Biomedical Engineering Reference
In-Depth Information
used to create microscale features
[50]
. The most widely used approach to create nanotopogra-
phy on the implant surfaces is the sandblasting and acid-etching (SLA) method
[51,52]
. The SLA
method has the advantages of both sandblasting and acid-etching techniques: creating both macro-
to microscale structures (sandblasting) and micro- to nanoscale structures (acid etching). The major
limitation of the SLA method is that it is a random process, thus it is hard to control the uniformity
and distribution of nanostructures on the implant surfaces. To overcome the limitation of SLA meth-
ods, both plasma-spray coating of inorganic materials including HA and electron beam evaporation of
calcium phosphate have also been commonly employed to generate nanostructures on the Ti implant
surfaces with a better uniform distribution of nanofeatures on the implant surfaces. However, these
methods are difficult to form the nanostructures on complex-shaped implants and inside the implant
cavities.
Another method is the deposition of a preselected molecule onto the implant substrate by che-
misorption. This method leads to the expression of selected functional groups at the cell implant
interface
[38]
that may aid in cellular attachment onto the substrate or even lead to directed cell dif-
ferentiation. One such popular molecule is the RGD peptide incorporated into a polyethylene glycol
(PEG) end group, which is adsorbed onto implant surfaces and aids in osseointegration
[53]
. Since
this method involves chemisorption of functional groups, it leads to a localized modification of the
chemical properties of the bulk substrate.
8.2.2
Surface Modifications of Ti Implants with Polymers
Coating the surface of metals such as Ti with functionalized polymer films has been shown as a use-
ful strategy to improve osseointegration while retarding metal corrosion
[54-60]
. Polymers possess
several advantages including ease of manipulation to vary their physical and chemical properties of
the implants. Another major advantage of polymers is that they can be used for loading and controlled
release of therapeutic reagents such as bone-promoting factors. However, they do possess some major
disadvantages like the potential of releasing harmful compounds such as degraded products into the
surrounding tissue and consisting of mechanical properties that are prone to wear and tear. Polymers
also easily adsorb proteins, which causes an alteration in the surface chemistry of such polymers.
Although these limiting factors prevent polymers from being more widely used in areas such as den-
tal and bone replacement implants, several techniques have been developed to modify the titanium
implant surfaces with various polymers to improve implant properties and enhance bone growth and
integration.
Electrochemical polymerization is a technique that has been often employed to deposit polymer
films on different metal surfaces. For example, homogenous passive polyacrylic acid films have been
synthesized onto the surfaces of pure Ti and Ti6Al4V substrates to provide improved anticorrosion
and new bioactive properties
[55,61]
. Various methods have also been devised to deliver biochemical
factors at the interface between the implant surface and bone tissue. These methods include adsorp-
tion or covalent binding of bioactive molecules onto the polymer films
[62,63]
. Besides provid-
ing corrosion resistance and a bioactive surface, polymer coating of metal bone implants have also
been shown to improve vascularization. Since metal implants show little or no support for revascu-
larization, polymer coatings have been performed to aid in vascularization of the new bone tissue
by entrapping and releasing biologically active molecules like vascular endothelial growth factor
(VEGF), a key protein involved in vasculogenesis and angiogenesis
[64]
. This was supported by
