How hairs affect fluid flow

Our hairy insides protect us from the full force of fluids racing through our bodies and may inspire future robotic design, according to researchers at the Massachusetts Institute of Technology.

As well as having hairs on the surface of our skin, did you know that the insides of our bodies are lined with microscopic hair-like structures?

These hairs range in shape from the short hairs inside our noses and ears to superfine protein chains projecting into our blood vessels.

So why do we have such hairy insides? Well, one thing that these hairs all have in common is their interaction with surrounding bodily fluids and therefore, scientists believed that these hairs may impact how these fluids move throughout our bodies.

To illuminate their potential role, Jose Alvarado, lead author of the study, created upscaled rubber replicas of the different types of these internal hair-like structures. The different hair types were then exposed to fluids at various velocities and the impact of hair structure on the fluid flow was analysed using a mathematical model.

The results, published in Nature Physics, show that stiff hairs remain upright in fluid flow, whilst more elastic hairs yield easily to the current. Interestingly though, hairs that are bent at just the correct angle impact the flow of the fluid in a more dynamic way. These angled hairs straighten only when fluid is flowing against them and act as a temporarily raised grate, slowing the flow of the surrounding fluid.

It is thought that these angled hairs protect tissues from the damaging force of fluids moving through our bodies, such as high pressure blood flow that occurs in the kidneys.

As well as shedding light on their biological role, these results could also potentially have an interesting application in the world of robotics and the production of small hydraulic machines.

Hydraulic systems use liquid pressure to generate force and, although this mechanism is well understood on a larger scale, controlling fluid flow at a small scale has thus far been a great limitation in creating small hydraulic systems. Co-author, Anette Hosoi, explains, ‘Computers and smartphones were made possible because of the invention of cheap, solid-state, electronics. On hydraulic systems, we have not seen that kind of revolution because all the components are complex in themselves.’

Inspired by the structure of these angled internal hairs, Jose created a device, called a microfluidic diode, that allows fluid flow in one direction but restricts it in the other direction. This permits the control of fluid movement on a very small scale and he believes that this will revolutionise the field of hydraulics, allowing the creation of small, fully autonomous hydraulic robots.