Elsevier

Applied Surface Science

Volume 254, Issue 15, 30 May 2008, Pages 4599-4605
Applied Surface Science

An XPS study on the attachment of triethoxsilylbutyraldehyde to two titanium surfaces as a way to bond chitosan

https://doi.org/10.1016/j.apsusc.2008.01.066Get rights and content

Abstract

A bioactive coating has the ability to create a strong interface between bone tissue and implant. Chitosan, a biopolymer derived from the exoskeletons of shellfish, exhibits many bioactive properties that make it an ideal material for use as a coating such as antibacterial, biodegradable, non-toxic, and the ability to attract and promote bone cell growth and organized bone formation. A previous study reported on the bonding of chitosan to a titanium surface using a three-step process. In the current study, 86.4% de-acetylated chitosan coatings were bound to implant quality titanium in a two-step process that involved the deposition of triethoxsilylbutyraldehyde (TESBA) in toluene, followed by a reaction between the aldehyde of TESBA with chitosan. The chitosan coatings were examined on two different metal treatments to determine if any major differences in the ability of titanium to bind chitosan could be detected. The surface of the titanium metal and the individual reaction steps were examined using X-ray photoelectron spectroscopy (XPS). Following the deposition of TESBA, significant changes were seen in the amounts of oxygen, silicon, carbon, and titanium present on the titanium surface, which were consistent with the anticipated reaction steps. It was demonstrated that more TESBA was bound to the piranha-treated titanium surface as compared to the passivated titanium surface. The two different silane molecules, aminopropyltriethoxysilane (APTES) and TESBA, did not affect the chemistry of the resultant chitosan films. XPS showed that both the formation of unwanted polysiloxanes and the removal of the reactive terminal groups were prevented by using toluene as the carrier solvent to bond TESBA to the titanium surfaces, instead of an aqueous solvent. Qualitatively, the chitosan films demonstrated improved adhesion after using toluene, as the films remained attached to the titanium surface even when placed under the ultra-high vacuum necessary for XPS, unlike the chitosan films deposited using an aqueous solvent, which were removed when exposed to the ultra-high vacuum environment of XPS.

Introduction

The ability of surrounding bone tissue to incorporate an implant, also called osseointegration, is a major issue with orthopaedic and dental/craniofacial implants. One way to improve osseointegration is to bond bioactive coatings to the implant surface. Several different bioactive materials are currently being investigated, such as enzymes and proteins [1], [2], [3], hydroxyapatite and calcium phosphate [4], [5], [6], [7], [8], and bioactive glass [9], which allow the attachment and growth of bone cells into the implant, improving the implant's stability [10]. Several of the bioactive coatings currently being investigated, including hydroxyapatite, calcium phosphate, and bioactive glass, are considered ceramics or glass–ceramics [5], [10]. During surgery to place the implant into the human body, stresses are placed upon the coatings that the brittle nature of ceramics and glass–ceramics cannot withstand, leading to cracking and flaking of the coatings [11]. Osseointegration of the implant is then reduced or prevented due to the removal of the coating, which allows fibroblast growth and prevents the production of ordered bone tissue [12].

Bioactive polymers may overcome issues regarding the brittle ceramic material coatings for bone implants. Chitosan is one such bioactive polymer that has shown promise as an implantable material [13]. Chitin is found in the exoskeletons of shellfish, arthropods, and the cell walls of fungi [14], [15]. Through chemical treatment, chitin can be de-acetylated, forming chitosan [15]. This de-acetylation of chitin into chitosan produces more amine groups in the chitosan chain, which then become protonated in solution [12]. The positively charged chitosan chains attract proteins and cells and promote cell adhesion [12], [16]. Chitosan also prevents the growth of fibroblasts, allowing for the growth and replication of osteoblasts that produce orderly bone tissue [12], [17]. Because chitin is produced biologically, the by-products of the degradation of chitosan are part of normal cellular metabolism, indicating that chitosan is both biodegradable and non-toxic, with an LD50 greater than 16 g/kg [15], [16], [18]. Chitosan also possesses bactericidal properties, with the ability to kill Staphylococcus epidermis, Staphylococcus aureous, and members of the yeast family, Candida, and bacterialstatic properties with the ability to prevent the replication of Pseudomonas aeruginosa [14], [19].

