Magnesium vs. sodium alginate as precursors of calcium alginate: Mechanical differences and advantages in the development of functional neuronal networks

Calcium alginate is one of the most widely employed matrices in regenerative medicine. A downside is its heterogeneity, due to the poorly controllable character of the gelation of sodium alginate (NaAlg), i.e. the commonly used alginate salt, with calcium. Here, we have used magnesium alginate (MgAlg) as an alternative precursor of calcium alginate. MgAlg coils, more compact and thus less entangled than those of NaAlg, allow for an easier diffusion of calcium ions, whereas Mg is exchanged with calcium more slowly than Na; this allows for the formation of a material (Ca(Mg)Alg) with a more reversible creep behaviour than Ca(Na)Alg, due to a more homogeneous - albeit lower - density of elastically active cross-links. We also show that Ca(Mg)Alg supports better than Ca(Na)Alg the network development and function of embedded (rat cortical) neurons: they show greater neurite extension and branching at 7 and 21 days (Tubb3 and Map2 immunofluorescence) and better neuronal network functional maturation / more robust and longer-lasting activity, probed by calcium imaging and microelectrode array electrophysiology. Overall, our results unveil the potential of MgAlg as bioactive biomaterial for enabling the formation of functional neuron-based tissue analogues.