We investigate by different complementary methods the processes occurring when a polydimethylsiloxane film is used as interlayer for a silver doped hydroxyapatite coating. The X-ray diffraction and Fourier Transform Infrared Spectroscopy measurements show that the hydroxyapatite doped with silver is in a crystalline form and some
Hydroxyapatite (HAp, Ca10(PO4)6(OH)2) is a biomaterial with a wide range of applications in medicine due to its biocompatibility, bioactivity, and osteoconductivity [
From an antibacterial point of view, silver nanoparticles are widely used in medical devices and supplies such as wound dressings, scaffold, skin donation, recipient sites, and sterilized materials in hospitals, medical catheters, contraceptive devices, surgical instruments, bone prostheses, artificial teeth, and bone coating.
Recently, the use of inorganic antibacterial agents, like silver or copper, incorporated in the structure of hydroxyapatite has been shown to be of great interest in the fight against microbes [
The most common technique to incorporate Ag into HAp structure is via an ion exchange method, in which the Ca ions in HAp are replaced by Ag ions while dipping the HAp coatings into AgNO3 for a period of time [
In our previous studies [
Coatings of HAp have been deposited as amorphous layers by various techniques like plasma-spray technique, pulsed laser deposition, electrodeposition, sol-gel processing, and radio-frequency magnetron sputtering [
In order to improve the delamination of HAp coatings different types of interlayers between the substrate and the HAp coating have been used as reinforcement agents. For example, SiO2 layers are known for their own excellent compatibility with the living tissues and their high chemical inertness [
The polydimethylsiloxane (PDMS) is an elastomer with biocompatible properties and is frequently used as substrate for biological studies [
In this paper a method for generation of a Ag:HAp-PDMS composite layer by thermal evaporation of Ag:HAp nanoparticles and their deposition on the surface of a pure Si disk substrate previously covered with a PDMS layer is presented. The role of the polymer layer and the associated physicochemical processes that take place during the interaction of the hydroxyapatite vapours with the polymer are investigated by Scanning Electron Microscopy (SEM), X-ray diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR), and Glow Discharge Optical Emission Spectroscopy (GDOES).
The PDMS layers have been generated in atmospheric air pressure corona discharges starting from liquid precursors of vinyl terminated polydimethylsiloxane. The method and the experimental conditions used for the deposition of thin PDMS layers (with an average thickness in the hundred nanometers range) on metallic substrates were presented in detail in [
In order to synthesize the silver doped hydroxyapatite (Ag:HAp) precursors of calcium nitrate [Ca(NO3)2·4H2O, Aldrich, USA], ammonium hydrogen phosphate ((NH4)2HPO4; Wako Pure Chemical Industries Ltd.) and AgNO3 (Alpha Aesare, Germany, 99.99% purity) were used. Controlled amounts of ammonium hydrogen phosphate and silver nitrate were dissolved in ethanol. After adding distilled water, the solution was stirred vigorously for 24 h at 40°C. In a separate container, a stoichiometric amount of calcium nitrate was dissolved in ethanol with vigorous stirring for 24 h at 40°C. The Ca-containing solution was added slowly to the P-containing solution and then aged at room temperature for 72 h and further at 40°C for 24 h. The composition ratios in the Ag:HAp (
Starting from an Ag:HAp (
The evaporation time measured during deposition is situated in the range of 20 sec for a maximum current intensity of
The morphology of the material was studied using a Quanta Inspect F Scanning Electron Microscope (SEM) equipped within gun beam in emission field, with wolfram filament and with a linear resolution of 5 nm. A part (10 mm) of a Si substrate with a diameter of 20 mm was covered with a PDMS layer by the method presented above. Then an Ag:HAp layer was deposited by thermal evaporation technique on the entire surface of the Si substrate. The XRD patterns were recorded in the range of 25–55° using a Philips PW1830 diffractometer with filtered Cu K
An accurate conversion of sputtering time into sputtered depth is not straightforward because the sputtering rate is material dependent and varies during the elemental depth profiling measurement.
In Figure
SEM image of (a) interface between the Ag:HAp-PDMS composite layer and the Ag:HAp layer and (b) Ag:HAp-PDMS composite layer.
XRD patterns of the Ag:HAp nanopowder and the Ag:HAp-PDMS composite layer are shown in Figure
XRD diffraction patterns of the Ag:HAp nanopowder (a) and the Ag:HAp-PDMS composite layer (b).
In the following section we will present the FT-IR analysis of each component that constitutes the Ag:HAp-PDMS composite layer.
First, we investigated the polymer structure. The IR spectra and the IR bands assignment of the polymer generated from liquid precursors of vinyl terminated PDMS on a silicon substrate function of the angle of the incident light on the sample are presented in Figure
FT-IR bands associated with the functional groups present in the structure of Ag:HAp-PDMS composite layer.
