Chitin and chitosan: Properties and applications

Chitosan—A versatile semi-synthetic polymer in biomedical applications

Chitosan-based biomaterials for tissue engineering

This review describes the most common methods for recovery of chitin from marine organisms. In depth, both enzymatic and chemical treatments for the step of deproteinization are compared, as well as different conditions for demineralization. The conditions of chitosan preparation are also discussed, since they significantly impact the synthesis of chitosan with varying degree of acetylation (DA) and molecular weight (MW). In addition, the main characterization techniques applied for chitin and chitosan are recalled, pointing out the role of their solubility in relation with the chemical structure (mainly the acetyl group distribution along the backbone). Biological activities are also presented, such as: antibacterial, antifungal, antitumor and antioxidant. Interestingly, the relationship between chemical structure and biological activity is demonstrated for chitosan molecules with different DA and MW and homogeneous distribution of acetyl groups for the first time. In the end, several selected pharmaceutical and biomedical applications are presented, in which chitin and chitosan are recognized as new biomaterials taking advantage of their biocompatibility and biodegradability.

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Chitin and Chitosan Preparation from Marine Sources. Structure, Properties and Applications

Chitin and its deacetylated derivative chitosan are natural polymers composed of randomly distributed � -(1-4)- linked D-glucosamine (deacetylated unit) and N-acetyl-D-glucosamine (acetylated unit). Chitin is insoluble in aqueous media while chitosan is soluble in acidic conditions due to the free protonable amino groups present in the D-glucosamine units. Due to their natural origin, both chitin and chitosan can not be defined as a unique chemical structure but as a fam- ily of polymers which present a high variability in their chemical and physical properties. This variability is related not only to the origin of the samples but also to their method of preparation. Chitin and chitosan are used in fields as different as food, biomedicine and agriculture, among others. The success of chitin and chitosan in each of these specific applica- tions is directly related to deep research into their physicochemical properties. In recent years, several reviews covering different aspects of the applications of chitin and chitosan have been published. However, these reviews have not taken into account the key role of the physicochemical properties of chitin and chitosan in their possible applications. The aim of this review is to highlight the relationship between the physicochemical properties of the polymers and their behaviour. A functional characterization of chitin and chitosan regarding some biological properties and some specific applications (drug delivery, tissue engineering, functional food, food preservative, biocatalyst immobilization, wastewater treatment, molecular imprinting and metal nanocomposites) is presented. The molecular mechanism of the biological properties such as biocompatibility, mucoadhesion, permeation enhancing effect, anticholesterolemic, and antimicrobial has been up- dated.

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Functional Characterization of Chitin and Chitosan

N-Deacetylation and depolymerization reactions of chitin/chitosan: Influence of the source of chitin

Application of MRI to monitor the process of ripening and decay in citrus treated with chitosan solutions.

The ability to form a gel through the physical or chemical crosslinking of chitosan has been well documented. In an attempt to mimic biological systems, thermal and pH-sensitive chitosan cylindrical hydrogels were produced by a combination of physical and chemical crosslinking processes. To this end, chitosan hydrogels prepared from alkali chitin were molded in cylinders and, once washed, were further crosslinked with glutaraldehyde at stoichiometric ratios, R (=[-CH=O]/[-NH 2 ]), of 1.61 and 3.22 × 10 -2 . Variation in swelling as a result of stepwise changes in temperature between 40 and 2 °C at pH values of 7.0, 7.6, and 8.0 revealed that the system responds in markedly different manners dependent upon the pH. At pH 7.0, cooling from 40 to 2 °C results in contraction of the gel network structure. While raising the temperature from 2 to 40 °C leads to a rapid swelling response (i.e., ca. a twofold increase in the amount of solvent uptake). Subsequent cooling to 2 °C is accompanied by a new contraction cycle. At pH ≥7.6 the temperature dependence of the swelling-contraction behavior is exactly the opposite of that observed at pH 7.0. Very similar trends were observed for the gels at both degrees of crosslinking. The swelling-shrinking behavior observed in gels of pH≥7.6, is similar in kind to that of uncrosslinked gels and is interpreted in terms of a lower critical solution temperature (LCST) volume phase transition, driven by hydrophobic association, presumably involving residual acetyl groups in the chitin. The results at pH 7.0 suggest that the slight ionization of the -NH + 3 groups leads to destruction of the hydrophobic hydratin thus effectively reversing the negative thermal shrinking.

Effect of Chemical Crosslinking on the Swelling and Shrinking Properties of Thermal and pH‐Responsive Chitosan Hydrogels

On heating, alkali chitin solutions undergo phase separation describing a characteristic "U-shaped" cloud point curve with a lower critical solution temperature (LCST) centered at ∼30∼°C. The process is accompanied by gelation of the polymer-rich phase. A different strategy to induce alkali chitin phase separation/gelation is by applying vacuum to the solution at room temperature during aprox. 72∼h. Once washed to neutrality, chitin gels had a degree of acetylation of ∼30-40% (i.e. they were converted into chitosan). On cooling, these gels exhibit an exothermic peak in micro-DSC and a depression in G " and tan δ traces, evidencing a volume phase transition centered at ∼20∼°C. This transition is observed only within a narrow range of pH ∼7.3-7.6. Variation in the mechanical response as a result of cyclic stepwise changes in temperature between 50 and 0∼°C at pH values from 7.3-7.6, revealed that the G' modulus of the gels increases on heating and decreases on cooling, a behavior that persists over at least four cycles of temperature change. Only marginal changes in G' at pH 8.0 and not at all at pH 12.0 are observed. By contrast, the variation of G " persists throughout the range of pH. This behavior is rationalized in terms of the existence of a fine balance between hydrophobic and hydrophilic interactions at varying temperature and pH, thus effectively controlling swelling and shrinking states of the gel network. The degree of swelling at pH 7.6 reaches a minimum at ∼22-25∼°C. © Springer-Verlag 2007.

Gemma Galed

Papers

Functional Characterization of Chitin and Chitosan

N-Deacetylation and depolymerization reactions of chitin/chitosan: Influence of the source of chitin

Application of MRI to monitor the process of ripening and decay in citrus treated with chitosan solutions.

Effect of Chemical Crosslinking on the Swelling and Shrinking Properties of Thermal and pH‐Responsive Chitosan Hydrogels

Temperature and pH-sensitive chitosan hydrogels: DSC, rheological and swelling evidence of a volume phase transition