Hydrophobic derivatives of chitosan: characterization and rheological behaviour

Synthesis and mechanical properties of quaternary salts of chitosan-based films for food application

A novel method is described to synthesize quaternary salts of chitosan with dimethylsulfate and subsequently cast films. In an attempt to improve both mechanical and hydrophobic characteristics, the chitosan was previously modified by N-alkylation, introducing 4, 8 and 12 carbons moieties into the polymeric chain. Analysis by FTIR and solid-state CP-MAS (13)C NMR spectroscopy confirmed the success of both alkylation and quaternization processes. The average degree of quaternization of these N-methylated derivatives was calculated to be 35%. DMA measurements indicated that chitosan and its derivative films are typically brittle materials, exhibiting similar non-linear viscoelastic behaviors. The films of unmodified chitosan have a very small strain (approximately 2.8%), though they were the most resistant films (Young's modulus=2283 MPa; tensile strength >44.0 MPa). In general, the alkyl-chitosan derivatives appear to be more plastic than chitosan films but less resistant, e.g., for butyl chitosan: maximum strain=13.1%; tensile strength=13.4 MPa and Young's modulus=171 MPa. Conversely the quaternization reaction increased the hardness of the parent sample, viz. for quaternary salt of dodecyl chitosan: maximum strain=2.6%; tensile strength=38.3 MPa and Young's modulus=1792 MPa.

Chitosan: a soft interconnect for hierarchical assembly of nano-scale components

Traditional microfabrication has tremendous capabilities for imparting order to hard materials (e.g., silicon wafers) over a range of length scales. However, conventional microfabrication does not provide the means to assemble pre-formed nano-scale components into higher-ordered structures. We believe the aminopolysaccharide chitosan possesses a unique set of properties that enable it to serve as a length-scale interconnect for the hierarchical assembly of nano-scale components into macro-scale systems. The primary amines (atomic length scale) of the glucosamine repeating units (molecular length scale) provide sites to connect pre-formed or self-assembled nano-scale components to the polysaccharide backbone (macromolecular length scale). Connections to the backbone can be formed by exploiting the electrostatic, nucleophilic, or metal-binding capabilities of the glucosamine residues. Chitosan's film-forming properties provide the means for assembly at micron-to-centimetre lengths (supramolecular length scales). In addition to interconnecting length scales, chitosan's capabilities may also be uniquely-suited as a soft component-hard device interconnect. In particular, chitosan's film formation can be induced under mild aqueous conditions in response to localized electrical signals that can be imposed from microfabricated surfaces. This capability allows chitosan to assemble soft nano-scale components (e.g., proteins, vesicles, and virus particles) at specific electrode addresses on chips and in microfluidic devices. Thus, we envision the potential that chitosan may emerge as an integral material for soft matter (bio)fabrication.

Synthesis and characterization of phosphorylcholine-substituted chitosans soluble in physiological pH conditions

A polymer analogous synthesis involving the reductive amination of phosphorylcholine (PC)-glyceraldehyde with primary amines of deacetylated chitosan (M(w) approximately 57000 g mol(-1)) was used to prepare phosphorylcholine-substituted chitosans (PC-CH) with a degree of substitution (DS) ranging from approximately 11 to approximately 53 mol % PC-substituted glucosamine residues. The PC-CH derivatives were characterized by (1)H NMR spectroscopy, FTIR spectroscopy, and multiangle laser light scattering gel permeation chromatography (MALLS-GPC). The pK(a) of the PC-substituted amine groups (pK(a) approximately 7.20) was determined by (1)H NMR titration. The PC-CH samples (1.0 g L(-1)) were shown to be nontoxic using an MTT assay performed with human KB cells. Aqueous solutions of PC-CH samples (4.0 g L(-1)) of DS >or= 22 mol % PC-substituted glucosamine residues remained clear, independently of pH (4.0 < pH < 11.0). The remarkable water solubility and nontoxicity displayed by the new PC-CH samples open up new opportunities in the design of chitosan-based biomaterials and nanoparticles.

