Magnesium Sensors In article number 2201854, Seungyong Han, Sang Min Won, and co-workers report a functional encapsulating structure that quantitatively measures the diffusion of fluids into the encapsulating barrier. The hydrolysis of magnesium electrode underneath the encapsulating barrier shows the capability of wireless monitoring of the water and/or biofluid penetration rate and the presence of defects, such as pinholes or cracks, in the encapsulating material.