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Bio-inspired supramolecular lubricating hydrogel induced by shear force Xuewei Zhang, Jian Wang, Hui Jin, Shutao Wang, and Wenlong Song J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b12886 • Publication Date (Web): 30 Jan 2018 Downloaded from http://pubs.acs.org on January 30, 2018
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Bio-inspired supramolecular lubricating hydrogel induced by shear force Xuewei Zhang,† Jian Wang,† Hui Jin,‡ Shutao Wang*,§,# and Wenlong Song*,† †
The State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130023 P. R. China § CAS Key Laboratory of Bio-inspired Materials and Interfacial Science, CAS Center for Excellence in Nanoscience, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190 P.R. China #University of Chinese Academy of Sciences, Beijing 100049, P. R. China ‡ Orthopaedic Institute, the Second Hospital of Jilin University, Changchun, 130041, P.R.China
Supporting Information Placeholder ABSTRACT: Bio-inspired lubricating materials are great challenge towards to the artificial joint. In this contribution we synthesize a bio-inspired hydrogel by combining thixotropic supramolecular network and polymer double network, exhibiting unique shear-responsive lubricating property. The disassembly of FT supramolecular network triggered by shear force will endow lubricating function to the hydrogel; meanwhile PAAm and PVA double network acts as the supporting skeleton with high mechanical property. This work will bring new insight on the design of artificial lubricating joint.
Bio-inspired smart hydrogel1 has aroused great attentions because of its potential applications in soft robot, 2 drug delivery,3 cells capture4 and so on. Especially in the field of artificial cartilage the bio-inspired smart hydrogels are generally fabricated by introducing the responsive factors to the polymer networks to pursue the controlled ultra-low frictional property.5 They are mainly focus on the regulation of surface chemical composition, topography and mechanical property. However cartilage lubrication is a very complex dynamic process,6 which is strongly related to not only the cartilage surface itself but also the external synovial fluid. Even as early as 1886, Reynolds had already pointed out that there was a fluidic film existed between two cartilage surfaces that performed the key lubricating characterizations.7 More important, the cartilage exhibits the dynamic lubricating function that the joint fluid will weep out from cartilage inner to the surface, and participate in the cartilage lubrication based on the weeping lubricating mechanism.8 The existence of dynamic fluidic lubrication allows human free motion, which is also offer a direction on fabricating the bioinspired smart artificial lubricating materials. Large quantities of efforts had been developed on fabricating artificial lubricating materials with low friction property. Tough hydrogels are concerned emphatically because cartilage is actually one kind of hydrogels.9 Especially the double network hydrogel developed by Gong et al. has been widely used in fabricating artificial cartilage due to the high mechanical property.10 Further-
more Zhou et al. prepared a bilayer tough hydrogel inspired by articular cartilage that exhibited low frictional property.9a However, it can be seen that most works are mainly focus on achieving the low frictional materials. It is still great challenge to develop the artificial lubricating materials that can generate lubricating layer under mechanical stimulus.
Figure 1. The bio-inspired lubricating principle of the supramolecular hydrogel. a) Before being sheared, the non-covalent supramolecular network introduced into covalent double polymer network. b) After being sheared, wept lubricating layer appears on the hydrogel surface. Herein inspired by the weeping lubricating phenomenon of articular cartilage we synthesize a bio-inspired hydrogel by combining supramolecular network and double polymer network, which exhibits unique shear-responsive lubricating property. Two key factors are involved in the hydrogel preparation: one offers dynamic lubricating ability during the shear process; the other is offering high mechanical support. First the non-covalent interaction in the supramolecular hydrogel leads easy gel-sol transition under external stimuli.11 The thixotropic supramolecular hydrogel is one kind of hydrogel that gel-sol transition can be regulated by
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shear force.12 Its viscosity decreases continuously with applied shear, and the viscosity will be subsequent recovery in time when the shear is discontinued. When hydrogel is disassembled triggered by shear force, it will change from gel to sol state. This will offer the possibility on the emergence of lubricating agent. Nfluorenylmethoxycarbonyl-L-tryptophan (FT) hydrogel is reported as a typical thixotropic supramolecular hydrogel.12a The quick gel-sol transition of FT (Figure S1) can be triggered by shear force due to the easy-destroyed balance of the weak non-covalent interactions including hydrogen bond and π-π stacking. Secondly tough hydrogel will undertake the role of support. We had reported a double network polyacrylamide/poly(vinyl alcohol) (PAAm/PVA) hydrogel that could be used as artificial cartilage due to the high mechanical strength.10c Therefore it can be predicted that the cooperation of the thixotropic FT supramolecular hydrogel and tough PAAm/PVA one would offer a possible way to fabricate the supramolecular lubricating artificial hydrogel triggered by shear force as exhibited in Figure 1.
