Multi-stable photochromic pigments for intradermal use

Abstract

Biocompatible bistable or multistable (P-type) photochromic nanoparticles or microparticles that can be embedded in the skin using techniques such as those used to create tattoos with tattoo inks. The color of a "tattoo" using biocompatible bistable or multistable photochromic microparticles can be written, erased, or changed and rewritten locally in the skin by irradiation with light of the appropriate wavelength. Some formulations, which have much higher sensitivity to UV light than visible light, can also be used as short-term intradermal colorimetric UV dosimeters that can be reset and reused on an hourly or daily basis. These particles can be homogeneous aggregates or polymers containing P-type photochromic dyes, microencapsulated or core-shell structures with a core containing P-type photochromic dyes, or solid particulate materials with P-type photochromic dyes adsorbed on their surface or within their pores.

Description

Technical Field

[0001] Cross - Reference to Related Applications This application claims the benefit of U.S. Provisional Patent Application No. 63 / 169,066, filed Mar. 31, 2021.

[0002] Field of the Invention The present invention generally relates to tattoo inks and their medical and cosmetic applications. More particularly, the present invention relates to compositions and methods for making biocompatible photochemically bistable or multistable nano - and microparticles and inks derived therefrom for optically rewritable tattoos and permanent makeup, biopsy markers, and intradermal use as colorimetric ultraviolet (UV) detectors and dosimeters.

Background Art

[0003] Background of the Invention Tattoo and permanent makeup inks containing nano - or micro - scale pigment particles (typically suspended in a water - or alcohol - based liquid) leave permanent or semi - permanent visible body markings when injected to a sufficient depth within the dermis. These pigments alter the color of the skin by modulating the frequencies of visible light absorbed and reflected in the dermis, and the pigments remain in place over the long term after the skin has healed from the injection procedure. While most intradermal pigments are used in body art and permanent makeup, they also have biomedical applications in preoperative delineation of anatomical biopsy sites, correction of pigmentary disorders, and medical aesthetic applications such as reconstructive surgery and hair removal camouflage.

Summary of the Invention

Means for Solving the Problems

[0004] Summary of the Invention The present invention provides biocompatible UV-activated bistable or multistable photochromic microparticles that can be embedded in the skin using techniques such as those used to create tattoos with tattoo ink. A "tattoo" using biocompatible photochromic microparticles will provide the skin with the ability to change color when exposed to light of a specific wavelength. Depending on the composition of the dyes and pigments contained within these particles, the color change can occur anywhere across the UV-visible-near infrared wavelength range, enabling visible-to-invisible, invisible-to-visible, or visible-to-visible conversions that can be controlled and programmed with an appropriate light source. If the photochromism of these particles has several orders of magnitude higher sensitivity to ultraviolet light than to visible light, they can be used for UV dosimetry. These particles can be homogeneous polymers to which or in which a P-type photochromic compound is bonded or embedded, crystalline or amorphous molecular aggregates containing a P-type photochromic compound, polymers or inorganic particles coated with a P-type photochromic compound, crystalline solids, amorphous solids, gels, liquids or solution cores containing a P-type photochromic compound, and core-shell (encapsulated) particles coated with a solid polymer or mineral-based shell or mesoporous particles containing a P-type photochromic compound, where the P-type photochromic compound can optionally be accompanied by other small molecule compounds such as stabilizers and dyes as light filters.

[0005] Exemplary biocompatible UV-absorbing microparticles are poly(methyl methacrylate) (PMMA) combined with commercial products. Some examples of materials that can be used as P-type photochromic dyes include Japanese Patent No. 3882746B2 and [Irie, M.; Fukaminato, T.; Matsuda, K.; Kobatake, S. Photochromism of Diarylethene Molecules and Crystals: Memories, Switches, and Actuators. Chem. Rev. 2014, 114, 12174-12277; Kobatake, S.; Takami, S.; Muto, H.; Ishikawa, T.; Irie, M. Rapid and reversible shape changes of molecular crystals on photoirradiation. Nature 2007, 446, 778-781; Irie, S.; Irie, M. Ultrahigh Sensitive Color Dosimeters Composed of Photochromic Diarylethenes and Fluorescent Metal Complexes. Chem. Lett. 2006, 35, 1434-1435; Kawamura, I; Kawamoto, H.; Fujimoto, Y.; Masanori, K.; Asai, K. Isomerization behavior of diarylethene-type photochromic compounds under X-ray irradiation: application to dosimetry, Jpn. J. Appl. Phys. 2020, 59, 046004; Jin, Y.; Qamar, I; Wessely, M.; Adhikari, A.; Bulovic, K.; Punpongsanon, P.; Mueller, S. Photo-Chromeleon: Re-Programmable Multi-Color Textures Using Photochromic Dyes.Diarylethenes as taught in [UIST’19 2019. 12 pp. New Orleans, LA, USA.], fulgides / fulgimides as described in [Yokoyama, Y. Fulgides for Memories and Switches. Chem. Rev. 2000, 100, 1717-1739], naphthopyrans as described in [Frigoli, M.; Maurel, F.; Berthet, J.; Delbaere, S.; Marrot, J.; Oliveira, M. M. The control of photochromism of [3H]-naphthopyran derivatives with intramolecular CH-π bonds. Org. Lett. 2012, 14, 4150-4153;Frigoli, M.; Marrot, J.; Gentili, P. L.; Jacquemin, D.; Vagnini, M.; Pannacci, D.; Ortica, F. P-Type Photochromism of New Helical Naphthopyrans: Synthesis and Photochemical, Photophysical and Theoretical Study. ChemPhysChem 2015, 16, 2447-2458] and acylhydrazones as described in [van Dijken, D. J.; Kovaricek, P.; Ihrig, S. P.; Hecht, S. Acylhydrazones as Widely Tunable Photoswitches. J. Am. Chem. Soc. 2015, 137, 14982-14991;Qian, H.; Pramanik, S.; Aprahamian, I. Photochromic Hydrazone Switches with Extremely Long Thermal Half-Lives. J. Am. Chem. Soc. 2017, 139, 9140-9143;Shao, B.; Qian, H.; Li, Q.; Aprahamian, I.Examples of hydrazones include those described in [Structure Property Analysis of the Solution and Solid-State Properties of Bistable Photochromic Hydrazones. J. Am. Chem. Soc. 2019, 141, 8364-8371]. Other suitable polymer capsule materials include polyacrylates and polyacrylamides, poly(dimethylsiloxane) (PDMS) and similar silicone rubbers, melamine-formaldehyde and other amino resins, epoxy resins, cross-linked polyethylene glycol (PEG) networks and related biocompatible networks, and poly(lactic acid) (PLA), poly(lactic-co-glycolic acid) (PLGA), methacrylamide chitosan, and many others.

[0006] In a first aspect, the present invention provides bistable photochromic particles comprising poly(methyl methacrylate) (PMMA) in combination with a P-type photochromic dye. The P-type photochromic dye can be a dye such as diarylethene, fulgide, fulgimide, naphthopyran, hydrazone, and combinations thereof.

[0007] In a second aspect, the present invention provides bistable photochromic particles comprising a polymer in combination with a P-type photochromic dye consisting of a diarylethene compound. In an advantageous embodiment, the polymer used in the composition of the second aspect can be PDMS and other silicone rubbers, melamine-formaldehyde and other amino resins, cross-linked PEG and other biocompatible networks, PLA, PLGA, methacrylamide chitosan, epoxy resins, PAA, PMMA, and other acrylate-based and acrylamide-based polymers and networks, and combinations thereof.

[0008] In a third aspect, the present invention provides a bistable photochromic particle composition comprising a polymer and a P-type photochromic dye in combination with a stable UV or visible light absorbing material, wherein the UV or visible light absorbing material acts as a filter for adjusting the spectral sensitivity or color appearance of the composition. The UV absorbing material can be hydroxybenzophenone, hydroxyphenyl-s-triazine, 2-(2-hydroxyphenyl)benzotriazole, oxalanilide, aminobenzoic acid, avobenzone, cinoxate, dioxybenzone, homosalate, meradimate, octocrylene, octinoxate, octisalate, oxybenzone, padimate O, ensulizole, sulisobenzone, cerium dioxide, titanium dioxide, trolamine salicylate, zinc oxide, layered double hydroxide, derivatives of the above compounds and combinations thereof. The visible light absorbing material can be an azo dye, perylene, anthraquinone, cyanine, triarylmethane, a commercially available pigment, pigment red, pigment orange, pigment yellow, pigment blue, pigment green, pigment violet, pigment black, pigment white and combinations thereof. In an advantageous embodiment, the bistable photochromic particle composition according to the third aspect comprises a light stabilizer for inhibiting the photodegradation of the polymer and thereby increasing the service life of the particles. The light stabilizer can be a hindered amine. In an advantageous embodiment, the hindered amine is 2,2,6,6-tetramethylpiperidine, a derivative of 2,2,6,6-tetramethylpiperidine or an alkylated or hydroxylamine analogue of 2,2,6,6-tetramethylpiperidine.

[0009] In a fourth aspect, the present invention provides a further bistable photochromic particle composition. The particles can be derived from any one of the particles of the aspects presented above. The particles are suitable for injection into the dermal layer of the skin and the particles form (A) polymer particles, (B) molecular aggregates, (C) surface-coated nano- or microparticles, (D) core-shell nano- or microparticles, (E) mesoporous nano- or microparticles or combinations thereof (see, for example, FIG. 1).

[0010] The bistable photochromic particles according to any of the above aspects can preferably be suspended in a biocompatible solvent. The biocompatible solvent can be water, alcohol (e.g., ethanol, isopropanol, glycerol, oligo and polyethylene glycol), oil (e.g., vegetable oil / triglyceride, geraniol, squalene, etc.) and combinations thereof. When alcohol is a biocompatible solvent, some advantageous alcohols are ethanol, isopropanol, glycerol, oligo and polyethylene glycol and combinations thereof. When oil is a biocompatible solvent, some advantageous oils are vegetable oil / triglyceride, geraniol, squalene and combinations thereof.

[0011] In a further advantageous embodiment, the bistable photochromic particle ink suspension comprises (i) a disinfectant (e.g., alcohol) to prevent bacterial contamination, (ii) a biocompatible surfactant (e.g., polysorbate) to stabilize the dispersion and adjust the surface tension, (iii) a thickener (e.g., xanthan gum, polyacrylate, polyglycol) to increase the viscosity and reduce the pigment sedimentation rate, (iv) a thixotropic agent (e.g., silica) to promote shear thinning, (v) a preservative / binder (e.g., polyether, polyvinylpyrrolidone) to assist in preventing the ink from drying and assist in binding of the ink to the needle, (vi) an astringent to minimize bleeding in the skin during injection, and (vii) an anesthetic to minimize pain during ink injection. Combinations of additives can be used to achieve multiple desired effects.

