Wall Shear Stress Nanosensor

When a river flows it is faster in the middle. This is because the edges are slowed down by a force. Wall Shear Stress (WSS) is the force a fluid exerts on a wall, or the wall on the fluid slowing the flow down at the edges. Normally maths tells us that it should be easy to find out this force, all we need to do is find how the gradient of the velocity changes (e.g. how much faster the flow is in the middle and how far away the middle is) and multiply that by the viscosity (e.g. blood is thicker than water) to get the force.

This force in blood vessels is important as it alters how the cells grow and react around the vessels. This, as one example, alters the permeability of blood vessels therefore altering the nutrients the tissues get. The problem is we can't measure the blood's gradient of velocity accurately near the wall nor the exact viscosity! More than this the walls are bendy, alter all the time and have particles in them (blood cells). I came up with an idea (not the same as doing it!!!) of how to measure WSS with enough resolution for blood vessels after seeing the adaptions Prof. Dafforn makes to a bacteriophage (a virus against bacteria). The adapted (see below for how it works) bacteriophage acts as a tiny piece of string tied to the surface, a bit like tell-tails in sailing or weeds in a river. Left is it tethered to a collagen surface. Note that it flaps around with 'Brownian' motion until the flow is applied. Then it flapping but around the direction of flow. The amount of flapping tells us the WSS.

Click here for scientific publication

A 4x real speed video of the adapted bacteriophage attached to collagen. The angle measured automatically is  also plotted.

A 6x real speed video of the adapted bacteriophage attached to an endothelial cell. There are two identical videos using different methods of detecting the angle, (both go wrong when it points towards us rather than left or right, something to work on...). There are two things of note, firstly the direction is not perfectly left and right due to the shape of the cell, second (maths not shown) the force is much higher than a flat surface (as expected by estimates) as the flow is squeezed over the cell.