Figure 2. a) The preparation of FT-PAAm/PVA supramolecular hydrogel. b) The broken strength of the prepared PAAm/PVA and FT-PAAm/PVA hydrogels. c) The oscillation stress dependency of the storage modulus G’ and loss modulus G’’ of FT, PAAm/PVA, FT-PAAm/PVA hydrogels. All the tests were done for three times for the parallel measurement. In this contribution, shear-responsive hydrogel combined with supramolecular FT network and PAAm/PVA double network (FT-PAAm/PVA) was prepared by using one-pot method (Figure 2a). Based on the concentration of FT in FT-PAAm/PVA hydrogel, the prepared hydrogels were marked as FTPAAm/PVA1.0, FT-PAAm/PVA2.0 and FT-PAAm/PVA3.0. The introduction of FT in PAAm/PVA double network was identified by FTIR spectrums (Figure S2). Because tough mechanical property is the prerequisite for constructing artificial cartilage hydrogel, the hydrogel mechanical property was investigated on a materials testing system. The broken strength of these FTPAAm/PVA hydrogels decreased from 2.32±0.18 MPa to 1.37±0.17 MPa with the increase of FT concentration (Figure 2b and Table S1). Compared to the strength (3.87±0.18 MPa) of PVA/PAAm hydrogel, the strength of all these FT-PAAm/PVA hydrogels became weaker. The reason might be attributed that the network of FT-PAAm/PVA hydrogel would become looser than that of PVA/PAAm hydrogel due to the interpenetration of supramolecular FT chains into double network; and simultaneously supramolecular FT network was easily destroyed by shear force. Although the strength reduced, the minimum value on FT-PAAm/PVA3.0 was still higher than 1.0 MPa. The strong mechanics of all these FT-PAAm/PVA hydrogels would insure the consequent frictional tests going smoothly when suffering shear action.
The following rheological measurement was performed for getting insight into the hydrogel thixotropy (Figure 2c). The storage modulus (G’) indicates the quantity of stored energy in the system, it will characterize the solid-liked property of the hydrogel; the loss modulus (G’’) presents the quantity of dissipated energy in the network, it will characterize the liquid-liked property of the hydrogel. For FT supramolecular hydrogel the thixotropic gel-sol transition occur at the shear stress increased to 27 Pa, the G’ and G’’ joined together (Star lines in Figure 2c). But for the PAAm/PVA hydrogel the G’ was much higher than G’’ that indicated no thixotropy was achieved. When FT was introduced in the polymer network, FT-PAAm/PVA2.0 exhibited more sensitive to shear compared to FT-PAAm/PVA1.0; and no gel-sol transition in the testing shear range compared to FT-PAAm/PVA3.0 respectively. Therefore FT-PAAm/PVA2.0 hydrogel was selected as proof-of-concept for demonstrating the shear-responsive lubrication on the hydrogel. The friction test was performed on a ball-on-disk reciprocating tribometer (Figure 3ai). The surface friction data was collected before and after being sheared. When the ceramic ball contacted the hydrogel surface with loading force, small deformation of the hydrogel would generate on the surface. The shear force would appear as the contact ball started moving. During this process, the contact area between contact ball and hydrogel is showed in Figure 3aii. Due to the shear stress at gel-sol transition is 27 Pa (Figure 2c), the calculated shear force at this moment should be 0.76 mN based on formula S1. It means that FT will disassemble if the shear force is higher than 0.76 mN in our experiment. The actual value may be lower than this one due to less contact area in measurement. With the shear applied continuously on the hydrogel surface, the friction coefficients decreased from 0.0372±0.0007 to 0.0233±0.0021 (Figure 3b). Compared to them, the friction coefficients of PAAm/PVA hydrogel were stable following the shear cycles (Figure S3). It means that FT in the prepared FTPAAm/PVA hydrogel plays key role on the supramolecular lubrication. Correspondingly the friction force decreased from 1.86±0.03 to 1.17±0.10 mN after being applied 30 shear cycles (Figure 3c). The reason might be attributed that the thixotropic FT supramolecular network would disassemble when the contact ball was sliding onto the hydrogel surface. The gel-state FT would change to sol-state and exude to the surface, and then form lubricating layer between hydrogel surface and contact ball. Meanwhile it also can be seen that, on the FT-PAAm/PVA2.0 hydrogel numerous nanofibrous were found on the micro-scaled loose and porous structures (around 10-20 µm) before being sheared (Figure 3ci). After being sheared the surface structures would become more porous, and few fibrous structures were found (Figure 3cii). This was similar with the structures of PAAm/PVA hydrogel (Figure S4). Therefore it evidenced that the disassembly of FT in the hydrogel. For further investigating the lubricating behaviors of the prepared FT-PAAm/PVA hydrogels, the loading force