WO2021111349A1

WO2021111349A1 - Photonic micro-fingerprints as anti-counterfeiting devices

Info

Publication number: WO2021111349A1
Application number: PCT/IB2020/061427
Authority: WO
Inventors: Maria Penelope DE SANTO; Riccardo Barberi; Mauro Daniel Luigi BRUNO; Gia Petriashvili
Original assignee: Universita' Della Calabria
Priority date: 2019-12-04
Filing date: 2020-12-03
Publication date: 2021-06-10
Also published as: IT201900022908A1

Abstract

The invention relates to a method for the preparation of micro-spheroids with unique optical fingerprint-type texture, having at least one minutiae, the method comprising the performance of the following steps: A. preparing a chiral liquid crystal material comprising alternatively: - one or more nematic liquid crystals and one or more chiral dopants; - one or more cholesteric liquid crystals; - one or more cholesteric liquid crystals and one or more chiral dopants; B. dispersing the chiral liquid crystal material prepared in step A in a liquid amorphous matrix immiscible with the chiral liquid crystal material, obtaining a dispersion; C. treating the dispersion obtained in step B so as to produce an emulsion comprising micro-spheroids of chiral material dispersed in the liquid amorphous matrix; the method being characterized in that it comprises the following further step: D. applying an electric field, having an intensity equal to or greater than 0.4 V/μm and a frequency between 100 kHz and 2 MHz, to the emulsion of step C; and by the fact that: - said liquid amorphous matrix is chosen so that in the emulsion of step C there is a planar alignment of the molecules of the chiral liquid crystal material with respect to the liquid amorphous matrix, in which the longest axis of the molecules is parallel to the interface between the chiral liquid crystal material and the liquid amorphous matrix; - a photo-polymerizable monomer and/or polymer as well as a photo- initiator are added to the chiral liquid crystal material of step A and a step E is performed, subsequent to step D, consisting in subjecting the emulsion to a lighting having a wavelength suitable for photo- polymerizing the monomer and/or polymer. The invention also relates to a method for the use of micro-spheroids in anti-counterfeiting devices.

Description

“PHOTONIC MICRO-FINGERPRINTS AS ANTI-COUNTERFEITING

DEVICES”

DESCRIPTION

Technical field of the invention

The present invention belongs to the field of anti-counterfeiting technologies, in particular it falls within the context of the development of advanced anti-counterfeiting labels, which use random production methods which are therefore impossible to reproduce. The present invention refers to microspheres with “unique” or “anti-counterfeiting” or “fingerprint” pattern or texture (term universally used in the field, see for example the book Textures of Liquid Crystals, Ingo Dierking Wiley-VCH, ISBN: 978-3-527-60527-9 March 2006) or “unique topological structure”, i.e., analogous to a fingerprint, and to the methods for the preparation thereof.

Background art

In recent years we have witnessed the change in counterfeiting from a purely artisan activity to a real industrial and commercial business with the consequence that it has become a widespread and dangerous criminal activity. The numerous victims caused by certain counterfeit products such as medicines, drinks, spare parts for aircraft and vehicles, are a direct testimony of the real nature of counterfeiting. Counterfeit copies are apparently so faithful that manufacturers themselves often have difficulty distinguishing the original product from an unauthorized copy. Although the copy is difficult to distinguish from the original, the basic functionality and/or properties of the original are generally no longer present. One of the simplest and most effective solutions for authenticating a product is to associate it with an anti-counterfeiting label. Unfortunately, most of the most common labels are made using production methods based on deterministic processes, the low complexity and high predictability of which make it possible to copy even the anti-counterfeiting system. This category includes, for example, labels based on printed codes, holograms, fluorescent inks or molecular tags.

The new anti-counterfeiting methods must be based on labels which cannot be copied, i.e., which are not clonable. In 2002 the concept of Physical Unclonable Function (PUF) was proposed for the first time, which is the basis of many anti-counterfeiting labels with high security levels. A PUF key is based on a unique physical structure generated in a stochastic manner. In fact, the PUF is a physical manifestation of a cryptographic key and is impossible to forge. The uniqueness of the PUF key is given by the disordered microstructure thereof, which is generated randomly and is therefore impossible to reproduce. Since non-clonable functions have high levels of coding, the information on the label need not be kept secret, it is based on the uniqueness of the physical features thereof.

In general, the structures or patterns of PUF labels can be one-, two- or three-dimensional and the label must be detected to allow a precise comparison with a conserved reference image or with a database describing the essential features thereof. PUF keys can also contain different markers with unique properties such as Raman emission and/or fluorescence, melting points or molecular mass and allow for multiple detection methods to enhance coding capacity (Nature Reviews Chemistry 2017, 1 , Riikka Arppe and Thomas Just Sorensen, ARTICLE NUMBER 0031 ).

The first PUF key created was formed by unique random patterns of micro-objects capable of “scattering” light, dispersed in a polymer matrix. Therefore, the light scattering was used as a means for coding information, which was then read by a special experimental apparatus (Ravikanth Pappu, Ben Recht, Jason Taylor, Neil Gershenfeld, Science 2002, 297, 2026). Flowever, such means proved difficult to use as it produced, among other things, a series of false positives and allowed a low coding capacity. When designing an anti-counterfeiting system, the key reading is just as important as the physical function used in the PUF. If the key is based on optical properties, the reading will be the structure’s optical response. Therefore, not only the creation of labels containing PUF keys is important, but also the development of the detection methodologies is required, as this allows to authenticate the product. For example, the detection can involve performing light scattering measurements, as in the previous example, or visualization using optical microscopy techniques. More complex detections can be based on the measurement of an absorption or transmission spectrum or on the measurement of a melting point. The detection method can also be the combination of several methods. Flowever, the ability of the detection system to record multiple responses and to be stable is of fundamental importance, i.e., to provide the same response to the first as well as to the nth reading.

