In recent years, natural adhesion has attracted increasing attention in the material engineering field. This can be mainly attributed to the marine mussel as it has a strong ability to attach to various surfaces in an aqueous environment where they reside. These surfaces vary from natural to synthetic, and inorganic to organic.[49-51] Previous studies on the mussel adhesive protein have discovered that 3,4-dihydroxy-L-phenylalanine-lysine sequences, may be the main contributor for the versatile nature of the marine mussel.[52, 53] Dopamine, having a similar structure with this sequence, may provide a new platform for bioengineers to physically or chemically enhance the performance of other biomaterials. Several papers have already been published regarding the use of dopamine to augment other biomaterials, such as poly (ethylene glycol), carbon nanotubes and nanofibers. The first part of this review will briefly introduce the basic properties of dopamine which will be followed by its applications
Dopamine’s properties can be divided into chemical and adhesive properties. The chemical properties mainly focus on the autopolymerization in aerated basic solutions and polymerization of dopamine based on vinyl groups. The adhesive property is dopamine’s most significant feature which gives dopamine its advantage as a biomaterial.
Messersmith and coworkers first reported that dopamine is able to auto-polymerize in aired Tris buffer of pH 8.5.. The process of dopamine autopolymerization with a pre-existing substrate results in polydopamine (PDA) films being deposited on the substrate surface. Longer substrate exposure times and higher reaction temperatures result in thicker PDA films being formed. Regardless of the surface type, the inserted PDA films can be coated on the desired surface, even poly(tetrafluoroethylene) (PTFE), known for its anti-adhesive property.
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Polymers carrying pendant dopamine are normally obtained by radical polymerization of vinyl monomers with protected or unprotected dopamine. When meditating protected dopamine carried by polymers with double bone, borax (Na2B4O7·10H2O) is widely used as the protecting reactant in order to keep dopamine from forming an annular bidentate catechol subunit. Normally, the polymerized reaction of protected dopamine happens in a liquid solution and forms linear chains. Deprotection reaction usually occurs in an acidic environment and results in the polymer carrying dopamine. Dimolybdenum trioxide, 1-dromotoluene and denzophenone chloride can also be used as protecting agents. Zhang et al. designed a novel polymer poly (n-acryloyl dopamine) that possesses high adhesion to wood, especially when mixed with polyethylenimine (PEI) at about 150°C. They used a protected double bond dopamine as a monomer and 2,2’-azobis(2-methylpropionitrile) as an initiator via radical polymerization, following the deprotection of dopamine in an acid solution. When meditating unprotected dopamine, Lee BP et al. was the first to report a creative hydrogel that copolymerizes modified dopamine with double bond and polyethylene glycol diacrylate via photo initiation by using a 2,20-dimethoxy-2-phenyl-acetonephenone (DMPA) initiator. As a result of this invention, greater attention has been given to hydrogels as a new artificial extracellular matrix (ECM) in the biomedical field. Dopamine belongs to the catechol family which leads to vinyled dopamine to act as an inhibitor.[61, 62], as a result they can react with radicals to inhibit polyreaction. The unprotected dopamine, modified with a vinyl group, is able to undergo free-radical polymerization. Several researches have done this experiment on radical polymerization to prove the reliability of this method.[63-75] The research group led by Metin Sitti, copolymerized a dopamine derivate (dopamine meth-acrylamide) with methoxyethylaceylate to obtain a reversible adhesion on the surface of nonflat glass under dry or wet condition. In another publication, 2-(meth-acryloyloxy) ethyl phosphate was used to copolymerize with dopamine methacrylamide, followed by a complicated cohesion in which the copolymer bonded with positively charged polymer, divalent calcium and magnesium. The chemical properties of dopamine provide the platform of its strong adhesive properties.
The adhesive property of dopamine is one of the most significant properties of dopamine as it has proved to be very versatile in adhering to various surfaces despite the surface chemistry. The bonding between dopamine and surfaces can be generally distributed to two parts: covalent and non-covalent.
