Development and Evaluation of MCC-SiO2/CMC-SiO2 Conjugates as Tablet Super-Disintegrants

In the present study, microcrystallinecellulose–colloidal silicon dioxide (MCC-SiO2) and carboxymethylcellulose–colloidal silicon dioxide (CMC-SiO2) conjugates have been investigated as superdisintegrants in fast dissolving tablets (FDTs). MCC-SiO2 and CMC-SiO2 conjugates were prepared and micromeritic studies, FTIR, SEM and XRD methods were utilized for characterizing the powdered conjugates. The conjugates were used for the preparation of domperidone FDTs by direct compression and the wetting time, water absorption ratio, disintegration time and in vitro drug release were evaluated. Effective pore radius of MCC-SiO2 and CMC-SiO2 conjugates for 1:1, 1:2.5 and 1:5 was found to be 13.35 ± 0.31 µm, 15.66 ± 0.17 µm and 18.38 ± 0.44 µm, and 16.81 ± 0.24 µm, 20.12 ± 0.39 µm and 26.37 ± 0.24 µm, respectively, compared to 12.21 ± 0.23 µm for MCC and 13.65 ± 0.21 µm for CMC. The results of effective pore radius indicate the wicking capability as well as the disintegration potential of MCC-SiO2 and CMC-SiO2 conjugates over pure MCC and CMC. The results of wetting time, water absorption ratio and disintegration time for MCC-SiO2 conjugates were found to be in the range of 19 ± 1.21 to 30 ± 1.33 s, 42 ± 0.28 to 49 ± 0.47% and 15 ± 2 to 40 ± 1 s, and for CMC-SiO2 conjugates were found to be in the range of 21 ± 1.13 to 40 ± 1.17 s, 42 ± 0.94 to 49 ± 0.57% and 12 ± 2 to 20 ± 3 s, respectively. Conjugation of MCC and CMC with SiO2 led to the formation of a complex with remarkable tablet superdisintegrant potential that could be used in preparing fast disintegrating tablets.


Introduction
Fast disintegrating tablets (FDTs) have emerged as prominent dosage forms in the recent past, as they disintegrate rapidly within the mouth in a matter of few seconds, showing rapid dissolution and fast onset of action. Pre-gastric absorption of the FDTs in the mouth and esophagus decreases the amount of drug undergoing the first-pass metabolism and thereby results in improved bioavailability [1,2]. These dosage forms provide various other advantages such as accurate dosing, easy portability, chemical and physical stability and are highly convenient for geriatric, pediatric, bedridden and uncooperative patients [3]. They are generally represented as "quick melt", "orally disintegrating", "melt in mouth", "quick dissolve", "fast disintegrating", "rapid melt", "oro-dispersible", "fast dissolve" and "mouth dissolving" etc. Disintegrants are added to solid oral dosage forms to promote the disintegration process of the compacted mass and further increase drug dissolution. Recently, newer disintegrants, known as superdisintegrants are being explored.
These agents are used in low concentrations, usually 1-10% of the total weight of the dosage form when compared to other disintegrants. Various formulations employ standard super disintegrants such as croscarmellose sodium, crosspovidone and sodium starch glycolate [4].
Microcrystalline cellulose (MCC) and carboxymethyl cellulose (CMC) are remarkable excipients that are widely used as disintegrants in tablet formulations and various modifications have been done to extend their properties as tablet super disintegrants. Studies have shown that PEGylated conjugates of microcrystalline cellulose as promising super disintegrant at a concentration above 1% which led to an enhancement in water vapor uptake, water penetration rate, super-disintegration power and dissolution rate [5]. In another study, optimized mouth dissolving tablets have been prepared using a combination of glycine, carboxymethyl cellulose and sodium alginate using Plackett-Burman design and the formulation was found to decrease the disintegration and wetting times and increase the porosity and water absorption ratio [6]. Chitosan-silicon dioxide [7], starch-silicon dioxide [8], chitin-metal silicates [9] have been reported as novel tablet super-disintegrants. In the present study, MCC-SiO 2 and CMC-SiO 2 conjugates have been synthesized and used in the formulation of domperidone FDTs. The conjugates were evaluated by SEM, FTIR and XRD techniques and the micrometric studies were conducted to explore the flowability of the conjugates. The mechanical and drug release characteristics of domperidone FDTs were also evaluated.

