Biomaterial thin film deposition and characterization by means of MAPLE technique

Biomaterial thin film deposition and characterization by means of MAPLE technique
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  Biomaterial thin film deposition and characterization by means of MAPLE technique F. Bloisi a, ⁎ , L. Vicari a  , R. Papa a  , V. Califano  b , R. Pedrazzani c , E. Bontempi c , L.E. Depero c a  CNR-INFM Coherentia  —   Napoli, Dip. Scienze Fisiche  —   Univ. Napoli  “   Federico II  ”  , P.le V.Tecchio, 80  —   80125 Napoli, Italy  b  Dip. Scienze Fisiche  —   Univ. Napoli  “   Federico II  ”  , P.le V.Tecchio, 80  —   80125 Napoli, Italy c  Laboratorio di Chimica per le Tecnologie, Universita' degli Studi di Brescia, Via Branze, 38  —   25123 Brescia, Italy Received 5 May 2006; accepted 5 December 2006Available online 10 January 2007 Abstract Polyethylene glycol (PEG) is a polymer with technologically important applications, especially as a biomaterial. Several biomedical applications(such as tissue engineering, spatial patterning of cells, anti-biofouling and biocompatible coatings) require the application of high quality PEG thinfilms. In order to have a good adhesion to substrate chemically modified polymer molecules have been used, but for some  “ in vivo ”  applications it isessential to deposit a film with the same chemical and structural properties of bulk PEG. Pulsed laser deposition (PLD) technique is generally able to produce highquality thin films but it isinadequate for polymer/organic molecules. MAPLE (Matrix Assisted Pulsed Laser Evaporation) is a recentlydeveloped PLD based thin film deposition technique, particularly well suited for organic/polymer thin film deposition. Up to now MAPLEdepositions have been carried out mainly by means of modified PLD systems, using excimer lasers operating in UV, but the use of less energeticradiations can minimize the photochemical decomposition of the polymer molecules. We have used a deposition system explicitly designed for MAPLEtechniqueconnectedtoaQ-switchedNg:YAGpulsedlaserwhichcanbeoperated atdifferentwavelengthrangingfromIR toUVinordertooptimisethedepositionparameters.ThecapabilityofMAPLEtechniquetodepositPEGhasbeenconfirmedandpreliminaryresultsshowthatvisible(532 nm wavelength) radiation gives better results with respect to UV (355 nm) radiation. Despite usually UV wavelengths have beenused and evenifmoresystematictestsmustbeperformed,itisimportanttounderlinethatthechoiceoflaserwavelengthplaysanimportantroleintheapplicationof MAPLE thin film deposition technique.© 2007 Published by Elsevier B.V.  Keywords:  MAPLE (Matrix Assisted Pulsed Laser Evaporation); Nd:YAG laser; PEG (polyethylene glycol); biomaterials; FTIR microscopy; X-ray reflectivity 1. Introduction Polyethylene glycol (PEG) is a polymer with technologicallyimportant applications, especially as a biomaterial (materialinterfacing with living tissues or biological fluids). Some of the biomedicalapplicationsofPEGare tissueengineering [1], spatial patterningofcells[2],anti-biofoulingandbiocompatiblecoatings[3], such as drug delivery coatings or orthopaedic implants.Biofouling is the cellular and proteinaceous adhesion on bio-material surface and is commonly avoided through the immobi-lization of anti-fouling polymers on the surface to protect.For these applications, thin films of high quality are required.In order to have a good adhesion of PEG film to substratechemically modified PEG has been used [4], but even thisapproachmaybeinadequateforsome “ invivo ” applications,suchas anti-biofouling: in these situations it is essential to deposit afilm with the same chemical and structural properties of bulk PEG. It is therefore required a technique allowing on one hand acontrol of the film morphology and thickness, and on the other hand the preservation of the chemical structure, the molecular weight distribution and the functionality of the bulk polymer.Matrix Assisted Pulsed Laser Evaporation (MAPLE) techniquewas shown to respond to both requirements [5 – 7].MAPLE is a recently developed thin film deposition tech-nique, slightly different from PLD (pulsed laser deposition), particularlywellsuitedfororganic/polymerthinfilmdeposition. Materials Science and Engineering C 27 (2007) 1185 – 1190www.elsevier.com/locate/msec ⁎  Corresponding author. Tel.: +39 081 76 82585; fax: +39 081 2391821.  