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Pattern Memory Surface (PMS) with Dynamic Wrinkles for Unclonable Anticounterfeiting Mingxuan Xie,† Gaojian Lin,‡ Dengteng Ge,§ Lili Yang,§ Luzhi Zhang,† Jie Yin,†,∥ and Xuesong Jiang*,†
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†
School of Chemistry & Chemical Engineering, State Key Laboratory for Metal Matrix Composite Materials, Shanghai Jiao Tong University, Shanghai 200240, China ‡ School of Mechanical Engineering, Beijing Institute of Technology, Beijing 100081, China § State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, Institute of Functional Materials, Donghua University Shanghai 201620, China ∥ School of Physical Science and Technology, ShanghaiTech University, Shanghai 201210, China S Supporting Information *
ABSTRACT: To avoid the losses caused by counterfeit commodity, documents and currency, various anticounterfeiting strategies have aroused an intensive interest and have been developed rapidly; however, obtaining unclonable anticounterfeiting technology that is sufficiently adaptable for industry-suitable authentication is still a challenge. Here, we report a facile yet effective anticounterfeiting strategy by utilizing pattern memory surface with NIR-driven dynamic wrinkles, which can be used as dynamic biomimetic fingerprints. The randomness and unpredictability of the wrinkles’ morphology enable them to be analogous to the minutiae of fingerprints. The dynamic nature of the fingerprint-like wrinkles endows them with hidden information to further improve security levels, and their NIRresponsiveness can help maintain a nearly identical topography during the cycles of erasure and regeneration, thus truly realizing unclonable anticounterfeiting. On the basis of these characteristics of NIR-driven dynamic wrinkles, such as simple preparation and easy readout, unique and unclonable identity tags were fabricated for antique authentication, which can be further combined with fluorescence patterns to achieve multiple anticounterfeiting technologies.
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methods have disadvantages of high preparation cost and requirement for professional equipment for readout. Because counterfeiting has important economic implications and is also a threat to security and health, obtaining low-cost and universally adaptable, unclonable anticounterfeiting methods is a necessity but remains a challenge. Surface wrinkles with wavy topography, as a pervasive and random pattern in natural and living organisms, have been utilized to create surfaces with unique functions.26−32 In particular, by mimicking human fingerprints based on the similarity between the structure of wrinkles and the minutiae of fingerprints, Park and Kwon’s group first demonstrated the wrinkle pattern as a biomimetic fingerprint for anticounterfeiting strategy,33 which is the result of creatively linking natural fingerprints with wrinkles. The fingerprint-like pattern of wrinkles can implement a higher level of anticounterfeiting because of the nondeterministic process, randomness, and
he development of surface patterns has attracted attention for their various fascinating properties and applications in wetting control,1−3 smart adhesion,4−6 7−10 optics, and especially, encoding information in the form of color or topography, which plays an important role in data recording, intelligent display, and anticounterfeiting.11−14 By taking advantage of optical information, such as fluorescence for anticounterfeiting, various printed tags with color information have been fabricated for products posing technical barriers for counterfeiting.15−17 However, these tags face the risk of being cloned by fast-growing counterfeiting technologies because of their predictable and deterministic encoding mechanisms. The same risk also exists in some nano- and microstructured surfaces, such as plasmonic surfaces.18−20 Thus, fabricating complex surface patterns with unpredictable formation mechanisms will improve the technical barriers to tag cloning, undoubtedly enhancing anticounterfeiting effectiveness.21,22 Various unclonable anticounterfeiting tags have been reported, such as fingerprint-like patterns based on randomly distributed nanowires or patterns generated by spherical cholesteric liquid crystals.23−25 However, these © 2019 American Chemical Society
Received: February 21, 2019 Accepted: May 15, 2019 Published: May 15, 2019 77
