Silk Fibroin/Bacterial Cellulose Nanofiber Ribbon Composite Scaffolds With Gradient Lamellae And Intercalation Structure

Nowadays,the regeneration of bone defect caused by diseases or injury is a big challenge.Clinically,the best therapy to this issue is autologous bone graft which is also the gold standard for treatment of critical size bone defects.However,the rare source of the autologous bone graft,re-operation and the infection of donor site restrict its further applications.Therefore,it is significant to develop bone tissue engineering scaffolds which can meet the medical requirements.Silk fibroin(SF)/ bacterial cellulose nanofiber ribbons(BCNR)composite scaffolds were prepared via lyophilization.The influence of SF concentration,precooling temperature,and BCNR content on the morphology of composite scaffolds were studied systematically.Scanning electron microscope(SEM)images showed that the morphology of the scaffolds were changed from “fiber mat-like” structure to lamellar structure with gradient gap,and finally to un-lamellar structure,when SF content increased.The morphology transformation between lamellar structure and porous structure can be tuned by changing the precooling temperature.The inter-lamellar gap distance and lamellar thickness were influenced by mechanical disturbance conditions.The addition of BCNR can form a intercalation structure between the SF lamellae,where BCNRs partially attach on the surfaces of lamellae and some BCNRs penetrated into the lamellae.The intercalation structure is very beneficial for the improvement of mechanical properties and biological properties of the scaffolds.As a natural polymer material,SF possesses many excellent properties such as good biocompatibility,tunable biodegradability,rare inflammatory response and so on.However,the SF scaffolds normally owned poor mechanical properties.In order to prepare SF-based scaffolds which can meet the bone tissue engineering requirements,the influence of BCNR addition on the mechanical properties of the scaffolds were investigated.Compared to the pure SF scaffolds,the composite scaffolds with SF/BCNR=20:3 exhibited a sixfold to eightfold enhancement in the compression strength(2.32 MPa)or in compression modulus(72.5 MPa),respectively.Both values were in the range of cancellous bone.Fourier Transform Infrared spectra and wide-angle X-ray diffraction indicated that BCNR had strong hydrogen bonding with SF and promoted the crystallization of SF.This is the microscopic reinforcing mechanism of BCNR on SF.Moreover,the addition of BCNR had positive impacts on water absorption and 3D size stability of the scaffolds.The porosity declined slightly with the addition of BCNR,but still remained at high level(>85%).Cytotoxicity experiment in vitro indicated that the composite scaffolds had excellent biocompatibility and the addition of BCNR improved the adherence and proliferation of sclerotomal cells.The simulated mineralization experiment in vitro showed that the addition of BCNR obviously improved the deposition of hydroxyapatite on the laminar surfaces,which indicated the enhancement of bone bioactivity.The radial lamellar pattern and gradient gap distance can transfer nutrient solution and metabolic waste by capillarity and radially guide cells forward from the outer part to the inner part of the scaffolds.The robust three-dimensional composite scaffolds with improved in vitro bioactivity,bone cell adhesion and proliferation are promising for bone defect repair and the scaffolds may improve the repair capacity by further loading multiple bone growth factors for bone regeneration.