Sequential Release Of Bioactive Factors From Core-shell Alginate/Chitosan Microspheres

The ability of articular cartilage to repair itself after injury is extremely limited.It is difficult to achieve stable long-term repair of articular cartilage function with the commonly used clinical methods.The use of scaffolds,seed cells and cytokines in tissue engineering for the repair and treatment of articular cartilage injuries has now shown promising clinically potential.In general,protein cytokines are prone to be inactive and have a short half-life in vivo.In cases of in vivo cytokine applications,the key issues include the effective dose control and sustained maintenance of their effects.In the field related to the repair and reconstruction of articular cartilage,how to use biodegradable and biocompatible natural polymer materials for locally controlling the dose of a single factor or a combination of factors has been one of the hot issues in this field.Sodium alginate and chitosan are two kinds of natural polymers that are very commonly used in tissue engineering.Due to their respectively anionic and cationic characteristics,they have been combined for use in preparing scaffolds,fibers,hydrogels and microspheres.In this study,they were intended to be used for the preparation of core-shell microspheres and some attempts were made to deliver two kinds of bioactive factors for the repair and reconstruction of damaged articular cartilage.An emulsion crosslinking method and a layer-by-layer self-assembly method were respectively used to prepare different alginate/chitosan microspheres with core-shell structures.In the self-assembly method,alginate microspheres with appropriate structure and particle size were first prepared via orthogonal design method and they were then used as the core portion of core-shell microspheres.The multilayer assembly on the surface of alginate microspheres was conducted by using alginate and chitosan as the shell materials.By selecting bovine serum albumin and ovalbumin as model drugs,the effects of alginate concentrations,calcium chloride amount and different assembled layers on the drug loading capacity and release patterns of composite microspheres were studied.The experimental results showed that the release rate of bovine serum albumin slowed down with the increase of the number of assembled layers,and the drug release rate of alginate/chitosan microspheres with a shell structure was significantly lower than that of alginate microspheres without assembly.However,the experimental results showed that the alginate/chitosan microspheres obtained by self-assembly technique could effectively load the model drug inside the core portion,but not in the shell portion.Core-shell alginate/chitosan microspheres were also prepared by direct emulsification method.The microspheres had good spherical shapes and suitable particle size as well as narrow size distribution.In comparison to the microspheres without genipin crosslinking,the drug loading and encapsulation efficiency of alginate/chitosan microspheres with genipin crosslinking were significantly increased,and the release rate for bovine serum albumin loaded inside the core portion was also significantly reduced.The circular dichroism analyses showed that the released bovine serum albumin could well maintain its second-level structure.It was concluded that the core-shell alginate/chitosan microspheres prepared by direct emulsion crosslinking method could control the release of drug loaded inside their core.The follow-up studies on the shell structure and the activity of active molecules loaded in the shell portion are now in progress.It can be expected that the core-shell alginate/chitosan microspheres are able to control the sequential release of bioactive factors and have potential in cartilage tissue engineering.