Patent Publication Number: US-2012042882-A1

Title: Particles with charged surface domains

Description:
BACKGROUND OF THE INVENTION 
     Spherical colloidal particles are ubiquitous in drug delivery, in vivo and in vitro diagnostics, as well as additives in almost every industry (food, cosmetics, paints, etc). The ability of these particles to accurately interact with biological organisms, cells and molecules in a complex mixture or in vivo is crucial in both basic research and clinical settings. The vast majority of particles used in suspension arrays are optically encoded latex microspheres with diameters between 0.3 and 10 microns (1 micron=10 −6  meters) that can be interrogated and decoded with laser-based flow cytometry (measurement of cell sized particles). Optical encoding is accomplished by swelling the spheres with fluorescent organic dyes with different emission spectra. While recent advances in the field of colloid synthesis have produced non-spherical particles, the ability to impart electronic charge density domains on portions of a single particle and to determine uses for such particles have not been demonstrated. 
     SUMMARY OF THE INVENTION 
     Techniques are provided for particles functionalized at least with one or more charged surface domains, such as can be used in adaptive aesthetic markings including reversible tattoos. 
     In a first set of embodiments, a particle comprises a core structure having a surface, a plurality of first linkers and a plurality of second linkers. Each of the first linkers includes a first end that binds to the surface of the core structure and a second end that includes a first functional group that has a first charge. Each of the second linkers includes a first end that binds to the surface of the core structure and a second end that includes a second functional group that has a different second charge. 
     In another set of embodiments, a method comprises forming a tattoo that is at least occassionally visible on a surface of skin by introducing, into a layer of the skin, a plurality of particles. Each particle includes a core structure and at least one surface domain that comprises a net charge. At least one of the core structure or the surface domain further comprises a dye. 
     In some of these embodiments, the method also includes placing an electrode near a surface of the skin and applying an electric field through the electrode sufficient to cause the plurality of particles to move from the layer of skin toward the electrode. 
     In another set of embodiments, a method comprises forming a tattoo that is at least occasionally visible on a surface of skin by introducing, into a layer of the skin, a plurality of particles. Each particle comprises a core structure and a first surface domain and a different second surface domain. The first surface domain comprises a first dye and a first net charge. The second surface domain comprises a different second net charge. The first dye is substantively absent in the core structure and in the second surface domain. 
     In some of these embodiments, the method also includes placing an electrode near a surface of the skin and applying an electric field through the electrode sufficient to cause the plurality of particles to orient in the skin based on the electric field. 
     In another set of embodiments, a particle comprises a core structure and a first surface domain and a different second surface domain. The first surface domain comprises a first dye and a first net charge. The second surface domain comprises a different second net charge. The first dye is substantively absent in the core structure and in the second surface domain. 
     Still other aspects, features, and advantages of the invention are readily apparent from the following detailed description, simply by illustrating a number of particular embodiments and implementations, including the best mode contemplated for carrying out the invention. The invention is also capable of other and different embodiments, and its several details can be modified in various obvious respects, all without departing from the spirit and scope of the invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as restrictive. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings and in which like reference numerals refer to similar elements and in which: 
         FIG. 1A  is a block diagram that illustrates an example particle with a surface domain different from a core structure, according to an embodiment; 
         FIG. 1B  is a block diagram that illustrates an example particle with two different surface domains, each different from a core structure, according to an embodiment; 
         FIG. 1C  is a block diagram that illustrates an example particle with multiple surface domains, each different from a core structure, according to an embodiment; 
         FIG. 2  is a flow diagram that illustrates an example method for making and using particles with at least one charged surface domain, according to an embodiment; 
         FIG. 3A  is a micrograph that illustrates an example population of particles that includes particles with at least one charged surface domain, according to an embodiment; 
         FIG. 3B  is a micrograph that illustrates an example population of particles in which one surface domain is randomly distributed over the particles&#39; surfaces, according to an embodiment; 
         FIG. 3C  is a micrograph that illustrates an example population of particles that includes particles with at least two charged surface domain, according to an embodiment; 
         FIG. 3D . is a micrograph that illustrates an example population of particles in which one surface domain is randomly distributed over the microparticles&#39; surfaces, according to an embodiment; 
         FIG. 4A  is a block diagram that illustrates an example color observed from a population of randomly oriented particles that each include two surface domains of differently net charge and different dyes, according to an embodiment; 
         FIG. 4B  is a block diagram that illustrates an example color observed from a population of particles of  FIG. 4A  in a first electric field, according to an embodiment; 
         FIG. 4C  is a block diagram that illustrates an example color observed from a population of particles of  FIG. 4A  in a second electric field perpendicular to the first electric field, according to an embodiment; 
         FIG. 4D  is a block diagram that illustrates an example color observed from a population of particles of  FIG. 4A  in a third electric field opposite to the second electric field, according to an embodiment; 
         FIG. 5  is a block diagram that illustrates an example range of colors observed in an electric field from a population of particles that each include two different surface domains but of different total charges, according to an embodiment; 
         FIG. 6A  is a block diagram that illustrates an example color observed from a population of particles that each include two different surface domains in an electric field of a first strength; 
         FIG. 6B  is a block diagram that illustrates an example color observed from a population of particles that each include two different surface domains in an electric field of a second strength; 
         FIG. 7A  is a block diagram that illustrates an example particle that includes two surface domains of differently net charge and different dyes and a core structure that includes a third dye, according to an embodiment; 
         FIG. 7B  is a block diagram that illustrates an example tattoo comprising a population of the particles of  FIG. 7A , according to an embodiment; 
         FIG. 7C  is a block diagram that illustrates an example tattoo comprising a population of the particles of  FIG. 7A  in the presence of an electric field, according to an embodiment; 
         FIGS. 8A through 8G  are photographs that illustrates colors observed from a population of the particle of  FIG. 7A  before and after applying an electric field, according to various embodiments; 
         FIG. 9A  is a block diagram that illustrates an example particle that includes a single surface domain of negative net charge and blue dye, according to an embodiment; 
         FIG. 9B  is a block diagram that illustrates an example tattoo comprising a population of the particles of  FIG. 9A , according to an embodiment; 
         FIG. 9C  is a block diagram that illustrates an example tattoo comprising a population of the particles of  FIG. 9A  after application of an electric field, according to an embodiment; 
         FIGS. 10A through 10H  are photographs that illustrate example migration of the particles of  FIG. 9A  after applying an electric field, according to various embodiments; 
         FIGS. 11A through 11C  are photographs that illustrate example deposition of the particles of  FIG. 9A  on a positive electrode, according to an embodiment; 
         FIGS. 12A through 12F  are photographs that illustrate example removal of the particles of  FIG. 9A  after applying an electric field using adhesive electrodes, according to various embodiments; and 
         FIG. 13  is a micrograph that illustrates example duration of particles after six months in an aqueous environment indicative of biodegradability, according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     A method and composition of matter are described for particles with one or more charged surface domains, such as for reversible tattoos. In some embodiments, one or more such charged domains are also functionalized with additional functions, such as with dyes and chemicals. In the following description, for the purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent, however, to one skilled in the art that the present invention may be practiced without these specific details. In other instances, well-known structures and devices are shown in block diagram form in order to avoid unnecessarily obscuring the present invention. 
     Some embodiments of the invention are described below in the context of particles with at least one colored surface domain to be injected into skin for a tattoo. However, the invention is not limited to this context. In other embodiments particles with one or more differently charged surface domains are used for other purposes, with or without dyes, including safe, painless, instantaneously or temporary removable color tattoos, flexible displays, cosmetic surgery, electronic paint for fashion and fabrics, adaptive camouflage, smart card, logos on products, optogenetics, delivery of compounds for wound healing, cardiology and pain medicine, medical circuits, protein transistors, medical tattoos (e.g., for radiotherapy), paint for artists, performer&#39;s clothing, smart cards for army applications, or voltage gates in neurosciences, among others, alone or in any combination. 
     1. Overview 
     The formation of multiple functional domain particles has been described elsewhere, e.g., in international patent application WO/2010/042555, the entire contents of which are hereby incorporated by reference as if fully set forth herein, except for terminology inconsistent with that defined herein. As described therein, the formation of such particles includes obtaining a core structure having a surface and a plurality of first linkers and a plurality of second linkers. Each first linker includes a first end that binds to the surface of the core structure, and a second end that has a first functional group. Each second linker includes a first end that binds to the surface of the core structure, and a second end that includes a second functional group, different than the first. The first ends of the linkers are bound to the surface of the core structure via their respective first ends. The first and second functional groups form an external mosaic of surface domains, each domain including a majority of one type of functional group. 
     For example, a polymeric nanoparticle having multiple functionalized surface domains are formed by dissolving a polymer in a volatile, water-miscible organic solvent to form a first solution; dissolving a plurality of first and second amphiphilic components bound to linkers in an aqueous solvent to form a second solution. The amphiphilic components each include a hydrophobic end and a hydrophilic end. The first linkers each include a first functional group, and the second linkers each include a second functional group. The first and second solutions are combined such that a polymeric nanoparticle is formed having a polymer core surrounded by the amphiphilic components. The linkers extend from the amphiphilic components and the first and second functional groups form an external mosaic of surface domains, each domain including a majority of one type of functional group. 
     In these methods, the first and second linkers can be the same except for the different functional groups, and the methods can further include obtaining a plurality of third (or fourth or more) linkers, different from the first and second linkers, and binding first ends of the third linkers to the surface of the core structure. In certain embodiments, the final particles can have mosaic patterns that include two surface domains, a first domain including a majority of the first functional groups, and a second domain including a majority of the second functional groups. In other embodiments, the mosaic pattern can have multiple surface domains each including a majority of first functional groups and multiple surface domains each including a majority of second functional groups. 
     According to various embodiments of the present invention, at least one of the functional groups includes a net charge. The charge density (charge per unit area) in each surface domain depends on the mix of first and second linkers in each surface domain. The net charge of the particle depends on the charge density and the size of each domain and any charge in the core structure. 
     In some of these embodiments, the charged functional group also includes a dye. As used herein, a dye includes any source of color, including any chromophore, dye, fluorescent protein, or other material that emits light of a limited wavelength range under at least some conditions, alone or in some combination. 
     In some embodiments, the dyes and other component materials of the particles are selected from among materials that have been approved by the United States Food and Drug Administration for use in human. 
       FIG. 1A  is a block diagram that illustrates an example particle  100  with a surface domain  120  different from a core structure  110 , according to an embodiment. The surface domain  120  comprises a plurality of first linkers with a first end bound to the core structure  110  and a second end that comprises a first functional group having a first charge. As a result of the net charge per first functional group, the surface domain  120  has a net charge density per unit area. The global charge of the particle&#39;s surface depends on the magnitude of the charge of the particle&#39;s surface domains and the charge, if any, on the core structure. 
     The core structure  110  has a minimum curvature (maximum radius of curvature) of C  112   a  at one location, and has a maximum curvature (minimum radius of curvature) of C  112   b  at a second location, separated from the location of minimum curvature by a distance d  114 . For spherical particles C  112   a =C  112   b;  and d=2πr/4, where r is the radius of the sphere. Any material may be used as the core structure  110 , including polymeric, metal and core-shell particles as well as reverse micelles, metal oxide, and quantum dots. 
     A particle  100  with a single surface domain  120  can be synthesized in any manner known in the art, including nanoprecipitation (also known as solvent displacement), emulsification-solvent evaporation, thin-film hydration, modified Stober method, layer-by-layer synthesis, ionotropic gelification, ultrasonication, and polymer-monomer pair reaction. In the illustrated embodiment, the surface domain  120  has a charge density that depends on the net charge of each functional group on each linker. In some embodiments, a dye is included in the functional group of the linkers in surface domain  120 , or in the core structure  110 , or both. 
       FIG. 1B  is a block diagram that illustrates an example particle  102  with two different surface domains  132  and  134 , each different from a core structure  110 , according to an embodiment. The number of different surface domains depends on the number of different components exposed to the core structure  110  during synthesis. In some embodiments, each of surface domain  132  and surface domain  134 , collectively referenced hereinafter as surface domains  130 , comprises one type of linker with one type of functional group. In some embodiments, each of surface domains  130  includes a different ratio of two or more linkers with associated functional groups. In some embodiments, one or more of the functional groups has a net charge. The charge density in each surface domain  130  depends on the net charge of each functional group and the ratio of the two or more functional groups in each surface domain. In some embodiments, a dye is included in the functional group of one or more of the linkers, or in the core structure  110 , or some combination; and the particle  102  is at least partially colored. 
       FIG. 1C  is a block diagram that illustrates an example particle  104  with multiple surface domains  142 ,  144 ,  146 , each different from a core structure  110 , according to an embodiment. In some embodiments, each of surface domain  142  and surface domain  144  and surface domain  146 , collectively referenced hereinafter as surface domains  140 , comprises one type of linker with one type of functional group from a set of three or more functional groups available during synthesis. In some embodiments, each of surface domains  140  includes a different ratio of two or more linkers with associated functional groups. In some embodiments, one or more of the functional groups has a net charge. The charge density in each surface domain  140  depends on the net charge of each functional group and the ratio of the two or more functional groups in each surface domain. In some embodiments, a dye is included in the functional group of one or more of the linkers, or in the core structure  110 , or some combination; and the particle  104  is at least partially colored. 
     The global charge of any of the particles  100 ,  102  or  104  is tunable by the selection of the net charge per functional group. Furthermore, in some embodiments, the charge of the particle is increased by adding sodium dodecyl sulfate (SDS) or hexadecyltrimethylammoniumbromide (CTBA), or some combination. To increase the charge of a particle&#39;s surface, SDS (negatively charged) or CTBA (positively charged) is added after the synthesis of the particles. In some embodiments, the magnitude of the particle&#39;s charge is modulated further by varying the molar ratio between pegylated lipids and the SDS or CTBA. The ability to manipulate the charge density of the microparticles allows control of the electrophoretic mobility of the particles. The better the electrophoretic mobility, the faster the response to an external electric field, e.g., for color formation or color switching capability of the particles. 
       FIG. 2  is a flow diagram that illustrates an example method for making and using particles with at least one charged surface domain, according to an embodiment. Although steps are depicted in  FIG. 2  as integral blocks in a particular order for purposes of illustration, in other embodiments, one or more steps, or portions thereof, may be performed in a different order, or overlapping in time, in series or in parallel, or one or more steps, or portions thereof may be omitted, or additional steps added, or the method may be changed in some combination of ways. For example, in some embodiments, steps  201 ,  203  and zero or more passes of step  207  are performed during the same time interval. 
     In step  201 , core structures that have a core charge density are formed. For example, one or more monomer components of the core structure are placed in solution and polymerized using any method known in the art. In some embodiments, the core structures are formed simultaneously with the surface domains, so that step  201  overlaps step  211 , described below. In some embodiments, the core structure  110  is electrically neutral so that the core charge density is zero. In some embodiments, the core structure  110  includes a dye. In some embodiments, the core particle is a microparticle with a maximum dimension no greater than 1000 micrometers (μm, also called microns, 1 micron=10 −6  meters). In some embodiments, the core particle is a nanoparticle with a maximum dimension no greater than 1000 nanometers (nm, 1 nm=10 −9  meters). In some embodiments, the core structure is biodegradable. 
     In step  203  first linkers are obtained, each with a first end that binds to the core structure and a second end that includes a first functional group having a first charge. In some embodiments, the first functional group is electrically neutral so that the first charge is zero. More detail on linkers and functional groups are described below with reference to particular embodiments. 
     In the illustrated embodiments, the charge density of a surface domain is characterized by a Zeta potential of a particle including that surface domain. A Zeta potential is the electrical potential difference between a dispersion medium and the stationary layer of fluid attached to a dispersed particle. A positive or negative value of 25 millivolts (mV, 1 mV=10 −3  Volts) is taken as an arbitrary value that separates low-charged surfaces from highly-charged surfaces. Zeta potential is not measurable directly but it can be calculated using theoretical models and an experimentally-determined electrophoretic mobility or dynamic electrophoretic mobility. Electrophoresis is used for estimating zeta potential of particulates, as is well known in the art. 
     All zeta potential measurements recited herein were performed using a Malvern Instrument of Westborough Mass. A linker component, such as a pegylated lipid functional group, dyes (red fluorescent, blue fluorescent) or colored pegylated lipid functional group, was suspended in 4% ethanol and placed in a folded capillary cell (disposable electrode). The Smoluchwski equation, given by Equation 1 was used to extract the zeta potential, ζ, from the measured particle electrophoretic mobility, 
       ζ=4 π 82  η/D    (1)
 
