Evaluation of Screen Printing Process in Fabrication of Small Profile Conductive Ink-based Contact Force Sensor

Incorporating contact force sensor in renal denervation device provides immediate sensing feedback in determining ablation quality and improves mechanical catheter control. Commonly available contact force sensor in catheter devices is either fiber optic or magnetic in nature. They are typically located at the distal tip of the catheter. In this work, we proposed to fabricate small profile contact force sensor using carbon conductive ink by screen-printing process. This process maximizes the sensing area of contact. Subsequently, three of these contact force sensors will be placed 120 degrees apart around the catheter wall. The force sensor consists of two-stacked polyimide flexible printed circuit boards (FPCB) that have the same dimension. One has gold over copper interdigital traces and solder pads while the other has screen-printed carbon conductive ink, i.e., the sensing element. The printed size of each force sensor is limited to
$\mathbf{2.4}\mathbf{mm}\times \mathbf{0.60}\mathbf{mm}\times \mathbf{0.06}\mathbf{mm}$
. The critical dimension is the width which is only a maximum of 0.6mm. The force sensor is piezoresistive in nature. It will be integrated directly under the micro-electromechanical systems (MEMS) sensor chip that has the heater electrode to ablate the renal artery. To fulfill the required alignment holes of the screen-printing machine, the substrate is designed in a wide array format with an external dimension of
$\mathbf{205}\mathbf{mm}\times \mathbf{100}\mathbf{mm}$
. Each substrate contains 3 arrays of 15 columns x 3 rows of a single sensor FPCB. The testing of the force sensor is conducted on a flat surface with a load range of 50mN to 500mN, which serves as the baseline to represent the degree sensor contact to artery wall. Repeatability tests showed that from 50mN - 150mN, the sensitivity of the force sensor is
$\mathbf{2.1}\ \mathbf{K}\mathbf{\Omega}/\mathbf{mN}$
, with a dynamic response of 70.1%, while 50mN - 500mN is 0.70
$1.7241\ \mu \Omega \cdot \text{cm}$
with a dynamic response of 91.4%. The sensor can differentiate various degrees of contact, which are defined from light contact (20mN), good contact (150mN), and over contact (500mN). The overall sensor structure, i.e., integrated MEMS chip, contact force sensor, and radiopaque material, passed biological evaluation for medical devices per ISO-10993-S.