The microfluidic chambers were fabricated using methods previously described in detail (Taylor et al., 2005; Taylor et al., 2006; Taylor et al., 2003). All microfluidic chambers were replica molded using poly(dimethylsiloxane) (PDMS) from masters which were patterned using the photosensitive epoxy SU-8 (Microchem) [reviewed in (Whitesides et al., 2001)]. All masters consisted of two permanent SU-8 layers on a 3″ silicon wafer and were made in the clean room facility in Michael Roukes' lab (Caltech). The first layer of SU-8 (3 μm depth) contained the microgrooves which were patterned by photolithography using a high resolution chromium mask (5 μm minimum feature size; Advance Reproduction Corp.). The second layer of SU-8 (100 μm depth) contained the compartments and perfusion channels which were patterned by photolithography using a 20,000 dpi printed transparency mask (CAD/Art Services, Inc.). To assess dendritic growth properties and synapse formation, we used previously described chambers with microgrooves 900 μm in length separating the compartments (Taylor et al., 2005). For the μLP chambers, we used the same mask design for the microgrooves, but the second layer mask was redesigned
