The use of central vascular catheter in the United States has caused 80,000 cases of catheter-related bloodstream infections (BSIs) in intensive care units each year.1 In addition, it is estimated that 250,000 cases of BSIs occur annually in assessing all hospitals.2 It has also been reported that the infections are associated with 2,400 to 20,000 deaths per year.3 Therefore, the development of antiinfective materials as a part of permanent implantable catheters is imperative to prevent intravascular catheter-related infections from the point of view of biomedical engineering. There are several types of commercially available medical products to reduce the risk of intravascular catheter-related infections, such as chlorhexidine-silver sulfadiazine–impregnated catheters4 and minocycline-rifampin–impregnated catheters.5
However, in Japan, 13 patients using chlorhexidine-silver sulfadiazine–impregnated catheters experienced serious anaphylactic shock, including one potentially associated death.6 It has been thought that chlorhexidine molecules interacting with IgE antibodies lead to anaphylaxis.7 As a result, catheters impregnated with strong antibacterial chemical agents have not been commercially available in Japan because of the previous accidents. From the point of view of infection control using vascular implanted catheters, it is necessary to develop a novel weak antibacterial material as a component of vascular catheter.
About a decade ago, cuffs coated with a monolayer of dispersible calcinated hydroxyapatite [HAp, Ca10(PO4)6(OH)2] nanoparticles were developed and equipped onto catheters.8 The percutaneous nano-HAp–coated cuff is expected to be able to prevent germ infection from the outside by adherence with skin tissue because of rapid ingrowth of fibrous tissue without severe inflammation, compared with commercial-grade polyester cuffs in animal implantation.9 However, the proliferation of microbacteria on the catheter in a living body was permitted when the HAp barrier was broken. Because HAp does not exhibit antibacterial activity, it is necessary to confer a weak antibacterial property on HAp.
This study focused on the preparation and characterization of nanosized fluorinated HAp (F-HAp) and the assessment of its antibacterial property and the improvement of its dispersible properties. Particle dispersibility is particularly indispensable for nanocoating on medical devices. Fluoride was selected because the chemical compound has been routinely used as an additive in toothpastes in oral care.10 Wet chemical processing was also selected because it allows easy control of the chemical composition of the final product by ion doping.11,12 To avoid calcination-induced F-HAp particle agglomeration, polyacrylic acid (PAA) and calcium salt were used as the antisintering agents during calcination (see Figure 1S, Supplemental Digital Content 1, http://links.lww.com/ASAIO/A85).13,14 The F-HAp nanoparticles were well separated, making them suitable as nanomaterials for coating on catheter substrates. Antibacterial examinations were also conducted on F-HAp using Escherichia coli and compared with original HAp and sodium fluoride (NaF).
Materials and Methods
Analytical-grade calcium nitrate tetrahydrate [Ca(NO3)2·4H2O], NaF, phosphoric acid (H3PO4), and ethanol (99.5%) were purchased from Wako Pure Chemical Industries Ltd. (Osaka, Japan), and used without further purification. Polyacrylic acid (molecular weight = 6,000–8,000 g/mol) was purchased from Toagosei Co., Ltd. (Tokyo, Japan).
Preparation of Dispersible F-HAp
The preparation reaction of F-HAp was performed in ethanol at 80°C, and the amount of reactants required was calculated based on a Ca2+:PO43−:F− molar ratio of 10:6:2. A total of 30 mmol Ca(NO3)2·4H2O and 6 mmol NaF were mixed with 200 ml ethanol for 30 min at room temperature in nitrogen atmosphere. A total of 1.8 mmol of H3PO4 in 50 ml ethanol was added to the solution and stirred at 80°C for 60 min. The solution was washed three times with deionized water using a centrifugal method to remove impurities. Then, the mixture was filtered using fine filter paper to produce a wet cake. To obtain dispersible nanoparticles after calcination, the antisintering agent was used. A 1.0-g sample of the dry powder was added and stirred in 1.0 g PAA/100 ml of deionized water and adjusted to pH 5.0 with ammonium solution. Ca(NO3)2·4H2O (16 mmol) in 370 ml of deionized water was mixed in the aforementioned solution. The powder was filtrated and dried for 1 h and calcinated at 800°C for 2 h. The final products were obtained by washing with NH4(NO3) solution and deionized water and drying. Original HAp containing no fluoride was prepared by the same method without the addition of NaF.
