Dr Daniel Chang explains the function of the epidermal barrier and provides an overview of how it is affected by aesthetic treatment
In this paper, I will review existing aesthetic treatments in relation to their effect on the epidermal barrier. Three main aesthetic treatment types will be considered: topical, energy-based and needle-based.
The epidermis is the outer layer of our skin. It consists of keratinocytes, melanocytes, Langerhan cells and corneocytes.
Langerhan cells are present in all layers of the epidermis except the stratum corneum. The stratum corneum, which is the outermost layer of the epidermis, primarily forms the epidermal barrier. It is made up of anucleated keratinocytes, known as corneocytes, and inter-corneocyte lipids. These lipids are synthesised by keratinocytes and are under strict regulation from subcellular processes, hence facilitating barrier requirements. The ratio of ceramides, cholesterol and fatty acids – of which an ideal ratio is yet to be fully established – in an equimolecular distribution, contributes to optimal barrier function.1,2
However, the epidermal barrier changes as a result of age, environmental and physiological factors. For example, low humidity reduces desquamation of corneocytes and improves barrier competence, while increased stress raises cortisol levels and disrupts the barrier.1,2
The key barrier function, on a macroscopic level, consists of the following actions:
At a subcellular level, functions of the epidermal barrier include:
The barrier maintains itself and achieves barrier homeostasis, which is attained by regulating corneocyte growth and desquamation, lipid production and cellular processes.9
So, what are the effects of the three main aesthetic treatment types on the epidermal barrier?
According to the Oxford Dictionary, the term cosmeceutical can be defined as ‘a cosmetic that has or is claimed to have medicinal properties’.3 The word itself is not officially recognised by the FDA4 or the Medicines and Healthcare product Regulatory Agency (MHRA).5 Cosmeceuticals serve two main functions with respect to the epidermal barrier. Firstly, to enhance delivery through the epidermal barrier to reach deeper layers to repair and maintain, and secondly, to protect the epidermal barrier and skin from external pollutants.
Types of cosmeceuticals include the following:
Moisturisers: A moisturiser aims to augment the function of epidermal lipids, hence enhancing barrier properties. For example, moisturisers containing ceramide help to repair and maintain the epidermal barrier. Glycerol and urea are humectants which work by absorbing and retaining moisture, hence reducing water loss.6
Sunscreen: These can be divided into physical and chemical agents. The delivery of sunscreen has evolved, and currently, many manufacturers now use micro-ionised nanoparticles, hence enhancing penetration.27 Sunscreens limit the free radical damage from UV rays and serve to protect the epidermal barrier and underlying skin. Topical green tea extract or epigallocatechin gallate (EGCG), has a vital role in protecting cells from free radical damage. Green tea extract, useful as an ingredient in sunscreen, can increase the SPF of the sunscreen.7
A formula of notable benefit is vitamin C, E and ferulic acid. Ferulic acid helps stabilise the vitamin derivatives, hence improving absorption
Antioxidants: Antioxidants play a significant role in anti-inflammation and antiageing.10,28 Vitamin C is a powerful antioxidant, but is water soluble, hence has difficulty penetrating the stratum corneum, so needs to be delivered via other means. Vitamin E is a lipid antioxidant and it protects the lipid environment by removing lipid-free radicals. This reduces the damage to the epidermal barrier and underlying skin. A formula of notable benefit is vitamin C, E and ferulic acid. Ferulic acid helps stabilise the vitamin derivatives, hence improving absorption.29 The use of nanoparticles has also improved the delivery of antioxidants.39,34
It is important to highlight key vehicles in the delivery of cosmeceuticals in order to identify the optimum delivery tool. There are studies currently looking at measures to enhance delivery whilst keeping the epidermal barrier intact.28,41,42
Delivery enhancers can be divided into solvents, delivery vectors and delivery devices.26,35,36
Chemical peels consist of superficial, medium and deep peels, in association to their depth of penetration. Peels work by inducing epidermolysis, through reducing corneocyte adhesion, reducing desmosomes, increasing desquamation and coagulation of epidermal proteins. All of these factors help to improve the overall penetration of the ingredients through the epidermal barrier.30
Beta hydroxy acid (BHA) peels, such as salicylic acid and tropic acid, have an additional comedolytic effect due to their lipophilic quality, enhancing their action on sebaceous glands, hence controlling acne
In addition, at a histological level, peels, for example alpha hydroxy acid (AHA) peels, can increase dermal thickness by 25% through speeding up keratinocyte renewal.13 Some examples of AHA peels include glycolic, lactic, oxalic and malic. Beta hydroxy acid (BHA) peels, such as salicylic acid and tropic acid, have an additional comedolytic effect due to their lipophilic quality, enhancing their action on sebaceous glands, hence controlling acne.12,13
2. Energy-based devices
Laser treatment is based on the principle of selective photothermolysis. This is when the target skin chromophore absorbs photons, leading to its destruction by thermal energy.
Lasers breach the epidermal barrier and reach the correct depth of penetration through varying the following factors:31
Lasers can be classified under two broad categories, namely ablative and non-ablative.
