Porous alginate hydrogels or scaffolds have applications in cartilage regeneration and the treatment of cartilage lesions or diseases, like osteoarthritis1,2. Cartilage producing cells, seeded in alginate scaffolds, have successfully shown cartilage extracellular matrix formation ex vivo and in vivo (in mice)2 . Magnetic Resonance Imaging (MRI) techniques provide a non-invasive method for monitoring this regenerated cartilage, both in vivo3 and ex vivo4 . Therefore, MR investigations of the hydrogels, prior to seeding, which relate quantitative MR characteristics of the gel (e.g. chemical shifts, relaxation rates) to the physical characteristics to be monitored (e.g. composition, structure, pore size) may be crucial for clinical evaluations4 and refined development of the scaffolds.
In this study, we use μ-MRI (with a 7 T static magnetic field) to investigate the longitudinal relaxation rates in alginate scaffolds with the potential for use in treating osteo-chondral defects. The cylindrical scaffolds (diameter ~ 8 mm, height ~ 4 mm) investigated were produced through ionotropic gelation with Ca2+ ions. Two types of scaffolds were investigated: the first is a homogenous alginate gel; the second contains channels, or pores (diameter ~ 150 μm), which run parallel to the cylindrical axis of the sample. Six scaffolds were imaged in total, three of each type. The samples were first imaged in an uncompressed state, in 3 mM CaCl2 solution, and then compressed by the application of Teflon plugs. We discuss the results in the context of water loss and gel deformation, with a focus on how the pores change during compression. Pore size is a crucial factor in the successful cultivation of cells in scaffolds5 and, given that any scaffold implanted in a joint would undergo compressive stresses, the extent to which pore sizes change may be the difference between an unsuccessful or successful scaffold.