This article was originally published in the the March 2007 issue of the South African Aesthetic Review journal by John Knowlton, who is a cosmetic scientist.
It was not so long ago that cosmetic skin care products, and the miraculous promises that they made, were perceived as nothing more than marketing hype, with the benefits actually being delivered ranking closely alongside “The Emperor’s New Clothes”. Now, firmly planted in the twenty-first century, this perception is changing radically, and rightly so. Recent advances in cosmetology, biotechnology and skin science has made the existent of topically applied skin care products that have meaningful and sustainable benefits on skin, become a reality. Contemporary skin care products must be multifaceted in order to obtain consumer acceptance and the desire to purchase. Efficacy, safety, cosmetic elegance and superior in-use characteristics all make vital contributions to products which will be successful in a very demanding marketplace.
Against this backdrop, what are the design parameters for a contemporary skin care cream, the ingredients it is composed of and the functions of each of them in the final product? Fundamentally, cosmetic skin care creams are emulsions, which can be subcategorised into two distinct types, oil-in-water creams and water-in-oil creams. More complex, multiphase emulsions do exist but these are reserved for special designs of cosmetic skin care creams, where the need to separate otherwise incompatible hydrophiles and lipophiles in the same formulation exist. The majority of commercially available skin care creams are of oil-in-water design, principally because the aesthetics of such products from a consumer perception are significantly superior. Arguments that water-in-oil emulsions are more effective in delivering lipophilic actives into the skin have some merit but the inherent oiliness of products in this category invariably compromise their cosmetic elegance. One of the most important ingredients in a skin care cream is the emulsifier used to stabilise the final product. Emulsifier choice not only determines whether the cream is oil-in-water or water-in-oil but also contributes significantly to the texture and absorbency characteristics of the product on skin. Historically, now antiquated emulsifier systems such as beeswax/borax and sodium soaps were frequently utilised.
Modern cosmetic creams have abandoned such systems in favour of nonionic or polymeric emulsifiers, which are far more flexible in use and have an excellent record of safety and skin compatibility. There are literally thousands of such materials available and the final choice will depend upon many factors. Amongst the important group of nonionic emulsifiers, there is a very wide range of ethoxylated and non-ethoxylated materials available, including sorbitan esters, fatty alcohols, fatty amine ethoxylates, fatty acid ethoxylates and their esters, alkyl phenyl ethoxylates and many more. Almost invariably, two or more of these emulsifiers are used in the same formulation to optimise the stability of the final product. Further advantages of such synergistic combinations include formulation flexibility, determination of rheological properties and control of product aesthetics, the latter being of vital significance to consumer acceptance and, ultimately, commercial success.
Frequently, the stability, activity and aesthetics of a cosmetic cream cannot be determined by emulsifier selection alone and, in these cases, a rheological modifier is incorporated. The function of these materials is to alter the flow characteristics of the emulsion, which translates commercially into the texture, appearance and cosmetic feel of the product in use. The most common types of rheological emulsifier include cellulose derivatives, natural gums such as xanthan, natural clays such as bentonite and laponite, or complex gels utilising acrylate-based, polymer technology. The choice of rheological modifier, and its level of incorporation, is of paramount importance to the spreadability, absorbency and feel of the final product on the skin. Unacceptably high levels of inclusion will result in inferior spreading characteristics, poor absorbency and residual stickiness on the skin after use. In cases where an active ingredient must be suspended in the final formulation, a rheological modifier with a high yield point must be incorporated. A good example of such a material is acrylates/C10-30 alkyl acrylate crosspolymer, which forms a hydrogen-bonded network in the formulation, resulting in high viscosity at low stress and the ability to keep particulate active ingredients in suspension. In addition, rheological modifiers are often included to act as colloid stabilisers, further enhancing the stability and shelf-life of the final product.
