22 Scientific American, June 2022AzmanL/Getty ImagesMATERIALS SCIENCESuper Gels
New ionogels are tough,
stretchable and easy to makeFlexible, squishy hydrogel materials are
used in a dizzying array of applications,
from soft contact lenses to Jell-O. But all
share a common structure: a tangled net-
work of polymer chains soaked in water.
Now researchers have developed their
own version of a hydrogel cousin that is
tougher and could perhaps find even more
uses, including longer-lasting batteries.
Meet a new ionogel.
Instead of water, ionogel polymers are
inundated with an ionic liquid. This fluid
is made up of positively and negatively
charged ions—like the composition of table
salt. But an ionic liquid does not assemble
into a crystalline solid at room temperature.
Nevertheless, the strong bonds between
ions ensure the liquid does not evaporate like
water does. And thanks in part to the stick-
ing power of ions, polymers soaked in an ion-
ic liquid can be tougher than hydrogels are.
Michael Dickey, a chemical engineer
at North Carolina State University, and his
colleagues have devised a new method for
creating ionogels with mechanical proper-
ties he describes as “best in class.” They
are harder to break than either cartilage
or natural rubber while remaining soft and
stretchy. (One type of ionogel can stretch
up to seven times its length—more than
twice as far as a rubber band can, Dickey
says.) Like other ionogels, his team’s new
ones conduct electricity and remain stable
amid shifting temperatures. If heat is
applied, they can self-heal a cut or tear.
The new material is described in a recent
study published in Nature Materials.
“These transparent ionogels have
remarkably tough mechanical properties
and are distinguished by how easy they are
to prepare,” says Xuanhe Zhao, a mechani-
cal engineer at the Massachusetts Institute
of Technology, who reviewed the study
but was not directly involved in the work.
Other researchers have developed their
own ionogels, but making them typically
involved multiple steps or complex chem-
istry. Using the new formula, Dickey and
his colleagues simply mixed an ionic liquidwith the building blocks (called monomers)
of two different types of polymers, and
then they shone light on the liquid to
trigger the monomers to link up into poly-
mer chains. “In this case, one plus one
equals 100,” Dickey says. “You take these
two materials that, by themselves, are
[common], but then you put them in this
new environment, and you get something
remarkably tough.”
Easy-to-make ionogels could have many
applications. Their toughness and stretchi-
ness make them a good candidate for
cushioning to protect against car accidents
or explosions. They solidify when exposed
to light, so an ionogel formulation could be
used to 3-D print hard-to-break objects.
And because their ions can carry a charge,
the gels could be used in a rechargeable
battery, where the researchers say they
would resist degradation better than the
liquid electrolytes currently in use.
Those are just a few possibilities, Dickey
says. “When you have a material that’s
so easy to make and has such great prop-
erties,” he adds, “it’s bound to find applica-
tions that are probably even beyond my
imagination.” — Sophie BushwickIonogels could aid 3-D printing and more.