Uniaxial Tensile Testing of Copy Papers and Ecoflex

The force-displacement relationships of the copy papers and Ecoflex 00-30 elastomers were measured using uniaxial tension tests (Instron 6800, Illinois Tool Works), with displacement controlled at a loading rate of 5 mm min⁻¹, as shown in fig. S3. Then, Young’s modulus E was calculated through E = FL/A∆L, where F is the tensile force, ∆L is the tensile displacement, L is the initial length, and A is the cross-sectional area of the samples. The adopted parameters are L = 80 mm and A = 1 mm² for copy paper, and L = 20 mm and A = 12 mm² for Ecoflex 00-30. Note that the force-displacement curves at small tensile displacements were used to calculate Young’s modulus.