Research On The Biomimetic Artificial Cervical Intervertebral Discs Constructed By Two Types Of Scaffold-reinforced Hydrogel

BackgroundTotal cervical disc replacement is an important technique for stability reconstruction after nerve decompression in cervical disc herniation.The technique has gone through decades.Currently,several artificial cervical discs,such as Bryan,Prodisc-C and MobiC are constructed by metal,ceramic or polymer materials.Limited by prosthetic structure design and material properties,the current artificial cervical intervertebral disc cannot simulate the natural intervertebral disc in biological properties and the motion qualities simultaneously.There exist the following drawbacks:(1)wears of the prosthesis cause the inflammation of surrounding tissues,leading to function failure;(2)bone hyperplasia surrounding the implant can result in gradual loss of mobility or segment fusion;(3)mismatch in mechanics between prothesis and cervical spine may lead to changes in cervical curvature and accelerate segmental degeneration.The key to these problems is that the prosthesis does not mimic the viscoelasticity of human intervertebral disc tissue,which is critical to shock absorption and stress distribution.At present,novel hydrogel materials not only show good biocompatibility,but also have mechanical viscoelasticity close to human intervertebral discs.By means of strengthening strategies,composite hydrogels with different scaffold structures can be constructed,which combine the stress dispersion characteristics of hydrogel matrix and the mechanical strength of scaffold,showing good toughness,elasticity and anti-fatigue.This provides the material basis for the construction of biomimetic artificial cervical disc.Objective: To design and construct a biomimetic artificial cervical disc prosthesis with hydrogel as matrix material and two kinds of scaffolds as reinforcement structure,to explore the biomechanical characteristics of the prosthesis and the biocompatibility of the materials.This study would provide a reference for the design and development of new artificial cervical disc prosthesis.I.Research on the AID constructed by lamellar scaffold-strengthened hydrogel Objective: To construct and evaluate the mechanical strength,viscoelasticity and internal mechanical response of an AID constructed by lamellar scaffold-strengthened hydrogel.Methods: 1.Design and construct artificial cervical disc prosthesis: Firstly,the 3D lamellar scaffold was designed in accordance to general size of intervertebral disc in human C6-7 and the multi-lamellar structure of annulus fibrosus.The designed model was then printed by a fused deposition modeling machine using medical grade thermoplastic polyurethanes,the annulus scaffold was then impregnated into hydrogel matrix through composite prototyping fluid process to construct the prothesis;2.In vitro mechanical tests:(1)mechanical strength: 3 samples for axial compression test and 3 samples for compression-shear test;(2)viscoelasticity: 3 samples were tested for creep response,and 3 samples were tested for stress relaxation behavior;3.Finite element analysis of the prothesis model was performed under axial compression and compression sheer conditions.Results: 1.Artificial cervical disc prosthesis: polyacrylic acid hydrogel was prepared for nucleus pulposus,and the hydrogel composite material was wrapped by threedimensional TPU scaffold to form a fibrous ring structure.The overall structure was 7mm×25mm×18mm(height×long axis×short axis).2.Mechanical test results:(1)strength test:(1)under the condition of axial compression strength of 1.2k N(50% strain),the structure of the three samples remained intact,and the stress-strain curve showed a "J" curve similar to that of human intervertebral disc,in which the modulus of linear segment was 12±2.2MPa.(2)compression shear test: shear stiffness was 103±11N/mm;(2)the viscoelasticity test:(1)creep curve was similar to that of human cervical intervertebral disc,creep rate reduce with the increase of the time,and gradually entered the equilibrium phase,creep strain after 2h constant loading was 13.8%(range 10.4-15.6%),(2)the stress relaxation curve was similar to human cervical intervertebral disc.Initial relaxation rate was higher,and gradually decreased with time.Finally reached the equilibrium phase.Stress relaxation at 2h was 95.28±2.32N;3.Finite element analysis: the stress peak of the model was in the nucleus pulposus region,and the stress gradually decreased in the annular fiber from inner to outer region.Conclusion: The artificial cervical intervertebral disc constructed based on hydrogel composite TPU scaffold material had suitable mechanical strength,viscoelasticity and mechanical response were similar to that of human cervical intervertebral disc.II.Study on the artificial cervical intervertebral disc based on hydrogel reinforced by 3D braided glass fiber