Photoprotection by stabilisation

Gary Neudahl and Justin Bill discuss substances that can interact with unstable sunscreens and antioxidants to help them retain their beneficial aspects and prevent the detrimental effects

Sunlight is an integral part of our existence and brings with it both the good and the bad. Among its detrimental effects are the promotion of skin cancer and the visible signs of skin photoageing, including age spots, loss of elasticity and increased wrinkling. It is a constant battle in the laboratories of top personal care scientists, and even within our own homes, to find the best methods to prevent these effects from negatively impacting our lives.

There are several ways to protect our skin from sunlight – or photoprotect our skin – but success can vary greatly depending on the options we choose.

Ready, set, protect

The first approach – the most common method, even though it doesn’t always yield the best results – is photoprotecting by simply covering the skin. The most primitive but sure-fire way is, unsurprisingly, through clothing. While nothing chemically can compete with a full-body snow suit, this doesn’t befit the beach going crowd, nor most other people. The next obvious skin covering solution, then, is sunscreens.

There are two different types of sunscreen active ingredients (UV filters) that mitigate the effects of the sun: light shielding agents and light absorbers. Both are utilised in typical sunscreen systems, but they photoprotect in distinct ways, neither of which is completely effective.

Light shielding agents used in sunscreens include titanium dioxide and zinc oxide.  These metallic oxides, particles just tenths of a micron in size, reflect and scatter the sun’s rays. So the greater the quantity that is used, the higher the performance of the sunscreen, but the more whitening the product on the skin. In essence, the skin is being painted and the whiter the ‘paint’ the better the coverage.

Nanotechnology allows sunscreen producers to achieve sun protection using zinc oxide and titanium dioxide with a lesser whitening effect. This is done by further reducing the size of the oxides to nano sized (less than 100nm, 0.1micron) particles, much smaller than the wavelengths of visible light (about 400nm-700nm). Because of lingering concerns regarding the toxicity of these tiny particles should they penetrate the skin, a number of governmental agencies require labeling that informs the consumer of nanoparticle content. However, while these products protect skin from the UV rays of the sun, they do nothing to protect it from visible light, which is also a concern.

The majority of UV filters in sunscreens are UV (light) absorbers. Simply put, UV absorbers intercept most of the ultraviolet rays in sunlight before they can hit our skin. Unfortunately, a number of the key absorbers used throughout the world are highly photolabile; that is, they degrade quickly upon exposure to light. When these photolabile UV filters degrade, sunscreen performance diminishes and there is a concurrent build-up of photochemical reaction products on the skin, neither of which is desirable. What photolabile ingredients may be found in sunscreens? At the top of the list is butyl methoxydibenzoylmethane (BMDM), the most widely used UVA filter in the world.

You might think that adding other UV absorbers to the sunscreen would help stabilise BMDM, but that is too often not the case. In fact, combining BMDM with ethylhexyl methoxycinnamate (OMC), a commonly utilised and powerful UVB filter, actually increases the photolability of both. So something other than UV absorbers is needed to ensure the stabilisation of BMDM in sunscreens. Plus, as noted for light shielding agents, products built on UV filters protect skin from the sun’s ultraviolet rays, but do nothing to protect it from visible light. To do so, they would necessarily absorb visible light and the skin would be coloured, as if by a stain.

Reacting to the reaction

Why is protection from both the ultraviolet and visible rays of the sun desirable? A primary cause of skin ageing is oxidative stress – perhaps half of the oxidative stress experienced by the skin originates from the sun’s visible rays[1]. So protecting the skin from both ultraviolet and visible light makes sense, since both are able to generate free radicals (atoms or groups of atoms with one or more unpaired electrons). These may react with ingredients in personal care products that have been applied to the skin and with biochemical compounds within the skin, destroying them in the process. Degradation often occurs through reactive oxygen species (ROS) and other radicals generated as a result of sun exposure. These radicals are highly reactive and bad for biological systems, as they can cause damage to DNA (strand breakage), lipids (polyunsaturated fatty acid peroxidation), proteins (amino acid oxidation), enzymes (co-factor oxidation), etc.

So how do we fight ROS or other reactive radicals? By adding antioxidants to neutralise them. This is like fire-fighting, as damage is already being done if ROS are present. Free radical scavengers are not preventing photodegradation, but rather are ‘mopping up’, to limit the damage that occurs after degradation has already begun. Anti-ageing skin care products rely on multifunctional ingredients with antioxidant activity, such as retinol, resveratrol and coenzyme Q10, to reverse premature skin ageing that is caused by ROS and other radicals formed due to sun exposure. Unfortunately, these compounds are highly photolabile, so their effectiveness is severely diminished after very brief exposure to sun.

