The new ingredient innovations in skin care and sunscreens have decreased in recent years due to the paucity of novel materials. The cosmetics industry refers to this as the “raw material glass ceiling.” Since the skin care industry thrives on newness, this has indeed been a marketing challenge. Yet, is it realistic to think that at some point all of the raw materials on the earth will have been discovered? How can this need for newness be fulfilled? One answer to this multimillion-dollar question is to rediscover old materials in a new form.
Nanoparticles represent such an opportunity. Well-known materials with established appearance and behavior can be rediscovered in a very small particle size that changes their characteristics. This offers a broad horizon for discovery, as small particles behave very differently from small particles due to alterations in surface-to-volume ratios.
For example, very small nano-sized particles have a very large surface area, but a very small volume. This changes the intermolecular attraction forces altering their electrical and magnetic behavior.
Nano-sized particles can also be generated smaller than a wavelength of light, allowing them to become invisible or possess colors different from expected. Macromolecular elemental gold has a characteristic yellow color while nanoparticle gold appears red. These significant changes offer tremendous opportunities for new skin care products and sunscreens.
WHAT ARE PARTICLES?
Many different types of particles are used in the skin care products and sunscreens discussed in this article. By definition, particles are ground solids. Their oldest use is in colored facial cosmetics applied with a brush. These cosmetics include powders, eye shadows, blushes, and the new mineral facial foundations. These products are composed of different-sized particles of various ingredients combined to create a unique color or surface texture.
For example, talc particles are the main component in facial powder, providing both camouflage to give the skin a more even appearance and photoprotection. If the talc is combined with iron oxide brown pigments, a facial mineral foundation is created. If the talc is combined with red pigments, a facial blush is created. If the talc is combined with greens, blues, and purples, an eye shadow is created. Particles are the basis for all colored cosmetics.
Particles also are the basis for the inorganic sunscreens, including zinc oxide and titanium dioxide. Zinc oxide and titanium dioxide are superb broad-spectrum sunscreens, able to reflect both UVB and UVA radiation. They are available as micronized and microfine particles. Crushing a solid to yield particles of many different sizes creates micronized particles. The different-sized particles reflect light better than smaller particles all of the same size, known as microfine particles.
The problem with microfine and micronized particles is that they are visible on the skin surface. While this is required in the realm of colored cosmetics, it is not a good quality for sunscreens that are expected to appear transparent. This has led to technology designed to produce particles of a smaller size, known as nanoparticles.
WHAT ARE NANOPARTICLES?
Nanoparticles are particles with a diameter between 1 nm and 100 nm. They are commonly found in the environment as a by-product of fire or combustion.
Nanoparticles are found in automobile exhaust, airplane exhaust, and air pollution in general. We breathe them every day in an urban environment, and over time develop nanoparticle deposits on the walls of our alveoli. Is this a problem? No one knows for sure. However, the effects of cigarette smoke nanoparticles on the lungs are well established.
Nanoparticles are so small that they escape detection by macrophages and cannot be eliminated from the body. Over time, after smoking many cigarettes, the number of nanoparticles increases, each forming a nidus for inflammation. This cumulative inflammation results in destruction of the alveoli, causing chronic obstructive pulmonary disease and, in some cases, lung cancer.
What are the problems associated with nanoparticles?
There is growing concern over the presence of nanoparticles in the environment. These particles are invisible to the human eye and can penetrate the skin and lung tissues, gaining access to the lymphatics and blood circulation. From there, these particles can be widely distributed throughout the body. Once these particles enter the body, they cannot be removed.
Some in the medical community have voiced concerns that nanoparticles of metals might be responsible for neurologic disease. Others have wondered if the chronic inflammation induced by nanoparticles might not cause other degenerative diseases.
The biggest present concern among those in the skin care industry is the use of nanoparticle zinc oxide and titanium dioxide. These white particles become invisible when manufactured in nanoparticle size, making them suitable for all skin types. Otherwise, persons of Fitzpatrick skin types III-VI cannot use these inorganic sunscreens.
These nanoparticle-based, highly effective sunscreens are poised to dominate the market, but environmental concerns arise when the disposition of nanoparticles in the environment is considered. After a day at the beach, the sunscreen is rinsed into the ocean water and finally completely removed in the shower. These zinc oxide nanoparticles find their way into the water supplies of the world.
