3DP insoles measure pressure in the shoe

In professional sports, fractions of a second sometimes make the difference between victory and defeat. To optimize performance, athletes use custom-​made insoles, but people with musculoskeletal pain also turn to insoles to combat discomfort.

Before specialists can accurately fit insoles, they must create a pressure profile of the feet. Athletes or patients must walk barefoot over pressure-​sensitive mats, where they leave their individual footprints. Based on this pressure profile, orthopedists create customized insoles by hand. The problem with this approach? Optimizations and adjustments take time. Also, pressure-​sensitive mats allow measurements only in a confined space, not during workouts or outdoor activities.

Now, an invention by a research team from ETH Zurich, Empa, and EPFL could greatly improve things. The researchers used 3DP to produce a customized insole with integrated pressure sensors, measuring the pressure on the sole of the foot directly in the shoe 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 in Switzerland.

These insoles aren’t just easy to use, they’re also easy to make. They’re produced in one step using an extruder. For printing, the researchers use various inks developed specifically for this application – a mixture of silicone and cellulose nanoparticles.

Next, they print conductors on this first layer using a conductive ink containing silver. Then they print the sensors on the conductors in individual places using ink that contains carbon black. Sensors are then placed exactly where the foot sole pressure is greatest. Researchers then coat them with another layer of silicone for protection.

As sensors for measuring normal and shear forces, researchers use piezo components, which convert mechanical pressure into electrical signals. They’ve also built an interface into the sole for reading out the generated data.

“With data analysis, we can actually identify different activities based on which sensors responded and how strong that response was,” Siqueira says.

Currently, Siqueira and his colleagues still need a cable connection to read out the data, as they’ve only installed a contact on the side of the insole. He says one of the next development steps will be to create a wireless connection.

In the future, 3DP insoles with integrated sensors could be used by athletes or in physiotherapy to measure training or therapy progress. Based on measurement data, training plans can then be adjusted and permanent shoe insoles with different hard and soft zones can be produced using 3D printing.

ETH Zürich
https://ethz.ch/en.html