Dextran-based Smart Nanogels: Their Self-assembly Assisted Fabrication And Biomedical Applications

Nanogels are nano-scaled three dimensional interpenetrating networksformed by hydrophilic or amphiphilic polymers, and their excellent stabilityin aqueous solutions and ability to deliver a wide range of cargoes haverendered them very popular in biomedical applications. Nanogels can bemodified with fluorescent or magnetic agents for bio-imaging, or loaded withdrugs, proteins and growth factors for environmental-sensitive release, orused for bioactive molecules detection in complexed system. In this work,we efficiently fabricated several dextran-based smart nanogels with aself-assembly assisted one-pot synthesis method (SAA method). Duringthis process, free radicals were initiated in dextran backbone by using cericammonium nitrate (CAN), afterwards, various functional monomerspolymerized from dextran backbone. With the proceeding of thepolymerization of monomers, the self-assembly between dextran backbonesand polymeric grafts was induced to form nano-aggregates. With theaddition of di-vinyl monomers in this process, the “polymerization”,“self-assembly”, and “crosslinking” were accomplished together with highefficiency. The functional nanogels can be fabricated by this SAA methodat the concentration of as high as10mg/mL. Various monomers andcrosslinkers were utilized for the preparation of nanogels with pH-, reducingenvironment-, or even glucose-responsiveness. The structure andcomposition of the as-prepared nanogels were confirmed and their potentialapplications in biomedical fields were explored. The main contributions inthis thesis are as follows: (1) Dextran-poly(acrylic acid) nanogels (Dex-PAA NGs) were fabricatedwith hydrogen force-assisted self-assembly, and parameters affectingnanogels size and Zeta potential were studied. Dex-PAA NGs were furtherconjugated with a small fluorescent molecule5-aminofluorescein (5-AF)through amide condensation and the acquired fluorescent NGs (FNGs) wereused for adipose-derived stem cells (ADSCs) imaging. Furtherinvestigations discovered that sizes and Zeta potentials were two importantfactors affecting FNGs ability to enter ADSCs cells, and the results showedthat charged nanogels with a diameter less than200nm could enter cellsmore efficiently. The preferred FNGs were then used for lymphaticendothelial cells (LECs) labeling, and the FNGs-labeled LECs functionssuch as Dil-Ac-LDL ingestion and micro-tube formation ability were notaffected. Owing to their capability of entering lymph vessel by lymphaticdrainage, FNGs could also be used for lymph node imaging when injectedsubcutaneously, and the imaging time lasted for2-4hours which was muchbetter than the currently used imaging agents. What s more, Dex-PAA NGswere used in loading vascular endothelial growth factor C (VEGF-C), a drugwhich has been proven useful for lymphedema treatment. Based on thestudy, Dex-PAA NGs are of potential in lymphedema treatment.(2) As is well known, the tumor microenvironment is more reductiveand acidic than normal human tissues, we fabricated reduction-sensitivedextran-based nanogels through the SAA method by using the disulfide bond(-SS-) containing diallyl disulfide (DADS) as crosslinker. The disulfidebond in the resultant nanogels could be reduced to thiols when reductant waspresent, as a result, the nanogels were de-crosslinkable in tumormicroenvironments. Here we fabricated Dex-SS-PAA NGs from acrylicacid (AA) based on the self-assembly force of hydrogen bond, andDex-SS-PMA NGs from methyl acrylate (MA) with hydrophobic interactions.The successful fabrication of Dex-SS-PMA NGs greatly expanded the scopeof the SAA method from hydrophilic monomers to hydrophobic ones, thusallowing the fabrication of a diverse range of dextran-based nanogels. After the successful fabrication of two kinds of reduction-sensitivenanogels and the studies on their fabrication mechanisms, the size,morphology, reduction-sensitiveness and anti-cancer drug delivery abilities ofthese two categories of nanogels were studied. Dynamic light scattering(DLS) and static light scattering (SLS) results showed that the size andmolecular weight (Mw) of the two nanogels changed with the addition ofreductive agent glutathione (GSH), so their reduction-sensitivede-crosslinkage behavior was verified. The reduction-sensitive carrierswere very useful and promising in anti-cancer drug delivery because of thereductive tumor microenvironment. With this in mind, we conjugated aneffective anti-cancer drug doxorubicin hydrochloride (DOX) onto theDex-SS-PAA NGs and Dex-SS-PMA NGs through an acid-labile hydrazonebond, and their release behaviors and anti-cancer effects on human cervicalcancer HeLa cells and human brease cancer MCF-7cells were studied.Compared with Dex-SS-PAA NGs, the Dex-SS-PMA NGs had a narrowersize distribution, smaller size than Dex-SS-PAA NGs, higher loading contentand encapsulation efficiency of DOX when load DOX through the hydrazonebond. Correspondingly, Dex-SS-PMA-DOX NGs also revealed a strongerinhibition ability toward HeLa cells and MCF-7cells. When GSH wasadded to the culture medium to simulate the tumor environment, the DADScrosslinked nanogels showed better anti-cancer performance than theirnon-degradable counterparts, especially toward MCF-7cells. The in vivoexperiments carried out on nude mice with MCF-7tumor xenograft showedthat Dex-SS-PMA-DOX retained the anti-cancer ability of free DOX butsignificantly reduced the side effects. Dex-SS-PMA-DOX was shown to bea very promising anti-cancer drug delivery system.(3) pH/glucose dual responsive nanogels based on dextran andpoly(3-acrylamidophenylboronic acid)(PAAPBA) were fabricated withhydrophobic force assisted self-assembly strategy. The as-preparedDex-PAAPBA NGs (short for DABA NGs) showed a narrow size dispersity.As the ionization equilibrium shift of the PAAPBA chains would shift to the formation of more negatively charged species when glucose was present,DABA NGs exhibited a volume increase and Zeta potential decrease uponglucose addition at slightly alkaline pH. The glucose sensitivity of DABANGs was visualized by introducing a stimuli-responsive fluorescent agent,2-[4-(3-hydroxy-4-oxo-4H-chromen-2-yl)phenoxy]ethylacrylamide (3HF-AM) into the system. The fluorescence color of the resultant FDABA NGschanged from greenish blue to a deeper blue when glucose was added, andthis fluorescence variation upon glucose addition might have potentialapplication for glucose detection. Moreover, DABA NGs could encapsulateinsulin-a drug used for diabetes treatment, and the insulin-loaded nanogelsshowed a glucose-responsive release behavior, which may have potential forin vivo insulin delivery.In conclusion, we proposed and realized an SAA method for thepreparation of various dextran-based smart nanogels with environmentalsensitiveness and good biocompatibility. The novelty of this thesis lies inthe development and the universal applicability of the SAA method, whichinclude the expandation of the SAA method from hydrophilic polyanionmonomers to hydrophobic ones, the introduction of disulfide-containingcrosslinkers to prepare reduction-sensitive nanogels, and the exploration ofthe biomedical applications of these dextran-based smart nanogels.