Adjustable Tribological Behavior of Glucose-Sensitive Hydrogels

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Adjustable Tribological Behavior of Glucose-Sensitive Hydrogels Jin Zhao, Pengxiao Liu, and Yuhong Liu Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b01388 • Publication Date (Web): 03 Jun 2018 Downloaded from http://pubs.acs.org on June 3, 2018

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Adjustable Tribological Behavior of GlucoseSensitive Hydrogels Jin Zhao, Pengxiao Liu, Yuhong Liu*

State Key Laboratory of Tribology, Tsinghua University, Beijing, 100084, China

Telephone: 86-10-62781385; E-mail: [email protected]

1

ACS Paragon Plus Environment

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ABSTRACT: Stimuli-responsive hydrogels have been considered to have various applications in numerous fields. In the present work, a double-network (DN) hydrogel has been synthesized. The copolymer of 2-acrylamide-2-methylpropane sulfonic acid (AMPS) and acrylamide (AM) [P (AMPS-co-AM)] are prepared as the 1st network and polyacryly acide (PAAc) as the 2nd network. This DN hydrogel is sensitive to glucose by introducing the glucose-sensitive group phenylboronic acid (PBA) to the network. The tribological properties of this glucose-sensitive DN hydrogel have been investigated using a universal mechanical tester (UMT-5). The tribological results show that the friction coefficient varied with the glucose solution. The friction coefficient increased to a maximum of 0.06, and finally decreased to 0.025 with the increase in the glucose concentration. An adjustable friction coefficient of the hydrogel, between 0.025 and 0.056, was achieved along with the change of lubricant. According to the tribological experimental results and the analysis of the DN structure, it can be deduced that a hydrated layer exists in the interface of the hydrogel. The hydrated layer consisting of water molecules are bounded with the hydrophilic group of the hydrogel network by hydrogen bonds. The change in the number of water molecules leads to the difference in the water content of the hydrogel, which further resulted in the various tribological properties. In addition, the hydrogel's mesh size also has an impact on the change in friction coefficient. In general, the adjustable friction of the hydrogel in a glucose environment is achieved. KEYWORDS: stimuli-responsive; double-network hydrogel; modified; hydrated layer; mesh size 2


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INTRODUCTION Hydrogel is a type of wet and soft material. It consists of a three-dimensional polymer network structure and a large quantity of water (50%–99%) inside the network structure1. Hydrogels have been widely researched because of their highly hydrated nature resembling the natural extracellular matrix (ECM), which is mostly comprised of highly hydrated biomacromolecular networks. Therefore, the synthetic hydrogel is the best option for artificial substitutes of tissues and organs, including cartilage2, blood vessels3, muscles4, and cells5. However, as a typical soft material, hydrogels have lower mechanical strength and toughness than common polymeric materials, which limit their applications. The improvement in the hydrogel strength and toughness are quite important. Therefore, in recent years, much attention has been paid in developing stiff and tough hydrogels, such as nanocomposite6–8, polyampholyte hydrogels9, hydrogen-bonding hydrogels10,11, and double-network (DN) hydrogels12,13. Due to the typical structure of the DN hydrogel, the diversity of its constituent monomers allows it to perform many other functions. The DN hydrogels are composed of a physically or chemically crosslinked primary network and secondary network formed by covalent14 or other interactions15,16. According to Gong et al., a PAMPS/PAM DN hydrogel that contains nearly 90% water is able to withstand large deformations during stretching and compression, and its fracture energy (102 to 103 J m-2) is 10–10000 times larger than the single-network hydrogel of PAMPS (ca. 10-1 J m-2) and PAM (ca. 10 J m-2)

17,18

.

