Drug Release Modulation from Cross-Linked Calcium Alginate Microdiscs, 2: Swelling, Compression, and Stability of the Hydrodynamically-Sensitive Calcium Alginate Matrix and the Associated Drug Release Mechanisms

Synthesis and Swelling Behavior of Sodium Alginate/Poly(vinyl alcohol) Hydrogels

Objectives

Hydrogels are macromolecular networks able to absorb and release water/biological fluids in a reverse-phase manner, in response to specific environmental stimuli. Such stimuli-sensitive behavior makes hydrogels interesting for the design of smart devices applicable to a variety of technological fields. They are able to absorb and retain 10-20% and up to 1000 times the water or biological fluids than their dry weight can. The aim of this study was to extend the work on drug delivery in the stomach at pH 2-2.2.

Materials and Methods

The authors synthesized sodium alginate (SA)/poly(vinyl alcohol) (PVA) hydrogels. These hydrogels were characterized by fourier transform infrared spectroscopy and scanning electron microscopy, and the swelling properties of the hydrogels were examined at different pH values, in different salts, at different temperatures, and in different acids and bases.

Results

The authors studied and reported the swelling effects or variations such as the effects of salts, acids, bases, temperature, and pH. The results for the crosslinking agent glutaraldehyde showed that 8 mL of glutaraldehyde had a higher swelling rate compared to that of 10 mL and 12 mL.

Conclusion

In this work the authors studied the swelling degree in different acids and bases. It is concluded that the degree of swelling decreases with increases in the concentration of glutaraldehyde and also depending on the concentrations of the acids. The swelling degrees of PVA/SA hydrogels gradually increase with increases in the concentrations of acids (low pH). The swelling of hydrogels decreases with increases in pH (>7) or at high alkaline. Based on the results for salt solutions the swelling behavior was found to be in the order: K⁺>Na⁺>Ca²⁺>Mg²⁺.

Effects of Microwave on Drug Release Property of Poly(Methyl Vinyl Ether-co-Maleic Acid) Matrix

The drug release behavior of beads made of poly(methyl vinyl ether-co-maleic acid) was investigated with respect to the influence of microwave irradiation. The beads were prepared by an extrusion method with sodium diclofenac as a model water-soluble drug. The beads were subjected to microwave irradiation at 80 W for 5 and 20 min, and at 300 W for 1 min 20 s and 5 min 20 s. The profiles of drug dissolution, drug content, drug-polymer interaction, and polymer-polymer interaction were determined by using dissolution testing, drug content assay, differential scanning calorimetry, and Fourier transform infra-red spectroscopy. Keeping the level of supplied irradiation energy identical, treatment of beads by microwave at varying intensities of irradiation did not bring about similar drug release profiles. The extent and rate of drug released from beads were markedly enhanced through treating the samples by microwave at 80 W as a result of loss of polymer-polymer interaction via the (CH(2))(n) moiety, but decreased upon treating the beads by microwave at 300 W following polymer-polymer interaction via the O-H, COOH, and COO(-) moieties as well as drug-polymer interaction via the N-H, O-H, COO(-), and C-O moieties. The beads treated by microwave at 300 W exhibited a higher level of drug release retardation capacity than those that were treated by microwave at 80 W in spite of polymer-polymer interaction via the (CH(2))(n) moiety was similarly reduced in the matrix. The mechanism of drug release of both microwave-treated and untreated beads tended to follow zero order kinetics. The drug release was markedly governed by the state of polymer relaxation of the matrix and was in turn affected by the state of polymer-polymer and/or drug-polymer interaction in beads.

Elastic, superporous hydrogel hybrids of polyacrylamide and sodium alginate

A novel approach was developed to prepare a superporous hydrogel with superior mechanical and elastic properties. According to this method, a synthetic monomer was polymerized and crosslinked in the presence of a water-soluble alginate polymer. Later in the process, the alginate part of the synthesized hydrogel was treated with metal cations, which resulted in a hydrogel hybrid with an interpenetrating network structure. In this article, a hydrogel hybrid of acrylamide and alginate is highlighted because of its unique swelling and mechanical properties. This hydrogel hybrid shows resilience and a rubbery property in its fully water-swollen state, which not previously been reported. To help understand the underlying mechanism responsible for such unique properties with hydrogel hybrids, the ionotropic gelation of the alginate polymer was also studied in more detail.

