•Endowing the capability of partly gel-sol transition to the hydrogel•Exhibiting reversible appearance of sol-lubricating layer triggered by UV/Vis light•Integrating responsive supramolecular network within polymer framework Supramolecular hydrogel with the unique capability of gel-sol transition responding to external stimuli has exhibited unprecedented superiority in constructing smart materials with exceptional features, such as lubrication, self-healing, and high mechanical strength. The cooperation between supramolecular non-covalent bonding and polymer covalent bonding may provide one new type of supramolecule-tuned functional materials that can switch their state between solid and liquid-solid under external stimulus. Here, we demonstrate a semi-convertible hydrogel composed of a stable covalent framework and the stimuli-responsive supramolecular network, which exhibits photoresponsive lubricating ability and high toughness. We believe that this concept is now emerging from the infant stage and may show more promising applications far beyond lubrication, leading to an emerging branch of interdisciplinary materials science that integrates supramolecular chemistry, hydrogel, and surface science. A semi-convertible hydrogel with reversible photoresponsive supramolecular lubricating ability is demonstrated by integrating a responsive supramolecular system of α-cyclodextrins/polyethylene glycol (α-CD/PEG) and competitive guest 1-[p-(phenylazo)benzyl]pyridinium bromide (AzoPB) within a poly(vinyl alcohol) (PVA) and polyacrylamide (PAAm) framework. The competitive host-guest interaction between α-CD/PEG supramolecular network and AzoPB after UV and visible-light irradiation enables the appearance and disappearance of the sol-state layer on the hydrogel surface while the PVA and PAAm consistently keeps the framework. This semi-convertible process realizes the reversible photoresponsive lubricating ability and variable toughness. We envision that this finding will offer a possible way for fabricating novel smart devices far beyond lubrication, leading to an emerging branch of interdisciplinary materials science that integrates supramolecular chemistry, hydrogel, and surface science. A semi-convertible hydrogel with reversible photoresponsive supramolecular lubricating ability is demonstrated by integrating a responsive supramolecular system of α-cyclodextrins/polyethylene glycol (α-CD/PEG) and competitive guest 1-[p-(phenylazo)benzyl]pyridinium bromide (AzoPB) within a poly(vinyl alcohol) (PVA) and polyacrylamide (PAAm) framework. The competitive host-guest interaction between α-CD/PEG supramolecular network and AzoPB after UV and visible-light irradiation enables the appearance and disappearance of the sol-state layer on the hydrogel surface while the PVA and PAAm consistently keeps the framework. 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The photoresponsive lubrication of the hydrogel is investigated after UV and Vis irradiation, and analysis of the mechanical properties and relevant spectroscopy are utilized to explore the photoresponsive lubricating mechanism. The photoresponsive semi-convertible hydrogel (PSCH) was fabricated by a simple one-pot method (Figure 1). In brief, the hydrogel was formed by mixing the solution I (mixture of α-CD, AzoPB, and PEG) and solution II (mixture of AAm monomers, PVA, N,N-methylenebisacrylamide, potassium persulfate, and glutaraldehyde) under UV irradiation. Based on the volume ratio of solutions I and II (1:20, 1:25, 1:50, 1:75, and 1:100), the prepared PSCHs are abbreviated as PSCH20, PSCH25, PSCH50, PSCH75, and PSCH100. After preparation by UV irradiation, three interpenetrating networks were formed in the hydrogel, which were the supramolecular α-CD/PEG non-covalent network, and PAAm and PVA covalent networks; the uncombined cis-AzoPB molecule was dissociative in hydrogel (Figure 1i). The supramolecular α-CD/PEG non-covalent network was flexible and entangled in the interpenetrating PAAm and PVA covalent networks. For supramolecular gelation, α-CDs assembled at the two ends of one PEG chain, and such complex domains aggregated with one from another α-CD/PEG chain to form the non-covalent physical crosslink.41Li J. Harada A. Kamachi M. Sol-gel transition during inclusion complex formation between α-cyclodextrin and high molecular weight poly(ethylene glycol)s in aqueous solution.Polym. J. 1994; 26: 1019-1026Crossref Scopus (281) Google Scholar,42Guan J.J. Xu X.F. Hu J.D. Peng W. 