Figure 7.

Long-term culture demonstrates that hydrogel-embedded PLGA µSpheres degrade over an 84-day period and leads to a significant decrease in biomechanical properties while maintaining a physiological pH range corresponding to that of a healthy IVD. (A) Complex shear stiffness (|G*|) and tangent phase angle (tan δ) at 1Hz for IPN hydrogels with 0% (w/v), 2.5% (w/v), and 5% (w/v) PLGA µSpheres over an 84-day culture period. (B) Weekly pH of culture solution over the 84-day culture period for all hydrogel conditions. (C) IPN hydrogel wet weight over the 84-day culture period for all hydrogel conditions. (D) Swelling ratio (Q_w) at day 84 for IPN hydrogels with 0% (w/v), 2.5% (w/v), and 5% (w/v) PLGA µSpheres. (E) SEM images of IPN hydrogel microstructure over the 84-day culture period for all hydrogel conditions. Yellow dashed box = regions corresponding to higher magnification SEM images in panel F. Objective = 70x; Scale bar = 500 µm. (F) Quantification of hydrogel pore size after 7, 14, 21, 42, 63, and 84 days in culture. (G) SEM images of IPN hydrogel microstructure over the 84-day culture period for IPN hydrogels with 2.5% (w/v), and 5% (w/v) PLGA µSpheres. Objective = 250x; Scale bar = 250µm. (H) Quantification of PLGA µSphere count embedded within the IPN hydrogels after 7, 14, 21, 42, 63, and 84 days in culture. PLGA = poly(lactic-co-glycolic acid); IVD = intervertebral disk; IPN = interpenetrating network; SEM = Scanning Electron Microscopy. #: P < 0.05 at t_n compared with t₀ for 0% (w/v) PLGA µSpheres; $: P < 0.05 at t_n compared with t₀ for 2.5% (w/v) PLGA µSpheres; &: P < 0.05 at t_n compared with t₀ for 5% (w/v) PLGA µSpheres.