Collagen and proteins from silk fibers have biomedical applications. Silk fiber proteins are created by arthropods like spiders and silkworms. These proteins have mechanical properties and are not toxic. The silk associated research today involves usage of silk in cell culture, tissue engineering, especially concentrating on cell surface adherence, mechanical properties , biodegradability, immunogenicity and toxicity. Apart from the elastic and mechanical properties of natural silk fibers, they are medically useful in inducing weak immune response.
Silk fibers are naturally produced by arthropods like silkworms (Bombyx mori and Anthereae pernyi) and spiders (Nephila clavipes and Araneus diadematus). The fibroins present in the silk fibers of B.mori consists of heavy and light chains linked by disulfide bonds and a glycoprotein called as P25. The glycoprotein is linked with the heavy or light chains with non-covalent hydrophobic interactions. Certain alternate procedures to generate proteins from spider silk were studied and they were dependent on recombinant DNA technology.
Investigation of expression of spider silk proteins was done in bacterial hosts, plants, yeast, mammalian cells and insect cells. The advantage of recombinant silk protein compared to the natural or regenerated silk proteins is the effortlessness in which they are modified genetically. Recombinant silk protein can lead to the formation of other material forms like foams, fibers, hydrogels, films, spheres and capsules.
The primary structure of the silk protein from fibroin of B.mori comprises of a repetitive sequence of a hexapeptide called GAGAGS. The sequence GPGXX is found to be present in the silk protein taken from the ampullate gland of spiders named as A. diadematus and N. clavipes. The variation in the amino acid sequence of silk protein will have an impact on the physicochemical and mechanical properties of it. These differences will influence the extensibility, strength and toughness of the silk protein. The use of biomaterials in the cell culture is by providing an environment for cell adhesion, differentiation and proliferation. These biomaterials also support the regeneration of organs and tissues.
Applications of silk protein
Cell anchorage to silk surface
Cell-matrix anchoring is supported by the membrane proteins called as integrins. These integrins have adherence domains that can bind to the amino acid sequence present in the extracellular matrix proteins. The adherence of cells to the matrix is dependent on the type of cell. The fibroin nanofibers from B.mori coated with collagen I, fibronectin III, elastin or laminin enhanced the spread of keratinocytes of the human mouth, epidermal keratinocytes, gingival fibroblasts and mesenchymal stem cells of human in the culture compared to the cells cultured on plain silk nanofibers.
The cell adhesion property of silk proteins is improved by adding the peptidic cell binding domains like RGD motif (tri-peptide arginine-glycine-aspartic acid) to the silk proteins. This specific motif is present as a adhesion sequence in integrins available in fibronectin III.
Elasticity of silk fibers
Elasticity of the matrix plays a vital role in the cell survival, differentiation and proliferation of the cells. The mechanical properties of the extra cellular matrix stimulates cell spreading and alterations in the morphology of the cell. The synthesis of the specific transcription factors is repressed or stimulated by the ECM. The transcription factors can induce the establishment of specific phenotypes compatible with the organ, generating these cells.
The scaffold elasticity utilized for cell culture play a vital role in migration of cells as well as in directing locomotion of the cell. The mechanical characteristics of the tissue might get altered due to the changed pathology which triggers abnormal responses in the cell. In the silk associated materials, the scaffold stiffness depends on the extent of beta sheets and water content present in them.
If the silk films are kept inside methanol for 10 minutes, the beta sheets are formed in it by increasing the elastic modulus to 40 MPa. If the silk is dipped for 60 minutes then the beta sheet formation enhances the elastic modulus of the films to 80 MPa. Except the bone, all other tissues in the human body have the elastic modulus between 1 and 200 kPa, while the silk based scaffolds can attain the elastic modulus greater than 400 kPa.
The distribution and diffusion of pore-size
The transport of nutrients, oxygen In Vitro using 3D matrices depend on the morphology of the matrix, variation in the concentration of the waste products, nutrients and oxygen between the matrix core and media. The transport also depends on the physicochemical interaction between these products and matrix.
The silk scaffolds will show the pore size of 0.5 to 1000 micrometers, which will allow the movement of massive amounts of nutrients, proteins and waste. The presence of large pores might allow the cells to migrate out of the scaffold. The large pores also might allow the antibodies inside resulting in the destruction of immobilized cells.
Toxicity of silk material
The viability of human endothelial cell line -1, proliferation of cells and metabolic activity of the cells on the degummed silk fibers of A. pernyi or B. mori showed that there was decreased cell spreading and proliferation than the control without silk fibers. The toxicity of silk obtained from A. pernyi might be due to the cytotoxic components present in these fibers.
Many In Vivo and In Vitro experiments have revealed that silk materials show lots of advantages like lower degradation and lower immunoreactivity.
Aldo Leal-Egana and Thomas Scheibel. Silk-based materials for biomedical applications. Biotechnol. Appl. Biochem. (2010) 55, 155-167. DOI: 10.1042/BA20090229.
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