Characterising pharmaceutical lotions by rheology

S Balakrishnan, Applications Specialist, TA Instruments explains the process of rheology and its significance.

In general, pharmaceutical creams and lotions fall under the category of complex fluids systems. Structured fluids or complex fluids are multiphase systems consisting of a dispersed phase (solid or fluid) in a surrounding fluid phase. After formulation of lotions and creams, its success in the market depends on the end use performance. Correct formulation of these ingredients allows creams and lotions to easily flow out of the container, no sedimentation of solid particles while storing and good spread ability on the surface. Most of the tonics available in the market have to be shaken well before use. Why? What is the reason behind this? Rheology gives answer for this and helps in characterising creams and lotions. Based on this technique, one could predict the behaviour of lotions or ointments in the end use performance. Improper formulations, on the other hand, result in sedimentation of solid particles, improper application on the skin or surface, and flow of ointment out of the tube due to gravity.

Rheology

Rheology is defined as the study of the deformation and flow of materials. When most chemists hear the word 'rheology', they immediately think of viscosity and, in fact, quantitative viscosity is one of the material properties that can be obtained from rheological measurements. However, a material's viscosity is not a discrete value, as is temperature. Rather, viscosity is a property that depends on the conditions of measurement, for example, the rate of deformation (shear rate). Newtonian fluid is one, in which the viscosity does not vary with shear rate and time example is water. For non-Newtonian fluid viscosity is a function of shear rate and time. Viscosity may increase or decrease with respect to shear rate and time. Example is viscosity of shampoo decreases with the increased shear rate, while viscosity of paints increases with time. A number of rheological properties play an important role in determining how a material behaves as it moves from a static to a dynamic environment and vice versa. For example, the force or stress required to initiate flow of fluids and semi-solids products plays a significant role in the storage, transfer, packaging, and end-use performance of those materials.

Apart from viscosity measurement, rheology is the correct technique for characterising visco-elastic materials. Liquids are viscous in nature and solids are elastic in nature. There are materials, which have both viscous and elastic property and are called visco-elastic materials. These materials are characterised by the parameters-storage modulus (G') and loss modulus (G"). These characterisations are significant for the end use performance. For example, better spread ability can be obtained if the loss modulus is greater than storage modulus. Sedimentation can be avoided, if the storage modulus is greater than loss modulus.

Instruments

There are two common approaches for rheological studies. They are controlled stress and controlled rate mode. In controlled rate mode, the material is placed between two surfaces. One surface is rotated at a fixed speed (strain rate) and the torsion force produced on the other surface is measured. In controlled stress mode, the material is again placed between two surfaces but a torque (stress) is applied to one surface and the resulting displacement or rotation speed of the same surface is measured. Although both approaches have inherent advantages, the controlled stress approach is most preferred because it allows measurements to be made with minimal destruction of the internal structure, which is responsible for those properties.

Here is an example for comparative study between two lotions for their spread ability. Rheological characterisation of creams and lotions in a controlled stress rheometer requires 0.2 millilitre of material. Furthermore, the material can be evaluated under conditions which stimulate the ultimate end-use. Fig. 1 and 2 show the difference between two lotions under oscillation and dynamic condition. Visco-elastic properties are studied under oscillation mode. In oscillation mode, the top plate oscillates with certain amplitude and frequency and the response of the sample is measured. The results in Fig. 1 were obtained when the lotions were exposed to a small oscillatory stress. Oscillatory experiments are particularly useful as a non-destructive method for investigating the structure of delicate materials over a short and medium scale of time. The result indicates that lotion A had a stronger elastic structure than lotion B which indicates lotion B spread well than lotion B.

In the Fig. 2, flow curve, the shear stress was varied linearly and the viscosity was measured. The viscosity of lotion A is greater than lotion B through out the range. Both the lotions show shear-thinning behaviour, viscosity decreases with the increase in shear rate. Of particular interest was the region of those curves at higher shear stresses which began to approach conditions that the lotions would see during rubbing on a surface.