A fuel cell is provided with an anode and a cathode. The anode is in electrical communication with an anode enzyme and the cathode is in electrical communication with a cathode enzyme. The anode enzyme is preferably an oxidase or a dehydrogenase. The cathode enzyme is a copper-containing enzyme, such as a lacasse, an ascorbate oxidase, a ceruloplasmine, or a bilirubin oxidase. Preferably, the cathode enzyme is operable under physiological conditions. Redox polymers serve to wire the anode enzyme to the anode and the cathode enzyme to the cathode. The fuel cell can be very small in size because it does not require a membrane, seal, or case. The fuel cell can be used in connection with a biological system, such as a human, as it may operate at physiological conditions. By virtue of its size and operability at physiological conditions, the fuel cell is of particular interest for applications calling for a power source implanted in a human body, such as a variety of medical applications.