Biofabrication of cancer cell-seeded collagen hydrogel constructs using gel aspiration-ejection. (a) Collagen disks were created using a dermal punch or extruding from a PDMS mold on gels polymerized in Petri dishes. (b) The punched disks were transferred to a gel aspiration system, under an inverted phase-contrast microscope to observe rapid gel deformation. (c) The aspiration pressure was calibrated to the rotation of the knob controlling plunger position of the oil-filled syringe by connecting the capillary tube to a second syringe with a fixed plunger, housing a pressure sensor. (d) Two breast cancer cell lines were cultured in aspirated (+Asp) and unaspirated (−Asp) gels for 3 days, with assessments taken at the indicated timepoints. H&E, hematoxylin and eosin; PDMS, polydimethylsiloxane; XTT, 2,3-Bis(2-methoxy-4-nitro-5-sulfophenyl)-2H-tetrazolium-5-carboxanilide salt