Structure and physical properties of silkworm cocoons

Abstract

Silkworm cocoons have evolved a wide range of different structures and combinations of physical and chemical properties in order to cope with different threats and environmental conditions. We present our observations and measurements on 25 diverse types of cocoons in a first attempt to correlate physical properties with the structure and morphology of the cocoons. These two architectural parameters appear to be far more important than the material properties of the silk fibres themselves. We consider tensile and compressive mechanical properties and gas permeation of the cocoon walls, and in each case identify mechanisms or models that relate these properties to cocoon structure, usually based upon non-woven fibre composites. These properties are of relevance also for synthetic non-woven composite materials and our studies will help formulate bio-inspired design principles for new materials.

Carbondioxide Gating in Silk Cocoon

“…It is of interest to understand how silk cocoon membrane performs such activity based on their porous structure, mechanical strength, and its potential for control of the gaseous environment and temperature inside. One should try to understand, how life inside a narrow confinement of cocoon breath for so many days and the most important aspect here is to address the structure of cocoons which is optimized to function in such a manner [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18].…”

Environmental effects on the construction and physical properties of Bombyx mori cocoons

“…Spinning takes place over several hours as the larva draws a continuous silk thread from labial glands in its head in a figure-of-eight fashion [3]. As a material, the cocoon is a random-fibre non-woven composite [4,5]. It consists of two principle proteins, heavy and light chain fibroin (which form the fibre) and at least five sericin 'gum' proteins (which coat and bind the fibres together and account for 20-30 % of the cocoon mass) [6][7][8].…”

The impact behaviour of silk cocoons

“…In nature, O. eucalypti seems to have tolerance of holes in the cocoon, as it fabricates woven holes around its attachment in the cocoon construction (Fig.1). This may be a compensating strategy for ventilation as it has very low porosity in the cocoon walls compared with other species (Chen et al, 2012c). These fabricated holes do not appear to interfere with the structure and mechanical properties of the rest of the cocoon wall, unlike other nonwoven cocoons.…”

“…Four types of silk cocoon were categorised according to their different mechanical behaviours in a previous paper (Chen et al, 2012c): (1) 'lattice' cocoons, with a loose scaffold structure, (2) 'weak' cocoons, with high porosity and weak interlayer bonding properties, (3) 'brittle' cocoons, with low porosity, strong interlayer bonding and brittle mechanical properties, and (4) 'tough' cocoons, with medium porosity, interlayer bonding and tough mechanical properties. Here a typical cocoon was selected from each of the latter (2-4) three nonwoven cocoon categories in order to investigate their failure mechanisms.…”

“…More applied silkworm research tends to focus on methods to best extract the silk from the cocoons of the domesticated silkworm Bombyx mori or from wild cocoons (Gheysens et al, 2011;Kundu et al, 2012), or indeed on turning those silks into useful textiles or other products. In contrast, the structure of the cocoon itself has largely been ignored, although recent research shows that cocoons can be considered as hierarchical composites composed of silk fibres and sericin bonds, which confers a number of unique properties to the cocoon (Chen et al, 2012a;Chen et al, 2012b;Chen et al, 2012c).…”