The High Mechanical Strength Microfibrillated Cellulose/Gelatin Composite Hydrogels With Tunable Network Structure

The diseases and injuries of human tissues and organs seriously endanger human health.The tissue engineering technology is expected to break the limitations of traditional medical treatment such as organ transplants and the biological material implant.Good biocompatibility is the premise condition of tissue engineering scaffolds,and scaffold materials should have the similar mechanical properties and threedimensional structure as the cartilage.In this paper,two kinds of natural polymer materials with good biocompatibility were used to prepare cartilage tissue engineering scaffold.Modified cellulose/Gelatin composite hydrogels were prepared,with a single network,an interpenetrating network and a giant network structure,respectively.(1)Cellulose was oxidized with TEMPO/NaBr/NaClO.TEMPO-oxidized cellulose was homogenized with a high-pressure homogenizer at the pressure of 1000 bar for 30 passes.The elementary fibrils with diameter of 3-5 nm(T-MFC)could be separated during the homogenization.The FTIR showed that the cellulose could be effectively oxidized by TEMPO/NaBr/NaClO system.Carboxyl functional groups were introduced.The effects of NaClO dosage,oxidation time and passes of homogenization on the product were investigated.The content of carboxyl group increased with the increase of NaClO concentration and oxidation time.SEM and TEM characterization showed that during the process of TEMPO system oxidation,with the NaClO concentration and oxidation time increasing,the diameter of the homogenized cellulose microfibrils decreased and the microfibrils were dispersed more uniformly.(2)T-MFC was mixed with gelatin and hydrogels with single network structure was prepared.Then EDC(1-(3-Dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride)was added to the mixture,hydrogels with interpenetrating network structure was obtained.The FTIR showed that with the addition of EDC,the free amino and carboxyl groups of gelatin reacted with each other to form amide bond.The SEM showed that the addition of T-MFC increased the pore size inside the composite hydrogel.With the increase of T-MFC content,the pores in the hydrogel changes denser and the pore size decreases gradually.The addition of EDC makes the pore structure more uniform and the pore size increased.The compression test showed that with the increase of T-MFC content,the maximum compressive strength of T-MFC/gelatin composite hydrogels could be increased by three times.The maximum compressive strength of composite hydrogel crosslinked by EDC was 16.78 times higher than that of pure gelatin hydrogel.Besides,the addition of T-MFC and EDC enhances the ability of the composite hydrogel to keep the original morphology and structure in the PBS buffer solution,and also extends the degradation cycle of the composite hydrogel in PBS+GSH enzymatic degradation fluid.(3)T-MFC was further oxidized by periodate to convert 1,2-dihydroxyl(glycol)groups to double aldehyde groups,thus the double oxidized MFC(D-MFC)was prepared.The content of aldehyde groups was determined by copper titration.The amount of the aldehyde group reached a maximum of 0.45 mmol/100 g at 9 h.Blending D-MFC with gelatin,the aldehyde groups of D-MFC reacted with the free amine groups of gelatin to form a giant network.The SEM showed that,with the increase of D-MFC content in composite hydrogel,the pores were more dense and the pore size decreased gradually.The compression test showed that with the increase of D-MFC content,the maximum compressive strength of hydrogels gradually increased.When the content of D-MFC reached 4%,the compression strength of hydrogel could reach 1.34 MPa,74 times higher than that of pure gelatin hydrogel.The addition of D-MFC and EDC enhanced the ability of the composite hydrogel to keep the original morphology and structure in the PBS buffer solution,and the degradation cycle of the composite hydrogel in PBS+GSH enzymatic degradation was prolonged.