Tuning the Hydration and Lubrication of the Embedded Load-Bearing

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Tuning the hydration and lubrication of the embedded load bearing hydrogel fibers Ran Zhang, Yange Feng, Shuanhong Ma, Meirong Cai, Jun Yang, Bo Yu, and Feng Zhou Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b03883 • Publication Date (Web): 15 Feb 2017 Downloaded from http://pubs.acs.org on February 17, 2017

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Tuning the hydration and lubrication of the embedded load bearing hydrogel fibers Ran Zhang†,‡, Yange Feng†,‡, Shuanhong Ma†, Meirong Cai†, Jun Yang†, Bo Yu*,†, Feng Zhou*,† †

State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Tianshui middle Rd, 730000 Lanzhou, China ‡

University of Chinese Academy of Sciences, 100049 Beijing, China

ABSTRACT: One of the most prominent properties of hydrogels is their excellent hydrolubrication that derives from the strong hydration of the gel network. However, excessive hydration makes hydrogels very poor mechanical property, which limits the practical applications. Here, the novel composite surface of hydrogel nanofibers embedded in anodic aluminum oxide (AAO) substrate was prepared and exhibited both excellent lubrication and high load bearing capacity. Through the copolymerization of acrylic acid (AA) and 3-Sulfopropyl methacrylate potassium salt (SPMA), gel network swelled sufficiently in aqueous solution and caused high osmotic pressure repulsion to bear heavy load, and hence performed excellent aqueous lubrication (µ~0.01). Notably, the friction coefficient of gels shows no dependence on the load in the experiment, while it is strongly influenced by the sliding velocity. Additionally, the electrolyte solution, and ionic surfactants all affect the conformation of polymer chains which make a significant impact on the friction properties of hydrogel fibers.

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1. INTRODUCTION The lubrication of synovial joints is one of the most efficient and sophisticated lubrication practices, with friction coefficient in the range of 0.001-0.01 under typical pressures of ∼5 MPa at human hips or knees.1,2 The structural organization of cartilage constituents provides the excellent mechanical properties and frictionless movement on the surfaces of articulating joints.3, 4

Inspired by this, hydrogels as a kind of ideal substitute materials of articular cartilage, are

widely researched and show abroad applications in many fields.5,6 One of the most prominent properties of hydrogels is their excellent lubrication, and the friction coefficient could even be as low as 10-3.7 However, the most serious problem of hydrogel for a long time is the poor mechanical behavior on its application field. Recently, the double network (DN) gels8, nanocomposite gels9 and sliding-ring gels10 have been developed and exhibit excellent mechanical properties. In addition, utilizing divalent or trivalent metal ions as the crosslink points to develop stretchable and tough hydrogel has also aroused wide interests.11-13 Lin et al. fabricated novel dualcrosslinked network hydrogels that combined both covalent bonds and multivalency Fe3+-acrylic acid coordination, which possessed ultrahigh mechanical strength (~6 MPa), excellent elongation (>7 times) and good self-recovery property.14 Moreover, a hydrogel with self-healing capability after inflicted damage will be of great significance to extend its application and lifespan. The healable polymer networks usually constructed through hydrogen bonding and electrostatic interactions exhibit both efficient self-healing properties and good mechanical properties.15, 16 Therefore, practical applications of hydrogels as biological lubricants are very much promising. In addition, the understanding and optimization of the friction of hydrogels have been important for practical applications. To date, the low friction mechanism of polymer hydrogels