     where μ, η and D are the electrophoretic mobility, viscosity and dielectric constant of the dispersion medium, respectively. The reported zeta potential values are an average of 3 measurements, each of which was obtained over 20 electric field cycles. The effective voltage was 151 Volts (V). 
     For example, some linkers include 1, 2-Distearoyl-sn-Glycero-3-Phosphoethanolamine-N-Polyethylene Glycol (DSPE-PEG) at a first end with a second end chosen from amine (NH 2 ) or maleimide (MAL). In some embodiments, a red fluorescent dye (PKH26 Red Fluorescent Cell Linker) was included in the amine functional group; and a blue fluorescent dye was included in the maleimide functional group. The formations of such particles are described in more detail below with reference to particular embodiments. Table 1 depicts the measured Zeta potentials of particles with a surface domain comprising the linker components listed in the Table. 
                     TABLE 1                  Charge density of each of the linkers that form example       charged microparticles                             Linker component   Zeta potential (mV)                       DSPE-PEG-MAL   No detectable           DSPE-PEG-NH 2     No detectable           Red fluorescent   +47 ± 2           Blue fluorescent   −10 ± 2           DSPE-PEG-MAL-Blue fluorescent   −31 ± 3           DSPE-PEG-NH 2- red fluorescent     16 ± 2                        
Thus in these embodiments, the dye contributes to the net charge provided by each linker. Other dyes and functional groups that provide net charges in various embodiments include Fluorescein Isothiocyanate (FICT), Alexa fluor 488, Alexa fluor 633, FITC-dextran, Rhodamine-B-dextran, SE (molecular probes), Alexa 647, Green Fluorescent Nucleic Acid Stain, Red Fluorescent Nucleic Acid Stain, Blue Fluorescent Nucleic Acid Stain, Orange Fluorescent Nucleic Acid Stain, Bodipy fluorescein (Bodipy FL), Bodipy rhodamine 6G (Bodipy R6G), Bodipy tetramethylrhodamine (Bodipy TMR), Bodipy texasRed fluorophores (Bodipy TR), Amine reactive bodipy dyes, Thiol-reactive bodipy dyes Bodipy succinimidyl esters, water-soluble bodipy sulfonated succinimidyl esters, bodipy carboxylic acids, bodipy lipids or bodipy receptor ligand conjugate, alone or in some combination. The following functional groups contribute to the net charge provided by each linker, in various embodiments: amine, maleimide, hydroxyl, carboxyl, methoxyl, pyridythiol, azide group, hydrocarbyls, ketone, aldehyde, acyl halide, alcohol, carbonate, carboxylate, carboxylic acid, ether, ester, hydroperoxide, peroxide, amide, amine, imine, imide, azo compounds, cyanates, isocyanates, nitrate, nitrile, nitrite, nitrocompound, nitroso compound, or pyridine derivatives, alone or in some combination.
 
