Researchers at ETH Zurich, Empa and EPFL are developing a 3D-printed insole with integrated sensors that allows the pressure of the sole to be measured in the shoe and thus during any activity. This allows athletes and patients to measure their performance and track their therapy progress.
Sometimes, a fraction of a second can make all the difference in deciding victory or defeat in elite sports. To optimize their performance, athletes use custom-made insoles. Insoles can also be used by people suffering from musculoskeletal pain to alleviate their discomfort.
Before they can fit insoles accurately, specialists must first make a pressure profile. To this end, athletes or patients have to walk barefoot over pressure-sensitive mats, where they leave their individual footprints. Orthopaedists create custom insoles based on the pressure profile. This method is slow and requires optimisations and adjustments. Another disadvantage is that the pressure-sensitive mats allow measurements only in a confined space, but not during workouts or outdoor activities.
A new invention from EPFL, Empa, and ETH Zurich could make things much easier. 3D printing was used to make a customized insole. The integrated pressure sensors can be used to measure pressure on the sole of your foot during various activities.
“You can tell from the pressure patterns detected whether someone is walking, running, climbing stairs, or even carrying a heavy load on their back — in which case the pressure shifts more to the heel,” explains co-project leader Gilberto Siqueira, Senior Assistant at Empa and at ETH Complex Materials Laboratory. These pressure patterns make tedious mat testing a thing of the passé.
One device, multiple inks
These insoles not only make it easy to use but also easy to create. They can be made in one step, including the integrated sensors or conductors. This is done with a single 3D printer called an extruder.
The researchers use a variety of inks specifically designed for printing. The insole is made from a combination of silicone and cellulose nanoparticles.
The conductors are printed on the first layer with a conductive ink that contains silver. The sensors are then printed on the conductors using carbon black ink. The sensors aren’t placed randomly; they are placed at the highest foot sole pressure. The researchers also applied another layer of silicone to protect the sensors.
The initial problem was to get good adhesion between different layers of material. Researchers solved this problem by heating the silicon layers with plasma.
They use piezo elements to measure shear and normal forces. These components convert mechanical pressure into electric signals. The researchers also built an interface to the sole to read the generated data.
Wireless reading of running data is possible soon
The researchers were able to confirm that the insole was additively made. Siqueira explains, “So we can identify different activities using data analysis based on which sensor responded and how strong that response is.”
Siqueira and his coworkers still require a cable connection in order to read the data. To this end they have placed a contact on the side. According to Siqueira, one of the next steps will be to make a wireless connection. “However, reading the data hasn’t been the main focus for our work so far.”
In the future, 3D-printed insoles with integrated sensors could be used by athletes or in physiotherapy, for example to measure training or therapy progress. This data can then be used to adjust training programs and create permanent insoles using 3D printing.
Although Siqueira believes their product has strong potential in the elite sports market, his team have not yet made commercialization.
Insole development was done by Empa, ETH Zurich, EPFL researchers. Danick Briand, an EPFL researcher, coordinated the project and provided the sensors. Empa and ETH researchers developed inks for the printing platform and the inks. The Lausanne University Hospital (CHUV), and Numo, an orthopaedics company, were also involved in the project.