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has been mainly studied by many groups. Gel friction strongly depends on the chemical structure, such as hydrophilicity, charge density, cross-linking density, water content and elasticity.15-17 The researchers systematically studied the friction behaviors of hydrogels in order to reveal friction mechanism and one of the most famous models is repulsion-adsorption model proposed by Gong. Their studies have revealed that the friction of hydrogels in water is strongly dependent on the interfacial interaction between the polymer network and the opposing surface.17-19 An attractive interaction led to a higher friction force than that of repulsion, and the stronger the repulsion, the lower the friction. Additionally, they used the resonance shear measurements (RSMs) to study the frictional properties of DN gel against a silica sphere and firstly quantitatively estimated the influence of the elastic deformation of gels on the friction force.7 As far as we know, the strong hydration ability of the gel led to the excellent lubrication, while excessive hydration usually drastically reduced the gel bearing capability.20 However, it has been reported that the combination of soft matter and hard substrate can achieve ultralow friction even at relatively high contact pressure (e.g., articular cartilage). Inspired by this, the novel composite surface of ordered hydrogel nanofiber embedded in AAO nanoporous substrate based on a soft/hard combination was developed in our previous work.21 Poly (acrylic acid) (PAA) gel fibers were capable of achieving very low friction coefficient (~0.005) under high load (~40 N). However, the ultralow friction of gels can be realized only in alkaline solution, which can be attributed to the deprotonation and hydration of carboxylic acid group occurred in high pH solution.22, 23 On the other hand, it greatly limits their applications in biological field. Therefore, we attempted to introduce another ionic monomer to maintain highly hydration under neutral conditions. Poly (3-sulfopropyl methacrylate potassium salt) (PSPMA), as a kind of strong


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polyelectrolyte, contains sulfonate group in the side chain while it can produce strong electrostatic repulsion between charged chains because of dissociation of the K+-sulfonate ion pair. Through the copolymerization of AA and SPMA, gel fibers could cause high osmotic pressure repulsion to bear heavy load and therefore performed excellent water lubrication property. 2. EXPERIMENTAL SECTION 2.1 Materials. Aluminum sheet (purity 99.999%, thickness 0.3 mm, Zhongnuo Advanced Material (Beijing) Technology Co., Ltd.) was used as received. The monomer acrylic acid (AA, Sigma-Aldrich) and 3-Sulfopropyl methacrylate potassium salt (SPMA, 95%, TCI), the crosslinker N, N′-methylenebisacrylamide (BIS, Sigma-Aldrich) and the initiator potassium persulfate (KPS) were commercially available and used without any purification. Other general reagents and solvents were used as received. Deionized water was applied for all polymerization and treatment processes. 2.2 Fabrication of the nanoporous anodic aluminum oxide (AAO) templates. AAO templates were prepared as described in the literatures.24, 25 Firstly, the aluminum sheet was cleaned by acetone in ultrasonic cleaner for 2 h and soaked in NaOH solution (0.1 M) for 1 min to remove the oxide film on the surface. Al sheet was then polished through an electrochemical method for 10 min in a voltage of 15 V. Finally, a two-step anodization method was used to prepare AAO template in 0.3 M oxalic acid with a voltage of 60 V. The first oxidation process lasted for 2 h and then was immersed into the mixture solution of H2CrO4 (0.3 M) and H3PO4 (0.4 M) for 1 h to remove the oxidation films. Then, a second anodic oxidation was performed under the same condition for 4 h. The obtained alumina substrate was activated in H3PO4 (5%) for 5 min to enlarge the pore size.


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2.3 Preparation of hydrogel fiber arrays First, variable molar ratios of SPMA (10, 2 and 1 mmol) and AAc (1.5 g, 20 mmol), initiator KPS (30 mg, 0.1 mmol), chemical cross-linker BIS (20 mg, 0.1 mmol) and the deionized water (10 g) were added to a 50 mL centrifuge tube to form a homogeneous solution. After vigorously stirred for 1 min with nitrogen, the resulting solution was poured into AAO membrane and filled the hole entirely through vacuum extraction. Then the substrate was exposed under high temperature for 5 h to initiate the free radical polymerization. In the end, the ordered gel fiber arrays underneath were formed after removing the top bulky gel layer. 2.4 Characterization The surface morphology of hydrogel fiber was obtained on a JSM-6701F field emission scanning electron microscope (FE-SEM) at 5-10kV. The absorption peaks of the samples were measured on a Perkin-Elmer Transform Infrared Spectrometer (Perkin-Elmer, USA). In order to analyze the structure accurately, the sample was scraped from AAO surface. The chemical composition about the samples were obtained by X-ray photoelectron spectroscopy (XPS); the measurement was carried out on an ESCALAB 250xi spectrometer (Thermon Scientific, USA) using Al Kα radiation and the binding energy of C1s (284.8 eV) was used as the reference. The swelling tests were conducted to evaluate the water content of gel fibers. First, the samples (1 cm ×1 cm ×0.4 mm) were immersed in deionized water for more than 24 h in order to ensure their saturation with water. The water flowing on the surface of samples was removed quickly with filter paper, and then the swollen weights (Ws) were measured by an analytical balance. The weights of dehydrated sample (Wd) were obtained after the samples were dried for 24 h at 60 °C in vacuum. The outside length of gel layer was obtained from the SEM images and then the


