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Journal Club for November 2020: Hydrogel Ionotronics: Principles, Materials and Applications

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Hydrogel Ionotronics: Principles, Materials and Applications

Canhui Yang

Department of Mechanics and Aerospace Engineering, Southern University of Science and Technology



1.     Introduction

Life on Earth for billions of years has conducted electricity using mostly ions, while machines created by humans in recent centuries have conducted electricity using mostly electrons. The two types of systems—natural and synthetic—have evolved sophisticated, but separate, ionic circuits and electronic circuits. Ions and electrons, however, do not always flow separately. They couple, for example, at human-machine interfaces in the electrophysiological study of brain, heart, and muscle (Fig. 1a). Hybrid circuits of ions and electrons have inspired the broad field of ionotronics (also called iontronics), where devices function using both mobile ions and mobile electrons.

Hydrogel ionotronics are ionotronic devices that use hydrogels as the ionic conductors. A hydrogel aggregates a polymer network and a large amount of water molecules (Fig. 1b). The polymer network makes the hydrogel a stretchy solid, and the water molecules make the hydrogel a transparent ionic conductor. Stretchable, transparent, ionic hydrogels have enabled ionotronic devices well beyond previously imagined.

We published a review article titled “Hydrogel ionotronics” in Nature Reviews Materials in 2018 [1], summarizing all hydrogel ionotronic devices that had been described in the literature by that time, along with the associated mechanical properties and the chemistry of materials. This article is the first review on hydrogel ionotronics. This emerging field is evolving rapidly, and many progresses have been made since then. In this Journal Club, I would like to share with you this exciting field. Definitely, the discussions here are not comprehensive, but to outline the basic working principles and the associated material researches of selected types of devices. For more details, please refer to the full version of the review.

Fig. 1 (a) Measuring the electrophysiology of brain, heart and muscle requires the synergy of mobile ions and mobile electrons. (b) A hydrogel aggregates a polymer network and water, which can dissolve salt into mobile ions to endow the hydrogel with ionic conductivity.

2.    Hydrogel ionotronics of various kinds

Animals are ionic machines, with biological signals mediated by the flow of ions. One theme of the early hydrogel ionotronics is to mimic neuromuscular and neurosensory systems, with the emphasis of mimicry being placed on the functions but not the anatomies. Working principles of selected hydrogel ionotronics are discussed in the following.

2.1 Electric double layer (EDL)

When a hydrogel and a metal are in contact, mobile ions in the hydrogel and mobile electrons in the metal meet at the interface to form an electric double layer (EDL) (Fig. 2a). The EDL functions as a capacitor of giant capacitance ~ 10-1 F/m2, couples the ionic current in the hydrogel and the electronic current in the metal, and remains stable so long as the sustained voltage is within the electrochemical window, typically on the order of 1 V.

2.2 Artificial muscle

An artificial muscle (commonly known as dielectric elastomer actuator) converts electric voltage into mechanical movements. Such devices were invented by Pelrine and co-workers [2]. An ionotronic version, firstly reported by Keplinger, Suo and co-workers [3], comprises a layer of elastomer sandwiched between two layers of hydrogel (Fig. 2b). See the Journal club regarding the milestone work:

When a voltage is applied, ions of opposite polarities accumulate on the two interfaces between the hydrogels and the elastomer, and their attraction causes the elastomer to reduce thickness and expand area. The stress σ exerted by the attraction scales as σ ~εE2, where ε is the permittivity of the elastomer and E is the electric field. Thus, the voltage needed for the elastomer to deform significantly scales as V~H(μ/ε)1/2, where V is the voltage applied across the thickness of the elastomer, and H and µ are the thickness and shear modulus of the elastomer. Substituting representative numbers into the scaling gives a voltage on the order of 104 V. The artificial muscle operates at high voltage, but low current. Careful design is needed to mitigate the concern for safety, by, e.g. reducing the thickness and modulus or increasing the permittivity of the elastomer.  The electromechanical coupling is highly nonlinear and, depending on geometry, gives rise to many designs of artificial muscles, as well as many modes of electromechanical instability. See a recent review on the mechanics of dielectric elastomer structures [4].

