Biomedical subjects

Alain Domard

Publications and source records attributed to Alain Domard.

At least 19 recordsLinked to original sources

PubMed2006

Morphology and mechanical properties of chitosan fibers obtained by gel-spinning: influence of the dry-jet-stretching step and ageing.

The morphology and mechanical properties of chitosan fibers obtained by gel-spinning are reported. The objectives were both to understand how the microstructure of the fibers could be modified and how the mechanical properties were improved by means of a dry-jet-stretching step. A highly deacetylated chitosan (degree of acetylation=2.7%) from squid-pens, with a high weight-average molecular weight (M(w)=540,000g/mol) was dissolved in an aqueous acetic acid solution, spun using gaseous ammonia as the coagulant, and then directly dried under hot air. A "post-drying" of 1week was necessary to stabilize the fibers in ambient atmosphere. A dry-jet-stretching ratio applied during the monofilament coagulation (maximal value of 4.3) allowed us to increase the density and favor the orientation of chains along the fiber axis. This allowed us to improve the mechanical properties of the fibers (Young's modulus of 82g/denier and tenacity of 2g/denier). The ageing in ambient atmosphere played an important role in the crystalline microstructure in relation to: the kinetics of ammonium acetate hydrolysis, the formation of a weak fraction of the anhydrous allomorph of chitosan, and an increase of the crystallinity index, whereas the Young's modulus was increased and the tenacity was slightly lowered. In addition, gel-jet-stretched or dry-jet-stretched fibers could be stored at least 3months in ambient atmosphere without any significant degradation.

Acetic Acid↗

PubMed2006

Pseudo-dry-spinning of chitosan.

A pseudo-dry-spinning process of chitosan without any use of organic solvent or cross-linking agent was studied. A highly deacetylated chitosan (degree of acetylation=2.7%) from squid-pens, with a high weight-average molecular weight (M(W)=540,000 g/mol) was used. The polymer was dissolved in an acetic acid aqueous solution in order to obtain a polymer concentration of 2.4% w/w with a stoichiometric protonation of the -NH(2) sites. The coagulation method consisted of subjecting the extruded monofilament to gaseous ammonia. The alkaline coagulation bath classically used in a wet-spinning process was therefore not useful. A second innovation dealt with the absence of any aqueous washing bath after coagulation. The gaseous coagulation was then directly followed by a drying step under hot air. When the chitosan monofilament coagulated in the presence of ammonia gas, ammonium acetate produced with the fiber could be hydrolyzed into acetic acid and ammonia, easily eliminated in their gaseous form during drying. The pseudo-dry-spinning process did not give rise to any strong degradation of polymer chains. After 2 months at ambient atmosphere, chitosan fibers could then be stored without any significant decrease in the M(W), which remained at a rather high value of 350,000 g/mol. The obtained chitosan fibers showed a smooth, regular and uniformly striated surface.

Animals↗

PubMed2005

Physico-chemical studies of the gelation of chitosan in a hydroalcoholic medium.

The formation of chitosan physical hydrogels without any external cross-linking agent was studied. This gelation took place in an acetic acid-water-propanediol solution. Static light scattering was used to detect the gel point and then, to study the gelation for different initial conditions. Thus, we investigated the influence of the degree of acetylation, the gelation temperature and the nature of the initial solvent. The variation of the solvent composition during gelation was determined from a simple weighting, and the ionisation state of the polymer at the gel point, by pH titrations. This work showed that it was possible to form a chitosan physical-hydrogel, whatever the degree of acetylation provided typical conditions were observed. The mechanism of gelation simply consisted in the modification of the hydrophilic/hydrophobic balance allowing the formation of both hydrophobic interactions and hydrogen bonding. Several parameters had an important role on this mechanism: 1--the apparent charge density of chitosan, modified by the degree of neutralisation, 2--the dielectric constant of the solvent, related to the composition of the medium, 3--the degree of acetylation, 4--temperature, playing a role on the interactions responsible for the physical cross-linking and the molecular mobility, and, 5--the molecular mobility depending on possible changes of conformation, steric hindrance and viscosity of the medium.

Acetic Acid↗

PubMed2005

Rheometric study of the gelation of chitosan in a hydroalcoholic medium.

