(163ah) The Effects of Different Silanes and Metal Surface Treatments on the Binding of Chitosan as Investigated by Mechanical and Biological Testing

            Implants are commonly made from strong, light, durable
metals, such as commercially pure titanium and titanium alloys.  These metals are usually chosen based on the
lack of interaction between the body and the surface because of the formation
of a passive oxide layer.  However,
interactions do occur between the implant surface and the bone cells
surrounding the implant.  These
interactions include the necrosis, or death, of the surrounding tissue and the
leeching of metal ions, both grouped in a broad category of biocorrosion.  One way to prevent biocorrosion is bond a
biocompatible material onto the surface of the metal implant before the implant
is placed in the body.  Currently,
coatings of several different variations are being investigated, including calcium
phosphate [1], hydroxyapatite [2], and biological molecules such as proteins
and enzymes [3-5].

            Two of the
most biologically compatible materials, calcium phosphate and hydroxyapatite,
which are precursors to bone formation, have been investigated as possible
coatings.  However, these materials are
considered ceramics.  By their nature,
these materials are very brittle and are easily sheared or flaked off when the
implant is placed into bones within the human body [6].  This shearing and flaking will ultimately
result in pitting and crevice corrosion, leading to the complete and
catastrophic failure of these implants [7]. 

            One method
to prevent the flaking and shearing of the coating is to use a biologically
compatible polymer, which can withstand the stresses incurred during
implantation.  The material being
investigated at Mississippi State University is chitosan, a de-acetylated form
of chitin, which is produced in biological systems.  Chitin is the second most abundant form of polymerized carbon in
nature [8] and is primarily found in the exoskeletons of arthropods [9] and
fungi [10].  Chitosan is a cationic
copolymer of glucosamine and N-acetylglucosamine [11] and is considered
biocompatible because specific enzymes can degrade it [9].  Currently, chitosan is being investigated
for use as implant material coatings, wound dressings, drug delivery systems,
and bone implants [11]. 

            At
Mississippi State University, we have been investigating four different
treatment combinations to bind chitosan to commercially pure titanium, grade
4.  The chemical, mechanical, and
biological features of these combinations were investigated to determine the
highest quality film produced.  Atomic
Force Microscopy was used to determine the roughness and thickness of the
chitosan films.  Nanoindentation was
used to determine the hardness of the films, while Scratch-Testing was
performed to determine the resistance to shearing of the different treatment
combinations.  In-Vitro
biological testing was performed to determine the interaction of cells to the
different films and chemicals used.

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