<p>Minimally invasive surgeries have replaced many open surgeries because of faster recovery time and reduced pain to the patients. In these surgeries, tiny incisions are made in a patient’s body through which small surgical needles are inserted into the target tissue. Some procedures in interventional radiology like brachytherapy, biopsy, and tissue ablation also use this surgical method. The success in these percutaneous interventional procedures relies heavily on the accuracy of surgical needles in reaching the target tissue/organ. This has brought growing research interests in designing these surgical needles. Advancements in imaging technologies like ultrasound and magnetic resonance imaging help surgeons track needles during insertion. However, due to needle deflection and tissue deformation, missing a target is inevitable unless the needle is steered during insertion for correction. The commonly used method of correction is by rotating or bending these flexible needles from the needle base and utilizing unbalanced force generated in the needle tip-tissue interface to change the needle path towards the target. The correction capacity in these passively steered needles is insufficient which has led the researchers to search for methods of active steering. Active steerable needles use actuation capability by employing cables, magnetic fields, Shape Memory Alloy (SMA). Very few active steerable needles have reached the commercial stage of development. These needles actuate either by cable or by the pre-curved stylet. While there are a lot of ongoing researches on SMA actuated needles, they are still not approved by the medical industry due to the difficulty in manufacturing the existing designs in a functional form in a true needle scale (i.e., about 1.5 mm in hollow needle diameter). The main goal of this work is to design and study an SMA-based active steerable needle that can be manufactured on a clinically applicable scale. This preliminary study proposes a design of an active needle with an outer collet and an inner stylet in which the stylet is modified at the tip region to install SMA wire and flexible polymer. The SMA wire acts as an actuating component that rotates the stylet tip when actuated and the flexible polymer acts as a flexible joint that facilitates the tip rotation. For SMA wire, a commercially available Nitinol wire actuator is used. Nitinol can produce a recoverable strain of 4 to 6% when stress is applied (i.e., pseudoelasticity) and can contract to its original length even on stressed conditions when heated above its actuation temperature (i.e., shape memory effect). Also, the high strain-to-weight ratio and bio-compatibility make nitinol a suitable SMA actuator for active needles. The SMA wire is heated by supplying DC power which contracts the wire causing the tip to rotate about the flexible polymer joint. An SMA with 70°C actuation temperature is used in this design but this temperature can be lowered by heat-treatment such that heat dissipation inside tissue is at a safe level. The tip rotation followed by needle insertion will bend the needle to steer in the desired direction. When the excitation is removed from the SMA wire, the bias force from the deformed polymer pushes back the tip and makes it straight. This study also investigates the effect of using a shape memory polymer as a flexible joint in bending the needle tip. The ‘Pseudoelasticity’ in shape memory polymer allows large strain that is recoverable once the SMA wire is deactivated. It eliminates the need to add additional actuators as an antagonistic element and thus helps to reduce the size of the needle. A prototype is fabricated and tested to demonstrate the behavior of the proposed active steerable needle design. Besides, a computational study is performed on the actuation of the needle tip by modeling the SMA wire using the Lagoudas model. This helps understand the interaction between different components of the needle, their stresses, and temperatures. As a result, the needle design can be optimized and its behavior predicted to allow for future design improvements.</p>

Towards Clinically-Relevant Shape Memory Alloy Actuated Active Steerable Needle

Paper Proceedings: https://asmedigitalcollection.asme.org/SMASIS/SMASIS2021/volume/85499

ASME 2021 Smart Materials, Adaptive Structures and Intelligent Systems (SMASIS2021)

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