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Here most of the coiled PEG chains were difficult to exude out to the surface due to their entanglement in the interpenetrating polymer network, which resulted in the polymer skeleton not changing much. Therefore, no significant change in structures could be observed by scanning electron microscopy (SEM) (Figure S1). However, some chain segments or ends of PEG might still have been exposed on the hydrogel surface participating in the lubrication. Therefore, the lubricating layer that appeared might be the sol composed of α-CD/trans-AzoPB complex and some chain segments or ends of PEG. When the surface was treated by UV again, trans-AzoPB transferred to the cis form that brought disassembly of α-CD/AzoPB, after which α-CD again assembled with PEG to gel. In the experiment, AzoPB was synthesized first according to previous work.36Liao X.J. Chen G.S. Liu X.X. Chen W.X. Chen F.E. Jiang M. Photoresponsive pseudopolyrotaxane hydrogels based on competition of host-guest interactions.Angew. Chem. Int. Ed. 2010; 49: 4409-4413Crossref PubMed Scopus (255) Google Scholar The 1H nuclear magnetic resonance (NMR) spectrum in D2O (Figure S2) demonstrated that AzoPB was synthesized successfully. The fracture strength of PSCHs increased with the decrease in supramolecular contents of the whole hydrogel (Figure S3). This could be attributed to the easily destroyed non-covalent supramolecular network of α-CD/PEG bringing weak molecular interaction and a relatively unstable polymer network. Consequently, the corresponding water content of these prepared PSCHs decreased slightly from 94.1% to 92.1% following the supramolecular content decrease in the hydrogel (Table S1). The friction test of PSCHs after UV and Vis irradiation was performed on a ball-on-disk reciprocating tribometer. A ceramic Al2O3 contact ball with diameter of 5 mm moved in a reciprocating rectilinear manner on the hydrogel surface under controlled sliding velocity and load force. Because the hydrogel surface is water rich, which is crucial to the lubrication on the surface,43Ma L.R. Gaisinskaya-Kipnis A. Kampf N. Klein J. Origins of hydration lubrication.Nat. Commun. 2015; 6: 6060Crossref PubMed Scopus (159) Google Scholar,44Klein J. Hydration lubrication.Friction. 2013; 1: 1-23Crossref Scopus (284) Google Scholar the friction coefficient (FC) was measured in controlled humidity from 20% to 98%, and with a thin water film added on the hydrogel surface, to investigate how water affects the photoresponsive lubricating process. In the presence of water film, by maintaining a sliding velocity of 0.1 mm/s and load force of 50 mN, after 2 h UV irradiation PSCH50 presented a stable FC of 0.0144 ± 0.0035 (red line in Figure 2A). When it was irradiated under Vis for 2 h, the FC fell to 0.0045 ± 0.0022 (black line in Figure 2A). Both of these values were lower than the FC (0.0538 ± 0.0002) of interpenetrating PVA and PAAm hydrogel (Figure S4), which might demonstrate that supramolecular contents play a dominant role in photoresponsive lubrication. Here, the treatment time was identified by the time-dependent rheological property of PSCH following Vis and UV irradiation time (Figure S5). The storage modulus (G′) illuminates the quantity of stored energy in the polymer system, which characterizes the solid-like property of the hydrogel. The G′ of PSCH20 decreased with the increase in Vis irradiation time and then became relatively stable after 120 min. This meant that the gel-sol transition of the supramolecular section was almost finished after 120 min of Vis treatment (Figure S5A). When the PSCH was further irradiated by UV light, the G′ increased along the treatment timeline, and also exhibited stability after 120 min (Figure S5B). This demonstrated that the sol-gel transition was almost complete after 120 min of UV irradiation. Here, PSCH20 was selected to investigate the mechanical property during gel-sol transition because the supramolecular content in PSCHs was the highest in the experiment, which might thus have the longest response time of all the samples under light treatment. By controlling the humidity, the FCs decreased obviously following the increase in humidity after either UV or Vis irradiation (Figure 2B). The photoresponsive lubrication could be observed at each relative