Adapting Formulas to Skin Moisture Needs: A Review

Moisturizing technologies are becoming increasingly sophisticated as consumers place more importance on the condition of their skin and its refinement through skin care products.¹ The skin is a living organ and moisturizers formulated according to important underlying physiological principles have begun to appear on the market. As such, the skin care industry is developing a growing understanding of skin functionality to formulate for different consumer target groups.

Clinical Benefits of Moisturizers

The stratum corneum (SC) is a biosensor that changes its structure in response to product application. Specifically, a swelling effect has clearly been demonstrated with the application of moisturizers,² and it is the architecture of the SC that influences skin hydration.³ Adequate hydration is essential not only for the maintenance of smooth, supple skin with a pleasing appearance, but also for normal functioning of the SC.⁴

Moisturizers are skin care products designed to maintain healthy skin and prevent or alleviate dehydrated and dry skin conditions. Application of a moisturizer induces immediate tactile and visual changes to the skin surface, and all moisturizers alleviate dry skin symptoms when formulated well. However, in recent years different moisturizing technologies have surfaced with different effects on the SC.⁵ Moisturizers will increase SC hydration short-term and improve desquamation medium-term, but it is their long-term effect that must be quantified in order to establish their efficacy.²

Dry skin conditions stem from a cyclical impairment of the barrier function, which can be induced by several external factors such as low environmental temperature, humidity, abrupt changes in environmental conditions, air conditioning and surfactants; or internal factors such as chronological aging, psychological stress and genetics. Clinically, dehydration causes a dramatic increase in hardness and brittleness of the SC that the individual perceives initially as tightness, cracking and associated discomfort of the skin with inflammation, irritation or itching. This discomfort eventually leads to thickening, visible dryness and scaling.⁴ In milder cases, clinical symptoms of dehydration are whiteness of the skin around micro-textural lines, ashing in individuals with darker pigmented skin and overall loss of radiance.⁶ Facial wrinkles also have been found to develop earlier in dry skin.⁷ Moisturization increases skin smoothness and leads to an even skin surface, providing more aesthetically pleasing appearance and touch.

Achieving adequate hydration of the SC depends on three key factors: the intercellular lamellar lipids, fully matured corneocytes and the presence of natural humectants known as the natural moisturizing factor (NMF). The SC must maintain a hydration level of more than 10% to prevent the clinical symptoms of dry skin and sustain key enzyme activities for the desquamation process.⁴

Mechanisms of Action

The efficacy of current moisturizing technologies differs depending on their mechanism(s) of action. There are four main mechanisms: external occlusion, via materials such as petrolatum; emolliency such as from plant oils and waxes; humectancy, obtained via glycerol; and the recently identified internal occlusion based on orthorhombic transition induced moisturization (OTIM), e.g., skin lipids.^{8, 9} Internal occlusion seems to be the most important skin moisturizing mechanism since it fortifies SC barrier function.⁸ Adequate skin hydration can only be maintained if the SC barrier is functional, and the efficacy of moisturizers in achieving the desirable clinical end points such as reduced wrinkles and reduced dry skin symptoms depends on the degree of skin barrier repair.

Measuring Skin Hydration

Substantiating the efficacy of moisturizing technologies and skin hydration typically involves the assessment of the SC water gradient, thickness and total water content. Various noninvasive in vivo methods are used, based on different biophysical approaches. Many devices measure electrical properties that are indirectly related to the water content of the SC, e.g. capacitance, impedance and high frequency conductance methods. Other methods are spectroscopic, such as near-infrared, Fourier-transformed infrared, opto-thermal transient emission radiometry, high-resolution magnetic resonance and confocal Raman. Most commonly, the easier and less expensive method based on electrical capacitance measurement, described in the guidelines and standardized protocols of the European Group of Efficacy Measurements of Cosmetics and Other topical products (EEMCO), is used.¹⁰

It is apparent that accurate skin hydration measurements require a number of complementary methods. Though electrical measurements are useful among noninvasive bioengineering methods, there is growing evidence of their limitations. A recent study has shown that optical methods such as confocal Raman spectroscopy (CRS) provide more sensitivity and a step-change in information about SC water content; this is particularly useful when measuring thinner SC areas such as the face.² Results from different methods cannot be directly compared. There are a vast number of uncontrolled individual, instrumental and external variables that influence these measurements.¹⁰ For example, the CRS data does not correlate with corresponding capacitance values.² Also, discrepancies between clinical severity of skin dryness and capacitance measurements previously have been reported.²

The quality of SC barrier function is assessed by measuring transepidermal water loss (TEWL).¹¹ An increase in TEWL is a marker of barrier disruption and skin damage that directly impacts SC hydration;¹² higher TEWL means the skin barrier is inadequate and the water loss is high, resulting in low skin hydration. Therefore, TEWL measurements should be considered when determining the efficacy of a moisturizer.¹³

Establishing Product Efficacy

Recent research has shown that some moisturizers may be detrimental to the skin’s barrier function by affecting the genes involved in keratinocyte differentiation and skin lipid formation, thereby altering TEWL and inducing a dysfunctional skin barrier.^14–16 The performance of commercially available moisturizers was examined in vivo in a long-term, blinded and randomized clinical study. It demonstrated that moisturizing efficacy depended on the mechanisms by which moisturizing technologies enhance the barrier function of multiple SC targets to consider the immediate, medium- and long-term effects beyond the regression time point.²

