Electrospinning, Crosslinking And Biomedical Properties Of Pectin Nanofibers

Pectin has great biomedical potential because of its extensive sources,good biocompatibility and unique biological activities.The nanofibers prepared by electrospinning has similar structure to that of the natural extracellular matrix and the characteristics of high porosity,good connectivity,high specific surface area,and thus show significant advantages for biomedical applications.However,pectin nanofibers for biomedical applications are still in lack due to the following reasons:?1?pure aqueous pectin solutions can not be electrospun directly,?2?high levels of synthetic polymer are needed to assist the electrospinning of pectin nanofibers,and?3?the as-spun pectin nanofibers cannot tolerate aqueous environment.In this study,we prepared pectin nanofibers containing low level of synthetic polymer through reasonable improvements of the electrospinning solution system.The pectin nanofibers were then crosslinked to construct water resistance so that they can maintain a stable structure in the aqueous physiological environment and possible to be used as biomedical materials.Subsequently,the biomedical application related properties of pectin nanofibers were characterized and the biomedical application potential of the pectin nanofibers was confirmed.The specific work of this study includes three parts:In the first part,in order to overcome the difficulty in electrospinning of aqueous pectin solutions and obtain pectin nanofibers,we used a flexible synthetic polymer polyethylene oxide?PEO?as removable carrier to enhance chain entanglements in aqueous pectin solutions.Moreover,we added surfactant Triton X-100 to the pectin aqueous solution to reduce the surface tension,and added co-solvent dimethyl sulfoxide?DMSO?or improved the molecular weight of PEO to further promote chain entanglements.PEO was further removed by washing with a selective solvent.By these ways,pectin nanofibers containing only 1.5% PEO were successfully prepared.In the second part,we used three methods to crosslink the nanofibers of three representative pectin?high-methoxylated citrus pectin?H-CP?,low-methoxylated sunflower pectin?L-SP?,low-methoxylated and amidated pectin apple pectin?LAm-AP??to construct water resistance for them.The first method was to crosslink the pectin nanofibers with calcium ions(Ca²⁺),making use of the carboxyl groups on pectin molecular chain.The second method is to further crosslink with glutaraldehyde?GLU?after Ca²⁺crosslinking,making use of hydroxyl groups.The third method is to further crosslink with adipic acid dihydrazide?ADH?after Ca²⁺ crosslinking,making use of carboxyl groups.The success of crosslinking was confirmed by morphology and chemical composition characterization.In the third part,we characterized the biomedical application related properties of crosslinked pectin nanofibers,including crystallization,mechanical,degradation and cellular properties,and also dissucssed the factors affecting these properties.Mechanical tests and degradation results indicated that for Ca²⁺-crosslinked nanofibers,those of H-CP with high degree of methoxylation had the lowest mechanical strength and the fastest degradation rate,because that the crosslinking network in them was most sparsely,and the molecular chain of H-CP is branched.The L-SP nanofibers had the highest mechanical strength and the slowest degradation rate because that L-SP was low-methoxylated and of high content of galacturonic acid?GalA?,causing denser crosslinking network,and also the molecular chain of L-SP is mostly linear.Subsequent GLU crosslinking after Ca²⁺crosslinking moderately improved the mechanical strength of pectin nanofibers but did not inhibit their degradation.This is because that although during the GLU crosslinking process new covalent crosslinking network was formed,the acidic condition caused dismiss of the Ca²⁺-mediated network partially and could also destroy the glycosidic bonds of pectin molecular chains.By contrast,subsequent ADH crosslinking after Ca²⁺crosslinking dramatically improved their mechanical strength and slowed down their degradation rate.This is because that ADH crosslinking was carried out under mild condition,which had no damage to pectin molecular chains and caused less or no effects on the preexisted Ca²⁺-mediated network.Cell culture experiments demonstrated that GLU crosslinking caused cytotoxicity of pectin nanofibers at high concentration but all pectin nanofibers crosslinked with other methods caused no cytoxicity.High degree of methoxylation and ADH crosslinking facilitated cell adhesion and proliferation on pectin nanofibers.To summarize,pectin nanofibers with water resistance was successfully prepared through the electrospinning and crosslinking of pectin nanofibers with low content of PEO,which fill in the blank in the biomedical fields.Characterization of the biomedical properties of pectin nanofibers showed that the mechanical,degradation and cell properties of pectin nanofibers were tunable.These water resistant pectin nanofibers with different features hold great potential for biomedical applications,such as tissue engineering,drug delivery and wound healing.