Design and experimental characterization of a small flexible TMR-based indentation probe for soft tissue hardness estimation

The endoscopic transsphenoidal approach is a surgical method used to remove pituitary adenomas—benign tumors located near the pituitary gland. The success of this procedure depends heavily on assessing the tumor’s consistency, which is currently based on the surgeon’s tactile feedback and experience. This study presents the development of a novel sensor-based probe designed to provide real-time measurements of tissue hardness, supporting more objective surgical decision-making. The probe integrates a tunneling magneto resistance (TMR) sensor with a deformable, 3D-printed tip made of thermoplastic polyurethane. The tip features micro-beams that deform upon contact with tissue; the extent of deformation, measured by the TMR sensor, correlates with tissue stiffness. The flexible structure was fabricated using fused filament fabrication, and the process was characterized to control micro-beam dimensions by adjusting print parameters such as layer height, print speed, and nominal line width. A regression model was developed to predict beam width with high accuracy and repeatability. Eighteen probe designs, combining different beam widths (0.42 mm-0.63 mm) and numbers (3-5), were tested through indentation experiments on samples with Shore 000 hardness ranging from 51 to 83 (ASTM D2240), including a cooked egg white sample as a biological tissue analogue. Results revealed that probes with 4 and 5 beams printed at lower speeds offered superior sensitivity, linearity, and uniformity. In these configurations, the probe was able to estimate the hardness of adenoma-like materials with errors below 2%. These findings validate the proposed system’s potential for clinical use in evaluating soft to moderately firm tumors during surgery.