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1Repair Windows errors before they cause bigger problems2Fix the driver behind crashes, sound loss and screen glitches3Clear out junk files and repair common Windows errorsA 2018 study showed that one ionic hydrogel layer can replace several separate parts of an electrochromic device. Deposited directly on tungsten oxide (WO3) over fluorine-doped tin oxide (FTO), the lithium-chloride hydrogel served simultaneously as a transparent electrode, electrolyte and ion-storage layer. The simplified stack changed color under a small electrical bias, and a hydrogel pen used the same chemistry to draw patterns that could be erased and rewritten electrically.
This is a laboratory architecture, not a finished smart-window product. The authors proposed windows as a possible use, while independent expert Jianguo Mei said commercial window products would face a demanding 10–30-year durability requirement. Later work continues to refine flexible, electrolyte-free hydrogel devices, but no commercial deployment is established by the cited studies.
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What the hydrogel changed in the device architecture
Conventional electrochromic stacks use separate functions
An electrochromic device changes its optical state when ions and electrons move through an electrochemically active material. In a conventional stack, separate layers typically provide the transparent conductor, ion-conducting electrolyte, electrochromic film and an ion-storage or counter-electrode function. Each interface adds manufacturing steps, alignment requirements and opportunities for leakage, delamination or electrical resistance.
Huajing Fang and colleagues’ 2018 paper in Materials Horizons addressed that complexity by integrating several jobs into one soft material. Their structure placed a multifunctional ionic hydrogel directly on a WO3 film coated onto an FTO substrate. The paper is titled “Multifunctional hydrogel enables extremely simplified electrochromic devices for smart windows and ionic writing boards”.
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One hydrogel, three device roles
The hydrogel was formulated with aqueous lithium chloride. Its water-rich polymer network provided high ionic conductivity, allowing lithium ions to move when a voltage was applied. At the same time, the hydrogel was optically transparent enough to act as a transparent electrode, supplied the electrolyte needed for ion transport and stored ions for the electrochemical cycle.
Combining those roles is the central simplification. Instead of fabricating and laminating distinct electrolyte, counter-electrode and ion-storage layers, the researchers deposited a single functional hydrogel over the electrochromic WO3 film. The result remained a three-layer arrangement—FTO, WO3 and hydrogel—but the top layer performed several jobs at once.
How the hydrogel electrochromic effect works
Coloring with a small bias
In the reported device, a voltage was applied between the hydrogel and the FTO substrate. The bias drove ionic and electronic charge compensation in the WO3 film, producing a visible, broadly uniform color change. The study’s abstract reports significant color change, transmittance modulation and coloration efficiency, although the cited summary does not provide a single set of operating values that can be compared directly with other devices.
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Why FTO and WO3 were used
- FTO substrate: fluorine-doped tin oxide is a transparent conducting coating. It provides an electrically addressable base while allowing light to pass through the device.
- WO3 electrochromic film: tungsten oxide is the color-changing active layer. Its optical absorption changes as ions and electrons are inserted or removed during biasing.
- Ionic hydrogel: the lithium-chloride-containing network supplies mobile ions, conducts and stores electrochemical charge, and forms the top transparent contact in the simplified stack.
This arrangement should not be confused with a hydrogel that merely protects an ordinary display. The layer is part of the switching mechanism and replaces multiple conventional functions.
The ionic writing board demonstration
Drawing with a biased hydrogel pen
The most distinctive demonstration was an ionic writing board rather than a window. The researchers built a hydrogel pen with an electrically biased nib. When the nib touched the WO3/FTO structure, it deposited a hydrogel pattern while applying the electrical conditions needed to change the local optical state.
The resulting letters or shapes remained visible after the bias was removed. That persistence means the board could display information without continuous power at every moment. A reverse bias erased the pattern, after which a new one could be written. The method does not print pigment onto a surface; it writes an electrochromic state into the device by placing and electrically addressing the ionic hydrogel.
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 quotation was reported by Chemistry World in 2018.
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What the demonstration does and does not show
- It shows that a hydrogel can be patterned locally to create rewritable electrochromic information.
- It shows electrical erasure by reversing the bias, rather than mechanically wiping the surface.
- It does not establish a consumer-ready paper substitute, a specified writing speed, a quantified lifetime or resistance to outdoor weather.
- It does not demonstrate a manufactured smart window with the scale, sealing and safety requirements of architectural glazing.
Why smart windows were proposed—and why that remains a proposal
A colored electrochromic window can, in principle, reduce the amount of sunlight and solar heat entering a building. Fang’s team therefore discussed smart windows as an application of the simplified hydrogel architecture. The paper’s title includes smart windows, but the cited work is a laboratory device demonstration, not a field installation or an energy-saving study.
Window products must survive repeated switching, ultraviolet exposure, temperature swings, humidity, mechanical stress, contamination and decades of operation while remaining optically uniform and sealed. Jianguo Mei, an electronic-polymers expert at Purdue University, told Chemistry World: “Commercialising this technology for window products will be challenging – it requires an extremely demanding durability, ie 10–30 years.” That 10–30-year figure is a durability requirement described by an expert, not a measured lifetime of the hydrogel prototype.
For a practical window, developers would also have to solve issues that a small laboratory sample can avoid: large-area coating uniformity, edge sealing, electrical bus design, switching consistency across panes, repair or replacement procedures and safe operation after the hydrogel dries or loses water. None of those engineering milestones is established by the 2018 demonstration.
