## 4 Topical Applications of Curcuminoids  
Turmeric and its components have been subjected to numerous studies investigating their potential application in various medical conditions, particularly those involving some oxidative and inflammatory mechanisms. In most of the studies, turmeric constituents are considered for oral administration and are supposed to exert systemic effects. However, the antioxidant, anti-inflammatory and anticancer properties of curcuminoids combined with their antimicrobial potential can be also utilized in topical formulations designed to treat the conditions localized in skin and mucous membranes. According to the literature reports, curcumin can be useful in various skin infections, psoriasis, acne and skin cancer [92]. It is noteworthy that some of the difficulties encountered in oral curcumin administration can be overcome with the use of topical route. As curcumin is a lipophilic compound revealing log P = 3.29 and relatively low molecular mass [93], it may be expected that transdermal products can be advantageous in terms of drug bioavailability. However, in vitro skin permeation studies performed by [PERSON] et al. [94] indicate that curcumin flux without any additional percutaneous absorption enhancers is low. On the other hand, in the case of topical formulations, other drawbacks of curcumin are mentioned, i.e., chemical instability leading to the decomposition upon the contact with UV and visible light and rapid elimination from the body [92]. Therefore, in the studies regarding topical administration of curcumin-based formulations, some approaches usually aim at minimizing the effects related to unfavorable properties of curcumin. Similarly to the formulations designed to be administered by other routes, nanostructured lipid carriers (NLC), liposomes, nanoparticles, nano -and microemulsions are taken into consideration [95]. The application of a proper drug delivery system allows for transporting the active ingredient through _stratum concneum_, the most important skin barrier, into the deeper skin layers. One of the most extensively investigated directions in the studies regarding topical formulations with turmeric-derived compounds is related to wound healing. Curcumin reveals numerous activities useful in terms of tissue regeneration. It displays antimicrobial activity preventing the infection of damaged tissues, and its anti-inflammatory properties also support the regeneration. It was shown that curcumin increases fibroblast proliferation and helps in epithelium regeneration [96]. The study performed by [PERSON] et al. [51] with the use of animal model revealed that burn wounds healed significantly faster with the aid  
Figure 4: Chemical structure of the common components of turmeric, other than curcumin. Reproduced from [78] with permission. of curcumin compared to non-treated burns which was also confirmed by histopathological and biochemical tests. In the presence of curcumin, the epithelialization and granulation rate were much higher, which indicates the potential usefulness of curcumin in burn wounds therapy. Apart from antioxidant, anti-inflammatory and antimicrobial effects presented by curcumin, it also affected the process of collagen crosslinking, indicating the enhanced rate of wound healing [97]. Similar effects were observed in the case of wounds localized in the mucous membrane. [PERSON] et al. [98] investigated healing process in rabbits with perforated nasal septum in the presence of curcumin. It was found that the active ingredient significantly enhanced the process, increasing the amounts of collagen and granulation tissue. Similar effects were observed in various mucosal damages, including oral mucositis occurring as a side effect of anticancer therapy [99] and corneal wounds observed as a complication of diabetes [100]. As important drawbacks of curcumin, its low solubility in water and low bioavailability are mentioned.