The introduction of hydrogels as stretchable and transparent electrodes has broadened the scope of applications, such as transparent loudspeaker [3], ionotronic fish [5] and hydraulically amplified self-healing electrostatic actuators [6].

2.3 Artificial skin

Human skin is a stretchable and large-area sheet of distributed sensors of pressure, deformation, temperature, and humidity. An ionotronic artificial skin comprises an elastomer sandwiched between two hydrogels (Fig. 2c) [7], which are connected to a capacitive meter through two metallic wires. The contacts between the hydrogels and metallic wires form two EDL capacitors, in series with the elastomer capacitor. Because charges are separated at a distance on the order of 10-9 m, the capacitance of the EDL is generally much larger than that of the elastomer, so the equivalent capacitance is primarily dominated by the elastomer capacitance, εEAE/dE, where εE, AE and dE are the permittivity, area and thickness of the elastomer respectively. Subject to a pressure or a stretch, the elastomer reduces thickness and expands area. The change in geometrical dimensions causes a change in capacitance, which is recorded by the capacitive meter.

The hydrogel ionotronic artificial skin can sense single touch and remain functional in the deformed state [7], sense multiple touches [8], self-heal [9], or sense changes in resistance rather than capacitance. Existing hydrogel ionotronic artificial skins only sense pressure and deformation. Using hydrogels to convert changes of temperature/humidity/chemicals into electrical signals can make the artificial skins one more step closer to or even superior the human skin.

2.4 Artificial axon

The axon transmits ionic signals that coordinate a living system to sense, decide, and act. Inspired by the myelinated axon, an ionotronic artificial axon is made of two lines of hydrogel separated by a layer of elastomer (Fig. 2d) [10]. The elastomer mimics the myelin sheath, and the electrolytic hydrogel mimics the body fluid. Subject to a time-dependent signal, the artificial axon transmits the signal from the input port to the output port. The voltage between the top and bottom hydrogel is a function, v(x,t), where t is the time and x is the location along the length of the artificial axon. The voltage obeys the diffusion equation, ∂v/∂t=D∂v2/∂x2, where D is the diffusivity of electrical signal. With suitable design, the diffusivity of electrical signal in the artificial axon can reach D ~ 107 m2/s, much higher than the diffusivity of ions in water, Dion ~ 10-9 m2/s. The artificial axon can transmit signal over long distance and at high frequency, readily serving as stretchable and transparent interconnects for wearable and implantable devices, as well as for soft robots.

Fig. 2 (a) Electric double layer forms at hydrogel/metal interface. (b) An ionotronic artificial muscle consists of an elastomer sandwiched between two hydrogels, connected to a source of electrical power by metallic wires. (c) An artificial skin consists of an elastomer sandwiched between two hydrogels, connected to a capacitor meter by two metallic wires. (d) In an artificial axon, two hydrogel layers are insulated by a layer of elastomer. The input port connects to a source of time-varying voltage, and the output port connects to a load of impedance Z. In a myelinated axon, myelin sheath is the dielectric, and body fluid is the electrolyte.


2.5 Other hydrogel ionotronics

Hydrogels have enabled other ionotronics of various functions. For example, hydrogels are transparent to let light through and conductive to apply voltage so that they are ideal candidates to replace conventional electrode, e.g. ITO, in electro-optical devices, replicating the primary electro-optical performances meanwhile achieving large stretchability. Examples include liquid crystal device (Fig. 3a) [11] and electroluminescent device (Fig. 3b) [12]. Both the responses of liquid crystal molecules in liquid crystal device and phosphors in electroluminescent device do not necessitate the injection of electrons from external power source, but rather relying on the application of electric field. It is this insight that enables the replacement of an electronic conductor with an ionic conductor (such as a hydrogel) in such devices.

