Alcohol Pretreatment to Eliminate the Interference of Micro Additive Particles in the Identification of Microplastics Using Raman Spectroscopy

Stress-induced phase separation in plastics drives the release of amorphous polymer micropollutants into water

Residual stress is an intrinsic property of semicrystalline plastics such as polypropylene and polyethylene. However, there is no fundamental understanding of the role intrinsic residual stress plays in the generation of plastic pollutants that threaten the environment and human health. Here, we show that the processing-induced compressive residual stress typically found in polypropylene and polyethylene plastics forces internal nano and microscale segregation of low molecular weight (MW) amorphous polymer droplets onto the plastic's surface. Squeeze flow simulations reveal this stress-driven volumetric flow is consistent with that of a Bingham plastic material, with a temperature-dependent threshold yield stress. We confirm that flow is thermally activated and stress dependent, with a reduced energy barrier at higher compressive stresses. Transfer of surface segregated droplets into water generates amorphous polymer micropollutants (APMPs) that are denatured, with structure and composition different from that of traditional polycrystalline microplastics. Studies with water-containing plastic bottles show that the highly compressed bottle neck and mouth regions are predominantly responsible for the release of APMPs. Our findings reveal a stress-induced mechanism of plastic degradation and underscore the need to modify current plastic processing technologies to reduce residual stress levels and suppress phase separation of low MW APMPs in plastics.

The released micro/nano-plastics from plastic containers amplified the toxic response of disinfection by-products in human cells

Micro- and nanoplastics (MNPs) released from plastic containers pose significant food safety concerns; however, their release patterns in different containers along with their impacts on other pollutants remain poorly understood. This study revealed that feeding bottles, food containers, and paper cups released approximately 104 microplastics and 107 nanoplastics after hot water treatment. These released MNPs were nontoxic in six cell types, whereas they significantly amplified the toxicity of disinfection by-products (DBPs), a widely existing contaminant in drinking water. The joint toxicity was influenced by MNPs size, the types of cells and DBPs, with a maximum synergistic efficiency of 57.89 ± 4.64 % in human hepatic carcinoma cells (HepG2) exposed to nanoplastics from feeding bottles and iodoacetamide. Additionally, the exposure assessment indicated that released MNPs posed greater risks to infants. These findings suggested that while MNPs alone were nontoxic, their interactions with DBPs presented potential risks, particularly for sensitive populations.

Advancements and challenges in microplastic detection and risk assessment: Integrating AI and standardized methods

Microplastics (MPs) pose significant threats to ecosystems and human health due to their persistence and widespread distribution. This paper provides a comprehensive review of sampling methods for MPs in aquatic environments, soils, and biological samples, assessing pre-treatment procedures like digestion and separation. It examines the application and limitations of identification techniques, including microscopic observation, spectroscopic analysis, and thermal analysis. The review highlights the potential of AI technology to enhance detection efficiency and precision. It underscores the necessity of standardized protocols for consistent sampling and detection, and the importance of systematic risk assessment methodologies for managing environmental and health risks associated with MPs. The paper concludes with recommendations for future research, emphasizing the standardization of methods, advancement of detection technologies, integration of AI, and comprehensive health risk assessments. This review will be helpful for researchers to comprehensively understand the current main detection technologies and risk assessment methods of the MP, and to accelerate the establishment of an artificial intelligence regulatory framework for MPs.

Microplastics in soils: A comparative review on extraction, identification and quantification methods

Microplastics (MPs) have been continuously accumulating in soil, posing significant environmental concerns due to their potential toxicity and role as pollutant vectors. Effective monitoring of MPs in soils requires appropriate methods for extraction, identification and quantification. This comparative review systematically examined various methods used in MP analysis from solid matrix environments, highlighting the advantages and disadvantages of each technique. Soils are heterogeneous and complex matrices, which can interact differently with MPs, rendering the separation of MPs from soils a significant challenge. Key challenges include the interactions between MPs and soil organic matter and detection limitations of smaller size MP which can interfere with accurate extraction. Specialized techniques and devices are required for precise identification and quantification. Digestion can be effective in removing organic matter, although harsh digestion can degrade MPs. To improve the liability of MP analysis, it is critical to minimize steps that may reduce accuracy such as filtration, harsh digestion and inappropriate sample size. Conversely, adjusting or combining methods can enhance the precision of MP analysis. This review offers a forward-looking perspective by advocating extraction and detection approaches, thus providing a more accurate, reproducible and holistic framework for MP analysis in soils. These insights are anticipated to guide future research promote standardized protocols and enhance environmental monitoring efforts.