Nanoparticle zinc is antibacterial—another possible use for this technology—but it is also toxic to plankton, the first line of the aquatic food chain. Imagine the consequences of large-scale aquatic organism death due to invisible nanoparticles. It is for this reason that many countries are taking a regulatory stance on nanoparticle use.
NANOPARTICLE USE AND SAFETY
The cosmetics industry in the United States has called for a voluntary halt on widespread nanoparticle use until more information can be obtained. No reputable manufacturer wants to put particles in the marketplace with long-term adverse health implications, even though there is no governmental organization forbidding their use in the United States.
This is not the case in Europe. Sweden has banned the use of nanoparticle formulations, and the European Union has asked that all presently marketed nanoparticle-containing products have the term “nano” on the packaging. Further, all products marketed with nanoparticles after 2013 must be registered with the government.
The US FDA has convened a task force to examine nanoparticles, but no documents have been produced.
The penetration of nanoparticles into the skin is perhaps the biggest immediate health concern. However, it is impossible to generalize. The skin barrier, composed of protein-rich corneocytes and intervening covalently bound lipid layers, is extremely effective at keeping out substances deleterious to the body.
Larger nanoparticles of 14 nm to 100 nm do not penetrate the skin, but smaller nanoparticles 13 nm in size and less can penetrate the skin. These extremely small nanoparticles in the range of 10 nm are labeled as quantum dots. Quantum dots are a specialized form of nanoparticle with unique electrical and magnetic properties. Currently, quantum dots are being investigated for their use in computing for the next generation of microcomputers.
To answer the question of nanoparticle penetration: It is theoretically less if the particles are smaller than 13 nm. However, in reality nanoparticles do not remain as nanoparticles in skin care and sunscreen formulations. Nanoparticles can be sprinkled into an emulsion and put into a zinc oxide sunscreen, but they do not remain nanoparticles.
Nanoparticles like to aggregate, meaning that they stick together. A clump of 15 10-nm nanoparticles is no longer a nanoparticle, since their cumulative size of 150 nm is larger than the 100-nm upper limits for nanoparticles. Further, nanoparticles placed in emulsions, which are the basis for the lotion that suspends the nanoparticles in sunscreens, tend to agglomerate. This means that the clumps of nanoparticles stick together even further when placed in an oil-in-water mixture. Ten clumps of 150-nm aggregated nanoparticles become 1,500 nm, well above the nanoparticle limit.
In theory, nanoparticles could exist singly and penetrate the skin, but in actuality the aggregation and agglomeration of particles found in nature protects both the body and the environment from deleterious effects. This is not to say, however, that further research is not needed to evaluate nanoparticle behavior and safety.
NANOPARTICLE USE IN SKIN CARE PRODUCTS
Nanoparticles could revolutionize skin care beyond creating invisible sunscreens composed of nanoparticle zinc oxide and titanium dioxide. The blending of nanoparticle pigments could create invisible camouflaging or pigmentation abnormalities on the skin. Nanoparticles that absorb and reflect in different wavelengths could alleviate unwanted red tones from the face more efficiently than current facial foundations.
Nanoparticles also present tremendous opportunities for the delivery of active agents into the skin, a challenge that the skin care industry would like to overcome. For example, nanoparticle-sized liposomes can fuse with the corneocytes of the stratum corneum and deliver a payload into the upper epidermis.
Liposomes are hollow spheres composed of phosphatidylcholine that can be loaded with many different water-soluble and oil-soluble ingredients. These ingredients can function in the skin rather than on the skin. Imagine delivering antioxidants into the stratum corneum to decrease photoaging and skin cancer formation.
Particles and their interactions with the skin are important to skin care and sunscreens. The physical and chemical properties of particles create cosmetic and photoprotection opportunities that are important and unique. Particles are an indispensable component of all colored cosmetics and some sunscreen formulations.
Controversy arises when extremely small nanoparticles are placed on the skin surface. Their safety remains to be established, but the opportunities nanoparticles create for novel formulations cannot be ignored.
Zoe Diana Draelos, MD, is a consulting professor at Duke University School of Medicine, Department of Dermatology, based in Durham, NC. She can be reached at (336) 841-2040.