DOI: 10.1021/acsmaterialslett.9b00039 ACS Materials Lett. 2019, 1, 77−82
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Cite This: ACS Materials Lett. 2019, 1, 77−82
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ACS Materials Letters unpredictability of wrinkle formation. Park and Kwon’s work revealed that each artificial fingerprint of a wrinkle has different minutiae by establishing a matrix model based on minutiae distribution and image-hashing-based correlation analysis. In addition, a theoretical simulation by Yin and Boyce confirmed the sensitivity of wrinkle patterns in films to geometrical imperfections, further proving that generating identical topography via wrinkling is very difficult.34 However, this does not mean that the fingerprint-like pattern of wrinkles is unclonable because the fingerprint can be reproduced in other methods, such as imprinting or molding. For example, by using wrinkled surface as a corresponding template, a pattern complementary to the original wrinkle can be obtained through imprinting, which can be utilized as a template to duplicate the original wrinkle and probably results in the failure of anticounterfeiting.35,36 It is, therefore, necessary to develop a complicated pattern of wrinkles, for example, dynamic wrinkles, which are more difficult to replicate and possess a higher anticounterfeiting level. Because the information hidden in the dynamic process is unpredictable, dynamic patterns with a responsive surface color or topography can effectively increase the level of anticounterfeiting. For example, surface patterns with photochromic or thermochromic luminescence and long-lived luminescence have been used for dynamic anticounterfeiting applications. As the responsive fluorescence of the surface comes from some unique chemicals, these dynamic patterns of color are deterministic and can still be cloned. Unlike dynamic fluorescent patterns, surface patterns with dynamic topography have so far been rarely reported as anticounterfeiting strategies, which might be ascribed to their difficult fabrication. Recently, our group developed a simple and general method for the fabrication of IR-driven dynamic wrinkles based on bilayer systems, in which carbon nanotubes containing polydimethylsilane (CNT-PDMS) were used as elastic substrates, and varieties of functional polymers served as the top stiff layers.37 The resulting dynamic wrinkle is highly reversible and can be erased temporarily by IR irradiation. It is of great interest that this random wrinkle can keep a nearly identical morphology during the cycles of erasure and regeneration as if the wrinkled surface has memories. We call this IR-driven dynamically patterned wrinkle surface as the pattern memory surface (PMS), whose topography can change reversibly without losing its original morphology. In this study, we utilized PMS with IR-responsive wrinkle as a dynamic biomimetic fingerprint for anticounterfeiting (Figure 1). Because of the random minutiae and hidden information, the dynamic wrinkle undoubtedly increases the anticounterfeiting level, even realizing unclonable anticounterfeiting based on the following three premises: (1) The wrinkled surface composed of artificial fingerprint minutiae cannot be duplicated through the nondeterministic process of bucking or wrinkling. (2) Although the minutiae of the wrinkle pattern can be identically copied by the other deterministic processes, such as imprinting, the duplicated pattern with the same topography cannot be dynamic and has no hidden information. (3) The PMS has memorability to maintain the minutia-relative position during the cycles of erasure and regeneration. These three premises are reasonable and are supported by the experimental and theoretical results. On the basis of the unique and memorable minutiae of the wrinkle system and the universal properties of the dynamic platform, various anticounterfeiting tags with different shapes or optical information can be fabricated using functional polymers as
Figure 1. Strategy for unclonable anticounterfeiting using NIRresponsive dynamic wrinkles as a reversible biomimetic fingerprint. (a) Erasure/regeneration of infrared light-responsive wrinkle and the dynamic evolution of minutiae. (b) Similar minutiae in fingerprints and analogous dynamic artificial fingerprints as unique identifiers. The minutiae, including ridge ending and ridge bifurcation, are marked by different symbols in the photographs.
