Fingerprintable Hydrogel from Dual Reversible Cross-Linking

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Applications of Polymer, Composite, and Coating Materials

A Fingerprintable Hydrogel from Dual Reversible Cross-linking Networks with Different Relaxation Time Haoqi Li, Fuyong Liu, Zhiyong Li, Shanfeng Wang, Renhua Jin, Chen-Yang Liu, and Yongming Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b06754 • Publication Date (Web): 23 Apr 2019 Downloaded from http://pubs.acs.org on April 23, 2019

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ACS Applied Materials & Interfaces

A Fingerprintable Hydrogel from Dual Reversible Cross-linking Networks with Different Relaxation Time Haoqi Li,† Fuyong Liu,Ω Zhiyong Li,*,† Shanfeng Wang,† Renhua Jin,‡ Chenyang Liu,§ and Yongming Chen*,† †School

of Materials Science and Engineering, Center of Functional Biomaterials, Key

Laboratory of Polymeric Composite Materials and Functional Materials of Ministry of Education, GD Research Center for Functional Biomaterials Engineering and Technology, Sun Yat-sen University, Guangzhou 510275, China. ‡Department

of Material & Life Chemistry, Kanagawa University, 3-27-1 Rokkakubashi,

Kanagawa-ku, Yokohama 221-8686, Japan. §CAS

Key Laboratory of Engineering Plastics, Institute of Chemistry, The Chinese Academy

of Sciences, Beijing 100190, China. ΩInstitute

of Environmental Science, Shanxi University, Taiyuan 030006, China.

Corresponding Authors: *Yongming Chen, School of Materials Science and Engineering, Sun Yat-sen University, Guangzhou 510275, China; E-mail: [email protected] *Zhiyong Li, School of Materials Science and Engineering, Sun Yat-sen University, Guangzhou 510275, China; E-mail: [email protected]

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ABSTRACT Most of chemical hydrogels are stretchable and the deformed hydrogels may be recovered when the strain is removed. Such hydrogel with viscoelastic property cannot be remoldable under mild conditions. Here we demonstrated that a combination of dual reversible crosslinking with different relaxation time scale could be used to develop a remoldable hydrogel responding to a mild external stress. We fabricated the hydrogel with the surface-primary amine-rich silica nanodots (SiND, ca. 2.0 nm) and benzaldehyde-terminated PEO-PPO-PEO triblock copolymers (BAF127) at low temperature (< 10 °C) to form the chemical cross-linking by Schiff-base bonding. Increasing temperature (>15 °C) induced formation of physical crosslinking between the hydrophobic PPO segments. The latter network is weak and shows a fast relaxation while the former one shows the slow relaxation. The unique structural characteristics brought this hydrogel with high stretchability and self-healability as well as moldability. In particular, we demonstrated that this transparent hydrogel can keep fine three-dimensional (3D) patterns of a fingerprint, which may be applied for collecting digit information of fingerprints for identification.

KEYWORDS: Hydrogel; Moldability; Pluronic F127; Reversible cross-linking; Fingerprint.

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1. INTRODUCTION Hydrogel is a water-swollen molecular network and may support its own weight containing a large amount of water. In general, the network of hydrogel may be obtained by cross-linking through physical interaction and such hydrogel normally is weak in mechanical properties, such as poor elasticity and toughness.1 Also, the covalent chemical bonding may be used for formation of networks and the hydrogels are tougher and shows better viscoelastic character.23

However, these hydrogels cannot be stretched to a large extent and may be easily fractured at

a high strain. Therefore, double networks (DN) have been fabricated to improve the toughness of hydrogels.4-6 Recently, dynamic covalent bonding has been applied to form hydrogels and the reversible character brings about the most important character, self-healability, to hydrogel materials.7-8 The soft hydrogels have shown many promising applications, especially in medicine for drug delivery, tissue regeneration and replacement.9-10 However, as far as we know, no report on application of printing fine three-dimensional (3D) imaging patterns onto a hydrogel has been found. For example, impression of finger friction ridges onto the surface of a hydrogel may leave a 3D pattern, which may be applied for identification. Such application requires the hydrogel being deformable at certain stress and keeping the fine patterns for some time. Also, it should be transparent for easy transforming 3D image information into digits. However, the physical hydrogels are normally fragile under a stress. Also, the hydrogels with permanent covalent cross-links are too tough for this application and the deformation will be recovered immediately once removal of the external stress.11-13 The hydrogels consisting of reversible cross-linking can be readily deformed in shape at conditions of high temperature, different pH, and large stress.14-16 As a result, these hydrogels are stretchable, moldable, injectable, and also self-healable. However, to our best knowledge, molding microscale architecture of a 3D pattern, e.g., fingerprints, on a hydrogel substrate has never been reported. To form a pattern on hydrogel by impression, duration and amplitude of the external stress should be sufficiently long or high to suppress the reorganization of reversible networks. It means that either the period of applying stress is longer than the relaxation time for reorganization of cross-linking, or the stress is high enough to break the dynamic networks. Different physical and dynamic chemical interactions respond to an external strain at different 3



