Biomimetic Lubrication and Surface Interactions of Dopamine-assisted

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Biological and Environmental Phenomena at the Interface

Biomimetic Lubrication and Surface Interactions of Dopamine-assisted Zwitterionic Polyelectrolyte Coatings Linbo Han, Li Xiang, Jiawen Zhang, Jingsi Chen, Jifang Liu, Bin Yan, and Hongbo Zeng Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b02473 • Publication Date (Web): 29 Aug 2018 Downloaded from http://pubs.acs.org on August 31, 2018

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Biomimetic Lubrication and Surface Interactions of Dopamine-assisted Zwitterionic Polyelectrolyte Coatings Linbo Han,1,2,† Li Xiang,2,† Jiawen Zhang,2 Jingsi Chen,2 Jifang Liu,2,3 Bin Yan,2,4,* Hongbo Zeng1,2,* 1

College of Health Science and Environmental Engineering, Shenzhen Technology University,

Shenzhen, 518118, China 2

Chemical and Materials Engineering, University of Alberta, Edmonton, AB, T6G 1H9, Canada

3

The Fifth Affiliated Hospital, Guangzhou Medical University, Guangzhou, Guangdong, 510700,

China 4

College of Light Industry, Textile & Food Engineering, Sichuan University, Chengdu, 610065,

China Corresponding Authors *E-mail: [email protected], Phone: 780-492-1044 *E-mail: [email protected]

1

ACS Paragon Plus Environment

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ABSTRACT Bioinspired zwitterionic polyelectrolyte coating with excellent hydration ability has been regarded as a promising lubricating candidate for modifying artificial joint cartilage surface. In physiological fluids, the ubiquitous proteins play an important role in achieving outstanding boundary lubrication; however, a comprehensive understanding of the hydration lubrication between polyelectrolyte coatings and proteins still remains unclear. In this work, a facile fabrication of ultra-smooth polyelectrolyte coatings was developed via the co-deposition of synthesized poly(dopamine methacrylamide-co-2-methacryloyloxyethyl phosphorylcholine) (P(DMA-co-MPC)) and dopamine (DA) in a mild condition. By optimizing the feeding ratio of P(DMA-co-MPC) and DA, the as-fabricated PDA/P(DMA-co-MPC) coatings exhibit excellent lubricating properties when sliding with each other (with friction coefficient µ=0.036±0.002, ~2.8 MPa), as well as sliding with a model protein (bovine serum albumin (BSA)) layer (µ=0.041±0.005, ~4.8 MPa) in phosphate-buffered saline (PBS, pH 7.4). Intriguingly, the lubrication in both systems shows Amontons-like behaviors: the friction is directly proportional to the applied load but independent of the shear velocity. Moreover, the PDA/P(DMA-co-MPC) coatings could resist the protein fouling (i.e., BSA) in PBS, which is crucial to prevent the surfaces from being contaminated when applied in biological media, thus maintaining their lubricating properties. Our results provide a versatile approach for facilely fabricating polyelectrolyte coatings with superior lubrication properties to both polyelectrolyte coatings and protein surfaces, with useful implications into the development of novel lubricating coatings for bioengineering applications (e.g., artificial joints).

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INTRODUCTION

Reducing the friction between sliding surfaces is highly desirable for achieving smooth and continuous operations in automobiles,1 mammal joints,2-3 heavy industries and turbines,4 since friction and wear have become two main critical issues in achieving eligible artificial devices,5 especially for joint replacement in orthopedic arthroplasty.6-7 In physiological systems, hydration lubrication has attracted great research interest, as water is a natural and extensively existed solvent for these surface interactions.8 Taking advantage of their excellent hydration ability, a variety of synthetic polyelectrolyte coatings have been developed as potential boundary lubricating layers to ease the sliding of artificial joints.9-13 Generally, “graft from” and “graft to” are two widely used strategies to immobilize polyelectrolyte on surfaces.14 “Graft from” method, such as surface-initiated atom-transfer radical-polymerization (SI-ATRP), has been employed to fabricate well-defined poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC) brush coatings exhibiting excellent lubricating properties.10, 15-16 However, the tedious steps and harsh reaction conditions such as oxygen-free environment limit the practical applications of “graft from” method at large-scale.17 On the other hand, “graft to” method is easy to carry out based on the interactions between the active chain end of polyelectrolyte and the target surface via physisorption18-20 or chemical bonds.21 With strong and substrate-independent interfacial adhesion, mussel-inspired polydopamine (PDA) has become one of the most popular anchoring agents for surface modification on various substrates,22 especially using DA-assisted co-deposition with polyelectrolytes.22-24 However, the as-fabricated polyelectrolyte coatings usually suffer from insufficient packing density, unstable bindings with substrates and relatively high surface roughness,25 affecting their lubricating properties, wear resistance and durability.21, 3