humidity setpoint of 20%, 40%, 60%, 80%, and 98%. The FC at 98% relative humidity (0.0018 ± 0.0014 after Vis irradiation and 0.0160 ± 0.0019 after UV irradiation) was similar to that in the measured condition of added water film on the hydrogel (0.0045 ± 0.0022 after Vis irradiation and 0.0144 ± 0.0035 after UV irradiation), which might be attributed to a thin water film being formed on the hydrogel surface under a closed saturated relative humidity. Therefore, one can conclude that the water film on the hydrogel surface would benefit to the lubricating behavior on PSCHs but not bring about photoresponsive change. The subsequent friction experiment was mainly performed in the presence of a thin water layer on the hydrogel surface. For further observation of the responsive lubricating behaviors of PSCHs, the load force and sliding velocity were regulated (Figures 2C and 2D). When the sliding velocity was kept stable at 0.1 mm/s and the load force was increased from 25 to 125 mN, the friction force at each load setpoint after UV irradiation was higher than that after Vis treatment (Figure 2C and Table S2), and the friction force after either UV or Vis treatment increased in line with the increasing load force (Figure 2C). These results are consistent with the classic friction theory f = μF (where f is friction force, μ is the frictional coefficient, and F is the load). The fitted line showed that the FCs after UV and Vis irradiation were 0.0177 ± 0.0032 and 0.0033 ± 0.0011, respectively. These theoretical values matched well with the experimental results in Figure 2A. In addition, the contact pressure between PSCH and contact ball can be calculated by Hertzian contact mechanism.45Fischer-Cripps A.C. The Hertzian contact surface.J. Mater. Sci. 1999; 34: 129-137Crossref Scopus (68) Google Scholar As the pressure between contact ball and hydrogel surface is crucial to understanding the lubricating process, the contact pressure p0, contact radius a, and penetration depth d were calculated by Hertzian contact mechanism:p0=(6FE∗2π3R2)1/3,(Equation 1) a=(3FR4E∗)1/3,(Equation 2) d=πap02E∗,(Equation 3) Here, F is the load, R is the radius of contact ball (5 mm), and E∗ is the relevant modulus approximated by the low-frequency storage modulus G′ of around 6 × 103 N/m2 (Figure 4B). As shown in Table 1, the maximal contact pressure was 3.27 kPa with the load of 125 mN in our experiment. Moreover, as the load force was kept at 50 mN, the sliding velocity was changed from 0.10 to 1.00 mm/s and the friction force increased with sliding velocity. This might be attributed to the FC on the hydrogel surface being proportional to the sliding velocity when liquid viscosity, contact area, and thickness of liquid layer were assumed to be constant.46Gong J.P. Iwasaki Y. Osada Y. Kurihara K. Hamai Y. Friction of gels. 3. Friction on solid surfaces.J. Phys. Chem. B. 1999; 103: 6001-6006Crossref Scopus (132) Google Scholar Photoresponsive lubrication could also be observed after UV and Vis irradiation (Figure 2D and Table S3). In addition, the responsive lubricating phenomenon was observed on all the PSCHs with different concentrations (Figure 2E). The difference in FC after UV and Vis irradiation decreased with the decrease in supramolecular non-covalent contents.Table 1Contact Pressure, Contact Radius, and Penetration Depth between PSCH50 and Contact BallLoad (mN)255075100125Contact pressure (kPa)1.912.412.753.033.27Contact radius (mm)2.503.153.613.974.27Penetration depth (mm)1.251.982.603.153.66 Open table in a new tab To identify the photoresponsive lubricating layer on PSCHs induced by the gel-sol transition, we performed a directly profiled observation of the PSCH surface after Vis irradiation (Figure 3A). Here, the hydrogel was treated by a routine histology process and cut into slices. Because the water evaporation on the surface could not be avoided during the preparation procedure, the observed thickness of sol layer on the surface following UV and Vis in Figures 3A and 3B were analyzed qualitatively. Following Vis irradiation, the liquid layer appeared on the surface very quickly, as soon as 3 min of irradiation (Figure 3A). The thickness of the liquid layer increased with Vis irradiation time (Figure 3B), and reached around 90 μm after 2 h of treatment. Simultaneously the liquid layer could also be identified in force-distance detection terms on an instron materials