Doo Yong Lee

Supervised super-resolution methods estimate high-resolution simulation results from low-resolution inputs. These methods typically train the neural networks using pairs of high-resolution data and their downsampled low-resolution counterparts, implicitly assuming the global similarity between the low- and high-resolution models. This assumption, however, often fails as the low-resolution models usually exhibit stiffer behaviour than high-resolution models due to numerical stiffening. This paper proposes a novel supervised super-resolution method to mitigate this problem for linear deformable object simulation. The method constructs training data pairs by matching low- and high-resolution simulation snapshots based on the similarity of the normalized strain energy, the normalized temporal change rate of strain energy, and the modal contributions to the overall strain energy. The loss function also incorporates the equation residuals derived from the finite-element method. The time-integration scheme is selected by examining the eigenvalue distributions of the neural tangent kernel associated with the finite-element residuals. Compared with previous downsampling methods, the proposed method reduces the maximum relative displacement error by 25%, 54%, and 38% for the beam, elephant, and armadillo models, respectively.

Active steering catheters and guidewires for vascular intervention, employing pressure-driven mechanisms, have been studied for their promise of safety, miniaturization capacity, and ease of operation. Manufacturing methods of pressure-driven catheters and guidewires at the microscale is challenging, and suffer from limitations such as low durability, restricted steering shape, cytotoxicity, and applicability. This paper proposes a novel manufacturing method of the hydraulically steerable actuator for micro guidewires utilizing closed molding and a laser-patterned core template. The cylindrical core template composed of stainless steel is employed to ensure biocompatibility and is patterned by femtosecond laser processing with relative errors of up to 15.04%. The proposed method enables a bonding-free manufacturing of the micro actuator which is an eccentric tube with integrated patterns on its inner surface. Trapezoidal patterns are designed to be symmetrically placed in the direction of the eccentricity on the proximal part. The steering shape of the micro actuator is designed by adjusting the geometry of the patterns engraved onto the core template. A steerable actuator of 600 μm diameter with a double bending curvature is manufactured, achieving a maximum bending angle of 132.93 degrees and a steering distance of 9.20 mm. Experiments of selective insertion with the steerable actuator are conducted using the model of cerebral vascular branching, which resembles the human blood flow. The experiment results show that the steerable actuator can navigate through the vascular bifurcations of 4.2 mm and 1.5 mm diameters with branching angles of 112 degrees.

The pressure-driven mechanisms for steerable guidewires and catheters are difficult to fabricate when miniaturized to submillimeter-scale. Micro bubbles resulting from molding or surface irregularities due to surface tension can affect the actuation performance as the outer diameter of the pressure-driven actuator decreases to the submillimeter-scale. This paper presents a novel fabrication method to manufacture pressure-driven actuators of submillimeter-scale. The proposed fabrication method utilizes magnetorheological (MR) elastomer and magnetic field to determine the geometric dimensions of the actuator with micro-scale precision. An actuator of the diameter of 0.7 mm and the eccentricity of 80 lm is designed and fabricated with absolute errors of 12 lm and 3 lm, respectively. The steering performance of the fabricated micro actuator is tested through experiments. The actuator can achieve a sharp bending angle of 124 degrees with a length of 5.41 mm, by optimizing the eccentricity through the finite-element analysis.

This paper presents a haptic actuator made of silicone rubber to provide both passive and active haptic forces for catheter simulations. The haptic actuator has a torus outer shape with an ellipse-shaped inside chamber which is actuated by hydraulic pressure. Expansion of the chamber by providing positive pressure can squeeze the inside passage to resist the catheter traveling through. Further expansion can hold and push back the catheter in the axial direction to render active haptic forces. The size of the catheter passage is increased by providing negative pressure to the chamber, allowing various diameters of the actual medical catheters to be used and exchanged during the simulation. The diameter of the catheter passage can be enlarged up to 1.6 times to allow 5 to 7 Fr (1 Fr = 1/3 mm) medical catheters to pass through. Experiment results show that the proposed haptic actuator can render 0 to 2.0 N passive feedback force, and a maximum of 2.0 N active feedback force, sufficient for the cardiac catheter simulation. The haptic actuator can render the commanded force profile with 0.10 N RMS (root-mean-squares) and 10.51% L2-norm relative errors.

Discretization of signals often generates additional energy in the haptic systems, and makes them unstable. The currently popular controls based on the passivity stabilize the systems by limiting the rendered force of the haptic systems so that the generated energy from the system stays below zero. The passivity-based methods are, however, often conservative and sacrifice the performance of haptic rendering. This paper proposes a control method to adjust the change rate of the stiffness of virtual environment connected to the haptic systems to satisfy the input-to-state stability (ISS) for better stability and transparency. The ISS conditions for the systems are derived by modeling the system as a linear time-varying system. The systems become dissipative if they satisfy the ISS conditions. The generated surplus energy of the dissipative system is less than a positive finite value while maintaining the stability. Since the passive system is a special case of the dissipative system, the proposed ISS-based control is less conservative than the passivity-based approaches. Performance of the proposed control is experimentally compared with the virtual coupling and the force-bounding method that are widely used passivity-based approaches. The energy generated from the system using the proposed method is positive finite whereas the generated energy using the passivity-based approaches is less than zero. This means that more energy is transferred to the operator. Increased stiffness corresponding to this additionally transferred energy can be rendered. Root-mean-square and maximum errors of the rendered forces are, therefore, reduced by at least 86 % and 50 %, respectively.