modify the interface of these emulsions by depos-

FIG. 3 Solvent displacement process.

iting, concomitantly with the particle formation step, an adequately functional-ized copolymer [18].

Homopolymers, as well as block copolymers (i.e., PEO-PLA), have been used to obtain particles [19] and nanocapsules [20]. For both systems, the mean diameters and colloidal stability were dependent on the ratio of the two chains in the block copolymers. Increasing the hydrophobic segment resulted in decreased stability.

C. Dialysis-Based Methods

To avoid the emulsification step, the dialysis method can be used. It consists of filling a dialysis tube with a polymer solution in a water-miscible solvent such as dimethylsulfoxide (DMSO), dimethylformamide (DMF), or acetone. This solution is put to dialysis against water and, as the organic solvent diffuses away from the dialysis tube and is substituted by water, the polymer precipitates under the form of spherical particles. PLA particles coated with poly(L-lysine) (PLL) were obtained by dialyzing a DMSO mixture of PLA and derivatized PLL against water [21]. At a fixed PLA concentration, increasing the polysaccharide-PLL copolymer resulted in a reduction of the average particle size from 300 nm to 80 nm. With this approach, PLA particles were surface functionalized with a galactose-carrying polystyrene to target hepatocytes via their galactose receptors [22].

Block copolymers of PLA [23] and poly(e-caprolactone) [24] with amphi-philic properties have led to nanoparticles with a hydrophobic core-hydrophilic shell structure whose particle size was less than 200 nm in both cases. Both amphiphilic copolymers were obtained by ring-opening polymerization of gly-colide in the presence of MPEO [23] or of poly(e-caprolactone) in the presence of PEO-PPO-PEO triblock copolymer [24]. Interestingly, since PEO-PPO-PEO triblock copolymers are known to be thermoresponsive, the authors investigated the role of the temperature on particle size and reported a marked decrease in particle diameter with increasing temperature. Recently, the elaboration, characterization, and application of block copolymer micelles for drug delivery was reviewed by Kataoka et al., who cited the dialysis method for the preparation of reactive polymeric micelles [25].

Logically, if amphiphilic block copolymers could be formulated under the form of particles by the dialysis method, this method should apply as well to hydrophobized water-soluble polymers. Pullulan acetates, as hydrophobized pullulans, were synthezised in a DMF suspension in the presence of acetic anhydride and pyridine. After purification, the white powder was dissolved in DMSO and dialyzed against water to obtain particles of size of less than 80 nm [26].

D. Supercritical Fluid Technology

Supercritical fluid technology is an environment-friendly alternative to process particles in high yield and exempt of traces of solvent that can be toxic in many pharmaceutical applications. The two main technologies for obtaining particles from preformed polymers will be quickly overviewed in this section.

In the rapid expansion of supercritical solution (RESS), the polymer is solu-bilized in a supercritical fluid. On rapid expansion of the solution, the solvent power of the critical fluid is drastically reduced, provoking the precipitation of the polymer under the form of particles [27]. This technique is very attractive because the precipitated particles are completely solvent free; however, unfortunately, most polymers exhibit low to no solubility in supercritical fluids. This method is, in fact, limited to low molecular weight polymers.

To address this solubility issue, the supercritical antisolvent method was developed. The idea was to dissolve the polymer in a solvent and use a nonsolvent supercritical fluid to induce the precipitation of the polymer and thus the particle formation. Practically, the polymer solution is introduced in the precipitation chamber, which is then charged with the supercritical fluid. At high pressure, enough nonsolvent can enter the liquid phase and induce formation of the particles. When the particle-forming step is complete, the supercritical fluid is used to remove the solvent (Fig. 4). In a modified version of this technique the polymer solution can be directly introduced into the supercritical fluid [28].

The use of organic solvents for the preparation of colloids for in vivo biomedical applications is an important issue because one has to use pharmaceutically ac-

Supercritical fluid—►

III. METHODS USING AN AQUEOUS MEDIUM

Chamber-^

Polymer in solution

'article in formation

'article in formation

FIG. 4 Supercritical fluid antisolvent process.

ceptable solvents and quantify the trace amounts in the final preparation. Thus, to address this safety issue, various formulation procedures have been, or are being, designed, avoiding the use of organic media.

A. Ionic Gelation

Polyelectrolytes can be formulated under the form particles by polyion-mediated gelation due to the formation of inter- and intramolecular cross-links.

