Those familiar with 3D printing materials know that polylactic acid (PLA) is an extremely popular material. It’s strong, inexpensive, and easy to print with. PLA is is great for a wide range of applications, but there are other material options out there as well. Nylon is known for its toughness, but it is notoriously difficult to print successfully. Other plastics like polyvinyl alcohol (PVA) can be dissolved in water, making it the perfect choice for support material because it can be so easily removed.
In the prosthetics industry, 3D printing is starting to become a viable manufacturing process. Since a high level of quality is required, there are some situations where PLA and other common materials don’t quite cut it. Prosthetics and bionics companies are looking at using more exotic materials so that they can improve their products.
Above all else, prosthetic limbs must be lightweight and comfortable. If a prosthetic is powered either electrically or mechanically, it is considered a bionic prosthetic. It is beneficial for bionic products to be as simple as possible, so flexible parts may be desired to reduce the complexity. Bionic prosthetics can also feature sensory capabilities for user-feedback, so electronics may be required. All of these design criteria can be achieved by using special materials that do a little bit more than just your average PLA.
Carbon Fiber Reinforced Filament
Carbon fiber reinforced (CFR) filament is a 3D printing material that contains short strands of carbon. It is stiffer than most filaments, and it is a great material choice if weight and rigidity need to be optimized. One such application is in the socket for prosthetic legs. The socket is the part that connects the person’s leg to their prosthesis. People with leg amputations need their prosthesis to be both lightweight and rigid, and CFR filament fits both of these criteria.
This material consists of short carbon strands suspended in a plastic such as PLA or Nylon. These plastics are all classified as thermoplastics, which means they can be remelted. This material property can be quite beneficial. Typically, an amputee’s residual limb will change shape slightly over the course of months or years. This can lead to discomfort if their socket does not change shape. If modifications to the socket need to be performed, CFR filament can simply be heated up to soften it and then reshaped.
Carbon fiber composite material is different from CFR filament. It consists of woven carbon sheets glued together with epoxy. It does not soften with heat. This is because epoxy is classified as a thermoset polymer. That means it undergoes a chemical reaction as it cures which causes it to permanently harden.
Prosthetic sockets made from carbon fiber composite are in fact stronger than 3D printed carbon fiber material but they are expensive, hard to manufacture, and difficult to re-shape. This is why some prostheses are now made by 3D printing with CFR filament. With a 3D scan of the amputee’s residual limb, a socket can be 3D printed which very accurately captures every detail. This makes for a more comfortable fit. 3D printing a socket is much quicker and also less expensive than the traditional method of creating a prosthetic socket.
Unlike most 3D printing materials, thermoplastic polyurethane (TPU) is soft and flexible. This material is perfect for creating flexible joints, and it sees use in applications like prosthetic fingers and as soft liners for prosthetic sockets.
Prosthetic sockets which are made from rigid materials (such as CFR filament) can become uncomfortable if pressure is not evenly distributed. Introducing a soft inner liner can provide cushioning and support, improving comfort for the user. Because 3D printing can create complex shapes, a mesh-like structure can be printed which allows airflow throughout the socket. This ventilation is very necessary because moisture build-up can cause discomfort.
TPU is also being used in bionic hands as a material for flexible fingers. Instead of using a rigid mechanism, using flexible and compliant mechanisms to transfer forces can result in a more natural motion. Using flexible materials in compliant mechanisms reduces the number of parts, removes the need for lubrication, and greatly speeds up the assembly and manufacturing process.
3D printing is usually used to produce mechanical components, but certain filaments are electrically conductive and can be used in a variety of interesting electronics applications. Magalie Darnis (M.Eng), made this the topic of their master’s thesis. Magalie used a material known as ETPU to create 3D printed sensors.
ETPU combines carbon powder and TPU to develop a flexible and electrically conductive polymer that can be 3D printed. Although ETPU contains carbon, it is very different from CFR filament. This is because it uses graphene powder instead of short carbon fibers. Graphene easily conducts electricity, but it does not add much mechanical strength. Other types of conductive filaments exist, but they are rigid and sometimes brittle. In other applications, this may be desired, but for bionics, the flexibility that comes with ETPU allows for flexible, form-fitting sensors to be embedded in wearable products.
Currently, 3D printed sensors are only found in bionics prototypes, but ETPU has proven to be effective in applications such as touch sensors in bionic fingertips. To create a touch sensor, two ETPU surfaces are printed with a small air gap between them. These surfaces will move closer together when pressure is applied to the fingertip. When these surfaces make contact, it closes a circuit, and this signal can be used to let the user know when they’ve firmly grasped an object.
This binary (on/off) touch sensor is one of the most basic 3D printed sensors, and it can be modified to make more complex sensors such as deformation sensors, vibration sensors, and force sensors.
One of the main benefits of 3D printing sensors is that it simplifies and speeds up production. With 3D printed sensors, pre-built components do not need to be manually attached to an object. The sensors can instead be part of the printing process itself.
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