Abstract:
Described herein is the synthesis of adhesive complex coacervates. The adhesive complex coacervates are composed of a mixture of one or more polycations, one or more polyanions, and one of more multivalent cations. The polycations and polyanions in the adhesice complex coacervate are crosslinked with one another by covalent bonds upon curing. The adhesive complex coacervates have several desirable features when compared to conventional bioadhesives, which are effective in water-based applicatgions. The adhesive complex coacervates described herein exhibit good interfacial tension in water when applied to a substrate (i.e., they spread over the interface rather than being beaded up). Additionally, the ability of the complex coacervate to crosslink intermolecularly increases the cohesive strength of the adhesive complex coacervate. The adhesive complex coacervates have numerous biological applications as bioadhesives and drug delivery devices. In particular, the adhesive complex coacervates described herein are particularly useful in underwater applications and situations where water is present such as, for example, physiological conditions.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority upon U.S. provisional application Ser. No. 61/023,173, filed Jan. 24, 2008. This application is hereby incorporated by reference in its entirety for all of its teachings. 
    
    
     BACKGROUND 
     Bone fractures are a serious health concern in society today. In addition to the fracture itself, a number of additional health risks are associated with the fracture. For example, intra-articular fractures are bony injuries that extend into a joint surface and fragment the cartilage surface. Fractures of the cartilage surface often lead to debilitating posttraumatic arthritis. The main determining factors in the development of posttraumatic arthritis are thought to be the amount of energy imparted at the time of injury, the patient&#39;s genetic predisposition (or lack thereof) to posttraumatic arthritis, and the accuracy and maintenance of reduction. Of the three prognostic factors, the only factor controllable by orthopedic caregivers is achievement and maintenance of reduction. Comminuted injuries of the articular surface (the cartilage) and the metaphysis (the portion of the bone immediately below the cartilage) are particularly challenging to maintain in reduced (aligned) position. This relates to the quality and type of bone in this area. It also relates to the limitations of fixation with titanium or stainless steel implants. 
     Currently, stainless steel and titanium implants are the primary methods of fixation, but their size and the drilling necessary to place them frequently interfere with the exact manipulation and reduction of smaller pieces of bone and cartilage. A variety of bone adhesives have been tested as alternatives to mechanical fixation. These fall into four categories: polymethylmethacrylates (PMMA), fibrin-based glues, calcium phosphate (CP) cements, and CP resin composites. PMMA cements, which are used in the fixation of protheses, have well-known drawbacks, one of the most serious being that the heat generated from the exothermic setting reaction can kill adjacent bone tissue. Also, the poor bonding to bone leads to aseptic loosening, the major cause of PMMA cemented prothesis failure. 
     Fibrin glues, based on the blood clotting protein fibrinogen, have been tested for fixing bone grafts and repairing cartilage since the 1970s and yet have not been widely deployed. One of the drawbacks of fibrin glues is that they are manufactured from pooled human donor blood. As such, they carry risk of transmitting infections and could potentially be of limited supply. 
     CP cements are powders of one or more forms of CP, e.g., tetracalcium phosphate, dicalcium phosphate anhydride, and β-tricalcium phosphate. When the powder is mixed with water it forms a paste that sets up and hardens through the entanglement of one or more forms of CP crystals, including hydroxyapatite. Advantages of CP cements include isothermal set, proven biocompatibility, osteoconductivity, and they serve as a reservoir for Ca and PO 4  for hydroxyapatite formation during healing. The primary disadvantages are that CP cements are brittle, have low mechanical strength and are therefore not ideal for stable reduction of small articular segments. CP cements are used mostly as bone void fillers. The poor mechanical properties of CP cements have led to composite cements of CP particles and polymers. By varying the volume fractions of the particulate phase and the polymer phase, the modulus and strength of the glue can be adjusted toward those of natural bone, an avenue that is also open to us. 
     Given the overall health impact associated with bone fractures and the imperfect state of current fixation methods, new fixation methods are needed. 
     SUMMARY 
     Described herein is the synthesis of adhesive complex coacervates. The adhesive complex coacervates are composed of a mixture of one or more polycations, one or more polyanions, and one of more multivalent cations. The polycations and polyanions are crosslinked with one another by covalent bonds upon curing. The adhesive complex coacervates have several desirable features when compared to conventional adhesives, which are effective in water-based applications. The adhesive complex coacervates described herein exhibit low interfacial tension in water when applied to a substrate (i.e., they spread over the interface rather than being beaded up). Additionally, the ability of the complex coacervate to crosslink intermolecularly increases the cohesive strength of the adhesive complex coacervate. The adhesive complex coacervates have numerous biological applications as bioadhesives and drug delivery devices. In particular, the adhesive complex coacervates described herein are particularly useful in underwater applications and situations where water is present such as, for example, physiological conditions. 
     The advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the aspects described below. The advantages described below will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate several aspects described below. 
         FIG. 1  shows a model of pH dependent coacervate structure and adhesive mechanisms. (A) The polyphosphate (black) with low charge density paired with the polyamine (red) form nm-scale complexes. The complexes have a net positive charge. (B) Extended high charge density polyphosphates form a network connected by more compact lower charge density polyamines and when present divalent cations (green symbols). The net charge on the copolymers is negative. (C) Oxidation of 3,4-dihydroxyphenol (D) by O 2  or an added oxidant initiates crosslinking between the quinone (Q) and primary amine sidechains. The coacervate can adhere to the hydroxyapatite surface through electrostatic interactions, 3,4-dihydroxyphenol sidechains, and quinone-mediated covalent coupling to matrix proteins. 
         FIGS. 2-7  shows several protein sequences produced by  P. californica  that can be used as polycations and polyanions in the present invention as well as synthetic polycations and polyanions useful in the present invention. 
         FIG. 8  shows different mechanisms of DOPA crosslinking. 
         FIG. 9  shows dual syringe systems for applying small “spot welds” of complex coacervates described herein to repair fractures (A), small bone injuries (B), or bonding synthetic scaffolds to bony tissue (C). 
         FIG. 10  shows the structure and UV/VIS characterization of mimetic copolymers. (A) The Pc3 analog, 1, contained 88.4 mol % phosphate, 9.7 mol % dopamide, and 0.1 mol % FITC sidechains. The Pc1 analog, 2, contained 8.1 mol % amine sidechains. The balance was acrylamide subunits in both cases. (B) A single peak at 280 nm characteristic of the catechol form of 3,4-dihydroxyphenol was present in the spectrum of 1. Following oxidation with NaIO 4  a peak at 395 nm corresponding to the quinone form appeared confirming the expected redox behavior of the 3,4-dihydroxyphenol containing polymer. 
