Received: 4 July 2017 | Revised: 23 August 2017 | Accepted: 18 September 2017

DOI: 10.1002/ppap.201700132

FULL PAPER

Energetics of reactions in a dielectric barrier discharge withargon carrier gas: VI PEG-like coatings

Bernard Nisol1 | Sean Watson1 | Anne Meunier2 | David Juncker2 |

Sophie Lerouge3 | Michael R. Wertheimer1

1Groupe des Couches Minces (GCM) andDepartment of Engineering Physics,Polytechnique Montréal, Box 6079, StationCentre-Ville, Québec H3C 3A7, Canada

2Micro and NanobioengineeringLaboratory, Biomedical EngineeringDepartment, McGill University, Montréal,Québec, Canada

3 Research Centre, Centre Hospitalier del’Université de Montréal (CRCHUM), andDepartment of Mechanical Engineering,École de technologie supérieure (ÉTS),Montréal, Québec, Canada

CorrespondenceMichael R. Wertheimer, Groupe desCouches Minces (GCM) and Department ofEngineering Physics, PolytechniqueMontréal, Box 6079, Station Centre-Ville,Montreal, Québec, H3C 3A7, Canada.Email: michel.wertheimer@polymtl.ca

Funding informationthe Natural Sciences and EngineeringResearch Council of Canada (NSERC); theFonds de recherche du Québec – Nature ettechnologies (FRQNT)

We have studied “PEG-like” plasma-deposited coatings of poly(ethylene glycol),

some of which prevent protein adsorption and cellular adhesion. This enables

inhibition of possible inflammatory reactions or rejection of an implant following

its insertion into living tissue. Our approach, based on electrical measurements in

atmospheric pressure Ar dielectric barrier discharges, enables precise measurements

of Em, the energy absorbed per monomer molecule. Here, we demonstrate

the importance of Em in preparing PEG-like coatings for biomedical

applications, for example by highlighting the great importance of molecular

weight of monoglyme (1G) or

diglyme (2G) monomers, and

by obtaining anti-fouling layers,

“PP-2G,” only with the di-

glyme. We demonstrate resis-

tance to protein adsorption and

cell adhesion of PP-2G surfaces

prepared with optimized Fd

(and Em) values.

KEYWORDS

anti-fouling, argon carrier, atmospheric pressure, dielectric barrier discharge, energetics,

polymerization, mono- and diglyme

1 | INTRODUCTION

Atmospheric-pressure (AP) cold plasma-enhanced chem-ical vapor deposition (PECVD) of thin coatings destinedfor biomedical applications is a vast, ever-growingfield.[1–4] One of the main reasons is that AP plasmas,like their low-pressure (LP) counterparts,[4] enable finesurface engineering but also promise more economical,easier implementation by obviating the need for costlyvacuum installations.[5–7]

This work focuses on anti-fouling poly(ethylene glycol)-like (PEG-like) coatings, known to inhibit protein adsorptionthanks notably to strongwater-PEG interactions[8]; as a result,they have the potential to prevent adhesion of cells andbacteria, inflammatory reactions and other undesired “wall-off” reactions from the biological environment (eg, collage-nous encapsulating response).[9] These particular interactionswith water render conventional PEG—also referred to aspoly(ethylene oxide) (PEO)—completely soluble in water, inall proportions and for all degrees of polymerization at room

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temperature.[8,10] As a result, PEG cannot be used as a singlematerial for manufacturing medical devices. Various sol-utions have been envisaged, all with the aim of introducingPEG chains either into (a) PEG-containing copolymers,[11,12]

or onto the surface of another material by way of (b)physisorption,[13,14] or (c) covalent grafting.[15,16] Materialsand surfaces produced in this way nevertheless have certaindrawbacks, namely low strength for (a)[9,17]; for the case of(b), low durability and high PEG concentrations are hard toattain,[9] while (c) requires multi-step, complex, and costlyprotocols.

Plasma polymerization (PP) is a sub-field of PECVDwhere the precursor gas or vapor is an organic compound; it isan elegant solution that allows one to deposit thin organiclayers, for example PEG-like ones, with relatively highdegrees of cross-linking and good adhesion to mostsubstrates. An important additional benefit of PP is theability to work with organic precursors that CANNOTpolymerize by conventional chemical routes, such as the fullysaturated compounds used in this present study(precursor≠monomer).

