Introduction

The hands and feet are prone to dryness and impaired barrier function because of their unique functional roles, predisposing the skin to heightened irritant sensitivity and the development of dermatoses. Protective and regenerative moisturizing skin care is the foundation for averting and treating dry skin‐associated skin diseases and disorders.

Effective therapeutic moisturizers provide occlusive lipophilic substances that act as protectants and barrier replenishers, hydrophilic agents that function as humectants to bind and hold water, and may include other ingredients to address symptoms of specific skin disorders (e.g. eczema). The importance of urea as the most commonly prescribed and multifunctional physiologic humectant and Natural Moisturizing Factor is discussed. Application of moisturizers containing urea is shown to increase its concentration in the epidermis, exert ultrastructural changes in the stratum corneum, hydrate severely compromised skin, and support and enhance barrier function by modulating gene expression. In addition, the role of aquaporins (AQP) and the underlying mechanisms of moisture homeostasis of the skin are discussed vis‐à‐vis new opportunities to create better actives and product formulations which can help regulate moisturization from within the skin.

Moisturization needs of the hand and foot

Skin of the hands and feet is different from other body sites. In particular, skin on the palms and soles is thicker and has a high density of eccrine sweat glands; however, it lacks apocrine glands. These sites are highly innervated and involved in most of the daily activities of life. Repetitive use of the hands and feet accompanied by pressure and friction can promote the formation of areas of thickened keratinized skin or calluses, which can crack and fissure. Site‐specific requirements for hygienic care and diseases common to these sites have been described [1]. In addition, the hands and feet have special skincare needs for efficacious moisturization as well as unique requirements for formulations that are aesthetically appropriate and compatible with their special sensory and functional roles and needs.

Hand skin is particularly susceptible to xerosis and dermatitis. Constant use of the hands, frequent washing, exposure to alcoholic sanitizers, and environmental, chemical, and irritant exposure as well as environmental factors (e.g. low relative humidity) can provoke these problems. Further, because the hands are especially prone to injury and exposure to irritants and pathogens, specific skin protectant formulations can be highly beneficial to prevent and treat irritation or occupational dermatoses such as hand eczema [2]. Recently, a simple 3‐step approach has been proposed as an effective prevention strategy for irritant contact dermatitis in the workplace [3].

While the feet may be less likely to suffer from deleterious occupational exposures, environmental factors can have an impact on the moisture status of the foot skin. Cold, dry weather in winter, bare feet in summer, and the confinement of shoes can compromise the hydration state. Occlusive shoes and socks can also trap moisture and render the foot susceptible to microbial infections, especially from fungus, damaging the barrier function and dehydrating the skin. In addition, certain metabolic diseases can impact circulation and innervation of the extremities, which in turn affects skin hydration. In particular, reduced circulation and eccrine sweat gland activity in diabetics cause severe xerosis which can spiral into other severe foot problems.

Protective and regenerative moisturizing skin care is the foundation for treating all dry skin‐associated skin diseases and disorders. While the underlying cause of dry skin in any specific skin disorder needs to be addressed, frequently the symptomatic control of severe xerosis by appropriate moisturizers may reduce the need for more potent treatments, such as prolonged use of topical steroids and immune modulators, which can have detrimental side effects.

Moisturizing creams containing urea have been reported to improve the physical and chemical nature of the skin surface, with the manifest benefits of smoothing, softening, and making dry skin more pliable [4]. Traditional moisturizing emulsions have utilized nonphysiologic emollients, humectants, and skin protectants to rehydrate the skin and reduce moisture loss. The identification and understanding of the structure and function of the stratum corneum barrier lipids and the role of water‐binding physiologic substances, collectively referred to as the natural moisturizing factor (NMF), has led to the development of topical formulations enriched in these actives. Recent studies have verified their influence in moisture homeostasis beyond direct effects on water retention. Expanding knowledge of the impact of NMF components, especially urea, and the role of AQP, water regulating channels in skin cells, provide the opportunity for therapeutic moisturization technologies based on the skin’s innate hydration system.

Moisturizing formulations and technologies

For thousands of years, oils, animal and vegetable fats, waxes, and butters have been used to moisturize the skin. Recognized for their emollient or skin smoothing and softening properties, these substances were used as occlusive agents to retard moisture loss and help restore dry skin to a more normal skin condition. The first significant advancement from these simple moisturizers occurred over a hundred years ago when emulsifiers were developed to create the first stable water‐in‐oil (W/O) cream emulsions [5].

A simple emulsion can be defined as a heterogeneous system that contains very small droplets of an immiscible (or slightly miscible) liquid dispersed in another type of liquid. These emulsions consist of a hydrophilic (water loving) and a lipophilic (oil loving) portion, either of which can make up the external or internal phases of the emulsion system. The external phase generally comprises the majority of the emulsion while the smaller internal phase consists of the dispersed droplets. Most commonly used moisturizer formulations are either oil‐in‐water (O/W) emulsion systems, where aqueous components predominate, or W/O, where the majority of ingredients are nonaqueous.

Emulsifiers are necessary components of emulsion systems as water‐soluble and oil‐soluble ingredients are not miscible. Emulsifiers are surface‐active agents that reduce the interfacial tension between the two incompatible phases to create stable cream and lotion emulsion systems. The properties of the chosen emulsifiers determine the final emulsion type.

