3D-Printed Mesh Facilitates Orthopedic Brace Manufacture

A new study suggests that additive manufacturing (AM) of biomechanically tailored flexible meshes could lead to personalized wearable and implantable devices.

Developed at the Massachusetts Institute of Technology (MIT, Cambridge, MA, USA), the meshes are fabricated by extrusion of thermoplastic polyurethane using a continuous AM tool path to tailor the elasticity of the mesh cells via slack modification and modulation of the filament–filament bonding. The resulting mesh configuration resembles a tough, pliable fabric with directionally specific inversion stiffness. The wider the spacing of the unit cells, the more the mesh can be stretched at low strain before becoming stiffer, a design principle that tailors the mesh's degree of flexibility and helps it mimic soft tissue.

The pliable mesh can also be hardened by printing stainless steel fibers over regions of the elastic mesh where stiffer properties are needed, and then printing a third elastic layer over the steel to sandwich the stiffer thread into the mesh. The combination of both stiff and elastic materials provides the mesh with the ability to stretch easily up to a point, after which it starts to stiffen. The meshes can also be designed as an auxetic structure, a structure that becomes wider when pulled. Auxetic structures can also support highly curved surfaces of the body.

To demonstrate the capabilities of the new mesh, the researchers fashioned an ankle brace with directionally specific inversion stiffness arising from the embedded mesh, which can provide stronger support to prevent, for instance, a muscle from overstraining. They mesh's structure prevents the ankle from turning inward, while still allowing the joint to move freely in other directions. The tensile mesh mechanics of the brace were engineered to match the nonlinear response of muscle. The researchers also fabricated a knee brace that conforms to the knee as it bends, and a glove with a 3D-printed mesh sewn into its top surface, which conforms to a wearer's knuckles. The study was published on June 19, 2019, in Advanced Functional Materials.

“We were trying to think of how we can make 3D-printed constructs more flexible and comfortable, like textiles and fabrics. One of the reasons textiles are so flexible is that the fibers are able to move relative to each other easily,” said lead author mechanical engineer Sebastian Pattinson, PhD. “There's potential to make all sorts of devices that interface with the human body. Surgical meshes, orthoses, even cardiovascular devices like stents; you can imagine all potentially benefiting from the kinds of structures we show.”

Additive manufacturing describes technologies that build 3D objects using computer-aided design (CAD) modeling software, machine equipment, and layering material. Once a CAD sketch is produced, the data is relayed to the printer, which lays downs or adds successive layers of liquid, powder, sheet material or other, in a layer-upon-layer fashion to fabricate a 3D object. Many technologies are included in this definition, such as rapid prototyping, direct digital manufacturing, layered manufacturing, and additive fabrication.

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