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A 2018 study showed that an electrochromic device could be reduced to a three-layer structure by using one ionic hydrogel as a transparent electrode, electrolyte and ion-storage medium. The prototype changed color under a small electrical bias and supported a rewritable “ionic writing board.” It was an important architectural simplification, not a finished smart-window product: the authors identified long-term stability as a problem, and Jianguo Mei’s 2018 commentary in Chemistry World put the durability requirement for commercial windows at 10–30 years.
What the hydrogel redesign changed
Electrochromic devices alter their optical state when ions and electrons move through an electrochemically active material. In a conventional arrangement, separate layers usually provide the transparent conducting electrode, electrolyte and ion reservoir. Each interface adds manufacturing steps, alignment requirements and possible failure points.
Huajing Fang and colleagues proposed a simpler stack in a 2018 Materials Horizons paper, “Multifunctional hydrogel enables extremely simplified electrochromic devices for smart windows and ionic writing boards.” The design placed a tungsten oxide (WO3) film on a fluorine-doped tin oxide (FTO) substrate and deposited an aqueous lithium-chloride hydrogel directly on the WO3. The result had three principal layers rather than a more elaborate collection of separately fabricated functional films. The paper is available through the publisher’s DOI: 10.1039/C8MH00856F.
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The three-layer stack
| Layer | Material in the 2018 prototype | Primary job |
|---|---|---|
| Substrate/electrode | Fluorine-doped tin oxide (FTO) | Transparent electrical conductor and counter-electrode connection |
| Electrochromic film | Tungsten oxide (WO3) | Changes optical absorption as ions and electrons are driven into or out of the film |
| Multifunctional hydrogel | Aqueous lithium-chloride ionic hydrogel | Acts simultaneously as a transparent electrode, electrolyte and ion-storage layer |
The novelty was not that WO3 is electrochromic; tungsten oxide has long been used for that purpose. The simplification came from asking one hydrogel layer to perform several jobs that are normally assigned to separate components.
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How a hydrogel layer makes the device switch
The hydrogel contains water, polymer structure and dissolved lithium chloride. Lithium chloride supplies mobile ions, while the hydrated network keeps the material in place as a solid-like, optically usable layer. Because the hydrogel is ionically conductive and can carry electrical charge, it can participate directly in the electrochemical reaction at the WO3 interface.
Coloring the WO3 film
- Apply a bias: A voltage is applied between the hydrogel side and the FTO electrode.
- Move ions and electrons: The electric field drives lithium ions through the hydrogel toward the WO3, while the external circuit supplies the corresponding electronic charge.
- Change optical absorption: The charged WO3 becomes more strongly absorbing, producing a visible color change. The study reported significant color modulation and coloration efficiency, although the 2018 paper does not provide a single switching-voltage, response-time or contrast value that can be transferred to other devices.
Bleaching the film
Reversing the polarity drives the electrochemical process in the opposite direction. The color change is reversed as ions leave the electrochromic film and the material returns toward its original optical state. In the reported demonstration, the researchers obtained a uniform color change in one direction and reversed it with negative bias.
Why lithium chloride matters
The aqueous lithium-chloride formulation gives the hydrogel high ionic conductivity. It also avoids the need to laminate a separate liquid or solid electrolyte against the WO3. That combination is what allows the layer to be both an ion pathway and an electrically functional, transparent component. The hydrogel’s softness can also help it conform to surfaces, although the 2018 report should not be read as proof that a large, outdoor panel can survive years of thermal, ultraviolet, moisture and mechanical stress.
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The ionic writing board demonstration
The most distinctive demonstration was not a window but a rewritable display surface. The team built a hydrogel pen whose electrically biased nib deposited a hydrogel pattern onto the electrochromic structure. A line or character written with the pen remained visible after the writing bias was removed. Applying a reverse bias erased the pattern, allowing the same area to be written again.
Hong Wang, the corresponding author, described the concept this way: “In our hydrogel-based ionic writing board, arbitrary text and patterns can be drawn readily with a hydrogel pen just like writing calligraphy, and the information on this board is totally erasable and rewritable.” The statement describes the laboratory prototype’s operating concept; it is not a claim that a consumer writing tablet was placed on the market.
What happens during writing
- The operator places the biased hydrogel nib against the board.
- The nib supplies hydrogel and the local electrical stimulus needed to alter the underlying electrochromic area.
- The contacted region develops a visible pattern while surrounding areas remain in their previous state.
- Removing the bias leaves the pattern visible, providing short-term information retention without continuous writing power.
- Applying the opposite electrical polarity erases the mark and restores the writing area for another pattern.
This operating mode differs from a conventional display that refreshes every pixel through a permanent transistor backplane. It is closer to electrically controlled, locally deposited electrochromic information: the user writes where the pen touches, and erasure is a separate electrical operation.
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Are hydrogel electrochromic devices smart windows?
The 2018 authors discussed smart windows as a possible application. In principle, a window that becomes colored could reduce the solar radiation entering a building and therefore lower cooling demand. A hydrogel-based architecture might also reduce fabrication complexity if the multifunctional layer could be made large, uniform and durable.
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Those points describe a proposed use, not a tested commercial window. The reported work demonstrated electrochromic behavior and a writing-board concept at laboratory scale; it did not establish building-scale energy savings, outdoor lifetime, code compliance, optical uniformity over a full pane or a manufacturing process for architectural glazing.
The durability barrier
Long-term stability was explicitly identified as needing improvement. Jianguo Mei of Purdue University, commenting on the work in Chemistry World in 2018, warned: “Commercialising this technology for window products will be challenging – it requires an extremely demanding durability, ie 10–30 years.” The 10–30-year figure is a durability requirement for a practical window, not a measured lifetime of Fang’s prototype.
