3D printing technologies are leading to impressive innovations in the medical sector, largely because of their ability to produce patient-specific models, implants and devices. A recent research development out of MIT could push the envelope even further for AM in healthcare, leading to new bespoke medical products, such as ankle or knee braces.
MIT engineers have developed 3D printed mesh materials whose properties can be tuned to mimic or support the toughness and flexibility of soft tissues in the human body, such as muscles and tendons. The 3D printed materials can be tailored by adjusting their intricate structures and could have applications in the production of personalized ankle or knee braces and even implantable devices like hernia meshes.
The team demonstrated the 3D printed mesh materials by creating an ankle brace made from the tunable material. The brace was designed to prevent the wearer’s ankle from turning inward, while simultaneously allowing the joint to move in other directions. The MIT researchers also 3D printed a knee brace designed to fit the knee, even as it bends, and a glove with a 3D printed mesh sewn onto the top to conform with the wearer’s knuckles and provides resistance against involuntary clenching that can occur to patients after a stroke.
“This work is new in that it focuses on the mechanical properties and geometries required to support soft tissues,” explained Sebastian Pattinson, a postdoc researcher at MIT and lead author of the study. “3D printed clothing and devices tend to be very bulky. We were trying to think of how we can make 3D printed constructs more flexible and comfortable, like textiles and fabrics.”
Inspired by collagen
In addition to finding inspiration in textiles, the research team also drew from natural materials and specifically collagen, the structural protein found in the body’s soft tissues like ligaments, tendons and muscles. When observed under the microscope, collagen looks like it is made up of curvy intertwined strands, which, when stretched, straighten out.
The researchers sought to emulate the molecular structure of collagen by designing wavy patterns which could be 3D printed using a thermoplastic polyurethane (TPU) material. A mesh configuration could then be designed to resemble stretch but tough fabric.
The tunable properties of the material are achieved by modifying the wave structure: the taller the waves, the more the mesh can stretch at low strain before becoming stiff.
In testing the 3D printed mesh, the MIT team printed a long strip of the material to see how it would support the ankles of a number of healthy volunteers. Each person had a strip of the mesh adhered along the length of the outside of their ankle in an orientation conducive to supporting the ankle if it turned inward.
The volunteers’ ankles were then placed into an ankle stiffness measurement robot—aptly called Anklebot—which was developed in the lab of Neville Hogan, Sun Jae Professor in Mechanical Engineering. The robot subsequently moved each ankle in 12 directions and measured the force the ankle used with ever movement with and without the mesh.
These tests shed light into how the 3D printed mesh impacted the ankle’s stiffness in various directions. Ultimately, the researchers found that the mesh increased the ankle’s stiffness during inversion, while leaving it unaffected for movement in other directions.
“The beauty of this technique lies in its simplicity and versatility,” said associate professor of mechanical engineering A. John Hart. “Mesh can be made on a basic desktop 3D printer, and the mechanics can be tailored to precisely match those of soft tissue.”
Though the 3D printed meshes used for the supportive braces were made using flexible material, it is also possible to work with stiffer materials for creating implantable hernia meshes and other devices.
At this stage, the team has come up with a way to incorporate stronger and stiffer fibers into a pliable mesh structure by printing stainless steel fibers over certain sections of an elastic mesh, and then printing a third elastic layer on top of the steel to sandwich the stiff metal threads into the mesh.
According to the researchers, this approach enables the creation of meshes that can still stretch up to a point but provide extra strength and support when it stiffens. The combination meshes could be especially useful for products that prevent muscles from overstraining.
The researchers also devised a technique which imparted a fabric-like quality to the meshes As Pattinson explained: “One of the reasons textiles are so flexible is that the fibers are able to move relative to each other easily. We also wanted to mimic that capability in the 3D printed parts.”
This was achieved by slightly adjusting the traditional 3D printing process. That is, Pattinson realized that if the print nozzle was raised slightly after printing the first layer of a structure, the extruded material could take longer to land on the underlying layer, giving the material more time to cool and lose some of its stickiness. Using this technique, the team was able to print meshes that were not fully bonded and could move relative to each other.
Finally, the MIT researchers created meshes that incorporate auxetic structures, which become wider as you pull on them instead of contracting. This ability could have applications in supporting highly curved surfaces of the body, such as the knee.
“There’s potential to make all sorts of devices that interface with the human body,” Pattinson concluded. Surgical meshes, orthoses, even cardiovascular devices like stents—you can imagine all potentially benefiting from the kinds of structures we show.”