*3.3. Comparative Studies of the Release of p-Anisic Acid from (p-AA-CH2-HP)n Oligoester and [(p-AA-CH2-HP)x-co-(HB)y] (Co)oligoesters*

The release studies of p-anisic acid from both (p-AA-CH2-HP)n oligoester and [(p-AA-CH2-HP)x-co-(HB)y] (co)oligoesters were performed in aqua solutions at 37 ◦C for the period of 14 days. To determine the amount and profile of the p-anisic acid release from [(p-AA-CH2-HP)x-co-(HB)y] copolymers, the mediums collected after a specific period of degradation of (co)oligoesters were analysed by high-performance liquid chromatography (HPLC) equipped with a DAD detector. The results of HPLC analysis, including the presence of p-anisic acid and calculated based on the calibration curve of the amount of acid released from the studied (co)oligoesters over time, are presented in Figure 5.

**Figure 5.** Comparative release profiles of p-anisic acid from (p-AA-CH2-HP)n homo(oligoester) (**a**) and [(p-AA-CH2-HP)x-co-(HB)y] (co)oligoesters (**b**).

The amount of p-AA released from (p-AA-CH2-HP)n oligoester and [(p-AA-CH2-HP)x-co-(HB)y] (co)oligoesters was determined for 14 days. Initially, a slightly higher release rate was observed, but after 3 days, p-AA showed a relatively slower release rate. From the p-AA release profile, it is clear that almost 50% of the bioactive compound studied was released from the carrier in the first 3 days in the case of [(p-AA-CH2-HP)x-co-(HB)y] and, in the case of (p-AA-CH2-HP)n, it was more than 75%. Additionally, we consider such concentrations as achievable in the epidermis, taking into account possible cosmetic formulations, as well as release and absorption rates described in our previous work in this field [23]. We should highlight that the release of a bioactive agent proceeded regularly. However, the release rate of p-AA was much faster for (p-AA-CH2-HP)n, which is an expected result as this carrier possessed a higher loading of p-AA per polymer macromolecule compared with (co)oligoester carrier (Figure 5). As we described previously, bioactive oligoesters that contain from 1 to 8 of p-AA molecules and (co)oligoesters that contain from 2 to 3 molecules of the p-AA moiety (as the bioactive end and side groups) were synthesized [25].

## *3.4. Cytocompatibility of (Homo)- and (Co)oligoesters Containing p-AA Moiety*
