| Polyetheretherketone(PEEK)is an excellent semi-crystalline polymer material with high temperature resistance,non-toxicity and good biocompatibility,and it is currently considered to be a kind of promising material for the preparation of artificial joints.However,PEEK has some disadcantages such as poor surface wettability,high friction coefficient of pristine PEEK material under liquid lubrication conditions,which seriously restricts its application in clinical practice.Therefore,how to improve the surface wettability and tribological properties of PEEK is a fundamental issue to be solved.Aiming at this problem,this paper starts from the composition and lubrication mechanism of the natural joint lubrication system.Through the bionic design of the structure and function of the natural joint system,we are going to find a simple and effective surface modification method to improve the surface wettability and tribological properties of PEEK,and the purpose of this research provides a theoretical basis for the development of new artificial joints.The specific research work is carried out from the following aspects:(1)The surface of natural articular cartilage has a brush-like structure composed of biomacromolecules,which can reduce friction.Inspired by that structure,we used the method of ultraviolet(UV)light irradiation to initiate graft polymerization and fabricated a layer of acrylic acid(AA)polymer brushes on the polished PEEK surface.The results showed that the concentration of AA monomer solution and the irradiation time had a great influence on the performance of the prepared AA-g-PEEK specimens.The AA-g-PEEK specimen prepared by the preferred grafted AA monomer parameters(AA monomer solution concentration of 1.00 mol/L,irradiation time of 30 min)had a relative low friction coefficient(0.021)when subjected to a ball-disk rotary friction test with a load of 2 N under deionized water lubrication.In addition,we used the same method to graft 3-dimethyl(3-(N-methacrylamido)propyl)ammonium propane sulfonate(MPDSAH)and acrylamide(AM)on PEEK surface,and we explored the effects of different polymer brushes on friction coefficients in different lubricating fluids.(2)In the natural joint lubrication system,a layer of soft cartilage covers the hard bone surface,which is a layered structure with a "soft surface-hard substrate".Inspired by that structure,we combined PEEK sulfonation treatment,UV-initiated graft polymerization,and freeze-thaw method to prepare a soft polyvinyl alcohol(PVA)hydrogel layer on PEEK surface.After the sulfonation treatment,the original morphology of PEEK surface was destroyed,and replaced by a three-dimensional interpenetrating pore structure was formed,which could act as a reservoir to store gel liquid.After introducing carboxyl groups to the surface of the SPEEK,the hydrogen bond was formed by the carboxyl groups and the hydroxyl groups in the PVA,and the SPEEK surface microporous storage gel was used to fix the PVA hydrogel.Finally,a "soft-hard" composite layered structure of "surface PVA gel layer-SPEEK porous layer-hard PEEK substrate”was constructed on PEEK.This structure not only had excellent hydrophilicity(contact angle could be reduced to 7°),but also exhibited a lubrication mechanism similar to that of natural joints in reciprocating friction testing in deionized water:The PVA hydrogel layer covering the PEEK surface acted as a friction-reducing lubricant,and the rigid PEEK substrate acted as a load bearing.The friction coefficient was only 0.050 when subjected to a reciprocating friction test under a load of 8 N,and the friction coefficient remained stable throughout the experiment.(3)In order to increase the bonding force between the hydrogel layer and PEEK substrate,firstly,we prepared a layer of acrylic acid-acrylamide(AA-AM)hydrogel layer with chemical cross-linking structure on the surface of PEEK by using UV polymerization method.Then the specimen with the hydrogel layer prepared before was immersed in an aqueous solution containing iron ions,iron ions could form a coordination bond with the carboxyl group in the AA-AM hydrogel,thus forming a second layer of physical cross-linking network.Finally,we obtained a layer of dual cross-linked hydrogel anchored by chemical bonds on PEEK surface,and the thickness of the hydrogel layer can be controlled by varying the thickness of the separator used in the preparation of the hydrogel.After constructing an AA-AM dual cross-linked hydrogel layer(AA 0.60 mol/L,AM 3.00 mol/L)with a thickness of 0.31 mm on the surface of PEEK,that specimen exhibited a low and stable coefficient(0.066)in the reciprocating friction test at a load of 8 N.Only a certain compression deformation occurred and no loss of gel appeared on the surface of the obtained wear scar.After the wear scar was soaked in water for a period of time,it exhibited a certain self-healing performance.In addition,we used a template-filtration method to construct a microporous structure on the surface of hydrogel.That hydrogel layer with a superficial porous structure could absorb and store more lubricating fluid in friction tests under a small load(2 N),and its friction coefficient could be as low as 0.033.(4)Combined with laser surface texture(LST)and UV copolymerization cross-linked hydrogel methods,a layered structure of“soft gel layer-porous storage layer-hard base layer"was constructed on PEEK surface,thereby,a biomimetic structural design of the natural joint is achieved on PEEK substrate.We used the LST method to fabricate micro-cavity structures with the same pore size(200 μm)and different areal densities on the surface of PEEK.The surface texture obtained by this method had a high degree of controllability,the shape and pore size of the obtained dimples were substantially uniform.The textured PEEK surface could have a certain friction reduction ability,the friction coefficients were gradually decreased as the increasing of areal densities.When the load was 8 N,the specimen with an areal density of 35%obtained the lowest friction coefficient(0.087)under water lubrication.In addition,we designed a new MPDSAH-AA-AM dual cross-linked hydrogel and fixed it to the textured PEEK substrate.The dimples on PEEK surface could increase the contact area between the gel liquid and the substrate,and they acted to store more gel.After a long-term reciprocating friction test(60 min)in deionized water under a load of 8 N,the obtained friction coefficient was 0.061,only a slight wear scar appeared and it showed good wear resistance. |
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