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Improved wetting properties:
Due to the fact that the liquid vehicle can either act as surface active agent or has a low surface tension, wetting of the liquisolid primary particles is improved. Wettability of these systems has been confirmed by measurement of contact angles and water rising times.

Fig:  Mechanism represents formulation of liquisolid system

Theoretical aspects for designing the liquisolid systems:
The amounts of excipients (carrier and coating materials) used to prepare liquisolid compacts depends on the flowable liquid retention potential values (?-value) and the liquid loading factors (Lf), Equation (1). The ?-value of a powder is the maximum amount of a given non-volatile liquid that can be retained inside powder bulk (w/w) while maintaining acceptable flowability whereas, Lf is the mass ratio (w/w) of the liquid medication to the carrier powder in the liquisolid formulation and it is given by equation 2. Therefore, in order to calculate the required weight of excipients, we need to determine the liquid retention potential value for both carrier (?CA-value) and coating (?CO-value) materials for each non-volatile liquid vehicle used, these values are constant for the given vehicle/powder system. Knowing the carrier: coating ratio (R), liquid loading factor (Lf) can be calculated by the following equation:

Lf = ΦCA+ ΦCO (1 / R)……..… [1]

Once liquid loading factors are obtained for such non-volatile liquid vehicle used in this study, the optimum weight of the carrier (Q), required for the respective vehicle could be calculated by using the Equation (2).

Lf = W / Q……………………. [2]

Where: W is the weight of the liquid medication (the drug + non-volatile liquid vehicle) Once the values for Q are obtained for the respective vehicle, the optimum weight of the coating material (q) could also be obtained (Equation 3).

R=Q/q……………………..…. [3]

Q = Amount of carrier material
q = Amount of coating material

Angle of Slide:
Required amount of carrier is weighed and placed at one end of a metal plate with a polished surface. The other end is gradually raised till the plate becomes angular to the horizontal at which powder is about to slide. This angle on which the material is about to slide from the plate is known as angle of slide. It is used as a measure of the flow properties of powders. Angle of 330 is regarded as optimum.12

Flowable liquid retention potential (Φ value):-
The Flowable Liquid Retention Potential of a powder is defined as the maximum amount of a given non-volatile liquid that can be retained inside its bulk (w/w) while maintaining acceptable flowability. It is denoted by Φ.

The Compressible Liquid Retention Potential (Ψ) of a powder is the maximum amount of liquid, the powder can retain inside its bulk (w/w) while maintaining acceptable compactability, to produce compacts of suitable hardness and friability, with no liquid squeezing out phenomenon during the compression process.

The Φ values are calculated according to equation:

              Weight of liquid
Φ value =  _______________
              Weight of solid

Contact Angle:
The contact angle is the angle, conventionally measured through the liquid, at which a liquid/vapor interface meets a solid surface. It quantifies the wettability of a solid surface by a liquid: if the contact angle is small, a drop of the liquid will spread on the solid; if the contact angle is large, the drop of liquid will bead up.

For assessment of wettability, con­tact angle of Liquisolid tablets is measured. To measure the contact angle, a drop of liquid is directly placed on a flat surface of the solid in the so called imaging method. The liquid drop is prepared using a saturation solution of the drug in Simulated Gastric Fluid, Simulated Intestinal Fluid media and an excessively large amount of pure drug is added to this media, shaken for 24 hours at a constant rate then the upper solution was centrifuged. A drop of this solution was placed on the surface of the tablet. By measuring the height and diameter of the sphere drop on the liquisolid tablets and direct compressed tablets, contact angle is measured. Liquisolid tablet contact angle is less than that of direct compressed tablets. Polysorbate 80 showed the lowest contact angle in the liquisolid tablets.

Liquisolid compact technique could be effectively used to prepare rapid release tablets of water insoluble drugs.

The liquisolid compacts were prepared using MCC and silica as carrier and coating material and PEG 400 was used as a liquid vehicle. In this technique drug is dissolved in a non volatile solvent and by this liquid medicament is converted to non adherent, dry looking and free flowing by using suitable carrier and coating material.

The flow properties of liquisolid compacts showed an acceptable flowability. The hardness, friability, weight variation and disintegration tests were within acceptable limit. The in vitro dissolution study confirmed enhanced drug release fromliquisolid compacts. The higher dissolution rate showed by Liquisolid compacts may imply enhanced oral bioavailability due to the increased wetting properties and surface of drug available for dissolution.

The solubility-dissolution behavior is the rate-limiting step to absorption from the gastrointestinal tract of poorly water soluble drugs and needs to be enhanced.

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5. Khalid M. El-Say, Ahmed M. Samy, Mohamed I. Fetouh., Formulation and evaluation of Rofecoxib liquisolid tablets, Int. J. of Pharmaceutical Sci. Research and Review (IJPSRR). 2010; 3:(1), 135-142.
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9. Kasture SV., Gondkar SB., Darekar AB., Dash P. and Bhambar KV., Enhancement of dissolution rate of Lansoprasole using liquisolid tablet technique, Int. J. of pharmaceutical research (IJPR). 2011; 3:(1), 27-31.
10. Rajesh K., Rajalakshmi R., Umamaheswari J., Kumar Ashok C.K., Liquisolid Technique a novel approach to enhance solubility and bioavailability, Int. J. of Biopharmaceutics (IJB). 2011; 2:(1), 8-13.
11. Yadav V.B., Yadav A.V., liquisolid granulation technique for tablet manufacturing: An overview, J. of pharmacy research (JPR). 2009; 2:(4), 670-674.
12. Tayel SA., Soliman I., and Louis D., Improvement of dissolution properties of carbamazepine through application of the liquisolid tablet technique, Eur J Pharm Biopharm. 2008; 69, 342-347.



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