NEWS & VIEWS
doi:10.1038/nature14380
MAT ERIALS SCIENCE
Unique wrinkles as identity tags Spontaneously generated, random wrinkles of coatings on microscale particles have been found to be analogous to fingerprints — unique patterns with a wavy topography that can serve as unclonable tags for anti-counterfeiting purposes. means of generating ordered surface patterns5 and of harnessing surface topography for a wide range of controllable and tunable applications, such as the measurement of material properties6, wetting7, adhesion7, photonics8 and electronics9. The authors’ wrinkled microparticles are produced by drying precursors consisting of a soft polymeric core coated with a stiff shell of silica — a process similar to the way that raisins are formed. The core (which corresponds to the soft pulp in a grape) shrinks as the microparticles dry, whereas the shell (corresponding to the grape skin) does not. This generates excess surface area for the shell, which wrinkles spontaneously to accommodate the shrinking core. Previous work10 studying the wrinkling of thin coatings on soft spherical substrates reported labyrinth patterns similar to those described in the current paper. But Bae and colleagues are among the first to exploit the random and heterogeneous features in disordered wrinkling patterns for applications8. The ridged micropatterns in Bae and co-workers’ particles are similar to human fingerprints because their major defect features belong to two types of minutia: ridge ending
JIE YIN & MARY C. BOYCE
G
ottfried Wilhelm Leibniz articulated his principle of the indiscernibility of identicals1 with the words “No two identical leaves exist in the garden”. Writing in Advanced Materials, Bae et al.2 demonstrate the same principle for wrinkled microscale particles that have disordered ‘labyrinth’ surface patterns — no two identical microparticles form even when generated under seemingly identical conditions. The authors have used the unique and irreproducible characteristics of these wrinkles as “artificial microfingerprints” for identification and encryption applications. Wrinkles are ubiquitous in our lives, from the wavy undulations of our skin, to the folds of hanging curtains, to wrinkled raisins. In materials science, wrinkling has historically been viewed as a failure mechanism — the sudden transition of a flat surface to a wavy one under some form of critical load3. The potential of wrinkling as a method for creating wavy surface patterns, particularly at the microscale, was first demonstrated4 in 1998 for thin, stiff coatings attached to soft substrates. Wrinkling has since been pursued as a versatile
a
Minutiae
b Ridge branching
Ridge ending
Figure 1 | Similarities between fingerprints and surface wrinkle patterns of microscale particles. a, Human fingerprints are unique labyrinth surface patterns characterized by two types of feature (minutia): ridge ends (purple) and ridge branches (green). b, Bae et al.2 report that spontaneously generated labyrinth surface patterns of wrinkles on microparticles are also unique and exhibit similar minutiae. (Images adapted from ref. 2.)
and ridge branching (Fig. 1). When the authors examined the locations and density of minutiae of hundreds of wrinkled microparticles that were generated under identical conditions, they were surprised to find no duplication of patterns — unlike the repeatable wrinkling patterns produced in much of the previously reported work4–7. Although the wavelength of the labyrinth ridges is deterministic (mechanistically predictable by theory), the meandering structure of each labyrinth is random and contains minutiae that can function as identifiers. The researchers characterized the minutiae using the same technique that is used to read conventional fingerprints. They found that the artificial microfingerprints on the particles contain more-randomly oriented minutiae than human fingerprints, and could therefore offer an even higher level of security for identification applications. Furthermore, the wrinkling patterns lock in once they are formed because the silica coating is inelastic, and they survive extreme conditions — such as temperatures of 200 °C and repeated swelling and shrinking in ethanol. Bae et al. report that the number of ridge defects per unit area is inversely proportional to the size of the characteristic wrinkle wavelength, and that more minutiae per surface area can be created using thinner silica coatings, thus enabling the security