Hybrid Meta-Biomaterials Used to Make Better, Longer-Lasting 3D Printed Hip Implants

3D printing is increasingly seeing applications using biomaterials and metamaterials – and now with a new meta-biomaterial. A team of researchers from TU Delft developed the hybrid materials – essentially the biomedical variant of metamaterials – which promote bone growth and can help increase the lifespan of 3D printed hip implants.

According to the university, the number of hip prostheses around the world should rise to 2.5 million annually by 2020, but about 10% of those implants will not be firmly fixed in place 10 years post-op, because current technology won’t allow it. The researchers put their heads together to develop a solution for loosening implants, and found it in the new meta-implant.

This new implant, created with a 3D printer and existing biomaterials, combines a conventional meta-biomaterial with an auxetic one. Auxetics become thicker perpendicular to the applied force when they are stretched, which means they have a negative Poisson’s Ratio. This means that the new material can keep 3D printed hip implants firmly in place for the long haul.

Amir Zadpoor

“Auxetic meta-biomaterial, designed using simple geometry and printed in titanium, displays the unique mechanical property of expanding when put under pressure,” explained TU Delft Professor Amir A. Zadpoor. “This makes it ideal for use alongside materials that do the opposite. When someone with a hip implant walks, the prosthesis is subjected to various forces. If too much pressure develops on one side of the prosthesis, it can become detached from the bone, which is extremely undesirable.”

3D printed meta-biomaterials display characteristics you won’t find in nature: they could display negative thermal expansion, be invisible, or be very strong but lightweight at the same time.

Zadpoor and the research team discussed how these unique characteristics have the potential to develop better medical implants in a paper, titled “Rationally designed meta-implants: a combination of auxetic and conventional meta-biomaterials,” recently published and on the cover of the Royal Society of Chemistry‘s peer-reviewed journal Materials Horizons; co-authors include Helena M. A. Kolken, Shahram Janbaz, Sander M. A. Leeflang, Karel Lietaert, Harrie H. Weinans, and Zadpoor.

(A) Meta-implants were manufactured with SLM, (B) Additively manufactured meta-implants, (C) The test set-up in which they were loaded including bone-mimicking materials.

The abstract reads, “Rationally designed meta-biomaterials present unprecedented combinations of mechanical, mass transport, and biological properties favorable for tissue regeneration. Here we introduce hybrid meta-biomaterials with rationally-distributed values of negative (auxetic) and positive Poisson’s ratios, and use them to design meta-implants that unlike conventional implants do not retract from the bone under biomechanical loading.”

The team believes that a hybrid prosthesis made out of 3D printed meta-biomaterials, some with a positive Poisson’s Ratio and others with a negative, will fixate much better in the body than current hip implants, which will increase their longevity.

Zadpoor said, “This will significantly improve the chances of bone growth onto the hybrid meta-biomaterials, holding the implant much more securely in place.”

The new material could even be used one day to address, as TU Delft puts it, “the most significant cause of implant loosening.”

“Since there will be fewer unnatural forces at work on the prosthesis, there is a smaller chance of plastic particles wearing off in the hip cup, which can increase the risk of loosening,” Zadpoor explained.

To test out their research, the team put vertical compression on an implant made with the new 3D printed hybrid meta-biomaterial, which was surrounded by special foam containing the mechanical properties of human bone. The force of the compression simulated the amount of stress put on an implant in the body, and the implant expanded as a result of the pressure, causing compression in the bone on either side of the implant. This compression is exactly what will improve the long-term fixation of the implants.

Compression on both sides of the hybrid hip implant.

“Innovation in the field of hip implants is badly needed, and Professor Zadpoor’s new method certainly has the potential to prolong the lifespan of implants,” said Professor Rob Nelissen, Medical Delta Professor and orthopaedic surgeon at the Leiden University Medical Centre. “A phased introduction starting with 3D measurements of this implant in patients will be necessary in order to guarantee the safe improvement of quality for patients.”

According to the research paper, these 3D printed meta-implants are the first application of the new meta-biomaterials in improving the longevity of implants. The next step is clinical trials, and the TU Delft team, who will be continuing their research with other academic hospitals, will be working to determine how these unique properties will help create better implants for the future.

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