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Hydrogel ionotronics, Canhui Yang and Zhigang Suo, Nature Reviews Materials, 2018

This review discusses their pioneering research work in hydrogels (tough, fatigue-resistant, and higd interfacial adhesion) and the 1st generation hydrogel ionotronics. Hydrogels are ion-conductive, transparent, and highly-stretchable; hydrogel-based electronics have key components of hydrogels and metal wires/pieces, making use of the coupling of ion- and electron-interaction/induction through ingenious designs. These simulates more the signal sensing and actuating systems in the living biology with favorable efficiencies, thus showing great potential for biomedical/human-machine integration applications. It is comprehensive, clear, and coherent in describing the ideas, the fabrication, and the fundamentals.     A very good read.

Major contents (what and how):

Artificial muscle, skin and axon

1st-generation hydrogel ionotronic devices mimic neuromuscular and neurosensory systems, the function not the anatomies. 

Artificial muscle (existing research, electric elastomer actuators, dielectric elastomer actuators) consist of an elastomer sandwiched by two layers of hydrogel, which are connected with metal wires and further to a power source. Applying voltage results in EDL between the metal wire and the hydrogel, and then two opposite-charged elastomer-hydrogel interfaces; these lead to deformation of the elastomer (decrease in thickness and increase in area). Electricity controls shape change via field force.

The metal wires can be outside the active area, so that the entire system can be made transparent. This is advantageous than most other stretchable conductors like carbon grease, graphene, aqueous electrolytes.

The electromechanical coupling allows shape changes and controllability through quantitative relationship V≈HE(μEεE)1/2 .

The voltage needs to be high, requiring elastomers to have high electric breakdown strength but not causing electrochemical reaction at the EDL as the area ratio of EDL and elastomer is ~10-4.

The hydrogel transmits electrical signals fast enough, with a super small resistive-capacitive delay 10-8 s, for artificial muscle operation.

Artificial skin consists of an elastomer sandwiched between two hydrogel layers that connect to a capacitive meter. All are transparent and stretchable. Two EDL capacitors in series with the elastomer capacitor form, and the equivalent capacitance is the elastomer one, ~εEAEHE .  Applying pressure or stretching the elastomer causes a capacitance change and a signal in the capacitive meter.

Artificial axon consists of two hydrogel layers separated by an elastomer, simulating the axon (saline, electrolyte) with outer myelin sheath (fatty, dielectric shell) for fast electrical signals. One end connects to input port and power source (signals), and the other end outputs signals/connects to external load. The voltage between the hydrogel layers obeys the diffusion equation, ∂v∂t=D∂2v∂2x . The estimated diffusivity is many orders of magnitude higher than that of ions in water, so ionic signals transmitting along the artificial axon just need ions to move locally. The transmit behavior does not change when the length and thickness of the hydrogel and elastomer proportionally decreased.

It allows long distance signal transmit at high frequency without depolarization. It also is versatile in types and ways of transmitting signals, e.g., power to resistors, capacitors, inductors and semiconductor components.

It is also transparent and stretchable.

Hydrogel ionotronic devices

Current electrodes for optoelectronic devices (optical transparent and electrical conductive) are indium tin oxide (ITO)-brittle and costly, carbon nanotubes etc.-low stretchability low transparency low fatigue. Hydrogels with high conductivity, transparency, stretchability are ideal for electro-optical devices.

An ionotronic luminescent device consists of a phosphor layer sandwiched by two elastomer layers and two hydrogel layers connected to power source. Alternating voltage causes alternating electric field, generates mobile electrons and holes, which subsequently recombine to produce light. The elastomer layers provide separation, protection, isolation. The device is stretchable and transparent, and can operate over large areas.

The high voltage does not lead to electrolysis of EDL nor breakdown of elastomer, while the electric field enabling luminescence of phosphor is one order of magnitude lower than the electric breakdown strength of elastomer.