Assessing the Efficacy of Pyrolysis-Gas Chromatography-Mass Spectrometry for Nanoplastic and Microplastic Analysis in Human Blood

Humans are constantly exposed to micro- and nanosized plastics (MNPs); however, there is still limited understanding of their fate within the body, partially due to limitations with current analytical techniques. The current study assessed the appropriateness of pyrolysis-gas chromatography-mass spectrometry (Py-GC-MS) analysis for the quantification of a range of polymers in human blood. An extraction protocol that reduced matrix interferences (false positives) of polyethylene (PE) and polyvinyl chloride (PVC) was developed and validated. Extraction recoveries ranged 7-109%, although surface-modified polystyrene (carboxylated) increased nanoparticle recoveries from 17 to 52%. Realistic detection limits were calculated for each polymer, accounting for matrix suppression and extraction recovery. These were up to 20 times higher than nominal detection limits calculated with Milli-Q water. Finally, the method was tested with a pilot study of the Australian population. PE interferences were reduced but still present, and no other polymers were above detection limits. It was concluded that Py-GC-MS is currently not a suitable analysis method for PE and PVC in biological matrices due to the presence of interferences and nonspecific pyrolysis products. Furthermore, while it is plausible to detect some polymers in blood, the estimated exposure concentrations needed are approaching the detection limits of the technique.

Extraction and analysis of microplastics in wastewater sludges of a multi-product pulp and paper mill

Pulp and paper wastewater sludges are waste streams produced in major quantities across the world. The recycling of these organic sludges, for example to soil amendments, is desired in the circular economy but carries the risk of potential pollutants to be also introduced into the environment. Pulp and paper wastewater sludges have been scarcely studied matrices in the microplastic research due to their complex composition. In this study, we optimized an extraction process for microplastics from pulp and paper wastewater sludges, and quantified and characterized microplastics down to 20 μm in primary sludge and biosludge generated at the wastewater treatment plant of a multi-product pulp and paper mill in Finland. The occurrence of microplastics was high in primary sludge, 900-1600 microplastics g-1 dry weight, while the maximum number of detected microplastics in biosludge samples remained at 210 g-1 dry weight. Biosludge samples suffered from larger amounts of remaining solids after the extraction process, thus compromising the detection of smaller microplastics (<100 μm) and increasing the uncertainty related to the interpretation of the results. The most prevalent microplastic shape in all samples was fragment, and the most recurring polymer types were polyethylene and polypropylene, while a polystyrene-based copolymer represented approximately 10% of identified microplastics in primary sludge. The present study advances the development of microplastic analysis of the challenging pulp and paper wastewater sludges and brings novel information to the progressing discussion of their circulation potential.

Rapid Generation of Microplastics and Plastic-Derived Dissolved Organic Matter from Food Packaging Films under Simulated Aging Conditions

In this study, we show that low-density polyethylene films, a prevalent choice for food packaging in everyday life, generated high numbers of microplastics (MPs) and hundreds to thousands of plastic-derived dissolved organic matter (DOM) substances under simulated food preparation and storage conditions. Specifically, the plastic film generated 66-2034 MPs/cm2 (size range 10-5000 μm) under simulated aging conditions involving microwave irradiation, heating, steaming, UV irradiation, refrigeration, freezing, and freeze-thaw cycling alongside contact with water, which were 15-453 times that of the control (plastic film immersed in water without aging). We also noticed a substantial release of plastic-derived DOM. Using ultrahigh-resolution mass spectrometry, we identified 321-1414 analytes with molecular weights ranging from 200 to 800 Da, representing plastic-derived DOM containing C, H, and O. The DOM substances included both degradation products of polyethylene (including oxidized forms of oligomers) and toxic plastic additives. Interestingly, although no apparent oxidation was observed for the plastic film under aging conditions, plastic-derived DOM was more oxidized (average O/C increased by 27-46%) following aging with a higher state of carbon saturation and higher polarity. These findings highlight the future need to assess risks associated with MP and DOM release from plastic wraps.

