Synthesis Of Different Functional Polyurethanes And Their Application At The Dentin-composite Resin Bonding Interface

Photopolymerizable composite resins(RC)have been quite popular in the field of dental treatment and restorations,but their service life is still concerned.According to a survey,the proportion of restorations requiring reintervention in the UK is as follows:?within 11% of fillings after 1 year of placement?within 20% of fillings after 3 years of placement?within 50% of fillings after 10 years of placement In the United States,about 200 million teeth are filled and repaired every year,and the failure rate reaches 50% in 10 years,while secondary caries is still one of the main reasons for failure of RC restorations.Secondary caries(SC)is a chronic infectious disease caused by bacteria and plaque biofilms.Its occurrence and development are affected by various factors,including the contraction stress of the RC,the aggregation of bacteria and plaque biofilm,and the degradation of the dentin-RC bonding interface(hydrolysis,enzymolysis,mechanical degradation),the factors above are related to each other.Firstly,the adhesion and aggregation of bacteria and plaque biofilm at the marginal gap of the dentin-RC interface.The acid metabolites(such as lactic acid)produced by bacteria(such as Streptococcus mutans)can cause demineralization of the dentin and activate matrix metalloproteinases(MMPs)to degrade dentin collagen fibers.In addition,some enzymes produced by bacteria will also destroy the dentin collagen fibers,thus damaging the integrity and durability of the dentin bonding interface.Even so,the presence of bacterial biofilms alone does not guarantee the occurrence of SC.It has been found that there are a lot of bacteria at the marginal gap of the dentin-RC interface,and the marginal gap is also considered as an important factor for the formation of SC.Even when the RC provide good sealing of the cavity with no marginal gaps as a result of perfect placement by the clinician.Polymerization shrinkage stresses,occlusal stress during exercise,and thermal stress caused by the mismatch between the thermal expansion coefficient(TEC)of the dental tissue and the RC act repeatedly upon the dentin-RC interface,which willcause a stress concentration.This can induce adhesion failure and marginal gaps of the restoration;in addition,degradation of the hybrid layer and adhesive layer may also result in marginal gaps.Once the marginal gap is formed,it will provide more adhesion sites for bacteria.In vitro studies suggest that a minimum gap size exceeding 15 ?m has been suggested for significant bacterial colonization of the RC restoration.The marginal caries of restorations is affected by the size of the marginal gap.Generally,the marginal gap is greater than 50 ?m to have the opportunity to form SC.It is important to note,however,that the presence of biofilm and a marginal gap does not guarantee the development of SC.This finding suggested that the role of stresses loading in SC formation is not only to simply create a marginal gap or grow it above a critical size,but also to play the role of pumping,allowing nutrients and bacteria to flow more easily into the narrow gaps while refreshing the fluid and removing waste products.This process accelerates the degradation of dentin and adhesives or RC materials,and makes the marginal gap more hospitable for bacteria colonization.In summary,the occurrence of SC is affected by various factors.How to ensure a good sealing of the dentin-RC interface,at the same time improve the water resistance of the bonding interface,reduce or eliminate the various stresses on the interface,and improve the antibacterial performance of the materials are of great significance to the durability and stability of the bonding interface and the prevention of SC.The purpose of this research is to study the application and prospect of the PU polymer in the dentin-RC interface.PU polymers with different functions were prepared,and then the physical,chemical,antibacterial properties,microleakage prevention and bonding strength were evaluated.Finally,the possible influencing factors were described in order to provide potential ideas and experimental basis for the prevention of SC.In Chapter two,5 groups of acrylate-modified hydrophobic PU elastic layers with special elasticity were prepared by adjusting different proportion of PU prepolymer using solution polymerization method.The hydrophobic PU elastic layer was applied between dentin(coated with adhesive)and RC.Firstly,the tensile strength and elongation at break of five groups after light-curing were tested,and the group with excellent tensile properties was selected.Next,two commercial adhesives were selected as the control groups,together with the selected PU elastic layer,performed the following experimental tests: degree of conversion,water absorption/ solubility,static water