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Postdoctoral Fellow
Last time, we talked about various testing methods for lipsticks, you can find them here. Now let’s talk about our lips!
Lips, those soft bits on our faces, do a lot more than help us chat or smile. They're different from the rest of our skin, which means they need a little extra love and care. If you're someone who loves lipstick, it's even more important to know more about the uniqueness of our lip skin.
The Distinctive Nature of Lip Skin
While we often consider our skin as a uniform organ, it varies significantly across different body parts. The skin on our lips, for instance, is incredibly distinctive:
Thin but mighty: Unlike the rest of our body which has between 15 to 16 cell layers, the lip skin is wafer-thin, boasting only 3 to 5 layers [1]. This minimal layering makes lips susceptible to external factors and potential irritants.
Missing moisture mechanisms: One reason your lips might feel dry more often is because they lack sebaceous and sweat glands. These glands are responsible for naturally moisturizing the skin. Plus, lips experience a heightened rate of water loss compared to other skin areas, making them prone to dryness.
![Figure 1. Anatomy of lip skin [1].](https://static.wixstatic.com/media/aa3cdc_f7d7a0b198b442588ec659768155ef07~mv2.png/v1/fill/w_76,h_44,al_c,q_85,usm_0.66_1.00_0.01,blur_2,enc_auto/aa3cdc_f7d7a0b198b442588ec659768155ef07~mv2.png)
Given these characteristics, it’s easy to see why lips demand special attention and regular hydration.
A Closer Look at Lip Hydration
At TRI Princeton, Dr. Xuzi Kang employed a corneometer to gain a deeper understanding of lip hydration. This device is both easy to use and non-invasive, making it optimal for accurately measuring the hydration levels in the superficial layers of the lip skin (the stratum corneum) [2]. This feasibility study engages 18 participants (9 male and 9 female) who are systematically chosen from a broad age range of 24 to 68 years old. Five testing points on the lips are chosen to test their upper and lower lip hydration. Participants are selected based on specific inclusion criteria which require them to be devoid of lip pathologies, skin lesions, or evident signs of lip dryness, thereby ensuring the integrity of the data pertaining to lip hydration. The objective was to determine differences in hydration levels between the upper and lower lips.
The Impact of Lipsticks on Hydration
Hydration is a significant concern for consumers, with some noting that while matte lipsticks do not transfer easily, they can be drying. To provide a deeper understanding of this phenomenon, the corneometer is used to evaluate lip hydration after a 2-hour application of the lipstick. Through a comparative feasibility study of the different types of lipsticks both on the arms and on the lips, distinct characteristics are identified and evaluated.
By analyzing the products' impacts both on the arm and lip skin, significant differences are found in hydration behavior based on the application site, a phenomenon illustrated in Figure 3. Interestingly, lip skin consistently shows a higher hydration level than arm skin, suggesting that lip skin is more susceptible to water evaporation, highlighting the fundamental differences between the two skin types in terms of composition and function.
It is found that all lipsticks tested, regardless of brand or type (including Vaseline - the positive control), enhance arm skin hydration to different extents, ranging from 16% to 67%; while lip skin. It is worth noting that the lip skin, which naturally holds more water, undergoes noticeable changes when in contact with different lipsticks (-13% to 57% hydration increase). This stresses the effect of lipstick formulations on hydration levels, and these data align with panelists' perceptions. Another interesting finding from this study is the hydration level in the upper lips compared to the lower ones, suggesting a possible difference in the anatomical and physiological attributes of the upper and lower lips. It’s worth noting that ATR-FTIR is available at TRI too to reliably measure hydration levels of our skin.
Figure 3. A: Arm and lip hydration levels are different;
B: arm and lip skin hydration change behaviors are different after 2h lipstick application.
Wrapping It Up
In conclusion, the lip skin, inherently thinner and lacking sebaceous and sweat glands, is predisposed to quicker drying. Using a corneometer, this study offers a comprehensive examination of hydration levels on the lips and compares them to arm skin after lipstick application. Notably, because of their thin structure, lip skin has a higher hydration level than arm skin, and is more sensitive to different formulations of lipsticks. The application of various lipsticks showcases an impact on lip hydration levels, which correlates with the perceptions of panelists. Furthermore, the study shows a difference between the upper and lower lips in terms of hydration, shedding light on potential variations in their anatomical and physiological makeup. These insights not only deepen our understanding of lip skin hydration dynamics but also emphasize the influence of cosmetic formulations on the skin.
For a deeper dive into this topic, Dr. Xuzi Kang will be presenting at the SCC annual meeting later this year. If you're keen to learn more, we highly recommend attending her session.
Reference
2. López Jornet, MarÃa PÃa, Fabio Camacho Alonso, and Ana Belén RodrÃguez Espin. "Study of lip hydration with application of photoprotective lipstick: influence of skin phototype, size of lips, age, sex and smoking habits." (2010).