[0012] The bistable photochromic particle composition can contain a biocompatible surfactant at a ratio of <1.0% (v / v) and polyethylene glycol (molecular weight 1000) added at a ratio of 10% to 30%. The biocompatible surfactant stabilizes the suspension while polyethylene glycol acts as a disinfectant, thickener and / or binder. An advantageous biocompatible surfactant is polyvinyl alcohol.

[0013] The particle size of the bistable photochromic particles in the composition is preferably in the size range of 10 nanometers to 10 micrometers.

[0014] In a fifth aspect, the present invention provides photochemically bistable nanoparticles or microparticles in a biocompatible solvent. The biocompatible solvent is suitable for injection into the dermal layer of the skin. The nanoparticles or microparticles exhibit P-type photochromic properties. The photochemically bistable nanoparticles or microparticles can include an ink or pigment, and the ink or pigment is suitable for skin injection. The photochemically bistable nanoparticles or microparticles can include (i) a disinfectant (e.g., alcohol) to prevent bacterial contamination, (ii) a biocompatible surfactant (e.g., polysorbate) to stabilize the dispersion and adjust the surface tension, (iii) a thickening agent (e.g., xanthan gum, polyacrylate, polyglycol) to increase the viscosity and reduce the pigment sedimentation rate, (iv) a thixotropic agent (e.g., silica) to promote shear thinning, (v) a preservative / binder (e.g., polyether, polyvinylpyrrolidone) to assist in preventing ink drying and assist in binding the ink to the needle, (vi) astringents to minimize bleeding in the skin during injection, and (vii) anesthetics to minimize pain during ink injection. Combinations of additives can be used to achieve multiple desired effects.

[0015] In a sixth aspect, the present invention provides bistable photochromic particles comprising poly(dimethylsiloxane) (PDMS) in combination with a P-type photochromic dye. The bistable photochromic particles according to the sixth aspect can be P-type photochromic dyes based on diarylethene, fulgide, fulgimide, naphthopyran, hydrazone, and combinations thereof.

[0016] In a seventh aspect, the present invention provides multistable photochromic particles comprising a polymer in combination with two or more P-type photochromic dyes.

[0017] In an eighth aspect, the present invention provides multi-stable photochromic particles comprising a polymer in combination with one or more P-type photochromic dyes and one or more T-type photochromic dyes.

[0018] In the multi-stable photochromic particles according to the seventh or eighth aspect, P-type photochromic dyes using diarylethene, fulgide, fulgimide, naphthopyran, hydrazone, or combinations thereof can be used. Similarly, in the multi-stable photochromic particles according to the seventh or eighth aspect, T-type photochromic dyes selected from the group consisting of spiropyran, spirooxazine, and combinations thereof can be used.

[0019] In the multi-stable photochromic particles according to the seventh or eighth aspect, polymers such as PDMS and other silicone rubbers, melamine-formaldehyde and other amino resins, cross-linked PEG and other biocompatible networks, PLA, PLGA, methacrylamide chitosan, epoxy resins, PAA, PMMA, and other acrylate-based and acrylamide-based polymers and networks, and combinations thereof can be used.

[0020] In a ninth aspect, the present invention provides bistable photochromic particles comprising a polymer in combination with a P-type photochromic dye composed of a diarylethene compound involving UV-activated cyclization and visible-light-activated reverse cyclization, and the cyclization quantum yield is more than four orders of magnitude greater than that of the reverse cyclization. The diarylethene compound can be 1,2-bis(2-methoxy-5-phenyl-3-thienyl)-perfluorocyclopentene or 1,2-bis[2-methyl-5-(4-phenylbuta-1,3-dienyl)thien-3-yl]-perfluorocyclopentene.

[0021] The bistable photochromic particles / particle compositions can be combined with a stable UV or visible light absorbing material, which acts as a filter to adjust the spectral sensitivity or color appearance of the composition. Advantageous UV absorbing materials include hydroxybenzophenone, hydroxyphenyl-s-triazine and 2-(2-hydroxyphenyl)benzotriazole, oxalanilide, aminobenzoic acid, avobenzone, cinoxate, dioxybenzone, homosalate, meradimate, octocrylene, octinoxate, octisalate, oxybenzone, padimate O, ensulizole, sulisobenzone, cerium dioxide, titanium dioxide, trolamine salicylate, zinc oxide, layered double hydroxides, derivatives of the above compounds and combinations thereof. Visible light absorbing materials can be azo dyes, perylene, anthraquinone, cyanine, triarylmethane, commercially available pigments, pigment red, pigment orange, pigment yellow, pigment blue, pigment green, pigment violet, pigment black, pigment white and combinations thereof.

[0022] In the bistable photochromic particles according to the ninth aspect, polymers such as PDMS and other silicone rubbers, melamine-formaldehyde and other amino resins, crosslinked PEG and other biocompatible networks, PLA, PLGA, methacrylamide chitosan, epoxy resins, PAA, PMMA, other acrylate-based and acrylamide-based polymers and networks and combinations thereof can be used.

[0023] The photochemically multistable formulations can be prepared by combining two or more photochemically bistable nanoparticles and / or microparticles according to any of the above aspects. The nanoparticles or microparticles can be combined with a biocompatible solvent suitable for injection into the dermal or intradermal layer of the skin and can exhibit different P-type photochromic spectral sensitivities.

[0024] Similarly, a photochemically multistable formulation of nanoparticles or microparticles can be prepared using two or more P-type photochromic dyes, such as those of the above-described embodiments, in combination with a biocompatible solvent suitable for injection into the dermal or intradermal layer of the skin.

[0025] In a tenth aspect, the present invention provides a photochemically multistable formulation of nanoparticles or microparticles, wherein the nanoparticles or microparticles are suspended in a biocompatible solvent suitable for injection into the dermal or intradermal layer of the skin and contain one or more P-type photochromic dyes in combination with T-type photochromic microparticles. The formulation may include an ink or pigment suitable for skin injection. The photochemically bistable nanoparticles or microparticles may include additives such as (i) a disinfectant (e.g., alcohol) to prevent bacterial contamination, (ii) a biocompatible surfactant (e.g., polysorbate) to stabilize the dispersion and adjust the surface tension, (iii) a thickening agent (e.g., xanthan gum, polyacrylate, polyglycol) to increase the viscosity and decrease the pigment sedimentation rate, (iv) a thixotropic agent (e.g., silica) to promote shear thinning, (v) a preservative / binder (e.g., polyether, polyvinylpyrrolidone) to assist in preventing drying of the ink and assist in binding of the ink to the needle, (vi) an astringent to minimize bleeding in the skin during injection, and (vii) an anesthetic to minimize pain during ink injection. Combinations of additives can be used to achieve multiple desired effects.

[0026] In a 11th aspect, the present invention provides a method of injecting a bistable or multistable photochromic formulation according to any one of the above compositions or formulations, the method comprising: (1) contacting the skin with a microneedle having a bistable or multistable photochromic formulation; and (2) penetrating the microneedle through the contacted skin. The microneedle can be a soluble microneedle. The soluble microneedle can comprise a suitable carrier, such as an aqueous solution of polyvinylpyrrolidone, polyvinyl alcohol (and their liquid prepolymers) or carboxymethylcellulose, trehalose, maltodextrin, galactose, glucose, hyaluronic acid and silk.

[0027] In a 12th aspect, the present invention provides a method of injecting a bistable or multistable photochromic particle dispersion or ink formulation (e.g., according to the above aspects) comprising contacting the skin with droplets of the formulation released from a needle-free tattoo machine, the droplets being released at a rate high enough to penetrate into the dermis.

[0028] In a 13th aspect, the present invention provides a method of injecting a bistable or multistable photochromic particle or ink formulation comprising a formulation according to the above aspects into the skin by contacting the skin with an (electrical) tattoo machine (rotary or coil) under conditions sufficient for the particles or ink of the formulation to penetrate into the dermis.

[0029] The above method may include selectively irradiating an injection site (photochromic region) having bistable or multistable photochromic particles or formulations with light of a specific frequency to control the color or shape pattern of a tattoo or permanent makeup. The photochromic region may be exposed to a monochromatic or polychromatic light source constrained by a transparent mask. Similarly, the photochromic region may be exposed to monochromatic or polychromatic light controlled by a polychromatic electronic projection device. One or more lasers can locally activate or inactivate small areas of the skin, and using an appropriate manual or computer numerical control system to direct the position and movement of the laser beam enables the implementation of a raster or pixel unit technique for writing color and shape information. UV dose measurements based on the quantification of tattoo color in photographs, compared against empirical data or simulation models, can be performed using intradermal tattoos derived from the particles taught herein and using the methods described above. UV dose measurements can also be based on a visual comparison of the tattoo color to a standard color chart correlating tattoo color to UV dose, or on video analysis of the rate of color change when a known UV or visible light dose is applied via a light source having a known output spectrum.

[0030] Detailed Description of the Drawings For a more detailed understanding of the present invention, reference is made to the following detailed description in conjunction with the accompanying drawings below.

Brief Description of the Drawings

[0031]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0032] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Tattoos are formed using intradermal nanoparticles (typically 20 nm to 900 nm in diameter) in the form of color additives, most often borrowed from the pigment manufacturing industry. Tattoo pigments are typically inserted into the dermis by repeatedly puncturing the skin with a needle or an array of needles that carry tattoo ink, which typically contains a dispersion of these pigments, although alternative needle-free injection methods are under development. Without intervention, tattoos leave permanent markings on the skin because the pigments undergo repeated cycles of capture and release by skin melanophages with minimal movement within the dermis. Long-term tattoo fading is caused by the elimination of pigments via drainage to the lymph nodes by these immune cells, and this process can be facilitated by laser tattoo removal treatments and pigment photodegradation associated with UV exposure in sunlight.

[0033] Tattoos, permanent makeup, and related biomedical applications typically rely on conventional industrial pigment colorants, although some pre-biopsy tattoo pigments are designed to exhibit fluorescence. Most tattoo and permanent makeup pigments are stable colorants that do not readily undergo color-changing chemical or photochemical reactions in the dermis.

[0034] "T-type" photochromic dyes undergo a photochemical reaction that produces a color change when activated by light of an appropriate wavelength. In T-type photochromic dyes, this photochemical reaction is thermally reversible, so the dye spontaneously returns to its original state when the activating light is removed.

