Water that stagnates at the edges of a drainage pipe eventually stops flowing altogether. The same happens in a catheter that drains excess fluid from a child's brain: if the flow near the hole walls is too weak, cells start to grow into them, and the tube gets clogged. This is the main reason for repeat surgeries in children with hydrocephalus, with 11% to 25% of shunts failing within the first year, and 20% to 84.5% of patients requiring revision within 10–15 years.
The engineers built a computer model of the brain ventricle as a cylinder 60 mm high and 30 mm in diameter, containing a proximal catheter with an inner diameter of 1.3 mm and a wall thickness of 0.6 mm. Cerebrospinal fluid was modeled as plain water with a density of 1000 kg/m³ and a viscosity of 1 mPa·s at body temperature (310 K), and the flow was treated as laminar due to low velocities. A pressure drop of 10 mm Hg (about 1333 Pa) was applied, as in a real shunt. Then, in COMSOL, the fluid motion equations were solved to compute how much fluid passes through each hole and what frictional force acts on its side walls.
It turned out that drainage is governed by the lowest pair of holes. If these two holes are moved apart perpendicularly from 0.5 mm to 2 mm, the total outflow increases by 22.2%, and the average wall shear stress on their side walls rises by 18.8%. In contrast, shifting the upper holes longitudinally changes outflow by only a few percent. This means the lower holes act as the main gateway: their geometry determines whether the flow is strong enough to wash away cells.
The final design, with two conical holes of 0.5 mm diameter in four rows spaced 1.4 mm apart, produced wall shear stress on the lower side walls 30 times higher than in the catheter lumen. That is like the difference between a strong current near the bank and barely noticeable water movement in the middle of a river. This contrast is the protection: cells cannot establish themselves where the flow is strongest.
Before this work, it was unclear which hole parameters matter most for preventing clogging. Now it is clear: the perpendicular distance between the two lowest holes is the main lever, while longitudinal shifts of the upper holes are almost useless. This gives engineers a direct path: they can space the lower holes wider, reduce their diameter to 0.5 mm, and make them conical, while adjusting the overall flow rate with the valve. Then water does not stagnate at the tube edges, and the catheter lasts longer.
The valve hole diameter that provides the required drainage turned out to be about 0.17 mm, comparable to the thickness of a human hair. Meanwhile, the minimum mesh element size was 0.021 mm, almost eight times smaller.