MUSEOS DE LA SEDA / SILK MUSEUMS

intense electric field exerts a electrodynamic force on the polymer jet, producing filaments of diameter in the range of hundreds of nanometres that are deposited on a collecting plate as a thin layer , similar to a piece of paper. Under the microscope, the material appears as a fabric of randomly oriented filaments, mimicking the configuration of the extracellular matrix of the connective tissue. This matrix is essential for the development of cells, as it provides the physical structure for their adhesion and support. That is why the mat of electrospun fibroin nanofibers is an excellent substrate for the development of cells, and it is widely used, mainly in applications to produce artificial skin grafts and dress- ings for wounds, ulcers and burned tissue. There are other clinical needs that require a scaf- fold with a three dimen- sional configuration and a specific shape. In this case, it is possible to fabricate porous sponges by means of mixing the aqueous fibroin solution with salt grains (NaCl) around 0.8 mm in diameter. The mix is poured in a mould of the required shape and size, an after being dried and solid, the salt grains are dissolved in water. This result in a structure with a network of connected pores of different sizes where cells can grow with an adequate diffusion of nutrients. This type of scaffold is very useful in the regeneration of bone tissue. After a mineralization step of the fibroin with hydroxyapatite and the seed- ing with cells from bone marrow, the differentiation of these stem cells to osteoblasts gives raise to bone tissue, while the fibroin is degraded, resulting in a new tissue, with similar healing effects to autologous bone grafts. Another possibility offered by silk fibroin is the fabrication of nanoparticles, namely, spherical particles of diameters in the range of 100 nanometres. These particles, can be loaded with therapeutic drugs of many different types, while at the same time, can be bonded to antibodies or ligands that direct the loaded particles to the specific cells and tissues where the drug is desired to act. In this way, it is possible a drug delivery more lo- calised and slow, improving its efficacy and minimizing secondary effects. This paradigm is the base of an entire new field, the Nanomedicine and Nanopharmacy, which shows an amazing development in the last years. Nanoparticles can be made of a great variety of biomaterials, but fibroin is a very adequate material for this use, due to the singularity of its molecular structure. The nanofibrils which constitute the dissolved fibroin contain domains of hydrophilic and hydrophobic amino acids that in presence of and organic sol- vent coalesce in the shape of spheroids of 100-150 nanometres. As a consequence, their fabrication is a very simple and inexpensive process. Micrograph of L919 fibroblasts growing on a mat of nanofibers of fibroin, 400 nanometres in diameter, obtained by means of electrospinning. 63