It would seem that there is no perfect solution to all our concerns about the impact of sunlight on our skin. But what if there were an answer to this dilemma – What if there were technology that provided a broader spectrum of benefits?


Figure 1

A stabiliser returns an unstable component to its ground state so it doesn\'t react


Imparting stability

HallStar has pioneered the concept of photoprotection by stabilisation. The company has designed substances capable of interacting with the various unstable components in skin care and sun care formulations. By stabilising them, their beneficial aspects are retained and any detrimental effects that result from their degradation are prevented. HallStar’s unique technologies also allow for efficient regeneration and reusing of the stabiliser molecules. Figure 1 illustrates the chemistry that is involved.

The key insight provided by this figure is that when the unstable component resolves the excited state energy it acquired through sun exposure via the stabiliser, undesired photochemical reactions do not occur and the unstable component remains intact and able to perform its intended function.

For maintaining the efficacy and effectiveness of photolabile UV absorbers and skin care active ingredients, utilising a stabiliser is the best solution. Simply put, stabilisers enable photolabile substances to be stable while exposed to sunlight. This is best achieved by a mechanism called excited state quenching. Excited state quenchers markedly reduce the period of time during which the photolabile molecules remain excited and so are susceptible to chemical reaction. Stabilisation occurs when a molecule in its excited state resolves its energy much more rapidly than it could in the absence of the stabiliser, hastening its return to its stable ground state[2].

Some photochemical reactions occur extremely rapidly, as is the case in the aforementioned reaction between BMDM and OMC, a combination of great interest to formulators given its potentially greater cost effectiveness when adequately stabilised. Now even sunscreens with very rapid photochemical reaction kinetics may be stabilised by using recently developed, unique and highly effective stabilisers such as ethylhexyl methoxycrylene and polyester-25. These stabilisers are part of a family of para-methoxy aryl containing tetra-substituted propenoic acid derivatives (trade name: SolaStay) that provide powerful and persistent stabilisation.

BMDM is capable of absorbing UV radiation, but without photostabilisation it quickly loses that capability in sunlight. With one of the new stabilisers added, BMDM retains its protective capability under identical light exposure.

HallStar stabiliser chemistry defines a unique way of achieving photoprotection. When used to protect unstable UV filters, it enables a manufacturer to deliver powerful yet cost effective photoprotection from the burning and ageing rays of the sun. When used to protect the aforementioned highly photolabile skin care active ingredients such as retinol, retinyl palmitate, resveratrol, cholecalciferol (vitamin D3) and ubiquinone (coenzyme Q10), these stabilisers enable marketers to move their restorative and rejuvenating night creams into the light of day as day creams[3]. Most significantly, the HallStar stabiliser chemistry can be used to protect the endogenous unstable factors in human skin, enabling the ultimate in dermal stability under broad spectrum solar irradiation, including UV and visible light.

Future formulations

Until recently, the available methods for providing photoprotection to skin had significant drawbacks: full body coverage by clothing is desired by few. Light shielding by sunscreens with non-nano microfine oxides relies on light reflection which imparts colour to skin. Nanoscale oxides may require special labeling and do not provide protection from visible light generated ROS. Sunscreens with UV absorbing filters protect skin from harmful UV rays, but only as long as they do not degrade and even then they do nothing to protect the skin from the detrimental effects of visible light induced ROS. Antioxidants may neutralise free radicals, but they do not prevent their formation.

Photoprotection by stabilisation is now at hand, using products that quickly resolve the excess energy from a diverse array of photolabile materials, enabling them to be stable while exposed to sunlight. These innovative technologies can photostabilise unstable UV filters in sunscreens, active ingredients in anti-ageing skin care products and many endogenous components in human skin. Comprehensive, proactive photoprotection is now available to fight against the full range of UV and visible electromagnetic radiation.

Authors

Gary Neudahl and Justin Bill
HallStar
www.hallstar.com

References

  1. http://www.ncbi.nlm.nih.gov/pubmed/18248499
  2. http://www.happi.com/contents/view_features/2009-10-05/sunscreen-photostability-101/
  3. http://www.hallstar.com/pdf/PhotostabilizingRetinol.pdf

Featured Companies

Hallstar (more information, website)