     In step  205 , it is determined whether there is another linker to be used in the synthesis of particles. If so, then in step  207 , similar to step  203 , next linkers are obtained, each with a first end that binds to the core structure and a second end that includes a next functional group having a next charge different from the first charge. In some embodiments in which the first net charge is non-zero, the next functional group is electrically neutral so that the next charge is zero. Steps  205  and  207  form a loop that is repeated until all linkers to be used in the synthesis of a population of particles are obtained. 
     If it is determined in step  205  that there is not another linker to be included during the synthesis, then in step  211 , the core structures and linkers are combined so that the first ends of multiple linkers from one or more of all the types of linkers bind to the core structure to form a mosaic of one or more surface charge density domains on each particle of a population of particles. 
       FIG. 3A  is a micrograph  300  that illustrates an example population of particles  310  that includes particles with at least one charged surface domain, according to an embodiment. The micrograph was obtained from a scanning electron microscope of particles functionalized with 50% DSPE-PEG-NH 2 (+) and 50% DSPE-PEG-MAL-blue fluorescent dye(−). The micrograph scale  302  depicts 10 microns. The population of particles  310  includes both microparticles and nanoparticles. Surface domains  304  on three particles are indicated. The population of particles  310  includes particles  312  of relatively high curvature and small size, and particles  314  of moderate curvature and size, and particles  316  of low curvature and large size. 
       FIG. 3B  is a micrograph  320  that illustrates an example population of particles  330  in which one surface domain  332  is randomly distributed over the particle&#39;s surface, according to an embodiment. The micrograph scale  332  depicts 10 microns. Some example surface domains  332  are surrounded by dot-dashed ovals. These polymer mciroparticles  330  were prepared by functionalizing PLGA with 100% DSPE-PEG-NH 2  using the emulsion method as described in the next section. 
       FIG. 3C  is a micrograph  350  that illustrates an example population of particles  360  that includes particles with at least two charged surface domain, according to an embodiment. The micrograph was obtained from a scanning electron microscope of particles functionalized with 50% DSPE-PEG-NH 2 -red fluorescent dye(+) and 50% DSPE-PEG-MAL-blue fluorescent dye(−). The micrograph scale  352  depicts 10 microns. The population of particles  360  includes both microparticles and nanoparticles. Multiple surface domains  364  on some five particles inside a bounding square are indicated by a pair of Xs. Outside of the bounding square there are many other particles showing at least one visible domain  362 . 
       FIG. 3D . is a micrograph  380  that illustrates an example population of particles  390  that have multiple charged surface domains  392  that are randomly distributed over the microparticles&#39; surfaces, according to an embodiment. The micrograph scale  382  depicts 10 microns. The micrograph  380  was obtained from a scanning electron microscope of particles functionalized with 100% DSPE-PEG-MAL-Blue fluorescent dye(−). These polymer microparticles were prepared by functionalizing PLGA with 100% DSPE-PEG-MAL using the emulsion method as described in the next section. 
     Returning to  FIG. 2 , in step  213 , the charge in one or more functional groups is enhanced, e.g., by adding SDS or CTBA, or some combination. Table 2 depicts the effects of enhancing the charge on particles in two embodiments. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Manipulating a particle&#39;s charge by adding SDS or CTBA 
               
            
           
           
               
               
               
            
               
                   
                 Zeta potential of 
                 Zeta potential of 
               
               
                   
                 particles without 
                 particles after 
               
               
                   
                 adding SDS or 
                 adding SDS or 
               
               
                   
                 CTBA to particle 
                 CTBA to particle 
               
               
                 linker 
                 preparation (mV) 
                 preparation (mV) 
               
               
                   
               
               
                 100% DSPE-PEG-MAL 
                 −25 ± 2 
                 −113 ± 2 (SDS) 
               
               
                 100% DSPE-PEG-NH 2   
                    5 ± 1 
                    63 ± 3 (CTBA) 
               
               
                   
               
            
           
         
       
     
