Effects of extracting conditions using calcium chloride-ethanol process on characteristics of Thai silk fibroin

Abstract

Effects of extracting conditions using calcium chloride-ethanol (Ajisawa’s reagent) and lithium bromide (LiBr) process on characteristics of Thai silk fibroin (SF) were studied using 1: 30 - 1: 10 (w/v) dissolving ratios at 50 - 80 °C. Yields of Thai SF extractions were at 63 ± 9 - 74 ± 8 %. The Thai SFs were different in their molecular structures and thermal properties although they were similar in their amino acid compositions, molecular weights, zeta potentials, and viscosities. The Thai SFs extracted with the Ajisawa’s reagent had their random coil contents increased with decreases in the dissolving ratios and increases in the dissolution temperatures. They had their thermal decomposition masses, water evaporation enthalpies, and water contents increased with the random coil contents. Thermal decomposition temperatures of the Ajisawa-derived Thai SFs were slightly higher than those of the LiBr-derived Thai SFs. Relative contents in glycine, alanine, serine, tyrosine, and valine of the Thai SFs were at 46.6 - 48.3 mol %, 29.2 - 32.1 mol %, 4.9 - 8.2 mol %, 4.5 - 5.1 mol % and 2.3 - 2.6 mol %, respectively. Weight average and number average molecular weights of the Thai SFs were at 336 - 340 kilo Daltons and 161 - 167 kilo Daltons, respectively. Heavy-chain was degraded while light-chain was preserved at 25 kilo Daltons by the two dissolution processes. As-casted Thai SF films had their β-sheet contents at 33.8 - 35.0 %. The β-sheet contents of the SF films increased to 38.9 - 40.0 % and 42.8 - 46.7 % after treated with water vapor annealing and ethanol immersion, respectively. All SF films had similar total surface energies. Water vapor annealed SF films exhibited more surface polarities than ethanol treated SF films. The water vapor annealed film fabricated from the LiBr-derived SF was detached after 48 h. Early cell attachments of NIH-3T3 mouse fibroblasts cultured on the water vapor annealed films were higher than those cultured on the ethanol treated films. Both of them had their specific growth rates and population doubling times comparable to the cells cultured on tissue culture plates and glass substrates. The preparation processes affected the molecular structures of the Thai SFs. The SF’s molecular structures further influenced other properties including biocompatibilities with cells.