From a consumer perspective, the acceptance of a cosmetic cream will depend largely upon how the skin feels after it has been applied. For this reason, cosmetic products include significant quantities of emollients,
materials specifically designed to modify or change the feel of the skin. The vast majority of emollients used are lipophilic in character, with factors such as molecular weight, structural composition and carbon chain length being the main determinants of their primary properties. One of the most popular and cost-effective emollients is isopropyl myristate, although this material is now being abandoned in all but the lowest cost products, due to its high commedogenic potential. The majority of emollients used in cosmetic creams are natural oils, emollient esters, or combinations of the two. Sweet almond oil exhibits a characteristically luxurious feel and is widely used as a skin conditioning agent, due to its penetrating, smoothing and softening properties. By contrast, hazelnut oil is lighter and more readily absorbed, and is nourishing and revitalising to all skin types. It is also rich in vitamins and oleic acid. More novel natural oils have recently found their way into cosmetic creams and amongst those of particular interest are strawberry seed oil, plum oil and blackcurrant seed oil, the latter being particularly rich is gamma-linolenic acid, an excellent nutrient for conditioning of the epidermis. One of the most commonly encountered emollients is caprylic/capric triglyceride which, although not a natural oil, demonstrates excellent skin compatibility, a high level of lubricity and superior emolliency. There is a plethora of emollient esters that may be incorporated into cosmetic creams and, depending upon the combinations of these materials used, almost any skin feel can be engineered into the final product. Both ethylhexyl palmitate and PPG-2 myristyl ether propionate are light, evanescent, non-occlusive emollients, which are excellent for leaving a lightly conditioned feel to the skin. By contrast, myristyl lactate is somewhat heavier in kind and leaves a characteristically velvety texture to the skin. In addition to providing skin feel, some emollient esters such as isocetyl stearate can modify the application characteristics of the final product on skin, by improving spreadability.
Most modern cosmetic skin creams will, by their very nature, have a pronounced moisturising benefit on the skin. Moisturisation of the skin can be achieved in one of two fundamental ways, either by applying an occlusive material to the skin’s surface, thereby reducing transepidermal water loss, or by the use of a humectant which absorbs into the skin and attracts moisture from the surrounding environment. Historically, one of the most popular occlusive moisturisers has been mineral oil, although use of this material has been curtailed somewhat more recently, due to consumer negativity associated with its petroleum-based derivation. Most vegetable oils exhibit some degree of occlusivity on the skin and can therefore contribute to the moisturising benefit of the final product. The correlation of oil type with occlusive properties is difficult although, as a rule of thumb, many of the more unsaturated natural oils exhibit a higher degree of water vapour permeability. The most frequently found humectants in cosmetic creams include glycerine, propylene glycol and sorbitol, all of which function in a similar way by attracting moisture vapour back into the skin, thereby altering the transepidermal equilibrium. In recent years more complex and efficacious humectants have been developed, many of them being derived from natural botanical sources. One such example, an extract of the root of the plant Imperata cylindrica found in Asia and Australia, is claimed to be able to provide moisture to the epidermis for up to 24 hours after application, when used in a cosmetic cream at a level of 3% (w/w) or more. Similar claims are made for Caesalpinia spinosa gum, which is naturally obtained from the Tara tree and contains high levels of galactomannan-type polysaccharides.