         FIG. 11  shows the pH dependent complex coacervation of mixed polyelectrolytes. (A) At low pH, a 50 mg/ml mixture of 1 and 2 having equal quantities of amine and phosphate sidechains formed stable colloidal PECs. As the pH increased the polymers condensed into a dense liquid complex coacervate phase. At pH 10 the copolymers went into solution and oxidatively crosslinked into a clear hydrogel. (B) The net charge of the copolymer sidechains as a function of pH calculated from the copolymer sidechain densities. (C) The diameter of the PECs (circles) increased nearly three-fold over the pH range 2-4. Above pH 4 the complexes flocculate and their size could not be measured. The zeta potential (squares) was zero near pH 3.6 in agreement with the calculated net charge. 
         FIG. 12  shows the liquid character of an adhesive complex coacervate. The solution of 1 and 2 contained equal quantities of amine and phosphate sidechains, pH 7.4. 
         FIG. 13  shows the phase diagram of polyelectrolytes and divalent cations. The amine to phosphate sidechain and phosphate sidechain to divalent cation ratios were varied at a fixed pH 8.2. The state of the solutions represented in a gray scale. The mass (mg) of the coacervate phase is indicated in the dark grey squares. The compositions indicated with an asterisk were used to test bond strength. 
         FIG. 14  shows the bond strength, shear modulus, and dimensional stability of coacervate bonded bones. (A) Bond strength at failure increased ˜50% and the stiffness doubled as the divalent cation ratio went from 0 to 0.4 relative to phosphate sidechains. Specimens wet bonded with a commercial cyanoacrylate adhesive were used as a reference. (n=6 for all conditions) (B) Bonds of adhered bone specimens fully submerged in PBS for four months (pH 7.2) did not swell appreciably. 
         FIG. 15  shows UV-vis spectra of dopamine copolymers before and after oxidation (pH 7.2). A catechol peak present before oxidation was converted into the quinone form. Top left: p(DMA[8]-Aam[92]). Bottom left: p(AEMA[30]-DMA[8]). Right: Hydrogel formation by oxidative crosslinking of dopamine copolymers. (A) p(DMA[8]-Aam[92]). (B) p(EGMP[92]-DMA[8]). (C) p(DMA[8]-Aam[92]) mixed with p(AEMA[30]-Aam[70]). (D) p(EGMP[92]-DMA[8]) mixed with p(AEMA[30]-Aam[70]). Bracketed numbers indicate mol % of sidechains. Arrows indicate direction spectra are changing over time. 
         FIG. 16  shows pH dependence of dopamine oxidation in poly(EGMP[92]-DMA[8]). Arrows indicate direction spectra change with time. Top: pH 5.0, time course inset. Bottom: pH 6.0. 
         FIG. 17  shows direct contact of (A) human foreskin fibroblasts, (B) human tracheal fibroblasts, and (C) rat primary astrocytes with adhesive (red auto-fluorescent chunks, white asterisks). Cell morphology, fibronectin secretion, and motility are indistinguishable from cells growing in the absence of glue. Green=intermediate filament proteins. Red=secreted fibronection. Blue=DAPI stained nuclei. 
         FIG. 18  shows a multi-fragment rat calvarial defect model. (A) Generation of defect. (B) Fragmentation of bone cap. (C) Replacement of fragments in defect. (D) Application of bone glue. (E-F) Curing (darkening) of glue. Fragments are firmly fixed in E and F. 
         FIG. 19  shows the effect of pH and normalized net charge with respect to forming adhesive complex coacervates. 
     
    
    
     DETAILED DESCRIPTION 
     Before the present compounds, compositions, articles, devices, and/or methods are disclosed and described, it is to be understood that the aspects described below are not limited to specific compounds, synthetic methods, or uses as such may, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular aspects only and is not intended to be limiting. 
     In this specification and in the claims that follow, reference will be made to a number of terms that shall be defined to have the following meanings: 
     It must be noted that, as used in the specification and the appended claims, the singular forms “a,” “an” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a pharmaceutical carrier” includes mixtures of two or more such carriers, and the like. 
     “Optional” or “optionally” means that the subsequently described event or circumstance can or cannot occur, and that the description includes instances where the event or circumstance occurs and instances where it does not. For example, the phrase “optionally substituted lower alkyl” means that the lower alkyl group can or can not be substituted and that the description includes both unsubstituted lower alkyl and lower alkyl where there is substitution. 
     Ranges may be expressed herein as from “about” one particular value, and/or to “about” another particular value. When such a range is expressed, another aspect includes from the one particular value and/or to the other particular value. Similarly, when values are expressed as approximations, by use of the antecedent “about,” it will be understood that the particular value forms another aspect. It will be further understood that the endpoints of each of the ranges are significant both in relation to the other endpoint, and independently of the other endpoint. 
     References in the specification and concluding claims to parts by weight, of a particular element or component in a composition or article, denotes the weight relationship between the element or component and any other elements or components in the composition or article for which a part by weight is expressed. Thus, in a compound containing 2 parts by weight of component X and 5 parts by weight component Y, X and Y are present at a weight ratio of 2:5, and are present in such ratio regardless of whether additional components are contained in the compound. 
     A weight percent of a component, unless specifically stated to the contrary, is based on the total weight of the formulation or composition in which the component is included. 
     Variables such as R 1 , R 2 , R 3 , R 4 , R 5 , X, m, and n used throughout the application are the same variables as previously defined unless stated to the contrary. 
     The term “alkyl group” as used herein is a branched or unbranched saturated hydrocarbon group of 1 to 25 carbon atoms, such as methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, t-butyl, pentyl, hexyl, heptyl, octyl, decyl, tetradecyl, hexadecyl, eicosyl, tetracosyl and the like. Examples of longer chain alkyl groups include, but are not limited to, an oleate group or a palmitate group. A “lower alkyl” group is an alkyl group containing from one to six carbon atoms. 
     Any of the compounds described herein can be the pharmaceutically-acceptable salt. In one aspect, pharmaceutically-acceptable salts are prepared by treating the free acid with an appropriate amount of a pharmaceutically-acceptable base. Representative pharmaceutically-acceptable bases are ammonium hydroxide, sodium hydroxide, potassium hydroxide, lithium hydroxide, calcium hydroxide, magnesium hydroxide, ferrous hydroxide, zinc hydroxide, copper hydroxide, aluminum hydroxide, ferric hydroxide, isopropylamine, trimethylamine, diethylamine, triethylamine, tripropylamine, ethanolamine, 2-dimethylaminoethanol, 2-diethylaminoethanol, lysine, arginine, histidine, and the like. In one aspect, the reaction is conducted in water, alone or in combination with an inert, water-miscible organic solvent, at a temperature of from about 0° C. to about 100° C. such as at room temperature. In certain aspects where applicable, the molar ratio of the compounds described herein to base used are chosen to provide the ratio desired for any particular salts. For preparing, for example, the ammonium salts of the free acid starting material, the starting material can be treated with approximately one equivalent of pharmaceutically-acceptable base to yield a neutral salt. 
