Biofabricated film with enzymatic and redox-capacitor functionalities to harvest and store electrons

Visualized sensing of erythritol using a simple enzyme-free catechol-based hydrogel film

Based on the versatile properties of bio-derived materials, non-enzymatic assays in combination with electronic devices have attracted increasing interest. Here, we report a novel enzyme-free visualization approach for the detection of erythritol, which is a zero-calorie natural sweetener and serves as an ideal sucrose substitute for diabetics or overweight people who need sugar control. The recognition element of the electrochemical biosensor was constructed by catechol modification on a chitosan-based hydrogel film. The signal transduction was achieved by the competitive binding assay of sweeteners. The results show that 2-fluorophenylboronic acid (FPBA) can form a cyclic boronate ester with the ortho-hydroxyls of both reduced catechol and oxidized quinone, impeding the electron transfer and leading to redox signal attenuation. The addition of sweeteners caused a competitive reaction resulting in bonding between the 1,2-diols and FPBA moieties, and in the recovery of the redox signals. Importantly, the pattern of redox signal changes of catechol can be detected optically, as the oxidized quinone state is darker in color than the reduced catechol state. Using a simple cell phone imaging application, we demonstrate that erythritol can be distinguished from other sweeteners in real samples using the oxidized catechol-Chit0/agarose hydrogel film. Thus, we envision that this method could allow diabetics and people who need to control their sugar intake to detect whether the product contains only erythritol in the field or at home. In addition, this work further illustrates the potential of bio-derived materials for performing redox-based functions and enzyme-free visualization assays.

Living electronics: A catalogue of engineered living electronic components

Biology leverages a range of electrical phenomena to extract and store energy, control molecular reactions and enable multicellular communication. Microbes, in particular, have evolved genetically encoded machinery enabling them to utilize the abundant redox-active molecules and minerals available on Earth, which in turn drive global-scale biogeochemical cycles. Recently, the microbial machinery enabling these redox reactions have been leveraged for interfacing cells and biomolecules with electrical circuits for biotechnological applications. Synthetic biology is allowing for the use of these machinery as components of engineered living materials with tuneable electrical properties. Herein, we review the state of such living electronic components including wires, capacitors, transistors, diodes, optoelectronic components, spin filters, sensors, logic processors, bioactuators, information storage media and methods for assembling these components into living electronic circuits.

Catechol-Based Capacitor for Redox-Linked Bioelectronics

A common bioelectronics goal is to enable communication between biology and electronics, and success is critically dependent on the communication modality. When a biorelevant modality aligns with instrumentation capabilities, remarkable successes have been observed (e.g., electrodes provide a powerful tool to observe and actuate biology through its ion-based electrical modality). Emerging biological research demonstrates that redox is another biologically relevant modality, and recent research has shown that advanced electrochemical methods enable biodevice communication through this redox modality. Here, we briefly summarize the biological relevance of this redox modality and the use of redox mediators to enable access to this modality through electrochemical measurements. Next, we describe the fabrication of a catechol-chitosan redox capacitor that is redox-active but nonconducting and thus offers a unique set of molecular electronic properties that enhance access to redox-based information. Finally, we cite several recent studies that demonstrate the broad potential for this capacitor to access redox-based biological information. In summary, we envision the redox capacitor will become a vital component in the integrated circuitry of redox-linked bioelectronics.

Redox-Based Synthetic Biology Enables Electrochemical Detection of the Herbicides Dicamba and Roundup via Rewired Escherichia coli

Synthetic biology is typically exploited to endow bacterial cells with new biosynthetic capabilities. It can also serve to create "smart" bacteria such as probiotics that detect and treat disease. Here, we show how minimally rewiring the genetic regulation of bacterial cells can enable their ability to recognize and report on chemical herbicides, including those routinely used to clear weeds from gardens and crops. In so doing, we demonstrate how constructs of synthetic biology, in this case redox-based synthetic biology, can serve as a vector for information flow mediating molecular communication between biochemical systems and microelectronics. We coupled the common genetic reporter, β-galactosidase, with the E. coli superoxide response regulon promoter pSoxS, for detection of the herbicides dicamba and Roundup. Both herbicides activated our genetic construct in a concentration dependent manner. Results indicate robust detection using spectrophotometry, via the Miller assay, and electrochemistry using the enzymatic cleavage of 4-aminophenyl β-d-galactopyranoside into the redox active molecule p-aminophenol. We found that environmental components, in particular, the availability of glucose, are important factors for the cellular detection of dicamba. Importantly, both herbicides were detected at concentrations relevant for aquatic toxicity.

