gradients or magnetic fields and are therefore often counted into the realm of smart soft matter. [1] In the early 1960s, Wichterle and Lim prepared functional prototypes of hydrogels in the form of contact lenses. [2] Their achievement is now considered as the first biomedical application of hydrogels. However, it was soon understood that hydrogels could have a much wider range of applications due to their highly porous microstructure, tunable mechanical properties and high surface area. Therefore, they could potentially have diverse applications in drug delivery, tissue engineering, energy storage, electronics, and biosensing. [3-5] Structural characterization of hydrogels is often achieved by imaging techniques likes scanning and transmission electron microscopy (SEM and TEM) and atomic force microscopy (AFM). Using these powerful methods, one can obtain information on the nature of the hydrogel, its porosity, surface morphology, and mechanical properties. However, there are some drawbacks to these methods. For example, during SEM measurements, the electron beam could destroy the samples. In electron microscopy, only cryo-TEM can be used to study hydrogels in the swollen state. [6-8] Hydrogels can also be found in the form of hybrid (composites) organic/polymeric-inorganic materials, which opens new pathways for the design of new smart materials through variation and tuning of the individual components. Among these composite hydrogels, metal-containing hydrogels are an interesting class of materials. On a macroscopic scale, metal-containing hydrogels combine the flexibility and solubility of organic polymers with properties particular to metals like redox, catalytic and magnetic activity. On the molecular scale, metal ions can interact with the polymeric/organic components through complexation and mainly through electrostatic interactions, providing new interaction patterns inside hydrogels leading to specific properties. [9] In the search of new, environmentally friendly and sustainable materials, Sun et al. [10] proposed poly acrylic acid (PAA) hydrogels, cross-linked by calcium cations. Using calcium minerals even in low concentrations improved the mechanical properties considerably (G′ ≈10 5 Pa). In another experiment by the same group, successful hydrogel formation took place in the presence of earth alkaline metal ions and some of the Incorporating metal ions into synthetic polymer hydrogels results in toughening of these hydrogels. Herein, it is demonstrated that addition of small volumes of a 0.1 m aq. solution of Cu 2+-salts to poly(acrylic acid)-based hydrogels (physically cross-linked by Ca 2+) increases their mechanical/rheological properties by several order of magnitude. Continuous wave (CW) and pulsed electron paramagnetic resonance (EPR) spectroscopic techniques reveal that the origin of the observed boost in mechanical properties is due to parts of the hydrogel network that interact with and through hydrated copper ions. With EPR spectroscopy, it is found that these complexes mainly have a six...
