Researchers at ETH Zurich, Empa and EPFL are developing a 3D-printed insole with built-in sensors that allow measuring the pressure of the sole in the shoe and thus during any activity. This helps athletes or patients determine performance and treatment progress.
In elite sports, fractions of a second sometimes make the difference between victory and defeat. To improve their performance, athletes use a custom-made sole. But people suffering from musculoskeletal pain also turn to slippers to cope with their discomfort.
Before specialists can precisely position such insoles, they must first create a compression profile for the foot. To this end, athletes or patients have to walk barefoot on pressure-sensitive mats, where they leave their individual footprints. Based on this pressure profile, orthopedic specialists hand-create custom insoles. The problem with this approach is that optimizations and adjustments take time. Another drawback is that pressure-sensitive mats only allow measurements in a confined space, but not during workouts or outdoor activities.
Now the invention by a research team from ETH Zurich, Empa and EPFL could improve things significantly. The researchers used 3D printing to produce a custom insole with built-in pressure sensors that can measure the pressure on the sole of the foot directly in the shoe during various activities.
“You can tell from the pressure patterns detected if someone is walking, running, climbing stairs or even carrying a heavy load on their back – in which case the pressure is transmitted more to the heel,” explains co-project lead Gilberto Sequeira, Senior Assistant at Empa and at ETH Complex Materials Laboratory. This makes tedious mat tests a thing of the past.
One device, multiple inks
These slippers are not only easy to use but also very easy to make. They are produced in just one step – including sensors and embedded connectors – using a single 3D printer, called an extruder.
For printing, researchers use different inks developed specifically for this application. As the basis for the sole, materials scientists use a combination of silicone and cellulose nanoparticles.
Then, they print the conductors onto this first layer using conductive ink that contains silver. Then they print the sensors onto the connectors in individual locations using ink that contains carbon black. The sensors are not distributed randomly: they are placed exactly where the pressure on the sole of the foot is greatest. To protect the sensors and connectors, the researchers covered them with another layer of silicon.
The initial difficulty was to achieve good adhesion between the layers of different materials. The researchers solved this by treating the surface of the silicon layers with hot plasma.
As sensors for measuring normal forces and shear forces, they use piezo components, which convert mechanical stress into electrical signals. In addition, the researchers built an interface into the sole to read the generated data.
Data runs soon to be read wirelessly
Tests have shown researchers that the additive-made sole works just as well. “So by analyzing the data, we can actually identify different activities based on which sensors responded and how strong that response was,” says Siqueira.
For now, Siqueira and his colleagues still need a cable connection to read the data; To this end, they installed a contact on the side of the sole. One of the next development steps, he says, will be to establish a wireless connection. “However, reading the data has not yet been the main focus of our work.”
In the future, 3D printed insoles with integrated sensors could be used by athletes or in physiotherapy, for example to measure training or treatment progress. Based on this measurement data, training plans can then be modified and permanent shoe soles with different hard and soft zones can be produced using 3D printing.
Although Siqueira believes there is a strong market potential for its product, especially in elite sports, his team has not yet taken any steps toward commercialization.
Researchers from Empa, ETH Zurich and EPFL took part in developing the sole. EPFL researcher Danick Briand coordinated the project, his group provided the sensors, while ETH and Empa researchers developed the inks and printing platform. Lausanne University Hospital (CHUV) and the orthopedic company Numo also participated in the project.
