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Stretching the lifetime of hip implants with honeycombs

TU Delft researchers have designed a material that could lengthen the lifetime of hip implants. The material gets thicker on both sides when bended, unlike any material we know of in daily life.

When talking about hip implants, there is much at stake. The number of hip prostheses around the world is expected to rise to 2.5 million annually by the year 2020. After 15 years, implants tend to start separating from the femoral bone and need revision.

To tackle this problem, a team led by Dr Amir Zadpoor of the Department of Biomaterials and Tissue Biomechanics has designed a new type of implant material, a so-called hybrid meta-biomaterial. When stretched, it becomes thicker perpendicular to the applied force. The researchers published their findings in the scientific journal Materials Horizons on 2 January 2018.

The material is made of titanium and consists of different types of honeycomb-like structures. "To be more precise, it consists of conventional honeycombs and their auxetic counterparts, also known as the re-entrant hexagonal honeycomb or ‘bowtie’", says first author of the paper, Eline Kolken, who wrote the article for her graduation project, which in itself is quite a feat.

The deformation characteristics are highly unusual. The volume of most materials tend to stay constant when deformed. In other words, when compressed, they expand sideways; when stretched, they become thinner. Described in terms of Poisson’s ratio, which relates lateral to axial strain, these materials have a positive value.

Hip implants must curve slightly within the femur

The magazine Chemistry World, a monthly chemistry news magazine published by the Royal Society of Chemistry in the UK, is highly optimistic about the feat. “A positive Poisson’s ratio is good for making pizza,” the magazine writes. “But it is a sore point for artificial hips. Hip implants must curve slightly within the femur to correctly position the leg and pelvis. Standing and walking compress the inner side of the implant’s curve, and stretch the outer side. With a positive Poisson’s ratio, all current implants thin on their stretched side, pulling on adjacent bone and creating tension. This is what loosens implants and typically leads to their failure.”

Zadpoor’s team solved this long-standing problem by building an implant that has a positive Poisson’s ratio on the side that gets compressed and a negative ratio on the side that gets stretched. As a result, the implant behaves like no other: it expands to compress bone on both sides as it bends.

With no tension between the implant and the bone, the implant should last much longer. But what about the tension that is now created inside the implant instead? Could this not lead to ruptures?

We didn't measure how the material behaves inside

“That is something we have to investigate,” says Kolken. “We didn’t measure yet how the material behaves inside.”

Another gap that needs tackling is the strength of the implant. “The implant can carry the weight of an adult person,” says Kolken. “But it should be able to carry six times as much, given that users may also want to run and jump.”

In her next project, which Kolken will perform as a PhD researcher, she will use algorithms to calculate the optimal configurations of the material to increase its strength. “We are still far off any actual application of the material in implants in patients”, she says. The researchers need to improve the material, then run several experiments before small-scale clinical trials even become an option. It may easily take twenty years before the implants become available on the market. It is still a long stretch.

The paper ‘Rationally designed meta-implants: a combination of auxetic and conventional meta-biomaterials’ was published in the Royal Society of Chemistry's peer-reviewed journal Materials Horizons on 2 January 2018, DOI: 10.1039/c7mh00699c.

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