The formation of chitosan physical hydrogels without any external cross-linking agent was studied. The gelation took place in an acetic acid-water-propanediol solution. The time to reach the gel point was determined by rheometry and gelations from different initial conditions could be compared. The influence of different parameters on gelation such as the polymer concentration, the degree of acetylation (DA) of chitosan and the composition of the initial solvent were investigated. The fractal morphology of the sample was not affected by the composition of the system. The number of junctions per unit volume at the gel point varied only with the initial number of chain entanglements per unit volume. Then, below an initial concentration of 1.5% (w/w), physical chain entanglements were insufficient and more junctions had to be formed to induce gelation. Over this value, only the kinetics allowing to replace entanglements by stable physical junctions played a key parameter. This kinetics was influenced by several parameters such as DA, temperature or the initial proportion water/alcohol. The acetyl groups played an important role in the formation of hydrophobic interactions, mainly responsible for gelation. The study of the influence of the gelation media revealed two critical points at 40% and 70% of water in the initial solvent, probably due to conformational changes and then to different modes of gelation. These physical hydrogels being used for cartilage regeneration, their final rheological properties were studied as a function of their degree of acetylation, the polymer concentration and the solvent composition in the initial solvent. Our results allowed us to define an optimal gelation condition for our application, corresponding to: DA=40%, a proportion water/alcohol of 50/50 and a polymer concentration of 1.5%.

Alcohols↗

PubMed2004

Formation and properties of positively charged colloids based on polyelectrolyte complexes of biopolymers.

Formation of colloids based on polyelectrolyte complexes (PECs) was mainly studied with synthetic polyelectrolytes. In this study, we describe the elaboration of positively charged PEC particles at a submicrometer level obtained by the complexation between two charged polysaccharides, chitosan as polycation and dextran sulfate (DS) as polyanion. The complexes were elaborated by dropwise addition of default amounts of DS to excess chitosan. Quasi-elastic light scattering was used to investigate in detail the influence of the characteristics of components (chain length, degree of acetylation) and parameters linked to the reaction of complexation (molar mixing ratio, ionic strength, concentration in polymer) on the sizes and polydispersity of colloids. Chain length of chitosan is the major parameter affecting the dimensions of the complexes, high molar mass chitosans leading to the largest particles. Variations of hydrodynamic diameters of PECs with the molar mass of chitosan are consistent with a mechanism of particle formation through the segregation of the neutral and then hydrophobic blocks of the polyelectrolyte complexed segments. Resulting particles display probably a structure constituted by a neutral core surrounded by a chitosan shell ensuring the colloidal stabilization. Such a structure was evidenced by measurements of electrophoretic mobilities revealing that the positive charge of particles was decreasing with pH, in relation with the neutralization of excess glucosamine hydrochloride moieties.

Acetylation↗

PubMed2003

Preparation and development of anti-chitosan antibodies.

Polyclonal antibodies directed against chitosan were produced using several immunogens, prepared by binding the polymer according to two ways (covalent and electrostatic) with a protein (bovine serum albumin or hemocyanin). It appeared that the presence of a carrier protein linked to chitosan was necessary to enhance the immune response and to obtain antibodies in a stable and reproducible way. Direct and inhibition enzyme-linked immunosorbent assay experiments were performed to assess the affinity and the specificity of the antibodies. The interactions of these antibodies with modified chitosans showed no influence of the degree of polymerization of the polymer in the range studied (from 24 to 2261), by contrast with the degree of acetylation. The affinity decreased when the degree of acetylation increased. Absence of cross-reactivity with glycosaminoglycans was observed whatever the antibody. The cationicity of the amine function along the polymer chains may have a role in the immunological recognition of the chitosan structure.

Acetylation↗

PubMed2002

Study of a chitin-based gel as injectable material in periodontal surgery.

The optimal conditions of injection of a chitin gel for applications in periodontal surgery have been studied as a function of various parameters. They correspond to a time of 2 min 15s (+/- 15"). They are achieved for acetylation parameters corresponding to: a molar ratio acetic anhydride/glucosamine residue, R = 1.5, a temperature of the master solution of 12 degrees C, a mixture of hydroalcoholic solution/acetylating reactive stirred for 45s at room temperature and a chitin concentration of 3.6%. This concentration allows us to limit the syneresis. to improve the mechanical properties of the gel and to obtain a viscosity suitable for the injection. Doping of the gel by means of chitosan powder insoluble under these conditions allows us to consider an improvement of the biological activity of the gel.

Biocompatible Materials↗

PubMedDate not supplied

Optimization of chitin extraction from shrimp shells.

The aim of this paper is to define optimal conditions for the extraction of chitin from shrimp shells. The kinetics of both demineralization and deproteinization with, in the latter case, the role of temperature are studied. The characterization of the residual calcium and protein contents, the molecular weights, and degrees of acetylation (DA) allows us to propose the optimal conditions as follows. The demineralization is completely achieved within 15 min at ambient temperature in an excess of HCl 0.25 M (with a solid-to-liquid ratio of about 1/40 (w/v)). The deproteinization is conveniently obtained in NaOH 1 M within 24 h at a temperature close to 70 degrees C with no incidence on the molecular weight or the DA. In these conditions, the residual content of calcium in chitin is below 0.01%, and the DA is almost 95%.