the top layer. In addition, two essential features of artificial dynamic fingerprints, the density of minutiae and dynamic characteristics, are actively configurable by regulating the NIRresponsive wrinkles. Because of the easy fabrication, unpredictable and memorable characteristics, and convenient authentication and readout, the NIR-driven dynamic wrinkle provides a robust and cheap approach for unclonable anticounterfeiting. The anticounterfeiting strategy based on NIR-responsive wrinkles as a dynamic biomimetic fingerprint is illustrated in Figure 1. The dynamic wrinkles are fabricated using a bilayer system with CNT-PDMS as the elastic substrate and functional polymers as the stiff top layer. When the direct thermal treatment or irradiation of IR is used, the mismatch between the moduli and thermal expansion ratios of the bilayers can cause compressive stress, resulting in the formation of wrinkle patterns with random topography, which possess similar minutiae as the fingerprint: ridge ending and bifurcation.33 These stress-controlled wrinkles are reversible and sensitive to the thermal expansion of the bilayer systems caused by the IR irradiation. When bilayer system is irradiated by infrared light, the temperature of bilayer system rises rapidly because of the high photon-to-thermal conversion efficiency of the CNTs, so that the bilayer expands and the applied strain (ε) decreases, resulting in the decreased amplitude of wrinkle. The minutiae on the fingerprint-like wrinkled surface disappears with the erasure of wrinkles. After removal of the infrared light source, the wrinkles recover to the initial stage, maintaining the same distributions of minutiae as those in the original image (Figure 1a). Thus, to combine memory behavior with wrinkle randomness, we can construct a dynamic biomimetic fingerprint for an anticounterfeiting strategy by utilizing an infrared non-contact response. To gain detailed insight into the features of these artificial dynamic fingerprints, we investigated the minutia and dynamics of wrinkle extinction and formation using both experiment and finite element simulation. The basic characteristics of dynamic fingerprints can be designed on-demand by tuning the bilayer system of IR-responsive wrinkles. The minutia density is determined by the characteristic wavelength 78
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randomness of their wavy topography and their responsiveness. We prepared 100 wrinkled surfaces using the same raw materials under the same conditions and analyzed their minutiae. All these fingerprint-like wrinkles had different minutiae (Figure S4a and b), illustrating that duplicating wrinkles through wrinkling is impossible. This result is consistent with the experimental result from Park and Kwon’s group33 and the theoretical stimulation by Yin and Boyce,34 confirming that premise (1) is reasonable. In addition, these fingerprint-like wrinkles had good infrared memory and were highly reversible. The disappearance/ formation process of the fingerprints could be conducted for more than 1000 IR on/off cycles without damaging the characteristic minutiae of the original wrinkle (Figure 2b), suggesting excellent pattern-memory behavior. We use FFT algorithm for recognition. The FFT transform results of wrinkle patterns are obtained by software ImageJ. The consistency of the transform results can also prove the consistency of wrinkles (Figure S4c). It should be noted that the dynamic characteristic of the infrared response of the wrinkle pattern produced using our method is distinctly different from those produced by other methods, such as imprinting, thus guaranteeing the uniqueness of our dynamic wrinkles. By using dynamic wrinkles as templates for the molding and curing of PDMS, the minutiae of wrinkles could be duplicated through imprinting (Figure S5). However, the duplicated pattern with the same topography as that of our dynamic wrinkle could not be erased by IR irradiation, suggesting that the fingerprint-like pattern duplicated using this method is not dynamic. Therefore, the random topography and dynamic nature of IR-responsive wrinkles cannot be simultaneously reproduced using the current methods, confirming that premise (2) is reasonable. The duplicated pattern illustrated that the uniqueness of fingerprint-like wrinkles could not be ensured only by the randomness of their topography but also needed to be combined with their dynamic feature. To further identify this mechanism of memory behavior, a finite element analysis of the wrinkle extinction/formation process was performed to predict the evolution of minutiae within the wrinkles. As shown in Figure 2, the simulation results illustrate that the wrinkle can be circularly erased and displayed while the minutiae remain unchanged. A