## 2 Materials and Methods
### Rheological Properties and Sun Protection Factor (SPF) of an Extract-Based Cream
Reducing the concentration to 5% did not reduce the absorption in the shorter wavelength range, including the UVB region (290-320 nm). However, in the UVA region (320-400 nm), the absorption value decreased together with the extract concentration. The absorption of UV radiation is mainly due to flavonoids present in the extract [47; 48; 51]. Available literature data [43; 48; 51] indicate that most flavonoids show absorption in the range from 240 to 280 nm and from 300 to 400 nm. The range of absorption bands closely corresponds to the structure of the polyphenol molecule, both the degree of conjugation and the number and position of substituents and OH groups. Spectral analysis (UV) of flavanols provides absorption bands in the range of 270-290 nm. Epicatechin provides an absorption band at 278 nm, catechin at 280 nm, and chrysin, hesperetin, eriodictyl, and taxifolin at 288 nm [51]. The range of absorption bands, both the position of the peaks and their value, is strictly dependent on the composition of the extract itself and can be very diverse even among extracts obtained from the same plant [48]. The results of the UV absorption tests of the _A. millefolium_ extract influenced the decision regarding the composition of the cosmetic cream, in particular, the content of the extract. Four cosmetic creams were prepared: E0, without extract, and E1, E5, and E10 containing 1%, 5%, and 10% extract, respectively. The SPF factor of the creams was calculated based on measurement of UV absorption. SPF is a measure of the amount of solar energy needed to induce redness on protected skin relative to the amount of solar energy needed to induce redness on unprotected skin. The higher the SPF value, the better the sun protection of the cosmetic. The recommended method for assessing SPF is based on in vivo tests with the participation of volunteers. The methodology was developed and published by [PERSON] [52] and has been introduced into standardized measurement systems in Europe, Japan, and South Africa [45]. However, the use of in vivo methods is very difficult and time-consuming. Therefore, scientists are continually proposing more convenient in vitro methods using laboratory equipment. One of these is the Mansur method [22], which was used in this study. The Mansur method is distinguished by simplicity and good repeatability. It also shows good correlation with in vivo tests [46]. The SPF designation is based on the relationship between the erythematogenic effect (EE) and the radiation intensity at each wavelength. A detailed description of the method and calculations is presented in the section on research methodology. Based on the absorbance measurements, SPF was determined for individual samples (Figure 12). The SPF was lowest for the base sample without the extract (E0), amounting to 0.9. This value is not classified as protecting against the sun [45]. Higher SPF values were obtained for the remaining samples: 10.2 for E1 (classified as low protection), 18.5 for E5, and 20.7 for E10 (both classified as medium protection). In summary, it can be stated that many authors are undertaking research on _A. millefolium_[2; 24; 29], some of them also pointing to cosmetic and dermatological aspects of the use of this plant [7; 53; 54]. The study presents an evaluation of the cosmetic potential of extracts from _Achillea millefolium_ herb of the \"paprika\" cultivar, which has previously not been studied in this regard. Research on evaluating the antimicrobial and anti-ageing properties of water-ethanol extracts of yarrow complements and extends the existing knowledge in this field. Another novel aspect of the research was the determination of the sun protection factor of a model formulation based on the extract, as well as the rheological characteristics of the formulation, which determine its potential applications.