A Parasitic Insensitive Catheter-Based Capacitive Force Sensor for Cardiovascular Diagnosis

This paper presents a catheter-based capacitive force sensor interface for cardiovascular diagnosis. The force sensor is implemented on a flexible printed circuit board (FPCB) substrate with a force-sensitive polydimethylsiloxane (PDMS), and a force-induced change in a capacitance of the sensor is measured by a precision capacitive sensor interface. To recover the performance degradation caused by the large parasitic capacitance ${\rm C}_{\rm P}$ of a long catheter, we present a parasitic insensitive analog front-end (AFE) with active ${\rm C}_{\rm P}$ cancellation, which employs a charge amplifier and a negative capacitor at the virtual ground of the charge amplifier. The prototype sensor was measured with a force loader in whole blood. The proposed AFE successfully cancels ${\rm C}_{\rm P}$ of 348 pF in a 0.9-m-long sensor and measurement results show the SNR of 53.8 dB and the capacitance resolution of 16 aF, a 19.6 dB improvement by canceling nonideal effect of ${\rm C}_{\rm P}$ . This corresponds to a force resolution of 2.22 gf, which is 9.29 $\times$ reduction compared to the work without the ${\rm C}_{\rm P}$ cancellation. The proposed sensor interface is insensitive to ${\rm C}_{\rm P}$ from hundreds to 1-nF level, and the force-dependent stiffness of two different tissues has been successfully distinguished with an ex-vivo experiment. The proposed sensor interface enables the integration of capacitive force sensors in a smart catheter.