Preparation of MCC-SiO 2 and CMC-SiO 2 Conjugates
One gram of MCC/CMC dispersed in 100 mL of 2 M NaOH and one gram of SiO 2 was dispersed in 100 mL of 2 M HCl. The silica suspension was poured into the MCC/CMC suspension with continuous stirring for an hour. The pH of the mixture was adjusted with concentrated HCl to 6.5. The resulting mixture was then transferred to a round bottom flask and 5 mL acetone was added and then freeze-dried. The freeze-dried powder was scraped off and sieved using mesh number 44 and stored in a properly sealed container. A similar procedure was used to prepare conjugate containing MCC/CMC: colloidal silicon dioxide ratios of 1:2.5 and 1:5, in which the concentration of SiO 2 increased to 2.5 g and 5 g, respectively.

Characterization of MCC-SiO 2 and CMC-SiO 2 Conjugates
The prepared MCC-SiO 2 and CMC-SiO 2 conjugates were investigated for several pre-compression parameters, including micromeritic studies, loss on drying and effective pore radius. Post-compression evaluation was carried out using techniques such as FTIR, SEM and XRD.

Loss on Drying (LOD)
The presence of solvents or moisture in the conjugates was determined using the loss on drying (LOD) method. The initial weighed of the conjugate was determined (W 1 ) and the weight of the cool sample after heating at over 100 ± 5 • C for 2 h (W 2 ) was determined. The percentage loss was calculated as presented in Equation (1): The effective pore radius (R eff.p ) of the powdered blend was evaluated by weighing a 2 mL micropipette tip filled with the powder (W i ). Then n-hexane was added in a dropwise manner on the bed top till the solvent filtered out at the bottom of the tip and was weighed again (W f ) [11]. The R eff.P was determined as described in Equation (2):

Attenuated Total Reflectance-Fourier Transform Infrared Spectroscopy (ATR-FTIR)
The Infrared spectra of the samples were obtained using an ATR-FTIR spectrophotometer (IFS66/S, Alpha Bruker, Ettlingen, Germany). The KBr pellet technique was used to analyses the samples with spectra 4000 cm −1 to 400 cm −1 .

X-ray Diffraction (XRD)
The X-ray diffractograms of different samples were obtained using an X-pert pro system (P analytical X'Pert Pro MRD, Malvern Panalytical Ltd., Malvern, UK) which is configured in Bragg Brentano geometry. The equipment consists of a copper anode in glass tubing and a graphite monochromator. The operating conditions were set at 40 mA and 40 kV. The powders (≤250 µm) were mounted on a glass slide arbitrarily and scanned through a range of 2θ angles.

Scanning Electron Microscopy (SEM)
The surface morphology was examined using scanning electron microscopy (4300 SE/N SEM, Hitachi, Santa Clara, CA, USA) at an accelerating potential of 10 kV. The sample was placed on the silver plate of the specimen stage in a vacuum evaporator.

Preparation of Tablets
The prepared conjugates (400 mg) were compressed for 30 s using a hydraulic press (Model CAP15T-1233, PCI Analytics, Mumbai, India) to form tablet and then stored over silica gel for 24 h. The relative densities (R) of the tablets were determined by using the following equation: where, m is mass, V r is the volume of tablet (cm 3 ) and P s is the particle density of solid material (g/cm 3 ).

Compression Studies
Mathematical compression models are generally normalized with the inclusion of initial volume or particle density to compare the compression properties of materials. Two widely used compression models-Heckel and Kawakita-were used to study the compression properties of the composites.

Heckel Function
The Heckel model uses relative density to determine the compaction properties of the material. It relates the densification of the powdered material with the applied pressure. The force and displacement values (as obtained from the simulator) were determined and the apparent density of the powder bed 1/(1 − D) was calculated using force values (converted to pressure), displacement values and true density of the material. The compression stage was subjected to linear regression analysis using the Heckel model and is denoted by the following Equations (4) and (5): where, D is the relative density of the powdered material, K (slope) is the reciprocal of the mean yield pressure of the material and P is the applied pressure. Further, the value of A (intercept) can be used to calculate the relative density: where, D B is the relative density at low pressures and D 0 is the relative density when no pressure is applied [12,13].