Year-round spatial and temporal distribution of microplastics in water and sediments of an urban freshwater system (Jungnang Stream, Korea)

Streams and tributaries can play a significant role in the transport of inland microplastics to rivers and oceans; however, research on microplastics in these water bodies is limited compared to riverine and marine environments. Analyzing microplastic abundance at higher spatial and temporal resolutions is crucial to comprehend the dynamics of microplastics in these water bodies. Therefore, this study investigated year-round spatiotemporal variations of microplastics monthly in surface waters and sediments along the Jungnang Stream, one of the main tributaries to the Han River in South Korea. The mean concentration of microplastics in the stream was 9.8 ± 7.9 particles L-1 in water and 3640 ± 1620 particles kg-1 in sediment. Microplastic concentrations in surface waters during summer were significantly higher than in other seasons, positively linked to increased precipitation and river discharges. Polymer compositions mainly consisted of polyethylene, polypropylene, and polyethylene terephthalate, with the majority of microplastics detected smaller than 200 μm. Fragment-shaped microplastics were predominant over fibrous ones. The estimated annual input and output of microplastics through surface waters were 1.2-207 kg (2.7-150 billion particles) and 11.3-272 kg (17-769 billion particles), with the summer months contributing more than 70% of the total output. The greater microplastics output in the Jungnang Stream's waters compared to its receiving waters (Han River) indicates microplastics transport from water to other environmental compartments, such as sediments. These findings highlight the importance of investigating microplastic abundances in surface waters and sediments with temporal resolution, at least across different seasons. Such investigations offer valuable insights into the spatiotemporal occurrence and dynamic transport of microplastics, providing essential information for water management and the development of policies in freshwater ecosystems.

Using artificial intelligence to rapidly identify microplastics pollution and predict microplastics environmental behaviors

With the massive release of microplastics (MPs) into the environment, research related to MPs is advancing rapidly. Effective research methods are necessary to identify the chemical composition, shape, distribution, and environmental impacts of MPs. In recent years, artificial intelligence (AI)-driven machine learning methods have demonstrated excellent performance in analyzing MPs in soil and water. This review provides a comprehensive overview of machine learning methods for the prediction of MPs for various tasks, and discusses in detail the data source, data preprocessing, algorithm principle, and algorithm limitation of applied machine learning. In addition, this review discusses the limitation of current machine learning methods for various task analysis in MPs along with future prospect. Finally, this review finds research potential in future work in building large generalized MPs datasets, designing high-performance but low-computational-complexity algorithms, and evaluating model interpretability.

Advanced optical photothermal infrared spectroscopy for comprehensive characterization of microplastics from intravenous fluid delivery systems

Growing attention is being directed towards exploring the potential harmful effects of microplastic (MP) particles on human health. Previous reports on human exposure to MPs have primarily focused on inhalation, ingestion, transdermal routes, and, potentially, transplacental transfer. The intravenous transfer of MP particles in routine healthcare settings has received limited exploration in existing literature. Standard hospital IV system set up with 0.9 % NaCl in a laminar flow hood with MP contamination precautions. Various volumes of 0.9 % NaCl passed through the system, some with a volumetric pump. Fluid filtered with Anodisc filters washed with isopropyl alcohol. The IV cannula was immersed in Mili-Q water for 72 h to simulate vein conditions. Subsequently, the water was filtered and washed. Optical photothermal infrared (O-PTIR) microspectroscopy is used to examine filters for MP particles. All filters examined from the IV infusion system contained MP particles, including MPs from the polymer materials used in the manufacture of the IV delivery systems (polydimethylsiloxane, polypropylene, polystyrene, and polyvinyl chloride) and MP particles arising from plastic resin additives (epoxy resin, polyamide resin, and polysiloxane-containing MPs). The geometric mean from the extrapolated result data indicated that approximately 0.90 MP particles per mL of 0.9 % NaCl solution can be administered through a conventional IV infusion system in the absence of a volumetric pump. However, with the implementation of a pump, this value may increase to 1.57 particles per mL. Notably, over 72 h, a single cannula was found to release approximately 558 MP particles including polydimethylsiloxane, polysiloxane-containing MPs, polyamide resin, and epoxy resin. Routine IV infusion systems release microplastics. MP particles are also released around IV cannulas, suggesting transfer into the circulatory system during standard IV procedures.

