Although Bombyx mori cocoon has a 3-D nonwoven structure
that is made from a single continuous strand, it has also a
multi-layer structure with fewer fibres connecting layers than
aligning in the individual layers. The inter-layer bonding is much
weaker than the intra-layer bonding, which makes it easy to separate
individual cocoon layers.
Fig. 2 shows the graded morphologies of cocoon components in
different layers throughout the thickness of a cocoon wall. From
the inner to the outer surface, the porosity and amount of sericin
increase while the amount of bonding between fibres decreases.
Although sericin is a bonding agent between the fibres and thus
maintains the cocoon’s structure, we note that in the outer layer
the sericin coating does not interconnect the fibres, so it does not
form additional bonding between fibres. In the inner layers, the
sericin covers the surface of fibroins efficiently to form a highly
bonded network.