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26-27

In

order

to

overcome

methacrylamide-co-2-methacryloyloxyethyl

such

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disadvantages,

phosphorylcholine)

poly(dopamine

(P(DMA-co-MPC))

was

synthesized to co-deposit with DA through dip-coating method to fabricate smooth PMPC-containing coatings on various substrates, including mica, polystyrene (PS) and TiO2. In this way, the PMPC moiety could be anchored stably on substrate surface and mediate the surface morphology of as-fabricated PMPC-containing coatings.25, 28-31 Under practical conditions, it has been reported that the orthopedic prosthesis used to substitute damaged natural joints could be lubricated by pseudosynovial fluid (PSF) or human synovial fluid (SF) containing abundant biological macromolecules.5,

32

Albumin, a major

synovial protein, is believed to be responsible for enhancing boundary lubrication and reducing surface wear when it is trapped between sliding surfaces.5, 32-35 When polyelectrolyte-containing coatings are applied in clinical joints replacement, the boundary lubrication between polyelectrolyte-albumin surfaces is as important as that between polyelectrolyte-polyelectrolyte coatings, and investigating the effects of albumin on its associated boundary lubrication is crucial to gain better understanding on the complex lubrication of mammal joints. Up to now, no report is available about the shearing interaction of zwitterionic polyelectrolyte coating against albumin layer at the nanoscale. Therefore, in this work, the lubricating performances of PMPC-containing coatings have been measured when sliding with each other in phosphate-buffered saline (PBS, 0.13 M, pH 7.4), and between a PMPC-containing coating and a model albumin (bovine serum albumin (BSA)) layer. Moreover, the anti-biofouling ability of PDA/P(DMA-co-MPC) coatings for BSA has been evaluated, which is crucial to prevent the surfaces from being contaminated when applied in biological fluid media. 4


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EXPERIMENTAL METHODS Materials.

Tris(hydroxymethyl)aminomethane

2-methacryloyloxyethyl

phosphorylcholine

(Tris, (MPC,

≥99.8 97.0

%, %,

Sigma-Aldrich), Sigma-Aldrich),

4-cyano-4-(phenylcarbonothioylthio)pentanoic acid (CTA, 97.0 %, Sigma-Aldrich), 4, 4’-azobis(4-cyanovaleric acid) (ACVA, 98.0 %, Sigma-Aldrich) were used as received. Dopamine methacrylamide (DMA) was synthesized according to the protocol described in the literature.36 Synthesis of P(DMA-co-MPC). As shown in Scheme 1, P(DMA-co-MPC) random copolymer was synthesized with feeding mole ratio of DMA/MPC at 1/5 and the detailed synthesis process is described: firstly, 0.44 g (1.5 mmol) of MPC, 0.66 g (0.3 mmol) DMA, 0.0054 g (0.02 mmol) of CTA and 0.0027 g (0.01 mmol) of ACVA were charged into a 25 mL flask, where 4.0 mL of methanol was added and stirred into a homogeneous mixture. The mixture was then bubbled with nitrogen gas for 20 min to establish a nitrogen atmosphere in the flask. Then, it was heated to 60 °C and kept at 60 °C for reaction overnight. The reaction solution was concentrated using rotary evaporation and then the polymer was purified by precipitation in tetrahydrofuran. The precipitated polymer was dried in vacuum overnight, and a light grey solid product was obtained. The successful fabrication and purity of P(DMA-co-MPC) was demonstrated (see Figure S1). The molecular weight (Mn ~20700 g/mol) and polydispersity index (Mw/Mn ~1.5) of P(DMA-co-MPC) was measured via gel permeation chromatography (GPC).

