Study On Low Wear And Long Term Lubrication Mimic Joint System Based On Thixotropic Hydrogel

Ultra-high molecular weight polyethylene（UHMWPE）is an important component material in artificial joints due to its good biocompatibility,chemical resistance,wear resistance and impact resistance.However,because artificial joints do not have joint capsules,exogenous lubricating fluid is easy to lose.Compared with metals and ceramics,UHMWPE is weak and easy to be worn.It produces wear debris during the use of artificial joints and induces osteolysis around the joint prosthesis,resulting in no Bacterial loosening and failure of the prosthesis.Therefore,constructing the nature and structure of imitating natural joints to improve the lubrication of UHMWPE in artificial joints is of great significance for prolonging the life of artificial joints and improving the quality of life of patients.The surface porous structure of natural articular cartilage helps to store natural synovial fluid.When the cartilage is compressed,the synovial fluid leaks out of the pores to lubricate the joint surface.After the pressure is removed,part of the synovial fluid can re-infiltrate the pores to make the cartilage It has the effect of long-term self-lubrication;the shear thinning characteristics of natural synovial fluid make it have a higher viscosity at low shear rates,and can form a good extrusion film to protect joints.With the increase of shear rate,natural synovial fluid The viscosity of synovial fluid decreases,which can better lubricate the joints and reduce the wear between the joints.In the early stage,the research group used natural biomass proteins,polysaccharides and salvianolic acid to synthesize a thixotropic hydrogel.When the hydrogel is subjected to shearing force,it changes from a gel state to a sol state and stops shearing.After the force is applied,it changes from the sol state to the gel state.In this paper,the above-mentioned thixotropic hydrogel is used as the lubricating fluid to simulate the lubricating performance of natural joint synovial fluid;the hydroxyapatite with good biocompatibility is used as the porogen,and the porogen-filtration method and thin-layer tiling are used.The surface bionic porous UHMWPE was prepared by the method,and the bionic artificial joint system was designed by combining the two.The shear thinning property of the thixotropic hydrogel in the friction process was used for lubrication.After the friction was stopped,the porous structure of the surface could store the thixotropy.The mechanism of hydrogel achieves long-term lubrication.The main research and results of the thesis are as follows:（1）Using hydroxyapatite powder（HA）as a pore-forming agent,the HA and UHMWPE powders were laid in a thin layer and hot pressed together,and then the HA was removed by acid to prepare surface bionic porous UHMWPE with different surface porosities.The observation results of light microscope and scanning electron microscope show that the pores are evenly dispersed,the porosity is 25%,36%and 44%,the pore size is in the range of 50-80μm,and the average thickness of the porous layer is about 70μm.The surface porous structure reduces the water contact angle of UHMWPE.As the porosity increases,the surface water contact angle decreases from 102.4±1.3°to 80.4±2.0°.As the porosity increases,the porous structure of the surface layer will significantly reduce the Young’s modulus,tensile strength and elongation at break of UHMWPE,but the effect on the yield strength is not obvious,all within the range of 19-21MPa.The mechanical properties of all samples The performance complies with GB/T 1971.2-2016/ISO 5834-2:2011 standard.The porous structure of the surface can reduce the friction coefficient of the surface under the lubrication conditions of thixotropic hydrogel（GTS）,physiological saline（NS）and 25%calf serum aqueous solution（CS）.As the porosity increases,the friction coefficient is lubricated in GTS Under the conditions,it is reduced from 0.069 to 0.036,and the porous structure on the surface can store GTS,that is,it has a certain liquid holding capacity,and the maximum mass of stored GTS can reach 2.1mg.（2）This paper evaluates the lubrication performance of the bionic joint lubrication system composed of GTS,NS,CS and surface bionic porous UHMWPE.Digital viscometer and rotational rheometer were used to measure the viscosity and rheological properties of the above three lubricating fluids.Their viscosities were 26.353,0.002 and 0.005 Pa·s,and GTS has shear thinning and Thixotropy.According to the Hertz contact theory,the maximum contact stress during the friction process under the load of 10N,20N and 30N is calculated.They are respectively 14.6,18.4 and 20.1MPa,which are all below the yield strength.Therefore,the maximum contact stress during the experiment meets Hooke’s elasticity.Strain requires that the surface porous structure undergoes elastic deformation,which can release the lubricating fluid stored in the pores through squeezing,providing"secondary lubrication".The minimum oil film thickness empirical formula is used to calculate the minimum oil film thickness and film thickness ratio,and the Stribeck curve is used to determine the lubrication state.The results show that under the condition of non-porous UHMWPE friction,under the GTS lubrication condition,the Stribeck curve has a sudden change area,and the friction coefficient increases with the friction speed.However,it decreases from about 0.11 to about0.07,and the lubrication state changes from boundary lubrication to mixed lubrication;while the Stribeck curve does not change significantly under the NS and CS lubrication conditions,and the friction coefficient is stable at about 0.08 and 0.09 respectively,both of which are boundary lubrication.When the surface bionic porous UHMWPE is rubbed,the Stribeck curve of the lubrication system composed of the three lubricating fluids all have a sudden change area,and the friction coefficient decreases with the increase of the friction speed.By calculating the minimum oil film thickness and the surface roughness arithmetic root mean square value The film thickness ratio is obtained from the film thickness ratio results.Under the lubrication conditions of GTS,NS and CS,the film thickness ratio increased from 0.51 to2.81,0.25 to 1.41 and 0.27 to 1.51,respectively,and the film thickness ratio rose from below1 to 1.51.Above 1,the lubrication state changes from boundary lubrication to mixed lubrication.The film thickness ratio of GTS under lubrication conditions is higher than that of NS and CS,so GTS exhibits more excellent load-carrying capacity than NS and CS,and has better lubricating performance.（3）This thesis carried out long-lasting wear experiments on the bionic joint lubrication system composed of GTS,NS,CS and surface bionic porous UHMWPE,respectively.The time is 3600,5400 and 7200min respectively.The friction coefficient results show that the average friction coefficients under the lubrication conditions of GTS,NS and CS are about0.037,0.059 and 0.054,respectively,and GTS has a lower friction coefficient.The results of wear volume and wear factor show that after 7200min friction experiment,the wear volume under GTS,NS and CS lubrication conditions are about 160×10^-3,190×10^-3 and 220×10^-3 mm³,respectively.Under the GTS lubrication condition,the wear amount in the short term is slightly higher than that under the NS and CS lubrication conditions.However,with the passage of time,the wear amount under the NS and CS lubrication conditions increases suddenly while the wear amount under the GTS lubrication condition changes smoothly.The change of wear morphology shows that the wear mechanism under GTS lubrication conditions is mainly abrasive wear,the wear mechanism under NS lubrication conditions is mainly abrasive wear and slight adhesive wear,and the wear mechanism under CS lubrication conditions is mainly abrasive wear.And severe adhesive wear.The rheological test of the thixotropic hydrogel GTS shows that after the long-term lubrication experiment,GTS still has shear thinning and thixotropy.The research results of this paper show that the surface biomimetic porous UHMWPE prepared by the thin-layer tiling method and the porogen-removing method,combined with the thixotropic hydrogel,has a good long-term lubrication and low friction and wear properties.It provides a new idea for prolonging the service life of artificial joints.