     In some embodiments, step  213  includes separating the particles in the population, e.g., by total charge or number of surface domains or size or weight or some combination. In some embodiments, step  213  is omitted. 
     In step  215 , at least a portion of the population of particles is introduced to a subject. For example, the particles are used as ink on paper, or food, or in tattoos applied to the skin of a subject. In various embodiments, the particles are introduced to other subjects for other applications, e.g., for air filter applications through medicinal camouflage applications, as described below. 
     In step  217 , an electric field is applied in the vicinity of the subject to change the behavior of the portion of particles introduced to the subject. For example, the electric field is applied to remove the particles (e.g., during removal of a tattoo) or to separate the particles with different net charges (e.g., to separate differently colored particles) or to orient the particles with non uniform charge densities (e.g., to change the perceived color at a fixed location), or to rotate in a 3D space, or some combination. Such functionality is particular advantageous for imaging applications especially when charged surface domains are envisaged to be used as a Foster Resonance Energy Transfer System as described below. 
     In step  221 , it is determined if end conditions are satisfied, e.g., that the desired behavior has been achieved (such as removing the tattoo). If not, the process returns to step  217  to again apply an electric field. Otherwise, the process ends. 
     Air filter applications. In some embodiments, charged surface domain particles are used to create a paint that coats a wide range of products to retain dust or other toxic materials via an electrostatic interaction. For example, it is known that facial protective masks are not efficient enough to prevent inhaling nanomaterials such as carbon nanotubes and other nanoparticles. Coating these masks with a paint made of charged surface domain particles diminishes this problem. Furthermore, by applying an electric field on these masks, carbon nanoparticles are removed from the masks; therefore the mask can be reused. This represents a mechanism to prevent and diminish the toxic effects of such nanomaterials as well as providing an environmentally friendly procedure. In various embodiments, this technology is applied to many clean wipe systems. The fact that multiple charged surface domains can have different surface chemistry allows customization of the chemical composition of the filters in various embodiments, according to the type of particles that are to be filtered. For instance, in some embodiments, a filter is placed in the human nose to prevent bacteria or virus from entering respiratory airways. Many bacteria and virus have a layer of carbohydrates on their surfaces. The property that the dual charged surface domains are functionalized virtually by any functional groups is utilized in various embodiments to prevent respiratory diseases caused by many pathogens that have different surface chemistries. 
     Anti-counterfeit applications. To date, there are several devices for detecting counterfeiting, including ultraviolet light-emitting diode (UV-LED) counterfeit detectors. Although these systems are efficient, one of the biggest drawbacks of these technologies is the harmful effect that UV can produce in users&#39; eyes. The use of charged surface multiple domain particles offers the possibility to create counterfeiting detection devices by using particles that have one or two domains which are colored and charged. In various embodiments, these particles are incorporated in a tag that displays a visible color in the presence of an electric field and becomes uncolored when the electric field changes. For example, in some embodiments, only in the presence of an electric field does the color of the particle appear. The use of an electric field which displays the particle&#39;s color represents an easy, intuitive design, offering cheap, efficient and instant visual detection. Most importantly, such anti-counterfeit embodiments are not harmful to the users&#39; eyes. Also, another major advantage of some such embodiments is simplicity, because hundreds of code sequences, registration and central database storage are not required. 
     Adaptive fashions. In some embodiments, such charged and colored particles are placed on different types of fabrics including cotton, for example, by using a tattoo machine. In some embodiments, the multiple domains or cores of the particles are made of dyes which are sensitive to light, as described below, then the color of the fabric changes under different lighting conditions. Also, by applying an electric field to the fabric one can modulate the strength of the color that is displayed. This means that, in some embodiments, a user can change the color of the fabric by changing the properties of the electric field, such as the magnitude of the voltage. This technology is applied to any type of clothing in various embodiments. For example, in some embodiments, shoes are painted with a color electronic ink that changes colors by applying an electric field. 
     Adaptive adhesives. In some embodiments, charged domain particles are used for the creation of adhesives with different magnitudes of adhesive strength. The adhesive is composed of particles with charged multiple domains. Since these domains have been charged, they are attached to the surface via an electrostatic interaction. The strength of the electrostatic interaction is regulated by the electric field. Also, in some embodiments, changes in magnitude of the electric field&#39;s strength are identified by a change of the particle&#39;s color. The visible color allows proper alignment of components before bonding. The increase of the adhesive strength is controlled by the magnitude of the electric field. In some embodiments, this procedure is less harmful to a user than the use of a UV lamp for affecting adhesion, making the procedure safe for medical applications, such as for patients that undergo surgery. In various embodiments, these adhesives are clean, easy and practical, in any combination. 
     Adaptive art. In some embodiments, a color electronic ink is used for paintings. Because electronic ink embodiments are made of particles with colored and charged surface domains, one can change the color of the ink by changing the properties of the electric field. Using this technology a painter can make local or global changes in a painting. 
     Instantaneously removable logos. Instantaneously removable logos are created using an electronic ink made of particles that have charged surfaces domains. Only one domain is colored, the other domain is uncolored. By applying an electric field, the domain that is colored will become visible or invisible as desired. This procedure saves money and time so that a logo visible on a product in the market is invisible during use of the product. 
     Protein transistors. In some embodiments, charged surface domains particle are used to bridge two metal electrodes. In some embodiments, these particles are synthesized to include a lipid bilayer. Embedded in that layer is an Adenosine-5′-triphosphate (ATP)-powered pump, which is widespread in cells and which mediates the exchange of sodium and potassium ions across membranes. The central part of the charged surface domain particles is exposed to an ATP solution. When the device is switched on, the pump pushes ions across the lipid membrane, changing the conductance of the lipid particle and boosting the transistor&#39;s current output. 
     Adaptive cell imaging. In some embodiments, charged surface domain particles are used as a double or multiple fluorescence imaging tool to investigate the nature of the conformational changes that are associated with substrate binding and transport phenomena within the cell. For example, in some embodiments, particles each with multiple charged surface domains are used to observe and quantify time-dependent changes in biological structures which could be masked by ensemble averaging in bulk measurements or suppressed during crystallographic conditions. In such embodiments, these particles work precisely like Forster resonance energy transfer (FRET) molecules by incorporating Cy3 and Cy5 in one of the charged surface domains. Cy3 works as a donor while the Cy5 functions as a reporter. The second charged surface domain serves as a rotational tool since its charge is orientated with an electric field. The rotational functionality of the FRET adds an extra feature of a conventional FRET system. This rotational functionality allows one to trace conformational changes in regions that have never been examined using crystallographic tools. 
     Adaptive pharmaceutical. In some embodiments, charged surface domain particles are used to create a new type of cosmetic, pharmaceutical product in a form of a cream, perfume, cosmetic pencil or spray product (e.g., a cosmetic product with biologically active ingredients purporting to have medical or drug-like benefits). For example, in some embodiments, the particles encapsulate in the core one or more of a collagen, anti-aging agent, anti-oxidant, free radical scavenger, moisturizer, depigmentation agent, reflectant, humectant, antimicrobial agent, antibacterial agent, allergy inhibitor, anti-wrinkling agent, antiseptic, analgesic, keratolytic agent, anti-inflammatory inhibitor, low molecular weight molecule, natural macromolecule such as protein or sugar or peptide or DNA or RNA, artificial macromolecules, polymer, dyes, colorant or inorganic ingredient. In these embodiments, the core of these particles release the pharmaceutical agent at different rate (e.g. low or fast) by varying the inherent viscosity of the PLGA polymer or other biodegradable and biocompatible polymers. In some embodiments, a light sensitive agent is incorporated in one charged surface domain while a temperature sensitive agent is incorporated in a second charged surface domain. Changes in light or temperature induce a change in the particle&#39;s pharmaceutical effect. 
     2. Materials and Methods for Example Embodiments 
     2.1 Particles with Single Colored Surface Domains. 
