Excessive exposure to UV radiation can cause skin damage and premature aging of
Dr. Howard Epstein, technical director, cosmetic actives innovation and application, SusonityCourtesy of Susonity
Inorganic filters work by scattering, reflecting and absorbing some UV wavelengths. Photostable insoluble metal oxides such as titanium dioxide and zinc oxide scatter and reflect UV radiation away from skin and physically block it from penetrating. Important parameters influencing performance of inorganic filters include dispersion, aesthetics, stability, photostability and particle size. The smaller the particle size, the less whitening of skin/increased transparency of dispersion resulting in more desirable aesthetics. Nano based materials (<100nm) have shown improved aesthetics as compared to larger particle materials.
Titanium dioxide materials do not all perform the same when formulated into sunscreens products. Some grades of titanium dioxide can result in unfavorable interface reactions creating unstable products, poor aesthetics and/or reaction with sensitive actives.
Reported compatibility concerns exist with antioxidants, vitamin derivatives, skin brighteners, avobenzone, carbomers and even DHA resulting in degradation of sensitive actives, loss of activity or formation of off odors.
Diagram of dual coating in Eusolex® T-EASY.Courtesy of Susonity
Eusolex® T-EASY (INCI: Titanium Dioxide (and) Silica (and) Cetyl Phosphate) was designed to eliminate many of these concerns. This material demonstrates superior compatibility with many hard to formulate actives with stable processability into both o/w and w/o emulsions. This material has a titanium dioxide core that is coated with a layer of silica followed by a layer of cetyl phosphate. The silica coating decreases the number of active sites on the titanium oxide hydrate layer. Also, the C16 alkyl chain present in cetyl phosphate provides steric inhibition. Decreasing reactions with other ingredients allows for more flexibility when formulating.
Titanium dioxide also has photocatalytic activity that can induce oxidative degradation pathways. The catalytic center of titanium dioxide acts as a semiconductor and can become excited when photons of high energy are absorbed. Oxidation, reduction and photostability/instability are properties of titanium as a semiconductor that can be regulated by the coating on the TiO2. Semiconductor activity can be tuned by altering the surface of the particle. Eusolex® T-EASY coating tunes the surface activity of TiO2 while maintaining its ability to attenuate UV light.
Eusolex® T-EASY compatibility with self-tanner molecule dihydroxyacetone (DHA).Courtesy of Susonity
Another benefit of Eusolex® T-EASY is its high photostability and low skin whitening. In a cream formulation Eusolex® T-EASY showed an increase of about two SPF units for each 1% added to the sunscreen. The material was dispersed up to 5% in an o/w emulsion. UV-VIS-NIR absorbance was recorded from 290–390 nm. SEM was used to determine morphology of dispersion. Particle size/polydispersity was determined using photon correlation spectroscopy. SPF measurements were confirmed with in vitro tape-substrate testing.
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