Chitosan (poly 1-4 glucosamine obtained by deacetylation of the natural chitin from crab shell) as a polycation was formulated under the form of beads by tripolyphosphate (TPP) [29]. A modification of the original procedure allowed the formation of nanoparticles of chitosan alone or in combination with other hydrophilic polymers. Thus, a mild and simple procedure was developed based on the mixture of two aqueous phases, one containing the chitosan and PEO or a PEO-PPO diblock copolymer, and the second containing the TPP [30]. Size (100-200 nm) and Z potential could be modulated by the chitosan-to-PEO polymer ratio. Porous chitosan beads have been obtained by using an aqueous solution of TPP at pH 8.9 [31]. The morphology of the particles was evidenced by scanning electron microscopy and compared to the dense structure of the colloids obtained at pH values lower than 6. The particles were stabilized by cross-linking using ethylene glycol diglycidyl ether and then could be further modified with reagents such as succinic anhydride and benzoic anhydride. Glu-taraldehyde-cross-linked chitosan nanospheres have been reported [32], but the dialdehyde had too many negative effects on cell viability to allow a general application of these particles as drug carriers. Sodium sulfate has been used as well to obtain nanoparticles by dropwise addition of sodium sulfate solution and a chitosan solution containing a nonionic stabilizer [33].

Alginates are sodium salts of a linear glycuronan composed of a mixture of P-mannosyluronic acid and a-gulosyluronic acid residures. They are often used in the elaboration of capsules for oral delivery of bioactive molecules. Upon interaction with calcium, alginates cross-link under the form of microcapsules by ionic gelation. An alginate solution (1-2 wt %) is slowly added under stirring to a calcium chloride solution in water. The resulting capsules can be collected and washed with water [34] before further use.

B. Polyelectrolytic Complexes and LbL Approach

Colloidal dispersions of interpolyelectrolyte complexes can be obtained in an aqueous medium by mixing dilute solutions of polycations and anions. An aqueous solution of the sodium salt of the alternating copolymer of maleic acid and propene (MAPE) was added to a dilute solution of poly(dimethyldiallylammon-ium chloride) (DMDAA). A threefold excess of cationic groups vs. anionic group was used to obtain positively charged colloids [35]. Particles in the 100-

to 200- nm range were obtained depending on the ionic strength of the complex-ation medium. On increasing the ionic strength, the particle size increased. The particle formed via the aggregation of the polyelectrolyte complexes due to hy-drophobic interactions. Electrostatic stabilization was brought by the excess of positive charges used in the formulation.

By slowly mixing equal volumes of a solution of negatively charged calf thymus DNA and an aqueous solution of polycation poly(L-lysine) (PLL), loaded in different syringes and driven to a T-mixer, DNA-PLL particles were obtained. The particles had a surface charge density depending on the +/- charge ratio. Here again, the colloidal stability depended on ionic strength. At low pH and salt concentration, monodispersed DNA-PLL particles of 200 nm could be obtained [36]. A similar approach was used for obtaining isodisperse chitosan/ DNA nanospheres; sodium sulfate was added to the DNA solution, and the mixing speed and temperature of the blending of the two solutions were controlled as well [37].

A template-assisted synthesis of hollow capsules was developed by Caruso et al. [38]. On a preformed charged colloid, alternate deposition of polyions of opposite charges was achieved constituting thus a shell of polyelectrolytic complexes (Fig. 5). The control of the deposition of each layer of opposite charge polymers was be achieved by measuring the surface charge inversion of the Z potential of the colloid. In a next step, the template particle was degraded either by chemical or by thermal treatment for, respectively, a melamine or a polystyrene core so as to give rise to the formation of hollow capsule [38].

Recently, the colloidal template was replaced by enzyme crystals. This process led to the reversible encapsulation of the enzyme with retention of its bioac-tivity, thus offering new perspectives in the production of tailored and optimized systems in biotech applications [39].

C. Self-Assemblies

Particle formation can arise from the self-association of a polymeric structure designed to develop segment-segment interactions rather than solvent-segment interactions that are responsible for the solubility of the macromolecules. The most common structures used for self-assembies are amphiphilic macromole-cules, bearing hydrophobic and hydrophilic segments. For instance, there are reports on the use of diblocks [25] or comblike polymers (a hydrophilic backbone grafted with hydrophobic pendant groups) [40]. In some cases, the polymeric structure can acquire the amphiphilic character after (1) interactions with an other counterpart, such as the formation of neutral polyelectrolyte complexes onto a ionic segment of a diblock copolymer [25]; (2) after stimulation by an external factor such as heat [41] or pH [41,42]. Though hydrophobic interactions between macromolecules are widely described to ensure the autoassocia-

Polyanion y

FIG. 5 Template-assisted capsule synthesis.

Poly cation

Template elimination 1

capsule

tion of polymers, just as phospholipids associate in uni- or multi-lamellar vesicles, hydrogen bonding has also been involved in the formation of colloids [43].

To come back to the amphiphilic structures described above, the role of the hydrophilic segments of the polymer is to ensure the colloidal stability during and after formation of the particles. The chemical nature of these segments can be ionic [25,41], when, for instance, a polyanion is grafted with hydrophobic groups [40], or nonionic but hydrophilic, such as in the case of PEO chains [25].

