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After a feasibility study analysing different technologies, at the end of 2010 a technical solution was discovered making it possible to start developing a sole that creates heat from pressure while walking. This discovery was based on a novel kind of polyurethane (PUR).

As a result of this, in 2011, several development teams were charged with the task of developing the sole. The main difficulty lay in optimizing the sole, so that a maximum of energy could be converted from pressure to heat. Another issue was that the sole needed to regain its old shape quickly between every step or else the heat production would have been insufficient.

After countless attempts one development team came up with a satisfactory solution:

In autumn 2011 a material was found capable of producing an average power of 2 Watts – the goal was achieved. The winter 2011/2012 was dedicated to extensive testing. And the results were very convincing.

For testing purposes a specially designed device was used. The testing procedure was as follows:

A pneumatic cylinder (http://upload.wikimedia.org/wikipedia/commons/0/0d/Pneumatic_cylinder_%28animation%29.gif) is connected to two magnetic valves (http://de.wikipedia.org/wiki/Magnetventil) which control the flow of the compressed air in and out of the cylinder.

The compressed air (adjustable between 0 kg and 100 kg) pushes the cylinder against the sample which deforms under the pressure. The whole procedure is controlled by a computer program for processing measured data called a LabView Routine.

A thermoelement is inserted into the sample to exactly measure the temperature increase during the periodic pressure changes. In LabView the readings for the distance, force and temperature are displayed in different graphs as a function of time.

LabView saves the readings of the testing device and sends them directly to the computer.


The force-distance diagram allows the display of the power of the sample relative to the area of the sample. The bigger the bright green area, the higher the power of the sample and therefore the production of energy.

Furthermore we measured how the temperature changes over time. The next diagram shows the exemplary process:


The temperatures rise of the 3mm/5mm chili-feet warming insoles were measured in a laboratory. Within 12 minutes the temperatures rose from 0°C to 5°/10°C. The following video shows such a test:

Those were the results for the sample in the testing rig.

But how would this change if we tested it on a real foot? A real foot is bigger than our testing sample and the pressure distribution while walking is much more complex.

To be able to accurately model the real pressure conditions, extensive tests were conducted at the Institute for Biomechanics at the ETH Zürich (IfB) to measure the reaction forces during walking or running. The results are shown in the picture below:


Compared to a sample with a diameter of around 4cm, a foot behaves a lot differently. The pressure is distributed differently. The highest pressures are on the heel and on the ball of the foot (see the yellow and red areas in the picture above). Such a distribution is quite individual for every person, because people don’t all weigh the same and have different walking styles.


(Christine Bachmann, Hans Gerber, Alex Stacoff: Messsysteme, Messmethoden und Beispiele zur instrumentierten Ganganalyse, Zürich 2008,33)

This diagram shows the reaction forces during walking or running as a function of the body weight. You can also see the different forces while walking compared to running.



The calculations based on the above mentioned extensive tests in the testing rig and on the real foot show that our chili-feet soles each produce around 2 to 2.5 Watts of power, depending on body weight (though at least 48 kg), on how your foot is built and on your walking style. Measurements in the lab show a possible temperature increase of up to 10°C.


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