The polyelectrolytes in the 1st network interact to provide a rigid skeleton owing to 3


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the repulsion between ionic groups14. To produce a hydrogel with good deformability, it is common to use a flexible polymer such as PAM as the 2nd network. By using different synthetic monomers, the DN hydrogel may be stimuli responsive to the changing environment conditions19, such as temperature20,21, magnetic field22, light23, electric field24, ion concentration25,26, and pH27-30. These stimuli-responsive hydrogels have been considered to have various applications in numerous fields, such as a promising smart material for sensors31,32 and in controlled drug delivery systems33,34. However, only a few studies reported the tribological properties of the stimuli-responsive hydrogels. It is meaningful to transform the passive state of friction to an active controllable state. The friction properties of the hydrogel mainly depend on its own chemical structure, mainly including the amount of charge35, network structure36, and the water content of the hydrogel37. The frictional interface of environment-sensitive hydrogel has reaction on certain external stimuli and provoke changes in surface chemistry, interfacial charge, and topography, which then lead to a change in the friction coefficient.38-40 Polymer brushes grafted on hydrogel surface can achieve a wide range of friction coefficient, which can change from 0.001 to 141. The combine of a glucose-sensitive hydrogel with glucose molecules under certain conditions is primarily used in insulin release microgels for people with diabetes.42 The interaction between glucose and phenylboronic acids (PBA) was identified mechanistically as a reversible covalent interaction between the cis-diol groups on polyols and the ionic boronate state of PBA.43 This reaction leads to fluorophores and volumetric changes in the PBA-modified hydrogels.44 When the glucose molecules 4


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are introduced into the three-dimensional network of hydrogels and are bound to the molecular chains, the hydrophilicity of the hydrogels will change to a certain degree depending on the amount of binding, and affect the swelling ability of the hydrogel.45 It may be suggested that the change in water swelling will influence the lubrication properties. Glucose-sensitive hydrogels are generally used for insulin drug delivery and sensors46, and there is little research on friction. When a drug carrier is transported into human bodies, friction occurs in many places and between each other. Therefore, it is important to magnify the reaction to study its mechanism clearly. In this study, a PBA-modified DN hydrogel was synthesized and the relationship between the structure of the PBA-modified hydrogel and its lubrication properties was investigated. According to the results, the friction coefficient is negatively correlated with the water content that varies with the change in the glucose concentration. The friction of the glucose-responsive hydrogel can be controlled by changing the external environment. Further, an adjustable friction system was achieved using the PBAmodified DN hydrogel.

MATERIALS AND METHODS Materials.

2-Acrylamido-2-methyl-1-propanesulfonicacid

(AMPS,

chemical

pure, >98%), acrylamide (AAm, chemical pure, >98.0%) and acrylic acid (AAc, analytical reagent, >99.9%) as monomers, and D-(+)-glucose (GC, analytical reagent, >99.5%) were supplied by Aladdin Reagent (Shanghai) Co., Ltd. N, N'Methylenebisacrylamide (MBAA, analytical reagent, >99%) as the covalent cross5


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linking agent and α-Ketoglutaric acid (chemical pure, >98%) as ultraviolet (UV)light initiator were purchased by Sigma Aldrich. 3-Aminobenzeneboronic acid (APBA, chemical pure, >98%) as modified molecules, 1-(3-dimethylaminopropyl)-3ethylcarbodiimide hydrochloride (EDC, chemical pure, >98%) as catalyst and phosphate buffer (25℃, pH 6.6-8.0) as pH adjust solution were also supplied by Sigma Aldrich. Synthesis. A DN hydrogel (P (AMPS-co-AAm)/PAAc) was prepared according to two steps. First, 10mmol of AMPS, 0.4mmol of MBAA, 0.5mmol of AAm, and 0.1mmol of the α-Ketoglutaric acid were dissolved in 10ml of deionized water. After fully stirring and bubbling in N2 for 30 min, the aqueous solution was poured into a rectangular reaction-cell made of two parallel glass plates with smooth surfaces, whose width is approximately 3mm. Similarly, the solution was added into the hemisphere mold (glass-made, diameter of 6mm) to prepare the upper sample. The preparation of the hemisphere hydrogel is exactly the same as that of the bulk hydrogel, except for the mold.