     The particles depicted in  FIG. 3B  were formed as described here. Briefly, Poly {D,L-lactide-co-glycolide acid} (PLGA) was dissolved in ethyl acetate. At the same time, PKH26 Red fluorescent cell linker (Sigma, Aldrich of St. Louis, Mo.) was added to 2 milliliters (ml, 1 ml=10 −3  liters) DSPE-PEG-NH 2  (1 milligram/ml, where 1 milligram, mg,=10 −3  grams) which was suspended in 4% ethanol. This colored pegylated lipid functional group was sonicated with a thin homogeneizer tip (outer diameter of 10.2 millimeters, mm, where 1 mm=10 −3  meters; inner diameter of 7.8 mm; length of 113 mm) for 1 minute at 1500 revolutions per minute (rpm). Immediately, PLGA was added to the colored pegylated lipid functional group. The thin homogeneizer tip was located right at the interface formed by ethanol and ethyl acetate. The mixture was sonicated for 1 minute at 4000 rpm. Fifty ml of H 2 O was added to each sample. Samples were stirred overnight. Remaining solvent was filtered out using 100 kiloDalton (kDa, 1 kDa=10 3  Daltons, 1 Dalton is the mass of a proton) amicon filters. 
     The particles depicted in  FIG. 3D  were formed as described here. Briefly, PLGA was dissolved in ethyl acetate. At the same time, BODIPY™ 630/650-X, SE from Invitrogen of Carlsbad, Calif., previously dissolved in 10 microliters (μl, 1 μl=10 −6  liters) of DMF was added to 2 ml of DSPE-PEG-MAL (1 mg/ml). This colored pegylated lipid functional group was sonicated with a thin homogeneizer tip for 1 minute at 1500 rpm. Immediately, PLGA was added to the colored pegylated lipid functional group. The homogeneizer tip was located right at the interface formed by ethanol and ethyl acetate. The mixture was sonicated for 1 minute at 4000 rpm. Fifty ml of H 2 0 was added to each sample. Samples were stirred overnight. Remaining solvent was filtered out using 100 kDa amicon filters. 
     2.2 Microparticle Synthesis with Charged and Colored Multiple Domains 
     Polymer microparticles were prepared by functionalizing PLGA with 50% DSPE-PEG-NH 2  and 50% DSPE-PEG-MAL using the emulsion method. Briefly, PLGA was dissolved in ethyl acetate. 6-(((4,4-difluoro-5-(2-thienyl)-4-bora-3a,4a-diaza-s-indacene-yl)styryloxy)acetyl)aminohexanoic acid, succinimidyl ester (BODIPY™ 630/650-X, SE from Invitrogen of Carlsbad, Calif.) previously dissolved in 10 μl of DMF was added to 1 ml of DSPE-PEG-MAL (1 mg/ml). At the same time, PKH26 Red Fluorescent Cell Linker (Sigma, Aldrich of St. Louis, Mo.) was also added to 1 ml DSPE-PEG-NH 2  (1 mg/ml). These colored pegylated lipids with functional groups were suspended in 4% ethanol, then mixed and sonicated with a thick homogenizer tip (outer diameter 16.1 mm; inner diameter 12.9 mm; length 165 mm) for 1 minute at 1500 rpm. Immediately, the PLGA was added to the lipid mixture. The homogeneizer tip was located right at the interface formed by ethanol and ethyl acetate. The mixture was sonicated for 1 minute at 4000 rpm. Fifty ml of H 2 0 was added to each sample. Samples were stirred overnight. Remaining solvent was filtered out using 100 kDa amicon filters. These samples were characterized with scanning electron and fluorescent microscopes. 
     2.3 Nanoparticle Synthesis with Dual Charged Color Domains 
     Poly (D,L-lactide-co-glycolide)-Lipid poly(ethylene glycol) nanoparticles (PLGA-Lipid-PEG NPs) were prepared as follows. PLGA polymer was dissolved in acetonitrile at a concentration of 2 mg/mL. BODIPY™ 630/650 previously dissolved in 10 μl of DMF was added to 120 μl of DSPE-PEG-MAL. At the same time, red fluorescent dye was added to 120 μl of DSPE-PEG-NH 2 . These pegylated color lipids with functional groups were mixed with Lecithin (Alfa Aesar) at a molar ratio of 8.5:1.5 and dissolved in 4% ethanol aqueous solution. The total lipid weight (lecithin+DSPE-PEG-COOH) was 15% of the PLGA polymer. The lipid solution was preheated at 65° C. for 3 minutes, and the PLGA solution was added dropwise under gentle stirring. The mixed solution was vortexed vigorously for 3 minutes followed by gentle stirring for 2 hours at room temperature. Finally the nanoparticles were washed three times using an Amicon Ultra-4 centrifugal filter (Millipore, Billerica, Mass.) with a molecular weight cutoff of 10 kDa. The same procedures were used to prepare the hybrid nanoparticles with other surface functional groups or a mixture of surface groups, as enumerated in more detail below. 
     2.4 Electrophoretic Mobility of the Charged Particle 
     In order to test the electrophoretic mobility of new charged particles, the particles were loaded and run through a native gradient gel (4-12%) at 200 V for several minutes. Non-denaturing buffer was used to run these samples. 
     2.5 Particle Synthesis for Adaptive Pharmaceutical Camouflage 
     Dual charged surface polymer domain microparticles that can be used as a medicinal cosmetic adaptive camouflage product were prepared by encapsulating collagen or indolepropiomamide in the core of PLGA microparticles. This polymer was dissolved in ethyl acetate. At the same time, PKH26 Red Fluorescent Cell Linker (Sigma, Aldrich) was added to 1 ml DSPE-PEG-NH2 (1 mg/ml) and mixed with a light sensitive polymer. Blue fluorescent dye (BODIPY® 630/650-X, SE) (Invitrogen) was added to 1 ml DSPE-PEG-MAL (1 mg/ml) and mixed with a temperature sensitive polymer. These pegylated lipids with functional groups were suspended in 4% ethanol, then mixed and sonicated with a thick homogenizer tip for 1 min at 1500 rpm. Immediately, the dissolved PLGA was added to the lipid mixture. The homogeneizer tip was located right at the interface formed by ethanol and ethyl acetate. The mixture was sonicated for 1 min at 4000 rpm. Fifty ml of H 2 0 was added to each sample. Samples were stirred overnight. Remaining solvent was filtered out using 100 kDa amicon filters. In other embodiments, other anti-aging active ingredients, such as melatonin, are encapsulated in the core of the polymer. 
     3. Color Changes 
       FIG. 4A  is a block diagram that illustrates an example color observed from a population of randomly oriented particles that each include two surface domains of different net charge and different dyes, according to an embodiment. In the illustrated example, the core structure  412  is uncolored (white). One surface domain  414  includes a yellow color and a positive charge in the functional group. The other surface domain  416  includes a blue color and a negative charge in the functional group. 
     When randomly oriented and viewed in a view direction  404  by an observer  402 , the population of particles appears with a view color  405  of green, since both yellow and blue surface domains face the observer  402 . Because the particles are randomly oriented, the population appears green when viewed in any direction. 
       FIG. 4B  is a block diagram that illustrates an example color observed from a population of particles  410  of  FIG. 4A  in a first electric field, according to an embodiment. The electric field  420 , depicted as arrows oriented from a positive electrode to a negative electrode, is perpendicular to the view direction  404  to the observer  402 . If the particles have sufficient mobility, or the field  420  is strong enough, even neutral particles will align so that the positive yellow surface domain is facing the negative electrode and the negative blue surface domain is facing the positive electrode. Viewed in view direction  404  from the side perpendicular to the electric field, both yellow and blue surface domains will be in view of observer  402  and the view color  405  of the population will appear green, as with the random orientations. However, unlike the random orientations, if viewed from above or below, the population will appear predominantly yellow or blue, respectively. 
       FIG. 4C  is a block diagram that illustrates an example color observed from a population of particles  410  of  FIG. 4A  in a second electric field perpendicular to the first electric field, according to an embodiment. The electric field  440 , depicted as arrows oriented from a positive electrode to a negative electrode, is in the view direction  404  to the observer  402 . The view color  445  of the population will appear yellow. This is accomplished by the alignment of the electric field and not the movement of the observer  402 . Therefore, to the observer  402  the view color of the population appears to have changed from green  405  to yellow  445 . 
       FIG. 4D  is a block diagram that illustrates an example color observed from a population of particles  410  of  FIG. 4A  in a third electric field opposite to the second electric field, according to an embodiment. The electric field  460 , depicted as arrows oriented from a positive electrode to a negative electrode, is opposite the view direction  404  to the observer  402 . The view color  465  of the population will appear blue. Again, this is accomplished by the alignment of the electric field and not by the movement of the observer  402 . Therefore, to the observer  402  the view color of the population appears to have changed from green  405  to yellow  445  to blue  465 . As depicted in  FIG. 4B  through  FIG. 4D , the color of the population of particles in the presence of an electric field are changed by changing the polarity of the electric field from positive to negative and vice versa. 
       FIG. 5  is a block diagram that illustrates an example range of colors observed in an electric field from a population of particles that each includes two different surface domains but of different total charges, according to an embodiment. This population includes not only the neutral particles  410  of  FIG. 4A  but also particles  510  and  520 . Particles  510  have an excess positive yellow surface domain compared to the negative blue surface domain, so that the particles  510  have a net positive charge. Particles  520  have an excess negative blue surface domain compared to the positive yellow surface domain, so that the particles  520  have a net negative charge. If the particles are sufficiently mobile or the electric field  530  is sufficiently strong, the negative particles  520  migrate toward the positive electrode and the positive particles  510  migrate toward the negative electrode. After some time, the total population will stratify into different sub-populations in different portions of the electric field. To an observer  402  in view direction  404  from the particles  410 , the view color is  405  is green, as in  FIG. 4B . However, in view direction  534  from the particles  510 , the view color  535  is more yellow, called yellow green herein. Similarly, in view direction  536  from the particles  520 , the view color  537  is bluer, called blue green herein. Thus, in some embodiments, mixed populations of particles are used to change colors in the presence of a changing electric field. 
