NMR relaxometry and imaging of water absorbed in biodegradable polymer scaffolds

Porous substrates made of poly(3-hydroxybutyrate-3-hydroxyvalerate) (PHBHV) were prepared by a particulate leaching method. After removing the salt by extraction in water, proton nuclear magnetic resonance (NMR) relaxometry and imaging were performed on sets of PHBHV substrates immersed in phosphate-buffered solution during 3 months at different time points. Polarized optical microscopy studies were performed on thin sections, 25 and 5 μm, of the PHBHV samples. The results of NMR relaxometry showed two 1H nuclei populations, well distinguishable on the free induction decay (FID), due to the different decay time constants, a factor of 102 apart. Thus, it was possible to separate the two populations, giving separate distributions of T1 relaxation times. One population could be associated with water protons in the pores and the other to macromolecular protons. The distributions of T1 and T2 of the water proton shifted to lower values with increasing immersion time to a constant value after 30 days. The results obtained by NMR imaging showed an initial increase in the apparent porosity, reaching a plateau after 25 days of immersion. This increase is attributed mainly to the absorption of water in the microporosity as supported by the results of the relaxometry measurements and shown by scanning electron microscopy. The average porosity measured by NMR imaging at the plateau, 78±3%, is slightly higher than that determined by optical microscopy, 73±9%, which may be due to the fact that the latter method did not resolve the microporosity. Overall, the results suggest that at early stages after immersing the scaffolds in the aqueous medium, first 30 days approximately, NMR imaging could underestimate the porosity of the substrate.