Acetylation↗

PubMedDate not supplied

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.

Acetylation↗

PubMedDate not supplied

Typical physicochemical behaviors of chitosan in aqueous solution.

Physicochemical properties of a homogeneous series of chitosans with different degrees of acetylation and almost the same degree of polymerization were investigated in an ammonium acetate buffer. Techniques such as interferometry, static light scattering (in batch or coupled on line with a chromatographic system), and viscometry were processed. All of the results agree with a unique law of behavior only depending on the degree of acetylation of the polymer. Indeed, values of the refractive index increment, radius of gyration, second viral coefficient, and intrinsic viscosity are decreasing in the same way as DA is increasing. Three distinct domains of DA were defined and correlated to the different behaviors of chitosans: (i) a polyeletrolyte domain for DA below 20%; (ii) a transition domain between DA = 20% and 50% where chitosan loses its hydrophilicity; (iii) a hydrophobic domain for DAs over 50% where polymer associations can arise. Conformations of chitosan chains were studied by the calculations of the persistence lengths (L(p)). The average value was found to be close to 5 nm, in agreement with the wormlike chain model, but no significant variation of L(p) with the degree of acetylation was noticed.

Chemical Phenomena↗

PubMedDate not supplied

Light scattering studies of the solution properties of chitosans of varying degrees of acetylation.

The use of two techniques, differential interferometry and quasi-elastic light scattering (QELS), allowed us to study solutions of chitosan varying in degree of acetylation (DA), degree of dissociation (alpha), and concentration (C(p)). With the first technique, we demonstrated the modification of the electric polarizability of the polymer chains, through a law of behavior of the variation of the refractive index increment dn/dC with DA and alpha. This brought us information on the various kinds of interactions (H-bonds, electrostatic, and hydrophobic) involved in the evolution of the solution properties. QELS experiments performed in dilute regime showed the presence of supramolecular structures depending on DA and alpha. The topology and the nature of these objects are discussed. The typical presence of aggregates and their evolution with concentration was also demonstrated in semidilute regime.

Acetylation↗

PubMedDate not supplied

Characterization of shrimp shell deproteinization.

The aim of this paper was to contribute to the interpretation of the mechanism of shrimp shell deproteinization. We used amino acid analysis to quantify the amount of proteins remaining in chitin. NaOH 1 M was added to a demineralized shrimp shell powder with a solution-to-solid ratio of 15 mL/g at ambient temperature. Because of the limited precision of the technique, after 24 h the protein content measured by elemental analysis had to be considered as negligible. However, with the use of amino acid analysis, it was still possible to determine with precision this content down to 0.25%. We also showed that among the peptides remaining linked to chitin after deproteinization, acidic amino acids were always in proportion higher than alkaline ones, but the balance between the two kinds of residues increased in favor of the latter with time. The kinetic study of the deproteinization clearly revealed a three-step mechanism with very different rate constants. The variation of these constants with temperature was used to calculate the energies of activation and the frequency factors of collision, thus allowing us to propose a new interpretation of the mechanism of deproteinization.

Amino Acids↗

PubMedDate not supplied

New aspects of the extraction of chitin from squid pens.

In this work, we studied the extraction of beta chitin from squid pens. The characterization by ICP of the raw material confirmed the significant absence of calcium carbonate and then the possibility to definitely avoid a step of demineralization responsible for chain degradation. The kinetics studies associated with the extraction of lipid parts showed the important role played by the latter structures on the limitation of the protein extraction by a simple treatment in an aqueous solution of 1 M NaOH. The role of various parameters such as the size of the particles, time and number of steps, and the concentration in sodium hydroxide was studied. The determination of the molecular characteristics (molecular weight and degree of acetylation) allowed us to define the optimal conditions of deproteinization in relation with the best preservation of these characteristics.

Acetylation↗

PubMedDate not supplied

Comparative study of the first heterogeneous deacetylation of alpha- and beta-chitins in a multistep process.

Heterogeneous deacetylation of alpha- and beta-chitins from shrimp shells and squid pens were comparatively studied. Each deacetylated sample, recovered after neutralization, was fractionated according to a water-soluble and insoluble fraction (pH 8.5). The systematic study of DAs, crystallinity changes, and distribution of N-acetylglucosamine residues were performed on the two kinds of fractions. For the two fractions resulting from a deacetylation in the presence of 50% (w/v) NaOH, for temperatures ranging from 80 to 110 degrees C, the activation energies of the reactions were found close to 39.9 +/- 1.0 and 42.8 +/- 1.8 kJ mol(-1) and the frequency factors of collision were of 7.2 +/- 2.4 and 54.4 +/- 18.5 10(3) min(-1), for alpha- and beta-chitins, respectively. Deacetylations of water-soluble and insoluble fractions were compared, and the major role played by the crystallinity level during deacetylation was evidenced and, thereafter, the role of the nature of the starting chitins on the chemical behavior. Thus, in some conditions, we observed critical values of DDA where the structures were becoming fully amorphous.

