Characterization of chitosan. Influence of ionic strength and degree of acetylation on chain expansion

Glutaraldehyde and glyoxal cross-linked chitosan microspheres for controlled delivery of centchroman

Glutaraldehyde and glyoxal cross-linked microspheres were prepared using chitosan with different molecular weights (MWs) and degrees of deacetylation (DDAs) for sustained release of centchroman under physiological conditions. The DDA in chitosan was determined by different methods, and the samples were categorized as chitosan with low (48%), medium (62%), and high (75%) DDA. The size and shape of the microspheres were determined by scanning electron microscopy (SEM), and hydrophobicity was determined by adsorption of Rose Bengal dye on microspheres cross-linked with glutaraldehyde or glyoxal. The effect of MW, DDA, and degree of cross-linking in microspheres was studied on the degree of swelling, as well as by the loading and release of centchroman. The glyoxal cross-linked microspheres were more compact and hydrophobic and showed better sustained release in companion to chitosan microspheres and glutaraldehyde cross-linked microspheres. The linear fractional release of centchroman with the square root of time indicated a Fickian behavior of centchroman, and the microspheres also showed zero-order release kinetics for centchroman.

Effect of organic and inorganic acids on concentrated chitosan solutions and gels

Rheological properties of concentrated chitosan aqueous solutions and gels in the presence of different organic and inorganic acids were investigated. Viscosities of the solutions increased with polymer concentration and degree of ionization. Strong gels were obtained at pH around 2 with oxalic, phosphoric and sulfuric acids. Gelation was favored by simple and short chain length acids and was governed by ionic interactions. The gels could be distinguished from solutions by the frequency independence of their dynamic moduli and their high apparent activation energy for flow.

Kinetics of Gelation and Thermal Sensitivity of N-Isobutyryl Chitosan Hydrogels

N-Acylation of chitosan with carboxylic anhydrides in dilute acetic acid/methanol has been a well documented strategy to selectively modify chitosan. Although this reaction is known to lead to irreversible gel formation, the kinetics and mechanism of this process have not so far been addressed. To this purpose, gel formation during the N-isobutyrylation of chitosan was investigated as a function of the reaction stoichiometry (R), chitosan concentration, and temperature by small deformation oscillatory rheology. Gel formation follows closely the chemical reaction and it proceeds predominantly under second-order kinetics as established from the dependence of critical gel time, t(gel), on R and concentration. The activation energy value derived from t(gel) vs 1/T data (E(a) = 68.29 +/- 1.80 kJ/mol) was almost identical to values reported for the chitosan N-acetylation reaction in previous studies. An excess isobutyric anhydride is suggested to be necessary for nucleation and hydrophobic association. The potential application of N-isobutyrylchitosan (NIBC) hydrogels in the design of thermally sensitive materials is also demonstrated.

Study of the Release Mechanism of Diltiazem Hydrochloride from Matrices Based on Chitosan−Alginate and Chitosan−Carrageenan Mixtures

The aim of this work was to establish the diltiazem hydrochloride release mechanism from the chitosan-alginate matrix tablet (MCB/AS) and chitosan-carrageenan matrix tablet (MCS/CSI). The weight loss for MCS/CSI is mainly due to the weight loss of the matrix while for MCB/AS it is mainly due to the diltiazem hydrochloride released from the tablet. Using the Peppa's model the release order for MCS/CSI was n = 1.07 +/- 0.13 and for MCB/AS was n = 0.76 +/- 0.02. Thus, MCS/CSI has a transport mechanism, and for MCB/AS the drug release mechanism is a combined process of diffusion and relaxation. MCB/AS has an elastic modulus (G' = 10(5) Pa) one order of magnitude higher than MCS/CSI (G' = 10(4) Pa). MCB/AS is able to uptake solvent without disrupting the microstructure due to its high elastic modulus. Instead MCS/CSI showed a quick erosion process, which conducted to the tablet disintegration due to a fast solvent uptake process.

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.

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.

