Immobilization of individual cells and collections of cells in well-defined, reproducible, nano-to-microscale structures that allow structural and functional manipulation and interrogation is important for developing new classes of biotic/abiotic materials, for establishing the relationship between genotype and phenotype, and for elucidating responses to disease, injury/stress, or therapy - primary goals of biomedical research. Although there has been considerable recent progress in investigating the response of cells to chemical or topological patterns defined lithographically on 2D surfaces, it is time to advance from two-dimensional adhesion on dishes/fluidic devices to three-dimensional architectures that better represent the nanoporous, 3-D extracellular matrix (ECM). 3D immobilization in nanostructured hosts enables cells to be surrounded by other cells, maintains fluidic connectivity/accessibility, and allows development of 3-D molecular or chemical gradients that provide an instructive background to guide cellular behavior. Although 3-D cell immobilization in polymers, hydrogels, and inorganic gels has been practiced for decades, these approaches do not provide for bio/nano interfaces with 3D spatial control of topology and composition important to both the maintenance of natural cellular behavior patterns and the development of new non-native behaviors and functions. This LDRD project exploited our discovery of the ability of living cells to organize extended nanostructures and nano-objects in a manner that creates a unique, highly biocompatible bio/nano interface, mimicking the ECM, and maintaining cell viability, accessibility, and functionality (Baca et al. Science, 2006). Briefly, we found that, using short chain phospholipids to direct the formation of thin film silica mesophases during evaporation-induced self-assembly (EISA, Lu, Brinker et al. Nature 1997), the introduction of cells (yeast, Gram negative and positive bacteria, and several mammalian cells) alters profoundly the inorganic self-assembly pathway. Cells actively organize around themselves an ordered, multilayered lipid-membrane that interfaces coherently with a lipid-templated silica nanostructure. This bio/nano interface is unique in that it withstands drying (even evacuation) without cracking or the development of tensile stresses - yet it maintains accessibility to molecules, proteins/antibodies, plasmids, etc - introduced into the 3D silica host. Additionally cell viability is preserved for weeks to months in the absence of buffer or a fluidic architecture, making these constructs useful as standalone cell-based sensors. (On this basis, our sensors were launched to the space station for viability studies after exposure to vacuum and UV). The bio/nano interfaces we describe do not form 'passively' - rather they are a consequence of the cell's ability to sense and actively respond to external stimuli. During EISA, solvent evaporation concentrates the extracellular environment in osmolytes. In response to this hyperosmotic stress, the cells release water, creating a gradient in pH (and presumably other molecular components), which is maintained within the adjoining nanostructured host and serves to localize lipids, proteins, plasmids, lipidized nanocrystals, and a variety of other components at the cellular surface. This active organization of the bio/nano interface, which we refer to as cell directed assembly (CDA) can be accomplished during ink-jet printing or selective wetting - processes allowing patterning of cellular arrays - what's more we find that cells printed onto preformed, fluid lipid/silica mesophases integrate themselves within the silica nanostructure, creating a 3D environment essentially indistinguishable from that in CDA. We refer to this latterprocess as cell-directed integration (CDI). The synthetic constructs we have developed have allowed us to explore several fundamental questions concerning the mechanisms by which cells actively control nanostructure formation and function and conversely the mechanisms by which nanostructured interfaces, matrices, and patterns can control cellular behavior.