In summary, the key must be unique and easy to create; both the key and the reading system must be robust and economical, the systems must be able to be read several times without being damaged; the materials used must tolerate mechanical and light stresses. Furthermore, the detection must be non-invasive (Flyung Jong Bae, Sangwook Bae, Cheolheon Park, Sangkwon Flan, Junhoi Kim, Lily Nari Kim, Kibeom Kim, Suk-Fleung Song, Wook Park, and Sunghoon Kwon, Advanced Materials, 2015, 27, 2083). Finally, multiple markers with orthogonal responses can be used to ensure multiple levels of coding. Some types of PUF keys have been made so that their structure resembles that of a fingerprint (“fingerprint” texture, see the quote at the beginning of this description).

Human fingerprints have a peculiar shape and are unique. A fingerprint is essentially composed of a set of wrinkle or ridge lines, which flow mainly parallel to each other, creating a pattern (ridge pattern). Sometimes the ridge lines produce local macro-singularities, called whorls (or vortices, in which the lines circularly form around a central point of the finger), loops (the lines enter from one side of the finger, form a curve and exit from the same side) and arch (the lines enter from one side of the finger, form a curve, and exit from the other side of the finger).

Referring to Fig. 1 , a number of micro-features can be identified in fingerprints: minutiae. The minutiae, or Galton features, consist of singularities in the ridge lines.

The most common minutiae are described as follows:

- ending, the abrupt interruption of a ridge line;

- bifurcation, a single ridge line divides into two ridge lines (or even trifurcations if the division is into three ridge lines);

- lake, a single ridge line forks and the two ridge lines soon converge again in a single ridge line, forming a small ellipse;

- independent ridge, a ridge line showing a beginning and an end, usually short;

- island, a small ridge line which is not connected to other ridge lines;

- hook, a bifurcation leading to a short and a long ridge line;

- crossing or bridge, a small ridge line which joins two parallel ridge lines;

- delta: point where the ridge lines take on a triangular shape; and

- core: the center of the fingerprint, where there may be a sharp bend or a ridge ending.

The delta and core features are found only in loop- and whorl-type (macro-singularity) fingerprints. Such minutiae are an important factor for the discrimination of fingerprints, since they are the points in which the lines have anomalous behaviors. Forensic science uses these minutiae to unambiguously recognize an individual, through image processing software, see for example: Feng J, Jain AK, “Fingerprint reconstruction: from minutiae to phase”. IEEE Transactions on pattern analysis and machine intelligence, Vol. 33, pp. 209-223, 2011 ;

Sumitha SM, Mukunda Rao AN, “Multiple features based fingerprint identification system”. International Journal of Research in Engineering and Technology, Vol. 4, pp. 142-150, 2015. elSSN: 2319-1163 I pISSN: 2321-7308;

Win KN, Li KL, Chen JG, Viger PF, Li KQ, “Fingerprint classification and identification algorithms for criminal investigation: A survey”. Future Generation Computer Systems-The International Journal of Escience, vol. 110, pp. 758-771 , 2020. DOI:

10.1016/j. future.2019.10.019; e

Maltoni D, Maio D, Jain AK, Prabhakar S. (April 21 , 2009). Handbook of Fingerprint Recognition. Springer Science & Business Media p. 216. ISBN 978-1-84882-254-2.

The background art is therefore oriented, in view of the production of univocal characterization devices, to the artificial production of materials with optical surfaces or properties having ridge patterns similar to human ones at least in a subset of the minutiae, so as to be able to successfully use the same recognition software and have the same degree of uniqueness.

Examples of this approach, with still unsatisfactory coding levels, are that of Nakayama and Ohtsubo (Keizo Nakayamaa and Junji Ohtsubob, Optical Engineering 2012, 51, 040506-1), who proposed the use of photo- polymerizable liquid crystals enclosed in optical cells to obtain structures with the desired shape, inside which the molecules are oriented to provide a random fingerprint-type texture, which gave rise to precise polarization patterns (another name for the fingerprint texture). Through this method, a fingerprint is obtained fixed on areas of around tenths of a millimeter. In this case the cell in which the structure is created is unique and multiple types of liquid crystal or dye cannot be used, reducing the possibility of coding.

This type of PUF key has the general property that it can be detected using the same recognition methods as real fingerprints. The latter involve the use of algorithms to improve the visualization of the fingerprint pattern, i.e., to identify the ridges and grooves which characterize it. From the crests, for example, two types of information called “minutiae” can be extracted: the end points of the single crest and the position and angular orientation of the bifurcation points. The detected minutiae are transformed into a binary matrix and compared with those contained in the database through cross correlations. Although fingerprint detection technologies are reliable in 80% of cases, the strength of the fingerprint texture is the impressive coding capacity thereof.

Object and subject-matter of the invention

In order to further increase such aforesaid coding capacity, and therefore to compensate for the imperfection of the individual pattern recognition methods, it is an object of the present invention to produce specific materials with further unique properties, produced for this purpose in specific processes. It is a further object of the invention to allow the generation of anti-counterfeiting labels, for example by using processes for the preparation thereof which do not have the disadvantages of the labels of the background art.

A subject-matter of the present invention is a method according to the accompanying claims.

A further subject-matter of the present invention is an authentication method of an anti-counterfeiting product or device of the materials obtained from such a method for anti-counterfeiting purposes (use for anti counterfeiting). Description of the drawings

The invention will now be described by way of example, with particular reference to the figures of the accompanying drawings, in which:

- Fig. 1 shows in (b) a human fingerprint and in (a) a table of a set of minutiae associated therewith;

- Fig. 2 shows a cholesteric liquid crystal microsphere observed under a confocal microscope;

- Fig. 3 shows a fingerprint texture observed under the confocal microscope;

- Fig. 4 shows fingerprint textures on two different spheres observed under the confocal microscope

- after the removal of the electric field.

