Biomedical Engineering Reference
In-Depth Information
the UVA range (up to 380 nm), and are also very effective at blocking UVB.
Their chief drawback is that when formulated to have a high sun protection
factor, they are visible as a white substance when applied to skin. Much
work has been conducted lately to alter their physical properties to decrease
scattering such that visibility is decreased. Some of this alteration has been
done by making nanoparticle formulations. Unfortunately, their capacity to
protect against light in the UVA range is decreased when the particle size
is decreased into the nanometer range. This problem is being addressed by
grouping some nanoparticles in the sunscreen vehicle, and also by coating the
particles with dimethicone or silica. A recent concern has been raised that
the capacity of nanoparticles to penetrate the skin may be different from that
of larger particles. Because of the character of the stratum corneum, it may
be they cannot penetrate readily into viable tissue in significant quantities.
Research is ongoing in this area [9].
UVB absorbing sunscreens are capable of harmlessly absorbing UV light,
and are generally degraded over time by their absorption of light. Sunscreens
of this type are wavelength selective, unlike the physical sunscreens. Therefore,
it is necessary to combine several chemical sunscreens to gain optimal coverage
across the UV spectrum. More consideration is also now being given to group-
ing sunscreen ingredients so that their absorption of UV can be transferred to
heat, thereby leaving the sunscreen ingredient intact and able to function in
its absorbing capacity longer. UVB blockers include paraaminobenzoic acid
(up to 15%), Padimate O, a PABA ester (up to 8%), Octocrylene (up to
10%), Cinnamates (such as octyl methoxycinnamate, up to 8% ), and Sali-
cylates (up to 5%). Some chemical sunscreens are broader in their protective
capacity, such as the benzophenones and oxybenzone (up to 6%), which are
useful for UVB protection, but also protect against the shorter wavelengths
in the UVA spectrum. Until recently, the only available absorbing sunscreen
protective in most of the short UVA spectrum was Parsol 1789, which is
utilized in concentrations up to 3%. New formulations of UV absorbing mole-
cules are now becoming available, which will greatly expand the capacity of
sunscreen manufacturers to make a product that protects broadly across UV
wavelengths.
16.4 Phototherapy: Use of Light for Treatment for Skin
Disease
Despite the problems associated with exposure to UV light, it has been recog-
nized for centuries that it is effective in the treatment of skin disease. Diseases
such as psoriasis, eczema, and vitiligo have been managed with moderate ef-
ficacy, using sun exposure for centuries [9]. With the advent of artificial light
sources, it became practical to expand the use of light as a treatment option,
as therapy could be administered throughout the year and in cold climates.