[0035] P-type photochromic dyes are not thermally reversible and, therefore, do not spontaneously return to their original color after activation. Indeed, the reversal of photochemical activation in P-type dyes is achieved by a second photochemical activation in a different wavelength range. Because of their thermal irreversibility, P-type dyes are bistable under appropriate illumination conditions and, therefore, suitable for dosimetry [Japanese Patent No. 3882746B2; Irie, S.; Irie, M. Ultrahigh Sensitive Color Dosimeters Composed of Photochromic Diarylethenes and Fluorescent Metal Complexes. Chem. Lett. 2006, 35, 1434-1435; Kawamura, I; Kawamoto, H.; Fujimoto, Y.; Masanori, K.; Asai, K. Isomerization behavior of diarylethene-type photochromic compounds under X-ray irradiation: application to dosimetry, Jpn. J. Appl. Phys. 2020, 59, 046004].

[0036] The present invention provides a photochemically patterning and rewritable pigment that can be used on the skin. In a first aspect, the technique utilizes a formulation of P-type photochromic nanoparticles and / or microparticles (see Example 1 below). In a further aspect, the present invention provides an ink that utilizes a dispersion of these particles, for example in the first aspect, that enables injection into the dermis (see Example 2 below). In a further aspect, the present invention provides techniques for injecting ink into the dermis, including conventional tattooing, permanent makeup, threading, and microneedle patches (see Example 3 below). In a final aspect, the present invention performs writing, erasing, and rewriting of specific colors and patterns on the skin for the purpose of body art, beauty, or biopsy site marking, or utilizes the wavelength sensitivity of a bistable or multistable photochromic tattoo to record in vivo UV dose measurement information in a preferred embodiment of a photochromic tattoo (see Example 4 below).

Examples

[0037] Example 1 - Materials and Methods The present invention provides a formulation for P-type photochromic nano- or microparticles for intradermal use (see, for example, FIG. 1). The average particle size advantageously falls within the range of about 20 nm to 10 μm in order to (i) facilitate injection into the dermis by tattooing or other means and (ii) maintain a semi-permanent or permanent position in the dermis. If the particle size is below this size scale, the particles are more easily eliminated by the immune system. On the other hand, larger particles (e.g., greater than about 10 μm) can cause excessive granuloma or keloid reactions. The particles can include "functional elements" represented as the darker spheres in FIG. 1. These functional elements minimally include a P-type photochromic dye. "P-type photochromic dye" means any compound that satisfies the following two criteria: (i) the compound undergoes a photochemical reaction that changes its spectral absorbance characteristics when activated by a specific wavelength or wavelength range of light, and (ii) the photochemical reaction undergone by the compound is not thermally reversible but can be reversed by photochemical deactivation at a wavelength or wavelength range different from that used for activation. A class of suitable P-type photochromic dyes that can be used as functional elements is diarylethene [Irie, M.; Fukaminato, T.; Matsuda, K.; Kobatake, S. Photochromism of Diarylethene Molecules and Crystals: Memories, Switches, and Actuators. Chem. Rev. 2014, 114, 12174-12277], fulgide / fulgimide as described in [Yokoyama, Y. Fulgides for Memories and Switches. Chem. Rev. 2000, 100, 1717-1739.], naphthopyran [Frigoli, M.; Maurel, F.; Berthet, J; Delbaere, S.; Marrot, J; Oliveira, M. M. The control of photochromism of [3H]-naphthopyran derivatives with intramolecular CH-π bonds.Org. Lett. 2012, 14, 4150 - 4153] and [Frigoli, M.; Marrot, J; Gentili, P. L.; Jacquemin, D.; Vagnini, M.; Pannacci, D.; Ortica, F. P-Type Photochromism of New Helical Naphthopyrans: Synthesis and Photochemical, Photophysical and Theoretical Study. ChemPhysChem 2015, 16, 2447 - 2458] and hydrazones [van Dijken, D. T; Kovaricek, P.; Ihrig, S. P.; Hecht, S. Acylhydrazones as Widely Tunable Photoswitches. J. Am. Chem. Soc. 2015, 137, 14982 - 14991; Qian, H.; Pramanik, S.; Aprahamian, I. Photochromic Hydrazone Switches with Extremely Long Thermal Half-Lives. J. Am. Chem. Soc. 2017, 139, 9140 - 9143; Shao, B.; Qian, H.; Li, Q.; Aprahamian, I. Structure Property Analysis of the Solution and Solid-State Properties of Bistable Photochromic Hydrazones. J. Am. Chem. Soc. 2019, 141, 8364 - 8371]. These P-type photochromic dyes can be used as the sole coloring element to produce a single type of color change (bistability). Furthermore, multiple P-type photochromic dyes can be combined to further tune the wavelength sensitivity of the particles and access multi-stable coloring forms through the selective activation and / or deactivation of subsets of the dye component mixture, e.g., through the application of multi-component P-type photochromic coatings [Jin, Y.; Qamar, I; Wessely, M.; Adhikari, A.; Bulovic, K.; Punpongsanon, P.; Mueller, S. Photo-Chromeleon: Re-Programmable Multi-Color Textures Using Photochromic Dyes. UIST’19 2019. 12 pp. New Orleans, LA, USA].

[0038] In addition to the P-type photochromic dye, the formulation may also include any combination of the following functional elements.

[0039] UV absorber. To adjust the spectral distribution of particles in the UV range, which can affect the kinetics and extent of photoactivation and the stability of the particles against UV photodegradation, a UV absorber can be included. Various classes of UV absorbers are suitable for encapsulation as UV absorption additives. Organic UV absorbers include over-the-counter sunscreens approved by the FDA ([see US Food and Drug Administration. Sunscreen Drug Products for Over-the-Counter Human Use: Proposed Rule. Federal Register 2019, 84, 6204-6275]), industrial additives for coating agents, such as benzophenone, benzotriazole, and phenyltriazine [Keck, J.; Kramer, H. E. A.; Port, H.; Hirsch, T.; Fischer, P.; Rytz, G. Investigations on Polymeric and Monomeric Intramolecularly Hydrogen-Bridged UV Absorbers of the Benzotriazole and Triazine Class. J. Phys. Chem. 1996, 100, 14468-14475; Schaller, C.; Rogez, D.; Braig, A. Hydroxyphenyl-s-triazines: advanced multipurpose UV-absorbers for coatings. J. Coat. Technol. Res. 2007, 5, 25-31] or polymers incorporating these moieties within their repeating units [Kim, E.; Cho, S. Y.; Yoo, M. J.; Ahn, K.-H. Vinyl group-containing diarylethene and polymer thereof having excellent optical properties. US Patent No. 6787621B2, filed Sep. 19, 2002].Inorganic / mineral UV absorbers include TiO2 [Allen, N. S.; Edge, M.; Ortega, A.; Liauw, C. M.; Stratton, J.; McIntyre, R. B. Behaviour of nanoparticle (ultrafine) titanium dioxide pigments and stabilisers on the photooxidative stability of water based acrylic and isocyanate based acrylic coatings. Polym. Degrad. Stabil. 2002, 78, 467-478], ZnO [Becheri, A.; Duerr, M.; Lo Nostro, P.; Baglioni, P. Synthesis and characterization of zinc oxide nanoparticles: application to textiles as UV-absorbers. J. Nanopart. Res. 2007, 10, 679-689], doped SiO2 [He, Q.; Yin, S.; Sato, T. Synthesis and photochemical properties of zinc-aluminum layered double hydroxide / organic UV ray absorbing molecule / silica nanocomposites. J. Phys. Chem. Solids 2004, 65, 395-402], CeO2 [Goubin, F., et al., Experimental and Theoretical Characterization of the Optical Properties of CeO2, SrCe03, and Sr2Ce04Containing Ce。 4+(fO) can include [Ions. Chem. Mater. 2004, 16, 662-669], which can be crystalline, polycrystalline, or amorphous. The UV absorber can be an organic / inorganic combination (see, for example, [Mahltig, B., et al., Optimized UV protecting coatings by combination of organic and inorganic UV absorbers. Thin Solid Films 2005, 485, 108-114]), such as layered double hydroxides [Feng, Y.; Li, D.; Wang, Y.; Evans, D. G.; Duan, X. Synthesis and characterization of a UV absorbent-intercalated Zn-Al layered double hydroxide. Polym. Degrad. Stabil. 2006, 91, 789- 794;Li, D.; Tuo, Z.; Evans, D. G.; Duan, X. Preparation of 5-benzotriazolyl-4-hydroxy-3-sec-butylbenzenesulfonate anion-intercalated layered double hydroxide and its photostabilizing effect on polypropylene. J. Solid State Chem. 2006, 179, 3114-3120;Cao, T.; Xu, K.; Chen, G.; Guo, C.-Y. Poly(ethylene terephthalate) nanocomposites with a strong UV-shielding function using UV-absorber intercalated layered double hydroxides. RSC Advances 2013, 3, 6282-6285].

[0040] Color filter. Other dyes that absorb visible light or near-infrared light wavelengths of light can also be added to further adjust the activation and / or inactivation sensitivity of the P-type photochromic dye. For example, in the case of a photochromic dye that is activated by UV / deactivated by visible light, the presence of the color filter can reduce the total dose of visible light applied to the photoactive dye in sunlight and extend the lifetime of the photoactivated state in an outdoor environment. Examples of a family of suitable colored dyes having an adjustable transmission wavelength include azo dyes [Ashida, T. Azo compounds, dyes containing them, and colored compositions, Japanese Patent Application Laid-Open No. 2013043969A, Mar. 4, 2013. Sumitomo Chemical Co., Ltd., Japan; Do Kim, Y. et al., Synthesis, application and investigation of structure-thermal stability relationships of thermally stable water-soluble azo naphthalene dyes for LCD red color filters. Dyes and Pigments 2011, 89, 1-8], perylene [Choi, J.; Sakong, C.; Choi, J.-H.; Yoon, C.; Kim, J. P. Synthesis and characterization of some perylene dyes for dye-based LCD color filters. Dyes and Pigments 2011, 90, 82-88], anthraquinone [Park, J.; Park, Y.; Park, J. Synthesis and physical property measurement of new red pigment based on anthraquinone derivatives for color filter pigments. Mol. Cryst. Liq. Cryst. 2011, 551, 116-122], cyanine [Kwon, H.-S.; Yoo, J.-S.; Lee, H.-Y.; Choi, J.-H. Synthesis of Innovative Colorants Based on Cyanine Dye and Their FRET Efficiency to Reduce the Emission of Fluorescence for LCD Color Filter. Bull. Kor. Chem. Soc. 2015, 36, 2545-2548], triarylmethine [Kong, N. S. et al, Development of dimeric triarylmethine derivatives with improved thermal and photo stability for color filters. Dyes and Pigments 2017, 144, 242-248] and many others. For further examples of suitable dyes and pigments that can be used as color filters, see [Zollinger, H. Color Chemistry: Synthesis, Properties, and Applications of Organic Dyes and Pigments. (3. rd Ed.) Weinheim: Wiley-VCH, 2001].