- Fig. 5 shows a thin polymeric film containing the liquid crystal microspheres.

- Fig. 6 shows two images of a container with the materials of the invention, before (left) and after (right) the application of the process according to the invention (more in detail, the figure refers to the preparation of the emulsion. In the figure on the left the two drops of liquid crystal are at the base of the bottle containing the glycerol in which the liquid crystal is immiscible. After vigorously shaking the bottle by hand, the two drops transformed into a myriad of smaller droplets);

- Fig. 7 shows a series of images acquired by applying an electric field to the droplets. In the first column, images are given in which there is no electric field applied to the microspheres. Starting from the second column, each row refers to a different intensity of the AC electric field: 0.3 V/mΐti (above), 0.4 V/mΐti (center) and 0.7 V/mΐti (below), showing the evolution of the texture observed by increasing the frequency of the field application: OkFIz (first column), 50Flz (second column), 100kHz (third column) and 1 MHz (fourth column). - Fig. 8 shows a graph of the textured surface as a function of the increasing frequency at three different applied voltages: 0.3 V/mΐh (squares), 0.4 V/mΐh (circles) and 0.7 V/mΐh (triangles).

- Fig. 9 shows images of a chiral droplet in an oscillating electric field at 20kFlz observed at increasing voltages (from left to right) 0 V, 0.25 V/mΐti, 0.5 V/mΐti and 0.7 V/mΐti.

- Fig. 10 shows two images acquired using the confocal microscope when an electric field is applied to a microsphere with a frequency outside the range 500Flz-1 5MFIz (left), and within the same range (right);

- Fig. 11 shows two microspheres according to the invention which exhibit a fingerprint-type texture essentially consisting of parallel lines and loop-type macro-singularities;

- Fig. 12 shows core-type singularities in (a) and delta-type singularities in (b), two bifurcation-type singularities in (c) and (d), in another microsphere produced with the method of the invention;

- Fig. 13 shows two microspheres obtained according to the prior art (Orlova et al.) with two different initial starting conditions; and

- Fig. 14 shows the results obtained with the prior art (Orlova et al.) as the applied electric field varies.

Detailed description of some embodiments of the invention

The present invention relates to a method for the preparation of microspheres with so-called fingerprint texture, to the microspheres obtainable with such a method and to the use of such microspheres in authentication systems. The system proposed here is for example based on the use of spheroidal structures (micro-spheroids) with dimensions ranging from a few microns to one millimeter and containing, among other things, liquid crystal material. The microspheres according to the invention are preferably less than one millimeter in size. If observed through an optical microscope in polarized light, they exhibit therein (in volume) a structure (optical or polarization pattern) similar to that of fingerprints. The microspheres are made of liquid crystal materials, as described below. Each microsphere has a different fingerprint, which is generated in a completely random manner and is therefore impossible to duplicate. To increase the degree of complexity, the fingerprint can be highlighted with molecules (dyes, for example fluorescent dyes) which emit a light of intense color when properly illuminated. Each single microsphere has a dimension thereof, has a unique fingerprint thereof with the fine features of the fingerprint which can be modulated (configurable security level) by the method according to the invention. The microspheres thus created can be used to make non-clonable labels.

Compared to the PUF keys based on fingerprint-type textures present in the background art, the present invention provides a higher level of configuration (different levels of information coding).

In a method according to an aspect of the invention, microspheres with an optical pattern or “fingerprint texture” are prepared in the volume thereof, comprising the following three steps.

In a first step (A) according to an aspect of the invention, a chiral material mixture can be prepared (chirality is the property of a rigid object, or of a spatial arrangement of molecules, of being non-superimposable on the mirror image thereof) comprising one or more nematic liquid crystals and one or more chiral dopants or one or more cholesteric liquid crystals (chiral native, possibly additionally doped with chiral dopants). In a second step (B) according to an aspect of the invention, the chiral mixture (material) prepared in the preceding step is added to a liquid amorphous matrix, for example immiscible with liquid crystals. In a third step (C) according to an aspect of the invention, the mixture (dispersion) obtained in the preceding step is treated so as to produce microspheres.

In a fourth step according to an aspect of the invention, an electric field is applied to the microspheres dispersed in the preceding step, so as to generate a fingerprint texture for said microspheres. The cholesteric liquid crystal molecules orient themselves due to the electric field and this leads to the formation of the fingerprint texture.

According to a further and different aspect of the invention, a method is applied for stabilizing the microspheres with fingerprint texture, for example as follows.

For example, it is possible to add a photo-initiator and a photo- polymerizable monomer to the chiral mixture, or to the native chiral liquid crystal mentioned above, and repeat what is described above. By then illuminating the microspheres with light of a suitable wavelength, through photopolymerization, the fingerprint texture of the microspheres is fixed and the microspheres solidified.

In an embodiment, the method according to the invention allows to obtain colored fingerprint texture microspheres, i.e., containing dyes, including fluorescent ones, to increase the information coding capacity. The preparation method is described below. In a first step, add a fluorescent or non-fluorescent dye to the chiral mixture, or to the native chiral liquid crystal.

In a second step, repeat what is described in points A, B, C possibly with the variants described above.

According to an aspect of the invention, it is possible to extract the microspheres from the original liquid amorphous matrix and disperse them in other suitable materials which act as support (solid or liquid, for example polymeric), for example in a polymeric film which provides mechanical support to the microspheres. The microspheres with fingerprint texture, dispersed in a liquid or solid matrix, will therefore be used, according to the invention, as PUF for the univocal authentication of a product.

