Letter pubs.acs.org/macroletters
Articular Cartilage Inspired Bilayer Tough Hydrogel Prepared by Interfacial Modulated Polymerization Showing Excellent Combination of High Load-Bearing and Low Friction Performance Peng Lin,†,‡ Ran Zhang,†,‡ Xiaolong Wang,*,† Meirong Cai,† Jun Yang,† Bo Yu,† and Feng Zhou*,† †
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, People’s Republic of China ‡ University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China S Supporting Information *
ABSTRACT: Articular cartilage is a load-bearing and lubricious tissue covering the ends of articulating bones in synovial joints to reduce friction and wear. It ideally combines the high mechanical property and the ultralow friction performance as a result of biphasic structure and lubricious biomolecules. A biomimicking hydrogel with bilayer structure of thin porous top layer covering a compact and tough hydrogel bulk is fabricated with interfacial modulated polymerization. The top porous layer ensures the ultralow friction toward its contact pairs, while the bottom renders the high load-bearing property. Therefore, with bilayer architecture, hydrogel achieves an outstanding combination of low friction and high load bearing performance with long wear life when sliding against either steel or silicone elastomer counterpair.
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annealing can further improve the mechanical strength of PVA hydrogels, while largely reduces the water content, resulting in poor lubrication. Many other hydrophilic polymers such as polyacrylamide11 and polyvinylpyrrolidone12 were also used to form semi-interpenetrating network hydrogel to enhance the lubricity yet the mechanical strength is not satisfied enough.13 Therefore, the critical challenge in simultaneous combination of excellent lubrication and high mechanical property still exists. Gong et al. proposed an easy hydrophobic substrates induced method to form a less cross-linked top layer on highly crosslinked hydrophilic polymers.14 Hydrogels with low friction are achieved, yet the polymers still have the low load-bearing capacity. The recent work is denoted to solve the poor loadbearing issue using a double-network hydrogel system.15 Inspired by the special biphasic structure of articular cartilage, we herein report an easy way to fabricate bilayer hydrogel with an innovative interfacial modulated polymerization method. Our recent dual-cross-linked hydrogel system shows the mechanical performance of tensile elastic modulus >30 MPa and compressive elastic modulus >6 MPa, which is superior to most of the double-network hydrogel systems.16 Therefore, the dual-cross-linked hydrogel was chosen as the basic ultrahigh mechanical substrate in present case to pursue ultrahigh loadbearing capacity. However, the preparation process was
rticular cartilage provides one of the most efficient aqueous lubrication systems with the characteristic of simultaneous high load-bearing and low-friction property.1 Unfortunately, due to the avascular nature and very limited capacity to repair, the articular cartilage is hard to heal once lesion or injury happens. Therefore, joint replacement has to be carried out in clinic.2 The knowledge of how articular cartilage functions is the key to acquire appropriate replacement materials. The last decades have seen extensive efforts, both experimentally and theoretically to elucidate its structure3 and the mechanisms4 behind. Biphasic model is very well-known which simplifies the articular cartilage structure as two phases, scaffold-like solid phase and fluid phase.5 The top surface of articular cartilage is porous fluid phase that can hold abundant interstitial fluid, providing low-friction and also bearing load in high speed situation.6 The solid phase is almost incompressible due to the electrostatic repulsion interaction of proteoglycans dispersed within collagen network to bear the load at low speed.7 Generally, the articular cartilage can be viewed as hydrogellike tissue, therefore, many researchers have paid much endeavor on fabricating hydrogel-based materials for repairing injured tissues.8 During the past decades, many gel-based materials have been developed. Poly(vinyl alcohol) (PVA) hydrogel, as a candidate of synthetic articular cartilage, has been widely studied. The methods to prepare PVA hydrogels include freezing/thawing,9 additive thermal annealing,10 and so on. However, the PVA hydrogels by repeated freezing/thawing normally exhibit inadequate strength and toughness. Thermal © XXXX American Chemical Society
