Enhancing Mechanical Properties Of Porous HA Ceramics By Employing HA Particle Shape And HA-modified Poly(Urethane-urea)

Natural bones are composites of inorganic minerals such as hydroxyapatite（HA）and biopolymers such as type collagen I.According to this fact,the compounding of organic polymers with sintered porous HA ceramics has become a widely employed strategy to prepare bioactive ceramics with high mechanical properties,where polymers are employed as a ceramic-reinforcing material to overcome the fragility of pure HA ceramis while HA ceramics provide osteoconductivity and stiffness.However,an effective approach to realize the strategy has not been well established,since the HA paricle shape significantly affects the porosity and compressive strength of sintered porous HA ceramics,which we found in this study and investigated for the first time;and a new ceramics-reinforcing polymer containing HA,poly（D,L-lactide）（PDLLA）and poly（urethane-urea）（PUU）was designed and synthesized to integrate good mechanical properties and strong interfacial interactions with HA ceramics.An effective approach to prepare strong bioactive HA ceramics was developed by enhancing the mechanical properties of both HA ceramics and ceramic-reinforcing polymer and the HA ceramics-polymer interfacial interactions.The main work and conclusions were as follows:（1）Preparation and characterization of HA porous ceramics.Rod shape HA（r-HA）and spherical HA（s-HA）were employed as model HA particles to prepare sintered HA porous ceramics with polyacrylamide（PAM）as the sacrificial template.The effects of r-HA/s-HA ratio on the porosity and compressive strength of HA ceramics were investigated via experiments and mathematical fitting.The potential mechanism by which the shape of HA particles influenced the porosity and mechanical properties of porous HA ceramics was clarified by using Thermal Gravimetric Analysis（TGA）,Fourier Transform Infrared Spectroscopy（FTIR）,Scanning Electron Microscope（SEM）and X-ray Diffraction（XRD）.（1）It was found for the first time that,with the increase of r-HA/s-HA ratio,the porosity of HA ceramics decreased exponentially and the mechanical strength increased exponentially.For HA ceramics with high porosity（75%-91%）,the compressive strength increased with the increase of r-HA/s-HA ratio even when the porosity was the same.These findings indicate that the shape of HA particle can influence the porosity and compressive strength,and the addition of r-HA into s-HA may be an effective way to enhance the mechanical properties of HA ceramics.（2）The mechanism by which the shape of HA particles regulates the porosity and compressive strength of HA ceramics lies in the restriction of s-HA to the grain formation and growth of r-HA during sintering and results in the fusion of r-HA with s-HA.（2）Preparation and characterization of HA-PDLLA-PUU as HA ceramics enforcing agents.（1）Firstly,HA particles were employed as an co-initiator to synthesize poly（D,L-lactide）（PDLLA）grafted HA（HA-PDLLA）via melt ring-opening polymerization of D,L-lactide using Sn（Oct）₂ as the initiator.The effects of particle shape（r-HA,s-HA）on the grafting activity was investigated and the potential mechanism was probed.FTIR、¹H-NMR and ¹H CP/MAS NMR detections showed that PDLLA was successfully grafted onto r-HA and s-HA surfaces to form r-HA-PDLLA and s-HA-PDLLA.TGA curves and anti-settling detection revealed that s-HA has higher grafting activity than r-HA.XRD analysis showed that the grafting reaction reduced the crystallinity of both r-HA and s-HA,but did not cause obvious changes in crystal structure.Further simulation of the crystal faces by using Materials Studio 7.0and MDI Jade 6.0 verified the higher reactivity of s-HA than r-HA might be attribed to the divergent number of Ca-triangles on the surface of HA particles.These findings may add or expand the reaction theories of HA and guide the design and synthesis of HA grafted by other polymers as well.（2）HA-PDLLA was further chain extended by using hexamethylene diisocyanate（HDI）as the coupling agent and piperazine（PPZ）as the chain extender,producing the poly（urethane-urea）（PUU）based ceramics-enforcing agents（r-HA-PDLLA-PUU and s-HA-PDLLA-PUU）.FTIR and ¹H-NMR results verified the successful synthesis of r-HA-PDLLA-PUU and s-HA-PDLLA-PUU,and similar content of HDI and PPZ segments,according to the TGA results,was introduced into r-HA-PDLLA-PUU（1.15%）and s-HA-PDLLA-PUU（1.27%）.s-HA-PDLLA-PUU and r-HA-PDLLA-PUU demonstrated significantly better dispersibility and stability in dichloromethane than the corresponding HA-PDLLA,which further confirmed the successful synthesis of s-HA-PDLLA-PUU and r-HA-PDLLA-PUU.In addition,the improved dispersibility of HA-PDLLA-PUU is beneficial to its following compounding with the porous HA-ceramics.The static water contact angle of s-HA-PDLLA-PUU and r-HA-PDLLA-PUU were both about 57^o,significantly lower than those of HA-PDLLA and PDLLA,implying that HA-PDLLA-PUU might have good interfacial interactions with hydrophilic HA ceramics.（3）Preparation and characterization of HA ceramics/HA-PDLLA-PUU composite scaffolds.Finally,s-HA-PDLLA-PUU was selected as the ceramic-reinforcing material to coat various HA ceramics by means of solution infiltration method with PDLLA and s-HA-PDLLA as the controls.（1）SEM results showed that the interfacial bonding of HA porous ceramics with s-HA-PDLLA-PUU was better than with PDLLA and s-HA-PDLLA.Compared with pure HA ceramics,the compressive strength and fracture energy of HA ceramic/s-HA-PDLLA-PUU composite scaffolds were markedly increased by 37%-90%and 1.0-5.0 times,respectively,though the porosity was only slightly decreased,verifying that s-HA-PDLLA-PUU is an excellent reinforcing material for HA ceramics.（2）An in-depth understanding of the compressive stress-strain curves reveals that s-HA-PDLLA-PUU can change the fracture feature of pure HA ceramics;the enhanced mechanical properties of the composite scaffolds are attributed to both the strong interfacial interaction between s-HA-PDLLA-PUU and HA ceramics and the high rigidity of s-HA-PDLLA-PUU.These finding are valuable to guide the design of ceramics-enforcing materials so to produce HA based composite scaffolds with enhance mechanical properties.In summary,to develop a composite HA ceramics with enhanced mechanical properties,we conducted a series of researches to increase the mechical properties of both pure HA ceramics and ceramic reinforcing materials,and their interfacial interactions.It was found that the shape of HA particles regulates the porosity and mechanical properties of sintered HA ceramics,which provides a new particle shape-based idea or method to improve the mechanical properties of pure HA porous ceramics.Furthermore,the shape of HA particles was found to affect the reactivity of HA particles,which is of great significance to guide the chemical modification of HA particles.The developed HA ceramics/s-HA-PDLLA-PUU composite scaffolds have obviously enhanced mechanical properties,so that they should have promising applications in bone tissue repair and regeneration after further systematic study of its biocompatibility.