A very important part of a cosmetic cream, and one that is highly charged with emotion, is the preservative system. Most cosmetic creams contain significant amounts of water, making them highly susceptible to microbiological attack from bacteria, yeast and moulds. The existence of such microbes in a cosmetic cream not only detracts from its appearance and odour but also has the potential of making the product dangerous to human health, particularly if the contaminants are pathogenic in nature. It is for this reason that all types of cosmetic that are prone to such attacks must be adequately preserved. Unfortunately, largely due to internet hoaxes, misinformed media coverage and the activities of green movements around the world, consumers perceive preservatives as being chemical additives which are unnecessary and harmful to human health. The choice of cosmetic preservatives is wide and varied and includes isothiazolinones, imidazolidinyl urea, diazolidinyl urea, iodopropynyl butylcarbamate, phenoxyethanol and, amongst many others, the ubiquitous and now infamous parabens. It is this last group of materials which has been the subject of the most significant case of bad consumer press for many years. The negative perceptions associated with the use of parabens in cosmetics, which have now extended to global proportions, can be traced back to a paper published in the Applied Journal of Toxicology in the UK, in January 2004. In this paper, the author Dr. Phillipa Darbre, claims to have identified an inextricable link between the use of parabens as cosmetic preservatives and the incidence of breast cancer in females. Although scientific cross examination of this paper reveals some flaws in Dr. Darbre’s arguments, the subsequent media handling of this issue caused massive negative consumer perceptions about the safety of parabens as cosmetic preservatives globally and the fall out of this reportage is still being felt at the time of writing. Notwithstanding emotively-driven consumer issues such as this one, the need for adequate preservation of cosmetic creams is scientifically unquestionable, the irony being that an unpreserved cosmetic is potentially more dangerous than any unwanted side effects that could possibly be associated with a regulatory compliant preservative system.
In the case of cosmetic creams which are treatment orientated, undoubtedly one of the most important aspects of the formulation design is the type, combination and level of active ingredients used in the product. Invariably, the active ingredients are included to address one, or more, of the symptoms associated with the premature ageing of skin. Typically, the conditions that these products seek to address include fine lines and wrinkles, loss of elasticity, compromised barrier properties and hyperpigmentation. Active ingredients available to treat such conditions are wide and varied but recent advances in biotechnology have given rise to the existence of materials that have scientifically proven efficacy. Amongst the active ingredients available to the cosmetic formulator, vitamins still play an important role and, in the context of “anti-ageing” treatments, vitamin A and its derivatives are still of paramount importance. The most powerful form of vitamin A legally permitted in cosmetic products is retinol, the popularity of which has been propagated not only because of its proven ability to reduce the appearance of fine lines and wrinkles but also because of the fact that it is now available in several novel delivery systems. Liposomal retinol, for example, is claimed to exhibit improved efficacy at lower doses, due to enhanced transdermal penetration, targeting the active where it is most beneficial. Additional claimed benefits are improved stability and reduced potential for adverse reaction. A newer trend is for vitamin A to be added to skin care products as a constituent of certain natural extracts. Peach juice, for example, is naturally rich in vitamin A and is claimed to be able to smooth, tone and refine the skin’s texture, without irritation or other deleterious effects. Rose hip oil is also claimed to provide significant anti-ageing benefits, when used in concentrated form on the skin. Interestingly, this oil contains trans-retinoic acid, itself an ingredient normally restricted to prescription products only.
Perhaps one of the most interesting and potentially powerful categories of skin care actives is antioxidants and there seems to be a never-ending selection available, each claiming to be more powerful than the last. The foundation for the importance of antioxidants is well established. An increase in global pollution, exposure to the sun’s rays and modern lifestyle are all contributory factors to the excess production of free radicals which, in turn, lead to premature ageing of the skin. Whilst established antioxidants such as vitamin C and vitamin E still abound, many newer and more effective antioxidants, often containing polyphenolic or flavonoid structures, are finding their way into treatment skin care products. Alpha lupaline for example, is a main constituent of the natural extract obtained from the lupin flower and is claimed to exhibit several thousand times the antioxidant capability of vitamin E, making it an ideal candidate for inclusion into treatment skin care products, where it provides protection for the skin against the harmful effects of free radical breakdown and environmental pollution. Many scientists believe that the future of skin care actives lies in the ability of an ingredient to manipulate the condition of the skin at cellular level. Although this inevitably raises a whole gambit of questions concerning the regulatory and ethical status of such an approach, many of the active ingredients used in treatment skin care products today, do just this.