[0041] Light stabilizers. It is often advantageous to mix plastic materials containing polymer particles as described in the present invention with light stabilizers that can inhibit photodegradation in order to increase their service life [see, for example, Muasher, M.; Sain, M. The efficacy of photostabilizers on the color change of wood filled plastic composites. Polym. Degrad. Stabil. 2006, 91, 1156-1165; Andrady, A. L.; Hamid, S. H.; Hu, X.; Torikai, A. Effects of increased solar ultraviolet radiation on materials. J. Photochem. Photobiol. B 1998, 46, 96-103]. Hindered amines, particularly those derived from 2,2,6,6-tetramethylpiperidine and its alkylated or hydroxylamine analogs, are a preferred class of light stabilizers. These light stabilizers remove unwanted radicals generated in organic materials under UVA and UVB irradiation and are then regenerated (see, for example, the Denisov cycle described in [Hodgson, J. L.; Coote, M. L. Clarifying the mechanism of the Denisov cycle: How do hindered amine light stabilizers protect polymer coatings from photo-oxidative degradation? Macromolecules 2010, 43, 4573-4583]), imparting a persistent light stabilizing function to them [Klemchuk, P. P.; Gande, M. E. Stabilization mechanisms of hindered amines. Polym. Degrad. Stabil. 1988, 22, 241-274].

[0042] Preferably, the particles are pharmaceutically acceptable and exhibit no or near-zero toxicity, immunogenicity, or mutagenicity. The particles also exhibit high chemical, physical, and optical stability in an aqueous medium within a temperature range of 20 to 40 °C, which is representative of intradermal conditions. Particles exhibiting these characteristics need to maintain their long-term functionality and biocompatibility in the dermis. The functional elements may also be insoluble in the aqueous medium (or made insoluble by chemical or encapsulation strategies, see below) to prevent their partitioning into the interstitial fluid. When one of the stable color states is meant to be invisible, it is preferred that, in addition to their visible light absorption, the particles minimize scattering, reflection, and refraction and minimize their visibility on the skin. Since scattering is highest at particle diameters of approximately 100 to 200 nm [Dawson, P. L.; Acton, J. C. Impact of proteins on food color. Proteins in Food Processing, Second Ed. 2018, Elsevier Ltd. pp. 599-638], the preferred particle size is at or above the size scale of visible light (e.g., 400 nm and above 400 nm). To minimize excessive reflection and refraction, which cause the particles to appear white (Mie scattering), the refractive index of the particles within the visible range can be approximately matched to that of the dermis (1.36 to 1.41, see [Ding, H.; Lu, J. Q.; Wooden, W. A.; Kragel, P. J.; Hu, X.-H. Refractive indices of human skin tissues at eight wavelengths and estimated dispersion relations between 300 and 1600 nm. Physics in Medicine and Biology 2006, 51, 1479-1489]).

[0043] Formulation A. Polymer particles. Functional elements can be incorporated into polymer or copolymer particles of appropriate size (about 20 - 10,000 nm) by several strategies that can be broadly classified into dispersion and polymerization approaches [Rao, J. P.; Geckeler, K. E. Polymer nanoparticles: Preparation techniques and size-control parameters. Prog. Polym. Sci. 2011, 36, 887-913]. The dispersion approach involves converting a pre-formed polymer from a homogeneous solution into nano- or microparticles by solvent evaporation or solvent exchange in a spray or emulsion, precipitation by salts, dialysis or supercritical fluids. By dissolving the functional elements in the polymer phase during these processes, they are (non-covalently) incorporated into the polymer matrix of the resulting nano- or microparticles. The polymerization approach to polymer particle synthesis typically relies on an emulsion, where nano- or micro droplets of a prepolymer resin (monomer), typically dispersed in an aqueous solution, are polymerized directly into particles at the start of polymerization. In this case, the functional elements can be dissolved in the monomer phase of the emulsion and incorporated into the polymer matrix during polymerization.

[0044] In both the dispersion and polymerization approaches, functional elements can be directly incorporated into the main chain, side chain, or cross-linking part of the polymer structure by encapsulating them in monomers during polymer synthesis. In most cases, functional elements can be covalently bonded to a polymer or copolymer by modifying them with reactive functional groups. For example, when a diarylethene-based P-type photochromic dye is functionalized with one or more acrylic or vinyl functional groups, its polymerization or copolymerization with other acrylic or vinyl monomers (such as PMMA and many silicone rubbers) by catalytic action or radical polymerization becomes possible [Kim, E.; Cho, S. Y.; Yoo, M. J.; Ahn, K.-H. Vinyl group-containing diarylethene and polymer thereof having excellent optical properties. U.S. Patent No. 6787621B2, filed on September 19, 2002]. Alternatively, functional elements can be coupled to a pre-synthesized polymer [Finden, J.; Kunz, T. K.; Branda, N. R.; Wolf, M. O. Reversible and Amplified Fluorescence Quenching of a Photochromic Polythiophene. Adv. Mater. 2008, 20, 1998-2002]. These covalent methods of incorporating functional elements are more expensive than the hybrid approach but reduce the risk of any functional elements leaching from the particles.

[0045] The advantageous polymer matrices in this formulation include poly(methyl methacrylate) (PMMA) and other methacrylate compounds (e.g., poly(methyl methacrylate), poly(isopropyl methacrylate), poly(isobutyl methacrylate)). PMMA is a biocompatible polymer [Frazer, R. Q.; Byron, R. T.; Osborne, P. B.; West, K. P. PMMA: An Essential Material in Medicine and Dentistry. Journal of Long-Term Effects of Medical Implants 2005, 15, 629-639]. Another class of advantageous polymers are poly(dimethylsiloxane) (PDMS) and other silicone rubbers, which are also biocompatible [Rahimi, A.; Mashak, A. Review on rubbers in medicine: natural, silicone and polyurethane rubbers. Plastics, Rubber and Composites 2013, 42, 223-230]. These polymer matrices are particularly suitable because (i) their biocompatibility is well established, (ii) their refractive indices are close to that of the dermis when less than 1.5 [Polymer Database. Refractive Index of Amorphous Polymers. Polymerdatabase.com], (iii) they exhibit high long-term stability, and (iv) they are relatively convenient and inexpensive to produce.

[0046] Formulation B. Molecular aggregates. Functional elements of small molecules or oligomers that are solid at biological temperatures and are sufficiently insoluble in aqueous media and of sufficient size for skin injection can be used directly as aggregated particles. Most conventional colored tattoo pigments (red, yellow, green, blue, etc.) consist of small molecules. Even if the molecules are individually too small to serve as tattoo pigments, they aggregate into crystalline or amorphous nano- or microparticles. Since the molecules are insoluble in water, they maintain an associated state with these "molecular aggregate" particles and the molecules do not dissolve. Depending on the functional element, some can be used in a manner similar to these normal pigments. The process of making low water-soluble compounds into small microparticles is known as nanosizing [Kesisoglou, F.; Panmai, S.; Wu, Y. Nanosizing - Oral formulation development and biopharmaceutical evaluation. Adv. Drug Deliv. Rev. 2007, 59, 631-644] or micronizing [Rasenack, N.; Mueller, B. W. Micron-Size Drug Particles: Common and Novel Micronization Techniques. Pharm. Dev. Technol. 2004, 9, 1-13].The molecular aggregates can be prepared as nano- or microparticles by (i) precipitation from a solvent to a non-solvent (ideally water) [Rabinow, B. E. Nanosuspensions in drug delivery. Nat. Rev. Drug Discov. 2004, 3, 785-796], (ii) the spray-drying process [Vehring, R. Pharmaceutical Particle Engineering via Spray Drying. Pharm. Res. 2007, 25, 999-1022], (iii) supercritical fluid technology [Martin, A.; Cocero, M. J. Micronization processes with supercritical fluids: Fundamentals and mechanisms. Adv. Drug Deliv. Rev. 2008, 60, 339-350], or (iv) milling [Merisko-Liversidge, E.; Liversidge, G. G.; Cooper, E. R. Nanosizing: a formulation approach for poorly-water-soluble compounds. Eur. J. Pharm. Sci. 2003, 18,113-120]. Using these methods, nano- or microcrystalline P-type photochromic particles or amorphous P-type photochromic particles containing a mixture of a UV absorber, a color filter and / or a stabilizer can be produced.

[0047] In this case, the advantageous functional element in the form of the P-type photochromic compound is the family of diarylethenes, since it surely undergoes its P-type photochromic reaction in the solid state [Kobatake, S.; Takami, S.; Muto, H; Ishikawa, T.; Irie, M. Rapid and reversible shape changes of molecular crystals on photoirradiation. Nature 2007, 446, 778-781;Irie, S.; Irie, M. Ultrahigh Sensitive Color Dosimeters Composed of Photochromic Diarylethenes and Fluorescent Metal Complexes. Chem. Lett. 2006, 35, 1434-1435;Kawamura, L; Kawamoto, H.; Fujimoto, Y.; Masanori, K.; Asai, K. Isomerization behavior of diarylethene-type photochromic compounds under X-ray irradiation: application to dosimetry, Jpn. J Appl. Phys. 2020, 59, 046004].

