Table 1.

List of potential Matrigel Mattress problems and solutions.

Problem	Possible Reason	Solution
Matrigel solidifies in pipette tip	Matrigel has reached room temperature	Work quickly
Mattress in oblique line	Pipetting too quickly	Slow down
Poor quality mattresses	Preparing too many mattress simultaneously	Prepare less until proper technique is established
Mattress implodes	Short incubation	Incubate mattresses for 8–10 minutes prior to seeding hiPSC-CMs
hiPSC-CMs do not contract on mattress	Mattresses are too thick or thin	Apply Matrigel at a thickness of 0.4 mm
Cells dissociate from mattress	Perfusion or aspiration too strong during daily media changes	Decrease aspiration strength
Mattress ruptures or dislodges from dish	Media forcefully applied to dish	Gently add media to the corner of the dish; not directly on the mattress