Female parasitic wasps pass their eggs through an organ called the ovipositor into their hosts, which sometimes hide in a solid substrate such as wood. The ovipositor has the shape of a tube and consists of three slender, parallel-positioned segments, called valves. The wasp can push and pull the valves with respect to each other in a reciprocating manner. A groove-and-tongue mechanism interlocks the valves along their length. The push-pull motion of the valves has two functions. First, it keeps the unsupported length of the individual valves low. Second, moving the individual valves forward one by one while pulling the others provides stability to the wasp’s ovipositor and prevents buckling. The push and pull forces produce a net force near zero, enabling a self-propelled motion.
Inspired by the wasp ovipositor, we developed a self-propelled Ovipositor MRI-Needle with a diameter of 0.8 mm that can be used inside an MRI system. Our needle consists of six parallel needle segments and an actuation unit. The design of the actuation unit is based on the so-called click-pen mechanism of a ballpoint pen. The actuation unit allows you to actuate the needle that consists of six parallel Nitinol segments by just a translating motion. We 3D-printed the components of this actuation to be able to test it inside an MRI system. The video below shows the movement of the needle segments actuated by the actuation unit:
The prototype was tested with success in ex-vivo human prostate tissue in a preclinical 7-Tesla MRI system at the Amsterdam University Medical Centres. The results showed that the needle tip was visible in MR images and that the needle was able to self-propel through tissue.
This project, in which we developed a self-propelled wasp-inspired needle that can be used inside an MRI system, is part of Project 4 of the MEDPHOT programme funded by the Netherlands Organization for Scientific Research (NWO). MEDPHOT focusses on the development of photonics-based technologies that can enable earlier diagnosis and tailored treatment of diseases in the pulmonology, urology, and gastroenterology fields and translate these technologies to their clinical environments. The goal of Project 4 is to develop a novel transperineal laser ablation platform for an accurate treatment of prostate tumours under MRI.
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 novel self-propelling Ovipositor Device designed for locomotion through the large intestine (colon). The device contains a miniature electric motor connected to a cylindrical cam. Six sliders are placed around the cam and move forward and backward following the path defined by the cam. Designed for motion through soft environments, the working principle of the propulsion mechanism is that multiple stationary sliders create sufficient friction to allow for a single slider to shuffle forward. In each step, one slider moves forward whereas the others remain stationary relative to the environment, generating a smooth and continuous motion at approximately 1/6 of the speed of a moving slider. The ovipositor mechanism allows a simple and robost construction that can be easily miniaturised to very small dimensions, see our research on self-propelled ovipositor needles.
Experiments were carried out with various flexible 3D-printed structures attached to the outer surface of each slider to generate direction-dependent friction for further enhancement of grip. Tests in plastic tubes showed fast and fluent self-propelled motion. Locomotion in a colon was succesfully achieved with an improved 3D-printed outer surface in which the tangential spacing between the sliding structures was decreased so that the colonic wall does not flex between them. The improved prototype was able to self-propel ex-vivo through a porcine colon without any visual damage to the colonic wall.