Surfaces which possess amine groups or thiol groups can covalently bind to dopamine via Michael addition or Schiff base reactions. However since most surfaces don’t have those groups, non-covalent bonding, like H-bond, π-π interaction and benzenediolcharge-transfer compounds are preferred to generate a valid layer and metallic chelating.[7, 53, 76-87] In a high pH environment, metal ions and medal oxides have a high chance of being hydroxylated or hydrated, which make chelate with catechol groups of dopamine much easier. This can be seen from many experiments done on polydopamine linking with metal oxides (such as Fe2O3, Fe4O3, ZrO2) through chelating bonding interaction.[82, 84, 85] This can be seen when polydopamine nanoparticle suspensions are added to a solution of KMnO4 with H2SO4. A core-shell nanoparticle structure is created in which the polydopamine act as the core and MnO2 act as shell, followed by blending the KOH solution to obtain MnO2 nanospheres. This adhesive property of dopamine provides promising opportunities for new bioengineered materials.
For decades, carbon nanotubes (CNT) have been attracting increasing attention because of their superior features, such as thermal conductivity, excellent tensile strength and remarkable conductivity. They have been applied in various different areas, from sensors to catalysis, and from semiconductors to inductors for osteocytes. In order for CNTs to have a wide range of applications, surface modification is necessary. However, during this modification various intermediate reactions steps are required which increase the complexity of the CNT’s fabrication. Dopamine modification has been viewed as an promising alternative, leading to a coated multifunctional CNT with a polymeric shell that has tunable thickness by time, pH value and temperature. The dopamine coating facilitates the addition of alternate modifications to the surface of CNTs, such as gold nanoparticles. What’s more, CNTPDAs, first, were modified with ATRP initiator and then polymerized with diethylamine methacrylateto to form brushes polymer — poly (dimethylamine-thyl methacrylate) (PDMAEMA) on the surface. Following that the functionalized CNT were quaternized in order to combine palladium nanoparticles on the CNTs’ surface. These two examples indicate the capability of dopamine coated CNTs to bind to metal complexes.
There are many different applications in which dopamine could be applied in; three of them will be the focus here including applications in hydrogels, nanofibers, and biosening. These fields are of great interest currently as they show great promise for dopamine in bioengineering.
The need of a viscous hydrogel, as a unique material, is dramatically increasing in various biomedical fields. The high performance requirements of adhesive hydrogels are strict and various. This includes being sufficiently adhesive in a wet environment, satisfactory elasticity of artificial tissue scaffold and biocompatible.[60, 90] Moreover, biomedical hydrogels also need a quick sol-gel conversion for avoiding surgical obstruction. Recently, adhesive hydrogel, inspired by strong wet adhesion of mussel and cross-bonding capabilities of dopamine, has been attracted increasing attention and considered as a hopeful candidate to fulfill this technologic niche.
Messersmith et al. reported the creation of four different adhesive hydrogels using dopamine derivative (L-3,4-dihydroxyphenylalanine (DOPA)) as end-groups and poly(ethylene glycol) (PEG) as a backbone. The difference of these four hydrogels can be divided into 2 subcategories, linear network and branched network. They applied multiple-angle laser light scattering to study the influences of different oxidative reagents on DOPA oxidation and hydrogel formation. The result showed that gelation time of PEG-DOPA gels relied on oxidative reagents, such as concentration and type. In Lee H.’s report, they also used DOPA and PEG to form hydrogels, but this time they used DOPA modified with methacryloyl chloride and PEG diacrylate instead of pure DOPA and PEG. In order to avoid introducing toxicity of oxidative reagent to the hydrogels and any loss of adhesion, the hydrogels underwent UV initiation. These photo-imitated gels demonstrate appreciable elastic properties for use as a promising biomedical material. Using a similar method Phillip B. Messersmith’s research group also synthesized an adhesive hydrogel, prepared by copolymerizing DOPA with hydrophobic segments of an amphiphilic block copolymer under photo-imitation. The adhesive property of the hydrogel was surprisingly improved in the presence of DOPA in wet condition. The elasticity of the hydrogel was found to be similar to that of soft tissues leading to consider it as a encouraging candidate for biomaterial.