E-mail address:  bloisi@na.infn.it  (F. Bloisi).0928-4931/$ - see front matter © 2007 Published by Elsevier B.V.doi:10.1016/j.msec.2006.12.005  InPLD,largelyappliedforinorganicthinfilmdeposition[8,9],a pulsedlaserbeamisfocusedontoasolidtargetwhosematerialisablated from target and deposited on a nearby substrate. Al-though some addition polymers [10] were successfully depos-ited, the PLD of addition polymers seems to proceed via a “ depolymerization – ablation of monomers – repolymerization ” mechanism. This is clearly not possible in general for conden-sation polymers or other organic molecules.In MAPLE deposition technique the pulsed laser beam isfocused on a target obtained as a frozen solution in a relativelyvolatile solvent of the molecules to be deposited. The advantageof MAPLE with respect to PLD technique is that a great part of the laser beam energy is transferred to the solvent and moleculesto be deposited are ejected from the target mainly due to solvent vaporization. Such technique is therefore especially well suitedfor deposition of large molecules as in organic or polymericcompounds, whose structure may be damaged by direct laser irradiation. It is therefore clear that great importance has thechoice of laser beam wavelength, even if up to now most MAPLE depositions have been carried out operating with ex-cimer lasers emitting either 248 nm [11,12] or 193 nm [12,13] wavelengths radiation. Using less energetic radiations canminimize the photochemical decomposition of the polymer molecules. MAPLE technique has already been successfullyapplied for PEG thin films deposition [5,7]. This work dealswith the optimisation of the parameters (mainly laser wave-length) for PEG deposition. Moreover, often MAPLE systemsare just a slight modification of an existing PLD system, whileour system has been explicitly designed for MAPLE technique,allowing the introduction of some useful specific characteristicssuch as  “ in situ ”  target freezing (controlled atmosphere or vacuum), use of different (currently 355 nm, and 266 nm) laser wavelength, 2D target movement (and consequent full target surface scanning). 2. The MAPLE deposition system In MAPLE technique (Fig. 1) the target is a frozen matrix,composed by a dilute solution of the material to be deposited inthe appropriate solvent. In this way most of the laser radiationenergy is absorbed by the solvent, limiting the damage to themolecules of interest. The process leads to the formation of a plume,composedofsolventmoleculesindifferentphases(i.e.gasmolecules and solid clusters [11,14]) which entrain the organic/  polymer molecules to deposit. During the target-to-substrate journey,the morevolatilesolventmoleculesarepumpedawaybya vacuum system, while the organic/polymer molecules aredeposited onto the substrate.Theidealsolventisthe onethathasnotendencytoformfilms,has a relatively high vapour pressure, is optically absorbing anddoes not photochemically interact with the organic/polymer molecules.MAPLE provides, respect to PLD, a softer desorptionmechanism since most of the incident radiation is absorbed bythe frozen solvent. With MAPLE technique it is possible toobtain thin, homogeneous, well adherent coatings over largesurfaces or selected areas with accurate thickness control, main-taining the chemical integrity and the physiochemical propertiesof the organic/polymer molecules deposited [5,11,12], thoughfor some polymers molecular weight distribution variationstowards lower molecular weights have been observed [11].Our MAPLE deposition system (Fig. 1) consists of a pulsedlaser deposition chamber with a target holder provided with a 2Dcomputer controlled movement system. Using this system, it is possible to perform a full target surface scanning, avoidingexcessive heating or erosion of a single spot on the target. Thetargetholder(insidevacuumchamber)isinthermalcontactwithaliquid nitrogen tank (outside vacuum chamber). This