Mohammad Honarvar

A model to predict deflection of bevel-tipped active needle advancing in soft tissue

Active needles are recently being developed to improve steerability and placement accuracy for various medical applications. These active needles can bend during insertion by actuators attached to their bodies. The bending of active needles enables them to be steered away from the critical organs on the way to target and accurately reach target locations previously unachievable with conventional rigid needles. These active needles combined with an asymmetric bevel-tip can further improve their steerability. To optimize the design and to develop accurate path planning and control algorithms, there is a need to develop a tissue-needle interaction model. This work presents an energy-based model that predicts needle deflection of active bevel-tipped needles when inserted into the tissue. This current model was based on an existing energy-based model for bevel-tipped needles, to which work of actuation was included in calculating the system energy. The developed model was validated with needle insertion experiments with a phantom material. The model predicts needle deflection reasonably for higher diameter needles (11.6% error), whereas largest error was observed for the smallest needle diameter (24.7% error).

Towards a Nitinol Actuator for an Active Surgical Needle

Surgical needles, for safe and accurate percutaneaous interventions, need to be navigated accurately through the tissue and placed precisely at the targets. A novel active needle, using Nitinol wires as actuators, has been proposed to navigate the needle within the tissue. In this design, when temperature of Nitinol wire was increased by Joule heating, the material undergoes a phase transformation that produces relatively large actuating forces and strains. Using both experimental and numerical simulations, the force-temperature response of the Nitinol wires were characterized. The results indicate that increasing the applied current decreases the response time to reach maximum force, but increases the maximum temperature reached. Therefore, the chosen applied current should be high enough to produce sufficient actuation force and shorter response time, but not too high such that the lower actuator temperatures are maintained to minimize tissue damage.Copyright

Application of SMA Wire for an Active Steerable Cannula

In this article we present the feasibility of using the shape memory alloy (SMA) wires, namely Nitinol, as an actuator for a steerable surgical cannula. A 3D finite element (FE) model of the actuated steerable cannula was then developed in ANSYS to show deflection of the surgical cannula under the actuation force. The behavior of SMAs was simulated by defining the isothermal stress-strain curves using the multi-elasticity capability of ANSYS. The transformation temperatures of the Nitinol wire at different levels of stress were gathered to form the transformation diagram. Using the one-dimensional Brinson model, the isothermal stress-strain response of the wire was obtained. The thermomechanical characteristics of SMAs were also studied completely by a series of experiments performed on the wires. Birth and death method was used in the solution procedure to have the prestrain condition on Nitinol wire prior to the actuation step. A prototype of the actuated steerable cannula was also developed to validate the numerical simulation. Finally a study was done on design parameters affecting the deflection such as Young’s modulus of cannula, SMA diameter and its offset from the neutral axis of the cannula which can be useful in design optimization.Copyright

Design optimization study of a shape memory alloy active needle for biomedical applications

Majority of cancer interventions today are performed percutaneously using needle-based procedures, i.e. through the skin and soft tissue. The difficulty in most of these procedures is to attain a precise navigation through tissue reaching target locations. To overcome this challenge, active needles have been proposed recently where actuation forces from shape memory alloys (SMAs) are utilized to assist the maneuverability and accuracy of surgical needles. In the first part of this study, actuation capability of SMA wires was studied. The complex response of SMAs was investigated via a MATLAB implementation of the Brinson model and verified via experimental tests. The isothermal stress-strain curves of SMAs were simulated and defined as a material model in finite element analysis (FEA). The FEA was validated experimentally with developed prototypes. In the second part of this study, the active needle design was optimized using genetic algorithm aiming its maximum flexibility. Design parameters influencing the steerability include the needles diameter, wire diameter, pre-strain and its offset from the needle. A simplified model was presented to decrease the computation time in iterative analyses. Integration of the SMA characteristics with the automated optimization schemes described in this study led to an improved design of the active needle.

Size Effect on the Critical Stress of Nitinol Wires

Nitinol has the best shape memory and superelasticity properties of all known polycrystalline shape memory alloys (SMAs) due to diffusionless Martensitic transformation. Due to these unique properties, Nitinol is increasingly used in different fields such as biomedical, structural and aerospace engineering. However, under certain stresses Nitinol exhibits unrecovered strain, or permanent set, that limits the applicability of Nitinol wire. This study showed that there exists a critical range of stress beyond which the permanent set is negligible. The goal of this paper is to determine range of critical stress using two different methods i.e. constant stress experiment and isothermal tensile test and to show variation of this range with changes in wire diameters.Copyright

Simulation and experimental studies of the SMA-activated needle behavior inside the tissue

Recently, the concept of developing an active steerable needle has gathered a lot of attention as they could potentially result in an improved outcome in various medical percutaneous procedures. Compared to the conventional straight bevel tip needles, active needles can be bent by means of the attached actuation component in order to reach the target locations more accurately. In this study, the movement of the passive needle inside the tissue was investigated using numerical and experimental approaches. A finite element simulation of needle insertion was developed using LSDYNA software to study the maneuverability of the passive needle. The Arbitrary-Eulerian-Lagrangian (ALE) formulation was used to model the interactions between the solid elements of the needle and the fluid elements of the tissue. Also the passive needle insertion tests were performed inside a tissue mimicking phantom. This model was validated for the 150mm of insertion which is similar to the depth in our needle insertion experiments. The model is intended to be based as a framework for modeling the active needle insertion in future.

Investigation of crystal structures of one-way shape memory Nitinol wire actuators for active steerable needle

Due to its outstanding properties of Nitinol, known as shape memory and superelasticity, Nitinol wires have been used as actuators in many medical devices. For the medical applications, it is critical to have a consistent strain response of Nitinol wires. This work focuses on studying the effect of parameters such as biased stress, maximum temperature, and wire diameters that influence the strain response of Nitinol wires. Specifically, Nitinol phase transformations were studied from microstructural point of view. The crystal structures of one-way shape memory Nitinol wires of various diameters under different thermomechanical loading conditions were studied using X-Ray Diffraction (XRD) method. The location and intensity of characteristic peaks were determined prior and after the thermomechanical loading cycles. It was observed that Nitinol wires of diameters less than 0.19 mm exhibit unrecovered strain while heated to the range of 70ºC to 80ºC in a thermal cycle, whereas no unrecovered strains were found in larger wires. The observation was supported by the XRD patterns where the formation of R-phase crystal structure was showed in wire diameters less than 0.19 mm at room temperature.

Electro thermomechanical behavior of a smart actuator for an active surgical needle

A large and increasing number of cancer interventions, including both diagnosis and therapy, involve precise placement of needles. The challenge in most of the existing needle-based procedures is the safe and accurate navigation of the needle through tissue to the desired target. This challenge is due to lack of proper actuation of the needle (i.e., actuated from the proximal end, which is far away from the needle tip). To overcome this challenge, we propose to bend the needle using a smart actuator that applies bending forces on the needle body; thereby, improving the navigation of the needle. The smart actuator is designed with shape memory alloy (SMA) wires due to their unique properties such as superelasticity, shape memory effect, and biocompatibility. These wires, connected on the needle, can be heated with resistance heating to produce relatively large forces that can bend the needle. However, predicting the actuation forces is difficult due to their coupled thermomechanical effects. Therefore, this study will experimentally and numerically evaluate the feasibility of the SMA wires for the smart actuator.