3D printing strategies to create gradients in macroscale structures. (A) An emulsion printing strategy to create stimuli-responsive multiplexed arrays of capsules within 3D hydrogel matrices (cylinder outer diameter is 8 mm; cube edge length is 10 mm) [62]. (B) Direct printing of vascular networks in granular media. Jamming of the media allows the printed network to be stabilized as it is printed [177]. (C) A carbohydrate glass is printed as a sacrificial scaffold for the vascular network. Once the gel matrix is cast, the scaffold is dissolved leaving behind open channels (scale bars are 1 mm, left; 2 mm, right) [179]. (D) In this example, the authors use a fugitive ink to create microfluidic channels in a hydrogel. After the channels are formed, the ink is removed by decreasing the temperature to fluidize the fugitive ink [178]. Reprinted with permission from Refs. [62], [177], [179], [178], respectively. Copyright 2015 American Chemical Society, 2015 American Association for the Advancement of Science, 2012 Nature Publishing Group, 2014 John Wiley & Sons.