adsorbent material has limited adsorption capacity, the chemical cartridge or the filter needs to be replaced before the adsorption capacity is exceeded. End of service life indicator (ESLI), a system that informs the users as to when to replace the filter, has been widely employed in consumer products such as air conditioners and water/air purifiers. For these products, replacing the filters on time is advisable but not mandatory. By contrast, for the filtration of toxic gases, such as chemical warfare agents (CWAs) and toxic industrial chemicals (TICs), the expired filter must be promptly replaced for the safety of the users. Developing an ESLI in those systems, however, has been sluggish. [2] ESLI for filtration of CWAs is not available, and ESLI developed for canisters against TICs uses colorimetric sensors that are slow and have low sensitivity as compared to electronic sensors. Occupational Safety and Health Administration (Department of Labor, USA) presents a guideline for replacing the filter based on mathematical models, [3] whose input parameters are the type of gases, and their concentrations, temperature, humidity, flow rate, etc. Application of such models, however, is not practical as the parameters not only are difficult to measure but also vary In a filtration system, a chemical cartridge packed with porous adsorbents needs to be replaced when its maximum adsorption capacity is exceeded. An end of service life indicator (ESLI) informs when to replace the cartridge. However, ESLI technology for filtration of toxic gases is overlooked despite high toxicity of the gases even at low concentrations. In this study, a real-time ESLI is demonstrated for filtration of chemical warfare agents by inserting four chemiresistors made of carbon nanotubes into the adsorbent layer of a canister for gas masks. The sensors are installed on a flexible printed circuit board with a 3D structure to minimize the channeling effect in the canister. As the breakthrough of dimethyl methylphosphonate, a nerve agent simulant, progresses in the canister, the resistance of the sensors increases sequentially with time. The inflection points in the responses from sensors designated #1-#4 indicate that the percentage of remaining service life is 75%, 50%, 25%, and 0%, respectively. The ESLI potentially applies to a wide range of toxic gases and cartridge designs, regardless of the sensitivity and selectivity of the sensors, as supported by the breakthrough of ammonia as well as the results from a smaller canister.