The most contemporary group of active ingredients which fall into this category are peptides, which are biotechnologically engineered molecules composed of specific sequences of amino-acids, purposely designed to have biologically active benefits on skin. The first big breakthrough in peptide technology was the development of palmitoyl pentapeptide-3, a molecule which has proven benefits in the stimulation of skin fibroblasts and the ability to help reconstruct the extracellular matrix. This material, which has the ability to reduce the appearance of fine lines and wrinkles by inducing the synthesis of collagen and glycosaminoglycans, has performed consistently well in clinical trials with improvements in skin condition being superior to those achieved with retinol, in some cases. In recent years, other peptides have been developed and incorporated into cosmetic creams, each specifically bioengineered with a specific skin care benefit in mind. One of these new generation peptides, acetyl hexapeptide-8, has been scientifically proven to have “botox-like” effects on the skin, without encountering the risks associated with the latter treatment. Another peptide which is valuable as an active ingredient in topically applied cosmetic applications is hexapeptide-10, which has been proven to promote cell adhesion, growth and migration, in addition to assisting in the regeneration of the basal lamina.
Perhaps more controversial than peptides, is the use of enzymes and even hormones in cosmetic skin care products, to confer beneficial effects on the skin. Enzymes have already been incorporated into cosmetic creams for some years and perhaps the best example of this is superoxide dismutase, a stable and efficacious enzymatic antioxidant. Papain, a proteolytic enzyme derived from papaya, has also been used in treatment skin care products to encourage desquamation, thereby gently exfoliating the skin and brightening the complexion. Wider use of enzymes in treatment skin care products has been hampered by the difficulties associated with their use in cosmetic preparations. By virtue of their reactivity, enzymes are often unstable and any undesirable by-products could result in a high level of allergic response, or even more serious toxicological effects on the skin. In addition, their reactivity gives them a “high-risk” status, when examining compatibility with other cosmetic ingredients. The use of hormones in skin care products is an even more contentious issue. Currently, hormones are prohibited for use in cosmetic products in most parts of the world. The reason for this seems clear; hormones are a vital part of the fundamental control mechanism of the human body and the inclusion of such ingredients in treatment skin care products would unquestionably violate the current legal definition of a cosmetic product in most parts of the world. Interestingly however, some natural extracts, most notably the wild yam, contain hormone analogues, in this case natural progesterone, which are claimed to exhibit significant benefits to post-menopausal women, when applied topically. Wild yam extract has no formal restriction for use in cosmetics and this material is currently finding its way into many skin care creams all around the globe. Amongst the benefits being claimed for such creams are weight loss, reduction in the likelihood of contracting osteoporosis, deceleration of the symptoms of ageing and improvement in energy and mental alertness.
Finally, one must not forget the aesthetics of a cosmetic cream and in this context both colour and fragrance play an important role. In the case of facial skin care products, consumers still demonstrate a strong preference for uncoloured, naturally white emulsions. This is linked to the psychology that facial products must be as pure as possible and the colouring of such a product with a cosmetic dyestuff represents an adulteration of an otherwise “pure” product. Fragrance, on the other hand, is a little different. Most consumers want products to smell pleasant in use and the conservative use of properly designed fragrances in cosmetic creams is still very popular, particularly if the base odour of the product is slightly unpleasant. Fragrances, of course, are one of the principal causes of allergic dermatological reactions and, for this reason, fragrances used in cosmetic creams must comply with specific safety requirements before they can be used commercially. In this context, compliance with the safety guidelines issued by IFRA (International Fragrance Association) is a minimum requirement.
Much water has passed under the bridge since the opinion that cosmetic skin creams are hype only. There is no question that contemporary product designs, using the correct active ingredients in a suitable selected excipient can confer demonstrable benefits to the condition of the skin. As such, it is likely that the future of cosmetic skin care will see cosmetic formulators and dermatologists working more closely together towards a common goal – to keep skin healthy and younger looking for as long as possible, in an attempt to help their clients in the never ending search for eternal youth.