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volume (Vg) can be calculated (1 cm×1 cm×4 µm). The equilibrium water content (EWC) was calculated by equations as followed: (/ ) =

( – )

The tribological tests of hydrogel fiber were performed on a conventional pin-on-disk reciprocating tribometry (UMT-2, CETR) by recording the friction coefficient (µ) at different sliding conditions. Elastomeric poly-(dimethylsiloxane) (PDMS) hemisphere with a diameter of 6 mm was employed as a pin against hydrogel in aqueous solution. The distance of one sliding was 5 mm and the friction coefficient was determined by dividing the friction force by applied normal load. Each friction test was carried out by sliding a silicone elastomer ball at a sliding velocity of 0.01 m/s under the load of 10 N. Unless otherwise mentioned, the friction tests of gel fibers were performed in pH 7.5 buffer solution. 3. RESULTS AND DISCUSSION The morphology and composition of gel fibers play important roles in their aqueous lubrication and load bearing capacity. According to previous work, poly(acrylic acid) (PAA) gel fibers were capable of achieving very low friction coefficient (~0.005) under high load (~40 N) in alkaline medium.21 By introducing another strong polyelectrolyte component PSPMA, the gel fibers are hopeful to realize high lubrication and low friction under neutral conditions. Figure 1 shows the schematic illustration of the fabrication process of hydrogel fibers. First, the mixture of SPMA and AAc monomer, BIS and KPS was poured into AAO templates and initiated polymerization both in the nanotubes and surface. After polymerization, the sample was placed into the air. The top gel layer was more easily to lose water and generated stronger shrink stress than gel fibers in pores. At this case, the top bulk gel trend to detach from the AAO substrate while the gel fibers were toughly confined in the pores. Finally, the top gel layer separated from


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the AAO substrate and the gel fibers were left in the pores. At the same time, the gel fibers were plucked out a few microns from the hole due to the instantaneous shrink stress, leading to the formation of ordered gel fiber arrays. By adjusting the molar ratio of monomer AA and SPMA, the lubrication property of gel fibers can be further optimized. The morphology of gel fibers with different molar ratio of monomer is demonstrated by the FESEM images of the top views and the respective magnified images as shown in Figure 2. The AAO membrane prepared by two-step anodization method possessed hexagonal structure in the outer edge and the inner pore with diameter of 80-100 nm. Generally, the thickness of AAO layer used in this study was about 40 µm (Figure S1). For PAA gel fibers, they were separated from each other even on the top and had an average diameter size of 100 nm. The addition of SPMA led to the aggregation of fibers on the top (see the top part of fibers in the magnified images as shown in Figure 2(a-d). The outside length of gel fibers was about 4 µm (Figure S2), and the inner gel fibers were tightly confined in nanoporous to ensure their long-term stability. In addition, the FT-IR analysis revealed adsorption band of S=O at 1195 cm-1 and the XPS spectrum showed characteristic peaks of S2s and S2p at 228 and 165 eV respectively, which all indicated the successful copolymerization of SPMA and AA (Figure S3).


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Figure 1. Schematic illustration of the fabrication process of hydrogel fibers.

