Researchers develop self-healing material for bone implants

Researchers from the Center for Composite Materials at the National University of Science and Technology MISiS (NUST MISIS) have recently developed a self-healing material for bone implants. The material was based on a shape memory polymer, which can be restored to its original structure upon the local application of heat. The findings of this research were presented during the Healthcare of the Future: The Latest Promising Innovations Moscow-Delhi video conference, which was held at Rossiya Segodnya International Multimedia Press Center.

The discussion centers around the possibility of replacing parts of fractured bones of small and large sizes, as well as surgically removing bone tissue fragments that are affected by malignant tumors. The human body lacks the required resources for self-healing large amounts of bone tissue and hence the need for implants arises.

If the implanted material is subject to cyclic loading (which commonly happens upon the replacement of bone fragments in limbs, especially in legs), cracks may form in the material. These cracks are very hard to manage and impossible to prevent, but it is possible to create an implant made from self-healing material.

“We are currently developing an approach toward using shape memory materials,” said Fyodor Senatov, Candidate of physical and mathematical sciences and a research assistant at NUST MISIS’ Center for Composite Materials. “Initially, the implant has its own defined shape. When a deformation appears caused by a crack, we can apply heat and the material returns to its original structure. In order to possess this shape memory effect, the polymer must contain both a fixed phase (either chemical or physical crosslinks, molecular entanglements or intermolecular interactions) and a soft phase, which is responsible for the entropic elasticity of macromolecules and allows the material to be temporarily deformed. Imagine the polymeric material, from which an implant is made, as being a spring placed inside gelatin. As you deform this piece of plastic, the spring gets stretched out inside. Thick gelatin will prevent the spring from recoiling. But if we heat the gelatin up, it will become soft, and the spring will be able to return to its original shape,” Senatov added.

The driving force behind the material’s ability to restore its shape is the change in the polymer’s molecular mobility and its switch from a more structured temporary configuration, following the deformation, to a thermodynamically favored configuration with higher entropy and less internal energy.

Today, researchers from various laboratories all over the world are carrying out tests on animals to study the possibility of applying heat to such implants locally, without damaging the neighboring tissues. In order to heat up the implant, researchers conduct minimally invasive surgery by making a small incision to bring the waveguide directly to the replaced bone fragment. The main problem is that the implant’s shape can only be restored at temperatures above 50ºC, a temperature that can cause serious damage to living cells. Moreover, the temperature activating the shape memory effect is way too high for the polymers that are used in bone implants.

If we decide to make implants more similar to bones, to make them able to withstand great cyclic loads, the heating temperature will be even higher – about 60-70ºC. These temperatures, without a doubt, will simply destroy the neighboring tissues.

“Unfortunately, so far humans have not developed a material, which is both solid and strong and able to change its structure under acceptable temperatures,” Senatov explained. “I think we will have to keep experimenting with careful heating technology or optimize implants by creating composite materials and altering their internal structure. We have already “fumble” for materials like this, but so far they can self-heal only at 50ºC.”

Now the researchers from the NUST MISIS’s Center for Composite Materials are using various polymers, mainly those which are bioresorbable, or biodegradable, as a foundation for the production of implants. Implants from these materials can be used to replace smaller bone fragments – one of the most in-demand operations in oral and maxillofacial surgery. For replacing larger bone fragments, medical professionals use implants made from ultra-high molecular weight polyethylene.

The polymer’s density can be increased via the introduction of other particles, such as hydroxyapatite – the mineral base of bones and teeth. To obtain the necessary temperature, researchers can use direct heat, electric current, ultrasound or an alternating magnetic field. In order to achieve heating by using a magnetic field, they introduce special magnetic nanoparticles into the polymer. Upon the application of an alternating magnetic field, these particles start warming up and begin transferring heat to the surrounding material. Now researchers are experimenting with the composition of these materials, trying to increase their density and reduce the temperature required to heat them up.