Research Paper
Development, mechanical evaluation and surface characteristics of chitosan/polyvinyl alcohol based polymer composite coatings on titanium metal

https://doi.org/10.1016/j.jmbbm.2014.08.014Get rights and content

Abstract

Mechanical properties of orthopedic implants play important role in the regeneration and cell growth of the diseased body part. The present investigation was aimed at the development of a biocompatible, biodegradable and mechanically stable coating of chitosan (CS)-polyvinyl alcohol (PVA) polymer composite on Titanium (Ti) metal by employing a simple methodology at ambient conditions. The PVA to CS concentrations were maintained in fixed ratios of 1:4 weight/weight (w/w) for the development of all the coatings on Ti metal. Four different concentrations of the polymers ranging in the order of 5%, 10%, 15% and 20% weight/volume (w/v) solution of CS were selected in an aim to test their efficacy on mechanical stability. The results obtained from the analysis confirmed considerable improvement in mechanical properties of the composite polymer film comprising CS and PVA on Ti metal with the four different concentrations showing variable elastic modulus and hardness. The difference in mechanical properties of both dehydrated and hydrated coatings demonstrates the effective and efficient shielding of high mechanical properties of Ti metal in physiological conditions. The scratch tests performed on the coated specimens also indicated a good adhesion of the polymer on the Ti metal surface.

Introduction

Orthopedic implants, such as fracture fixations, hip, artificial shoulder, spinal and dental implants play important roles in improving the quality of life of injured or aged individuals. Titanium (Ti) made orthopaedic implants are the ultimate choice because of its noncorrosive nature and biocompatibility (Sjostrom et al., 2012). Mechanical properties of an implant material are considered essential factors as it greatly influences phenomenon starting from the cell adhesion to the complete regeneration of the tissues at the implant site. The modulus and hardness of Ti are much higher than bone and it is a well-known fact that the mechanical properties of bones vary at different structural level. For example, values of the modulus of trabecular bone range from 1 to 20 GPa (Choi et al., 1990, Rho et al., 1993, Rho et al., 1997a, Rho et al., 1997b), the modulus of osteon lamellar bone is 22 GPa (Rho et al., 1997a, Rho et al., 1997b), the modulus of large tensile cortical bone is in the range of 14–20 GPa (Rho et al., 1998). The elastic modulus and hardness are the essential mechanical features of orthopedic implants and they should be in the same range like biological bone (Hollister, 1995). It is also demonstrated that the Young’s modulus of an implant material should vary in the range of 1 to 30 GPa for varied bone replacement applications and this data is found lower than the modulus of Titanium metal (Black and Hastings, 1998). Due to this mechanical incompatibility between Ti and biological bone, bone cells around implant are reabsorbed and become dead, which result in loosening of implant. In this context, coating of a synthetic material that possesses modulus comparable to the replacing bone could be a major prerequisite for an orthopedic implant. Such implant is expected to provide a major support for the surrounding tissues to regenerate. Hence, the noncorrosive, biocompatible coatings with supportive mechanical properties are important for expanding the convenient use of implantable devices.

In recent years, chitosan (CS) has shown remarkable advantages for biomedical applications due to its non-toxicity, biodegradability, biocompatibility, bioadhesive and absorption enhancing properties (Ohkawa et al., 2006). CS, a derivative of chitin is a natural cationic heteropolyaminosaccharide consisting of 2-amino-2-deoxy-d-glucopyranose and N-acetyl-2-amino-2-deoxy-d-glucopyranose linked together by a β-(1→4) glycosidic bond. CS is soluble in acidic solution and its solubility depends on the degree of deacetylation, molecular weight and the nature of acid used for the protonation of amino group of CS (Dragan et al., 2009, Park et al., 2002). The other advantages of CS are its excellent bio-stimulation properties which facilitate the reconstruction and vascularization of the damage tissues and also possess the ability to compensate the shortcomings of cells components, which are conductive for small scar forming. The cationic property of CS forms the basis of many of its potential applications that can be considered as a linear polyelectrolyte with a high charge density that can interact with negatively charged surfaces, like proteins and anionic polysaccharides (Martino et al., 2005, Gaharwar et al., 2010, Pillai et al., 2009). Generally, pure CS films are known for its poor flexibility, brittle nature and fragile behavior. Under these circumstances, addition of suitable plasticizers is expected to reduce the frictional forces between the polymer chains thus resulting in the improvement of mechanical properties (Ligler et al., 2001, Aryaei et al., 2012, Suyatma et al., 2005).

