Mechanisms of polymer ca2+ interaction and their effects on the characteristics of alginate microspheres and films 2

Part II. Influence of different cross-linking methods on the properties of alginate microspheres produced by emulsification. A. Production of the alginate microspheres by the emulsification/external crosslinking method A1. Preparation of partially cross-linked alginate for microsphere production A small amount of calcium chloride was incorporated into sodium alginate to produce a partially cross-linked alginate in solution. The calcium chloride solution was added slowly with vigorous stirring to avoid rapid clumping of the alginates due to cross-linking (Figure 4). Concentrations of the calcium chloride above 0.08% w/w in calcium chloride-alginate mixture led to an almost instantaneous sol-gel tranformation in the absence of high shear stirring, making it impossible to use the mixture for microsphere formation by emulsification. Therefore, 0.08 % w/w of calcium chloride was carefully incorporated as it allowed the mixture to remain in the fluid state for a sufficient period of time after stirring had stopped. During the shearing process, heat was generated which might cause degradation of the macromolecular alginate polymer into lower molecular weight fragments. An exponential decrease in the viscosities of alginate solutions with decreasing molecular weight of the polymer had been reported (Kong et al., 2003). Hence a change in the viscosity of the solution could be used to indicate polymer degradation. No significant difference in viscosities was observed between SA solution and S1 mixture (p>0.05), indicating that the shearing action and associated heat produced was unlikely to cause degradation of the alginate used (Figure and Table 13). 98 Table 13. Viscosities of alginate solutions subjected to high shear with/without partial cross-linking and the properties of resultant microspheres. Code Solution/ mixture viscosity (mPas) Properties of microspheres Mean size (µm) Sphericity Drug content (mg) t75% (min) SA 169.62 ± 0.98 192.64 ± 3.85 1.04 ± 0.001 14.02 ± 0.03 17.1 S1 172.28 ± 0.59 197.6 ± 3.85 1.04 ± 0.002 14.13 ± 0.02 22.9 S2 166.20 ± 3.17 216.62 ± 4.86 1.05 ± 0.003 13.35 ± 0.01 14.3 S3 242.76 ± 1.16 219.82± 7.96 1.05 ± 0.002 13.87 ± 0.03 14.3 99 99 A2. Properties of microspheres produced from the partially cross-linked alginate Different amounts of Ca2+ were added to the partially cross-link sodium alginate before it was used to produce microspheres by the emulsification/external cross-linking method. The microspheres obtained were compared with those prepared from sodium alginate (i.e. without partial cross-linking). This study was aimed at evaluating the effect of the cross-linking technique on the properties of the microspheres produced. The microspheres produced from the different alginate mixtures were spherical, with mean diameters ranging from 192 to 220 µm (Figure 13 and Table 13). S3 microspheres had the widest size distribution among the different batches. SA and S1 microspheres were significantly smaller than S3 microspheres (p[...]... 51.03 ± 1 .23 88 .23 ± 3.16 29 74.40 ± 1 12. 15 65.71 ± 0.39 CAI 0 .25 59.00 ± 1. 62 86.56 ± 4.67 26 46.34 ± 79.91 73.55 ± 0.48 CAI 0.35 66 .22 ± 2. 21 87 .23 ± 2. 13 22 10.71 ± 22 0.17 76 .24 ± 0. 72 CAI 0 .25 S - 84.51 ± 3 .26 323 0.33 ± 199.01 - CAI 0 .25 N - 90 .22 ± 1.61 327 0 .21 ± 28 7.63 - Film Code 1 12 SA CAE 0.15 CAI 0.15 CAE 0 .25 CAI 0 .25 CAE 0.35 CAI 0.35 Figure 17 Scanning probe microscope images of alginate films. .. strong peaks around 1737 cm-1 were absent in the FTIR spectra of the calcium cross-linked alginate films, indicating potential interactions involving C=O groups (Figure 16) It could therefore be inferred that the pH of the medium used promoted interactions between Ca2+ and alginate The displacement of Ca2+ by H+ could be controlled by careful manipulation of the pH of the