Unlocking Wearable Microbial Fuel Cells for Advanced Wound Infection Treatment.

Better infection control will accelerate wound healing and alleviate associated healthcare burdens. Traditional antibacterial dressings often inadequately control infections, inadvertently promoting antibacterial resistance. Our research unveils a novel, dual-functional living dressing that autonomously generates antibacterial agents and delivers electrical stimulation, harnessing the power of spore-forming Bacillus subtilis. This dressing is built on an innovative wearable microbial fuel cell (MFC) framework, using B. subtilis endospores as a powerful, dormant biocatalyst. The endospores are resilient, reactivating in nutrient-rich wound exudate to produce electricity and antibacterial compounds. The combination allows B. subtilis to outcompete pathogens for food and other resources, thus fighting infections. The strategy is enhanced by the extracellular synthesis of tin oxide and copper oxide nanoparticles on the endospore surface, boosting antibacterial action, and electrical stimulation. Moreover, the MFC framework introduces a pioneering dressing design featuring a conductive hydrogel embedded within a paper-based substrate. The arrangement ensures cell stability and sustains a healing-friendly moist environment. Our approach has proven very effective against three key pathogens in biofilms: Pseudomonas aeruginosa, Escherichia coli, and Staphylococcus aureus demonstrating exceptional capabilities in both in vitro and ex vivo models. Our innovation marks a significant leap forward in wearable MFC-based wound care, offering a potent solution for treating infected wounds.

Combined electrical-electrochemical phenotypic profiling of antibiotic susceptibility of in vitro biofilm models.

More than 65% of bacterial infections are caused by biofilms. However, standard biofilm susceptibility tests are not available for clinical use. All conventional biofilm models suffer from a long formation time and fail to mimic in vivo microbial biofilm conditions. Moreover, biofilms make it difficult to monitor the effectiveness of antibiotics. This work creates a powerful yet simple method to form a target biofilm and develops an innovative approach to monitoring the antibiotic's efficacy against a biofilm-associated infection. A paper-based culture platform can provide a new strategy for rapid microbial biofilm formation through capillary action. A combined electrical-electrochemical technique monitors bacterial metabolism rapidly and reliably by measuring microbial extracellular electron transfer (EET) and using electrochemical impedance spectroscopy (EIS) across a microbe-electrode interface. Three representative pathogens, Pseudomonas aeruginosa, Escherichia coli, and Staphylococcus aureus, form their biofilms controllably within an hour. Within another hour their susceptibilities to three frontline antibiotics with different action modes (gentamicin, ciprofloxacin, and ceftazidime) are examined. Our antibiotic susceptibility testing (AST) technique provides a quantifiable minimum inhibitory concentration (MIC) of those antibiotics against the in vitro biofilm models and characterizes their action mechanisms. The results will have an important positive effect because they provide immediately actionable healthcare information at a reduced cost, revolutionizing public healthcare.

All-electrical antibiotic susceptibility and resistance profiling of electrogenic Pseudomonas aeruginosa.

There is a pressing need for evidence-based, non-surgical therapy guidance for biofilm-based infections. Conventional phenotypic or genotypic or emerging antibiotic susceptibility testing (AST) techniques cannot provide clinically relevant guidelines and widely adaptable stewardship for effective biofilm treatment because they are mainly limited to planktonic bacteria and suffer from many technical and operational challenges. Here, we created an all-electrical, reliable, rapid AST device to monitor antibiotic efficacy in bacterial biofilms that can be practically translatable to clinical settings and industrial antibiotic developments. The electrons metabolically produced by a Pseudomonas aeruginosa biofilm provided a strong signal for monitoring bacterial growth and treatment efficacy while a 3-D paper-based culturing platform provided a new strategy for rapid biofilm formation through capillary action. When antibiotics are effective against the pathogenic biofilm, their metabolic activities are inhibited, decreasing their electron transfer reactions. The changes in electrical outputs can be measured to assess the treatment effectiveness against pathogenic biofilms. Within 100 minutes, our six-well AST device successfully distinguished antibiotic-susceptible and -resistant P. aeruginosa biofilms, provided a quantifiable minimum inhibitory concentration (MIC) of antibiotics, and characterized the bacterial antibiotic action mechanisms.

