Fig. 4. Mechanical reinforcement via in situ mineralization in other metal-coordinate crosslinked networks.

Using noncovalent metal-coordinate networks crosslinked by Ni- or Cu-histidine-coordination, the same four processing pathways (i.e., Mineral-Free—blue violet, skyblue; In Situ—red violet, navy; Ex Situ—magenta, sapphire blue; Ligand-Free—purple, blue, respectively for Ni- and Cu-systems) as in Fig. 1 were tested. The Mineral-Free samples were formed via mixing the metal ions with 4hPEG in 1:2 stoichiometric ratios of metal ions:histidine, then raising pH to ~12 using NaOH. The in situ samples were prepared by inducing Ni(OH)₂ or Cu(OH)₂ nucleation via additional introduction of Ni²⁺ and Cu²⁺ ions to the Mineral-Free systems through reaction with the excess OH⁻(final metal ions:histidine = 1:1). a Linear viscoelastic frequency sweeps (storage modulus-G′, loss modulus-G″) and b plateau moduli (G_p) of the samples from the four different processing pathways of the Ni-histidine system. c Linear viscoelastic frequency sweeps and d G_p of the Cu-histidine system. Inset images are representative photographs of each type of sample. Note that precipitation of particles is visible in the Ex Situ gels. The rheological measurements were performed at 1% strain and 25 °C. The asterisk indicates missing values when G′ is not detectable.