Filtration & fluid control
medical conduits (IMCs) – catheters, shunts and other small tubes used in the brain, liver and other organs. The challenge is that IMCs often need to be smaller in order to be used more effectively in less invasive procedures, but a decrease in size exacerbates problems with fluid flow. “Being an ear doctor, I have thought about a lot of ear possibilities with the redesign of TTs, but here at MEE we also work on eyes, and there are IMCs that can work as shunts,” Remenschneider explains. “The existing technology has the same problems with fouling and occlusion.”
A myringotomy is a procedure to create a hole in the ear drum to allow fluid trapped in the middle ear to drain out.
“Adding something like that to TTs would avoid some of the problems, so we explained what TTs are and the engineers immediately asked why they are the size and shape they are, why the flanges have a certain diameter and so on,” Remenschneider remarks. “I didn’t know. They have been designed that way for 50 years and have worked well enough, so nothing has changed, but the engineers immediately thought about geometry, coating, porosity and many other factors.”
The trouble with TTs
Though tympanostomy procedures are only performed in patients with chronic or treatment-resistant middle ear infections, the procedure is relatively common, so the major problems are well understood. The function of plastic or metal TTs is known to be far from perfect, largely due to bacterial biofilms and local tissue growing on their surfaces. This blocks a TT’s lumen – the interior space through which fluids flow – and can cause them to extrude. This often occurs around one year after being implanted, though many are intended to stay in place far longer. If squamous epithelial cells – a type of flat cell found throughout the body – accumulate beneath the tube’s outward flange, they can force the inward flange through the tympanic membrane into the external auditory canal. Sometimes, TTs fall out without notice, but even when they remain in place, they do not necessarily perform well.
One major limitation is that they are very poor at fluid flow. Fluid is sometimes unable to flow because of pinning, where fluid beads build up on the surface and needs high pressure to be driven through. If the flow is obstructed, antibiotic ear drops applied in the ear canal may not reach the site of infection. If frequent replacement surgeries are consequently required, costs rise significantly. Such problems are also common in other fluid-transporting implantable
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Through collaboration with the Wyss Institute, and the Harvard John A. Paulson School of Engineering and Applied Sciences (Seas), Remenschneider and the MEE team in Boston embarked on a project to completely overhaul IMC design. The result is a broadly applicable strategy that solves many of the problems that arise in making IMCs both smaller and more effective for uni and bi-directional fluid transport at the millimetre scale.
Total TT redesign
Remenschneider worked closely with Dr Joanna Aizenberg, an associate faculty member of the Wyss Institute, who has pioneered bioinspired materials including Slippery Liquid-Infused Porous Surfaces (Slips). With Slips, a thin layer of oil-based liquid is used to prevent biofouling.
Slips has already been used in many industrial applications, but Remenschneider and other physicians are now opening the door to its use in healthcare through a complete design overhaul of IMCs. TTs manufactured with liquid-infused material – known as infused tympanostomy tubes (iTTs) – enable rapid drug delivery into the middle ear and improved fluid drainage. Furthermore, they limit water from crossing and entering from the outside, while also preventing bacterial and cell adhesion. This is partly due to a change in geometry, as the new design features a curved lumen. Until now, all TTs have been designed with straight and cylindrical lumens, but the team hypothesised that specific curvatures could allow IMCs to discriminate between different fluids at a small scale. “We took nothing for granted in the redesign,” Remenschneider says. “We went back to basics, and the engineers came up with many overly engineered solutions that were too complicated to manufacture or for clinicians to use, including designs with two lumens, but we arrived at a specific curved geometry and used an infused liquid material to pair with a particular fluid viscosity.”
The geometry decreases pinning, which happens mostly at sharp angles, and promotes fluid flow in one direction.
Medical Device Developments /
www.nsmedicaldevices.com
PepermpronShutterstock.com
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