rates.17-19 Therefore, we proposed that a combination of multiple reversible cross-linking in a hydrogel may be a feasible strategy to achieve the above-mentioned striking property. 2. RESULTS AND DISCUSSION In this work, we used the surface -NH2 enriched silica nanodots (SiND) and the benzaldehyde-terminated PEO-PPO-PEO triblock copolymer (BAF127) to fabricate the super moldable SiND-BAF127 hydrogel under a mild condition (Figure 1A). Formation of SiNDBAF127 hydrogel from BAF127 and SiND is contributed by imine formation and hydrophobic interaction of PPO segments of BAF127. The former one is strong but dynamic chemical bonding and the latter one is weak hydrophobic physical interaction. Ultra-small SiND was prepared according to a literature report.20 The dynamic light scattering (DLS) trace of SiND in ethanol (Figure S1) gave a monomodal distribution and the mean particle diameter of SiND was ~2.0 nm with a narrow polydispersity. The content of surface primary amines was determined to be 9.7 mmol/g by the titration in water using methyl thymol blue as the indicator. BAF127 was prepared by terminal modification of F127 according to a previous procedure and the terminal functionalization degree was ca. 99% as determined by 1H NMR.21 It is known that F127 forms physical hydrogel by chain entanglement above the lower critical solution temperature (LCST), typically at concentration of above 15 wt%. The hydrogel was prepared first by mixing the aqueous solutions of BAF127 and SiND at 5 °C to form the chemical crosslinking. The hydrogel thus obtained at low temperature was sticky and very soft. Then, by increasing temperature, the second cross-linking was generated and the hydrogel was transparent and became strong. After an optimization of the feed ratio, we prepared the SiNDBAF127 hydrogel at BAF127 concentration (CBAF127) of 19 wt% and SiND concentration (CSiND) of 1 wt% and, thus, the solid content of the hydrogel (CSolid) was 20 wt%. Since formation of the Schiff-base bond is dependent upon pH, the solutions to prepare the hydrogels was made with buffer solutions of different pH. While no gelation observed when pH 9, the hydrogels were obtained at pH=10, 11, 12 and 13 (Gel10, Gel11, Gel12, and Gel13). By Raman spectra (Figure S2), formation of the Schiff-base between the amine groups and the benzaldehyde groups was confirmed by appearance of C=N stretching band at 1643 cm-1. We monitored the gelation process of the SiND-BAF127 hydrogel Gel12 using time sweep 4


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oscillatory shear measurements at 5 °C (Figure S3) and 25 °C (Figure 1B) by fixing 1 rad/s of frequency and 5% of strain. For the SiND-BAF127 hydrogel Gel12, the storage modulus (G') quickly exceeded the loss modulus (G") within a short period. G' and G" reached a plateau and thus G"/G' was constant (~50 min at 5 °C, ~150 min at 25 °C), indicating formation of an elastic hydrogel. Since the hydrogel of F127 is temperature sensitive, we evaluated G' and G" of the Gel12 dependent upon temperature. Figure 1C shows that the Gel12 was always a gel in the entire temperature range, i.e. G' was always larger than G". Both G' and G" of Gel12 showed a step increase in range of 15 to 25 °C. Above this temperature, the hydrogel became even stronger and storage modulus G' reached 50 kPa, roughly one magnitude higher than that at lower temperature. In contrast, the loss modulus G" of BAF127 alone at a range lower than 30 °C was always higher than the storage modulus G', suggesting that the BAF127 in pH12 at a low temperature was a fluid (Figure 1C). Starting from 30 °C, both G' and G" increased and G' upturned sharply and exceeded G" at ~39 °C, indicating that BAF127 also formed a hydrogel, similar to the gelation behavior of the parent F127. Apparently, the gelation of SiND-BAF127 hydrogel Gel12 at lower temperature was attributed to formation of the imine bonding. The increase of both G' and G" upon temperature was attributed to the hydrophobic interaction of PPO segments of BAF127 to form the second cross-linked network. The transmission of temperature was lower than the LCST of BAF127 alone, which should be due to the influence of chemical cross-linking to hydrophobic interaction. Thus, above 15 °C both chemical and physical interactions contributed formation of the Gel12. Attributed to the dual networks, the storage modulus of Gel12 at high temperature increased for roughly one magnitude than that of either the Gel12 at low temperature or the BAF127 alone. pH influence on the properties of SiND-BAF127 hydrogels was checked at a high temperature (Figure 1D). G' as a function of pH and frequency at stain of 5% demonstrated that the hydrogel at pH12 showed the highest storage G' among the four hydrogels of different pH. The decrease of G' at pH 10 and pH 11 should be attributed to the shift of equilibrium of imine bonding towards the cleavage side.22 It is noteworthy that the modulus of Gel13 is the lowest. This might be due to the hydrolysis of siloxane bonds of SiND at a more basic condition.23

