The Cell–Surface Interaction - Tissue Engineering: Cell-Surface Interactions for Tissue Culture - page 9

do promote integration of an implant with surrounding bone, this osseointegration
is relatively slow and results in poor mechanical anchorage when the device is
inserted into osteoporotic bone [ 42 , 43 ]. Furthermore, coating techniques with
commercial hydroxyapatite or CaP are demanding and problematic to obtain the
maximal biological response [ 44 ]. Additionally, in long-term implantation they
can delaminate [ 45 ]. Thus, large variability in the quality of different hydroxy-
apatite coatings from different companies or even from different batches causes
concerns about the long-term reliability of these coated implants [ 46 ]. Recently,
Schlegel et al. [ 47 ] used anodic-plasma-chemical (APC) treatment to improve the
adhesive strength of the CaP layer. APC treatment is an anodization technique that
allows porous oxide layer formation with incorporation of CaP directly into the
oxide. Results from Schlegel et al. showed no significant difference in bone
remodelling and removal torque between APC-treated implants and surfaces
coated in a standard manner. However, the histological results indicated some
delamination of standard coated CaP and hydroxyapatite surfaces. Thus, the APC
treatment results in higher strength of bonding to the implant surface and allows
the drawbacks of standard CaP coatings to be overcome.
More recently, O'Hare et al. [ 48 ] investigated a novel surface modification for
incorporating biomolecules such as hydroxyapatite within the oxide layer of a
metal substrate. The CoBlast method [ 49 ] is an advanced version of microblasting
in which both an abrasive and a dopant are applied to the substrate surface
simultaneously without the need for any form of presurface treatment. O'Hare
et al. [ 48 ] showed that CoBlasting resulted in a stable surface that was observed to
support enhanced osteoblast attachment and viability in vitro compared with
hydroxyapatite alone or metal substrate controls. Implantation of the CoBlast
surface in a rabbit femoral model confirmed that the surface promoted in vivo
formation of early-stage lamellar bone growth after 28 days. Techniques such as
this may provide a way for chemical modifications to become more reliable and
reproducible, which is advantageous in bone-compromised patients in need of
rapid osseointegration, although testing in a larger-animal model would be

required.

3.1.4 Surface Biological Modifications

Currently available biochemical surface modifications include immobilization of
ECM proteins such as collagen or peptide sequences modulating bone cell adhe-
sion; immobilization of DNA for structural reinforcement; deposition of cell
signalling agents (bone growth factors) to trigger new bone formation; and
enzyme-modified titanium surfaces for enhanced bone mineralization [ 46 ].
Presently, coating implant surfaces with the RGD sequence is the most common
peptide-based strategy. The RGD sequence has been identified as a cell attachment
motif present on several plasma and ECM proteins, including collagen
type I, fibronectin, vitronectin, bone sialoprotein and osteopontin. These proteins
interact with integrins, including the predominant osteoblast integrin a 5 b 1 [ 50 ].