     As another example of mixed populations, in some embodiments, a first population comprising particles with a negative charge and a first color, e.g., a core structure with a first color and a surface domain with a negative charge, is mixed with one or more other populations of particles. For example, one other population of particle has a core structure with a different color and a surface domain with a different charge, such as a positive charge or a more negative charge or a less negative charge. The colors become separated in a steady electric field, and mixed in an alternating electric field. Thus an observer will see the combined color (e.g., green) or the separate colors (e.g., blue and yellow) based on the strength and persistence of the electric fields. 
     In some embodiments, a reversal of the electric field is not needed, but a change in field strength is sufficient to change the color of the particles.  FIG. 6A  is a block diagram that illustrates an example color observed from a population of particles  610  that each include two different surface domains in an electric field  620  of a first strength. The particles  610  include a non-symmetric mosaic of surface domains, such as a white core  612  with a positive yellow surface domain  614  and an adjacent negative blue surface domain  616 . In the electric field  620  of low strength, or with particles of low mobility, the particles align somewhat with the field  620 ; but do not achieve a complete alignment. In view direction  604  to observer  602  the blue surface domains are not apparent; and the view color  605  is yellow. 
       FIG. 6B  is a block diagram that illustrates an example color observed from the population of particles  620  of  FIG. 6A  in an electric field  630  of a different second strength. In the electric field  630  of high strength (e.g., indicated by extra field lines), the particles align more completely with the field  630 . In view direction  604  to observer  602  the blue surface domains are apparent; and the view color  635  is green. As depicted in  FIG. 6A  through  FIG. 6B , the color of the population of particles in the presence of an electric field are changed by changing the magnitude voltage. For example, a green ink is displayed when the voltage is 100 mV, and a yellow ink is displayed when the voltage is 75 mV. Similar results may be obtained with other colors. 
     4. Reversible Tattoos 
     In some embodiments, a single-color tattoo is created with a color electric ink particle made of charged, biodegradable and biocompatible particles, by encapsulating the dye in the core of the particles. In some embodiments, two reversible color tattoos are created with charged polymer particles with two surface domains, by encapsulating one dye in the core and placing the other dye on one of the charged surface domains, or simply by coloring the charged surface domains with two different dyes. By changing the polarity of the electric field and depending on the charge of the surface domains, the desired color will be displayed. This can be repeated at will many times. 
     In some embodiments, tattoos with three reversible color are created with polymer particles with three functionalized surface domains, by coloring the positive charged surface domain with two different dyes and by coloring the negative charged surface domain with a third dye. By increasing the magnitude of the voltage, the color surface domain that is more positive charged will be displayed when voltage of a specific magnitude is applied. The same rationale is used for the second positive color surface domain. The third color negative surface domain will be displayed when the polarity of the electric field is changed. This can be repeated at will many times. 
     In some embodiments, tattoos with three super-imposed reversible colors are created with single charged particles containing a dye in their core. The electronic ink is formed by mixing two sets of particles of different charge and color. The color of the tattoo will be the one that emerges from the combination of the particles of different colors. By applying an electric field and changing the polarity of the electric field, the desired color will correlate with the charge of the particle. For example, mixing yellow positive charged particles with blue negative charged particles give rise to green charged particles, and by changing the polarity of the electric field one can produce yellow or blue tattoos. 
     For example tattoo embodiments, chicken skins were used as a skin model. Before applying the colored charged surface domain particles into the skin, the chicken&#39;s leg was cleaned with MQ water to remove any large impurities. A tattoo was made on the chicken skin using a tattoo machine and one of the charged surface domain ink particles. A tissue was passed over the skin in order to determine that the ink did not come off the chicken leg by wiping. 
       FIG. 7A  is a block diagram that illustrates an example particle  710  that includes two surface domains  714  and  716  of different net charges and different dyes and a core structure  712  that includes a third dye, according to an embodiment. In this illustrated embodiment, the core structure  712  includes a yellow dye, one surface domain includes a negatively charged blue dye functional group and the other surface domain includes a positively charged red dye functional group. A population of particles  710  was produced by functionalizing a core labeled with NBD dye, a yellow green color, with 50% DSPE-PEG-MAL linkers and 50% DSPE-PEG-NH 2  linkers. The first linker was labeled with blue fluorescent dye(−) while the latter linker was colored with red fluorescent dye(+). The overall charge of these particles is negative because of a negative charge contribution from the core structure  712 . The functionalized surface domains do not completely cover the particle&#39;s surface; therefore the charge of the core contributes to the global charge of the particle. 
     Although surface domains  714  and  716  are shown on opposite sides of particle  710  for purposes of illustration, in various embodiments, the population includes particles with differently arranged surface domains, of varying relative positions, sizes and total charges. In some embodiments, the population includes some particles with a single surface domain and some particles with no surface domains. 
     These particles are safe for use in humans because the particles&#39; compositional components are biocompatible. The core is made of a biodegradable and biocompatible polymer (PLGA) that is already approved by the FDA. The charged surface domains are formed by pegylated lipid functional groups which are also biocompatible. For example, PLGA particles functionalized with 100% DSPE-PEG-NH 2 , 100% DSPE-PEG-COOH, and 100% DSPE-PEG-OCH 3  were reported to be blood compatible (Salvador-Morales, C.; Zhang, L.; Langer, R.; Farokhzad, O. C. “Immunocompatibility properties of lipid-polymer hybrid nanoparticles with heterogeneous surface functional groups,” Biomaterials v. 30 pp 2231-2240, 2009). According to that paper, in several measured combinations, such particles activate the complement system in blood to a lesser extent than does Zymosan, a well-known activator of the complement system. The complement system is part of the immune system and is the most important system in the blood for recognition of foreign materials. To date, it is known that the activation of the complement system is mainly due to the surface chemistry of the charged surface domains microparticles. Because the surface chemistry of these particles induces low levels of complement system activation in blood, it is unlikely that these microparticles cause any major adverse reaction on skin such as inflammation, keloids (scars that grow beyond normal boundaries) or formation of granulomas. The formation of granulomas, as well as inflammation, is closely related to the activation of the complement system not only in the blood stream but also in other tissues including the skin. Therefore, the surface chemistry of these particles is not expected to cause a harmful effect on the skin. Nevertheless, other toxicity assays in vitro and in vivo are underway in order to assess the toxicity profiles of the other elements involved in this electronic ink—such as the use of dyes and pigment. 
     To date, the toxicity profile of tattoos is closely related to the chemical composition of the tattoo ink. This is because the tattoo ink in most of the cases is made of pigments which are completely and permanently exposed to the skin. In some embodiments described herein, the dyes or pigments are encapsulated in the core of the microparticle and the rate of biodegradability of such particles is extremely low, as described below. Thus the particles in such embodiments represent an advantage because such particles display colors while the contact with the skin is minimal. The toxicity of the electronic ink particles for tattoos is related to the nature of the pigments and not to the composition of the polymer carriers. The versatile method of synthesis allows the encapsulation of a wide range of dyes and pigments. Thus it is advantageous to select pigments or dyes that are minimally harmful. In some embodiments with dyes in the charged surface domains, the dye is exposed to the skin; however, in temporary tattoos that are removed by electric field, the exposure can be controlled. In addition, in some embodiments, the colored particles are applied to the epidermis layer of the skin; and, not to the hypodermis layer. This reduces the migration of the pigments to the nodes of the adjacent tissues. 