Chitosan originally was investigated as bone filler for holes produced by wisdom teeth extraction [20], [21], [22], [23] and for wound dressings [24]. Very little has been done, however, to investigate the bonding of chitosan to implant quality metals, despite the ability of chitosan to produce ordered bone tissue. The few tests that have been performed on bonding chitosan to a substrate have been performed on plastic or glass dishes [25], [26], [27]. An understanding of the surface chemistry needed to bond chitosan to a material has not been developed. The research efforts that do involve coating a substrate with chitosan did not examine the surface chemistry involved in the bonding process, but instead focused on building a film on top of the substrate. The most fundamental method to attach chitosan to a substrate is evaporation, where a chitosan solution is poured over the substrate and the solvent is allowed to evaporate [28]. Chitosan films can also be created by reacting the substrate with a silane molecule, followed by a linker molecule, and finally through evaporation of the chitosan solution [16], [29]. The silanation reaction produced an increase in the bond strength of the chitosan film to the substrate (1.5–1.8 MPa) as compared to the simple evaporation method (0.5 MPa) [16]. The reported bond strengths of hydroxyapatite coatings (6.7–26 MPa), however, are much greater as compared to the chitosan film attached through the silanation reaction [16].

3-Aminopropyltriethoxysilane (APTES) is one silane molecule commonly used in the biomedical literature to bond an assortment of materials because of the primary amine group [8], [9], [10]. However in order to actually bond chitosan and titanium, a linker molecule, such as gluteraldehyde, must be used to modify the terminal amine group to an aldehyde group [16], [29], [30]. In previous research, a three-step process was used to bond chitosan to titanium, which qualitatively improved the bond strength [30]. One way to reduce the time required to attach a coating is to reduce the number of steps. Through careful selection of a silane molecule, the three-step process was reduced to a two-step process. As with previous research, toluene was used as the carrier solvent [30]. By using toluene as the solvent instead of an aqueous solution, loss of the reactive terminal groups and formation of the polysiloxane layer has been prevented [31], [32].

The issue with linker molecules and the solvent used to deposit these molecules is not the only issue, however. Titanium is commonly used as an implant metal because it can become highly unreactive through a process called passivation [33]. This unreactive surface is highly desirable in the human body, as it prevents negative reactions [10]. However, this unreactive surface can also reduce the ability to bond a coating to the titanium implant. Piranha is a method to remove any carboneous materials [34], which may be introduced to the titanium surface because of the manufacturing and passivation processes [33]. Piranha has also been shown to etch titanium, which can help produce more surface area for the linker molecules to bond to the surface. Because of piranha's ability to react with both carbon and titanium, it is believed that the piranha-treated surface will have more reactive areas, thereby increasing the amount of linker molecules bound to the titanium surface and increasing coating bond strengths.

Following each reaction step, X-ray photoelectron spectroscopy (XPS) was used to determine if the chemical changes in the titanium surfaces were consistent with the anticipated reaction series. The effect of using toluene as the solvent on the reactive terminal aldehyde groups and the amount of TESBA bound by the two treated titanium surfaces were also examined using XPS. Therefore, the aim of this study was to evaluate the surface chemistry involved in the deposition of a covalently bound chitosan film on implant quality titanium using triethoxsilylbutyraldehyde (TESBA).

Section snippets

Reagents

99.7+% ACS grade glacial acetic acid, gluteraldehyde, 35% aqueous solution hydrogen peroxide, 95–98% ACS grade sulfuric acid, 99% min. semiconductor grade toluene, and HPLC grade ultra-pure water were purchased from Alfa Aesar (Ward Hill, MA). 99.5% ACS grade acetone and 200 proof ethanol were purchased from Sigma–Aldrich (St. Louis, MO). ACS grade nitric acid and ACS grade isopropyl alcohol were purchased from Acros Chemical (Morris Plains, NJ). TESBA was purchased from Gelest (Morrisville,

Results and discussion

The samples were scanned using XPS after each reaction step. The passivated and piranha-treated titanium surfaces were first scanned using XPS. TESBA was then deposited (Fig. 1, reaction step 1) and XPS was performed on the TESBA treated surfaces. The chitosan film was then deposited (Fig. 1, reaction step 2) and XPS was run on the final film.

Conclusions

XPS was used to document the deposition of TESBA on two different titanium surfaces. There were significant changes in the amounts of oxygen, carbon, silicon, and titanium detected by XPS, which were consistent with the anticipated reaction steps. It was demonstrated that more TESBA was bound to the piranha-treated surface as compared to the passivated surface, based on the lower amount of titanium and the higher amount certain species of silicon. XPS was also used to document the attachment of

Acknowledgement

Financial support from the Bagley College of Engineering at Mississippi State University is gratefully acknowledged.

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