Elastic protection
Hearing aids, dental crowns and prosthetic limbs are some of the medical devices that can now be digitally designed taking into account the individual characteristics of the patient and created using three–dimensional printing. Such devices are usually designed to replace or support bones and other solid structures of the body, therefore they are printed from a rigid, low-elastic material.
A group of engineers at the Massachusetts Institute of Technology (Massachusetts Institute of Technology) has developed a flexible mesh material, the elasticity and strength of which can be customized to simulate and support softer tissues such as muscles and tendons. In medicine, its field of application is the creation of personalized assistive devices, including knee and ankle fixators, as well as implantable structures, such as surgical meshes for hernia prevention.
As a demonstration of the technique, the authors printed a flexible mesh for the ankle retainer. They set up the seal in such a way that the mesh structure prevents the foot from turning inward – a common cause of injuries – and at the same time allows the joint to move freely in other directions. The researchers also made a knee pad that fits against the knee even when it bends. Another example is a glove with a printed mesh sewn into the upper part of it so that it fits against the knuckles (metacarpophalangeal joints) of an athlete's fist, providing resistance to excessive flexion that can occur on impact.
The innovation of the invention is that it focuses on the mechanical properties and geometry needed to support soft tissues.
Catch the collagen wave
When creating an elastic, durable mesh, engineers were inspired by the flexible, malleable nature of tissues, especially collagen, a structural protein that makes up most of the soft tissues of the body, ligaments, tendons and muscles. Under the microscope, collagen fibers look like twisting intertwined threads, similar to elastic bands with a weak braid. When stretching, first the bends in the collagen structure straighten, and then it is quite difficult to stretch the fibers.
Inspired by the molecular structure of collagen, Sebastian Pattinson and colleagues designed a structure with a sinuous pattern, which was then printed on a 3D printer using thermoplastic polyurethane as ink. The more waves and bends there were, the more the mesh could stretch at low strain before becoming more rigid – a design principle that helps to change the degree of flexibility of the mesh and simulate soft tissues.
The researchers printed a long strip of mesh and tested its ability to fix the ankle joint in several healthy volunteers. For each subject, a long grid was printed in an orientation that, according to forecasts, would keep the joint from turning inward. The volunteer's leg was then placed in an Anklebot device specially designed to measure joint mobility. The volunteer performed foot movement in 12 different directions, and then the strength of each movement was measured. The experiment was carried out with and without a fixing grid.
In general, the mesh increases the stiffness of the joint while turning inward, leaving it relatively free when moving in other directions.
Durable and comfortable
The retainer was made of a relatively elastic material. For other purposes, for example, the creation of an implantable hernia mesh, you need to use a tougher material, which is also convenient. The group has developed a way to weave stronger and stiffer threads into a flexible mesh, printing stainless steel fibers on top of an elastic mesh where tougher properties are required. The third elastic layer on top of the steel thread completed the sandwich structure of the implant.
The combination of rigid and elastic materials makes it possible for the mesh to stretch easily to the point after which it firmly retains the structure, preventing, for example, muscle overstrain.
The researchers went further and developed two more techniques for giving the grid qualities that make it easy to adapt to the features of the body even while moving.
With traditional three-dimensional printing, the material is created by feeding "ink" through a heated nozzle layer by layer. The heated polymer binds to the layer lying under it. The engineers found that if you slightly lift the printing nozzle, the material will come out of it a little longer, having time to cool down before it falls on the previous layer. Such fibers are loosely bonded and can move freely relative to each other.
Finally, the group developed nets that became wider when put on. This property is useful for supporting curved body surfaces, for example, the knee brace created in this way tightly fitted the joint area.
Sports medicine is far from the full potential of nets. Surgical meshes, orthoses, even cardiovascular devices (stents) - all of them can be created using a new three–dimensional printing technique.
Article by S. W. Pattinson et al. Additive Manufacturing of Biomechanically Tailored Meshes for Compliant Wearable and Implantable Devices is published in the journal Advanced Functional Materials.
Aminat Adzhieva, portal "Eternal Youth" http://vechnayamolodost.ru based on MIT News: Engineers 3-D print flexible mesh for ankle and knee braces.