A breathable, stretchable nanofiber sensor uses a framework-enhanced polymer to harvest motion energy and capture precise finger and joint movement for advanced wearable communication and monitoring applications.
(Nanowerk Spotlight) A person signing a single word can shift several fingers by only a few degrees. Those small changes carry meaning, yet most sensing technologies miss them. Camera systems lose track when hands turn or when lighting shifts. Gloves with rigid joints resist natural motion and change the way signs are formed. Batteries run low at inconvenient moments. These limits affect not only communication but also fields where precise movement matters, including rehabilitation and athletic training. Engineers who work on wearable systems have tried repeatedly to capture detailed motion without altering how the body moves.
Triboelectric sensors promise an alternative because they generate electrical signals from contact and separation. The challenge is that materials with strong output often feel stiff or trap sweat, while soft breathable structures tend to produce weak signals. This tradeoff has slowed progress toward sensors that match the mechanics of real skin and function without external power.
At the heart of the work is a triboelectric nanogenerator, or TENG. A TENG produces electricity when two materials touch and then separate. Electrons move from one surface to the other during contact. When the surfaces pull apart, a difference in charge builds up and drives current through an external circuit. Here, the researchers design a TENG that operates in a contact separation mode, using two soft fiber mats as the active layers and a stretchable textile as the electrode.
Illustration of the functionality overview of the stretchable and breathable and TENG. (Image: Reproduced with permission from Wiley-VCH Verlag)
The key advance lies in the positive triboelectric material. The team starts with polyurethane, a common flexible polymer used in many soft products and combines it with a covalent organic framework called TpDq COF. A covalent organic framework is a crystalline porous polymer built from light elements such as carbon, nitrogen and oxygen. Its building blocks link in a regular grid, creating aligned pores and a high internal surface area. The TpDq COF in this device carries amine groups with lone pair electrons. These groups tend to donate electrons during contact, which makes the material strongly tribopositive.
To turn this composite into a usable layer, the researchers dissolve polyurethane, add different weight percentages of TpDq COF and then form the mixture into nanofibers by electrospinning. Electrospinning uses a high voltage to pull thin jets of liquid from a needle. As the jets dry, they become long fibers that collect as a porous mat. The result is a soft, flexible fabric-like layer called PU@TpDq COF, in which TpDq COF particles are embedded throughout the polyurethane.
The negative triboelectric layer uses a blend of polyvinylidene fluoride hexafluoropropylene, abbreviated PVDF HFP, and thermoplastic polyurethane, TPU. PVDF based polymers tend to gain electrons in contact with many other materials and are widely used as tribonegative surfaces. Blending PVDF HFP with TPU makes the layer more stretchable while preserving its electrical behavior. This second nanofiber mat is labeled PVDF HFP@TPU.
Both nanofiber layers are laminated onto knitted conductive fabric that serves as a stretchable electrode. An elastic silicone spacer keeps the layers apart until pressure or bending brings them into contact. When pressure is applied, the PU@TpDq COF and PVDF HFP@TPU layers touch and exchange charge. When pressure is released and the layers separate, a potential difference forms and electrons flow through the fabric electrodes.
Material analysis confirms that the covalent organic framework changes the structure and properties of the polyurethane. X ray diffraction shows that adding TpDq COF sharpens and strengthens the main diffraction peak of the polymer, which indicates higher crystallinity and more ordered chains. X ray photoelectron spectroscopy and infrared spectroscopy reveal new features linked to quinone and amine groups from the framework and show evidence of hydrogen bonding between the framework and polyurethane. These interactions help stabilize charge carrying sites.
The composite also behaves like a network of tiny capacitors. Each TpDq COF particle, surrounded by the insulating polymer, acts like a small electrode embedded in a dielectric medium. Together they form many microcapacitors. This microcapacitor network, along with charge buildup at the interfaces between framework and polymer, raises the effective dielectric constant. The dielectric constant, which measures how much charge a material can store for a given electric field, increases as TpDq COF loading rises and reaches a maximum at 20 wt.%. At higher loadings the particles begin to aggregate and form partial conductive paths, which lowers performance.
Surface potential measurements show the same trend. A PU@TpDq COF layer with 20 wt.% framework reaches a surface potential of 0.61 kV, about six times that of pure polyurethane. The composite also retains charge longer, likely due to the high surface area and porous nature of the framework. These properties translate directly into triboelectric performance.
When assembled into a working device, the optimized triboelectric layers generate strong electrical signals from simple motion. The sensor produces high voltage and current when the layers press together and separate, and it performs best when the composite contains the optimal amount of covalent organic framework. This version delivers far more power than the same device made from unmodified polyurethane. It also stores enough energy to run a small hygrometer and to charge common capacitors, which shows that the output is practical for lightweight wearable electronics.
The sensor also meets key requirements for comfort. Its nanofiber structure allows water vapor to escape much faster than skin produces sweat, which helps prevent moisture buildup. The material stretches easily, and the full device can extend well beyond the natural strain of skin on the arm without damage. These properties allow the sensor to move with the body during bending, twisting and stretching while maintaining stable performance.
The researchers also study temperature and humidity effects. Between 25 °C and 38 °C the output remains nearly constant. Above 38 °C the signal begins to fall. As relative humidity increases from 25% to 63%, the output decreases because water layers on the surface provide conductive paths that let charge leak away.
To highlight practical use, the team mounts the sensor on different body joints. When placed on the elbow, knee and wrist, the TENG generates clear voltage waveforms during bending and straightening. On a finger, the output increases stepwise as the bending angle changes from 30° to 90°. The sensor shows an angle sensitivity of 0.01439 V degree⁻¹ over this range. This sensitivity allows it to distinguish different finger positions.
Finally, the researchers attach five TENG sensors to the five fingers of one hand and record patterns during simple sign language shapes and gesture like symbols. Each bent finger produces a voltage pulse. Each straight finger produces little or no signal. By reading combinations of on and off signals across the five channels, the system can represent words such as HELLO and phrases such as I LOVE YOU, as well as hand shapes that resemble common emojis. The device can also encode binary sequences, using bent fingers as ones and straight fingers as zeros, which could be transmitted through a wireless link in future systems.
Taken together, the Advanced Energy Materials work shows how a covalent organic framework can boost the electrical performance of a soft polymer while preserving breathability and stretchability. The nanofiber based triboelectric sensor behaves like a fabric but delivers voltages and power densities high enough for both sensing and small-scale energy harvesting. Its ability to monitor joint and finger motion without external power points toward wearable systems that support communication, training and health monitoring while remaining close to the feel of everyday textile products.
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