Received: September 1, 2016 Accepted: October 4, 2016
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DOI: 10.1021/acsmacrolett.6b00674 ACS Macro Lett. 2016, 5, 1191−1195
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ACS Macro Letters
Scheme 1. (a) Schematic Depiction of the Preparation of Bilayer Hydrogel by a Three-Step Method; (b) Chemical Structure of Monomers; (c) Briefly Schematic Demonstration of Bilayer Hydrogel Containing Two Layers: Low Density Crosslinked Network for Lubrication Covers on the High Density Crosslinked Network for Load-Bearing
hydrogels have a poorly cross-linked top surface layer on a highly cross-linked substrate. The bottom highly cross-linked network has low water content and excellent mechanical strength for load-bearing, while the top poorly cross-linked network has high water content with softness and serves for lubrication. The bilayer structure was confirmed by the SEM Figure 1a−c showing the SEM images of gel-25%-2% sample treated by freeze-drying method. As observed by the cross-section SEM image (Figure 1a), the bilayer structure is obvious with a clear boundary in between. The top layer exhibits a large porous structure (Figure 1b), while the underlying substrate exhibits a compact structure (Figure 1c). Formation of the bilayer structure arises from different cross-linking density between the surfaces near ABS and the bulk. The bulk monomer polymerizes completely and thus forms a very tight physical network after Fe3+ cross-linking process. Meanwhile the polymerization of the monomers near ABS is interrupted because of the successive deactivation of free radicals from the active hydrogens of ABS, leading to short propagation chains and smaller polymerization degree. Consequently, the layer near ABS is in a low cross-linking degree, which largely limits the formation of tight network during the Fe3+ loading process. The bilayer hydrogels exhibited excellent mechanical strength as shown in Figure 1d,e. Taking gel-25%-2% as an example, a typical tensile stress−strain and compressive stress−
modified. Scheme 1 shows the concrete fabrication procedure of the bilayer hydrogel. Specifically, a 3D printed ABS (abbreviation for acrylonitrile butadiene styrene copolymer, Figure S1) mold polished with metallographical sand paper prior to use, was used as the reaction container to prepare the hydrogels. The monomers (Scheme 1b) used for the hydrogels are [2-(methacryloyloxy)ethyl] trimethylammonium chloride (METAC), acrylic acid (AAc) and acrylamide (AAm). An abbreviation of gel-X%-Y% is used in which the X% represents the molar ratio of acrylic acid (AAc)/acrylamide (AAm) and the Y% represents the molar ratio of METAC/(AAc+AAm). The mixing monomers can be polymerized by free radical polymerization. However, the ABS is hydrophobic polymer with abundant active hydrogens on the polymer chains that can significantly affect the radical polymerization due to the radical chain transfer reaction. Therefore, the polymerization speed near ABS plate is quite low, compared with that far from the ABS walls. As a result, after a given polymerization time when the bulk precursor monomers were fully polymerized, the polymerization near the ABS remained insufficient. This was evidenced by the fact that the resultant polymer near the wall of ABS mold assumes viscous state. The chemically cross-linked hydrogel sheets were soaked into Fe3+ solution to proceed the secondary ionic cross-linking, followed by 3 days’ immersion in deionized water to remove the superfluous ferric ions. Bilayer hydrogels were obtained. As illustrated by Scheme 1c, 1192
DOI: 10.1021/acsmacrolett.6b00674 ACS Macro Lett. 2016, 5, 1191−1195
Letter
ACS Macro Letters
Figure 1. (a−c) SEM images of bilayer hydrogel: (a) cross-section of integral bilayer hydrogel; (b) surface porous structure; (c) surface of the loading-bearing substrate; typical tensile stress−strain (d) and compressive stress−strain (e) curve of the bilayer hydrogel.