     In another aspect, if the compound possesses a basic group, it can be protonated with an acid such as, for example, HCl, HBr, or H 2 SO 4 , to produce the cationic salt. In one aspect, the reaction of the compound with the acid or base is conducted in water, alone or in combination with an inert, water-miscible organic solvent, at a temperature of from about 0° C. to about 100° C. such as at room temperature. In certain aspects where applicable, the molar ratio of the compounds described herein to base used are chosen to provide the ratio desired for any particular salts. For preparing, for example, the ammonium salts of the free acid starting material, the starting material can be treated with approximately one equivalent of pharmaceutically-acceptable base to yield a neutral salt. 
     Described herein are adhesive complex coacervates and their applications thereof. In general, the complexes are a mixture of cations and anions in balanced proportions to produce stable aqueous complexes at a desired pH. The adhesive complex coacervate comprises at least one polycation, at least one polyanion, and at least one multivalent cation, wherein at least one polycation or polyanion is a synthetic compound, and the polycation and/or polyanion are crosslinked with one another upon curing the complex coacervate. Each component of the coacervate and methods for making the same are described below. 
     The adhesive complex coacervate is an associative liquid with a dynamic structure in which the individual polymer components diffuse throughout the entire phase. Complex coacervates behave rheologically like viscous particle dispersions rather than a viscoelastic polymer solution. As described above, the adhesive complex coacervates exhibit low interfacial tension in water when applied to substrates either under water or that are wet. In other words, the complex coacervate spreads evenly over the interface rather than being beading up. Additionally, upon intermolecular crosslinking, the adhesive complex coacervate forms a strong, insoluble, cohesive material. 
     Conversely, polyelectrolyte complexes (PECs), which can be a precursor to the adhesive complex coacervates described herein, are small colloidal particles. For example, referring to  FIG. 11A , a solution of PECs at pH 3.1 and 4.2 exists as a milky solution of colloidal particles having a diameter of about 300 nm Upon raising the pH to 7.2 and 8.1, the PEC condenses into a liquid phase of concentrated polymers (the coacervate phase) and a dilute equilibrium phase. In this aspect, the PEC can be converted to an adhesive complex coacervate described herein. 
     An exemplary model of the differences in phase behavior between the polyelectrolyte complex and the adhesive complex coacervate is presented in  FIG. 1 . At low pH the oppositely charged polyelectrolytes associate electrostatically into nano-complexes with a net positive surface charge that stabilizes the suspension to produce PEC 1. With increasing pH the net charge of the complexes changes from positive to negative but remains near net neutrality. The PEC can form a loose precipitate phase, which can be converted to a complex coacervate 2 by raising the pH further ( FIG. 1 ). Thus, in certain aspects, the conversion of the PEC to complex coacervate can be “triggered” by adjusting the pH and/or the concentration of the multivalent cation. For example, the PEC can be produced at a pH of less than or equal to 4, and the pH of the PEC can be raised to greater than or equal to 7.0, from 7.0 to 9.0, or from 8.0 to 9.0 to convert the PEC to a complex coacervate. Subsequent crosslinking between the polycation and polyanions (e.g., oxidation and covalent crosslinking as shown in  FIG. 1C ) results in the formation of the adhesive complex coacervate described herein. 
     The polycations and polyanions contain groups that permit crosslinking between the two polymers upon curing to produce new covalent bonds and the adhesive complex coacervate described herein. The mechanism of crosslinking can vary depending upon the selection of the crosslinking groups. In one aspect, the crosslinking groups can be electrophiles and nucleophiles. For example, the polyanion can have one or more electrophilic groups, and the polycations can have one or more nucleophilic groups capable of reacting with the electrophilic groups to produce new covalent bonds. Examples of electrophilic groups include, but are not limited to, anhydride groups, esters, ketones, lactams (e.g., maleimides and succinimides), lactones, epoxide groups, isocyanate groups, and aldehydes. Examples of nucleophilic groups are presented below. 
     In one aspect, the crosslinkable group includes a hydroxyl-substituted aromatic group capable of undergoing oxidation in the presence of an oxidant. In one aspect, the hydroxyl-substituted aromatic group is a dihydroxyphenol or halogenated dihydroxyphenol group such as, for example, DOPA and catechol (3,4 dihydroxyphenol). For example, in the case of DOPA, it can be oxidized to dopaquinone. Dopaquinone is an electrophilic group that is capable of either reating with a neighboring DOPA group or another nucleophilic group. In the presence of an oxidant such as oxygen or other additives including, but not limited to, peroxides, periodates, or transition metal oxidants (e.g., NaIO 4  or a Fe +3  compound), the hydroxyl-substituted aromatic group can be oxidized. In another aspect, crosslinking can occur between the polycation and polyanion via light activated crosslinking through azido groups. Once again, new covalent bonds are formed during this type of crosslinking. 
     The stability of the oxidized crosslinker can vary. For example, the phosphono containing polyanions described herein that contain oxidizable crosslinkers are stable in solution and do not crosslink with themselves. This permits nucleophilic groups present on the polycation to react with the oxidized crosslinker. This is a desirable feature of the invention, which permits the formation of intermolecular bonds and, ultimately, the formation of a strong adhesive. Examples of nucleophilic groups that are useful include, but are not limited to, hydroxyl, thiol, and nitrogen containing groups such as substituted or unsubstituted amino groups and imidazole groups. For example, residues of lysine, histidine, and/or cysteine can be incorporated into the polycation and introduce nucleophilic groups. An example of this is shown in  FIG. 8 . DOPA residue 1 can be oxidized to form a dopaquinone residue 2. Dopaquinone is a reactive intermediate and can crosslink (i.e., react) with a DOPA residue on another polymer or the same polymer to produce a di-DOPA group. Alternatively, the dopaquinone residue can react with nucleophiles such as, for example, amino, hydroxyl, or thiol groups via a Michael-type addition to form a new covalent bond. Referring to  FIG. 8 , a lysyl group, cysteinyl group, and histidyl group react with the dopaquinone residue to produce new covalent bonds. Although DOPA is a suitable crosslinking group, other groups such as, for example, tyrosine can be used herein. The importance of crosslinking with respect to the use of the adhesive complex coacervates described herein will be discussed below. 
