Chitosan adsorption on hydroxyapatite and its role in preventing acid erosion Hyun-Su Lee a , Shannon Tsai a , Chin-Chen Kuo a , Alice W. Bassani a , Brian Pepe-Mooney a , Davide Miksa b , James Masters b , Richard Sullivan b , Russell J. Composto a,⇑ a Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, PA 19104, United States b Colgate-Palmolive Company, Piscataway, NJ 08855, United States article info Article history: Received 21 April 2012 Accepted 25 June 2012 Available online 7 July 2012 Keywords: Chitosan Hydroxyapatite HA Acid erosion In situ quartz-crystal microbalance with dissipation QCM-D Atomic force microscopy AFM Polymer adsorption abstract Polymer adsorption onto an artificial saliva (AS) layer is investigated using quartz-crystal microbalance with dissipation (QCM-D) and chitosan as the model polymer. QCM-D is utilized in an innovative manner to monitor in situ adsorption of chitosan (CH) onto a hydroxyapatite (HA) coated crystal and to examine the ability of the adsorbed layer to ‘‘protect’’ the HA upon sequential exposure to acidic solutions. After deposition of a thin AS layer (16 nm), the total thickness on the HA substrate increases to 37 nm upon exposure to CH at pH 5.5 for 10 min. Correspondingly, the surface charge changes from negative (i.e., AS) to positive, consistent with the adsorption the polycationic CH onto or into the AS layer. Upon expo- sure to an oxidizing agent, the chitosan cross-links and collapses as noted by a decrease in thickness to 10 nm and an increase in the shear modulus by an order of magnitude. Atomic force microscopy (AFM) is used to determine the surface morphology and RMS roughness of the coated and HA surfaces after citric acid challenges. Both physisorbed and cross-linked chitosan are demonstrated to limit and prevent the erosion of HA, respectively. Ó 2012 Elsevier Inc. All rights reserved. 1. Introduction Over the last decade, dental enamel erosion in all age groups has increased due to a variety of dietary factors [1]. One of the main causes has been attributed to an increase in the consumption of beverages containing citric acid, such as citrus fruit juices and carbonated soft drinks [2]. Permanent dental enamel is primarily composed of mineralized carbonated hydroxyapatite (Ca 10x Na x (PO 4 ) 6y (CO 3 ) 2 (OH) 2u F u ), and can be modeled using calcium hydroxyapatite (HA), Ca 10 (PO 4 ) 6 (OH) 2 , due to their similar compo- sition and structure. As with enamel, direct exposure of HA to acids results in demineralization [3,4]. In vivo, saliva provides limited protection against this erosion by forming a pellicle layer that coats the HA surface [5,6]. In order to provide a better means of protec- tion, the present study examines the use of a polymer coating to further deter the effects of acid erosion of HA. Of particular interest is the ability of polymers to modify the native properties of the host surface by changing the wettability [7], surface topography [8], or chemical reactivity [9]. One goal of this study is to show that model experimental systems can allow for the elucidation of poly- mer interactions with surfaces [10,11]. Normally, HA coated with a pellicle layer undergoes demineral- ization and re-mineralization reactions depending on the ion prod- uct. In the idealized chemical reaction, as the pH decreases, positive hydrogen ions from the acid bind with the negative phos- phate and hydroxyl ions from the HA (enamel). As a result, the io- nic solution in the pellicle layer becomes unsaturated resulting in a shift that favors demineralization, which leads to the loss of cal- cium and phosphate ions from the crystal until a solubility equilib- rium is reached (Fig. 1). Dissolution, however, only occurs after diffusion of the acid through the pellicle and the protein-lipid coat- ing. The details of acid erosion in enamel and dentin are described by Featherstone and Magalhães et al. [12,13]. To prevent dental erosion, several strategies have been utilized including increasing saliva or plaque calcium by calcium treat- ments to counteract inherent deficiencies in the crystal structure [13], promoting re-mineralization and strong teeth by fluoride exposure [14], and using laser irradiation to improve the resistance of enamel to acid [15]. Recently, however, many products have been developed that utilize polymers as a protective coating against acid exposure. Polymer coatings serve as an efficient meth- od to protect surfaces from erosion because they can be biocom- patible and have been known to be effective agents against 0021-9797/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2012.06.074 ⇑ Corresponding author. Address: Department of Materials Science and Engi- neering, University of Pennsylvania, Laboratory for Research on the Structure of Matter, 3231 Walnut Street, Philadelphia, PA 19104, United States. E-mail addresses: Hyun-Su.Lee@uphs.upenn.edu (H.-S. Lee), shantsai@seas. upenn.edu (S. Tsai), janson925@gmail.com (C.-C. Kuo), awbassani@gmail.com (A.W. Bassani), bpepemoo@gmail.com (B. Pepe-Mooney), fiume_2000@yahoo.com (D. Miksa), Jim_Masters@colpal.com (J. Masters), Richard_Sullivan@colpal.com (R. Sullivan), composto@seas.upenn.edu (R.J. Composto). Journal of Colloid and Interface Science 385 (2012) 235–243 Contents lists available at SciVerse ScienceDirect Journal of Colloid and Interface Science www.elsevier.com/locate/jcis