There already exists a body of literature relating to LPplasma deposition of PP-PEG coatings, some of the mostimportant being references.[18–22] In the most relevantexample, published in 2005, Johnston et al[22] reported asystematic LP PP study using various saturated PEG-likeprecursors (including oligoglymes, dioxane, and crownethers). The resulting PP-PEG coatings were compared interms of concentration (%) of “ethylene glycol (EG) units,”something hereafter referred to as “PEG character.” Thoseauthors observed that the highest PEG character was achievedusing tetra(ethylene glycol) dimethyl ether (tetraglyme, 4G),in comparison with the lighter analogous oligoglymes,namely monoglyme (1G), diglyme (2G), and triglyme(3G); the authors concluded that increasing the number ofethylene glycol units in the precursor significantly decreasedthe chances for them to be fragmented in the discharge.Breakage of a C─O bond obviously results in disappearanceof one EG building block from the initial molecule in favor ofother species contributing to film growth, thus leading to adecrease in the PP's PEG character. In the case ofmultiple EGbuilding blocks, the probability of conserving PEG characteris therefore obviously higher. Also, it has been demonstratedthat the non-fouling properties of PP-PEG strongly dependedon maximizing the PEG character.[22,23] Following this logic,4G was selected in the very first studies reporting successfulAP plasma deposition of PP-PEG coatings.[10,24] However,due to the low vapor pressure of 4G, entraining a sufficientamount of its vapor requires heating (up to 100°C), includingthe gas lines and reactor walls. Despite such heating,condensation on conduits, and reactor walls can occur withinseconds after initiating the process, which can detrimentallyaffect its control and reproducibility.

Such experimental complications led several researchgroups, including the present authors, to prefer lighterprecursors. For example, vinyl-containing compounds suchas tri(ethylene glycol) divinyl ether,[25] or di(ethylene glycol)vinyl ether[26,27] were found to yield quite high PEG characterin resulting PP. One of the above-cited light oligoglymes,namely diglyme (2G) was used by Bhatt et al[28] in AP PPexperiments with an argon plasma jet; they reported partially-reduced cell adhesion (human ovarian carcinoma cell line[NIH:OVCAR-3]) on their coatings; those coatings possesseda PEG character of ca. 56%, below the accepted anti-foulingthreshold of ca. 65% for plasma polymers,[24,27] to be clarifiedfurther below.

Some of the present authors have recently shown thatdielectric barrier discharge (DBD) plasmas with Ar as carriergas can be used to deposit PP coatings from a wide variety ofmonomers, with precise knowledge of the energy perprecursor molecule, Em, absorbed from the plasma's “energyreservoir.”[29–32] In recent joint communications with D.Hegemann,[33,34] this approach was even shown to permitcomparisons between AP and LP PP processes, based on anew “energy conversion efficiency” (ECE) parameterreminiscent of the “Yasuda parameter—W/FM” conceptfrom the 1970s and 80s.[35]

In this present research, we adopted our above-cited APDBD methodology as a routine tool for depositing PP-PEGusing low molecular weight monoglyme (1G) and diglyme(2G) as precursors (see Figure 1), and we henceforth refer tothe resulting coatings as PP-1G and PP-2G, respectively. Finetuning of Em once again enabled unprecedented processcontrol, the goal being to achieve anti-fouling properties bymaximizing the coatings’ PEG character.

2 | EXPERIMENTALMETHODOLOGY

2.1 | Plasma polymerization process andmaterial characterization

The DBD plasma reactor consists of two top electrodes(6 × 18 cm2) and a single large bottom electrode separated bya 2 mm gap between top and bottom dielectrics, respectively

FIGURE 1 Low molecular weight glymes, used as PP-PEGprecursors in this study

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Macor® ceramic (3.50 ± 0.05 mm thick) and glass(3.00 ± 0.02 mm) plates, for a total plasma volume ofv= 43.2 cm3. For more details about the complete plasmareactor system the reader is kindly invited to consult previouswork.[29]

