research activities in 2007 - CSEM

Composite Materials for Bone Implants
M. Giazzon, M. Liley, G. Weder
CSEM is developing methods to nanostructure (‘texture’) composite materials for use in orthopaedic implants. Photolithography and bead
lithography methods are used to create master templates that are then replicated in a resin matrix. The structures will be tested for their effect on
bone cells in vitro.
The use of orthopaedic surgery to replace joints and repair
bone defects is increasing rapidly. Due to a more active aging
population, the demands made on the materials used in these
operations are also increasing: improved robustness, longer
lifetimes and easier surgical procedures are expected for hip
implants and other joint replacements.
Metals have been used with enormous success in orthopaedic
surgery, but there are still some problems associated with
their use. While issues such as weight and thermal
conductivity can be important for specific applications such as
cranial repair, one major issue of almost universal importance
is that of Young’s modulus. The high stiffness of metallic
implants results in a mismatch in its mechanical properties
and surrounding bone. This in turn leads to “stress shielding”:
loss of bone mass and weakening of the surrounding bone
that may lead to implant failure. In this context, the repair of
osteoporotic bone remains a major challenge.
As a partner in the EU project Newbone, CSEM is working to
develop composite materials that can be used in orthopaedic
surgery. Glass-fibre reinforced composites (FRCs) based on
polymeric materials currently used in dentistry are being
tested and optimised for applications in bone repair. CSEM’s
contribution is to test the effect of surface coatings and
modification, with the goal of developing surface micro- and
nanostructures that enhance cell growth, cell adhesion and
the integration of the implant into the surrounding bone.
Figure 1: Hemispherical pits fabricated by replication in a bis-
GMA/TEGDMA resin matrix
The micro- and nanostructures are fabricated in the surface of
the resin composite matrix by replication. The original
(“master”) structures are made by photolithography, in the
case of microstructures (Figure 1), and bead lithography for
the nanostructures (Figure 2). In a first stage, the two types of
structures will be tested separately for their effect on cultured
bone cell lines. Previous studies on dental implants have
shown that multi-length scale topographies give the best bone
60
integration. So, combinations of the individual structures that
give the best results will also be tested.
Figure 2: Pillar structures in a silicon master
CSEM is also developing methods to determine the reaction
of bone cells to structured surfaces. In addition to previously
reported new methods to directly measure cell adhesion are
being investigated. One of the most promising of these
methods uses a specially adapted atomic force microscope
(AFM) to measure the force necessary to pull individual cells
from the surface (see Figure 3).
Figure 3: Direct measurement of cell adhesion: a) A cell attached to
an AFM cantilever is brought towards the surface. b) The cell
contacts the surface. c) The cell is pulled away from the surface. The
deflection of the cantilever is measured to determine the force
applied to the cell. d) The cell is released from the surface.
Initial results with this technique are promising, and it is hoped
that it will give new insights into the influence of surface
modification on the interactions between bone cells and
surfaces.
This work was partially financed by the European Commission
in the context of the Project Newbone (NMP3-CT-2007-
026279). CSEM thanks them for their support.