Functional Materials Based On The Mucopolysaccharide Of Hyaluronan And Their Applications

Mucopolysaccharide or Glycosaminoglycan is widely distributed inanimals’body and presents multiple physiological functionalities. Hyaluronan (HA) isone of the most representative mucopolysaccharides and supposed to be the onlypolysaccharide that exists in almost all animal species, from bacteria to human beings.The biological functions of HA include maintenance of the elastoviscosity of liquidconnective tissues such as joint synovial and eye vitreous fluid, control of tissuehydration and water transport, supramolecular assembly of proteoglycans in theextracellular matrix, and numerous receptor-me-diated roles in cell detachment,mitosis, migration, tumor development and metastasis, and inflammation. Moreover,as one natural biopolymer, it is inherently biocompatible, biodegradable, bioactive,non-immunogenic and nonthrombogenic. For these merits of HA, this bioactivepolysaccharide has been an attractive building block for the fabrication of functionalmaterials. Nowadays, HA and the materials based on it are widely used in the fields ofpharmaceutical, food and cosmetics, tissue engineering and nanomaterials, etc.Among the stuies of HA hydrogels most work focused on the chemicallycrosslinked hyalruoan or HA derivatives. In the process of synthesizing HA hydrogelsby chemical reaction, the use of crosslinking agent or organic solvent, and theexistence of reacting by-products in final hydrogels are inevitable. These will impairthe biological compatibility in both short and long-term applications of HA, especiallyin the biomedical aspects. Therefore, the study on the physically crosslinked HAhydrogels will be very important and promising. Moreover, HA can be used asbioactive component for construction of nanomaterials, which has gained researchers’enormous concern and interest. The main work in this dissertation thus revolves around the construction of hyluroan-based functional materials with various nano-andmicro-structures and different potential applications. HA in these processes acts asimportant building block for creating materials and presents multiple functionalitieswith regulating the nanostructure and promoting the biocompatibility etc. The maincontents and conclusions are summarized as follows:Firstly, physically crosslinked hydrogels from HA were prepared by freeze-thawtechnique. The effects of processing steps (freezing time and number of freeze-thawcycles), molecular weight of HA, and some small molecular additives such asdicarboxylic acids and polyols as probes on the formation of the HA cryotropichydrogels were investigated by dynamic rheometry, optical microscopy, X-raydiffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR) and gelpermeation chromatography (GPC). The experimental results showed that the longperiod of the freezing time and repeated freeze-thaw cycles benefited the alignment ofthe polymer chains in the unfrozen polymer phase and thereby promoted theformation of intermolecular aggregations and densely fibrillar network structures. HAwith a higher molecular weight was favorable to the formation of HA cryogel with astronger mechanical performance. The influences of various small molecular additiveson the gelation of HA had relevance to the intermolecular associations in the gelnetwork. Experimental findings showed that both succinic acid and glutaric acid werecapable of participating in the formation of HA cryogel whereas the addition of oxalicacid, malic acid and tartaric acid as well as some polyols (glycol, butanediol andglycerol) inhabited the gelation of HA. Protonation of the polyanion of HA at properlow pH served as a prerequisite of the association of multiple interchain hydrogenbonding among groups of-COOH and-NHCOCH3and the formation of networkafterwards. HA cryogels showed fibrillar network in which HA chains were associatedinto bundles. It was reavled that the contribution of the crystallization of HA was notobvious and the gel formation was essentially related to the HA conformation changesand the molecular aggregations driven by the hydrogen bonds among–COOH and-NHCOCH3groups. While in HA acidified cryogels hydrogen bonding between-COOH played dominating roles the in stabilizing the network, hydrogen bonding involving–COO-and NHCOCH3became the main driving force stabilizing thenetwork in neutral cryogels. All the experimental results in the present stronglysuggested that in either acidified or neutral cryogels hydroxyl groups of HA made nonoticeable contributions to such hydrogen bonding in the network. In comparison withnative HA in aqueous solution at the same concentration, the neutral HA cryogelsobtained by in situ neutralizing the acidified HA cryogel, showed higherviscoelasticity, better