All PP experiments were carried out in cold (T< 35 °C)AP DBD plasma sustained by audio-frequency power atconstant frequency and applied voltage, f = 20 kHz,Va fð Þ= 2.8 kVrms (=8 kVpp, peak-to-peak). Argon (Ar)was used as the inert carrier gas (99.9 + % purity, AirLiquide Canada, Ltd., Montreal), controlled by a rotameter-type flowmeter (Matheson, model 7642H, tube 605). Twoliquid reagents, 1,2-dimethoxyethane (1G, Sigma-Aldrich,purity > 99.5%) and di(ethylene glycol) dimethyl ether (2G,Sigma-Aldrich, purity > 99%) were the selected precursormolecules. Coating homogeneity was ensured by operatingthe AP DBD reactor in “dynamic” mode, the moveablegrounded electrode (+dielectric) platen, on which werefixed the substrates being displaced back-and-forth at a15 mm s−1 scan rate.

A glass bubbler (∅= 30 mm, height = 70 mm) containedseveral cm3 of either 1G or 2G. Vapor flows, Fd, in standardcm3 per minute (sccm), corresponding to‰ concentrations inthe 10 slm Ar carrier gas stream, were entrained by a smaller,separately metered parallel flow of Ar through the bubbler. Ineach such case, a calibration was first carried out bymeasuring steady-state rate of mass change of monomer inthe bubbler with time. The temperatures of the bubbler andconduits were appropriately controlled: 1G was cooled to0 °C (ice water bath), while 2G was thermalized at60 °C (warm water bath), both at standard (atmospheric)pressure.

Without repeating details presented earlier,[29,36] meas-urements of the absorbed energy difference, ΔEg, and theresulting Em value (in eV/molecule)—which plays a key role—were carried out. This was possible by using an equivalentcircuit model along with the MATLAB® program forcomputing Eg, energy dissipated in the AP plasma duringeach applied high-voltage cycle.[29,36] The uncertainty of Em

is dictated by that of Fd, being lowest (<5%) for larger Fd

(≥1 sccm) and highest (up to 15%) for the very smallest Fd

(<1 sccm); data points on the graphs are an average of at leastthree separate experiments.

Techniques used for characterizing the PP-PEG coatingswere the following:

1. Spectroscopic ellipsometry (SE) measurements, to evalu-ate film thickness and deposition rate, were carried out oncoated fragments of single-crystal silicon (c-Si) wafers.The parametersΨ and Δ were determined at 45°, 55°, 65°,and 75° incidence angles using a J. A. Woollam RC2®instrument operating in the 193-1700 nm wavelengthrange, and they were interpreted using CompleteEASE™

software with a Gaussian oscillator-based opticalmodel.[32,37]

2. Attenuated total reflectance (ATR) Fourier-transforminfrared (FTIR) spectroscopy measurements wereperformed using a Digilab® FTS 7000 spectrometer,equipped with a UMA 600 microscope and a hemispheri-cal Ge ATR crystal, for operation in the region from 4000to 700 cm−1. Spectra with a resolution of 4 cm−1, 128 co-added measurements, were acquired for PP-PEG filmsdeposited on KBr (99 +%, Fisher Scientific IR grade) discsubstrates of ca. 0.5 mm thickness and 12.9 mm diameter.Deposits of PP films on KBr were d≥ 200 nm thick.

3. X-ray photoelectron spectroscopy (XPS) analyses werecarried out in a VG ESCALAB 3 MKII spectrometer.Spectra were acquired using a Mg anode (1253.6 eV)operating at 300W. The pass energy was set to 100 eV forsurvey spectra (not presented here), and 20 eV for high-resolution (HR)C 1s peak shape analysis. TheHRC1s peakfitting was performed using CasaXPS (CasaSoftware Ltd.),by considering the lowest number of physically meaningfulcomponents; for the sake of clarity, these were restricted tothe “first neighbor”-induced chemical shift (C─C/C─H,C─O─R, CO/O─C─O, COOR [R= alkyl or H]).[38] TheFWHM (20 eV pass energy) of these components wasconstrained to 1.7 ± 0.1 eV, and binding energies werecharge-referenced by setting the hydrocarbon (C─C/C─H)component to 285.0 eV.

