The lightweight construction, robust mechanical properties, and novel hardening mechanisms of invertebrate biting and piercing structures highlight their potential as models for advanced polymer systems. The mechanical properties of these structures often surpass those of synthetic polymers and can be comparable to mineralized composite tissues found in higher organisms. In addition, the natural structures also demonstrate mechanical gradients tuned for specific functions. Spatial and reconfigurable control of material mechanical properties would have many applications from shock-resistant electronic platforms to smart, responsive bandages and sutures. We have demonstrated the ability to express and purify large scale quantities of the protein Nvjp-1 (identified as the primary protein in marine worm jaw structures) and to process the purified protein into macro-scale hydrogels. Through a hierarchical sequence of ion treatments, involving exposure to divalent zinc cations, the mechanical properties of Nvjp-1 hydrogels can be modulated over a >100 fold range. Under certain conditions Nvjp-1 hydrogel sclerotization is very specific for zinc cations. However, it was observed that other transition metal cations (such as Cu2+, Ni2+ and Co2+) could induce a significant increase in elastic modulus if the hydrogel pH was modulated after metal binding. As with zinc-induced sclerotization, these changes are completely reversible and can be modulated over many cycles. Thus, modulation of polymer parameters through the incorporation of transition metal cations will potentially enable structures and materials with graded and reconfigurable properties.