Research advances on production and application of algal biochar in environmental remediation

Algae, comprising microalgae and macroalgae, have emerged as a promising feedstock for the production of functional biochar. Recently, the application of algal biochar in environmental remediation gains increasing attention. This review summarizes research advancements in the synthesis and application of algal biochar, a versatile and sustainable material for environmental remediation ranging from wastewater treatment to soil improvement. Algal biochar can be prepared by pyrolysis, microwave-assisted pyrolysis, and hydrothermal carbonization. Physical and chemical modifications have proven to be effective for improving biochar properties. Algal biochar is promising for removing diverse pollutants including heavy metals, organic pollutants, and microplastics. The role in soil improvement signifies a sustainable approach to enhancing soil structure, nutrient retention, and microbial activity. Research gaps are identified based on current understanding, necessitating further exploration into variations in biochar characteristics, the performance improvement, large-scale applications, and the long-term evaluation for environmental application. This review provides a better understanding of algal biochar as a sustainable and effective tool in environmental remediation.

Micro and nano plastics release from a single absorbable suture into simulated body fluid

Synthetic polymers are widely used in medical devices and implants where biocompatibility and mechanical strength are key enablers of emerging technologies. One concern that has not been widely studied is the potential of their microplastics (MPs) release. Here we studied the levels of MP debris released following 8-week in vitro tests on three typical polyglycolic acid (PGA) based absorbable sutures (PGA 100, PGA 90 and PGA 75) and two nonabsorbable sutures (polypropylene-PP and polyamide-PA) in simulated body fluid. The MP release levels ranked from PGA 100 > > PGA 90 > PGA 75 > > PP ∼ PA. A typical PGA 100 suture released 0.63 ± 0.087 million micro (MPs > 1 µm) and 1.96 ± 0.04 million nano (NPs, 200-1000 nm) plastic particles per centimeter. In contrast, no MPs were released from the nonabsorbable sutures under the same conditions. PGA that was co-blended with 10-25% L-lactide or epsilon-caprolactone resulted in a two orders of magnitude lower level of MP release. These results underscore the need to assess the release of nano- and microplastics from medical polymers while applied in the human body and to evaluate possible risks to human health.

Assessing the inconsistency of microplastic measurements in foods and beverages

The widespread occurrence of microplastics (MPs) in the food chain has gained substantial recognition as a pressing concern, highlighting the inevitability of human exposure through ingestion of foodborne MPs, coupled with the release of MPs from plastic packaging. However, there are notable disparities in the reported numbers of MPs in foods and beverages, warranting a thorough investigation into the factors contributing to these discrepancies. Table salt is one of the major sources of MPs, and there was an approximately hundred-fold difference between the reviewed studies that reported the highest and lowest number of MPs. In addition, more noticeable discrepancies were discovered between studies on MPs released from teabags. One study reported that approximately 15 billion MPs were released into a cup of tea from a single teabag, whereas another research paper found only approximately 106.3 ± 14.6 MP/teabag after brewing. This comprehensive review focuses on the inconsistencies observed across studies examining MPs, shedding light on the plausible factors underlying these variations. Furthermore, the review outlines areas in analytical procedures that require enhancement and offers recommendations to promote accuracy and standardization in future research efforts, such as employing analytical methods capable of confirming the presence of MPs, using appropriate filter sizes, considering representative sample sizes when extrapolation is involved, and so on. By pinpointing the detection processes leading to the inconsistent results observed in MP studies, this comparative analysis will contribute to the development of reliable analytic methods for understanding the extent of microplastic contamination in the human food chain.

From cracks to secondary microplastics - surface characterization of polyethylene terephthalate (PET) during weathering

Secondary microplastics are a product of the fragmentation of plastic debris. Despite concerns regarding the omnipresence of microplastics in the environment, knowledge about the mechanics of their actual formation is still limited. Fragmentation is usually linked to weathering, which alters the properties of the plastic and allows fragmentation to occur. Therefore, in this study, polyethylene terephthalate (PET) samples were exposed to artificial UV light or a combination of UV light and water for a total of three months to simulate natural weathering. The samples included three forms of PET with different production histories: pellets, yarns, and films. The surface alterations to the samples during weathering were characterized using scanning electron microscopy and Raman spectroscopy. Results indicated that the three different types of PET developed markedly different surface defects and also exhibited signs of weathering within different time frames. Differences were also found between samples exposed only to UV and those exposed to UV and submerged in water. In water, the first surface changes were spotted within 30 days of initial submersion and later developed into an organized crack network. Upon the introduction of mild mechanical forces, pieces of the weathered surface started to delaminate. The fragments from films had an elongated shape with a median size of 16.1 × 2.1 × 1.8 μm, resembling a fibre. If the weathered surface of a film were to detach completely, it could create 1.4-7.9 million microplastic fragments/cm2. For pellets, this number would range between 0.4 and 2.2 million microplastics/cm2. In addition to particle formation by surface delamination, particles also emerged on the weathered surfaces of all studied samples, presenting another possible source of micro-sized particles during weathering. Overall, the results of this work show that the weathering of plastics and the formation of microplastics are heavily influenced not only by the weathering mechanism but also by the type and production history of the polymers.

