The language used to describe physical surface modi cations can be misleading. For example, Flemming et al. (1999) have used “micro-and nanostructured surfaces” to include surfaces with grooves up to 120 μm deep (Brunette, 1986a, 1986b). For the purposes of this book, micro-and nano-structured surfaces are reserved for physical modi cation with aspect ratios such that cells cultured thereon remain in a monolayer and the structure imparts its effect by interaction with only the bottom side of the cells. For example, nanotube surfaces present wells with nanoscale dimensions such that cells with micron-scale sizes interact with the surfaces as monolayers (Oh et al., 2009). The word “scaffold” is used to refer to structures with high aspect ratio architecture that allow cells to be embedded into the structure pores and in some cases to form microtissues. For example, Cheng et al. (2008) fabricated polystyrene structures with random pores of more than 60 μm that supported growth and differentiation of neural aggregates of progenitor cells.