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A window product would have to tolerate repeated coloration cycles, sunlight, temperature swings, humidity, pressure differences, cleaning, scratches and edge sealing failures. Hydrogels add additional engineering questions: water retention, drying or freezing, swelling, ionic contamination and adhesion to the WO3 and transparent conductor. None of those challenges is solved merely by showing that a small sample changes color.
What the 2018 study established—and what it did not
- Established: A multifunctional ionic hydrogel can be deposited directly on WO3 over FTO to form a simplified electrochromic structure.
- Established: The lithium-chloride hydrogel can serve as transparent electrode, electrolyte and ion-storage layer.
- Established: A small electrical bias can produce a visible, reversible color change in the prototype.
- Established: A biased hydrogel pen can create patterns that remain after writing and can be erased with reverse bias.
- Not established: A 10–30-year operating lifetime, commercial window deployment, quantified building energy savings or mass-production economics.
- Not established: A universal switching voltage, response time or optical contrast applicable to every hydrogel electrochromic device.
How this design compares with other electrochromic approaches
The useful comparison is architectural rather than a contest over isolated headline numbers. Different studies use different film thicknesses, electrode areas, ion concentrations, measurement voltages, illumination conditions and cycle definitions. Without matching those conditions, numerical comparisons can be misleading.
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| Comparison axis | 2018 hydrogel/WO3/FTO prototype | Conventional multilayer electrochromic device | What can responsibly be concluded |
|---|---|---|---|
| Functional layers | Three principal layers; hydrogel combines three roles | Typically uses separate electrode, electrolyte and ion-storage functions | The hydrogel design simplifies the stack and may reduce interfaces |
| Switching voltage and response time | Small bias and reversible switching were demonstrated; exact values are not stated in the 2018 report | Varies by material and device design | No fair numerical ranking is possible without matched test conditions |
| Optical behavior | Significant color change, transmittance modulation and coloration efficiency were reported | Depends on electrochromic material, thickness and optical stack | The prototype demonstrated electrochromism, not a standardized window rating |
| Mechanical flexibility | Hydrogel architecture is compatible with soft or conformal concepts, but the 2018 account does not establish a flexible commercial panel | Ranges from rigid glass devices to flexible laboratory films | Flexibility must be measured on the complete device, including electrodes and seals |
| Cycle and environmental durability | Long-term stability remained an acknowledged issue | Product lifetimes depend on encapsulation and qualification testing | The prototype cannot be treated as a 10–30-year window |
| Deployment status | Laboratory ionic writing board and electrochromic sample | Some electrochromic products exist in other architectures | The hydrogel implementation was not shown as a deployed product |
What later hydrogel work adds
A 2024 paper by Zhang and colleagues, titled “All-in-One fast response electrochromic devices based on PAAm/P2W18 hydrogel without additional electrolytes,” describes another attempt to simplify the electrolyte arrangement. Its polyacrylamide/tungsten-polyoxometalate (PAAm/P2W18) hydrogel was designed to operate without an additional electrolyte and the abstract reports flexibility, tensile properties, transparency and a simple preparation process.
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That later result indicates that researchers continue to investigate all-in-one and mechanically compliant hydrogel electrochromic structures. It does not prove that the 2018 architecture reached commercialization, nor does an abstract-level description establish field lifetime, production yield or smart-window energy performance. The appropriate interpretation is continuing laboratory development, not a product announcement.
How to evaluate a claim about “hydrogel electrochromic” technology
When a paper, press release or product page uses this phrase, check the following details before accepting a performance claim:
- Layer definition: Is the hydrogel genuinely serving multiple functions, or is it only an electrolyte added to a conventional stack?
- Electrolyte chemistry: Which ions provide conductivity, and does the device require a separate counter-electrode or ion reservoir?
- Test conditions: What voltage, current density, temperature, humidity, optical path and sample area were used?
- Switching metrics: Are response time, transmittance change and coloration efficiency reported with measurement methods?
- Retention and erasure: Does the colored state persist without power, and how is it erased?
- Durability evidence: How many cycles were completed, and were the samples exposed to heat, ultraviolet light, moisture and mechanical handling?
- Scale: Is the result a small coupon, a flexible strip, a writing-board demonstration or a full-size window?
- Encapsulation: How are water loss, leakage, freezing, swelling and edge degradation controlled?
These checks separate an elegant materials demonstration from evidence that a technology is ready for buildings or consumer electronics.
Why the 2018 work still matters
The paper’s lasting contribution is an architectural idea: combine ionic conduction, transparent charge transport and ion storage in one hydrogel rather than building each function as a separate layer. That can make experiments easier to assemble and opens unusual interaction modes, including a pen that writes electrochemically active patterns directly onto a board.
The same simplification creates the central engineering trade-off. A layer that performs several jobs must remain transparent, conductive, mechanically stable and chemically compatible at the same time. Improving one property can hurt another: more water may improve ion mobility but worsen drying or sealing; a softer network may improve writing contact but reduce dimensional stability; stronger encapsulation may improve lifetime but complicate fabrication.
Bottom line
Hydrogel electrochromic devices were “reinvented” in the 2018 work by collapsing several conventional functions into one lithium-chloride ionic hydrogel on WO3/FTO. The result switched color electrically and enabled a genuinely distinctive rewritable ionic writing board. Smart windows were a plausible future application, but the evidence supports a laboratory proof of concept—not a commercial window—and durability on the order of 10–30 years remains the key hurdle identified for real-world deployment.
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