Chitosan (deacetylated chitin) is a cationic natural polysaccharide and is regarded as generally recognized as safe (GRAS) material [13]. It is composed of α-1, 4-linked 2- amino- 2-deoxy- α-d-glucose (N-acetyl glucosamine) [14]. Chitosan possesses excellent biocompatibility and biodegradability [15, 16]. In drug delivery industry, chitosan has been exploited as a very promising biopolymer for use as carrier material for encapsulation of drugs [17, 18]. Though Chitosan is a promising carrier material in the formulation of polymer-based oral drug delivery systems, it displays poor mechanical behavior and limited capability for controlling drug release due to its greater solubility in lower pH through promoting faster dissolution of in the stomach pH [19]. The pH-sensitivity of Chitosan is because of the protonation of free amino groups in response to external pH alterations [20]. Because Chitosan is a cationic polymer, it interacts with the negatively charged silicic acid residues in mucin, the glycoprotein that makes up mucus [21]. Chitosan's mucoadhesive properties are due to this interaction. Additionally, Chitosan's hydroxyl and amino groups may form hydrogen bonds with mucus [22]. During past few years, numerous Chitosan-based PECs with a number of anionic polymers are being researched as drug delivery carrier materials by several research groups [1, 2, 6, 23]. In the present investigation, PEC beads were prepared using cationic Chitosan and anionic polymer-blends of Gellan Gum–Gum Ghatti or Gellan Gum–Gum Karaya for prolonged drug release. DS, a non-steroidal anti-inflammatory drug (NSAID) of about 1–2 h biological half-life [9, 12, 24], was investigated as model drug to assess the prolonged drug release properties. The proposed Chitosan-based PEC beads were expected to retard the release of encapsulated DS in alkaline pH (6.8) with minimal release in the acidic environment of stomach (pH 1.2).
Confirmation of Chitosan–Gellan Gum–Gum Ghatti and -Gum Karaya PEC beads formation
Because of the protonation of amino groups on the backbone, chitosan becomes a cationic polyelectrolyte in acidic medium, which is able to form PECs with negatively charged polyelectrolytes [23]. Considering the versatility of Chitosan-based PECs, in the present study, an attempt was made to prepare PEC beads comprised of cationic Chitosan and anionic polymers like Gum Karaya and Gum Ghatti in combination with Gellan Gum. During the preliminary trial experimentation, it was observed that PEC beads was formed using Chitosan along with Gum Ghatti–Gum Karaya blends were very weak; therefore, to provide mechanical strength to the beads, Gellan Gum was added into the system. For the confirmation of PEC formation, these formed spherical beads were characterized by DSC analyses and FTIR Spectroscopy analyses.
DSC analysis
Chitosan–Gellan Gum–Gum Ghatti beads
The change(s) in appearance and temperature of the endothermic peak(s) in the DSC thermograms of dried blank Chitosan–Gellan Gum–Gum Ghatti beads compared to DSC thermograms of Chitosan, Gellan Gum and Gum Ghatti suggested the formation of new polymeric system of PEC between Cationic Chitosan and anionic Gellan Gum–Gum Ghatti.
Chitosan–Gellan Gum–Gum Karaya beads
Similarly, when comparing DSC thermograms of dried Chitosan–Gellan Gum–Gum Karaya beads to DSC thermograms of Chitosan, Gellan Gum, and Gum Karaya, the change(s) in appearance and temperature of the endothermic peak(s) in the DSC thermograms of dried Chitosan–Gellan Gum–Gum Karaya beads suggested the formation of PEC between cationic Chitosan and anionic Gellan Gum–Gum Karaya.
Fourier transform-infrared (FTIR) spectroscopy analyses
Chitosan–Gellan Gum–Gum Ghatti beads
The FTIR spectrum of dried blank Chitosan–Gellan Gum–Gum Ghatti beads showed shifting of bands due to amide I of Chitosan to lower wave-number, whereas bands due to amide II and III were found disappeared in the FTIR spectrum of dried blank Chitosan–Gellan Gum–Gum Ghatti beads. This phenomena suggested PEC formation between cationic Chitosan and anionic polymers (here Gellan Gum–Gum Ghatti) in the dried blank Chitosan–Gellan Gum–Gum Ghatti beads.
FTIR spectrum of dried blank Chitosan–Gellan Gum–Gum Karaya beads
The band due to amide I of Chitosan was found to be shifted to a lower wave-number in the FTIR spectrum of dried blank Chitosan–Gellan Gum–Gum Karaya beads, whereas bands due to amide II and III were found to have vanished. These shifting and disappearances of bands suggested PEC formation between cationic Chitosan and anionic polymers (here Gellan Gum–Gum Karaya) in the dried blank beads.
