Figure 2: Specialized exemplar tools of epithelial tissue engineering. Chemical signals can be presented as immobilized growth factors (a) that promote differentiation of airway basal cells to specific cell types in a pattern that is reminiscent of in vivo airway epithelium, or (b) a mobile chemokine gradient of CXCL12 that promotes airway epithelium polarity in the presence of Wnt5a, based on the work of Witze et al., 2008 [68]. Mechanical signals can be presented as a constant force that organizes epithelial cells cultured on nanogrooved and flat substrates (c) based on the work of Texeira et al., 2003 [90], or as a reversible force that mimics the transluminal pressure gradient applied to airway epithelium during normal tidal breathing to modulate ciliary beat frequency [3].