Silk fibroin has a unique and useful combination of properties, including good biocompatibility and excellent mechanical performance. These features provided early clues to the utility of regenerated silk fibroin as a scaffold/matrix for tissue engineering. The silk fibroin scaffolds used for tissue engineering should degrade at a rate that matches the tissue growth rate. The relationship between secondary structure and biodegradation behavior of silk fibroin scaffolds was investigated in this study. Scaffolds with different secondary structure were prepared by controlling the freezing temperature and by treatment with carbodiimide or ethanol. The quantitative proportions of each secondary structure were obtained by Fourier transform infrared spectroscopy (FTIR), and each sample was then degraded in vitro with collagenase IA for 18 days. The results show that a high content of βsheet structure leads to a low degradation rate. The random coil region in the silk fibroin material is degraded, whereas the crystal region remains stable and the amount of β-sheet structure increases during incubation. The results demonstrate that it is possible to control the degradation rate of a silk fibroin scaffold by controlling the content of β-sheet structure.
Cell-microstructure surface interactions play a significant role in tissue engineering to guide cell spreading and migration. However, the mechanisms underlying cell-topography interactions are complex and remain elusive. To address this topic, microsphere array patterns were prepared on silk fibroin films through polystyrene microsphere self-assembly, followed by culturing rat bone marrow derived mesenchymal stem cells on the films to study cell-substrate interactions. Filopodia sensed and anchored to the microspheres to form initial attachments before spreading. Importantly, the anchored filopodia converted into lamellipodia, and this conversion initiated the directional formation of lamellipodia. Therefore, the conversion of exploratory filopodia into lamellipodia was the main driving force for directional extension of the lamellipodia. Correspondingly, cell spreading, morphology, and migration were modulated by pseudopodial recognition and conversion. This finding demonstrated that filopodia not only act as an antenna to detect microenvironment but also serve as skeleton to guide lamellipodial extension for directing cell motions. The micropatterned films promoted cell adhesion and proliferation due to accelerated lamellipodia formation and cell spreading, with recognition and conversion of filopodia into lamellipodia as a critical role in cell response to surface topography.
Although regenerated silk fibroin (SF), which has excellent biocompatibility, biodegradability, and a low inflammatory response in vivo, has promising applications in tissue engineering, the mechanical properties and biofunctionality must be further improved to satisfy tissue-engineering applications. Cellulose nanofibrils (CNFs) are promising candidates for bionanocomposite production due to their ultrahigh strength and excellent biocompatibility. In this study, CNFs were extracted directly from microcrystalline cellulose using an aqueous lithium bromide solution, a typical solvent for dissolving SF fibers. As a result, SF/cellulose nanocomposite films with improved tensile strength were fabricated using aqueous lithium bromide solution as a novel solvent system for the dissolution and blending of SF and cellulose. The extracted CNFs were homogeneously dispersed within the composite films through the rapid gelation of cellulose. The degradability of the composite films in a protease XIV solution was strongly dependent upon the SF component, which significantly promoted the degradation rate of composite films. Adhesion and proliferation results showed that SF/cellulose nanocomposite films promoted cell viability. Our work suggests a facile and effective approach for designing SF/cellulose nanocomposites that may have wide potential applications in tissue engineering.
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