Water-absorbing material inspired by plant roots could power robots

Plants may have no muscles, nonetheless they can grow upwards against any risk of strain of gravity and their roots may also shift soil and rocks – because their cells can absorb water to form strong structures. Now an artificial material which mimics this ability may help to create better soft robots and medical implants.

Shelby Hutchens and her colleagues at the University of Illinois Urbana-Champaign formed so-called plant tissue analogues (PTAs) by fabricating closed cells from a compound of silicon called polydimethylsiloxane, which is semi-permeable like plant cell walls.

Researchers used varying salt levels in the cells to control just how much pure water from outside was absorbed through the polydimethylsiloxane cell walls via osmosis. The higher the salt concentration, the more water was absorbed and the stiffer and larger the cells became. This had to be tuned carefully, as at high salt concentrations the artificial cells ruptured.

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The team discovered that if a layer of this material was bonded to a less expandable substance, the upsurge in size of the PTA caused it to move and bend into an arched condition as you side expanded and the other remained at its original size. This movement required no electrical energy, only a source of moisture, and could be utilized to power soft robots or medical devices in the future.

Previous materials like hydrogels have already been shown to demonstrate the same expanding behaviour, nonetheless they lose stiffness because they swell. On the other hand, the PTA was found to strengthen since it took in water.

In a single experiment, the team showed how this influences the potential application of the materials. They took a strip of PTA and a strip of hydrogel, each bonded to a material that expanded less, and exposed them to water. Both exhibited the same swelling and deformation, curling upwards. However, when the experiment was repeated with the help of a 5-gram weight to the end of each strip, the PTA curled upwards while “holding” the weight, but the hydrogel lacked the inner strength to take action.

In another experiment, small discs of PTA and of hydrogel each weighing 0.15 grams were located in beakers and covered with 5 centimetres of wet sand. The PTA swelled to twice its initial surface, displacing the sand, as the hydrogel only grew to 56 % of the PTA’s final surface area.

Journal reference: Matter , DOI: 10.1016/j.matt.2021.10.015

More on these topics:

  • plants
  • materials science

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