A flexible gel material improves the durability and safety of tiny zinc batteries by controlling ion movement and protecting the surface, making them better suited for wearable electronics.
(Nanowerk Spotlight) A body-worn sensor that tracks heart rate, muscle movement, or brain activity must do more than collect data. It has to flex and stretch with the skin, operate safely near biological tissues, and keep working through thousands of charge and discharge cycles without breaking down or running out of power. Meeting all of these demands in a single, compact battery remains a difficult engineering problem.
Zinc-based micro-batteries are one of the most promising candidates for powering wearable devices. Their water-based chemistry avoids the fire risks of traditional lithium-ion cells, and zinc itself is biocompatible and abundant. But these systems suffer from a pair of interrelated failures.
First, when zinc ions move unevenly through the electrolyte during charging, they tend to deposit in jagged, branching structures called dendrites that can short-circuit the battery. Second, the interface between the zinc anode and the electrolyte is chemically unstable. Without a reliable barrier to regulate reactions at the surface, the battery degrades with each cycle, losing both capacity and safety.
Attempts to address these issues have mostly treated them in isolation. Some strategies focus on mechanical coatings to suppress dendrites, others on modifying the electrolyte to reduce side reactions. But wearable batteries must do more than store charge. They need to remain chemically stable under strain, operate safely over long periods, and deliver power in systems that are constantly moving. Most current designs fall short because they do not manage the relationship between ion transport and interfacial chemistry with enough precision or adaptability.
The team developed a flexible hydrogel that incorporates zwitterionic molecules, which contain both positive and negative charges in a single structure. These molecules organize into internal pathways that guide zinc ions through the material. At the same time, the electrolyte promotes the formation of a protective layer on the zinc surface that resists breakdown. This dual function improves the stability, safety, and flexibility of the battery, offering a path toward power systems that can operate reliably inside wearable or skin-integrated electronics.
The new hydrogel is made by combining a natural polymer (CMC) with a small molecule (AMP) that carries both positive and negative charges. This design creates tiny, organized channels that help zinc ions move smoothly, while also changing how water molecules interact inside the gel. Microscopy and spectroscopy show that the material has a porous structure, stable bonding, and improved ion transport compared with conventional gels. These features make it well suited for use in flexible zinc batteries. (Reprinted with permission by Wiley-VCH Verlag) (click on image to enlarge)
The core of the design is a hydrogel based on carboxymethyl cellulose (CMC) modified with adenosine monophosphate (AMP), a naturally occurring molecule with zwitterionic properties. When AMP is introduced into the CMC network, it creates self-assembling pseudo-ionic channels that allow zinc ions to move in an orderly way through the hydrogel. These channels reduce ion scattering, which helps prevent the formation of dendrites. The phosphate group on AMP attracts positively charged zinc ions, while the amine group engages with sulfate ions from the electrolyte. This balance of interactions forms a structured environment that regulates how ions flow during battery operation.
Just as important as guiding ion movement is the formation of a stable interfacial layer, known as the solid electrolyte interphase or SEI. In this system, the zwitterionic AMP environment facilitates the spontaneous formation of a bilayer SEI on the zinc anode. The outer layer is rich in organic compounds that cushion mechanical stress and suppress side reactions. The inner layer contains inorganic compounds that form a strong bond with the zinc surface. Together, they protect the anode and support uniform zinc deposition, which is essential for preventing capacity loss and structural failure.
To understand how these molecular features affect performance, the researchers carried out a series of structural and electrochemical characterizations. Spectroscopy revealed that AMP disrupts the hydrogen bonding network of water within the hydrogel. This reduces the number of water molecules coordinating with zinc ions, limiting unwanted reactions. The material also showed a porous microstructure with high ionic conductivity, measured at 30.1 millisiemens per centimeter. The transference number for zinc ions reached 0.86, indicating that the majority of current is carried by zinc rather than by other ions. This is a key factor in achieving uniform plating and reducing the risk of dendrite formation.
Mechanically, the hydrogel was both soft and strong. It stretched to nearly twice its length before breaking and demonstrated consistent elasticity during stress testing. These properties come from multiple interactions within the hydrogel structure, including hydrogen bonding, electrostatic attraction, and coordination between metal ions and ligands. The result is a material that can deform without losing its structural or electrochemical performance, which is essential for wearable use.
To evaluate the system in a real-world format, the team used 3D printing to fabricate zinc-ion micro-batteries. Printable inks containing manganese dioxide and zinc particles were prepared with additives that ensured both flow and structural stability. The AMP-modified hydrogel served as both electrolyte and separator. These printed batteries delivered high specific capacities across a range of current densities and maintained stable performance even under mechanical stress.
At a current of 1.5 amperes per gram, the batteries retained 99.3 percent of their original capacity after 1000 charge-discharge cycles. Under repeated bending, they lost less than one percent of their capacity after 100 deformation cycles. Compared to batteries using conventional hydrogel electrolytes, those built with the AMP-modified version showed smoother zinc deposition, thinner surface layers, and lower surface roughness.
The same hydrogel material was also used to create a skin-contacting electrode, enabling it to function as both electrolyte and biosensing interface. In a fully integrated wearable device, the 3D-printed batteries powered a system that recorded electrophysiological signals including heart activity, muscle movement, brain waves, and eye motion. The hydrogel electrodes picked up these signals with accuracy comparable to commercial silver chloride gel electrodes, while offering greater flexibility and mechanical resilience.
Tests on volunteers showed that the system could monitor heart rate under different physical and emotional conditions, record muscle activity during gripping and standing tasks, and detect eye movement patterns such as blinking and gaze shifts. Signal quality remained stable after repeated bending, twisting, and mechanical impact. The hydrogel maintained close contact with the skin and continued to collect reliable data throughout extended use.
What sets this work apart is the way it combines structural design and chemical function within a single material. Instead of treating the battery’s internal dynamics and surface reactions as separate challenges, the researchers engineered a hydrogel that handles both. By directing ion movement through self-assembled pathways and supporting the spontaneous formation of a protective interfacial layer, this system achieves a rare combination of electrochemical stability and mechanical adaptability.
For wearable electronics to become truly autonomous and body-integrated, their power sources must match the softness, resilience, and complexity of the biological systems they serve. The hydrogel system developed in this study suggests that such power sources are not only feasible but within reach. It offers a clear example of how precise molecular design, inspired in part by biological structures, can overcome long-standing obstacles in flexible energy storage.
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