Moisture-Enabled Germination of Heat-Activated Bacillus Endospores for Rapid and Practical Bioelectricity Generation: Toward Portable, Storable Bacteria-Powered Biobatteries.

Small-scale battery-like microbial fuel cells (MFCs) are a promising alternative power source for future low-power electronics. Controllable microbial electrocatalytic activity in a miniaturized MFC with unlimited biodegradable energy resources would enable simple power generation in various environmental settings. However, the short shelf-life of living biocatalysts, few ways to activate the stored biocatalysts, and extremely low electrocatalytic capabilities render the miniature MFCs unsuitable for practical use. Here, heat-activated Bacillus subtilis spores are revolutionarily used as a dormant biocatalyst that can survive storage and rapidly germinate when exposed to special nutrients that are preloaded in the device. A microporous, graphene hydrogel allows the adsorption of moisture from the air, moves the nutrients to the spores, and triggers their germination for power generation. In particular, forming a CuO-hydrogel anode and an Ag2 O-hydrogel cathode promotes superior electrocatalytic activities leading to an exceptionally high electrical performance in the MFC. The battery-type MFC device is readily activated by moisture harvesting, producing a maximum power density of 0.4 mW cm-2 and a maximum current density of 2.2 mA cm-2 . The MFC configuration is readily stackable in series and a three-MFC pack produces enough power for several low-power applications, demonstrating its practical feasibility as a sole power source.

Integrated Papertronic Techniques: Highly Customizable Resistor, Supercapacitor, and Transistor Circuitry on a Single Sheet of Paper.

Humanity's excessive production of material waste poses a critical environmental threat, and the problem is only escalating, especially in the past few decades with the rapid development of powerful electronic tools and persistent consumer desire to upgrade to the newest available technology. The poor disposability of electronics is especially an issue for the newly arising field of single-use devices and sensors, which are often used to evaluate human health and monitor environmental conditions, and for other novel applications. Though impressive in terms of function and convenience, usage of conventional electronic components in these applications would inflict an immense surge in waste and result in higher costs. This work's primary objective is to develop a cost-effective, eco-friendly, all-paper, device for single-use applications that can be easily and safely disposed of through incineration or biodegradation. All electronic components are paper-based and integrated on paper-based printed circuit boards (PCBs), innovatively providing a realistic and practical solution for green electronic platforms. In particular, a methodology is discussed for simultaneously achieving very tunable resistors (20 Ω to 285 kΩ), supercapacitors (∼3.29 mF), and electrolyte-gated field-effect transistors on and within the thickness of a single sheet of paper. Each electronic component is completely integrated into functionalized paper regions and exhibits favorable electrical activity, adjustability, flexibility, and disposability. A simple amplifier circuit is successfully demonstrated within a small area and within the thickness of a single sheet of paper, displaying component versatility and the capability for their fabrication processes to be performed in parallel for efficient and rapid development.

Accelerated antibiotic susceptibility testing of pseudomonas aeruginosa by monitoring extracellular electron transfer on a 3-D paper-based cell culture platform.

Pseudomonas aeruginosa is the most important opportunistic pathogen leading to serious and life-threatening infections, especially in immunocompromised patients. Because of its remarkable capacity to resist antibiotics, the selection of the right antibiotics with the exact dose for the appropriate duration is critical to effectively treat the infections and prevent antibiotic resistance. Although conventional genotypic and phenotypic antibiotic susceptibility testing (AST) methods have been dramatically advanced, they have suffered from many technical and operational issues as a generalized antibiotic stewardship program. Furthermore, given that most microbial infections are caused by their biofilms, the existing AST methods do not provide evidence-based antibiotic prescribing guidance for biofilm-based infections because the results are based on individual bacteria traditionally grown in their planktonic form. In this work, we create an innovative susceptibility testing technique for P. aeruginosa that offers clinically relevant guidelines and widely adaptable stewardship to effectively treat the infections and minimize antibiotic resistance. Our approach evaluates the antibiotic efficacy by continuously monitoring the accumulated electrical outputs from the bacterial extracellular electron transfer (EET) process in the presence of antibiotics. Our innovative paper-based culturing 3-D scaffold promotes surface-associated growth of bacterial colonies and biofilms. The platform replicates a natural habitat for P. aeruginosa where it can grow similarly to sites it infects. Our technique enables an all-electrical, real-time, easy-to-use, portable AST that can be easily translatable to clinical settings. The entire procedure takes 96 min to provide evidence-based antimicrobial prescribing guidance for biofilm-based infections.

