Agarose

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 (link)). GelMA was synthesized according to an established protocol (55 (link)). Detailed preparation methods of both materials can be found in the Supplementary Materials.

Sera from immunized mice were tested for neutralizing activity against authentic SARS-CoV-2 infection using plaque reduction neutralization assay as described below.²⁵ (link)^,26 (link) Specifically, serially diluted sera were incubated with the SARS-CoV-2 original strain, Delta (B.1.617.2) or Omicron (B.1.1.529) variant (40–80 plaque-forming unit (PFU)/well) at 37°C for 1 h. The virus-serum mixture was then added to Vero E6 (for original strain and Delta variant) or Vero E6 cells in the presence of ACE2 and TMPRSS2 (for Omicron variant) at 37°C for 45 min. The inoculum was removed, followed by overlaying the cells with 0.6% agarose (Research Products International) and culturing them for three days. The plaques were stained with 0.1% crystal violet (Fisher Scientific). The neutralizing antibody titer was calculated as NT₅₀ (representing highest serum dilution that reduced the number of virus plaques by 50%).