Journal Club Theme of September 2013: Stretchable Ionics
In a paper just published in Science, we describe a class of devices fabricated using stretchable, transparent, ionic conductors. These devices are highly deformable and fully transparent. They can operate at frequencies above 10 kHz and voltages above 10 kV. We demonstrate a fully transparent loudspeaker that plays music. See a YouTube video. The Supplementary Materials contain experimental methods, theory, and more movies. The Science magazine conducted a podcast interview, which covered some of the same ground in this post.
Motivation. The past century has seen the rise of electronics—engineered devices in which electrons carry electrical charge. The successful rise of electronics, however, does not diminish another success with a much longer history—ionics, Nature’s solution to charge transport, based on ions and water. We live in two parallel worlds: the world of electronics and the world of ionics.
The two worlds communicate with each other. We watch TVs and listen to radios. We drive cars, guided by GPS. More intimate communication between these parallel worlds—the engineered (using electrons) and the natural (using ions)—is creating a hybrid field: bioelectronics. Examples of applications include electrode arrays, where the electronics of medical instruments meet the ionics of tissues and cells, and brain-machine interfaces, through which cortical ionic impulses control prosthetic arms. One person's brain can control the other person's fingers, through electronics and the Internet. The thought of a human can move the tail of a rat.
The emergence of bioelectronics highlights a fundamental challenge: Electronic devices are mostly made of hard materials, and human bodies are mostly made of soft tissues. To enable electronics to meet skin, heart and brain, we need stretchable conductors. Stretchable conductors are also needed in non-biomedical applications, such as in soft robotics and tunable optics. See a perspective written by John Rogers in the same issue of Science.
Limitations of existing stretchable conductors. Existing stretchable conductors are mostly electronic conductors, including carbon grease, micro-cracked gold films, serpentine-shaped metallic wires, carbon nanotubes, graphene sheets, and silver nanowires. Attributes other than conductivity and stretchability are also important in specific applications. Conductors may need to operate at high frequencies and high voltages, remain conductive whilst undergoing areal expansions of 1000% or more, be biocompatible, and be transparent.
While electronic conductors struggle to meet these demands, ionic conductors meet most of them readily. Many ionic conductors, such as hydrogels and ionogels, take a solid form, and are stretchable and transparent. Many hydrogels are biocompatible and conformal to tissues and cells down to the molecular scale.
What are stretchable, transparent, ionic conductors? When salt dissolves in water and forms ions, the ions are electric charges mobile in water. Thus, saltwater is a transparent, ionic conductor.
But saltwater is a liquid, and our devices need a solid electrical conductor. We make a hydrogel by combining saltwater with a polymer network. Saltwater enables electrical conduction, and the polymer network provides the solid form. The hydrogel is like jello, but is far more stretchable. The hydrogel can be stretched more than five times its length.
We have used hydrogels in much of this work, but we have also used nonaqueous ionic conductors.
How to use ionic conductors to make high-voltage devices. The interface between an electronic conductor and an ionic conductor will undergo electrochemical reactions when the voltage across the interface exceeds a value on the order of 1 V. How can we use the ionic conductor to make a high-voltage device without causing electrochemical reactions? Our device consists of two capacitors in series: one capacitor is the interface between the electrode (electronic conductor, copper) and the electrolyte (ionic conductor, hydrogel), and the other capacitor is the dielectric. The two capacitors have vastly different capacitances. When a high voltage is applied cross the device, the voltage drop across the electrode/electrolyte interface is tiny, so that electrochemical reaction will not occur. Our device achieve electromechanical transduction without electrochemical reaction.
How to use ionic conductors to make high-frequency devices. The conductivity of the ionic conductor is about 6 orders of magnitude smaller than the conductivity of copper. How can we use the ionic conductor to make a high-frequency device, such as a loudspeaker? In our device, the ionic conductor and dielectric form a layered structure. The ionic conductor provides the resistance R, and the dielectric provides capacitance C. Their product RC gives the time delay of the device. Even though the resistance R of the ionic conductor is large, the capacitance C of the dielectric is very, very small. Consequently, the RC time delay is very short. Our actuators are not limited by the electrical resistance, but by mechanical inertia.
How does voltage cause deformation? A dielectric elastomer is an electrical insulator, and is a highly stretchable elastomer. When an external circuit applies a voltage to a membrane of the dielectric elastomer across its thickness, the membrane behaves like a capacitor. Positive charge accumulates on one face of the membrane, and negative charge on the other face. The positive and negative charges attract each other, causing the membrane to reduce its thickness and expands its area. In the last decade, intense effort has been devoted to the development of dielectric elastomer transducers. See an animation of dielectric elastomer actuator at Wikipedia.