Figure 2. The FE-SEM images of gel fibers with different ratio of monomer (A) PAA, (B) P(AA/SPMA0.05), (C) P(AA/SPMA0.1), (D) P(AA/SPMA0.5); (a)–(d) were the respective magnified images.

Figure 3. (A) Friction coefficients of the gel fibers with different molar ratio of AA and SPMA in buffer solution (pH 7.5) and (B) the corresponding friction curve of P(AA/SPMA0.5) gel fibers. The friction test was carried out by sliding a silicone elastomer ball at a sliding velocity of 0.01 m/s under the load of 10 N.


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The friction coefficients of the gel fibers with different molar ratio of AA and SPMA in buffer solution are shown in Figure 3A. The inset displays the conventional pin-on-disk reciprocating tribometry and the elastic PDMS hemisphere adapted in the holder. The PAA gel fibers exhibited higher friction coefficient (~0.34) under neutral media owing to the weaker dissociation of carboxylic acid group. While with increasing the amount of SPMA, the friction coefficients of the gel fibers gradually decreased to 0.21, 0.09 and even 0.01, respectively. Obviously, the addition of SPMA made gel fibers hydrate easier under aqueous medium and therefore performed excellent lubrication property. The increase of water content with the addition of SPMA also confirmed it (Figure S4). This water might form a thin film layer and serves as a lubricator to decrease the friction.17 Furthermore, the contact interface between PDMS ball and gel fibers was mutually exclusive since they were both negatively charged.26 On the basis of repulsion-adsorption model, the composite interface in sliding was separated due to a hydration layer retained at the interface, to give a very low friction, and the stronger the repulsion, the lower the friction. In addition, the wear test of P(AA/SPMA0.5) gel fibers was performed for 10000 reciprocating cycles in order to evaluate the wear-resisting property of fibers. As shown in Figure 3B, the friction coefficient vs time curve verified their superior wear-resisting performance while the friction coefficient increased slightly. For comparison, P(AA/SPMA0.5) bulk hydrogel with the same chemical composition was prepared and investigated the friction performance (Figure S5). The bulky gel was embedded in deeply by the PDMS ball even at a lower applied load 3N and thus exhibited a higher friction than the confined hydrogel. After 2000 reciprocating cycles, the friction coefficient reached up to 0.21, leading to severe wear of the bulky gel.


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We also characterized the wear morphology of PDMS pin and gel fibers after 10000 friction cycles. First, the contact area at 10 N load was obtained by dyeing the PDMS pin. According to the contact area (14.5 mm2), the maximum contact pressure under a load of 10 N was calculated to be 1.03 MPa. As shown in Figure S6, there was no obvious abrasion mark on the surface of PDMS pin under the high contact pressure, and only little small gel fragments was observed on the counterface, but which should not affect the lubrication of interface. Besides, after testing, the ordered gel array structure disappeared, along with the appearance of the polymer gel film (Figure S6D, top view) with the thickness decreased from 4 µm to 1.0-1.5 µm (Figure S6E). From the cross-section view, the friction process led to the fracture of gel fibers and the formation of the denser polymer gel film under high load. Moreover, the formed gel film could be tightly confined onto the substrate, otherwise it would be easily sheared off by the high speed. Notably, the inner gel fibers played a significant role in anchoring the gel film onto porous substrate. Taken together, these results suggest that the strong hydration ability of the gel fibers and tight confine onto nanoporous substrate made it possible to sustain the low friction even at a very low sliding velocity and under a high load. Next, we explored the changes of friction coefficients of gel fibers after immersing in different pH buffer as shown in Figure 4. It’s well known that carboxylic acid group can deprotonate and make the polymer chains highly swell in solution with high pH value.27 Hence, the strong electrostatic repulsion between chains can bear high load and show excellent lubricating property. In regard to PAA hydrogel fibers, the friction coefficients decreased rapidly from 0.40 to 0.007 with the increase of pH values. Previous work has shown that the pH-driven electrostatic interaction between PDMS and gel surface is proportional to the product of their surface potential ζgelζPDMS.28 As the pH increases, ζgelζPDMS starts to increase and crosses zero at


