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.