Spherical microparticles

A large variety of drug delivery systems are described in the literature, such as liposomes (Torchilin, 2006), micro and nanoparticles (Kumar, 2000), polymeric micelles (Torchilin, 2006), nanocrystals (Muller et al., 2011), among others. Microparticles are usually classified as microcapsules or microspheres (Figure 8). Microspheres are matrix spherical microparticles where the drug may be located on the surface or dissolved into the matrix. Microcapsules are characterized as spherical particles more than Ipm containing a core substance (aqueous or lipid), normally lipid, and are used to deliver poor soluble molecules... 

Spherical microparticles are more difficult to manufacture and can be prepared by several methods. One method prepares silica hydrogel beads by emulsification of a silica sol in an immiscible organic liquid [20,21,24,25]. To promote gelling a silica hydrosol, prepared as before, is dispersed into small droplets in a iater immiscible liquid and the temperature, pH, and/or electrolyte concentration adjusted to promote solidification. Over time the liquid droplets become increasingly viscous and solidify as a coherent assembly of particles in bead form. The hydrogel beads are then dehydrated to porous, spherical, silica beads. An alternative approach is based on the agglutination of a silica sol by coacervation [25-27], Urea and formaldehyde are polymerized at low pH in the presence of colloidal silica. Coacervatec liquid... 

Spherical microparticles have been preferred in modem column technology since they form more hMiogeneous, stable and permeable column beds. Irregular microparticles are less expensive and still widely used, largely because in practice, it has not been shorn that their properties are significantly inferior. Particle shape may become more important as the particle size is reduced, and spherical microparticles are considered superior for particle dieuaeters less than 5 micrometers [33]. 

Determination of femtosecond dephasing times of organic dyes confined in a single spherical microparticle... 

Fig. 3. Panel (A) gives the fluorescence decay curves of 9AAHH ( 5 x 10-4 M) in PMMA single spherical microparticle of radius (a) 7.5 pm (b) 3.5 pm (c) 2.5 pm and (d) 1.5 pm. Panel (B) gives the observed enhancement ratio as a function of microparticle radius.

In calculating is is important to use the proper units those most commonly used by chromatographers are AP in bar (105 Pa), rM in seconds, dp in pm, t] in centipoise (mN s/m2), and L in centimeters. The most permeable columns are the most desirable, and they will have the smallest values of values ranging from about 500 for spherical microparticles to 1000 for irregular microparticles. 

Masters and Domb [250] reported on an injectable drug delivery system that uses liposomes [251] to release the local anesthetic, bupivacaine, from a liposomal matrix that is both biodegradable and biocompatible to produce SLAB. Bupivacaine due to its minimum vasodilating properties was preferred to other local anesthetics (e.g., lidocaine) allowing the released drug to remain at the site of injection longer [252]. Lipospheres are an aqueous microdispersion of water insoluble, spherical microparticles (0.2 to 100 pm in diameter), each consisting... 

R. Nastke and E. Neueunschwander, Process for the Preparation of Spherical Microparticles Containing Biologically Active Compounds, United States Patent 5,908,632, 1999. 

Liapis and McCoy [63] have assumed that the bimodal pore structure of per-fusive adsorbent particles is made of a macroporous region, in which mass transfer takes place through intraparticle convection and pore diffusion, and a microp-orous region made of spherical microparticles in which mass transfer takes place through pure diffusion. Frey et al. [61] developed a model for the analysis of mass transfer in spherical particles having a bimodal pore distribution and derived the following expression for h nt in perfusion chromatography. 

Thompson and Compton investigated, from a theoretical standpoint, the case of a spherical microparticle with an electroactive compound on its surface and attached to a solid electrode surface [33, 34]. The movement of charge was assumed to start exclusively from the contact point (or line) between the microparticle and electrode (i.e., at the three-phase boundary, if an electrolyte phase is considered) and to proceed over the particle surface only (see also Section 6.3.1). In Ref. [33], the idealized microparticle geometries of a full sphere, a hemisphere, and an inverted hemisphere have been considered (cf. Figure 6.8). 

Neo-amylose is suited for food and non-food applications. Due to its resistant starch properties it is suited for use as dietary fiber [139]. Beside this food application, smooth spherical microparticles with a size of 10-100 pm are accessible by recrystaUization of Neo-amylose in dimethylsulfoxide (DMSO). These are suited as cosmetics additive in creams, lotions, or as UV-reflectors [140]. Furthermore, Neo-amylose has proved to be an advantageous constituent in hard or soft films in order to generate gelatin free capsules [139]. 

The surface area of chitin can be expanded to large values. Rapid expansion techniques with SC-CO2 were used by SaUnas-Hemandez et al. [69] to form chitin microstmctures. Depending on the experimental conditions, they found that spherical microparticles with average diameters of 1.7-5.3 pm are obtained with expansion of supercritical solutions, whereas continuous microfibers with average diameters of 11.5-19.3 pm are obtained with expansion in a liquid solvent. Lower concentrations and smaller diameter nozzles favored the production of smaller diameter microstmctures and narrow size distributions. 

In 1998, Avnir and coworkers patented the interfacial polymerization process in which the mild sol-gel production of silica glass is combined with emulsion chemistry to form sol-gel spherical microparticles in place of irregular granules. In brief, the emulsion droplets provide a microreactor environment for the hydrolysis and condensation reactions of Si alkoxides. All sorts of molecules can be entrapped and stabilized in similar ceramic microparticles with broad control over the release rate for a range of applications (such as drug delivery, release of specialty chemicals and cosmeceutical/ nutraceutical, and beyond, Figure 18.1), with no need for reformulation for different molecules. 

Microspheres prepared by spray drying maintain their spherical geometry with a narrow size distribution with a mean diameter of 2-5 pm. Calceti et al. used suspension solvent evaporation, double emulsion-solvent evaporation, and suspension/double emulsion-solvent evaporation for the preparation of insulin-loaded polyphosphazene microspheres [80], These preparation procedures produced spherical microparticles with a porous surface and a honeycomb internal structure (Figure 11.11). 

By droplet-based microfluidic techniques, spherical microparticles can be produced. In this process, a polymer solution or a two-component system is separated by an inert nonmiscible fluid to obtain droplets in the 10-200 om range. In most cases, spherical particles are obtained however, also rods or ellipsoids have been realized [108]. For droplet miCTofluidics, either glass capillary devices can be used or devices made by lithography techniques, commonly consisting of PDMS. Figure 3.69 shows a flow scheme for the... 

In these equations, is the microparticle space coordinate and its half-dimension, is the non-dimensional concentration of the adsorbate in the micropores, the micropore diffusion coefficient and the microparticle shape factor (cr = 0 for plane, = 1 for cylindrical, and o- j, = 2 for spherical microparticle geometry). is the adsorption isotherm relation (generally nonlinear), which is again replaced by its Taylor series expansion (the coefficients of which, Op, b, ... depend on the steady-state pressure and concentration). The meaning of the boundary condition (11.33) is that the concentration profile in the microparticle is symmetrical, and of the boundary condition (11.34) that adsorption equilibrium is established at the micropore mouth. 

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