Branched chitosans: Effects of branching parameters on rheological and mechanical properties

Chitosan continues to be studied as a promising biomaterial for tissue repair and regeneration applications. However, chitosan structures show a large reduction in tensile strength in the wet state. Methods for improving the wet strength of chitosan materials may broaden its applicability as a tissue scaffold for applications requiring significant load bearing capacity. In this study, the role of molecular architecture in defining the mechanical properties of hydrated chitosan membranes was examined. Specifically, branched chitosan molecules were synthesized with a range of branch lengths and branch densities. Physical and mechanical properties were characterized using viscometry, FTIR spectroscopy, and tensile testing measurements, and the results were correlated with the postulated architecture of the linear and branched chitosan materials. Both branch density and branch length were found to influence the mechanical properties of chitosan membranes. For example, high-molecular-weight (600 kDa) chitosans grafted with 80 kDa branches exhibited up to twofold increases in both tensile strength and extensibility. FTIR results indicated that these increases correlated with enhanced levels of hydrogen bonding in the branched materials. Vascular smooth muscle cells cultured on cast membranes of the branched chitosans exhibited no differences in adhesion or spreading as compared to the linear polymer. The results indicate that the mechanical properties of chitosan materials can be improved by the induction of a branched molecular architecture.

Novel chitosan-derived nanomaterials and their micelle-forming properties

Six novel N-alkyl-N-dimethyl and N-alkyl-N-trimethyl chitosan derivatives were chemically synthesized and characterized using FT-IR, 1H NMR, 13C NMR, differential scanning calorimetry (DSC), and X-ray diffraction spectrometry (XRD). The alkyl groups included octyl (C8H17-), decanyl (C10H21-), and lauryl (C12H25-). These chitosan derivatives were also evaluated for their micelle-forming properties using dynamic light scattering (DLS) and transmission electron microscopy (TEM) techniques. All six chitosan derivatives were capable of forming polymeric micelles in water with an average particle diameter ranging from 36 to 218 nm. Both N-octyl-N-dimethyl and N-octyl-N-trimethyl chitosan derivatives formed nanomicelles under the experimental conditions, with an average particle diameter of 36.0 and 52.5, respectively. Both the length of alkyl group and the N-trimethylation degree of the chitosan derivatives altered the size of their polymeric micelles. To further understand the effect of N-alkyl substitution degree of chitosan derivatives on size of their micelles, additional five N-octyl-N-trimethyl chitosan derivatives with N-alkyl substitution degree ranging from 8 to 58% were prepared and their micelle sizes were determined. The results showed that the diameter of the nanomicelles was proportional to the degree of N-octyl substitution. These data suggest that novel N-alkyl-N-dimethyl and N-alkyl-N-trimethyl chitosan derivatives may form nanomicelles. Additional research is required to further investigate the potential value-added utilization of these chitosan derivatives in controlled release and targeted delivery of hydrophobic bioactive food factors.

Enhancement of fungicidal and insecticidal activity by reductive alkylation of chitosan

A series of N-alkyl chitosan (NAC) derivatives were synthesized using a reductive alkylation reaction to examine their fungicidal and insecticidal activity. The chemical structures were characterized by IR and (1)H NMR spectroscopy, and the degree of substitution (DS) ranged from 0.02 to 0.37. Their fungicidal activity was evaluated against the grey mould Botrytis cinerea Pers ex Fr (Leotiales: Sclerotiniaceae) and the rice leaf blast pathogen Pyricularia grisea Sacc [Teleomorph: Magnaporthe grisea (Hebert) Barr] by a radial growth bioassay. It was of interest that most of the NAC derivatives were more active against both fungi than chitosan itself. The most active derivative was N-(2,2-diphenylethyl)chitosan with EC50 values of 0.031 and 0.23 g L(-1) against B. cinerea and P. grisea respectively. In addition, some derivatives, at higher concentrations up to 1.0 g L(-1), inhibited the mycelial growth and spore formation of P. grisea. Bioassays against larvae of the cotton leafworm, Spodoptera littoralis (Boisd.) (Lepidoptera: Noctuidae) with the NAC derivatives at a rate of 5.0 g kg(-1) in artificial diet demonstrated that N-(3-phenylbutyl)chitosan was the most active compound. In addition, N-propylchitosan, N-undecanylchitosan and N-(3-phenylpropyl)chitosan derivatives strongly inhibited larval weight gain in S. littoralis, with respective reductions of 76, 66 and 65% after 4 days of feeding on treated diet.

