Scalp skin is exposed to surfactants, UV radiation, pollution and mechanical injury from hair styling techniques on a daily basis. Nevertheless, this area of skin has historically been overlooked by consumers, leading to many scalp-related ailments. In fact, an internal global consumer survey was conducted to assess current scalp care needs and revealed a range of factors including relief from itch, prevention of flaking and maintenance of a healthy scalp. Many of these needs can be linked to scalp moisturization, which emphasizes the strong demand for technologies supporting this claim.
Scalp skin has not previously received worthy attention due to a lack in understanding its physiology, although advances in equipment such as the in vivo Raman probe have improved its characterization. For example, reduced amounts of natural moisturizing factor (NMF) exist on scalp skin, compared with the volar forearm, since regular cleansing of the hair removes these hygroscopic components.1
However, conflicting opinions exist in the literature regarding trans-epidermal water loss (TEWL). Several sources state TEWL is higher in scalp skin than volar forearm skin, while others state the contrary. The present authors found, in an internal study (data not shown), that the TEWL on the scalp was approximately 35 g/m2/hr, compared with the volar forearm at approximately 15 g/m2/hr. Furthermore, the lipid composition was found to differ and the keratinization cycle was faster in scalp skin, compared with the volar forearm.2 These evident differences have revealed scalp skin is truly unique in its attributes, highlighting the importance of developing specialized personal care products for it.
In recent years, the increased awareness for scalp care has been reflected in the growth of scalp-related claims from hair care products; growing from 1,170 in 2008, to 2,305 in 2014.3
Yet, improving scalp skin while imparting consumer-perceivable benefits to hair is a challenging task. The average surface area of the scalp is 700 cm2 whilst that of the hair is 6 m2 for women and 1 m2 for men.4 Thus, a considerable amount of any applied product therefore will remain on the hair and not reach the scalp surface. Moreover, the claims substantiation of scalp care ingredients is difficult due to the presence of hair, which limits the accessibility of traditional skin testing methods.
Subsequently, it is crucial for any scalp care ingredient to not impose a detrimental effect on hair. In fact, ideally, it would provide added benefits to meet the increasing demand for multifunctional products. As such, a study was designed to determine whether scalp moisturization and sensory benefits for both the scalp and hair could be delivered from a wash-off regime.
A test moisturizing ingredienta was incorporated into a shampoo and conditioner at 1% and used on sample hair tresses and by panelists. This moisturizing ingredient was selected based on previous research showing its ability to improve the skin bilayer structure and packing parameters of skin lipids.5 Additionally, the ingredient is oil-based, increasing the likelihood of its deposition on the scalp during cleansing. The ingredient’s effects were then evaluated instrumentally and by trained panelists, as described next.
Materials and Methods
Panel studies: Thirty-six healthy Caucasian panelists (18 male and 18 female, ages 22 to 45 years) were recruited to use test shampoos and conditioners (see Formula 1 and Formula 2, respectively) daily for 16 days. To provide consumer-relevant data, the male panelists received shampoo only whereas the female panelists received both shampoo and conditioner products.
For all panelists, the study consisted of a three-day wash-out phase with a basic SLES-based commercial shampoo and, in the case of the women, a basic commercial conditioner. This was followed by the 16-day treatment phase in which half the panel received formula regime A and the other half received formula regime B.
Thus, in this randomized, blind, unpaired test, eighteen panelists received formulas containing the test moisturizing ingredient (regime B), while the others received the control formulations (regime A). During both the wash-out and treatment phases, panelists were instructed to wash their hair daily using their normal routine with the provided formulations only; the use of any other hair or scalp product was strictly prohibited.
Panelists were assessed at a temperature- and humidity-controlled facility (20°C ± 1.0, 50% RH ± 5.0) on days 0 (baseline), 2, 8,14 and 16, with an acclimation period of 20 min prior to testing. Instrumental skin evaluations included TEWLb and scalp hydrationc. A sensory questionnaire also was provided to panelists to indicate daily, on a scale from 1-10, the moisturization level of their scalp and softness of their hair.
Tress evaluations by experts: A trained panel of 15 sensory experts assessed a number of attributes of bleached European hair tresses treated either with shampoo A and conditioner A, or shampoo B and conditioner B. This study was performed to determine the impact of the scalp moisturizing ingredient on hair. Assessments were carried out in three independent sessions in order to avoid panelist sensorial fatigue. These sessions comprised:
Session 1: assessment of detangling, wet-combing and wet smoothness characteristics;
Session 2: assessment of clean appearance, shine, residue and dry-combing characteristics; and
Session 3: assessment of dry smoothness, dry softness and flyaway control.
Statistical analyses: Grubbs Outlier tests followed by parametric two-tailed, un-paired t-tests were used to analyze the instrumental moisturization data. A Tukey test was used for analyzing the expert hair sensory data. Statistical significance was set at 95% for all tests.
Results and Discussion
Scalp hydration: As noted, scalp hydration is difficult to measure due to the presence of hair interfering with traditional skin capacitance instruments such as the corneometer. An eight-pin hydration probeb therefore was used to measure values of conductance in panelists, to measure the water-binding capacity of their scalp stratum corneum.
