Cannula assemblies and ambulatory infusion systems with pressure sensors made of stacked coplanar layers

An ambulatory infusion system for the infusion of a liquid drug into a patient's body over an extended time period, the ambulatory infusion system comprising:

• an infusion device an infusion cannula fluidically coupled to the infusion device;
• a measuring unit;
• a pressure sensor electrically coupled to the measuring unit and fluidically coupled to the infusion cannula with a fluid channel, the pressure sensor comprising a stack of coplanar layers comprising:
• a rigid top layer and a rigid base layer, wherein at least a portion of the fluid channel is positioned between the top layer and the base layer;
• an electrode layer positioned between the top layer and the base layer and flexibly coupled to the fluid channel;
• a counter-electrode layer positioned between the top layer and the electrode layer, the counter-electrode layer and the electrode layer forming electrodes of a sensing capacitor;
• and
• a spacer layer defining an electrode cavity disposed between the top layer and the base layer such that the electrode layer extends across the electrode cavity, wherein positive fluidic pressure in the fluid channel flexes the electrode layer into the electrode cavity thereby modifying a capacitance of the sensing capacitor;
• and wherein the electrode layer and the spacer layer that are arranged between the top layer and the bottom layer are of substantially the same size.