testing system. As shown in Figure 3C, a typical force-distance curve was measured on the PSCH surface that was irradiated under Vis for 60 min. A thin-tipped probe moved to the PSCH surface vertically with a velocity of 8.3 μm/s. The force was close to zero before the tip contacted the sol-state surface (Figure 3Ci); there was a sharp dropoff as the tip touched the sol layer due to the action of capillarity (Figure 3Cii); then the force become relatively stable while the tip was moving in the sol layer, and subsequently increased when the tip touched the gel surface (Figure 3Ciii). Therefore, the distance corresponding to the stable force curve might demonstrate the appearance of the sol-lubricating layer. Comparing these results, the force-distance curve on the hydrogel after UV irradiation showed that the force was almost zero before the tip reach the hydrogel surface (Figure 3Di) and increased when the tip contacted the surface (Figure 3Dii), with no force plateau appearing. This indicates that there was no sol layer on the surface after UV irradiation. This reversible lubrication of PSCH is shown in Figure 4A. Following the UV and Vis irradiation, the FC showed good repeatability even after five cycles. The lubricating sol layer could be observed reversibly as shown on the right side of Figure 4A. During this photoresponsive process, the accompanying gel-sol transition might have engendered change in mechanical properties. The rheological behavior of PSCHs after UV and Vis irradiation is shown in Figure 4B. It can be seen that the storage modulus G′ of PSCH50 after Vis irradiation was lower than that after UV irradiation. The decreased mechanical property might be attributed to disassembly of α-CD/PEG supramolecular networks among the PSCHs. This mechanical change exhibited good reversibility following UV and Vis treatment (Figure 4B). This photoresponsive mechanical property of the PSCHs could also be verified in fracture strength corresponding to UV and Vis irradiation. The fracture strength of PSCH50 after UV irradiation was higher than that after Vis treatment (Figure S6). This result was consistent with the rheological behavior in Figure 4B, which demonstrated the assembly and disassembly of α-CD/PEG induced by AzoPB photoisomerization under UV and Vis. On the other hand, PSCH50 after UV irradiation exhibited higher enthalpy value (ΔH = 21.50 J/g) and melt temperature (Tmelt = 67.9°C) than those (ΔH = 13.45 J/g; Tmelt = 65.3°C) after Vis irradiation (Figure 4C), and also demonstrated that PSCHs after UV irradiation showed higher mechanical strength than after Vis irradiation. For further understanding of the lubricating mechanism, a controlled experiment was designed by preparing three kinds of coatings on polydimethylsiloxane (PDMS) surface: (1) α-CD, PEG, and AzoPB solution onto PDMS substrate; (2) the α-CD/AzoPB solution; and (3) α-CD/PEG hydrogel coating. The FC of coating (1) after UV and Vis irradiation was 0.0238 ± 0.0007 (gel state) and 0.0011 ± 0.0002 (sol state), respectively (Figure 5A). At the same time, the FC of both α-CD/AzoPB solution and α-CD/PEG hydrogel coating on PDMS substrate exhibited no difference after UV or Vis irradiation (Figure S7), indicating that the photoresponsive lubricating change could be attributed to the gel-sol transition of the supramolecular system (α-CD, PEG, and AzoPB). More interestingly, the FC (0.0024 ± 0.0006) of coating (2) (pink line in Figure 5A) was lower than that of gel-state coating (1), but slightly higher than that of the sol-state coating (1), which showed that the superficial PEG chains might participate in the formation of the lubricating layer. In addition, the FC of water on PDMS was 0.0187 ± 0.0025, lower than FC on PDMS without water (0.0259 ± 0.0036) and FC of gel-state coating (1) (0.0238 ± 0.0007), but still higher than the FCs of sol-state supramolecular contents coatings (1) and (2). Simultaneously there was also no change in FC after UV and Vis irradiation, indicating that the water layer could be a lubricant but not generating the photoresponsive lubricating change. Generally, UV-visible spectroscopy can be utilized to investigating the trans/cis photoisomerization of azobenzene derivatives. In this case, the hydrogel was immersed in water after each irradiation of UV and Vis light, after which the extracted solution was measured. After Vis irradiation, two adsorption peaks