Water-swelling "weightlifter" — Development of cubic polymer gel particles by Osaka Institute of Technology

A research group at Osaka Institute of Technology has developed cubic polymer gel particles that can swell by absorbing water and lift a weight placed on top of them, much like a "weightlifter." While water-absorbent materials are already widely used in products like disposable diapers, particles in shapes other than spheres are rare, and their manufacturing processes have traditionally been labor-intensive. This new gel is unique because it can be produced simply by surrounding a liquid with plastic plates. Since it lifts weights without using electricity, the research group describes it as an "environmentally friendly way to generate power."

Professor Syuji Fujii of the Department of Applied Chemistry, Faculty of Engineering at Osaka Institute of Technology, who specializes in interfacial colloid chemistry and polymer chemistry, has long researched "liquid marbles," i.e., spherical structures that contain a liquid. These liquid marbles have structures where a liquid core is surrounded by small solid particles and can serve as "reaction vessels" for synthesizing polymer gels.

Typically, polymer gel particles take a spherical shape due to surface tension, which makes them prone to rolling. At the microscopic level, polymers form a network-like structure, trapping water and oil molecules within it.

Non-spherical gels are attracting attention because their movement, deformation, and assembly patterns differ significantly from spheres. Due to their large surface area and the way their interactions change depending on their orientation, they are expected to be useful in various fields, including medicine, robotics, and micro-machinery.

The research group focused on polyethylene glycol and polyglycerin-based monomers, which have a high affinity for water. They discovered that when a few microliters of the monomer were sandwiched between polyethylene terephthalate (PET) plates, the liquid naturally conformed to the shape of the plates, whether circular or square. By polymerizing the liquid and then peeling off the plates, they obtained polymer particles in the exact shape of the original monomer.

By manipulating the shape and number of plates, as well as the volume of the monomer, the group succeeded in creating stable liquid marbles in the form of regular polyhedra, such as tetrahedrons. Furthermore, by polymerizing the monomer inside these liquid marbles, they synthesized polyhedral polymer particles identical in shape to the marbles themselves. When water was dropped onto these polyhedral particles, the polymer absorbed the water and swelled into a gel particle.

Four dry cubic gel particles were arranged in a square, and a glass plate with a weight, totaling approximately 10 grams, was placed on top. Upon adding water, the particles expanded to about 1.6 times their original height, lifting the weight against gravity.

While lifting objects usually requires electricity or magnetism, this mechanism requires only the addition of water. Fujii noted, "By making them cubic, friction with the ground increases, making them less likely to move and providing higher positional stability. This is an advantage that spherical gel particles do not have." Moving forward, the team aims to develop gels that swell in acidic or alkaline solutions, in addition to pure water, to expand their potential applications.

This research was supported by the Japan Society for the Promotion of Science (JSPS) Grants-in-Aid for Scientific Research and Sakamoto Yakuhin Kogyo (Osaka City). Sakamoto Yakuhin Kogyo provided the monomer. The results were published in the American Chemical Society's online edition Langmuir on March 7 and announced by Osaka Institute of Technology on March 12.