Category Archives: Ovipositor Needle II

Ovipositor Needle II – Self-Propelling & Steering through Tissue

Developed in 2016-2019, diameters ranging from 1.2 mm to 0.4 mm. 

A wasp ovipositor is a needle-like structure composed out of three elements, called valves. A female wasp uses this structure to drill into wood or fruit and deposit eggs inside a living host. The propagation of the ovipositor through the substrate is achieved by a smart push-pull mechanism, in which one of the valves is pushed while the other two are pulled, using the surface-dependent friction properties with the soft substrate to move forward. Inspired by the ovipositor of parasitoid wasps, we developed a series of self-propelled steerable Ovipositor Needles with ultrathin diameters ranging from 1.2 mm to 0.4 mm.

Our first Ovipositor Needle prototype consists of six super elastic Nickel Titanium (NiTi) wires concentrically arranged around a seventh NiTi wire. The seven wires are interconnected at the tip with a small flower-shaped ring (Ø 1.2 mm) manufactured for minimal resistance during propulsion. The ring has a central hole to which the central wire is glued, surrounded by six concentric holes through which the six other wires can slide back and forth. Each proximal end of the six movable wires is connected to a miniature stepper motor, in which a leadscrew-slider mechanism converts rotational motion into linear motion.

We performed a series of experiments in which the needle was inserted in tissue-mimicking gel phantoms. The wires were sequentially moved back and forth, resulting in the needle moving forward inside the phantom using the surface-dependent friction properties between the wires and the gel. Different sequences of wire actuation were used to achieve both straight, curved and S-shaped trajectories.

In our second Ovipositor Needle prototype we changed the shape of the interlocking ring from cylindrical to conical to investigate the effect of pre-curved wires. We found out that pre-curved wires facilitate steering, however, at the drawback of a slightly larger tip diameter due to the use of a conical flower-ring.

In a final series of Ovipositor Needle prototypes, the flower-shaped ring was replaced by an thin-walled shrinking tube, glued to one of the outer wires, ultimately resulting in ultrathin 0.4 mm needle diameters three times the size of a human hair. The prototypes were tested in multi-layered gel phantoms with varying stiffness properties and artificial membranes, representing different organs and tissues. In a final series of ex-vivo experiments the needles were evaluated with success in porcine liver, kidney and brain tissue.

This project, in which we developed world’s thinnest self-propelled-steerable needles, shows the strength of a novel bio-inspired approach leading to a new generation of needles that can be used to reach deep targets inside the body without a risk of buckling and with the possibility to correct the trajectory. Our needles were developed within the WASP project that focused on the development of steerable needles for localized therapeutic drug delivery or tissue sample removal (biopsy). In a follow-up project, funded by the Netherlands Organization for Scientific Research (NWO) we will develop the needles further towards clinical application in urological interventions under MRI.

(Picture at the top adapted from “Braconid Wasp Ovipositing” by Katja Schulz is licensed under CC BY 2.0.)