Bioactive glass coating for hard and soft tissue bonding on Ti6Al4V and silicone rubber
using electron beam ablation
(by J. Schrooten, S.V.N. Jaecques, R.Eloy, C. Schultheiss,
J. vn Humbeeck, J. vander Sloten)
In contrast to the bone bonding potential of
existing implants, the adhesion of soft tissue with any commonly used implant material
is poor. Therefore an important problem of percutaneous implants is the
infectious passage of bacteria into the body at the interface between tissue
and implant. Subcutaneous implants, e.g. access ports for injections used in
chemotherapy, suffer under low fixation and should be anchored with the surrounding
soft tissue.
The main objective of this research is to
develop a method, which allows the growth of living soft tissue on medical
implants ranging from metals to temperature sensitive polymers. The surface of
the implant is to be coated with bioactive glass (BAG), which has proven
bonding potential to hard and soft tissue. A new coating procedure based on
electron beam ablation (ELBA) will be used.
A first project goal was to deliver a “proof of
principle” of the ELBA technique by coating Ti6Al4V-alloy and silicone rubber
(poly-dimethylsiloxane, PDMS) substrates with micrometer thin BAG layers.
BAG targets (Ć30mm, thickness d = 5-10mm) with the following composition (weight%) were produced: 60%
SiO2, 9% CaO, 28% Na2O and 2% P2O5. Through ELBA this BAG was deposited on two different
substrate materials: (i) Ti6Al4V, cylindrical disks (Ć10mm, d =
1mm), with a glass-blasted surface finish prior to coating and (ii) PDMS cylindrical disks (Ć10mm, d =
1mm), in as-received condition. The following properties
of the coated samples were evaluated: composition, amorphous character,
thickness, roughness, adhesion strength, short term (<24h) in vitro dissolution kinetics in Hanks’ solution and fibroblast
cell adhesion with L929 mouse fibroblast cells (ISO-10993-5 standard). For the
latter, tissue culture polystyrene (TCPS) was used as a control. The number of
cells per ml medium in contact with the TCPS was taken as 100%.
The ELBA process changed the composition of the final BAG
coating to 54% SiO2, 13% CaO, 31% Na2O and
2% P2O5. The
deposited BAG layers, with a thickness ranging from 4 to 14 µm depending
on the ELBA process parameters, were amorphous. The coating roughness had an Ra
value of approximately 614 nm. The
adhesion strength decreased with increasing coating thickness: 6.8 MPa for
2.4 µm thickness, versus 2.5 MPa for 8.3 µm. However, the
adhesion strengths obtained with the thickest coatings are still sufficient for
the intended use.
The in vitro
dissolution experiments in Hanks’ solution showed a compositional change that
is expected for BAG, related to leaching, dissolution and reprecipitation.
After these in vitro tests the
average coating roughness was significantly decreased (from 882 nm Ra to 797 nm Ra, p<0.05 for Ti6Al4V substrates
and from 1470 nm Ra to 531 nm Ra, p<0.01 for PDMS substrates) and
crystalline phases (calcite) were formed.
The fibroblast cell adhesion test showed that
after 24 hours, on the coated PDMS substrates 750% cells per ml medium were
counted and 120% on the coated Ti6Al4V substrates, compared to 100% for the
control. After 48 hours the results were 800% and 350% respectively. Afterwards
the coating was completely covered with fibroblasts. There is good cell
adhesion and proliferation on the treated PDMS samples and also on the treated
Ti6Al4V samples, but on a lower level.
The coating of metallic and polymeric substrates
with BAG of composition 54% SiO2,
13% CaO, 31% Na2O and 2% P2O5 was successful: the BAG coatings were amorphous, homogeneous
and well adherent. In comparison to cell growth on untreated TCPS control
surfaces, coated Ti6Al4V-alloy and PDMS samples showed an increased cell
population at the surface after 48 hours. It can be concluded that the reported
BAG coatings produced by ELBA have the ability to promote cell growth. These
results will be used as a basis for investigating other BAG compositions more
rigorously and understanding the reaction mechanism of thin BAG coatings with
relation to soft and hard tissue bonding.