3D Printed Scaffolds Based On Novel Linear Polyurethane And ?-tricalcium Phosphate For Bone Repair

Bone defects are a kind of common diseases caused by trauma,inflammation,tumor resection,etc.One key barrier to the repair of defected bones is the lack of artificial scaffolds possessing both high mechanical properties and bioactivity.Construction of polymer/calcium phosphate composite scaffolds by mimicking the chemical composition and architecture of natural bones is a potentially effective strategy to provide bioactive scaffolds.Calcium phosphate acts to endow osteoconductivity and enforce the composite scaffolds as well.High interfacial interactions between calcium phosphate and polymers is beneficial to the enforcement of calcium phosphate in composite scaffolds.Linear polyurethanes,a kind of block copolymers with good biocompatibility and processibility,have strong interfacial interactions with calcium phosphate,since their urethane groups can form hydrogen bonds with calcium phosphate.Unfortunately,most of the currently empolyed biomedical linear polyurethanes are not suitable for bone defect repair since they are elastomers with low modulus.Development of a new linear polyurethane with high modulus is essentially neccessary.Besides,hierarchical pore structure with connected pores is also indispenssable for bone repair scaffolds,which might be realized by using3D plotting/low-temperature deposition/freeze drying technique.In this study,a novel linear polyurethanes(ISO-PUs)with high mechanical properties was designed and synthesized,which was then composited with?-tricalcium phosphate(?-TCP)by using3D plotting/low-temperature deposition/freeze drying technique to form a novel composite scaffolds.The obtained ISO-PUs/?-TCP scaffolds should be featured with high mechanical properties and good bone repair ability.The physiochemical properties and pore structures of ISO-PUs and ISO-PUs/?-TCP scaffolds were characterized by using Fourier transform infared spectroscopy(FTIR),nuclear magnetic resonance(NMR),gel permeation chromatography(GPC),differential scanning calorimetry(DSC),scanning electron microscope(SEM)and mechanical test;the in vitro biocompatibility and in vivo osteogenesis ability of ISO-PUs and ISO-PUs/?-TCP scaffolds were evaluated.The main contents and conclusions are summarized as follows:(1)Preparation and characterization of ISO-PUs with high mechanical propertiesEnhancement of chain rigidity and prevention of crosslinking were supposed to improve the mechanical properties of linear polyurethane.Taking this in mind,we selected isosorbeide(ISO),a diol with rigid double-ring structure,as a co-initiator and a chain-extender,by which the chain rigidity of both soft segments and hard segments of polyurethane could be increased and a novel linear polyurethan ISO-PUs with high mechanical properties could produced.(1)First,ISO as a co-initiator of Sn(Oct)₂ was employed to initiate the melt ring-opening of D,L-lactide,giving poly(D,L-lactide)-based macrodiol(ISO-PDLLA)as the soft segment,which was then chain extended by using ISO as the chain-extender and hexamethylene diisocyanate(HDI)as the coupling agent,producing the desired ISO-PUs.Results from FTIR and ¹H NMR verified the successful synthesis of ISO-PDLLA and ISO-PUs;DSC results revealed the increased rigidity of both ISO-PDLLA and ISO-PUs chains by ISO;Detection of crosslinking degrees confirmed the effective elimination of crosslinking by using ISO as the chain extender and thus the positive contribution to increased M_n of ISO-PUs.As a result,the tensile modulus and strength of ISO-PUs were significantly improved;increase of the content of hard segments(i.e.ISO-PDLLA/HDI/ISO molar ratio)could further enhance the mechanical properties.(2)When ISO-PDLLA/HDI/ISO was 1.0/1.5/0.5,the resulting ISO-PU1.5(T_g=59.48^oC,M_n=84.79 kDa,PDI=1.86)had a tensile modulus of 2.57 GPa and a tensile strength of 42.68 MPa,which were obviously higher than those of PDLLA with a M_n of100 kDa,suggesting that ISO-PU1.5 might be a suitable polymer with high mechanical properties for bone defect repair.(2)in vitro biocompatibility evaluation of ISO-PUsThe biocompatibility of ISO-PUs was evaluated by using primary osteoblasts as model cells and PDLLA as control.It was observed that the adhesion,spreading,cell survival,ALP expression and osteogenesis-related gene expression of osteoblasts on ISO-PUs were significantly better than on PDLLA,indicating that the osteoblast compatibility of ISO-PUs is better than PDLLA.Moreover,the increase of hard segments could further improve the biocompatibility of ISO-PUs.(3)Fabrication and characterizaiton of ISO-PUs/?-TCP scaffolds(1)The preparation process of 3D plotting/low-temperature deposition/freeze drying was established and optimized.The as-prepared scaffolds including PDLLA scaffolds,ISO-PU1.5 scaffolds and ISO-PU1.5/?-TCP scaffolds had hierarchical pore structures with porosity of>70%,macropore size of 400?m⁶00?m and micropore size of about 15?m.(2)The compression strength(?)values of ISO-PU1.5/?-TCP at 5%,20%and80%strain were 7.41 MPa,13.42 MPa and 312.39 MPa,respectively,which were 1.6times,1.36 times and 1.66 times of the?values of ISO-PU1.5 scaffolds;the initial compressive modulus(E)and the E value at 80%strain were 5.89 MPa and 1.68 GPa,respectively,which were 1.88 times and 1.62 times of the E values of ISO-PU1.5scaffolds.These results indicate that the compositing of?-TCP with ISO-PU1.5 can effectively improve the compressive mechanical properties of the composite scaffolds.(4)in vitro biocompatibility and in vivo bone repair ability of ISO-PU1.5/?-TCPThe in vitro biocompatibility and in vivo bone repair ability of ISO-PU1.5/?-TCP were evaluated by using PDLLA scaffolds as the conrol,primary osteoblasts as the model cell,and a?2.5 mmí5 mm bone defect in the middle of SD rat tibial below the growth plate as the model animal.(1)It was found that,compared with PDLLA scaffolds and ISO-PU1.5 scaffolds,ISO-PU1.5/?-TCP scaffolds were more effective in promoting the migration and in-growth of osteoblasts into the pore structure,and induced better cell adhesion,spreading,proliferation and differentiation,revealing the good cytocompatibility of ISO-PU1.5/?-TCP scaffolds with osteoblasts.(2)The results from micro-CT and tissue section staining showed that there were more new bone,thicker trabecular bone,and narrower space between trabecular bones in ISO-PU1.5/?-TCP scaffolds than in PDLLA scaffolds and ISO-PU1.5 scaffods,indicating that?-TCP particles can endow the composite scaffolds with osteoconductivity and promote the integration of the scaffolds with the surrounding tissues.In addition,the histological staining results revealed that all scaffolds had no obvious negative effects on heart,liver and kidney,suggesting that all of them have good biosafety.In summary,ISO-PU1.5/?-TCP scaffolds have good mechanical properties and osteogenic ability and may become a promising bone repair scaffolds.In addition,ISO-PUs may be a promising polymer for bone repair as well due to its good mechanical properties and biocompatibility.