Each individual microsphere which will be incorporated according to an aspect of the invention into an anti-counterfeiting product or device exhibits a different texture and, since each microsphere is generated by a random process, it is not possible to replicate it. The matrix containing the microspheres can also use different types of microspheres, each of them prepared, for example, using a different fluorescent material capable of emitting, under suitable lighting conditions, fluorescence light in different wavelength ranges, thus increasing the degree of complexity and coding of the system.

It must be said that chiral liquid crystals can be prepared by combining a nematic liquid crystal with a chiral dopant (properly called “chiral nematic liquid crystals”) but there are also cholesteric liquid crystals which have an intrinsic chirality not induced by the presence of a chiral dopant. The difference between the two is that in the first case we can, by varying the concentration of chiral dopant, vary the periodicity of the medium and this leads to being able to increase and decrease the distance between the lines of the fingerprint texture as desired. In the second case, the natural cholesteric materials already have a preset step. Surely it can be varied by adding a chiral dopant.

Some materials are listed below which can be used to produce the fingerprint microspheres object of the present invention. The list is not exhaustive and is for the purposes of explanation and not by way of limitation.

Examples of natural cholesteric liquid crystals are: cholesteryl benzoate, cholesteryl nonanoate, cholesteryl chloride.

Examples of nematic liquid crystals are: BL-036, BL-090, 5CB (Merck, USA), HPC21700-000, HAE625484 (HCCH, China). Other examples (non- exhaustive list) of nematic liquid crystals consist of cyanobiphenyls and analogues, fluorinated biphenyls and analogues, carbonates, phenyl esters, Schiff bases, azoxybenzenes.

Examples of chiral dopants for nematic liquid crystals are: R-2011 , S- 2011 HCCH (China), ZLI-4571 , ZLI-811 (Merck, USA). Examples of photoluminescent dyes are Pyrrometene, DCM, Rhodamine (Exciton), Red Nile (Sigma Aldrich).

Examples of monomer/photoinitiator coupling are: RM257 (Merck) and Irgacure 2100 (BASF), RM257 (Merck) and Darocur 1173 (BASF).

The chiral mixture can be added to a liquid amorphous matrix in which it is immiscible, for example glycerol, or polyvinyl alcohol solutions at high concentrations in water, and subjected to a treatment, such as an emulsification (in fact, treatment which consists of creating a system consisting of a liquid dispersed in another liquid or a solid dispersed in a gelatinous medium, for example, a system consisting of droplets of one liquid dispersed in another in which they are completely or almost insoluble) or through simple mechanical shaking which produces a type of emulsion, capable of creating chiral microspheres dispersed in the amorphous matrix.

According to an aspect of the invention, the liquid amorphous matrices which can be used are all those having an aqueous base. These include glycerol and solutions containing water and a polymer which is soluble therein such as polyvinyl alcohol or polyvinylpyrrolidone or even polyacrylic acid. By definition, an emulsion is a dispersion of a liquid in the form of micrometer-sized droplets in another liquid in which they remain completely insoluble. The key element for creating an emulsion is the non-solubility of one medium in another. In this particular case, the liquid crystal is a hydrophobic material, which does not like being in contact with water, and which, immersed in a water-based solution, spontaneously aggregates and collects in micrometric-sized drops. Since they range in size from a few units of microns to hundreds of microns, emulsions (in general) are usually investigated using light microscopy, from simple to more advanced.

According to an embodiment, this treatment can be performed by microfluidic techniques known to those skilled in the art, which also produce a type of emulsion. Microfluidics deals with the control and manipulation of fluids on a sub-millimeter scale, using capillaries which manage the transport of mass. The fluids are handled by active micro-components, such as micropumps and microvalves: the micropumps are used continuously or to dose, while the microvalves determine the direction or type of movement of the pumped fluid. A microfluidic particular which can be used is drop- based microfluidics, which allows the manipulation of discrete volumes of fluids in immiscible phases.

Multiple chiral mixtures can be created at the same time, containing different quantities of chiral dopant and/or nematic liquid crystal and/or different types of photo-luminescent dyes, to be joined to the same amorphous matrix.

The amorphous medium in which the liquid crystal microspheres are dispersed preferably meets three requirements:

- it provides a suitable alignment of liquid crystal molecules at the interface between the liquid crystal itself and the immiscible liquid of the amorphous matrix;

- it is viscous enough to prevent the coalescence of the microspheres and the displacement under the application of the electric field; and

- it allows to apply an electric field without creating discharges or short circuits.

As known per se, the above alignment can be planar or homeotropic, see for example Ingo Dierking (2003) Textures of Liquid Crystals (Wiley) Print ISBN: 9783527307258, Online ISBN: 9783527602056, DOI:10.1002/3527602054; and/or Jones L.P. (2012) Alignment Properties of Liquid Crystals. In: Chen J., Cranton W., Fihn M. (eds) Handbook of Visual Display Technology. Springer, Berlin, Heidelberg, https://doi.orQ/10 1007/978-3-540-79567-4 86.

The alignment of the molecules at the interface can be obtained as a function of the material used for the amorphous matrix.

For example, in the case of planar alignment at the interface (conveniently obtained using glycerol), a uniform radial orientation of the cholesteric helix inside the micro-spheroidal structures can be obtained. Planar alignment is intended as the following. The liquid crystal molecules used have an anisotropic structure, similar to a long, narrow cylinder (a stick). Therefore, in a molecule it is possible to define a long axis and a short axis. Planar alignment means that all the molecules are oriented, near the separation interface between the liquid crystal and the amorphous liquid matrix, so as to have the long axis parallel to the interface itself.

