Fig. 2: b-FDM printed digital materials with tunable and multi-functional properties.

a Schematic of the extrusion of a DM filament with two materials (illustrated in cyan and magenta). While a DM filament (diameter d_f) is supplied at a feed rate of C and passes through a heated region (length of L) in the nozzle (outlet diameter d_out), polymer interdiffusion (illustrated in purple) takes place across the material interface with a depth of Δx. b DM filament design with interdigitated multi-layer arrangements and corresponding homogeneity parameter η. c Schematic of different DM filament designs with varying η (top). Optical images of extrudates (bottom) were obtained by extruding DM filaments through a heated nozzle without deposition on the bed. d Mechanical responses of CPLA-TPU blends printed using DM filaments with varying η. e Elongation at break as a function of η. All error bars represent the standard deviation (n = 5). f Electrical responses during stretching of the specimens produced with DM filaments with varying η. g Designs of CPLA-TPU DM filaments with varying mixing ratios, while η was kept constant at 6. Numbers indicate the volume fraction of TPU in each design. h Elongation at break as a function of TPU fraction. All error bars represent the standard deviation (n = 5). i Representative electrical resistance change with respect to the base line resistance, ΔR/R₀, during stretching. j The strain sensor that is b-FDM printed using a TPU75 DM filament. The sensible gauge had overall dimensions of 20 mm × 1.6 mm × 1.2 mm, while handles attached to the grips were printed on both sides. k Electrical resistance variation during triangular strain cycles with the peak strains of 10%, 20% and 40%. l Electrical resistance variation during cyclic test with the peak strain of 20%, demonstrating a robust performance of the strain sensor.