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Figure 1. (A) Construction of fingerprintable hydrogel based on dual reversible cross-linking. (B) Storage modulus (G') and loss modulus (G") of Gel12 as a function of incubation time at a strain = 5 %, frequency = 1 rad/s, and T = 25 °C. (C) G' and G" of Gel12 and BAF127 as a function of temperature at strain = 5 % and frequency = 1 rad/s. (D) G' as a function of frequency for the hydrogels at different pH, Gel10, Gel11, Gel12, and Gel13, at a strain = 5% and T = 37.5 °C.

Then properties of the Gel12 hydrogel dependent upon shear, tensile stress and compression stress were explored. As for viscosity dependent upon shear rate (Figure 2A), at 10 °C, the Gel12 hydrogel showed a slightly increased viscosity when shear rate was less than 5 s-1, indicating an elasticity of chemical cross-linking. At 20 °C, the viscosity increased one magnitude but showed no shear dependent, implying formation of physical network. At 30 and 40 °C, the viscosities increased for another magnitude but decreased quickly with shear rate, showing a shear-thinning phenomenon which should be due to failure of hydrophobic interaction under shear. The Gel12 hydrogel was extremely stretchable at room temperature and it could be stretched for more than 300 times as much as its original length, beyond the strain limit of a universal testing machine (Figure 2B). The Young’s modulus of the hydrogel was ~24 kPa at ca. 500% strain and the maximum stress was ~147 kPa at stain of 4000 %. It is 6


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interesting that tensile stress experienced three periods during stretching: 1) the stress increased with the strain at the low strain regime, 2) the stress became constant in the strain of 1000 to 2500 %, and 3) again the stress increased above the stain of 3000 %. This should correspond to the structural change from a mixed chemical and physical cross-links as function of strain change as well as the time length. Because of the irreversible breaking physical interaction, the highly stretched hydrogel could not return its original state when the stress was removed. Then compression of the hydrogel was tested (Figure 2C). Upon subjecting compression stress, the hydrogel was deformed gradually and at rate of 10 mm/min the compression stress reached 300 kPa (76 % strain), a moderate strength for a hydrogel. With increase of rate, it may reach the same compression stress at a smaller strain. It is interesting that the cylinder hydrogel was compressed into an integral flat disc but did not recover its shape when unloading (Figure 2D). Thus, the deformation was irreversible. This property allowed to remold hydrogel Gel12 from one shape into another shape in different molds at mild external stress (Figure 2E). Furthermore, the present hydrogel may be self-healable. When two halves of hydrogels were positioned in a mold, they could self-heal rapidly at the interface (Figure 2F). Even the fractured hydrogel pieces may form a complete cylinder by stress in the mold (Figure 2G). These characters should be attributed to the dynamic networks that may demonstrate self-healability and deformability.

Figure 2. (A) Viscosity of Gel12 as a function of shear rate. (B) Tensile stress curve of Gel12 at speed of 30 mm/min. (C) Compression stress curves of Gel12 at different speeds. (D) Photographs of the Gel12 before and after compression. (E) Photographs of the remoldable property of Gel12. The hydrogel was remolded to circular, square and triangle shape at 25 °C. (F) Self-healing of Gel12 by putting two cut hydrogels into a 7



circular mold at 25 °C for 3 min. The red cracked hydrogel contained Rhodamine B (0.02 mg/mL). (G) Photographs of the fractured Gel12 before and after compression.