Instead, in the case of homeotropic alignment, as reported in the article TETIANA ORLOVA ET AL: "Creation and manipulation of topological states in chiral nematic microspheres", NATURE COMMUNICATIONS, vol. 6, no. 1 , 6 July 2015 (2015-07-06), XP055594637, D01 : 10.1038/ncomms8603, even by applying an electric field (however at low frequencies, see the effect of this in the present description below) it is not possible to obtain the fingerprint texture of the invention. The inventors were able to confirm that in the conditions and with the materials of the cited article the fingerprint effect claimed herein is not obtained, but it can be obtained with the electric fields claimed if the materials of the invention are chosen.

The method according to the invention allows to obtain the formation of microstructures with a spheroidal shape (even very flattened), containing the cholesteric mixture, from a few microns to a millimeter in size.

The chiral structure of the cholesteric helices determines a periodic variation of the refractive index inside the microsphere (M. Humar and I. Musevic, Optics Express 2010, 18, 26995). Observed under the microscope, before applying the electric field, each sphere exhibits a periodic pattern of concentric circles of alternating color, often presenting defect lines. The periodicity of the structure is closely related to the amount of chiral dopant used to create the cholesteric mixture. For example, periodicities of a few hundred nanometers can be obtained, from 400nm to 800nm for chiral dopant quantities ranging from 23% to 19% by weight, and periodicities ranging from one to a few microns for chiral dopant quantities ranging from 4% and 0.5%. Advantageously in general, there will be useful periodicities for dopant quantities ranging from 0.1 to 25% depending on the type of chiral material used, more preferably ranging from 2 to 12% by weight to obtain more spaced ridge lines which are better visible to optical detection.

In an embodiment according to the invention, to prepare the emulsion, about 20 pi of the mixture containing the liquid crystal, the chiral dopant, the dye and the photopolymer are deposited on the bottom of a glass container which is then filled with 5 ml of glycerol. The container is closed and shaken vigorously by hand (but nothing prevents an automation in this sense). Since glycerol and liquid crystal are immiscible, this leads to the formation of liquid crystal droplets in the amorphous matrix of glycerol. The spheres obtained are of very variable dimensions, with an average diameter of 50 pm.

Device and further conditions of the microsphere production process

According to one embodiment, the optical cell of the microspheres’ texture observation apparatus is prepared using two pieces of glass coated with indium tin oxide or other conductive material. The pieces of glass are assembled so that the parts covered with conductive material face each other. The two pieces of glass are separated by two thin strips of polymeric or solid material with a thickness ranging from a few microns to a few hundred microns. In place of plastic strips, spacers of other shape can be used as well. Electrodes are suitably welded to the pieces of glass which allow to apply an electric field in a direction perpendicular to the surface of the glass. The electric field is applied by a function generator and a signal amplifier.

The electric field produces a distortion of the chiral structure and leads to the formation of the fingerprint texture.

Preferably, such a texture has a loop macro-singularity and at least one minutiae is selected from the group: core, delta and bifurcation.

The electric field which determines the formation of the fingerprints in the microspheres has an intensity of at least 0.01 V/pm, preferably greater than 0.4 V/pm, still preferably 0.7 V/pm or greater. There are various disadvantages over 0.8 or better over 0.9 or 1 V/pm, such as the increased risk of damaging the emulsion; given the high electric field, the cell containing the emulsion overheats and, moreover, there may be discharges therein which literally “burn” the material. In fact, the problem related to the application of high electric fields is the heating of the cell, which contains the emulsion. The temperature increases both as a function of the time in which the field is applied to the cell and the intensity of the electric field itself. As mentioned above, the amorphous fluid matrix in the emulsion can be water based. In extreme conditions, smoke is observed exiting the containment cell, and in the worst case completely burned areas of the cell, which compromises the functionality thereof. For electric fields of intensity up to 0.7 V/pm, this effect is not immediate and it is possible to carry out all the stabilization operations of the microspheres described in claim 1 D without triggering any thermal effects. As the electric field increases, the formation of the unique optical patterns in the microspheres is still visible but the average life time of the cells containing the mixture becomes around a few minutes before the emulsion it contains burns out and the cell becomes completely unusable.

The electric field preferably has a frequency equal to or greater than 100 KHz, in particular between 100 and 2000 kHz, or between 200 and 2000 Hz, even more preferably between 500 and 1500 kHz, reaching the optimum frequency around 1 MHz.

The production step of the microspheres can be carried out:

- shaking the container containing the amorphous matrix the chiral material; and/or

- emulsifying the dispersion mixture prepared in step B; and/or

- applying microfluidic techniques to the amorphous matrix and chiral material.

According to a specific aspect of the invention, in order to fasten the fingerprint texture (which is metastable per se) with the electric field still present, the optical cell is illuminated with ultraviolet radiation. The radiation is absorbed by the photo-initiator material, which is activated to create a polymeric network using the monomeric structures present in the cholesteric mixture.

The method can further comprise another step in which after the formation and stabilization of the fingerprint-type texture, the optical cell can be disassembled and the microspheres can be separated or not from the liquid matrix.

Once the fingerprint texture microspheres have been prepared, they can be enclosed in a liquid, solid or polymeric matrix to form agglomerates with specific and anti-counterfeiting features, i.e., real anti-counterfeiting labels. In particular, they can be enclosed in a polymeric film. The microspheres can be collected inside a polymer matrix so that each microsphere can be arranged in a structure which can contain microspheres of different diameters.

Making a copy or an exact imitation of the individual microspheres or sets thereof will therefore be impossible. The microspheres can be randomly arranged or positioned to obtain a precise pattern through the use of micromanipulators or other positioning techniques. Examples of solid matrix in which the microspheres can be enclosed are films of polyvinyl alcohol or polydimethylsiloxane, the latter can be treated to obtain polymeric matrices with different degrees of flexibility.