       FIG. 7B  is a block diagram that illustrates an example tattoo comprising a population  720  of the particles  710  of  FIG. 7A , according to an embodiment. The population  720  is deposited in the skin  790  with a tattoo machine. The skin  790  includes an epidermis layer  792 , a dermis layer  794  and a hypodermis layer  796 . For purposes of illustration, the population is shown between the epidermis layer  792  and dermis layer  794  of the skin  790 , but in other embodiments, the population of ink particles is distributed in a different location in the skin  790 . After insertion, the population  720  of ink particles in the tattoo is evident as a yellow brown area as a result of the contributions from the red, blue and yellow dyes in various portions of the randomly oriented particles  710 . 
       FIG. 7C  is a block diagram that illustrates an example tattoo comprising a population  720  of the particles  710  of  FIG. 7A  in the presence of an electric field  730 , according to an embodiment. The population  720  and skin  790  are as described above. The electric field  730  is illustrated as field lines extending from a positively charged electrode  732  to a negatively charged electrode  734 . If the particles  710  are mobile enough or the field  730  is strong enough, the particles in population  720  will orient, at least to some extent, with the electric field. Because the skin  790  is conductive, a current will flow between the electrodes  732  and  734 . In the depicted electric field, the particles will orient with the positive red surface domains directed to the surface, and the tattoo will appear more red. In a reversed electric field, the particles will orient with the negative blue surface domains directed to the surface, and the tattoo will appear more blue. In an even stronger electric field  730 , if the particles are mobile enough, the negatively charged particles  710  in population  720  will migrate toward the positively charged electrode  732 . 
       FIGS. 8A through 8G  are photographs that illustrates colors observed from a population of the particles  710  of  FIG. 7A  before and after applying an electric field, according to various embodiments.  FIG. 8A  is a fluorescent micrograph of microparticles synthesized as described above for particle  710 . The microparticles  812  fluoresce green due to the NBD dye in the core structure  712 . Red and blue fluorescence were also observed due to the dyes in the two surface domains.  FIG. 8B  is a photograph  820  of the population of particles  822  deposited in a gel. In this configuration the population appears a brownish yellow due to contributions from all three colors of the randomly oriented particles  710 . 
       FIG. 8C  is a photograph  830  that shows the dye after being driven through the gel by an electric field of 200 volts upward for five minutes. The particles with predominant positively charged red surface domains appear in regions  832 , while the negatively charged predominately yellow and blue particles appear in region  834 . 
       FIG. 8D  is a photograph  840  of a chicken skin  849  with a M-shaped tattoo  844  just outside the dashed M shape  842 . The tattoo comprises the population  720  of particles  710  before orientation in an electric field, and appears a dark brownish yellow as in photograph  820  of  FIG. 8B . 
       FIG. 8E  is a photograph  850  of the chicken skin  849  with the M-shaped tattoo  854  just outside the dashed M shape  842 , after placing two adhesive electrodes and passing a current of 25 milliAmperes (mA, 1 mA=10 −3  Amperes) for five minutes. The tattoo color  854  is becoming slightly reddish as the skin  849  lightens during drying.  FIG. 8F  is a photograph  860  of the chicken skin  849  with the M-shaped tattoo  864  just outside the dashed M shape  842  after again applying 25 mA for five minutes. The color  864  is more reddish.  FIG. 8G  is a photograph  870  of the chicken skin  849  with the M-shaped tattoo  874  just outside the dashed M shape  842  in which the color  874  is reddish. This demonstrates that by applying an electric field the color of the tattoo is changed; in this embodiment from yellow to red. 
     In various other embodiments, other populations of particles are used as tattoo inks For example, in some embodiments, microparticle surfaces were functionalized with 100% DSPE-PEG-NH 2 (+) and blue fluorescent dye was encapsulated in particle&#39;s core. These particles have a positive charge. In some embodiments, microparticle surfaces were functionalized with 100% DSPE-PEG-MAL; and NBD dye was encapsulated in particle&#39;s core. These particles have a negative charge and appear green. In some embodiments, microparticle surfaces were functionalized with 100% DSPE-PEG-NH 2 ; and a red fluorescent dye was encapsulated in the particle&#39;s core. These particles have a positive charge. In some embodiments, microparticle surfaces were functionalized with 100% DSPE-PEG-MAL(−); and NBD dye (yellow-green color) was encapsulated in the particle&#39;s core. In some embodiments, microparticle surfaces were functionalized with 50% DSPE-PEG-MAL(−) and 50% DSPE-PEG-NH 2 (+). The 50% DSPE-PEG-NH 2  surface domain was labeled with red fluorescent dye while NBD dye was encapsulated in the particle&#39;s core. The overall charge of these particles is negative because of the core&#39;s charge contribution. In some embodiments microparticle surfaces were functionalized with 50% DSPE-PEG-MAL(−) and 50% DSPE-PEG-NH 2 (+). The first linker was labeled with blue fluorescent dye(−) while the latter was colored with red fluorescent dye(+). The core was not labeled but had a net negative charge. The overall charge of these particles is negative because of the core&#39;s charge contribution. The functionalized surface domains do not completely cover the particle&#39;s surface; therefore the charge of the core contributes to the global charge of the particle. 
     The electrophoretic mobility of these particles depends on the particle&#39;s surface domains and/or color. Nine populations of particles were synthesized and subjected to electrophoretic mobility measurements. The nine particle populations include the following. 1. PLGA-50% DSPE-PEG-NH 2 -Red fluorescent dye/50% DSPE-PEG-MAL. 2. PLGA-50% DSPE-PEG-NH 2 -Red fluorescent/50% DSPE-PEG-MAL+PLGA-50% DSPE-PEG-MAL-Blue fluorescent/50% DSPE-PEG-NH 2 -red fluorescent. 3. PLGA-50% DSPE-PEG-MAL-Blue fluorescent dye+PLGA-50% DSPE-PEG-NH 2 . 4. PLGA-50% DSPE-PEG-MAL-Blue fluorescent dye+50% PLGA-DSPE-PEG-NH 2 . 6. PLGA-50% DSPE-PEG-NH 2 -red fluorescent-dye+PLGA-50% DSPE-PEG-MAL-Blue fluorescent dye+Core-NBD. 7. PLGA-50% DSPE-PEG-NH 2 -red fluorescent-dye+PLGA-50% DSPE-PEG-MAL-Blue fluorescent dye+Core-NBD. 9. PLGA-50% DSPE-PEG-NH 2 -red fluorescent-dye+PLGA-50% DSPE-PEG-MAL-Blue fluorescent dye+unlabeled core. The samples were run at 200V for 3 minutes and the polarity of the electric field was changed several times from positive to negative and vice versa. Particles whose two functionalized surface domains are colored with red and blue and that also have a yellow core display three colors: red (top), blue (bottom), yellow (bottom). The yellow core dominates over blue so the blue color is often not visible. Particles that have two colored surface domains (e.g. red and blue) display blue on top and red on bottom. When the polarity changed the colors inverted themselves. The separation of the two colors was clearly observed. 
     In some embodiments, the electrophoretic mobility of the particles is used to remove them from skin and hence provide removable tattoos.  FIG. 9A  is a block diagram that illustrates an example particle  910  that includes a single surface domain  916  of negative net charge and blue dye, according to an embodiment. The particle&#39;s surface was functionalized with DSPE-PEG-MAL(−) and a blue fluorescent dye was encapsulated in particle&#39;s core structure  912 . The overall charge of this particle is negative. 
       FIG. 9B  is a block diagram that illustrates an example tattoo comprising a population  920  of the particles  910  of  FIG. 9A , according to an embodiment. The skin  790  is as described above for  FIG. 7B . The tattoo appears blue when viewed from above the surface of the skin  790 . 
       FIG. 9C  is a block diagram that illustrates an example tattoo comprising a population  922  of the particles  910  of  FIG. 9A  after application of an electric field  930 , according to an embodiment. The skin  790  is as described above for  FIG. 7B . The electric field  930  is illustrated as field lines extending from a positively charged electrode  932  to a negatively charged electrode  934 . Because the skin  790  is conductive, a current will flow between the electrodes  932  and  934 . If the electric field  930  is strong enough, and if the particles  910  are mobile enough, the negatively charged particles  910  in population  920  will migrate toward the positively charged electrode  932  to become the migrated population  922  adjacent to the positive electrode  932 . If the electric field is reversed repeatedly, the population of particles will migrate to both electrodes (not shown). At the surface of the skin  790 , the migrating negatively charged particles  710  will contribute to the current flow and become affixed to the positive electrode  932 . 
       FIGS. 10A through 10H  are photographs that illustrate example migration of the particles of  FIG. 9A  after applying an electric field, according to various embodiments.  FIG. 10A  is a photograph of chicken skin  1010  with a tattoo  1020  comprising a population of the negatively charged blue particles  910 . The tattoo spells the letters “MIT”.  FIG. 10B  is a photograph depicting the chicken skin being wiped by a paper towel  1012 .  FIG. 10C  is a photograph depicting the wiping surface of the paper towel  1012  without a blue mark.  FIG. 10D  is a photograph that depicts the chicken skin  1010  and tattoo  1020  after wiping. Thus  FIGS. 10B through 10D  demonstrate that the population of particles  910  formed a tattoo in the skin and is not easily removed from the surface. 
