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

Holly J. Martin, Dave C. Swalm School of Chemical Engineering, Mississippi State University, 330 Swalm - President's Circle, Mississippi State, MS 39762, Kirk H. Schulz, Vice President for Research and Economic Development, Mississippi State University, 617 Allen Hall, Mississippi State, MS 39762, and Joel D. Bumgardner, Biomedical Engineering Department, Herff College of Engineering, University of Memphis, Memphis, TN 38152.

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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