Stretchable, Transparent, Ionic Conductors

. This paper shows that stretchable, transparent, ionic conductors (STICs) enable the fabrication of devices with new attributes. In demonstrations of performance, a transparent actuator generates an area strain of 167% at 18 kV, and a transparent loudspeaker produces sound over the entire audible range. These devices achieve high-voltage and high-frequency electromechanical transduction without electrochemical reaction. The ionic conductors exhibit transmittance of 99.99% at 550 nm, linear strain beyond 500%, and sheet resistance below 200 (cid:525) /sq. At large stretch and high transmittance, the ionic conductors have lower electrical resistance than all existing stretchable and transparent electronic conductors. STICs offer new opportunities for scientific exploration, and for applications.

The past century has seen the rise of electronics-engineered devices in which electrons carry electrical charge. The successful rise of electronics, however, has overshadowed another success with a much longer history-ionics, Nature's solution to charge transport, based on ions and water. Combining 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 (1), and brain-machine interfaces, through which cortical ionic impulses control prosthetic arms (2).
A current challenge to bioelectronics is to bridge a common gap-in mechanics-between electronics and ionics. Electronic systems are usually made of hard materials, while tissues and cells are soft. The field of "stretchable electronics" is developing rapidly, and promises to make electronics conformal to the skin, heart and brain (3,4). Stretchable conductors are also needed in non-biomedical applications, such as electromechanical transduction (5) and solar energy conversion (6). Existing stretchable conductors are mostly electronic conductors, including carbon grease (7), micro-cracked gold films (8), serpentine-shaped metallic wires (3), carbon nanotubes (9,10), graphene sheets (11,12), and silver nanowires (13)(14)(15). Stretchable devices have also been made using corona discharge (16), liquid metals (17), and saline solutions (18); these conductors, however, are not solid, and their uses are limited. Attributes other than conductivity and stretchability are, of course, also important in specific applications. For example, for some applications stretchable conductors must operate at high frequencies and high voltages (7), remain conductive under repeated expansion in area beyond 1000% (19), be biocompatible (5), and be transparent (9)(10)(11)(12)(13)(14)(15).
While electronic conductors struggle to meet these demands, ionic conductors meet most of them readily. Many ionic conductors, such as hydrogels (20) and gels swollen with ionic liquids (21), take a solid form, and are stretchable and transparent. Ionic conductors constitute a 7/15/13 3 large class of materials; their diversity enables them to meet requirements in addition to conductivity, stretchability and transparency. Many ionic hydrogels, for example, are biocompatible and conformal to tissues and cells down to the molecular scale (22). This paper demonstrates, surprisingly, that ionic conductors can even be used to fabricate devices that operate at high voltage and high frequency. We build an actuator achieving large deformation at high voltage, and a loudspeaker producing sound across the entire audible range. These devices are essentially perfectly transparent to light across the entire visible range. We study the fundamental limits of such electromechanical transduction by a combination of experiment and theory.
We derive the conditions under which the ionic conductors enable electromechanical transduction without electrochemical reaction. The strain of actuation is not limited by the elasticity of the soft ionic conductors, but by electromechanical instability.
The frequency of actuation is not limited by electrical resistance, but by mechanical inertia. At large stretch and high transmittance, elastomeric ionic conductors have lower electrical resistance than all existing stretchable and transparent electronic conductors.
Stretchable, transparent, ionic conductors (STICs) enable many devices, which we call "stretchable ionics" for brevity. One basic design of stretchable ionics places two electrodes (electronic conductors), an electrolyte (ionic conductor) and a dielectric (insulator) in series To demonstrate the remarkable properties of STICs, we build a transparent, high-speed, large-strain actuator using a design which we call a "layered electrolytic and dielectric elastomer" (LEADER). A membrane of a dielectric elastomer is sandwiched between two membranes of the electrolytic elastomer ( Fig. 2A and fig. S1). The electrolytes and the dielectric are stretchable and transparent, but the electrodes need not be. So long as the electrodes are placed outside the active area of the device, the actuator is stretchable and transparent. When a voltage is applied between the electrodes, ions of different charge polarities collect on the two electrolyte/dielectric interfaces; the oppositely charged interfaces attract each other, and cause the sandwich to reduce its thickness and enlarge its area (Fig. 2B).
We demonstrate this design using 1-mm-thick 3M™ VHB™ 4910 as the dielectric, and 100-m-thick polyacrylamide hydrogel containing 2.74 M NaCl as the electrolyte. To compare the performance of the ionic conductor to an electronic conductor, we use the electrolytic elastomer to replace carbon grease in an existing design of an electrostatic actuator (7). We stack three layers of the VHB together, stretch them radially to three times their initial radius, and fix them to a circular acrylic frame (Figs. 2C and 2D). We then sandwich the dielectric stack between two layers of heart-shaped hydrogels, which are linked through thin hydrogel lines to copper electrodes placed on the frame. When a voltage is applied and removed, the heart expands and contracts (movie S1). The beating heart is transparent to all colors (Figs. 2E and 2F).