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Scheme 1. Schematic of the synthesis route of P(DMA-co-MPC). Fabrication of PDA/P(DMA-co-MPC) Coatings on Substrates. Dopamine hydrochloride (DA) and P(DMA-co-MPC) with certain feed ratio were dissolved in Tris buffer solution (pH 8.4, 10 mM), where nascent substrates were immersed for 12 hrs at static state under ambient condition. Then, the obtained substrates were thoroughly rinsed with the same buffer solution and dried by nitrogen. Surface Characterization. The surface topographies of as-fabricated PDA/P(DMA-co-MPC) coatings with various feeding ratios of P(DMA-co-MPC) and DA were determined by atomic force microscopy (AFM) imaging using a Bruker ICON AFM system (Bruker, Santa Barbara, CA) operated in tapping mode under ambient condition (23 °C). At least three different positions on each surface were imaged, and the typical AFM images were presented. The static water contact angles (CAs) of the fabricated coatings were measured by a contact angle goniometer (Ramé-hart instrument co., Succasunna, NJ) using a sessile drop method. The average CA was presented by measuring at least three different positions of each coating. The surface elemental contents of the untreated mica, PDA-coated mica and PDA/P(DMA-co-MPC) coated mica 6


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surfaces were analyzed by X-ray photoelectron spectroscopy (XPS). The XPS spectrum was obtained using an XPS spectrophotometer (Kratos AXIS 165) at the pressure lower than 1×10-9 Torr at ~130 K. The collected data was calibrated by the standard C1s peak at 284.6 eV.

Friction Test by Surface Forces Apparatus (SFA). The lubricating performances of as-fabricated PDA/P(DMA-co-MPC) coatings with extremely low surface roughness were investigated using an SFA. For a typical friction or normal force test using SFA, two back-silvered mica sheets of the same thickness (1-5 µm) were first glued onto two cylindrical silica disks (radius R = 2 cm). The PDA/P(DMA-co-MPC) polymers or BSA were coated on mica surfaces by immersing the substrates into DA/P(DMA-co-MPC) or BSA solutions. Two desired surfaces were mounted in the SFA chamber in a crossed-cylinder configuration, with desired aqueous solution injected into the confined space between the surfaces.37-40 The lower surface was driven to shear laterally past the upper surface at varying sliding speeds using a piezoelectric bimorph slider, using an experimental setup as reported previously.41 The friction forces were detected using a force sensor composed with four semiconductor strain gauges attached symmetrically to the bending arms of the springs suspending the upper surface.42-43 Evaluation the Antifouling Properties of PDA/P(DMA-co-MPC). A quartz crystal microbalance with dissipation monitoring (QCM-D) was employed to quantitatively monitor the protein adsorption on silica sensors with/without PDA/P(DMA-co-MPC) coating in real-time, and the adsorbed mass of protein was determined based on frequency change of QCM-D sensor.44-45 RESULTS AND DISCUSSION

Characterization of PDA/P(DMA-co-MPC) Coatings on Mica: Surface Morphology, Wettability and Composition. P(DMA-co-MPC) with both catechol and zwitterionic moieties 7


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has been synthesized for the fabrication of substrate-independent lubricating coatings. Though PDMA itself could help P(DMA-co-MPC) adsorb on mica surface via hydrogen bonding (Figure S2),40 the formation of covalent bonding between DA and PDMA segment within P(DMA-co-MPC), cation-π interaction between DA and zwitterions within P(DMA-co-MPC) would enhance the anchoring of PMPC segment on mica surface.25, 29, 31, 46 Upon oxidation, DA could not only polymerize itself, but also covalently couple with catechol groups within P(DMA-co-MPC) to form crosslinked surface coatings.22, 24, 47-49 Therefore, the catechol groups acted as anchor ligands to directly tether the PMPC segments onto substrates within one step. The solution pH, initial feeding concentration of DA and reaction time were kept constant (Tris buffer (10 mM, pH 8.4), 4 mg/mL, 12 hrs) in this work.50 The influence or contributions of P(DMA-co-MPC) to the fabricated coating films could be characterized in terms of surface wettability and surface morphology. Without the addition of P(DMA-co-MPC), a relatively rough PDA coating was formed on mica from pure DA solution with a root mean square (RMS) roughness ~16.5 nm (Figure 1a). The resultant rough PDA coating was mainly contributed by the agglomerate deposition and inhomogeneous stacking of the PDA molecules.51-53 The water contact angle of the as-fabricated PDA coating was 41.0° (Figure 2), which was in good agreement with the previous study.22 Intriguingly, by adding a small amount of P(DMA-co-MPC) (1 mg/mL) into DA solution, the surface roughness of as-fabricated coating was significantly reduced (RMS ~2.5 nm) (Figure1b), with the water contact angle decreased to 20.8° (Figure 2). Introducing the P(DMA-co-MPC) during DA deposition could reduce the PDA aggregates, thus forming the relatively small PDA/P(DMA-co-MPC) moieties deposited on the substrate.24-25 Generally, the PDA aggregates can grow up through the covalent polymerization and 8


