Research On Mechanical Strengthening And Degradation Regulation Mechanism Of Poly (L-lacti Acid) Bone Scaffolds

Poly-l-lactic acid（PLLA）possesses great applied potentialities,because of extensive source,natural degradability,favorable biocompatibility.However,its mechanical strength is too low to meet the requirements of bone tissue repair.Meanwhile,the excessive acidic degradation product of PLLA usually forms local acidic environment in the body,which cause adverse inflammatory reactions.In addition,the degradation rate of PLLA is relatively slow,which is not conducive to the growth of new bone tissue.In this manuscript,PLLA scaffolds were fabricated by selective laser sintering（SLS）technology.Magnesium oxide nanoparticles（nMgO）were used to enhance the crystallinity of PLLA,thus promoting the mechanical properties of PLLA scaffold.Moreover,nMgO would release alkaline substances to neutralize the acidic substances caused by PLLA degradation.Poly（l-lactic acid-co-malic acid）（PLMA）surface modified nMgO enhancement could improve the interface bonding between PLLA and MgO,and promote the dispersion of nanoparticles in matrix,thus further enhancing the mechanical properties of the scaffold.Finally,the incorporation of Zeolitic imidazolate framework-8（ZIF-8）in PLLA scaffold could enhance its mechanical strength and degradation rate.The main work and innovation of this paper are as follows:1.To counter the acidic degradation product,magnesium oxide nanoparticles（nMgO）were used to neutralize the acidic environment,thus and avoiding inflammatory reaction.It was because the hydration process of nMgO would form alkaline products（Mg（OH）₂）,which effectively neutralized the excessive acidic degradation product of PLLA.Meanwhile,the hydration process of nMgO resulted in the release of Mg ions,which can promote the adhesion and proliferation of cells on the scaffolds.Furthermore,it was found that nMgO as heterogeneous nucleating agent can effectively accelerate the crystallization rate and increase the crystallinity of PLLA,thus improving the mechanical properties of PLLA.2.To counter the insufficient mechanical strength,PLMA surface modified nMgO（mMgO-NPs）enhancement was utilized to improve the interface bonding between PLLA and MgO,and promote the dispersion of nanoparticles in matrix.It was found that PLMA possessed a hydrophilic end with carboxyl group and an l-lactic acid（LA）chain.On the one hand,the carboxyl group may form hydrogen bonding with the hydroxyl groups of nMgO.On the other hand,the LA chain possessed similar structure and composition to PLLA matrix,which may form chemical combination with PLLA matrix.Therefore,the mMgO-NPs enhancement would effectively improve their interfacial bonding.In addition,PLMA could reduce the affinity between nMgO,thus hindering the formation of oxygen bridge bond,and promoting the dispersion of nMgO in the matrix.The results revealed that PLLA/mMgO-NPs scaffold exhibited significantly enhanced compressive strength and modulus by 47.1%and 237.7%,respectively.3.To counter the slow degradation rate,ZIF-8 was introduced as reinforcement to accelerate the degradation rate of PLLA through its unique p H-response dissolution behavior.The degradation experiment showed that the degradation of ZIF-8 would cause numerious cavities on matrix surface,which would promote the immersion solution to invade the scaffold.In this case,PLLA released acid products,which in turned promoted the hydrolysis ZIF-8nanoparticles,thus synergistically accelerating the degradation rate of PLLA scaffold.Besides,it was found that the organic ligands of ZIF-8 could form a good interface with PLLA,and ZIF-8 could be used as an effective reinforcing phase to enhance the mechanical strength of the scaffold.