The preparation of the striated substrate is based on the soft lithography and micromoulding techniques as generally described in [35 (link)], and specifically applied to electrospun nanofiber mechanical measurements in [31 (link),32 (link),33 (link),34 (link)]. First, a PDMS (polydimethylsiloxane) stamp (negative (inverted) relief structure of the striated substrate) was created by pouring dimethyl siloxane mixed with a catalyst (Sylgard® 184, Sigma-Aldrich, St. Louis, MO, USA) onto an SU-8-silicon master grid in a large plastic Petri dish. A 1 cm × 1 cm stamp can then be excised with a scalpel. Excised stamps were stored in a 2% sodium dodecyl sulfate (SDS) solution to keep them clean; the stamps can be stored for at least several months and used repeatedly. To create the striated substrate, a drop of Norland Optical Adhesive-81 (NOA-81, Norland Products, Cranbury, NJ, USA) was placed on a 60 mm × 24 mm, #1.5 microscope cover slide (Thermo Fisher Scientific, Waltham, MA, USA). The PDMS stamp was pressed into the NOA-81 drop on the slide and cured with 365 nm UV light (Benchtop 3UV transilluminator, UVP, Upland, CA, USA) for several minutes. The substrate had ridges of width 7.3 μm and height 6 μm. The gaps between the ridges were 6 μm across.
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