NOVELTY - An electrochemical reactor comprises an electrode (100), microfluidic channel(s) (106) on the electrode, and a gas-permeable polymer membrane on the microfluidic channel. The distance between the electrode and the membrane is less than 500 microns.    USE - The electrochemical reactor is used for measuring small quantities or concentrations of analyte, e.g. for detection of glucose in diabetes patient and for detection of small amount of gas in liquid or atmospheric samples. It may be used to perform microchemical synthesis or microfluidic pumping or to pattern a surface. It is useful in electrochemical device, microfluidic electrochemical sensor, microfluidic actuator, or microfluidic fuel cell.    ADVANTAGE - The inventive electrochemical reactor has improved reactivity for reactions occurring at the microelectrodes.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are also included for the following:    (A) an electrochemical device comprising the above electrochemical reactor, and a second electrode electrochemically connected with the electrode of reactor;    (B) a microfluidic electrochemical sensor comprising the above electrochemical reactor including a working electrode, a counter electrode electrochemically connected with the working electrode, and a reference electrode electrochemically connected with the working electrode and counter electrode;    (C) a microfluidic actuator comprising the above electrochemical reactor, a second electrode electrochemically connected with the electrode of the reactor through the microfluidic channel, where application of electric potential to the electrode of the reactor causes a net flow of electrolyte along the channel, and a reference electrode electrochemically connected with the electrode of reactor and the second electrode;    (D) a microfluidic fuel cell comprising a first electrochemical reactor, a second electrochemical reactor connected with the first electrochemical reactor and comprising a second electrode, microfluidic channel(s) on the second electrode and a gas-permeable polymer membrane on the microfluidic channel, an inlet for an oxidant to the first electrochemical reactor, an inlet for a reductant to the second electrochemical reactor, and a proton-permissive membrane between the electrode of first reactor and the second electrode; and    (E) a method of making a microfluidic electrochemical reactor by forming a gas-permeable polymer layer having a first side defining interconnected channels and a second side comprising a membrane covering the interconnected channels, and contacting the first side of gas-permeable polymer layer with a substrate and an electrode on the substrate, where the channels, the portions of electrode and substrate exposed to the channels, and the membrane covering the channels form a microfluidic network.    DESCRIPTION OF DRAWING(S) - The figure is a schematic top view of the inventive electrochemical reactor.    Electrode (100)    Active region (101)    Lead attachment region (103)    Microfluidic network (104)    Connection region (105)    Microfluidic channels (106)