resistances to the acidic decomposing and enzymaticdegradation.Secondly, novel nanocomposite hydrogels composed of HA, poly(vinyl alcohol)(PVA), and silver nanoparticles were prepared by several cycles of freezing andthawing. The nanocomposite was then characterised using FTIR, differential scanningcalorimetry (DSC), XRD, and scanning electron microscopy (SEM). The complexhydrogels consisted of semi-interpenetrating network structures, with PVAmicrocrystallines as junction zones. By increasing the HA content, the crystallinityand melting temperature of the complex hydrogels decreased, whereas the glasstransition temperatures of these materials increased because of the steric hindrance ofHA and the occurrence of intermolecular interactions through hydrogen bondingbetween HA and PVA in the complex hydrogels. Swelling studies showed that thoseof the complex hydrogels can be significantly improved in comparison with theswelling properties of the cryogels from PVA alone and presented a pH-sensitivemanner. In addition, silver nanoparticles were synthesised through UV-initiatedphotoreduction with HA functioning as a reducing agent and stabiliser. The silvernanoparticles were then incorporated into the HA/PVA complex hydrogel matrix. Thesize and morphology of the as-prepared Ag nanoparticles were investigated throughUltraviolet-visible light spectroscopy, transmission electron microscopy, XRD, andthermogravimetric analysis (TGA). The experimental results indicated that silvernanoparticles20nm to50nm in size were uniformly dispersed in the hydrogel matrix.The antibacterial effects of the HA/PVA/Ag nanocomposite hydrogel againstEscherichia coli were evaluated by Kirby-Bauer and LB Broth method. The resultsshowed that this nanocomposite hydrogel possessed high antibacterial property and has a potential application as a wound dressing material.In the third section, HA-silver nanoparticles (HSNPs) were prepared byUV-initiated photoreduction and protein hemoglobin (Hb) was then alternatelyassembled with the prepared negatively charged HSNPs into layer-by-layer (LBL)films on solid surface. The electrochemical behavior and electrocatalytic activitiestoward oxygen and hydrogen peroxide of the resulting films were studied. It wasfound that the HSNPs greatly enhanced the electron transfer reactivity of Hb as abridge between protein and electrode and that the assembled films showed a pair ofnearly reversible redox peaks with a formal potential of0.32V (vs. Ag/AgCl) for theheme Fe(III)/Fe(II) redox couple. The immobilized Hb in the films maintained itsbiological activity, showing a surface controlled process with a heterogeneouselectron transfer rate constant (ks) of1.0s-1and displaying the same features of aperoxidase in the electrocatalytic reduction of oxygen and hydrogen peroxide. Thiswork provides a novel model to fabricate LBL films with protein, polysaccharide andnanoparticles and may establish a foundation for fabricating new type of biosensorsbased on the direct electron transfer of redox proteins immobilized in nanocompositemultilayer films with underlying electrodes.Fourthly, pyrene-conjugated HA (Py HA) was synthesized and characterized byATR-FTIR, NMR, UV-vis, flurorecent spectroscopy and surface tension. Py HAfacilitated the exfoliation of low-dimensional nanomaterials including graphene,hexagonal boron nitride (h-BN), molybdenum disulfide (MoS2), carbon nanotubes(CNTs) and carbon nano-onions (CNOs) in water (and PBS solutions), with theassistance of sonication. The materials were stabilised by the Py HA as uniformlydispersed suspensions in water, with excellent crystallinity. Moreover, thefunctionalized hybrid nanomaterials had biocompatibility factored in, and offeredscope for applications in biomedical fields, and beyond.And finally, we have developed a versatile platform involving reduced grapheneoxide and amphiphilic Py HA for fabricating composite nanomaterials involvingnoble metals (Au, Ag, Pd and Pt) using facile and green methods, with UV irradiationor introduction of hydrogen gas as the benign reducing process/agent. Py HA not only acted as a stabilizing agent for RGO, but also facilitated and controlled thedecoration of the metal on the Py-HA-RGO substrate. The hybrid nanocomposites canserve as sensing material for different analytes, depending on the specific propertiesof metals. The Au-graphene hybrid material was employed as the electrochemicalenhanced material for H2O2sensing, with wide linear ranges and low detection limits,and presented different sensing behaviours depending on the different nanostructures.The Pd-graphene hybrid material gave a change in conductivity in the presence ofhydrogen gas when assembled in an interdigitated electrode, which was the basis of ahydrogen gas sensing.