Physicochemical Behavior of Homogeneous Series of Acetylated Chitosans in Aqueous Solution: Role of Various Structural Parameters

Physicochemical properties of four different homogeneous series of chitosans with degrees of acetylation (DA) and weight-average degrees of polymerization (DP(w)) ranging from 0 to 70% and 650 to 2600, respectively, were characterized in an ammonium acetate buffer (pH 4.5). Then, the intrinsic viscosity ([eta](0)), the root-mean-square z-average of the gyration radius (R(G,z)), and the second virial coefficient (A(2)) were studied by viscometry and static light scattering. The conformation of chitosan, according to DA and DP(w), was highlighted through the variations of alpha and nu parameters, deduced from the scale laws [eta](0) = K(w)and R(G,z) = K', respectively, and the total persistence length (L(p,tot)). In relation with the different behaviors of chitosan in solution, the conformation varied according to two distinct domains versus DA with a transition range in between. Then, (i) for DA < 25%, chitosan exhibited a flexible conformation; (ii) a transition domain for 25 < DA < 50%, where the chitosan conformation became slightly stiffer and, (iii) for DA > 50%, on increasing DP(w) and DA, the participation of the excluded volume effect became preponderant and counterbalanced the depletion of the chains by steric effects and long-distance interactions. It was also highlighted that below and beyond a critical DP(w,c) (ranging from 1 300 to 1 800 for DAs from 70 to 0%, respectively) the flexibility of chitosan chains markedly increased then decreased (for DA > 50%) or became more or less constant (DA < 50%). All the conformations of chitosan with regards to DA and DP(w) were described in terms of short-distance interactions and excluded volume effect.

Preparation of low-molecular-weight and high-sulfate-content chitosans under microwave radiation and their potential antioxidant activity in vitro

In the present paper microwave radiation has been used to introduce N-sulfo and O-sulfo groups into chitosan with a high degree of substitution and low-molecular weight. The sulfation of chitosan was performed in microwave ovens. It was found that microwave heating is a convenient way to obtain a wide range of products of different degrees of substitution and molecular weight only by changing reaction time or/and radiation power. Moreover, microwave radiation accelerated the degradation of sulfated chitosan, and the molecular weight of sulfated chitosan was considerably lower than that obtained by traditional heating. There are no differences in the chemical structure of sulfated chitosan obtained by microwave and by conventional technology. FTIR and 13C NMR spectral analyses demonstrated that a significantly shorter time is required to obtain a satisfactory degree of substitution and molecular weight by microwave radiation than by conventional technology. In this present paper, we also determined antioxidant activity of low-molecular-weight and high-sulfate-content chitosans (LCTS). The results showed LCTS could scavenge superoxide and hydroxyl radical. Its IC50 is 0.025 and 1.32 mg/mL, respectively. It is a potential antioxidant in vitro.

Study of the interpolyelectrolyte reaction between chitosan and alginate: influence of alginate composition and chitosan molecular weight

The interpolyelectrolyte reaction between chitosan (CHI) and alginate (ALG) was followed by conductimetry and potentiometry. Five chitosan samples, all with almost the same degree of N-acetylation (DA approximately 0.20) and molecular weights ranging from 5 x 10(3) to 2.5 x 10(5) Da were used. The polyelectrolyte complex was formed using alginate samples with three different M/G values (0.44, 1.31 and 1.96). The composition of the complex, Z (Z = [CHI]/[ALG]) resulted 0.70 +/- 0.02, independently of the molecular weight of chitosan and the composition of the alginate used. The degree of complexation was 0.51 with no dependence on the alginate composition.

Non-specific depolymerization of chitosan by pronase and characterization of the resultant products

Pronase (type XXV serine protease from Streptomyces griseus) efficiently depolymerizes chitosan, a linear beta-->1,4-linked polysaccharide of 2-amino-deoxyglucose and 2-amino-2-N-acetylamino-D-glucose, to low-molecular weight chitosans (LMWC), chito-oligomers (degree of polymerization, 2-6) and monomer. The maximum depolymerization occurred at pH 3.5 and 37 degrees C, and the reaction obeyed Michaelis-Menten kinetics with a Km of 5.21 mg.mL(-1) and Vmax of 138.55 nmoles.min(-1).mg(-1). The molecular mass of the major product, LMWC, varied between 9.0 +/- 0.5 kDa depending on the reaction time. Scanning electron microscopy of LMWC showed an approximately eightfold decrease in particle size and characterization by infrared spectroscopy, circular dichroism, X-ray diffractometry and 13C-NMR revealed them to possess a lower degree of acetylation, hydration and crystallinity compared to chitosan. Chitosanolysis by pronase is an alternative and inexpensive method to produce a variety of chitosan degradation products that have wide and varied biofunctionalities.

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.