X-ray diffraction (XRD) was performed on a RAD-X diffractometer (MiniFlex, Rigaku Co., Tokyo, Japan), with Cu kα (λ = 1.5418 Å) radiation. The diffraction patterns were collected in the 2θ range 10–90°, with a step size of 0.01°. The cell parameters were calculated using the free software “unit cell” with values of 2θ obtained from the measured XRD peaks.15 Samples for Fourier transform infrared spectroscopy (FT-IR; Spectrum One, Parkin-Elmer Japan Co., Ltd., Yokohama, Japan) measurements were prepared by pressing the powder mixture with KBr, and the spectra were captured in the range 400–4,000 cm−1, with a scanning resolution of 16×. The morphology of the samples were observed using a scanning electron microscopy (SEM; JSM-6301F, JEOL Ltd., Tokyo, Japan), operating at 5 kV with an emission current of 8 mA. The average major and minor axes of the particles (n = 50) were determined from the SEM images. In addition, the size distribution and average size of F-HAp nanoparticles dispersed in ethanol were measured at a 10 ppm concentration by dynamic light scattering (DLS; ELS-8000, Otsuka Electronics Co., Kyoto, Japan). The fluoride ion content of the F-HAp was determined using a fluoride meter (F-10Z, Kasahara Chemical Instruments Co., Saitama, Japan). The value was determined from fluoride ion concentration of 50 mg from the sample powder in 100 ml of the analytical solution. The composition of Ca and P ions in the F-HAp was determined using inductively coupled plasma atomic emission spectroscopy (ICP-AES; Optima, 2000DV, Perkin-Elmer, Japan, Co., Ltd., Yokohama, Japan). Acid resistance test was conducted using a Ultraviolet–visible (UV-Vis) spectrometer V-550 (JASCO Co., Tokyo, Japan) at 595 nm. The transmittance of 0.04 w/v% sample solutions with dispersed F-HAp powders was measured at room temperature in phosphate buffer solutions of different pH adjusted by adding 1N HCl.
To evaluate antimicrobial activity of the nanoparticles, F-HAp and HAp were milled between two glass plates to disperse aggregates and washed with 70% ethanol and twice with 140 mM NaCl. Both washed F-HAp and HAp were resuspended within 140 mM NaCl to a concentration of 500 mg/ml. Wild-type E. coli K-12 W3110 was cultivated in 2 ml Luria-Bertani (LB) liquid medium (1% tryptone, 1% NaCl, and 0.5% yeast extract) at 37°C for 20 h, and the culture was diluted with 140 mM NaCl to adjust the OD600 to 0.1. To prepare 50, 20, and 0 mg F-HAp in 200 µl of 140 mM NaCl with E. coli, 100 µl of the culture was mixed with 100 µl of the 500 mg/ml F-HAp suspension, 40 µl of the 500 mg/ml F-HAp and 60 µl of the 500 mg/ml HAp suspensions, and 100 µl of the 500 mg/ml HAp suspension, respectively. The samples were incubated at 25°C for 2 h with gentle agitation. After preparation of a 10-fold dilution series of the E. coli suspension with 140 mM NaCl, 5 µl of each sample was dropped onto a plate containing LB agar medium (LB medium with 1.5% agar), and viable cells were visualized as colonies. Cell viability assay were carried out and concluded on the basis of three independent experiments. This work was fully approved by the Kinki University Bio-safety Committee and was conducted in accordance with the regulations for use of recombinant pathogens.
