## 2 Materials and Methods
### Extraction  
Initially, 50.0 to 200.0 g of chopped and dried birch bark (kindly provided by Birken AG, Niefern-Oeschelbronn, Germany) was placed in a high-pressure extraction unit with an extraction volume of 3.3 L (SITEC GmbH, Maur/Zurich, Switzerland). Supercritical carbon dioxide (Westfalen AG, Muenster, Germany) was conveyed for three hours via a high-pressure pump to the incubator to obtain a solution of the components in supercritical CO\({}_{2}\). Monitoring of the extraction parameters was done for mass flow and density with a mass flow meter (MassFlo 2100 di 1.5, Danfoss, Ebersbach, Germany) located between the high-pressure pump and the entrance of the extraction vessel, temperature (SITEC Thermocouple Pt100, SITEC GmbH, Maur/Zurich, Switzerland) and pressure (SITEC pressure \begin{table}
\begin{tabular}{c c} \hline \multicolumn{2}{c}{**Pharmacological Effects of Titirepenoids**} & \multicolumn{1}{c}{**References**} \\ \multicolumn{2}{c}{**Contained in Birch Bark**} \\ \hline antibacterial, antiviral, anti-parasite & BE: [5; 6]; BA: [7; 8; 9]; BE/BA: [10; 11]; BE/BA/OA: [12]; LU/BA: [13]; OA/BA: [14] \\ \hline hepatoprotective & BE: [15; BE/BA: [16]]; OA: [17; 20]; LU: [2]; BA: [22] \\ \hline antitumor & BE: [23; 24]; BA: [25; 26] BE/BA: [27; 28]; LU: [29]; OA: [18; 19; 30] \\ \hline wound healing & extract [31; 32; 33]; BA: [34]; OA: [35; 36] \\ \hline anti-inflammatory & BE: [5; 37]; LU: [38; 9]; OA: [18; 40] \\ \hline \end{tabular}
\end{table} 
Table 1: Pharmacological effects of triterpenoids obtained from bark of birch. BE: betulin, BA: betulinic acid, OA: oleanolic acid, LU: Lupeol. transducer, SITEC GmbH, Maur/ Zurich, Switzerland). The extraction conditions were optimized in pilot experiments in the same setup, obtaining an extraction yield of about 5.8% \(\pm\) 1.4%, and were found to be a pressure of 35.0 MPa and a temperature of 333.2 K (Supplementary Materials, Table S1). Three ways of separating the extract from the supercritical solution were tested:  
* Supercritical fluid extract (SCF-E): The supercritical solution was expanded through an automatic valve (pneumatic) into a separator at 4.0 MPa. Under these conditions, the supercritical CO\({}_{2}\) separated into a liquid and a gaseous phase. In the liquid phase, the extract accumulated and when the pressure was completely released, particles deposed and could be removed as a dry extract (Figure 1a). In parallel, the CO\({}_{2}\) was recycled by the high-pressure pump and refed to the incubator. For this setup, the mass flow of CO\({}_{2}\) was between 12.5 and 15.0 kg/h, and the CO\({}_{2}\) density at 35 MPa and 333.2 K was 850.0 \(\pm\) 2.0 kg/m\({}^{3}\). * RESS-Extract (R-E): The supercritical solution here was expanded through a capillary nozzle (diameter: 150 \(\upmu\)m) into a vessel where the precipitated particles could be collected (Figure 1b). To avoid the formation of ice near the nozzle due to a massive temperature drop caused by the expansion of the supercritical solution (Joule-Thompson effect), the CO\({}_{2}\) mass flow was reduced to 1.0-2.0 kg/h for this setup. Additionally, the vessel was heated so that the mean deposition temperature remained at 317.5 \(\pm\) 3.5 K. In the expansion unit, the pressure was maintained at 4.4 \(\pm\) 0.3 MPa. A manual micro valve controlled the CO\({}_{2}\) flow through the nozzle. The pressure in the incubation chamber was kept at 35 MPa and the temperature at 333.2 K, as in the SCF-E experiments. The CO\({}_{2}\) density was measured at 888.3 \(\pm\) 7.2 kg/m\({}^{3}\), being slightly higher due to the reduced mass flow compared to the SCF-E conditions. * A third extract was prepared by a modified RESS-procedure (MR). Here, the supercritical solution was expanded through the nozzle again, but directly into a lipophilic phase (jojoiba oil) to form a gel in a single step production method, including particle deposition and gel formation (Figure 1c).