Further research conducted by Messersmith and coworkers focused on the biological capabilities of dopamine-PEG adhesive gels. In 2010 they reported that DOPA as end-caps covalently bonded with an amine-terminated 4-arm PEG. The PEG was the core in which oxidative reagents (NaIO4) were added to form an adhesive hydrogel in less than 1 minute. The results of the in vivo test, performed in a murine model, showed the adhesive gels caused minimal inflammation and were stably interfaced with the surrounding tissues for more than 24 months. To form a catena degradable adhering polymer, three materials were reacted to form a semblable branched polymer, including dopamine derivative as end-group, PEG and polycaprolactone (PCL) as a backbone. These polymers are able to form films whose properties, such as swelling capacity and biodegradation, were flexible by changing the ratio, or concentration of these reactants or by adding other additive agents. After coating these adhesive polymers on a biologic meshes, stronger water-resistant was exhibited when compared with fibrin sealant or cyanoacrylated polymers. Applications for this biomaterial can be extended in the surgical field for hernia repair.
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Stewart’s group published several papers about adhesive hydrogels based on complex cohesion. In 2010 they created a bio-mimic hydrogel blending with revised gelatin and a copolymer which is obtained by a dopamine derivative reacting with monoacryloxyethyl phosphate in an alkaline condition. The addition of Ca2+ and Mg2+ to the bio-mimic hydrogel could significantly improve the coacervation of the hydrogel, which was applied to tune agglomeration temperature to body temperature. The result demonstrated that the cohesion interaction was biodegradable, perfectly suited for medical applications. In another similar research, an adhesive hydrogel was synthesized by complicated cohesion of a positively charged copolymer and a terpolymer involving a dopamine derivative when its pH was higher than 4. The bonding property of the hydrogel to hydroxylapatite was around 40% of common cyanoacrylate glue. T.G. Park’s group developed a temperature sensitive and injectable tissue-attachable hydrogel. The hydrogel was synthesized by conjugating hyaluronic acid and dopamine, following by cross-linking with thiol tail-ended Pluronic F127 via Michael addition. The hydrogel precursor exists at room temperature, and a cured hydrogel is formed when brought to a temperature of 37°C. In a later paper, they used a similar strategy forming hydrogel by blending a dopamine derivative modified chitosan with thiol-capped Pluronic F127 at body temperature. The adjustable gelation time of this block copolymer made it suitable for tissue-repair at 37°C. The resulting hydrogel dedicated excellent in vivo results, where chitosan served as hemostatic agent and dopamine derivative group acted as adhesive agent to soft tissues.
Tissue engineering tends to use nanofiberous biomaterials instead of a micropores matrix since the filiform and polyporous nanolevel structure allow for artificial extracellular matrix to enhance the fundamental cellular procedures. Nanotechnology reformation have aided in the development of techniques for the production of such a nano-composite materials.
Electro-spinning has recently obtained increasing attention, attributing to its briefness and facility for nanofiber fabrication. Through this technique, fibrous structures are easily tuned in order to coordinate it with the extracellular matrix (ECM).[99, 100] So far, this technique has been studied in a range of biological fields, such as bone and skin regeneration.
The artificial polymer ECMs usually have difficulties with interfaced reactions between tissues and materials. For electro-spinning nanofibers in applications of biomedicine, it is necessary to physically and chemically combine them with biomolecules or cell-recognizing ligands. This subsequently provides bio-modulating or biomimetic micro- environments to contacting cells and tissues. Dopamine coating can be considered as a simple and versatile approach to modify various synthetic polymers so that they are able to serve in biomedical applications.[49-51] Ku and coworkers firstly reported culturing human endothelial cells on a polydopamine treated electro-spun polycaprolactone (PCL) nanofiber membrane. They used two control groups, pure PCL nanofibers and PCL nanofibers coated with gelatin, to investigate the ability of cell attachment of dopamine. The result of the water contact angle demonstrated that polydopamine uniformly was coated on the PCL nanofibers. Polydopamine also significantly improve endothelial cells’ attachment on the nanofiber, compared with other non-adhesive substrates. Moreover, endothelial cells culture on PCL nanofibers coated by dopamine had developed cytoskeleton, positive PECAM-1 and vWF expressions and high cell extend.Rim and coworkers designed dopamine functionalized electro-spinning poly(L-lactide) (PLLA) nanofibers with minimal influence on its mechanical performances, like wetting capability and roughness. The polydopamine coated PLLA nanofibers significantly enhanced cell attachment and the degree of spread, contradistinguishing with pure PLLA nanofibers. Meanwhile, its fibrous morphology had changed to more of a polygon shape instead of sphere after the polydopamine coating, which lead to higher DNA content of polydopamine treated PLLA nanofibers. The higher gene expressions of cells cultivated on polydopamine treated fibers indicated better osteogenic differentiation and vasculogenesis.
Extensive research regarding the chemical or physical coating of metal on the surface of scaffolds to increase tensile strength has been done. Jungki Ryu et al. used dopamine to process hydroxyapatite deposits on PCL nanofiber by coating it. The result demonstrated a combination of surface activation through dopamine coating and hydroxyapatite mineralization allowing the hybridization of various shapes and surfaces. In other reported, Xie and coworkers considered dopamine as a ‘superglue’, allowing minerals to easily attach to fibrous surfaces. The mechanical properties of mineral functionalized electro-pinning PCL nanofibers, such as stiffness, durability and tensile strength, were near to that of natural bone. Dopamine coated nanofibers show an improvement on existed biomaterials such as their mechanical performances, and cell adhesion. This makes them quite suitable for tissue regeneration and other related bioengineering applications.
There is an enormous demand to design highly sensitive and selective biosensors for multiple applications, such as diagnostics, drug screening, and drug discovery. Biosensors usually are in the microscale or nanoscale and there are numerous methods to develop them, such as DNA and antibody-based sensor[111, 112]. Scientists employ dopamine in order to optimize biosensor’s capabilities which have been reported by several research groups.
Lui and coworkers first reported that dopamine could be used in a biosensor. They used electricity to oxidize dopamine to form polydopamine on a gold electric pole with existing nicotine. The dopamine-imprinted sensor showed outstanding selectiveness of nicotine and excellent repeatability. Furthermore, Ouyang and coworkers developed a one-step well-defined structure of a dopamine-imprinted sensor. They applied electro-polymerization of o-phenylenediamine (o-PD) and dopamine with existing glutamic acid (Glu). By using a potentiostatic time scan, the sensor exhibited satisfactory stereo selectiveness of bonding L- or D-Glu because their relative synthetic receptor. In a different publication, they designed protein imprinted nanowires which dopamine was also involved. First, the protein-coupled alumina membrane was immersed in dopamine solution followed by an ammonium persulfate solution in order to self-polymerize polydopamine; in which afterward the removal of the attached protein is necessary. The nanowires demonstrated constant bonding capability and selectiveness of template proteins due to their cavity structure with bonding spots (like amino group, hydroxyl, π-π stacking and van der Waals force) that can bind with protein. In another research, Zhou et al. display the creation of magnetic nanoparticles coated by imprinted polymer with a pre-existing template protein. The nanoparticles are able to separate target protein from the mixture. In order to investigate the versatility of the imprinted nanoparticles, they operated on a binding test by using five different proteins excluding the template protein. The result indicated that more than 80% of target proteins were rebinding with imprinted nanoparticles, suggesting imprinted nanoparticles have a bright future to be employed for separating and detecting specific protein.
One of the greatest difficulties for biosensors is how to immobilize enzymes on the surface of an electric pole and preserve the enzymes’ functionalities. Wei et al. designed a novel glucose electrochemical sensor, prepared by using a polydopamine film to entrap glucose oxidase and gold nanoparticles. Their research displayed a polydopamine matrix embedded with gold nanoparticles that had high efficiency of immobilizing glucose oxidase. The dopamine film embedded gold nanoparticle biosensor showed a superior sensitivity, good repeatability, linear over broad dynamic range and a low detective threshold. Furthermore, in order to assess adaptability of this sensor, they use it to test glucose concentration in attenuated human serum. The result suggested this biosensor is an attractive material for clinical applications
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