Ionogels: The Key to Soft Machines?

Some of the biggest differences between manmade and living machines (if you consider the human body a machine) are in the flexibility, elasticity, and self-healing properties of natural systems. Polymers are the materials that can most closely mimic the properties of the human body: They can be super-flexible, highly elastic, and lately, engineered to self-heal from damage. Machines that come into direct contact with the human body (such as wearable monitors), often lack needed elasticity due to the rigidity typical of electronic components. While polymers can easily provide a flexible foundation for electronics, making stretchable electronic components remains a challenge many researchers are trying to address.

Conductive Liquids

Hydrogels, an interpenetrating network of polymer chains and water, probably have the most potential for use in “soft machines.” The only trouble is they rapidly lose moisture, shrink, and turn brittle. Substituting the water component of a hydrogel with a non-evaporating liquid make the gels air-stable, while using a conducting ionic liquid results in conductive, soft, and elastic ionogels, possibly opening the door to the soft electronics of the future.

Scientists from the Xi’an Jiaotong University in China and Harvard University in the U.S. have created and tested new conductive ionogels using polyacrylic acid, crosslinked with polyethyleneglycol diacrylate, and “filled” with an ionic liquid, ethylmethylimidazolium ethylsulphate.

The conductive component is the ionic liquid, which is literally a salt of bulky organic ions existing in a liquid state. Ionic liquids are important components of batteries and capacitors, as they are highly conductive, usually nonflammable, and have low vapor pressure (i.e., they do not evaporate easily). The scientists presented their findings in ACS Applied Materials and Interfaces:

Large deformation of soft materials is harnessed to provide functions in the nascent field of soft machines. This paper describes a new class of systems enabled by highly stretchable, transparent, stable ionogels. We synthesize an ionogel by polymerizing acrylic acid in ionic liquid 1-ethyl-3-methylimidazolium ethylsulfate ([C2mim][EtSO4]). The ionogel exhibits desired attributes of adequate conductivity (0.22 S m-1), low elastic modulus (3 kPa), large rupturing stretch (4.6), and negligible hysteresis and degradation after cyclic stretches of large amplitude. Using the ionogel and a dielectric elastomer, we fabricate electromechanical transducers that achieve a voltage-induced areal strain of 140%. The ionogel is somewhat hygroscopic, but the transducers remain stable after a million cycles of excitation in a dry oven and in air.

Testing Promising

For testing, a voltage-responsive dielectric polymer was sandwiched between two layers of the ionogel, forming an actuator. Ionogel electrodes performed beautifully, stretching and relaxing along with the polymer for more than a million cycles without a problem. The conductive ionogel was 96% transparent, as was determined by UV-visible spectroscopy. Mechanical characterization, which included tensile testing, showed that the ionogel could be stretched more than four times and showed good mechanical reversibility.

The authors expect such ionogels to be used in tunable lenses, transparent loudspeakers, and active noise-cancellation windows. Artificial muscles seem like a logical application too. Obviously it will take time, additional research, and biocompatibility adjustments before ionogels can become part of biocompatible human monitoring systems.