simulation of the corresponding stress distribution was also performed (Figure S6). When the wrinkle system was exposed to infrared light, the strains/stresses in the film were gradually released in situ, and the amplitude of the wrinkles gradually decreased with the disappearance of the minutiae. This kind of stress cannot be completely eliminated at a certain temperature. After the source of infrared light is removed, the wrinkles regenerate in the original location of the residual stress; thus, the distribution of minutiae can be maintained to be consistent with the original distribution. Interestingly, the initial formation process and stress distribution of the wrinkles are different from those after erasure in the finite element stimulation, which also illustrates the effect of this residual stress. We also observed a morphological transition during the initial formation of the wrinkles and found that the stimulation result is consistent with our experimental observation (Figure S7). During the initial formation process, punctiform protrusions first appeared on the smooth surface and then developed into a sinusoidal wrinkle. In the second generation process, the wrinkle maintained a similar morphology with
of the wrinkle and can thus be tuned by the thickness and modulus of the top layer. The photo-cross-linkable copolymer containing anthracene (PAN, detailed synthesis shown in Supporting Information) was spin-coated on a PDMS substrate containing little CNT as the skin layer, which can be cross-linked through photodimerization of anthracene. It is noteworthy that the glass transition temperature of the top layer and adhesion force between the skin layer and substrate also increased after photo-cross-linking, which might improve the stability of the wrinkle. By controlling the thickness of the top PAN layer and the irradiation time of 365 nm UV light, wrinkle patterns with different wavelengths and minutia distributions were generated. The characteristic wavelength of the wrinkle decreased with the increasing thickness of the top PAN layer. As the wavelength of the wrinkle decreases from 30 to 2 μm, the minutia density increases from 400 to 10000 mm−2 (Figure 2). The resulting wrinkles were dynamic
Figure 2. Minutiae information and dynamic characteristic of the dynamic wrinkle. (a) Minutiae density of the wrinkle as a function of the characteristic wavelength. (b) One thousand extinction/ formation cycles with the corresponding photograph of minutiae during 100 cycles. (c) Height image of wrinkles predicted through finite element simulation during the erasure/regeneration of wrinkles with the extinction/formation of minutiae. The wavelength was tuned by changing the thickness of the skin layer. The thicknesses of the top layer are 60, 100, 200, and 300 nm. The fingerprints with different wavelengths showed different encoding capacities, and the cycles can be repeated at least 1000 times without damaging the characteristic minutiae. The scale bar represents 50 μm, and the IR light intensity is 1.5 W/cm2.
and could be erased temporarily by IR irradiation, which could cause the CNT-PDMS elastic substrate to thermally expand because of photothermal conversion. The erasure speed of the dynamic wrinkles can be tuned by the CNT content in the PDMS, and a higher CNT content will lead to faster wrinkle erasure by IR irradiation. When the CNT content in the PDMS substrate reached 0.1 wt %, IR irradiation (λ = 808 nm, 1.5 W/cm2) for 15 s could heat the bilayer system to 65 °C, resulting in the erasure of the wrinkles (Figure S3). After removing the IR source and cooling to room temperature, the smooth surface is restored to the original wrinkled state. One unique characteristic of these dynamic wrinkles is the 79
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gradually blurred until it disappeared because of the extinction of the wrinkle. After the source of the IR light was removed, the tripod image reappeared, accompanied by the regeneration of the wrinkle because the bilayer sample shrank to the original state as the system temperature returned to room temperature. We monitored the morphological evolution of the wrinkles by optical microscopy in six different regions of the same tripod. As shown in Figure 3i−vi, the morphology of the wrinkles in different regions exhibits different orientations with individual minutiae. In regions vi and iv, the wrinkle was mildly ordered and slightly perpendicular to the boundary of the tripod due to the boundary effect, and there were disordered wrinkles in the region relatively far from the boundary. The wrinkles in all regions showed similar dynamic evolution upon exposure to infrared light, and the corresponding temperature variation was recorded by an infrared camera (Figure S8). The wrinkle of the