Surface Science and Adhesion in Cosmetics. Группа авторов

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capacitance values [32, 33]. When examining the correlation between TEWL and the ceramides from different body locations, Ishikawa et al. observed an inverse relationship between TEWL and CER[NS], CER[NP] and CER[NDS] [33]. Conversely, a positive relationship between TEWL and CER[AS] was found [33]. Interestingly, seasonal variations have very little impact on the TEWL, total ceramide and CER composition on the lips; however, an increase in capacitance value was shown in the summer from the spring [33]. This work was followed by another publication in 2016, which studied the relationship between lip roughness and ceramide profile in an attempt to clarify the biological contributing factors to lip chapping (Figure 1.5) [40]. In the work conducted by Tamura et al., CER content, TEWL, and capacitance were measured from forty-one Japanese subjects with different classifications of lip roughness [40]. In their work, an inverse correlation between lip roughness and capacitance was found, while TEWL showed no significant trend between the different lip roughness scores [40]. Lip roughness also correlated negatively, albeit with relatively low correlation coefficients, with the level and average carbon number of CER[NH] and CER[NP] [40]. Hikima et al. investigated the lip surface corneocytes and desquamation regulating proteinase of normal and chapped lips and found that reduced levels of cathepsin D activity tended to be a factor for chapped lips [41]. In this work, apricot extracts were applied topically to chapped lips, resulting in a decerease in lip roughness accompanied by an increase of cathepsin D activity [41]. The effect of season was investigated by Gubanova and Caisey where they followed the lip surface properties of 20 women in Moscow between April and September [16]. In the study, lip surface properties were evaluated by examining four key criteria: dryness, cracking, scaling, and pleating. Not surprisingly, dryness, cracking and scaling were found to be more significant in April and May compared to July and August, while pleating was less visible in July than in September [16]. In a separate study, Gubanova and Caisey investigated the effects of lip balm application using the same criteria for a four-week study during December in Irkutsk [16]. The application of lip balm showed significant improvement in the drying, scaling, and pleating scores of the lips over the four-week period [16].

Photos depict the Lip roughness score.

      Throughout the development of a lipstick product, in vitro performance analysis is conducted in order to develop and optimize the best-performing product to be launched in market. Typically, both standardized and productspecific characterization tests are conducted in order to properly rank formulations and identify product weaknesses. Formulation characterization tests can be conducted by applying the formulation onto lips, skin, artificial skin, or glass, depending on the test method being used. This section shall provide a brief overview as to the types of tests available and used in the development of lipstick formulations, more specifically for long-wear lipstick formulations.

       1.3.1 Stability Testing of Lipstick Formulations

      It is important to note that the priority in lipstick development is product stability. Not only does stability impact product performance, but it can also directly influence consumer perception of the brand. Stability testing is typically conducted for different time periods at varying temperatures at which samples are observed. Common observations may include lipstick sweating, in which the oil within the formulation forms visible droplets on the exterior of the formulation, in addition to discoloration, crystallization, separation, or cracking. A Design of Experiment can be utilized during lipstick formulation stages in order to identify the best optimized formulation that passes stability testing. Currently, there is no in vitro method to predict a formulation performance in regards to stability. Therefore, utilizing a Design of Experiment is the best way to identify the optimal formulation within a particular batch.

      Stability testing of long-wear lipstick formulations is especially important due its volatile solvent component. Isododecane, which has a flashpoint of 45°C, constitutes nearly half of the volatile solvents in both the liquid and stick forms of long-wear lipstick. Therefore, the stability readout at 45°C is crucial in identifying and optimizing potential formulations. In the liquid format, should the volatile solvent flash off and fail stability, there would be an increase in viscosity; meanwhile, the stick would become dry. The packaging design and seal of such formulations is also tested through stability to help identify any loss of volatile solvent over time. Should a formulation pass these stability tests, further formulation characterization will be conducted through the use of in vitro test methods and consumer studies.

       1.3.2 Physical Properties of Lipstick Formulations

       1.3.2.1 Lipstick Hardness and Deposition

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