Alginate in drug delivery systems

Alginates are established among the most versatile biopolymers, used in a wide range of applications. The conventional use of alginate as an excipient in drug products generally depends on the thickening, gel-forming, and stabilizing properties. A need for prolonged and better control of drug administration has increased the demand for tailor-made polymers. Hydrocolloids like alginate can play a significant role in the design of a controlled-release product. At low pH hydration of alginic acid leads to the formation of a high-viscosity "acid gel." Alginate is also easily gelled in the presence of a divalent cation as the calcium ion. Dried sodium alginate beads reswell, creating a diffusion barrier decreasing the migration of small molecules (e.g., drugs). The ability of alginate to form two types of gel dependent on pH, i.e., an acid gel and an ionotropic gel, gives the polymer unique properties compared to neutral macromolecules. The molecule can be tailor-made for a number of applications. So far more than 200 different alginate grades and a number of alginate salts are manufactured. The potential use of the various qualities as pharmaceutical excipients has not been evaluated fully, but alginate is likely to make an important contribution in the development of polymeric delivery systems. This natural polymer is adopted by Ph.Eur. It can be obtained in an ultrapure form suitable for implants. This review discusses the present use and future possibilities of alginate as a tool in drug formulation.

Preparation and properties of glutaraldehyde cross-linked whey protein-based microcapsules containing theophylline

Whey protein-based microcapsules containing a model drug, theophylline, were prepared in organic phase, using glutaraldehyde-saturated toluene. In all cases, spherical microcapsules, ranging from <400 to 1000 microm in diameter, were obtained. Results indicated that core crystals were embedded throughout the wall matrix. In all cases, retention efficiency of theophylline was higher than 74% and was not affected by cross-linking conditions. Results of theophylline release in simulated intestinal and gastric fluids at 37 degrees C indicated that the diffusion-governed core release was significantly affected by size of microcapsules, cross-linking conditions, and by type of dissolution medium. In all cases, core release in simulated intestinal fluid was faster than in simulated gastric fluid.

In vitro release modulation from crosslinked pellets for site-specific drug delivery to the gastrointestinal tract. I. Comparison of pH-responsive drug release and associated kinetics

Multiple unit dosage forms for oral delivery of bioactive agents offer many advantages over single unit products (e.g., site-specific delivery, predictable gastrointestinal transit time and less localized adverse effects). In view of such benefits, this paper investigates the crosslinking of sodium alginate, low methoxylated pectin and their novel binary mixture with calcium ions through ionotropic gelation to pelletize the model drug, diclofenac sodium, using "environmentally benign" solvents and processing techniques. Crosslinked pellets of the above polymers in 2% (w/v) aqueous calcium chloride solution were prepared and evaluated for their structural and release behavior. The average size of the different pellets was 1.3 mm and drug entrapment capacity was optimized by reducing the calcium chloride solution pH to 1.6. Three types of pellet formulations were subjected to dissolution studies using the USP 23 Apparatus 2 and 3 over a pH range simulating the human gastrointestinal tract. Negligible drug release occurred in pH 1-4. However, rate of drug release in pH 6.6 ranged from rapid to slow (i. e., 100% drug release in 4 to 10 h, respectively) but always in a controlled manner. Weight change/erosion studies and swelling measurements were used to provide experimental correlation of kinetic model analysis for each of the three pellet systems. From model fitting studies and statistical treatment, the modified Hopfenberg equation {Mt/M infinity =1-[1-k1(t-tL,min)]n} best described the release kinetics for calcium-pectinate pellets. The model assumes heterogeneous erosion with kinetic constant k1=k0/C0r0, in which k0 is the erosion rate constant, C0 is the uniform initial concentration of drug in the matrix, r0 is the initial radius and tL, min is the lag time. The n values of 1, 2 and 3 apply to a slab, cylinder and sphere, respectively. In addition, the exponential models, namely the Power Law (Mt/M infinity =k1tn) and its derivative containing the lag time [Mt/M infinity =k1(t-tL,min)n], employed in the statistical treatment of data provided n values of approximately 0.8-1 in the case of the calcium-alginate and calcium-alginate-pectinate release kinetics. It is concluded that the proper selection of rate-controlling polymers and their interactive potential for crosslinking is important, and will determine the overall size and shape of pellets, the duration and pattern of dissolution profiles, pH sensitivity, drug loading capacity and mechanism of drug release.