## 3 Results and Discussion
### Ultrasound Micromolding of PLA:PEG:NaCl Mixtures. Salt Leaching and Scaffold Characterization  
PLA:NaCl mixtures with salt contents lower that 50 wt-% could be micromolded under the previously determined conditions (i.e., amplitude, force and time of 24 \(\upmu\)m, 400 N and 1.2 s, respectively) without a significant degradation. Namely, GPC curves showed only a minor molecular weight loss for the PLA component as above discussed for the PLA:NaCl specimens (Figure 3). Scaffolds were therefore prepared with PLA, PEG and NaCl contents of 50 wt-%, 10 wt-% and 40 wt-% (abbreviated as PLA:PEG-10:NaCl-40), respectively, and a NaCl particle size lower than 45 \(\upmu\)m in order to favor the leaching process. PEG content was found enough to dissolve the salt as demonstrated by thermogravimetric analyses. Note also that the low PEG percentage (i.e., 10 wt-%) should lead to a relatively homogeneous distribution of both polymers in the specimen. FTIR spectra (Figure 10) clearly indicated that PEG could be effectively removed after immersion of micromolded specimens for 1 day into water. Results were positive for both selected PLA:PEG 70% and PLA:PEG-10:NaCl-40 specimens. Thermogravimetric (TGA) assays were useful to quantify the amount of NaCl incorporated in the processed specimens, since the residue determined after heating the samples up to 600 \({}^{\circ}\)C should be associated to the salt content due to the expected polymer decomposition. Figure 11a shows TGA curves of two representative specimens before and after the washing process. In the case of the PLA:NaCl-35 sample the TGA curve was characterized by a single decomposition step with a maximum at 370 \({}^{\circ}\)C, which is associated to the PLA decomposition [49] and a high residual weight. This is nevertheless lower that the theoretical salt content (i.e., 30.2% with respect to 35%), suggesting that the flow of NaCl was hindered and consequently the salt was preferably located in the sprue. This feature was corroborated also from the 40% char yield determined from TGA experiments performed with the sprue (not shown). Char yield decreased to 18.9% after the washing process,  
Figure 10: FTIR spectra of PLA:PEG 70% and PLA:PEG-10:NaCl-40 specimens after immersion in a water medium. For comparison purposes spectra of PLA and PEG micromolded specimens are also given. a value that was significant and indicative of a considerable amount of NaCl retained in the specimen. In this case the effectiveness of the leaching process was only 37%. Figure 11b demonstrates the effectiveness of the leaching process (e.g., 95%) when PEG was incorporated. The TGA curve of the non-washed samples showed two decomposition steps associated to PLA and PEG at 370 \({}^{\circ}\)C and 430 \({}^{\circ}\)C (423 \({}^{\circ}\)C is the value reported in the literature for PEG [50]) and a high residue that was lower, as expected, than the theoretical value. After washing, salt was practically eliminated, and a small trace of PEG could still be detected. In addition to the residue quantification by the TGA assay, a visual inspection of SEM micrographs showed that the internal surface of PLA:PEG-10:NaCl-40 specimens after washing (Figure 11d) was rough and porous and practically free of NaCl particles. A clear contrast exists in related specimens where PEG was not incorporated (Figures 4c and 11c). Hydrophilicity of the specimen surface is an important factor that favors the salt leaching process to get a porous scaffold. Contact angle measurements of representative ultrasound processed samples are shown in Figure 12.  
The initial PLA specimen was characterized by an angle of 69\({}^{\circ}\), which is low probably as a consequence of the relatively low molecular weight of selected PLA (effect of terminal groups) and the smooth surface of processed samples. In fact, angles close to 90-100\({}^{\circ}\) are usually determined for PLA due its hydrophobic character, instead higher contact angles of 131.9\({}^{\circ}\) were reported for PLA electrospun scaffolds [51].