[0048] Formulation C. Surface-coated particles. A monolayer or multilayer of a P-type photochromic compound and other functional elements can be adsorbed onto the surface of nano- or microparticles by chemical or physical means. The functional elements are added to the particle surface by covalent bonding of the functional elements to the particles. For example, in the case of surface-coated particles, silica particles can be used as a substrate. Silica is suitable because (i) it is already used as a thixotropic agent in tattoo inks [Piccinini, P.; Pakalin, S.; Contor, L.; Bianchi, I; Senaldi, C. Safety of tattoos and permanent make-up: Final report. European Commission Joint Research Centre Science for Policy Report 2016, 1-118], (ii) it can be biocompatible (see [Gerion, D.; Pinaud, F.; Williams, S. C.; Parak, W. J.; Zanchet, D.; Weiss, S.; Alivisatos, A. P. Synthesis and properties of biocompatible water-soluble silica-coated CdSe / ZnS semiconductor quantum dots. J. Phys. Chem. B 2001, 105, 8861-8871]) and (iii) it can be easily functionalized by silylation with a wide variety of alkoxysilanes and halosilanes [Voort, Der, P. V.; Vansant, E. F. Silylation of the Silica Surface A Review. J. Liq. Chromatogr. R. T. 2006, 19, 2723-2752]. In the case of those functional elements, it is necessary to be modified to present these silane functional groups for covalent bonding to SiO2. Polymer particles can also be formulated for surface modification on the condition that they present reactive functional groups that can be coupled to the functional elements.However, due to the low mass and volume ratio of the functional elements in this formulation, it is expected to be less effective in achieving a high optical density in the photoactivated state due to the low dye loading compared to formulations A and B and formulations D and E presented below.

[0049] Formulation D. Core-shell particles. The core-shell particles include formulations of core fluid / polymer shell, core fluid / inorganic shell, core polymer or gel / polymer shell, and core polymer or gel / inorganic shell. A convenient inorganic shell in this formulation is silica, as it makes the inorganic particles more biocompatible [Gerion, D.; Pinaud, F.; Williams, S. C.; Parak, W. J.; Zanchet, D.; Weiss, S.; Alivisatos, A. P. Synthesis and properties of biocompatible water-soluble silica-coated CdSe / ZnS semiconductor quantum dots. J. Phys. Chem. B 2001, 105, 8861-8871]. The core or shell polymer can consist of the same polymers as considered in Formulation A above, and PMMA and PDMS are preferred for their transparency and biocompatibility.Core-shell particles, especially when they contain a fluid core, are also known as nanocapsules or microcapsules and can be produced by various emulsion polymerization techniques [Jamekhorshid, A.; Sadrameli, S. M.; Farid, M. A review of microencapsulation methods of phase change materials (PCMs) as a thermal energy storage (TES) medium. Renew. Sust. Energy Rev. 2014, 31, 531-542] and microfluidic reactor approaches [Wang, J.-T.; Wang, J.; Han, J.-J. Fabrication of Advanced Particles and Particle-Based Materials Assisted by Droplet-Based Microfluidics. Small 2011, 7, 1728-1754] or spray drying techniques [Gharsallaoui, A.; Roudaut, G.; Chambin, O.; Voilley, A.; Saurel, R. Applications of spray-drying in microencapsulation of food ingredients: An overview. Food Research International 2007, 40, 1107-1121].

[0050] In the case of the advantageous composition of formulation D, to provide a protective barrier, it contains a crystalline or amorphous molecular-aggregate core of the type described in formulation B having a thin PMMA or PDMS shell. In the case of a second advantageous composition of formulation D, it contains a biocompatible liquid or gel core containing a certain concentration of a P-type photochromic dye optimized for visual sensitivity in the photoactivated state, and the biocompatible liquid or gel matrix may include water, vegetable oil, a biocompatible oil such as geraniol, or an organogel or hydrogel network of crosslinked polyacrylate generally used in biomedical applications [Esposito, C. L.; Kirilov, P.; Roullin, V. G. Organogels, promising drug delivery systems: an update of state-of-the-art and recent applications. J. Contr. Release 2018, 271, 1-20].

[0051] Formulation E. Mesoporous silica nanoparticles. Mesoporous silica nanoparticles (MSNP) have been highly developed as nanocarriers for drug delivery applications [Slowing, I.I.; Vivero-Escoto, J. L.; Wu, C.-W.; Lin, V. S.-Y. Mesoporous silica nanoparticles as controlled release drug delivery and gene transfection carriers. Adv. Drug Deliv. Rev. 2008, 60, 1278-1288]. Similarly, due to their wide use and biocompatibility in many situations, they are attractive carriers for photochromic compounds and other functional elements [Asefa, T.; Tao, Z. Biocompatibility of mesoporous silica nanoparticles. Chem. Res. Toxicol. 2012, 25, 2265-2284; Tam, D. et al, Mesoporous silica nanoparticle nanocarriers: biofunctionality and biocompatibility. Acc. Chem. Res. 2013, 46, 792- 801]. However, in contrast to drug delivery, when the contents of the particles are intended to be released, the functional element must be permanently included, as in the case of intradermal photochromic microparticles. Therefore, an advantageous method is to covalently bond the functional element to the SiO2 surface using alkoxysilanes and halosilanes [Voort, Der, P. V.; Vansant, E. F. Silylation of the Silica Surface A Review. J. Liq. Chromatogr. R. T. 2006, 19, 2723-2752]. However, it is also possible to encapsulate the functional element inside the pores as long as the pore openings on the surface are sufficiently blocked so as to lose bulk transport (cargo release). The advantages of MSNP over silica nanoparticles (Formulation C) are (1000m 2Their much higher surface areas (which can exceed 1000 m² / g) enable adsorption onto the surface of each particle in a higher density of functional elements, resulting in inks and tattoos that ultimately appear stronger. Methods for preparing particles containing photoactive dyes and silica or other ceramic particles are taught in U.S. Patent No. 9,163,145 B2.

[0052] Example procedure for the preparation of bistable photochromic PMMA microparticles. Solid powder of 1,2-bis(2-methyl-5-phenyl-3-thienyl)-perfluoro-cyclopentene diarylethene (photochromic dye DAE-0001, Yamada Chemical Industry) was mixed with PMMA at a mass ratio of 10:90, and this mixture was dissolved in dichloromethane at a concentration of 6% m / v. This solution was added dropwise to a solution of poly(vinyl alcohol) (PVA) (0.1 wt%) in water at RT until a concentration of 7.5% v / v was reached. The resulting two-phase mixture was shaken briefly to form an emulsion, which was then sonicated with a horn for 15 minutes. The emulsion was transferred to a flask with a stir bar and stirred vigorously at room temperature. After 12 hours, the reaction was brought back to room temperature and the particle suspension was transferred to a centrifuge tube. The particles were rinsed with several cycles of centrifugation, the supernatant was decanted, and the purified water was refilled. The size distribution of the particles (Figure 2A) was evaluated using an Accusizer 780 optical particle sizer (NICOMP Particle Sizing Systems), and their absorption data (Figure 3B) were collected using a Cary 5000 UV-Vis-NIR spectrophotometer (Agilent). The microparticles can be stored as a wet or dry slurry after synthesis.

[0053] Example procedure for the preparation of bistable photochromic PDMS microparticles. Ultraviolet-absorbing microparticles of formulation A containing a P-type photochromic dye dispersed in a PDMS matrix were prepared. The PDMS prepolymer resin was prepared using a 2-part Sylgard 184 silicone elastomer kit (Dow Inc.) at a base: catalyst mass ratio of 10:1. In an organic solution containing the diarylethene dye DAE-0001 (Yamada Chemical Industry), the concentration of the dye was 1 mg / mL in the prepolymer. After vigorously mixing for 5 minutes, reverse osmosis (RO) purified water was added to this prepolymer / dye resin to obtain a two-phase mixture at a water: resin mass ratio of 4:1. A TWEEN-80 (Sigma-Aldrich) surfactant was added to this two-phase mixture at a mass ratio of 1%. The mixture was sonicated in an ultrasonic bath (Branson M-1800) at room temperature for 5 minutes to create an emulsion. A stir bar was added to the emulsion vessel and the solution was stirred at a temperature of 80 °C and approximately 1000 rpm. After 12 hours, the reaction was returned to room temperature and the particle suspension was transferred to a centrifuge tube. The particles were rinsed with several cycles of centrifugation, the supernatant was decanted and refilled with purified water. The microparticles can be stored as a wet or dry slurry after synthesis.

[0054] Example 2 - Multistable Photochromic Microparticle Ink Bistable or multistable photochromic microparticles (see Example 1 above) can be dispersed in a solvent or ink to prepare a multistable photochromic ink. The ink formulation can be prepared for intradermal delivery methods such as those described below, which may include various tattooing / permanent makeup methods and microneedles or needle patches.

[0055] Tattoos and permanent makeup inks. To produce a liquid ink suitable for skin injection, multi-stable photochromic microparticles are suspended in a fluid, with or without additives. An exemplary fluid is water, but other biocompatible solvents such as alcohols (e.g., ethanol, isopropanol, glycerol, oligo- and polyethylene glycols) or oils (e.g., vegetable oil / triglyceride, geraniol, squalene, etc.) can also be used. Suitable additives for these inks are: (i) disinfectants (e.g., alcohol) to prevent bacterial contamination, (ii) biocompatible surfactants (e.g., polysorbate) to stabilize the dispersion and adjust the surface tension, (iii) thickeners (e.g., xanthan gum, polyacrylate, polyglycol) [Petersen, H; Roth, K. To Tattoo or Not to Tattoo? Chem. Unserer Zeit 2016, 50, 44 - 66] to increase the viscosity and reduce the pigment sedimentation rate, (iv) thixotropic agents (e.g., silica) [Piccinini, P.; Pakalin, S.; Contor, L.; Bianchi, I; Senaldi, C. Safety of tattoos and permanent make-up: Final report. European Commission Joint Research Centre Science for Policy Report 2016, 1 - 118] to promote shear thinning, (v) preservatives / binders (e.g., polyether, polyvinylpyrrolidone) to assist in preventing ink drying and assist in binding the ink to the needle, (vi) astringents to minimize bleeding in the skin during injection, and / or (vii) anesthetics to minimize pain during ink injection. The resulting ink can be sterilized by treatment with gamma rays (preferred) or other means, such as autoclaving, heat, ultraviolet irradiation, X-ray radiation, or ethylene oxide, before packaging and storage.

[0056] Dual-stable or multi-stable photochromic microparticles can be mixed with other dual-stable or multi-stable photochromic microparticles during ink formulation by pre-mixing dry or wet slurries of different microparticles before ink formulation or by mixing separately formulated inks together to obtain an ink composition containing two or more types of dual-stable or multi-stable P-type photochromic microparticles. These mixed particle formulations may enable spectral tuning of the color response upon activation and selective deactivation at specific wavelengths of light to approach photo-multi-stable inks [Jin, Y. et al, Photo-Chromeleon: Re-programmable Multi-Color Textures Using Photochromic Dyes. UIST’19 2019. 12 pp. New Orleans, LA, USA].

[0057] Dual-stable or multi-stable photochromic microparticles can also be mixed with T-type photochromic microparticles suitable for tattoo ink by pre-mixing dry or wet slurries of different microparticles before ink formulation or by mixing separately formulated inks together to obtain an ink composition containing a mixture of P-type and T-type photochromic microparticles.