Kawakita Function
The Kawakita model uses the relative volume change to determine the compressibility of pharmaceutical powders. It is calculated using the following formula: where, C is the degree of volume reduction, V 0 is the initial bulk volume of the powder, V p is the bulk volume under pressure, 'a' is a constant related to minimum porosity of material before compression and 'b' is a constant associated with the plasticity of the material [12,13]. The simplified equation, in practice, is given by:

Preparation of Fast Disintegrating Tablets
Direct compression technique was used to prepare FDTs of domperidone. Table 1 incorporating MCC, MCC-SiO 2 conjugates and CMC, CMC-SiO 2 conjugates as superdisintegrant were prepared in three different ratios viz. 1:1, 1:2.5 and 1:5 according to the formulae. Fast disintegrating tablets each containing 10 mg of the drug were prepared and Avicel 102 was used as a diluent. The amount of drug, diluent and MCC/MCC-SiO 2 conjugates or CMC/CMC-SiO 2 conjugates were carefully weighed, sieved through the mesh of size 40 and finally mixed for 15-20 min using the tumbling mixer. To the mixture, talc and magnesium stearate were added as lubricants and then compressed into tablets using a multi-punch tableting machine (AK Industries, Nakodar, Punjab, India).

Wetting Time
A culture dish was filled with a 6 mL solution of Eosin (water-soluble dye) and twicefolded tissue paper (10.75 m × 12 m). The tablet was kept on top of the tissue paper and the wetting time was recorded as the time which the tablets took to absorb the solution to its top surface [14].

Water Absorption Ratio
The water absorption ratio was determined by a method similar to the wetting time technique. The tablet was weighed before and after complete wetting took place. The following formula was used for calculating the water absorption ratio: where W a refers to the weight of tablet before absorption of water and W b refers tothe weight of tablet after absorption of water [15].

In Vitro Disintegration Time
The disintegration test for the fast disintegrating tablets was conducted using USP disintegration apparatus (EI Product, Panchkula, India) in 900 mL of 0.1N HCl.

In Vitro Dissolution Study
The in vitro dissolution was investigated using the USP dissolution apparatus II (Paddle type -Lab India DS 8000, Mumbai, India). For the study, 0.1 N HCL (900 mL, 37 ± 0.5 • C) was used as dissolution medium and the paddles was rotated at the speed of 50 revolutions per min. Sampling was done by drawing out 5 mL samples after particular periods and filtering them using Whatman filter paper. The samples were then appropriately diluted and analyzed with the help of a UV-Visible spectrophotometer (Systronics, Mumbai, India) at λ max 284 nm. The drug concentration was determined and expressed as cumulative percent drug released. The similarity factor (f 2) is considered to be the logarithmic reciprocal square root transformation of the sum-squared error. It is a convenient way of comparing the dissolution rates if the dissolution time points are greater than three or four. The similarity factor is calculated as presented Equation (9): where W j is an optional weight factor, R j is the percentage of reference sample dissolved at time t and T j is the percentage of test sample dissolved at time t. The f 2 value rangefrom 0 to 100. If the f 2 value is 100 then both the test as well as reference samples are identical whereas it tends to be 0 if they are non-identical. The f 2 values should be nearer to 100, to depict a similar dissolution profile.

Friability Test
The friability (F) of 20 tablet were measured using a Rocha friabilator (Campbell Electronics, Mumbai, India). As per USP 30-NF 25, tablets were weighed and then rotate at 25 rpm for 4 min. Tablet were taken out, dedusted and reweighed. The limit of the friability test shall not be more than 1.0% 2.5.6. Hardness The tablet was placed in a vertically holding edges of the anvil of a Monsanto Hardness Tester (Model VMT-1, VinSyst Technologies, Mumbai, India). The pointer is adjusted at zero position on the scale, then the screw rotated till the break point of the tester. The breakage of tablet shows hardness on the scale. The test was performed on six tablets and the average was calculated.