level of identifier particles to be controlled. Moreover, they show that microfingerprints can be generated on particles that have complex geometries, such as the shapes of letters of the alphabet. This could help to categorize particles to enable their efficient identification by comparison with patterns recorded in databases, and thereby allow a large variety and number of products to be labelled with identifiers. The authors also show that the microscale wrinkling patterns can be decoded using a technique called con focal laser scanning microscopy when particles are attached to products. The new findings open up the possibility of using defect information in labyrinth wrinkling patterns as unique identifiers for encryption, identification and security-check applications. They also raise several questions and suggest future research opportunities. For example, can we better understand and harness the irreproducibility? And what factors govern the randomness? One possibility is that the observed labyrinth patterns are sensitive to small geometrical or material imperfections in the coating. Similar disordered labyrinth patterns have been observed in thin film coatings on planar soft substrates under ‘equi-biaxial’ loading4, and so we conducted some quick numerical simulations of three such systems that differ | NAT U R E | 1
©2015 Macmillan Publishers Limited. All rights reserved
RESEARCH NEWS & VIEWS JIE YIN & MARY C. BOYCE
patterns at the microscale opens up many opportunities for encryption, and so a simple, portable technique for reading the microminutiae information should be developed. ■
y z
x
1 µm
Figure 2 | Sensitivity of wrinkle patterns in films to geometrical imperfections. These computational simulations (conducted by the News & Views authors using the finite element method) depict the wrinkle patterns that form from three systems in which a 250-nanometre-thick film on a planar soft substrate is subjected to the same ‘equi-biaxial’ compression. Each starting system contained a different thickness imperfection corresponding to just a 0.1% variation of the film’s thickness at a few random locations. These seemingly negligible differences result in three distinct wrinkled patterns with different distributions of ridge ends (purple) and ridge branches (green).
only by the presence of a few randomly placed imperfections (at which the thickness is 0.1% thicker than at other, perfect, locations). As shown in Figure 2, our simulations reveal that these almost identical systems containing random, negligible geometrical imperfections render three different labyrinth wrinkling patterns under the same loading conditions. It remains to be seen whether the same is true for Bae and colleagues’ curved systems. The wrinkle patterns on planar substrates are also
unique, and could therefore also be applicable for identification applications. Widespread application of the artificial microfingerprints might currently be limited, because decoding them requires the use of a specialized high-resolution microscope. However, one could use the same wrinkling technique at larger length scales, which would enable patterns to be read more easily (as human fingerprints are). Nevertheless, the availability of irreproducible, random wrinkling
2 | NAT U R E |
©2015 Macmillan Publishers Limited. All rights reserved
Jie Yin is in the Department of Mechanical Engineering and at the Temple Materials Institute, Temple University, Philadelphia, Pennsylvania 19122, USA. Mary C. Boyce is in the Fu Foundation School of Engineering and Applied Science, Columbia University, New York, New York 10027, USA. e-mails: [email protected]; [email protected] 1. de Risi, V. Geometry and Monadology: Lebniz’s Analysis Situs and Philosophy of Space (Birkhäuser, 2007). 2. Bae, H. J. et al. Adv. Mater. http://dx.doi. org/10.1002/adma.201405483 (2015). 3. Timoshenko, S. P. & Gere, J. M. Theory of Elastic Stability 2nd edn (McGraw-Hill, 1961). 4. Bowden, N., Brittain, S., Evans, A. G., Hutchinson, J. W. & Whitesides, G. M. Nature 393, 146–149 (1998). 5. Genzer, J. & Groenewold, J. Soft Matter 2, 310–323 (2006). 6. Chung, J. Y., Nolte, A. J. & Stafford, C. M. Adv. Mater. 23, 349–368 (2011). 7. Yang, S., Khare, K. & Lin, P.-C. Adv. Funct. Mater. 20, 2550–2564 (2010). 8. Kim, J. B. et al. Nature Photon. 6, 327–332 (2012). 9. Rogers, J. A., Someya, T. & Huang, Y. Science 327, 1603–1607 (2010). 10. Cao, G., Chen, X., Li, C., Ji, A. & Cao, Z. Phys. Rev. Lett. 100, 036102 (2008).