Then, we explored its application in keeping fingerprint pattern on hydrogels, which requires materials remoldable to a small spatial deformation. When the press stress of a finger, ca. 10 kPa, was subjected to the Gel12 at 25 °C for 1 s, the impressions disappeared after 150 s (Figure 3A). When the duration time was extended to 10 s, the fingerprint impression became more evident and remained within 300 s. With a stronger stress (ca. 100 kPa), the impression was even more clear and the duration time did not show obvious difference in the image-persistence. Thus, the fingerprintability was influenced mainly by the amplitude of an external stress. Then the precision of 3D image reproducing was checked. Figure 3B1 shows the photo of a human friction ridge of one finger captured by a 3D microscopy and Figure 3B2 shows its impression on the dyed hydrogel Gel12. Observed by naked eyes, the impression could clearly retrieve the spatial architecture of the corresponding friction ridges. Since the hydrogel was transparent, the fingerprint may be observed from the other side of hydrogel (Figures 3B3). The width and depth of the finger friction ridges (wF and hF) and the 3D impression (wIM and hIM) (Figure 3C), collected by 3D microscopy, matched well as shown in Figure 3D. From the center of the whorl, as the count (N) of friction ridges increased, the width wF fluctuated in a range of 54.2-150.1 μm (for wIM 78-156.7 μm), and the hF 8.6-14.3 μm (for hIM 8.1-17.8 μm). Impressed at 25 °C, the patterns of finger patterns may be kept for several hours. But, at 30 and 40 °C, it was kept for at least 24 h. However, when it was cooled to 5 °C, the pattern dispersed in 30 s. Therefore, the finger patterns can be impressed onto the hydrogel surface and also may be erased at a low temperature.

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Figure 3. (A) Photographs of fingerprint impressions on Gel12 changed over time under different pressure for different time. (B) 3D optical microscopy images of a human friction ridge (B1) and its impression on the dyed Gel12 contained Rhodamine B (0.02 mg/mL). Photographs of impression on Gel12 of positive side (B3, left) and negative side (B3, right). (C) Diagram of finger friction ridge (width, wF and depth, hF) and impression on Gel12 (width, wIM and depth, hIM). (D) Width (left) and depth (right) of friction ridge and impression calculated by 3D digital microscope and plotted by the software Origin 9.0. All tests were performed at 25 °C.

In order to understand the reason of deformation, we investigated the influence of temperature in range of 10 to 40 °C on the modulus of the hydrogel Gel12 (Figure 4A). At 10 °C, G' and G" were almost parallel and remained constant during all range of 0.01 to 100 %. This further supports the elastic nature of Gel12 over the entire strain range. Here, the Schiffbase bonding governed the cross-linking of hydrogel. At 20, 30, and 40 °C, G' and G" were still linear at the strain ranged from 0.1 to 10 %. However, from strain 20 % the G' turned to decrease rapidly, a hint of the gel transformation from a high-elastic state to a viscous state. Then, we fixed the strain at 5 % to evaluate the modulus properties of Gel12 as function of frequency (Figure 4B). At 10 °C, the G' remained constant in the measured window, supporting a stable elastic property from the Schiff-base interaction. At 30 and 40 °C, the G' showed no dependent upon the frequency of 1 to 100 rad/s; however, decreased gradually at a low frequency. Therefore, the Gel12 at higher temperature also behaved as a typical elastic solid but may be 9



fast relaxed partly, which should be derived from the weak hydrophobic interaction of BAF127. It is noteworthy that the G' at 20 °C decreased throughout the frequency window, indicating an immature gelation from physical interaction. Above results demonstrated that the gelation of BAF127 components should play an important role to the hydrogel. Furthermore, the strain relaxation results at higher temperature (37.5 °C) presenting as a double logarithmic cure of stress-time demonstrated a two-mode relaxation, corresponding to two relaxation processes. This is contrast to the single mode relaxation at low temperature (10 °C). The relaxation curve at 37.5 °C was fitted well by the modified Maxwell spring-dash pot model, Eq. 2.24 According to the calculation of Eq. 1, the relaxation time of weak physical interaction was much shorter than the time of chemical interaction.

Figure 4. (A) Storage modulus G' and loss modulus G" as a function of strain for Gel12 at a frequency of 1 rad/s. (B) G' and G" as a function of frequency for Gel12 at a strain of 5%. (C) Relaxation stress dependent upon time at 37.5 °C and a strain of 20%. Experimental data: red circle; Fitted curves from single relaxation model given by Eq. 1: for chemical interaction (green dot line, with τ = 475 s) and physical interaction (blue dash line, with τ = 50 s); Fitted curve from double relaxation model given by Eq. 2: red solid line. Relaxation cure at 10 °C was made for comparison (black square).

