Wasps may be the bane of many a summer picnic but they are now the inspiration behind a new type of flexible medical probe.
The biomimetic instrument is being developed by a team at the Department of Mechanical Engineering, Imperial College London, UK. Its design is based on the ovipositor (egg-laying tube) of a female wood wasp.
The wood wasp’s ovipositor looks like two hollow needles, one inside the other, each of which is lined with backward-facing teeth to give purchase as the wasp ‘drills’ progressively into the bark of a tree. This takes surprisingly little force, making it attractive for carrying out minimally invasive procedures such as brain biopsies without having to exploit a natural orifice.
Unlike the wood wasp’s ovipositor, however, the probe ‘displaces’ the tissue rather than removing it. The current prototype consists of four interlocking probe segments lined with 50μ teeth and propelled by small motors and actuators. The ultimate aim is to be able to steer the probe in three dimensions through soft tissue and monitor its progress.
Dr Ferdinando Rodriguez, at Imperial, explains that existing percutaneous instruments can be divided into two main groups – thick and non-flexible probes (bioposy probe and laparoscopes) and thin and flexible needles (such as a brachytherapy needle).
He says, ‘Thick and non-flexible probes can be pointed to the target with the aid of a visualisation system and will not deform under load, but their manipulation causes significant pressure on the tissue, limiting the surgeon’s degrees of freedom.
‘Conversely, thin and flexible probes tend to be less damaging to the surrounding tissue, but deflect and buckle against tissue resistance, resulting in placement accuracy which is inversely proportional to the depth of the target’.
Furthermore, the ‘underlying technological and functional limitation’ of both devices is that they cannot be guided along curvilinear trajectories. This ‘limits their application to surgical procedures where a straight-line approach is viable’.
A ‘scaled up’ prototype of the team’s flexible probe, measuring 12mm-wide and about 30cm long, has been demonstrated but a final prototype will be no more than a few millimetres in diameter.
‘Developing a multi-part probe of a few millimetres outer diameter will mean identifying materials, manufacturing methods and surface treatments that will work at this scale,’ remarks Rodriguez. ‘Particularly challenging will be [ensuring] that the probe can sustain the loading conditions incurred during actuation, while being sufficiently flexible not to damage surrounding tissue during insertion.’
The team is exploring medical-grade silicon, latex rubber and possibly even polytetrafluoroethylene (PTFE) from which to manufacture their device.
Professor Tony Anson, of UK biomedical materials research company Diameter Ltd, says, ‘There’s a significant global market for such a device, as there are many patients with problematic physiology who would benefit here.’
He agrees, though, that finding a suitable material will be challenging. He says, ‘I would discount latex, as there have been too many problems with latex gloves. Medical-grade silicon could work but it has a possible demerit because of its high surface friction, and PTFE is a good biomaterial, although in some cases it lacks the right mechanical properties – both have potential.
‘I notice that polyurethane (PU) isn’t mentioned. I think PU shows promise, but then so could some combination of materials, coatings or polymer composites,’ he says. Anson suggests that the manufacturing technology may ultimately influence the choice of material.
The prototype of a few millimetres should be available before the end of 2015 for clinical trials to take place. Rodriguez says, ‘The feasibility study focused on probe design, actuation and control. It also laid the foundation for an “intelligent probe,” where the insertion process is guided interactively by pre-operative image data, allowing deep lesions of the brain and other regions of the human body to be accessed with greater accuracy and repeatability’.