Hydrogel balls

Shapeshifting hydrogel could have soft robotics applications

Image credit: Dreamstime

Engineers at John Hopkins University have used DNA to induce hydrogels to shapeshift. This method could be used to produce devices – including soft robots – which do not rely on wiring or batteries.

Rapid and gradual shapeshifting is taken for granted in nature: plants and animals have grown, developed and metamorphosed within their lifetimes for billions of years. Inducing manmade objects to change their shape, however, has proved far from a simple task.

Now, a team of researchers at John Hopkins’ Whiting School of Engineering may have found a method which allows hydrogels - high absorbant gels containing water as the liquid constituent - to shapeshift in various ways.

The process makes use of specific DNA sequences nicknamed “hairpins” or “stem loops”, which are formed when two parts of the same DNA strand base-pair form, creating a double helix ending in an unpaired loop. These were used to induce a chickpea-sized blob of hydrogel to expand to 100 times its original volume.

The expansion was halted using a “terminator hairpin”: a different DNA sequence.

More controlled and complex changes in shape can be achieved using a photo-patterning technique similar to that used in the computer industry to make microchips: embedding patterns in different parts of the gel resulted in the different parts responding to specific DNA triggers. This can cause bending, folding or other responses.

“DNA sequences can be thought of as an analogue to computer code,” said Professor David Gracias, a senior author of the study, published in Science. “Just as computer software can direct specific tasks, DNA sequences can cause a material to bend or expand in a certain way at a specific site.”

The team created flower-shaped hydrogels which responded to these “hairpin” DNA sequences. When exposed to different sequences, some or all of the flower’s petals folded.

They also created moving crab-shaped hydrogel devices, which had antennae, claws and legs which could curl up in response to the appropriate DNA sequences.

The researchers hope that their new method could make it possible to weave moving hydrogel parts into soft materials and play a role in the development of metamorphic devices, smart medical devices and soft robotics with medical and marine applications. The induced shapeshifting achieved by the researchers could mean that wires, batteries and tethers – which limit the practicality of a device – are no longer required for some devices.

Soft robotics is an emerging field which offers the promise of more flexible machines capable of landing without shattering, handling delicate objects such as fruit, squeezing through gaps, and which could be more pleasant for humans - particularly elderly patients or children - to physically interact with.

Due to their cheapness and useful properties, hydrogels have formed the basis for much cutting-edge research into new medical devices, including a drug delivery system, slippery coatings for cathetars and condoms, and surgical glue.

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