and shear velocity would be managed in the subsequent experiments. In addition since the friction force of FT-PAAm/PVA hydrogel would tend to be stable after 15 shear cycles (Figure 3c), the subsequent friction data would be collected for the first 15 shear cycles. When keeping the loading force constant at 50 mN, the shear velocity of the contact ball changed from 0.1 mm/s to 0.5 mm/s, the friction force on FT-PAAm/PVA2.0 increased with the shear velocity after the 1st shear cycle (Figure 3d and Table S2). It is because that the liquid viscosity, contact area and liquid layer thickness at this moment can be assumed to be definite value, the friction force will be proportional to shear velocity based on the formula S2.13 Subsequently it can be seen that all the friction
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force would reduce after 15 shear cycles corresponding to each shear velocity (Figure 3d and Table S2). When keeping the shear velocity constant at 0.1 mm/s, the loading force was controlled from 30 mN to 110 mN, the friction force on FT-PAAm/PVA2.0 hydrogel after the 1st shear cycle also raised (Figure 3e and Table S3). Due to the enlarged hydrogel deformation induced by the raised loading force the contact area between hydrogel surface and contact ball will increase. Consequently the friction force enhanced following the loading force according to the formula S2. After 15 shear cycles, the friction force in each loading force was reduced (Figure 3e and Table S3). Therefore one can find that the
being sheared according to the mechanical results (Figure 2b). To verify this analysis, the mechanical property of FT-PAAm/PVA hydrogel was tested before and after being sheared. The broken strength (2.19±0.03 MPa) of FT-PAAm/PVA hydrogel after 15 shear cycles became higher than that (1.83±0.05 MPa) of the unsheared one (Figure 4b). This changed mechanical results also demonstrated that FT was wept out from the hydrogel network.
Figure 4. a) The fluorescence spectrum of the FT-PAAm/PVA2.0 hydrogel following the shear cycles; b) The broken strength on FT-PAAm/PVA2.0 hydrogel before and after being applied 15 shear cycles. In conclusion, bioinspired supramolecular FT-PAAm/PVA hydrogels with shear force responsive lubrication were fabricated by one-pot method. The friction coefficients on the FT-PAAm/PVA hydrogel surface would reduce when shear force was executed continuously by controlling the loading force and shear velocity. Shear force induced lubrication might be attributed to two aspects: i) the supramolecular sol-gel transition of thixotropic FT hydrogel triggered by shear force performs lubricating function; ii) the tough double network PAAm/PVA hydrogel acts as the role of physical support. Moreover the mechanical property of the FTPAAm/PVA hydrogels would increase after being applied shear force. This study will offer great potential in fabricating forceinduced lubricating surfaces and materials, such as cartilage substitutions.
ASSOCIATED CONTENT Supporting Information Experiment details and data. Figure 3. a) Photo of a typical tribological test on the FTPAAm/PVA hydrogel (i) and detailed scheme on friction of the contact ball on the hydrogel (ii). b) The friction coefficients before and after being sheared. Experimental condition: shear velocity of 0.1 mm/s, loading force of 50 mN. c) Friction force on the FT-PAAm/PVA2.0 hydrogel following 30 shear cycles. i and ii SEM images of the hydrogel before and after being sheared. d) The relation between friction force and shear velocity when loading force was 50 mN. e) The relation between friction force and loading force when shear velocity was 0.1 mm/s.
AUTHOR INFORMATION
friction force would decrease after being sheared in different shear velocity and loading force, which demonstrated that the dynamic lubrication could be achieved successfully on the FT-PAAm/PVA hydrogels. As reported12a FT supramolecular system exhibits aggregation induced emission property, the disassembly of FT will lead decreased fluorescence intensity. Herein FT-PAAm/PVA hydrogel would be tested by the fluorescent spectrum (excited wavelength at 365 nm) in solid mode following the shear cycles. The fluorescence intensity reduced with the increase of shear cycles (Figure 4a). It indicated that the FT supramolecular would disassemble acted by shear force. Moreover due to the weeping of the disassembled FT, its quantities in the hydrogel would decrease. Therefore it can predict that mechanical strength should improve after
This work is supported by the National Natural Science Foundation of China (21774044, 21425314, 21434009 and 21421061), MOST (2013YQ190467), and the Top-Notch Young Talents Program of China.
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[email protected] Notes The authors declare no competing financial interests.
ACKNOWLEDGMENT
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