     In other aspects, the crosslinkers present on the polycation and/or polyanion can form coordination complexes with transition metal ions. For example, a transition metal ion can be added to a mixture of polycation and polyanion, where both polymers contain crosslinkers capable of coordinating with the transition metal ion. The rate of coordination and dissociation can be controlled by the selection of the crosslinker, the transition metal ion, and the pH. Thus, in addition to covalent crosslinking as described above, crosslinking can occur through electrostatic, ionic, or other non-covalent bonding. Transition metal ions such as, for example, iron, copper, vanadium, zinc, and nickel can be used herein. 
     The polycation and polyanion are generally composed of a polymer backbone with a plurality of chargeable groups at a particular pH. The groups can be pendant to the polymer backbone and/or incorporated within the polymer backbone. The polycation is any biocompatible polymer possessing cationic groups or groups that can be readily converted to cationic groups by adjusting the pH. In one aspect, the polycation is a polyamino compound. The amino group can be branched or part of the polymer backbone. The amino group can be a primary, secondary, or tertiary amino group that can be protonated to produce a cationic ammonium group at a selected pH. For example, the amino group can be derived from a residue of lysine, histidine, or imidazole attached to the polycation. Any anionic counterions can be used in association with the cationic polymers. The counterions should be physically and chemically compatible with the essential components of the composition and do not otherwise unduly impair product performance, stability or aesthetics. Non-limiting examples of such counterions include halides (e.g., chloride, fluoride, bromide, iodide), sulfate and methylsulfate. 
     The polycation can be a synthetic polymer or naturally-occurring (i.e., produced from organisms). In one aspect, when the polycation is naturally-occurring, the polycation is a positively-charged protein produced from  P. californica .  FIGS. 2-6  show the protein sequences of several cement proteins produced by  P. californica  (Zhao et al. “Cement Proteins of the tube building polychaete  Phragmatopoma californica” J. Biol. Chem . (2005) 280: 42938-42944). Table 1 provides the amino acid mole % of each protein. Referring to  FIGS. 2-5 , Pc1, Pc2, and Pc4-Pc8 are polycations, where the polymers are cationic at neutral pH. The type and number of amino acids present in the protein can vary in order to achieve the desired solution properties. For example, referring to Table 1, Pc1 is enriched with lysine (13.5 mole %) while Pc4 and Pc5 are enriched with histidine (12.6 and 11.3 mole %, respectively). 
     In the case when the polycation is a synthetic polymer, a variety of different polymers can be used; however, it is desirable that the polymer be biocompatible and non-toxic to cells and tissue. In one aspect, the polycation includes a polyacrylate having one or more pendant amino groups. For example, the backbone can be a homopolymer or copolymer derived from the polymerization of acrylate monomers including, but not limited to, acrylates, methacrylates, acrylamides, and the like. In one aspect, the backbone of the polycation is polyacrylamide. In other aspects, the polycation is a block co-polymer, where segments or portions of the co-polymer possess cationic groups depending upon the selection of the monomers used to produce the co-polymer. 
     In one aspect, the polycation is a polyamino compound. In another aspect, the polyamino compound has 10 to 90 mole % tertiary amino groups. In a further aspect, the polycation polymer has at least one fragment of the formula I 
                                
wherein R 1 , R 2 , and R 3  are, independently, hydrogen or an alkyl group, X is oxygen or NR 5 , where R 5  is hydrogen or an alkyl group, and m is from 1 to 10, or the pharmaceutically-acceptable salt thereof. In another aspect, R 1 , R 2 , and R 3  are methyl and m is 2. Referring to formula I, the polymer backbone is composed of —CH 2 —C(R 1 )—C(O)X—, which is a residue of an acrylate, methacrylate, acrylamide, or methacrylamide. The remaining portion of formula I (CH 2 ) m —NR 2 R 3  is the pendant amino group.  FIG. 3  (structures C and D) and  FIG. 6  ( 4  and  7 ) show examples of polycations having the fragment of formula I, where the polymer backbone is composed acrylamide and methacrylate residues. In one aspect, the polycation is the free radical polymerization product of a cationic tertiary amine monomer (2-dimethylamino-ethyl methacrylate) and acrylamide, where the molecular weight is from 10 to 20 kd and possesses tertiary monomer concentrations from 15 to 30 mol %.  FIG. 4  (structures E and F) and  FIG. 6  ( 5 ) provide examples of polycations useful herein, where imidazole groups are directly attached to the polymer backbone (structure F) or indirectly attached to the polymer backbone via a linker (structure E via a methylene linker).
 
     Similar to the polycation, the polyanion can be a synthetic polymer or naturally-occurring. In one aspect, when the polyanion is naturally-occurring, the polyanion is a negatively-charged protein produced from  P. californica .  FIGS. 2 and 7  show the sequences of two proteins (Pc3a and Pc3b) produced by  P. californica  (Zhao et al. “Cement Proteins of the tube building polychaete  Phragmatopoma californica” J. Biol. Chem . (2005) 280: 42938-42944). Referring to Table 1, Pc3a and Pc3b are essentially composed of polyphosphoserine, which is anionic at neutral pH. 
     When the polyanion is a synthetic polymer, it is generally any biocompatible polymer possessing anionic groups or groups that can be readily converted to anionic groups by adjusting the pH. Examples of groups that can be converted to anionic groups include, but are not limited to, carboxylate, sulfonate, phosphonate, boronate, sulfate, borate, or phosphate. Any cationic counterions can be used in association with the anionic polymers if the considerations discussed above are met. 
     In one aspect, the polyanion is a polyphosphate. In another aspect, the polyanion is a polyphosphate compound having from 10 to 90 mole % phosphate groups. In a further aspect, the polyanion includes a polyacrylate having one or more pendant phosphate groups. For example, the backbone can be a homopolymer or copolymer derived from the polymerization of acrylate monomers including, but not limited to, acrylates, methacrylates, acrylamides, and the like. In one aspect, the backbone of the polyanion is polyacrylamide. In other aspects, the polyanion is a block co-polymer, where segments or portions of the co-polymer possess anionic groups depending upon the selection of the monomers used to produce the co-polymer. In a further aspect, the polyanion can be heparin sulfate, hyaluronic acid, chitosan, and other biocompatible and biodegradable polymers typically used in the art. 
     In one aspect, the polyanion is a polyphosphate. In another aspect, the polyanion is a polymer having at least one fragment having the formula II 
                                
wherein R 4  is hydrogen or an alkyl group, and n is from 1 to 10, or the pharmaceutically-acceptable salt thereof. In another aspect, wherein R 4  is methyl and n is 2. Similar to formula I, the polymer backbone of formula II is composed of a residue of an acrylate or methacrylate. The remaining portion of formula II is the pendant phosphate group.  FIG. 7  (structure B), shows an example of a polyanion useful herein that has the fragment of formula II, where the polymer backbone is composed acrylamide and methacrylate residues. In one aspect, the polyanion is the polymerization product ethylene glycol methacrylate phosphate and acrylamide, where the molecular weight is from 10,000 to 50,000, preferably 30,000, and has phosphate groups in the amount of 45 to 90 mol %.
