base for skin, there is limited existing literature on the properties of lips. With the development of new and sophisticated analytical tools, gaining a strong understanding of lip surface and biophysical properties can provide key information towards the future innovation of lipstick and lip care products. Towards that goal, this book chapter has two main objectives: to provide an overview of biophysical properties of the lips and to provide a review of conventional lipstick evaluation methods.
1.2 Overview of Lip Anatomy & Lip Surface Properties
1.2.1 Lip Anatomy and Biophysical Properties
The lips are two pliable and mobile muscular folds which surround the oral cavity, and act functionally to support food intake, articulation, facial expression as well as being an important tactile organ [10–14]. The upper lip (Labium superioris) extends from the base of the nose to the nasolabial folds, while the lower lip (Labium inferioris) lies between the mouth and labiomental groove (Figure 1.3a) [15, 16]. The interface between the outer versus inner surface of the lip is defined as the vermilion border, where, histologically, the transition between the keratinized vs. the non-keratinized epithelium can be visualized. The two paramedian elevations of the vermilion of the upper lip are defined as the Cupid’s Bow [15, 16]. The two raised vertical columns form the philtrum, which is a midline depression located between the paramedian elevations of the vermilion and the columella. The lips have three distinct surfaces: outer surface, edge, and inner surface. The outer surface of the lip is composed of stratified squamous keratinized epithelium (Figure 1.3b) while the inner surface of the lip is composed of stratified squamous non-keratinized epithelium. The inner surfaces are lubricated with saliva to facilitate chewing and swallowing, while the keratinized outer surface of the lip serves to protect against friction, drying, and potential microbial invasion.
Figure 1.3 (a) The anatomical landmarks of the lips [15] (b) Histological sections of human skin (left) and lip (right) as stained by H&E [66].
The tissue structure of lip skin is illustrated in Figure 1.3b. The tissue of lip skin is made of epidermis, subcutaneous tissue, orbicularis oris muscle, and mucosa [17]. Compared to the human skin (Figure 1.3b, left), the keratinized epithelium of the lips is much thinner (3-5 cell layers averaging to only a few micrometers in thickness in the lip versus 16-20 cell layers averaging to 10-20 micrometers for the skin). In terms of the extracellular matrix content, both the skin and the lips contain similar levels of epithelial glycogen, elastin and collagen [17]. The lips, however, are richer in blood vessel networks, contain many sensory nerve endings from the mandibular and maxillary nerves and show absence of hair, sebaceous glands, and sweat glands [10]. These histological features of the lips provide the unique tissue properties that can be perceived. The thin keratinized epithelium is part of the reason why the lips are more susceptible to drying and chapping, as the reduced barrier layer makes the lips more susceptible to water loss. The rich vascular network, combined with the thinness of epidermis and the lack of pigments, is what makes the lips appear red. The lips receive blood supply from external carotid arteries [10, 18, 19]. The facial artery branches into an inferior and a superior labial artery, which courses beneath the orbicularis oris muscle. The superficial artery reach the skin of the upper lips and form a vascular plexus [18]. The lower lip receives blood supply from three labial arteries: the inferior labial artery, horizontal and vertical labiomental arteries [18]. Under extremely cold temperatures, the blood vessels in the lips will undergo vasoconstriction as well as having reduced level of oxygen, thus turning the lips purple. Similarly, the sensory nerve endings in the lips can make the lips sensitive to touch and temperature. The upper lip receives sensory innervations from the infraorbital nerve, while the lower lips receive its sensory innervation from the mandibular nerves. The buccal nerve, a sensory branch of the mandibular nerve, provides innervation to the entire cheek mucosa and skin around the mouth [20]. In the context of makeup, the sensitive lip surface is what allows the consumers to perceive tightness and drying after the application of lipsticks.
A 2,023-subject study on the soft tissue 3D landmarks around the lips was conducted using a Three-Dimensional Facial Morphometry system and provided valuable insight into the development process of the lips [21]. This study investigated the differences in lip dimensions (linear distance and ratio as well as area and volume of vermilion) as a function of gender and aging. Their measurements, taken from subjects aging from 6-7 years old all the way through adulthood, illustrated that male subjects have on average higher lip volume, greater vermilion area, and greater lip heights thorough all ages [21]. Linear distance in girls has similar dimension as measured in adults, while in the boys in the 13-14 year old age group reported a period where a large year to year increase was observed [21].
The motions of the lips are controlled by the orbicularis oris muscle, which subdivides into pars marginalis and pars peripheralis [10, 11, 13, 16, 17] The pars marginalis consists of the muscle fiber group lodged within the vermilion zone of the lips, while the pars peripheralis is located in the cutaneous lip. When the mouth is closed, the orbicularis oris is tonically contracted; while in motions such as kissing and whistling, the orbicularis oris muscle undergoes active contraction. During chewing, four other muscle groups (masseter, temporalis, medial pterygoid and lateral pterygoid) are responsible for the adduction of the jaw. Many of the biophysical properties of the lips have been reported by Fogel and Stranc in a 500-subject study representing different genders and age groups [22]. In this study, the lips of the subjects were evaluated for intercommissural distance, perioral tissue strength & elasticity and lip sensitivity. Logically, the age and gender of the subjects have profound effects on the biophysical properties of lips. The soft tissue gape and oral aperture of the lips generally peak in the age group of 16-30 and have consistent reducing trends in older age groups [22]. In 2011, Sjogreen et al. used 3D imaging systems to capture the mobility of the lips for their subject at rest and during open mouth smile and lip puckering [23]. Not surprisingly, the lips are subjected to very large range of mechanical motions. During open mouth smile, the lips experience 34% extension over rest, while during lip puckering the lips experience 37% contraction over rest. Using a pommeter, Fogel and Stranc were able to measure the strength of the orbicularis oris muscle of the subjects and reported higher strength values in age groups ranging from 16-30 to 46-60, while subjects of advanced age (61-75) had much reduced value of muscle strength [22]. The mechanical properties of the lips were measured using a device called LASTIC (Light Aspiration device for in vivo Soft TIssue Characterization) which functioned very similarly to a cutometer [24]. In this study, Luboz et al. reported that the measured Young’s modulus of the lips ranged from 18 to 46 kPa [24]. The measured Young’s modulus values of the lips are on par with the measured value of skin from the cheek, cheekbone and forehead [24]. Such biophysical understanding of the lip motion can be very important information for designing lipstick products that have ideal comfort and durability.
1.2.2 Surface Properties of the Lips
In order to understand the surface properties of the lips, it is essential to understand the process of barrier assembly in the keratinized epithelium. This process is controlled by the differentiating epithelial cells in a tightly regulated manner in order to form a functional physical barrier and to maintain skin and lip epithelium [25, 26]. In the process of barrier formation, epidermal cells accumulate lamellar bodies containing keratins, ω-hydroxy-ceramides, free fatty acids, cholesterol, and other barrier lipids. At a late stage of differentiation, these organized extracellular lipids assemble into