2.2 | Evaluation of anti-fouling properties

All solutions used for protein adsorption and cell adhesionassays were prepared with ultrapure water from a MilliporeMilli-Q system (resistivity: 18MΩ cm) and passed through a0.2 µm filter. Phosphate buffered saline (PBS 1X, pH = 7.4)was procured from Fisher Scientific and contained11.9 × 10−3, 137.0 × 10−3, and 2.7 × 10−3 mol L−1 of phos-phates, NaCl and KCl, respectively. Trypsin-EDTA andfluorescently labeled bovine serum albumin-fluoresceinisothiocyanate (BSA-FITC) were obtained from Sigma-Aldrich. Triton X–100 and paraformaldehyde (PFA) werepurchased from Fisher Scientific. Dulbecco's modified Eaglemedium (DMEM), fetal bovine serum (FBS) and 4′,6-diamidino-2-phenylindole (DAPI) were purchased from LifeTechnologies. Antibiotics (penicillin/streptomycin) wereobtained from Invitrogen.

For protein adsorption assays, substrates consisted of highdensity polyethylene (HDPE) disks (∅= 15mm, thick-ness = 0.85mm). Cell adhesion assays were conducted on c-Si substrates. In both types of assays, samples were half-maskedduring the PP process to obtain a clear, well-defined borderbetween the PP-PEG coating and the uncoated substrate. Alldeposits of PP films on HDPE and c-Si were ca. 200 nm thick.

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2.2.1 | Protein adsorption assays

All protein adsorption experiments were performed at roomtemperature. Sampleswere immersed in PBS1X for 15min, thenthe PBS solution was removed and the samples were incubatedfor 120min in BSA-FITC solution (1.0mgmL−1). Afterincubation, samples were rinsed twice, first with PBS thenwithultrapurewater, and finallydriedunderadrynitrogenstream.

2.2.2 | Cell adhesion assays

Normal human IMR-90 lung fibroblasts, stably expressingthe fluorescent marker mCherry and kindly provided by

Dr. M. Park (McGill University, Montreal, Canada), werecultured in DMEM supplemented with 10% FBS and 1%(v/v) antibiotics, and maintained in 5% CO2 at 37 °C in25 cm2 flasks (Corning, NY, USA). For all samples, apoly(dimethylsiloxane) (PDMS) well (I.D.≈ 5 mm) wascentered on the coating/substrate border, delimiting a ca.20 mm2 area, half-coated with PP-PEG. Once (80-90%)confluent, cells were harvested from flasks using trypsin.Cells in suspension were rinsed by dilution in PBS, thencentrifuged at 4600 rpm for 5 min and finally re-suspendedin culture medium. About 5 × 105 cells mL−1 were seeded ineach well and incubated in culture medium overnight. Cellswere gently rinsed by replacing culture medium with PBS,then PBS was removed and cells were fixed with PFA (4%)for 10 min, rinsed again, twice with PBS for 5 min, andpermeabilized with Triton X–100 (0.2%) for 5 min. Cellswere rinsed twice with PBS (5 min) and their nucleus wasstained with DAPI (0.1 µg mL−1) for 5 min. Samples wererinsed one last time with PBS (5 min) and imagedimmediately thereafter.

FIGURE 3 Deposition rate, r, for 1G (blue diamonds) and 2G(red triangles), plotted as a function of precursor flow rate, Fd

TABLE 1 Identification and numerical values (Fd [in sccm] and Em

[in eV]) of the deposition conditions for all characterized PP-1G andPP-2G samples

PP sample Fd (sccm) Em (eV)

1G-1.4 1.4 57.3

1G-2.3 2.3 56.2

1G-5 5 27.2

1G-7 7 18.9

2G-2 2 43.3

2G-3.5 3.5 44.3

2G-5 5 33.9

2G-7 7 24.5

FIGURE 2 Plots of (a) Em (average energy absorbed permolecule in eV, see text) versus precursor vapor flow, Fd. Labels onthe plots identify the conditions selected for this study (see text andTable 1); (b) Em versus 1=Fd, for monoglyme (1G, blue diamonds)and diglyme (2G, red triangles). Regions (i), (ii), and (iii) areidentified for 1G (see text). The inset presents numerical values ofslopes (in watts), Emð Þmax (in eV), and Fdð Þcrit (in sccm) (see text).The uncertainty of Em is dictated by that of Fd, being lowest (<5%)for larger Fd (≥1 sccm) and highest (up to 15%) for the very smallestFd (<1 sccm); data points are an average of at least three separateexperiments

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2.2.3 | Fluorescence microscopy

After protein adsorption and cell adhesion assays, samples wereplaced upside down on the platform of an inverted microscope(TE-2000-E, Nikon) connected to a CCD camera (QuantEM512SC, Photometrics); fluorescence images were recorded withNIS-Elements Advanced Research software (Nikon) andanalyzed with ImageJ software.[39] Images were collected usinga mercury arc lamp, and 41001 (blue, for FITC), 41004 (green,formCherry), and 31000v2 (UV forDAPI) filter cubes (ChromaTechnology Corp.) were used.

