Abstract:
Compositions, kits, and methods for repairing bonds, for example, disulfide bonds, in hair or on the skin are disclosed. The compositions provide improved conditioning benefit for dry hair or moisturize the skin. The compositions also provide a long lasting moisturized feel and smooth feel to the skin or hair, without feeling greasy. The compositions contain one or more compounds that covalently crosslink at least two thiol groups in the hair or on the skin. Use of the crosslinking compositions prevents reversion of the repaired bonds to their reduced (thiol) state, for at least one week, one month, six months, or one year, after a single application of the composition. Improved methods of styling hair, for example permanent hair waving, hair curling, and hair straightening are also provided.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. Provisional Application Ser. No. 61/861,281, filed Aug. 1, 2013 and U.S. Provisional Application Ser. No. 61/885,898, filed on Oct. 2, 2013. The disclosures of which are incorporated herein by reference in their entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention generally relates to compositions and methods for treating hair or skin, particularly for repairing disulfide bonds in hair or on the skin. 
       BACKGROUND OF THE INVENTION 
       [0003]    Hair consists of many long, parallel chains of amino acids. These chains, or polymers, of amino acids are bound to each other via 1) hydrogen bonding, 2) salt bridges between acid and base groups, and 3) disulfide bonds. Water reversibly cleaves the hydrogen bonds. This makes wet hair easy to shape and set. When the water evaporates, hydrogen bonds form at new positions, holding the hair in this set. In strongly acidic solutions, such as where the pH is 1.0 to 2.0, both hydrogen bonds and salt bridges are broken. The disulfide bonds, however, can still hold the protein chains together in the strand of hair under such conditions. 
         [0004]    At a slightly alkaline pH of 8.5, some disulfide bonds are broken (Dombrink et al.,  Chem Matters,  1983, page 8). Repeated washing with slightly alkaline shampoo damages the hair by breaking more and more of the disulfide bonds. This causes the cuticle or outer surface of the hair strands to become ruffled and generally leaves the hair in a wet, tangled, and generally unmanageable state. This is one cause of “split ends.” Once the hair dries, it is often left in a dry, rough, or frizzy condition. Additionally, rough hair catches light unevenly and makes the hair look lusterless and dull. The hair can also be left with increased levels of static upon drying, which can interfere with combing and result in a condition commonly referred to as “fly-away hair.” 
         [0005]    Disulfide linkages are also ruptured due to heating or use of various reducing treatments. Current compositions and methods for waving and straightening mammalian hair use reducing agents such as thioglycolic acid, particularly as the ammonium salt, to cleave the hair&#39;s cystine disulfide bonds. Once the disulfide bonds are broken, and the hair is placed in stress to establish the final style (e.g., straight, wavy, or curly) the disulfide bonds are reestablished. Oxidation to restore the reduced bonds can be achieved by simply exposing the hair to atmospheric oxygen, but this oxidation step is very slow and is of very little practical use. Generally, hydrogen peroxide or sodium bromate is used as the oxidizing agent. However, the newly formed disulfide bonds are under stress to maintain the hair&#39;s new shape, thus, they break easily resulting in a reversion of the hair style over time. In addition, the use of peroxides in the hair styling process can result in damaged hair, removal of non-natural color from the hair, and/or leave the hair frizzy. Furthermore, some latent reduced thiols may remain in the hair even after oxidative treatment. 
         [0006]    Treatment with peroxides used in the hair styling process results in the following reaction: 
         [0000]      2K—S—H+H 2 O 2 →K—S—S—K+2H 2 O  (Rxn I)
 
         [0000]    where K represents keratin in the hair. However, if two K—S—H groups are not present for the reaction (Rxn I) to take place, it is believed that the following reaction takes place, which results in damaged hair. 
         [0000]      K—S—H+H 2 O 2 →K—SO 2 —OH  (Rxn II)
 
