It is a protein-based biomaterial, highly biocompatible, with excellent mechanical properties, biodegradable and it can be processed in different configurations (dissolutions, hydrogels, films, sponges, particles and nanofibers) 2, 3 according to the needs of the clinical condition to be treated or the tissue to be repaired. The advantages offered by fibroin are numerous, in relation to the most commonly used materials. It has been proven that SF, apart from its obvious textile application, present uses of great interest in the field of biomedicine and tissue engineering. This molecular structure is what gives the material a unique combination of mechanical strength and elasticity. These crystalline regions alternate with other non-repetitive primary sequences, and therefore, less organised. The heavy chain contains 12 domains of repeated amino acid motifs, which constitute the crystalline region of the fibre. At the same time, six of these structural blocks are connected together through their interaction with the glycoprotein P25, also named as fibrohexamerin (25 kDa) 1. Moreover, SF is composed of two protein components: a heavy chain (390 kDa) and a light chain (26 kDa) present in a ratio 1:1 and linked by disulphide bonds. The silk filament is constituted by fibroin and sericin, the second one, is the water-soluble protein, with a globular structure that holds the filaments of fibroin together. The silk of Bombyx mori (silkworm) is composed of two proteins that present interesting biological properties. Silk fibroin (SF) biomaterials are attracting the attention of numerous researchers around the world, especially over the past two decades.
On the other hand, and contrary to expectations, the proliferation of fibroblasts growing on the materials was improved by all the different stifling methods, compared to negative control, being this improvement, especially accentuated, on the films produced with fibroin purified from cocoons treated with dry heat. Tensile strength and strain at break values were detected as significantly lower when this procedure was carried out by means of dry heat (85 ☌) and sun exposure. It is also exposed the impact of the stifling on the mechanical properties of the materials. The β-sheet content, analysed by means of infrared spectroscopy, was significantly higher when stifling was performed at higher temperature (70 ☌ and 85 ☌).
Structural changes are also described for annealed silk fibroin films. The protein degradation (visualised by SDS-PAGE) was dramatically increased in all the fibroin dissolutions produced from stifled cocoons heavy and light chains of fibroin were specially degraded, reducing their presence along the lanes of the gel compared to the negative control (untreated fresh cocoons). In this work, the consequences of the stifling treatments most commonly used by the silk producing countries and companies are explored in depth, using fibroin films as biomaterial model. None of the scientific articles related to silk fibroin biomaterials has previously taken into account this fact in its section of materials and methods. All of them involve the application of aggressive steps, such as sun exposure, hot steam from boiling water or hot air, during hours or even days. Stifling treatments are applied to silk cocoons in order to kill the pupae, preventing the emergence of moths and allowing to preserve the silk during long periods of time.
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