JOURNAL OF COSMETIC SCIENCE 554 diameter of 0.003 μm to 1 μm. For micro/nanospheres, the agents are either dissolved in the sphere matrix or absorbed onto the surface of the particles, while for micro/nanocap- sules the actives are either attached to the surface or trapped within the capsules. Incorporating agents into micro/nanoparticles has demonstrated to be advantageous in improved sustained drug release (37) and increased drug uptake (38), which has won these particles broad attention in the cosmetic industry. Specifi cally, nanoparticles have been used to encapsulate a wide range of ingredients that are intended to provide cos- metic benefi ts (39). Many studies focusing on the topical use of microparticles and nanoparticles have been carried out, and actives encapsulated into particles include vitamin A and some precious metal elements: vitamin A has skin-softening and anti-wrinkle functions. One compari- son experiment of in vitro and in vivo drug release of a microencapsulated vitamin A cream with a non-microencapsulated formulation showed that microspheres were able to remain on the skin for a longer period of time, and as a consequence were able to prolong the release of vitamin A (40). Free radicals including reactive oxygen species (ROS) are con- sidered to be one of the main hazards to the skin. A decrease in ROS production was observed in platinum-nonparticle-treated HaCaT kertinocytes. Pretreatment of the cells with nano-platinum also caused a signifi cant inhibition of UVB- and UVC-induced apoptosis. These results suggested that nano-platinum effectively protected against UV- induced infl ammation by decreasing ROS production and inhibiting apoptosis in kerati- nocytes (41). Gold nanoparticles enhanced the proliferation of keratinocytes (42), while silver nanoparticles demonstrated preservative effects against mixed bacteria and mixed fungi, and did not penetrate normal human skin, suggesting that silver nanoparticles may have a potential for use as a preservative in cosmetics (43). The use of nanomaterials gives rise to controversy: While cosmetologists claim that nano- particles are able to penetrate the skin, which is attributed to their size, some academics question the potential dangers of the contact of nanoparticles with human skin. Recently, Friends of the Earth, an international environmental organization, warned against the use of nanoparticles in cosmetic and sunscreen products due to a possible uptake of particles by the human skin into the circulation: “If nanoparticles penetrate the skin, they can join the bloodstream and circulate around the body with uptake by cells, tissues and organs.” (44). The safety of nanoparticles in cosmetic products needs to be addressed. SOLID LIPID NANOPARTICLES (SLN) AND NANOSTRUCTURED LIPID CARRIERS (NLC) Solid lipid nanoparticles (SLN) were developed at the beginning of the 1990s as an alter- native carrier system to emulsions, liposomes, and polymeric nanoparticles (45). With SLN, a solid lipid or a blend of solid lipids is used to substitute the oil phase of an oil-in- water space (o/w) emulsion. Nanostructured lipid carriers (NLC) were developed to over- come the low loading capacity of SLN. Both SLN and NLC can be loaded with actives as carriers in cosmetic and cosmeceutical products. It is reported that SLN and NLC are advantageous in dermal application in the following ways: (a) SLN and NLC are composed of biocompatible and biodegradable lipids exhibit- ing low toxicity (46) (b) SLN and NLC are reported to be able to enhance dermal pene- tration (47) (c) SLN and NLC provide controlled release profi les for many substances

ADVANCED CARRIER SYSTEMS 555 (48) and (d) lipid nanoparticles are able enhance the chemical stability of compounds sensitive to light, oxidation, and hydrolysis (49). Researchers successfully incorporated active ingredients, e.g., vitamin A (55), Coenzyme Q10 (57), ascorbyl palmitate (58), etc., into SLN and NLC. The outcome of the study in vitamin A-loaded glyceryl behenate SLN has shown a sustained release of vitamin A from SLN compared to a vitamin A nanoemulsion (50,51). Experiments compar- ing SLN and conventional o/w emulsions as carrier systems for the molecular sunscreen oxybenzone found that release rates could be decreased by up to 50% with the SLN for- mulation. SLN was also able to improve UV protection when applied together with or- ganic sunscreens such as 2-hydroxy-4-methoxy benzophenone (52). Coenzyme Q10 is a potent antioxidation enzyme and is a popular component in many cosmetic and cosme- ceutical products. It was reported that in contrast to the Coenzyme Q10-loaded nano- emulsion, Coenzyme Q10-loaded NLC possessed a favorable biphasic release pattern. The NLC release patterns were defi ned by an initial fast release followed by a prolonged re- lease, while the nanoemulsion showed a nearly constant release (53). Ascorbyl palmitate (AP), a cosmetically effective ingredient in skin-whitening, is unstable under normal conditions, but was proven to be more stable in both the NLC and SLN stored at 4°C (54). The mechanism is explained as the occlusion effect of the fi lm formed by lipid nano- particles on the skin. Film formation on the skin may increase stratum corneum hydra- tion, resulting in reduced corneocyte packing and an open intercellular gap for drug penetration. Although SLN and NLC are promising carrier systems in cosmetics and cosmeceutics, they suffer drawbacks. Low loading capacity and instability during storage are two prob- lems concerning SLNs. To overcome these issues, NLCs were developed with their high- loading capacity and long-term stability, making them favorable in many cosmetic applications. However, NLCs are not suitable for purposes that may require a high level of crystallinity, such as in UV protection (55). These problems need to be addressed in the future. MICROEMULSIONS AND NANOEMULSIONS The concept of microemulsions (Figure 3) was introduced for the fi rst time in the 1940s by Hoar and Schulman (56), who produced a transparent single-phase solution by titrating a milky emulsion with hexanol. The term “microemulsion” was coined in 1959 (57). Mi- croemulsions consist of an aqueous phase, an oil phase, a surfactant, and a cosurfactant. They are colloidal, thermodynamically stable dispersion systems with a droplet diameter usually in the range of 10–100 nm (58). Compared to microemulsions, nanoemulsions have a droplet diameter smaller than 100 nm, are in a metastable state, and are easily val- ued in skin care because of their sensorial and biophysical properties (59). Stability studies of nanoemulsions indicate that they are stable for 15 years, which is an advantageous prop- erty for carriers in cosmetic and cosmeceutical products (60). In addition, they exhibit several advantages in topical drug delivery, i.e., control of drug release (61), protection of labile agents, increase of bioavailability, and enhancement of actives penetration (62). They are considered as potentially good carriers for cosmetic and cosmeceutical use. Drugs incorporated into microemulsions and nanoemulsions range from antioxidants, such as hesperetin and quercetin, to moisturizing compounds, like ceramides. Hesperetin