The above rheological properties appeared for the SiND-BAF127 hydrogel at above 15 °C is particularly interesting. On one hand, it showed strong storage modulus (50 kPa), moderate tensile stress (150 kPa) and compression stress (300 kPa). On the other hand, the hydrogel may be highly stretched or deformed without fracture at a moderate strain, but the deformation of hydrogel is irreversible. The latter one differs from normal hydrogels that may recover the shape. These characters should be attributed to the structure of the present hydrogel with two networks (Figure 1A). The imine bonding, a chemical nature, supplies the first network to maintain the structure of hydrogel. But this structure in this system is mechanically weak and shows slow 10


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relaxation time to a strain. At higher temperature, the PPO segments become hydrophobic and aggregate to form the second network, which increases the mechanical properties greatly. Since hydrophobic interaction is a weak physical interaction, it may reorganize easily under a certain stain, namely the relaxation of the second network is a fast process (Figure 4C). Therefore, a combination of strong but dynamic interaction and weak hydrophobic interaction into this hydrogel materials demonstrates unique characters, i.e. deformable, remoldable and selfhealable. We demonstrated the use of the hydrogels in molding the biometric signals, i.e., 3D architecture of human fingerprints. The obtained 3D fingerprint pattern may nicely copy the spatial architecture of the friction ridges and valleys of the finger skin, of which the dimension is in the scale of micrometers. 3. CONCLUSIONS We have developed an interesting hydrogel with strong mechanical properties as well as a unique character of deformation under certain strain. The hydrogel was made from a reactive F127 and functional silica nanodots through both dynamic chemical and physical cross-linking. The first cross-link supported the hydrogel to hold its own weight and maintained its structure under deformation, while the second one may give a fast relaxation when a stress was given. We applied this hydrogel to collect the 3D patterns of finger impression, which can be erased simply by decrease of temperature. This material may be applied to transform the 3D patterns of fingerprints into digit information for identification. 4. EXPERIMENTAL SECTION 4.1. Materials: Pluronic F127 (F127, Mn = 12,600 g/mol) and 3-(trimethoxysilyl)-1propanamine (APS) were purchased from Sigma-Aldrich Chemical Co. Silica was purchased from Qingdao Ocean Chemical Plant. All other reagents were of analytical grade and used without further purification. F127 was modified into benzaldehyde-terminated PEO-PPO-PEO triblock copolymer via a two-step reaction according to our published protocols.21 The degree of benzaldehyde functionality of BAF127 was close to 99%, as determined using 1H NMR (Brucker Ascend 400MHz). SiND was prepared using the method from literature.20 4.2. Preparation of Hydrogels: The BAF127 was dissolved in buffer solvents at different 11



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pH (pH=10, 11, 12, and 13) to give a concentration of 255 mg/mL. The SiND was dissolved in deionized water to give a concentration of 90 mg/mL. Two solutions of BAF127 and SiND at 5 °C were mixed at a volume ratio of 10:1 and then sonicated for 1 min and preserved at 5 °C for 3 h. The final solid content of hydrogel was 20 wt%. The hydrogels were obtained at pH=10, 11, 12 and 13 named Gel10, Gel11, Gel12, and Gel13. 4.3. Characterization: The size and size distribution of SiND were characterized by transmission electron microscopy (TEM) and dynamic light scattering (DLS). The surface density of amine groups of the SiND was determined by the titration method with methylthymol blue as the indicator. The Raman spectra of BAF127 hydrogel and Gel12 were recorded at 25 °C on the Raman Spectrometer (Renishaw, Britain) with a laser wavelength of 514.5 nm. The 3D structures of fingerprints on the surface of the hydrogels were observed using a 3D digital microscope (VHX-1000C, Keyence, Japan). 4.4. Rheological tests: The dynamic strain and frequency tests were performed on the ARES-G2 rheometer (TA Instruments) equipped with 10.0 mm parallel plate (gap, 2.0 mm). The dynamic strain sweep experiment was performed at oscillatory frequency of 1 rad/s to determine the linear viscoelastic region. The dynamic frequency sweep (strain control) experiment was performed at frequency ranging from 0.1 to 100 rad/s at 5 % strain to indicate the storage modulus (G') and loss modulus (G"). The temperature sweep experiments were carried out on a HAAKE Mars 3000 rheometer from 5 to 55 °C with 20.0 mm parallel plates attached to a transducer (gap, 0.5 mm), at a strain = 5 % and frequency = 1 rad/s. For single relaxation model, the stress-relaxation experiment was performed at a strain = 20% at 10 and 37.5 °C. The stress-relaxation line was fitted using Maxwell spring-dash pot model Eq. 1:24 𝑮(𝒕) = 𝑮(𝟎)𝐞𝐱𝐩 [ ― (𝒕/𝝉)]