 
     As described above, the polycation and polyanion contain crosslinkable groups. For example, the polyanion can include one or more groups that can undergo oxidation, and the polycation contains on or more nucleophiles that can react with the oxidized crosslinker to produce new covalent bonds. Polymers 3 and 7 in  FIG. 6  provide examples of DOPA residues incorporated into a polyanion and polycation, respectively. In each of these polymers, an acrylate containing the pendant DOPA residue is polymerized with the appropriate monomers to produce the polyanion 3 and polycation 7 with pendant DOPA residues. 
     It is contemplated that the polycation can be a naturally occurring compound (e.g., protein from  P. californica ) and the polyanion is a synthetic compound. In another aspect, the polycation can be a synthetic compound and the polyanion is a naturally occurring compound (e.g., protein from  P. californica ). In a further aspect, both the polyanion and polycation are synthetic compounds. 
     The adhesive complex coacervates also contain one or more multivalent cations (i.e., cations having a charge of +2 or greater). In one aspect, the multivalent cation can be a divalent cation composed of one or more alkaline earth metals. For example, the divalent cation can be a mixture of Ca +2  and Mg +2 . In other aspects, transition metal ions with a charge of +2 or greater can be used as the multivalent cation. In addition to the pH, the concentration of the multivalent cations can determine the rate and extent of coacervate formation. Not wishing to be bound by theory, weak cohesive forces between particles in the fluid may be mediated by multivalent cations bridging excess negative surface charges. The amount of multivalent cation used herein can vary. In one aspect, the amount is based upon the number of anionic groups and cationic groups present in the polyanion and polycation. In the Examples, the selection of the amount of multivalent cations with respect to producing adhesive complex coacervates and other physical states is addressed. 
     The adhesive complex coacervate can be synthesized a number of different ways. In one aspect, the polycation, the polyanion, and at least one multivalent cation, can be mixed with one another to produce the adhesive complex coacervate. By adding the appropriate amount of multivalent cation to the mixture of polyanion and polycation, the adhesive complex coacervate can be produced. In another aspect, the adhesive complex coacervate can be produced by the process comprising: 
     (a) preparing a polyelectrolyte complex comprising admixing at least one polycation, at least one polyanion, and at least one multivalent cation, wherein at least one polycation or polyanion is a synthetic compound, and the polycation and/or polyanion comprises at least one group capable of crosslinking with each other; and
 
(b) adjusting the pH of the polyelectrolyte complex, the concentration of at least one multivalent cation, or a combination thereof to produce the adhesive complex coacervate.
 
In this aspect, the polyelectrolyte complex is converted to the adhesive complex coacervate. Methods for producing the adhesive complex coacervate in situ are described below.
 
     The adhesive complex coacervates described herein have numerous benefits with respect to their use as biological cements and delivery devices. For example, the coacervates have low initial viscosity, specific gravity greater than one, and being mostly water by weight, low interfacial tension in an aqueous environment, all of which contribute to their ability to adhere to a wet surface. An additional advantage with respect to the bonding mechanism (i.e., crosslinking) of the adhesive complex coacervates includes low heat production during setting, which prevents damage to living tissue. The components can be pre-polymerized in order to avoid heat generation by in situ exothermic polymerization. This is due for the most part by the ability of the adhesive complex coacervates to crosslink intermolecularly under very mild conditions as described above. 
     The adhesive complex coacervates described herein can be applied to a number of different biological substrates. The substrate can be contacted in vitro or in vivo. The rate of crosslinking within the adhesive complex coacervate can be controlled by for example pH and the presence of an oxidant or other agents that facilitate crosslinking. One approach for applying the adhesive complex coacervate to the substrate can be found in  FIG. 9 . The techniques depicted in  FIG. 9  are referred to herein as “spot welding,” where the adhesive complex coacervate is applied at distinct and specific regions of the substrate. In one aspect, the adhesive complex coacervate can be produced in situ. Referring to  FIG. 9A , a pre-formed stable PEC solution 1 composed of polycations and polyanions at low pH (e.g., 5) is simultaneously applied to a substrate with a curing solution 2 composed of an oxidant at a higher pH (e.g., 10) with the use of syringes. Upon mixing, the curing solution simultaneously produces the adhesive complex coacervate by crosslinking the polymers on the surface of the substrate. 
     In another aspect, referring to  FIG. 9B , a solution of polyanions 3 and polycations 4 are applied simultaneously to the substrate. One of the solutions has a pH higher than the other in order to produce the adhesive complex coacervate. Referring to  FIG. 9B , polyanion 3 is at a lower pH than the polycation solution 4; however, it is also contemplated that the polyanion can be in solution having a higher pH than the polycation. The solution having the higher pH can include an oxidant in order to facilitate crosslinking. 
       FIG. 9C  depicts another aspect of spot welding. In this aspect, the substrate is primed with polycation at a particular pH. Next, a solution of the polyanion at a higher pH is applied to the polycation in order to produce the adhesive complex coacervate in situ. It is also contemplated that the substrate can be primed with polyanion first followed by polycation. An oxidant can then be applied separately on the complex coacervate to facilitate crosslinking to produce the adhesive complex coacervate. Alternatively, the solution applied after the substrate has been primed can contain the oxidant so that the adhesive complex coacervate is formed and subsequently crosslinked in situ. 
     The adhesive complex coacervates described herein can be used to repair a number of different bone fractures and breaks. The coacervates adhere to bone (and other minerals) through several mechanisms (see  FIG. 1C ). The surface of the bone&#39;s hydroxyapatite mineral phase (Ca 5 (PO 4 ) 3 (OH)) is an array of both positive and negative charges. The negative groups present on the polyanion (e.g., phosphate groups) can interact directly with the positive surface charges or it can be bridged to the negative surface charges through the cationic groups on the polycation and/or multivalent cations. Likewise, direct interaction of the polycation with the negative surface charges would contribute to adhesion. Additionally, when the polycation and/or polyanion contain catechol moieties, they can facilitate the adhesion of the coacervate to readily wet hydroxyapatite. Other adhesion mechanisms include direct bonding of unoxidized crosslinker (e.g., DOPA or other catechols) to hydroxyapatite. Alternatively, oxidized crosslinkers can couple to nucleophilic sidechains of bone matrix proteins. 
