Titanium is commonly used as implant materials for its mechanical properties, including strength, weight, and durability.  It also is easily passivated and does not react with the physiological fluids surrounding the implant.  However, titanium does not promote bone cell growth, preventing the integration of the implant into the bone.  One way to improve osseointegration is to coat the implant with a bioactive material, which promotes bone cell attachment and growth [1].  Several different bioactive materials are currently being examined, including calcium phosphate [2], hydroxyapatite [3], and biological molecules, such as enzymes and proteins [4-6].

            Two precursors to bone formation, hydroxyapatite and calcium phosphate, exhibit high bioactive behavior, but are considered ceramics.  When shear stress is applied to ceramics, the material fractures before plastic deformation, or an irreversible change in shape, can occur [7].  Because ceramics cannot absorb stress, these materials are likely to crack and/or flake when stressed during implantation, reducing the effectiveness of the coating and preventing osseointegration of the implant.  One way to avoid the cracking or flaking of a ceramic coating is to use a coating created from a polymer.  Polymers are better suited to absorb the stresses applied during implantation because of their ability to stretch, or elongate, which is a type of plastic deformation [7]. 

            Polymers can be created both in the laboratory and biologically.  Currently, Mississippi State University is investigating the use of chitosan, a de-acetylated form of chitin, which is found in the exoskeletons of shellfish and some insects [8].  Because of the ability of invertebrates to form chitin, the polymer is the second most abundant form of polymerized carbon found in nature, behind only cellulose [9].  Besides being readily available, chitosan exhibits many properties that make it an ideal material for implantation.  The material is non-toxic and the degradation products are also non-toxic, since the by-products are part of normal cellular metabolism [8,10].  The biopolymer is also cationic, so it attracts proteins and cells and encourages attachment and growth of bone, a necessity for the osseointegration of the implant [11].  Finally, chitosan has been shown to be antibacterial and bacteriostatic [12,13].

            At Mississippi State University, four treatment combinations have been designed to attach chitosan to implant quality titanium.  These treatment combinations involve two different metal treatments and two different silanation reactions [14,15].  X-Ray Photoelectron Spectroscopy (XPS) was run on the final films, which demonstrated no significant differences between the films produced using the four treatment combinations [14,15].  In order to confirm that the four treatment reactions did not affect the bulk properties of the chitosan films, mechanical tests were performed.  Nano-indentation tests were performed to determine the hardness and elastic modulus of the films.  Both the hardness values (0.19 ± 0.08) and the elastic modulus values (4.90 ± 1.82) showed no significant differences between the four treatment combinations or published literature values [16].  Scanning Electron Microscopy (SEM) of the nano-indentation marks showed that the chitosan films could absorb the stress applied by the diamond shaped nano-indenter, as the cracking of the film did not extend outside of the nano-indentation marks [16].  The purpose of the four treatment combinations is to create a chitosan film that is more strongly attached to titanium than previous research.  Because the nano-indentation did not show any delamination events, tensile testing was performed to determine the bond strength of the chitosan films on the titanium surface.  While there were no significant differences between the four treatment combinations, the results indicate that the bond strengths were significantly higher (19.50 ± 1.63 MPa) than previous results (1.5 MPa) [8,16].  Overall, mechanical testing showed that the bulk properties of the chitosan films were not changed by the four treatment combinations, but the bulk adhesion of the chitosan films was greatly increased.  

[1] B.D. Ratner and A. S. Hoffman. In: Biomaterials Science: An Introduction to Materials in Medicine. California: Academic Press, Inc., 1996, Foreword, 1-8.

[2]  Y. Yang, C.M. Agrawal, K.H. Kim, H. Martin, K. Schulz, J.D. Bumgardner, J.L. Ong.  Journal of Oral Implantology, 29, 6, 270-277, 2003.

[3] R.J. Friedman, T.W. Bauer, K. Garg, M. Jaing, Y.H. An, R.A. Draughn.  Journal of Applied Biomaterials.  6, 231-235, 1995.

[4] A. Nanci, J.D. Wuest, L. Peru, P. Brunet, V. Sharma, S. Zalzal, M.D. McKee.  Journal of Biomedical Materials Research, 40, 324-335, 1998.

[5] D. A. Puleo.  Journal of Biomedical Materials Research, 37, 222-228, 1997.

[6] D. A. Puleo.  Journal of Biomedical Materials Research, 29, 951-957, 1995.

[7] W.D. Callister, Jr. In: Materials Science and Engineering: An Introduction. New York: John Wiley & Sons, Inc., 2000, Appendix B, 789-816.

[8]  J.D. Bumgardner, R. Wiser, P.D. Gerard, P. Bergin, B. Chestnutt, M. Marini, V. Ramsey, S.H. Elder, J.A. Gilbert.  Journal of Biomaterials Science: Polymer Edition, 14, 5, 423-438, 2003.

[9]  G. Haipeng, Z. Yinghui, L. Jianchun, G. Yandao, Z. Nanming, Z. Xiufang.  Journal of Biomedical Materials Research, 52, 285-295, 2000.

[10]  M. Prasitsilp, R. Jenwithisuk, K. Kongsuwan, N. Damrongchai, P. Watts. Journal of Materials Science: Materials in Medicine, 11, 773-778, 2000.

[11]  C. Jarry, C. Chaput, A. Chenite, M.A. Renaud, M. Buschmann, J.C. Leroux.  Journal of Biomedical Materials Research (Applied Biomaterials), 58, 127-135, 2001.

[12]  A.K. Singla, M. Chawla. Journal of Pharmacy and Pharmacology, 53, 1047-1067, 2001.

[13]  R.A.A. Muzzarelli, R. Tarsi, O. Filippini, E. Giovanetti, G. Biagini, P.E. Varaldo.  Antimicrobial Agents and Chemotherapy, 34, 2019-2023, 1990.

[14]  H.J. Martin, K.H. Schulz, J.D. Bumgardner, K.B. Walters.  Langmuir, 2007, In Press.

[15]  H.J. Martin, K.H. Schulz, J.D. Bumgardner, K.B. Walters.  Langmuir, 2007, Under Review.

[16]  H.J. Martin, K.H. Schulz, J.D. Bumgardner, J.A Schneider.  Thin Solid Films, 2007, Under Review.