52 TESTING
as enable high levels of bioavailability after application to the skin. Another goal for this design phase was to ensure that prototype formulations were developed to match consumer desirable rheological and sensorial properties as closely as possible. Considering the basic chassis components
for each of the product formats, the most appropriate active ingredient solubilisers previously identified were trialled in an iterative manner to establish the highest weight percentage incorporation of each of the active ingredients without impacting on the prototypes’ physical characteristics, such as phase separation, discolouration, or visible inhomogeneity.
Prototype characterisation Rheological properties From a physical characterisation perspective, the rheological properties of the formulation prototypes were critical. In this context, rheological properties are a feature of formulations that dictates how they flow or deform in response to the application of force or stress. To a consumer this is typically noted as how creamy the products look and feel, or how easily it is spread over the skin; can it be poured, or does it hold its shape in the packaging? In regard to materials characterisation, two
basic parameters dictate a significant portion of this consumer perception: the dynamic viscosity and the yield stress. Viscosity is a measure of how resistant a
material is to flowing: honey has a high viscosity while water has a relatively low viscosity. Measuring how the viscosity of a material changes as a function of the force being applied to it can provide useful information on how likely a product, for example, will pour out of its container. A skin care product that has a high viscosity
when exposed to forces typically experienced when spreading a product onto the skin will feel very stiff and unpleasant to apply. Conversely, a product that has a low viscosity may run off the skin when applied, such that the product is wasted. Either extreme can lead to a negative
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Figure 1: Hansen solubility parameters sphere2
consumer perception. The dynamic viscosity, meaning how the viscosity changes upon the application of different forces, is therefore a critical parameter both for ensuring high quality during manufacture as well as achieving a high level of consumer acceptance. Yield stress is the other critical rheological
parameter to evaluate. The yield stress is often defined as the amount of force required for a material to display flow; below this critical value the material is said to deform elastically or display solid-like flow characteristics. Together, the viscosity and the yield stress define the overall physical characteristics of a formulation and its organoleptic properties. These properties are derived from the interactions of the ingredients present in the formulation at a molecular level and can be
Gel Cream ■ Lotion ■ Cream ■ Serum ■
probed using a rotational rheometer. A rotational rheometer expresses this critical
information by standardising parameters such as the temperature and the volume of the test sample and then by varying a force applied to the sample in a controlled fashion. The sample’s response to the force applied allows the critical viscosity and yield stress data to be extrapolated. During our product development trials
this information was used in two ways: to investigate the impact of changing the formulation ingredients on the rheological properties of the prototype formulations, and to also ensure that our prototype materials had rheological properties similar to benchmark market products, with the goal of preventing negative consumer perception of the prototype formulations.
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100 10-2 10-1
Oscillation strain R (%) 101
100 Figure 2: Rheological evaluation of four product formats PERSONAL CARE November 2023 102 103 Figure 3: Rotational rheometer
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Complex modulus (Pa)
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