tripod has good stability, and both the minutia distribution and dynamic characteristics remained nearly unchanged under ambient conditions for 3 months, which might be ascribed to cross-linking caused by the photodimerization of anthracene and noncontact of infrared light (Figure S9). The longer-term stability test is still being conducted. By forming a hierarchical wrinkled pattern, the practical and aesthetic aspects are improved. More importantly, dynamically evolving minutiae can be recorded by attaching an amplifier to a smartphone because the wrinkle wavelength is on the microscale, which is sufficiently adaptable for public authentication compared with other technologies that need professional equipment.33 The feasibility of our infrared dynamic wrinkle makes it possible to combine with other anticounterfeiting techniques. The difficulty in counterfeiting can be further improved by tuning the components of the functional layer to endow more information, such as fluorescence, to the identity tags. Replacing PAN with anthracene (AN) and naphthalene diimide (NDI) moieties containing a copolymer (PAN-NDI, detailed synthesis shown in Supporting Information) as the top layer allows fluorescence information to be added on the original dynamic identity tag for anticounterfeiting.40 The red fluorescence and cross-linking density of the PAN-NDI top layer can be simultaneously controlled by the photodimerization of anthracene, which can weaken the CT interaction and increase the modulus, consequently resulting in the formation of a wrinkle pattern with green fluorescence in the exposed region. Therefore, we can fabricate a fluorescent tag with dynamic wrinkling for the anticounterfeiting of some targets that are difficult to identify. For example, the identification of ancient Chinese chinaware from Ruyao in the Song dynasty is very difficult because the surface of porcelain is often smooth, lacking obvious texture to help distinguish between real and fake. As shown in Figure 4, an identity tag with a bottle shape was obtained under natural light after 365 nm UV irradiation with a photomask. The pattern of the bottle could temporarily be erased by IR irradiation and swiftly restored to its original state after the infrared light source was removed (Movie S3). Under illumination with UV light, the exposed and wrinkled areas emitted blue green fluorescence, which resembles the color of celadon, whereas the unexposed and smooth regions remained red (Figure 4d). The corresponding evolution of the wrinkle was recorded via fluorescence microscopy, simultaneously displaying the dynamic information on the wrinkles with fluorescence (Figure 4e and f and Movie S4). The identity tags have truly become the exclusive labels of this celadon and cannot be duplicated by anyone, even the producers of the
increasing amplitude. The disappearance process is influenced by the initially formed wrinkles and stress distribution, and the second formation and disappearance process are completely reversible. The results from our experiments and finite element simulation indicated that the regenerated wrinkle exhibited the same topography and minutiae as the original wrinkle, confirming the truth of premise 3. Pattern-memory behavior is very important and critical to the authentication of dynamic wrinkles. Therefore, the randomness, dynamic nature and pattern-memory behavior of our NIR-responsive wrinkles make them dynamic biomimetic fingerprints with undeterministic encoding mechanisms and feasible readouts that can truly realize unclonable anticounterfeiting. Utilizing the photo-cross-linkable PAN as a top layer,38,39 we can fabricate hierarchical wrinkles with a photomask, where cross-linking occurs in the region exposed to UV light and leads to a mismatch between the modulus of the rigid skin layer and that of the soft CNT-PDMS, resulting in the selective generation of wrinkles. Dynamic fingerprint-like wrinkles with individual minutiae and hidden information are obtained as identity tags for anticounterfeiting. These tags have the characteristic of being uniquely suitable for the anticounterfeiting of antiques. Here, we selected a tripod that is a kind of a cultural relic for sacrifice in ancient China as the object of protection. After the tripod was irradiated by 365 nm UV for 15 min through a photomask with thermal treatment, a specific wrinkling pattern of the tripod with minutiae was generated when the system was cooled down to room temperature. Thus, a unique exclusive identity tag for the tripod was fabricated, and the dynamic information could be observed either with a microscope or with the unaided eye because of light scattering caused by the wrinkle pattern (Movies S1 and S2). The dynamic evolution of the identity tag is shown in Figure 3. Upon irradiation with IR light, the pattern of the tripod
