Medical sting

A probe that can inch its way into the brain without causing any damage could be the result of research based at London’s Imperial College. Based on the egg-laying organ of a species of wasp that drills into the bark of trees, the probe could be used for investigations or surgery, claims lead researcher Ferdinando Rodriguez y Baena.

The wasp in question is the European wood wasp, Sirex noctilio, which drills into pine trees to lay its eggs beneath the bark. The peculiar design of its ovipositor attracted biomimetics expert Julian Vincent of Bath University, who spent several years investigating how such a small animal could make deep holes in tough wood without being able to exert much force.

Vincent found that the ovipositor has two parts that interlock along their length, much like the seal on a zip-lock plastic bag. When the wasp drills into wood, these parts reciprocate — one moves forward while the other remains stationary, then the other moves forward while the first stays still. Each part is coated with backward-facing, hair-like structures, which mean they generate less friction moving forward — into the wood — than backwards, a property known as anisotropy. ’Pushing them in requires less force than pulling them out,’ said Rodriguez.

’The key to the natural design is the fact that the drill doesn’t need to rotate and it doesn’t need to be loaded axially. The reciprocating motion pulls the drill into the tissue or substrate without any need to push from the back of the drill,’ he explained.

It is this property that has led him to work with Vincent to design the medical probe based on the wasp’s drill.

Rodriguez is about to embark on a three-year project to develop the device. The probe itself, he says, would be made of a flexible material, about 40cm long and 1-3mm in diameter, roughly the same diameter as a syringe needle.

’It wouldn’t rip any tissue on the way in,’ he said. The probe would have a microtexture coating that would give it the same anisotropic properties as the wasp’s ovipositor.

To be useful as a medical probe, the device would have to reach any point within the brain, which means it must be steerable. This, said Rodriguez, can be achieved by offsetting one of the two parts of the probe so it is positioned ahead of the other before drilling starts.

As the probe pulls its way into the tissue, the offset causes the tip to curve along the axis of the probe; the longer the offset, the tighter the curve. Rotating the probe around its axis would steer the curve through three dimensions.

To produce this motion, Rodriguez is designing a drive unit consisting of ’at least three’ actuators. ’One will produce the reciprocating motion, possibly using some kind of crankshaft between the two halves of the probe. Then another motor changes the offset between the two, which increases the degree of curve. And the third would twist the probe along its axis.’

The surgeon using the probe would need to know exactly where the device was, so Rodriguez plans to incorporate a magnetic tracker into the probe tip. ’There are plenty of these on the market now, at very small sizes, which allow you to track their position in 3D space using technologies which are already associated with medical robotics,’ he said.

The work is at an early stage, stressed Rodriguez; the material selection and the design of the anisotropic microsurface coating still needs to be done and the steering concept needs to be proven.

He aims to produce 5mm-long prototypes capable of tunnelling through synthetic mimics of various types of tissues by the end of the project. ’We’ll try to prove the feasibility of the concept with this, then see where it takes us,’ he said.