Electrobiofabrication: electrically based fabrication with biologically derived materials

While conventional material fabrication methods focus on form and strength to achieve function, the fabrication of material systems for emerging life science applications will need to satisfy a more subtle set of requirements. A common goal for biofabrication is to recapitulate complex biological contexts (e.g. tissue) for applications that range from animal-on-a-chip to regenerative medicine. In these cases, the material systems will need to: (i) present appropriate surface functionalities over a hierarchy of length scales (e.g. molecular features that enable cell adhesion and topographical features that guide differentiation); (ii) provide a suite of mechanobiological cues that promote the emergence of native-like tissue form and function; and (iii) organize structure to control cellular ingress and molecular transport, to enable the development of an interconnected cellular community that is engaged in cell signaling. And these requirements are not likely to be static but will vary over time and space, which will require capabilities of the material systems to dynamically respond, adapt, heal and reconfigure. Here, we review recent advances in the use of electrically based fabrication methods to build material systems from biological macromolecules (e.g. chitosan, alginate, collagen and silk). Electrical signals are especially convenient for fabrication because they can be controllably imposed to promote the electrophoresis, alignment, self-assembly and functionalization of macromolecules to generate hierarchically organized material systems. Importantly, this electrically based fabrication with biologically derived materials (i.e. electrobiofabrication) is complementary to existing methods (photolithographic and printing), and enables access to the biotechnology toolbox (e.g. enzymatic-assembly and protein engineering, and gene expression) to offer exquisite control of structure and function. We envision that electrobiofabrication will emerge as an important platform technology for organizing soft matter into dynamic material systems that mimic biology's complexity of structure and versatility of function.

Selective assembly and functionalization of miniaturized redox capacitor inside microdevices for microbial toxin and mammalian cell cytotoxicity analyses

We report a novel strategy for bridging information transfer between electronics and biological systems within microdevices. This strategy relies on our "electrobiofabrication" toolbox that uses electrode-induced signals to assemble biopolymer films at spatially defined sites and then electrochemically "activates" the films for signal processing capabilities. Compared to conventional electrode surface modification approaches, our signal-guided assembly and activation strategy provides on-demand electrode functionalization, and greatly simplifies microfluidic sensor design and fabrication. Specifically, a chitosan film is selectively localized in a microdevice and is covalently modified with phenolic species. The redox active properties of the phenolic species enable the film to transduce molecular to electronic signals (i.e., "molectronic"). The resulting "molectronic" sensors are shown to facilitate the electrochemical analysis in real time of biomolecules, including small molecules and enzymes, to cell-based measurements such as cytotoxicity. We believe this strategy provides an alternative, simple, and promising avenue for connecting electronics to biological systems within microfluidic platforms, and eventually will enrich our abilities to study biology in a variety of contexts.

Biofabricating Functional Soft Matter Using Protein Engineering to Enable Enzymatic Assembly

Biology often provides the inspiration for functional soft matter, but biology can do more: it can provide the raw materials and mechanisms for hierarchical assembly. Biology uses polymers to perform various functions, and biologically derived polymers can serve as sustainable, self-assembling, and high-performance materials platforms for life-science applications. Biology employs enzymes for site-specific reactions that are used to both disassemble and assemble biopolymers both to and from component parts. By exploiting protein engineering methodologies, proteins can be modified to make them more susceptible to biology's native enzymatic activities. They can be engineered with fusion tags that provide (short sequences of amino acids at the C- and/or N- termini) that provide the accessible residues for the assembling enzymes to recognize and react with. This "biobased" fabrication not only allows biology's nanoscale components (i.e., proteins) to be engineered, but also provides the means to organize these components into the hierarchical structures that are prevalent in life.

Connecting Biology to Electronics: Molecular Communication via Redox Modality

Biology and electronics are both expert at for accessing, analyzing, and responding to information. Biology uses ions, small molecules, and macromolecules to receive, analyze, store, and transmit information, whereas electronic devices receive input in the form of electromagnetic radiation, process the information using electrons, and then transmit output as electromagnetic waves. Generating the capabilities to connect biology-electronic modalities offers exciting opportunities to shape the future of biosensors, point-of-care medicine, and wearable/implantable devices. Redox reactions offer unique opportunities for bio-device communication that spans the molecular modalities of biology and electrical modality of devices. Here, an approach to search for redox information through an interactive electrochemical probing that is analogous to sonar is adopted. The capabilities of this approach to access global chemical information as well as information of specific redox-active chemical entities are illustrated using recent examples. An example of the use of synthetic biology to recognize external molecular information, process this information through intracellular signal transduction pathways, and generate output responses that can be detected by electrical modalities is also provided. Finally, exciting results in the use of redox reactions to actuate biology are provided to illustrate that synthetic biology offers the potential to guide biological response through electrical cues.

Review of Electrochemically Triggered Macromolecular Film Buildup Processes and Their Biomedical Applications

Macromolecular coatings play an important role in many technological areas, ranging from the car industry to biosensors. Among the different coating technologies, electrochemically triggered processes are extremely powerful because they allow in particular spatial confinement of the film buildup up to the micrometer scale on microelectrodes. Here, we review the latest advances in the field of electrochemically triggered macromolecular film buildup processes performed in aqueous solutions. All these processes will be discussed and related to their several applications such as corrosion prevention, biosensors, antimicrobial coatings, drug-release, barrier properties and cell encapsulation. Special emphasis will be put on applications in the rapidly growing field of biosensors. Using polymers or proteins, the electrochemical buildup of the films can result from a local change of macromolecules solubility, self-assembly of polyelectrolytes through electrostatic/ionic interactions or covalent cross-linking between different macromolecules. The assembly process can be in one step or performed step-by-step based on an electrical trigger affecting directly the interacting macromolecules or generating ionic species.