The microspheres and labels described here can therefore be used in anti-counterfeiting devices and for the authentication of a product in general. Being of millimeter or sub millimeter size, they are difficult to detect with the naked eye and can be easily hidden inside and/or on the outside of the object to be protected. The individual microspheres or agglomerations thereof can be used to protect, for example, documents such as passports, identity cards or driving licenses or for the authentication of commercial products such as jewelry or watches, they can be inserted in the packaging or directly incorporated in the product to be authenticated. The microspheres or agglomerates can advantageously be comprised in films made with biocompatible materials in order to be used, for example, to protect foods or commonly used products such as tobacco or clothing. The dimensions of the micro-spheroids are for example of dimensions between 1 and 1000 mΐti, preferably between 10 and 100 mΐti. The optimal size is approximately 50mΐti (40-60 mΐti). The smaller they are, the more difficult they are to detect with a low magnification microscope, thus they are better hidden. On the other hand, if they are small the number of lines contained in the fingerprint texture is low. If, on the other hand, they are larger (for example 100mΐti and greater) they will also be viewable with a low magnification microscope but there will be more lines to render the fingerprint texture much more complex.

The anti-counterfeiting labels thus obtained can be read with optical microscopy techniques. The more complex the detection technique, the greater the amount of information it will be able to detect. In particular, the detection systems of such structures are of an optical nature, from the simple visualization of the fingerprint-type texture of the microsphere using a portable optical microscope, to the visualization of birefringence colors by a polarized light microscope, to the visualization of fluorescence using a fluorescence or confocal microscope. Furthermore, more complex structures can be obtained by reducing the spacing between the ridges of the fingerprints. This leads to an increase in the minutiae present and an increase in the coding capacity. In the latter case, a confocal optical microscope or equivalent optical system will be used for detection.

EMBODIMENT EXAMPLES

EXAMPLE 1

Cholesteric liquid crystal microspheres were made starting from a mixture of nematic liquid crystal HAE625484 (Jiangsu Hecheng Advanced Materials Co., Ltd.) to which a chiral dopant R2011 (Jiangsu Hecheng Advanced Materials Co., Ltd.) was added and a fluorescent dye PM650 (Exciton, USA), in the following percentages by weight:

99.7% (96% HAE + 4% R2011 ) + 0.3% PM650

This will be referred to as the cholesteric mixture in the following.

Once the chiral dopant was added, the mixture was heated on a small oven (scientific VELP) to the transition temperature of the liquid crystal in the isotropic state by mixing it with a magnetic shaker, so as to ensure the homogeneity of the sample. The PM650 dye (Pyrromethene 650, Sigma Aldrich) was added following the same method. The starting cholesteric mixture was doped at 4% with a mixture of monomer and a photo-initiator, in the following percentages by weight:

90% RM257 + 10% D1173. The mixture of RM257 (4-(3-Acryloyloxypropyloxy)-benzoesure 2- methyl-1 , 4-phenylester, Merck) and D1173 (Darocur, BASF) was mixed on a small oven at about 90°C and subsequently added to the cholesteric mixture. To ensure the homogeneity of the sample, the final mixture was heated to 90°C and at the same time mixed with a magnetic shaker. Subsequently, about 20 pi of the final mixture was deposited on the bottom of a glass container which was then filled with 5 ml of glycerol (Marco Viti). Microspheres of cholesteric mixture were obtained by magnetic shaking, with an average size of around 50 pm; from here on we will define the system obtained as an emulsion. An electric field must be applied to the microspheres to obtain the fingerprint texture. This is obtained by making optical cells the inner surfaces of which are coated with a layer of indium tin oxide (ITO) due to the conductive properties thereof (however, when only confocal microscopy is used, it would be enough for only one of the two confinement surfaces). The pieces of glass were cut with dimensions of about 2 cm x 2.5 cm, and the resistivity measured with a multimeter (In Line) was found to be equal to about 0.40 W; the cell was assembled by positioning the glass so that the surfaces of the ITO faced each other; the pieces of glass were separated using 200 pm spacers to avoid an excessive crushing of the microspheres. The pieces of glass were locked in place using metal clips. The cells were then sealed along two edges using a two-component glue (Evo-Stik), which acts over the course of 24 hours. Once the glue has dried, the external clips can be removed. Two electric wires must be welded to the cell in order to be able to apply an electric field, which were positioned along the short sides of the cell, by means of a welder (Weller) using a tin alloy and derivatives (Omodeo); given the low adhesion of the alloy to the glass, once welded, super attak glue (Loctite) was applied in the welding area. Infiltrating the cell with the emulsion from the short side of the cell, the sample was analyzed under a confocal microscope (Leica STP6000) which revealed the presence of a radial texture with concentric circles (Fig. 2). An electric field was then applied to the cell. The experiment was conducted by varying the voltage applied by means of a generator (Agilent) and an amplifier (Khrohn-Hite 7500) at a constant frequency, equal to 1 MHz. By increasing the applied voltage, the texture of the microspheres visibly distorts, and around a voltage of 0.7 V/pm it was possible to observe a fingerprint texture. (Fig. 3).

Once the fingerprint texture was obtained, the cell was irradiated with a mercury lamp (Jelosil HG 200L) for one minute at maximum power. The analyses carried out with the confocal microscope and the optical microscope in polarized light (DMRX, Leica) (Fig. 4) showed that the fingerprint texture is present even after removing the electric field and is stable even after several days.

The distance between the crests of the fingerprint-type texture is around a micron. Once polymerized, the microspheres can be collected and incorporated into an additional flexible polymer film (Fig. 5).

EXAMPLE 2

It is possible to obtain systems containing cholesteric liquid crystal microspheres with different pitch doped with different dyes (figure 5). The dye-doped cholesteric liquid crystals are prepared in separate containers by the same method described in example 1 . Each cholesteric liquid crystal inside the single container contains a different concentration of chiral dopant and a different photo-luminescent dye.