Preparation of Chitosan–Gellan Gum–Gum Ghatti and -Gum Karaya PEC beads containing DS
During the preparation of beads, it was observed that when DS was added to the homogenous aqueous dispersion of anionic polymer(s), the viscosity of the system decreased and the system became fluid. This has resulted in easy extrusion of drug-polymer liquid mix through the hypodermic syringe even though high concentrations of anionic polymer were used. This could be explained as when Gellan gum/Gum Ghatti or Gellan gum /Gum Karaya were added to the deionized water, the polymer blend became fully hydrated due to application of heat (90 °C). This has resulted in their ionization (pH of aqueous dispersion was around 6.0) and because of this they carry a negative charge due to COO−. This led to increased charge density and thus increased electrostatic repulsion leading to thicker hydrogel.
Diclofenac, 2-{2-[(2, 6-dichloro phenyl) amino] phenyl}acetic acid, is a weak acid having the pKa of 4.15 [24]. It is more soluble in alkaline pH than acidic pH. Its solubility at neutral pH is around 1.13 g/L (DS) [25]. In the present study, the pH of the aqueous dispersion of polymeric blends was 6.0, which is 2 pH units above the pKa of Diclofenac. Therefore, more of the drug is expected to be ionized as Diclofenac anions. This might have resulted in a decrease in degree of swelling due to the screening of electrostatic repulsion between charged groups on the polymer chains and, thus, a reduction in viscosity.
Drug-excipient interaction analyses of Chitosan–Gellan Gum–Gum Ghatti and -Gum Karaya PEC beads containing DS
No drug-polymer interaction was observed in both the chitosan-based PEC beads containing DS.
Drug entrapment efficiency
Highest drug entrapment efficiency (%) was found in case of P6 Chitosan–Gellan Gum–Gum Karaya PEC beads containing DS (81.03 ± 4.22%). This occurrence could be attributed to the increased viscosity of the drug-polymer mixture, which hindered drug migration towards the acidic Chitosan-based gelation medium during the formation of Chitosan–Gellan Gum–Gum Karaya PEC beads containing DS. The high drug entrapment efficiencies shown by the Chitosan-based PEC beads P3 and P6 could also be attributed to the stoichiometry related with the polyanion to polycation ratio, which might provide strong PEC structure that could entrap more amount of the drug.
In vitro swelling
Overall swelling index values of these Chitosan-based PEC beads were found low. This could be attributed to the formation of more compact structure due to PEC formation. The Chitosan-based PEC beads swelled to a significantly greater extent at pH 6.8. The fluctuation in the degree of ionization of functional groups controlled by the pH of the swelling media could explain the pH responsive swelling behavior of these PEC beads. An acid–base type PEC system is formed when the negatively charged –COO– groups of anionic polysaccharide(s) bond to the positively charged amino groups of Chitosan in an acidic solution. The amino groups are deionized when the pH rises, and the binding affinity between two polyelectrolyte molecules decreases, causing the PEC to inflate and disintegrate, modulating the release. [26]. This pH responsive swelling behavior should be advantageous for the PEC beads in sustained drug release applications, where inhibition of the drug release in the gastric environment (low pH values) is desired. It is possible to precisely tune medication release at the target spot by altering elements that produce PEC's swelling features [26].
In vitro drug release
The in vitro release of DS from various PEC bead formulations was found faster initially. Within the first hour, about 59–64% of the entrapped drug were found to be released from these Chitosan-based PEC beads containing DS. The ionic interactions between cationic Chitosan and anionic polymer-blends might have been reduced at pH 6.8 forming a loose network of the tested beads with increased porous surface, which could allow a large amount of dissolution medium to enter into the PEC matrix. This might have resulted in rapid dissociation of the PEC bead-matrix, leading to drug release with a burst effect. Another possible explanation of the initial burst drug release from these PEC beads within the first hour of drug release study could most likely be due to the presence of drug crystals onto bead surface. The drug crystals might be formed onto these PEC bead surfaces because of their migration along with water to the surface during drying [27]. At pH 6.8, because of higher solubility of entrapped drug (here DS) and suppressed ionization of polyelectrolytes within these formulated PEC beads, the in vitro drug release was found increased [1, 18]. After the first hour, comparatively slower DS release was observed. About 94 to 97% of in vitro DS release was released from these Chitosan-based PEC beads at the end of 6 h. The release kinetics of the PEC bead formulations P3-P5 were zero-order. When a release rate is basically constant over time, the zero-order release is appropriate. The PEC bead formulation P6 followed first-order drug release pattern. In this case, an initial burst release effect was evidenced where the entrapped drug (here DS) was being rapidly dissolved and released from the bead-surface before the formation of gel-layer. After that the drug release rate of PEC bead formulation P6 was found decreased continually until the end of the drug release process. Since more than 60% of DS were released within the first hour, then values were calculated by Korsmeyer–Peppas model using drug release result up to first hour (Table 3). The n value of different PEC bead formulations ranged between 0.97 and 0.99, indicating a super case-II transport mechanism. This type of release describes a transport in which the rate of dissolution medium uptake into the polymer matrix is substantially dictated by the rate of swelling and relaxation of the polymer chains, and in which the drug diffusion varies on both concentration and time.