Artificial neuromuscular system. A dielectric elastomer functions as an artificial muscle, capable of large deformation in response to electrical stimulation. An ionic conductor functions as an artificial neuron, capable of bringing in the electrical stimulation. That is, the ionic conductor innervates the dielectric elastomer. The pair of materials together mimic the functions, but not the anatomy, of a neuromuscular system.
Artificial neurosensory system. This paper focuses on the use of ionic conductors in devices operating at high frequency and under high voltage, but the layered electrolytic and dielectric elastomer also works for applications that require low voltage or low frequency. When stretched mechanically, the layered material increases area and reduces thickness, so that its capacitance increases. This characteristic will enable transparent sensors operating at low voltage, capable of measuring strains over a large range, and conformal to soft tissues. Such devices mumic functions of neurosensory systems.
Mechanics of artificial neurons. Ionic conductors mimic neurons, capable of sending electrical signals between brain, muscle and sensory organ. The brain, muscle and sensory organ can each be real (e.g., in bioelectronics), or artificial (e.g., in soft robotics).
The Supplementary Materials of the paper describes the mechanics of artificial neurons. As mentioned above, the frequency of actuation is not limited by the RC delay, but by mechanical inertia. The actuation strain decreases as the frequency increases. The RC delay does limit the length of ionic interconnect. The amplitude of electrical signal decays over distance. It is intriguing to compare an ionic interconnect to a long neuron.
We also show that viscoelasticity causes creeping movement of the artificial muscle in response to a suddenly applied voltage. The elasticity of the ionic conductors can also be important. We make the ionic conductors of low elastic modulus (~kPa) and small thickness (~0.1 mm), so that the ionic conductors do not constrain the deformation of the dielectric elastomers. We show that, at low frequencies, the actuation strain is limited by electromechanical instability.
Potential applications. Here are some obvious targets:
- Stretchable, ionic conductors can be used to make artificial neuromuscular and neurosensory systems for soft robots
- Stretchable, biocompatible, ionic conductors can be used to make biomedical devices.
- Stretchable, transparent, ionic conductors can be used to make tunable optical devices.
- Transparent loudspeakers might be attached to windows to achieve active noise cancellation.
Every device designer can ask this question: Can I replace the electronic conductor in an existing device with an ionic conductor? The device may lose some performance, but may gain other attributes, such as stretchability, transparency, and biocompatibility. What can I do with these attributes? Can I start from scratch, and design a stretchable ionic device that does not even have an electronic counterpart?
Our theoretical estimate has shown that ionic conductors can operate at much higher frequencies than 10k Hz. One should be able to make devices much faster than loudspeakers. What high-frequency, ionic devices make sense?
Hydrogels dry as water evaporates. We use hydrogels as stretchable, transparent, ionic conductors because they are easy to make and inexpensive. Hydrogels are the ionic conductors of choice to demonstrate concepts, and to fabricate devices that require biocompatibility. We also note that the diversity of ionic conductors creates a large pool of candidates, some of which avoid this problem. For example, ionic liquids and gels swollen with ionic liquids are nonvolatile ionic conductor. We show that ionic liquids can indeed be used as conductors for dielectric elastomers.
Challenges and opportunities. The development of ionic conductors for stretchable devices raises many questions in mechanics and materials science. Here are a few examples:
- Will ionic conductors have long lifetimes?
- Will ionic conductors be compatible with electronic conductors and dielectrics?
- How do we ensure adhesion when the devices are stretched repeatedly?
There is only one kind of electron, but there are infinite many kinds of ions. This diversity will enable ionic conductors to be designed for many applications. Life uses primarily ions—rather than electrons—to carry electrical charge. In creating biomedical and engineering devices, it is well to consider the opportunity: the hard and the soft do not necessarily have to meet through electronic conductors; they may as well meet through ionic conductors.
We love to hear your thoughts. Many of you have been working on soft materials, stretchable electronics, sensors and actuators. Some of you are experts on large deformation of soft materials, or large displacement of flexible structures. We would love to hear from you about opportunities and challenges. Also, we would love to learn about your work in related areas. Please leave your comment below. As always, your comment can be on anything related to the topic, and will be especially valuable if it connects to your own work, or the work you know well. We love to discuss with you.
Links cited in this post
- YouTube video of transparent loudspeaker.
- Christoph Keplinger, Jeong-Yun Sun, Choon Chiang Foo, Philipp Rothemund, George M. Whitesides, Zhigang Suo. Stretchable, transparent, ionic conductors. Science 341, 984-987 (2013).
- Supplementary Materials ( Materials and Experimental Methods. Theory. Four movies).
- Podcast interview.
- John A. Rogers, A clear advance in soft actuators. Science 341, 968-969 (2013).
- Wikipedia page on dielectric elastomer.