Rheological properties of chitosan–tripolyphosphate complexes: From suspensions to microgels
Highlights
► The method to prepare chitosan nanosuspensions and microgels has been developed. ► The effect of particle sizes on the rheological properties of chitosan microgels has been investigated. ► The attempt to qualitatively understand the interparticle interactions and particle packaging has been carried out.
Introduction
Chitosan (CS), the second most abundant biopolymer in nature next to cellulose, is one of the very few positively charged natural biopolymers existing in the world. It is derived from the exoskeleton of shrimps and other crustaceans, and has a linear structure composed of glucosamine unit and N-deacetylated glucosamine unit, also known as 2-amino-2-deoxy-(1 → 4)-β-d-glucopyranan. Chitosan has received broad attention from researchers of different backgrounds due to its unique structure and natural abundance (Yi et al., 2005). Previous literatures show that chitosan has been used to form complex coacervates (Espinosa-Andrews, Baez-Gonzalez, Cruz-Sosa, & Vernon-Carter, 2007), biocomposites (Luo et al., 2008), bio-carbon nanotubes (Zhang, Smith, & Gorski, 2004), and scaffolds for tissue engineering (Skotak, Leonov, Larsen, Noriega, & Subramanian, 2008). Other applications of chitosan include drug delivery systems, nanofibers, biosensors, and edible films (Han et al., 2007, Zhang et al., 2006, Zhang et al., 2008). Among the above research areas, chitosan-based delivery system is one of the most important applications due to its biodegradability, biocompatibility, bioadhesion and non-toxicity (Janes et al., 2001, Pillai and Panchagnula, 2001, Sogias et al., 2008). Many investigations of chitosan-based delivery systems have been carried out previously. For example, Jang and Lee (2008) succeeded in improving the heat stability of l-ascorbic acid during processing by utilizing chitosan–TPP nanoparticles (Jang & Lee, 2008). Richardson, Kolbe, and Duncan (1999) conjugated chitosan to DNA backbone for protecting DNA from endonuclease degradation and promoting DNA's cell targeting (Richardson et al., 1999). Wu, Yang, Wang, Hu, and Fu (2005) applied CS–TPP nanoparticles for loading the drug ammonium glycyrrhizinate. The release profile of their CS–TPP nanoparticles followed the rule of first burst release and then steady release, suggesting that CS–TPP nanoparticle was a suitable oral delivery agent (Wu et al., 2005).
In order to meet different demands, distinct methods were used to produce chitosan nanoparticles. Chemical modification provides us with series of methods for producing stable chitosan nanoparticles. For instance, amphiphilic micellar structure of linolenic acid-modified chitosan could be immobilized with trypsin by using glutaraldehyde as the crosslinker, which greatly improved trypsin's thermal stability and enzymatic activity (Liu, Desai, Chen, & Park, 2005). Other researchers functionalized chitosan with multiple functional groups, such as octyl, sulfate and polyethylene glycol monomethyl ether (mPEG) groups to target both polymeric micelle structure and brain-targeting function, and the resulted chitosan nanoparticles could improve the water solubility of hydrophobic drug paclitaxel by 4000 times (Yao, Zhang, Ping, and Yu, 2007). Very recently, a novel chitosan-based amphiphile, octanoylchitosan–polyethylene glycol monomethyl ether (acylChitoMPEG), has been synthesized using both hydrophobic octanoyl and hydrophilic polyethylene glycol monomethyl ether (MPEG) substitutions (Huang, Yu, Guo, & Huang, 2010). The synthesized acylChitoMPEG exhibited good solubility in either aqueous solution or common organic solvents such as ethanol, acetone, and CHCl3. Cytotoxicity results showed that acylChitoMPEG exhibited negligible cytotoxicity even at the concentration as high as 1 mg/mL (Huang et al., 2010).
