A new color-changing material can remember and forget like a brain cell, creating self-erasing images that hide information after a short time without using extra power.
(Nanowerk Spotlight) Screens mediate almost everything people know and trust. Financial transactions, identity checks, and private messages all depend on what can be seen on a surface of light and color. Yet visibility is both a strength and a weakness. What can be shown can also be recorded, copied, or forged.
Traditional cybersecurity protects information through algorithms and encryption keys, but far less attention is given to how that information appears in the physical world. As digital systems extend into wearable electronics, flexible displays, and connected devices, the question of how to control what information looks like has become increasingly important.
Materials scientists have started treating visibility itself as part of the security challenge. Their goal is to create surfaces that can change, hide, or erase information at will, adding a material layer of protection to complement software-based security.
One promising direction uses electrochromic materials, substances that change color when small electrical charges move ions through them. These materials already serve in smart windows and low-power displays, but most act as simple switches. When the power is removed, they either revert to their original state immediately or remain fixed indefinitely. They have no way to remember or adapt over time. Without memory, they cannot support displays that vary or protect data according to timing or sequence.
A new approach is emerging at the intersection of electrochromics and neuromorphic electronics, a field that takes cues from the adaptive behavior of the human brain. In biological systems, synapses strengthen or weaken based on how signals arrive in time, a process known as plasticity. Embedding similar timing-dependent behavior in color-changing materials could lead to displays that learn when to reveal or hide information, using memory as part of the visual response.
The paper describes a hybrid electrochromic device that connects fast optical switching with memory-like persistence. It acts as a learning pixel that records color intensity based on brief electrical pulses and gradually forgets as the color fades.
a) Schematic of biological neurons and synapses, where action potentials from presynaptic neurons trigger the release of chemical neurotransmitters to postsynaptic neurons, leading to ion flow, accompanied by electrical signal output. b) Structure design of the bio-inspired self-coloring electrochromic artificial synaptic device, accompanied by optical signal output. c) Schematic diagram of device operation: colored (initial state), partially bleached (intermediate state), and fully bleached (final state) states. d) Schematic diagram of encryption for electrochromic artificial synapse array device. (Image: Reprinted with permission by Wiley-VCH Verlag) (click on image to enlarge)
The system merges two materials with complementary properties: PEDOT:PSS, a conductive polymer known for rapid color change, and hydrated tungsten oxide (WO₃·H₂O), which holds its color long after the power is off. Together they form a thin film that can display, store, and erase information without complex circuitry.
The device is assembled on a transparent electrode and paired with a magnesium-based gel that carries charged magnesium ions. A thin zinc foil acts as a counter electrode. When the circuit briefly connects, magnesium ions move into the film while electrons flow through the circuit.
PEDOT:PSS turns blue almost instantly, while WO₃·H₂O changes more slowly but retains part of that charge, keeping its color after the circuit opens. This self-coloring and gradual self-erasing behavior makes the device energy-efficient and simple to operate.
The mechanism relies on how ions and electrons interact inside the materials. In tungsten oxide, magnesium ions occupy spaces within the crystal lattice and cause some tungsten atoms to gain an extra electron. This change in charge state alters how the material absorbs visible light, producing a deep blue tint. The hydrated form contains thin layers of water between oxide sheets, which gives ions more room to move and reduces stress during repeated cycling.
PEDOT:PSS forms a conductive network that spreads charge evenly and ensures fast and uniform color change. Microscopy reveals a porous nanosheet structure with large surface area for ion exchange, and spectroscopy confirms that both materials retain their key properties in the hybrid film.
The magnesium gel electrolyte underpins the device’s performance. It is made from gelatin, water, and glycerol to keep it flexible, with dissolved magnesium chloride and magnesium sulfate providing mobile ions. The best balance between conductivity and chemical stability occurs when both salts are present in equal concentrations. This composition achieves high ionic conductivity of more than six siemens per meter and a wide voltage window of over four volts.
Hydrogen bonding strengthens the gel’s internal structure, and glycerol slows evaporation, keeping the electrolyte stable in air. It remains flexible up to about 46 degrees Celsius and retains nearly 90 percent of its mass after a month of exposure, which is critical for long-term durability.
Optical measurements show that the hybrid film changes light transmission by as much as 82.9 percent at a wavelength of seven hundred nanometers, which is much greater than either component alone. Short electrical pulses of increasing duration produce progressively deeper shades of blue, each representing a distinct optical memory level. The film can maintain up to eight such states and holds them for hundreds of seconds before fading back to transparency.
Even after one thousand cycles, the color contrast remains stable. Spectroscopic analysis confirms that these changes stem from magnesium ion insertion and corresponding shifts in tungsten oxidation states, validating the intended mechanism.
The researchers also found that the device exhibits synaptic behavior similar to biological learning. When two voltage pulses occur in quick succession, the second response is stronger than the first, mirroring a phenomenon called paired-pulse facilitation. As the interval between pulses increases, the effect weakens. The response follows two characteristic time scales of about three and seven seconds, which correspond to rapid reactions in the polymer and slower ion diffusion in the oxide.
Repeated pulses deepen the color and slow its fading, reflecting a transition from short-term to longer-term memory. These time-dependent optical responses form the basis for what the authors call an electrochromic artificial synapse.
Using this property, the team built a time-sensitive encryption display. They patterned the hybrid film into a grid of tiny pixels and wrote messages by touching selected areas with a stylus coated in the gel electrolyte. Short touches created light blue pixels and longer touches made darker ones.
Arranged in Morse code, these patterns appeared clearly when first written but gradually blurred and disappeared as the color faded. Because darker pixels lost their color faster than lighter ones, each message had a built-in lifetime that could be tuned from seconds to minutes by adjusting the writing pulse.
This coupling of spatial pattern and timing created a form of spatiotemporal encryption, information visible only for a defined window before erasing itself.
The approach offers several advantages. It consumes power only during writing, since maintaining or erasing color requires no energy input. The same material acts as both memory and display, reducing system complexity. The magnesium gel is transparent, stable, and flexible, making it suitable for thin or curved devices. The team demonstrated arrays with one hundred micrometer pixels, and finer resolutions appear feasible with existing microfabrication techniques. Such materials could support temporary authentication tags, time-limited access indicators, or visual codes that disappear automatically after viewing.
Some challenges remain. The rate of fading depends on humidity and oxygen exposure, which could complicate precise timing outside laboratory conditions. Pixel uniformity and scaling also require further refinement for more complex displays. Yet the central achievement stands: a hybrid material system that combines learning-like dynamics with visible information, transforming time and memory into active components of security.
This study shows how PEDOT:PSS, hydrated tungsten oxide, and a magnesium gel can be integrated to create an optical device that learns and forgets in color. It points to a broader movement in materials science toward systems that sense, compute, and communicate within a single structure. As digital and physical technologies continue to converge, materials that control not only how data is stored but also how it appears could redefine the meaning of secure information.
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