Moisturizer Types	Functions	Ingredients
Occlusive	Trap water on the skin surface and enhance lipid component of epidermal barrier.	Oils (petrolatum, mineral oil) Cholesterol, squalene Waxes (beeswax, carnauba wax, liquid wax) Silicones (dimethicone, cyclomethicone) Stearic acid, cetyl alcohol, stearyl alcohol Lanolin, lanolin alcohol
Humectant	Attract water from the dermis.	Glycerin Urea Hyaluronic acid Sorbitol Propylene glycol
Emollient	Smooth roughened epidermis.	C12-15 alkyl benzoate Cetyl stearate Glyceryl stearate Octyl octanoate Decyl oleate Isostearyl alcohol

Moisturizers are formulated in oil-in-water emulsions as creams or lotions, and their heaviness depends on the relative amounts of oil and water. Ointments contain nearly all oil, creams contain more oil relative to water, and lotions contain more water relative to oil. Creams and lotions tend to be less irritating and drying than gels and solutions. The following list shows product formulations from most to least hydrating:

• Ointment (most hydrating)

• Cream

• Lotion

• Gel

• Solution (least hydrating).

Barrier repair creams are a newer class of moisturizer that contain occlusive and humectant ingredients at ratios that closely mimic the content of normal, healthy stratum corneum: ceramides (50%), fatty acids (30%), and cholesterol (30%). They enhance barrier function, reduce transepidermal water loss, and increase the skin’s resistance to environmental irritants.⁹ These products may be used by patients with compromised barriers, such as those exposed to cleansers and chemicals, and conditions such as atopic dermatitis. Barrier repair products include nonprescription CeraVe^® and TriCeram^®, and prescription Mimyx^®, Atopiclair^®, and EpiCeram^®.

Dry skin may also be associated with hyperkeratosis as seen with ichthyosis, keratosis pilaris, and callouses. Topical hydroxy acids such as lactic acid 5% to 12% (e.g., generic ammonium lactate available in the brand names of Lac-Hydrin^® and AmLactin^®) and urea 10% to 40% (e.g., Carmol^® and Vanamide^®) are keratolytic and function to reduce stratum corneum thickness and increase hydration.¹⁰

Exfoliation treatments, which superficially resurface the skin by removing the topmost layers of the epidermis, can also be used to treat dry skin. Exfoliation stimulates the keratinocyte maturation process with regeneration of a healthier epidermis. This results in a compacted stratum corneum and increased production of ceramides, which improves epidermal barrier function and skin hydration.^11,¹² Exfoliation, where the outermost layers of skin are removed, can be achieved using mechanical methods (e.g., microdermabrasion) with gentle abrasion, or using chemical methods (e.g., chemical peels) with application of topical products such as acids (see Chapter 23, Microdermabrasion, and Chapter 22, Chemical Peels, respectively).

Photoaging

Most of the changes seen with skin aging are due to years of cumulative UV exposure, referred to as photoaging.^13,¹⁴ Skin is affected by ultraviolet A radiation (UVA, 320 to 400 nm), long-wavelength solar radiation that penetrates into the dermis, and ultraviolet B (UVB, 290 to 320 nm), shorter wavelength radiation that is absorbed by the epidermis.

Skin Rejuvenation Products

Skin rejuvenation products are aimed at preventing and treating cosmetic issues associated with photoaging, which include textural changes, dyschromia, and vascular ectasias. A simple three-part regime of topical products can be used to cleanse, treat, and protect the skin, whereby each product provides a specific benefit.

Products used for the treatment of photoaging must address several aspects of the aging process, including increasing skin exfoliation and stimulating epidermal renewal, increasing synthesis of collagen and other dermal matrix components, reducing hyperpigmentation, and increasing hydration. Products used for skin protection are aimed at preventing damage from UV exposure and free-radical oxidation. Skin care regimes usually consist of multiple topical products, including prescription medications and cosmeceuticals, to address these various aspects of skin aging. Table 24-2 lists topical products commonly used for skin rejuvenation and their functions.¹⁵

TABLE 24-2 Skin Rejuvenation Products and Their Functions ¹⁶^–¹⁹

CLEANSE
Cleanser selected based on skin oiliness/dryness
TREAT
Textural changes Fines lines Rough skin	Retinoids, hydroxy acids, N-acetyl glucosamine, growth factors, kinetin, peptides, moisturizers
Dyschromia Solar lentigines Postinflammatory hyperpigmentation	Retinoids, hydroquinone, kojic acid, azelaic acid, arbutin, and others (Tables 24-3 and 24-4)
Vascular ectasias Telangiectasias Rosacea	Bisoprolol, allantoin, aloe, algae extract, borage and evening primrose oil, vitamin C, chamomile, green tea extracts
PROTECT
Antioxidants	Vitamin C, vitamin E, ferulic acid, coffee berry, idebenone, gluconolactone, polyhydroxy acids, resveratrol, tea and red wine extracts
Sunscreens	Zinc oxide, titanium dioxide, and others (Figure 24-1)

Prevention of Photoaging

Antioxidants

UV light generates free radicals in skin which oxidize nucleic acids, proteins, and lipids, leading to the development of skin cancers and signs of photoaging.¹³ The skin protects itself by preventing free-radical damage with endogenous antioxidants consisting primarily of vitamin C in the aqueous compartment, and vitamin E in the lipid compartment such as cell membranes and stratum corneum. Vitamins C and E can be applied topically to enhance antioxidant protection.^16,¹⁷ These two antioxidants act synergistically. Ferulic acid is another antioxidant that can improve the protective effects of vitamins E and C.¹⁷ In addition to antioxidant properties, vitamin C also directly increases collagen synthesis and clinically reduces fine lines.²⁰ The greatest percutaneous absorption of topical vitamin C is obtained with L-ascorbic acid in acidic formulations at pH 3.5 or less, with concentrations of 10% to 20%.¹⁷

