Article Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
pubs.acs.org/IECR
Engineering Lubricious, Biopassive Polymer Brushes by SurfaceInitiated, Controlled Radical Polymerization Ella S. Dehghani, Shivaprakash N. Ramakrishna,* Nicholas D. Spencer, and Edmondo M. Benetti* Laboratory for Surface Science and Technology, Department of Materials, ETH Zürich, Vladimir-Prelog-Weg 1-5/10, CH-8093 Zurich, Switzerland S Supporting Information *
ABSTRACT: Surface-initiated controlled radical polymerization enables the fabrication of biopassive polymer brushes with interfacial, physicochemical properties that can be independently varied across a single substrate. Poly[(oligoethylene glycol) methacrylate] (POEGMA) brushes were synthesized by surfaceinitiated atom transfer radical polymerization (SI-ATRP), locally varying the exposure of initiator-functionalized surfaces to the polymerization solution to yield POEGMA brush thickness gradients. A combination of variable-angle spectroscopic ellipsometry (VASE) and atomic force microscopy (AFM) demonstrated that brush swelling, grafting density, nanomechanical properties, and biopassivity towards protein adsorption all remained constant within a thickness range between 20 and 90 nm. However, the nanotribological properties of POEGMA brushes, investigated by lateral force microscopy (LFM), were found to vary progressively along the gradient, thinner brushes showing significantly lower friction than thicker and more viscoelastic grafts. The independent variation of lubricity across a biopassive brush gradient shows how SI-ATRP can be used to tailor surfaces destined for applications involving both contact with biological media and exposure to shear stresses, as is the case for tissue-replacement implants and scaffolds for tissue engineering.
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INTRODUCTION Polymer grafting has been increasingly applied as an extremely versatile and efficient approach to modulate the interfacial physicochemical properties of a broad variety of surfaces, enabling the functionalization of inorganics, metals, and polymers.1−6 Exploiting both “grafting-from”7−9 and “graftingto”10 methods, polymer “brushes” with various compositions and nano-/micromorphologies have been applied to tune the interaction of materials with the surrounding environment or to provide function to otherwise inert surfaces. Surface-initiated, controlled polymerization has been applied from immobilized initiators to synthesize dense brushes featuring tunable thickness and a broad range of chemistries. Among the different polymerization methods, controlled-radical processes have proven to be the most suitable for fabricating brush surfaces, given their compatibility toward a wide range of monomers and the relatively mild conditions required for their application. In particular, surface-initiated atom transfer radical polymerization (SI-ATRP)11−13 arose as the most common method of choice, since it enables full control over brush thickness within a broad range of values, from few nanometers to several micrometers. SI-ATRP has been successfully applied for designing polymer-brush-based sensors,14−17 responsive membranes,18,19 microfluidic reactors where catalysts are supported by brush layers,20,21 and surfactants22 or, alternatively, for modifying scaffolds for tissue engineering and cellculture platforms.23−31 Within this wealth of applications, two © XXXX American Chemical Society
distinctive properties of densely grafted polymer brushes have been of special technological interest, namely, their capability of reducing friction and preventing unspecific biological contamination on surfaces. These unique traits characterize hydrophilic and overall neutral brushes, including those based on poly(ethylene glycol)s (PEGs), 32,33 poly(zwitterion)s (PZWs),34−36 poly(acrylamide)s (PAAms),37,38 and poly(2alkyl-2-oxazoline)s (PAOXAs),39−41 and both derive from the interplay between the entropic barrier provided by dense assemblies of chain-end-grafted polymers and the energetic shield generated by their highly hydrated nature.42,43 It should be noted that the application of polymer brushes simultaneously providing lubrication and reducing bioadhesion is of great relevance for implants aimed at replacing articular joints or for coating materials that need to withstand shear stresses, or which are potentially subject to wear within biological media. We report how precise brush thickness modulation, by means of SI-ATRP, can be exploited to tune the nanotribological properties of biopassive brushes. Specifically, poly[oligo(ethylene glycol) methacrylate] (POEGMA) brushes presenting a thickness gradient in the range 20−90 nm were synthesized by locally tuning the exposure time of initiatorReceived: Revised: Accepted: Published: A