Biofabrication and characterization of multispecies electroactive biofilms in stratified paper-based scaffolds.

Bioelectrochemical technologies have attracted significant scientific interest because the effective bacterial electron exchange with external electrodes can provide a sustainable solution that joins environmental remediation and energy recovery. Multispecies electroactive bacterial biofilms are catalysts that will drive the operation of bioelectrochemical devices. Unfortunately, there is a lack of understanding of key mechanisms determining their electron-generating capabilities and syntrophic relations within microbial communities in biofilms. This is because there are no universally standardized models for simple, rapid, reliable, and cost-effective fabrication and characterization of electroactive multispecies biofilms. The heterogeneous and long-term nature of biofilm formation has hampered the development of those models. This work develops novel biofabrication and analysis platforms by creating innovative, paper-based 3-D systems that accurately recapitulate the structure, function, and physiology of living multispecies biofilms. Multiple layers of paper containing bacterial cells were stacked to simulate different layered 3-D biofilm models with defined cellular compositions and microenvironments. Overall bacterial electrogenic capabilities through the biofilm structures were characterized by thoroughly monitoring collective electron flows through different external resistors. Changes in the type of species and order of stacking created biofilm modeling which allowed for the study of their electrogenic performance via variation in electron flow rate output. Furthermore, multi-laminate structures allowed for straightforward de-stacking and layer-by-layer separation for analyses of pH distribution and cellular viability. Our multi-laminate structures provide a new strategy for (i) controlling the biofilm geometry of 3-D bacterial cultures, (ii) monitoring the microbial electoral properties, and (iii) constructing an artificial biofilm layer by layer.

A sweat-activated, wearable microbial fuel cell for long-term, on-demand power generation.

In this work, we enabled on-demand, long-functioning, sweat-based power generation through a wearable paper-based microbial fuel cell (MFC) using a novel spore-forming biocatalyst, Bacillus subtilis. The MFC is sustainable and survivable even in the extreme environmental conditions of human skin. B. subtilis, usually found on the skin, was able to form endospores that endure extreme dryness or nutrient limitation when sweat access was limited or unpredictable for humans at rest, offering long-term operation and stable storage. When human sweat was introduced, spore germination and gradual power generation were observed without adding nutrient germinants. Through repeated sporulation and germination depending on the sweat availability, B. subtilis provided a sustainable solution for an innovative sweat-activated power source that can result in the long-lasting vision of self-sustaining wearable electronics. Even after the 48-h operation, the device generated a maximum power density of 24 µW/cm2 and a maximum current density of 175 µA/cm2, which is comparable to or even higher than the previously reported paper-based MFCs using well-known strong exoelectrogens in an optimized bacterial medium. Furthermore, B. subtilis in sweat was shown to be commensal with other skin microorganisms while producing antibiotic substances that were effective against potential pathogens, exhibiting a great potential for seamless and intimate integration with skin-mountable applications.

Electrogenic Bacteria Promise New Opportunities for Powering, Sensing, and Synthesizing.