doi:10.1038/nature14380
MAT ERIALS SCIENCE
Unique wrinkles as identity tags Spontaneously generated, random wrinkles of coatings on microscale particles have been found to be analogous to fingerprints — unique patterns with a wavy topography that can serve as unclonable tags for anti-counterfeiting purposes. means of generating ordered surface patterns5 and of harnessing surface topography for a wide range of controllable and tunable applications, such as the measurement of material properties6, wetting7, adhesion7, photonics8 and electronics9. The authors’ wrinkled microparticles are produced by drying precursors consisting of a soft polymeric core coated with a stiff shell of silica — a process similar to the way that raisins are formed. The core (which corresponds to the soft pulp in a grape) shrinks as the microparticles dry, whereas the shell (corresponding to the grape skin) does not. This generates excess surface area for the shell, which wrinkles spontaneously to accommodate the shrinking core. Previous work10 studying the wrinkling of thin coatings on soft spherical substrates reported labyrinth patterns similar to those described in the current paper. But Bae and colleagues are among the first to exploit the random and heterogeneous features in disordered wrinkling patterns for applications8. The ridged micropatterns in Bae and co-workers’ particles are similar to human fingerprints because their major defect features belong to two types of minutia: ridge ending
JIE YIN & MARY C. BOYCE
G
ottfried Wilhelm Leibniz articulated his principle of the indiscernibility of identicals1 with the words “No two identical leaves exist in the garden”. Writing in Advanced Materials, Bae et al.2 demonstrate the same principle for wrinkled microscale particles that have disordered ‘labyrinth’ surface patterns — no two identical microparticles form even when generated under seemingly identical conditions. The authors have used the unique and irreproducible characteristics of these wrinkles as “artificial microfingerprints” for identification and encryption applications. Wrinkles are ubiquitous in our lives, from the wavy undulations of our skin, to the folds of hanging curtains, to wrinkled raisins. In materials science, wrinkling has historically been viewed as a failure mechanism — the sudden transition of a flat surface to a wavy one under some form of critical load3. The potential of wrinkling as a method for creating wavy surface patterns, particularly at the microscale, was first demonstrated4 in 1998 for thin, stiff coatings attached to soft substrates. Wrinkling has since been pursued as a versatile
a
Minutiae
b Ridge branching
Ridge ending
Figure 1 | Similarities between fingerprints and surface wrinkle patterns of microscale particles. a, Human fingerprints are unique labyrinth surface patterns characterized by two types of feature (minutia): ridge ends (purple) and ridge branches (green). b, Bae et al.2 report that spontaneously generated labyrinth surface patterns of wrinkles on microparticles are also unique and exhibit similar minutiae. (Images adapted from ref. 2.)
and ridge branching (Fig. 1). When the authors examined the locations and density of minutiae of hundreds of wrinkled microparticles that were generated under identical conditions, they were surprised to find no duplication of patterns — unlike the repeatable wrinkling patterns produced in much of the previously reported work4–7. Although the wavelength of the labyrinth ridges is deterministic (mechanistically predictable by theory), the meandering structure of each labyrinth is random and contains minutiae that can function as identifiers. The researchers characterized the minutiae using the same technique that is used to read conventional fingerprints. They found that the artificial microfingerprints on the particles contain more-randomly oriented minutiae than human fingerprints, and could therefore offer an even higher level of security for identification applications. Furthermore, the wrinkling patterns lock in once they are formed because the silica coating is inelastic, and they survive extreme conditions — such as temperatures of 200 °C and repeated swelling and shrinking in ethanol. Bae et al. report that the number of ridge defects per unit area is inversely proportional to the size of the characteristic wrinkle wavelength, and that more minutiae per surface area can be created using thinner silica coatings, thus enabling the security level