scaffolds.Objective: To study the biomechanical strength,viscoelasticity and durability of an artificial cervical disc based on hydrogel reinforced by 3D braided glass fiber scaffolds.Methods: 1.Design and construction: according to the average size of human C6-7 intervertebral disc and the fiber arrangement,a scaffold simulating human fiber ring structure was constructed by 3D braiding technology using glass fiber,and the scaffold was then impregnated into hydrogel through twice perfusion molding to complete the construction of artificial cervical disc prosthesis.2.In vitro mechanical tests:(1)mechanical strength: 3 samples for axial compression test and 3 samples for compression-shear test;(2)viscoelasticity: 3 samples for creep response,and 3 samples for stress relaxation behavior;3.axial mechanical fatigue test: 3 samples.Results: 1.Artificial cervical intervertebral disc prosthesis: polyacrylic acid hydrogel was used as nucleus pulposus,and microfiber reinforced hydrogel composite was wrapped to form the fiber ring structure.The overall structure was 7mm×25mm×18mm(height×long axis×short axis).2.Mechanical test:(1)strength tests:(1)under the condition of axial compression strength of 1.54 k N(50% strain),the structure of the three samples remained intact,and the stress-strain curve presented a "J" curve similar to that of human intervertebral disc,in which the modulus of linear segment was 34±2.9MPa.(2)compression shear test: the load-displacement curve was "J" type,in which the shear stiffness of the linear segment was 90±13N/mm.(2)viscoelastic test: the creep curve was similar to that of human cervical intervertebral discs.The creep rate decreased with the increase of time and gradually entered the equilibrium stage.The creep strain under the 2h constant loading was 23.7%(range 21.4-25.1%).The stress relaxation curve was similar to that of human cervical intervertebral disc.The relaxation rate was higher at the beginning,and gradually decreased.Finally,the curve entered the equilibrium stage.The stress relaxation at 2h was 99.8±3.48 N.3.Fatigue test(150N-1500 N,5Hz): After 5 million axial cyclic loading,the samples remained intact with no exposure of the scaffold.Conclusion: The artificial cervical intervertebral disc based on hydrogel reinforced by 3D braided glass fiber scaffold had suitable mechanical strength,anti-fatigue property,and the viscoelasticity was similar to that of human cervical intervertebral disc.III.Study on the biocompatibility of new polyacrylic acid hydrogel matrix materials in vitroObjective: To evaluate the biocompatibility of a novel polyacrylic acid hydrogel matrix material in vitro.Methods: 1.Preparation of sample extracts: 6 well plate was used as the mold to prepare the corresponding size of polyacrylic acid hydrogel samples and the material extracts.2.Cell culture: Mouse fibroblasts(L929)were inoculated into 96-well plates and divided into 3 groups,which were cultured with 100% extracts,50% extracts and standard medium,respectively.3.Cell viability detection: The cell activity was detected by CCK-8 kit from the 1st to 5th day of culture.Results: 1.L929 cells showed good viability under the culture of polyacrylic acid hydrogel extracts,and no significant morphology difference was found between L929 cells under the culture of extracts and standard medium.2.The viability of L929 cells cultured in the extracts were significantly increased at day 5 compared with that at day 1 and day 3,the differences were of statistically significant(p< 0.01,p< 0.01).3.The cell proliferation rates were more than 80% from day 1 to day 5 under the culture of material extracts.Conclusion: The polyacrylic acid hydrogel matrix materials met the requirements of biocompatibility.To sum up,this study constructed two kinds of artificial cervical intervertebral discs based on composite hydrogel materials reinforced by three-dimensional scaffolds.The biomimetic scaffolds were constructed at micro and macro scale,respectively.Comparing the two prosthesis,the prosthesis strengthened with glass fiber has a fiber arrangement more similar to that of human anulus fibrosus at the micro scale,while the prosthesis supported by TPU had a layered structure similar to that of human anulus fibrosus at the macro scale.In terms of mechanical properties,the prosthesis constructed by glass fiber reinforced composite gel had better strength and fatigue resistance,and the viscoelasticity were closer to human cervical intervertebral disc.In summary,both prostheses had biomechanical properties similar to those of human intervertebral discs,thus minimizing the wear and degeneration of adjacent segments currently associated with artificial disc prostheses.Based on biomimetic structure design and materials,the artificial cervical disc constructed with hydrogel reinforced by three-dimensional scaffolds will provide reference for future research and clinical trials.