at 330 and 430 nm were observed, which were assigned to the π-π∗ transition of trans-AzoPB and n-π∗ transition of cis-AzoPB (Figure 5B).47Xie G.H. Li P. Zhao Z.J. Zhu Z.P. Kong X.Y. Zhang Z. Xiao K. Wen L.P. Jiang L. Light-and electric-field-controlled wetting behavior in nanochannels for regulating nanoconfined mass transport.J. Am. Chem. Soc. 2018; 140: 4552-4559Crossref PubMed Scopus (61) Google Scholar, 48Liu Y. Zhao Y.L. Zhang H.Y. Fan Z. Wen G.D. Ding F. Spectrophotometric study of inclusion complexation of aliphatic alcohols by β-cyclodextrins with azobenzene tether.J. Phys. Chem. B. 2004; 108: 8836-8843Crossref Scopus (72) Google Scholar, 49Yang L. Takisawa N. Kaikawa T. Shirahama K. Interaction of photosurfactants, [[[4′-[(4-alkylphenyl)azo]phenyl]oxy]ethyl]trimethylammomium bromides, with α- and β-cyclodextrins as measured by induced circular dichroism and a surfactant-selective electrode.Langmuir. 1996; 12: 1154-1158Crossref Scopus (22) Google Scholar When the hydrogel was treated by UV light, the strength of trans-AzoPB peak at 330 nm clearly decreased and the peak of cis-AzoPB at 430 nm increased compared with the adsorption after Vis irradiation, demonstrating the trans-to-cis transition of AzoPB after UV irradiation. After subsequent Vis irradiation, the spectrum returned to its previous state. This reversible change of UV spectrum was fully consistent with the photoresponsive lubricating behavior on the PSCH surface. Based on the above analysis, one can conclude that the supramolecular contents in this semi-convertible hydrogel played an important role in lubrication, and the photoresponsive supramolecular assembled-disassembled process resulted in lubricating change on the hydrogel surface. To demonstrate the photoresponsive lubricating property, we fixed two pieces of prepared PSCH50 on the respective arm tips of tweezers. A hollow aluminum ball was placed for grasping after UV and Vis irradiation (Figure 6). After UV irradiation, the ball could be easily grasped the PSCH-loaded tweezers, possibly because of the high static FC (0.0403 ± 0.0045) between hydrogel and ball in the boundary range based on the Stribeck curve.50Gleghorn J.P. Bonassar L.J. Lubrication mode analysis of articular cartilage using Stribeck surfaces.J. Biomech. 2008; 41: 1910-1918Crossref PubMed Scopus (125) Google Scholar This was around a 3-fold increase in friction compared with the sliding FC (0.0144 ± 0.0035 after UV irradiation). When treated by Vis, the ball slid down or could not be grasped due to the lubricating layer that appeared between the PSCH surface and the ball. The corresponding video is presented in Supplemental Information (Videos S1 and S2). https://www.cell.com/cms/asset/cd18684b-55f9-4e4c-9737-2b0cc1016782/mmc2.mp4Loading ... Download .mp4 (2.71 MB) Help with .mp4 files Video S1. PSCH after UV Irradiation https://www.cell.com/cms/asset/44bf6a12-5186-4428-a02a-f398b65f35bc/mmc3.mp4Loading ... Download .mp4 (2.57 MB) Help with .mp4 files Video S2. PSCH after Vis Irradiation In conclusion, a PSCH was demonstrated by integrating the non-covalent supramolecular network (α-CD/PEG), covalent polymeric network (PVA and PAAm), and dissociative competitive photoresponsive guest AzoPB. The photoresponsive AzoPB molecule was introduced into the hydrogel network with the aim of tuning the assembled process of α-CD/PEG under UV and Vis irradiation. Due to the competitive host-guest interaction, the disassembly of supramolecular α-CD/PEG network after Vis irradiation resulted in partial gel-sol transition on PSCH. This sol-state layer composed of α-CD/trans-AzoPB composite and some chain segments or ends of PEG on the hydrogel surface acted as the main lubricating layer. Therefore, the FC values after Vis irradiation decreased obviously compared with those after UV irradiation, and exhibited good photoresponsive reversibility. In addition, the mechanical property of the prepared hydrogel also exhibited switchable change following the UV and Vis treatment. The presented PSCH is just emerging from the infant stage and may show more promising applications in novel smart devices far beyond lubrication, such as soft robotics, anti-fouling, and microfluidic devices. We believe that it will be developed to become a burgeoning branch of interdisciplinary materials science through the integration of surface science, supramolecular chemistry, and hydrogel networks.