Machine learning driven methodology for enhanced nylon microplastic detection and characterization

In recent years, the field of microplastic (MP) research has evolved significantly; however, the lack of a standardized detection methodology has led to incomparability across studies. Addressing this gap, our current study innovates a reliable MP detection system that synergizes sample processing, machine learning, and optical photothermal infrared (O-PTIR) spectroscopy. This approach includes examining high-temperature filtration and alcohol treatment for reducing non-MP particles and utilizing a support vector machine (SVM) classifier focused on key wavenumbers that could discriminate between nylon MPs and non-nylon MPs (1077, 1541, 1635, 1711 cm-1 were selected based on the feature importance of SVM-Full wavenumber model) for enhanced MP identification. The SVM model built from key wavenumbers demonstrates a high accuracy rate of 91.33%. Results show that alcohol treatment is effective in minimizing non-MP particles, while filtration at 70 °C has limited impact. Additionally, this method was applied to assess MPs released from commercial nylon teabags, revealing an average release of 106 particles per teabag. This research integrates machine learning with O-PTIR spectroscopy, paving the way for potential standardization in MP detection methodologies and providing vital insights into their environmental and health implications.

Plastic bottles for chilled carbonated beverages as a source of microplastics and nanoplastics

Carbonated beverages are characterized by low temperatures, multiple microbubbles, high pressure, and an acidic environment, creating ideal conditions for releasing contaminants from plastic bottles. However, the release patterns of microplastics (MPs) and nanoplastics (NPs) are poorly understood. We investigated the effects of plastic type, CO2 filling volume, temperature, sugar content, and additive on the leakage of MPs/NPs and heavy metals. Our results showed that polypropylene bottles released greater MPs (234±9.66 particles/L) and NPs (9.21±0.73 × 107 particles/L) than polyethylene and polyethylene terephthalate bottles. However, subjecting the plastic bottles to 3 repeated inflation treatments resulted in 91.65-93.18% removal of MPs/NPs. The release of MPs/NPs increased with increasing CO2 filling volume, driven by the synergistic effect of CO2 bubbles and pressure. After 4 freeze-thaw cycles, the release of MPs and NPs significantly increased, reaching 450±38.65 MPs and 2.91±0.10 × 108 NPs per liter, respectively. The presence of sugar leads to an elevation in MPs release compared to sucrose-free carbonated water, while the addition of additives to carbonated water exhibits negligible effects on MPs release. Interestingly, actual carbonated beverages demonstrated higher MPs concentrations (260.52±27.18-281.38±61.33 particles/L) than those observed in our well-controlled experimental setup. Our study highlights the non-negligible risk of MPs/NPs in carbonated beverages at low temperatures and suggests strategies to mitigate human ingestion of MPs/NPs, such as selecting appropriate plastic materials, high-pressure carbonated water pretreatment, and minimizing freeze-thaw cycles. Our findings provide insights for further study of the release patterns of the contaminants in natural environments with bubbles, pressure, low temperature, and freeze-thaw conditions.

A microfluidic approach for label-free identification of small-sized microplastics in seawater

Marine microplastics are emerging as a growing environmental concern due to their potential harm to marine biota. The substantial variations in their physical and chemical properties pose a significant challenge when it comes to sampling and characterizing small-sized microplastics. In this study, we introduce a novel microfluidic approach that simplifies the trapping and identification process of microplastics in surface seawater, eliminating the need for labeling. We examine various models, including support vector machine, random forest, convolutional neural network (CNN), and residual neural network (ResNet34), to assess their performance in identifying 11 common plastics. Our findings reveal that the CNN method outperforms the other models, achieving an impressive accuracy of 93% and a mean area under the curve of 98 ± 0.02%. Furthermore, we demonstrate that miniaturized devices can effectively trap and identify microplastics smaller than 50 µm. Overall, this proposed approach facilitates efficient sampling and identification of small-sized microplastics, potentially contributing to crucial long-term monitoring and treatment efforts.