contact angle,in vitro cytotoxicity,and micro tensile strength,microleakage.The results show that the selected PU elastic layer represented the maximum tensile strength and elongation at break,which were 22.75 MPa and 112.09% respectively.Compared with commercial adhesives,PU elastic layer had higher degree of conversion,lower water absorption/solubility,larger static water contact angle and no difference in vitro cytotoxicity.Compared with the conventional RC restoration system,the immediate micro tensile strength of the RC restoration system with PU elastic layer was unchanged or slightly lower.After the thermocycling,the micro tensile strength of the latter was significantly higher than that of the former(p < 0.05),and the microleakage depth was significantly lower than that of the former(p < 0.05).In Chapter three,the broad-spectrum antibacterial agent triclosan(TCS)was modified to obtain the triclosan derivative(TCS-IH),which was endowed light-curable properties.Then,according to the mass percentage,TCS-IH was added to the PU elastic layer selected in Chapter 2,and finally 4 groups(0wt%,1wt%,3wt%,5wt%)of the antibacterial PU elastic layers were formed.The physical and chemical properties(thermal stability,degree of conversion,water absorption/solubility,static water contact angle,etc.),antibacterial properties,in vitro cytotoxicity and microleakage of the 4 groups of antibacterial PU elastic layers were evaluated.The results shown that the physical and chemical properties decreased with the increase of TCS-IH content except for the static water contact angle;1wt%,3wt%,5wt% groups exhibited antibacterial effectsand the antibacterial rates were over 90% and >99% for the 3wt% and 5wt% groups,respectively;each group had no significant cytotoxicity in vitro;compared with the conventional RC restoration system,the effect of the RC restoration system using antibacterial PU elastic layer to prevent microleakage was significantly better than the former,except for 5wt% group with slightly stained;in summary,3wt% group was selected for evaluating micro tensile bonding strength.The results shown that the immediate micro tensile bonding strength of RC restorations with 3wt% group of antibacterial PU elastic layer was lower than the control group(p < 0.05),but the strength of RC restorations with the antibacterial PU elastic layer increased after thermocycling.The antibacterial PU elastic layer,which has both hydrophobic and antibacterial effects,is applied between the dentin(coated with an adhesive)and the RC to absorb the stresses of dentin-RC interface.However,the presence of the water at the dentin surface and in the dentin tubules after tooth preparation may still affect the stability of the dentin-RC interface.Therefore,in Chapter four,by adjusting the molecular weight and feed ratio of the raw materials and introducing new groups,the hydrophilic PU was synthesized using solution polymerization method,then components such as water absorbing agent and nano-hydroxyapatite were added to obtain water-absorbing and light-curing PU primer.Then,water absorption,mechanical properties(surface hardness and tensile strength),in vitro cytotoxicity and micro tensile strength were tested.The results shown that by adjusting the ratio of the components,the water absorption and diameter increase rate of the water-absorbing PU primer reach the maximum and continue to be flat after 1.5 hours,about 0.8 g/g and 20% respectively.The mechanical property results shown that the water-absorbing PU primer after light curing had a certain surface hardness(maximum about 5 HV)and tensile strength(maximum about 40 MPa).Finally,the water-absorbing PU primer was applied to the surface of dentin slices,the longitudinal and cross section of the dentin slices was observed using FE-SEM.The observation revealed that resin tags with length ranging from 50 to 100 ?m were formed and most area of the dentin tubule was sealed.The application of the water-absorbing PU primer had no effect on the immediate micro-tensile bonding strength of the RC restoration system(p > 0.05);after thermocycling,the micro-tensile bonding strength of the RC restoration system using PU primer was higher than that of control groups.This indicates that the application of the water-absorbing PU primer could slow down the aging rate of the bonding strength.In summary,PUs were synthetized with special functions by adjusting the molecular weight,feeding ratio of raw materials and adding different new monomers and then applied to different parts of the dentin-RC interface,so as to effectively play the functions of hydrophobicity,absorbing or buffering various stresses,antibacterial,sealing the dentin tubules immediately and preventing the flow of dentin tubules.It provides more research ideas and experimental basis for the maintenance of the stability and durability of dentin bonding interface and the prevention of SC.