It is likely that arrhythmias should be avoided for therapies based on human 
pluripotent stem cell (hPSC)-derived cardiomyocytes (CM) to be effective. 
Towards achieving this goal, we introduced light-activated channelrhodopsin-2 
(ChR2), a cation channel activated with 480 nm light, into human embryonic stem 
cells (hESC). By using in vitro approaches, hESC-CM are able to be activated 
with light. ChR2 is stably transduced into undifferentiated hESC via a 
lentiviral vector. Via directed differentiation, hESC(ChR2)-CM are produced and 
subjected to optical stimulation. hESC(ChR2)-CM respond to traditional 
electrical stimulation and produce similar contractility features as their 
wild-type counterparts but only hESC(ChR2)-CM can be activated by optical 
stimulation. Here it is shown that a light sensitive protein can enable in vitro 
optical control of hESC-CM and that this activation occurs optimally above 
specific light stimulation intensity and pulse width thresholds. For future 
therapy, in vivo optical stimulation along with optical inhibition could allow 
for acute synchronization of implanted hPSC-CM with patient cardiac rhythms.