Figure 2. Tribological measurements of bilayer hydrogel: (a) schematic of tribological measurement mode; (b) coefficient of friction of bulk hydrogel and porous surface layer with different METAC contents (applied load 10N, reciprocating frequency 5 Hz); (c) effect of applied load on the coefficient of friction at reciprocating frequency 5 Hz; (d) effect of frequency on the coefficient of friction (applied load 10 N; All measurements were performed in the presence of deionized water using 6 mm diameter steel ball as friction contact pair).
strain are presented. From Figure 1d, the gel-25%-2% can be stretched 3.8× to its original length, and its nominal tensile stress can be as high as ∼9 MPa. The gel-25%-2% also has excellent compressive mechanical strength. It can be compressed to 90% without rupture with the compressive stress reaching as high as 45 MPa (Figure 1e), indicating the excellent toughness and strength. The ferric ions play a vital role in enhancing the mechanical strength. For comparison, the mechanical strength of control samples without ferric ions is shown in Figure S2. It shows that the tensile stress of gel-25%2% is as low as 0.11 MPa with a tensile strain ∼380%, and its compressive stress is also very low ∼0.32 MPa. Evidently, the high mechanical strength is due to the formation of coordinates between Fe3+ and carboxyl groups. The ultrahigh mechanical strength provides a solid basis for the high load-bearing strength. It should be noted that the top lubricious layer has quite high water content and certainly exhibits low mechanical strength, but it is very thin so it does not influence the integral mechanical strength of the gel-25%-2%. Tribological measurements were conducted to reveal the low-friction property and lubrication mechanisms. Figure 2a shows the tribological measurement performed on a ball-onplate mode in reciprocating way sliding against a 6 mm diameter steel ball. Figure 2b shows the lubricating properties of the substrate surface and the lubricious surface at 10 N load and 5 Hz slide frequency. It is found that the lubricious surface shows much lower friction than the substrate for a given hydrogel in all cases. The friction performance of the lubricious and supporting substrate is highly dependent on the content of METAC. METAC has high hydrophilicity and thus water content of the bulk hydrogel increases with the content of METAC. As shown in Figure S3, when there is no METAC in the network, water content of the bottom substrate is as low as ∼50 wt %, however, when the molar ratio of METAC/AAc +AAm increases to 1 mol % and 3 mol %, water content becomes 53.6 and 78.6 wt %, respectively, and as a result, friction coefficient of the substrate surface decreases from 0.348 to 0.156 and 0.109, respectively (Figure 2b). It can be
concluded that the existence of hydrophilic METAC improves friction-reduction of the bulk hydrogel, but what’s more important for the present bilayer hydrogel is the effect of METAC content on friction property of the top lubricious layer, which is more relevant to articular cartilage replacement. It can be seen that when the molar ratio of METAC/AAc +AAm increases from zero to 1 mol % and 2 mol %, the top lubricious layer exhibited COF as low as 0.023 and 0.021, respectively. The much better lubricating properties of the top lubricious layer is attributed to its high water trapping capability with large porous structure. However, superfluous water may result in negative effect on friction-reduction. It can be seen that the friction coefficient of 3 mol % METAC sample is 0.037 (Figure 2b), which is higher than that of 1 mol % and 2 mol % METAC samples. It can be explained as when too much water is trapped, the thickness of the soft lubricious layer increases, which exhibits negative effect on friction-reduction because of severe deformation during the test. Hydrogels are viscoelastic and their complex surface physicochemical properties always lead to complex tribological performance.17 So the tribological property of hydrogels highly depends on the applied load, slide frequency and external stimulation.18 The effect of applied loads and sliding frequencies on friction property of the resultant hydrogels is given in Figure 2c,d. The interface interaction increases with the applied load, as a result, the friction coefficient increased from 0.015 to 0.053 when the load was increased from 5 to 30 N with maximum Hertz contact stress estimated at ∼2.03 and ∼3.68 MPa, respectively. Besides, the sliding velocity also affects the friction coefficient. When the slide frequency was increased from 1 to 10 Hz, the friction coefficient were 0.028, 0.025, 0.021, 0.021, and 0.013, respectively. The decreasing trend of friction was depicted from data. It is attributed to different deformations under applied loads. Sinking depth of the contact area has closed relationship with the contact time. The increased elastic deformation caused by the applied load should increase the friction force when deformation has an increasing trend. In low slide frequency situation, the steel ball 1193
DOI: 10.1021/acsmacrolett.6b00674 ACS Macro Lett. 2016, 5, 1191−1195
Letter
ACS Macro Letters
experience large deformation under normal load of 20 N, leading to a much smaller maximum Hertz contact stress, ∼1.2 MPa estimated. This explains why the friction coefficient remains