         [0007]    Keratin is also a major component in skin. Damage to the disulfide bridges of keratin can cause skin to look unhealthy or flaky. Maintaining the disulfide bridges of keratin keeps the skin healthy and prevents cracking and splitting. 
         [0008]    A variety of approaches have been developed to alleviate these problems, including post-shampoo application of hair conditioners, such as leave-on and rinse-off products. Typically, conditioning rinses put back the oily coating, especially to the damaged portion of the hair where the cuticle has become ruffled since conditioners cling best to these portions. However, too much or too heavy a conditioner will make the hair stickier, thus attracting dirt and often may make more shampooing treatments necessary. Typically conditioners do not crosslink the reduced thiols in hair. 
         [0009]    The use of cationic polymers to form coacervates to provide conditioning benefits to the hair is known, such as described in International Published Applications WO 93/08787 to King et al. and WO 95/01152 to Napolione et al. Commonly used cationic deposition polymers include natural polymers, such as guar gum polymers, that have been modified with cationic substituents. The selection of a cationic guar polymer with sufficient charge density and molecular weight results in sufficient deposition of conditioning agents when incorporated in a shampoo or body wash. However, a relatively high level of such cationic guar polymer generally must be deposited on the hair or skin. Moreover, the cost of such cationic guar polymer is relatively high. As a result, incorporation of cationic guar polymer can increase the manufacturing costs of such shampoo compositions. Additionally, these shampoo compositions typically are useful for wet hair conditioning, but are not capable of delivering satisfactory dry hair smooth feel. Furthermore, these conditioners do not crosslink the reduced thiols in hair. 
         [0010]    U.S. Pat. No. 5,656,265 to Bailey et al., discloses a hair styling conditioning process for use after treating the hair with a reducing agent. The process involves contacting the hair with a compound having an electrophilic group and at least one hydrophobic group. According to Bailey, the electrophilic groups react with the thiol groups to provide a plurality of hydrophobic groups on the hair. However, these conditioners do not crosslink the reduced thiols in hair. 
         [0011]    There is a need for hair formulations and treatments that can provide improved conditioning benefit for hair. Specifically, there is a need to provide long lasting moisturized feel, smooth feel, and manageability control to hair when it is dried. There is also a need for hair formulations and treatments that repair latent reduced thiols in the hair. 
         [0012]    There is a need for hair formulations and treatments that repair and/or strengthen damaged hair and rebuild stronger bonds in hair treated with reducing agents. 
         [0013]    There is also a need for skin formulations and treatments that provide improved conditioning and/or moisturizing benefit to the skin. In particular, there is a need to provide a long lasting moisturized and smooth feel to the skin. There is also a need for skin formulations and treatments that repair reduced thiols in the skin. 
         [0014]    Therefore, it is an object of this invention to provide improved compositions and methods for repairing and/or strengthening damaged hair. 
         [0015]    It is also an object of this invention to provide compositions and methods for using these compositions that repair and/or strengthen hair after a washing or reducing treatment. 
         [0016]    It is also an object of this invention to provide compositions and methods for conditioning, moisturizing, and/or otherwise treating the skin. 
       SUMMARY OF THE INVENTION 
       [0017]    Compositions, kits, and methods for repairing bonds, for example, disulfide bonds, in hair or on the skin that have been damaged are disclosed. The compositions provide improved conditioning benefit for dry hair or moisturize the skin. Specifically, the compositions provide long lasting moisturized feel and smooth feel without leaving the hair greasy, improved appearance (e.g., sheen), increased dry strength (tensile strength), ease of combing the hair when wet or dried, less hair breakage, and decreased frizz. The compositions also provide a long lasting moisturized feel and smooth feel to the skin. 
         [0018]    The compositions contain one or more compounds that covalently crosslink at least two thiol groups in the hair or on the skin. Under normal hair washing conditions, including shampooing and conditioning, the covalent crosslinks formed are not succeptable to reduction or hydrolysis. Use of the crosslinking compositions prevents reversion of the hair&#39;s repaired bonds to their reduced (thiol) state, for at least one week, two weeks, three weeks, four weeks, one month, two months, three months, six months, eight months, or one year, after application of the composition. Improved methods of styling hair, for example permanent hair waving, hair curling, and hair straightening are also provided. The crosslinking compositions can be applied each time the hair is washed or daily, once-weekly, twice-weekly, biweekly, once-monthly, every other month, or at less frequent intervals. Preferably, the crosslinking compositions are applied once-monthly to achieve the desired results. 
         [0019]    Traditional methods of permanent hair waving, hair curling, or straightening use hydrogen peroxide to rebuild the disulfide bonds after a reducing treatment. The process generally takes about three days to complete. The methods disclosed herein use crosslinking agents to repair the hair; these crosslinking agents are washed from the individual&#39;s hair on the same day that they are applied to the hair. In some embodiments, the crosslinking agents and the reduced thiol groups form a carbon-sulfur covalent bond. Under the same conditions, such as temperature and moisture, hair treated with the crosslinking agents takes a longer time to revert to its prior state compared to the same hair that is treated with hydrogen peroxide. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION 
     I. Definitions 
       [0020]    The term “hair” refers to one or more than one strand of hair, as well as the natural components of hair, such as oil from a body. Hair also refers to virgin hair or processed hair, for example hair that has been exposed to hair waving or hair straightening formulations. 
         [0021]    An “effective amount”, e.g., of the crosslinking agent or compositions described herein, refers to an amount of the crosslinking agent in a composition or formulation which, when applied as part of a desired dosage regimen oxidatively crosslinks free thiols in the hair. 
         [0022]    “Pharmaceutically acceptable” and “cosmetically acceptable” are used interchangeably and refer to those compounds, materials, compositions, and/or dosage forms which are, within the scope of sound medical judgment, suitable for use in contact with the tissues of human beings and animals without excessive toxicity, irritation, allergic response, or other problems or complications commensurate with a reasonable benefit/risk ratio. More specifically, pharmaceutically acceptable refers to a material, compound, or composition which is suitable for use in contact with the skin, scalp, or hair. Pharmaceutically acceptable materials are known to those of ordinary skill in the art. 
         [0023]    “Shampoo”, as used herein, generally refers to a liquid or semi-solid formulation applied to the hair that contains detergent or soap for washing the hair. 
         [0024]    “Conditioner”, as used herein, generally refers to a formulation (e.g., liquid, cream, lotion, gel, semi-solid) applied to the hair to soften the hair, smooth the hair, and/or change the sheen of the hair. 
         [0025]    “Analog” and “Derivative” are used herein interchangeably and refer to a compound that possesses the same core as the parent compound, but differs from the parent compound in bond order, the absence or presence of one or more atoms and/or groups of atoms, and combinations thereof. The derivative can differ from the parent compound, for example, in one or more substituents present on the core, which may include one or more atoms, functional groups, or substructures. In general, a derivative can be imagined to be formed, at least theoretically, from the parent compound via chemical and/or physical processes. 
         [0026]    “Electrophilic group” or “electrophilic moiety” are used interchangeably and refer to one or more functional groups or moieties that have an affinity for or attract electrons. 
         [0027]    “Michael acceptor”, as used herein, is a species of electrophilic groups or moieties that participates in nucleophilic addition reactions. The Michael acceptor can be or can contain an α,β-unsaturated carbonyl-containing group or moiety, such as a ketone. Other Michael acceptors include pi-bonds, such as double or triple bonds conjugated to other pi-bond containing electron withdrawing groups, such as nitro groups, nitrile groups, and carboxylic acid groups. 
         [0028]    “Alkyl”, as used herein, refers to the radical of saturated or unsaturated aliphatic groups, including straight-chain alkyl, alkenyl, or alkynyl groups, branched-chain alkyl, alkenyl, or alkynyl groups, cycloalkyl, cycloalkenyl, or cycloalkynyl (alicyclic) groups, alkyl substituted cycloalkyl, cycloalkenyl, or cycloalkynyl groups, and cycloalkyl substituted alkyl, alkenyl, or alkynyl groups. Unless otherwise indicated, a straight chain or branched chain alkyl has 30 or fewer carbon atoms in its backbone (e.g., C 1 -C 30  for straight chain, C 3 -C 30  for branched chain), more preferably 20 or fewer carbon atoms, more preferably 12 or fewer carbon atoms, and most preferably 8 or fewer carbon atoms. In some embodiments, the chain has 1-6 carbons. Likewise, preferred cycloalkyls have from 3-10 carbon atoms in their ring structure, and more preferably have 5, 6 or 7 carbons in the ring structure. The ranges provided above are inclusive of all values between the minimum value and the maximum value. 
         [0029]    The term “alkyl” includes both “unsubstituted alkyls” and “substituted alkyls”, the latter of which refers to alkyl moieties having one or more substituents replacing a hydrogen on one or more carbons of the hydrocarbon backbone. Such substituents include, but are not limited to, halogen, hydroxyl, carbonyl (such as a carboxyl, alkoxycarbonyl, formyl, or an acyl), thiocarbonyl (such as a thioester, a thioacetate, or a thioformate), alkoxyl, phosphoryl, phosphate, phosphonate, a phosphinate, amino, amido, amidine, imine, cyano, nitro, azido, sulfhydryl, alkylthio, sulfate, sulfonate, sulfamoyl, sulfonamido, sulfonyl, heterocyclyl, aralkyl, or an aromatic or heteroaromatic moiety. 
         [0030]    Unless the number of carbons is otherwise specified, “lower alkyl” as used herein means an alkyl group, as defined above, but having from one to ten carbons, more preferably from one to six carbon atoms in its backbone structure. Likewise, “lower alkenyl” and “lower alkynyl” have similar chain lengths. Preferred alkyl groups are lower alkyls. 
         [0031]    The alkyl groups may also contain one or more heteroatoms within the carbon backbone. Examples include oxygen, nitrogen, sulfur, and combinations thereof. In certain embodiments, the alkyl group contains between one and four heteroatoms. 
         [0032]    “Alkenyl” and “Alkynyl”, as used herein, refer to unsaturated aliphatic groups containing one or more double or triple bonds analogous in length (e.g., C 2 -C 30 ) and possible substitution to the alkyl groups described above. 