[0058] The bistable or multistable photochromic microparticles can also be mixed with the standard pigments used in tattoos and permanent makeup inks. Examples of standard tattoo and permanent makeup pigment colors include white (e.g., titanium dioxide, zinc sulfide, barium sulfate), red (e.g., pigment red 22, 101, 122, 146, 170, 184, 188, 202, 210, 254), orange (e.g., pigment orange 13, 16 or 73), yellow (e.g., pigment yellow 14, 65, 74, 83, 97 or 194), blue (e.g., pigment blue 15 or 61), green (e.g., pigment green 7 or 36) and magenta / violet (e.g., pigment violet 1, 19, 23, 37). The combination of these standard pigments with bistable or multistable photochromic microparticle tattoo inks changes the color response of the ink towards the color of the standard pigment in all its optically accessible states. The mixed multistable particle / pigment ink can be obtained by (i) directly dispersing the multistable particles as a wet or dry slurry into a pre-formulated standard tattoo and permanent makeup ink, (ii) directly dispersing the standard pigment as a wet or dry slurry into a pre-formulated bistable or multistable microparticle ink, or (iii) pre-mixing the pigment and the multistable microparticles as a wet or dry slurry and then formulating the ink of these mixtures.

[0059] Example of a procedure for the preparation of a bistable photochromic microparticle ink. The bistable photochromic PDMS microparticle tattoo ink of formulation A (see Example 1 above) was made by suspending the wet slurry in reverse osmosis purified water at a mass ratio of 30% in the presence of PVA (0.1% w / v). The suspension was shaken vigorously by hand for 30 seconds in a scintillation vial. The ink was characterized by photography (Figs. 3A - B), UV - Vis spectroscopy (Fig. 3C), and shear rheology (Fig. 3D). The ink was maintained in a well - dispersed state on a time scale of several hours. Although not used in this example, advantageous formulations include glycerol or polyethylene glycol (molecular weight 1000, Sigma - Aldrich) added at a ratio of 1% - 30% as a disinfectant, thickening agent, and binder to improve the stability and transferability of the bistable photochromic nanoparticle ink.

[0060] Microneedle tattoo ink. Emerging technologies that need to demonstrate suitability for delivery of materials such as bistable or multistable photochromic microparticle inks to the dermis are typically of the type of devices having many possible three-dimensional arrangements of microstructured protrusions that penetrate the epidermis and target transdermal drug delivery and vaccine application, the microneedle patch [Prausnitz, M. R. Engineering Microneedle Patches for Vaccination and Drug Delivery to Skin. Annual Rev. Chem. Biomol. Eng. 2017, 8, 177-200]. U.S. Patent No. 6,565,532B1 teaches microneedle devices used to mark the skin and dispense semi-permanent subdermal makeup. These devices have not appeared on the market, but it may be possible to use them for intradermal injection of photochromic microparticles. Ink formulations for these microneedle patches are composed of suspensions of photochromic microparticles (optionally mixed with pigments) in fluids containing polymers, prepolymers or molecular precursors for the matrix of the microneedle delivery method. For example, if a favorable formulation is used, a dissolvable microneedle array is utilized [see, for example, Bediz, B. el al., Dissolvable Microneedle Arrays for Intradermal Delivery of Biologics: Fabrication and Application. Pharm. Res. 2013, 31, 117-135]. This is because this formulation of the microneedle patch is optimized to deliver a relatively large amount of material compared to other microneedle patch formulations. The carrier matrix for the dissolvable microneedle array is preferably a non-toxic material that is strong enough to penetrate the epidermis but rapidly dissolves in the interstitial fluid of the dermis and is thus water-soluble enough to release its contents.Examples of suitable carriers for the invisible ultraviolet absorbing microparticle ink of the micron needles include polyvinylpyrrolidone or polyvinyl alcohol and their liquid prepolymers or carboxymethylcellulose, trehalose, maltodextrin, galactose, glucose, hyaluronic acid, and aqueous solutions of silk (which solidify inside the micron needle mold during curing or drying).

[0061] Microneedles with needle widths and depths dimensions < 1 mm may be too small to permanently inject material into the dermis. This is because it has an average thickness of about 2 mm and can reach a maximum thickness of 4 mm [Oltulu, P.; Ince, B.; Kokbudak, N.; Findik, S.; Kilinc, F. Measurement of epidermis, dermis, and total skin thicknesses from six different body regions with a new ethical histometric technique. Turk. J. Plast. Surg. 2018, 26, 56 - 61], and tattoo machines penetrate up to 4 mm into the skin [Petersen, H; Roth, K. To Tattoo or Not to Tattoo? Chem. Unserer Zeit 2016, 50, 44 - 66]. Soluble needles with larger dimensions (> 1 mm) can be prepared in a similar manner using masters and molds with larger features and may be more suitable for use in the applications presented in the present invention.

[0062] Example 3 - Injection Method for Multistable Photochromic Microparticle Tattoos A safe and effective amount of multi-stable photochromic microparticle tattoos can be injected by various methods, typically including needles or arrays of needles, immersed in a multi-stable photochromic microparticle dispersion (see Example 2 above). Ink-coated needles can pierce the skin repeatedly to break through the epidermal barrier and deliver the ink material to the dermis. Inserting one or more needles into the skin can be performed manually, following several ancient indigenous tattooing traditions, including tapping (tatau, Polynesia), raking (hand-carved, Japan), threading / stitching with needles and thread (North America), and scratching followed by ink rubbing (Europe) [Krutak, L.; Deter-Wolf, A. (Eds.). Ancient Ink: The Archaeology of Tattooing 2017. Seattle; London: University of Washington Press]. An advantageous method is to attach the needle array to a modern motor-driven tattoo or permanent makeup machine, which improves efficiency and minimizes pain compared to manual methods.Needle-free tattoo machines that inject tattoo ink droplets intradermally at a speed high enough to penetrate the dermis have been described in the academic literature [Oyarte Galvez, L.; Brio Perez, M.; Fernandez Rivas, D. High speed imaging of solid needle and liquid micro-jet injections. J. Appl. Phys. 2019, 125, 144504-13;Cu, K.; Bansal, R.; Mitragotri, S.; Rivas, D. F. Delivery Strategies for Skin: Comparison of Nanoliter Jets, Needles and Topical Solutions. Ann. Biomed. Eng. 2019, 2028-2039] and are taught in U.S. Patent No. 6,689,095 B1 by Garitano and Garitano, L. These machines can also be used in the present application, to the extent they are compatible with standard tattoo inks.

[0063] Alternatively, the ink can be formulated into dissolvable microneedles or needle patches in a PDMS mold, as described by [Bediz, B. et al., Dissolvable Microneedle Arrays for Intradermal Delivery of Biologics: Fabrication and Application. Pharm. Res. 2013, 31, 117-135]. Patches that are inserted only once intradermally and maintained in place for a time sufficient to release the multi-stable photochromic microparticle ink into the interstitial fluid of the dermis can be used.

[0064] Example procedure for the injection of multi-stable photochromic microparticle ink. Using an ex vivo porcine skin model, a multi-stable photochromic tattoo was injected using a rotary tattoo machine (Dragonhawk) equipped with a steel 9RS tattoo needle array dipped in tattoo ink containing an aqueous dispersion of approximately 10 wt% PDMS-based bistable photochromic microparticles (described in Examples 1 and 2 above) with a driving output of 7V exceeding an area of 1 cm2 until a tattoo with a uniform appearance was obtained. The skin samples were washed with isopropanol before and after tattooing. A photograph of this photochromic particle tattoo that has undergone cycles of writing and erasing with UV and red light respectively is shown in Figure 4 to verify that the tattoo functions as a photochemically bistable tattoo.

[0065] Example 4 - Application of Ultraviolet Absorbing Microparticle Tattoo Use and Advantages of the Present Invention The multi-stable photochromic microparticle tattoo can be used in a new form of semi-permanent or permanent body art that is frequently reprogrammable using light of different wavelengths, photo-switchable anatomical markers, and short-term colorimetric analysis UV detectors and dosimeters in specific formulations.

[0066] Multi-Stable Photochromic Tattoo for Photo-Chemically Rewritable Body Art Pigment tattoos and permanent makeup inks are used by hundreds of millions of people worldwide, including approximately one-quarter of the adult population in the United States, for permanent body art and cosmetics applications [Piccinini, P.; Pakalin, S.; Contor, L.; Bianchi, I; Senaldi, C. Safety of tattoos and permanent make-up: Final report. European Commission Joint Research Centre Science for Policy Report 2016, 1-118]. Bistable or multistable photochromic tattoos and permanent makeup inks, which are injectable like conventional tattoos and permanent makeup, can be used in place of, in combination with, or in addition to these widely used inks to create body art or permanent cosmetic markings that change color in response to different types of illumination. For example, the images in FIG. 4 show that any tattoo design can be reversibly programmed, erased, and reprogrammed using ultraviolet light, red light, and ultraviolet light respectively, demonstrating that users can change their tattoo design using only light irradiation rather than more invasive irreversible procedures such as tattooing (known as "cover-up" in this context) or laser excision.

[0067] Body art and permanent makeup tattoos created using bistable or multistable photochromic microparticle inks can be programmed with a specific design or pattern using (i) a single-color or multi-color light source combined with a transparent mask that covers the tattoo area portion during exposure (such as in the case of FIG. 4), (ii) a projector that projects a pattern or image of light of different wavelengths onto the skin, such as the protocol described by Jin et al. [Jin, Y.; Qamar, L; Wessely, M.; Adhikari, A.; Bulovic, K.; Punpongsanon, P.; Mueller, S. Photo-Chromeleon: Re-Programmable Multi-Color Textures Using Photochromic Dyes. UIST’19 2019. 12 pp. New Orleans, LA, USA], or (iii) a laser that enables local activation or inactivation of small areas of the tattoo and allows for the implementation of a raster or pixel-by-pixel approach to programming the tattoo design or "hand-drawn" design when manually operated by a suitable computer numerical control system or when programmed by hand.

