## 2 Materials and Methods
### Rheological Properties and Sun Protection Factor (SPF) of an Extract-Based Cream  
In this part of the study, the _A. millefolium_ extract was incorporated into a base cream developed by the authors. Rheological tests were conducted for the cream, and its SPF was determined by comparing it with the UV protection factor of yarrow extract. Rheological properties have a major impact on the quality of a cosmetic. An appropriate viscosity enables convenient dosing of the product and affects sensory parameters, such as spreadability and the cushion effect, ensuring comfort of use. In the case of skin care cosmetics, viscosity also significantly affects the effectiveness of the product. Previous studies show that the release of active substances from the cosmetic is more effective at low viscosity values [41]. The viscosity of the formulation as a function of shear rate is shown in Figure 10.  
For all samples tested, the viscosity curves have a similar shape typical of non-Newtonian fluids thinned by shear. The viscosity of such fluids decreases as the shear rate increases. This shape of the viscosity curve is typical for emulsion formulations [42]. This is highly advantageous in terms of application. High viscosity at low shear rates enables  
Figure 10: Viscosity of samples vs shear rate. E0, base cream; E1, cream with 1% extract; E5, cream with 5% extract; E10, cream with 10% extract. dosing of the product. However, reduced viscosity in combination with high shear force facilitates the release of active particles from the formulation as it is spread over the skin. Chronic exposure of the skin to UV radiation leads to many adverse effects, such as sunburn, premature skin ageing, and in extreme cases, even skin cancer. The task of sunscreen cosmetics is to counteract these unfavourable processes. The main ingredient in cosmetic formulas is UV filters, which are responsible for radiation protection properties. Currently, two types of filters are commercially used: inorganic (physical) and organic (chemical). The mechanism of action of inorganic filters is based on typical physical reflection and scattering of radiation, while organic filters act mainly as radiation absorbers [43; 44; 45]. Both types of filters have their limitations, associated with low efficiency, potential phototoxicity, or photosensitization. They can also adversely affect the natural environment. Therefore, intensive searches for effective and ecological UV filters are ongoing. Due to the current trend of sustainable development and care for the natural environment, plant materials are the most suitable alternative to synthetic UV filters. One proposal that fits into this ecological trend is the use of plant extracts both as stand-alone filters and as substances supporting photoprotection. Plant extracts have many compounds containing aromatic rings and other functional groups, which usually have a wider absorption spectrum, covering a wavelength range of 200-400 nm [44; 46]. Plant ingredients used as UV filters usually also exhibit strong antioxidant properties. In addition, phytochemicals, which are considered non-toxic and pharmacologically safe, have been shown to have genoprotective and anticancer properties. They can prevent cell mutations associated with skin cancer and ageing by regulating UV-induced mutations [47; 48; 49; 50]. The UV absorption of _A. millefolium_ extract was analysed. To select the optimal concentration of the extract for the cosmetic cream, the absorption of aqueous solutions of extracts was tested at various concentrations, from 100%wt. to 0.5%wt. The initial extract solution of 2 g of dry matter dissolved in 60 mL of water-ethanol solvent was assigned a value of 100%. The UV absorption spectra for the aqueous solutions are presented in Figure 11.  
The _A. millefolium_ extract showed a high absorption capacity in the UV range, which indicates potential protection against both UVB (280-320 nm) and UVA (320-400 nm). Figure 11: UV absorption spectra of yarrow extracts.