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non-covalent self-assembly of small PDA moieties,54 in which non-covalent self-assembly is contributed from the intermolecular interactions such as cation-π, π-π, and hydrogen bonding interactions. The hydrated phosphate group and quaternary amino group in PMPC moiety are presumably to mediate the deposition of catechol segments via phenol-phospholipid hydrogen bonding and cation-π interactions between the quaternary ammoniums and aromatic benzene rings.25, 28-31, 55 At the same time, the incorporation of hydrophilic/zwitterionic PMPC segments could help to enhance the surface wettability of the obtained composite coatings. By further increasing the feeding dosage of P(DMA-co-MPC) to 4 mg/mL, the RMS of formed PDA/P(DMA-co-MPC) coating was further reduced to as low as ~0.6 nm (Figure 1d). The corresponding water contact angle was decreased to ~13.0°, implying the successful immobilization of PMPC segments on the obtained coating. The PDA/P(DMA-co-MPC) coating was stable after being washed with water, evidenced by the remained water contact angle even after 5 cycles of washing and drying, as described in Figure S3. In addition, no obvious change was observed on both water contact angle and surface morphology of obtained PDA/P(DMA-co-MPC) coating after 5-day incubation in PBS, demonstrating its excellent long-term stability in physiological environment (Figure S4). Such PMPC-induced roughness reduction could effectively optimize the surface morphology and hydrophilicity of the PDA/P(DMA-co-MPC) coatings.25 The obtained ultra-smooth PMPC-containing coatings are expected to act as promising boundary lubricating layers to reduce the friction dissipation due to the strong surface hydration of PMPC moieties, as well as the minimized chain interpenetration and asperity contacts under high compression, as confirmed by SFA force measurements.

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Figure 1. Topographic AFM images of (a) PDA coating on mica obtained from aqueous solution with feeding dosage of DA at 4 mg/mL (RMS ~16.5 nm), and PDA/P(DMA-co-MPC) coatings on mica obtained with different feeding ratios of DA : P(DMA-co-MPC): b) 4 mg/mL : 1 mg/mL (RMS ~2.5 nm), c) 4 mg/mL : 2 mg/mL (RMS ~0.9 nm), and d) 4 mg/mL : 4 mg/mL (RMS ~0.6 nm) in Tris buffer (pH 8.4) at ambient condition for 12 hrs.

The dry thickness of PDA/P(DMA-co-MPC) coating on mica was determined as ~4.3 nm by SFA, in good agreement with that measured on silica using ellipsometry (~3.9 nm). The elemental compositions of PDA and PDA/P(DMA-co-MPC) coatings were characterized by XPS as shown in Figure 3. The successful incorporation of PMPC on mica surface via co-deposition was confirmed by XPS full spectrum presented in Figure 3a, evidenced by the distinct observation of characteristic peaks of PMPC at ~402.0 eV (N1s) and ~133.3 eV (P2p).56 Moreover, the broad signal of N1s in high resolution XPS spectrum (Figure 3b) could be deconvoluted into ~402.5 and ~399.9 eV, corresponding to the characteristic quaternary ammonium in PMPC and related amine complex within PDA, respectively.25,

57

The

high-resolution phosphorous P2p signal at ~133.3 eV also confirmed the successful immobilization of PMPC segments on mica (Figure 3c).58

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Figure 2. Static water contact angles (CAs) of as-fabricated PDA/P(DMA-co-MPC) polymer coatings on mica with different feeding ratio of DA : P(DMA-co-MPC) in Tris buffer (pH 8.4) at ambient condition for 12 hrs.