Eq. 1

where 𝝉 and 𝑮(𝒕) denote the relaxation time of hydrogel and the relaxation modulus over time. For double relaxation model, the stress-relaxation line was fitted using modified Maxwell spring-dash pot model Eq. 2: 𝑮(𝒕) = 𝑮𝟏(𝟎)𝐞𝐱𝐩 [ ― (𝒕/𝝉𝟏)] + 𝑮𝟐(𝟎)𝐞𝐱𝐩 [ ― (𝐭/𝝉𝟐)] 12


Eq. 2

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where 𝝉𝟏 and 𝝉𝟐 denote the relaxation time of chemical interaction and physical interaction, 𝑮(𝒕), 𝑮𝟏(𝒕) and 𝑮𝟐(𝒕) denote the relaxation modulus of hydrogel, chemical interaction and physical interaction over time. The steady shear sweep experiments were performed in the shear rate ranging from 0.001 to 100 s-1. 4.5. Mechanical Tests: Tensile and compression tests were performed using a KJ-1065A testing machine (Kejian-Tech, China) equipped with 50 N load cell. For tensile tests, the samples were cut into a dumbbell shape with inner width 5.0 mm, gauge length 8.0 mm and thickness 2.0 mm at 25 °C. The crosshead speeds used in the tensile tests was 30 mm/min. For compression tests, the samples were molded into 10.0 mm diameter disks with a thickness of 5.0 mm at 25 °C. The compression tests speeds were 10, 30 and 90 mm/min. 4.6. Remoldable property and self-healing property tests: For the remoldable property of hydrogels, the incubated hydrogel was fitted into a mold with shape of circular (diameter = 10 mm), square (side = 10 mm) and equilateral triangle (side = 10 mm). The hydrogel was demolded after 30 s recorded by camera. For the self-healing property, the incubated hydrogel was demolded from circular mold. It was cut into two pieces by a fresh scalpel blade. One cracked hydrogel was dyed by rhodamine B (0.02 mg/mL). Two cracked hydrogels were fitted into circular mold at 25 °C waiting for 3 min. The merged hydrogel was demolded from mold and recorded by camera. 4.7. Fingerprintable experiments: For the finger impression recovery tests of hydrogels, the hydrogels were put on slides. The hydrogels were applied by different pressure (10 kPa, 100 kPa) for different time (1 s, 10 s). The pressure (p) was measured by following equation 3: p=F/S

Eq. 3

where F is the stress of finger and S is the area of fingerprint. F was measured by electronic balance. S was measured from picture of fingerprint. The impression on hydrogels changing over time were recorded by camera. The width and 13



depth of finger friction ridge and impression were calculated by the software of 3D digital microscope (VHX-1000C, Keyence, Japan). The experiment was measured three times. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.xxxxxx. TEM image and size distribution (Figure S1), Raman spectra (Figure S2), and rheology (Figure S3) (pdf). AUTHOR INFORMATION Corresponding Authors *Yongming Chen, E-mail: [email protected] *Zhiyong Li, E-mail: [email protected] ORCID Yongming Chen: 0000-0003-2843-5543 Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS Financial support from the Guangdong Innovative and Entrepreneurial Research Team Program (No. 2013S086) and National Natural Science Foundation of China (Grant No. 51820105004) is gratefully acknowledged. REFERENCES (1) Zhang, Y. S.; Khademhosseini, A. Advances in Engineering Hydrogels. Science 2017, 356, eaaf3627. (2) Haraguchi, K.; Takehisa, T. Nanocomposite hydrogels: A Unique Organic-inorganic Network Structure with Extraordinary Mechanical, Optical, and Swelling/De-swelling Properties. Adv. Mater. 2002, 14, 1120-1124. (3) Yuk, H.; Zhang, T.; Lin, S. T.; Parada, G. A.; Zhao, X. H. Tough Bonding of Hydrogels to Diverse Non-porous Surfaces. Nat. Mater. 2016, 15, 190-196. 14


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Fingerprintable Hydrogel from Dual Reversible Cross-Linking

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