Considerable research efforts into the promises of electrogenic bacteria and the commercial opportunities they present are attempting to identify potential feasible applications. Metabolic electrons from the bacteria enable electricity generation sufficient to power portable or small-scale applications, while the quantifiable electric signal in a miniaturized device platform can be sensitive enough to monitor and respond to changes in environmental conditions. Nanomaterials produced by the electrogenic bacteria can offer an innovative bottom-up biosynthetic approach to synergize bacterial electron transfer and create an effective coupling at the cell-electrode interface. Furthermore, electrogenic bacteria can revolutionize the field of bioelectronics by effectively interfacing electronics with microbes through extracellular electron transfer. Here, these new directions for the electrogenic bacteria and their recent integration with micro- and nanosystems are comprehensively discussed with specific attention toward distinct applications in the field of powering, sensing, and synthesizing. Furthermore, challenges of individual applications and strategies toward potential solutions are provided to offer valuable guidelines for practical implementation. Finally, the perspective and view on how the use of electrogenic bacteria can hold immeasurable promise for the development of future electronics and their applications are presented.

Bioelectricity production from sweat-activated germination of bacterial endospores.

A microbial fuel cell is created that uses a bacterium's natural ability to revive from dormancy to provide on-demand power for next-generation wearable applications. In adverse conditions, Bacillus subtilis responds by becoming endospores that serve as a dormant biocatalyst embedded in a skin-mountable paper-based microbial fuel cell. When activated by nutrient-rich human sweat, the germinating bacteria produce enough electricity to operate small devices, such as the calculator that we operated to test our methodology. The spore germination is artificially accelerated by nutritious germinants, which are pre-loaded on the skin-contacting bottom layer of the device, absorb the released sweat, and deliver a mixture of the dissolved germinants and sweat to the spores. When the skin-mountable device is applied to the arm of a sweating volunteer, it can generate a maximum power density of 16.6 µW/cm2 through bacterial respiratory activity. A potential risk of bacteria leakage from the device is minimized by packaging with a small pore size paper so that bacterial spores and germinated cells cannot pass through. When three serially connected devices are integrated into a single on-chip platform and energized by sweat, a significantly enhanced power density of 56.6 µW/cm2 is generated, powering an electrical calculator. After three weeks of dormant storage, the device exhibits no significant decrease in electrical output when activated by sweat. After use, the device is easily incinerated without risking bacterial infection. This work demonstrates the promising potential of the spore-forming microbial fuel cell as a disposable and long storage life power source for next-generation wearable applications.

Miniature microbial solar cells to power wireless sensor networks.

Conventional wireless sensor networks (WSNs) powered by traditional batteries or energy storage devices such as lithium-ion batteries and supercapacitors have challenges providing long-term and self-sustaining operation due to their limited energy budgets. Emerging energy harvesting technologies can achieve the longstanding vision of self-powered, long-lived sensors. In particular, miniature microbial solar cells (MSCs) can be the most feasible power source for small and low-power sensor nodes in unattended working environments because they continuously scavenge power from microbial photosynthesis by using the most abundant resources on Earth; solar energy and water. Even with low illumination, the MSC can harvest electricity from microbial respiration. Despite the vast potential and promise of miniature MSCs, their power and lifetime remain insufficient to power potential WSN applications. In this overview, we will introduce the field of miniature MSCs, from early breakthroughs to current achievements, with a focus on emerging techniques to improve their performance. Finally, challenges and perspectives for the future direction of miniature MSCs to self-sustainably power WSN applications will be given.

Characterization of Electrogenic Gut Bacteria.

While electrogenic, or electricity-producing, Gram-negative bacteria predominantly found in anaerobic habitats have been intensively explored, the potential of Gram-positive microbial electrogenic capability residing in a similar anoxic environment has not been considered. Because Gram-positive bacteria contain a thick non-conductive cell wall, they were previously believed to be very weak exoelectrogens. However, with the recent discovery of electrogenicity by Gram-positive pathogens and elucidation of their electron-transfer pathways, significant and accelerated attention has been given to the discovery and characterization of these pathways in the members of gut microbiota. The discovery of electrogenic bacteria present in the human gut and the understanding of their electrogenic capacity opens up possibilities of bacterial powered implantable batteries and provide a novel biosensing platform to monitor human gastrointestinal health. In this work, we characterized microbial extracellular electron-transfer capabilities and capacities of five gut bacteria: Staphylococcus aureus, Enterococcus faecalis, Streptococcus agalactiae, Lactobacillus reuteri, and Lactobacillus rhamnosus. A 21-well paper-based microbial fuel cell array with enhanced sensitivity was developed as a powerful yet simple screening method to accurately and simultaneously characterize bacterial electrogenicity. S. aureus, E. faecalis, and S. agalactiae exhibited distinct electrogenic capabilities, and their power generations were comparable to that of the well-known Gram-negative exoelectrogen, Shewanella oneidensis. Importantly, this system was used to begin a large-scale transposon screen to examine the genes involved in electrogenicity by the human pathobiont S. aureus.