Comparative studies on polyelectrolyte complexes and mixtures of chitosan–alginate and chitosan–carrageenan as prolonged diltiazem clorhydrate release systems

The aim of this work was to evaluate the possibility of using mixtures and/or polyelectrolyte complexes from both chitosan-alginate and chitosan-carrageenan as prolonged drug release systems. Different dissolution profiles were obtained by changing the polymer matrix system (chitosan-alginate or chitosan-carrageenan) and the method used to include these polymers into the formulation (physical mixture or polyelectrolyte complex). Drug dissolution profiles from the matrices have been discussed by considering the swelling behavior of the polymers used. The swelling behavior of the chitosan-carrageenan and chitosan-alginate systems was analyzed by using the Hopfenberg model which permits to separate the diffusional contribution, kf, from the relaxational contribution, kr, involved in solvent penetration/sorption in glassy polymers. The chitosan-alginate system is better than the chitosan-carrageenan system as prolonged drug release matrix because the drug release is controlled at low percentage of the polymers in the formulation, the mean dissolution time is high, and different dissolution profiles could be obtained by changing the mode of inclusion of the polymers. Good agreement between td and kf/kr values for the system chitosan-alginate was found, which means that the swelling behavior of the polymers controlled the drug release from the matrix. In the case of the system chitosan-carrageenan, the high capacity of carrageenan promotes the entry of water into the tablet and therefore the main mechanism of drug release would be the disintegration instead of the swelling of the matrix.

Caracterização de quitosanas comerciais de diferentes origens

A quitosana é um polissacarídeo linear obtido da desacetilação da quitina. As propriedades físico-químicas da quitosana são função do grau médio de acetilacão <img border=0 id="_x0000_i1026" src="../../../../img/revistas/po/v13n4/19880x1.gif" align=absmiddle>e da massa molar média (<img border=0 id="_x0000_i1027" src="../../../../img/revistas/po/v13n4/19880x2.gif" align=absmiddle>). Várias aplicações da quitosana foram propostas na literatura, principalmente no tratamento de águas, produção de cosméticos, drogas e medicamentos, aditivos alimentícios, membranas semipermeáveis e no desenvolvimento de biomateriais. Neste trabalho, três diferentes tipos de quitosanas comerciais foram caracterizados por ressonância magnética nuclear de hidrogênio (RMN ¹H), espectroscopia de absorção na região do infravermelho (IV), titulação condutimétrica e análise térmica (TG e DSC) com o objetivo de comparar as características de quitosanas provenientes de diferentes fornecedores. A massa molar média das amostras foi determinada a partir de medidas de viscosidade intrínseca. Os resultados obtidos por estas técnicas mostraram diferenças quanto ao grau médio de acetilação (ou desacetilação), em função da procedência do material. Os resultados obtidos da análise térmica permitiram acompanhar os processos de desidratação, decomposição e a temperatura de transição vítrea (Tg).

Low molecular weight chitosans--preparation by depolymerization with Aspergillus niger pectinase, and characterization

The viscosity of a chitosan solution was rapidly lowered in the presence of pectinase from Aspergillus niger at pH 3.0 and 37 degrees C. The low molecular weight chitosans (LMWC) had a molecular weight in the range 20,000-5000 Da. Circular dichroism spectra showed a decrease in the segment of acetylated glucosamine units, whereas X-ray diffraction and CP-MAS 13C NMR indicated higher crystallinity and polymorphism in LMWC. The latter on thermal drying resulted in structural alterations, and yielded an insoluble product. FT-IR and X-ray diffraction showed no evidence of either Schiff's base linkage or any annealed polymorph. CP-MAS 13C NMR showed marked changes in the chain conformations of LMWC, which are believed to be responsible for its loss of solubility and functionality.

Thermoreversible gelation of aqueous mixtures of pectin and chitosan. Rheology

The synergistic interaction between pectin and chitosan in aqueous acid solution and in the gel phase has been studied by oscillatory shear measurements. Mixtures of pectin and chitosan form thermoreversible gels over a broad composition range by lowering the temperature. The value of the gelation temperature depends on the composition of the mixture, with low values for mixtures with low pectin contents. For incipient gels, a power law can describe the frequency dependence of the complex viscosity, with power law exponents close to -1. The gel evolution of pectin-chitosan mixtures upon a temperature quench below the gel point has been studied. Evidence is provided for a relation between gelation and phase separation in the process of temperature-induced gelation of pectin-chitosan mixtures. A simple model is proposed to rationalize the gelation process in these systems.