Further, they were placed at room temperature

(approximately 25°C) under ultraviolet (UV) lighting (125W, wavelength of 365nm). After 10 h, the 1st network hydrogels were synthesized. They were then taken out and placed in deionized water for 48h to swell equilibrium and remove unreacted monomers. The water was changed every 12h. Second, 0.3mol of AAc, 1.2mmol of MBAA, and 0.3mmol of the α-Ketoglutaric acid were dissolved in 150ml of deionized water. The 1st network hydrogels were put into the mixed solution under 25°C, allowing the 2nd monomer molecules to go into the 6


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1st network structure. After 48 h, the hydrogels were taken to the UV lighting for 10 h. Finally, the 2nd network hydrogels were synthesized based on the 1st network. The new DN hydrogels were synthesized, and also placed in deionized water for 48h. The water was changed every 12h. As for the modification, the aqueous solution with 0.05M APBA and 0.05M EDC was prepared and cooled to 0°C. The DN hydrogels were immersed in the solution for 4 h at 0°C, and finally the APBA-modified DN hydrogels (BMDN, APBA-P (AMPS-coAAm)/PAAc) were completed.42, 47-49

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Figure 1. Illustration of hydrogel preparation process and the reaction mechanism. (a) the synthesis and the schematic of the 1st network hydrogel; (b) the synthesis and the schematic of the DN hydrogel (monomers distributed in the 1st network was directly polymerized);(c)

representation

of

the

complexation

between

the

ackylamidophenylboronic acid (APBA) and glucose in aqueous solution; (d) modification of the DN hydrogel.

8


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Characterization Microstructure Analysis. The microstructures of these hydrogels were investigated using a scanning electron microscope (SEM, FEI Quanta 200 FEG). Four types of hydrogels—1st network, 2nd network, the DN, and the BMDN hydrogels were observed. The hydrogels need to be freeze-dried to remove water molecules and platinum-sputtered before taken to SEM. Chemical Composition Analysis. The BMDN hydrogels before and after APBA modification was observed by Fourier transform infrared (FTIR, Nicolet 6700FTIR) spectroscopy at the ambient environment. The composition of the hydrogel was detected by X-ray photoelectron spectroscopy (XPS, ESCALAB 250 XI, Thermo Scientific Instrument, and USA). XPS measurements were performed with a monochromatized Al Kα X-ray source. Before being taken for XPS, four types of hydrogels as mentioned before were dried in the drying oven at 100°C for 24 h and ground into powder, and then were pressed into a 1-mm thick wafer. Mechanical Property Analysis. The dynamic compressive elastic modulus of the hydrogels was determined by a tensile-compressive tester (Tensilon RTC-1310A, Orientec Co.) at 25°C. The cuboid hydrogel sample of 15 mm ×10mm×10mm was set on the lower plate and compressed by the upper plate at a strain rate of 2mm/min. The hydrogels were compressed until they were completely broken and an average of three experiments was taken. Surface roughness measurement. Sa is used to represent the surface roughness. Five points on the surface of hydrogel were measured by 3D white-light Interfering 9


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profilometers (Nexview, Zygo, Middlefield, OH, USA), and the average of Sa was taken. Scan range is 800µm×800µm. Water Content Measurement. The degree of swelling, q, of the hydrogel at 25°C is estimated by q=(m1-m2)/m1 as the ratio of the water weight of the swollen hydrogel to the weight of the xerogel, where m1 is the weight of the swollen hydrogel at 25°C and m2 is the weight of the xerogel. Water Molecule State Analysis. The water state in the hydrogel was investigated via Raman spectrometer (Jobin Yvon HR800) at 0.2 cm-1 repeatability and a 0.65 cm-1 resolution. The laser wavelength was 633nm. The modified hydrogels immersed in different glucose ratio solutions were observed in the region of 2800–4000 cm-1 at ambient conditions. Moreover, thermogravimetric analysis (TGA) was performed by the TGA/DSC1 (Mettler Toledo) from 30 °C to 200°C with a heating rate of 10°C/min under an argon atmosphere. Friction test General tests. A conventional ball-on-disk tribometer was used to determine the friction coefficients. Measurements were conducted on a universal micro tribometer (UMT-5, Bruker), with an elastomeric BMDN hydrogel hemisphere of diameter 12 mm against the BMDN hydrogel bulk (33 mm ×33 mm ×10mm). The distance of a reciprocating trip, the frequency, and the loading of a hemisphere exerted on the bulk were set to 5 mm, 1Hz, and 1 N (apparent contact pressure = 34kpa), respectively. Each friction coefficient was obtained from three independent measurements. Before the friction test, the BMDN hydrogels were immersed in a series of glucose solutions 10