of identifier particles to be controlled. Moreover, they show that microfingerprints can be generated on particles that have complex geometries, such as the shapes of letters of the alphabet. This could help to categorize particles to enable their efficient identification by comparison with patterns recorded in databases, and thereby allow a large variety and number of products to be labelled with identifiers. The authors also show that the microscale wrinkling patterns can be decoded using a technique called con focal laser scanning microscopy when particles are attached to products. The new findings open up the possibility of using defect information in labyrinth wrinkling patterns as unique identifiers for encryption, identification and security-check applications. They also raise several questions and suggest future research opportunities. For example, can we better understand and harness the irreproducibility? And what factors govern the randomness? One possibility is that the observed labyrinth patterns are sensitive to small geometrical or material imperfections in the coating. Similar disordered labyrinth patterns have been observed in thin film coatings on planar soft substrates under ‘equi-biaxial’ loading4, and so we conducted some quick numerical simulations of three such systems that differ | NAT U R E | 1
©2015 Macmillan Publishers Limited. All rights reserved
RESEARCH NEWS & VIEWS JIE YIN & MARY C. BOYCE
patterns at the microscale opens up many opportunities for encryption, and so a simple, portable technique for reading the microminutiae information should be developed. ■
y z
x
1 µm
Figure 2 | Sensitivity of wrinkle patterns in films to geometrical imperfections. These computational simulations (conducted by the News & Views authors using the finite element method) depict the wrinkle patterns that form from three systems in which a 250-nanometre-thick film on a planar soft substrate is subjected to the same ‘equi-biaxial’ compression. Each starting system contained a different thickness imperfection corresponding to just a 0.1% variation of the film’s thickness at a few random locations. These seemingly negligible differences result in three distinct wrinkled patterns with different distributions of ridge ends (purple) and ridge branches (green).
only by the presence of a few randomly placed imperfections (at which the thickness is 0.1% thicker than at other, perfect, locations). As shown in Figure 2, our simulations reveal that these almost identical systems containing random, negligible geometrical imperfections render three different labyrinth wrinkling patterns under the same loading conditions. It remains to be seen whether the same is true for Bae and colleagues’ curved systems. The wrinkle patterns on planar substrates are also
unique, and could therefore also be applicable for identification applications. Widespread application of the artificial microfingerprints might currently be limited, because decoding them requires the use of a specialized high-resolution microscope. However, one could use the same wrinkling technique at larger length scales, which would enable patterns to be read more easily (as human fingerprints are). Nevertheless, the availability of irreproducible, random wrinkling
2 | NAT U R E |
©2015 Macmillan Publishers Limited. All rights reserved
Jie Yin is in the Department of Mechanical Engineering and at the Temple Materials Institute, Temple University, Philadelphia, Pennsylvania 19122, USA. Mary C. Boyce is in the Fu Foundation School of Engineering and Applied Science, Columbia University, New York, New York 10027, USA. e-mails: [email protected]; [email protected] 1. de Risi, V. Geometry and Monadology: Lebniz’s Analysis Situs and Philosophy of Space (Birkhäuser, 2007). 2. Bae, H. J. et al. Adv. Mater. http://dx.doi. org/10.1002/adma.201405483 (2015). 3. Timoshenko, S. P. & Gere, J. M. Theory of Elastic Stability 2nd edn (McGraw-Hill, 1961). 4. Bowden, N., Brittain, S., Evans, A. G., Hutchinson, J. W. & Whitesides, G. M. Nature 393, 146–149 (1998). 5. Genzer, J. & Groenewold, J. Soft Matter 2, 310–323 (2006). 6. Chung, J. Y., Nolte, A. J. & Stafford, C. M. Adv. Mater. 23, 349–368 (2011). 7. Yang, S., Khare, K. & Lin, P.-C. Adv. Funct. Mater. 20, 2550–2564 (2010). 8. Kim, J. B. et al. Nature Photon. 6, 327–332 (2012). 9. Rogers, J. A., Someya, T. & Huang, Y. Science 327, 1603–1607 (2010). 10. Cao, G., Chen, X., Li, C., Ji, A. & Cao, Z. Phys. Rev. Lett. 100, 036102 (2008).
