2.5. Calcium imaging analysis

In order to validate the progressive synchronous contraction along with tissue maturation in bioprinted cardiac tissues, calcium imaging analysis was performed at 1 and 3 weeks in culture. Bio-printed cardiac tissue constructs were incubated with 3 μM Fluo-8 AM (Abcam) for 15 min at 37 °C. The constructs were subsequently washed with DMEM medium supplemented with 10% FBS for 3 times and placed on a heating adaptor on a Zeiss inverted microscope system (Zeiss Axiovert 200). Calcium influx fluorescent videos were recorded by a Cannon G11 camera with the Zeiss microscope under high-quality video mode (640 480 resolution, 30 fps). Fluorescence intensity (F) was normalized based on basal tissue fluorescence after dye loading (F0). IamgeJ software was used for plotting and quantifying the beating of the bioprinted cardiac tissues.

Cardiac drug response on bioprinted cardiac tissues was examined by calcium imaging analysis. Known cardiac drugs, epinephrine (200 nM, Sigma-Aldrich Co.) and carbachol (10 μM, Sigma-Aldrich Co.) were exposed to bioprinted cardiac tissues at 3 weeks in culture, respectively. Calcium imaging was monitored to detect the synchronous contraction of bioprinted cardiac tissues, as described above.