A Disposable, Papertronic Three-Electrode Potentiostat for Monitoring Bacterial Electrochemical Activity.

Bacterial electrochemical activities can promote sustainable energy and environmental engineering applications. Characterizing their ability is critical for effectively adopting these technologies. Conventional studies of the electroactive bacteria are limited to insensitive, time-consuming, and labor-intensive two-electrode microbial fuel cell (MFC) techniques. Even the latest miniaturized MFC array is limited by irreproducibility and uncontrollability. In this work, we created a 4-well electrochemical sensing array with an integrated, custom-made three-electrode potentiostat to provide a controllable analytic capability without unwanted perturbations. A simple potentiostat circuit used two operational amplifiers and one resistor, allowing chronoamperometric and staircase voltammetric analyses of three well-known electroactive bacteria species: Shewanella oneidensis MR1, Pseudomonas aeruginosa PAO1, and Bacillus subtilis. Portability and disposability were emphasized by integrating all the functions into a paper substrate, which makes analyses possible at the point-of-use and in resource-limited settings without a bulky and expensive benchtop potentiostat. After use, the papertronic system was disposed of safely by incineration without posing any bacterial cytotoxic risks. This novel sensing platform creates an inexpensive, scalable, time-saving, high-performance, and user-friendly platform that facilitates the study of fundamental electrocatalytic activities of bacteria.

A simple, inexpensive, and rapid method to assess antibiotic effectiveness against exoelectrogenic bacteria.

A sufficiently fast and simple antimicrobial susceptibility testing (AST) is urgently required to guide effective antibiotic usages and to surveil the antimicrobial resistance rate. Here, we establish a rapid, quantitative, and high-throughput phenotypic AST by measuring electrons transferred from the interiors of microbial cells to external electrodes. Because the transferred electrons are based on microbial metabolic activities and are inversely proportional to the concentration of potential antibiotics, the changes in electrical outputs can be readily used as a transducing signal to efficiently monitor bacterial growth and antibiotic susceptibility. The sensing is performed by directly measuring the total energy, or all the accumulated microbial electricity, generated by microbial fuel cells (MFCs) arranged in a large-capacity disposable, paper-based testbed. A common Gram-negative pathogenic bacterium Pseudomonas aeruginosa wild-type PAO1 and first-line antibiotic gentamicin (GEN) are used in our experiments. The minimum inhibitory concentration (MIC) values generated from our technique are validated by the gold standard broth microdilution (BMD). Our new approach provides quantitative, actionable MIC results within just 5 h because it measures electricity produced by bacterial metabolism instead of the days needed for growth-observation methods. Moreover, as the equipment needed is simple, common, and inexpensive, our test has immense potential to be adopted in the field or resource-limited hospitals and labs to provide insightful assessments for research and clinical practices.

A portable papertronic sensing system for rapid, high-throughput, and visual screening of bacterial electrogenicity.

Electrogenic bacteria or exoelectrogens can transfer electrons to extracellular electron acceptors and thus have a wide range of applications to the ever-emerging fields of bioenergy, bioremediation, and biosensing. Standard state-of-the-art techniques for screening of electrogenic bacteria are inefficient, and often prevent rapid, high-throughput analyses. Herein, we created a simple, rapid, and straightforward papertronic 4- and 16-channel sensing platforms that is connected to a visual readout, allowing the naked eye to evaluate and quantify direct bacterial electrogenic capabilities. Our system integrated multiple 2-electrode sensing units into a signal amplifier circuit connected to light-emitting diode (LED) reporting units. The current generated from electrogenic bacteria in the sensing unit was amplified by the transistor and was transduced into LED illumination. The sensing units incorporated on the paper-based printed circuit boards (PCBs) absorbed bacteria-laden suspensions through capillary action, allowing for a rapid assessment (<2 min) of their electrogenic potential. Two well-known exoelectrogens, Shewanella oneidensis MR1 and Pseudomonas aeruginosa PA01, and many other mutants of the latter were selected to demonstrate the practicality of the proposed sensor. The effectiveness for on-site and portable measurements was validated by testing solid wastewater samples randomly obtained from the environment. Thus, the system described in this work highlights a novel form of a scalable, high-throughput sensing array for simple and rapid quantification of bacterial electrogenicity.

