Chitosan is proving to be an effective biopolymer within drug delivery systems for biomedical applications, especially for the controlled and sustained release of therapeutic agents. In formulating an optimal chitosan-based drug delivery system, many important parameters of the chitosan must be considered. These factors include, but are not limited to its; degree of deacetylation, molecular weight (Mw), molecular mass (MM), viscosity, solubility, pH, purity, protein content, ash content, contaminants, etc. This short summary focuses on chitosan degree of deacetylation (DDA), its determination via several testing methods, and its behavior under different conditions.
The Degree of Deacetylation (DDA) of chitosan can be determined by various methods including Elemental Analysis, Titration, Hydrolytic, HPLC–UV and NMR techniques, etc.1-6 Each method has its particular advantages and disadvantages.
Elemental Analysis
This method is usually not preferred for determining DDA of chitosan as it’s not precise. It is on the percentage of nitrogen content in completely deacetylated chitosan. When chitosan samples with different DDA are compared they present with variations in their nitrogen contents which resulted in inaccurate results.
Titration
This method can be categorized into the following four titration techniques;
i) Acid–base
Values are estimated from the titration curve aided by inflection points. Here, the viscosity of the chitosan solution and the formation of precipitates are responsible for the accuracy of the results.
ii) Potentiometric
Values are estimated by measuring the number of amino groups at two inflection points. The purity and weight of chitosan needs special attention with this method as it has been shown that it’s not suitable for measuring DDA of low–grade chitosan.7
iii) Colloid
DDA measurements are carried out using stoichiometric calculations based on the ions. This method is not suitable for the samples having low DDA as the end point is difficult to locate and in some cases neither colour change nor precipitation occurs.8
iv) Conductometric
This method is applicable when using chitosan with low DDAs, as opposed to a high DDA, due to the low solubility of chitosan in HCl.1 In this method the solubility of chitosan is essential for the estimation of DDA.9
Hydrolytic
This is a method is inexpensive as it requires simple chemicals for DDA determination. But it’s a somewhat complex and tedious method requiring multiple procedural steps (hydrolysis, acidification and distillation).10
HPLC-UV
This is a spectroscopic technique which is affordable method. It can determine chitosan DDA as well as its molecular mass (MM). This method is appropriate for use with often formulated acidic acid soluble chitosan. With this method, special care in the preparation of chitosan and its calibration is required.11,12
Gas Chromatography
This method is works for the entire chitosan DDA ranges. It could be used as a routine analysis as it requires a small sample size and less time for testing. Inaccurate results has been cited with this method due to contaminants caused by the carbohydrates.13
Nuclear Magnetic Resonance (NMR) spectroscopy
This method is widely used as has demonstrated more reliability and reproducibility than other methods and techniques.14 It can be carried out with a small sample in a short time. This technique can be separated into:
i) Liquid state analysis
Requires little time to perform and a small sample can be used. This method does not perform well when analyzing insoluble chitosan.
ii) Solid state analysis
Though this technique is more expensive and time consuming, it can determine the DDA of insoluble chitosan. A disadvantage is it has demonstrated cases of non-reproducible results due to contaminants caused by carbohydrates.6,15-19
Overall, Nuclear Magnetic Resonance (NMR) spectroscopy this method for determining chitosan DDA is effective for characterization of modified chitosan, such as chitosan oligosaccharides, Chitosan HCI, Carboxymethyl chitosan, Chitosan Lactate, Chitosan Acetate, Chitosan Glutamate, etc. Interpretation of the DDA determination with this method is easier compared to other methods. Importantly, it’s a non-invasive method in that it does not destroy/alter the underlying chitosan molecule.
The degree of deacetylation (DDA) is one of the important factors which governs the changes in chitosan’s properties (e.g., solubility, crosslinking, particle size, shape, degradation, release profile of the drug, etc.) of CS.
The degradation rate and radiation stability of chitosan depends on crystallinity and thus on its DDA. In general, the increase of DDA results in low crystallinity of chitosan giving it a loose structure which in turn results in high degradation rate. A study conducted on the radiation degradation of chitosan proved its importance20, indicating the degradation rate decreases as the crystallinity of CS increases.
In another study of CS degradation behaviour with respect to DDA and MM, the results showed that higher DDA values slow down the degradation of CS as compared with low DDA values for samples with similar MM.21 The solubility of CS in acetic acid, crosslinking of CS with a suitable crosslinking agent and hydrophobicity of microspheres were found to increase with an increase in the DDA. The addition of free amino groups, due to increasing DDA, changes the properties of CS and helps to attain more solubility and better crosslinking without a change in MM.22 This factor also affects the size and morphology of particles, ultimately play a role in the release profile of the drug from the matrix. A study conducted to investigate the effect of DDA (90% and 75%) of CS on particle size, encapsulation efficiency and release of the anti–metabolite drug Fluorouracil (5–FU) with CS NPs proved that the particle size decreases as the DDA values of CS increases, while encapsulation efficiency increases with the increase of DDA values of CS. The release profile of the 5–FU in PBS (pH=7.40 and at T= ±37 °C) showed a burst release (≥55%) in 10 h, and a sustained drug release (≥75%) in 100 h from the 5–FU–CS NPs prepared by 90% DDA CS. On the other hand, the 5–FU–CS NPs prepared by 75% DDA exhibited initially a burst release (≥75%) in 10 h and a sustained release (≥85%) in 100 h. The electrostatic interactions are based on the charge density and in this case the charge density of the NPs prepared with the higher DDA CS (90%) was found to be higher.
The strong interaction between the CS and polyanionic TPP lead to a slow and sustained release of 5–FU.21 The release profiles of 5–FU from 5–FU–CS NPs (CS having DDA=90% and 75%). As discussed earlier, the size, shape, surface charge, zeta potential and type of interaction between the CS and drug are the significant factors which have the ability to change the release profile, which in turn are dependent on DDA of CS. The DDA value of CS is usually determined by the various methods which have certain limitations and sometimes results in incomparable DDA values. Therefore, it would be necessary to consider that the DDA value of same CS sample may have different numbers depending on the method of determination and hence resulting in a different release profile for the same CS sample.
The higher DDA values of CS result in more spherical and compact NPs which degrade slowly and ultimately results in slow drug release. It is important to consider the effects of DDA while formulating the CS-drug matrix for controlled release applications especially if a higher degree of drug adsorption is required. The high drug adsorption on to the NPs can be achieved by using the higher DDA CS.
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