Mechanical properties of biopolymer films
Hard gelatin capsules have been used for drug delivery for a long time. The current production process takes advantage of the very unusual properties of gelatin: gelation, very low viscosity, film mechanical properties and film solubility. Although the hard gelatin capsules present many advantages compared to other drug delivery systems, their uses are restricted because of the animal origin of the gelatin. A HPMC gelling agent system is currently used for producing animal product free hard capsules. This work examines the possibility of using a different system in a similar production process. The gelling conditions of the mixed system, the potential of various film formers and the mechanical properties of some films are considered. Gelling agent filler mixed systems were prepared, and the limit concentration of filler that allowed gelation was noted. It was shown that none of the gelling agents would always gel and gelation was never prevented by the maltodextrin (up to a concentration of 14%). The gelation inhibition obtained is likely to be due to phase separation. The charge densities of the various products were also measured. It showed that when there is little charge density difference, gelation is inhibited. Polymer compatibility is increased by increasing the charge density differences. However, an asymmetry is observed. This is explained by the necessary shift of the binodal that would predict prevention of incompatibility. Many films were cast from various biopolymers. The films were screened via sensory analysis. The process allowed to define terms that discriminate the films. The results showed that cellulose derivatives, alginate and alginate derivative films had sensory analysis scores similar to gelatin. Although none of the starch derivatives had such good scores, some presented some promising results. Alginate and caseinate films were selected for further analysis. The mechanical properties of gelatin and HPMC films were compared by puncture tests. The results at a relative humidity of 44% are similar. However, the effect of the moisture content on both films' mechanical properties showed differences. The fracture patterns and polarised microscopy observation were also very different. Alginate films' mechanical properties were similar to gelatin. However, alginate films are not soluble in acidic environments. The effects of molecular weight on the mechanical properties of cellulose derivatives and alginates films were different. Increasing the calcium content of the alginate sample gave similar results to those obtained by increasing the molecular weight. It is proposed that ultimate deformation occurs through different processes in various films. Alginate/gelatin films are thought to deform through crazing, and the fracture process generates many surfaces (lines). Molecular weight and crosslinking would stabilise the crazes. On the other hand, cellulose derivative would deform through slippage and the energy is dissipated during deformation. This is consistent with the orientation observed after fracture, the lack of new surfaces and the high hydrophobicity of these polymers. Caseinate films of sodium, potassium, calcium and magnesium were studied. Sodium caseinate presented the best mechanical properties. Glycerol proved to be the best plasticiser. Glyoxal crosslinking or increase in pH did not improve the mechanical properties of these films. Caseinate films are poorer than alginate, HPMC or gelatin films. Caseinate deformation processes might occur through both slippage and crazing owing to the low molecular weight and high hydrogen bonding ability. Overall, different deformation processes can lead to similar mechanical behaviour. None of the films studied is likely to replace gelatin or HPMC. More complex systems are proposed for further study.