Adapting formulations to different levels of moisturization is not a novel concept, yet a true understanding of the impact that specific technologies have on SC biology is in its infancy. Moisturizers are most often emulsions such as multiphase systems comprised of hydrophilic or lipophilic phases in which one phase contains droplets of the other. The formulator’s choice of emulsification system depends on the technologies used in the formulation, and the aesthetics of the formulation drive the consumer-perceived benefits.⁹

Moisturizing technologies have been shown to influence SC thickness.² Lipophilic (w/o) moisturizers increase SC thickness, whereas hydrophilic (o/w) moisturizers reduce SC thickness.¹⁷ This is most likely due to osmotic effects of high concentrations of some humectants or activation of SC proteases that result in more efficient desquamation.² Hydrophilic emulsions are more common in skin care⁵ due to consumer aesthetic preferences. Formulating with NMF derivatives such as niacin in both lipophilic and hydrophilic forms increases SC thickness.^{2, 18} TEWL would be expected to decrease with a thicker SC due to the corresponding increase in tortuosity,² but the link between increased skin hydration and TEWL is not always clear.⁵

Most moisturizers alleviate visible dry skin conditions but external occlusive, emollient and humectant technologies alone offer only a temporarily solution.⁸ Only moisturizers containing NMF-based technologies act via internal occlusion and improve the barrier function by inducing keratinocyte differentiation and increased ceramide synthesis.² Also, the introduction of skin-identical lipid technologies such as phytosphingosine–containing ceramides⁶ promotes skin barrier recovery if used in the correct ratios.¹⁹

Facial Skin Care

When developing moisturizers for facial skin, formulators must consider that the anatomy of the face is unique, complex and continuously exposed to environmental factors.²⁰ The face has distinct zones with different clinical needs for hydration. It has the thinnest SC, particularly around the eyes.²¹ Michael Cork et al. introduced the concept of a low barrier reserve in the face in 2006. This concept reflects that facial skin theoretically has a lower resistance to mechanical or chemical insult due to its thinness.²²

In terms of facial skin care, it is important to formulate moisturizing products that repair the lipid phase in the SC barrier and promote its maturity.²¹ When adapting formulations to different moisturization levels, many individual factors such as skin condition, skin type, age group, ethnicity and season as well as external factors such as method of measurement, type of equipment, anatomic site, resting conditions, climate and sample size must be taken into consideration.

The link between TEWL and age is debated but, in general, the strength of the skin barrier improves with age. Also, facial skin hydration has overall been found to decrease with age, which is supported by the aging population’s need for richer moisturizers. Facial zones have been mapped recently but hydration, measured by capacitance, shows large regional variations across the face.²³ The nasolabial and perioral areas, in particular, show the weakest SC barrier and low hydration in all age groups. The cheeks and the eye region have a strong barrier function, and although the eyelid SC is thin, it is well-hydrated.

There is not yet a consensus within the scientific community as to what constitutes dry or oily skin types in order to develop objective and broadly accepted definitions²⁴ and research into facial zones has not taken skin type into consideration. Facial sebum defines skin oiliness and is an important parameter in considering skin type, but dry skin should not be viewed as the opposite of oily skin; the two are unrelated.²⁵ Further studies are required to elucidate the specific clinical needs of different skin types and targeted facial areas.

Efficacy Measurement on Facial Skin

The efficacy testing of facial moisturizers should be assessed on the face rather than the volar forearm, where it typically is conducted. It is important to note that the face and typical volar forearm testing sites show different biological rhythms, i.e. daily physiological changes that have a reproducible waveform.²⁶ The TEWL on the cheeks shows a bimodal circadian (daily) rhythm with a peak in the morning and late afternoon; this is in agreement with clinical observations that the skin is more reactive in the late afternoon.²⁷ The daily variation in skin hydration prevents conclusions from being drawn.²⁸ Also, internal factors such as hormonal status and stress impact the integrity of the SC barrier, and ingested substances such as caffeinated drinks alter skin hydration due to the pharmacological effect of caffeine.^29–31

Conclusion

Consumers consider the condition of their skin of vital importance in defining their attractiveness, and recent research indicates that some parts of the face, i.e. the forehead and the eye region, influence the perception of age and attractiveness more strongly than others. Even small changes in skin surface topography are detectable by the human eye.³²

Consumers are ultimately interested in both visual and instrumental validation of product efficacy including the subjective opinion of trial participants, the clinical opinion of dermatologists, and an objective scientific measurement. Product efficacy should be substantiated with scientific data such as long-term, in vivo, controlled, randomized clinical trials, which is meaningful in everyday life. Some larger corporations have led the way in substantiating product efficacy as a part of a product regimen with benchmark studies that show reduced wrinkles with improved SC thickness and hydration.³³

Well-formulated moisturizers increase skin hydration and repair the barrier function by inducing functional changes in the SC. It is the combination of different moisturizing technologies and their specific mechanisms of action that work on multiple SC targets and determine product efficacy.8 Thus, adapting skin care formulas to different levels of moisturization also requires a detailed understanding of clinical needs and relevant biological targets of the intended customer group. Well-designed, long-term efficacy studies are required to further the industry’s understanding of this interesting field.

References

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