Figure 3. Identity tags with NIR-responsive dynamic wrinkles for anticounterfeiting of cultural relics. (a) Schematic illustration of identity tag preparation with photo-cross-linkable PAN as the top layer. (b) Photographs of the tripod during the on/off irradiation by infrared light. The scale bar represents 5 mm. (i−vi) The optical images illustrate the evolution process of the erasure/ regeneration of wrinkles in the corresponding areas. The scale bar represents 20 μm, and the IR light intensity is 1.5 W/cm2. 80
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predict, they have great potential application in anticounterfeiting.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmaterialslett.9b00039. Experimental details, characterization, tuning of the dynamic process, unpredictable minutiae and memory of fingerprints, difference in hidden information between IR responsive fingerprint and fingerprint fabricated by molding, finite element simulation to predict the evolution of wrinkles and stress, difference between the first formation of wrinkles and regeneration after erasure, evolution of surface temperature, long-term stability for the wrinkled pattern, and identity tags for anticounterfeiting with multi-information (PDF) Evolution process of tripod identity tag (MP4) Minutiae in tripod identity tags (MP4) Evolution process of bottle identity tags (MP4) Minutiae in bottle identity tag (MP4)
Figure 4. Fluorescent identity tags based on the dynamic wrinkle for anticounterfeiting. (a) Schematic illustration for the preparation of identity tags with PAN-NDI as the top layer. (b, c) Photograph of identity tags with fluorescence and dynamic wrinkles under natural light during the irradiation of IR light. (d) Photograph of identity tags under UV light. (e, f) Photograph of identity tags under a fluorescence microscope; the scale bar represents 50 μm. PAN-NDI was spin-coated on CNT-PDMS as the top layer with a thickness of 200 nm and then irradiated by 365 nm UV light for 15 min through a photomask.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
anticounterfeiting labels. In addition, by selective exposure with a photomask of a school badge to the bilayer and adapting the photolithography process, identity tags with the specific wrinkling pattern of the school badge were obtained (Figure S10a). In contrast to the fluorescence of the bottle pattern, these tags only displayed the blue green school badge without a red background under UV illumination, which caused a change in the fluorescence information (Figure S10d). The identity tags produced by photolithography also have similar dynamic characteristics in both the macropattern and wrinkles (Figure S10b, c, e, f). In our daily life, the QR code is widely used in commodity anticounterfeiting, to which the specific anticounterfeiting information is linked when the customers scan it. With our strategy, dynamic QR codes with specific information as identity tags could also be realized by replacing a photomask of the QR code (Figure S10g). The twodimensional code formed through wrinkling had the ability to scan and read information, while it would be hidden under the irradiation of infrared light (Figure S10h). In this case, the infrared light is no longer only the switch of the hidden information on the dynamic fingerprint but is also the switch of the QR code information. In summary, we demonstrated a facile and robust strategy for the fabrication of novel reversible biomimetic fingerprints by utilizing IR-responsive dynamic wrinkles to improve anticounterfeiting levels and even realize unclonable anticounterfeiting because of the unpredictable minutia distribution and hidden information. The key point of this dynamic fingerprint-like wrinkle is that the minutiae of the wrinkle remain unchanged during multiple cycles of wrinkle erasure/ regeneration. Various functional materials can be used as a skin layer for fabricating identity tags possessing more hidden information, suggesting the good feasibility of our anticounterfeiting technology. Because our IR dynamic wrinkles are easy to produce and read but impossible to copy and
Xuesong Jiang: 0000-0002-8976-8491 Author Contributions
The manuscript was written through contributions of all authors. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank National Nature Science Foundation of China (51773114, 11774049, and 21704062) and Shanghai Municipal Government (17JC1400700 and 17ZR1440000) for their financial support.
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ABBREVIATIONS NIR, near infrared light; PMS, pattern memory surface; CT, charge transfer. REFERENCES
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DOI: 10.1021/acsmaterialslett.9b00039 ACS Materials Lett. 2019, 1, 77−82