[0068] By programming body art and permanent makeup tattoos created with bistable or multistable photochromic microparticle inks containing a mixture of P-type and T-type photochromic dyes, more dynamic color changes can be achieved that evolve over time in response to lighting changes. For example, a T-type photochromic ink that appears yellow upon activation by UV irradiation, rapidly thermally inactivates upon removal of the UV irradiation, and returns to a colorless state, in combination with a P-type photochromic ink that is colorless in the ground state and cyan in the UV-activated photoequilibrium state, will exhibit any of (i) colorless (colorless + colorless) before UV irradiation, (ii) green (yellow + cyan) under active UV irradiation, and (iii) cyan (colorless + cyan) at the time of removal of the UV, but before inactivation of the cyan ink via red light. According to these principles, numerous combinations are possible, which can be further combined with conventional tattoo inks as well as the above-described methods to provide additional color combinations and design complexities.

[0069] Photo-chemically activated anatomical markers for medical applications Dermatologists routinely use intradermal pigments to demarcate the boundaries of biopsy sites that test positive for cancer or other diseases, which may require complete removal by a surgeon at a later date [Goldman, L.; Richfield, D.; Kubitz, D. Small Biopsy With Tattoo Identification of Tissue. Archives of Dermatology 1964, 90, 195-196; Jalgaonkar, A. et al., Preoperative biopsy tract identification using india ink skin tattoo in tumous surgery. Orthopaedic Proceedings 2012, 94- B:SUPP_XXXVII, 321; Chuang, G. S.; Gilchrest, B. A. Ultraviolet Fluorescent Tattoo Location of Cutaneous Biopsy Site. Dermatol. Surg. 2012, 38, 479; Choi, J. et al., Cross-Linked Fluorescent Supramolecular Nanoparticles as Finite Tattoo Pigments with Controllable Intradermal Retention Times. ACS Nano 2017, 11, 153-162]. Since the patient's surgery can be performed several months after the biopsy, these intradermal markings are intended to reduce the uncertainties and errors associated with the accurate identification of the surgical site by the surgeon. In particular, on areas of the skin that are highly visible to the public, physicians can use "invisible" intradermal pigments that fluoresce under appropriate illumination, such as ultraviolet or "black" light. These fluorescent pigments minimize the visibility of biopsy site markers on the patient's skin under normal indoor and outdoor lighting conditions. However, the composition and safety of these pigments are often unknown, and it can be difficult to detect fluorescence with the naked eye in a well-lit environment.The bistable or multistable photochromic particles, inks, and tattoos described in Examples 1-3 above can provide a convenient and potentially safer alternative to these biopsy marking procedures.

[0070] Intradermal pigments are also routinely used in radiation oncology to contribute to beam alignment at anatomical sites. For the same reasons as in the above dermatological procedures, the bistable or multistable photochromic pigments described in the present invention can be used to alter the visibility of these anatomical markers.

[0071] Particularly, (as demonstrated with the photochromic dye 1,2-bis(2-methyl-5-phenyl-3-thienyl)-perfluorocyclopentene in the above Examples and Figures 1-4) particles and ink formulations containing only P-type photochromic dyes that are colorless in the ground state and visible in the UV-activated state are advantageous for application as anatomical biopsy site markers in that the corresponding photobistable biopsy markers can remain colorless until activated by a UV lamp. Thus, when appropriate bistable or multistable photochromic inks are administered according to any of the procedures described in Example 3 above, these photochromic intradermal particles have low visibility on the patient's skin under indoor lighting and can be made visible, facilitating identification of the biopsy site by the physician only after a short period of exposure to a UV lamp or sunlight.

[0072] Intradermal UV dosimetry. Ultraviolet (UV) radiation is a major risk factor for skin cancer (both melanoma and non-melanoma), which is the most common malignancy in the United States and among other predominantly light-skinned populations worldwide (more common than all other cancers combined) [Diepgen, T. L.; Mahler, V. The epidemiology of skin cancer. Br. J. Derm. 2002, 146, 1-6; Rogers, H. W.; Weinstock, M. A.; Feldman, S. R.; Coldiron, B. M. Incidence Estimate of Nonmelanoma Skin Cancer (Keratinocyte Carcinomas) in the US Population, 2012. JAMA Dermatol. 2015, 151, 1081-1086]. Most of the UV light passing through the Earth's atmosphere is UVA (wavelengths of 320 - 400 nm), while a small amount of UVB light (wavelengths of 280 - 320 nm) also reaches the Earth's surface. Exposure to both UVA and UVB causes cumulative skin damage over time, increasing the risk of skin cancer and the rate of aging [Taylor, C. R.; Stern, R. S.; Leyden, J. J.; Gilchrest, B. A. Photoaging / Photodamage and Photoprotection. J. Am. Acad. Dermatol. 1990, 22, 1-15; Koh, H. K.; Geller, A. C.; Miller, D. R.; Grossbart, T. A.; Lew, R. A. Prevention and Early Detection Strategies for Melanoma and Skin Cancer: Current Status. Arch. Dermatol. 1996, 132, 436-443].According to these sources, while UVB radiation is the main cause of sunburn and a major risk factor in melanoma (one of the less common but most deadly skin cancers) and other skin cancers, the UVA rays that penetrate deeper have been verified to be associated with skin aging and further increase the risk of the most common keratinocyte cancers.

[0073] By providing quantitative data on cumulative UV exposure at the site of the detector of a wearable UV dosimeter through personal UV dose measurement, protective interventions against skin cancer and other UV-related diseases are promoted [Foller, P.; Fritz, T; Olguin, C.; Wrobel, S.; Le Maitre, C.; Kang, E. R.; Tibbits, S. J. E. Sensing of solar ultraviolet radiation by wearable colorimetry. U.S. Patent Application Publication No. 20200149960A1, filed on June 18, 2018; Davis, A.; Deane, G. H. W.; Diffey, B. L. Possible dosimeter for ultraviolet radiation. Nature 1976, 261, 169-170]. Wearable UV dosimeters can be based on various materials including polysulfone, UV-reactive dyes embedded in polymer films, and zinc oxide nanowires [Zou, W.; Sastry, M.; Gooding, J. J.; Ramanathan, R.; Bansal, V. Recent Advances and a Roadmap to Wearable UV Sensor Technologies. Adv. Mater. Technol. 2020, 5, 1901036]. However, wearable dosimeters are subject to several limitations. That is, electronic dosimeters are relatively large and expensive and require batteries that can run out, while thin-film wearable dosimeters have a limited shelf life and single-use design that lead to long-term accumulated depletion and cost. Furthermore, in wearable dosimeters, UV detection is performed at the surface of the skin, and therefore, the UV dose below the skin surface where most UV-vulnerable tissues are located can be overestimated. Intradermal UV dosimeters can overcome these limitations. Ideally, the information recorded by an intradermal UV dosimeter can be read by colorimetric measurement and then reset and reused, avoiding repeated skin injections of new dosimetric materials, but existing wearable UV dosimeter materials do not meet this requirement.

[0074] To enable reproducible UV dose measurements using intradermal P-type photochromic microparticles, in an advantageous formulation, a colorless P-type photochromic dye is used that forms a colored photoequilibrium state that can be deactivated only by visible or near-infrared light when activated by ultraviolet light. In this case, since the ultraviolet light of sunlight is accompanied by a large amount of broad-spectrum visible light and near-infrared light that can deactivate the photochromic dye before its color can be measured for the purpose of dose measurement, the efficiency of UV activation must be significantly higher than in the case of deactivation by visible or near-infrared light (reverse cyclization in the case of diarylethenes). For convenience, when the photochemical rate constants, quantum yields, and wavelength-dependent molar absorptivities of the dye are known, the coloration of the dye can be predicted as a function of the spectral distribution, intensity, and illumination time of the light source, as shown in FIG. 5. For example, in a dye that is activated to a colored state by ultraviolet light and deactivated to a colorless state by visible light, the present inventors modeled the relative concentration of the colored state (photoactivated or photoequilibrium state) as a function of time using the AM1.5G standard and simulated the solar spectral distribution of sunlight in the Earth's atmosphere near the sea surface using the following equation.

Equation

[0075] When changing the constants defining the molar absorptivity, rate and quantum yield in the activation and deactivation reactions, the quantum yield of the visible light-driven deactivation (reverse cyclization) reaction is <10 -4 and it has been found that when the UV and visible light irradiances are reduced by 0.66 times and 0.5 times respectively, the kinetics of activation become slow enough (minutes to hours, as shown in FIG. 5F) in the actual UV dose measurement in sunlight. In fact, as described in Example 1 above, these low deactivation quantum yields can be achieved by appropriate selection of the photochromic dye, while the reduction of UV and visible light irradiances can be achieved by mixing the bistable photochromic dye or particles with UV and color filters in the form of dyes and pigments.

[0076] P-type photochromic dyes suitable for UV dose measurement have a quantum yield of 0.44 for their UV-activated cyclization, while their visible-light-activated reverse cyclization has a quantum yield of <0.00002, and the absorbance rates of both the open-ring and closed-ring states are of the same order of magnitude. Thus, it can be 1,2-bis(2-methoxy-5-phenyl-3-thienyl)-perfluorocyclopentene (DAE-Cl, Yamada Chemical Industry) [Shibata, K.; Kobatake, S.; Irie, M. Extraordinarily low cycloreversion quantum yields of photochromic diarylethenes with methoxy substituents. Chem. Lett. 2001, 30, 618-619]. The simulations illustrated in FIGS. 5E and 5F are based on the empirical absorbance and quantum yield data for this compound. Another suitable P-type photochromic dye for UV dose measurement has a quantum yield of 0.6 for its UV-activated cyclization, while its visible-light-activated reverse cyclization has a quantum yield of 0.00003, and the absorbance rates of both the open-ring and closed-ring states are of the same order of magnitude. Thus, it can be 1,2-bis[2-methyl-5-(4-phenylbuta-1,3-dienyl)thien-3-yl]-perfluorocyclopentene [Bens, A. T.; Frewert, D.: Kodatis, K.; Kryschi, C.; Martin, H.-D.; Trommsdorff, H. P. Coupling of Chromophores: Carotenoids and Photoactive Diarylethenes - Photoreactivity versus Radiationless Deactivation. Eur. J Org. Chem. 1998, 2333-2338]. Many other P-type dyes with similar quantum yield ratios of activation and deactivation are candidates suitable for UV dose measurement by the methods described herein.