Sunscreens

Sunscreens protect the skin by reducing UV exposure. There are two main types of sunscreen ingredients each of which has a different mechanism of action. Physical sunscreens, composed of inorganic compounds, reflect and scatter UV light, whereas chemical sunscreens, composed of organic sunscreens, absorb UV light.¹⁸ To achieve broad-spectrum UVA and UVB coverage, multiple sunscreens are often combined. Figure 24-1 shows common sunscreen ingredients and their UV blocking capabilities. Certain sunscreen combinations offer greater photostability. For example, avobenzone alone has poor photostability; however, the addition of oxybenzone (HelioPlex^®) renders it highly photostable. Sunscreens with broad-spectrum UVA and UVB coverage with an SPF 30 are generally recommended for daily use. Because sunscreens do not block all UV radiation, maximal protection from UV damage is provided by using an antioxidant in addition to a sunscreen.¹⁷

FIGURE 24-1 Sunscreens and their ultraviolet light blocking capabilities.

SPF stands for sun protection factor (soon to be renamed as sunburn protection factor by the FDA) and represents the ability of a sunscreen to delay erythema on sun-exposed skin, primarily by blocking UVB. For example, if erythema develops after 10 minutes of sun exposure in a given individual with unprotected skin, after application of an SPF 30 sunscreen, the individual will develop the same degree of erythema in 10 × 30 or 300 minutes (5 hours) of sun exposure. The standard SPF testing dose for sunscreens on human skin, and the basis for a sunscreen product’s SPF designation, is 2 mg/cm², regardless of the formulation (cream, lotion, spray, powder, makeup, etc.), which equates to tsp for the face and 1 oz for the body of cream sunscreens.

Treatment of Photoaging

Retinoids

Topical retinoids are used to treat virtually all cosmetic aspects of photoaged skin, including: fine lines, dry and rough skin, hyperpigmentation, as well as acne.^19,^21,²² Retinoids are vitamin A derivatives, which range from potent prescription products such as tretinoin and synthetic tretinoin derivatives such as tazarotene, to less active cosmeceutical products such as retinol (Figure 24-2). Two topical retinoid products are FDA approved to treat photoaging: Renova^® (tretinoin) and Avage^® (tazarotene). Several other topical prescription retinoids approved to treat acne are also used off-label to treat photoaging such as Retin-A^® (tretinoin), Retin-A^® Micro (a less irritating formulation of tretinoin), Differin^® (adapalene), and other generic versions of these.

FIGURE 24-2 Topical retinoids.

Prescription retinoids have an associated local skin reaction of erythema, sensitivity, peeling, and flaking, called a retinoid dermatitis, which usually occurs within a week of initiating therapy. While this irritative response spontaneously resolves for most patients in 4 to 6 weeks, it can persist for the duration of use. The rejuvenating effects of prescription retinoids are more rapid than cosmeceuticals. Results are usually apparent by 1 month, with improved texture due to compaction of the stratum corneum, and by 3 months reduction of hyperpigmentation and fine lines are evident. Combining prescription retinoids with other aesthetic procedures such as lasers and chemical peels can be challenging, because retinoids must be discontinued 1 week prior to treatment and restarted 2 weeks after these treatments. This repeated initiation and termination of therapy can cyclically induce a retinoid dermatitis. For this reason, some providers use prescription retinoids as a stand-alone intervention for photoaged skin. Prescription retinoids are a particularly useful treatment option for skin rejuvenation in patients with darker Fitzpatrick skin types (IV through VI), where more aggressive procedures may be contraindicated.

Cosmeceutical retinoids include retinol (vitamin A), retinal (vitamin A aldehyde), and retinol storage forms (retinyl acetate and retinyl palmitate).²³ They are less irritating than prescription retinoids and, when used in recommended doses, do not cause a retinoid dermatitis. However, they are much less effective than prescription retinoids in reducing the signs of photoaging.

Matrix Metalloproteinase Inhibitors

Matrix metalloproteinases (MMPs) are naturally occurring enzymes that degrade the skin’s extracellular matrix to facilitate recycling of collagen, elastin, and glycoaminoglycans.²⁴ In young, healthy skin, production of matrix components exceeds destruction caused by MMPs. In photoaged skin, matrix degradation is accelerated because UV light upregulates MMPs. Enhanced matrix degradation combined with age-related decline in collagen synthesis, decreases the structural integrity of the skin’s matrix and contributes to the formation of wrinkles, skin laxity, and telangiectasias.²⁵ Products that inhibit MMPs, such as zinc, calcium, idebenone, and lactobionic acid (polyhydroxy acid) reduce destruction of dermal matrix, increase dermal collagen and elastin, and clinically improve textural changes in photoaged skin.^26,²⁷

Human-Derived Growth Factors

Endogenous growth factors are regulatory proteins that initiate wound healing by mediating signaling pathways within and between cells.²⁸ Multiple growth factors migrate to wounds and interact synergistically to promote wound healing and tissue regeneration including synthesis of new dermal matrix components. Topical application of growth factor products, such as Nouricel-MD^®, which is derived from human fibroblast cultures, twice daily for 3 months has been shown to improve skin hydration and reduce roughness, hyperpigmentation, and wrinkles in photoaged skin.^29,³⁰ Growth factors are large protein molecules and their size may limit penetration through an intact epidermis. Combining them with skin resurfacing procedures that thin or disrupt the skin’s barrier, such as laser resurfacing, chemical peels, and microdermabrasion, or topical exfoliants such as hydroxy acids, may improve penetration of these active proteins.³¹