Tensile Strength
Tensile strength of the tablet is the force required to break the tablet by compression in the radial direction or it is measure of stress necessary to Couse diametric compaction of the compact and it is measure by a Monsanto Hardness Tester. For measuring the hardness of tablet, the plunger of the hardness tester is driven down at the speed of 20 mm/min. Tensile strength were calculated by following formula: where F is the crushing load, D diameter and t thickness of tablet.

ATR-FTIR Analysis
Analysis of the spectrum of microcrystalline cellulose (MCC) shown in Figure 1. revealed the existence of a wide band in the 3336 cm −1 region signifying a large number of hydrogen bonds, formed by-OH groups contributing to the polymer's stiffness and tight chain packing. In the 2913 cm −1 region the band corresponding to the symmetric and asymmetric vibrations of the CH 2 group is observed. The 1440-1255 cm −1 range indicates the deformation vibrations of CH 2 and CH groups as well as the angular deformation vibrations of C-O-H. The peak at 1032 cm −1 represents C-O-C bending vibrations. The MCC-SiO 2 conjugates exhibited a sharp peak near 1076 cm −1 , which indicates a strong interaction between the surface OH groups of cellulose and silica particles, with the emergence of Si-O-C bridging bond. With an increase in the concentration of SiO 2 , this sharp peak transforms into a broad band. Further, a decrease in the intensity of the peaks at 3336 cm −1 and 2913 cm −1 validates the presence of intermolecular bonding between silicon dioxide and MCC, which could be responsible for the increased disintegration behavior of MCC-SiO 2 conjugates. The IR spectra of CMC shown in Figure 2 revealed a wide indistinct band at 3200-3400 cm −1 representing -OH stretching and a sharp peak at 2921.40 cm −1 which represents the symmetric and asymmetric vibrations of -CH 2 groups. The peaks at 1586.04 cm −1 and 1414.8 cm −1 represent the asymmetric and symmetric stretching of the carboxylate group, respectively. Further, the peak at 1323.5 cm −1 represents -CH 2 scissoring and the one at 1044.42 cm −1 represents C-O-C vibrations. The CMC-SiO 2 conjugates exhibit a decrease in the intensity of all the peaks. The sharpening of the peak at 1044 cm −1 indicates the formation of the Si-O-C bridge between the -OH group of CMC and silicon of SiO 2 . This interaction can be held responsible for the reduction in the time for tablet disintegration.

XRD Analysis
The XRD patterns for MCC and MCC-SiO 2 conjugates are depicted in Figure 3.

SEM Analysis
The SEM micrographs of MCC (Figure 4a,b) showed discrete and irregularly shaped structures. Cellulose microfibrils with unequal distribution are also evident in the images. Conjugation of MCC with SiO 2 damages the microfibrillar structure of cellulose moiety into small fragments, which enhances the total surface area of the conjugates. MCC shows a non-porous structural appearance whereas MCC-SiO 2 conjugates (Figure 4c,d) had a more porous structure. Increased surface area and pores in the surface morphology of MCC-SiO 2 conjugates could be held responsible for their improved tablet super disintegrant property.
The SEM micrographs of CMC (Figure 4e,f) showed tubular rod shaped microfibrils of cellulose, having smooth surface morphology. The conjugates (Figure 4g,h) on the other hand showed fragmentation of cellulose microtubules into tiny particles, thereby increasing the surface area. The presence of inter-particle voids in the prepared CMC-SiO 2 conjugates as depicted by the SEM micrographs could be the reason for the wicking action, which enhanced water uptake of the conjugates.

