(331b) Comparing the Attachment and Growth of Bone Cells on Chitosan Bound by Two Silane Molecules to Titanium for Use In Joint Replacements

            Metals that are used for implantation, such as
titanium, are chosen because of advantageous mechanical properties, such as
strength, weight, and durability.  These metals also possess the ability to be
passivated, meaning the implant is unreactive with the surrounding
physiological fluids.  Unfortunately, osseointegration, or the attachment and
growth of bone cells, does not occur on metal implants, preventing the
integration of the implant into the bone.  By attaching bioactive materials,
such as calcium phosphate/hydroxyapatite [1,2] or biological molecules [3-5],
osseointegration can be achieved [6]. 

            One implant coating that has been shown to be
bioactive is chitosan, a de-acetylated form of chitin [7].  Found in the
exoskeletons of shellfish and insects, chitin is the second most abundant form
of polymerized carbon in nature, behind only cellulose [7,8].  Chitosan is
being investigated as an implant coating because it is non-toxic and the
by-products produced are considered a normal part of metabolism [8,9].  In
addition to the non-toxic nature, chitosan is cationic, or positively charged,
meaning it will attract negatively charged proteins and cells [10].  This
attraction has been shown to encourage the attachment and growth of bone cells
[10].  Along with the attraction, chitosan is useful as an implant coating
because the biopolymer encourages proper bone formation [11].  The cells
responsible for bone production retain the desired cell shape, which influences
cell-specific functions [11].

            At Mississippi State University, methods to
improve the bond between chitosan and titanium are being studied.  A two step
process to deposit chitosan consisted of two titanium surface treatments,
passivation and piranha, followed by the deposition of triethoxsilylbutyraldehyde
(TESBA) [12].  A three step process to deposit chitosan also consisted of the
two titanium surface treatments followed by the deposition of
aminopropyltriethoxysilane (APTES) and gluteraldehyde [13].  Four treatment
combinations were produced, by varying the titanium surface treatment and the
silane deposited.  The titanium surface was documented following each reaction
step using X-Ray Photoelectron Spectroscopy, which showed that more silane was
deposited on the piranha treated surface as compared to the passivated titanium
surface [12,13].  The chitosan deposited using one of the four treatment
combinations were also documented using XPS, which showed no differences in the
films [14].  Other bulk properties, including hardness and elastic modulus,
were unchanged from films deposited on glass slides [14].  An increase in the
adhesion strength of the chitosan coating to the titanium surface was seen as
compared to other methods of deposition, although no differences were seen
between the four treatment combinations [7,14]. 

            The individual steps of the reaction series have
been documented using XPS, while the effects of the reaction series on the
mechanical properties have been documented using nano-indentation and tensile
testing [12-14].  Some research on the effects of the silane molecule on the
biological properties of chitosan has been performed.  Using APTES to bond
either chitosan [7] or other biological molecules [4,5], no decrease in the
ability of the bone cells to attach to the coating was seen [4,5,7].  However,
these studies were conducted using the 5% water ? 95% ethanol mixture as a
carrier for APTES [4,5,7].  By changing the solvent from a 5% water - 95%
ethanol mixture to toluene, the adhesion strength between the chitosan coating
and the titanium surface was increased at least 10 fold [14].  However, the
effects of changing the solvent to toluene on the biological properties of
chitosan have not been determined.  Since toluene is considered mutagenic,
there is a chance that any trapped solvent could negatively affect the
attachment and growth of the bone cells.  The effects of changing the silane
deposited from APTES to TESBA have also not been studied.  The research
presented will cover the effects of bonding chitosan to the titanium surface
using two silanes and toluene as the solvent. 

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