       FIG. 10E  is a photograph that depicts a positive electrode  1034  and a negative electrode  1032  placed on the surface of the chicken skin  1010 . Electrodes  1032  and  1034  were placed on the chicken skin  1010  in order to pass a current. Positive electrode  1034  is over the left edge of the letter “M” of the tattoo  1020 ; and, the negative electrode  1032  is over the letter “I” of the tattoo  1020 . FIG.  1 OF depicts the electrodes  1032  and  1034  after passing 25 mA of current through the skin  1010 . Positive electrode  1034  showed blue particles. No blue particles were found on negative electrode  1032 .  FIG. 10G  depicts the chicken skin  1010  after passing the electric current. The approximate positions of the positive and negative electrodes are depicted as rectangles  1044  and  1042 , respectively. Part of the “MIT” tattoo has been removed leaving portion  1022  of the tattoo and collecting blue particles in the skin between the approximate locations  1042  and  1044  of the negative electrode and positive electrode, respectively. The letter “I” of the tattoo  1020  is essentially absent. Thus, it is apparent that the particles have moved away from the position  1042  of the negative electrode and toward the position of the positive electrode  1044 . 
       FIG. 10H  is a photograph depicting another tattoo  1054  on a different chicken skin  1050  after being stored in moist conditions for 1½ months. The tattoo  1024  is evident, and demonstrates that the particles are long lived without electric current removal. 
       FIGS. 11A through 11C  are photographs that illustrate example deposition of the particles  910  of  FIG. 9A  on a positive electrode, according to an embodiment.  FIG. 11A , like  FIG. 10F , depicts the electrodes  1032  and  1034  after passing 25 mA of current through the chicken skin  1010 .  FIG. 11B  is a micrograph  1120  showing particles  1122  on the surface of the positive electrode  1032 .  FIG. 11C  is a fluorescent micrograph  1130  showing blue fluorescent particles  1132  that correspond to the particles  1122  in the micrograph  1120 . The blue fluorescent microparticles are consistent with the particles  910  used in the tattoo. This corroborates that the electric ink can be removed from a tattoo using an electric field. 
     In order to improve the efficiency of removing the electronic ink from chicken skin, in some embodiments, professional adhesive electrodes were used. Two dura-stick self-adhesive electrodes were placed on a tattooed chicken leg. A current of 25 mA was applied to the chicken leg using a DC power supply. Electrodes were removed after 5 min. 75% of the mark was removed by this method. The previously used electrodes  1032  and  1034  left many of the particles on the skin surface, forming a blue stained area, due to lack of proper contact. The blue stained areas were not observed using the self adhesive electrodes, and a more clean tattoo removal was achieved. 
       FIGS. 12A through 12F  are photographs that illustrate example removal of the particles  910  of  FIG. 9A  after applying an electric field, according to various embodiments using adhesive electrodes.  FIG. 12A  is a photograph of chicken skin  1210  with a tattoo  1220  comprising a population of the negatively charged blue particles  910  in the shape of an M just outside the dashed lines  1214 . 
       FIG. 12B  depicts the chicken skin  1210  with positively charged adhesive electrode  1232  and negatively charged adhesive electrode  1234  connected to voltage source to drive a 25 mA current for five minutes.  FIG. 12C  depicts the chicken skin  1210  after passing the electric current. The approximate positions of the positive and negative electrodes are depicted as rectangles  1242  and  1244 , respectively. Part of the M tattoo outside the dashed lines  1214  has been removed leaving portion  1222  of the tattoo.  FIG. 12D  depicts the adhesive electrodes  1232  and  1234  after passing 25 mA of current through the skin  1210 . Positive electrode  1232  showed blue particles. No blue particles were found on negative electrode  1234 . The chicken skin  1210  and the remaining M shaped tattoo portion  1222  outside the dashed lines  1214  are also shown.  FIG. 12E  depicts the chicken skin  1210  after passing another electric current. Another portion of the M tattoo outside the dashed lines  1214  has been removed leaving portion  1224  of the tattoo.  FIG. 12F  depicts the chicken skin  1210  after applying  25  mA three consecutive times for 5 min each time. About 75% of the ink particles were removed from the chicken skin  1210 , leaving 25% of the blue particles in the M shaped tattoo portion  1226  outside the dashed lines  1214 . This further corroborates that the electric ink can be removed from a tattoo using an electric field. 
     In still other embodiments, marks were made on a cotton fabric using a tattoo machine and a negative blue ink particle. Adhesive electrodes were placed on the marks and 37 mA were applied for 5 min. The marks were partially removed after applying the current. 
     5. Biodegradable Markings 
     The charged particles can be rather permanent.  FIG. 13  is a micrograph  1300  that illustrates example duration of particles  1310  after six months in an aqueous environment indicative of biodegradability, according to an embodiment. The scale  1302  depicts 10 microns. Both microparticles and nanoparticle are included. These microparticles were functionalized with 100% DSPE-PEG-MAL(−). This demonstrates that these particles degrade at an extremely low rate. After being stored for six months in H 2 0, the morphology of the particles is intact. 
     In some embodiments, it is desirable that the particles degrade more rapidly. In such embodiments, the degradation is speeded by modifying the synthesis procedure. For example, By changing the lactide:glycolide ratio and inherent viscosity range of the PLGA polymer, the degradation rate of the polymer can be tuned. It was found that 50:50 lactide:glycolide has one of the fastest degradation rates of the polymer via hydrolysis. Furthermore, nanoparticles are expected to degrade faster than microparticles. In such conditions, tattoos made with nanoparticles with this lactide: glycolide ratio will likely last 1-3 months, thus imparting a subject with a temporary tattoo. 
     6. Other Embodiments 
     Other particles for reversible tattoos that depend on temperature, light, humidity, conductivity or pH, or some combination, are also used in some embodiments. 
     Sensitivity to pH and temperature. Dual charged surface polymer domain microparticles sensitive to pH and temperature were prepared by functionalizing a polymer solution mixture composed of poly(N,N-dimethylamino)ethyl methacrylate (DMAEMA)-co-ethyl acrylamide (EAAm) with 100% DSPE-PEG-MAL, 100% DSPE-PEG-NH2, or 50% DSPE-PEG-MAL/50% DSPE-PEG-NH2 using the emulsion method. Briefly, polymer solution mixture composed of poly(N,N-dimethylamino)ethyl methacrylate (DMAEMA)-co-ethyl acrylamide (EAAm) was dissolved in ethyl acetate. This mixture of polymer formed the core of the particle. At the same time, 2 ml of DSPE-PEG-MAL or DSPE-PEG-NH2 was suspended in 4% ethanol, mixed and sonicated with a thick homogenizer tip for 1 minute at 1500 rpm. Immediately, the polymer solution mixture composed of polyDMAEMA, poly EAAm was added to the lipid mixture. The homogeneizer tip was located right at the interface formed by ethanol and ethyl acetate. The mixture was sonicated for 1 minute at 4000 rpm. Fifty ml of H 2 O was added to each sample. Samples were stirred overnight. Remaining solvent was filtered out using 100 kDa amicon filter. 
     Sensitivity to light. Dual charged light sensitive surface domains microparticles were prepared by functionalizing polymer solution mixture composed of poly(N,N-dimethylamino)ethyl methacrylate (DMAEMA)-co-ethyl acrylamide (EAAm) with 100% DSPE-PEG-MAL, 100% DSPE-PEG-NH 2 , or 50% DSPE-PEG-MAL/50% DSPE-PEG-NH 2  using the emulsion method. Briefly, polymer solution mixture composed of poly(N,N-dimethylamino)ethyl methacrylate (DMAEMA)-co-ethyl acrylamide (EAAm) was dissolved in ethyl acetate. At the same time, 2 ml of DSPE-PEG-MAL or DSPE-PEG-NH 2  were suspended in 4% ethanol, mixed them and sonicated with a thick homogenizer tip for 1 minute at 1500 rpm. Immediately, the polymer solution mixture composed by polyDMAEMA, poly EAAm was added to the lipid mixture. The homogeneizer tip was located right at the interface formed by ethanol and ethyl acetate. The mixture was sonicated for 1 minute at 4000 rpm. Fifty ml of H 2 O was added to each sample. Samples were stirred overnight. Remaining solvent was filtered out using 100 kDa amicon filters. It is important to mention that the light sensitive polymer can be used to form the core of the particles or can serve to form one of the domains. This means that polymers such as poly(2,5-bis(3-sulfonatopropoxy)-1,4-phenylene, disodium salt-alt-1,4-phenylene) can replace the DSPE-PEG-MAL or DSPE-PEG-NH 2 . The light sensitive polymer is incorporated in the reaction exactly the same as pegylated lipids are incorporated in the formulation. In other embodiments, other sensitive light polymers are used to form the core of the particles or are encapsulated in the core, such as: poly[(O-phenylenevinylene)-alt-(2-methoxy-5-(2-ethylhexyloxy)-p-phenylenevinylene)], poly[(p-phenylenevinylene)-alt-(2-methoxy-5-(2-ethylhexyloxy)-p-phenylene vinylene)], poly[9,9-bis-(2-ethylhexyl)-94-fluorene-2,7-diyl]. 
     Sensitivity to conductivity. Dual charged surface polymer domain microparticles that are conductive sensitive were prepared forming the core of the particle with poly(3-butylthiophene-2,5-diyl). This polymer was dissolved in ethyl acetate. At the same time, 2 ml of DSPE-PEG-MAL or DSPE-PEG-NH 2  was suspended in 4% ethanol, mixed and sonicated with a thick homogenizer tip for 1 minute at 1500 rpm. Immediately, the polymer solution mixture was added to the lipid mixture. The homogeneizer tip was located right at the interface formed by ethanol and ethyl acetate. The mixture was sonicated for 1 miute at 4000 rpm. Fifty ml of H 2 O was added to each sample. Samples were stirred overnight. Remaining solvent was filtered out using 100 kDa amicon filters. In other embodiments, other conductive polymers are used to form the core of the particle or are encapsulated in the particle, including: poly(3-cyclohexyl-4-methylthiophere-2-5-diyl) and poly(9,9-di-(2-ethylhexyl)-9H-fluorene-2,7-vinylene). 
     In the foregoing specification, the invention has been described with reference to specific embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense.