The ability to develop multiple bioreagent-loaded coatings, and to tune their release profiles provides an important technology that can create numerous pathways to new medical applications. Ideally, one “smart” coating capable of delivering a drug with a relatively complex release profile, or of releasing two or more different drugs in localized areas via a choice of sequential, parallel or gradient profiles. Layer-by-layer (LbL) assembly is the alternating adsorption of materials onto a surface using complementary interactions, one layer of material at a time, creating nanometer-scale thin films. This process enables fine control over the assembly of functional materials into ultra-thin coatings that exhibit a wide range of interesting properties and have found diverse applications in reactive membranes, drug delivery systems, and electrochemical and sensing devices. These advances enable the implementation of promising LbL systems at the speed and throughput needed for commercial products and processes; however, to date there is no approach capable of assembling libraries of LbL multi-layered nanofilms. As research on LbL films continues to expand and researchers pursue new discoveries along with translation in the pharmaceutical industry, several challenges need to be overcome. These include simplifying in vitro analysis of films, improving material conservation, and making the LbL process more accessible to a broader scientific community. To address these challenges and enable more thorough investigations of nanofilm assemblies, we propose to develop a simple microfluidic approach for the high-throughput construction of multiple LbL films in parallel. We have termed this approach “capillary flow layer-by-layer” (CF-LbL) as we harness capillary action to fill microchannels in which LbL films are assembled. The CF-LbL device consists of an array of these microchannels formed by bonding a polydimethylsiloxane (PDMS) mold to an oxygen plasma treated substrate (e.g., glass, silicon, etc.). This approach can be operated as a simple lab bench-top apparatus or combined with liquid-handling robotics to extend the library size. We demonstrate the CF-LBL technique to identify key parameters for controlling film growth and morphology. We use it to assay biological properties such as cell adhesion and proliferation and provide two examples of the use of this approach: 1) to identify LbL films for surface-based DNA transfection of commonly used cell types; and 2) preparation and screening multi-layer nanofilms for combinational delivery of multiple bioreagents.
The layout of channels is modular by design and can be based on 96- and 384-well plate dimensions for high-throughput screening or consists of only a few microchannels for easy bench-top use. The characterization of films assembled within the microchannels is performed similarly to those created by existing LbL methods using ellipsometry, profilometry, optical microscopy, and atomic force microscopy. Fluorescently labeled polyacrylic acid is incorporated into LbL films with the various polycations to screen LbL film architectures for material incorporation. Various type of cells are cultured on (BPEI/DNA-GFP labeled)10 film within the microchannels. We also perform a systematic study using heat maps of cell density, cell spreading area, and fraction GFP of DNA transfection of cells cultured on films made by DNA-GFP with multiple type of polycations. This platform requires as little as 0.1% as much material per film compared to conventional methods, while enabling the simultaneous assembly of nearly 1000 times as many independent films. LbL films with varying compositions and architectures can be rapidly produced in a format that suits a number of physico-chemical and in vitro biological assays, and screened for meaningful material properties.