Ionotronic liquid crystal device consists of liquid crystal sandwiched by two elastomer layers and two hydrogel layers. With voltage off, the liquid crystal scatters light and opaque, while with voltage on the liquid crystal molecules align and become transparent. The device is transparent and stretchable. The high stretchability allows for a new operation mode: it is switchable in response to a combination of electrical voltage and mechanical force.

A hydrogel ionotronic touchpad consists of a strip of hydrogel with metallic electrodes at both ends that connect to alternating voltage. When a finger touches the hydrogel, two measured electronic currents (by ionic and the capacitive coupling at EDL) determine the locations of the touch point. The RC delay is much smaller than the sampling instant. This touchpad is soft (stretchable) and transparent.

A hydrogel iononic triboelectric generator consists of a hydrogel layer encapsulated in an elastomeric cell and connected to an external load by a metal piece. When a dielectric approaches and leaves the elastomer cyclically, ions flow in the hydrogel, and the capacitive coupling of EDL causes electron flow between the metal and the ground, generating electricity. The device is highly stretchable and transparent.

An artificial eel can be created by stacking a high-salinity hydrogel, a cation-selective hydrogel, a low-salinity hydrogel, an anion-selective hydrogel and a second high-salinity hydrogel in sequence (mimicking the ionic gradients in series in an electric eel). Upon contact, an ionically conductive pathway is established to result in a significantly scaled up open-circuit voltage.

A gel-elastomer-oil (GEO) device has potential in various engineering applications.

Materials for hydrogel ionotronics

Hydrogels: PAAm, PVA, PAAc/alginate

Salts: NaCl, LiCl

Elastomers: VHB, PDMS, Ecoflex

The electrochemistry at the interfaces between electronic conductors and hydrogels is important, and a working hypothesis is that it is similar to electronic conductors-aqueous electrolytes interfaces.

Hydrogels can be designed and fabricated to achieve high toughness, strong and stretchable and to resist fatigue fracture under cyclic loading.

At molecular scale, the hydrogel/or elastomer) is a solid-liquid hybrid (solid for along chain-covalent bonds, liquid for between chains-physical interactions), and can be described by the Lake-Thomas model.

Hydrogels with high fracture toughness can be made through introducing sacrificial bonds, which breaks when the hydrogel is stretched to an intermediate level and elicits hysteresis to dissipate energy. This is inherent to tough materials, such as metals and composites, and also biological materials such as bone/mineralized collagen.

Adhesion, water retention and fatigue

Adhesion. To achieve the Lake-Thomas mechanism (at crack tip, breaking the chain across the crack plane consumes the strength and energy of a chemical bond and triggers the stored, covalent energy of a layer of chains, ~10-100 J/m2), (1) strong inter-network bonds, (2) sacrificial bonds, (3) third network via topological entanglement with pre-​existing networks and/or strong bonds with pre-​existing networks (topological adhesion).

The adhesion can be improved by oxidization, modifying the –CH3 groups into –OH ones, but these do not lead to strong adhesion.

High adhesion involves interfacial bonding, chain breaking, and hysteresis in the bulk.

High interfacial toughness can be achieved through using cyanoacrylate glues, benzophenone, and coupling agents.

Water retention. Stretchability and permeability are inextricably linked at the molecular level. Adding hygroscopic salts inside, applying a thin coating of elastomer can help to retain water. Sealing with an elastomeric film also prevents mass exchange.

Fatigue. Hydrogel fatigue can be studied by applying cyclic loading to uncut samples (fatigue damage, causing decreased modulus) or pre-cut samples (fatigue fracture). Hydrogel can be designed through chemical ways, e.g., introducing both strong ionic bonds/crosslinks to enable elasticity and weak ionic bonds that can reversibly form and break for energy dissipation and toughness.

Outlook

Next generation hydrogel ionotronics embrace applications such as soft robots and wearable and implantable devices. New adhesion methods and manufacturing, better study of the metal-hydrogel interface electrochemistry, batch synthesis, etc., are along the road for hydrogel ionotronics as the integration between the natural and the artificial.

It is both challenging and exciting.

Here is the link of the paper: https://www.nature.com/articles/s41578-018-0018-7