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pH 9, which is associated with the weak acid nature of the carboxylate group.29 Beyond this pH, the sliding surfaces were in repulsion and the frictional stress has been decreased, leading to ultralow friction coefficients. With the addition of strong electrolyte SPMA, there was a greatly reduced dependence on pH value. By contrast, P(AA/SPMA0.5) gel fibers presented a slight decrease in friction coefficients from 0.02 to 0.003, which could be attributed to the strong sulfonate group that can keep a high dissociation state and not be affected by solution pH.30 The sliding surfaces were always in repulsion and then kept the lower friction within the scope of pH from 4.0 to 11, which could have great potential to be used in the body. For P(AA/SPMA0.05) and P(AA/SPMA0.1) gel fibers, however, due to the low amount of sulfonate groups in gel network that hardly played a critical role, and hence the friction coefficients declined rapidly with the increase of pH values. Importantly, the effect of pH has revealed that above pH 9, the strong electrical double-layer repulsion between two negatively charged surfaces led to low friction.

Figure 4. (A) Schematic diagram illustrating the effects of pH on the swelling properties of the polymer chains and (B) the corresponding friction coefficients of the gel fibers with different molar ratio of AA and SPMA in media with different pH values. The friction test was carried out by sliding a silicone elastomer ball at a sliding velocity of 0.01 m/s under a load of 10 N.


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We have also investigated the effects of the sliding velocity and load on the lubricating properties of P(AA/SPMA0.5) gel fibers. Figure 5A shows the friction coefficients of P(AA/SPMA0.5) gel fibers at different velocities, for a applied load of 10 N. The values of friction coefficients slightly increased from 0.010 to 0.016 while the sliding speed changed from 0.01 to 0.05 m/s. At low speeds, there is enough time for water molecules to diffuse to the contact interface and thus water makes a good lubricative effect. As the speed increases, there is not enough time for water to diffuse to the wear zone, leading to partially dehydrated of gel fibers and an increase in friction.31 Figure 5B shows the friction coefficients at different applied loads, for a sliding speed of 0.01 m/s. It was obvious that the friction coefficients did not change obviously and remained low values when the normal loads changed from 7 to 40 N. In conclusion, the frictional behaviours of hydrogel fibers did not conform to Amonton’s law F= µW. Instead, the friction coefficients of gels showed no dependence on the load in the investigated load range, while it was strongly influenced by the sliding velocity. According to previous paper, the velocity dependence of the gel friction suggests a hydrodynamic lubrication mechanism.16, 17, 32


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Figure 5. (A) Friction coefficients of the P(AA/SPMA0.5) gel fibers for different sliding velocities (at F = 10 N). (B) Friction coefficients of the P(AA/SPMA0.5) gel fibers for different loads (at ʋ = 0.01 m/s). The interactions between metal ions and polymer chains have been widely discussed for a long time. One of the most common research is to utilise divalent or trivalent metal ions as the crosslink points to develop self-healing and tough hydrogels.5,11, 13, 14 Therefore, both electrolyte solution and metal ions could make a significant impact on the interfacial property of hydrogel fibers. The friction coefficients of the P(AA/SPMA0.5) gel fibers in various salt concentrations are shown in Figure 6A. After immersing in different concentration of NaCl, the friction coefficients of gel fibers increased gradually from 0.01 to 0.29. In general, the ionic strength of salt solution affects the conformation of the polymer chains.33 In high ionic strength solution, the polymer chains collapsed and dehydrated, thus a higher friction of interface was obtained. In addition, the protonation equilibrium of carboxylic group was also influenced by the ionic strength of the medium, which may weaken the screening effect.29 Figure 6B shows changes of friction coefficients of P(AA/SPMA0.5) gel fibers after coordinated with divalent Ca2+, Cu2+ and trivalent Fe3+. The friction coefficients sharply increased from 0.01 to 0.20, 0.27 and 0.31, respectively. As previously reported, Cu2+ ions can be coordinated to both oxygen atoms of the carboxylate ligand and form chelating bidentate complexes.34 The strong interaction neutralized the charges on polymer chains and thus forced them to collapse and dehydrate, which explained the ultrahigh friction of hydrogel after coordinated with Cu2+. Similarly, the friction coefficients of gel fibers increased significantly after coordinated with Ca2+ and Fe3+. In addition, the interaction of hydrogel fibers with surfactants was also explored. The interaction of a polyelectrolyte gel with oppositely charged surfactants has been studied widely