     Examples of such breaks include a complete fracture, an incomplete fracture, a linear fracture, a transverse fracture, an oblique fracture, a compression fracture, a spiral fracture, a comminuted fracture, a compacted fracture, or an open fracture. In one aspect, the fracture is an intra-articular fracture or a craniofacial bone fracture. Fractures such as intra-articular fractures are bony injuries that extend into and fragment the cartilage surface. The adhesive complex coacervates may aid in the maintenance of the reduction of such fractures, allow less invasive surgery, reduce operating room time, reduce costs, and provide a better outcome by reducing the risk of post-traumatic arthritis. 
     In other aspects, the adhesive complex coacervates described herein can be used to join small fragments of highly comminuted fractures. In this aspect, small pieces of fractured bone can be adhered to an existing bone. It is especially challenging to maintain reduction of the small fragments by drilling them with mechanical fixators. The smaller and greater number of fragments the greater the problem. In one aspect, the adhesive complex coacervate or precursor thereof may be injected in small volumes to create spot welds as described above in order to fix the fracture rather than filling the entire crack. The small biocompatible spot welds would minimize interference with healing of the surrounding tissue and would not necessarily have to be biodegradable. In this respect it would be similar to permanently implanted hardware. 
     In other aspects, the adhesive complex coacervates can be used to secure scaffolds to bone and other tissues such as, for example, cartilage, ligaments, tendons, soft tissues, organs, and synthetic derivatives of these materials. Using the complexes and spot welding techniques described herein, the development of scaffolds is contemplated. Small adhesive tacks composed of the adhesive complex coacervates described herein would not interfere with migration of cells or transport of small molecules into or out of the scaffold. In certain aspects, the scaffold can contain one or more drugs that facilitate growth or repair of the bone and tissue. For example, the scaffold can be coated with the drug or, in the alternative, the drug can be incorporated within the scaffold so that the drug elutes from the scaffold over time. 
     The adhesive complex coacervates and methods described herein have numerous dental applications. For example, the adhesive complex coacervates can be used to repair breaks or cracks in teeth, for securing crowns, or seating implants and dentures. Using the spot weld techniques described herein, the adhesive complex coacervate or precursor thereof can be applied to a specific points in the mouth (e.g., jaw, sections of a tooth) followed by attaching the implant to the substrate. 
     In other aspects, the adhesive complex coacervates can adhere a metal substrate to bone. For example, implants made from titanium oxide, stainless steel, or other metals are commonly used to repair fractured bones. The adhesive complex coacervate or a precursor thereof can be applied to the metal substrate, the bone, or both prior to adhering the substrate to the bone. In certain aspects, the crosslinking group present on the polycation or polyanion can form a strong bond with titanium oxide. For example, it has been shown that DOPA can strongly bind to wet titanium oxide surfaces (Lee et al., PNAS 103:12999 (2006)). Thus, in addition to bonding bone fragments, the adhesive complex coacervates described herein can facilitate the bonding of metal substrates to bone, which can facilitate bone repair and recovery. 
     It is also contemplated that the adhesive complex coacervates described herein can encapsulate one or more bioactive agents. The bioactive agents can be any drug that will facilitate bone growth and repair when the complex is applied to the bone. The rate of release can be controlled by the selection of the materials used to prepare the complex as well as the charge of the bioactive agent if the agent is a salt. 
     EXAMPLES 
     The following examples are put forth so as to provide those of ordinary skill in the art with a complete disclosure and description of how the compounds, compositions, and methods described and claimed herein are made and evaluated, and are intended to be purely exemplary and are not intended to limit the scope of what the inventors regard as their invention. Efforts have been made to ensure accuracy with respect to numbers (e.g., amounts, temperature, etc.) but some errors and deviations should be accounted for. Unless indicated otherwise, parts are parts by weight, temperature is in ° C. or is at ambient temperature, and pressure is at or near atmospheric. There are numerous variations and combinations of reaction conditions, e.g., component concentrations, desired solvents, solvent mixtures, temperatures, pressures and other reaction ranges and conditions that can be used to optimize the product purity and yield obtained from the described process. Only reasonable and routine experimentation will be required to optimize such process conditions. 
     Mimetic Copolymer Synthesis and Characterization. 
     Pc3 Analogs. 
     The dopa analog monomer (dopamine methacrylamide, DMA) was prepared by slight modification of a published procedure. (Lee B P, Huang K, Nunalee F N, Shull K R, Messersmith P B. Synthesis of 3,4-dihydroxyphenylalanine (DOPA) containing monomers and their co-polymerization with PEG-diacrylate to form hydrogels. J Biomater Sci Polym Ed 2004; 15(4):449-464). Briefly, a borate-dopamine complex was reacted at pH&gt;9 with methacryloyl chloride. After disrupting the borate-catechol bond by acidification, the product was washed with ethyl acetate, recrystallized from hexane, and verified by  1 H NMR (400 MHz, DMSO-TMS): d8.8-8.58 (2H, (OH) 2 —Ar—), 7.92 (1H, —C(═O)—NH—), 6.64-6.57 (2H, C 6 H 2 (OH) 2 —), 6.42 (1H, C 6 H 2 H(OH) 2 —), 5.61 (1H, —C(═O)—C(—CH 3 )═CHH), 5.30 (1H, —C(═O)—C(—CH 3 )═CHH), 3.21 (2H, C 6 H 3 (OH) 2 —CH 2 —CH 2 (NH)—C(═O)—), 2.55 (2H, C 6 H 3 (OH) 2 —CH 2 —CH 2 (NH)—C(═O)—), 1.84 (3H, —C(═O)—C(—CH 3 )═CH 2 ). 
     Before polymerization monoacryloxyethyl phosphate (MAEP, Polysciences) was diluted in MeOH and extracted with hexane to remove dienes. Copolymer 1 was prepared by mixing 90 mol % MAEP, 8 mol % DMA, 2 mol % acrylamide (Aam, Polysciences), and 0.1 mol % FITC-methacrylamide in MeOH at a final monomer concentration of 5 wt %. Free radical polymerization was initiated with azobisisobutyronitrile (AIBN) and proceeded at 60° C. for 24 hrs in sealed ampules. A similar procedure was used to make polymers 3-7 as shown in  FIGS. 2-7 . Copolymer 1 ( FIG. 10 ) was recovered by size exclusion chromatography (SEC) in MeOH on a Sephadex LH-20 column (Sigma-Aldrich), concentrated by rotary evaporation, dissolved in DI water, and freeze dried. 
     The MW and polydispersity index (PDI) of 1 were determined by SEC in DMF on a PLgel column (Polymer Labs) connected to a small angle light scattering detector (Brookhaven BI-MWA) and refractive index monitor (Brookhaven BI-DNDC). The column was calibrated with polystyrene standards. The MW of 1 was 245 kda with a PDI of 1.9. The dopamine sidechain concentration and reactivity was verified by UV/VIS spectroscopy (e 280 =2600 M −1  cm −1 ). The phosphate sidechain concentration were determined by titration with 0.005 M NaOH using an automated titrator (Brinkmann Titrando 808). The UV/vis spectrum of 1 contained a single absorption peak at 280 nm characteristic of the catechol form of dopamine ( FIG. 10B ). Addition of a 1:1 molar ratio of NaIO 4  to 1 at pH 5.0 oxidized the dopa catechol to dopaquinone with an absorption peak near 395 nm as expected. The dopaquinone peak was stable for several hrs at pH&lt;5. 