In addition to chemical synthesis, physical methods were also used to create chitosan complexes or nanoparticles with milder processing conditions. Since chitosan has hydroxyl and amino groups on the backbone, chitosan can interact with other negatively charged hydrocolloids or small molecular weight compounds to form complexes. These complexes could potentially be used for mouth-feel improvement in food industry (Carvalho et al., 2006) and drug delivery in pharmaceutical industry (Weinbreck, Tromp, & de Kruif, 2004). Gum arabic is a thickening agent commonly used in food product development, such as flavor encapsulation. Espinosa-Andrews et al. (2007) investigated the interactions between gum arabic and chitosan by examining the influence of gum arabic/chitosan ratio, total polymer concentration, pH and ionic strength upon the electrostatic complexes formation. Their turbidity and electrophoretic mobility results showed that the optimized gum arabic/chitosan mass ratio was 5 for coacervate formation. The maximized gum arabic–chitosan interaction could be obtained within the pH range between 3.5 and 5 (Espinosa-Andrews et al., 2007). Another negatively charged compound worth noting is sodium tripolyphosphate (TPP), a small molecular weight crosslinker carrying five negative charges in each molecule. TPP has been approved as a GRAS (“generally recognized as safe”) reagent by FDA. Chitosan (CS) and TPP can form nanoparticles through electrostatic interaction, which has previously been investigated for different delivery applications (Gan and Wang, 2007, Hu et al., 2008, Jang and Lee, 2008, Ko et al., 2002, Wu et al., 2005). One interesting formulation among them is CS–TPP nanoparticles developed through an O/W emulsion route for entrapping hydrophobic felodipine (Ko et al., 2002). After felodipine was entrapped into CS–TPP nanoparticles, the control release of felodipine could be achieved by tuning pH, initial concentration, and molecular weight during nanoparticles preparation.
Previous studies suggest that CS–TPP nanoparticles are very useful carriers for drug and nutraceutical delivery. It is known that the CS–TPP particles were formed mainly through the electrostatic interaction between positively charged chitosan and negatively charged TPP molecules. However, how the CS–TPP particle sizes affect their packing, as well as the rheological properties of the resulted complex fluids (either chitosan–TPP particle suspensions or microgels) have been scarcely reported. In this paper, chitosan particles of different sizes were prepared through the use of TPP and ultrasonication. Depending on particle sizes, either CS–TPP particle suspensions or microgels were obtained after centrifugation at 11,000 × g, and their corresponding rheological properties were investigated by both static and dynamic rheological measurements. The static rheological technique measured the apparent viscosity (η) of polymer solution as a function of shear rate, while dynamic frequency test determined the storage modulus (G′) and loss modulus (G″) as a function of angular frequency (ω). The correlation between particle sizes and particle packing profiles was also explored through rheological measurements.
Section snippets
Materials
Chitosan with deacetylation degree (DD) of 98.0% and molecular weight (Mw) of 330 kDa was purchased from Kunpoong Bio. Co., Ltd. (South Korea). Sodium tripolyphosphate (TPP, 85%, technical grade) was purchased from Acros Organics (Morris Plains, NJ). Acetic acid, glacial (ACS grade) was purchased from Fisher Scientific (Fair Lawn, NJ). All of these reagents were used as received. Milli-Q (18.3 MΩ) water was used in all experiments.
Preparation of chitosan–sodium tripolyphosphate (CS–TPP) nanoparticles
Different amounts of chitosan (CS, 330 kDa) were dissolved in 2 wt%
Morphology and sizes of CS–TPP nanoparticles
Fig. 1A displays the tapping mode AFM height image of CS–TPP nanoparticles in dry state on the silicon wafer, which indicates the pseudo-spherical morphology of CS–TPP nanoparticles. Some particles were overlapped with each other. Section analysis embedded in the software Nanoscope5.30 was applied to calculate the particle size on the wafer surface. The vertical distance from the upper edge of the particle to the bottom of the silicon wafer was taken as particle size, which was previously
Discussion
Chitosan formed complexes/particles after being crosslinked by TPP. Calvo's classical procedure was commonly adopted by many researchers to directly prepare CS–TPP nanoparticles (Calvo et al., 1997a, Calvo et al., 1997b). Due to electrostatic interaction between chitosan and TPP, many physicochemical factors, including pH, ionic strength, CS/TPP mass ratio, initial chitosan concentration and processing methods can affect the stability of CS–TPP particle suspensions. Here we optimized the
Conclusion
CS–TPP particles with controlled particle sizes have been successfully prepared through the electrostatic interaction between amino groups of chitosan and phosphate groups of sodium tripolyphosphate. Different techniques including dynamic light scattering, atomic force microscopy, rheology, and Fourier transformed infrared spectroscopy were applied to characterize the structure and rheological properties of CS–TPP particles. The CS/TPP mass ratio of 3.75 was found to be the optimum condition to
Acknowledgements
We thank Drs. Yunqi Li and Songmiao Liang for the insightful discussion. This work was supported by US Department of Agriculture National Research Initiative Program (#2009-35603-05071).
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