Results and Discussion
The HAp doped with fluoride was calcinated at 800°C for 2 h with PAA-Ca as the antisintering agent to prevent particle aggregation during calcination. The fluoride ion content of the F-HAp was 90%, corresponding to conversion of F− from OH− in the chemical structure of HAp, determined using fluoride meter (Table 1). The fluoride ion could be doped well into HAp lattice. The OH group was supplied by Ca(NO3)2·4H2O of the feed in ethanol. Figure 1 shows the XRD profile of the F-HAp calcinated with the antisintering agent. Identification of phases was achieved by comparing the diffraction patterns of fluorine HAp obtained in the laboratory with the International Center for Diffraction Data-Powder Diffraction File (ICCD-PDF) standards. The major phase, as expected, was fluorapatite, which was confirmed by comparing the data obtained here with the ICCD-PDF card: 15-0876. Table 1 shows unit cell parameters of standard HAp and F-HAp calculated using the “unit cell” software. The unit cell parameter of the a-axis of F-HAp was smaller than that of standard HAp, i.e., 9.367 and 9.418, respectively, although the parameters of the c-axis were quite similar, i.e., 6.878 and 6.884, respectively. This is because fluoride ions substitute the OH sites.16 The cell parameter of the c-axis almost did not change with the substitution of OH groups by fluoride ions, because the ions lined up in the channel along the direction of the c-axis in the HAp structure.17 Meanwhile, the parameter of the a-axis was changed because of the smaller ionic radius of the fluoride ion (1.32 Å) than that of OH− (1.68 Å).18 Thus, highly crystalline F-HAp containing 90% fluoride was obtained because there was no contamination of other calcium phosphate based on the XRD profile and the determination of the unit cell parameters.
Fourier transform infrared spectroscopy was performed to investigate the functional groups present in the F-HAp. These data clearly revealed that the presence of the various vibrational modes corresponded to the presence of phosphate and carbonate groups. The spectra of the sample powder of F-HAp and original HAp calcinated at 800°C for 2 h with an antisintering agent are shown in Figure 2. The bands of F-HAp at 610/580 and 475 cm−1 were assigned to ν4(PO43−) and ν2(PO43−) in the lattice, respectively. The peaks at 965 and 1,105/1,060 cm−1 reflected ν1(PO43−) and ν3(PO43−), respectively. The bands at around 890 and 1,420–1,470 cm−1 indicated the presence of B-type carbonate CO32−, which were substituted for PO43− in the apatite lattice.19 The ratio of Ca and P of the F-HAp powder was determined to be 1.73 by ICP-AES, indicating a phosphate deficiency. The carbonate bands of both samples prepared using PAA-Ca as an antisintering agent are shown in Figure 2, A and B, where the organic carbon in PAA heated at 800°C was converted to carbonate by reacting with atmospheric oxygen. The presence of hydroxyl ions in the F-HAp lattice was confirmed by the absorption band at 3540/750 cm−1 as shown in Figure 2A, attributed to OH…F depending on hydrogen bonding between fluoride ions and hydrogen atoms in OH groups.20 Thus, these data supported to successful substitution of fluoride ion into HAp structure.
The SEM observations were performed on the F-HAp powder sample (Figure 3). The particles were mostly separate with few agglomerates consisting of several particles in the SEM images. The primary F-HAp particles generally showed rod-shaped morphology with nearly round corners. The average particle sizes were 367 ± 67 nm in length and 223 ± 21 nm in width, as determined by the SEM images. As the reaction temperature used in the wet method was 100°C, the c-axis was elongated. In addition, the dispersibility of the F-HAp nanoparticles was evaluated from the dispersed particle size, which was measured in ethanol by DLS (Figure 2S, see Supplemental Digital Content 2, http://links.lww.com/ASAIO/A86). The dispersed particle size (313 ± 51 nm) was almost the same as that obtained from the SEM images. This is because the antisintering agent in the calcination process at 800°C was effective in avoiding particle agglomeration.
It is well known that fluorapatite increases acid resistance in teeth.21 The acid resistance test was conducted using a UV-Vis spectrometer by measuring the turbidity in solutions containing of F-HAp powders with 90% fluoride content at different pH values. In Figure 4, a low value of light transmittance indicates insoluble state of the sample powders, whereas conversely, a high value shows a soluble state. The F-HAp and original HAp started to dissolve at around pH 3.4 and 4.2, respectively. Thus, the acid resistance of F-HAp is higher than that of original HAp.
The dissolution rate of synthetic F-HAp with various fluoride content in acetate buffer solution at pH 4.0 decreased with degree of fluoridation.22 The solubility of fluoridated CO3 apatite in acetate buffer solutions at pH 4.0 decreased with increasing degree of fluoridation.23 The initial dissolution of F-HAp consisted of the preferential removal of F and Ca. The final destruction of F-HAp resulted from the breaking of Ca-O bonds in the Ca- and F-depleted surface.24 Chen and Miao25 explained about chemical stability of F-HAp compared with HAp by considering crystal structure. The hydrogen ions in HAp arrange in the atomic interstices neighboring the oxygen ions of hydroxyl groups. Then, the hydrogen ions orient randomly in the HAp crystal structure. When the fluoride ions partially substitute the hydroxyl groups, they interact strongly with the hydrogen ions because of their higher affinity, resulting in well-ordered crystalline structures. Thus, F-HAp obtains higher chemical stability, such as acid resistance, compared with original HAp. The F-HAp coating nanolayer on medical devices is expected to form stable surface in a living body.