Figure 3. a) XPS survey spectrum of PDA/P(MDA-co-MPC) coated mica and the associated high-resolution XPS spectrum of (b) N1s, (c) P2p, respectively. 11


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Characterization of PDA/P(DMA-co-MPC) Coatings on Polystyrene (PS) and TiO2 Substrates: Surface Morphology, Wettability and Composition. Besides mica, PS and TiO2 substrates were used for coating tests to verify the versatility and feasibility of the co-deposition method. After being incubated in Tris buffer (pH 8.4) containing 4 mg/mL of DA and 4 mg/mL of P(DMA-co-MPC) for 12 hrs, the obtained substrates were then characterized in terms of surface morphologies and water CAs (Figure 4a-4h). As shown in Figure 4a and 4c, the water CA of PS substrate decreased significantly from 85.9° to 18.5° after incubation, indicating the successful immobilization of PMPC on PS substrate. The successful immobilization of PMPC on PS was further confirmed by the characteristic peaks of quaternary ammonium and phosphate (N1s at ~402.5 eV and P2p at ~133.3 eV) in its high-resolution XPS spectrum (Figure 5a and 5b). Similarly, the characteristic peaks (quaternary ammonium and P2p) of PMPC were detected after TiO2 substrates being incubated, also confirming the successful polymer attachment on TiO2 surfaces (Figure 5c and 5d).

Figure 4. The water contact angles (CAs) and corresponding topographic AFM images of polystyrene (PS) before (a, b) and after (c, d) being coated with PDA/P(DMA-co-MPC), and TiO2 surfaces before (e, f) and after (g, h) being coated with PDA/P(DMA-co-MPC). The RMS

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roughness of PS, modified PS, TiO2 and modified TiO2 were 1.9, 1.8, 1.6 and 1.8 nm, respectively.

Figure 5. High-resolution XPS spectra of characteristic peaks of PDA/P(DMA-co-MPC) coated on PS (a, b) and TiO2 (c, d), respectively. Normal and Lateral Forces between PDA/P(DMA-co-MPC) Coatings. Adopting an optimized feeding

ratio

of

DA : P(DMA-co-MPC)

at

4 mg/mL :

4

mg/mL,

PDA/P(DMA-co-MPC) coatings were successfully fabricated on mica surfaces with a relatively low surface roughness (RMS ~0.6 nm) and excellent surface wettability (water CA ~13.0°). The associated normal force interactions and lateral lubricating performances were investigated using an SFA. SFA has been widely used for measuring the normal and lateral forces between opposite substrates, and the detailed working mechanisms of SFA were described elsewhere.43 The normal forces between two PDA/P(DMA-co-MPC) coatings (symmetric configuration) were measured in PBS, and the results are shown in Figure 6a. Pure repulsion was detected 13


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during both surface approach and separation with negligible hysteresis. In contrast, obvious adhesion could be detected on the normal force-distance profiles for the case of PDA-PDA coatings (see Figure S5 and Figure S6 in Supporting Information for surface morphology of PDA coatings for SFA measurement and the adhesion between PDA-PDA coatings in PBS). Figure 6a and Figure S6 indicate that for the interactions of two PDA/P(DMA-co-MPC) coatings, there was no direct contact between the PDA-PDA anchoring layers even under a relatively high compression load (25 mN/m). The observed repulsions were mainly contributed by the well-confined and highly-hydrated PMPC chains at the outmost layer (Figure 6a).15, 59-60 To evaluate the lubricating properties of as-fabricated PDA/P(DMA-co-MPC) coatings, different loads were applied to measure the friction forces between two PDA/P(DMA-co-MPC) surfaces (symmetric configuration) in PBS (Figure 6b). The measured friction coefficient was determined to be as low as µ=0.036±0.002 under a normal load up to 2.8 MPa (Figure 6b), which was at the same order of magnitude as that (µ=0.0115±0.0003) measured between bottle-brush PMPC polymers adopting multiple loops or train-like conformation under a normal pressure of 2.1 MPa.61 (see Figure S7 for the determination of compressive load between sliding surfaces.) The PMPC segments in the PDA/P(DMA-co-MPC) coating were suggested to adopt a similar loop conformation due to the multiple anchoring sites (PDMA groups) on each P(DMA-co-MPC) copolymer. The proposed conformations of PMPC on mica surface are illustrated as the inset of Figure 6a. Within the PDA/P(DMA-co-MPC) coatings, the PMPC chains were chemically immobilized on the crosslinked structure of PDA layer, preventing the polyelectrolyte chains from being sheared off during friction tests even under high pressure. Meanwhile, the as-formed loop-structure PMPC chains could help to bear the load due to their 14