Glucose biosensor based on the layer-by-layer self-assembling of glucose oxidase and chitosan derivatives on a thiolated gold surface

The work proposed here deals with the design and characterization of biorecognition layers for the amperometric glucose determination based on the self-assembling of new chitosan derivatives, Nafion and glucose oxidase onto thiolated gold electrodes. The supramolecular multistructure is obtained by deposition of a layer of chitosan derivative (quaternized or hydrophobic) onto the gold surface modified with the sodium salt of 3-mercapto-1-propansulfonic acid, followed by the deposition of a layer of Nafion (as anti-interference barrier) and by the alternate deposition of the chitosan derivative and glucose oxidase (as biocatalytic layer). The influence of the deposition time and concentration of polyelectrolytes, organization and number of layers, and nature of the chitosan derivative on the sensitivity and selectivity of the bioelectrode is examined and optimized in order to obtain a rational design of the biosensor. The system is studied electrochemically from the oxidation at 0.700 V of the hydrogen peroxide enzymatically generated using gold as substrate, and spectrophotometrically from the protein absorption at 277 nm using quartz as substrate. The selected biosensor containing five quaternized chitosan/glucose oxidase bilayers exhibits very good analytical performance with a sensitive ((4.9+/-0.2) x 10(2) nA mM(-1)) and highly selective response (0% interference for maximum physiological levels of ascorbic acid and uric acid), demonstrating that the alternate electrostatic adsorption of conveniently selected polyelectrolytes allows noticeable improvements in the selectivity and sensitivity of a biosensor.

Non-Covalent Interactions in Polysaccharide Systems

[Chemical structure: see text] This paper describes the behavior of some polysaccharides with well-known chemical structures and in which the influence of cooperative secondary interactions play an important role. The roles played by hydrophobic and ionic interactions (including ionic selectivity) on polysaccharide conformation and gelation are discussed. Electrostatic attractions are also important in the complexes formed between surfactants and polyelectrolytes of opposite charge. Finally, van der Waals dipolar interactions and particularly hydrogen-bond formation are examined. The role of hydrogen bonds in solubility, conformation, and especially the local stiffness of polysaccharides, but also in polymer-polymer complexes frequently obtained with polysaccharides, is developed. Repeat unit for a number polysaccharides.

Physicochemical properties of low molecular weight alkylated chitosans: A new class of potential nonviral vectors for gene delivery

Low molecular weight chitosans grafted with N-/2(3)-(dodec-2-enyl)succinoyl groups (HM-LMW-chitosans) with a mean molecular mass of 5 kDa, a degree of acetylation of 3% and a degree of tetradecenoyl substitution (TDC) of 3-18 mol% have been synthesized. These molecules are monodisperse and soluble in water at neutral pH. Using tensiometry and Nile Red fluorescence, the HM-LMW-chitosans were found to form micelles through hydrophobic interactions involving their tetradecenoyl chains and nonprotonated glucosamine monomers. Their critical micelle concentration decreases with increasing TDC values but varies little with pH and salt. Interaction with large unilamellar vesicles taken as model membranes indicated that HM-LMW-chitosans interact mainly with vesicles mimicking the inner leaflet of biomembranes both through electrostatic and hydrophobic interactions. This preferential interaction may destabilize endosomal membranes and favor the DNA release into the cytoplasm in gene delivery applications. Moreover, since this interaction significantly decreased the membrane fluidity of these vesicles, the HM-LMC-chitosans are thought to exhibit limited lateral mobility and flip-flop ability, and thus, limited cytotoxicity. These properties suggest that the HM-LMW-chitosans may constitute a promising new class of nonviral vectors for gene therapy.

Bioinspired vesicle restraint and mobilization using a biopolymer scaffold

Biology employs vesicles to package molecules (e.g., neurotransmitters) for their targeted delivery in response to specific spatiotemporal stimuli. Biology is also capable of employing localized stimuli to exert an additional control on vesicle trafficking; intact vesicles can be restrained (or mobilized) by association with (or release from) a cytoskeletal scaffold. We mimic these capabilities by tethering vesicles to a biopolymer scaffold that can undergo (i) stimuli-responsive network formation (for vesicle restraint) and (ii) enzyme-catalyzed network cleavage (for vesicle mobilization). Specifically, we use the aminopolysaccharide chitosan as our scaffold and graft a small number of hydrophobic moieties onto its backbone. These grafted hydrophobes can insert into the bilayer to tether vesicles to the scaffold. Under acidic conditions, the vesicles are not restrained by the hydrophobically modified chitosan (hm-chitosan) because this scaffold is soluble. Increasing the pH to neutral or basic conditions allows chitosan to form interpolymer associations that yield a strong, insoluble restraining network. Enzymatic hydrolysis of this scaffold by chitosanase cleaves the network and mobilizes intact vesicles. Potentially, this approach will provide a controllable means to store and liberate vesicle-based reagents/therapeutics for microfluidic/medical applications.

