Beads as Drug Carriers
Beads are often used as drug carriers in passive, as well as active drug targeting, making this a highly relevant topic in today’s research. Major general topics covered in the first part of this chapter include controlled drug release, gels in drug-deliv
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Beads as Drug Carriers
8.1 Introduction Beads are often used as drug carriers to study both passive and active drug targeting. When discussing drug delivery by beads, topics related to the method of drug incorporation, size and density of the bead, extent and nature of the crosslinking, physicochemical properties of the drug, interactions between the drug and the matrix material, and concentration of the matrix material and release environment, such as the presence of enzymes, are of the utmost importance. Several beads will be described in this chapter including, but not limited to, those that are based on alginate, chitosan, gelatin, gellan, guar, pectin, dextran, and cellulose, and biodegradable hydrogels based on polyesters and polyvinylpyrrolidone (PVP) crosslinked with functionalized albumin.
8.2 Controlled Drug Release Diffusion through a rate-controlling membrane or matrix, osmosis [i.e., the movement of water across a semi-permeable membrane from an area of high water potential (low solute concentration) to an area of low water potential (high solute concentration)], ion exchange, and matrix degradation are all means of controlling drug release (Heller 1980; Park et al. 1993). Biodegradable drug-delivery systems have a unique advantage in that they do not need to be removed from the site of application once the delivery has been made. In addition, degradation of polymers to low molecular weight fragments that can be eliminated from the body is preferred in many systemic applications (Heller 1980; Park et al. 1993). Surface (heterogeneous) and bulk (homogeneous throughout) degradation are two different modes of biodegradation based on the site of polymer breakdown (Heller et al. 1978). In bulk polymer degradation, drug diffusion occurs prior to or concurrent with matrix degradation (Chang 1976). In surface polymer degradation, drug release is determined by the relative contributions of drug diffusion and matrix degradation (Heller 1985). In many cases, when biodegradable hydrogels are used, the drugs are in contact with water and their solubility is a main factor in their A. Nussinovitch, Polymer Macro- and Micro-Gel Beads: Fundamentals and Applications, DOI 10.1007/978-1-4419-6618-6_8, C Springer Science+Business Media, LLC 2010
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8 Beads as Drug Carriers
release. If solubility is not a limiting factor, release is rapid and independent of the matrix degradation rate (Park et al. 1993). As a result, most low molecular weight drugs are not transported by hydrogels; instead, these gels are used for proteins and peptides that are entrapped within the gel network until its degradation (Park et al. 1993). Diffusion-controlled and swelling-controlled drug-release mechanisms have been discussed extensively in previous reports (Jost 1960; Baker and Lonsdale 1974; Crank 1975; Baker 1987). Diffusion-controlled systems are divided into reservoir or monolithic devices (Zeoli and Kydonieus 1983; Kost and Langer 1987). In the reservoir system, the drug core is encapsulated by an inert membrane (Park et al.
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