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in recent years.35, 36 The absorption of cationic surfactant results in anionic gel network collapse and the interaction is governed mainly by electrostatic interactions at lower critical micelle concentration (CMC).37 In order to investigate the electrostatic attraction, the friction tests were performed in weak basic media (pH 10) to make sure the fully dissociation of carboxylate group. As shown in Figure 7A, the friction coefficients of P(AA/SPMA0.5) gel fibers increased gradually from 0.003 to 0.11 after immersing in different concentration of cationic surfactant hexadecyltrimethyl-ammonium bromide (CTAB). Adsorption of oppositely charged surfactants reduced the net charge of polymer chains and thereby weakened the repulsion between the sliding interfaces, leading to the increase in friction. Inversely, no significant change in friction coefficients was observed upon exchanging with anionic surfactant sodium dodecyl sulfate (SDS) (Figure 7B), which illustrates there are no interactions on the molecular level between anionic PAA gels and similarly charged surfactant SDS.38, 39

Figure 6. (A) Friction coefficients of the P(AA/SPMA0.5) gel fibers for various salt concentrations. (B) Friction coefficients of the P(AA/SPMA0.5) gel fibers after coordinated with divalent Ca2+, Cu2+ and trivalent Fe3+ ions. The friction test was carried out by sliding a silicone elastomer ball at a sliding velocity of 0.01 m/s under a load of 10 N.


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Figure 7. Friction coefficients of the P(AA/SPMA0.5) gel fibers after exchanging with different concentration of cationic surfactant CTAB (A) and anionic surfactant SDS (B). The friction test was carried out by sliding a silicone elastomer ball at a sliding velocity of 0.01 m/s under a load of 10 N. 4. CONCLUSIONS The novel composite surface of ordered hydrogel nanofibers embedded into AAO nanoporous substrate has been developed and exhibits both low friction coefficient and high load bearing capacity under neutral solution. The copolymerization of AA and SPMA make the gel network high osmotic pressure repulsion to bear heavy load and therefore perform excellent lubrication property (µ~0.01) under high load (10 N). Study on the effect of pH has shown that above pH 9, the composite interfaces in sliding are in repulsion since they are both negatively charged, to give a very low friction even under high loads. Moreover, the friction coefficient of gels is strongly influenced by the sliding velocity, while it shows no dependence on the load in the investigated load range. In addition, as the conformation of the polymer chains can be significantly affected by medium factors, including electrolyte solution, metal ions and ionic surfactants, thus the tuning of the friction behaviors of hydrogel fibers can be achieved as well.


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This novel composite interface may provide important guidance for the development of biomimetic cartilage articular materials.

ASSOCIATED CONTENT Supporting Information. The Support Information is available free of charge via the Internet at http://pubs.acs.org. The FE-SEM images of AAO and gel fibers; The FT-IR and XPS spectra of gel fibers; The equilibrium water content of gel fibers; The friction curve of bulk hydrogel P(AA/SPMA0.5); The FE-SEM images of the test tracks. AUTHOR INFORMATION Corresponding Author *E-mail [email protected] (B.Y.). *E-mail [email protected] (F.Z.). Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was financially supported by NSFC (51573198, 51335010, 21434009), and Ministry of Science and Technology (2016YFC1100401). REFERENCES 1.

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Tuning the Hydration and Lubrication of the Embedded Load-Bearing

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