     Pc1 Analogs. 
     The lysine sidechains of Pc1 were mimicked with N-(3-aminopropyl)methacrylamide hydrochloride (APMA, Polysciences). Copolymer 2 ( FIG. 10 ) was synthesized by dissolving 10 mol % APMA and 90 mol % Aam in DI water, degassing with N 2  and initiating polymerization with 2 mol % ammonium persulfate (Polysciences). Polymerization proceeded at 50° C. for 24 hrs in sealed ampules. Polymer was recovered by dialysis against water for 3 days, and then freeze dried. The primary amine sidechain mol % was determined by  1 H NMR (400 MHz, DMSO-TMS) from the ratios of d 13.45 (3H, —CH3) and d 51.04 (1H, RC(═O)CHR2). The MW and PDI of 2 were determined by SEC in PBS (20 mM PO 4 , 300 mM NaCl, pH 7.2) on a Superose 6 column (Pharmacia). The column was calibrated with poly-2-hydroxypropyl methacrylate standards. The MW of 2 was 165 kd and PDI was 2.4. 
     Coacervate Formation and Characterization. 
     A 5 wt % aqueous solution of 2 was added dropwise while stirring to a 5 wt % aqueous solution of 1 until reaching the target amine/phosphate ratio. Total copolymer concentration was 50 mg/ml. After mixing for 30 min the pH was adjusted with NaOH (6M). Compositions at pH (&lt;4) conducive to polyelectrolyte complex (PEC) formation were diluted to 1 mg/ml in DI H 2 O and the zeta potentials and size distribution of PECs were measured on a Zeta-Sizer 3000HS (Malvern Instruments). At higher pH, coacervated compositions were centrifuged at 2500 rpm in a microfuge (Eppendorf), at 25° C. for 2 min to collect the coacervate phase. The volume of both phases was measured. The coacervate phases were freeze dried and weighed to determine their mass and concentration. 
     The phase behavior of 1 and 2 mixed at a 1:1 molar ratio of phosphate to amine sidechains (50 mg/ml combined concentration) over the pH range 3-10 is shown in  FIG. 11A . The calculated net copolymer charge normalized to the total ionizable sidechain concentration is shown in  FIG. 11B . Ascorbate, a reductant, was added at a 1:5 molar ratio to dopa to retard oxidation of dopa by O 2  and subsequent crosslinking at elevated pH. At low pH, the polyelectrolytes formed a stable milky solution of colloidal polyelectrolyte complexes (PECs). The mean diameter of the PECs at pH 2.1, determined by dynamic light scattering, was 360 nm with a narrow dispersity and increased to 1080 nm at pH 4.0 ( FIG. 11C ). The crossover of the zeta potential from positive to negative at pH 3.6 fit well with the calculated pH dependent net charge of the complexes ( FIG. 11B ). The particle size could not be measured accurately above pH 4 because the complexes flocculated. As the net charge increased due to the deprotonation of the phosphate sidechains, the copolymers condensed into a dense second phase. At pH 5.1 the separated phase had the character of a loose low density precipitate. At pH 7.2 and 8.3 the dense phase had the character of a cohesive liquid complex coacervate ( FIG. 12 ). The copolymers were concentrated about three-fold to 148 and 153 mg/ml, respectively, in the coacervated phases. At pH 9.5 the polyelectrolyte mixture formed a dense non-liquid ionic gel. At pH 10 the copolymers went into solution and spontaneously crosslinked through the dopaquinone and amine sidechains into a clear hydrogel. 
     Extraction of divalent cations with the chelator EDTA resulted in a 50% decrease in compressive strength of  P. californica  tubes, a ten-fold decrease in adhesiveness, and collapse of the glues porous structure. The effect of divalent cations on the phase behavior of the mimetic polyelectrolytes was investigated by mixing 1 and 2 at amine to phosphate sidechain ratios ranging from 1:1 to 0:1 with divalent cation to phosphate sidechain ratios ranging from 0:1 to 1:1 to create a coacervate phase diagram ( FIG. 13 ). The pH was fixed at 8.2, the pH of seawater, and divalent cations were added as a 4:1 mixture of Mg 2+  and Ca 2+ , the approximate Mg 2+ /Ca 2+  ratio in the natural glue determined by elemental analysis. The highest mass of coacervate (dark gray squares) occurred in mixtures with higher amine to phosphate sidechain ratios and lower divalent cation to phosphate sidechain ratios. Mixtures with lower polyamine ratios were clear (clear squares) even at higher divalent cation/phosphate sidechain ratios. At higher amine/phosphate and divalent cation/phosphate ratios the solutions were turbid (light gray squares) with slight precipitates but much less turbid than solutions containing PECs (medium gray squares). 
     Mechanical Bond Testing. 
     Bone test specimens, ˜1 cm 3 , were cut with a band saw from bovine femur cortical bone, obtained from a local grocery store, sanded with 320 grit sandpaper, and stored at −20° C. NaIO 4  at a 1:2 molar ratio to dopa sidechains was evenly applied to one face each of two wet bone specimens. Forty ml, a volume sufficient to completely fill the space between 1 cm 2  bone interfaces, of the test coacervate solution was applied with a pipette, the bone specimens were pressed together squeezing out a small excess of adhesive, clamped, and immediately wrapped in PBS (20 mM PO 4 , 150 mM NaCl, pH 7.4) soaked gauze. The applied coacervate contained ascorbate at a 1:5 molar ratio to dopa to prevent premature crosslinking. The bonded specimens were incubated at 37° C. for at least 24 hr in a sealed container containing soaked sponges to maintain 100% humidity. Reference specimens were bonded with 40 ml Loctite 401 superglue in exactly the same manner. A commercial non-medical grade cyanoacrylate was used because there are no hard tissue medical adhesives available for comparison. Mechanical tests were performed on a custom built material testing system using a 1 kg load cell. The instrument was controlled and data acquired using LabView (National Instruments). One bone of a bonded pair was clamped laterally 1 mm from the bond interface. The second bone was pressed with a cross-head speed of 0.02 mm/s against a dull blade positioned 1 mm lateral to the bond interface. Bond strength tests were performed at room temperature immediately after unwrapping the wet specimens to prevent drying. After testing, the bonds were examined for failure mode. The bonded area was measured by tracing an outline of the bone contact surface on paper, cutting out the trace, and determining its area from the weight of the paper cut-out. At least 6 specimens were tested for each condition. 