Transition from unilamellar to bilamellar vesicles induced by an amphiphilic biopolymer

We report some unusual structural transitions upon the addition of an amphiphilic biopolymer to unilamellar surfactant vesicles. The polymer is a hydrophobically modified chitosan and it embeds its hydrophobes in vesicle bilayers. We study vesicle-polymer mixtures using small-angle neutron scattering (SANS) and cryotransmission electron microscopy (cryo-TEM). When low amounts of the polymer are added to unilamellar vesicles of ca. 120 nm diameter, the vesicle size decreases by about 50%. Upon further addition of polymer, lamellar peaks are observed in the SANS spectra at high scattering vectors. We show that these spectra correspond to a co-existence of unilamellar and bilamellar vesicles. The transition to bilamellar vesicles as well as the changes in unilamellar vesicle size are further confirmed by cryo-TEM. A mechanism for the polymer-induced transitions in vesicle morphology is proposed.

Physical Gelation of Chitosan in the Presence of β-Glycerophosphate: The Effect of Temperature

When adding beta-glycerophosphate (beta-GP), a weak base, to chitosan aqueous solutions, the polymer remains in solution at neutral pH and room temperature, while homogeneous gelation of this system can be triggered upon heating. It is therefore one of the rare true physical chitosan hydrogels. In this study, physicochemical and rheological properties of chitosan solutions in the presence of acetic acid and beta-GP were investigated as a function of temperature in order to gain a better understanding of the gelation mechanisms. The gel structure formed at high temperature was only partially thermoreversible upon cooling to 5 degrees C because of the existence of remaining associations, confirmed by the spontaneous recovery of the gel after breakup at low temperature. Increasing temperature had no effect on the pH values of this system, while conductivity (and calculated ionic strength) increased. Values from the pH measurements were used to estimate the degree of protonation of each species as a function of temperature. The decreasing ratio of -NH3+ in chitosan and -OPO(O-)2 in beta-GP suggested reduced chitosan solubility along with a diminution of ionic interactions such as ionic bridging with increasing temperature. On the other hand, the increased ionic strength as a function of temperature, in the presence of beta-GP, enhanced screening of electrostatic repulsion and increased hydrophobic effect, resulting in favorable conditions for gel formation. Therefore, our study suggests that hydrophobic interactions and reduced solubility are the main driving force for chitosan gelation at high temperature in the presence of beta-GP.

New Aspects of the Formation of Physical Hydrogels of Chitosan in a Hydroalcoholic Medium

New aspects concerning the mechanism of formation of chitosan physical hydrogels without any cross-linking agent were studied. The gelation took place during the evaporation of a hydroalcoholic solution of chitosan. We first demonstrated that it was possible to form a physical hydrogel from a hydrochloride form of chitosan. Chromatographic methods showed that during the gel formation, when the initial concentration is over C, the critical concentration of chain entanglement, the water and acid used for the solubilization of the polymer were both eliminated. This particular situation contributed to decrease the dielectric constant of the medium and the apparent charge density of chitosan chains, thus inducing the formation of a three-dimensional network through hydrophobic interactions and hydrogen bonding. In the gelation process, this step was kinetically determining. The speed of evaporation of water and acid were determined and different initial conditions were compared. Thus, we investigated the influence of: the initial polymer concentration, the nature of the counterion and the alcohol, the temperature and the geometry of the reactor. Our results allowed us to confirm the existence of a second critical initial concentration C, from which the evaporation of water became more difficult. We suggested that C corresponded to a reorganization of the solution involving the presence of gel precursors. Then, a mechanism of formation of physical hydrogels of chitosan in a hydroalcoholic medium could be proposed. For the first time, we demonstrated that it was possible to generate physical hydrogels in the presence of various diols, which size of the carbonated chain appeared as a limiting factor for the gelation process. These physical hydrogels of chitosan are currently used in our laboratory for tissue engineering in the treatment of third degree burns with the possibility to adapt their mechanical properties from the choice of both the acid or the alcohol used.

