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Heyy... I am also working on tbe same project and vibration energy harvesting from piezo.. I will be making this shoe ready in few days.. For the problem you mentioned, you can use diodes for every piezo in order to get a steady and enhanced response.. But it will dissipate more energy due to its biasing need.. Using piezo array will also do..but it depends how have you connected the piezo sensors.. All the best for your project.. You can discuss related topics on piezo on firstname.lastname@example.org