     The shear modulus and strength at failure were measured with bovine cortical bone specimens bonded while wet with the three coacervating compositions marked with an asterisk in  FIG. 13 . The coacervate density in the three compositions increased with increasing divalent cation ratios (to 120, 125, and 130 mg/ml, respectively). Both the modulus and bond strength of the fully hydrated specimens increased with increasing divalent cation concentration, reaching 37% of the strength of wet bones bonded with a commercial cyanoacrylate adhesive ( FIG. 14A ). The cyanoacrylate adhesive was used as a reference point because there are no bone adhesives in clinical use for comparison. The strength of the mimetic adhesive is also about ⅓ the strength of natural  P. californica  glue estimated to be 350 kPa and mussel byssal glue estimated to range from 320 to 750 kPa dependent on the season. In almost all cases the bonds failed cohesively leaving adhesive on both bone interfaces, which suggested the compositions formed strong interfacial bonds with hydroxyapatite. The bonds were dimensionally stable, neither shrinking nor swelling appreciably after complete submersion in PBS pH 7.2 for several months ( FIG. 14B ). Dimensional stability during cure and long term exposure to water is an important requirement for a useful bone adhesive. 
     Dopamine-Mediated Copolymer Crosslinking. 
     Addition of NaIO 4  to solutions of 3 at a 1:1 molar ratio immediately and quantitatively oxidized DOPA (280 nm) to dopaquinone (392 nm). Within a few minutes the quinine peak decayed into broad general absorption as the reactive quinones formed covalent diDOPA crosslinks ( FIG. 15 , top left). Crosslinking between the quinones and primary amines ( FIG. 15 , bottom left) led to a broader general absorption than diDOPA crosslinking Dopamine oxidation and crosslinking chemistry therefore behaved as expected in the dopamine copolymers. The dopamine copolymers rapidly formed hydrogels as a result of oxidative crosslinking ( FIGS. 15 , A&amp;C). Oxidized phosphodopamine 3 did not gel by itself ( FIG. 15B ) but when mixed with 4 it gelled rapidly ( FIG. 15D ). Intermolecular diDOPA crosslinking between PO 4  copolymers was inhibited but not intermolecular DOPA-amine crosslinking. This provides a crosslinking control mechanism that may be useful for formulating and delivering a synthetic adhesive. 
     pH Triggered DOPA-Mediated Crosslinking. 
     To explore, the pH dependence and kinetics of DOPA oxidation, crosslinking of the dopamine copolymers were evaluated by UV-Vis spectroscopy. Results with p(EGMP[92]-DMA[8]) (3) are shown in  FIG. 16 . UV-vis spectra were acquired at increasing time after addition of a stoichiometric amount of NaIO 4 . At pH 5.0 (top), dopaquinone absorbance (398 nm) was maximal in ˜15 min and remained stable for several hrs (inset). At pH 6.0, absorbance at 398 nm peaked in &lt;1 min and evolved into broad absorbance with peaks at 310 and 525 nm. The broad absorbance is not due to dopaquinone crosslinking since gels do not form ( FIG. 16 ). For comparison, 6 was oxidized at low pH crosslinked but at a significantly slower rate (not shown). 
     The results show that the dopaquinone is stable at low pH and diDOPA crosslinking was inhibited at higher pH in the phosphodopamine copolymers. In the presence of the polyamine, the covalent crosslinking was channeled toward intermolecular amine-DOPA bonds. This is an important observation because it lays out a path to controlled delivery and setting of the synthetic adhesive. 
     In Vitro Cytotoxicity. 
     Solutions of 3 and 4, 40 wt % each, were mixed at low pH to form a polyelectrolyte complex. The solution was partially oxidized with NaIO 4  and basified with NaOH just before application to sterile glass coverslips. The adhesive-treated coverslips were placed in the bottom of culture plate wells and human foreskin fibroblasts, human tracheal fibroblasts, and rat primary astrocytes in serum containing media were added to separate wells at 30K cells/well ( FIG. 17 ). After 24 hr, the cells were fixed with 4% para-formaldehdye, then immunostained for the intermediate filament protein, vimentin, to visualize cell morphology (green, A-B), pericellular fibronectin to assess ECM secretion (red, B), glial fibrillary protein to visual primary astrocyte morphology (green, C), and DAPI to visualize nuclei (blue, C). The granular globs of adhesive auto-fluoresced orangish-red (A-C). 
     In the representative images ( FIG. 17 ), all cell types had morphologies indistinguishable from cells growing on glass without adhesive. The cells had normal motility and in several cases extended processes that directly contacted the adhesive. No toxicity was apparent. 
     Rat Calvarial Defect Model. 
     Production of the fragmented defect and repair with an adhesive complex coacervate is shown in  FIGS. 18A-F . Male Sprague Dawley rats (256-290 g) (Harlan) were anesthetized with a mixture of ketamine (65 mg/kg), xylazine (7.5 mg/kg), and acepromazine (0.5 mg/kg). At full depth of anesthesia, the eyes were covered with ophthalmic ointment, the head shaved, and the scalp disinfected with isopropanol and butadiene. With the prepped rats in a stereotactic frame, a compressed air-driven drill operating at ˜5000 RPM was lowered using a stereotactic fine toothed manipulator. Sterile saline or PBS was continuously applied at the craniotomy site while the custom made trephine tool was lowered 600 microns (previously determined as the skull thickness of rats the age of which were used in the experiment). The result is a round, accurate hole through the skull with little observable effect on the underlying dura or vasculature ( FIG. 18A-B ). The bone plug was recovered with fine curved forceps and broken into fragments using a hemostat and fine rongeur ( FIG. 18B ). The bone fragments were returned to the defect ( FIG. 18C ) and 5 μl of test adhesive (3 and 4 mixed immediately prior to the application of the fracture) was applied with a micropipettor ( FIG. 18D ). The low viscosity adhesive solution (pre-formed PECS mixed with curing solution just before delivery) readily and cleanly wicked into the fractures. Within 5 min the fragments were sufficiently fixed that they could be tapped sharply with the forceps without displacement. The adhesive continued to turn dark reddish brown as it cured ( FIG. 18E-F ). 
     Throughout this application, various publications are referenced. The disclosures of these publications in their entireties are hereby incorporated by reference into this application in order to more fully describe the compounds, compositions and methods described herein. 
     Various modifications and variations can be made to the compounds, compositions and methods described herein. Other aspects of the compounds, compositions and methods described herein will be apparent from consideration of the specification and practice of the compounds, compositions and methods disclosed herein. It is intended that the specification and examples be considered as exemplary.