Major progress in recent decades has enabled the formulation of increasingly complex emulsions (e.g. water‐in‐oil‐in‐water emulsions, multilamellar emulsions), which combine and stabilize many incompatible ingredients for moisturizing products with unique delivery characteristics that are both highly effective and aesthetically pleasing [6, 7]. However, it is beyond the scope of this chapter to discuss the multitude of emulsion technologies which have been developed since the advent of the simple W/O system [8].

Occlusive materials and humectants are two major classes of moisturizing ingredients in many current moisturizers (Table 19.1). Occlusive materials coat the stratum corneum to inhibit transepidermal water loss (TEWL). In addition, cholesterol, ceramides, and some essential and nonessential free fatty acids present in oils can help to replenish the natural lamellar barrier lipids that comprise the envelope surrounding the corneocytes in the stratum corneum, fortifying the barrier function of the skin. Some common examples of occlusive materials are petrolatum, olive oil, mineral oil, soybean oil, lanolin, beeswax, and jojoba oil. Petrolatum, lanolin, and mineral oil are considered occlusive materials, yet they also serve as emollients on the skin [9, 10].

Humectants are materials that are capable of absorbing high amounts of water from the atmosphere and from the epidermis, drawing water into the stratum corneum for a smoother skin feel and look. Examples of well‐known humectants include glycerin (or glycerol), sorbitol, urea, sodium hyaluronate, and propylene glycol. Glycerin is a widely used humectant with strong water binding capacity and holding ability, making it ideal for dry skin moisturizing formulations. Because of its importance in moisturizing products, and its ability to create a moisture reservoir effect in skin, it has been extensively reviewed elsewhere [6, 11].

A number of commercially available hand and foot moisturizers incorporate combinations of both humectants and occlusive materials to deliver the optimal skin benefits (Table 19.2).

Natural moisturizing factor

The NMF are a collection of hygroscopic substances in the skin that act synergistically to confer effective water‐binding and retention properties to the stratum corneum. The NMF is composed of approximately 40% amino acids, 12% pyrrolidone carboxylic acid, 12% lactates, 7% urea, 18% minerals, and sugars, organic acids, citrates, and peptides [12]. These substances, derived from the catabolism of proteins in sweat and also largely from the breakdown products of the insoluble protein filaggrin, have an important role in maintaining moisture in the nonviable layers of the epidermis. Discovery of loss‐of‐function mutations in the filaggrin gene in many individuals with xerotic skin disorders including atopic dermatitis and psoriasis and associated diminished levels of the NMF have confirmed the critical importance of filaggrin processing and these humectant substances in maintaining skin hydration and plasticity [13]. Due to the moisture gradient that exists from the well‐hydrated dermis to the relatively moisture‐deprived stratum corneum, the cutaneous moisturization state is highly influenced by the occlusive barrier lipids in the stratum corneum and the humectant properties of the NMF [14]. Both are critical to retain moisture and resist TEWL and the dehydrating effects of the environment. Therefore, qualitative or quantitative changes in either the barrier lipids or the NMF components can alter skin hydration.

Urea is an end‐product of protein metabolism and major constituent of the water‐soluble fraction of the stratum corneum [14], largely from the breakdown of filaggrin (Figure 19.1) [13]. Because of the high water‐binding capacity of urea, the water content in the skin depends on its concentration. In dry skin and in keratinization disorders, a deficit of urea is often found in the stratum corneum, confirming its importance in skin moisture balance. The concentration of urea has been reported to be reduced by approximately 50% in clinically dry skin compared to healthy skin and diminished with age [15, 16]. The stratum corneum of unaffected psoriatic skin reveals no deficit in urea content, but levels in psoriatic lesions are reduced by 40% [17]. However, in patients with atopic dermatitis, there is a deficit of about 70% in unaffected skin and about 85% in involved skin [18]. Urea has been demonstrated to be an effective moisturizer for a range of dry skin conditions [19], especially xerosis of the elderly [20, 21].

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Nov 13, 2022 | Posted by admin in Dermatology | Comments Off on 19: Hand and Foot Moisturizers

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Key ingredients		Functions and claims
		Hand cream	Foot cream
I	Glycolic acid, mineral oil, petrolatum	Exfoliation and moisturization by “occlusives” to both smooth and soften skin	Exfoliation and moisturization by “occlusives” to both smooth and soften skin
I	Glycerin, shea butter, almond oil, olive oil	Moisturization of hands and softening of cuticles	Moisturizes, soothes, and protects dry, cracked, and callused heels
III	Caprylic/capric triglycerides, glycerin, sunflower oil, olive oil, almond oil	Moisturization of hands, nails, and cuticles	Soothes and heals severely dry, cracked heels
IV	Beeswax, sweet almond oil	Moisturizes and softens dry skin	Prevents and heals cracked heels, calluses, corns, blisters
V	Lanolin, allantoin, glycerin, sunscreens: avobenzone, octinoxate	Moisturizes skin and helps treat the signs of aging
VI	Glycerin, petrolatum, dimethicone, mineral oil	Helps form a protective moisture barrier; heals and protects dry hands with 24‐hour moisturization