3.5.4. The Swelling Studies

The swelling degree of the gel samples was determined in PBS (phosphate-buffered saline) (0.01 M at pH = 5.4) and at 25 ◦C room temperature. The amount of the adsorbed solution was gravimetrically determined: the swollen gels were regularly taken out from the swelling medium, soaked on filter paper surface, weighed, and placed after every weighing in the same swelling medium. The measurements were continued until a constant weight was reached for each studied sample. All swelling experiments were performed in triplicate for S1, S2, and S3 samples. The degree of swelling (SD) was calculated by the following equation:

$$SD\_\prime \%= \frac{M(t) - M\_0}{M\_0} \times 100$$

where *M*(*t*) is the weight of the swollen particles at time *t*, and *M*<sup>0</sup> is the weight of the sample before swelling (the weight of the dry sample).

The experiment was carried out in triplicate.

#### 3.5.5. The Release Study

The in vitro release studies of carvacrol were carried out in a 708–DS Dissolution Apparatus coupled with a Cary 60 UV–VIS spectrophotometer (from Agilent Technologies, Germany) in 100 mL medium (85 mL 7.4 pH PBS/15 mL ethanol) at 25 ± 0.5 ◦C with a rotation speed of 100 rpm. Aliquots of the medium withdrawn at predetermined time intervals were analyzed with a λmax value of 273 nm. The carvacrol concentrations were calculated based on the calibration curves determined at the same wavelengths.

The experiment was carried out in triplicate.

#### **4. Conclusions**

New gel network systems based on PITAU and alginate were developed in order to observe their characteristics, and to predict their potential applications in the biomedical and pharmaceutical fields.

According to SEM microscopy, the synthesized gels presented a continuous and porous morphology. SEM analyses also confirmed the important role of the ratio between PITAU and Alg co-partners in generating the bioconjugated structures with improved crosslinked networks. The also study underlined the dependence of the swelling properties of the synthesized networks on the ratio between PITAU and Alg co-partners. We found a better swelling capacity for the S2 sample (PITAU\_Alg\_2\_1), justified by the larger pores generated by the intermolecular physical bonds between the macromolecular chains of the system. The study also demonstrated that the increase in the alginate content in the PITAU– Alg structures determined a smooth growth of the thermal stability, due to the additional crosslinking bridges that occur with increasing the amount of alginate. Moreover, the increase in the synthetic polymer content in the gel network structure ensured a slower release of carvacrol, the encapsulated bioactive compound.

**Author Contributions:** Conceptualization, A.E.S. and L.E.N.; methodology, A.E.S.; validation, L.E.N., A.P.C. and A.G.R.; formal analysis, A.E.S.; investigation, N.T. and D.P.; resources, L.E.N. and A.E.S.; writing—original draft preparation, L.E.N. and A.P.C.; supervision, L.E.N.; funding acquisition. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was financially supported by the grant of the Romanian National Authority for Scientific Research, CNCS–UEFISCDI, project number 339PED⁄2020, PN–III–P2–2.1–PED–2019– 2743 "New hybrid polymer/peptide hydrogels as innovative platforms designed for cell cultures applications", within PNCDI III.

**Conflicts of Interest:** The authors declare no conflict of interest.
