28 CHAPTER 2 Energy in the small: Integrated micro-scale energy sources fabricated using polypyrrole (PPy) as electrodes and solidified polyvinyl alcohol (PVA) as electrolyte. The lateral dimensions of the capacitor were 2 Â 2 mm and the thickness was much less than 1 mm. The capacitor structure could be deformed, rolled up, etc., without changes in performance. The structure had a capacitance of several mF and was tested at a maximum voltage of 0.6 V. In principle, such an all- solid polymer capacitor could have better scalability due to relaxed encapsulation constraints. It should also be noted that embedded miniature supercapacitors would find many applications in semiconductor microelectronics. For example, in order to respond to the power integrity problem in microprocessors, integrated thin-film high capacitances are needed [29]. Currently existing super- capacitors are not available in a planar format suitable for on-chip integration. The potential for integrated miniature supercapacitors for microprocessors was recently discussed in [29]. In conclusion, embedded miniature supercapacitors appear to be a potentially feasible power source, whose estimated upper bound for power lies between 0.1 and 1 W. 2.3 ENERGY FROM RADIOISOTOPES 2.3.1 Radioisotope energy sources