Researchers at Imperial College London are working on a project to make steerable needles for medical applications, particularly procedures involving the brain. When tissue deep in the brain needs to be accessed, such as a brain tumor for a biopsy, it is usually a problem, because the act of inserting a tool into the brain such as a biopsy needle — let along trying to operate to remove a tumor — could obviously cause more brain damage. The objective of the project, led by Ferdinando Rodriguez y Baena, is to make certain procedures possible that are currently considered not by the use of a flexible needle.
This is where the horntails come in. Horntails, or woodwasps in the genus Sirex, drill into tough wood using their ovipositors, in order to lay eggs. Their larvae feed inside trees, and in fact are considered a serious forest pest in areas where they have been introduced outside their natural range. Horntails (which are an evolutionary more basal relative of parasitic wasps such as the walnut fly parasitoid Diachasmimorpha juglandis) have a particular construction to their ovipositors, the tubes through which they lay their eggs, because they drill up to 20 mm through wood with them, although they are less than 0.5 mm wide. At the same time, the ovipositors are quite flexible, and can be directed.
The engineers in London used a paper from 1995 detailing the physiological mechanism by which S. noctilio completes this task (J. F. V. Vincent and M. J. King, 1995. The mechanism of drilling by wood wasp ovipositors. Biomimetics, 3(4):187–201) to design a flexible needle which can be directed around obstacles in the brain. Their needle, like the woodwasp’s ovipositor, consists of two interlocking valves which slide relative to each other. Each valve has external teeth which allows it to be anchored while the second valve slides. That stabilization allows great force in movement of the long thin tube.
Some other wasp species demonstrate that it doesn’t matter how long such a flexible tube is, it can easily penetrate long distances. A parasitoid of Sirex (in the family Stephanidae, right) must find a woodwasp larva deep in the wood to lay an egg on it, and also needs a maneuverable ovipositor that can follow the larval tunnels to the host. So it needs even a longer ovipositor to find the larvae which have tunneled deeply — I have seen such ovipositors up to 15 cm long. They likely use a similar principle as the ovipositors of Sirex.
The research has now reached the stage of animal testing, after development of an algorithm to determine the best paths to take to a tumor through healthy brain tissue. Hopefully in a few years this breakthrough will be available to people with brain tumors who might have otherwise been told that it would be unsafe to investigate their tumors further. With a better diagnosis through biopsy, perhaps the chances for a better outcome will increase for these patients.