Synthesis of biocompatible surfaces by nanotechnology methods

More documents

Recommendations

Info

SYNTHESIS OF BIOCOMPATIBLE SURFACES BY NANOTECHNOLOGY METHODS 697Albumin(1)Fibrinogen(2)Carbon cluster(3)K ads K desK ads K desK ads K desK ads Kdes1of albumin molecules in competition with fibrinogenmolecules.As can be seen from Fig. 1a, both albumin andfibrinogen molecules adsorb on the initial polymersurface. Fibrinogen molecules, which are 4–5 timesgreater than albumin molecules (~50 nm against~9 nm, respectively), are, for thermodynamic reasons,almost irreversibly adsorbed so that they stick to thesubstrate with their maximum surface area. In contrast,part of the modified surface (Fig. 1b) is occupiedby carbon clusters so that fibrinogen molecules have(a)(b)Fig. 1. Schematic diagram illustrating the adsorption–desorptionof (1) albumin and (2) fibrinogen molecules on the(a) initial and (b) modified polymer surface containingcarbon clusters (3). Arrows indicate the relative rate constantsof adsorption (K ads ) and desorption (K des ) for albuminand fibrinogen.32no area to settle and stick, whereas significantlysmaller albumin molecules can readily adsorb on carbon-freesurface sites. Practical implementation ofthis adsorption scheme would improve the hemocompatibilityof polymer surfaces and eliminate the formationof thrombuses.In order to provide conditions for the selective(competitive) adsorption of albumin and fibrinogenmolecules as depicted in Fig. 1, it is necessary to producethe nanostructurization of the substrate surfaceby carbon clusters so that the dimensions of open(adsorption-accessible) surface regions between theclusters are be on the order of several units to severaldozen nanometers.As is known, the ion–plasma deposition of nanostructuralcarbon coatings in a pulsed rather than continuousregime makes it possible to quite simply controlthe degree of carbon supersaturation (Δg) in thegas phase and, hence, the mechanism of initial growthstages [5]. Indeed, as low Δg values, the carbon filmgrowth may proceed via the layered or two-dimensional(2D) mechanism of nucleation and growth. Theprobability of this process is characterized by the isobaric–isothermalpotential ΔG 2 ~ 1/Δg. As the Δgvalue increases, the mechanism can change from 2Dto 3D, in which case ΔG 3 ~ 1/Δg 2 (power dependence)and the conditions are more nonequilibrium. This circumstanceimplies a greater possibility that the carbon-coatedsurface morphology will be influenced bythe varying conditions of the technological process. Inaddition, the pulsed regime facilitates the removal ofheat from the substrate surface, since the period oftime between pulses is an order of magnitude greater(as characterized by the off/on ratio) than the pulseduration. The parameters of this technological processcan be optimized by studying the surface morphologyand introducing changes in the process so as to obtainthe desired surface relief.Figure 2 schematically illustrates the dynamics offilm nucleation and growth according to the Volmer–Weber (island growth) mechanism. At the stage of car-(a) (b) (c) (d)Fig. 2. Schematic diagram of the island nucleation and film growth mechanism: (a) nucleation and growth of carbon clusterswithout mutual interactions; (b) lateral growth and island formation; (c) coalesce of islands at a sufficiently large surface coverageand the formation of a network with electrically continuous structure capable of conducting electric current (percolation threshold);(d) formation of a continuous film.NANOTECHNOLOGIES IN RUSSIA Vol. 5 Nos. 9–10 2010
698ALEKHIN et al.bon coating formation, it is necessary to determine theso-called percolation threshold (see Fig. 2c), wherebya carbon network that forms an electrically continuousstructure capable of conducting electric current isformed, typically when the substrate is covered up to70–80%. Why is determining the percolation thresholdso important? Because this point at the stage ofcarbon deposition corresponds to the formation of amosaic carbon nanostructure with cluster dimensionsin a nanometer range. This situation leads to a changein the physicochemical and biomedical properties,thus making it possible to control the hemocompatibilityof the carbon-modified surface [7, 8]. It is highlyprobable that the percolation threshold determines thefinal stage of formation of a hemocompatible mosaiccluster structure on the substrate surface. In this study,the percolation threshold was determined as a functionof the number N and the repetition frequency f ofpulses of the carbon plasma generator. The thicknessof the deposited carbon coating varied within 0.3–15 nm. The proposed technology was