Relation between solution properties and degree of acetylation of chitosan: role of aging

Aging of solutions of chitosan varying in degree of acetylation (DA) and degree of dissociation (alpha) was studied using two techniques. The first concerned potentiometric experiments performed during 3 days on solutions having the same concentration of amino groups (5.2% < DA < 70.6% and 0 < alpha < 1.1). The presence of aggregates at low alpha certainly depends on electrostatic interactions for low DA values and on hydrophobic interactions and H-bondings for high values. When alpha increases, the role of the cationicity of the amine groups, which depends on DA, seems to play a more important role on the behavior of the polymer chains. The second regarded capillary viscometric experiments performed during 5 days on solutions of the same polymer concentration (5.2% < DA < 70.6% and 0 < alpha < 0.30). The observations mentioned above and the results obtained in a previous paper (Biomacromolecules 2001, 2 (3), 765) are confirmed, and the influence of the electroviscous effects is discussed.

Disentangling alpha from beta mechanical relaxations in the rubber-to-glass transition of high-sugar-chitosan mixtures

The occurrence of molecular motions in addition to those of the glass-transition region (alpha mechanism) were investigated in chitosan and a branched derivative substituted with alkyl chains having eight carbon atoms. Once hydrophobic interactions of the alkyl groups in aqueous solution were demonstrated, polymers were mixed with glucose syrup at high levels of solids. The real (G') and imaginary (G") components of the complex dynamic modulus in high-solid mixtures were measured between 0.1 and 100 rad s(-1) in the temperature range from -55 to 50 degrees C. The method of reduced variables gave superposed curves of G' and G", which unveiled an anomaly in the dispersion of the alkylated derivative both in terms of higher modulus values and dominant elastic component of the polymeric network, as compared with the glass-transition region of chitosan. It was proposed that the new mechanical feature was due to beta mechanism, and master curves of viscoelastic functions and relaxation processes were constructed to rationalize it.

Viscosity of semiflexible chitosan solutions: influence of concentration, temperature, and role of intermolecular interactions

The influence of polymer concentration and temperature on the rheological behavior of chitosan solution was studied. The threshold concentrations for the different viscometric regimes were determined and the different power laws exponents were calculated and compared with those predicted from models. Different observations and the high values of these exponents within the high concentration region lead to consideration of the presence of intermolecular interactions as soon as the polymer concentration is larger than the overlap concentration. The activation energy was determined as a function of the polymer concentration, and its evolution was compared with theoretical predictions. A gel-sol transition was demonstrated at high concentrations.

Relationship between composition and structure in chitosan-based hybrid films

Chitosan/poly(aminopropylsiloxane) hybrid films were obtained by blending 3-(aminopropyl)siloxane oligomers (pAPS) with chitosan (CHI). The pAPS oligomers were prepared by the sol-gel method starting from 3-(aminopropyl)triethoxysilane. These hybrids were characterized by chemical, spectroscopic and morphological methods. Scanning electron micrographs of hybrid films of different composition revealed an organized microscopic pattern suggesting the existence of systematic interactions among their components. Comparison of the thermal stabilities and X-ray diffraction patterns as well as FT-IR spectra of the films with those of the pure components revealed that nanocomposites were formed. Similar studies of films including lithium perchlorate, as a third component, showed that addition of certain amount of lithium ions affected the structure of the CHI/pAPS films. When addition of the lithium salt exceeded the homogeneous incorporation limit, a little excess generated anisotropically oriented patterns in the hybrid films.

Efeito de Aditivos na Desacetilação de Quitina

Reações de desacetilação de quitina comercial em suspensão aquosa de hidróxido de sódio foram realizadas em etapa única de 6 horas a 115°C. Os efeitos de aditivos (boro hidreto de sódio ou antraquinona) e de borbulhamento de gases inertes (nitrogênio ou argônio) sobre as características das amostras desacetiladas foram avaliados. A espectroscopia de ressonância magnética nuclear de hidrogênio e viscosimetria capilar foram empregadas para determinações de graus médios de acetilação e de viscosidades intrínsecas de quitosanas, respectivamente. A difração de raiosX foi empregada para comparar as amostras quanto à cristalinidade e os espectros no infravermelho foram comparados para avaliar modificações estruturais decorrentes da reação de desacetilação. As quitosanas mais cristalinas foram obtidas quando um dos gases inertes foi borbulhado no meio durante a reação de desacetilação. Amostras ligeiramente mais desacetiladas foram obtidas na ausência de qualquer aditivo, mas severa despolimerização ocorreu nesses casos. A adição de boro hidreto de sódio ao meio reacional reduziu significativamente a despolimerização, mas a presença de antraquinona e o borbulhamento de nitrogênio, ou de argônio, não surtiu qualquer efeito, sugerindo que a presença de oxigênio não é um pré-requisito para a ocorrência de despolimerização.