Improving prosthetic liners using wearable sensors, 3D printing and deep learning
Prosthetic liners are usually made of silicone, or another type of elastomer. They sit in the stump-socket interface and improve the comfort of using a prosthetic for users. But there is still a lot that can be improved in the design of prosthetic liners, to take account of differing stump shapes and the growth that happens in children who are amputees or who have congenital limb difference.
In order to understand how prosthetic liners could be improved, it is important to understand more about what is happening at the stump-socket interface itself.
Creating sensor skins
In response to this, Dr Ben Oldfrey focused his PhD research on identifying the detail of what is happening at the stump-socket interface.
“Tracking growth and tracking the topology of the stump over time is crucial to our understanding of the human-device interaction, and I thought a combination of wearable sensors, 3D printing and deep learning could help.”
First, he created nanocomposite stretch sensors, using a mixture of carbon nanotubes and a silicone elastomer. These sensor skins were 3D printed, and were created to enable 2D mapping across the whole of the surface. The behaviour of these sensor skins is highly complex, so in order to calibrate them, Oldfrey developed a computer vision system that could watch the skin via cameras, tracking the position of the skin over time, while it was stretched in different ways. This allowed the rapid collection of big data sets of thousands of readings of position versus sensor output.
The purpose of getting these big data sets was to develop an AI algorithm that could learn from and make sense of the data. He used a special type of AI that has memory called ‘Long Short Term Memory Neural Networks (LSTM NN)’, which could ‘remember’ the recent history of the sensor readings. Only with this memory element could accuracy be achieved. As a result, the system can now track stretching in real time in a 2D surface, which could allow the tracking of growth of a residual limb either across the day or seasons, or in the long term for growing children.
To apply this technology, he also developed a method to 3D print bespoke prosthetic liners, by producing a liner that was designed specifically for a 10 year old boy who has congenital limb difference.
At the same time as doing this research, Oldfrey explored the option for printing fluidic cooling channels into prosthetic liners. Many amputees or people with congenital limb difference suffer from stumps that feel hot, which can also cause the skin to feel sore. Although the desirable thickness of a silicone prosthetic liner is around 4mm, depending on the patient, Oldfrey devised a way to print channels into the liner that coolant could be pumped through to cool the stump.
Having improved the understanding of what is happening at the stump-socket interface through his PhD research, Oldfrey is now looking at creating and 3D printing functionally graded materials. These are materials that would gradually change from stiff to soft through the material, without a sudden step change.
“This is something that nature does really well, and it’s what gives natural structures great strength,” said Oldfrey. “An example of this is bone, tendon and muscle in the human body. One of the reasons they're incredibly strong and have amazing force is that they grade between materials, from cellular bone structure to tendons and muscles. It’s not like two pieces of wood glued together. This means that when the whole system is stretched, the stresses are spread out through that whole section and there isn't a weak point.”
Applying this concept to the stump-socket interface could help to alleviate the issues caused by rubbing and friction. Together with collaborators at Southampton University, Oldfrey has applied for further funding to develop a material that could mimic the way nature handles similar scenarios.
Creating prosthetic liners that are more comfortable and durable has the potential to help prosthetic users across the world. “The literature that exists on this area shows that the major deciding factor in the success of a prosthetic is comfort,” Oldfrey explained. “My research has made progress towards being able to produce custom made prosthetic liners more easily, which provide greater comfort and less stress on the stump-socket interface. This has an impact on durability too, as it means liners will wear down less quickly. Overall, it brings very strong benefits in terms of the actual use and impact of prosthetic devices.”
Funded by: Engineering and Physical Sciences Research Council (EPSRC), Starworks, Medical Devices and Vulnerable Skin Network (MDVS).