Portable, Disposable, Paper-Based Microbial Fuel Cell Sensor Utilizing Freeze-Dried Bacteria for In Situ Water Quality Monitoring.

Water quality monitoring is becoming an essential part of our lives as increasing human activities continue to spill unknown and unexpected contaminants into our water systems. To ensure the provision of safe and clean water to the public and the ecosystem, the development of rapid and sensitive in situ early warning systems for water toxicity monitoring is crucial. In this work, an entirely paper-based microbial fuel cell sensor utilizing freeze-dried bacteria is demonstrated as a portable and disposable water toxicity sensor. The bacterial cells were preinoculated on the anode reservoir of the device, and they were freeze-dried, making their on-site and on-demand applications possible. Upon rehydration of the bacteria with the water samples, current readings were obtained, and inhibition ratios (IRs) were calculated for different concentrations of formaldehyde as a model toxin. For 0.001, 0.01, and 0.02% of formaldehyde, IRs of 7.88, 16.08, and 23.14% were obtained, respectively. These IRs showed a very good linearity with the formaldehyde concentrations at R 2 = 0.995. Additionally, the shelf life of the freeze-dried microbial fuel cell sensor was investigated. Even after 14 days of storage in the desiccator, at 4, and at -20 °C, the performance outputs compared to the new device were all at 96%.

A 96-well high-throughput, rapid-screening platform of extracellular electron transfer in microbial fuel cells.

Microbial extracellular electron transfer (EET) stimulates a plethora of intellectual concepts leading to potential applications that offer environmentally sustainable advances in the fields of biofuels, wastewater treatment, bioremediation, desalination, and biosensing. Despite its vast potential and remarkable research efforts to date, bacterial electrogenicity is arguably the most underdeveloped technology used to confront the aforementioned challenges. Severe limitations are placed in the intrinsic energy and electron transfer processes of naturally occurring microorganisms. Significant boosts in this technology can be achieved with the growth of synthetic biology tools that manipulate microbial electron transfer pathways and improve their electrogenic potential. In particular, electrogenic Pseudomonas aeruginosa has been studied with the utility of its complete genome being sequenced coupled with well-established techniques for genetic manipulation. To optimize power density production, a high-throughput, rapid and highly sensitive test array for measuring the electrogenicity of hundreds of genetically engineered P. aeruginosa mutants is needed. This task is not trivial, as the accurate and parallel quantitative measurements of bacterial electrogenicity require long measurement times (~tens of days), continuous introduction of organic fuels (~tends of milliliters), architecturally complex and often inefficient devices, and labor-intensive operation. The overall objective of this work was to enable rapid (<30 min), sensitive (>100-fold improvement), and high-throughput (>96 wells) characterization of bacterial electrogenicity from a single 5 µL culture suspension. This project used paper as a substratum that inherently produces favorable conditions for easy, rapid, and sensitive control of an electrogenic microbial suspension. From 95 isogenic P. aeruginosa mutant, an hmgA mutant generated the highest power density (39 µW/cm2), which is higher than that of wild-type P. aeruginosa and even the strongly electrogenic organism, Shewanella oneidensis (25 µW/cm2). In summary, this work will serve as a springboard for the development of novel paradigms for genetic networks that will help develop mutations or over-expression and synthetic biology constructs to identify genes in P. aeruginosa and other organisms that enhance electrogenic performance in microbial fuel cells (MFCs).

Paper-Supported High-Throughput 3D Culturing, Trapping, and Monitoring of Caenorhabditis Elegans.