         [0033]    “Aryl”, as used herein, refers to 5-, 6- and 7-membered aromatic rings. The ring may be a carbocyclic, heterocyclic, fused carbocyclic, fused heterocyclic, bicarbocyclic, or biheterocyclic ring system, optionally substituted as described above for alkyl. Broadly defined, “Ar”, as used herein, includes 5-, 6- and 7-membered single-ring aromatic groups that may include from zero to four heteroatoms. Examples include, but are not limited to, benzene, pyrrole, furan, thiophene, imidazole, oxazole, thiazole, triazole, pyrazole, pyridine, pyrazine, pyridazine and pyrimidine. Those aryl groups having heteroatoms in the ring structure may also be referred to as “heteroaryl”, “aryl heterocycles”, or “heteroaromatics”. The aromatic ring can be substituted at one or more ring positions with such substituents as described above, for example, halogen, azide, alkyl, aralkyl, alkenyl, alkynyl, cycloalkyl, hydroxyl, alkoxyl, amino, nitro, sulfhydryl, imino, amido, phosphonate, phosphinate, carbonyl, carboxyl, silyl, ether, alkylthio, sulfonyl, sulfonamido, ketone, aldehyde, ester, heterocyclyl, aromatic or heteroaromatic moieties, —CF 3 , and —CN. The term “Ar” also includes polycyclic ring systems having two or more cyclic rings in which two or more carbons are common to two adjoining rings (the rings are “fused rings”) wherein at least one of the rings is aromatic, e.g., the other cyclic rings can be cycloalkyls, cycloalkenyls, cycloalkynyls, aryls and/or heterocycles, or both rings are aromatic. 
         [0034]    “Alkylaryl”, as used herein, refers to an alkyl group substituted with an aryl group (e.g., an aromatic or hetero aromatic group). 
         [0035]    “Heterocycle” or “heterocyclic”, as used herein, refers to a cyclic radical attached via a ring carbon or nitrogen of a monocyclic or bicyclic ring containing 3-10 ring atoms, and preferably from 5-6 ring atoms, containing carbon and one to four heteroatoms each selected from non-peroxide oxygen, sulfur, and N(Y) wherein Y is absent or is H, O, (C 1-4 ) alkyl, phenyl or benzyl, and optionally containing one or more double or triple bonds, and optionally substituted with one or more substituents. The term “heterocycle” also encompasses substituted and unsubstituted heteroaryl rings. Examples of heterocyclic ring include, but are not limited to, benzimidazolyl, benzofuranyl, benzothiofuranyl, benzothiophenyl, benzoxazolyl, benzoxazolinyl, benzthiazolyl, benztriazolyl, benztetrazolyl, benzisoxazolyl, benzisothiazolyl, benzimidazolinyl, carbazolyl, 4aH-carbazolyl, carbolinyl, chromanyl, chromenyl, cinnolinyl, decahydroquinolinyl, 2H,6H-1,5,2-dithiazinyl, dihydrofuro[2,3-b]tetrahydrofuran, furanyl, furazanyl, imidazolidinyl, imidazolinyl, imidazolyl, 1H-indazolyl, indolenyl, indolinyl, indolizinyl, indolyl, 3H-indolyl, isatinoyl, isobenzofuranyl, isochromanyl, isoindazolyl, isoindolinyl, isoindolyl, isoquinolinyl, isothiazolyl, isoxazolyl, methylenedioxyphenyl, morpholinyl, naphthyridinyl, octahydroisoquinolinyl, oxadiazolyl, 1,2,3-oxadiazolyl, 1,2,4-oxadiazolyl, 1,2,5-oxadiazolyl, 1,3,4-oxadiazolyl, oxazolidinyl, oxazolyl, oxindolyl, pyrimidinyl, phenanthridinyl, phenanthrolinyl, phenazinyl, phenothiazinyl, phenoxathinyl, phenoxazinyl, phthalazinyl, piperazinyl, piperidinyl, piperidonyl, 4-piperidonyl, piperonyl, pteridinyl, purinyl, pyranyl, pyrazinyl, pyrazolidinyl, pyrazolinyl, pyrazolyl, pyridazinyl, pyridooxazole, pyridoimidazole, pyridothiazole, pyridinyl, pyridyl, pyrimidinyl, pyrrolidinyl, pyrrolinyl, 2H-pyrrolyl, pyrrolyl, quinazolinyl, quinolinyl, 4H-quinolizinyl, quinoxalinyl, quinuclidinyl, tetrahydrofuranyl, tetrahydroisoquinolinyl, tetrahydroquinolinyl, tetrazolyl, 6H-1,2,5-thiadiazinyl, 1,2,3-thiadiazolyl, 1,2,4-thiadiazolyl, 1,2,5-thiadiazolyl, 1,3,4-thiadiazolyl, thianthrenyl, thiazolyl, thienyl, thienothiazolyl, thienooxazolyl, thienoimidazolyl, thiophenyl and xanthenyl. 
         [0036]    “Heteroaryl”, as used herein, refers to a monocyclic aromatic ring containing five or six ring atoms containing carbon and 1, 2, 3, or 4 heteroatoms each selected from non-peroxide oxygen, sulfur, and N(Y) where Y is absent or is H, O, (C 1 -C 8 ) alkyl, phenyl or benzyl. Non-limiting examples of heteroaryl groups include furyl, imidazolyl, triazolyl, triazinyl, oxazoyl, isoxazoyl, thiazolyl, isothiazoyl, pyrazolyl, pyrrolyl, pyrazinyl, tetrazolyl, pyridyl, (or its N-oxide), thienyl, pyrimidinyl (or its N-oxide), indolyl, isoquinolyl (or its N-oxide), quinolyl (or its N-oxide) and the like. The term “heteroaryl” can include radicals of an ortho-fused bicyclic heterocycle of about eight to ten ring atoms derived therefrom, particularly a benz-derivative or one derived by fusing a propylene, trimethylene, or tetramethylene diradical thereto. Examples of heteroaryl include, but are not limited to, furyl, imidazolyl, triazolyl, triazinyl, oxazoyl, isoxazoyl, thiazolyl, isothiazoyl, pyraxolyl, pyrrolyl, pyrazinyl, tetrazolyl, pyridyl (or its N-oxide), thientyl, pyrimidinyl (or its N-oxide), indolyl, isoquinolyl (or its N-oxide), quinolyl (or its N-oxide), and the like. 
         [0037]    “Halogen”, as used herein, refers to fluorine, chlorine, bromine, or iodine. 
         [0038]    The term “substituted” as used herein, refers to all permissible substituents of the compounds described herein. In the broadest sense, the permissible substituents include acyclic and cyclic, branched and unbranched, carbocyclic and heterocyclic, aromatic and nonaromatic substituents of organic compounds. Illustrative substituents include, but are not limited to, halogens, hydroxyl groups, or any other organic groupings containing any number of carbon atoms, preferably 1-14 carbon atoms, and optionally include one or more heteroatoms such as oxygen, sulfur, or nitrogen grouping in linear, branched, or cyclic structural formats. Representative substituents include alkyl, substituted alkyl, alkenyl, substituted alkenyl, alkynyl, substituted alkynyl, phenyl, substituted phenyl, aryl, substituted aryl, heteroaryl, substituted heteroaryl, halo, hydroxyl, alkoxy, substituted alkoxy, phenoxy, substituted phenoxy, aroxy, substituted aroxy, alkylthio, substituted alkylthio, phenylthio, substituted phenylthio, arylthio, substituted arylthio, cyano, isocyano, substituted isocyano, carbonyl, substituted carbonyl, carboxyl, substituted carboxyl, amino, substituted amino, amido, substituted amido, sulfonyl, substituted sulfonyl, sulfonic acid, phosphoryl, substituted phosphoryl, phosphonyl, substituted phosphonyl, polyaryl, substituted polyaryl, C 3 -C 20  cyclic, substituted C 3 -C 20  cyclic, heterocyclic, substituted heterocyclic, aminoacid, peptide, and polypeptide groups. 
         [0039]    Heteroatoms, such as nitrogen, may have hydrogen substituents and/or any permissible substituents of organic compounds described herein that satisfy the valences of the heteroatoms. It is understood that “substitution” or “substituted” includes the implicit proviso that such substitution is in accordance with permitted valence of the substituted atom and the substituent, and that the substitution results in a stable compound, i.e. a compound that does not spontaneously undergo transformation such as by rearrangement, cyclization, elimination, etc. 
         [0040]    “Polymer”, as used herein, refers to a molecule containing more than 10 monomer units. 
         [0041]    “Water-soluble”, as used herein, generally means at least 50, 75, 100, 125, 150, 200, 225, or 250 g is soluble in 1 L of water at 25° C. 
       II. Crosslinking Formulations 
       [0042]    The formulations disclosed herein are concerned with treating hair or skin. In particular, the formulations can rebuild latent disulfide bonds in hair or skin. Additionally, the formulations may also react with free amines in the hair to provide a conditioning effect. 
         [0043]    The formulations contain one or more crosslinking agents (also referred to herein as “compounds” or “active agents”). Generally, the formulation, when applied as part of a desired treatment regimen, oxidatively crosslinks at least 1%, 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, or 100% of the free thiols in the hair or on the skin. 
         [0044]    The crosslinking agents can be combined with one or more pharmaceutically acceptable carriers and/or excipients that are considered safe and effective to human hair, skin, and/or human scalp, and may be administered to an individual&#39;s hair without causing undesirable side effects, such as burning, itching, and/or redness, or similar adverse reactions. The formulations may further contain an excipient that renders the formulations neutral pH, or a pH ranging from about pH 3 to about pH 12, preferably from pH 5 to pH 8. 
         [0045]    The crosslinking agent is typically present in an amount ranging from about 0.01 wt % to about 50 wt % of the formulation, preferably from about from about 1 wt % to about 25 wt % of the formulation, more preferably from about 1 wt % to about 15 wt %, most preferably from about 1 wt % to about 10 wt %. Typically, the crosslinking agent is about 2.5-3 wt % of the formulation. 
         [0046]    The crosslinking agent is stable in aqueous solution for a period of at least 2, 3, 4, 5, 6, 8, 9, 10, 11, or 12 months or longer at pH of 6 to 8 and a temperature of about 25-30° C., preferably about 25° C. “Stable” as used herein with respect to shelf-life means that at least 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, or 95% of the reactive moieties are intact or to the extent that the reactive moieties react with water, the resulting product is also electrophilic. 
         [0047]    a. Crosslinking Agent 
         [0048]    The crosslinking agent contains at least two reactive moieties capable of reacting with a thiol. The crosslinking agent optionally contains a linker between the two reactive moieties. The reactive moieties, upon reaction with thiol groups on the hair follicle, form bonds that are stable, for example, hydrolytically stable. “Stable”, as used in reference to the crosslinks formed between thiol groups on hair follicles means the bonds remain intact for at least 10, 15, 20, 25, 30, 45 or 60 days, 3, 4, 5, 6, 7, 8, 9, 10, 11, or 12 months, or longer when exposed to water at pH 6-8 at a temperature from about 25° C. to about 100° C., preferably from about 25° C. to about 75° C., more preferably from about 25° C. to about 50° C., more preferably from about 25° C. to about 40° C., most preferably from about 25° C. to about 30° C. In some embodiments, the temperature is about 25° C. It is also preferred that the crosslinking reaction occur around room temperature, for example, from about 20° C. to about 35° C., preferably from about 20° C. to about 30° C., more preferably from about 25° C. to about 30° C. 
         [0049]    The crosslinking agents typically have a low molecular weight and are compatible with aqueous or solvent delivery systems. In some embodiments, the compound is water-soluble. The low molecular weight is preferred, as it allows the molecule to diffuse in and out of hair at a reasonable rate. Molecular weights of less than 10,000 Da, 8,000 Da, 6,000 Da, 5,000 Da, 4,000 Da, 3,000 Da, 2,000 Da, or 1,000 Da are preferred. In some embodiments, the molecular weight is less than 1500 Da, preferably less than 800 Da, more preferably less than 500 Da, most preferably less than 350 Daltons to achieve sufficient diffusion rates in conventional aqueous hair care systems. 
         [0050]    i. Crosslinking Agents Defined by Formula I 
         [0051]    In some embodiments, the crosslinking agents have a structure according to Formula I: 
         [0000]    
       
