Ecoflex 00 10

Elastic chambers with a culture surface area of 7 × 23 mm² or 7 × 26 mm² (Figure 5B) were generated by curing silicone rubber (Ecoflex 00–10, Smooth-On Inc.) in customized molds. The chambers were coated with 10 µg/ml fibronectin after sterilization with 70% ethanol and dried. Heart explants were plated as described above. Three days after plating, the culture chambers were mounted in a bioreactor (Figure 5C) with a linear actuator (UltraMotion) operated by LabVIEW software via a motor controller (Motion Mind 3, Solutions Cubed LLC). The culture surface was uniaxially stretched at a constant elongation rate for 1 h to extend 50%, 100% or 200% of the initial 7 mm dimension (10.5 mm, 50% stretch, 14 mm, 100% stretch or 21 mm, 200% stretch; Figures 5C and S5), and then held at the stretched length for 18 h. Stretching at 50% caused a milder increase in multinucleation than stretching at 100%. With 200% stretch, migrated epicardial cells lost connections with each other (Figure S5). Thus, only 100% stretched samples were chosen for further analysis. Fluorescent images were taken by using a Zeiss Axiozoom microscope. Alexa Fluor® 594-conjugated Wheat germ agglutinin (WGA, ThermoFisher, #W11262) was incubated with explant culture at 2.5 µg/ml for 5 min in PBS right before imaging.

The force sensor contains two functional layers: a sensing layer and a force amplification layer. The silicone insulated wires were loaded in the 3D printed mold first. Then two parts of the sensing layer were molded with the silicone mixture, a 3:1 ratio of 10:1 PDMS (Slygard 184, Dow Corning, MI, USA) and Ecoflex 0030. The force amplification base layer was molded with Ecoflex 0010 (Smooth-on, PA, USA). After curing, the micro-fillers were placed along the microchannel embedded in the sensing layer manually. These cylindrical fillers were prepared by cutting 0.5 mm diameter PMMA optical fiber in to 1mm length cylinders with a diode pumped solid state laser (Oxford Laser, Industrial System, UK). The two sensing layer bases were then plasma bonded together, and the conductive liquid was injected into the microchannel with syringes. After finishing the sensing layer, Ecoflex 0030 was spin-coated onto the sensing layer to wet bond the previously molded force amplification layer. Finally, the UV curable adhesive (Henkel 36480, Loctite 3943, CT, USA) was injected into the chamber between force amplification layer and the sensing layer with syringes, and was cured under UV light for 1 min. Figure S5 in supporting information shows the force sensor fabrication process.