Heckel Function Analysis
The D o value, which is the extent of packing when the powder is being filled in the die for MCC was found to be 0.791 and it reduced from 0.786 to 0.569 for MCC: SiO 2 conjugates in the ratios of 1:1 to 1:5, respectively. Generally, increased porosity of powders is related to lower values of D o . In addition, the D B value represents the densification of powder bed at low pressure, further reflecting particle rearrangement and fragmentation (plastic/elastic) when pressure is applied. The MCC conjugates prepared in the ratio 1:5 exhibited the highest D B value (0.224) while pure MCC exhibited the lowest value (0.126). This demonstrates that more fragmentation takes place within the 1:5 conjugates when compared to the other ratios. D A exhibits the overall degree of densification at zero and low pressures. It was found to be in the order of 1:1 > 1:2.5 > 1:5 for MCC-SiO 2 conjugates, thereby indicating the highest degree of packing in 1:1conjugates. Further, P y is the mean yield pressure and is inversely proportional to the plastically deforming ability of the material which undergoes pressure. The results conclude that conjugates in the ratio of 1:5 depict the fastest onset of plastic deformation whereas the conjugates in the ratio of 1:1 depict the slowest onset. Figure 5 show representative Heckel plots for MCC and MCC-SiO 2 conjugates, and CMC and CMC-SiO 2 conjugates, respectively, prepared in different ratios, 1:1, 1:2.5 and 1:5. The D o of CMC was found to be 0.772 and its value reduces from 0.748 to 0.679 for MCC-SiO 2 ratios of 1:1 to 1:5, respectively. The CMC conjugates prepared in the ratio 1:5 exhibited the highest D B value (0.203) while pure MCC exhibited the lowest value (0.101). This demonstrates a higher degree of fragmentation taking place in the 1:5 conjugates. The values of D A were in the rank order of 1:1 > 1:2.5 > 1:5 for CMC-SiO 2 conjugates, with a 1:1 ratio showing a higher degree of packing at low pressures. Further, in terms of mean yield pressure (P y), the conjugates of 1:5 exhibit the fastest onset of plastic deformation whereas the conjugates of 1:1 exhibit the slowest onset [17,18].

Kawakita Function Analysis
The Kawakita plots for the MCC and MCC-SiO 2 conjugates, and CMC and CMC-SiO 2 conjugates prepared in different ratios 1:1, 1:2.5 and 1:5 presented in Figure 6. The results of D I and P k are listed in Tables 4 and 5. The D I value was 0.733 for MCC and 0.510, 0.562, 0.582 for MCC-SiO 2 conjugates of ratio 1:1, 1:2.5, 1:5, respectively. The value of P k is inversely related to the extent of plastic deformation taking place during the compression process and was observed to be 1.789 for MCC and 2.758, 3.995 and 5.141 for MCC-SiO 2 conjugates of ratio 1:1, 1:2.5, 1:5, respectively. Therefore, the ability of material to undergo plastic deformation during compression could be rated as MCC > MCC-SiO 2 ; 1:1 > MCC-SiO 2 ; 1:2.5 > MCC-SiO 2 ; 1:5. Further, the value of P y indicates the onset of plastic deformation during the compression process. The D I was 0.697 for CMC and 0.513, 0.521, 0.562 for CMC-SiO 2 conjugates of ratio 1:1, 1:2.5, 1:5, respectively. The value of P k was found to be 2.541, 4.519, 6.122 and 6.904 for CMC and CMC-SiO 2 conjugates of ratio 1:1, 1:2.5, 1:5, respectively. The ability of material to undergo plastic deformation during compression could be rated as CMC > CMC-SiO 2 ; 1:1 > CMC-SiO 2 ; 1:2.5 > CMC-SiO 2 ; 1:5 [19,20].  All the fast disintegrating tablets were prepared under identical circumstances to prevent any variation in the formulation process. The diameter varied from 6.72 to 6.73 mm for all the formulations of MCC-SiO 2 tablets and 6.73 to 6.74 mm for all the formulations of CMC-SiO 2 tablets. The thickness varied from 3.03 to 3.04 mm for all the samples of MCC and CMC. Further, the hardness of MCC-SiO 2 tablets and CMC-SiO 2 tablets was found to be in the range of 3.11-2.90 Kg/cm 2 , and 3.30-3.01 Kg/cm 2 , respectively, as compared to 2.65 and 2.95 Kg/cm 2 of tablets formulated using pure MCC and pure CMC, respectively. The percentage friability of all the samples was found to be less than 1%, thereby demonstrating sufficient physical properties. Tensile strength was observed to be high in case of MCC-SiO 2 and CMC-SiO 2 when compared to that of pure MCC and pure CMC. This can be attributed to the fact that addition of SiO 2 forces the particles to come closer together during compression, resulting in tighter packing and further increasing the tensile strength of the tablets. Table 6 lists the parameters including diameter, thickness, friability, hardness and tensile strength of the formulated FDTs.  Table 7 lists various values depicting the wetting time, water absorption ratio, disintegration time and drug content of the prepared FDTs. Wetting time was found to be 35 s for the tablets incorporating pure MCC; it decreased from 30 to 19 s as the ratio of MCC-SiO 2 conjugates increased from 1:1 to 1:5. In addition, disintegration time reduced from 40 s for the tablets constituting MCC to 23, 18 and 15 s for the tablets constituting MCC-SiO 2 conjugates in the ratios 1:1, 1:2.5 and 1:5. Similarly, wetting time and disintegration time were observed to be less in case of tablets constituting CMC-SiO 2 conjugates when compared to tablets constituting pure CMC. Further, a decrease in time was observed with increase in the ratio of the CMC-SiO 2 conjugates, i.e., 1:1 > 1:2.5 > 1:5. Water absorption ratio was observed to be inversely related to the wetting and disintegrating time of the tablets of all batches. The fast disintegration of tablets comprising the conjugates can be attributed to the formation of voids which promote quick penetration of the dissolution medium into the tablets. This further leads to swelling and wicking action, creating hydrodynamic pressure inside the tablets and destroying the physical bonds between the particles [21]. The formulated FDTs were compared with the marketed formulation of Domperidone (MKTD). Figure 7 depicts the in vitro dissolution results. The order of drug dissolution for the different ratios of conjugates was found to be in the order of 1:1 > 1:2.5 > 1:5. For the FDTs prepared by using the conjugates, the percentage cumulative drug release was observed to decrease with increasing concentration of SiO 2 . This was due to the fact that SiO 2 acts as a viscosifying agent. It forms a viscous layer around the drug, which in turn decreased the rate of drug dissolution. The f 2 values obtained from the in vitro dissolution studies of the prepared FDTs are mentioned in Table 8.