medium The FTIR spectra of the. .. (20 02) attributed this to the displacement of Ca2+ by H+ from the acid added While the acid liberated Ca2+ from the insoluble salt, it also competed with Ca2+ for interaction with the alginate Therefore, the pH range of the cross-linking media had to be carefully controlled Addition of glacial acetic acid enabled liberation of Ca2+ from the insoluble calcium salt was attempted, but it also reduced the. .. calcium-induced gelling zone resulted in a dense “membrane” of high alginate concentration at the Ca2+- alginate interface (Skjåk-Bræk et al., 1989) This phenomenon would retard the diffusion of acetaminophen out of the micropellets On the other hand, the release of CO2 from CaCO3 within the alginate matrix gave rise to matrix disruption for the internal cross-linking method When the CO2 escaped into the surrounding... cross-linked films, the increased availability of Ca2+ was accompanied by higher level of CO2 liberation Hence, formation of films of comparable tensile strength resulted as the positive effect of cross-linking was counterbalanced by the negative effect of CO2 generated The detrimental effect of CO2 generation was further illustrated by the formation of stronger externally cross-linked films compared to the. .. matrices The thickness of internally cross-linked matrices was dependent on the balance between opposing effects of cross-linkage and CO2 produced, whereas the thickness of externally cross-linked films was controlled by the distribution pattern of crosslinkage Cross-linking of alginate by Ca2+ would bring the polymer chains closer together leading to film contraction and decreased film thickness On the other... mechanical properties and calcium content of dried calcium alginate films Ra Value (nm) Tensile Strength (N/mm2) Elastic Modulus (N/mm2) Ca2+ content (mg/g of film) SA 44.68 ± 3.00 78.78 ± 2. 13 1674.66 ± 81.41 - CAE 0.15 45.09 ± 0.77 89 .28 ± 3.55 3141.97 ± 91 .28 67.01 ± 0.89 CAE 0 .25 53.53 ± 1.16 95.89 ± 2. 78 3017.95 ± 127 .73 76. 42 ± 1. 72 CAE 0.35 52. 62 ± 1 .26 99.86 ± 4. 92 2533 .22 ± 163.79 79.48 ± 1.84... complete liberation of Ca2+ from the insoluble CaCO3 Protons, which are smaller in size than Ca2+ diffuse more rapidly and with greater ease into the alginate matrix (Quong et al., 1998) Hence, all the CaCO3 used for internal gelation was available for cross-linkage No significant difference (p>0.05) was observed in the Ca2+ content of CAI 0 .25 and CAI 0.35 films indicating that 0 .25 g of CaCO3 was sufficient... CAI 0 .25 CAI 0 .25 S and CAI 0 .25 N was observed (p>0.05), indicating that size and number of cavities were equally important to mechanical strength of the film The two cross- linking mechanisms gave rise to similar extent of binding between Ca2+ and alginate as illustrated by an insignificant difference in Ca2+ contents between CAE 0.15 and CAI 0.15 and between CAE 0 .25 and CAI 0 .25 (Table 14) The cross-linked... cross-linked alginate matrices for retarding or controlling drug release C2 Alginate micropellets Calcium alginate micropellets were produced to examine the effects of different cross-linking mechanisms on the properties of calcium alginate particles for the encapsulation of drugs Micropellets were used rather than microspheres as they were easily prepared and had similar shape as microspheres C2.1 Formation and . 26 46.34 ± 79.91 73.55 ± 0.48 CAI 0.35 66 .22 ± 2. 21 87 .23 ± 2. 13 22 10.71 ± 22 0.17 76 .24 ± 0. 72 CAI 0 .25 S - 84.51 ± 3 .26 323 0.33 ± 199.01 - CAI 0 .25 N - 90 .22 . differently and the properties of the matrix to different extents. Unravelling the cross-linking mechanisms and their effects on matrix integrity would assist in the control of the interaction between. eliminate the adverse effects of in vacuo filtration and oven drying on the structural integrity of the alginate microspheres. The M5 and M6 microspheres produced with 0.5 g and 0.8 g of CaCO 3