In addition, the architecture of device should be carefully engineered to assure that the device can function stably without electrolyzing hydrogels, namely the magnitude of voltage drop across EDL should be < 1 V. Equivalent circuit of the device can be constructed, followed with the concerns of charge balance between different capacitors.

Additional hydrogel ionotronics include soft touchpad (Fig. 3c) [13], stretchable triboelectric generator (Fig. 3d) [14], artificial eel (Fig. 3e) [15], stretchable diode (Fig. 3f) [16] and more recently artificial spiderweb (Fig. 3g) [17]. Refer to corresponding paper for more details.

Fig. 3 Schematic of the working principles of various hydrogel ionotronics.


3.    Hydrogel ionotronics as a new context for materials research

Hydrogel ionotronics integrate dissimilar materials, e.g. hydrogels, elastomers, and metals. The dissimilarities between materials arise many issues. Consequently, the development of hydrogel ionotronics has motivated many areas of materials research, including but not limited to the engineering of strong adhesion for hydrogels (particularly strong, stretchable and sometime transparent adhesion between hydrogel and elastomer) and the improvement of fatigue resistance of hydrogels and hydrogel adhesion under prelonged cyclic loads.

For adhesion, because the dense water molecules barely carry load while the sparse polymer network often interacts weakly with the adherend, which can neither activate the Lake-Thomas mechanism nor elicit much hysteresis in the bulk, so that the adhesion between a hydrogel and an adherend is typically small (below 1 J/m2). It is conceivable that a device will not work properly if the hydrogels were to detach/fall easily from the device. Encouragingly, significant advances have been made recently in achieving tough hydrogel adhesion, an emerging field across multi-disciplines. See a recent review on hydrogel adhesion [18]. Also refer to related Journal club led by Zhenwei Ma and Prof. Jianyu Li:

In practice, a hydrogel ionotronic device may operate for a long time under static or cyclic conditions, exerting prelonged static or cyclic loads on hydrogels. Most existing hydrogels are prone to fatigue. Whereas the study on the fatigue of hydrogel is recent, many fundamental studies have been done, providing valuable fundamental insights towards practical deployments such as the design and synthesis of fatigue-resistant hydrogels/hydrogel adhesion. See the first comprehensive review [19] and the Journal club led by Dr. Ruobing Bai: Also refer to the Journal club led by Dr. Shaoting Lin themed on fatigue-resistant hydrogels and adhesion:

Other aspects of materials research include but not limited to water retention of hydrogel, manufacturing, fatigue under electromechanical load, synthesis and combination of hydrogel/dielectric elastomer with ideal properties oriented with applications, among others.


4.    Opportunities

The development of hydrogel ionotronics opens up an ample space for research. Many immediate opportunities exist, for example, in inventing new concepts of hydrogel ionotronic devices, in advancing the science of hydrogels and in developing integration and manufacturing.

Current hydrogel ionotronic devices mainly rely on manual assembly. Advanced manufacturing technique such as 3D printing would allow for rapid prototyping and miniaturization of devices to facilitate innovation and production.

Existing hydrogel adhesion strategies are mostly proposed without taking the advantages of 3D printing and often require molding or preformed hydrogels. Also, the wetting issue between a hydrophilic precursor and a hydrophobic surface has been overlooked in these strategies, which greatly restricts the allowable design and functions of device.

New adhesion methods and manufacturing processes will improve the integration of hydrogels and elastomers and allow for the design of devices with complex shapes and novel functions. We recently made some preliminary explorations towards this direction [20], by fulfilling the requirements of adhesion and wetting simultaneously to allow a multi-step dip coat process.

Hydrogels are susceptible to dehydration in air. One may replace water by an ionic liquid, a special type of salt that is in liquid state at room temperature, to obtain an ionogel, which can be non-volatile at a wide range of temperatures and even under vacuum [21], and thus can extend the range of possible applications. Moreover, the solvent can be eliminated, resulting in an ionoelastomer [22]. For both types of materials, high stretchability, appropriate ionic conductivity, and sometimes high transparency are of preferential concerns.

