Given its proximity to the human body (sometimes even inside), many medical devices have high safety battery applications. Energy density and specific energy are important; however, for cost, the battery may represent only a small fraction of the total medical device cost. The energy delivered over the battery's useful lifetime per mass (i.e. lifetime specific energy) could be another important measure for implantable medical devices; this is essentially a combined measure of cycle life and specific energy. Some implantable medical devices require operation for 10 or more years . Improving the performance of batteries could potentially lengthen time between surgeries. Cycle life requirements depend on the nature of the device: some are single-use, whereas others (even heart-related devices) can be charged directly or remotely. Examples of medical devices include heart pumps, blood-glucose (BG) monitors, and hearing aids.