Control of Heterogeneous Nucleation and Growth Kinetics of Dopamine-Melanin by Altering Substrate Chemistry

Dopamine-melanin (DM or “polydopamine”) can be deposited on virtually any substrate from solution through autoxidation of dopamine. The versatility of this process has allowed surface-mediated assembly of DM for a wide variety of functional coatings. Here we report the impact of well-defined surface chemistries on the nucleation and growth of such films. DM was deposited on silicon dioxide (SiO₂) and SiO₂ substrates modified with self-assembled monolayers (SAMs) bearing octadecyl (C18), phenethyl, and aminopropyl functional groups. Atomic force microscopy revealed three-dimensional islands whose areal density and surface coverage are lowest on bare SiO₂ substrates and highest on the neutral aromatic and aliphatic substrates. Increasing the pH of the solution from 8.2 to 10 dissociates catechol moieties in DM and inhibits adsorption on negatively charged SiO₂ substrates. The growth rate of DM films on SAM-modified SiO₂ is maximized at pH 9.5 and almost completely abolished at pH 10 because of increased DM solubility. The initial rates of DM adsorption were measured using quartz crystal microbalance with dissipation measurements. The initial adsorption rate is proportional to the nucleation density, which increases as the hydrophobicity of the substrate increases. Taken together, these data provide insight into the rates of heterogeneous nucleation and growth of DM on substrates with well-defined chemistries

Facile Arm-First Synthesis of Star Block Copolymers via ARGET ATRP with ppm Amounts of Catalyst

Star polymers with block copolymer arms were prepared by atom transfer radical polymerization (ATRP) via an arm-first method. Several macroinitiators based on block copolymers (MIs), PBA-<i>b</i>-P<i>t</i>BA–Br, P<i>t</i>BA-<i>b</i>-PBA–Br, PSAN-<i>b</i>-PBA–Br, and PBA-<i>b</i>-P<i>t</i>BA–Br, were prepared by activators regenerated by electron transfer (ARGET) ATRP to maintain high chain-end functionality. Then the MIs were reacted with divinylbenzene as a cross-linker to form star-shaped polymers via ARGET ATRP. Several parameters including concentration of reducing agent, copper catalyst concentration, degree of polymerization (DP) of MIs, and composition of MIs were investigated. A high level of control was achieved by sequential feeding of the reducing agents for DP_MI ≤ 100. Stars in >95% yield and with narrow molecular weight distributions (<i>M</i>_w/<i>M</i>_n < 1.3) were obtained under the optimized polymerization condition

Elastomeric Conducting Polyaniline Formed Through Topological Control of Molecular Templates

A strategy for creating elastomeric conducting polyaniline networks is described. Simultaneous elastomeric mechanical properties (<i>E</i> < 10 MPa) and electronic conductivities (σ > 10 S cm^–1) are achieved <i>via</i> molecular templating of conjugated polymer networks. Diblock copolymers with star topologies processed into self-assembled elastomeric thin films reduce the percolation threshold of polyaniline synthesized <i>via in situ</i> polymerization. Block copolymer templates with star topologies produce elastomeric conjugated polymer composites with Young’s moduli ranging from 4 to 12 MPa, maximum elongations up to 90 ± 10%, and electrical conductivities of 30 ± 10 S cm^–1. Templated polyaniline films exhibit Young’s moduli up to 3 orders of magnitude smaller compared to bulk polyaniline films while preserving comparable bulk electronic conductivity. Flexible conducting polymers have prospective applications in devices for energy storage and conversion, consumer electronics, and bioelectronics

Biologically Derived Soft Conducting Hydrogels Using Heparin-Doped Polymer Networks

The emergence of flexible and stretchable electronic components expands the range of applications of electronic devices. Flexible devices are ideally suited for electronic biointerfaces because of mechanically permissive structures that conform to curvilinear structures found in native tissue. Most electronic materials used in these applications exhibit elastic moduli on the order of 0.1–1 MPa. However, many electronically excitable tissues exhibit elasticities in the range of 1–10 kPa, several orders of magnitude smaller than existing components used in flexible devices. This work describes the use of biologically derived heparins as scaffold materials for fabricating networks with hybrid electronic/ionic conductivity and ultracompliant mechanical properties. Photo-cross-linkable heparin–methacrylate hydrogels serve as templates to control the microstructure and doping of <i>in situ</i> polymerized polyaniline structures. Macroscopic heparin-doped polyaniline hydrogel dual networks exhibit impedances as low as <i>Z</i> = 4.17 Ω at 1 kHz and storage moduli of <i>G</i>′ = 900 ± 100 Pa. The conductivity of heparin/polyaniline networks depends on the oxidation state and microstructure of secondary polyaniline networks. Furthermore, heparin/polyaniline networks support the attachment, proliferation, and differentiation of murine myoblasts without any surface treatments. Taken together, these results suggest that heparin/polyaniline hydrogel networks exhibit suitable physical properties as an electronically active biointerface material that can match the mechanical properties of soft tissues composed of excitable cells