Diversity in structure, composition and properties of silk from African wild silkmoths

Abstract

Wild silk production is a unique eco-friendly agro-practice with enormous potential for income generation and additional benefit of compatibility with conservation goals. African wild silkmoths occur in three major families; Saturniidae, Lasiocampidae and Thaumetopoeidae and have significant economic importance mainly due to their exceptional quality fibers and aesthetic values. Silk cocoon shells from these insects are non-woven structures majorly composed of two proteins, fibroin and sericin, and depending on their origin, exhibit extensive variation in structure, property and composition directly influencing their ecological adaptive functions. Hence, this study was aimed at investigating the extent of variability in structure, composition and properties of silk cocoon shells and fibers from the major African wild silkmoth species: Gonometa postica, Anaphe panda, Argema mimosae and Epiphora bauhiniae. Scanning Electron Microscope (SEM) was used to study the surface and cross sections of the cocoon shells and degummed fibers. The components on cocoon shell surfaces were determined by Fourier Transform Infrared spectroscopy (FTIR). The composition of major fibroin amino acids was established with High Pressure Liquid Chromatography (HPLC). Moisture regain and weight loss of fibers and cocoons were determined by oven dry method. The dissolution properties were studied using 9M aqueous solutions of Cacium chloride (CaCl2), Lithium bromide (LiBr) and Sodium thiocynate (NaSCN). Mechanical properties were measured with Instron tensile testing machine. Thermal decomposition behaviours were determined using Thermogravimetric Analyzer (TGA) at temperature range of 25 - 800 and 25 - 900ºC for fibers and cocoon shells, respectively. SEM micrographs showed that G. postica and A. panda cocoons had thorny spines and hairs on their surface, while A. mimosae cocoons were highly perforated. The FTIR spectra peaks around 1312 and 712cm-1 for outer surfaces revealed the presence of calcium oxalate crystals on G. postica cocoons. Cocoon surfaces also showed great cross bindings, wrinkles and networking of twisting filaments in different shapes and forms conferring rough outer surfaces. Fiber surfaces were irregular with several fibriliar sub-structures running along the fiber axis in A. mimosae and E. bauhiniae. The cross sectional view of degummed fibers revealed elongated, rectangular, triangular, globular and wedge shaped fibers with variable number and size of voids. HPLC results showed that African silk fibroins were characterized by the presence of high proportions of the three amino acids, glycine, alanine, and serine. These amino acids represented 71-74 and 82% of the total amino acids present in wild and Bombyx mori silk fibroins, respectively. No significant difference (P < 0.05) was observed among the moisture regain of the wild silk degummed fibers. However, there were significant differences (P > 0.05) in weight loss and moisture regain among cocoons as well as shell layers. E. bauhiniae cocoons had the lowest weight loss and moisture regain of 23.2 and 5.6%, respectively while G. postica and A. mimosae had the highest weight loss and moisture regain, 56.8 and 9.1%, respectively. In both A. panda and E. bauhiniae, the outer cocoon layers had the lowest moisture regain and highest weight loss while the inner layer of E. bauhiniae and middle layer of A. panda lost the least weight. African wild silk fibers and cocoons also showed significant variability (P > 0.05) in their dissolution behaviours. Degummed fibers were more readily soluble than the cocoon shells. B. mori had higher solubility than the wild silk cocoon shells (51.5%) and fibers (59.3%). Among the wild species, G. postica cocoons and degummed fibers had the highest solubility (37.3 and 51.7%, respectively). LiBr was the most effective dissolving agent for both the cocoons and fibers (41.2 and 84.5%, respectively). The tensile test measurements showed that G. postica and A. mimosae cocoons had lower breaking stress and breaking energy and higher breaking strain than B. mori. Gonometa postica fibers had the highest breaking strain (41.3%), while E. bauhiniae had the lowest breaking stress (237MPa). Wild silk fibers had breaking energy ranging between 34.5-76.4J/cm3. The TGA curves showed A. mimomsae fibers and cocoons had higher temperature for water loss (113 and 154ºC, respectively). G. postica cocoons underwent multistep decomposition between 296-413, 483-506, and 710-730ºC. B. mori had the highest total weight loss for degummed fibers (93.4%), while E. bauhiniae cocoons lost the highest total weight (97.2%). The overall results demonstrated the extensive variability present in structure, composition and properties among silks of the four African wild silkmoths. This could be associated with the selection pressure for environmental adaptation experienced by each species and the difference in molecular organization and composition of silk spun by each species. The physical structure and chemical composition of the cocoon shells and fibers have contributed towards the variations observed in the properties and these features have commercial and industrial implications. Hence, the value of silks when compared across taxa should be based on suitability of the fiber properties for specific application as well as potential silk products.