                 
         
             
             
         
       
     
         [0052]    wherein 
         [0053]    A, B, C, and D are reactive moieties, 
         [0054]    R is a linker, 
         [0055]    n is an integer that is ≧1, and 
         [0056]    each occurrence of p, q, r, and s is independently an integer from 0 to 25, preferably from 0 to 10, more preferably from 0 to 2. The sum of p+q+r+s is equal to or greater than 2. 
         [0057]    The reactive moieties may be present on any atom of the linker. In some embodiments, the reactive moieties are the same. In some embodiments, one or more of the reactive moieties is different. 
         [0058]    ii. Linker 
         [0059]    The reactive moieties on the crosslinking agents are preferably linked via a linker. The term “linker”, as used herein, refers to one or more polyfunctional, e.g. bifunctional molecules, trifunctional molecules, tetrafunctional molecules, etc., which can be used to covalently couple the two or more reactive moieties and which do not interfere with the reactive properties of the crosslinking agents. The reactive moieties may be attached to any part of the linker. 
         [0060]    Linkers can be a single atom, such as a heteroatom (e.g., O or S), a group of atoms, such as a functional group (e.g., amine, —C(═O)—, —CH 2 —), or multiple groups of atoms, such as an alkylene chain. Suitable linkers include but are not limited to oxygen, sulfur, carbon, boron, nitrogen, alkoxy, alkyl, alkenyl, cycloalkyl, cycloalkenyl, aryl, heterocycloalkyl, heteroaryl, ether, amine, and a polymer. 
         [0061]    The linker is optionally independently substituted with one or more substituents including hydrogen, halogen, cyano, alkoxy, alkyl, alkenyl, cycloalkyl, cycloalkenyl, aryl, heterocycloalkyl, heteroaryl, amine, hydroxy, formyl, acyl, carboxylic acid (—COOH), —C(O)R 1 , —C(O)OR 1 , carboxylate (—COO − ), primary amide (e.g., —CONH 2 ), secondary amide (e.g., —CONHR 11 ), —C(O)NR 1 R 2 , —NR 1 R 2 , —NR 1 S(O) 2 R 2 , —NR 1 C(O)R 2 , —S(O) 2 R 2 , —SR 1 , and —S(O) 2 NR 1 R 2 , sulfinyl group (e.g., —SOR 11 ), and sulfonyl group (e.g., —SOOR 11 ); wherein R 1  and R 2  may each independently be hydrogen, alkyl, alkenyl, alkynyl, cycloalkyl, aryl, heterocycloalkyl and heteroaryl; wherein each of R 1  and R 2  is optionally independently substituted with one or more substituents selected from the group consisting of halogen, hydroxyl, cyano, nitro, amino, alkylamino, dialkylamino, alkyl optionally substituted with one or more halogen or alkoxy or aryloxy, aryl optionally substituted with one or more halogen or alkoxy or alkyl or trihaloalkyl, heterocycloalkyl optionally substituted with aryl or heteroaryl or ═O or alkyl optionally substituted with hydroxyl, cycloalkyl optionally substituted with hydroxyl, heteroaryl optionally substituted with one or more halogen or alkoxy or alkyl or trihaloalkyl, haloalkyl, hydroxyalkyl, carboxy, alkoxy, aryloxy, alkoxycarbonyl, aminocarbonyl, alkylaminocarbonyl and dialkylaminocarbonyl. 
         [0062]    In some embodiments, the linker may be an alkoxy, ether, alkyl, alkenyl, cycloalkyl, cycloalkenyl, aryl, heterocycloalkyl, heteroaryl, amine, or a polymer. In some embodiments, the linker is not a polymer. 
         [0063]    iii. Polymeric Crosslinking Agents 
         [0064]    The crosslinking agent can be a polymer. In this form, the linker forms or is the polymer backbone having covalently attached thereto to two or more reactive moieties. Optionally, the polymeric crosslinking agent can have a structure according to Formula I. In some forms, for each occurrence of a monomer unit in the polymer, zero, one, two, three, four, or more reactive moieties can be covalently linked to the monomer. The reactive moieties on each monomer unit in the polymer can be the same or different. 
         [0065]    In some embodiments, at least one reactive moiety is present on each monomer unit. Alternately, the reactive moieties may be present on alternate monomer units. In some embodiments, reactive moieties are present on a minimum percentage of the monomer units in the polymer. For example, at least one reactive moiety can be present on 0.1%, 1%, 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, or 100% of the monomer units in the polymer. The reactive moieties can be present on any atom on the monomer. 
         [0066]    Polymers 
         [0067]    The polymer may be functionalized at the termini with one or more of reactive moieties, A-D. Alternatively, the polymer may be functionalized within the polymer backbone. One or more monomers in the polymer may be functionalized so that one or more reactive moieties, A-D, may be introduced (e.g., covalently bound to) using techniques known in the art. The reactive moieties can be introduced onto the monomers before polymerization or grafted onto the polymer backbone after polymerization. 
         [0068]    A wide variety of polymers and methods for forming the polymers are known in the art of polymer science. Polymers can be degradable or non-degradable polymers. Polymers can be natural or unnatural (synthetic) polymers. Polymers can be homopolymers or copolymers comprising two or more monomers. In terms of sequence, copolymers can be random, block, or comprise a combination of random and block sequences. The polymers can in some embodiments be linear polymers, branched polymers, or hyperbranched/dendritic polymers. The polymers may also be present as a crosslinked particle or surface functionalized inorganic particle. Suitable polymers include, but are not limited to poly (vinyl acetate), copolymers of styrene and alkyl acrylates, and copolymers of vinyl acetate and acrylic acid, polyvinylpyrrolidone, dextran, carboxymethylcellulose, polyethylene glycol, polyalkylene, polyacrylates, and polymethacrylates; polyanhydrides; polyorthoesters; polysytyrene (PS), poly(ethylene-co-maleic anhydride), poly(ethylene maleic anhydride-co-L-dopamine), poly(ethylene maleic anhydride-co-phenylalanine), poly(ethylene maleic anhydride-co-tyrosine), poly(butadiene-co-maleic anhydride), poly(butadiene maleic anhydride-co-L-dopamine) (pBMAD), poly(butadiene maleic anhydride-co-phenylalanine), poly(butadiene maleic anhydride-co-tyrosine), poly(bis carboxy phenoxy propane-co-sebacic anhydride) (poly (CCP:SA)), alginate; and poly(fumaric anhydride-co-sebacic anhydride (p[FA:SA]), copolymers of p[FA:SA], polyacrylates and polyacrylamides, and copolymers thereof, and combinations thereof. In some embodiments, the polymeric linker is preferably water-soluble. 
         [0069]    If the linker is a polymeric linker, the polymer is not a polysiloxane, such as an acrylic funtionalized polysiloxane. If the polymeric linker is or contains polyethylene glycol (PEG), the number of ethylene oxide units is less than 20, 15, 10, 9, 8, 7, 6, 5, or 4. In some embodiments, it is one, two, or three. 
         [0070]    The linker may have one of the following general structures: 
         [0000]    
       
