TESTING


pH value The pH values of each of the prototype formulations were checked. This is a standard quality control check to ensure that the introduction of new ingredients, including the actives, does not negatively impact the overall pH balance of the formulation. Drastic changes in pH can also be indicative of ingredient degradation. Many skincare formulations are formulated


to have a slightly acidic pH similar to that of the skin. This ensures that any products applied to the skin won’t have potentially harmful pH interactions, and also supports market claims regarding whether a product’s pH is ‘balanced’ in line with the natural pH of skin. Any variation in pH seen during formulation


prototyping was adjusted using suitable pH modifiers or pH buffers. This must be carried out judiciously since the addition of pH modifiers, which tend to be salt-based or display high ionic potential, can have a dramatic impact on inter-ingredient interactions and thus product viscosity in particular. This is especially the case if the product format is based on an emulsion, where any significant change in salt level can significantly affect the structure-building micelles present.


Permeability testing Given that solubilisation of the poorly water soluble active was such an important determining factor in this development programme, testing for active release and


Figure 4: Typical Franz Cell setup


potential bioavailability was also assessed. A Franz Cell apparatus is a well-known and


effective way of evaluating active permeation kinetics for ingredients delivered from topical products. A bottom chamber acts as the active receiving fluid and is often designed to mimic biological conditions – an appropriate phosphate buffer is typically used. This fluid is often maintained at a body


relevant temperature and recirculated. On top of this fluid sits a semi-permeable membrane, which is intended to act as a mimic for the skin. The choice of material used as the membrane is critical for validating the system as a model to


53


reflect the in vivo environment, and as a result this is an active area of research. Common materials used as the membrane


include synthetic polymers, animal skin or human skin. The use of human skin gives a much more representative guide to in vivo ingredient permeability as compared with a synthetic membrane, but there are considerably more ethical, regulatory, and technical challenges to consider. The test sample or product is placed


onto the membrane and the receiving fluid periodically sampled to assess how much of the ingredient of interest has permeated across the membrane. There are several different analytical


techniques available for quantifying the amount of ingredient that has diffused into the receiving fluid, signifying permeation through the ‘skin’ and into the bloodstream or lower dermal layers. The choice of technique mostly depends upon the physicochemical characteristics of the ingredient of interest. If the ingredient that is being quantified is


volatile or has a high vapour pressure then a gas chromatographic technique typically works well, otherwise a liquid chromatographic method would be favoured. Use of chromatography is critical for assessing ingredient concentrations in the receiving fluid since it is likely that other materials present in the test product will also diffuse through the semi-permeable membrane along with the material to be quantified.


www.personalcaremagazine.com


November 2023 PERSONAL CARE


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