Calcium phosphate formation on plasma immersion ion implanted low density polyethylene and polytetrafluorethylene surfaces
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The flexible structure of polymers has enabled them to be useful in a wide variety of medical applications due to the possibility to tailor their properties to suit desired applications. For a long time, there has been an increasing interest in utilizing polymers as matrices for calcium phosphate-based composites with applications in hard tissue implants. On the other side, polymers with application as heart valves, urea catheters and artificial vessels present a case where the formation of minerals (namely calcification) should be avoided. The modification of polymer surfaces by various ion beam treatments for reducing the calcification, as for example plasma immersion ion implantation (PIII), is well known and has a long time effect. This work is part of a wider investigation of the ability of plasma immersion ion implanted polymers to induce calcium phosphate formation from an aqueous solution resembling the human blood plasma. In the experiment described in this paper, topographical and chemical changes were inserted on the surfaces of two conventional polymers (low density polyethylene and polytetrafluorethylene) by PIII with nitrogen ions, and under conditions mimicking the natural mineral formation processes. The effect of the plasma modification on the calcium phosphate nucleation and growth from the aqueous solution was ambiguous. We suppose that the complex combination of surface characteristics influenced the ability of the plasma treated polymer films to induce the formation of a calcium phosphate layer.
Ion beam implantation (IBI) is known as a method for controlled and selective surface modification of polymer materials without altering their bulk properties. Modified polymer surfaces find specific applications in the human body as heart valves, artificial vessels, urea catheters, mammary, face and finger prosthesis [1, 2, 3, 4, 5]. Among IBI, plasma immersion ion implantation (PIII) is frequently used to convert hydrophobic polymers to hydrophilic. This process can also improve the adhesion strength, biocompatibility, and other pertinent properties, as well as to change a polymer hydrophilic behavior to hydrophobic [1, 6, 7, 8]. It is known that three processes take place when a material (polymer or other) is introduced into a living organism, where the ambient media are generally aqueous solutions. These are: rapid protein absorption as monolayers on the surfaces, cell attachment [9, 10, 11, 12] and mineralization of the surface . Mineralization in nature is a process involving complex interactions between inorganic ions, crystals and organic molecules in an aqueous media. The organic and inorganic counterparts are continuously interacting with each other and the result is the formation of mineralized tissues with convoluted architecture and distinguished microscopic design [13, 14, 15, 16]. In the case of polymers introduced into a living organism as soft tissue implants, the biomineralization process, including the formation of calcium or calcium phosphate deposits, on their surface is not desired. For example, calcium deposits or protein layers formed on the internal sides of artificial vessels can obstruct the blood circulation; calcification can decrease the elasticity of soft tissue implants like mammary prosthesis, which brings pain to the patient and limits the implant life-time. Heart valve operations can be unsuccessful after time due to calcification of the valve walls which make them non-elastic [4, 17, 18]. Therefore, the prevention of the mineralization on the polymer surfaces in the body is attributed to the success of some medical polymer implants.
This work is part of a wider investigation of the ability of PIII treated polymers (low density polyethylene, polytetrafluorethylene, 2103 and 2363 commercial Pelletane materials with medical applications and polyurethane synthesized from polypropyleneglycol and toluenediisocyanate, PPG-TDI) to induce calcium phosphate formation from an aqueous solution resembling the human blood plasma. In this paper we study the modification of two conventional polymers, low density polyethylene and polytetrafluorethylene, by PIII with nitrogen ions, and their ability for calcium phosphate formation after the modification upon immersion in an aqueous solution of inorganic salts. This approach was chosen because it mimics the natural mineral formation processes.
Low density polyethylene (LDPE; Goodfellow, England) and polytetrafluorethylene (PTFE; Halogen, Russia) films with thickness of 50 and 20 μm, respectively, were used in the experiments. The surface of the films was cleaned by distilled water, alcohol and acetone, and air dried before surface modification.
LDPE and PTFE films were modified by PIII with various doses of 20 keV nitrogen ions (5 × 1014, 5 × 1015, 2 × 1016 and 5 × 1016 ions/cm−2). Doses were varied by the pulse frequency. The depth of the modified layer on LDPE and PTFE surfaces at 20 keV nitrogen ion implantation is equal to 90 nm according to calculations with TRIM code. Sample holder had additional electrode and a metal grid that excluded direct contact of samples with radio frequency plasma between the high voltage pulses and also avoided charging effect during high voltage pulse. The grid was placed on 40 mm distance over the polymer films which excluded shadow effect on the modified surfaces. Residual atmosphere and working pressure of nitrogen at PIII were 1 × 10−3 and 5 × 10−1 Pa, respectively. Other conditions of PIII were 5 μs pulse duration, 200 W plasma power, 13.56 MHz radio frequency plasma, and full time in plasma discharge of 10 min for all doses. Pulse repetition frequency from 0.2 Hz to 100 Hz was used to exclude overheating during the PIII treatment. Non-implanted films were used as control samples.
Calcium phosphate growth
Ion concentrations of the human blood plasma and the SBF used for calcium phosphate growth on the polymers films
Ion concentrations (mM)
The analysis of the polymer films and of the effect of the ion doses on the calcium phosphate formation was examined by contact angle measurements, AFM, optical microscopy, FTIR in ATR and transmission modes, and micro-Raman spectroscopy.
The surface wettability of the polymer films after PIII and before layer growth was studied by a contact angle measuring system DSA10 (Kruss, Germany) with water drops and by using the method of sessile drop. The contact angle was determined by analysis of the drop geometry through video-imaging and calculation by Rabel approximation model of water drop form.
AFM observations were carried out on DMA scanning probe optical microscope DualScope C-21 in tapping mode under ambient conditions.