                 
         
             
             
         
       
     
         [0071]    For the polymeric structures above, each occurrenace of m is independently an integer greater than or equal to 1, such as 1-10 (e.g., oligomer) or greater than 10 (e.g., polymer), such as 10-1000 or greater. 
         [0072]    Each occurrence of o is independently an integer greater than or equal to 0, such as 0-100, 0-75, 0-50, 0-40, 0-30, 0-25, 0-20, 0-15, 0-10, or 0-5. In one embodiment, o is from about 1 to about 20, about 1 to about 15, or about 1 to about 10. 
         [0073]    iv. Reactive Moieties that React with Thiols 
         [0074]    The crosslinking agent contains at least two reactive moieties that react with thiols to form covalent bonds. The reactive moieties are capable of reacting with a thiol group in the hair or on the skin to form a stable covalent bond. The reactive moiety can be an electrophilic moiety. Alternately, the reactive moiety can be a free radical forming moiety. 
         [0075]    The crosslinking agent contains at least two reactive moieties. However, the crosslinking agent may contain three, four, five, six, or greater than six reactive moieties. 
         [0076]    The reaction between the reactive moiety and the thiol groups may be initiated at room temperature and pressure when the reactive moiety contacts a thiol group in the hair or on the skin. In some embodiments, the reaction may require an initiator, such as heat, catalyst, basic conditions, or a free radical initiator. The rate of reaction between the reactive moiety and the thiol may be increased by changes in temperature, pH, and/or addition of one or more excipients, such as a catalyst; however, this is generally not required. 
         [0077]    The two or more reactive moieties on the crosslinking agent can be the same. In some embodiments, the two or more reactive moieties are different. 
         [0078]    In some embodiments, the reactive moieties are capable of undergoing a conjugate additional reaction. The reactive moieties can independently be a Michael acceptor, a succinimidyl-containing group, a maleimido-containing group, azlactone, a benzoxazinone derivative, vinyl sulfone, vinyl sulfoximine, banzoxazinone, isocyanate, epoxide, an electrophilic moiety containing a leaving group, an electrophilic thiol acceptor, acrylate group, a methacrylate group, a styrene group, an acryl amide group, a methacryl amide group, a maleate group, a fumarate group, an itaconate group, a vinyl ether group, an allyl ether group, an allyl ester group, or a vinyl ester group. In some embodiments, the reactive moiety or moieties are not an aldehyde or carboxylic acid, particularly an unconjugated aldehyde or carboxylic acid. 
         [0079]    Michael Acceptor 
         [0080]    A “Michael acceptor,” as used herein, is a compound with at least one Michael acceptor functional group with the structure below: 
         [0000]    
       
                 
         
             
             
         
       
     
         [0081]    where R 3 , R 4 , and R 5  taken independently, are hydrogen or a group or grouping selected from, but not limited to, alkyl, substituted alkyl, alkenyl, substituted alkenyl, alkynyl, substituted alkynyl, phenyl, substituted phenyl, aryl, substituted aryl, heteroaryl, substituted heteroaryl, halo, hydroxyl, alkoxy, substituted alkoxy, phenoxy, substituted phenoxy, aroxy, substituted aroxy, alkylthio, substituted alkylthio, phenylthio, substituted phenylthio, arylthio, substituted arylthio, cyano, isocyano, substituted isocyano, carbonyl, substituted carbonyl, carboxyl, substituted carboxyl, amino, substituted amino, amido, substituted amido, sulfonyl, substituted sulfonyl, sulfonic acid, phosphoryl, substituted phosphoryl, phosphonyl, substituted phosphonyl, polyaryl, substituted polyaryl, C3-C20 cyclic, substituted C3-C20 cyclic, heterocyclic, substituted heterocyclic, aminoacid, peptide, and polypeptide groups. 
         [0082]    Some suitable Michael acceptors include, but are not limited to molecules in which some or all of the structure above are residues of (meth)acrylic acid, fumaric acid, or maleic acid, substituted versions thereof, or combinations thereof, attached to the Michael acceptor molecule through an ester linkage. 
         [0083]    The linker is attached to the Michael acceptor via R 3 , R 4 , or R 5 . In some embodiments, R 3 , R 4 , or R 5  may be the linker. 
         [0084]    Vinyl Sulfone 
         [0085]    The chemistry of vinyl sulfones with respect to attack by nucleophiles is analogous to that of α,β-unsaturated ketones in that they can undergo a 1,4-type Michael addition without releasing any undesirable by-products. 
         [0086]    Vinyl Sulfoximines 
         [0087]    The chemistry of vinyl sulfoximines is similar to vinyl sulfones. The N-tosyl sulfoximine group is more electron withdrawing than the phenyl sulfone and therefore the vinyl groups will be more susceptible towards nucleophilic attack. N-substituents can be used to alter the electrophilic potential of the vinyl group. 
         [0088]    Electrophilic Moiety Containing a Leaving Group 
         [0089]    The reactive moiety may be an electrophile with a leaving group. Electrophile, as used herein refers to one or more functional groups or moieties that have an affinity for or attract electrons. Suitable electrophiles include, but are not limited to, ester moieties (—(CO)—O—R, wherein R is lower alkyl or the like), carbonyl moieties (—C(O)), carboxylic acid or carbonic acid (—COOH or —OCOOH), carbonate moieties (—O—(CO)—O—R, wherein R is lower alkyl or the like), urethane moieties (—O—(CO)—NH—R, wherein R is H, lower alkyl, or the like), substituted urethane moieties (—O—(CO)—NR′—R, where R′ is a nonhydrogen substituent such as alkyl, aryl, alkaryl, or the like), amido moieties (—(CO)—NH—R, wherein R is H, lower alkyl, or the like), substituted amido moieties (—(CO)—NR′—R where R′ is as defined previously), thioester moieties (—(CO)—S—R, wherein R is H, lower alkyl, or the like), sulfonic ester moieties (—S(O) 2 —O—R, wherein R is H, lower alkyl, or the like), and the like. Other electrophiles will be known to those of ordinary skill in the art of organic chemistry and polymer science and/or can be readily found by reference to the pertinent texts and literature. 
         [0090]    The electrophiles preferably contain a leaving group. Suitable leaving groups are well known in the art, see, e.g., “Advanced Organic Chemistry,” Jerry March, 5th Ed., pp. 445-448, John Wiley and Sons, N.Y. Examples of leaving groups include, but are not limited to, halogen, sulfonyloxy, optionally substituted alkylsulfonyloxy, optionally substituted alkenylsulfonyloxy, optionally substituted arylsulfonyloxy. Specific examples of leaving groups include chloro, iodo, bromo, fluoro, methanesulfonyloxy (mesyloxy), tosyloxy, triflyloxy, nitrophenylsulfonyloxy (nosyloxy), bromophenylsulfonyloxy (brosyloxy), hydroxyl, carboxylate, carbonate, phosphate, phosphonate, phosphinate, phosphonium, urethane, urea, amide, imide, amine, ammonium, sulfonato, —N 3 , CN, RO—, NH 2 O—, NHRO—, N(R 4 ) 2 O—, R 4 CO 2 —, R 4 OCO 2 —, R 4 NCO 2 —, R 4 S—, R 4 C(S)O—, R 4 CS 2 —, R 4 SC(O)S—, R 4 SCS 2 —, R 4 SCO 2 —, R 4 OC(S)O—, R 4 OCS 2 —, R 4 SO 2 —, R 4 SO 3 —, R 4 OSO 2 —, R 4 OSO 3 —, R 4 PO 3 —, R 4 OPO 3 —, an N-imidazolyl group, an N-triazolyl group, an N-benzotriazolyl group, a benzotriazolyloxy group, an imidazolyloxy group, an N-imidazolinone group, an N-imidazolone group, an N-imidazolinethione group, an N-imidazolinethione group, an N-succinimidyl group, an N-phthalimidyl group, an N-succinimidyloxy group, an N-phthalimidyloxy group, ON═C(CN)R 4 , and a 2-pyridyloxy group. R 4  is preferably an alkyl group or an aryl group. 
         [0091]    Preferably, the leaving group is removed from the reactive moieties and does not result in the formation of side product that disadvantageously affects the reaction between the reactive moieties and the thiol groups or form a material or compound that is unsuitable for contact with skin or hair. 
         [0092]    In some embodiments, the leaving group is a halogen. 
         [0093]    Electrophilic Thiol Acceptors 
         [0094]    Electrophilic thiol acceptors, as used herein, refer to a chemical moiety that reacts with a thiol group so that the sulfur atom of the thiol group becomes covalently bonded to the thiol acceptor. Thiol acceptors are well known in the art. Koval (Reactions of Thiols, Russian Journal of Organic Chemistry, 2007, 43:319-349) discloses several electrophilic thiol acceptors, the disclosure of which is incorporated herein by reference. 
         [0095]    Electrophilic thiol acceptors, in addition to those listed above, include but are not limited to an alpha-substituted acetyl group with the formula Y—CH 2 —CO— wherein Y is a leaving group. Examples of leaving groups include, but are not limited to, Chlorine, Bromine, Iodine, mesylate, tosylate, and the like. If the thiol acceptor is an alpha-substituted acetyl group, the thiol adduct after covalent linkage to the acceptor forms the bond —S—CH 2 —. 
         [0096]    Free Radical-Forming Groups 
         [0097]    The crosslinking agent may contain at least two free radical-forming groups that can react with thiols. The free radical-forming groups on the crosslinking agent can be the same. Alternately, the free radical-forming groups may be different. Suitable free radical forming groups include, but are not limited to acrylate groups, methacrylate groups, styrene groups, acryl amide groups, methacryl amide groups, maleate groups, fumarate groups, itaconate groups, vinyl ether groups, allyl ether groups, allyl ester groups, and vinyl ester groups. For example, suitable crosslinking agents include ethylene glycol dimethacrylate, diethylene glycol diacrylate, allyl methacrylate, trimethylolpropane triacrylate, triallylamine, tetraallyloxyethane, and di- and triacrylates, mixed acrylates which, as well as acrylate groups, comprise further ethylenically unsaturated groups. Other examples of crosslinking agents include N,N′-methylenebisacrylamide and N,N′-methylenebismethacrylamide, esters of unsaturated mono- or polycarboxylic acids of polyols, such as diacrylate or triacrylate, for example butanediol diacrylate, butanediol dimethacrylate, ethylene glycol diacrylate, ethylene glycol dimethacrylate and also trimethylolpropane triacrylate and allyl compounds, such as allyl (meth)acrylate, triallyl cyanurate, diallyl maleate, polyallyl esters, tetraallyloxyethane, triallylamine, tetraallylethylenediamine, allyl esters of phosphoric acid and also vinylphosphonic acid derivatives, pentaerythritol diallyl ether, pentaerythritol triallyl ether, pentaerythritol tetraallyl ether, polyethylene glycol diallyl ether, ethylene glycol diallyl ether, glycerol diallyl ether, glycerol triallyl ether, polyallyl ethers based on sorbitol, and also ethoxylated variants thereof. Other examples of crosslinking agents include di- and triacrylates of 3- to 15-tuply ethoxylated glycerol, of 3- to 15-tuply ethoxylated trimethylolpropane, of 3- to 15-tuply ethoxylated trimethylolethane, especially di- and triacrylates of 2- to 6-tuply ethoxylated glycerol or of 2- to 6-tuply ethoxylated trimethylolpropane, of 3-tuply propoxylated glycerol, of 3-tuply propoxylated trimethylolpropane, and also of 3-tuply mixed ethoxylated or propoxylated glycerol, of 3-tuply mixed ethoxylated or propoxylated trimethylolpropane, of 15-tuply ethoxylated glycerol, of 15-tuply ethoxylated trimethyllolpropane, of 40-tuply ethoxylated glycerol, of 40-tuply ethoxylated trimethylolethane and also of 40-tuply ethoxylated trimethylolpropane, ethylene glycol dimethacrylate, diethylene glycol diacrylate, allyl methacrylate, trimethylolpropane triacrylate, triallylamine, tetraallyloxyethane, N,N-methylenebisacrylamide, N,N′-methylenebismethacrylamide, butanediol diacrylate, butanediol dimethacrylate, trimethylolpropane triacrylate, triallyl cyanurate, diallyl maleate, a polyallyl ester, tetraallylethylenediamine, pentaerythritol diallyl ether, pentaerthyritol triallyl ether, pentaerythritol tetrallyl ether, polyethylene glycol diallyl ether, ethylene glycol diallyl ether, glycerol diallyl ether, glycerol triallyl ether, di- and triacrylates of 3- to 15-tuply ethoxylated glycerol, di- and tri-acrylates of 3- to 15-tuply ethoxylated trimethylolpropane, and di- and tri-acrylates of 3- to 15-tuply ethoxylated trimethylolethane. 
         [0098]    The reactive free radical moieties may require the presence of one or more initiators. Suitable initiators include, but are not limited to peroxides, hydroperoxides, hydrogen peroxide, persulfates, azo compounds, and redox initiators. Suitable organic peroxides include acetylacetone peroxide, methyl ethyl ketone peroxide, tert-butyl hydroperoxide, cumene hydroperoxide, tert-amyl perpivalate, tert-butyl perpivalate, tert-butyl perneohexanoate, tert-butyl perisobutyrate, tert-butyl per-2-ethylhexanoate, tert-butyl perisononanoate, tert-butyl permaleate, tert-butyl perbenzoate, di(2-ethylhexyl) peroxydicarbonate, dicyclohexyl peroxydicarbonate, di(4-tert-butylcyclohexyl) peroxydicarbonate, dimyristil peroxydicarbonate, diacetyl peroxydicarbonate, allyl peresters, cumyl peroxyneodecanoate, tert-butyl per-3,5,5-trimethylhexanoate, acetylcyclohexylsulfonyl peroxide, dilauryl peroxide, dibenzoyl peroxide, and tert-aryl perneodecanoate. Suitable azo compounds include 2,2′-azobisisobutyronitrile, 2,2′-azobis(2,4-dimethylvaleronitrile) and 2,2′-azobis(4-methoxy-2,4-dimethylvaleronitrile), preferably water-soluble azo initiators, such as, but not limited to, 2,2′-azobis{2-[1-(2-hydroxyethyl)-2-imidazolin-2-yl]propane}dihydrochloride, 2,2′-azobis-(2-amidinopropane) dihydrochloride, 2,2′-azobis[2-(2-imidazolin-2-yl)propane]dihydrochloride and 2,2′-azobis[2-(5-methyl-2-imidazolin-2-yl)propane]dihydrochloride. For the redox initiators, the oxidizing component is at least one of the peroxo compounds indicated above and the reducing component is, for example, ascorbic acid, glucose, sorbose, ammonium bisulfite, ammonium sulfite, ammonium thiosulfate, ammonium hyposulfite, ammonium pyrosulfite, ammonium sulfide, alkali metal bisulfite, alkali metal sulfite, alkali metal thiosulfate, alkali metal hyposulfite, alkali metal pyrosulfite, alkali metal sulfide, or sodium hydroxymethylsulfoxylate. 
         [0099]    The crosslinking agent may have any one of the chemical structures shown below: 
         [0000]    
       