[0077] By quantifying the tattoo color, quantitative UV dose measurement can be performed using an intradermal P-type photochromic dye having a low inactivation / recyclization quantum yield. To quantify the tattoo color, the tattoo is photographed with a camera, which includes those found in webcams and mobile phone devices, and can be subjected to image processing operations. The image processing operations can be performed manually or automated by a software application. In a manual operation, the area of the tattoo can be characterized within a specific digital color space before UV activation and after complete UV activation. Examples of suitable color spaces include the RGB, CMYK, HSV, CIE1931, and CIELAB systems. In FIGS. 5A and 5B, the empirical data on color change is plotted as a quantification by the ΔE * ab method using the CIELAB chromaticity space. Two photographs of the completely inactivated tattoo and the completely activated tattoo define the full range of sensitivity in the tattoo of the dosimeter, where they are used to calibrate the photographs against the modeled or empirically measured "standard" sensitivity curve as shown in FIG. 5. If this calibration is complete, the UV dose introduced into the intradermal tattoo can always be quantified when the UV dose is known as a function of color and the color change of the tattoo (e.g., ΔE * ab value) is less than its maximum value by mapping to the calibration curve of its observed value. The simulation curve in FIG. 5F shows that a properly designed system can reach 85% activation within 1092 seconds (18.2 minutes), which is longer than the time it takes for unprotected skin under direct sunlight on the earth's surface to receive the standard erythema dose of solar radiation at the ASTM G03-173 standard irradiance level. To facilitate intradermal UV dose measurement, a software tool can be developed to automate the manual calibration and colorimetric analysis processes described herein using a mathematical model or a machine learning / artificial intelligence platform.

[0078] The above-described intradermal UV dose measurement procedure can also be qualitatively achieved by visual comparison of the tattoo with a color chart correlating color with UV dose and by video analysis of the tattoo color (as contrasted with photographic analysis) that accounts for the rate of color change when known UV or visible light doses are applied via a light source having a known output spectrum.

[0079] As demonstrated by the inventors' models, the activation kinetics of these intradermal UV dosimeters can be tailored using the density and wavelength sensitivity of the UV and color filters to meet the needs of the user. For example, a tattoo that reaches full activation after one standard erythema dose may be useful for managing UV exposure for vitamin D production without exceeding the limit (beyond which the risk of skin cancer begins to increase). The dosimeter tattoo can then be "reset" with red light (in the case of DAE-C1) or light of another wavelength that promotes inactivation on an hourly or daily basis, and the dose measurement monitoring process can be repeated as needed.

[0080] Definitions The term "administer" and variations thereof (e.g., "administering" a compound) as related to the compounds of the present invention mean introducing the compound into the system of a subject in need of treatment, e.g., via injection into the dermal layer of the subject's skin. When the compounds of the present invention are provided in combination with one or more other active agents, it is understood that "administer" and variations thereof include the simultaneous and sequential introduction of the compound and the other agents, respectively.

[0081] As used herein, the term "composition" is intended to encompass a product containing specific ingredients in specific amounts and any product directly or indirectly obtained from a combination of specific amounts of the specific ingredients.

[0082] A "pharmaceutically acceptable" ingredient is one that is suitable for use in humans and / or animals without undue adverse side effects (such as toxicity, irritation, and allergic response) commensurate with a reasonable benefit / risk ratio.

[0083] "Safe effective amount", as used in the context of the present invention, refers to an amount of a component that is sufficient to obtain a desired therapeutic response without undue adverse side effects (such as toxicity, irritation or allergic response) and that results in a reasonable benefit / risk ratio.

[0084] As used throughout this application, the terms "a" and "an" are used in the sense that they mean "at least one", "at least first", "one or more" or "a plurality" of the referenced components or steps, unless the context clearly indicates otherwise. For example, the term "cell" includes a plurality of cells, such as a mixture thereof.

[0085] The term "and / or", as used herein, always includes the meanings of "and", "or", and "all or any other combination of the elements connected by said term".

[0086] The term "about" or "approximately", as used herein, means within 20%, preferably within 10% and more preferably within 5% of a given value or range.

[0087] As used herein, the term "comprising" is intended to mean that the product, composition and method include the referenced components or steps, but do not exclude others. "Consisting essentially of", when used to define a product, composition and method, is intended to mean excluding any other component or step of any essential significance. Thus, in the case of a composition consisting essentially of the recited components, trace contaminants and pharmaceutically acceptable carriers are not excluded. "Consisting of" is intended to mean excluding more than trace amounts of other components or steps.

[0088] As used herein, the term "bistable" refers to a color having two stable color states. As used herein, the term "multistable" refers to a color having two or more stable color states.

[0089] As used herein, the term "photochromic" refers to a substance or composition that has the ability to change color upon exposure to radiant energy (such as light). Photochromism is a reversible conversion of a chemical species between two forms by the absorption of electromagnetic radiation, and the two forms have different absorption spectra. Briefly, this can be described as a reversible change in color upon exposure to light.

[0090] As used herein, a "P-type photochromic dye" is a compound that meets the following two criteria: (i) the compound undergoes a photochemical reaction that changes its spectral absorbance characteristics when activated by a specific wavelength or wavelength range of light, and (ii) the photochemical reaction undergone by the compound is not thermally reversible but can be reversed by photochemical deactivation at a wavelength or wavelength range different from that used for activation.

[0091] "Suitable for injection" means that the particles are pharmaceutically acceptable and exhibit no or near-zero toxicity, immunogenicity, or teratogenicity.

[0092] A kit for carrying out the method of the present invention is further provided. A "kit" is intended to mean any product (such as a package or container) that contains at least one reagent, for example, the pH buffer of the present invention. The kit can be promoted, distributed, or sold as a unit for carrying out the method of the present invention. In addition, the kit can include an accompanying document that describes the kit and the method for its use. Any or all of the kit reagents can be provided within a container that protects them from the external environment, for example, within a sealed container or a pouch. The advantages and the advantages apparent from the above description are efficiently achieved. In the above configuration, since specific modifications can be made without departing from the scope of the present invention, all the content included in the above description or shown in the accompanying drawings is intended to be construed by way of illustration and not in a limiting sense.

[0093] All references cited in this application are hereby incorporated by reference in their entirety to the extent not inconsistent with the present specification.

[0094] It is recognized that the above advantages and the advantages made apparent from the above description are efficiently achieved, and in the above configuration, since specific modifications can be made without departing from the scope of the present invention, all the content included in the above description or shown in the accompanying drawings is intended to be construed as illustrative and not in a limiting sense.

[0095] It should also be understood that the following claims are intended to embrace all of the general and specific features of the invention described herein, as well as all statements of the scope of the invention that may be said to be included therebetween in terms. The invention has been described as above.

Claims

1. A tattoo ink comprising: (a) P-type photochromic particles comprising poly(methyl methacrylate) (PMMA) or poly(dimethylsiloxane) (PDMS) nanoparticles and microparticles, said nanoparticles and microparticles containing a P-type photochromic dye in the range of about 10% by weight, the P-type photochromic particles; (b) an aqueous fluid matrix in which the P-type photochromic particles are suspended; comprising; The tattoo ink, wherein the proportion of the P-type photochromic particles in the tattoo ink is about 30% by weight.

2. The tattoo ink according to claim 1, wherein the P-type photochromic dye is 1,2-bis(2-methyl-5-phenyl-3-thienyl)-perfluoro-cyclopentene diarylethene.

3. The P-type photochromic particles contain additional additives, The additional additives in the P-type photochromic particles are (a) a UV absorbing material selected from the group consisting of hydroxybenzophenone, hydroxyphenyl-s-triazine, 2-(2-hydroxyphenyl)benzotriazole, oxalanilide, aminobenzoic acid, avobenzone, cinoxate, dioxybenzone, homosalate, meradimate, octocrylene, octinoxate, octisalate, oxybenzone, padimate O, ensulizole, sulisobenzone, cerium dioxide, titanium dioxide, trolamine salicylate, zinc oxide, layered double hydroxide, and derivatives of the above compounds or combinations thereof; (b) a visible light absorbing material selected from the group consisting of azo dyes, perylene, anthraquinone, cyanine, triarylmethane, commercially available pigments, pigment red, pigment orange, pigment yellow, pigment blue, pigment green, pigment violet, pigment black, and pigment white or combinations thereof; and (c) a light stabilizer in the form of a hindered amine such as 2,2,6,6-tetramethylpiperidine, a derivative of 2,2,6,6-tetramethylpiperidine or an alkylated or hydroxylamine analogue of 2,2,6,6-tetramethylpiperidine; The tattoo ink according to claim 1, selected from the group consisting of.

4. The aqueous fluid matrix contains additives, The additives in the aqueous fluid matrix are (a)A biocompatible oil selected from the group consisting of vegetable oil, triglyceride, geraniol, and squalene, or a combination thereof, (b)A disinfectant selected from the group consisting of ethanol, isopropanol, glycerol, oligoglycol, and polyethylene glycol, or a combination thereof, (c)A biocompatible surfactant selected from the group consisting of polysorbate and polyvinyl alcohol, or a combination thereof, for stabilizing the dispersion and adjusting the surface tension, (d)A thickener selected from the group consisting of xanthan gum, polyacrylate, and polyglycol, or a combination thereof, for increasing the viscosity and reducing the pigment sedimentation rate, (e)Silica as a thixotropic agent for promoting shear thinning, (f)A preservative or binder selected from the group consisting of polyvinylpyrrolidone and polyether, or a combination thereof, (g)An astringent for minimizing bleeding in the skin during injection, and (h)A local anesthetic for minimizing pain during ink injection, or (i)A combination thereof, The tattoo ink according to claim 1, selected from the group consisting of.

5. A method of injecting the tattoo ink according to any one of claims 1 to 4, comprising the step of contacting the skin with a tattoo machine under conditions sufficient for the particles or ink of the formulation to penetrate into the dermis.

6. A method of injecting the tattoo ink according to any one of claims 1 to 4, comprising the step of contacting the skin with droplets of the formulation released from a needle-free tattoo machine, wherein the droplets are released at a speed high enough to penetrate into the dermis.

7. A method of injecting the tattoo ink according to any one of claims 1 to 4, comprising the steps of contacting the skin with a micro-needle having the ink or the pigment of the ink; and penetrating the micro-needle through the contacted skin The method according to claim 7, wherein the micro-needle is a soluble micro-needle comprising a suitable carrier selected from the group consisting of polyvinylpyrrolidone, polyvinyl alcohol, a liquid prepolymer of polyvinyl alcohol, or an aqueous solution of carboxymethylcellulose, trehalose, maltodextrin, galactose, glucose, hyaluronic acid, and silk.

8. The method according to claim 7, wherein the micro-needle is a soluble micro-needle comprising a suitable carrier selected from the group consisting of polyvinylpyrrolidone, polyvinyl alcohol, a liquid prepolymer of polyvinyl alcohol, or an aqueous solution of carboxymethylcellulose, trehalose, maltodextrin, galactose, glucose, hyaluronic acid, and silk.

Images