Both HDPE and PP-PEG exhibited small auto-fluores-cence signal intensities. For protein assays, all imagespresented in this work were normalized to a unique colorscale (800-13 500 a.u., from blue to yellow); the low auto-fluorescence from HDPE (ca. 2% of the color scale) andPP-PEG regions (ca. 10% of the color scale) did not preventqualitative estimation of protein adsorption by directcomparison of the fluorescence intensity of each regionbefore and after immersion in BSA-FITC. For quantitativeanalyses, HDPE and PP-PEG regions were imaged before andafter BSA-FITC incubation and auto-fluorescence intensitiesfrom both regions were subtracted, respectively. Each imagewas acquired with a 10× objective, a gain of one and anexposure time of 1 s. The mean fluorescence intensity before(IHDPE or IPP�PEG) and after (IHDPEþBSA or IPP�PEGþBSA)incubation was determined by averaging on five (5) areas andtwo (2) replicated experiments. The variation of fluorescenceintensity due to BSA-FITC adsorption (ΔIfluo) was thusobtained as follows:

ΔIfluo ¼ ½IHDPEþBSA� � ½IHDPE�

ΔIfluo ¼ ½IPP�PEGþBSA� � ½IPP�PEG�

Cell adhesion was estimated byqualitatively comparing the cell surfacecoverage on c-Si and on PP-PEGregions. IMR-90 that already expressedmCherry (red fluorescence) were furtherstained with DAPI (blue fluorescence,nucleus). Multi-layering of cells in someregions disabled quantitative analysis.Auto-fluorescence from PP-PEG coat-ings (green) was very low, and highexposure times (ca. 4 s) were used tolocalize the c-Si/PP-PEG border regions.

3 | RESULTS ANDDISCUSSION

3.1 | Energy measurements

Figure 2 shows plots of Em, (a) versus Fd, the flows of 1G and2G vapors (in standard cm3 per minute, sccm); (b) versus1/Fd. For both compounds, three distinct regimes can be noted(see Figure 2b):

(i) a very narrow region, corresponding to the lowest Fd

values, reveals a sharp rise in Em as Fd is increased and isreferred to as “monomer-lean.”[40] The upper limit of thisregion is bounded by a first peak, defined as Emð Þmax[Fd = Fdð Þcrit], which corresponds to the particularcondition where maximum transfer of energy from theplasma resulted in presumed total fragmentation,breakage of all covalent bonds in the molecule. Here,Emð Þmax values for 1G and 2G were 61 eV and 76.1 eV,respectively. In agreement with previous work,[32] theheavier molecule exhibits a significantly higher value ofEmð Þmax;

(ii) a “monomer-rich” region spreading over high Fd values,where Em smoothly decreases with rising Fd.

[40]

Figure 2b also shows plots of Em, this time versus1/Fd, and it reveals quasi-linear behavior in themonomer-rich region, something that we had alreadyamply discussed in our earlier reports.[31,32,40] The initialslopes for 1G and 2G were 8.8W and 11.1W,respectively. These are closely related to the propensityof a precursor molecule to absorb energy available fromthe plasma; a higher value in the case of 2G, heavier than1G but from the same chemical family (here, ethers), isalso in agreement with previous observations[32];

(iii) a “dip region” separating (i) and (ii) is a transition featurethat was already reported (and interpreted) in our earlierarticle dealing with a series of esters.[31] It is suggested

FIGURE 4 ATR FTIR spectra of (a) liquid 1G precursor (top) and PP-1G deposits; and(b) 2G (top) and PP-2G, all normalized with respect to the area of the CH2/CH3 stretchingband. Labels to the left identify the PP conditions (see Table 1); numbers in brackets: Em

(in eV). Also shown are spectral assignments of the dominant absorption bands

NISOL ET AL. | 5 of 10

that this region, neighboring Emð Þmax and in which Em

drops drastically, results from amarked fragmentation ofoxygen-bearing hydrocarbon groups, dominated byliberation of CO2.