                 
         
             
             
         
       
       
                 
         
             
             
         
       
     
         [0100]    wherein R is the linker. 
         [0101]    In one embodiment, the crosslinking agent has the chemical structure: 
         [0000]    
       
                 
         
             
             
         
       
     
         [0102]    The top structure above is referred to as bis-(maleimidoethoxy) ethane. The bottom structure above is referred to as tris-(maleimidoethyl) amine. 
         [0103]    b. Excipients 
         [0104]    The formulations typically contain one or more cosmetically acceptable excipients. Cosmetically acceptable excipients include, but are not limited to, water, preservatives, antioxidants, chelating agents, sunscreen agents, vitamins, dyes, hair coloring agents, proteins, amino acids, natural extracts such as plant extracts, humectants, fragrances, perfumes, oils, emollients, lubricants, butters, penetrants, thickeners, viscosity modifiers, polymers, resins, hair fixatives, film formers, surfactants, detergents, emulsifiers, opacifying agents, volatiles, propellants, liquid vehicles, carriers, salts, pH adjusting agents (e.g., citric acid), neutralizing agents, buffers, hair conditioning agents, anti-static agents, anti-frizz agents, anti-dandruff agents, absorbents, and combinations thereof. 
         [0105]    The formulations can contain at least two or more cosmetically acceptable excipients. In some forms, the formulations contain the crosslinking agent, water, and optionally a preservative and/or fragrance. 
         [0106]    The formulation for treating hair may be in any suitable physical form. Suitable forms include, but are not limited to low to moderate viscosity liquids, lotions, milks, mousses, sprays, gels, creams, shampoos, conditioners, and the like. Suitable excipients, such as those listed above, are included or excluded from the hair care formulation depending on the form of use of the formulation (e.g., hair spray, cream, conditioner, or shampoo). 
         [0107]    The formulation for treating skin may be in any suitable physical form. Suitable forms include, but are not limited to low to moderate viscosity liquids, lotions, milks, mousses, sprays, gels, creams, ointments, and the like. Suitable excipients, such as those listed above, are included or excluded from the skin formulation depending on the form of use of the formulation (e.g., lotion, gel, ointment, or cream). 
         [0108]    The pharmaceutical excipient is typically present in an amount ranging from about 10 wt % to about 99.99 wt % of the formulation, preferably about 40 wt % to about 99 wt %, more preferably from about 80 wt % to about to about 99 wt %. 
         [0109]    i. Surfactants 
         [0110]    Surfactants are surface-active agents that are able to reduce the surface tension of water and cause the formulation to slip across or onto the skin or hair. 
         [0111]    Surfactants also include detergents and soap. The surfactants may be amphoteric, anionic, or cationic. Suitable surfactants that may be used in the formulation include, but are not limited to, 3-aminopropane sulfonic acid, almond amide, almond amidopropyl betaine, almond amidopropylamine oxide, aluminum hydrogenated tallow glutamate, aluminum lanolate, aminoethyl sulfate, aminopropyl lauryl glutamine, ammonium C 12-15  alkyl sulfate, ammonium C 12-15  pareth sulfate, ammonium C 12-16  alkyl sulfate, ammonium C 9-10  perfluoroalkylsulfonate, ammonium capryleth sulfate, ammonium capryleth-3 sulfate, ammonium monoglyceride sulfate, ammonium sulfate, ammonium isothionate, ammonium cocoyl sarcosinate, ammonium cumene sulfonate, ammonium dimethicone copolyol sulfate, ammonium dodecylbenzenesulfonate, ammonium isostearate, ammonium laureth sulfate, ammonium laureth-12 sulfate, ammonium laureth-5 sulfate, ammonium laureth-6 carboxylate, ammonium laureth-7 sulfate, ammonium laureth-8 carboxylate, ammonium laureth-9 sulfate, ammonium lauroyl sarcosinate, ammonium lauryl sulfate, ammonium lauryl sulfosuccinate, ammonium myreth sulfate, ammonium myristyl sulfate, ammonium nonoxynol-30 sulfate, ammonium nonoxynol-4 sulfate, ammonium oleate, ammonium palm kernel sulfate, ammonium polyacrylate, ammonium stearate, ammonium tallate, ammonium xylene sulfonate, ammonium xylene sulfonate, amp-isostearoyl gelatin/keratin amino acids/lysine hydroxypropyltrimonium chloride, amp-isostearoyl hydrolyzed collagen, apricot kernel oil PEG-6 esters, apricot amide, apricot amidopropyl betaine, arachideth-20, avocadamide, avocadamidopropyl betaine, babassuamide, babassuamidopropyl betaine, babassuamidopropylamine oxide, behenalkonium chloride, behenamide, behenamide, behenamidopropyl betaine, behenamine oxide, sodium laureth sulfate, sodium lauryl sulfate, a polyoxyether of lauryl alcohol or ceteareth-20, or combinations thereof 
         [0112]    Suitable anionic surfactants include, but are not limited to, those containing carboxylate, sulfonate and sulfate ions. Examples of anionic surfactants include sodium, potassium, ammonium of long chain alkyl sulfonates and alkyl aryl sulfonates such as sodium dodecylbenzene sulfonate; 
         [0113]    dialkyl sodium sulfosuccinates, such as sodium dodecylbenzene sulfonate; 
         [0114]    dialkyl sodium sulfosuccinates, such as sodium bis-(2-ethylthioxyl)-sulfosuccinate; and alkyl sulfates such as sodium lauryl sulfate. Cationic surfactants include, but are not limited to, quaternary ammonium compounds such as benzalkonium chloride, benzethonium chloride, cetrimonium bromide, stearyl dimethylbenzyl ammonium chloride, polyoxyethylene and coconut amine Examples of nonionic surfactants include ethylene glycol monostearate, propylene glycol myristate, glyceryl monostearate, glyceryl stearate, polyglyceryl-4-oleate, sorbitan acylate, sucrose acylate, PEG-150 laurate, PEG-400 monolaurate, polyoxyethylene monolaurate, polysorbates, polyoxyethylene octylphenylether, PEG-1000 cetyl ether, polyoxyethylene tridecyl ether, polypropylene glycol butyl ether, Poloxamer 401, stearoyl monoisopropanolamide, and polyoxyethylene hydrogenated tallow amide. Examples of amphoteric surfactants include sodium N-dodecyl-.beta.-alanine, sodium N-lauryl-β-iminodipropionate, myristoamphoacetate, lauryl betaine and lauryl sulfobetaine. 
         [0115]    More than one surfactant may be included in the formulation. 
         [0116]    The surfactants are optionally included in an amount ranging from about 0.1% to about 15% by weight of the formulation, preferably about 1% to about 10% by weight of the formulation. 
         [0117]    ii. Emollients 
         [0118]    Emollient refers to a material that protects against wetness or irritation, softens, soothes, coats, lubricates, moisturizes, protects, and/or cleanses the skin. Suitable emollients for use in the formulations include, but are not limited to a silicone compound (e.g., dimethicone, cyclomethicone, dimethicone copolyol or a mixture of cyclopentasiloxane and dimethicone/vinyldimethicone cross polymer, cyclopentasiloxane polysilicone), polyols such as sorbitol, glycerin, propylene glycol, ethylene glycol, polyethylene glycol, caprylyl glycol, polypropylene glycol, 1,3-butane diol, hexylene glycol, isoprene glycol, xylitol; ethylhexyl palmitate; a triglyceride such as caprylic/capric triglyceride and fatty acid ester such as cetearyl isononanoate or cetyl palmitate. In a specific embodiment, the emollient is dimethicone, amidodimethicone, dimethiconol, cyclopentasiloxane, potassium dimethicone PEG-7 panthenyl phosphate, or a combination thereof. More than one emollient may be included in the formulation. 
         [0119]    The emollient is optionally included in an amount ranging from about 0.5% to about 15% by weight of the formulation, preferably from about 1% to about 10% by weight of the formulation. 
         [0120]    iii. Emulsifiers 
         [0121]    The formulations may also contain one or more emulsifiers. Suitable emulsifiers include, but are not limited to, copolymers of an unsaturated ester and styrene sulfonate monomer, cetearyl alcohol, glyceryl ester, polyoxyethylene glycol ether of cetearyl alcohol, stearic acid, polysorbate-20, ceteareth-20, lecithin, glycol stearate, polysorbate-60, or polysorbate-80, or combinations thereof. More than one emulsifier may be included in the formulation. 
         [0122]    The emulsifier is optionally included in an amount ranging from about 0.05% to about 15% by weight of the formulation, preferably from about 0.1% to about 10% by weight of the formulation. 
         [0123]    iv. Preservatives 
         [0124]    One or more preservatives may be included in the formulations to prevent microbial growth in the formulations. Suitable preservatives include, but are not limited to, glycerin containing compounds (e.g., glycerin or ethylhexylglycerin or phenoxyethanol), benzyl alcohol, parabens (methylparaben, ethylparaben, propylparaben, butylparaben, isobutylparaben, etc.), sodium benzoate, ethylenediamine-tetraacetic acid (EDTA), potassium sorbate, and/or grapefruit seed extract, or combinations thereof. More than one preservative may be included in the formulation. Other preservatives are known in the cosmetics industries and include salicylic acid, DMDM Hydantoin, Formaldahyde, Chlorphenism, Triclosan, Imidazolidinyl Urea, Diazolidinyl Urea, Sorbic Acid, Methylisothiazolinone, Sodium Dehydroacetate, Dehydroacetic Acid, Quaternium-15, Stearalkonium Chloride, Zinc Pyrithione, Sodium Metabisulfite, 2-Bromo-2-Nitropropane, Chlorhexidine Digluconate, Polyaminopropyl biguanide, Benzalkonium Chloride, Sodium Sulfite, Sodium Salicylate, Citric Acid, Neem Oil, Essential Oils (various), Lactic Acid, and Vitamin E (tocopherol). 
         [0125]    The preservative is optionally included in an amount ranging from about 0.1% to about 5% by weight of the formulation, preferably from about 0.3% to about 3% by weight of the formulation. Preferably, the formulations are paraben free. 
         [0126]    v. Conditioning Agents 
         [0127]    One or more conditioning agents may be included in the formulations. Suitable conditioning agents include, but are not limited to, silicone-based agents (e.g., silicone quaternium-8), panthenol, hydrolyzed wheat and/or soy protein, amino acids (e.g. wheat amino acids), rice bran wax, meadowfoam seed oil, mango seed oil, grape seed oil, jojoba seed oil, sweet almond oil, hydroxyethyl behenamidopropyl dimonium chloride, aloe leaf extract, aloe barbadensis leaf juice, phytantriol, panthenol, retinyl palmitate, behentrimonium methosulfate, cyclopentasiloxane, quaternium-91, stearamidopropyl dimethylamine, and combinations thereof. 
         [0128]    The conditioning agent(s) is optionally included in an amount ranging from about 0.1% to about 5% by weight of the formulation, preferably from about 0.3% to about 3% by weight of the formulation. 
         [0129]    vi. Diluents 
         [0130]    Diluent, as used herein, refers to a substance(s) that dilutes the crosslinking agent. Water is the preferred diluent. The formulations typically contains greater than one percent (wt) water, preferably greater than five percent (wt) water, more preferably greater than 50% (wt) water, and most preferably greater than 80% (wt) water. Alcohols, such as ethyl alcohol and isopropyl alcohol, may be used at low concentrations (about 0.5% by weight of the formulation) to enhance hair or skin penetration and/or reduce odor. 
         [0131]    vii. Viscosity Modifying Agents 
         [0132]    The formulations may contain one or more viscosity modifying agents, such as viscosity increasing agents. Classes of such agents include, but are not limited to, viscous liquids, such as polyethylene glycol, semisynthetic polymers, such as semisynthetic cellulose derivatives, synthetic polymers, such as carbomers, poloxamers, and polyethyleneimines (e.g., PEI-10), naturally occurring polymers, such as acacia, tragacanth, alginates (e.g., sodium alginate), carrageenan, vegetable gums, such as xanthan gum, petroleum jelly, waxes, particulate associate colloids, such as bentonite, colloidal silicon dioxide, and microcrystalline cellulose, surfactants, such as PPG-2 hydroxyethyl coco/isostearamide, emulsifiers, such as disteareth-75 IPDI, and salts, such as sodium chloride, and combinations thereof. 
         [0133]    viii. Antioxidants 
         [0134]    The formulations may contain one or more antioxidants. Examples include, but are not limited to, tocopheryls, BHT, ascorbic acid,  camellia sinensis  leaf extract, ascorbyl palmitate, magnesium ascorbyl phosphate, carotenoids, resveratrol, triethyl citrate, arbutin, kojic acid, tetrahexydecyl ascorbate, superoxide dismutase, zinc, sodium metabisulfite, lycopene, ubiquinone, and combinations thereof. 
         [0135]    ix. Opacifying agents 
         [0136]    The formulations may contain one or more opacifying agents. Opacifying agents are added to the formulations to make them opaque. Suitable opacifying agents include, but are not limited to, glycol distearate and ethoxylated fatty alcohols. 
         [0137]    c. Forms of the Formulation 
         [0138]    i. Sprays 
         [0139]    The formulation may be in the form of a spray. The spray typically includes the crosslinking agent and a cosmetically acceptable carrier. In some embodiments, the carrier is water or a water and alcohol mixture. The spray formulation optionally includes an antioxidant, sunscreen agent, vitamin, protein, peptide, plant extract, humectant, oil, emollient, lubricant, thickener, hair conditioning agent, polymer, and/or surfactant. Preferably, the spray formulation includes a preservative. In some embodiments, the formulation includes a fragrance. In some embodiments, the formulation includes a surfactant. In some embodiments, the formulation contains water, fragrance, a preservative, and a crosslinking agent. In some embodiments, the formulation contains water, fragrance, a preservative, and a crosslinking agent. In some embodiments, the formulation contains water, a preservative, fragrance, the crosslinking agent, and an anti-static agent. In some embodiments, the formulation contains water, a preservative, fragrance, the crosslinking agent, and a hair conditioning agent. In some embodiments, the formulation contains water, a preservative, fragrance, the crosslinking agent, and a surfactant. 
         [0140]    The hair spray formulations may be dispensed from containers that include aerosol dispensers or pump spray dispensers. Such dispensers are known in the art and are commercially available from a variety of manufacturers. 
         [0141]    Propellant 
