Dr Michele Pozzi
UK scientists have developed an energy-harvesting knee strap capable of powering electrical devices whilst on the move. It is thought that this technology could spell the beginning of the end for mobile, battery-powered gadgets.
The strap, created by researchers from Cranfield University, uses kinetic energy supplied by the wearer’s knee joint during walking to generate an electrical charge capable of powering equipment such as heart rate monitors, pedometers and accelerometers.
The materials that we are using are currently very expensive. We are buying 10 or 20 pieces at a time. However, when produced on a mass scale, similar components are sold for around £0.01 each. Producing the device on a large scale would drive down costs across the board. It is just a matter of how many components are made and how many you purchase.
Dr Michele Pozzi
The device, which consists of an outer ring and a central hub, fits to the outside of its wearer’s knee. The outer ring is fitted with 72 plectra, and rotates along with the motion of walking. The plectra pluck four energy-generating arms called bimorphs, which are attached to the inner hub. This process causes the bimorphs to vibrate, thus producing electrical energy.
The knee strap is presented in the latest issue of IOP Publishing’s journal Smart Materials and Structures
, and is currently capable of harvesting 2mW of power. However, its creators believe that it has the potential to generate in excess of 30mW of power. If their planned improvements are successful, the device could enable battery-free GPS tracking, signal processing and wireless transmission technologies.
I spoke to Dr Michele Pozzi, Research Fellow at Cranfield University, to find out more about this energy-harvesting knee strap. I began by asking why the knee was so well suited to this type of energy-harvesting device.
"Essentially, the knee offers a wide angle of displacement at a good speed," Dr Pozzi answered. "There are other joints in the body that could support a device like this; the elbow, for example. However, when you run, you tend not to bend your elbows so much. The knee goes through a wider angle and it is also bigger, so you can attach a larger device in order to generate more energy."
Dr Pozzi went on to explain that whilst running offers a greater potential for energy generation, walking is still sufficient to get the most out of the knee strap.
"A person running would generate more energy than somebody who was walking," he explained. "However, the tests that we performed were based upon a subject walking at a normal speed. I must stress that we have not tested the device with running data, so whilst an increase in power would be likely, it has neither been confirmed nor quantified."
Researchers from the Universities of Liverpool and Salford carried out biomechanical measurements, allowing Dr Pozzi to test the efficacy of the harvester. The team took accurate measurements of knee movement during walking, and used a motion simulator to reproduce the gait pattern of a human being. The minute control offered by the simulator enabled the scientists to observe in intricate detail, the movement of a knee joint during walking. The simulator was also fitted with different backpack loads in order to test how the knee joint functions under varying levels of stress. I asked Dr Pozzi whether the knee strap resulted in any additional energy expenditure for its wearer, and whether the generated power could be doubled by wearing a device on each knee.
"It will involve some metabolic cost, although from the calculations that we’ve made, this will be negligible," he said. "The torque required to turn the device is approximately 0.5 per cent of the torque of a knee joint during kneeling. It is quite small. It would therefore be possible to have two – one per knee – and double your power output."
Dr Pozzi is confident that by making some adjustments to the next iteration of the knee strap, its power output can be increased significantly.
Dr Pozzi is confident that the device can be improved to deliver a much greater power output
"The most obvious improvements involve the way in which the device is manufactured," he said. "I have noticed that a lot of energy is wasted at the point of contact between the plectra and the bimorphs. This contact is not as clean as it might be and energy is dissipated here. One easy route to increasing energy generation is therefore to improve this interaction.
"We must also factor in the materials used to produce the straps, as I didn’t really optimise these during the design process. We used bimorphs that were commercially available. Moreover, if we had the opportunity to create a version at a larger scale, we could optimise its geometry. This could result in a more compact product."
Although Dr Pozzi contends that the large-scale production of these devices could result in individual unit prices as low as £10, cost effectiveness was not necessarily a priority during development.
"What we have so far published is mostly an advanced proof of concept," he said. "We have demonstrated that you can obtain useful energy from this type of device. At this incubatory stage, the strap was not designed to be cheap to manufacture. Indeed, at present, it is very expensive because of development costs. However, I am currently leading the development of an improved version that should be cheaper to manufacture.
"The materials that we are using are currently very expensive. We are buying 10 or 20 pieces at a time. However, when produced on a mass scale, similar components are sold for around £0.01 each. Producing the device on a large scale would drive down costs across the board. It is just a matter of how many components are made and how many you purchase."
I went on to ask Dr Pozzi whether or not he had an idea of when the knee strap would reach the market.
"That is a difficult question," he replied. "The improved version should be ready in a few months’ time. This is essentially a commercial matter as it depends very much on the interest we receive from industry, from the people who might actually want to invest in and manufacture this product. As a university, we are not really interested in making the product. We deliver a certain level of technological readiness and then it is down to the companies to come in and to show their interest. At this stage, we can work together towards producing the product. Whilst this could happen within a short space of time, it depends on how interested companies are."
The study originally received EPSRC and DSTL funding and aimed to reduce the battery burden carried by soldiers on foot patrol
The study was originally funded by the Engineering and Physical Sciences Research Council (EPSRC) and the Defence Science and Technology Laboratory (DSTL), as part of a project aimed at mitigating the weight burden that batteries place on members of the armed forces. If Cranfield Nano – the group supporting the development of the improved version – is able to increase the power-generating capacity of the knee strap, it will be welcomed by soldiers who have to carry up to 10kg of power equipment whilst on foot patrol. I asked Dr Pozzi about the device’s other potential applications.
"I think that the strap could have a wide range of applications. Take, for example, its potential medical utility. It could be used during rehabilitation to monitor the gait of a patient and to provide valuable information for the healthcare practitioner. Also, I see it as a consumer product. Joggers could power their pedometers, their heart-rate monitors or their MP3 players from their own movement. There are many possibilities in this respect."