Earlier experience[37] taught us that the upper limit ofregion (iii) (lower limit of [ii], see Figure 2) corresponds to theone of interest for most PP experiments. For both precursors afew such conditions were therefore selected, with Em valuesranging upward from ca. 20 eV. These are identified bysuitable labels in Figure 2a and in Table 1; they once againunderline the advantages of this approach as a routine tool thatenables one to pinpoint a “sweet spot” in the PP process,before any material characterization is undertaken.

3.2 | Characterization of PP-PEG coatings

Figure 3, a plot of PP deposition rate, r, versus Fd, reveals twodistinct behaviors for PP-1G and −2G. While PP-2G shows apseudo-linear increase from about r = 9 to ca. 32 nmmin−1, r

remains low for PP-1G, reaching an asymptotic limit near8 nmmin−1. The presence of a “plateau” in the r(Fd) curvecould be explained by the following: in order to contribute tofilm growth, a given precursor molecule must be activated inthe discharge; here, activation via at least one bond breakage(radical or ion formation) is required because the presentprecursors, 1G and 2G, are fully saturated. As one increasesFd to reach the “deeper”monomer-rich region (Fd > 5 sccm inthe 1G case), probability for the molecules to pass through thedischarge without being activated also increases. Since it wasfelt to stray from the main objectives of this study, we did notinvestigate whether or not the plot for 2G would level off, asone might expect. At Fd = 7 sccm, r values of the two differby a factor of ca. 4, signaling particular fragility of the 1Gmolecules in plasma. Deposition rates are seen to be low forboth 1G and 2G close to region (iii) (Fd ∼2 sccm, see Figure2a), namely r < 5 and r < 10 nmmin−1, respectively.

It is noteworthy that r ∼20–30 nmmin−1 values observedfor PP-2Gare adequate formanybio-technological applications,because thicknesses in the 20-100 nm range usually suffice.

Figure 4 showsATRFTIR spectra of(a) PP-1G; and (b) PP-2G, all beingnormalized to the area of the CH2/CH3

stretching (str.) band (2800-3000 cm−1).The upper spectra in (a) and (b)respectively correspond to the liquid1G and 2Gprecursors. They allow one toidentify the characteristic absorptionbands of glymes, and to note that theseare preserved in PP-PEG; they are: C-Ostr. (1100-1200 cm−1), CH2/CH3 str.(2800-3000 cm−1) and the multipleC─H bending modes in the 1200-1500 cm−1 range.[10]

Clearly, for PP-1G the C─O str.band intensity is quite low, even under“mild” deposition conditions (1G-7,Em = 18.9 eV), and is much decreasedwhen Em is raised. This is a characteris-tic of intense precursor fragmentation,responsible for decrease in “ethyleneglycol” and increase in “hydrocarbon”content in the coating.

For PP-2G coatings (Figure 4b), highrelative intensity of the C─O str. bandsignals fair conservation of the “ethyleneglycol” content, in particular for 2G-7(Em = 24.5 eV) and 2G-5 (33.9 eV), forwhich precursor fragmentation wasquite low. However, under the harshestcondition, 2G-2 (43.3 eV), carbonyl(CO str. near 1715 cm−1) and hydroxyl(OH str., 3200-3500 cm−1) groups gain

FIGURE 5 High-resolution XPS C 1s spectra of PP-1G (left, blue); and PP-2G (right, red),and corresponding peak fittings. Labels to the left identify the PP conditions (see Table 1);numbers in brackets are corresponding Em values (in eV)

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significance. Surprisingly, these were minimally present in thecase of PP-1G, confirming major differences in the plasma-inducedbehaviors of the twoprecursors. It is noteworthy that thehydroxyl peaking at around 3350 cm−1 remains quite low for allconditions except 2G-2; consequently, we assign minimalimportance to that condition. For comparison, anFTIR spectrumof poly(vinyl alcohol), PVA,[41] manifests a gigantic OHstretching band, which dominates all other features, counter tothe situation observed here. Therefore, we feel comfortablestating that OHgroups play a relativelyminor role here, and thatXPS data hereunder are reliable in their interpretation.