A double sliding-window method for baseline correction and noise estimation for Raman spectra of microplastics

When measuring microplastics of environmental samples, additives and attachment of biological materials may result in strong fluorescence in Raman spectra, which increases difficulty for imaging, identification, and quantification. Although there are several baseline correction methods available, user intervention is usually needed, which is not feasible for automated processes. In current study, a double sliding-window (DSW) method was proposed to estimate the baseline and standard deviation of noise. Simulated spectra and experimental spectra were used to evaluate the performance in comparison with two popular and widely used methods. Validation with simulated spectra and spectra of environmental samples showed that DSW method can accurately estimate the standard deviation of spectral noise. DSW method also showed better performance than compared methods when handling spectra of low signal-to-noise ratio (SNR) and elevated baselines. Therefore, DSW method is a useful approach for preprocessing Raman spectra of environmental samples and automated processes.

Structure evolution of air cathodes and their application in electrochemical sensor development and wastewater treatment

Cathode structure and material are the most important factors to determine the performance and cost of single chamber air-cathode microbial fuel cell (MFC), which is the most promising type of MFC technology. Since the first air cathode was invented in 2004, five major structures (1-layer, 2-layer, 3-layer, 4-layer and separator-support) have been invented and modified to fit new material, improve power performance and lower MFC cost. This paper reviewed the structure evolution of air cathodes in past 18 years. The benefits and drawbacks of these structures, in terms of power generation, material cost, fabrication procedure and modification process are analyzed. The practical application cases (e.g., sensor development and wastewater treatment) employed with different cathode structures were also summarized and analyzed. Based on practical performance and long-term cost analysis, the 2-layer cathode demonstrated much greater potential over other structures. Compared with traditional activated-sludge technology, the cost of an MFC-based system is becoming competitive when employing with 2-layer structure. This review not only provides a detailed development history of air cathode but also reveals the advantages/disadvantages of air cathode with different structures, which will promote the research and application of air-cathode MFC technology.

Application of 3D Printing Technology in Sensor Development for Water Quality Monitoring

The development of sensors for water quality monitoring is crucial to protect water quality, aquatic biota and human health. Traditional sensor manufacturing methods have significant drawbacks, such as low fabrication freedom, limited material choice and expensive manufacturing cost. As a possible alternative method, 3D printing technologies are increasingly popular in sensor development due to their high versatility, fast fabrication/modification, powerful processing of different materials and ease of incorporation with other sensor systems. Surprisingly, a systematic review examining the application of 3D printing technology in water monitoring sensors has not yet been conducted. Here, we summarized the development history, market share and advantages/disadvantages of typical 3D printing techniques. Specifically focused on the 3D-printed sensor for water quality monitoring, we then reviewed the applications of 3D printing in the development of sensors' supporting platform, cell, sensing electrode as well as all-3D-printed sensors. The fabrication materials and processing, and the sensor's performances regarding detected parameters, response time and detection limit/sensitivity, were also compared and analyzed. Finally, the current drawbacks of 3D-printed water sensors and potential directions for future study were discussed. This review will substantially promote the understanding of 3D printing technology used in water sensor development and benefit the protection of water resources.

Recent Implementations of Hydrogel-Based Microbial Electrochemical Technologies (METs) in Sensing Applications

Hydrogel materials have been used extensively in microbial electrochemical technology (MET) and sensor development due to their high biocompatibility and low toxicity. With an increasing demand for sensors across different sectors, it is crucial to understand the current state within the sectors of hydrogel METs and sensors. Surprisingly, a systematic review examining the application of hydrogel-based METs to sensor technologies has not yet been conducted. This review aimed to identify the current research progress surrounding the incorporation of hydrogels within METs and sensors development, with a specific focus on microbial fuel cells (MFCs) and microbial electrolysis cells (MECs). The manufacturing process/cost, operational performance, analysis accuracy and stability of typical hydrogel materials in METs and sensors were summarised and analysed. The current challenges facing the technology as well as potential direction for future research were also discussed. This review will substantially promote the understanding of hydrogel materials used in METs and benefit the development of electrochemical biosensors using hydrogel-based METs.