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Despite the promise of engineered tissue implants for the treatment of organ failure, scaling of these constructs to sizes of therapeutic relevance remains a barrier to clinical translation. Here, we propose a strategy to circumvent this limitation: to instead implant a small-scale construct and then induce it to grow in situ after its engraftment into a host. Using engineered liver tissue as a proof-of-concept application, we integrated synthetic biology and tissue engineering tools to build liver tissues that can be expanded on-demand after implantation in vivo. To achieve this goal, we first identified the combination of Yes-associated protein (YAP) and growth factor (GF) signaling as sufficient to drive human hepatocyte proliferation in dense, three-dimensional engineered tissues. We then engineered control of these signaling axes using synthetic biology tools to drive human liver tissue expansion both in vitro and in vivo. As such, this work establishes a genetic strategy for generating large organ implants through bioengineered on-demand outgrowth via synthetic biology triggering (BOOST).