Further, the amine functionality in acidic solution is a unique behavior of CS, which can be tailored for derivatization to improve mechanical properties (Sasson et al., 2012). The addition of plasticizer like polyvinyl alcohol (PVA) increases toughness and reduces the brittleness of CS coatings. Plasticizer chains interpose in between the polymer chains and also interact with functional groups to reduce interaction and intermolecular cohesive forces between the polymer chains (Boateng et al., 2009, Ayensu et al., 2012). PVA is not a natural polymer, but it is a biocompatible, biodegradable and water soluble polymer (Scotchford et al., 1998, Asran et al., 2010). Thus the essential features of PVA could be combined with CS to form a mechanically stable polymer composite and this resultant composite is expected to form a strong film on the metallic implant. The compatibility with wet-processes which can be performed under the atmospheric pressure is one of the most fascinating features of polymeric materials. The dip-coating and spin-coating techniques are very familiar to the researchers working in the laboratory, since the apparatus required is simple and cheap. However, the poorness in the materials efficiency of these methods may be a problem for the mass-production. The electrophoretic deposition is a classical room temperature coating method widely used in the industrial coating process. It is demonstrated that the electrophoretic deposition of pure CS on a metallic surface is not possible due to its low ionic charge and ionic mobility.

To the best of our knowledge the coating of polymers based on CS and PVA composite on metallic surface is not demonstrated before and thus the present investigation was aimed at the development of polymer composite coatings on Ti metallic surface through a simple drop casting method of layering the polymer composite solution and allowing them to polymerize at ambient conditions. The resultant polymer composite coatings were tested for its chemical characteristics using suitable analytical methods. The mechanical properties and the adhesion ability of polymer composite on the metallic surface were determined through nanoindentation technique.

Section snippets

Materials

Medium molecular weight chitosan (CS) with degree of deacetylation more than ~80% and cold water soluble Polyvinyl alcohol (PVA) were commercially procured (Hi-media, INDIA) for the present investigation. The dissolution of CS was done with acetic acid (Hi-media, INDIA) and Millipore water was used for all the processes in the present study.

Preparation of metal

Titanium (Ti) metal as a metallic implant was commercially procured for use in the present investigation. The percentage by weight composition of Ti metal

Chemical analysis

FT-IR spectrum of pure CS, pure PVA, and four different CS/PVA composite films were recorded to elucidate the interaction between CS and PVA (Fig. 1). FT-IR spectroscopic measurements exhibited the existence of relevant functional groups of both CS and PVA in the composite films. Pure CS powder showed the characteristic peak at about 3430 cm−1 that accounts for the hydroxyl (OH) and amino (NH2) stretching, 2920 cm−1 for –CH stretching, 1080 cm−1 for C–O–C stretching, 1593 cm−1 for free NH2 and 1658 

Conclusions

The present investigation led to the following conclusions. A simple room temperature process involving the drop casting of polymeric solutions comprising chitosan and polyvinyl alcohol on Ti metal resulted in the establishment of strong polymer metal binding. The characterization results from the XRD, TG-DSC, FT-IR confirmed the strong intercalation that occurred between the chitosan and polyvinyl alcohol. The use of PVA as plasticizer with CS overcame key drawbacks of CS namely fragile

Acknowledgments

The financial assistance received from DST (SERB, no. SB/FT/CS-101/2012), India is greatly acknowledged. The Instrumentation availed from the Central Instrumentation Facility (CIF) of Pondicherry University and the Atomic Force Microscopy (AFM) facility availed from Department of Physics, Pondicherry University is greatly acknowledged.

References (37)

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