Preparation And Application Of Polyetheretherketone-based Microspheric Bone Filling Materials For Bone Repair

Bone tissue is a dynamically growing,highly vascularized tissue with unique healing and remodeling abilities.Bone defect is one of the most frequent problems in clinical bone injuries and tumors in which application biomaterials filling is necessary.The reconstruction of bone defects remains a challenge,especially the treatment of large,irregular bone defects due to severe trauma or systemic disease,pathological fractures（metastatic or primary malignancies）,tissue infection and poor blood supply is often a major challenge for clinicians and patients.At the same time,with the increase of life expectancy and population aging,the bone tissue suffers an increased risk of injury or disease,such as fracture and osteoporosis,which requires surgical intervention to achieve better clinical results.At present,bone tissue has become the most common transplant tissue second to blood transplants.With the development of science and technology and medical philosophy,orthopedic transplantation has benefited many patients.Current therapies are mainly focus on the use of autografts,allografts,xenografts,engineered synthetic or biomaterial substitutes to replace bone defects.The gold standard among reconstruction materials is autografts which have to be obtained from other bone tissues of the host.It causes donor site morbidity and is usually difficult to fit the desired shape.Allograft and xenograft exhibit the potential risk of disease transmission and immune response.In order to overcome the limitations of autografts,allografts or xenografts,in the past ten years,a variety of biocompatible bone graft substitutes have been developed,which can be used alone or combined with other materials to improve osteogenic ability through bone induction and conduction activity.Factors such as tissue viability,defect size,graft shape and volume,graft processing method and cost will affect the selection of materials.Various bone grafts with different structures and functions have become the focus of research for the replacement of autografts,allografts or xenografts.Polyetheretherketone（PEEK）,an artificial,fully aromatic,semi-crystalline polymer possesses excellent biocompatibility,chemical durability and favorable mechanical properties.PEEK does not release ions,by-products and does not corrode or degrade,so it can be applied in the medical field as permanent bone implants.However,available PEEK implants require shape in advance and cannot fill irregular bone defects perfectly.In addition,PEEK is a bioinert polymer with weak osteointegration property.As a non-degradable and permanent implant material,good bone integration will benefit long-term stability of the implant.Much work has been conducted to improve the osteointegration property and the bioactivity of PEEK mainly including coating deposition,direct surface modification or preparation of PEEK-based composites where the surface topography and chemistry of PEEK could be altered.Current available PEEK implants require shape in advance and cannot fill irregular bone defects perfectly.At the same time,combined with the clinical widely used non-invasive diagnosis and minimally invasive surgery,including injectable methods,microspheric bone tissue engineering materials have gradually appeared in the field of medicine.To avoid the large open surgery caused by traditional scaffolds transplantation,microspheric bone fillers combine the advantages of microcarriers and scaffolds.As injectable carriers,microspheres can be applied conveniently into the tissue defect region to repair complex and irregular bone defects.In addition,owing to the large specific area of microspheres,which facilitates the sufficient exchange of nutrients,oxygen and metabolic waste,microspheres are quite efficient for cell amplification.Overall,the microspheric materials can be easily adapted to any required shape of defect with high surface area to volume ratio（SA/V）and act as a micro-Noah’s Ark to transport cells safely to defective tissues,promoting bone regeneration through the establishment of"bone conductive bridge".The microspheres play a key role in tissue regeneration and stem cell therapy.Nevertheless,the poor mechanical property of biodegradable microspheres limits their application and it is also difficult to control their degradation rate to match the speed of bone reconstruction.In this study,we prepared PEEK-based microspheric bone filling materials and then improve the osseointegration ability by modifying the surface or the matrix.Dealing with any shape of bone defects,especially large and irregular bone defects,PEEK-based microspheres could reduce bone mass and speed up the integration process through the establishment of"bone