Study On The Sterilization And Antimicrobial Properties Of Biodegradable Polyesters

Since the devices are employed in environments of body fluids such as blood and urine,they are usually single-use in order to prevent infection with viruses and bacteria.Up to now,nearly all the commercial available devices are made of non-biodegradable polymers such as polyvinyl chloride,polypropylene,and thermoplastic elastomer.Moreover,disposal of these plastics needs incineration to reduce biological contamination risk,which has a huge environmental impact.Hopefully,the problem could be resolved by replacement of non-biodegradable polymers with biodegradable alternatives.For medical devices,in order to avoid contact infection,sterilization treatments are the prerequisite of sterility assurance.However,polymer-based medical devices may suffer from physiochemical modifications upon sterilization that could limit their clinical applications.On the other hand,in the applications such as interventional surgery,the contact time of the medical devices with human tissue is relatively long,which may easily suffer from severe infection.Therefore,it is necessary to evaluate the suitability of various sterilization treatments for specific polymers and fabricate the matrix materials with antimicrobial function.In this thesis,biodegradable polyesters including polylactic acid(PLA),poly(butylenes adipate-co-terephthalate)(PBAT)and PLA/PBAT blends were selected for investigation.Firstly,the suitability of various sterilization methods for these polyesters was evaluated.Then,the effect of EB dose on the properties of the polyesters was focused.For the investigation of the polyester systems with antimicrobial function,the influence of two different kind of antimicrobial agents,one is easy to dissolve in water while the other is hard,on the antimicrobial ability of the polyesters is examined.Meanwhile,controlled release of the antimicrobial agent was achieved through the addition of nanofillers like montmorillonite(MMT)and graphene nanoplatelets(GNP).The contents in details and the main results are as follows:1)The effect of diverse sterilization methods on the properties of PLA,PBAT and PLA/PBAT blends.The samples were prepared by hot-compression.Then,the samples were sterilized by four different methods like ethylene oxide(EtO),electron beam(EB)irradiation,high temperature and high pressure steam(SS)and hydrogen peroxide plasma sterilization(HPGP),respectively.The effect of sterilization treatment on the molecular weight and its distribution,crystal structure,thermal behavior,transparency,dimensional stability,yellowing,chemical structure,and mechanical properties of the polyesters.It is found that the non-sterilized PLA presents transparency.This feature makes it a promising candidate for the medical devices like the barrel of syringes where transparency is required.After EtO,EB or HPGP treatment,the transparency of PLA remains.However,after SS treatment,PLA gets opaque and shrinkage occurs,due to the crystallization of PLA at high temperature.Shrinkage also occurs for PLA after EtO treatment,which can be ascribed to the relaxation of the molecular chains near the glass transition temperature.For PBAT and PLA/PBAT blends,yellowing takes place after HPGP treatment,although apparent changes of chemical structures are not detected by Fourier transform infrared spectroscopy.After sterilization,the mechanical properties of these polyesters are acceptable and sterilization has in general no remarkable effect on the mechanical properties of the polyesters.From above results,it can be concluded that EB treatment is suitable for PLA,PBAT,and PLA/PBAT blends.However,SS and EtO treatments are not recommended for PLA,while HPGP is not suitable for PBAT and PLA/PBAT blends.2)The effect of EB irradiation dose on the properties of PLA,PBAT,and PLA/PBAT(42/58)blends.The EB doses are in the range of 25～100 kGy.Wide angle X-ray diffraction results show that the amorphous structure of PLA and the crystalline structure of PBAT are not affected by EB radiation dose.With the increase of irradiation dose,the yellowing index(YI)of PLA and PBAT samples increase significantly,and the YI of PLA/PBAT blend also shows an increasing trend.Under high irradiation dose,the mechanical properties of PLA decrease dramatically,due to the decrease in the molecular weight induced by irradiation.However,the molecular weights and mechanical properties of PBAT and PLA/PBAT blends are almost not affected by the radiation dose.This indicates that PBAT and PLA/PBAT blends have good EB resistance,while the PLA samples should not be irradiated with an overdose of electron beam.3)Tailoring the antimicrobial ability of PLA/PBAT(30/70)blends containing ciprofloxacin hydrochloride hydrate(CFX·HCl),which is easy to dissolve into water,through the incorporation of nanofillers such as GNP and MMT.The powder of antimicrobial composite of MMT,GNP and CFX·HCl was prepared by ultrasonic treatment and freeze-drying.Then,the PLA/PBAT based antimicrobial composites were fabricated by melt-blending the powder with PLA and PBAT.The testing samples were prepared by hot-compression,followed by EB sterilization treatment.The results of antimicrobial test show that,in the case of without leaching,the inhibition zones of the PLA/PBAT systems containing CFX·HCl/MMT,CFX·HCl/MMT/GNP,and CFX·HCl/GNP are all smaller than that of the system with CFX·HCl,indicating that the existence of the nanofillers retards the diffusion of the antimicrobial agent.However,after leaching for a certain period(1～15 days),the inhibition zones of the antimicrobial system with nanofillers are larger than that of the system containing antimicrobial agent only.The release kinetics of the antimicrobial agent shows that the addition of nanofillers reduces the initial burst release effect.Moreover,the cumulative release increases continuously with time and reaches 8.7%and 8.0%in 15 days for the systems containing CFX·HCl/MMT,CFX·HCl/MMT/GNP,respectively.However,the cumulative release approaches equilibrium very soon and the value is 3.7%only for both the systems containing CFX·HCl and CFX·HCl/GNP.The evaluating results of coagulation time and hemolysis rate show that all the neat PLA/PBAT blends and the systems containing CFX·HCl and/or nanofillers have good blood compatibility.The results of mechanical testing show that the introducing of MMT and GNP increases the modulus and tensile strength of the antimicrobial matrix decreases its elongation at break.4)The effect of nanofiller MMT on the antimicrobial ability of PLA,PBAT,PLA/PBAT(70/30),and PLA/PBAT(30/70)containing ciprofloxacin(CFX),which is hard to be dissolved into water.The polyesters based antimicrobial composites were fabricated by melt-blending.The testing samples were prepared by hot-compression,followed by EB sterilization treatment.The antimicrobial results show that all the materials without leaching have good antimicrobial ability.After leached for 1 day,the antimicrobial ability of PLA with CFX disappears.While PBAT,PLA/PBAT(70/30),and PLA/PBAT(30/70)blends containing CFX keep good antimicrobial ability even after leaching for 15 days.The release kinetics show that the release rate of the antimicrobial agent is slow in the PLA matrix.After leached for 15 days,the cumulative releases are only 0.22%and 0.28%for PLA containing CFX and the one with CFX/MMT,respectively.While for PLA/PBAT(70/30)with CFX,the value is 0.73%,and it increases further to 2.57%with the addition of MMT.Meanwhile,the values for PLA/PBAT(30/70)and PBAT containing CFX are 12.67%and 8.78%,respectively,and the values increase further with the addition of MMT.This indicates that the polyester matrix has remarkable effect on the cumulative release of CFX.The sequence of the cumulative release is PLA/PBAT(30/70)>PBAT>PLA/PBAT(70/30)>PLA.The results of mechanical tests show that PLA/PBAT blends balance the rigidity and brittleness of PLA and the flexibility and toughness of PBAT.Further addition of CFX or MMT enhances the rigidity but decreases the toughness of the matrixes.