The antibacterial effects of fluoride-releasing dental materials, such as glass ionomer cements, resin-modified glass ionomer cements, and polyacid-modified composites, have reviewed elsewhere.26 Stanić et al.27 reported the antibacterial activity of the fluorine-substituted HAp nanopowders without calcination increased with the increase in fluoride concentration and the decreased pH of saline. In our study, crystalline and dispersible F-HAp nanoparticles prepared using an antisintering agent during calcination at 800°C for 2 h were subjected to antibacterial evaluation. To evaluate the antimicrobial effects of F-HAp, wild-type E. coli cells were incubated with F-HAp nanoparticles for 2 h, and cell sterilization rates were measured (Figure 5; see Figure S3A, Supplemental Digital Content 3, http://links.lww.com/ASAIO/A87). It was remarkably shown that >99% of the E. coli was sterilized in 50 mg/100 µl F-HAp (corresponding to 90 mM of fluoride if F-HAp thoroughly dissolved), whereas no diminution was observed with original HAp (Figure 5) and with 90 mM NaF under the same conditions (see Figure S3B, Supplemental Digital Content 3, http://links.lww.com/ASAIO/A87). However, the E. coli strain could not grow in the medium containing 90 mM NaF (see Figure S3C, Supplemental Digital Content 3, http://links.lww.com/ASAIO/A87). Thus, it seemed that F-HAp could provide the combined synergistic antimicrobial effect of fluoride and HAp nanoparticles under 2 h of contact.
Fluoride is widely used as an antimicrobial agent affecting metabolic enzymes in multiple pathways of bacteria, such as glycolysis enzymes, heme-containing enzymes, and F1/0-ATP synthases.28,29 However, the mechanism of antimicrobial effects are still unknown. A clear size-dependent antimicrobial activity was reported with the magnesium fluoride nanoparticles.30 Therefore, the critical mechanism of F-HAp nanoparticles for the synergistic antimicrobial effects might depend on the particle size and distribution. The relationship between the antibacterial activity and the acid resistance of F-HAp is also unknown. After the F-HAp nanoparticles have penetrated into the cell membrane of bacteria, it was thought that fluoride ions released from the nanoparticles, which were dissolved in the cytoplasm, might damage the metabolic enzymes in bacteria.
Regarding antibacterial concentration of ions, it has been reported that silver, which is one of the most popular antibacterial regents, can exhibit effective antibacterial activity at a concentration level of 0.1 ppb.31 In comparison with the concentration values, the antibacterial activity of F-HAp was much lower at approximately 1/107 than that of silver. Additional works for antibacterial and proliferation tests on F-HAp powder and the coating sheets are now conducted by using other bacteria for catheter-related infection, such as Staphylococcus aureus, Staphylococcus epidermidis, Serratia marcescens, and Pseudomonas aeruginosa besides E. coli. The dispersed F-HAp nanoparticles possessing weak antimicrobial activity produced herein can be potentially useful without severe damage to living tissue for coating nanomaterials on substrates of implantable medical devices, such as catheter, left ventricular assist system, artificial blood vessel, and artificial bone.
Crystalline and dispersible F-doped HAp nanopowders, F-HAp, were prepared using antisintering agent in a wet chemical reaction method via calcination. The average morphology of the primary F-HAp nanoparticle was rod shape with a length of 367 ± 67 nm and a width of 223 ± 21 nm, as determined by SEM images. The nanoparticles were also shown to be well separated in ethanol. The F-HAp with a fluoride content of 90% obtained higher acid tolerance compared with the original HAp. To evaluate the antibacterial activity of F-HAp, E. coli was incubated with F-HAp nanoparticles for 2 h. Remarkably, >99% of the E. coli was sterilized by F-HAp compared with the original HAp. From the antibacterial activity, which was higher than that at the same concentration of NaF, F-HAp seemed to show combined synergistic antimicrobial effects of fluoride and HAp. The antibacterial activity proved to be much weaker than that of popular metallic antibacterial regents, such as silver.
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