'Gel sheet' absorbs 3x more liquid than a paper towel

Paper towels and rags are normally sufficient for cleaning up spills in the kitchen, while gauze is typically used to soak up blood in the operating room. Now, researchers at the University of Maryland have used a hydrogel — a gelatin-like material in the form of a dry sheet — to craft so-called ‘gel sheets’ that absorb and hold significantly more liquid, while also being foldable and cuttable. Their results have been published in the journal Matter.

There are generally two types of materials that absorb liquids: porous materials and hydrogels. Porous materials like cloth and paper are flexible, foldable and easy to use, but not very absorbent. On the other hand, superabsorbent hydrogels that are made of polymer, a web of large molecules, can soak up more than 100 times their weight in water. However, when dried, these hydrogels become brittle solids that crumble.

“We reimagined what a hydrogel can look like,” said Srinivasa Raghavan of the University of Maryland. “What we’ve done is combine the desired properties of a paper towel and a hydrogel.”

To craft the gel sheets, the research team first mixed in acid, base and other ingredients for the hydrogel in a zip-lock bag. Like vinegar meeting baking soda, the mixture released carbon dioxide bubbles within the gel, creating a porous and foam-like material. Next, the bag was sandwiched between glass slabs to form a sheet and then exposed to UV light, which sets the liquid around the bubbles, leaving pores behind. Lastly, the team dipped the set sheet in alcohol and glycerol and air-dried it.

The result was that the dried gel sheet felt soft and flexible, similar to a fabric’s texture. The sheet also stayed soft and flexible in ambient conditions for a year, indicating stability.

“We are trying to achieve some unique properties with simple starting materials,” Raghavan said.

“To our knowledge, this is the first hydrogel that has been reported to have such tactile and mechanical properties.”

Compared to a commercial cloth pad and a paper towel, a gel sheet the same size can absorb more than three times the amount of liquid than others. When researchers placed the gel sheet over 25 mL of spilled water, the sheet swelled and soaked it up within 20 seconds, holding the water without dripping. The cloth pad only absorbed about 60% of the water, leaving drips behind.

The gel sheet also performed well with thick liquids, such as syrup, blood and even fluids that are a million times thicker than water. The researchers found that the gel sheet could absorb nearly 40 mL of blood within 60 seconds, while gauze dressing soaked up only 55% of the blood. The gel sheet also holds its liquid well, whereas the blood-soaked gauze trickles. Compared to sanitary pads, sponges and gauze, the gel sheet absorbed over two times more blood than the others.

Next, the team plans to optimise their gel sheets by increasing absorbency, strengthening the material, lowering the cost and making the sheets reusable. The researchers are also looking to develop gel sheets for absorbing oil.

“In principle, the gel sheets could be a superior form of paper towels,” Raghavan said. He envisions the gel sheets picking up spills in kitchens and laboratories, as well as cleaning up blood from surgeries and menstrual bleeding. Because of their flexible and absorbent nature, gel sheets also have the potential to stop bleeding from severe wounds as dressing.

“I’m always interested in taking our inventions further than just publishing a paper,” Raghavan said. “It would be wonderful to take it to actual practical use.”

Image caption: The commercial gauze (left) only absorbs 55% of the blood and, moreover, the blood drips out when held vertically. The gel sheet (right) absorbs 99% of the blood and the swollen sheet does not drip when held vertically. Scale bars = 2 cm. Credit: Matter/Choudhary et al.