Reinforcement And Modification Of PVA-based Hydrogels Biomimetic Articular Cartilage Materials

Hydrogels,which show structural similarity to the natural articular cartilage,high water content and excellent biocompatibility,are considered as the potential biomaterials for cartilage repair.The articular cartilage operates in a harsh environment with high alternating physiological shock,complex multi-direction movement,high load friction and wear.So investigation of the novel hydrogel cartilage,which can meet the demand of biomechanics and tribological properties,is a challenging scientific and technical issue.The lacks of mechanical and tribological properties of hydrogels have limited their development as articular cartilage materials.To solve these problems,in the present work,PVA hydrogels were modified by physical blending,double-network gels and by filling with nano-materials.PVA/PVP blend hydrogels,PVA/PAAm IPN hydrogels.PVA/graphene composite hydrogels were prepared by the incorporation of polyvinylpyrrolidone(PVP),acrylamide(AAm)and various graphene(including graphene oxide(GO),monolayer graphene(SG)and multilayer graphene(MG)),and the composite hydrogels were further modified by y-ray irradiation with various irradiation dose(0-200 kGy).The micro-structure,morphology,composition,crystallinity and thermal stability were characterized by SEM,FT-IT,XRD,Raman spectra,XPS,DSC,and TGA,and were compared with the natural cartilage.The swelling behaviors of PVA/PVP blend hydrogels were investigated in both non-osmotic solution and osmotic solutions,which mimic the osmotic environment within the natural articular cartilage.The mechanical performances of uniaxial tension and unconfined compression of PVA composite hydrogels were studied.The dynamic mechanical properties of PVA composite hydrogels were tested by using DMTA and rotational rheometer.The fatigue properties of PVA/PVP blend hydrogels were evaluated under large load,and the recovery behaviors of hydrogels during and after the fatigue tests.The tribological properties of PVA composite hydrogels against various counterfaces in stationary and migrating contact were evaluated under rotary and reciprocating motion conditions,and the effects of movement modes,lubricating conditions,load,sliding speed,diameter and material of the friction counterfaces on the tribological properties of PVA composite hydrogels were investigated.The water-locking and cross-linking effects of graphene in the composite hydrogels were elaborated,and the influence of the layers of graphene on the mechanical properties was analyzed.It was the first to propose and validate that GO could be reduced,and the defects on graphene could be repaired in situ in the composite hydrogels by ?-ray irradiation,followed by the significant improvement in mechanical strength of PVA/graphene composite hydrogels.PVA/PVP blend hydrogels,PVA/PAAm IPN hydrogels,PVA/graphene composite hydrogels exhibited the similar three-dimensional porous network structure of the natural cartilage.The addition of hydrophilic polymers of PVP and AAm,and graphene increased the hydrophily of the PVA hydrogel network,which increased the water contents of the composite hydrogels from 81%of the pure PVA hydrogel to 84-90%,and the porous structure of hydrogels became denser.The addition of PVP,AAm and graphene increased the crystallinity of PVA.The crystallinity of PVA composite hydrogels decreased with increasing AAm content,and increased first and then decreased with increasing GO content.The incorporation of PVP,AAm and graphene improved the tensile strength,compressive strength and tangent modulus of PVA hydrogels.The tensile strength of the composite hydrogels increased first and then decreased with increasing GO content,and increased linearly with increasing SG and MG content.The maximum tensile strength of PVA/GO composite hydrogels was obtained when the GO content was 0.15wt%,which was 109%higher than that of pure PVA hydrogel,and was comparable to the values of PVA/SG and PVA/MG composite hydrogels(1.29MPa and 1.36MPa),but the value was lower than that of the natural cartilage(3.17MPa).The compressive strength of the composite hydrogels were increased first and then decreased with increasing graphene content.When the GO content was 0.10wt%,the compressive strength of PVA/GO composite hydrogels reached the maximum value(0.95MPa),which was similar to the value of PVA/SG composite hydrogels(0.90MPa),and was larger than that of PVA/MG composite hydrogels.Swelling in osmotic solution promoted the crystallization of hydrogels,and significantly decreased the swelling ratio and equilibrium water content,and remarkably increased the compressive tangent modulus.After swelling in osmotic solution for 28 days,the compressive tangent modulus increased significantly,and the friction coefficient of hydrogels decreased by 50%.The addition of PVP,AAm and graphene improved the load-bearing capacity.The deformation of indentation creep of PVA composite hydrogels decreased from 0.45mm of pure PVA hydrogel to 0.28-0.35mm,which was close to the creep deformation of cartilage(0.16mm).The normalized compressive relaxation modulus of the composite hydrogels increased from 0.53 of pure PVA hydrogel to 0.63-0.92.The addition of PVP,AAm and GO increased the storage modulus from 0.12MPa of pure PVA hydrogel to 0.21-0.70MPa,and decreased the loss tangent from 0.037 to of pure PVA hydrogel to 0.035,0.010 and 0.007,respectively,which was similar to the loss tangent of the natural cartilage(0.01-0.10).The mechanical properties of PVA composite hydrogels could recover after 50,000 of fatigue cycles.The addition of hydrophilic polymers of PVP and AAm significantly decreased the friction coefficient of PVA.When the AAm content was 5wt%,the prepared PVA/PAAm IPN hydrogel-on-natural cartilage contact under reciprocating motion showed the lowest friction coefficient(0.016).The incorporation of GO improved the friction properties of PVA/GO hydrogels by enhancing the load-bearing capacity of hydrogels and by the formation of transfer film on the counterface surface,and the friction coefficient of the composite hydrogels decreased with increasing GO content.With the increase of diameter of counterface,the friction coefficient of PVA/PAAm IPN hydrogels decreased gradually.The friction coefficient of PVA composite hydrogels obtained under the bovine serum lubrication was lower than that under deionized water lubrication.Hydrogel sliding against itself showed the highest friction coefficient.The friction reduction effect of MG and GO was better than that of SG.?-ray irradiation made the structure of PVA composite hydrogels denser,and reduced the water content,the crystallinity and thermal stability.y-ray irradiation treatment remarkably increased the compressive strength of PVA composite hydrogels,but showed little effect on the tensile strength.The optimal irradiation dose for the SG reinforced PVA composite hydrogels,and the obtained tensile strength was 107%higher than that of pure PVA.For the GO and SG reinforced PVA composite hydrogels,the compressive strength improved by about 697%(2.87MPa)and 658%(2.73MPa),respectively,when the irradiation dose was 150kGy and 200kGy.Irradiation treatment increased the friction coefficient of the composite hydrogel,but the wear resistance of hydrogels was improved with the increase of irradiation dose.