Scalp hydration increased for both regime A and regime B (see Figure 1), indicating the formulation chassis developed for the treatment phase were milder to the scalp than those used in the wash-out phase—i.e., the commercial formulations in the wash-out phase might have contained harsher surfactants, resulting in lower scalp hydration due to the removal of lipids and NMF.
Although both test regimes increased scalp hydration, the average increase was greater for regime B, which was significant on days 14 and 16 (p < 0.05). The net changes for the control and test formulas were 11.9 and 21.8, respectively, on day 14; and 10.5 and 20.4, respectively, on day 16. This showed the moisturizing ingredient improved scalp hydration even from a wash-off regime, which as noted is challenging since shampoos contain high levels of surfactants, disrupting corneocyte structure and removing NMF. This is particularly true of anionic surfactants such as the SLES used in this study.6
Scalp TEWL: Like other skin, corneocytes in scalp stratum corneum are surrounded by lipids, providing the skin barrier with a brick-and-mortar-like structure. Loss of these lipids lessens the strength of the scalp skin barrier, which leads to higher TEWL and, in turn, lower moisturization. A relatively large panel (n = 36) was selected for this study to increase the reliability of results since the TEWL for the scalp is much more variable than that of the volar forearm.
Again, the TEWL for both regimes decreased on average, showing the test formulas more strongly supported the scalp barrier than those used in the wash-out phase (see Figure 2). The TEWL for test regime B showed a larger decrease than control regime A, with the decrease being significant at 5% on day 16 (p < 0.05). This indicated the moisturizing ingredient improved the skin barrier.
Further work generated internally by X-ray diffraction showed the ingredient has the ability to form a lipid bilayer on the surface of the skin, with lateral packing of the lipid head groups being in a tight orthorhombic packing configuration. This tighter packing and bilayer formation may be the reason for the observed decrease in TEWL.
Sensory effects on scalp, panelists: Panelists completed a sensory questionnaire daily to rate, on a scale from 1-10, the moisturization level of their scalps. The results (see Figure 3) indicated panelists on test regime B perceived a greater increase in scalp moisturization than those on control regime A. The results therefore show the ingredient is providing perceived scalp moisturization as well as clinical improvement, shown through the hydration and TEWL measurements.
In relation, to understand how the panelists assessed scalp moisturization, they were asked to define how they would portray it. The wide range of attributes given strongly suggest scalp moisturization is indeed the key to a healthy scalp and moreover the basis for healthy hair.
Sensory effects on hair, panelists: As noted, panelists also completed a sensory questionnaire daily to rate, on a scale from 1-10, the softness of their hair. Similarly to the sensory effects on scalp moisture, the panelists on test regime B indicated a larger increase in hair softness than panelists on control regime A (see Figure 4). This indicated the moisturizing ingredient also had a positive effect on hair.
As mentioned, it is vital for any ingredients developed for scalp moisturization to also deliver either a neutral or positive benefit to hair. Many consumers feel a healthy scalp will lead to healthy hair and the present data shows the panelists perceived these positive benefits on both the scalp and hair. These positive effects on the scalp and hair increased as the study continued, which may suggest deposition of the moisturizing ingredient. Furthermore, the perceived positive effect suggests the appropriate amount of moisturizing ingredient was delivered, since too little would provide no clinical effect whereas too much would likely give a heavy or greasy feel to the scalp and hair.
Sensory effects on hair, expert graders: In addition to the sensory questionnaires completed by panelists, hair tresses treated with regimes A and B also were submitted to an expert hair sensory panel. This enabled a deeper understanding of whether the ingredient impacted a wide range of hair attributes (see Figure 5).
Expert analysis revealed, on average, that test regime B outperformed control regime A on all attributes with regime B statistically outperforming regime A for wet hair detangling and dry hair softness (p < 0.05). This data complements the results obtained from the moisturization study and suggests hair conditioning effects. These sensory results highlight the ingredient’s potential in providing both scalp and hair benefits.
This study has shown that a test moisturizing ingredient increases scalp hydration as well as improves scalp skin TEWL. These improvements were also consumer-perceivable—which is essential if a product is to be successful on the cosmetics market.
In addition, expert hair graders indicated the test moisturizing ingredient delivered benefits to the hair including wet hair detangling and dry hair softness. Finally, to further benefit wash-off systems, additional internal data (not shown) proved the ingredient was mild to skin and thus ideal for leave-on applications, therefore giving formulators the capability to incorporate scalp moisturization into a diverse range of hair and scalp products.
- PD Pudneyet al, A new in vivo Raman probe for enhanced applicability to the body, Appl Spectroscopy 66(8) 882-891 (2012)
- K O’goshi, M Iguchi and H Tagami, Functional analysis of the stratum corneum of scalp skin: Studies in patients with Alopecia areata and androgenetic alopecia, Arch Derm Res 292(12) 605-611 (2000)
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- K Papp, J Berth-Jones, K Kragballe, G Wozel and M Brassinne, Scalp psoriasis: A review of current topical treatment options, JEADV 21 1151-1160 (2007)
- G Pennick, B Chavan, B Summers and AV Rawlings, The effect of an amphiphilic self-assembled lipid lamellar phase on the relief of dry skin, Intl J Cos Sci 34 567-574 (2012)
- E Uhoda, JL Lévêque and GE Piérard, Silicon image sensor technology for in vivo detection of surfactant-induced corneocyte swelling and drying, Dermatology 210(3) 184-188 (2005)