Hydraulically steerable micro guidewire capable of distal sharp steering for accessing 1 mm diameter vessels

Abstract

Vascular intervention is a procedure that diagnoses or treats disease by inserting a catheter into blood vessels. Since the catheter with a fixed tip shape cannot overcome the bifurcations with various diameters and branching angles, guidewires with a shapable tip are used together for guidance. The conventional guidewires, however, have limitation that its tip should be shaped manually by hand. Medical doctors often extract the guidewire out of the patient’s body, shape the tip according to the branch geometry, and then try insertion again. These steps of manual shaping are repeated a few times at difficult branches, and this increases operation time and risk to the patients. This dissertation proposes a hydraulically steerable guidewire which can access 1 mm diameter vessels with 128 degrees branching angle. Since the proposed guidewire has $400 \mu m$ diameter and can bend up to 90 degrees at the distal short 2 mm segment, it can access small diameter vessels with sharp branching angle larger than 90 degrees. Only the biocompatible materials are used in the proposed guidewire to be actually applicable in medical field. This dissertation presents the details on the hydraulically steerable guidewire such as steering shape, hydraulic actuation mechanism, fabrication method, and guidewire body with gradually increasing stiffness. The steering shape of the proposed guidewire is designed to have double bending curvatures, which is known in medical field as the adequate shape for accessing largely angulated branches. The two curvatures are realized in a single degree of freedom (DOF) for miniaturization, and their detail dimensions are decided based on diameters and branching angles of a hepatic artery, which is our target vessel. The proposed steering shape can access small diameter vessels because its distal sharp curve in a short length segment provides a large steering angle even in confinement inside the narrow vessels. It also can access large diameter vessels because its proximal gradual curve in a long length segment provides a large steering distance to overcome the remoteness of side branch. The proposed steerable guidewire employs a hydraulic actuation mechanism to achieve the designed steering shape. It consists of an eccentric tube and inner micro patterns around an internal cavity for steering in double bending curvatures. The distal segment bends in the sharper curve than the proximal segment because the triangular micro patterns placed only at the proximal segment of the thin-walled side of the eccentric tube reduces the bending deformation. The fabrication process is developed to make the eccentric tube and inner micro patterns using medical-grade silicone. A cylindrical template with micro patterns is prepared using 3D printing and stamping, and the patterned template is removed chemically after coating it with silicone. The fabrication method allows the flexible structure having inner patterns without using adhesion and division of the overall structure. This makes the fabricated hydraulic actuation mechanism safe from puncture, which increases reliability of the steerable tip. The guidewire body is designed to have the increasing stiffness for better trackability, torquability, and guiding capability. It consists of micro coils with varying pitches inside PEBAX tube to achieve the requirements for bending and torsional stiffness. The bending stiffness of the guidewire body is designed to increase gradually to prevent the prolapse for better trackability and reach larger than the catheter for guidance. The ratio of torsional to bending stiffness is designed to be large for better tolerability. The selective insertion performance of the proposed steerable guidewire is evaluated using the blood circulatory system that mimics human arterial environments. The bifurcation model is fabricated to have the target diameter of 1 mm and target branching angle of 128 degrees. Since the hydraulic steering mechanism can be affected by blood pressure and velocity, the blood flow of the hepatic artery is also emulated in the model.