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steric repulsion, originated from the configurational entropy effects.61-62 In addition, the water molecules tenaciously bound to PMPC chains (15 water molecules were formed in the primary hydration shell of each phosphorylcholine (PC) segment).63 These water molecules were identified as remarkable lubricating elements, which could not only support a high pressure due to their resistance to be dehydrated from charged segments,15 but also form hydration layer to ease the surfaces sliding with a fluid-like response to shear.15, 60, 64-65 The lubricating properties of PDA/P(DMA-co-MPC) coatings remained similar when the surrounding aqueous medium was changed from PBS to pure water (see Figure S8 in Supporting Information for the friction forces measured in pure water), which was mainly contributed by the largely fixed conformation of PMPC chains of the PDA/P(DMA-co-MPC) coatings in aqueous media. The measured friction behaviors were found to obey the Amontons’ laws: the force of friction is directly proportional to the applied load and the kinetic friction is independent of the shear velocity (Vs=0.01 to >1 µm/s) (Figure 6b and 6c). Such Amontonian lubricating behavior was generally observed between solid surfaces with the confined fluid owning shear thinning properties.61, 66 Consisting of zwitterionic PMPC and PDA layer, the structure of PDA/P(DMA-co-MPC) coating was similar to that of proposed cartilage boundary lubricating configuration.67 At the same time, the as-fabricated PDA/P(DMA-co-MPC) coatings remained their excellent lubricating performances under a high compression ( ~2.8 MPa), demonstrating their outstanding mechanical properties. Such a biomimetic lubrication configuration is believed to be able to enhance the lubrication and anti-wear ability, thus has great potential for the development of artificial joints.68

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Figure 6. a) Normal interaction forces between two PDA/P(DMA-co-MPC) coated mica surfaces in PBS buffer. The inset shows the proposed conformation of PDA/P(DMA-co-MPC) coating with zwitterionic PC head groups (red) facing to aqueous solution. b) The lateral shearing interactions between two PDA/P(DMA-co-MPC) coated mica surfaces in PBS buffer. The friction forces were directly proportional to the applied load in PBS under the applied sliding velocity at Vs=0.15 to 1.2 µm/s. c) The kinetic friction was negligibly dependent of the sliding velocity (Vs=0.01 to >1 µm/s) under compressive load of ~1.5 MPa in PBS.

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Lateral Friction Interactions between PDA/P(DMA-co-MPC) Coating and BSA Layer. PDA/P(DMA-co-MPC) coating on mica surface was also used to test its lateral friction interactions with a BSA layer. 2 mg/mL of BSA/PBS solution was used to fabricate BSA layer on mica surface via dip-coating method. The BSA coating was thoroughly rinsed by PBS before use, and its surface morphology was imaged before SFA measurement (Figure S9). The lateral shearing interaction and tribological behavior between the PDA/P(DMA-co-MPC) coating and BSA layer were investigated in PBS buffer (0.13M, pH 7.4) using an SFA. Their lubricating properties were evaluated through measuring the friction forces under a series of applied normal loads. The friction coefficient was determined as µ=0.041±0.005 under a normal pressure up to ~4.8 MPa (Figure 7a). Such excellent resistance to high pressure was mainly attributed to the formation of water-containing BSA layer69 as well as the highly hydrated loop-structure PMPC in aqueous medium.15, 61-63 PMPC chains adopting loop-like conformation within the obtained PDA/P(DMA-co-MPC) coating could suppress the penetration of BSA molecules due to configurational entropy effects.61-62

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Figure 7. a) The lateral shearing interactions between PDA/P(DMA-co-MPC) coatings deposited on mica and BSA coated mica surface in PBS buffer. The friction force was directly proportional to the applied load in PBS with sliding velocity at Vs=0.12 to 1.23 µm/s. b) The kinetic friction force was found to be independent of the sliding velocity under a normal compression ~1.0 MPa in PBS.

Figure 7b showed that the kinetic friction between the BSA layer and PDA/P(DMA-co-MPC) coating was independent of the shear velocity, obeying the Amontons’ law. Such tribological phenomenon was ascribed to the shear thinning behavior of the confined fluid between surfaces as proposed previously,15, 70 and the overall surface interactions were dominated by the osmotic repulsive forces. Our experiment results provide new insights into the design and development of novel lubricating, compressive and wear-resistant PMPC-containing coatings under asymmetric interaction configuration in artificial medical devices. 19


