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 is useful for athletes or patients who want to track their progress in therapy and performance.
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. To ease musculoskeletal discomfort, people also use insoles.
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. An orthopaedist then designs custom insoles using this 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.
The invention of a research team comprising EPFL, Empa, and ETH Zurich is a great improvement. Researchers used 3D printing to create a custom insole that could be worn during different 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 can be used easily and are also very easy to make. 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. Next, they print the sensors on conductors individually using carbon black ink. The sensors are not randomly distributed. They are placed where the foot pressure is highest. Researchers coat the conductors and sensors with another layer silicone.
It was difficult to establish good adhesion among the various material layers. Researchers solved this problem by heating the silicon layers with plasma.
The sensors measure normal and shear forces using piezo components. They convert mechanical pressure to electrical signals. Researchers also created an interface in the sole to read the generated data.
Soon, wireless reading of running data will be possible
The researchers were able to confirm that the insole was additively made. “So, with data analysis we can actually identify various activities based upon which sensors responded to and how strong that response has been,” Siqueira states.
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 insole. According to Siqueira, one of the next steps will be to make a wireless connection. “However, reading the data has not 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 be used to adjust training programs and create permanent insoles using 3D printing.
Siqueira believes that their product has strong market potential, particularly in elite sports. However, his team has not yet made any commercialization steps.
The insole was developed by researchers from Empa, ETH Zurich, and EPFL. 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.