Fuel cells that run on glucose could be ideal alternatives to batteries for powering implanted medical devices, such as pacemakers and brain stimulators. But fuel cells alone cannot generate the quick, high-energy pulses such devices need. Researchers have now made a compact power source that delivers such bursts by combining a glucose fuel cell with a supercapacitor (ACS Energy Lett. 2016, DOI: 10.1021/acsenergylett.6b00225). Shelley D. Minteer of the University of Utah and colleagues coated carbon electrodes with enzymes that catalyze redox reactions to produce an electric current. The anode is loaded with glucose dehydrogenase and coated with polyethylenimine, which stores charge and acts as the supercapacitor. As glucose becomes oxidized, electrons travel through an external circuit to the cathode, which is coated with bilirubin oxidase. Some of the electrons charge the capacitive polymer layer. During spikes in power demand, the supercapacitor discharges, providing the needed high-energy bursts. The researchers encased the device in a metal mesh so that blood can reach the electrodes and provide a constant supply of glucose. The new device has a capacitance of 300 faradays per gram, more than four times as high as previous biobased supercapacitors delivering comparable voltage.