         [0142]    When the hair spray formulation is dispensed from a pressurized aerosol container, a propellant may be used to force the composition out of the container. Suitable propellants include, but are not limited to, a liquefiable gas or a halogenated propellant. Examples of suitable propellants include dimethyl ether and hydrocarbon propellants such as propane, n-butane, iso-butane, CFCs, and CFC-replacement propellants. The propellants may be used singly or admixed. 
         [0143]    The amount of propellant may range from about 10% to about 60% by weight of the formulation. The propellant may be separated from the hair repair formulation as in a two compartment container. Other suitable aerosol dispensers are those characterized by the propellant being compressed air, which can be filled into the dispenser using a pump or equivalent device prior to use. Conventional non-aerosol pump spray dispensers, i.e., atomizers, may also be used to apply the hair strengthening formulation to the hair. 
         [0144]    ii. Conditioners 
         [0145]    The formulation may be in the form of a conditioner. The conditioner typically includes the crosslinking agent in a suitable carrier. Additionally, the conditioner may include cationic polymers derived from polysaccharides, for example cationic cellulose derivatives, cationic starch derivatives, cationic guar derivatives and cationic locust bean gum derivatives, synthetic cationic polymers, mixtures or combinations of these agents. The formulation may comprise other synthetic or natural polymers or polymers derived from biological preparation processes, which are functionalized, where appropriate, for example with cationic or neutral groups. These polymers may have a stabilizing or strengthening action on the compositions, and/or a conditioning action (deposition on the surface of the skin or the hair). 
         [0146]    The crosslinking agent may be included in any suitable concentration. Typical concentrations of the crosslinking agent in the conditioner range from small amounts such as approximately 0.01% (wt), preferably at least 0.1% (wt), to large amounts, such as up to 50% (wt). Preferably the conditioner contains the crosslinking agent in a concentration ranging from 0.1% (wt) to 5% (wt), more preferably from 0.1% wt to 3% (wt). While greater concentrations of crosslinking agent could be present in the conditioner, they are generally not needed to achieve the desired results. 
         [0147]    iii. Shampoos 
         [0148]    The hair repair formulation may be in the form of a shampoo. The shampoo typically includes the crosslinking agent in a suitable carrier. The crosslinking agent may be included in any suitable concentration. Typical concentrations of the crosslinking agent in the shampoo range from small amounts such as approximately 0.01% (wt), preferably at least 0.1% (wt), to large amounts, such as up to 50% (wt). Preferably the shampoo contains the crosslinking agent in a concentration ranging from 0.1% (wt) to 5% (wt), more preferably from 0.1% wt to 3% (wt). While greater concentrations of crosslinking agent could be present in the shampoo, they are generally not needed to achieve the desired results. 
         [0149]    Additionally, the shampoo may include from about 0.5% to about 20% of a surfactant material. Surfactants utilized in shampoo compositions are well-known in the art and are disclosed, for example, in U.S. Pat. No. 6,706,258 to Gallagher et al. and U.S. Pat. No. 7,598,213 to Geary et al. 
         [0150]    iv. Creams 
         [0151]    The formulation may be in the form of a cream. The cream typically includes the crosslinking agent in a suitable carrier. The crosslinking agent may be included in any suitable concentration. Typical concentrations of the crosslinking agent in the cream range from small amounts such as approximately 0.01% (wt), preferably at least 0.1% (wt), to large amounts, such as up to 50% (wt). Preferably the cream contains the crosslinking agent in a concentration ranging from 0.1% (wt) to 5% (wt), more preferably from 0.1% wt to 3% (wt). While greater concentrations of crosslinking agent could be present in the cream, they are generally not needed to achieve the desired results. 
         [0152]    Additionally, the cream may include an oil, a hair conditioning agent, and/or a thickening agent. The cream may also include a fragrance, a plant extract, and/or a surfactant. The cream may be packaged in a tube, tub, bottle, or other suitable container. 
         [0153]    v. Liquid Crosslinking Formulations 
         [0154]    In some embodiments, a liquid crosslinking formulation is provided, which is mixed at the time of use with a second formulation, such as a coloring or highlighting formulation. In these embodiments, the liquid crosslinking formulation may contain any suitable concentration of crosslinking agent in a suitable carrier, typically a diluent, such as described above. The concentration of the crosslinking agent is suitable to provide a mixture with the appropriate final volume and final concentration of crosslinking agent. 
         [0155]    For example, a liquid crosslinking formulation can contain a concentration of crosslinking agent ranging from about 5% (wt) to about 50% (wt) or greater. In a preferred embodiment, the liquid crosslinking formulation contains about 20% (wt) crosslinking agent. 
         [0156]    For highlighting applications, prior to use, a sufficient volume of a liquid crosslinking formulation is mixed with a sufficient volume of a highlighting formulation to form a highlighting mixture having the desired concentration of crosslinking agent. Typical concentrations of the crosslinking agent in the highlighting mixture range from small amounts, such as approximately at least 0.01% (wt), preferably at least 0.1% (wt), to large amounts, such as up to 50% (wt). Preferably the highlighting mixture contains the crosslinking agent in a concentration ranging from 0.1% (wt) to 5% (wt), more preferably from 0.1% wt to 3% (wt). While greater concentrations of crosslinking agent could be present in the highlighting mixture, they are generally not needed to achieve the desired results. 
       III. Kit 
       [0157]    Kits for treating hair typically contain a crosslinking formulation containing an effective amount of a crosslinking agent to covalently crosslink latent reduced thiol groups in hair. 
         [0158]    Instructions for use of the kit are also typically provided. 
         [0159]    The kit may further contain a formulation, also referred to herein as the reducing formulation, capable of reducing the disulfide bonds in the hair and producing reduced thiol groups. 
         [0160]    a. Reducing Formulation 
         [0161]    A reducing formulation contains a reducing agent capable of reducing the disulfide bonds in hair and producing reduced thiol groups. The reducing formulation may differ depending on the hair styling treatment desired (such as hair waving or hair straightening), the texture of the hair, the sensitivity of the user&#39;s skin, and the like. 
         [0162]    Formulations containing reducing agents and their selection are well known to those skilled in the cosmetic industry. Suitable reducing agents include, but are no limited to, thioglycolic acid and thioglycolic acid salts and esters, thiolactic acid and thiolactic acid salts and esters, cysteine thioglycerol, thioglycolic hydrazide, thioglycolamide, glycerol monothioglycolate, sodium metabisulfite, beta-mercaptopropionic acid, N-hydroxyethyl mercapto-acetamide, N-methyl mercapto-acetamide, beta-mercapto-ethylamine, beta-mercaptopropionamide, 2-mercapto-ethanesulfonic acid, dimercaptoadipic acid, dithiothreitol, homocysteinethiolactone, cysteine derivatives, polythiol derivatives formed by the addition of cysteamine onto a maleic anhydride-alkylvinylether copolymer, inorganic sulfites, inorganic bisulfites, cysteamine and its derivatives, dithioerythritol, organic phosphines, and Japanese relaxers. 
         [0163]    In some embodiments, the kit contains a reducing formulation, which contains a reducing agent for permanent hair waving and hair curling such as acid perms, alkaline perms, perms having neutral pH, or perms using buffered alkaline waving lotions. Such reducing agents include, but are not limited to thioglycolic acid and its derivative salts and esters, thiolactic acid and its derivative salts and esters, cysteine and its derivatives, cysteamine and its derivatives, inorganic sulfites, and inorganic bisulfites such as sodium metabisulfite, dithiothreitol, dithioerythritol, organic phosphines, and Japanese relaxers. 
         [0164]    In other embodiments, the kit contains a reducing formulation, which contains a reducing agent for straightening hair. Such reducing agents include, but are not limited, to inorganic bisulfites such as sodium metabisulfite, inorganic sulfites, and ammonium thioglycolate, dithiothreitol, dithioerythritol, organic phosphines, and Japanese relaxers. 
         [0165]    The amount of the reducing agent in the reducing formulation is sufficient to rupture a sufficient number of disulfide bonds for effective hair waving, hair curling, or hair straightening as would be appreciated by one of skill in the art. 
         [0166]    b. Crosslinking Formulation 
         [0167]    The crosslinking formulation contains an effective amount of a crosslinking agent to crosslink reduced thiols in the hair. Suitable formulations containing the crosslinking agents are discussed above. The crosslinking formulation may be in any suitable form. Suitable forms include, but are not limited to, low to moderate viscosity liquids, lotions, milks, mousses, sprays, gels, creams, shampoos, conditioners, and the like. The crosslinking formulation will be present in a suitable container, which depends on the form of the formulation. 
         [0168]    In one embodiment, the crosslinking formulation is provided as two or more separate ingredients. For example, the crosslinking agent may be provided as a dry powder in a sealed package and the excipient provided in a vial or other container. A suitable mixing container for the crosslinking agent and the excipient may be provided. 
         [0169]    Optionally, the crosslinking agent is premixed with a shampoo or conditioner. 
         [0170]    c. Other Materials in the Kit 
         [0171]    The kit optionally contains shampoos and conditioners. Suitable shampoos and conditioners include, but are not limited to LiQWd® Hydrating Shampoo and LiQWd® Hydrating Conditioner. 
         [0172]    The kit may further contain an odor eliminator. The odor eliminator can be incorporated into the reducing formulation. Alternately, the odor eliminator is present in a suitable container for use before or after washing the crosslinking formulation from the hair. Some suitable odor eliminators are known to those of ordinary skill in the art. 
       IV. Methods of Use 
       [0173]    The methods disclosed herein are concerned with treating hair with reduced thiol groups. 
         [0174]    A. Treating Damaged Hair with Reduced Thiol Groups 
         [0175]    In one embodiment, prior to treatment with a crosslinking agent, the hair has been damaged and the thiol groups in the hair are reduced. The crosslinking agent can be applied to the hair to crosslink the reduced thiol groups. Preferably, the crosslinking agent is applied at least within one week of the hair being damaged, preferably within three days, more preferably within two days, most preferably, the same day. 
         [0176]    a. Rinse or Wash the Hair 
         [0177]    Optionally, the hair may be shampooed and/or conditioned prior to applying the crosslinking formulation. Alternately, the hair may only be rinsed with water prior to application of the crosslinking formulation. 
         [0178]    b. Apply the crosslinking formulation to the hair 
         [0179]    Subsequent to shampooing, conditioning, and/or rinsing the hair, the crosslinking formulation is applied to the hair. Alternately, the hair does not have to be washed or rinsed prior to application of the crosslinking formulation. In this embodiment, the crosslinking formulation is applied to dry hair. 
         [0180]    The crosslinking formulations may be used as a daily conditioning treatment for hair. 
         [0181]    Typically, the amount of crosslinking formulation applied is sufficient to saturate the hair. 
         [0182]    The crosslinking formulation may be applied to the hair as a single application, or application of the crosslinking agent may be repeated one or more times. Typically, the amount of crosslinking formulation applied in each application is sufficient to saturate the hair. The volume of crosslinking formulation applied to the hair in each application may be about 1 to about 100 mL per person depending on their length and volume of hair. In some embodiments, application of the crosslinking agent could be repeated immediately (e.g. within about 10 to 15 seconds) or between about one and five minutes, greater than five minutes, between about five and ten minutes, greater than ten minutes, between about ten and twenty (20) minutes after the first application. 
         [0183]    c. Remove the Crosslinking Formulation from the Hair 
         [0184]    Preferably, the hair is washed or rinsed subsequent to the application of the crosslinking formulation. The hair may be rinsed and subsequently washed immediately (e.g. within 10, 15, 25, 30, 45, 60 seconds (one minute), two minutes, three minutes, four, or five minutes following application) after final application of the crosslinking agent. Alternatively the hair may be rinsed and washed about within about 30 minutes following application, preferably between about 5 minutes and about 20 minutes, more preferably about 10 minutes after the final application of the crosslinking agent to the hair, depending on the hair type. 
         [0185]    Alternately, the hair does not have to be washed or rinsed subsequent to application of the crosslinking formulation. 
         [0186]    The crosslinking agent covalently crosslinks latent reduced thiols in the hair. The thiols remain crosslinked for at least one week, preferably for at least one month following application of the crosslinking agent. The thiols may remain crosslinked for longer periods of time, such as for about three months or greater than three months, such as for at least one year following application of the crosslinking agent. The crosslinking reaction is a stable reaction, such that the thiols remain crosslinked even if subjected to a hair coloring treatment (simultaneous or subsequent to the crosslinking reaction). 
         [0187]    B. Chemical Treatment of Hair with a Reducing Agent 