Stability Study
The formulated tablets were assessed for their stability by storing them at temperature and relative humidity of 40 ± 2 • C and 75% RH, respectively, for time intervals of 0 day, 1, 2 and 3 months. The samples were characterized for parameters including, hardness, friability, drug content and disintegration time. As evident from Tables 9 and 10, the formulations containing the polymers in ratio 1:5 exhibited the best results. on drying were performed on the powder samples. All the samples containing conjugates possessed good powder flowability. The effective pore radius of pure MCC and CMC were found to be 12.21 ± 0.23 µm and 13.65 ± 0.21 µm, respectively, whereas the MCC-SiO 2 and CMC-SiO 2 conjugates developed in the ratio 1:1, 1:2.5 and 1:5 showed effective pore radius 13.35 ± 0.31 µm, 15.66 ± 0.17 µm and 18.38 ± 0.44 µm, and 16.81 ± 0.24 µm, 20.12 ± 0.39 µm and 26.37 ± 0.24 µm, respectively. ATR-FTIR, XRD and SEM were used to characterize the conjugates. FTIR spectra showed intermolecular bridging in MCC-SiO 2 conjugates as well as CMC-SiO 2 conjugates, contributing to be the major reason for faster disintegration of the conjugates. SEM micrographs revealed that the fragmentation of cellulose microtubules into tiny particles along with the interparticulate spaces potentiated the wicking action as well as the superdisintegrant property of the conjugates. Additionally, compression studies, based on the Heckel and Kawakita methods were carried out for the formulated MCC-SiO 2 and CMC-SiO 2 conjugates. FDTs containing domperidone were formulated utilizing pure samples (MCC and CMC) and their conjugates (MCC-SiO 2 and CMC-SiO 2 ). The tablets were assessed for their physical properties, in vitro disintegration time, water absorption ratio and wetting time. Stability studies showed no alteration in the appearance as well as the performance of the formulations.