FTIR transmission spectra were recorded on Nicolet Magna 750 spectrometer, using 100 scans and resolution of 2 cm−1. FTIR ATR spectra were recorded with ZnSe crystal in ATR accessory and angle of light beam accident 45°. Spectra were analyzed by OMNIC Nicolet software.
Micro-Raman spectra were obtained in a backscattering mode (λ = 532,14 nm) on diffraction double monochromator spectrometer HR800, Jobin Yvon, LabRam System 010. Spectral resolution was 4 cm−1. Number of scans and integration time were varied for sufficient signal-to-noise ratio. Optical microscope Olympus BX40 with objectives of 50× and 100× was used for imaging of the surfaces. LabRam software was used for spectra analysis.
Optical microscope Nikon Eclipse ME600 with magnification objectives of 5×, 20× and 50× was used to observe the topography of the polymer films before and after their immersion in the SBF. Digital video camera was used for microphoto image recording.
Results and discussion
Change of the wettability properties (contact angle θ, °) of the LDPE and PTFE films with the PIII doses
Doses of PIII (N+/cm2)
1 × 1014
1 × 1015
1 × 1016
92.8 ± 2.0°
56.8 ± 5.1°
58.3 ± 7.7°
78.3 ± 3.1°
112.4 ± 0.3°
97.5 ± 0.3°
81.2 ± 5.5°
126.9 ± 0.1°
FTIR ATR spectra of the modified LDPE films (Fig. 2a, spectra 2–5) revealed significantly increased intensity of the C–O, C=O, C=C and O–H stretching vibrations (the regions at 980–1260 cm−1, 1660–1780 cm−1, 1600–1630 cm−1 and 3120–3700 cm−1, respectively). This result shows that the concentration of the hydrophilic groups (C–O, C=O, and O–H) was increased. In addition, lines at 910 and 966 cm−1 due to unsaturated C–H groups (vinyl and vinylene) appeared which correspond to the presence of end-groups of polyethylene macromolecules as a result of polymerization process from ethylene. The increased content of oxygen-containing and double carbon-carbon groups was assigned to the PIII treatment which has broken polymer chains and made the surfaces highly reactive through the energetic ion bombardment. Subsequently, chemical reactions between the unstable reactive radicals, and oxygen and carbon from the air took place, resulting in stronger oxidation of the top surface (increased concentration of surface hydrophilic functional groups, such as C=O, C–O and O–H) under storage in air after the PIII treatment and carbonization of the underlying surface layers [21, 27].
FTIR ATR spectra of the control PTFE film (Fig. 2b, spectrum 1) revealed basic C–F stretching modes of fluorethylene macromolecules as very strong lines at 620/640 (doublet) and in the region of 1100–1300 cm−1. C–O and C=O stretching modes were present with very low intensity in the regions of 830–970 and 1600–1800 cm−1. Very weak lines were also observed at 1420, 1459 and 1599 cm−1 as bending vibrations due to hydrocarbon traces. In the spectra of the PIII modified PTFE films (spectra 2–5), the lines due to oxygen-containing groups (C=O stretching modes in carbonyl, carboxyl, ketone, aldehyde, ether and esther) appeared in the region 1600–1810 cm−1 with increased intensity which means that the concentration of the hydrophilic groups becomes higher. Stretching due to unsaturated C = C groups was also detected in this region. These features reflect oxidation and carbonization processes in the PTFE in a similar manner to the LDPE surface, increased under the energetic bombardment [21, 27].
In summary, the results showed explicitly that the modification of the LDPE and PTFE films by PIII with nitrogen ions induced topographical and chemical changes on the polymer surfaces. One explanation for the measured higher contact angle of the films implanted with the highest dose is that the implantation with the highest dose could have inserted significant changes in the polymer topography (deep and narrow rough features), which do not allow the water drop to bridge the gaps and spread well on the surfaces . In addition, the highest dose treatment may has removed weakly bonded layers that may contribute to the restored hydrophobicity . Good wettability is known as a prerequisite for the crystal nucleation from aqueous solutions and for the compatibility of the biomaterials with aqueous biological media [37, 38]. However, the contact angle measurement and the optical microscopy images for both polymers showed increased calcium phosphate concentration on the hydrophobic surfaces. The PIII treatment broke polymer chains and created unstable radicals. With the highest dose this effect is expected to be stronger. Upon immersion in the SBF these surfaces could have been more reactive in respect to active species and/or water molecules in the supersaturated aqueous environment (post reaction of the free radicals) which has probably yielded the nucleation of more crystals. Our current work on the modification of 2103 and 2363 Pelletane materials, and PPG-TDI polyurethanes with PIII of nitrogen and the subsequent growth of calcium phosphate layers from SBF is expected to give more details on the observed processes.
In this study, topographical and chemical changes were inserted on the surfaces of LDPE and PTFE films by PIII with nitrogen ions in order to examine their ability to induce the formation of calcium phosphate at biological conditions (from aqueous solution, resembling the blood plasma, at pH of 7.4 and 37 °C). The results obtained showed that the effect of the PIII modification on the crystal nucleation and growth of calcium phosphate from the SBF solution is ambiguous. The effect is most probably based on a complex combination of surface characteristics which influences the ability of the PIII treated polymer films to induce the formation of a calcium phosphate layer. Our current work on the PIII with nitrogen ions of two types of Pelletanes and of newly synthesized polyurethanes is expected to clarify the observed processes.
This research was supported partly by the Bulgarian National Scientific Research Fund through Grant L1213/2002. Plasma implantation chamber at the Institute of Ion Beam Physics and Materials Research, Forschungszentrum Rossendorf, Dresden, Germany was used in the experiment.
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