         [0188]    In one embodiment, prior to treatment with a crosslinking agent, the hair has been subjected to a reducing agent used for waving (also referred to herein as hair penning or permanent waves), curling, and/or straightening of the hair. 
         [0189]    a. Apply a Reducing Agent to the Hair 
         [0190]    The first step in waving, curling, or straightening hair is breaking the cystine disulfide bonds to form reduced thiol moieties. The process for breaking the cystine disulfide bonds is via application of a reducing agent. The process for applying the reducing agent involves following normal perming or hair straightening procedures, that are known to those skilled in the art. For example, to perm a hair, the hair is first washed and set on perm rods of various sizes. Second, a reducing agent, such as thioglycolate reducing solution or lotion is applied to the hair. The hair is allowed to set for a specified period of time, and then the thioglycolate solution is rinsed from the hair. 
         [0191]    The application of hydrogen peroxide in this process is optional. In some processes, such as when treating previously chemically treated hair, hydrogen peroxide is generally not used. In other processes, such as when penning virgin hair, hydrogen peroxide may be added. In these embodiments, hydrogen peroxide is typically added after the reducing agent is rinsed out. Then the hydrogen peroxide is rinsed from the hair prior to adding the crosslinking agent. 
         [0192]    b. Apply the Crosslinking Agent 
         [0193]    Subsequent to the reducing treatment, one or more of the crosslinking agent, or a formulation thereof is applied to the hair. Although the crosslinking agent is typically applied on the same day as treatment with the reducing agent, it may be applied later such as within 1 to 2 weeks following treatment with the reducing agent. 
         [0194]    Typically, the amount of crosslinking formulation applied is sufficient to saturate the hair. The crosslinking agent is generally rinsed and shampooed from the hair after the desired level of hair waving, curling, or straightening is achieved. In some embodiments, the crosslinking agent is rinsed from the hair immediately (e.g. within 10, 15, 25, 30, 45, or 60 seconds following application) following the final application of the crosslinking agent. Alternatively the hair may be rinsed and washed about within about 30 minutes following application, preferably between about 5 minutes and about 20 minutes, more preferably about 10 minutes after the final application of the crosslinking agent to the hair, depending on the hair type. The crosslinking agent can be rinsed from the hair within 10, 15, 25, 30, 45, 60 seconds from the hair after application, and still achieve a desired level of hair waving, curling, or straightening. 
         [0195]    The crosslinking agent may be applied to the hair as a single application, or application of the crosslinking agent may be repeated one or more times. Typically, the amount of crosslinking formulation applied in each application is sufficient to saturate the hair. In some embodiments, the volume of crosslinking formulation applied to the hair in each application is about 1 to about 10 mL per perm rod. In some embodiments, application of the crosslinking agent could be repeated immediately (e.g. within 10 to 15 seconds) or approximately 1, 5, 7.5, 10, 12.5, 15, 17.5, or 20 minutes after the first application. In some embodiments, the second application is about 7 minutes to about 10 minutes after the first application. 
         [0196]    The crosslinking agent is rinsed from the hair after its application. The hair may be rinsed and washed immediately (e.g. within 10 to 15 seconds following application) after final application of the crosslinking agent. Alternatively the hair may be rinsed and washed about 10 minutes or later after the final application of the crosslinking agent, such as about 15 minutes to about 30 minutes, preferably about 20 minutes after repeated application of the crosslinking agent to the hair. 
         [0197]    The crosslinking agent covalently crosslinks the reduced thiols in the hair. The thiols remain crosslinked for at least one week, two weeks, three weeks, four weeks, one month, two months, three months, four months, five months, six months, seven months, eight months, nine months, ten months, eleven months, or one year. 
         [0198]    The crosslinking agents are generally washed from the individual&#39;s hair on the same day as they are applied. In contrast, traditional perms which use only hydrogen peroxide (and do not involve the addition of a cross-linking agent) are generally not washed for at least 48 hours following application (washing the hair prior to 48 hours following a traditional permanent treatment may result in significant loss in the amount of curl in the hair and/or cause damage to the hair). 
         [0199]    The compositions described herein improve hair quality, such as appearance (e.g., sheen) and feel, increase dry strength (e.g., tensile strength), and decrease hair breakage when the hair is subjected to subsequent treatments, such as coloring. In some embodiments, the tensile strength increases 5, 10, 15, 20, 25, 30, 35, 40, 45, or 50% or higher compared to untreated hair from the same individual. Tensile strength of hair can be tested using known techniques in the art. For example, an apparatus for measuring the tensile strength of hair is described in U.S. Pat. No. 4,628,742. Instron also described techniques and apparatus for measuring the tensile strength of hair. 
         [0200]    In other embodiments, hair breakage decreases by 5, 10, 15, 20, 25, 30, 35, 40, 45, or 50% or higher after crosslinking compared to untreated hair from the same individual. Hair breakage is a significant problem encountered during coloring and other treatments. 
       EXAMPLES 
     Example 1 
     Comparison of Traditional Perm Versus Perm Using Bismaleimide Crosslinking Agent 
       [0201]    General 
         [0202]    Hair samples were obtained from a human subject and cut in ½ inch wide wefts. 
         [0203]    Reducing Agents: 
         [0204]    Ammonium thioglycolate (ATG) was obtained from a permanent wave kit manufactured by Zotos. 300 mg of Dithiothreitol in a 10 g solution was also used as the reducing agent. 
         [0205]    Crosslinking Formulation: 
         [0206]    A bismaleimide crosslinking agent (bis-(maleimidoethoxy) ethane) at a concentration of 300 mg in 10 g total solution (water) was used. 
         [0207]    Methods 
         [0208]    Method for Penning Hair Using the Crosslinking Agents 
         [0209]    The hair was washed with clarifying shampoo, towel dried, and then rolled around a perm rod. Ammonium thioglycolate or dithiothreitol was then applied to the hair and left on the hair for 10 minutes to 1 hour. The hair was then rinsed for 30 seconds to 1 minute and then blotted dry with a towel. 
         [0210]    The crosslinking formulation was applied to the hair, via a needle nose applicator, drenching the hair. The crosslinking agent was left on the hair for a period of about 7.5 minutes. The hair was drenched for a second time with the crosslinking formulation and left for a second 7.5 minutes, for a total of 15 minutes. The hair was then rinsed with water for about 1-2 minutes then unrolled from the perm rods. After the hair was removed from the perm rods, the hair was shampooed and conditioned with various salon shampoo and conditioner brands, including LiQWd® Hydrating Shampoo and Hydrating Conditioner. The washing and drying steps were repeated 40 times. 
         [0211]    A second portion of hair was permed as described above, except, hydrogen peroxide was used instead of the crosslinking formulation. 
         [0212]    Results 
         [0213]    Both perms (utilizing the crosslinking formulation or hydrogen peroxide) showed only slight reduction in the overall curl after 40 cycles of washing and drying with the same shampoo and conditioner. However, the appearance and texture of the perm using the crosslinking formulation showed more sheen and less frizz compared to the perm using hydrogen peroxide. 
       Example 2 
     Comparison of Hair Breakage Due to Repeated Application of Traditional Perm and the Crosslinking Formulations 
       [0214]    Methods 
         [0215]    Two hair samples were obtained. Both samples were treated with dithiothreitol or ammonium thioglycolate as described in Example 1. One of the hair samples was subsequently treated with the crosslinking formulation, while the other was neutralized with hydrogen peroxide. The process was completed the same day for the hair treated with the crosslinking formulation. The process was completed in three days with hydrogen peroxide (traditional perm). 
         [0216]    The procedure was repeated three times for each hair sample over a 48 hour time period. 
         [0217]    Results 
         [0218]    Upon visual inspections, the second hair sample treated with the crosslinking formulation showed little or no signs of breakage. However, the first hair sample treated with hydrogen peroxide showed significant breakage. 
       Example 3 
     Comparison of the Extent of Damage to Hair Previously Relaxed with a Japanese Relaxer 
       [0219]    Methods 
         [0220]    Two samples of hair, the first previously straightened with a Japanese relaxer (Yuko), and the second previously straightened with a no lye relaxer (African Pride Miracle Deep Conditioning) were obtained. The samples were treated as described in Examples 1 and 2 using the crosslinking formulation. 
         [0221]    Another hair sample, previously straightened with a no lye relaxer (African Pride Miracle Deep Conditioning) was obtained. The sample was treated with a traditional hair straightening perm (Zotos). 
         [0222]    Results 
         [0223]    The hair samples treated with the crosslinking formulation showed no noticeable damage. However, the sample treated with a traditional perm showed significant breaking, even during application. 
       Example 4 
     Hair Sheen and Texture after Treatment with Crosslinking Formulation 
       [0224]    General 
         [0225]    A sample of untreated virgin gray hair was obtained from a human subject. 
         [0226]    Crosslinking Formulation: 
         [0227]    The bismaleimide crosslinking agent (300 mg) was dissolved in water (10 g). The resulting solution was mixed with LiQWD Volumizing Conditioner® in a 1:1 ratio. 
         [0228]    Methods 
         [0229]    A section of the virgin gray hair was washed with LiQWD® Hydrating Shampoo and then blotted dry with a towel. The hair was then combed with a wide tooth comb followed by combing with a fine tooth comb for 2 minutes. 
         [0230]    After combing, the crosslinking formulation (about 4 mL) was applied to the hair sample by hand and then the sample combed through for approximately 1 minute. The hair sample was left undisturbed for a period of about 10 minutes, after which it was rinsed with water, and then washed with LiQWD® Volumizing Shampoo and Conditioner before being examined. 
         [0231]    The hair sample was washed and conditioned for an additional five (5) times with LiQWD® Volumizing Shampoo and Conditioner. 
         [0232]    A second section of the virgin gray hair, the control, was treated identically as above, except the crosslinking formulation was not applied to the control hair sample. Thus after the hair was combed, LiQWD Volumizing Conditioner® (without a crosslinking agent) was applied to the hair sample by hand. 
         [0233]    Results: 
         [0234]    The hair sample treated with the crosslinking formulation had more shine and felt softer to the touch that the original untreated sample. The treated hair sample gave an overall healthier appearance compared to the control sample. 
         [0235]    The shine, texture, and overall appearance remained intact after five shampoo and conditioning treatments. 
       Example 5 
     Hair Sheen and Texture after Treatment with Crosslinking Formulation 
       [0236]    General 
         [0237]    A sample of untreated virgin blonde hair described as highly porous and difficult to comb through was obtained from a human subject. 
         [0238]    Crosslinking formulation: The bismaleimide crosslinking agent (300 mg) was dissolved in water (10 g). The resulting solution was mixed with LiQWD Enhancing Conditioner® in a 1:1 ratio. 
         [0239]    Methods 
         [0240]    A section of the virgin blonde hair was washed with LiQWD® Hydrating Shampoo and then blotted dry with a towel. The hair was then combed with a wide tooth comb followed by combing with a fine tooth comb for 5 minutes. 
         [0241]    The crosslinking formulation (about 7 mL) was then applied to the hair sample by hand and the sample combed through for approximately 2 minutes. The hair sample was left undisturbed for a period of about 5 minutes after which the hair was treated again with the crosslinking formulation (about 4 mL). The hair sample was combed through for approximately 10 seconds and left undisturbed for about 5 minutes. 
         [0242]    The hair sample was then rinsed with water then washed with LiQWD® Sulfate Free Enhancing Shampoo and Conditioner before examination. 
         [0243]    Following initial examination, the sample was washed and conditioned for an additional two (2) times with LiQWD® Sulfate Free Enhancing Shampoo and Conditioner. 
         [0244]    A second section of the virgin blonde hair, the control, was treated identically as above, except the crosslinking formulation was not applied to the control hair sample. Thus, after the hair was combed, LiQWD Volumizing Conditioner® (without a crosslinking agent) was applied to the hair sample by hand. 
         [0245]    Results: 
         [0246]    The hair sample treated with the crosslinking formulation had more shine and felt softer to the touch than the original untreated sample. The treated hair sample gave an overall healthier appearance compared to the control sample. 
         [0247]    The shine, texture, and overall appearance remained intact after two shampoo and conditioning treatments. 
         [0248]    Unless defined otherwise, all technical and scientific terms used herein have the same meanings as commonly understood by one of skill in the art to which the disclosed invention belongs. Publications cited herein and the materials for which they are cited are specifically incorporated by reference. 
         [0249]    Those skilled in the art will recognize, or be able to ascertain using no more than routine experimentation, many equivalents to the specific embodiments of the invention described herein. Such equivalents are intended to be encompassed by the following claims.