Front and top views of the 3D-printed REPS film holder. Film holder diameter, X = 22 mm, and height, Y = 75 mm, are indicated in the figure as dimensions X and Y. As illustrated in the top view, one piece of polarized, ferroelectric poly(vinylidene fluoride) (PVDF) film (130 × 70 mm) is spiraled through the openings. This results in the formation of multiple layers of film with sides of opposite polarization facing each other across air channels of optimized width (2.25 mm) (Therkorn et al. 2017).