“Muscled skin”: Simple formulas describe complex behaviors

SEM image of the membrane.

SEM image of the membrane. © MPIKGF

In 0,1 seconds, the membrane curls if in contact with acetone vapour (blue).

In 0,1 seconds, the membrane curls if in contact with acetone vapour (blue). © MPIKGF

HZB researchers help chemists understand polymeric "biomimetic" materials' mechanical properties

Sea cucumbers change the stiffness of their skin, Venus flytraps roll up their leaves and even pine cones are capable of closing up their scales at increasing levels of humidity. In the course of evolution, Nature has managed to give rise to complex materials capable of responding to external stimuli by way of mechanical movement. Which is exactly what chemists are now trying to do as well - and with considerable success! Dr. Jiayin Yuan's team at the MPI of Colloids and Interfaces in Golm, Germany, recently scored a particularly exciting breakthrough. The researchers managed to synthesize a membrane capable of rolling up extremely rapidly when exposed to fumes.

Now, Prof. Dr. Joe Dzubiella, a theoretical physicist at the HZB, has managed to identify those factors that are responsible for the high speed.

The porous material, which looks like a sponge, consists of interconnected polymers. Here, polymers in the top layers are visibly more tightly interconnected than is true of the bottom layers. And when the membrane takes up certain gas molecules like acetone, it bulges more strongly at the top than it does at the bottom, so that it starts to warp, ultimately coiling up.

Simple diffusion and geometry

"Jiayin Yuan and his team have already characterized the phenomena in much depth, and we were able to simply pick up where they left off," explains Dzubiella, prompting him to propose that the gas molecules initially cross the membrane by simply "diffusing" across it. The time it takes them to penetrate the membrane is described by the law of diffusion and depends both on the size of the pores as well as on the thickness of the membrane. The larger the pores and the thinner the membrane, the faster the gas molecules are able to cross. The chemists had already witnessed this behavior, which the law of diffusion describes in quantitative terms, in the lab.

Dzubiella also managed to show why it is that the membrane literally curls up when exposed to the vapour, in other words, why it exhibits a particularly small radius of curvature: "We're talking simple geometry," he says. "If the membrane is really thin, very small expansions of the top layers are enough to cause strong bending." Within one-tenth of a second, the membrane bends into a full circle; within a half a second, it is rolled up multiple times. Which is ten times faster than is true of similar materials.

Membrane also reacts on perfume

Together with his postdoc, Dr. Jan Heyda, Joe Dzubiella is currently in the process of continuing his work using a computer to simulate the movement and embedding of gas molecules within the membrane's network. The reason being that, at a microscopic level, the processes are rather complex and especially between the polymeric molecules and the gases, very different types of interactions are able to take place. As such, the polymer network also takes up water molecules from the humidity in the atmosphere. If the membrane now comes into contact with acetone, the acetone molecules migrate into the network, displacing the water molecules. "Often times, it is only by way of simulations that we're able to show the ways in which this could be happening and which processes and factors are important here. These insights in turn are helping the chemists optimize a given parameter in the lab in order to reach the desired property," explains Dzubiella.

In terms of potential applications, really, the sky's the limit. For example, you might coat other types of materials with these kinds of membranes, which begin to fold up as soon as they come into contact with certain molecules. Already, the chemists were able to document that the rolling up not only works with pungent acetone but even with French perfume!

The results are reported online in Nature communications, 1. Juli 2014; DOI: 10.1038/ncomms5293: An instant multi-responsive porous polymer actuator driven by solvent molecule sorption
Qiang Zhao, John W.C. Dunlop, Xunlin Qiu, Feihe Huang, Zibin Zhang, Jan Heyda, Joachim Dzubiella, Markus Antonietti und Jiayin Yuan


You might also be interested in

  • Scientists Develop New Technique to Image Fluctuations in Materials
    Science Highlight
    Scientists Develop New Technique to Image Fluctuations in Materials
    A team of scientists, led by researchers from the Max Born Institute in Berlin and Helmholtz-Zentrum Berlin in Germany and from Brookhaven National Laboratory and the Massachusetts Institute of Technology in the United States has developed a revolutionary new method for capturing high-resolution images of fluctuations in materials at the nanoscale using powerful X-ray sources. The technique, which they call Coherent Correlation Imaging (CCI), allows for the creation of sharp, detailed movies without damaging the sample by excessive radiation. By using an algorithm to detect patterns in underexposed images, CCI opens paths to previously inaccessible information. The team demonstrated CCI on samples made of thin magnetic layers, and their results have been published in Nature.
  • High-energy X-rays leave a trace of destruction in bone collagen
    Science Highlight
    High-energy X-rays leave a trace of destruction in bone collagen
    A team of medical researchers at Charité has analyzed damage by focused high energetic X-rays in bone samples from fish and mammals at BESSY II. With a combination of microscopy techniques, the scientists could document the destruction of collagen fibres induced by electrons emitted from the mineral crystals. X-ray methods might impact bone samples when measured for a long time they conclude.
  • Neutron experiments reveal what maintains bones in good function
    Science Highlight
    Neutron experiments reveal what maintains bones in good function
    What keeps bones able to remodel themselves and stay healthy? A team from Charité Berlin has discovered clues to the key function of non-collagen protein compounds and how they help bone cells react to external load. The scientists used fish models to examine bone samples with and without bone cells to elucidate differences in microstructures and the incorporation of water. Using 3D neutron tomography at the Berlin research reactor BER II, they succeeded for the first time in precisely measuring the water diffusion across bone material - with a surprising result.