Membrane and electrode structure for implantable sensor

A measuring device for determining concentration of a first material in an environment in contact with the device, which first material reacts within said device in presence of a catalyst with a second material to form a third material;

• and which device comprises:
• a membrane comprising a body, a surface in contact with said environment, and at least one discrete hydrophilic region in communication with said body, wherein said hydrophilic region is permeable to said first and second materials, and said body is otherwise permeable to said second material and essentially impermeable to said first material, wherein both said first and second materials diffuse into said device from said environment through said surface;
• a catalyst within said hydrophilic region wherein reaction of said first and second materials occurs;
• at least one critical zone within said hydrophilic region containing said catalyst;
• at least one sensor, having a surface communicating with at least one said hydrophilic region and sensitive to either said second material or said third material and producing a signal indicative of the concentration of said second or third material in said region;
• and
• a control responsive to said signal for comparing said signal to a reference to determine the concentration of said first material in said environment.