It looks like a huge microwave. However, instead of bringing water to the boil it creates bones made of metal. The laser sintering machine is the centre piece of research at the Fraunhofer institute: it copies the porous structures of human bone through the process of RP. This technique comes to use in order to produce prototypes quickly based upon 3D computerised data instead of modeling objects cumbersome by hand. The method has been developed 20 years ago and it found its way into medical technology in the mid 90s when the first models were produced in order to plan surgical interventions in a visual way– today, this is an established practice.
Especially facial surgeons have profited from objects created with RP: „Elementary functions like swallowing, breathing or speech are to be considered carefully when changing any facial structures. Surgery has to be carefully planned “, Professor Hans-Florian Zeilhofer, head of facial surgery at the university hospital of Basel in Switzerland, explains. Therefore, a 3D model of the patient’s skull enables the surgeon to plan surgery step by step theoretically and with a real prototype practically.
Selective laser sintering is one of many possible RP techniques. The specialty of this rather young technology: laser sintering machines can work with metal. Older RP technologies work with ceramics, plastics or even paper. „However, all RP technologies have one thing in common: instead of forming material by clearing away material bit by bit from the outside as happens with drilling or milling an object is built up in layers“, Professor Hermann Seitz of the university of Rostock, former head of the Rapid Prototyping Group of the research institute Caesar, points out. This way, it is possible not only to form external but also internal structures.
Metal is important material for implants
Important for the artificial bone created by researchers of the Fraunhofer institute: computer tomography data and a special software produce a virtual 3D model of a real bone which acts as the basis on how to shape an about 20 micrometers thin layer of metal powder which is applied and evened on a plate in the laser sintering machine. „Then, a laser enters into the picture “, Claus Aumund-Kopp, research assistant at the Fraunhofer institute, explains. The software that broke down the virtual 3D model into single layers sends the relevant information to the laser that glides over the metal powder, melts and hardens it where the bone needs firm structures as specified by the computer model. Porous sites are created by the laser not working upon the metal powder so that the loose powder can simply be shaken off after the procedure. „After finishing one layer, it starts all over again“, Aumund-Kopp says. The plate inside the laser sintering machine is lowered, new powder is applied, and the laser starts again.
The researchers aim to create artificial bones in future that could one day be used as individually produced implants for any body part. In this respect, laser sintering is especially important since it can work on biocompatible materials such as the metal titan – very suitable for bone substitutes: „Hence, laser sintering opens quite new possibilities for implant research“, Zeilhofer believes.
Creating individually tailored bone implants is already possible when it substitutes real bone that is not a supporting part. An example: Zeilhofer already uses implants substituting the upper cranial bone for his patients. Until recently they have all been made out of ceramics but due to laser sintering metal implants could also be used soon. However, incorporating bone implants into the body when it needs to connect to muscle fibers because it is a supporting body part is much more difficult: In spite of highly efficient computers and machines, it is still not possible to reproduce real bones one-to-one yet. „It will still take some time until a bone comes out of a machine that is exactly like the real one“, Aumund-Kopp says. On top of that: „The crack point above all is that the artificial material is not perceived as a foreign object by the body and that it adheres to surrounding tissue“, says Seitz.
Connecting bone implants with living cells
In order to reach more biocompatibility for implants, many researchers connect rapid prototyping with tissue engineering: The RP implant is coated with human cells. A bloody liquid human cells runs covers the rough surface and leaches into hollow cavities creating cell structures at the inside and outside which are to connect with the body tissue. Creating such living implants is still easier said than done.
Zeilhofer currently studies living implants in animal experiments. He created ceramic implants covered in bone cells and inserted them into sheep jaws. „We beforehand observed that the living implants were fully biocompatible in cell culture tests. Therefore, we expect the implants in sheep to behave like material belonging to the animal “, he says. Nevertheless, it still is a long way: „Tissue engineering must first prove to be superior when compared to other methods like the growing tissue out of the body’s own bone“, Seitz believes. „I guess that it will take another five to ten years until living implants will be ready for use.“