Once an arbitrary number of cholesteric mixtures were made, a drop of each mixture is deposited on the bottom of a glass container in several places so that they do not overlap and, subsequently, a glycerol matrix is added. A large number of microspheres with different radial texture pitch and containing different fluorescent dyes dispersed randomly in the fluid matrix are obtained by mechanical shaking. Applying the electric field as above, fingerprint-type textures are obtained in the microspheres with variable distances between the lines and with different fluorescence colors.

EXAMPLE 3 - Effect of the electric field on the method

Referring to Fig. 7-10, the results of experimental acquisitions on the texture of the microspheres as the electric field and frequency vary are shown. As regards the frequency range of the invention, it varies from 100kHz to 2MHz, preferably from 0.5 to 1.5 MHz, with an optimum value around 1 MHz (800-1200 kHz).

Fig. 7 shows images acquired under various electric field conditions. In the first column, images are given in which there is no electric field applied to the microspheres. Starting from the second column, each row refers to a different intensity of the AC field: 0.3 V/mΐti (above), 0.4 V/mΐti (center) and 0.7 V/mΐti (below), showing the evolution of the texture observed by increasing the frequency of the field application: OKHz (first column), 50Hz (second column), 100kHz (third column) and 1 MHz (fourth column). This experimental graph shows that the applied electric field must be at least 0.4 V/mΐti at a frequency of at least 100 kHz in order for the fingerprint texture of the invention to show, with an optimal value for 0.7 V/mΐti at 1 MHz.

Fig. 8 shows the dependence of the surface area as a function of the increasing frequency at three different applied voltages: 0.3 V/mΐti (squares), 0.4 V/mΐti (circles) and 0.7 V/mΐti (triangles). As can be seen in the figure, as the intensity of the electric field increases, the surface area changes and, given that the microsphere volume must remain constant (compared to before the application of the electric field), the thickness thereof decreases. Therefore, the microsphere showing the fingerprint texture will be rather an oval structure or even a flattened “pancake” shape, thus in the present description microsphere is intended as any spheroid, even very flattened.

Fig. 9 shows images of a chiral droplet in an oscillating electric field at 20kHz observed at increasing voltages (left to right) 0 V, 0.25 V/mΐti, 0.5 V/mΐh and 0.7 V/mΐh. It can be deduced from this that by using electric fields at lower frequencies than those of the invention, the fingerprint effect is not obtained, which is instead obtained in the conditions of the invention.

Regarding the electric field, in the article TETIANA ORLOVA ET AL: "Creation and manipulation of topological states in chiral nematic microspheres", NATURE COMMUNICATIONS, vol. 6, no. 1 , 6 July 2015 (2015-07-06), XP055594637, D01 : 10.1038/ncomms8603, the liquid crystal is dispersed in an immiscible fluid which provides an anchor perpendicular to the sphere surface, in the case of our example it is a planar anchor. The electric field they apply is 10V on 250mΐti, approximately 0.04n/mΐti. With the fields and frequencies applied, and also the anchor type, they cannot achieve fingerprint texture.

In WO 2018/033584 the reference to kV refers to the acceleration voltage of an imaging instrument, but they do not apply an electric field to their emulsion.

Fig. 10 shows two images of micro-spheres acquired using the confocal microscope when an electric field with a frequency outside the range 100Hz- 2MHz (left), and within the same range (right) is applied to a microsphere. It is clearly seen that the obtainable texture also depends on the frequency for any given intensity of the electric field.

According to the invention, it is also possible to go beyond 2 MHz. However, in this regard, the Inventors have performed further experimental tests applying an electric field by increasing the frequency from 1 MHz to 5 MHz. Once formed, the characteristic texture with parallel lines remains stable up to a frequency value of 2 MHz. Beyond this value, it is observed that the lines begin to fluctuate. The pattern becomes unstable, although parallel lines are still visible. The rate at which the pattern changes increases as the frequency increases. At 5 MHz, the vast majority of the microspheres appear irreversibly damaged. Characterization properties of the microspheres

Fig. 11 shows two images of microspheres obtained with the method of the invention, characterized by a set of lines running parallel to each other in configurations showing the presence of a considerable set of loop-type macro-singularities. The unique patterns proposed are characterized, like human fingerprints, by numerous lines running parallel to each other, creating patterns with local macro-singularities, in particular loops. Furthermore, they have the characteristic core and delta singularities. Finally, they also present a minutiae, singularity of the ridge lines: the bifurcation.

Fig. 12 provides details of other microspheres, in which several minutiae are simultaneously present: core, delta and bifurcation.

Referring to Fig. 13, in this sense, it is useful here to again speak of the results of the aforementioned article by TETIANA ORLOVA ET AL. In the set of experiments carried out by Orlova, in the planar anchoring conditions at the interface between the liquid crystal and the amorphous fluid matrix (molecules with the long axis perpendicular to the interface), the same initial pattern is found from which the experiments of the inventors described above started (Fig. 13 (a)), while in perpendicular anchoring conditions (molecules with the long axis perpendicular to the interface) the initial pattern shown in figure 13(b) is found.

In both cases parallel lines and a whorl-type configuration are observed, which is only a starting configuration both in the case of the prior art and in the case of the present invention. In the present invention, a loop macro singularity is finally obtained as well as other singularities which can include at least one choice from the group: core, delta and bifurcation.