set-upallows an  “ in situ ”  target freezing procedure: the target holder is Fig. 1. Schematic of the MAPLE apparatus. The inset shows the MAPLE deposition process: the plume is composed by solvent and polymer molecules; the morevolatile solvent is pumped away by the vacuum system while polymer molecules are deposited onto the substrate.1186  F. Bloisi et al. / Materials Science and Engineering C 27 (2007) 1185  –  1190  filled with about 2 ml of solution and the chamber is closed; thechamber is filled with a dry inert gas (e.g. nitrogen or helium) inorder to avoid humidity condensation on target surface and toavoid solvent evaporation (the gas pressure can be set to therequired value); the target temperature is then reduced by fillingthe tank with liquid nitrogen and, only after solidification, thechamberpressureisreduced.Thedepositionchamberisprovidedwitha windowfor plume observation/analysisbyoptical (i.e. fast CCD with system upgrade) or spectroscopic (LIBS) techniques.The substrate ismountedparalleltothefrozentargetsurface,onadevice that allows a manual movement so that the target-to-substrate distance can be opportunely varied as required. A Q-switched Nd:YAG pulsed (pulse duration 6 ns, pulse repetitionrateupto10pulsespersecond)laseroperatingwitheitherthefirst (1064 nm wavelength) or the successive (532 nm, 355 nm)harmonicsisconnectedtothechamberbymeansofanarticulatedarm. The possibility of varying the laser wavelength offers theadvantage of choosing the more appropriate wavelength for thesolvent  – organic/polymer pair. In this set of MAPLE depositionsresults obtained UV (355 nm wavelength) and visible (532 nm,green) laser radiation have been compared. 3. Experimental PEG (chemical formula is reported in Fig. 2) depositions werecarried out at either 355 nm or 532 nm laser wavelength. About 2mloftargetsolutionwereplacedintothetargetholderwithinthevacuum chamber. PEG target was a solution of PEG (4.1 wt.%)andpropanol(4.0wt.%)inbidistilledwaterobtainedstartingfroma Polyethylene glycol 3000 monodisperse solution 10% in H 2 O(PEG solution is from Fluka, propanol is from Romil, bidistilledwater is from Carlo Erba). After target holder was filled, thevacuumchamberwasclosedandplacedinadrygas(helium)flowin order to reduce humidity. Target temperature was slowlyreduced up to a value ( ∼− 10 °C) below target melting point ( ∼ 0°C),fillingtheliquidnitrogentank.Whilethetemperaturestilllowered, pressure was reduced up to about 5 10 − 7 Torr. The finalvalues, before starting pulsed laser deposition, were  T  = − 187 °C,  p =5.3 10 − 7 Torr.At this time target movement and laseremissionstarted, while substrate was far (about 10 cm) apart from target inorder to clean target surface. Substrate was then placed close(about 1 cm) to the target and deposition started. Duringdeposition, the pressure inside vacuum chamber rose to about 2 10 − 6 Torr. Deposition parameters used for different samples aresummarized in Table 1.Deposed films were evaluated by means of FTIR microscopyand X-ray reflectivity, so as to control polymer chemical inte-grity and film thickness and homogeneity. A Hyperion 2000infrared microscope (Bruker) was used; spectral measurementsof surface were performed in reflectance mode, using a 15xSchwarzschild IR objective. X-ray reflectivity (XRR) spectrawere collected by a Bruker   “ 'D8 Advance ”  diffractometer equipped with a Goëbel mirror. 4. Results and discussion In Fig. 3 IR spectra of a drop of PEG dried on a microscopyglass are shown in comparison with the spectra of the glasssubstrate: the whole range and the fingerprint window 1600 – 900cm − 1 areshown.Figs.4,5and6reportspectraofsamplesS1,S2andS3,respectivelyobtainedbydifferentmeasurementsonthewhole area. In Fig. 7 the X-ray reflectivity (XRR) spectra of thesamples are shown.The spectral measurements were performed in reflectancemode, as previously stated; normally, reflectance spectra are of  poorer quality thantransmittance spectra, exceptfor thincoatingson metallic substrates. In fact, a typical non-metallic materialreflectsonlylessthan5%ofthe light,while the remaininglight isabsorbedbythesubstrate.Therefore,measurementsinreflectancemode