Stimuli responsive hydrogels (SRHs) are attractive bioscaffolds for tissue engineering. The structural similarity of SRHs to the extracellular matrix (ECM) of many tissues offers great advantages for a minimally invasive tissue repair. Among various potential applications of SRHs, cartilage regeneration has attracted significant attention. The repair of cartilage damage is challenging in orthopedics owing to its low repair capacity. Recent advances include development of injectable hydrogels to minimize invasive surgery with nanostructured features and rapid stimuli-responsive characteristics. Nanostructured SRHs with more structural similarity to natural ECM up-regulate cell-material interactions for faster tissue repair and more controlled stimuli-response to environmental changes. This review highlights most recent advances in the development of nanostructured or smart hydrogels for cartilage tissue engineering. Different types of stimuli-responsive hydrogels are introduced and their fabrication processes through physicochemical procedures are reported. The applications and characteristics of natural and synthetic polymers used in SRHs are also reviewed with an outline on clinical considerations and challenges.

In this study, the possibility of keratin extraction from wool and feather by an enzymatic treatment along with a reducing agent has been investigated. The effects of different parameters, that is, enzyme loading, type of substrate and surfactant, hydrolysis time, and reducing agent concentration, have been examined in order to optimize the enzymatic hydrolysis. The optimal condition for maximum keratin extraction was attained by making use of 1 g/L sodium dodecyl sulfate (an anionic surfactant) and 2.6% (v/v) protease (Savinase), along with 8.6 and 6.4 g/L sodium hydrogen sulfite (a reducing agent) for wool and feathers, respectively, at liquor to fiber ratio of 25 mL/g for 4 hr. The obtained results indicated higher degradation of wool fiber in comparison with feathers, which might be due to the higher hydrophilic nature of the former. Sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) patterns revealed that the molecular weights of the extracted proteins from wool and feather were lower than those for the untreated fibers. Scanning electron micrographs showed fibers fibrillation and degradation upon enzymatic treatment. Besides, Fourier-transform infrared (FTIR) spectra indicated no evident changes in the chemical structure of the hydrolyzed fibers. However, wool and feather remainders were mostly composed of α-helix and β-sheets conformations, respectively.

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