Here, we introduce an engineered, tunable genetic switch that couples repressor proteins and an RNAi target design to effectively turn any gene off. We used the switch to regulate the expression of EGFP in mouse and human cells and found that it offers >99% repression as well as the ability to tune gene expression. To demonstrate the system's modularity and level of gene silencing, we used the switch to tightly regulate the expression of diphtheria toxin and Cre recombinase, respectively. We also used the switch to tune the expression of a proapoptotic gene and show that a threshold expression level is required to induce apoptosis. This work establishes a system for tight, tunable control of mammalian gene expression that can be used to explore the functional role of various genes as well as to determine whether a phenotype is the result of a threshold response to changes in gene expression.

Combining synthetic biology and materials science will enable more advanced studies of cellular regulatory processes, in addition to facilitating therapeutic applications of engineered gene networks. One approach is to couple genetic inducers into biomaterials, thereby generating 3D microenvironments that are capable of controlling intrinsic and extrinsic cellular events. Here, we have engineered biomaterials to present the genetic inducer, IPTG, with different modes of activating genetic circuits in vitro and in vivo. Gene circuits were activated in materials with IPTG embedded within the scaffold walls or chemically linked to the matrix. In addition, systemic applications of IPTG were used to induce genetic circuits in cells encapsulated into materials and implanted in vivo. The flexibility of modifying biomaterials with genetic inducers allows for patterned placement of these inducers that can be used to generate distinct patterns of gene expression. Together, these genetically interactive materials can be used to characterize genetic circuits in environments that more closely mimic cells' natural 3D settings, to better explore complex cell-matrix and cell-cell interactions, and to facilitate therapeutic applications of synthetic biology.T he complexity of cell signaling can be simplified by considering genetic networks composed of subsets of simpler parts, or modules. This simplification is the foundation of synthetic biology, where engineering paradigms are applied in rational and systematic ways to produce predictable and robust systems for understanding mechanisms of cellular function (1-6). The majority of work in synthetic biology has been in simple organisms, such as yeast and bacteria. However, as synthetic biology starts to expand to mammalian systems, it becomes increasingly more important to consider the environment in which the cells are grown. Biomaterials will play an important role in advancing synthetic biology within mammalian systems, because they provide highly controllable and tunable microenvironments where cells can behave as they do in vivo, in addition to organizing and delivering therapeutic cells to locations of interest. Our results show that interfacing synthetic biology and biomaterials can catalyze synthetic biology applications through engineered biomaterials that actively control genetic circuits in 3D scaffolds to more closely mimic the cells' natural settings, in addition to providing mechanisms for translating synthetic biology for clinical applications (Fig. S1).Biomaterials provide 3D environments for cell growth and have rapidly advanced our ability to investigate the coordinated interactions of many cellular phenomena because biomaterials recapitulate the in vivo setting better than traditional 2D cultures, where cells are grown in monolayers (7,8). Physiological development, homeostasis, and regeneration each require a complex interplay of multiple signals that originate from the extracellular matrix (ECM) and from the intrinsic cellular control of gene products (9). Wh...

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