1

Introduction

Resin adhesives may offer a unique platform to combat recurrent decay at the sealed interface between the resin and tooth interface ( i.e. , hybrid layer) with the incorporation of antimicrobial agents [ ]. Literature shows the incorporation of low concentrations of chlorhexidine into primers and dentin adhesives with the intent to prevent caries [ ]. However, the integration of antimicrobials into a resin-based material’s matrix commonly result in adverse effects on the material mechanical properties [ ] and, when in higher concentrations, increase cytotoxic effects. The use of a drug-carrier such as the Halloysite® clay nanotubes [HNTs, Al ₂ Si ₂ O ₅ (OH) ₄ ∙ nH ₂ O] allows the encapsulation of different antimicrobials without compromising the performance of the materials [ , ]. The lumen of the nanotube can serve as a container for the loading and sustained delivery of guest molecules [ , ].

The effect of incorporating nanotubes loaded with different guest molecules into the resin matrix has been previously evaluated for the inactivation of matrix-metalloproteinases, bacterial growth inhibition, cytotoxicity, and mechanical properties [ , , ]. While encapsulating antimicrobials into the nanotubes does not affect the material’s mechanical properties, it does not allow for an extended drug release as anticipated [ ]. Therefore, the addition of a functionalizing agent ( i.e. , an organosilane) was sought to further regulate and extend the release of the loaded drugs. The organosilane, 3-aminopropyltriethoxysilane [APTES, H ₂ NCH ₂ CH ₂ CH ₂ Si ₄ (OCH ₂ CH ₃ ) ₃ ], may be used for the amine functionalization of HNTs before the encapsulation of guest molecules to enhance the loading capacity and extend drug release time [ , ]. Studies in different areas have suggested that nanotube functionalization can result in increased loading and releasing capacity of nanotubes [ ] and depending on the silane, improve numerous mechanical properties of dental composites [ , ].

In respect to the above-mentioned background, the objectives of the present study were first to synthesize and determine optimal component ratios in a resin-based dental material containing APTES surface-modified halloysite-clay nanotubes (HNTs) for long-term delivery of chlorhexidine. Second, to characterize the optimized resin-based material. Third, to determine antimicrobial sensitivity and drug release properties of the modified materials. The hypotheses were that the APTES functionalization of nanotubes would positively affect the mechanical properties of the experimental resin-based material and that APTES functionalization of nanotubes loaded with chlorhexidine would extend the release of chlorhexidine when incorporated into resin.

2

Materials and methods

A resin-based material was prepared by mixing the monomers bis-GMA (Bisphenol A dimethacrylate), TEGDMA (Triethylene glycol dimethacrylate) and HEMA (2-Hydroxyethyl methacrylate); and CQ (Camphorquinone) and DMAEMA [2-(Dimethylamino)ethyl methacrylate] as photo-initiators. The optimal concentrations of silane (0, 2, 4 vol.%) and silanized HNT (10, 15, 20 wt.%) to be incorporated into the resin were determined in relation to each other through analysis of degree of conversion, flexural strength and flexural modulus, and ultimate tensile strength. Then, the HTN powder, functionalized or not, was loaded with chlorhexidine (CHX10%) and added to the resin-based material and these materials had their antimicrobial sensitivity and drug release properties analyzed.