January 31, 2018 March 16, 2018 March 17, 2018 March 18, 2018 DOI: 10.1021/acs.iecr.8b00494 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
APTES-functionalized substrates were later on incubated in 20 mL of dry DCM, to which 0.4 mL of TEA and 0.4 mL of BiBB were added. The mixture was left under gentle shaking for 3 h. After the formation of the ATRP-initiator layer, the substrates were rinsed with DCM, Milli-Q water, ethanol, and acetone, and finally dried with nitrogen gas. SI-ATRP of OEGMA. A solution of OEGMA (10 g, 0.48 mmol), Milli-Q water (6 mL), and methanol (1.5 mL) was deoxygenated by nitrogen bubbling for 20 min. This solution was subsequently transferred to a flask containing CuCl (16 mg, 0.17 mmol), CuCl2 (1.9 mg, 0.014 mmol), and 2,2′bipyridine (61 mg, 0.38 mmol), which was kept under nitrogen. The obtained mixture was stirred at room temperature for 30 min to allow the formation of the organometallic complex. By slow addition of polymerization solution into a flask containing the ATRP initiator-functionalized substrate, the polymer brush thickness gradients were fabricated, over a total period of 4.5 h using a syringe pump with a controller (pump speed 0.128 μL min−1). Following SI-ATRP the samples were rinsed with water and ethanol and finally dried by nitrogen flow. Surface Characterization. Static water contact angle measurements (CA) were performed using a drop shape analysis system (DSA 10, Krüss, Germany) at room temperature and using Milli-Q water. Three measurements were conducted on each sample, and the values of CA were obtained by applying the tangent method for fitting. Variable angle spectroscopic ellipsometry (VASE) was used to measure the dry and swollen thickness of POEGMA brushes (J.A. Woollam Co., Lincoln, NE). The accumulated spectra, reporting amplitude (Ψ) and phase (Δ) components, were modeled using SpectraRay 3 software. The dry polymer thickness was measured as a function of wavelength (275− 827 nm), using three-layer model featuring Si, SiO2, and a Cauchy layer (n = A + B/λ2), where the known refractive indices of Si and SiO2 were used (software WVASE32, LOT Oriel GmbH, Darmstadt, Germany).50 Measurements in MilliQ water were performed using a custom-made liquid cell presenting tow opposite windows positioned at a fixed angle, θ = 70°. A graded effective medium approximation (EMA) model was applied in order to estimate the swollen thickness of POEGMA brushes.51 The degree of swelling (%) was calculated according to
functionalized silicon oxide substrates to ATRP polymerization mixtures.44,45 The formation of continuous brush thickness gradients on substrates is enabled by the highly controlled nature of ATRP, providing a platform where interfacial properties are gradually varying and whose characterization is readily conducted across a single substrate.46 The swelling, biopassive, nanomechanical, and nanotribological properties were studied along the POEGMA brush gradients by a combination of variable angle spectroscopic ellipsometry (VASE), fluorescence microscopy, atomic force microscopy (AFM)-based indentation, and lateral force microscopy (LFM). POEGMA brushes featuring constant grafting density, swelling properties, and biopassivity, but displaying a gradual variation of thickness, were obtained. Friction was shown to increase progressively with tethered-chain length, longer grafts within a swollen brush causing an increment in frictional dissipation across the gradient substrates. This nanotribological behavior was further confirmed by applying single-asperity, contact-mechanics models to describe the friction vs load profiles (FfL) recorded by LFM. Thinner brushes followed Amontons’ law of macroscopic friction,47 showing linear FfL profiles, while they comparatively displayed lower friction with respect to thicker grafts. In contrast, an increment of brush thickness was accompanied by an increase in friction, the thicker brushes displaying sublinear FfL profiles and showing a progressive variation of contact-mechanical behavior from that described by Derjaguin−Muller−Toporov (DMT),48 which refers to stiffer and less deformable materials, to that of Johnson−Kendall−Roberts (JKR),49 which typically applies to materials with high surface energy and undergoing large elastic deformations. Overall, the comprehensive analysis of brush physicochemical characteristics highlights how SI-ATRP represents a powerful means to engineer polymer surfaces by independently varying technologically relevant properties, such as friction.