We model the electromechanical transduction of the actuators, and compare the theory with experiments using the dielectric sandwiched between layers of hydrogels of circular shape ( fig. S2). An area strain of 167% is achieved at 18 kV (Fig. 2G). This voltage-induced strain is limited by electromechanical instability, and the soft hydrogels do not constrain the dielectric ( fig. S3). The area strain of our actuator reduces as the frequency of applied voltage increases, and becomes vanishingly small at a frequency on the order of 10 3 Hz (Fig. 2H). These characteristics of the actuator using the hydrogel are comparable to those of actuators using carbon grease (7), but the carbon grease is an opaque electronic conductor, while the hydrogel is a transparent ionic conductor.
Our analysis shows that the frequency of actuation is not limited by electrical resistance, To demonstrate that the STICs can enable electromechanical transduction much beyond the fundamental resonance, we build a transparent loudspeaker that produces sound from 20 Hz to 20 kHz-that is, across the entire audible range. The fabrication process is similar to that We study the fidelity of the sound reproduction by feeding the loudspeaker with a 20 s test signal of constant amplitude and a linear sine sweep from 20 Hz to 20 kHz (Figs. 3C and 3D). The sound generated by the loudspeaker is recorded by the webcam (movie S3). In the first few seconds, the amplitude of the recorded sound is large (Fig. 3E). This interval reflects the resonance of the frame of the loudspeaker, which is not optimized to suppress vibration.
The amplitude varies only slightly over the remainder of the recording. The spectrogram of the recorded sound displays the successful reproduction of the main signal of the original test sound throughout the audible frequency range (Fig. 3F). In the lower part of the tested frequency range, vibrations of the frame and the membrane are visible (movie S4).
At large stretch and high transparency, the ionic conductors have lower resistance than existing stretchable and transparent electronic conductors, such as silver nanowires (AgNWs),   Fig. 4A), corresponding to a transmittance of 99.99% for a 100-m-thick hydrogel used in constructing actuators and loudspeakers. The resistivity of the hydrogel is almost identical to that of water containing the same concentration of NaCl when the hydrogel is not stretched, and increases when the hydrogel is stretched (Fig. 4B). Among all electrical conductors, these hydrogels show the highest transmittance (Fig. 4C). For a conductor whose resistivity is independent of deformation, the resistance of the conductor increases with stretch as , where is the resistance before the conductor is stretched, and R is the resistance after the conductor is stretched times its initial length. This prediction closely approximates the measured resistance-stretch curve for the elastomeric ionic hydrogel (Fig.   4D). By contrast, when SWNTs on VHB are stretched, the resistance increases faster by orders of magnitude than the prediction of the square law, indicating that the resistivity of the SWNTs is increased greatly by the stretch.
Our design of layered electrolytic and dielectric elastomers should be compared with existing actuators in which ionic conduction plays essential roles. Examples include actuators made of carbon nanotubes and conducting polymers in electrolytes (23,24), actuators made of ionic polymer-metal composites (25), and resistive strain sensors made of elastomeric ionic hydrogels (26). Our design introduces a dielectric in series with an electrolyte, and the small capacitance of the dielectric enables high-speed and large-strain actuation. In a design of electrowetting devices, electrolytes and dielectrics are in series, but the electrolytes are liquids, and the dielectrics do not deform (27).  (29) and carbon nanotubes (30). We have shown that ionic conductors allow the fabrication of loudspeakers having almost perfect transparency. Transparent loudspeakers might also, for example, be attached to windows to achieve active noise cancelation (30). The LEADER also works for applications that require low voltage and low frequency. When stretched mechanically, the LEADER increases area and reduces thickness, so that its capacitance increases. This characteristic will enable transparent capacitive strain sensors conformal to soft tissues and operating at low voltage.
There is only one kind of electron, but there are 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.    Electrical resistivities of hydrogels of several concentrations of NaCl were measured as functions of stretch, and compared with the resistivities of water containing the same concentrations of NaCl. (C) Transmittance (at 550 nm) is plotted against sheet resistance for ITO (14), AgNWs (13), SWNTs (10), Graphene (12) and hydrogels (100 m thickness, this work). (D) Stretch is plotted against normalized resistance for ITO (15), AgNWs (14), Graphene (11), SWNTs (9) and hydrogel (this work).