In elite sports, fractions of a second sometimes mean the difference between victory and defeat. To optimize their performance, athletes use custom orthotics.

But even people who suffer from musculoskeletal pain are turning to orthotics to combat their discomfort.

Before specialists can precisely fit these orthotics, they have to create a pressure profile of 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, orthopaedists then create custom insoles by hand. The problem with this approach is that optimizations and adjustments take time.

Another disadvantage is that pressure sensitive mats only allow you to take measurements in a confined space, but not during workouts or outdoor activities.

Now an invention by a research team from ETH, EPFL and the Federal Laboratory for Materials Science and Technology could improve things dramatically.

Researchers have used 3D printing to produce a custom insole with built-in pressure sensors, which can measure pressure on the ball of the foot directly in the shoe during various activities.

The invention was recently published in the journal "Scientific Reports".

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“From the pressure patterns detected, it is possible to understand if a person is walking, running, climbing stairs or even carrying a heavy load on the back, in which case the pressure shifts more to the heel”, explains co-designer Gilberto Siqueira.

“That way, tedious mat tests become a thing of the past”, concludes the senior assistant at EMPA and the Complex Materials Laboratory of ETH Zurich.

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One-step production with one extruder and multiple conductive silver-based inks

These insoles are not only easy to use, but also easy to make.

They are produced in one step, including the integrated sensors and conductors, using a single 3D printer, called an extruder.

For printing, the researchers use various inks developed specifically for this application.

As a basis for the insole, material scientists use a mixture of silicone and cellulose nanoparticles.

Next, they print the conductors on this first layer using a silver-containing conductive ink.

Then they print the sensors onto the conductors in individual dots, using an ink containing carbon black.

The sensors are not distributed randomly: they are positioned exactly where the pressure on the sole of the foot is greatest.

To protect the sensors and conductors, the researchers coat them with another layer of silicone.

An initial difficulty was to obtain a good adhesion between the different layers of material.

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Good adhesion between the different layers of silicone material is ensured with hot plasma

The researchers solved the problem by treating the surface of the silicone layers with hot plasma.

As sensors for measuring normal and shear forces, they used piezoelectric components, which convert mechanical pressure into electrical signals.

Furthermore, the researchers integrated an interface for reading out the generated data into the sole.

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Tests have shown that the additively manufactured insole works well.

“By analyzing the data, we can identify different activities based on the response of the sensors and their intensity”Siqueira explains.

Right now, Siqueira and his colleagues still need a patch cord to read the data; to this end, they installed a contact on the side of the slab.

One of the next development steps, he says, will be to create a wireless connection.

"However, data reading has not been the main focus of 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.

Based on this measurement data, training plans can be adapted and permanent insoles with different hard and soft zones can be produced three-dimensionally.

Although Gilberto Siqueira believes the product has strong market potential, especially in elite sports, his team has not yet taken any steps towards commercialization.

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Under the leadership of Danick Briand, the University Hospital of Lausanne and the Numo company

EPFL researcher Danick Briand coordinated the project and his team provided the sensors, while researchers from ETH and EMPA developed the inks and printing platform.

The University Hospital of Lausanne (CHUV) and the orthopedic company Numo also participated in the project.

The project was funded by the ETH sector's Advanced Manufacturing Strategic Focus Areas program.

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