Silanization of halloysite nanotubes was not detrimental to the mechanical properties of the resin-based material.
Silanization of halloysite nanotubes improved the antimicrobial activity of the chlorhexidine.
Silanization of nanotubes before encapsulation with chlorhexidine is a promising technique.
To synthesize and characterize a novel resin-based dental material containing 3-aminopropyltriethoxysilane (APTES) surface-modified halloysite-clay nanotubes (HNTs) for long-term delivery of guest molecules.
The optimal concentrations of HNT (10, 15, 20 wt.%) and silane (0, 2, 4 vol.%sil) to be incorporated into the resin-based materials were determined (15 wt.%HNT, 4 vol.%sil) after assessment of the mechanical properties (DC%, degree of conversion; FS, flexural strength; FM, flexural modulus; and UTS, ultimate tensile strength). The HNTsil-powder was loaded with chlorhexidine (CHX) to evaluate the effect of the silanization on drug release. Resin-discs were prepared for the following groups: RES (resin), HNT (resin+15 wt.%HNT), HNTsil (resin+15 wt.%HNT silanized), HNT-CHX (resin+15 wt.%HNT loaded with chlorhexidine), HNTsil-CHX (resin+15 wt.%HNTsil-CHX), and 0.2 vol.%CHX (resin+0.2 vol.%CHX solution). Specimens were stored in water for 1, 3, 5, 10, and 15 days at 37 °C. Aliquots from each time point and the final 15-day specimens were evaluated for the zone of inhibition (ZOI) against Streptococcus mutans . CHX release was analyzed using spectrophotometry at absorbance of 300 nm. Data were statistically analyzed ( α = 0.05).
All materials presented similar DC%. Reduced FS but increased FM was detected for 20 wt.%HNT–4%APTES. Groups with 15 wt.% and 20 wt.%HNT with/without APTES presented higher values of UTS. Agar diffusion data indicates that the HNTsil-CHX had a greater ZOI than all other groups over 15 days. HNTsil-CHX had the highest absorbance for day 1 but presented similar values to other groups every time point after.
Silanization of nanotubes followed by encapsulation of chlorhexidine is a promising technique for long-term delivery of guest molecules.
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.
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.
Synthesis of experimental resin-based material
The resin based material was prepared by mixing the monomers bis-GMA, TEGDMA and HEMA (50/25/25 wt.% ratio respectively) followed by the photo-initiator system CQ and DMAEMA (0.4 wt.% and 0.8 wt.% respectively) [ ]. Resin was divided into nine groups ( Fig. 1 ) according to: (1) the concentration of HNTs (10, 15 or 20 wt.%); and (2) APTES concentration (0%, 2% or 4%). Ethanol was the solvent used to dissolve the APTES.
APTES functionalization of HNTs
Nanotubes were dispersed in a suspension of silane (2 or 4%) and a solvent solution (ethanol) and vortexed. The resulting suspension was submitted to a sequence of vacuum (25 inHg) and end-to-end mixing. The silanized HNTs (HNTsil) were stored in an oven at 37 °C for 15 h. The suspension was centrifuged and the pellets, containing HNTsil, were washed (2×) using ethanol. The HNTsil were dried in an incubator at 37 °C for approximately 7 days. Dried HNTsil were sieved (45 μm) before incorporation into the resin-based material. To guarantee a proper dispersion of the HNTsil into the resin matrix, up to 3 mL of resin with 10, 15 or 20 wt.% HNTsil were mixed at the time using a stir bar/stir plate.
Characterization of the resin-based material modified with functionalized HNTs
For the following procedures, a light-emitting diode curing system (DEMI LED, Kerr, Orange, CA, USA) with an output intensity of 1100 mW/cm 2 was used. The unit’s intensity was periodically monitored using a radiometer (Cure Rite Visible Curing Light Meter, DENTSPLY Caulk). For the degree of conversion (DC%), experimental resin-based materials were evaluated using a Fourier Transform Infrared Spectrometer (model 4100, JASCO International Co., Tokyo, Japan) equipped with an attenuated total reflection device (ATR-MIRacle, Pike Technologies, Madison, WI) in the absorbance mode (8 cm −1 resolution and 2.8 mm/s mirror speed) [ , ]. The ATR had a 1.8 mm diameter diamond crystal plate. For that, uncured and cured (light activated for 20 s on the top only) specimens ( n = 3/group) were prepared and measured [ ] (3 measurements per specimen) using a single emission peak LED light-curing unit (LCU, Demi Ultra, Kerr Corporation, Orange, CA). The absorbance was measured using 64 scans and 4 cm resolution. To calculate the DC (expressed in %), the absorbance bands at 1637 cm −1 (methacrylate group) and 1607 cm −1 (aromatic ring in bis-GMA) were used according to the following equation [ ]: