Recent advances in polydopamine synthesis are described with a particular focus on biomedical applications. Prospects and future challenges for the application of polydopamine as a biomaterial are also described.

Hydrogel-based electronics are ideally suited for neural interfaces because they exhibit ultracompliant mechanical properties that match that of excitable tissue in the brain and peripheral nerve. Hydrogel-based multielectrode arrays (MEA) can conformably interface with tissues to minimize inflammation and improve the reliability to enhance signal transduction.However, MEA substrates composed of swollen hydrogels exhibit low toughness and low adhesion stability when laminated on the tissue surface and also present technical challenges for processes commonly employed in MEA fabrication. Here, we describe a new strategy to fabricate ultracompliant MEA based on aqueous-phase transfer printing. This technique employs redox active adhesive motifs in hygroscopic polymer precursors that simultaneously form hydrogels through sol-gel phase transitions and bond to underlying microelectronic structures. Specifically, in situ gelation of 4-arm-polyethylene glycol-grafted catechol [PEG-Dopa]4 hydrogels induced by oxidation using Fe 3+ produces conformal adhesive contact with the underlying MEA, robust adhesion to electronic structures, and rapid dissolution of watersoluble sacrificial release layers. MEA are then integrated on hydrogel-based substrates to produce free standing ultracompliant neural probes, which are then laminated to the surface of the dorsal root ganglia in feline subjects to record single-unit neural activity.

The separation of lithium from magnesium ions in salt brines is an important step in producing raw lithium for prospective use in electrochemical storage systems. Liquid–liquid extraction of Mg2+ ions from Li+ ions is challenging because of comparable thermodynamic behavior in aqueous solutions. Removing Mg2+ ions from brines using consumable ion‐exchange membranes is also a challenging prospect due to poor chemical selectivity and compromised sustainability. Here, we propose the use of redox‐active catechols in the form of melanin pigments for selective removal of Mg2+ ions from aqueous solutions. Synthetic melanin films are oxidatively polymerized on stainless steel meshes from aqueous solutions of dopamine precursors to create electrochemically functionalized polydopamine membranes. The binding selectivity of redox‐active catechol‐bearing polydopamine melanin for Mg2+ ions in aqueous solutions was measured as a function of electrochemical cycling. The binding kinetics of Mg2+ ions to polydopamine pigments was measured. Mg2+ ions binding selectivity to polydopamine in mixed aqueous solutions containing both Li+ and Mg2+ ions were also measured. Additionally, the equilibrium binding concentrations of Mg2+ ions to polydopamine were achieved very rapidly (<1 min). Redox‐active polydopamine membranes therefore show promise as functional materials for the separation of Mg2+ and Li+ ions in aqueous solutions. © 2016 Society of Chemical Industry

The conformal nature of in situ polymerization of adhesive dopamine molecules permits the strong underwater adhesion between polydopamine (PDA) nanomembranes and the target substrates. However, the adhesive interaction between the postdeposit PDA nanomembranes and other macrobodies is strongly influenced by the texture of PDA nanomembranes. Here we report the texture-dependent adhesion of PDA nanomembranes both in air and aqueous environments. Despite the nanometer-scale roughness of PDA nanomembranes, interfacial adhesion between PDA nanomembranes and elastomeric bodies are the strong function of the root-mean-square roughness of PDA nanomembranes, root-mean-square gradient of PDA nanomembranes, and the elasticity of the bulk materials. Reduced adhesion due to increased texture is intensified in hydrated conditions, possibly hinting that the conventional explanation of the negative effect of water to adhesion from a molecular level needs to be revisited. These findings can inform the role of adhesive interaction in conformal coatings and provide an explanation for the differential adhesion observed in freestanding PDA nanomembranes.