Leveraging one of the game-changing capabilities of 3D printing, engineers at the Massachusetts Institute of Technology (MIT; Cambridge, MA) have designed pliable, 3D-printed mesh materials in which the flexibility and toughness can be fine tuned to support soft tissues such as muscles and tendons. The intricate mesh structures can be tailored to match individual anatomies in ankle and knee braces and even implantable devices, such as hernia meshes, according to a press release on the MIT website.
|The 3D-printed flexible mesh structures can be tailored to match individual anatomies in ankle and knee braces and even implantable devices. Image courtesy Felice Frankel/MIT.|
As a demonstration, the research team printed a flexible mesh for use in an ankle brace. They tailored the mesh’s structure to prevent the ankle from turning inward—a common cause of injury—while allowing the joint to move freely in other directions. The researchers also fabricated a knee brace design that could conform to the knee even as it bends and a glove with a sewn-in 3D-printed mesh that conforms to the wearer’s knuckles, providing resistance against involuntary clenching that can occur following a stroke.
“This work is new in that it focuses on the mechanical properties and geometries required to support soft tissues,” said Sebastian Pattinson, who conducted the research as a postdoc at MIT. Pattinson, now on the faculty at Cambridge University, is the lead author of a study published in the journal, Advanced Functional Materials.
“3D-printed clothing and devices tend to be very bulky,” said Pattinson. “We were trying to think of how we can make 3D-printed constructs more flexible and comfortable, like textiles and fabrics.”
Inspired by collagen
Inspired by the molecular structure of collagen, which makes up much of the body’s soft tissues and resembles “curvy, intertwined strands” under a microscope, Pattinson designed wavy patterns that he 3D-printed using thermoplastic polyurethane. He then fabricated a mesh configuration to resemble a stretchy yet tough, pliable fabric. The taller he designed the waves, the more the mesh could be stretched at low strain before becoming more stiff, a design principle that can help to tailor a mesh’s degree of flexibility and helped it to mimic soft tissue, explained the article on the MIT website.
The researchers tested the mesh on the ankles of several healthy volunteers. In general, they found the mesh increased the ankle’s stiffness during inversion (i.e., when the ankle turns inward), while leaving it relatively unaffected as it moved in other directions.
“The beauty of this technique lies in its simplicity and versatility. Mesh can be made on a basic desktop 3D printer, and the mechanics can be tailored to precisely match those of soft tissue,” said Associate Professor of mechanical engineering A. John Hart, who participated in the research.
The ankle brace was made using relatively stretchy material, but for other applications, such as implantable hernia meshes, it might be useful to include a stiffer material that is also conformable. To this end, the team developed a way to incorporate stronger and stiffer fibers and threads into a pliable mesh, by printing stainless-steel fibers over regions of an elastic mesh where stiffer properties would be needed, then printing a third elastic layer over the steel to sandwich the stiffer thread into the mesh.
The combination of stiff and elastic materials can give a mesh the ability to stretch easily up to a point, after which it starts to stiffen, providing stronger support to prevent, for instance, a muscle from overstraining.
3D-printing innovation adjusts material's stickiness
The team also developed two other techniques to give the printed mesh an almost fabric-like quality, enabling it to conform easily to the body, even while in motion.
“One of the reasons textiles are so flexible is that the fibers are able to move relative to each other easily,” Pattinson says. “We also wanted to mimic that capability in the 3D-printed parts.”
In traditional 3D printing, a material is printed through a heated nozzle, layer by layer. When heated polymer is extruded, it bonds with the layer underneath it. Pattinson found that, once he printed a first layer, if he raised the print nozzle slightly, the material coming out of the nozzle would take a bit longer to land on the layer below, giving the material time to cool. As a result, it would be less sticky. By printing a mesh pattern in this way, Pattinson was able to create layers that, rather than being fully bonded, were free to move relative to each other. He demonstrated this in a multilayer mesh that draped over and conformed to the shape of a golf ball.
Finally, the team designed meshes that incorporated auxetic structures, patterns that become wider when you pull on them. For instance, they were able to print meshes, the middle of which consisted of structures that, when stretched, became wider rather than contracting as a normal mesh would. This property is useful for supporting highly curved surfaces of the body. To that end, the researchers fashioned an auxetic mesh into a potential knee brace design and found that it conformed to the joint.
“There’s potential to make all sorts of devices that interface with the human body,” Pattinson said. "Surgical meshes, orthoses, even cardiovascular devices like stents—you can imagine all potentially benefiting from the kinds of structures we show.”