We developed an innovative paper-based platform for high-throughput culturing, trapping, and monitoring of C. elegans. A 96-well array was readily fabricated by placing a nutrient-replenished paper substrate on a micromachined 96-well plastic frame, providing high-throughput 3D culturing environments and in situ analysis of the worms. The paper allows C. elegans to pass through the porous and aquatic paper matrix until the worms grow and reach the next developmental stages with the increased body size comparable to the paper pores. When the diameter of C. elegans becomes larger than the pore size of the paper substrate, the worms are trapped and immobilized for further high-throughput imaging and analysis. This work will offer a simple yet powerful technique for high-throughput sorting and monitoring of C. elegans at a different larval stage by controlling and choosing different pore sizes of paper. Furthermore, we developed another type of 3D culturing system by using paper-like transparent polycarbonate substrates for higher resolution imaging. The device used the multi-laminate structure of the polycarbonate layers as a scaffold to mimic the worm's 3D natural habitats. Since the substrate is thin, mechanically strong, and largely porous, the layered structure allowed C. elegans to move and behave freely in 3D and promoted the efficient growth of both C. elegans and their primary food, E. coli. The transparency of the structure facilitated visualization of the worms under a microscope. Development, fertility, and dynamic behavior of C. elegans in the 3D culture platform outperformed those of the standard 2D cultivation technique.

A Paper-Based Biological Solar Cell.

A merged system incorporating paperfluidics and papertronics has recently emerged as a simple, single-use, low-cost paradigm for disposable point-of-care (POC) diagnostic applications. Stand-alone and self-sustained paper-based systems are essential to providing effective and lifesaving treatments in resource-constrained environments. Therefore, a realistic and accessible power source is required for actual paper-based POC systems as their diagnostic performance and portability rely significantly on power availability. Among many paper-based batteries and energy storage devices, paper-based microbial fuel cells have attracted much attention because bacteria can harvest electricity from any type of organic matter that is readily available in those challenging regions. However, the promise of this technology has not been translated into practical power applications because of its short power duration, which is not enough to fully operate those systems for a relatively long period. In this work, we for the first time demonstrate a simple and long-lasting paper-based biological solar cell that uses photosynthetic bacteria as biocatalysts. The bacterial photosynthesis and respiration continuously and self-sustainably generate power by converting light energy into electricity. With a highly porous and conductive anode and an innovative solid-state cathode, the biological solar cell built upon the paper substrates generated the maximum current and power density of 65 µA/cm2 and 10.7 µW/cm2, respectively, which are considerably greater than those of conventional micro-sized biological solar cells. Furthermore, photosynthetic bacteria in a 3-D volumetric chamber made of a stack of papers provided stable and long-lasting electricity for more than 5 h, while electrical current from the heterotrophic culture on 2-D paper dramatically decreased within several minutes.

A Portable, Single-Use, Paper-Based Microbial Fuel Cell Sensor for Rapid, On-Site Water Quality Monitoring.

Human access to safe water has become a major problem in many parts of the world as increasing human activities continue to spill contaminants into our water systems. To guarantee the protection of the public as well as the environment, a rapid and sensitive way to detect contaminants is required. In this work, a paper-based microbial fuel cell was developed to act as a portable, single-use, on-site water quality sensor. The sensor was fabricated by combining two layers of paper for a simple, low-cost, and disposable design. To facilitate the use of the sensor for on-site applications, the bacterial cells were pre-inoculated onto the device by air-drying. To eliminate any variations, the voltage generated by the microorganism before and after the air-drying process was measured and calculated as an inhibition ratio. Upon the addition of different formaldehyde concentrations (0%, 0.001%, 0.005%, and 0.02%), the inhibition ratios obtained were 5.9 ± 0.7%, 6.9 ± 0.7%, 8.2 ± 0.6%, and 10.6 ± 0.2%, respectively. The inhibition ratio showed a good linearity with the formaldehyde concentrations at R2 = 0.931. Our new sensor holds great promise in monitoring water quality as a portable, low-cost, and on-site sensor.