5.2 Materials and preparation of model samples
The motivation of the work is to improve the active incorporation of bone implants into living bones. The bone cells accept some materials without repulsion, a condition called biocompatibility. The next step towards a faster and more permanent healing would be a modification of the implant surface into a bioactive layer. Bioactivity in the context of biomineralization means an improved and durable formation of bone tissue on the implant, caused by an appropriate surface. This can partly be accomplished with a tailored surface morphology (Kenar, 2013), but a much stronger effect can be achieved with a supportive chemistry especially with materials that best imitate the mineral phase of bone (Suchanek, 1998).
Two kinds of calcium alkali phosphates were applied for the experiments. These substances are among the most promising candidates able to stimulate bone cell growth. They were developed at the BAM Federal Institute for Materials Research and Testing, Berlin (Berger, 1995, Gildenhaar, 2007, Krahl, 2009). One has the main crystalline phase Ca2KNa(PO4)2, called GB14, the other one is Ca10[K/Na](PO4)7 with the short name Ca10. They can be classified as ceramic materials. Both substances were grinded with a ball mill to the following grain sizes of D50 = 1.8 µm for GB14 and of D50 = 1.3 µm for Ca10. This fine ceramic powder is still crystalline and will remain in this state for the following treatment. A thin layer of this ceramic powder needs to be fixed onto the metal surface. Medical grade Ti6Al4V was used being the most common alloy for bone implants (Narayan, 2009). The metal samples (size 2 mm × 20 mm × 30 mm) were grinded to a final roughness of less than one micrometer. It is not necessary to polish the metal further simply because the contact between the fine powder and the supporting metal suffices. Some initial tests were performed with sandblasted Ti6Al4V. The corundum grains of the blasting process partly were stuck in the metal. This reduces the surface available for binding phosphate powder. Since additional compounds in the prosthesis must be avoided, sandblasted samples were not used anymore.
Prior to the laser treatment a ceramic powder layer of defined and constant thickness has to be applied to the metal. The dry powder cannot be handled properly for the laser fixation. So it was necessary to prepare a slurry, containing the ceramic particles. This liquid was a mix of water with 20 – 40 % of GB14 or Ca10. Up to 5 % polyvinyl alcohol (PVA, Merck) served as a binder for the small particles after dip coating and subsequent drying. An individual dip coating cycle resulted in a homogeneous white layer on the metal. Repeated dip coatings produced layers of different thickness. The best results of the laser fixing process were found with thicknesses between 20 µm and 40 µm. The viscosity of the slurry varied between 30 and 80 mPas. The velocity of the vertical dip coating ranged from 20 to 100 mm/min. The samples rested in the slurry for 10 seconds. After the dip coating process the samples were heated to 100 °C to minimize the water content. Water can evaporate during the laser treatment and thus damage the ceramic layer.