Synthesis of chitosan-based polymeric surfactants and their adsorption properties for heavy metals and fatty acids

Chitosan-based polymeric surfactants (CBPSs) were prepared by N-acylation of chitosans (chitosan 10 and 500) with several acid anhydrides such as hexanoic (C6), lauric (C12), and palmitic (C16) anhydrides. Among the CBPS samples, CBPSs having a good solubility at pH 4.0 were selected and observed for viscosity, surface tension, and adsorption of heavy metals (Cd2+, Co2+, Cr2O7(2-), and Pb2+) as well as the fatty acid (n-octanoic acid). The 1H NMR spectrum of chitosan 10 modified with C16 at the substitution ratio of 0.4 (CBPS10-C16,0.4) showed 85% of acylation in 1% DCl/D2O solutions. CBPS10 with the substitution ratio less than 0.4 showed a good solubility because of shorter repeating units and lesser amounts of hydrophobic substituents. The intrinsic viscosity of CBPS10 was slightly increased, while that of CBPS500 was decreased. As the substitution ratio and length of the carbon chain increased, the surface tension of CBPS10 tended to decrease. CBPS10-C16,0.2 had high adsorption ability for cationic metal ions such as Cd2+, Co2+, and Pb2+ comparable to chitosan. Interestingly, CBPS(10)-C(16,0.2) showed a unique pH optimum for the anionic metal ion such as Cr2O7(2-). In addition, CBPS10-C16,0.2 exhibited the highest adsorption ability for n-octanoic acid among the tested CBPS10 with different carbon chains.

Diethyl methyl chitosan as an intestinal paracellular enhancer: ex vivo and in vivo studies

Chitosan exhibits favorable biological properties such as no toxicity, biocompatibility and biodegradability; therefore, it has attracted great attention in both pharmaceutical and biomedical fields. Chitosan exhibits poor solubility at pH values above 6 that prevents enhancing effects at the sites of absorption of drugs. In the present work, N-diethyl methyl chitosan (DEMC) was prepared and the enhancing effect of this polymer was investigated. Ex vivo studies have shown a significant increase in absorption of brilliant blue in the presence of diethyl methyl chitosan in comparison with chitosan. DEMC with positive charges is able to interact with tight junctions of colon epithelial cells and hence increases permeability of brilliant blue across the tight junctions. In vivo investigations have exhibited the absorption enhancer effects of DEMC on the colon absorption of insulin in normal and diabetic rats. The insulin absorption from the rat's colon was evaluated by its hypoglycemic effect. A significant decrease in blood glucose was observed, when mixture of insulin and DEMC was introduced in ascending colon of rats.

Chitosan−Cholesterol and Chitosan−Stearic Acid Interactions at the Air−Water Interface

We report in this work the isotherms of cholesterol and stearic acid at the air-water interface modified by different chitosans (chitosan chloride, hydrophobic modified chitosan, and medium and high molecular weight chitosans) in the aqueous subphase. The Langmuir-Blodgett films of the complexes cholesterol-chitosan and stearic acid-chitosan are analyzed by atomic force microscopy (AFM), and a molecular simulation was performed to visualize the chitosan-lipid interactions. Strong modifications are obtained in the isotherms as a result of the chitosan interactions with cholesterol and stearic acid at the air-water interface. These modifications were dependent on the type and concentration of chitosan. Severe modifications of all phases were noticed with larger molecular areas, and the observed changes in the compressional modulus were dependent on the type of chitosan used. The complexes of chitosan-stearic acid were more flexible than the ones of chitosan-cholesterol. The AFM images demonstrated that chitosan was disaggregated by the cholesterol and stearic acid interactions producing more homogeneous surfaces in some cases. The hydrophobic chitosan showed more affinity with stearic acid, while both medium and high molecular weight chitosans produced homogeneous surfaces with cholesterol. The simulated chitosan chains interacting with cholesterol and stearic acid demonstrated the possibility of specific sites of electrostatic bonds between these molecules. Adsorption of cholesterol on the different powdered chitosans, performed by HPLC, showed that the medium and high molecular weight chitosans could retain higher proportions of cholesterol compared with the other analyzed samples.

