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].
Figure 1.5 Lip roughness score [40].
Thus far, this book chapter has provided an overview of the lip anatomy and the surface properties of lip skin. Similarly to the skin, with age there are dynamic changes that can occur to the lips as part of the multifactorial aging process of the face. The topic of face aging is well documented [42–49] and is not the focus of this book chapter; however, it is important to understand the process of lip aging in the context of facial aging. Around the nasolabial and labiomental crease areas, aging subjects experience subcutaneous fat atrophy, changes in bone support in the pyriform area, significant loss of collagen and elastin fibers, and changes to the platysma and orbicularis oris muscle [16, 46, 49, 50]. These changes in the tissue structure contribute significantly to the development of bitterness fold (lines from the corner of the mouth down to the chin), sagging jowls, shriveled skin, and double chin around the chin and neck areas [49]. Changes to the lip volume and thickness have been elucidated by a number of research teams, but most impressively by Ramaut et al. where they conducted magnetic resonance imaging on the upper lips of 200 Caucasian subjects [51]. In their study, they segregated the population by gender (100 men and 100 women) and by age (20-30 and 65-80 years old) and evaluated for lip length, lip thickness, nasolabial tissue thickness and volume [51]. The older population reported to have significantly greater lip length, thinner lip thickness and lesser lip volume [51]. As for the comparison between genders, the female population had thinner lips and lesser lip volume compared to the males amongst the younger population; however, this difference was not as obvious in the older population [51]. Injection of hyaluronic acid based dermal fillers, botulinum toxin, fractional laser resurfacing and autologous fat transfer are amongst the most popular methods to reverse the signs of aging [52, 53]. However, these dermatological procedures still pose varying health risk to aging consumers. This opens an important opportunity for the cosmetics industry to offer an alternative solution to aging consumers or consumers seeking lip plumping innovations.
1.3 In Vitro Evaluation Methods for Lipsticks
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 is conducted by placing formulation samples in a controlled environment for a specified period of time in order to relate to shelf-life and consumer environments in which the finished product may be stored. This indicates what effects different temperatures and humidity levels might have on the product following a certain time frame. Initial stability tests are carried out in a 25°C chamber and samples are observed for signs of degradation or separation at particular time points [54]. According to a patent issued to L’Oreal, inspections for this initial stability testing are done at 24 hours, 3 days, 1 week, 2 weeks, 4 weeks, and 8 weeks, and the sample is observed for phase separation, bending, sweating, and other physical conditions [54]. Once a formulation passes this stage, a second stability test is conducted for 8 weeks in varying temperature chambers in addition to a freeze-thaw condition. Such temperatures disclosed in a patent issued to L’Oreal include 37°C, 40°C, 45°C, and 50°C [54].
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
Lipstick hardness helps provide some insight as to the stability of lipstick for throughout the product lifecycle, from manufacturing to transportation, use