Ablative treatment is in the form of CO2 and Erbium-YAG lasers. It involves the use of electromagnetic energy to induce damage to the entire epidermis and a portion of the upper dermis – the chromophore here is water. Their function lies in skin resurfacing. CO2 works at a 10600 nm wavelength, which targets water both intra- and extracellularly.
Erbium-YAG works on a 2940 nm wavelength, targeting water both intra- and extracellularly, but because it is more selective for water, its energy is mainly confined to the epidermis and superficial papillary dermis, hence there is less depth of penetration compared to CO2, so side effects are reduced, but peak efficacy is less rapid than the CO2 machine.
A subset of ablative lasers is fractional ablative lasers, namely the fractional CO2 and fractional Erbium lasers. They work through inducing microthermal zones, with healthy skin cells inbetween, therefore promoting faster healing and recovery. The potential for improvement in skin resurfacing is dependent on the depth and extent of tissue damage.14,15,16,17
Non-ablative resurfacing treatments target structures in the underlying dermis, while protecting the overlying epidermis. This is achieved through varying the wavelength, energy, pulse duration and spot size. Some examples include Q-switched Nd:YAG laser, intense pulsed light (IPL) and light-emitting diode (LED).
Q-switched Nd:YAG and IPL can be used to treat pigmentation, pores and acne and, to a small degree, skin texture. Q-switched Nd:YAG 1064 nm lasers, through a process of selective photothermolysis, target the chromophore and melanin in the epidermis, hence reducing damage to the overlying epidermis.12 Long pulsed 1064 nm Nd:YAG lasers, through increasing pulse duration, protect the epidermis, while targeting chromophores, water and oxyhaemoglobin in the dermis. They work to stimulate collagen and reduce vascularity in deeper vessels.12
IPL via a broad-spectrum wavelength of 515 nm to 1200 nm is based on the principle of selective photothermolysis, selectively targeting melanin and haemoglobin in the superficial skin layer.11-14
3. Needle-based treatments
Botulinum toxin, dermal fillers and microneedling make use of a needle to breach the epidermal barrier.
Knowing that the epidermis has varying depths throughout the face, for example 0.05mm over the eyelids and 0.3-1mm for the rest of the face,25 when performing needle-based treatments, the right needle depth is vital to ensure treatment efficacy.
The use of microneedles supports the intradermal injection and deposition of the treatment in the dermis. The physical nature of the needle helps breach the epidermal barrier
When injecting botulinum toxin, depth of injection is important in order to target the right area. This is due to the fact that injections need to be intramuscular for the forehead and glabellar region and intradermal for the crow’s feet to achieve optimum results.24
When using a device for microneedling, the average injection depth is 1mm, but individual variations exist. However, due to presence of needle bevel, actual injection depth is less than 1mm.23
Of note are skinboosters, a form of combination therapy, which are hyaluronic acid fillers delivered through a microneedling technique to achieve skin hydration and healing. The use of microneedles supports the intradermal injection and deposition of the treatment in the dermis. The physical nature of the needle helps breach the epidermal barrier.
It is postulated that microneedling and the deposition of hyaluronic acid can stimulate fibroblasts to increase collagen stimulation, hence improving skin thickness and texture.22
The current trend for skin rejuvenation is moving towards the use of combination therapies, which could potentially enhance the delivery process through the epidermis and improve results. Here are some demonstrable combination therapies:Microdermabrasion with AHA chemical peel: This is used to increase treatment efficacy. AHA peels reduce corneocyte adhesion, hence improving the abrasion.19
Laser and IPL: Laser, by virtue of its longer wavelength, targets deeper layers of the skin, and coupled with IPL, which works on a more superficial layer, enables a more efficient skin rejuvenation.20
Fractional laser before Q Switched ND Yag laser: It is postulated that fractional lasers, through inducing microthermal zones, form channels for the broken-down products of melanin to be eliminated through.21
Laser-assisted delivery of cosmeceuticals: Fractional laser, through its action on ablating tissue, can bypass the epidermal barrier and enhance delivery of cosmeceuticals.32
Ultrasound energy: Ultrasound through formation of temporary cavitation, can improve delivery of cosmeceuticals.33
Platelet rich plasma (PRP) is gaining more evidence in its effectiveness in skin healing. A Korean study has indicated that topically-applied PRP can help fibroblast proliferation and collagen production to boost skin elasticity and healing post-fractional laser. Results noted increased subject satisfaction and skin elasticity, as well as a decrease on the erythema index. PRP increased the length of the dermoepidermal junction, the amount of collagen, and the number of fibroblasts.18
Presently, there is no standardised way to deliver PRP into the skin. More research through robust clinical trials needs to be conducted to validate its efficacy in the arena of antiageing and regenerative medicine.
The effectiveness of aesthetic treatment on the epidermal barrier lies in the penetration through the stratum corneum.
Further research is needed to identify the most effective types of combination treatments, the sequence, timing, and number of sessions required for optimal results. Longer follow-up time and larger sample size clinical studies would help to answer these questions.