Ultra-thin steerable needle for solid-organ interventions
After the success of minimally invasive techniques in abdominal surgery, the next challenge is to reach deep anatomical structures in a minimally invasive manner. Reaching deep anatomical locations requires a steerable flexible needle to manoeuvre around vessels, nerves, and other vulnerable structures. We approach the major challenge of manoeuvring a needle through complex solid organs with poorly known properties by considering an analogue found in nature: the ovipositor (egg-laying probe) of wasps (see Figure). In several wasp species, the ovipositor is very thin (100–200 μm), long (up to 10 cm in some species) and bendable (minimum radius of 500 μm), and yet it penetrates deep into structures without buckling. Moreover, although it does not contain any musculature, it generates curved trajectories allowing the wasp to carefully choose the location inside the penetrating object for laying its eggs. Of particular interest for solid organ interventions are parasitic wasps that penetrate the soft body of host larvae without damaging it. Instead of being pushed inside a structure, the ovipositor consists of two semi-cylinders that slide against each other. Advancement occurs by reciprocally pulling one semi-cylinder so that it grips, while advancing the other semi-cylinder forward. The offset between the two sliding parts defines the steering direction of the tip. Importantly, this mechanism does not have theoretical dimensional constraints. The wasp ovipositor has already been used by others as inspiration in the development of a 4.4-mm neurosurgical prob.
Based on the advancing and steering mechanism of the wasp ovipositor we will develop needles as thin as 0.5 mm, and evaluate their use in the context of the following functional demands: (1) the needle should be able to follow a desired 3D path without Euler buckling, (2) the semi-cylinders should be coupled along their length, but free to slide along one another with minimal friction, (3) the needle should be able to cope with an inhomogeneous solid organ, with anisotropic and nonlinear elastic properties.
WP3.3.1: Advancing mechanism of the needle along a straight path through solid organs
A theoretical model will be developed by PhD1 (WU), integrating the mechanical and control parts of the needle as derived from the functional demands. Next, a series of prototypes of the needle system will be developed by PhD2 (TUD). The miniaturized cable-ring mechanism developed by co-applicant Breedveld  will be used as a basis, in which the cable ring will be replaced by two sliding semi-cylinders. Complexity will be raised gradually by designing systems of decreasing diameter (from 1.5 mm to 500 μm) and increasing length (from 50 to 150 mm). The first material choice will be nitinol, due to its super-elastic properties. A specific challenge is the design of the control system with two motors functioning at high frequency, with adaptive phase difference and amplitude. The prototypes will be first tested in homogeneous gel-structures, followed by in vitro tests in animal solid organs. Evaluation criteria will include accuracy of the approximation of a straight path and penetration speed. Multi-camera video will be used to follow the needle path in the gel; X-ray fluoroscopy for testing in organs.
WP3.3.2: Advancing mechanism of the needle along a curved path through solid organs
Closed-loop control is needed to navigate through inhomogeneous solid structures. A kinematic model and feedback control algorithm will be developed by PhD1 based on the required accuracy of the needle (current estimate ±1 mm). Offset will be experimentally measured in solid structures with distinct material properties (stiffness, relaxation, etc) to define the appropriate offset correction to be introduced in the feedback loop. Two 3D navigation strategies will be investigated: (1) by rotating the two semi-cylinders as in the wasp ovipositor, and (2) by splitting the cylinder into three instead of two parts. To realize the needle system, closed-loop controls previously developed based on fluoroscopy, stereo cameras, and electromagnetic sensors will be used as starting point. A 200-μm optical fibre will be inserted (cf. the "egg canal" in Figure) in the thinnest prototype developed, to measure deflections during penetration and provide feedback to the control system. These data will be used for calibrating and validating the algorithm developed. Two more diodes will be opened, for delivery and intake of fluids. The developed needle system will be tested initially in gel-structures and later in vitro. Additional evaluation measures will include 3D space coverage (i.e. what is the ensemble of points that the needle is able to reach in a solid structure?), ability to deliver (drug) and aspire (biopsy) fluids without tip displacement, and ease of manipulation.
Deliverables of both WPs: This project will generate fundamental knowledge on a new method of steering ultra-thin needles, in which both advancing and control mechanisms are served by the same elements: two reciprocally sliding semi-cylinders. This approach allows for extreme size minimization, while letting space for multiple diodes suitable for optical fibres and functional drug-delivery and biopsy channels.