Massachusetts Institute of Technology engineers have designed pliable, 3-D-printed flexible mesh
materials whose flexibility and toughness they can tune to emulate and support softer tissues such
as muscles and tendons. They can tailor the intricate structures in each mesh, and they envision the
tough yet stretchy fabric-like material being used as personalised, wearable supports, including
ankle or knee braces, and even implantable devices, such as hernia meshes, that better match to a
person’s body.
As a demonstration, the team printed a flexible mesh for use in an ankle brace. They tailored the
mesh’s structure to prevent the ankle from turning inward, 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, they produced a glove with a 3-D-printed mesh sewn into its top
surface, which conforms to a 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,” says 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.
The team’s flexible meshes were inspired by the pliable, conformable nature of fabrics.
“3-D-printed clothing and devices tend to be very bulky,” Pattinson says. “We were trying to think
of how we can make 3-D-printed constructs more flexible and comfortable, like textiles and
fabrics.”
Pattinson found further inspiration in collagen, the structural protein that makes up much of the
body’s soft tissues and is found in ligaments, tendons, and muscles. Under a microscope, collagen
can resemble curvy, intertwined strands, similar to loosely braided elastic ribbons. When
stretched, this collagen initially does so easily, as the kinks in its structure straighten out. But once
taut, the strands are harder to extend.
Inspired by collagen’s molecular structure, Pattinson designed wavy patterns, which he 3-D
printed using thermoplastic polyurethane as the printing material. He then fabricated a mesh
configuration to resemble 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.
The researchers printed a long strip of the mesh and tested its support on the ankles of several
healthy volunteers. For each volunteer, the team adhered a strip along the length of the outside of
the ankle, in an orientation that they predicted would support the ankle if it turned inward. They
then put each volunteer’s ankle into an ankle stiffness measurement robot – named Anklebot –
that was developed in Hogan’s lab. The Anklebot moved their ankle in 12 different directions, and
then measured the force the ankle exerted with each movement, with the mesh and without it, to
understand how the mesh affected the ankle’s stiffness in different directions.
In general, they found the mesh increased the ankle’s stiffness during inversion, 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 3-D printer, and the mechanics can be tailored to precisely match those of soft tissue,”
Hart says.
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 3-D-printed parts.”
In traditional 3-D 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 a layers
that, rather than being fully bonded, were free to move relative to each other, and 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 says.
Surgical meshes, orthoses, even cardiovascular devices like stents — you can imagine all
potentially benefiting from the kinds of structures we show.”
Picture caption: 3-D meshes are designed to be lightweight and conformable, similar to fabric and
textiles. Image: M. Scott Brauer
Source: Massachusetts Institute of Technology
Reference: http://www.opnews.com/2019/07/engineers-3d-print-flexible-mesh-for-ankle-and-knee-braces/15574