NOVELTY - An electrochemical cell (100) for positioning in a biological system having oxygen, comprises: a substrate comprising a first face and a second face; an anode (102) disposed on the first face of the substrate, and comprises an anode enzyme and a mediator, where the enzyme and the mediator are crosslinked; and a cathode (104) disposed on the second face of the substrate.    USE - As an electrochemical cell for positioning in a biological system having oxygen (claimed), particularly for generating electrical power which is used to operate a variety of devices, including medical or other devices implanted in a human or animal. The medical devices include e.g. pacemakers, nerve growth stimulators, nerve stimulators for relief of chronic pain, stimulators for regrowth of bone or other tissue, drug-release valves or microvalves, and fluid-flow control valves, such as a valve in a duct or in the urinary tract. The electrical power can also be used to operate external devices connected to the fuel cell (e.g. a fuel cell implanted in a plant or tree). Also, the electrical power can be stored in a storage device, such as a capacitive storage element or battery, for later use.    ADVANTAGE - The present electrochemical fuel cells have electrocatalysts that are resistant to fouling and that can operate using compounds found in biological systems. The anode and cathode are separated to avoid shorting.    DETAILED DESCRIPTION - An electrochemical cell (100) for positioning in a biological system having oxygen, comprises: a substrate comprising a first face and a second face; an anode (102) disposed on the first face of the substrate, and comprises an anode enzyme and a mediator, where the enzyme and the mediator are crosslinked; and a cathode (104) disposed on the second face of the substrate. At least a portion of the electrochemical cell is configured to be subcutaneously positioned in a subject. The electrochemical cell further comprises a mass transport limiting membrane disposed over the anode and cathode. The mass transport limiting membrane is configured to limit contact of macromolecules of greater than or equal to 5000 daltons with the anode and cathode.    DESCRIPTION OF DRAWING(S) - The figure shows a not-to-scale cross-sectional view of a fuel cell.    Fuel cell (100)    Anode (102)    Spacers (103)    Cathode (104)    Anode electrolysis layer (106)    Cathode electrolysis layer (108)    Non-fouling coating (109)