Wavenumber (cm−1) | IR bands assignment | |
---|---|---|
PDMS layer | Ag:HAP-PDMS layer | |
490 | — |
|
498, 512, 543, 550 | — |
|
562, 590, 602, 611 | — | P-O bending vibrations in |
620 |
|
— |
695 | — | Crystallinity of SiO2 type materials [ |
780 | — | Si-O-P vibration [ |
784 | Si-C stretching vibration [ |
— |
814, 816, 817.5 | — | Si-O-Si bonds [ |
828 | — | Si-O-Ag [ |
860 | Si-CH3 rocking vibration [ |
— |
862 | — | Si-O, Si-C vibrations [ |
870 | — |
|
877 | — |
|
889 | Si-O bending vibration [ |
— |
925, 945, 975 | — | P-O vibrations in |
1004, 1071 | Si-O-Si vibrations [ |
— |
1000, 1044, 1067, 1085 | — |
|
1140 | — | Si-O vibrations [ |
1152 | Si-O vibrations [ |
— |
1254, 1400 | CH3 vibrations in Si=CH3 group [ |
CH3 vibrations in Si=CH3 group [ |
1650 | H2O, OH, Si-OH [ |
H2O, OH, Si-OH [ |
2912, 2964 | C-H vibrations [ |
— |
IR spectra of a polymer generated from a vinyl terminated polydimethylsiloxane liquid precursors on a silicon substrate for different reflection angles.
A possible pathway of the polymerization mechanism of the vinyl terminated PDMS precursors involves the breaking of the double bond of the end group and thus the further linkage of the polymeric chains [
As we increase the angle of incident light on the sample up to 60°, it can be observed that the band from 1152 cm−1 increases and the band from 860 cm−1 decreases appearing like a shoulder on the new 880 cm−1 band. The increase of the intensities of the 1152 cm−1 and 880 cm−1 IR bands accompanied by the decrease of 860 cm−1 IR band can be understood knowing that, during the polymerization process of PDMS in negative corona discharge, the formation of the new Si-O bonds due to the injection of the negative ions in the liquid precursor bulk is associated with the diminishing of Si-CH3 bonds [
In the same time, in [
Therefore, the band from 1650 cm−1 present in the IR spectrum of the PDMS polymer, assigned to partly hydrated silica and to the stretching vibrations of adsorbed OH group respectively (Si-OH) [
Second, we performed FT-IR analysis on the Ag:HAp-PDMS composite layer. The IR spectra are presented in Figure
FTIR spectra of the Ag:HAp-PDMS composite layer function of the angle of the incident light on the sample.
The IR band from 1152 cm−1, Figure
The formation of
The 1004 and 1071 cm−1 IR bands (Figure
In comparison with the IR spectrum of the PDMS layer (Figure
The IR band from 1650 cm−1 observed in the spectrum of the PDMS layer (Figure
According to previous studies [
FT-IR deconvoluted spectra of the Ag:HAp-PDMS composite layer in the spectral regions: (a) 450–650 cm−1, (b) 900–1200 cm−1, (c) 810–820 cm−1, (d) 822–835 cm−1, and (e) 850–890 cm−1, for an incident angle of the light on the sample of 60°. The experimental curve is plotted in red and calculated theoretical bands are in blue.
In Figures
The results of the peak fitting analysis presented in Figure
In Figure
The peaks associated with all the functional groups present in the IR spectra of Ag:Hap-PDMS composite layer are summarized in Table
In Figure
GDOES spectra of (a) PDMS layer and (b) Ag:HAp-PDMS composite layer, generated on a Si substrate.
The depth profiles of the P, Ca, Ag, and O atoms in a Ag:HAp layer were presented elsewhere [
In Figure
Compared to Figure
This observation is an indication of silicon atoms involvement not only in the silicon oxide structures, Si-O-Ag or Si-O-P bonds formation, but also in the structure of the Ag:HAp-PDMS composite layer, in accordance with FT-IR investigations. A plateau in a profile curve followed by an increase is an indication of substrate interface. This kind of behaviour of a certain depth profile curve when all the other depth profile curves decrease is generally observed when an element contained in the investigated layer is also present in the substrate material [
In the current study we have presented for the first time the formation of an Ag:HAp-PDMS composite layer by deposition of Ag:HAp nanoparticles on a substrate previously coated with a PDMS layer via a simple and reproducible method. The studies of the Ag:HAp-PDMS composite layer are captivating, mainly because of their interesting physicochemical properties and unique structure, which make them attractive for many applications. Due to the presence of Ag in the layer structure, this compound presents a great interest in future biological and biochemical studies. In previous studies [
In this paper a method for the deposition of a Ag:HAp coating on a substrate previously covered with a PDMS layer is presented. As the SEM investigations showed, the PDMS layer acts as a matrix in which the Ag:Hap is incorporated. In this way, a Ag:HAp-PDMS composite layer is formed. The XRD measurements showed that the crystalline form of the Ag-HAP is maintained in the composite layer.
By FT-IR and GDOES spectral techniques we investigated the physicochemical processes that take place during the interaction of Ag:HAp with the PDMS layer. The FT-IR analysis, in agreement with the XRD measurements, showed that the physical procedure used for the generation of the Ag:HAp-PDMS composite layer is useful for the formation of
The GDOES depth profiling curves of the Ag:HAp-PDMS composite layer indicate that its composition is homogeneous which can be explained by the formation of the Si-O-Ag and Si-O-P bonds, respectively, and the Si involvement in Ag:HAp structure.
The authors declare that there is no conflict of interests regarding the publication of this paper. The authors also declare that they do not have any other conflicts to declare.
The financial and encouragement support provided by the Ministry of Education of Romania, Project nos. PN-II-ID-PCE-2011-3-0958 and PT-PCCA-2011-3.1-1136, is acknowledged.