Solution and Surface Composition Gradients via Microfluidic Confinement: Fabrication of a Statistical‐Copolymer‐Brush Composition Gradient

Materials and surfaces with composition gradients are of great importance for applications such as manipulating the motion of liquids or cells, directing the growth of neurons, and guiding the path of radiation. Within the last decade, the application of combinatorial methods in materials science has further increased interest in composition gradients. For example, gradient surfaces of poly(ethylene glycol) (PEG) and proteins have been used to study the interaction between cells and surfaces. The study of phase behavior in polymer blends and metal alloys has also been greatly accelerated by gradient specimens. Several methods have been developed to make chemicalcomposition gradients, including the diffusion of molecules in gases or gels. Gradients have also been prepared by ion/ metal deposition or ultraviolet–ozone irradiation. The use of scanning probe microscopy to form gradients at the nanometer scale has been explored. Recently, microfluidic networks were applied to generate complex discrete gradients in solution. Through surface-active components, gradients established in solutions were readily transferred onto surfaces. Here, we introduce a facile method to generate solutions with continuous composition gradients using microfluidic techniques. The uniqueness of this approach is the ability of a microchannel to preserve solution gradient profiles over a long period of time. The solution gradient inside the microchannel is generated by continuously changing the relative flow rates of the input solutions as the channel is filled. Once the flow inside the microchannel ceases, the only mixing is diffusive. Since liquid diffusion is relatively slow at room temperature, it is possible for certain reactions to complete before any significant change in the gradient profile takes place. Therefore, the microchannel provides a reaction environment in which different locations within the channel have different solution compositions. To demonstrate the power of this technique, a gradient in a statistical-copolymer-brush composition was synthesized from a solution gradient of two monomers (n-butyl methacrylate (BMA) and 2-(N,N-dimethylamino)ethyl methacrylate (DMAEMA)). Our fabrication of a statistical-copolymerbrush gradient is an important advance beyond existing techniques because it enables the intimate mixing of different monomers, leading to broad sequence distributions in the polymer chains and the trapped expression of complex chemistries at the air interface. Polymer brushes also provide a thicker layer of protection for the hydrolytically unstable silicon–oxygen–carbon bonds at the substrate interface as compared to self-assembled monolayers (SAMs) alone. Surfacegrafted polymers provide a robust means to control the surface properties such as wetting, cell adhesion, and alignment of block-copolymer microdomains. In this work, surfaceinitiated atom transfer radical polymerization (ATRP) was used to transfer the characteristics of the solution gradient onto the surface. The setup employed to generate solution gradients is illustrated in Scheme 1. Two syringes containing different solutions (Ia or Ib and II) were mounted on separate syringe pumps, which were individually programmed to control the infusion rate and relative volumes of the solutions. Each syringe was connected to one of two inlets of a microfluidic passive mixer. The outlet of the mixer was connected to the bottom of a channel that was held upright. The channel had inner dimensions of 0.5 mm × 15 mm × 68 mm and was formed by placing a silicon wafer into a slender cuboid glass vessel. The solution gradient inside the channel was established by manipulating the relative infusion rates of the two solutions. In our experiments, the infusion started with pumping only solution I at a rate of 0.3 mL min. The pumping rate C O M M U N IC A IO N S