## 3 Results and Discussion
### GPC Analysis  
Supplementary Table S1 reports the molecular weight distributions, intrinsic viscosity (IV) and weight fraction of the polymeric part, named peak 1, ranging from 9 to 15 min retention volume, and the low-molecular-weight fraction comprehensive of degraded chains (oligomers, monomers, etc. ), named peak 2, ranging from 17.5 to 19 min retention volume (see Supplementary Figure S1). It is important to underline that the values reported (Mw, Mn and IV) must be attributed to a mix of undegradable POLOX and PLGA. Comparing the 20-100 and 24-100 series, it is possible to note that the molecular weights and IV decreased over time, with the same trend (Figure 5), as expected due to the similar porous appearance of MPs (see Figure 1). Instead, the ratio between peak 1 and peak 2 was different, possibly due to the large amount of degraded PLGA (entries 4 and 16 in Supplementary Table S1). In any series, except 24:100, after 14 days, PLGA decomposed significantly slower, in line with the release kinetics and initial release data (Figures 2 and 3), therefore indicating an erosion-driven release mechanism. In the 20:90 series, it is possible to note that Mw was halved at the final point and, at the same time point, IV was higher than in previous samples, with a value close to pure POLOX F127. This could be explained by the fact that peak 1 (9-15 min retention volume) was composed mainly of POLOX as consequence of the \"high\" degradation rate of PLGA. This may indicate that POLOX cannot exert a satisfactory \"shielding effect\" due to their low amount in the 20-90 formulation. This hypothesis is supported by the values of peak 1 and peak 2. Figure 4: Trends of PLGA glass transition temperatures. (**A**): 20—70, 20—90, 20—100 formulations; (**B**): 24—70, 24—90, 24–100 formulations. Figure 5: Time trend of for 20–100 and 24–100 (PLGA—only) MPs. In the 20:70 series, the higher concentration of POLOX led to a \"slower\" degradation, as indicated by peaks 1 and 2; however, at the final point IV slightly increased outside of the trend, showing unsatisfactory protection of PLGA from degradation. In the 24:90 and 24:70 series, the higher concentration of polymers led to lower molecular weights at the starting points. In both series, and more evidently in the 24:70 series, it is possible to see (Figure 6) the presence of POLOX in peak 1 at 14, 21 and 28 days, and at the last point, it is possible to note a shoulder at 10.3 min attributable to un-degraded PLGA. Overall, the results indicate that the formulation conditions had a relatively weak effect on the MP degradation kinetics. More specifically, the decrease in overall MP molecular weight in the absence of POLOX was faster for the 24-100 compared to the 20-100 formulations. This indicates that, despite the porous morphology of microdevices, autocatalysis is enhanced at the highest polymer concentration. Moreover, 20-90 and 24-90 MPs showed a slightly increased degradation rate, to a higher extent in the case of the 24-90 formulation. This can be explained considering that POLOX facilitate water penetration, with autocatalysis being very likely due to the high polymer concentration. Consequently, the formation of short PLGA chains in the MP bulk is promoted by the enhanced water penetration, while the transport of degradation products is hampered. At the same time, the increased MP hydrophilicity is expected to facilitate base penetration in MPs, which neutralizes the generated acids. The overall balance suggests that acid generation is probably faster than the neutralization rate [25; 26], which is related to a drop in local pH within MPs. The presence of a higher POLOX concentration in 20-70 and 24-70 MPs, on the contrary, induced a degradation rate closer to that of 20-100 and 24-100 MPs. In fact, an increasing POLOX amount is expected to promote water intrusion in MPs, and this causes the enhanced mobilization of the acidic degradation products of PLGA.