Sodium alginate solution was made by dissolving 1% (w/v) sodium alginate (Sigma-Aldrich, UK) in deionized (DI) water. Calcium chloride solution was prepared by dissolving 4% (w/v) calcium chloride (CaCl2) (Sigma-Aldrich, St. Louis, MO) in DI water. For constructing fibrin scaffolds, fibrinogen (6 mg/ml) (Sigma-Aldrich) and thrombin (2.4 U/ml) (Sigma-Aldrich) were prepared for microvalve bioprinting. For bioprinting of electrocytes, agarose (A20070-100, Research Products International, IL) was dissolved in DI water to obtain a solution of 1% (w/v) agarose at 90°C. COL I was extracted from rat tails according to a published protocol (54). GelMA was synthesized according to an established protocol (55). Detailed preparation methods of both materials can be found in the Supplementary Materials.