A sensor comprising: a first substrate having a sensing portion configured to be percutaneously inserted into a patient and an extracorporeal circuit mounting portion, the sensing portion defining a sensor (18A, 18B) thereon and the circuit mounting portion having at least one electrically conductive pad (22A, 22B) defined thereon that is electrically connected to the sensor, the sensor (18A, 18B) configured to produce a signal indicative of a condition of the patient, characterized by a second substrate (80, 110) mounted to the first substrate and defining thereon a first number of electrically conductive pads (34A, 34B, 99; 112A, 112B, 114A, 114B), at least one of the first number of electrically conductive pads being aligned with the at least one electrically conductive pad defined on the first substrate and at least another of the first number of electrically conductive pads defined on the second substrate (80, 110) being electrically connected to the at least one of the first number of electrically conductive pads defined on the second substrate (80, 110), means for establishing electrical contact between the at least one electrically conductive pad defined on the first substrate and the at least one of the first number of electrically conductive pads defined on the second substrate, a third substrate (88, 133) mounted to the second substrate (80, 110) and defining thereon the at least one electrical terminal (98, 134), a first number of electrical components (40, 43, 44, 46, 48) mounted to the third substrate and a second number of electrical components (40, 43, 44, 46, 48) mounted to the second substrate (80, 110), the second number of electrical components (40, 43, 44, 46, 48) and the first number of electrical components (40, 43, 44, 46, 48) together forming a sensor control circuit (32, 132), the sensor control circuit (32, 132) being electrically connected to the at least one electrical terminal (98, 134) defined on the third substrate (88, 133), and means for establishing electrical contact between the at least another of the first number of electrically conductive pads defined on the second substrate (80, 110) and the at least one electrical terminal defined on the third substrate (88, 133), the sensor (18A, 18B) being thereby electrically connected to the sensor control circuit (32, 132); wherein the second substrate (80, 110) is flexible and the third substrate (88, 133) is rigid. a first substrate having a sensing portion configured to be percutaneously inserted into a patient and an extracorporeal circuit mounting portion, the sensing portion defining a sensor (18A, 18B) thereon and the circuit mounting portion having at least one electrically conductive pad (22A, 22B) defined thereon that is electrically connected to the sensor, the sensor (18A, 18B) configured to produce a signal indicative of a condition of the patient, characterized by a second substrate (80, 110) mounted to the first substrate and defining thereon a first number of electrically conductive pads (34A, 34B, 99; 112A, 112B, 114A, 114B), at least one of the first number of electrically conductive pads being aligned with the at least one electrically conductive pad defined on the first substrate and at least another of the first number of electrically conductive pads defined on the second substrate (80, 110) being electrically connected to the at least one of the first number of electrically conductive pads defined on the second substrate (80, 110), means for establishing electrical contact between the at least one electrically conductive pad defined on the first substrate and the at least one of the first number of electrically conductive pads defined on the second substrate, a third substrate (88, 133) mounted to the second substrate (80, 110) and defining thereon the at least one electrical terminal (98, 134), a first number of electrical components (40, 43, 44, 46, 48) mounted to the third substrate and a second number of electrical components (40, 43, 44, 46, 48) mounted to the second substrate (80, 110), the second number of electrical components (40, 43, 44, 46, 48) and the first number of electrical components (40, 43, 44, 46, 48) together forming a sensor control circuit (32, 132), the sensor control circuit (32, 132) being electrically connected to the at least one electrical terminal (98, 134) defined on the third substrate (88, 133), and means for establishing electrical contact between the at least another of the first number of electrically conductive pads defined on the second substrate (80, 110) and the at least one electrical terminal defined on the third substrate (88, 133), the sensor (18A, 18B) being thereby electrically connected to the sensor control circuit (32, 132); wherein the second substrate (80, 110) is flexible and the third substrate (88, 133) is rigid.