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Three-dimensional (3D) printing can be beneficial to tissue engineers and the regenerative medicine community because of its potential to rapidly build elaborate 3D structures from cellular and material inks. However, predicting changes to the structure and pattern of printed tissues arising from the mechanical activity of constituent cells is technically and conceptually challenging. This perspective is targeted to scientists and engineers interested in 3D bioprinting, but from the point of view of cells and tissues as mechanically active living materials. The dynamic forces generated by cells present unique challenges compared to conventional manufacturing modalities but also offer profound opportunities through their capacity to self-organize. Consideration of self-organization following 3D printing takes the design and execution of bioprinting into the fourth dimension of cellular activity. We therefore propose a framework for dynamic bioprinting that spatiotemporally guides the underlying biology through reconfigurable material interfaces controlled by 3D printers. 

Engineered living materials combine the advantages of biological and synthetic systems by leveraging genetic and metabolic programming to control material-wide properties. Here, we demonstrate that extracellular electron transfer (EET), a microbial respiration process, can serve as a tunable bridge between live cell metabolism and synthetic material properties. In this system, EET flux from Shewanella oneidensis to a copper catalyst controls hydrogel cross-linking via two distinct chemistries to form living synthetic polymer networks. We first demonstrate that synthetic biology-inspired design rules derived from fluorescence parameterization can be applied toward EET-based regulation of polymer network mechanics. We then program transcriptional Boolean logic gates to govern EET gene expression, which enables design of computational polymer networks that mechanically respond to combinations of molecular inputs. Finally, we control fibroblast morphology using EET as a bridge for programmed material properties. Our results demonstrate how rational genetic circuit design can emulate physiological behavior in engineered living materials. 

Abstract Organic electrochemical transistors (OECTs) are ideal devices for translating biological signals into electrical readouts and have applications in bioelectronics, biosensing, and neuromorphic computing. Despite their potential, developing programmable and modular methods for living systems to interface with OECTs has proven challenging. Here we describe hybrid OECTs containing the model electroactive bacteriumShewanella oneidensisthat enable the transduction of biological computations to electrical responses. Specifically, we fabricated planar p-type OECTs and demonstrated that channel de-doping is driven by extracellular electron transfer (EET) fromS. oneidensis. Leveraging this mechanistic understanding and our ability to control EET flux via transcriptional regulation, we used plasmid-based Boolean logic gates to translate biological computation into current changes within the OECT. Finally, we demonstrated EET-driven changes to OECT synaptic plasticity. This work enables fundamental EET studies and OECT-based biosensing and biocomputing systems with genetically controllable and modular design elements. 

Significance Metabolic engineering benefits from the tunable and tightly controlled transformations afforded by biological systems. However, these reactions have generally been limited to naturally occurring pathways and products. In this work, we coopt metabolic electron transfer fromShewanella oneidensisto control the activity of an exogenous metal catalyst in an abiotic reaction scheme: atom-transfer radical polymerization. In the presence ofS. oneidensis, polymerizations exhibited well-defined kinetics and yielded polymers with controlled molecular weights and low polydispersities. Additionally, polymerization activity was dependent on electroactive metabolism and specific electron transport proteins, both of which provide handles to control material synthesis. This work serves as a proof-of-principle toward expanding the scope of reactions available to metabolic engineers to include previously discovered transition-metal–catalyzed reactions.