experimentallyimplemented and patented [6].DETERMINING THE TECHNOLOGICALPARAMETERS AND PHYSICALAND BIOMEDICAL PROPERTIESOF PLASMA-DEPOSITED CARBONNANOSTRUCTURESWe have experimentally determined the optimumtechnological parameters for the process of LDPE andPU modification by carbon clusters to reach a percolationthreshold as the pulse repetition frequency f =1 Hz and their number N = 50, which corresponded toan effective carbon coating thickness of ~7.5 nm. Thesurface morphology of samples was studied by scanningelectron microscopy (SEM) and scanning probemicroscopy (SPM).The SEM measurements were performed on aCamscan 4 (Cambridge Instruments) microscopewith a typical magnification of ×1000–5000, whichcorresponds to the size of visible fields on a level ofseveral tens of microns. By studying the characteristicfeatures of morphology, it is possible to evaluate theoptimum technological parameters, introduce thenecessary corrections, and obtain the surface reliefnecessary for further research. It was established thatthe relief of modified polymer surfaces was significantlydependent on the number N of carbon plasmapulses and their repetition frequency f. The dynamicsof variation of the surface relief exhibited the followingtrends. At a small amount of the deposited material(N = 2–10), the necessary surface morphology (correspondingto Fig. 2c) is not formed. In this case, thecarbon-modified polymer surface has a globular structure,i.e., exhibits separate clusters with dimensionswithin 100–200 nm. Large values of the pulse repetitionfrequency and number ( f ≥ 10 Hz, N ≥ 100) correspondedto the formation of structures representingeither fibrils (3000–4000 nm long) or continuouschains. The most favorable regimes were those with f =0.3–1.0 Hz and N = 30–50.Direct evidence for the formation of a carbon clusterstructure on the LDPE surface was provided by theresults of SPM investigations in air at room temperature.These measurements were performed on aSolver TM Pro M (NT-MDT Company) instrumentoperating in a tapping mode with 2D and 3D imaging.The cantilever had an elasticity coefficient of about20 N/m and a resonance frequency of 131.851 kHz;the point probe had a tip radius of several nanometers.The minimum spatial resolution of the microscope, asevaluated by the size of the minimum visible elementsof the sample surface, was about 10 nm.Figures 3 and 4 present typical 2D images obtainedby an SPM examination of LDPE and PU modified atvarious repetition frequencies ( f = 0.1–1.0 Hz) andnumbers (N = 30–50) of carbon plasma pulses. As thepulse repetition frequency was increased from 0.1 to1.0 Hz and/or their number was increased from 2 to50, the size of the deposited carbon clusters or theiraggregates, as well as the degree of surface coverage bycarbon, increased to 70–80%. This led to the formationof a mosaic carbon network with an electricallyconducting structure.Thus, based on our results, it was concluded thatthe optimum pulse repetition frequency and carbondeposition velocity for the formation of the desiredcluster structure on the polymer surface are 0.3–1 Hzand 0.1–0.2 nm/s, respectively.Additional information on the structure of modifiedcarbon coatings on polymer substrates wasobtained by mathematically processing their Ramanspectra [9]. An analysis of parameters such as theRaman shift and the heights and widths of model spectrumcomponents (Fig. 5), which were determinedusing the results of Tamba et al. [10], allowed us tomake the following conclusions concerning the structureof layers formed on polymer substrates. It was evidentthat both sp 2 and sp 3 hybridization is present incarbon deposits. According to the general principles ofthermodynamics, structural elements with the sp 2 typeof hybridization form small-size clusters. Of all thepossible clusters of this kind, the most probable aresix-member rings, which can be either separate orconnected into sets of several rings. These rings maypossess angular misorientation and can be shaped likeirregular hexagons. These clusters are bound by elementsof the sp 3 hybridization and can be chaoticallyoriented relative to each other.Medico-technical investigations were performed atthe Research Institute of Transplantology and ArtificialOrgans (Ministry of Public Health of the RussianFederation, Moscow), which involved (i) an analysisof the thrombocyte adhesion with allowance for theirmorphological characteristics and (ii) a determinationNANOTECHNOLOGIES IN RUSSIA Vol. 5 Nos. 9–10 2010
Page 4 and 5: SYNTHESIS OF BIOCOMPATIBLE SURFACES

Synthesis of biocompatible surfaces by nanotechnology methods

Create successful ePaper yourself

Delete template?

Save as template?