| In recent years,the development of new bioactive and biocompatible materials has received great attention.It has been previously reported that bioactive materials have the ability to induce direct bonding to living bone,which is related to the formation of a carbonate hydroxyapatite layer on bioactive materials surfaces when in contact with body fluids or solutions mimicking properties of human plasma(simulated body fluid,SBF).Bioactive hydroxyapatite-polymer composites are a wide and very interesting alternative for bone tissue regeneration,as they combine the properties of polymer and hydroxm yapatite.In our study,Poly(vinyl alcohol)(PVA) was selected as polymeric substrate,biomimetic method,alternating soaking process,co-precipitation were adopted to prepare HAP/ PVA composites.Morphology,mechanical properties,bioactivity of the componete were investigated.HAP/PVA composite was prepared by a biomimetic method.PVA films were pretreated with calcium chloride solution and dipotassium hydrogen phosphate solution(Ca/P treatment),and then were immersed in 1.5 SBF for different time intervals(1 to 6 days) to induce the growth of HAP onto the films.A great amount of HAP was found on the surface of PVA films after a short period of time.The thickness of HAP deposited on PVA films reached 0.5μm after 3 days immersion. The PVA film without the Ca/P treatment could not generate HAP at the same period immersion. We also study the effect of different organic components in 1.5SBF on the growth of HAP.Citric acid,collagen and ATP(adenosine triphosphate) were added into 1.5 SBF to prepare different incubation media.We found that collagen and citric acid can promote the growth of HAP on the surface of PVA films,while ATP was hard to promote the generating of HAP.The obtained HAP was confirmed by XRD and infrared analysis as B-HAP in which some positions of PO43" were replaced by CO32-.PVA was wet-spun into fiber.HAP / PVA composite fibers were prepared by alternative soaking process.The amount of HAP which deposited on PVA fibers was increased along with interaction cycles.After 10 cycles of alternative soaking,the surfaces of PVA fibers were covered with HAP.The mechanical properties of PVA fibers were also improved owing to the existing of HAP.Breaking strength,Young's modulus of PVA fibers were enhanced by the coating of HAP on their surface.Silk fibroin(SF) was blended with PVA to improve its mechanical properties and bioactivity. PVA/SF(silk fibroin) films were prepared by solution blending.The addition of SF was not only improved mechanical performance of the films,such as significantly enhanced the film's modulus and fracture strength,but also promoted HAP growth on the film surface,which was verified by alternate soaking process.With the same reaction cycles,the amount of deposited HAP was increased with the content of SF in the film.PVA/PAA films of different PVA:PAA ratios were prepared,their HAP forming ability was investigated by alternate soaking process.Under the same circumstance,HAP growth was decreased with the content of PAA in PVA/PAA films.And it was found that the size of the HAP was reduced with the presence of PAA.It could be deduced from the results that on one hand the dissolution of acid PAA into solution lowered pH value which went against the formation of HAP; on the other hand,PAA formed hydrogen bonds with HPO42- and PVA which could introduce HPO42- into polymer matrix and then adsorpted Ca2+ existed in the solution through the electrostatic interaction for the completion of HAP nucleation process.In other words,PAA was nucleating agent in the polymer phase and HAP inhibitors in the solution phase.Co-precipitation methods could be used to deposit phosphate-type materials on PVA/PAA films.Different rates of sedimentation and the addition of citric acid in the solution could induce the formation of varied shape and compositions of calcium salts.It could be concluded that HAP (Ca10(PO4)6(OH)2),OCP(Ca8(HPO4)2(PO4)4·5H2O octacalcium phosphate),β-TCP(β-Ca3(PO4)2β-tri-Calcium phosphate) and DCPA(CaHPO4 dicalcium phosphate anhydrate) mixture were formed during the co-precipitation process.OCP,P-TCP and DCPA were precursor of HAP and could be transformed to HAP easily under certain conditions.It has been known that,the formation of HAP includes two steps:nucleation and increase. The barrier for nucleation was relatively high and hard to overcome in common circumstance.The infiltration between PVA film surface and simulated body fluid was increased and the contact angle between them was reduced by Pre-Ca/P processing.After that step,HAP nucleating became easily and the growth of HAP was accelerated.Collagen,citric acid and other organic molecules in 1.5 SBF combined with PVA through hydrogen bonds firstly.-COO- groups in these molecules could easily form some chelate with Ca2+ and PO43- ions from 1.5SBF,thus promote the nucleation of HAP and also facilitate its growth in PVA substrates.The co-precipitation process includes three steps:the hydrolization and decomposition of urea,the acid-base reaction and salt precipitation process.The decomposition of urea directly affected the speed of the pH variations,citric acid would also join in the effect of pH value,and different calcium phosphate precipitation occurred with different pH value.The composition and patterns of these precipitations were influenced by the conditions.In this study,different factors of the preparation of HAP/polymer were studied in many aspects,such as polymeric substrates,approaches of depositing HAP and the composition of incubation media.The results would be useful in the research of HAP formation on the surface of polymer and help people to get a better understanding of preparation of HAP/polymer.And these also provide valuable information to the study and development of bone tissue engineering.
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