Figure 5 shows the XPS C 1s high resolution spectra ofPP-1G (left, blue); and PP-2G (right, red), with correspondingpeak fittings. The calculated area ratios of the fittingcomponents are listed in Table 2. We remind the readerthat an “ideal” PEG XPS C 1s peak (100% PEG character)would solely comprise the C─OR component; % C─OR is

thus considered a semi-quantitative marker of the PEGcharacter, one that may include in this case some small C─OHcontributions.[23]

In agreement with the corresponding ATR-FTIRspectra, XPS data confirm that low PEG character(C─OR ≤ 29%) is associated with high hydrocarbon content(C─C/C─H= 60-70%) for all of the PP-1G depositionconditions investigated.

Now, focusing on PP-2G, the PEG character is observedto have varied significantly, between ca. 31% for the highestEm = 43.4 eV (2G-2), in region (iii), to the more encouragingvalue of ca. 59% for the case of 2G-7 (24.5 eV). Interestingly,even at high Em, for example 2G-2, diglyme yielded coatingswith higher PEG character than any of those obtained withmonoglyme; this agrees well with ATR-FTIR and depositionrate data, and it confirms the earlier-mentioned extremefragility of 1G in the PP process.

FIGURE 6 Fluorescence micrographs of partly coated (PP-1G and PP-2G on bottom half) HDPE substrates, after BSA-FITC treatment. Allof the presented images focus in the same orientation, centered on the coating/substrate border. Labels at the top/bottom identify PP conditions(see Table 1)

TABLE 2 XPS C 1s peak fitting values for PP-1G and PP-2G samples

PP sample C─C/C─H C─OR CO/O─C─O COOR

1G-1.4 74.7 ± 2.1 19.0 ± 1.6 5.3 ± 0.4 1.0 ± 0.1

1G-2.3 69.1 ± 0.1 23.2 ± 0.7 6.0 ± 0.7 1.7 ± 0.4

1G-5 64.3 ± 1.2 27.1 ± 0.3 6.8 ± 0.6 1.8 ± 0.4

1G-7 63.5 ± 0.3 28.9 ± 0.3 6.6 ± 0.2 1.0 ± 0.2

2G-2 56 ± 0.1 31.0 ± 0.1 10.8 ± 0.2 2.2 ± 0.1

2G-3.5 39.5 ± 0.9 45.8 ± 0.8 11.5 ± 0.3 3.2 ± 0.4

2G-5 30.0 ± 0.3 53.6 ± 0.1 12.7 ± 0.1 3.7 ± 0.2

2G-7 26.2 ± 0.1 59.3 ± 0.4 11.3 ± 0.1 3.2 ± 0.6

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On hand of the PP characterization data presented inpreceding sections, we rejected conditions in region (iii) fromthose selected for preparing samples evaluated by biologicaltests.

3.3 | Anti-fouling properties versus PEGcharacter

Fluorescence microscopy was used to evaluate anti-foulingproperties of selected PP-1G and PP-2G samples. First, proteinadsorption tests were conducted using fluorescently labeledbovine serum albumin (BSA-FITC). As shown in previouswork, the fluorescence intensity, Ifluo, can be used as a quasi-direct indicator of protein adsorption[25]: Figure 6 showsfluorescence micrographs of partly coated (coating on thebottom-half) polyethylene (HDPE) substrates after BSA-FITCtreatment. For clarity, emphasis is on the coating/substrateborder region, so as to better highlight differences in thefluorescence intensity. Qualitatively, one clearly notes twoextremes: 1G-2.3 (low PEG character≈ 23%) exhibits asignificant increase in Ifluo in comparison with bare HDPE; inthe case of 2G-7 (PEGcharacter≈ 59%), Ifluo is greatly reduced,down to PP-PEG's auto-fluorescence value (see Section 2.2).

Quantitative data forΔIfluo, the difference in fluorescenceintensity before (auto-fluorescence) and after adsorptiontests, is shown in Figure 7a; it confirms the great differencebetween PP-1G and PP-2G coatings. PP-1G samples (PEGcharacter≤ 29%) adsorbed significant amounts of BSA-FITC, comparable to—or even significantly higherthan—HDPE (twice as much in the case of 1G-2.3). AllPP-2G samples tested showed a marked decrease in ΔIfluo,near-zero under the “mild” 2G-7 (24.5 eV) depositioncondition.