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with different glucose concentrations and pH (0.25wt%-5wt%, pH=7.4; 3wt%, pH=6.6–8.0) for 4 h. The pH of glucose solution is adjusted by adding diluted phosphate buffer solution. The hydrogel is completely immersed into the lubricating fluid during the rubbing process. To ensure the same environment, the soaking solution used before was chosen to be the lubricant during the friction experiment. Each pair of hydrogels rubbed for 15 minutes and three experiments were performed to obtain the average friction coefficient. Regulation tests. The same pair of sliding solids was chosen, and the soaking solution as well as the lubricant solution for the friction experiments was changed during the tests. The BMDN hydrogels were immersed in water for 1 h, rubbed for 20 min, and then the water was replaced by 1.25wt% glucose solution (pH=7.4). The BMDN hydrogels were also soaked for 1 h, rubbed for 20 min. Next, the soaking solution was replaced by the phosphate buffered water (pH=6.5). The steps above were repeated eight times. The distance of a reciprocating trip, the frequency, and the loading of a hemisphere exerted on the bulk were set to 5 mm, 1Hz, and 1 N, respectively.

RESULTS AND DISCUSSION The polymer monomers are combined with each other into a three-dimensional network structure through the cross-linking agent. Differences in the contents of monomers and cross-linker cause the three-dimensional network structure to vary greatly. The SEM images of freeze-dried hydrogels are shown in Figure.2 with a 11


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comparison between the 1st network, 2nd network, DN, and BMDN hydrogels (P (AMPS-co-AAm), PAAc, P (AMPS-co-AAm)/PAAc and APBA-P (AMPS-coAAm)/PAAc).

Figure 2. SEM micrographs of (a)1st network hydrogel, (b)2nd network hydrogel, (c) DN hydrogel, (d) BMDN hydrogel.

Through the mechanical properties test, the 1st network hydrogel has a high stiffness but is quite easy to break, and the compressive ultimate stress is only 50kPa; this 2nd cross-linking network has a high flexibility, giving this final staggered network hydrogel (DN hydrogel) good mechanical extensibility. The compressive ultimate stress of the DN hydrogel increases by 40 times to 2MPa. The compressive modulus 12


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of the 1st network hydrogel is about 120kpa and the 2nd is only 0.8kpa. The microstructure of the DN hydrogel shown in Figure.2(c) has a more compact and dense network. The DN hydrogel can react with APBA under the catalysis of EDC to produce the final BMDN hydrogel. The 1st, 2nd, and DN hydrogels are all transparent colloids, and because of the introduction of the benzene ring, the BMDN hydrogel turned yellow. To determine whether the modification is successful, the FTIR spectra of the DN hydrogel before and after APBA modification were observed, as shown in Figure.3. The reduction in the carboxylic acid peak at 1667 cm-1 suggests that the carboxylic acid groups have reacted with APBA. Further, the proportions of the elements in various hydrogels prepared by XPS are shown in Table1 to demonstrate the effectiveness of the synthesis of each step.

Figure 3. FTIR spectra of the DN hydrogel and the corresponding BMDN hydrogel.

