How to Test the Physical Properties of Medical Ointments

A customer’s perception of a medical ointment can be gaged from results obtained by a test using the ointment. The test can determine whether the ointment comes out with sufficient ease, when the customer applies it to the skin. It can also determine whether it comes out too quickly, squirting out more than is actually required.

Another test result can show whether the customer feels the ointment spreads on the skin with a reasonable amount of effort, whether it offers comfort on application and so on. These data points form the basis of marketing surveys conducted by the manufacturer of the ointment.

Testing Physical Properties of Medical Ointments

A range of tests need to be conducted before the product reaches the customer, to ensure customer satisfaction towards the final product. One of the tools of choice among research teams are rheometers with plate/cone geometry. They can help assess the physical properties of ointments – such as squeezing behavior and spreadability, as seen in Figure 1.

Brookfield RS Cone/Plate Rheometer

Figure 1. Brookfield RS Cone/Plate Rheometer

To do this, a small quantity of test sample, usually less than 1 ml, is placed on a temperature-controlled plate, and the cone spindle is brought down to establish contact. To measure the amount of torque needed to allow the spindle to rotate, a controlled stress mode of operation is used. This torque value can be equated to the human force needed to squeeze ointment out of a tube.

Another test method is controlled rate mode of operation, which helps to characterize and assess the ointment’s spreading behavior. Rate ramps running from 5 sec-1 to 50 sec-1 offer a rapid analysis of the viscosity for the ointments. Viscosity tends to be high when the customer begins the spreading action. However, as the customer continues to rub, the ointment offers less resistance to moving and hence spreads more easily. Figure 2 depicts viscosity "flow curves" for three different formulations of an ointment.

Viscosity Flow Curves for 3 Formulations of Ointment

Figure 2. Viscosity Flow Curves for 3 Formulations of Ointment

Both the above tests are performed at a specific temperature. In the controlled stress test to assess squeezing force, the temperature is the ambient condition in the storage location for the tube. Whereas the controlled rate test may use a test temperature related to the area of human skin where the product is applied.

Finally, a consideration with regards to the controlled rate test is the time sensitivity of the ointment to the shearing action. For instance, if the spindle speed is reversed after reaching 50 sec-1 and returned back to 5 sec-1, resulting viscosity values may be respectively lower at each speed. This condition is called thixotropy.

However, in situations where the two viscosity curves have a high degree of separation, then the ointment can be said to be extremely time sensitive to shearing. Marketing teams usually establish typically acceptable limits for ointment behavior during human trials, which are a way of evaluating how the product handles and feels. These results are correlated with the test data to produce maximum and minimum values for torque and viscosity that will qualify the product during manufacturing.

Conclusion

Such methods are continually practiced by large pharmaceutical companies. New entrants can, on the other hand, learn quickly by consulting the body of published work on viscosity test methods presented in technical papers by the medical world.

Fortunately, the expense for this type of instrumentation is affordable and the learning curve for proper use is relatively quick. Most young scientists have undoubtedly received some level of exposure and training at university level on these instruments. However, the bottom line is to make sure that the company meets the needs of the customer when it comes to first impressions on look and feel of ointments.

About AMETEK Brookfield

Brookfield, a business unit of AMETEK Inc, is the world leader in viscosity measurement and control of liquids and semi-solids for 80+ years!

We manufacture and distribute globally viscometers the AST 100 for advance sensor technology for simple, direct in-line viscosity measurement, DV2T Touch Screen with temperature measurement, rheometers DV3T for measuring yield stress and viscosity, RST Controlled Stress for challenging rheological measurements, CT3 Texture Analyzers which features compression and tension mode for measuring firmness, springiness and chewiness and PFT Powder Flow Testers which measure yield stress and viscosity flow index and arching dimension used in R&D, QC and inline applications for rheological fluid analysis, tension and compression and powder flow analysis.

Markets include: food cosmetics, pharmaceuticals, oil & gas, personal care products, adhesives, printing, inks & coatings, paper coatings, chemicals, packaging, plastics, construction materials, and asphalt.

Brookfield AMETEK manufacturers and services a full range of viscometers and rheometers, texture analyzers, and powder flow testers. Additional services include: testing for viscosity, texture and powder properties, calibration and certification program, custom fixture development, free technical papers, seminars, videos, application notes, manuals, and educational and training programs.


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Last updated: Jan 22, 2019 at 11:24 AM

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