In Figure 7b, ΔIfluo is plotted as a function of PEGcharacter expressed as XPS C 1s % C-OR. Based on BSAadsorption assays, we note that a PEG character close to 60%may suffice for anti-fouling, a value somewhat below the so-far accepted 65% threshold for plasma polymers.[24,27] Thecurrent results are somewhat inferior to those of Nisol et al[10]

(ca. 83% using 4G aerosol; ca. 63% using 4G vapor), Da Ponteet al[24] (65% using 4G aerosol), but comparable to Bhattet al[28] (56% using 2G vapor).

The anti-fouling character of PP-1G and PP-2G sampleswas then further assessed with human IMR-90 lung fibroblastadhesion assays: c-Si substrates, partly coated with PP-PEGlayers, were examined using fluorescence microscopy aftercell adhesion (24 h). All of the images presented in Figure 8are taken in the same orientation, focused at the coating(bottom-half)/substrate (top-half) border.

Except for 2G-7 (Figure 8c,d), on all investigated PP-1Gand PP-2G deposits—only two shown here—, both thecoating and substrate surfaces were covered with densemono-, or even multi-layers of cells, as seen in Figure 8a,b.

On the contrary, 2G-7 (24.5 eV, PEG char.≈ 59 eV)manifested strong anti-fouling character, because almost noIMR-90 fibroblasts could be seen to adhere (see Figure 8c).This strongly endorses the protein adsorption assay as a probefor anti-fouling surface properties and also confirms thenecessity to maximize the coating's PEG character by way ofwell-controlled PP processing. The present methodology alsoallowed us to achieve a new (lower) minimum threshold valueof PPs’ PEG character, namely≈ 59% compared with thehitherto accepted 65% value.[24,27]

FIGURE 7 (a) Variation of fluorescence intensity (ΔIfluo) afterBSA-FITC adsorption on HDPE substrates, PP-1G and PP-2Gcoatings (labeled as in Table 1). The error bars represent the standarderror calculated from all samples of each type. (b) ΔIfluo correlated tothe PEG character, here expressed as % C─OR of the XPS C 1s high-resolution spectrum

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4 | CONCLUSIONS

In this present work we have, once again, demonstrated thatmethodology exemplified in Figure 2 lends excellent controland reproducibility over the energetics of AP PECVD (PP)experiments. They were used here to create PP-PEG coatingsfrom fully-saturated, low-MW monoglyme (1G), anddiglyme (2G) precursors.

For 2G, a narrow Em range in Figure 2, roughly between25 and 45 eV/molecule, yielded PP-2G coatings with variablechemical/biological properties. Low Em resulted in satisfac-tory retention of the precursor's PEG character (ca. 59%, forsample 2G-7) and remarkable anti-fouling properties, acondition that co-applies to both proteins and cells. Now,unlike that ready ability of 2G to form useful PP coatings, thiswas not the case with 1G as precursor: even within the widerEmrange, ca. 19-57 eV/molecule, it was impossible to obtain aPP-1G deposit with anti-fouling properties, the mildestcondition (1G-7; 19.8 eV) resulting in PEG character <30%.

To conclude, it is believed that the current researchestablishes PP-2G as a promising anti-fouling coating that

merits further testing and experimentation,notably for use with blood and other bio-fluids that can lead to rapid surface fouling.

ACKNOWLEDGMENTS

The authors are grateful for financial supportfrom the Natural Sciences and EngineeringResearch Council of Canada (NSERC) andfrom the Fonds de recherche du Québec—Nature et technologies (FRQNT) via PlasmaQuébec. We thank Yves Leblanc for skilledtechnical support. DJ and AM acknowledgefunding from NSERC and CIHR (CanadianInstitutes of Health Research) as part of theCollaborative Health Research Projects(CHRP) program. SL wishes to thank theCanada Research Chairs Program.

ORCID

Bernard Nisol http://orcid.org/0000-0001-7785-6307

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How to cite this article: Nisol B, Watson S,Meunier A, Juncker D, Lerouge S, Wertheimer MR.Energetics of reactions in a dielectric barrierdischarge with argon carrier gas: VI PEG-likecoatings. Plasma Process Polym. 2017;e1700132,https://doi.org/10.1002/ppap.201700132

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