The self-assembly method is fairly versatile in terms of structures that can be obtained. The formation of micelles [25], particles [43], and vesicles [44] has been reported. From a practical standpoint, colloids can be obtained by redispersion of the dried amphiphilic polymer in an aqueous mixture [40,43] with or without a sonication step to improve the colloid size dispersion. In the presence of cholesterol, vesicles can be elaborated with the amphiphilic copoly-mer being at the oil-water interface [45]. The dissolution-sonication procedure was used for the elaboration of particles via intermolecular hydrogen bonding of PEO-grafted chitosan molecules [43] and the obtained particles were in the 90- to 150-nm range, depending on the mole percent of PEO chains per sugar unit. A pH-induced modification of the solubility of the polymer has been used to obtain particles from modified polyamino acids [42] or polyacrylamide [41], the latter also exhibiting thermosensitive swelling properties.

IV. PHYSICAL METHODS

Following is a short overview of physical methods that are continuous processes for the production of microsphere whose main applications are found in the pharmaceutical industry or food industry. Two main processes will be devel-oped—extrusion and spray-drying—though these procedures lead mainly to the formation of micrometer-sized particles.

Extrusion consists in the production of droplets from a polymer solution that will be further hardened to solid microspheres. This latter step can be achieved by melt solidification, gelation, chelation, solvent extraction, or evaporation. For instance, alginate microparticles can be obtained by extrusion into an aqueous solution of calcium chloride, with the electrolyte serving as the gelation agent to yield the hardened particles [46].

Spray-drying relies on the atomization of a polymer solution, which is then spray-dried to yield the particles to be collected (Fig. 6).

The spray-drying step requires the use of a hot chamber in which the solvent can evaporate. This is fairly easy to achieve when the solvent is volatile such as methylene chloride for PLGA particles [47], but requires higher temperature (between 110°C and 180°C) when polysaccharides are used [48]. For these

FIG. 6 Spray-drying method.

polymers requiring aqueous solutions, an alternate method was developed that consisted of spraying the polymer solution into a liquid coagulating agent [48]. The separation of the microspheres from the coagulation medium was achieved by centrifugation. The mass ratio of air to polymer liquid, the relative velocity of air to polymer liquid and the viscosity of the polymer solution affected the particle size distributions.

V. CONCLUSIONS AND PROSPECTIVES

Most of the methods described in this chapter barely meet the requirements for large-scale production. Thus, applications for these procedures are mainly found in the pharmaceutical domain, for which the added value is very high and the production scale lower than for some other industries. Therefore, micelles [25], nanocapsules [49], and nanoparticles [50] obtained from polymeric materials have been used as drug delivery devices. Molecules larger than drugs, such as proteins [51] or DNA [52], have been loaded into or onto colloidal vectors. The variety in chemical nature of the polymers for the manufacture of these colloids, though limited due to toxicity constraints, includes polyesters [50], polycyano-acrylates [50], and polysaccharides [53]. For the sustained delivery of bioactive molecules, tuning the degradation kinetics of the carrier to reach the expected goals or performances is still a challenging issue, as much for small drugs as for larger antigens used for vaccination.

Since capsules with either lipophilic or hydophilic compartments can be made, as we have seen in this chapter, one can envision new colloids that would contain various types of cavities. For instance, one could think of the presence of a hydrophilic compartment to contain water-soluble drugs and a lipophilic one filled with hydrophobic drugs. These colloids could be well suited in multi-therapy approaches.

Another direction in the improvement of bioactive molecule delivery systems will probably be the stimuli-responsive carriers. We can imagine potential synthetic vectors which, once injected in a patient, could be driven from the outside, with a magnet, for instance, to the targeted organ. Once the target is reached, a second stimulus would burst the carrier and thus release the active compound.

The ultimate carrier should be capable of recognizing the cellular target, binding and/or entering the target, and delivering the bioactive molecule to a specific site of the target. This could be achieved by building multilayered colloids that could bear a variety of successive information. These colloids would mimic viruses during the infection process. Viruses specifically bind to the targeted cells and enter them. Fusion of the cell membrane and the viral membrane induces the unwrapping of the viral genome associated with proteins essential for viral replication. Then, intracellular trafficking takes place to drive the viral genome to the nucleus, which is then entered. Integrases allow the intercalation of the viral genome into the host cell and a complete takeover of the cell functions is achieved by the virus. Thus, to achieve efficient gene delivery to cells, multifunctional colloids would be needed, bearing a variety of information essential for each stage of the process but that should be removed to allow progression to the next step.

Most applications dealt with so far were focused on the delivery of bioactive molecule. A new developing field is the detoxification of the body by specific removal of defined substances. It could be cancer cells, as already achieved ex vivo, as well as small molecules like cholesterol, hormones, sugar, or larger assemblies such as circulating viruses in chronic infections. These colloids should circulate long enough in the body to adsorb the specific toxin and then to be eliminated in the natural ways.

Applications other than in the pharmaceutical domain can be envisioned for "sophisticated colloids." For instance, smart garments have appeared on the market, made of fabrics containing encapsulated perfumes or antiperspirants or antiwrinkle creams. Surely human imagination will provide numerous perspectives to colloids and new challenges for the chemistry of materials.

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