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EXPERIMENTAL SECTION Materials. Oligo(ethylene glycol) methyl ether methacrylate (OEGMA, Sigma-Aldrich, average Mn ∼ 475) was purified from inhibitors by passing it through a basic alumina column (ϕ = 0.22 μm). Copper(I) chloride (CuCl, Sigma-Aldrich, 98%) was purified by stirring in glacial acetic acid overnight, then filtered and washed with ethanol three times, and finally dried under vacuum. Copper(II) chloride (Sigma-Aldrich, ≥99%), methanol (Sigma-Aldrich, ≥99.8%), ethanol (VWR Chemicals, absolute), dichloromethane (DCM, dry, Acros, ≥99.8%), acetone (dry, Merck KGaA, ≥99.8), triethylamine (TEA, Sigma-Aldrich, ≥99.5%), 2-bromoisobutryl bromide (BiBB, Sigma-Aldrich, 98%), 3-aminopropyltriethoxysilane (APTES, Sigma-Aldrich, 99%), and 2,2′-bipyridyl (BiPy, Sigma-Aldrich, ≥99%) (Aldrich, Germany) were used as received. Silicon wafers (P/B ⟨100⟩) were obtained from SiMat Silicon Wafers (Germany). Water used in all the experiments was Millipore Milli-Q grade. Functionalization of SiO2 Surfaces with ATRP Initiator. Silicon oxide substrates were treated with piranha solution (H2O2:H2SO4 = 1:3 v/v for 20 min; caution: piranha solution reacts violently with organic matter, so please use caref ully!), rinsed extensively with Milli-Q water and ethanol, and finally dried with nitrogen gas. The substrates were subsequently functionalized with APTES via vacuum deposition and finally washed with toluene and Milli-Q water to remove any unbound silane.
S = (Twet − Tdry )/(Twet) × 100
(1)
where Twet and Tdry are the values of the swollen and dry thickness measured by VASE. The POEGMA brush grafting density (σ) was estimated according to eq 2:52 σ = ρ0 hdry NA((0.227(Twet)1.5 (Tdry (Å2))−0.5 )M 0)−1
(2) −3
where ρ0 is the density of POEGMA (1.40 g cm ), NA Avogadro’s number, M0 the monomer molecular weight (475 g mol−1), and 0.227 a constant related to the excluded volume parameter (ω = 7 Å3) and a constant ν = (a2/3)−1, where a refers to the Kuhn length of the monomer unit (10.5 Å for POEGMA).53 The adhesive properties (pull-off forces) and apparent Young’s modulus of POEGMA brushes along the fabricated gradients were measured by colloidal-probe atomic force microscopy (CP-AFM), while friction was recorded by lateral force microscopy (LFM). In both cases, an Asylum Research AFM (MFP-3D, Santa Barbara, CA) was used, and the B
DOI: 10.1021/acs.iecr.8b00494 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Industrial & Engineering Chemistry Research
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RESULTS AND DISCUSSION Fabrication of POEGMA brush gradients by SI-ATRP and their structural characterization. Thickness gradients of POEGMA brushes were synthesized by SI-ATRP, using the experimental setup depicted in Scheme 1. SI-ATRP polymerization was carried out according to
measurements were performed in Milli-Q water. Colloidal AFM probes were prepared by gluing silica colloids (Kromasil, Brewster, NY, diameter = 20 μm) onto tipless, Au-coated silicon cantilevers (Micromash, San Jose, CA) by means of a homemade micromanipulator using UV-curable adhesive (Norland optical adhesive 61). The normal spring constant of the cantilevers was measured by applying the thermal-noise method,54 while the torsional spring constant was estimated using Sader’s method, in both cases before gluing the colloids onto the cantilevers.55 The nanomechanical characteristics of POEGMA brushes were measured through the analysis of force vs separation (FS) curves. Thirty forces curves were recorded over an area of 20 × 20 μm2, in at least three spots on each position along the POEGMA brush gradients. The apparent Young’s modulus of POEGMA brushes was estimated by applying the Hertz model:56 F=
4ER0.5δ1.5 3 − 3ν 2
Article
Scheme 1. (a) POEGMA Brush Gradients Were Fabricated by SI-ATRP, Subjecting ATRP-Initiator-Functionalized Silicon Surfaces to a Polymerization Mixture for a Gradually Increasing Time; (b) Composition of ATRP InitiatorBearing Silicon Surfaces and the Subsequently Synthesized POEGMA Brushes
(3)
where F is the applied force, R is the radius of the colloidal probe, and ν is the Poisson’s ratio of the polymer (considered as 0.5). The approaching profiles from FS curves were fitted with eq 3, within the range between the contact point and an indentation depth corresponding to less than 10% of Twet. The values of Twet recorded by VASE were confirmed by AFM step-height measurements. Plastic tweezers were used to mechanically remove POEGMA brushes along the gradient films, exposing the underlying silicon oxide substrate. Tappingmode AFM was subsequently performed in Milli-Q water and under dry conditions (Bruker Dimension Icon, cantilever used BL- AC40TS-C2, Olympus, Japan, k = 0.09 N m−1, f = 110 kHz in air, and tip radius
