Lubricate with mucus

By constantly secreting a lubricating layer of mucus, earthworms move through soil without any dirt sticking to them. This pretty ingenious system has inspired researchers at the Leibniz Institute for New Materials (INM) to develop a material with a surface structure that provides itself with lubricant whenever pressure is applied. Because the lubricated material reduces friction and prevents the growth of microbes, numerous applications in the food and other industries and biomedicine are envisioned.

The new material is made of soft plastic, with droplets of silicone oil as a lubricant on the inside of the material.

“When we put pressure on the material, the droplets change shape and migrate to the surface. The silicone oil then spreads evenly on the surface to create a water- and dirt-repellent sliding layer,” explained Jiaxi Cui, head of the research group Switchable Microfluidics.

As the pressure decreases, the droplets will reform. In addition, the sliding layer can also be removed and formed again and again when pressure is again applied to the material.

“So it reacts dynamically to pressure — like a ‘breathing’ system,” Cui said.

The surface structure of the new material also plays an important role: “Again, we were inspired by the earthworm. Its skin surface is not smooth, but rough. That’s what we took into account in our material and roughened the surface,” Cui explained.

Precisely because of this roughness, a uniform lubricating film can form and adhere well. It depends on how friction-reducing the new material can behave.

“The surface structure is also important for the longevity of the lubricating effect. We compared the sliding film on our ‘earthworm structures’ with a sliding film on a smooth surface: our structures survive 10,000 cycles of friction, whereas sliding films on smooth structures have only 300 friction cycles,” Cui said.

It is precisely this combination of rough surface and the lubricant droplets inside that is special about the new material.

There are already some structures that reduce friction, including those that are modelled on the functioning of animal skins. Even systems that release lubricants themselves have been investigated by researchers. However, they all only work in a fluid environment.

“For the first time, we’re introducing an application that reduces friction in a solid environment, and we’ve been inspired by the earthworm because it also glides through a solid environment, Earth,” Cui said.

Researchers can imagine many applications in industry or biomedicine, whenever a device has to glide smoothly through something solid.

INM scientists have published their results in the journal Advanced Materials: Zhao, Huaixia; Sun, Qiangqiang; Deng, Xu; Cui, Jiaxi. Earthworm-Inspired Rough Polymer Coatings with Self-Replenishing Lubrication for Adaptive Friction-Reduction and Antifouling Surfaces. Advanced Materials 2018, 30 (29), 1802141. https://doi.org/10.1002/adma.201802141