Growing a living implant may seem futuristic, but it already works in the lab. "Tissue engineers" can make mini-tissues out of living cells. They then bring them together to create missing bone or cartilage. The next step is to make the biological prosthesis roll out of the 3D printer on an industrial scale.
If you break a bone, it usually turns out fine; our body is capable of a lot and often repairs itself. But sometimes that does not work. Take, for instance, cancer patients who have lost a large piece of bone due to a tumour, or people with serious problems in the cartilage of the knee. In those cases, an artificial prosthesis is implanted.
It is perfectly possible to live with such an artificial prosthesis, although it may not the ideal solution for every patient. A prosthesis made of metal or plastic only lasts 15 to 20 years. A younger patient therefore has to go under the knife every few years for a new spare part. A prosthesis is also not an option for children. Because they are not yet fully grown, the successive operations to implant ever larger versions would be near impossible to manage.
What if we could make living prosthetics that simply become part of our body and can grow with it? The interdisciplinary Prometheus platform is a division of Leuven Research & Development that has been working on bone repair for years and has exactly that goal: to develop a living implant.
Professor Liesbet Geris is scientific coordinator at Prometheus. Alongside her colleagues she leads a very diverse team of engineers, doctors and biomedical scientists. They are investigating how we can replace the classic titanium sphere with a piece of living tissue that fits perfectly and integrates into the bone.
Nature offers the "tissue engineers" inspiration: bone tissue consists of cells, calcium, fibres and growth factors. Growth factors are proteins that cause cells to divide – and therefore grow – and can cause cells to change function. For example, a stem cell can become a bone cell. "The idea arose to recreate bone using growth factors as a chemical stimulus to grow tissue, whether in a test tube or in the human body itself."
The researchers hope to automate the production of biological bone tissue in the future using fast robots and 3D printers instead of laboratory technicians and culture dishes
And with that, the recipe for biological bone tissue was known: cells are taken from the patient's periosteum (the fibrous membrane surrounding a bone), multiplied in the lab and enriched with growth factors. This combination is placed on a supporting structure of calcium and fibres.
“That biological bone tissue contains many cells that we have to grow manually, which is very labour-intensive and expensive. Moreover, there’s a chance that it will fail if there is poor blood supply into the implant, in which case the cells can die before they produce bone.”
To remedy that shortcoming, the researchers again consulted Mother Nature. “If we look at how bone is formed in the embryonic phase and how a bone fracture heals spontaneously, we see that cartilage is formed first, which is later converted into bone,” Geris explains. "An important first step is for cells to stick together and form clumps or spheres."
This insight led to a new strategy for the second generation of biological bone tissue. Starting with the cells of a patient, small cell spheres are first produced. These are cultured with growth factors until they transform into cartilage spheres and eventually form pieces of microtissue. “You get an organoid: a 3D structure of human cells in a culture dish. We then assemble these organoids layer by layer in a mould until they take the shape of the missing piece of bone.”
This new technique is very promising: “If we place such an implant of about 2,000 organoids in a mouse that has a large bone defect, we can bridge the defect." The piece of tissue behaves as in nature, growing in six to eight weeks into a perfectly integrated piece of bone tissue with the right shape. Of course, a mouse obviously has small bones in comparison to humans. "We are now investigating how we can make larger pieces of tissue from, for example, a million organoids."
The new generation of living implants offers several advantages: “We start from cartilage spheres, and cartilage is more resistant to poor circulation. This gives the implant a better chance of survival. With this technique we now have the building blocks to solve bone problems as well as cartilage and bone problems.”
Now it’s a matter of producing living implants on an industrial scale. To this end, the researchers are experimenting with bioprinters, 3D printers that work with biological ink. Bioink is made up of a combination of living cells, growth factors and hydrogel. This hydrogel consists of proteins that retain lots of water. The material serves as a soft support structure for the other ingredients in the ink. The implant design is perfectly tailored to the bone defect. The printhead of the bioprinter injects the bioink by layers into the required form.
We want to test whether children with neurofibromatosis type 1 can be helped with a living implant. In terms of quality of life, that would make a big difference for these children
Cheese with holes
And then a piece of tissue comes out of the printer: a tube that looks something like cheese with holes. "We mimic the inside of bone, which is made up of a spongy tissue, flexible yet strong." Although there are still challenges: “The printhead is very important, because the biological ink must not be damaged. If the cells in the ink are compressed or stretched too much, they die.” If the technical problems are solved, the researchers hope to automate the production of biological bone tissue in the future using fast robots and 3D printers instead of laboratory technicians and culture dishes.
Nevertheless, we can’t expect that living implants will completely replace artificial prostheses. This has to do with the high price tag of a bio-implant. “At the moment the costs can run into the multiple tens of thousands of euros. We will have to consider for which patients the costs outweigh the benefits. If a 70-year-old breaks a hip, it is better to keep using a normal artificial joint, because faster rehabilitation is paramount. And for the time being that’s only possible with metal prostheses."
Geris hopes that in a few years' time living implants can provide a solution for children with neurofibromatosis type 1. This is a rare genetic disorder that sometimes causes children to have a spontaneous fracture in a bone at a very young age. “That fracture no longer heals. Because artificial prostheses are not an option in children, treatment is now done by stretching the remaining bone over the fracture, a procedure that is very time consuming and often painful. We want to test whether these children can be helped with a living implant. In terms of quality of life, that would make a big difference for these children.”
Geris and her team work on bone and cartilage, but what about other organs? Given that there are not enough donors to meet the demand, 3D printed organs would be very welcome. “This technique is already being used for skin transplants, and on that front we’ve already come a long way. However, large printed organs will take a while, as blood circulation in these applications is an even more crucial. It is one thing to print tissue, but it is another thing to keep it alive.”