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Antifouling Properties of PDA/P(DMA-co-MPC) Coatings. The antifouling performances of polymer coatings are significant to maintain the surface properties of as-fabricated polyelectrolyte coatings in biological fluids during their bioengineering applications. The adsorption/fouling of biomolecules on surfaces would alter the surface properties, thereby affecting their lubricating properties. A globular BSA has been widely regarded as an easy-fouling molecule on various biomedical substrates,44, 71-73 and it has been employed in this work to test the antifouling performances of as-fabricated PDA/P(DMA-co-MPC) coatings. Without being coated with PDA/P(DMA-co-MPC), the bare silica sensor could be severely contaminated by BSA with a significant adsorption (446.0±3.5 ng/cm2), evidenced by the sharp decrease of frequency and positive dissipation shift after the introduction of BSA solution (Figure 8a). In stark contrast, both frequency and dissipation remained their initial states during the exposure of PDA/P(DMA-co-MPC) coated silica sensor to BSA/PBS solution, indicating that negligible amount of BSA was adsorbed on PDA/P(DMA-co-MPC) coated silica sensor. The excellent protein-repellent properties of PDA/P(DMA-co-MPC) coatings were mainly contributed by the antifouling and densely-packed PMPC segments on the outmost layer of coating.74 To gain better understanding about its excellent protein-resistant performances, the normal force interactions between PDA/P(DMA-co-MPC) coating and BSA layer were also investigated and the experimental configuration was illustrated as the inset (Figure 8b). The measured force-distance profiles showed pure repulsion and overlapped during surface approach and separation, indicating that there was no adhesion hysteresis.75 The pure repulsion was mainly contributed from the tightly bounded hydration layer around charge-balanced PMPC segments, acting as a physical and energetic barrier to prevent proteins from direct contact with the substrate surfaces.76-77

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Figure 8. a) Changes of frequency and dissipation associated with the BSA adsorption on bare silica sensor (blue circle) and PDA/P(DMA-co-MPC) coated silica sensor (red square) during the introduction of BSA solution (1 mg/mL in PBS), followed by PBS rinse, as measured by a QCM-D

device.

b)

Force-distance

profiles

measured

between

BSA

layer

and

PDA/P(DMA-co-MPC) coated mica surface in PBS buffer, and the experimental configuration is illustrated in the inset. 21


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CONCLUSIONS

In summary, this work reports a facile and one-step dip-coating strategy to fabricate ultra-smooth zwitterionic polyelectrolyte coatings using a dopamine-assisted co-deposition method. By adjusting the feeding ratio of P(DMA-co-MPC) and DA, the desirable PMPC coatings with loop-like conformation could be obtained with excellent load-bearing capabilities and lubricating performances under physiological condition. The low friction coefficient (µ=0.036±0.002) achieved between PMPC-containing coatings under high applied normal load (~2.8 MPa) mainly arises from the strong osmotic repulsion between the robustly anchored zwitterionic polyelectrolyte coatings, the as-formed polymer coatings exhibit superior lubricating properties when sliding over a model albumin (i.e., BSA) layers with the friction coefficient down to 0.041±0.005 under normal loading pressure up to 4.8 MPa. Intriguingly, the as-prepared polyelectrolyte coatings also show Amontons-like behaviors under both symmetric and asymmetric configurations (i.e., PDA/P(DMA-co-MPC) to PDA/P(DMA-co-MPC), and PDA/P(DMA-co-MPC) to BSA layer). It is found that the obtained polyelectrolyte coatings could significantly resist bio-fouling (i.e., BSA), preventing the surfaces from being contaminated by proteins when applied in biological media, which thereby further demonstrates the great potential of this fabrication strategy to be utilized in complex biological systems. Our work presents a useful approach for readily fabricating functional coatings with striking lubricating properties, and provides new insights into the development of novel lubricating surfaces in bioengineering applications (e.g., artificial joints).

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ASSOCIATED CONTENT

Supporting Information is available free of charge on the ACS Publications website at DOI: XXX-XXX-XXX. Supporting information includes typical normal force interactions between PDA-PDA coatings in PBS buffer and typical lateral interactions between PDA/P(DMA-co-MPC) coatings in pure water.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected], Phone: 780-492-1044 *E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. †

These authors contributed equally.

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC), Canada Foundation for Innovation, the Canada Research Chairs Program (H. Zeng), China Scholarship Council, and Sichuan Science and Technology Program 2018G20381.

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Biomimetic Lubrication and Surface Interactions of Dopamine-assisted Zwitterionic Polyelectrolyte Coatings

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