are only suitable for samples with a very smooth surfacesuch as chips, polymers, and thin coatings on metal (as in our specific case, although substrate is glass). For this reason, besidethe studied molecule curve, CO 2  peak at about 2400 cm − 1 can beclearly identified and the background curve of microscopy glassremarkably affects molecule typical pattern. Nevertheless, themain identification peaks of PEG can be evidenced (Fig. 3): CH 2 antisymmetric stretching vibrationsbands appear between 2940 – 2855 cm − 1 and CH 2  antisymmetric scissoring vibrations bands between 1475 – 1445 cm − 1 ; the C – OH deformation vibrationyields an undefined absorption at 1400 – 1300 cm − 1 ; alcoholshavethe C – O singlebondincommonwithethers; thus,likewise,they show a strong band in the region of 1210 – 1000 cm − 1 ;moreover, C – O – C aliphatic ether antisymmetric stretchingvibration bands (1150 – 1060 cm − 1 ) can be identified; addition-ally,inphaseC – C – Ostretchingvibrationofprimaryalcoholscan be identified by 900 – 800 cm − 1  peak.IR analyses performed on sample S1 (Fig. 4) highlight itshomogeneity, since all curves have the same shape. Neverthe-less, the quantity of deposed PEG seems to be scarce, becausethe pattern of glass is prevailing. Anyhow, we can state that deposed PEG is unmodified with respect to the reference drieddrop, in fact all the main peaks can be recognised.SampleS2(Fig.5)appearslesshomogeneous(measurements performed on different areas yield a different response), but, at the same time, PEG peaks are more evident with respect tosampleS1.Thisphenomenoncanbeduetothehigheramountof  Fig. 2. Chemical formula of polyethylene glycol (PEG).Table 1MAPLE deposition parameters of polyethylene glycolSample S1 S2 S3Target solution PEG PEG PEGWavelength (nm) 355 355 532Pulse repetition rate (pulse/s) 10 10 10Pulse duration (ns) 6 6 6Pulse energy (mJ) 70 – 94 94 68Beam spot size  ∼ 2 mm 2 ∼ 2 mm 2 ∼ 0.8 cm 2 Total pulses  ∼ 20,000  ∼ 60,000  ∼ 60,000Scanned target area (cm 2 )  ∼ 1  ∼ 1  ∼ 21187  F. Bloisi et al. / Materials Science and Engineering C 27 (2007) 1185  –  1190  PEG locally deposed (optical microscopy investigations con-firmed this result).MapledepositionperformedonsampleS3allowedtoachievea significant homogeneity and a higher amount of PEG withrespect to samples S1 and S2 (Fig. 6).X-ray reflectivity (XRR) is a surface-sensitive technique that  provides information on mass density, thickness, and roughnessof very thin films that are deposited on flat substrates. Thismeasurement is based on the specular reflection of X-rays from planarsurfaces.Thereflectedintensitiesshowfringesthatdepend Fig. 3. IR spectra of microscopy glass substrate clean (black line) and with a dried drop of PEG (red line). (For interpretation of the references to colour in this figurelegend, the reader is referred to the web version of this article.)Fig. 4. IR spectra of sample S1. The inset shows the fingerprint window 1600 – 900 cm − 1 .1188  F. Bloisi et al. / Materials Science and Engineering C 27 (2007) 1185  –  1190  on the film thickness, and different modulation lengths corres- pondtothe existence ofdifferentlayers.The criticalangleoftotalreflection is related to the mass density [15].XRR spectra, reported in Fig. 7, show large modulationsindicating that the films thickness is of the order of some nano-metres. Because of the film roughness and the low difference betweenthesubstrateandthefilmdensitythequantificationoftheerror in the thickness evaluation is very high. For all spectra thecritical angles result the same and are very near to the glass one;this is due to the low mass density of the PEG. 5. Conclusions The capability of MAPLE technique to deposit PEG (poly-ethylene glycol) has been confirmed using a deposition systemexplicitly designed for this technique. Preliminary results show Fig. 6. IR spectra of sample S3. The inset shows the fingerprint window 1600 – 900 cm − 1 .Fig. 5. IR spectra of sample S2. The inset shows the fingerprint window 1600 – 900 cm − 1 .1189  F. Bloisi et al. / Materials Science and Engineering C 27 (2007) 1185  –  1190
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