Fig. 13 also provides the opportunity to discuss the concentrations of the one or more chiral materials mentioned above. This concentration is closely linked to the distance between the lines and, consequently, to the number which a microsphere can contain. In particular, the number of lines increases with the increase in concentration. Having more lines confined in a microsphere increases the complexity of the pattern obtained and, consequently, increases the probability of obtaining macro- and micro singularities in the single microsphere. In the microspheres made by Orlova et al., according to the same authors, when the cholesteric mixture is confined in microspheres of about 50mΐti, in planar anchoring conditions of the liquid crystal molecules at the interface, it is observed that the distance between the lines is 12.3mΐti. In the case of the present invention, the images presented show that the distance between the lines is 1 mΐti. With the same size sphere, this ensures that the formulation according to the invention is optimal for obtaining patterns of greater complexity. As a reference, a 4% concentration by weight of the chiral material used leads to a pitch of 2.5mΐti. On the other hand, increasing the concentration to values greater than those mentioned above would make the lines too close to each other and complicate the observation thereof under the optical microscope.

Fig. 14 shows the images extracted from the Orlova article linked to the patterns obtained by applying an electric field to an emulsion of cholesteric liquid crystal microspheres. The experimental conditions used are different from those used by us in terms of: applied voltage intensity (only 10n/250mΐti = 0.04 V/mΐti), applied voltage frequency (2kHz), voltage application time (a few minutes and then abruptly turned off), homeotropic anchoring of the molecules at the interface (long axis of the molecules perpendicular to the interface and not parallel). The patterns obtained by applying the electric field do not show the presence of either parallel lines crossing the sample, nor of macro-singularities and even less of singularities in the ridge lines.

Advantages of the invention

The advantages of the invention include the following:

- Absolute irreproducibility of the texture in the micro-object, even by those who produce it. - A micro-object of this type, or a set thereof, can be used as a PUF key.

- The use of fluorescent materials, a different dye for each object or small set, can increase the level of randomness of the system.

- The creation procedure is low-cost both in terms of the quantity of materials used and instrumentation used.

- The fingerprint-type texture is confined to an area of micrometric size and is not visible to the naked eye.

- The fingerprint-type texture is similar to the pattern of a human fingerprint, therefore, biometric recognition software can be used to interpret the data acquired with a microscope.

Claims

1. A method for the preparation of micro-spheroids with unique optical fingerprint-type texture, having at least one minutiae, the method comprising the performance of the following steps:

A. preparing a chiral liquid crystal material comprising alternatively:

- one or more nematic liquid crystals and one or more chiral dopants;

- one or more cholesteric liquid crystals;

- one or more cholesteric liquid crystals and one or more chiral dopants;

B. dispersing the chiral liquid crystal material prepared in step A in a liquid amorphous matrix immiscible with the chiral liquid crystal material, obtaining a dispersion;

C. treating the dispersion obtained in step B so as to produce an emulsion comprising micro-spheroids of chiral material dispersed in the liquid amorphous matrix; the method being characterized in that it comprises the following further step:

D. applying an electric field, having an intensity equal to or greater than 0.4 V/mΐti and a frequency between 100 kHz and 2 MHz, to the emulsion of step C; and by the fact that:

- said liquid amorphous matrix is chosen so that in the emulsion of step C there is a planar alignment of the molecules of the chiral liquid crystal material with respect to the liquid amorphous matrix, wherein the longest axis of the molecules is parallel to the interface between the chiral liquid crystal material and the liquid amorphous matrix;

- a photo-polymerizable monomer and/or polymer as well as a photo initiator are added to the chiral liquid crystal material of step A and a step E is performed, subsequent to step D, consisting in subjecting the emulsion to a lighting having a wavelength suitable for photo- polymerizing the monomer and/or polymer.

2. A method according to claim 1 , comprising the addition of a fluorescent or non-fluorescent dye to the chiral liquid crystal material in step A.

3. A method according to claim 1 or 2, wherein a loop macro-singularity is present in the microspheres.

4. A method according to claim 3, wherein the at least one minutiae is selected from the group: core, delta and bifurcation.

5. A method according to claim 4, wherein the liquid amorphous matrix is made with glycerol.

6. A method according to any one of claims 1 to 5, wherein the treatment of step C is obtained:

- shaking the container containing the liquid amorphous matrix and the chiral liquid crystal material; and/or

- emulsifying the dispersion prepared in step B; and/or

- applying microfluidic techniques to the liquid amorphous matrix and to the chiral liquid crystal material.

7. A method according to any one of claims 1 to 6, comprising a further step F following step E, wherein the micro-spheroids of chiral material are dispersed in a further liquid or solid matrix, for example a polymeric matrix.

8. A method according to any one of claims 1 to 7, wherein in step D the electric field is equal to or greater than 0.7 V/mΐti.

9. A method according to any one of claims 1 to 8, wherein in step D the electric field is equal to or less than 0.9 V/mΐti.

10. A method according to claim 9, wherein in step D the electric field is less than or equal to 0.8 V/mΐti.

11. A method according to one or more claims 1-10, wherein in step D the electric field has a frequency between 200 and 2000 kHz.

12. A method according to claim 11 , wherein in step D the electric field has a frequency between 500 and 1500 kHz.

13. A method process according to claim 12, wherein in step D the electric field has a frequency of about 1 MHz.

14. A method according to any one of claims 1 to 13, wherein in step A said one or more chiral dopants are supplied in a quantity ranging from 0.1 to 25% by weight.

15. A method according to claim 14, wherein in step A said one or more chiral dopants are supplied in a quantity ranging from 2 to 12% by weight.

16. A method for authenticating an anti-counterfeiting product or device, comprising performing the following steps:

- providing a plurality of micro-spheroids with unique optical fingerprint-type texture according to the method of one or more of claims 1 to 14; and

- applying or incorporating said plurality of micro-spheroids to an anti counterfeiting product or device.

17. A method according to claim 16, wherein the micro-spheroids of said plurality of micro-spheroids have an average size between 1 and 1000 pm.

18. A method according to claim 17, wherein the micro-spheroids of said plurality of micro-spheroids have an average size between 10 and 100 pm.