Moisture-trapping film reduces heat stress in protective suits

Researchers from the National University of Singapore (NUS) have developed a novel super-hygroscopic material that enhances sweat evaporation within a personal protective suit, to create a cooling effect for better thermal comfort for users such as healthcare workers and other frontline officers. Their invention has been described in the journal Small.

Medical protective suits have excellent antibacterial and waterproof properties; however, this high level of protection stops the venting of water vapour produced by evaporated sweat and impedes heat loss from the body. This is why users such as healthcare workers who need to don protective suits for long hours, especially in tropical environments, often report of occupational heat strain.

“Under room temperature of about 35°C, a healthcare worker who dons a protective suit for one hour typically experiences a heat index of about 64°C,” noted NUS research team leader Assistant Professor Tan Swee Ching. “This causes discomfort, and prolonged thermal strain can result in heat stroke and even death.”

Thermal management solutions such as air-cooling garments with electrical fans or ingestion of ice slurry are impractical due to limitations such as bulkiness, heavy weight and limited effectiveness — and while advanced textiles and coatings are promising solutions, they are difficult to fabricate and production costs are high. The NUS team came up with a practical strategy to overcome the current challenges by leveraging the principle of evaporative cooling — by increasing sweat evaporation from the skin.

The solution involves using a super-hygroscopic composite film to control the humidity level in the micro-environment in the protective suit. When the moisture-trapping composite film absorbs water vapour within the protective suit, the humidity level drops. This in turn speeds up sweat evaporation from the skin. As a result, more heat is dissipated from the human body through sweating, providing thermal comfort for users.

The new desiccant film, which is biocompatible and non-toxic, has a fast absorption rate, high absorption capacity and good mechanical properties, meaning it is robust and durable for practical applications such as for protective suits worn by healthcare workers. It is also affordable, lightweight, easy to fabricate and reusable.

To examine the effectiveness of their solution, the NUS team conducted tests in collaboration with researchers from the Home Team Science and Technology Agency (HTX) — a statutory board under Singapore’s Ministry of Home Affairs — using a 20-zone ‘Newton’ manikin within a climatic chamber. By combining NUS’s scientific knowledge of advanced hydrogel materials and HTX’s engineering capabilities, the joint research team was able to customise and optimise the novel moisture-trapping material to enhance the performance and productivity of frontline officers.

With the composite film, relative humidity (RH) under moderate sweating condition dropped by about 40% — from 91% to 48.2% after one hour of sweating and to 53.2% after two hours of sweating. The team also found that within the first hour of sweating, the heat index or ‘felt air temperature’ dropped significantly from 64.6°C to 40°C at air temperature of 35°C. At this level, while users still feel hot, the likelihood of getting heat stroke, heat cramps and heat exhaustion is remarkably reduced.

In another laboratory experiment, the research team showed that body temperature (or skin temperature) could be significantly reduced by 1.5°C through evaporative cooling. This further proves that the composite film can potentially help users — such as healthcare workers, soldiers or firefighters — relieve thermal stress, especially during strenuous activities.

Regeneration of the team’s composite film is also more energy efficient, as it requires a lower temperature to release the trapped moisture. At 50°C, the composite film releases 80% of its water contents after 10 minutes and this reaches 95% after 40 minutes. Most hygroscopic materials regenerate at a temperature of more than 100°C, over a duration of more than an hour.

“From the findings of various studies in this project, we are hopeful that the moisture-trapping film can be embedded to personal protective equipment (PPE) and/or personal protective clothing (PPC) of the Home Team officers, to enhance thermal comfort and improve performance of the frontline officers,” said Ying Meng Fai, Director of the Human Factors & Simulation Centre of Expertise at HTX.

Encouraged by the results of their study, the NUS team is now working to improve their hygroscopic material so that it can absorb more and faster, while also planning to apply their cooling strategy to other types of protective apparel such as those for firefighters. In addition, Asst Prof Tan and his team are looking for opportunities to commercialise this technology.

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