Specific Interactions in Modified Chitosan Systems

This paper concerns the bulk and interfacial properties of a series of alkylated chitosans having different alkyl chain lengths grafted randomly along the main chitosan chain. Chitosan has a low degree of acetylation (5%); on chitosan derivatives, the role of the degree of grafting and of length of the alkyl chains are examined. The optimum alkyl chain length is C12 and the degree of grafting 5% to get physical gelation based on the formation of hydrophobic domains. The cross-linking is essentially controlled by the salt concentration: it is shown that 0.025 M AcONa is needed to screen electrostatic interchain repulsions. Hydrophobic interactions produce highly non-Newtonian behavior with large thinning behavior; this behavior is suppressed in the presence of cyclodextrins able to cap the hydrophobic alkyl chains. The interfacial properties of the chitosan derivatives were tested for the air/aqueous solution interfaces. Specifically, the role of their structure on the kinetic of film formation was examined showing that excess of external salt favors the stabilization of the interfacial film. The derivatives with a higher degree of substitution and longer alkyl chains are more efficient and give a higher elastic modulus compared to the model surfactant as a result of the chain properties.

An improved sonochemical reactor

The design and optimization of sonochemical apparatus are still open to advancement. Under high-intensity ultrasound reaction rates and yields are mainly influenced by the characteristics of transducer and reactor. Several useful improvements are introduced and described. In order to achieve uniformity of the acoustic field and optimal acoustic streaming in every part of the reaction vessel (a Teflon tube), the reactor can be made to rotate eccentrically around the horn axis and the probe to move alternatively up and down by a pre-determined excursion at a chosen speed. Continuous high-power irradiation is feasible without any time limit because the whole probe system is refrigerated by an oil forced-circulation circuit connected to a chiller. The apparatus can control a number of important reaction parameters: modified atmosphere, reaction temperature, tunable frequency and constant amplitude. Excellent performance was observed on several reactions, such as the chemical modification of chitosan, a poorly soluble biopolymer.

Degradability of Polysaccharides Multilayer Films in the Oral Environment: an in Vitro and in Vivo Study

Biomedical devices and modified biomaterial surfaces constitute an expanding research domain in the dental field. However, such oral applications have to face a very particular environment containing specific physiological conditions and specific enzymes. To evaluate their suitability in the development of novel oral applications, the degradability of polyelectrolyte multilayer films made of the natural polysaccharides chitosan and hyaluronan (CHI/HA) was investigated in vitro and in vivo in a rat mouth model. The films were either native or cross-linked using a water-soluble carbodiimide (EDC) in combination with N-hydroxysulfosuccinimide. The in vitro degradation of the films by different enzymes present in the oral environment, such as lysozyme and amylase, was followed by quartz crystal microbalance measurements and confocal laser scanning microscopy observations after being film labeled with CHI(FITC). Whereas native films were subjected to degradation by all the enzymes, cross-linked films were more resistant to enzymatic degradation. Films were also put in contact with whole saliva, which induced a slow degradation of the native films over an 18 h period. The in vivo degradation of the films deposited on polymer disks and sutured in the rat mouth was followed over a 3 day period. Whereas film degradation is fast for native films, it is much slower for the cross-linked ones. More than 60% of these films remained on the disks after 3 days in the mouth. Taken together, these results suggest that the multilayer films made of natural polysaccharides are of high potential interest for oral applications, especially as drug release systems, offering various degradation rates and consequent release characteristics.

Vesicle--biopolymer gels: networks of surfactant vesicles connected by associating biopolymers

The effect of adding an associating biopolymer to surfactant vesicles and micelles is studied using rheology and small-angle neutron scattering (SANS). The associating polymer is obtained by randomly tethering hydrophobic alkyl chains to the backbone of the polysaccharide, chitosan. Adding this polymer to surfactant vesicles results in a gel; that is, the sample transforms from a Newtonian liquid to an elastic solid having frequency-independent dynamic shear moduli. SANS shows that the vesicles remain intact within the gel. The results suggest a gel structure in which the vesicles are connected by polymer chains into a three-dimensional network. Vesicle-polymer binding is expected to occur via the insertion of polymer hydrophobes into the vesicle bilayer. Each vesicle thus acts as a multifunctional junction in the network structure. Significantly, gel formation does not occur with the native chitosan that has no hydrophobes. Moreover, adding the hydrophobically modified chitosan to a viscous sample containing wormlike micelles increases the viscosity further but does not give rise to a gel-like response. Thus, the formation of a robust gel network requires both the presence of hydrophobes on the polymer and vesicles in solution.

