How Researchers Used Kapton® polyimide film to Harvest Energy from Human Movement
Every year, millions of batteries are thrown away from wearable health monitors, fitness trackers, and body sensors. It is one of the quieter sustainability challenges of the connected healthcare era — and researchers at the University of Exeter have been working to solve it.
In a study published in Smart Materials and Structures, Yang Kuang and Meiling Zhu demonstrated a knee-joint energy harvester that generates electricity from the natural motion of walking, and used it to power a wireless sensor node entirely without a battery.
A key component in making it work was Kapton® polyimide film supplied by Advent Research Materials.
Science Made Simple
Wearable sensors — like the kind used to monitor heart rate, track activity, or send health data to a hospital — need electricity to run. Most of them use batteries.
The problem is that batteries run flat, need replacing, and if the sensor is implanted inside the body, replacing the battery means surgery.
The dream is to power these sensors from the body itself — specifically from the energy of everyday movement, like walking. Your legs already produce plenty of mechanical energy every time you take a step. The question is whether you can convert enough of that movement into electricity to run useful electronics.
The challenge is a speed mismatch. The material typically used to convert movement into electricity — called a piezoelectric — works best when it vibrates fast, around 50–100 times per second. Your knee bends about once per second when you walk. That is far too slow.
Here, flexible plastic strips (the plectra, made from Kapton® film) are arranged around a ring attached to the knee. As the knee moves, they flick small piezoelectric elements, making them vibrate fast and generate electricity — even though the knee itself is moving slowly.
That electricity is stored in a small capacitor (like a rechargeable cell) and used to send data wirelessly from a sensor node. No battery anywhere in the system.
The Problem with Wearable Power
Piezoelectric materials — the kind used to convert movement into electricity — work best when they vibrate quickly, at their natural resonant frequency. The human knee, by contrast, bends slowly: around once per second at a comfortable walking pace. That mismatch is the central challenge of biomechanical energy harvesting. If you can bridge it, you can turn walking into useful electricity. If you cannot, the device barely produces a trickle.
Kuang and Zhu's solution was elegant: instead of trying to drive the piezoelectric directly from the slow rhythm of the knee, they used a plucking mechanism. Imagine a music box — a rotating drum whose pins pluck the tines of a metal comb, each one vibrating at its own high frequency after being flicked.
The knee-joint harvester works on the same principle.
As the knee rotates, a ring of small flexible strips — the plectra — repeatedly flick the tips of the piezoelectric elements, each snap producing a short burst of vibration at exactly the right frequency.
Seventy-three of these plectra are arranged around the outer ring, creating a steady stream of energy pulses throughout each gait cycle.
Why Kapton® Polyimide Film Was the Right Choice
The plectra are where Advent Research Materials comes in. Each one is a small tab — approximately 3 × 2 mm — cut from Kapton® polyimide film (product IM8031, 125 µm thick), embedded in a PTFE ribbon within the rotating assembly.
Kapton® is a well-established material in precision engineering, and for good reason. It combines flexibility with toughness: it bends cleanly under load and springs back immediately, without creep or permanent deformation. It is stable across a wide range of temperatures, chemically resistant, and highly consistent from batch to batch. In this application, those properties translated into plectra that could flex and snap thousands of times every hour, throughout the life of the device, without losing their shape or stiffness.
That consistency matters beyond the device itself. When researchers model the forces involved in a mechanism like this — predicting exactly how hard each plectrum flicks the piezoelectric, and how that affects power output — they need to know that the material on the test bench behaves exactly as the datasheet says.
A Working, Self-Powered Wireless Sensor
The results were impressive. Under simulated walking at 0.9 Hz, the complete system generated an average of 1.76 mW — enough to charge a capacitor in 28.4 seconds, then keep a wireless communication node transmitting a data burst every 1.25 seconds. That is a genuinely useful duty cycle for remote health monitoring or body area network applications.
What made this study significant was the integration. Many energy harvesting papers demonstrate power generation in isolation. Kuang and Zhu showed a complete, working loop: walking motion → energy harvester → capacitor → wireless sensor → transmitted data, with no battery anywhere in the circuit. The paper has since been widely cited as an early demonstration that plucking-type biomechanical harvesters can power real electronics.
As wearable technology becomes more embedded in healthcare and personal monitoring, the materials that enable it matter more than ever. Advent Research Materials supplies Kapton® polyimide film in a range of thicknesses and formats to research groups working at exactly this frontier — where the quality of a small, precise material choice determines whether an ambitious experiment succeeds.
CITATION
Title: Characterisation of a knee-joint energy harvester powering a wireless communication sensing node
Journal: Smart Materials and Structures, Vol. 25, No. 5, Article 055013
Authors: Yang Kuang, Meiling Zhu (University of Exeter)
Published: April 2016
DOI: 10.1088/0964-1726/25/5/055013