Acetylation↗

PubMedDate not supplied

Formation of polyelectrolyte complex particles from self-complexation of N-sulfated chitosan.

This work investigated the elaboration of biocompatible nanoparticles from the pH-induced self-complexation of the amphoteric polysaccharide N-sulfated chitosan. The acidification of aqueous solutions of chitosan having a degree of acetylation of 24% and a degree of sulfation of 34% or 56% was followed stepwise by turbidimetry, dynamic light scattering, and electrophoresis. With the highest sulfated chitosan, no turbidity was recorded between pH = 7.8 and 2.0, traducing a high apparent solubility of the polymer chains in this domain of pH. With the lowest sulfated chitosan, a steady increase in turbidity was monitored from pH = 6.90 to 6.15 followed by the flocculation of the polymer at pH approximately 6.0. In this range of pH, the polymer phase separated to yield particles having hydrodynamic diameters decreasing from 350 to 260 nm and an almost constant negative charge. These particles were assembled by electrostatic interactions between the protonated amino residues and the sulfate functions and stabilized by an excess of surface sulfate groups. The particles could be separated from the reaction medium and concentrated by centrifugation-redispersion cycles without alteration of their structure.

Chitosan↗

PubMedDate not supplied

New route of deacetylation of alpha- and beta-chitins by means of freeze--pump out--thaw cycles.

1st and 2nd heterogeneous deacetylations of alpha- and beta-chitins in a multistep process by means of freeze-pump out-thaw (FPT) cycles in the presence of 50% (w/v) NaOH, for temperatures ranging from 80 to 110 degrees C, were compared to the classical method using argon as the medium atmosphere. It was clearly demonstrated that FPT cycles extensively improved the reaction effectiveness by opening the crystalline structure of the two chitins and made them more permeable to alkaline solutions. The acetylated groups being more accessible, the preexponential factors of the reaction consequently enhanced, whereas the activation energies slightly increased, attesting to a more sensitive reaction to temperature. Compared to the usual method that mainly led to block copolymers for intermediate DAs, the statistical copolymers proceeded by FPT cycles improved the solubility of the samples and strongly modified the reaction mechanisms. For the very first time, the quasi-full absence of dioxygen in the reaction medium allowed us to estimate the non-oxidative degradation of the copolymer in alkaline conditions, independently of the oxydo-reductive process. Thus, heterogeneous deacetylation by means of FPT cycles exhibited all of the advantages of a reaction in homogeneous conditions (high solubility of the samples produced, reaction independent of the starting crystalline structure) without the drawbacks (low efficiency, severe degradation). It allowed us to generate fully deacetylated chitosans with the highest molecular weights never published in the literature.

Acetylation↗

PubMedDate not supplied

Rheometric study of the gelation of chitosan in aqueous solution without cross-linking agent.

A new process of formation of chitosan physical hydrogels in aqueous solution, without any organic solvent or cross-linking additive, was studied. The three conditions required for the physical gelation were an initial polymer concentration over C*, a critical value of the balance between hydrophilic and hydrophobic interactions, and a physicochemical perturbation responsible for a bidimensional percolating mechanism. The time necessary to reach the gel point was determined by rheometry, and gelations were compared according to different initial conditions. Thus, we investigated the influence of the polymer concentration and the degree of acetylation (DA) of chitosan on gelation. The number of junctions per unit volume at the gel point varied with the initial polymer concentration, i.e., the initial number of chain entanglements per unit volume or the number of gel precursors. The time to reach the gel point decreased with both higher DAs and concentrations. For a chitosan of DA = 36.7%, a second critical initial concentration close to 1.8% (w/w) was observed. Above this concentration, the decrease of the time to reach the gel point was higher and fewer additional junctions had to be formed to induce gelation. To optimize these physical hydrogels, to be used for cartilage regeneration, their final rheological properties were studied as a function of their degree of acetylation and their polymer concentration. Our results allowed us to define the most appropriate gel for the targeted application corresponding to a final concentration of chitosan in the gel of near 1.5% (w/w) and a DA close to 40%.

Biocompatible Materials↗

PubMedDate not supplied

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.

Acetylation↗