Layer by layer buildup of polysaccharide films: physical chemistry and cellular adhesion aspects

The formation ofpolysaccharide films based on the alternate deposition of chitosan (CHI) and hyaluronan (HA) was investigated by several techniques. The multilayer buildup takes place in two stages: during the first stage, the surface is covered by isolated islets that grow and coalesce as the construction goes on. After several deposition steps, a continuous film is formed and the second stage of the buildup process takes place. The whole process is characterized by an exponential increase of the mass and thickness of the film with the number of deposition steps. This exponential growth mechanism is related to the ability of the polycation to diffuse "in" and "out" of the whole film at each deposition step. Using confocal laser microscopy and fluorescently labeled CHI, we show that such a diffusion behavior, already observed with poly(L-lysine) as a polycation, is also found with CHI, a polycation presenting a large persistence length. We also analyze the effect of the molecular weight (MW) of the diffusing polyelectrolyte (CHI) on the buildup process and observe a faster growth for low MW chitosan. The influence of the salt concentration during buildup is also investigated. Whereas the CHI/HA films grow rapidly at high salt concentration (0.15 M NaCl) with the formation of a uniform film after only a few deposition steps, it is very difficult to build the film at 10(-4) M NaCl. In this latter case, the deposited mass increases linearly with the number of deposition steps and the first deposition stage, where the surface is covered by islets, lasts at least up to 50 bilayer deposition steps. However, even at these low salt concentrations and in the islet configuration, CHI chains seem to diffuse in and out of the CHI/HA complexes. The linear mass increase of the film with the number of deposition steps despite the CHI diffusion is explained by a partial redissolution of the CHI/HA complexes forming the film during different steps of the buildup process. Finally, the uniform films built at high salt concentrations were also found to be chondrocyte resistant and, more interestingly, bacterial resistant. Therefore, the (CHI/HA) films may be used as an antimicrobial coating.

Structure and interactions in chitosan hydrogels formed by complexation or aggregation for biomedical applications

The aim of this review was to provide a detailed overview of physical chitosan hydrogels and related networks formed by aggregation or complexation, which are intended for biomedical applications. The structural basis of these systems is discussed with particular emphasis on the network-forming interactions, the principles governing their formation and their physicochemical properties. An earlier review discussing crosslinked chitosan hydrogels highlighted the potential negative influence on biocompatibility of covalent crosslinkers and emphasised the need for alternative hydrogel systems. A possible means to avoid the use of covalent crosslinkers is to prepare physical chitosan hydrogels by direct interactions between polymeric chains, i.e. by complexation, e.g. polyelectrolyte complexes (PEC) and chitosan/poly (vinyl alcohol) (PVA) complexes, or by aggregation, e.g. grafted chitosan hydrogels. PEC exhibit a higher swelling sensitivity towards pH changes compared to covalently crosslinked chitosan hydrogels, which extends their potential application. Certain complexed polymers, such as glycosaminoglycans, can exhibit interesting intrinsic properties. Since PEC are formed by non-permanent networks, dissolution can occur. Chitosan/PVA complexes represent an interesting alternative for preparing biocompatible drug delivery systems if pH-controlled release is n/ot required. Grafted chitosan hydrogels are more complex to prepare and do not always improve biocompatibility compared to covalently crosslinked hydrogels, but can enhance certain intrinsic properties of chitosan such as bacteriostatic and wound-healing activity.

Two types of hydrophobic aggregates in aqueous solutions of chitosan and its hydrophobic derivative

The aggregation phenomena in aqueous solutions of hydrophobically modified (HM) chitosan, containing 4 mol % of n-dodecyl side chains, were studied by viscometry and fluorescence spectroscopy with pyrene as a probe. The results are compared with those for unmodified chitosan. Surprisingly, fluorescence data reveal the appearance of intermolecular hydrophobic aggregates both in chitosan and in HM chitosan. Nevertheless, these polymers exhibit quite different rheological properties: upon the formation of aggregates the viscosity of HM chitosan sharply increases, while that of unmodified chitosan raises only slightly. The aggregation models for both chitosan and its hydrophobic derivative were proposed. It was shown that in solutions of HM chitosan two types of hydrophobic domains exist: hydrophobic domains typical for different associating polymers with hydrophobic side chains and hydrophobic domains inherent to chitosan itself.