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Table 1. Proportions of the elements in the various hydrogels as measured by XPS Chemical B%

C%

N%

O%

S%

Reactants formula

1st

2nd

-

-

60.25

68.55

8.46

2.02

25.39

5.9

29.43

AMPS

C7H13NO4S

AAm

C6H11NO

MABM

C7H10N2O2

PAA

C3H4O2

MBAM

C7H10N2O2

-

P(AMPS-coDN

-

66.7

3.26

28.78

1.26

1st+2nd AAm)/PAA P(AMPS-co1st+2nd

BM 1.02

66.36

4.35

27.22

1.05

AAm)/PAA+ +C6H7BO2

DN APBA

Sa of DN hydrogel is 316nm (std. error is 33.4) and Sa of BMDN hydrogel is 399nm (std. error is 28.7). The difference between the DN and BMDN hydrogels is clear and glucose has essentially no effect on the DN hydrogel, as can be seen in the Figure.4. However, the friction coefficient of the BMDN hydrogel first increases, then decreases and tends to be stable with the increase of the glucose solution concentration as shown in Figure.5(a). The water content of the BMDN hydrogel in Figure.5(b) shows the exact opposite trend. In addition, it has been demonstrated that the degree of equilibrium reaction is affected by the pH of the solution. The 3 wt% glucose solution concentration was used to set the different pH of the glucose solution 14


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(ranging from 6.6 to8.0). This series of solutions was used as the lubricant, and the experimental results are shown in Figure.5(c). The water content of these BMDN hydrogels is shown in Figure. 5(d), which also shows the opposite trend of the friction coefficient.

Figure 4. Comparison test results of the DN and BMDN hydrogels. (a) COF of the DN and BMDN hydrogels immersed in solutions of different glucose concentrations; (b) COF of the DN and BMDN hydrogels immersed in solutions of different pH.

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Figure 5. COF and water content of the hydrogels immersed in different solutions (a) COF of the hydrogels immersed in solutions of different glucose concentrations; (b) water content of the hydrogels changed with the increase in the glucose concentration; (c) COF of the hydrogels immersed in glucose solutions of different pH; (d) water content of the hydrogels changed with the increase of the glucose solutions pH values.

Because the stiffness of hydrogel and the viscosity of the lubricating fluid are quite stable (Figure. S1-S3), as well as the friction conditions are constant in the friction tests, it is indicated that the change in friction coefficient is related to changes in the friction interface after the combine of glucose and PBA. The introduction of glucose will change the structure of the molecular chain by combining with APBA, which will change the hydrogel’s hydrophilic properties. Before binding with glucose, the end of 16


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the molecular chain is the hydrophobic benzene ring. The combination of glucose will bring three hydroxyls and an ether bond, which will enhance the hydrophilic to a certain degree. Because glucose contains two cis -diols and the reaction between APBA and glucose is reversible, a glucose molecular can bind with two APBA molecules generating a 2:1 APBA–glucose complex and forming a cross linker at low glucose concentrations. As the glucose concentration increases, the complex breaks into two PBA–glucose complexes (molecule ratio=1:1) resulting in a further swelling of the hydrogel. The schematic diagram of the reaction structure is shown in Figure. 6.

Figure 6. Schematics of the 1:1 and 2:1 complex formed between glucose and immobilized PBAs. D (+)-glucopyranose represents glucose in this paper. The number of B atoms in the BMDN hydrogel can be calculated according to the proportion of the B element in the xerogel shown in Table1. The amount of the substance of glucose that is to be combined with the BMDN hydrogel in the equilibrium solution can also be obtained. The hydrogels were immersed into a 50-ml 1.25wt% glucose solution, and when it reached the swelling equilibrium, the combination of glucose and borate groups inside the hydrogel is 1:2. When the concentration of glucose increases, there will be a 1:1 combination between glucose 17


