MUSEOS DE LA SEDA / SILK MUSEUMS

groups, that allow the covalent bonding of a great variety of ligands and proteins, such as cellular growth factors, antibodies, enzymes, and so on. This feature is very relevant for the success of the material, given that those ligands act in the activation of cellular signal- ling pathways that promote stem cell differentiation to specific cell types, and also regulate their adhesion and movements. This biochemical dialogue between cells and biomaterial is a key element in Tissue Engineering. A considerable advantage of the fibroin is its good mechanical behaviour. The silk of B. mori is one of the organic biomaterials with the highest resistance to traction. This aspect is relevant for applications where resistance is needed, such as scaffolds for ligaments, tendons and bone. By contrast to traditional prosthetic devices, novel scaffolds need to degrade and disap- pear as the growing cells develop their own extracellular matrix structures and form a complete tissue. The degradability of the biomaterial should be controlled in order to modulate the development of the new tissue. The silk fibroin has a clear advantage in this sense, as its degradation rate, directly related to its proportion of beta-sheet secondary structure, can be modulated by the post processing of the material. This degradation is usually slow in silk fibroin scaffolds, allowing time for a correct regeneration of the tissue before disappearing. Finally, it has to be mentioned the fact that silk fibroin is produced in a very simple, cheap and scalable way. Practically all its processing until constituting a biomaterial is made in aqueous media without the need of toxic chemicals or reagents. 3.2) Formats of silk fibroin biomaterials Silk fibroin can be processed in an enormous diversity of formats and configurations as biomaterial: films, sponges, gels, electrospun mats, nanoparticles and fibres. Each one of these formats is suitable for diverse clinical applications. The fabrication of silk fibroin biomaterials starts by the removal of the pupa from the cocoon and a subsequent de- gumming step employing hot water to remove the sericin coat of the fibre. The resulting fibrous material of pure fibroin is dissolved in a salt such as concentrated lithium bromide, which is later removed by dialysis in ultrapure water. This process results in an aqueous dissolution of fibroin which is the starting point for all the successive processing steps towards different formats. The most simples of them is the film. It is made by means of pouring a certain amount of liquid fibroin on a flat surface and waiting for the evapora- tion of water. The resulting lamina (film) of pure fibroin is totally transparent and once soaked in a cell culture medium, can be seeded with cells of diverse types that grow to confluence and form monolayers similar to epithelial tissue. This material is being used at present to fabricate biomimetic scaffolds for corneal stroma in ocular reconstruction. Given that the cornea has a multilayer structure formed by collagen where the corneal keratocytes grow, the piling and compression of films of fibroin seeded with these cells can imitate the structure of a native cornea. Another format with a two-dimensional configuration is a non-woven mat of fibroin na- nofibers obtained by the technology of electrospinning. This technique is based on the projection of a jet of a polymer in a strong electric field of thousands of kilovolts. This 62