# Functionalization Strategies and Fabrication of Solvent-Cast PLLA for Bioresorbable Stents
## 3 Results and Discussion
### Surface Characterization  
Surface roughness modifications due to the activation treatments were characterized by white light interferometry (WLI) (Figure 2a). SC surfaces displayed the lowest value of R\({}_{\text{a}}\), but it was statistically equal to TT, OP, and ET suggesting that these three treatments had a negligible effect on the surface roughness. Contrarily, the effect of NaOH treatment was more aggressive and modified the surface roughness parameters by doubling the parameter Ra compared to the other films. In addition to the fact that the NaOH treatment also decreased the PLLA molecular weight, it is evidenced that the alkaline activation process was a more aggressive treatment than the plasma or enzymatic treatment, affecting a few microns depth from the surface of PLLA films. The wettability of the different fabricated PLLA films was characterized by a water contact angle (Figure 2b). Moreover, the wettability evaluation revealed that the thermal treatment significantly increased the water contact angle compared to SC. This difference could be explained by the higher crystallinity of TT films, inducing a lower percentage of amorphous phase groups and, consequently, lower hydrophilicity. On the one hand, NaOH etching did not change the wettability as SH presented a water contact angle of 84.2 \(\pm\) 4.3\({}^{\circ}\) with no-statistical differences when compared to TT. On the other hand, plasma and enzyme activation treatments significantly reduced the water contact angle, obtaining values \(\approx\)20\({}^{\circ}\) and \(\approx\)25\({}^{\circ}\) lower than TT, respectively. Other authors have already reported the increase in hydrophilicity after the plasma treatment and enzymatic hydrolysis [31; 54; 55; 56] on PLA, or other polyesters. This is usually attributed to the generation of new surface functional groups [22; 55; 57]. The surface chemical composition of TT and activated films is shown in Table 3. The XPS analysis revealed the presence of C 1s, O 1s, N 1s, and Na 1s. The atomic percentage of O 1s increased, while C 1s decreased for all the activated surfaces compared to TT films. Consequently, the O/C ratio was higher on the activated surfaces (>0.64) compared to TT (0.61). The increase confirmed the incorporation of O 1s on the PLLA film surfaces to form functional groups. The O/C ratio was statistically higher on OP and ET films, 0.75 and 0.97, respectively compared to SH (0.64), suggesting a higher efficiency of plasma and enzyme to create functional groups. Moreover, the presence of N 1s was observed in small quantities on OP and ET films 0.8 \(\pm\) 0.1 and 0.5 \(\pm\) 0.1%, respectively. For the OP samples, the detection of N 1s can be associated with air contaminations in the plasma chamber, while the ET samples can be ascribed to residues of cutinase, as cutinase contains nitrogen [58]. Finally, the SH films showed the presence of low amounts of Na 1s (0.5 \(\pm\) 0.1%). These residues of NaOH could explain the cytotoxicity observed on these films. The decomposition of the high-resolution C 1s peak of TT and the activated samples (Figure 3 and Table 3) revealed three peaks on all surfaces corresponding to: (1) C-C and C-H bonds at 284.7 eV; (2) C-O bonds at 286.6 eV; and (3) O = C-O bonds at 288.8 eV [56]. The surfaces with any of the three activation treatments presented a significantly higher atomic percentage of O = C-O bonds (>30.7 \(\pm\) 1.0%) than TT films (29.2 \(\pm\) 0.6%) indicating that all three activation treatments were able to create -COOH functional groups on the surface. Plasma and enzymatic treatments presented 36.9 \(\pm\) 0.1 and 36.6 \(\pm\) 0.4% of O = C-O bonds, respectively, meaning a higher amount of carboxyl groups were created