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and borate with sufficient glucose. As shown in Figure.5, the friction coefficient and the water content are negatively correlated to a certain degree. The modified hydrogel will affect the water content because it binds to glucose and affects the number of hydrophilic groups in the molecular chain. In general, the increase in water content will enhance the hydrogel’s lubrication performance. As for the pH series, the 3wt% glucose solution was used to ensure that the reaction occurred under the 1:1 ratio. The degree of the equilibrium reaction determines how many glucose molecules are attached to the molecular network, and the number of glucose molecules attached to the molecular network determines the water molecules bound to the BMDN hydrogels, which finally influences the lubrication ability. When the pH of the glucose solution is 7.4, the most complete reaction is performed with the best combination of glucose and APBA. Based on previous experiments and analysis, the adjustable tribological experiment was completed. The friction test was performed on the same hydrogel and the lubricant was changed systematically. As shown in Figure.7, the friction coefficient in water is approximately 0.033 initially. Next, the friction coefficient increased by 70% to 0.056 when the water was replaced by a 1.25wt% glucose solution, showing a significant change. From the previous analysis, the boronic acid groups on the hydrogel molecular chain will bind with the glucose molecules at a 2:1 ratio after adding a 1.25wt% glucose solution. When the soaking solution was changed to phosphate buffered water again, the glucose molecules that were previously bound to the hydrogel would be detached to reach a swelling equilibrium owing to the 18


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reversible reaction. The FTIR spectra of the BMDN hydrogel immersed in the 1.25wt% glucose solution previously and after being soaked in water, demonstrates that the glucose that was once bound to the benzene boric acid dropped. In the end, the friction coefficient in water and glucose solution was 0.029 and 0.054, respectively. An adjustable friction coefficient of the hydrogel was achieved through the change in lubricants. It is meaningful to transform the passive state of friction to an active controllable state. In contrast, the glucose level can be identified and detected by the change of friction. This achievement also contributes to the research and application of adjustable friction in many other fields.

Figure 7. Friction signal after lubrication with water and 1.25wt% glucose solutions. After each friction, the soaking solution was changed and the BMDN was immersed for 1 h before the next friction; the friction coefficient changed several times and 19


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achieved the goal of regulation.

According to the previous discussion about the binding ratio between APBA and glucose, it is speculated that two reasons can result in the increase in the friction coefficient when the ratio was 2:1. On the one hand, the decrease in the hydrophilic groups on the molecular chain results in a decrease of water molecules that are bound to the molecular chains by the hydrogen bonds; on the other hand, the glucose molecules would act as cross-linkers at 2:1 ratio combining and the network structure becomes dense and the mesh size is reduced . To investigate the water morphology and its effect on the friction behavior of hydrogels, Raman experiments were performed to determine the content of binding water molecules by observing the peaks of water in the Raman spectra. The Raman spectra of water in the BMDN hydrogels, which occurred at different glucose solutions were observed, as shown in Figure.8. The peaks at 3250 cm-1and 3400 cm-1 represent the stretching vibration of O–H. As is generally accepted, the lowest wavenumber of the O–H stretching is assigned to the water molecules of the strongest hydrogen bond.45 The O–H stretching vibration band at ca. 3250 cm-1 of the water represents the hydrogen bond formed by the water molecule bound to the hydrogel network, and the band at ca. 3400cm-1 represents the hydrogen bond between free water molecules. As shown in Figure.8(a), with the increase in glucose concentration, the peak at 3250 cm-1 first increases and then weakens with respect to the peak at 3400 cm-1, and is weakest when the soaking glucose concentration is 1.25wt%. The 20


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change in the ice-like spectra proves the state of the water molecules. Water molecules are confined to the polymer chain by hydrogen bonds except for free water, which demonstrates the previous deduction. In the pH-varying glucose solution, the structure of the hydrogel is not substantially changed because of the super saturation reaction, and the amount of glucose bound is 1:1. The difference due to the degree of the equilibrium reaction will result in a change in the number of water molecules bound to the BMDN hydrogel. Figure.8(b) shows that at pH 7.4, the peak at ca.3250cm-1 is the highest relative to the peak at 3400 cm-1, meaning that the combined water results in a decrease in the friction coefficient when the hydrogel is rubbed.

