Titanium foam - inspired by and replacement for human bone
Presentation held at Indo-Swedish International Workshop on Design of Materials, Manufacturing & Products, 24 & 25 November 2011, Kolkata, India
The ongoing progress in the area of health, hygiene, nutrition, housing and working conditions and the increase in material wealth, life expectancy increased appreciably in the last hundred years. In 1998/2000 a 60-year-old man, could expect to live something around another 19 years (remaining life expectancy). A hundred years ago he had six years to live and even in the 1970's it would have been an almost four years remaining life expectancy. The increase in life expectancy signs responsible for a change in the age structure of human. The number of 60-year-olds has increased absolutely and relatively, there are potentially more retirees and retirement lasts longer. Along with this positive trend comes a problem in the engineering plan of the human body. The bones and especially the joints are highly stressed for a longer period and fail more often then in former times. That's why the development of implants has been pushed during the last years. Common materials for implants are several ceramics, cobalt and chromium alloys, or titanium. Each year 400.000 artificial joints are implanted every year in Germany. But still, some problems need to be solved because in many cases the human life extends the life cycle of the implant and revision surgery is necessary. Another problem is caused by the mechanical properties of the implant material compared to human bone. Titanium as well as cobalt and chrome alloys show a much stiffer mechanical behavior. On a high load or impact e.g. while jumping, the weakest point will be just below the implant and that's the predetermined breaking point. Additionally the human body is working very resource efficient. Once recognized the loss of the bone typical amount of flexible bending bone material will be degraded. The so called Osteoporosis will finally cause loosening of the implant. By using new topological structures with rough surfaces such as foams the bone grows into the top layer of the implant and signs for a strong connection. One objective of current research activities of the Fraunhofer Institute IWU is to reduce the Young's modulus of the implant and to adapt it to that of the surrounding bone. Most of the metal foam applications in implants are still subject of research and investigation but the rough surfaces are already in serial production. Generative processes based upon electron beam or laser melting technology in selective 3D printing devices are the basis of 30.000 acetabular cups produced in Italy every year. The serial production of cellular Titanium is not yet state of the art. Main reason is the complex and difficult processing of the material. As the machining of Titanium is well understood and realized in several specified technologies and tools, the primary shaping is still very complex and expensive caused by the high chemical affinity to many other elements once in liquid state. Molten Titanium is easily reacting with Oxygen, Nitrogen, Carbon and many other elements and in a matter of high often explosive reactivity. Most ceramics get dissolved and reduced while releasing Oxygen so the processing in crucibles and moulds is very difficult. Especially for the primary shaping of Titanium a special casting furnace has been developed at the Fraunhofer IWU working with inert gas atmosphere in a high vacuum. Molten at around 1.700°C the process time is limited to a couple of minutes but anyway, it is possible. After reproduction of several patents and publications on open porous Titanium foams by sintering or coating of plastic cellular structures current investigations are focused on closed cellular Titanium foam. The objective is a Titanium foam with higher strength, cellular surface and if feasible a minor interconnectivity of the pores. With an adjustable density the foam could be adapted to the mechanical properties of the bone. The cellular surface (exposed by machining) will support a secure locking of the implant inside the bone providing a larger surface for bone cells to grow. If realizable, pore interconnectivity allows the supply of nutrients directly through the implant. Based upon these research projects some new ideas for additional function integration inside the implant and so an enlarged functionality has been developed. One example that is already patented and therefore free for advancements and further development is the integration of channel like structures realized in generative processes offering a chance for long term medication. Most recently the investigations on closed cellular Titanium foam at the Fraunhofer IWU succeeded at a fundamental level. By combining several Titanium and space holder powders with certain heat treatment and defined atmosphere the first samples of closed cellular Titanium foam have been produced. Still at laboratory scale the technology will need further investigations as we have just started to understand the process. Anyway, it has been proved to be realizable and so the first step on the way to reproduction of a bone like structure has been done. The final objective will be a combination of an open porous core inside a closed porous structure analogue to human bone that is adjustable in its mechanical properties. The puzzle pieces towards that aim are almost realized but the composition will still be a great challenge.