Determination of swelling behavior of calcium alginate in deionized water, isotonic NaCl, simulated gastric and simulated interstinal fluids

The use of biopolymer based materials in pharmaceutical applications is continuously increasing nowadays. Although biopolymers are composed of a large number of monomers linked together, such as all existing polymers, the difference between them is considerably greater than that of synthetic polymers. Biopolymers can be seen to be more advantageous than synthetic polymers due to their non-toxicity, biodegradability, biocompatibility with biocompatible, edible, ecologically compatible and low cost in the abundance in nature. Biopolymers consist of a wide range of materials, including polysaccharides and polypeptides. By the means of abundance, polysaccharides form an important percentage among the biopolymers. Cellulose, starch and glycogen are among the most common polysaccharides. These polysaccharides already find use in pharmaceutical, medicinal and environmental applications. One of the most important polysaccharides is alginate. Hydrogels are hydrophilic polymer webs and have the ability to absorb water from 10-20% to 1000 times their dry weight depending on the type of hydrogel. Hydrogels are called physical gels if they are held together with molecular entanglements or secondary forces such as ionic, hydrogen bonding or hydrophobic interaction. These gels can be reversible. Physical gels are not homogeneous, free chain ends may be present. A polyelectrolyte can form a physical gel with ionotropic gelation with oppositely charged polyions. Calcium alginate is an example of such a gel. Also, when oppositely charged polyelectrolytes are mixed they form a gel or precipitate depending on their concentration, ionic strength and pH of the medium. These are known as polyion or polyelectrolyte complexes. These interactions are reversible and can be degraded by changing the physical conditions such as the ionic strength, pH, temperature, materials added to the medium. The water in the hydrogels determines the permeability of the character, food, and gelatin of cellular products. If the network chains are fragile, the gel is dispersed or dissolved at the end of the swelling. For example, in this study, it was observed that spheres were disintegrated after a certain period of time during inflation with isotonic NaCl solution. Alginate ensures that algae are resistant to harsh environmental conditions (such as pressure and wave in the ocean). Which is extracted from the cell walls of algae species, a diverse group of aquatic organisms. Being extracted as alginic acid, this material is easily converted to its sodium salt and is commercially available. This non-toxic and edible polysaccharide is currently used in food formulations as a thickening and coating agent. Besides this, sodium alginate is used in anti-acid drug formulations. This biopolymer is composed of homopolymeric blocks of guluronate and mannuronate. Alginate is soluble in solutions with a pH above ~3.5, and when it is dissolved it is assumed that it forms an anionic polyelectrolyte. When dissolved alginate comes into contact with most divalent and trivalent cations, an immediate exchange reaction occurs and a hydrophilic gel is obtained. Most common alginate gel is calcium alginate. This gel is stable, can be easily adapted for many applications and very easy to prepare. Thanks to these properties, it finds many uses in various scientific and technological areas: From food industry to pharmaceuticals and water purification agents. In water purification agents, it generally uses as an adsorbent in the removal of pollutant toxic substances in the water by adsorption. In medicine and pharmaceuticals, development of controlled release agents gained interest in last decades. In the classical form of medication, the medication should be taken at frequent intervals. Because the activity of the drug cannot be maintained for a long time. Therefore, the amount of time it takes to metabolize the drug substance is not dependent on the parameters to be set in conventional drug forms. Controlled drug release systems eliminate these situations. The principle of controlled release is based on the sustained delivery of an active drug molecule over time, and preferably, under a predictable behavior. Controlled release of drugs from a release matrix is based on two principles: Chemical erosion of the support material (polymer) and diffusion of the drug. Since most drugs are water soluble, determination of the drug diffusion in water is crucial. Diffusion laws govern the drug transportation through the support material and it is strongly related to the swelling behavior. Recently there are many reports on development of calcium alginate based controlled release systems. As a hydrophilic gel, calcium alginate swells especially in basic medium. The swelling properties of this material are well known. Most common technique to characterize the swelling ability of a controlled release material is to measure its wet mass and calculate the ratio to the dry mass. This procedure is repeated for certain time intervals. While the swelling capacity of calcium alginate is generally measured by classic conventional gravimetric method, due to the disadvantages observed on these methods like low precision and long experiment time, various research have been made on the methods that will yield higher efficiency. In the present study, a novel analysis technique has been tried using a moisture analyzer. The amount of water in the swollen alginate beads was measured quickly with high precisions at the same time when the beads were dried in the device. Moisture analyzer is basically a precision scale with a halogen heater. It gives a fast and accurate mass for moist materials. It is possible to preselect all parameters of a measurement (drying temperature, drying time, etc.). The cost of the device is very low. The device does not have consumable material. It is only necessary maintenance to be done at regular intervals. In experimental section, calcium alginate beads were prepared by adding 1% (w/v) sodium alginate solution into 3% (w/v) calcium chloride solution using a syringe. While dropping, calcium alginate gel beads are immediately formed. After gelation, the calcium alginate spheres were filtered with filter paper. The filtered spheres were washed several times with deionized water. If the spheres are not washed with water, it is observed that the constructions of the spheres deteriorate with time. After filtration, the calcium alginate beads were dried at a certain temperature in the drying chamber. The spheres were made ready to be swelled in the determined solutions. After which the spheres were made ready to be inflated in the specified solutions. Swelling measurements were conducted by treating dried calcium alginate beads at specific time intervals with solutions that are important in drug release systems such as deionized water, isotonic NaCl solution, simulated gastric fluid (SGF), and simulated intestinal fluid (SIF). These solutions were selected because isotonic sodium chloride solution, SGF and SIF are commonly used in in vitro drug release studies. The solution remaining on the surface of the spheres was carefully removed with a napkin. After this analytical process, the amount of solvent in the swollen calcium alginate beads was measured by means of a moisture analyzer, based on the rate of swelling. The device has reduced the analysis to one step and reduced the possibility of analysis errors. High precisions have been obtained for the swelling ratios of calcium alginate beads in four different medium. In response to time of the analysis at least two analyses were made and the reproducibility of each point was measured and the maximum RSD value found was six. This value is a good result when compared to the other classical method. When the swelling percentages were compared, the results decrease in the following order: SIF, isotonic NaCl, SGF and pure water. While the swelling ratios were close to each other in the water and gastric fluid, the results in the salt solution and the intestinal fluid were close to each other. The relations of swelling ratios with the ionic strength and pH of the environment was discussed. In SIF solution, the affinity of phosphate towards calcium leads to a rapid swelling of calcium alginate matrix and following this, disintegration. Similarly, in sodium chloride solution sodium ions compete with calcium for an ion-exchange like reaction and due to the increase in the ionic strength of the medium, calcium alginate gel is rapidly expanded by absorbing the solvent and this is followed by the disintegration of the gel structure. By the means of disintegration, it is meant an irreversible expansion and deformation of the capsule. Due to the chemical balance, in SGF conditions calcium alginate gel beads only absorb the solvent however no chemical alteration is observed. This is due to the strong acidity (~1.2) of the SGF solution. Since the pKa of the alginate is around 3.5, the equilibrium tends towards calcium alginate, therefore the swelling is limited. Another limited swelling is observed for pure water, however in that case the reason is mostly the ionic strength instead of the pH. It is observed that the result difference of the experiments done on solutions with pH values close to each other is determined by ionic strength; while the result difference of the experiments done on solutions with ionic strength values close to each other is determined by pH values. It has been identified that the measurement done with humidity device coincided with the one in the theoretical information. Further studies with this technique might be beneficial for the characterization swelling-deswelling properties of the alginate beads as well. This technique can be used not only for alginate, but also other hydrophilic gels.