The fundamental challenges posed here cross-cut many applications beyond hydrogel ionotronics. We envision that the advances in hydrogel ionotronics will help to create deep integration between the natural and the artificial.

We love to hearing your feedbacks. Feel free to leave your comments on anything related to this topic below.


5.    References

[1] Yang, C., Suo, Z., Hydrogel ionotronics. Nature Reviews Materials 3, 125-142 (2018).

[2] Pelrine, R., Kornbluh, R., Pei, Q. & Joseph, J., High-speed electrically actuated elastomers with strain greater than 100%. Science 287, 836–839 (2000).

[3] Keplinger, C., et. al, Stretchable, transparent, ionic conductors. Science 341, 984–987 (2013).

[4] Lu, T., Wang, C., Wang, T., Mechanics of dielectric elastomer structures: A review. Extreme Mechanics Letters 38, 100752 (2020).

[5] Li, T. et. al, Fast-moving soft electronic fish. Science Advance 3, e1602045 (2017).

[6] Acome, E., Keplinger, C., et. al, Hydraulically amplified self-healing electrostatic actuators with muscle-like performance. Science 359, 61-65 (2018).

[7] Sun, J. Y., Keplinger, C., Whitesides, G. M. & Suo, Z., Ionic skin. Advanced Materials 26, 7608–7614 (2014).

[8] Sarwar, M. S. et. al, Bend, stretch, and touch: locating a finger on an actively deformed transparent sensor array. Science Advances 3, e1602200 (2017).

[9] Lei, Z., et. al, A bioinspired mineral hydrogel as a self-healable, mechanically adaptable ionic skin for highly sensitive pressure sensing. Advanced Materials 29, 1700321 (2017).

[10] Yang, C., et. al, Ionic cable. Extreme Mechanics Letter 3, 59–65 (2015).

[11] Yang, C., Zhou, S., Shian, S., Clarke, D. R. & Suo, Z., Organic liquid-crystal devices based on ionic conductors. Materials Horizons 4, 1102–1109 (2017).

[12] Yang, C.,  et. al, Electroluminescence of giant stretchability. Advanced Materials 28, 4480–4484 (2016).

[13] Kim, C.-C., et. al, Highly stretchable, transparent ionic touch panel. Science 353, 682–687 (2016).

[14] Pu, X. et. al, Ultrastretchable, transparent triboelectric nanogenerator as electronic skin for biomechanical energy harvesting and tactile sensing. Science Advances 3, e1700015 (2017).

[15] Schroeder, T. B. et. al, An electric-eel-inspired soft power source from stacked hydrogels. Nature 552, 214 (2017).

[16] Lee, H. et. al, A stretchable ionic diode from copolyelectrolyte hydrogels with methacrylated polysaccharides. Advanced Functional Materials 1806909 (2018).

[17] Lee, Y. et. al, Ionic spiderweb, Science Robotics 5 eaaz5405 (2020).

[18] Yang, J. et. al, Hydrogel adhesion: A supramolecular synergy of chemistry, topology, and mechanics. Advanced Functional Materials 30, 1901693 (2019).

[19] Bai, R., Yang, J.,  & Suo, Z., Fatigue of hydrogels. European Journal of Mechanics-A/Solids 74, 337-370 (2019).

[20] Yang, C. et. al, Ionotronic luminescent fibers, fabrics, and other configurations. Advanced Materials 2005545 (2020).

[21] Chen, B. et. al, Highly stretchable and transparent ionogels as nonvolatile conductors for dielectric elastomer transducers. ACS Applied Materials & Interfaces 6, 7840-7845 (2014).

[22] Kim, H. J., et. al, Ionoelastomer junctions between polymer networks of fixed anions and cations. Science 367, 773-776 (2020).

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