What is genuinely new about the 2018 design
- Layer reduction: a single multifunctional hydrogel replaced separate transparent-electrode, electrolyte and ion-storage functions.
- Direct deposition: the hydrogel was deposited directly on the WO3 film over FTO, avoiding a separately assembled electrolyte layer in the demonstrated structure.
- Reversible operation: a small bias colored the device and negative bias reversed the change.
- Local writing: a biased hydrogel pen created patterns that stayed after the field was removed and could be erased electrically.
- Material flexibility: a soft, water-containing network offered a route toward mechanically compliant devices, although the cited study does not establish a finished flexible product.
How this design compares with other electrochromic approaches
The fairest comparison is architectural rather than a ranking of numerical performance. Switching voltage, response time, optical contrast and cycle life depend on film thickness, electrolyte composition, electrode geometry, temperature, test protocol and device area. The cited summaries do not provide matching measurement conditions for a numerical head-to-head table.
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| Approach | Functional layers and roles | Mechanical form | Evidence level | Durability or performance information |
|---|---|---|---|---|
| 2018 ionic-hydrogel device | Three-layer FTO/WO3/hydrogel stack; the hydrogel combines transparent electrode, electrolyte and ion storage | Hydrogel-based laboratory structure; flexibility was not quantified in the cited summary | Laboratory color switching and rewritable ionic writing-board demonstration | Abstract reports significant color change, transmittance modulation and coloration efficiency; matched numerical conditions are not stated |
| Conventional multilayer electrochromic device | Separate transparent conductor, electrochromic film, electrolyte and counter-electrode or ion-storage components | Depends on substrate and layer materials | Established device architecture, but no single benchmark is specified here | Values vary by material system and test protocol; no directly comparable figures are stated |
| PAAm/P2W18 hydrogel study (2024) | Hydrogel electrochromic system designed to operate without an additional electrolyte | The abstract describes flexibility, tensile properties and transparency | Laboratory study reported in Dyes and Pigments | Simple preparation is reported; commercial deployment and field-life performance are not established |
The table’s “not stated” entries are deliberate. Quoting an isolated switching time or contrast value without the associated test conditions would create a misleading comparison.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What the 2024 hydrogel study adds
A later paper by Zhang and colleagues, “All-in-One fast response electrochromic devices based on PAAm/P2W18 hydrogel without additional electrolytes,” shows that the design direction continued beyond the 2018 WO3 demonstration. Its abstract describes a polyacrylamide/tungsten-oxide-cluster hydrogel with flexibility, tensile properties, transparency and a simple preparation process.
The phrase “without additional electrolytes” points to the same broad objective: incorporate ion transport into the active hydrogel rather than adding a separate liquid or gel electrolyte layer. However, a newer laboratory composition is not evidence that the earlier architecture has become a commercial window or writing product. The 2024 study provides evidence of ongoing materials research, not proof of outdoor lifetime, manufacturing scale or market availability.
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Durability questions that determine whether it can become a product
Water retention and environmental sealing
An aqueous hydrogel depends on its water content for ionic transport. A practical device would need packaging that limits evaporation while tolerating thermal expansion and contraction. Humidity, freezing, heat and ultraviolet radiation could all alter the hydrogel’s conductivity or optical clarity.
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Repeated switching and mechanical stress
Every color change moves ions through the material and changes local chemical and mechanical conditions. A product developer would need long cycle testing, measurements across the whole pane and tests after bending, vibration or impact where flexible substrates are used. The 2018 report does not supply a 10–30-year cycle or environmental qualification.
Uniformity at useful size
A small sample can switch evenly with short electrical paths. A window-sized panel introduces resistance gradients, edge effects and thickness variations. Uniform coloring, reliable bus bars and consistent erasure would have to be demonstrated over the intended area.
Manufacturing and repair
Directly depositing a multifunctional hydrogel simplifies the layer count, but production still requires controlled coating, adhesion, encapsulation and inspection. A commercial glazing system would also need a way to replace or isolate a failed pane without exposing the hydrogel or electrical contacts.
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- “Simplified” refers to fewer distinct functional layers, not necessarily a simple manufacturing line.
- “Transparent electrode” describes the hydrogel’s role in the demonstrated structure; it does not mean every hydrogel is sufficiently conductive or clear.
- “Rewritable display” describes the ionic writing-board proof of concept, not a specified consumer display standard.
- “Smart-window application” is a proposed use until large-area optical, thermal, safety and lifetime data are available.
- “Fast response” in a later paper should be interpreted with that paper’s own test conditions; it cannot be transferred automatically to the 2018 WO3/FTO device.
Bottom line for readers
The hydrogel layer matters because it collapses three electrochromic functions into one transparent, ion-conducting material. Fang’s 2018 team used that simplification to build both a uniformly switching WO3 device and a pen-written board whose patterns could be retained, erased and rewritten electrically. Those results make the architecture scientifically interesting and potentially useful for low-power optical control.
They do not yet make it a commercial smart window. The decisive missing evidence is long-term, large-area durability under real environmental conditions—the 10–30-year requirement highlighted by Jianguo Mei. Subsequent hydrogel studies show that researchers are still improving flexibility, transparency and electrolyte integration, but the technology remains at the laboratory-development stage in the evidence cited here.
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