Figure 8. (a) Raman spectra of BMDN hydrogels immersed in different concentrations of the glucose solution; (b) Raman spectra of BMDN hydrogels 21


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immersed in different pH solutions; The peak at 3250 cm-1 represents the hydrogen bond between APBA and glucose; 3400 cm-1 represent the O–H of free water; (c)– (d)differential thermal analysis(DTG-curve) of BMDN hydrogels soaked in different solutions.

In the TGA experiment, because of the different degrees of binding between the water molecules and hydrogels, the evaporating temperature of the bound water is higher than 100 °C of the general water, and the temperature at the time of rapid dehydration will increase. Some information can be obtained from the temperature at the peak of rapid dehydration and the temperature at the end of dehydration50. Three DTG-curves in Figure.8(c) demonstrate that the BMDN hydrogel in the 1.25wt% glucose solution has the least amount of bound water. It can be concluded from Figure.8(d) that the BMDN hydrogel soaked in a glucose solution at pH 7.4 has the largest amount of bound water, which confirms the previous hypothesis.

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Figure 9. Lubrication model of the BMDN hydrogel. A hydrated layer including bound water play the roles of bearing and lubricating.

According to the results and analysis above, it is acknowledged that the hydrated layer51, 52 is a key factor to the changing friction coefficient. A lubrication model that explains the friction mechanism is shown in Figure.9. Considering the large number of hydroxyl groups on the BMDN hydrogel molecular chains, these hydroxyl groups provide favorable conditions for the formation of hydrated layers by hydrogen bond. Besides, when glucose combines with APBA at 1:2 ratio, the glucose molecules would act as cross-linkers and the network structure becomes dense and the mesh size is reduced43,

46

(Figure.6). At low speeds, the non-Newtonian shear effects are

negligible and thermal fluctuation may dominate the lubrication mechanism53, 54. The 23


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mesh size is also approximately the amplitude of dynamic chain fluctuation55. Polymer chains at the surface of larger mesh size will fluctuate with increasing amplitudes. The random thermal fluctuations of polymers at the interface rapidly relieves shear strain during the sliding process. It provides a blurred interface over which sliding barriers are effectively reduced56. Therefore, when the glucose concentration is 1.25wt%, the mesh size decreases and the friction coefficient increase.57 In conclusion, the change in friction is caused by the formation of the hydrated layer and changes in the mesh size of hydrogel. A hydrated layer exists during the hydrogel friction for the bearing and lubrication. The water molecules form the hydrated layer between the interfaces. The water in the hydrated layer consists of water molecules that are bound with the hydrophilic group of the hydrogel network by hydrogen bond. Moreover, when glucose binds with APBA at 1:2 ratio, the glucose molecules act as cross-linkers and the mesh size is reduced, which also causes reduction in friction.

CONCLUSION In summary, a glucose-responsive hydrogel was synthesized, and an adjustable friction coefficient of the hydrogel was achieved. With APBA modification, the friction properties can be regulated by the change in the glucose solution, and the friction coefficient is negatively correlated with the change in water content. Furthermore, the change in friction is caused by the formation of the hydrated layer and changes in the mesh size of the hydrogel. In the future, through the analysis of the 24


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water composition and the adjustment of the hydrogel structure, the controllable effect of hydrogel friction in different environments can be achieved through the similar principle. The glucose level can be identified and detected by the change of friction.

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AUTHOR INFORMATION Corresponding Author *Email: [email protected]

ORCID liuyuhong: 0000-0001-9364-0039

SUPPORTING INFORMATION Five figures included in supporting information.

ACKNOWLEDGMENT This work was financially supported by the National Science Fund for Excellent Young Scholars (51522504) and the National Natural Science Foundation of China (51527901).

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Adjustable Tribological Behavior of Glucose-Sensitive Hydrogels

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