bridge".When combining PEEK-based microspheres with degradable microspheres as filling implants for bone defect repair,it is predicted that the applied amount of degradable materials and the technical difficulties for controlling their degradation rate could be reduced.And then the impact of degradation by-products to bone reconstruction microenvironment might also be minimized.This study mainly includes the following three parts:Part I:Biomimetic polyetheretherketone microspheres with specific surface topography and self-secreted extracellular matrix for large-scale cell expansionReusable microspheres with appropriate surface topography,mechanical properties,as well as biological modification through decellularization facilitating repeated cell culture are crucial for tissue engineering applications.Herein,we report the preparation of topological polyetheretherketone（PEEK）microspheres via gas-driven and solvent exchange method followed by hydrothermal treatment at high temperature and pressure.After hydrothermal treated for 8 h,the resulting topological PEEK microspheres exhibit walnut-like surface topography and good sphericity as well as uniform size distribution of 350.24±19.44μm.And the average width between ravine-patterned surface of PEEK microspheres is 780±290 nm.After repeated steam sterilization by autoclaving for 3 times,topological PEEK microspheres show nearly identical results compared with previous ones indicating strong tolerance to high temperature and pressure.This is a unique advantage for large-scale cell expansion and clinical applications.Moreover,PEEK microspheres with special topography possess higher mechanical properties and protein adsorption efficiency.In addition,the reutilization and biofunctionalization with repeated decellularization of topological PEEK microspheres show highly beneficial for cell adhesion and proliferation.Therefore,our study is of great importance for new generation microspheres with micro-and nano-scale surface feature for a broad application prospect in tissue engineering.Part II:Preparation of calcium sulfate/polyetheretherketone composite microspheres and their biological properties in vitroMicrospheres combining the excellent mechanical strength of polyetheretherketone（PEEK）and the good bioactivity of calcium sulfate（CaSO₄）are highly required for orthopedic applications.In addition,the advantage of CaSO₄in the preparation process is that it is relatively stable in concentrated sulfuric acid while other calcium salts are dissolved in concentrated sulfuric acid.Herein,we first prepared CaSO₄/PEEK microspheres by incorporating different amounts of CaSO₄（10%,20%,30%,and 40%）via gas-driven and solvent exchange method followed by hydrothermal treatment.And then the physicochemical properties,biomineralization ability and in vitro biological properties were studied.The results showed that CaSO₄could be effectively incorporated into PEEK matrix,and both the hydrophilicity and the biological mineralization ability of the CaSO₄/PEEK composite microspheres could be enhanced with higher contents of CaSO₄.Subsequently,in vitro biological properties and cellular responses of CaSO₄/PEEK microspheres were investigated systemically.With the increase of CaSO₄ contents,cell adhesion,proliferation and differentiation ability of CaSO₄/PEEK composite microspheres were significantly improved,especially the 40%CaSO₄/PEEK microspheres,which could be used as bone filling materials to investigate the bone repair effect in vivo.Part III:Non-biodegradable and biodegradable microspheres promoting bone regeneration in vivoHerein,we report a strategy by blending CaSO₄/PEEK microspheres with biodegradable hydroxyapatite/poly（lactic-glycolic acid）（HA/PLGA）microspheres to fabricate multi-material microspheres for bone filling and regeneration in vivo.The CaSO₄/PEEK microspheres were perfectly filled in a critical-sized rat calvarial defect model together with different proportions of HA/PLGA microspheres to evaluate their bone regeneration ability through CT,micro-CT and histological analysis.It is expected to acquire a synergistic effect of structural support from non-degradable materials and bone ingrowth ability from degradable matrix in the bone remodeling process.The bone defect repair results revealed that well-developed new bone tissues extended or stretched along the surface of microspheres and grew into the space from the biodegradable microspheres.The bone defect region was completely connected to the margin of host bone after 8 weeks of implantation for the mixtures of CaSO₄/PEEK and HA/PLGA microspheres in 7:3 and 5:5 ratios.Therefore,the combination of non-degradable and biodegradable microspheres provides a promising direction of PEEK-based materials as filling and repair implants for orthopedics applications.