By mimicking the water management system of a cactus, researchers from Pohang University of Science and Technology (POSTECH) have created a material that efficiently absorbs and stores water while maintaining its physical structure. As reported in ACS Macro Letters, the material can absorb more than 900× the amount of water that it loses by evaporation while undergoing little change in volume.

“The ability of materials to absorb and store liquid inside a well-defined structure is highly important not only to high-tech fields in materials science, nanotechnology, and bioengineering, but also to various engineering applications,” says Sang Joon Lee, who developed the material with Hyejeong Kim and Junho Kim.

Such research efforts have primarily focused on hydrogels and fiber-based materials, but there are challenges to their adoption. Hydrogels are brittle and need continuous hydration. Many fiber-based materials undergo physical changes upon absorbing water and common chemical treatments can hinder their water management abilities. 

The POSTECH team took a new approach, inspired by cacti that rapidly absorb water during rainfall, resist loss during droughts, and readily absorb more water during the next rainfall. The trick lies in the structure and features of the cactus root.

To explore how the morphology of a cactus root facilities these abilities, the team first imaged the roots of common cacti. The images revealed a complicated structure. The vascular tissue inside of a cactus root is surrounded by an epidermis made of concentric layers. The outside surface is rough, covered in clumps of soil particles that stick to the root through hairs and mucilage, a sugary substance secreted by the cactus.

To elucidate the role of the layered epidermis, soil particles, mucilage, and root hair in water management, the research team created three different models. Each model consisted of an agarose-based cryogel cylinder covered in a different presolution and cryogelated into a concentric layered structure. A control cylinder was left uncoated.

In the model most morphologically similar to cacti roots, the CCM model, the presolution included cellulose fibers in place of root hairs, a cryogel solution in place of mucilage, and hydrophilic silver-coated hollow microparticles in place of soil particles. The second model incorporated the cryogel solution and microparticles only (the CM model), and the third incorporated the cryogel solution and cellulose fibers only (the CC model).

The researchers compared the water management performance of the models using three indicators: water absorption capacity, rate of absorption, and water loss due to evaporation. Experimental results showed that the CCM model managed water most effectively.

Using measurements taken after one minute of water immersion and one minute of air exposure, the researchers calculated the swelling-to-evaporation ratio for the CCM model to be 933. This implies, they say, that the material can absorb 933× the amount of water that it loses to evaporation. The CM and CC models had ratios of 326 and 189, respectively.

As expected based on their individual properties, the fibers and cryogel in the CCM model absorbed water and the microparticles resisted evaporation, but the interaction of the three components contributed to the model’s efficiency in an important way. Cross-sectional images of the CCM model revealed air pockets in the rough surface created by the microparticle-clotted, interconnected fibers. When exposed to water, these pockets quickly filled with water that was rapidly absorbed by the surrounding fibers and cryogel. Similarly, the layered structure created by unidirectional freezing during cryogelation led to alignments and cracks in the surface that enhanced absorption.

In addition, the interaction provided structural support to the material as shown by its small change in volume after absorption. “As efficient water management without physical damage to the material is crucial in the design of practical water-handling devices, the cactus root-inspired material may have wide engineering applications,” Lee says. Furthermore, he says, the presolution elements can be changed to achieve different functionalities. For example, replacing the hydrophilic microparticles with hydrophobic microparticles could enable applications in oil-based systems.

Wonjung Kim, an expert in fluid mechanics and biomimicry at Sogang University, calls this research “an excellent example of biomimetics.”  Noting the possibilities, he says, “This material provides a new platform for water management in the cosmetics, agriculture, textiles, and pharmaceuticals industries. Further study of a mathematical model for quantitative analysis will be useful for maximizing the effects of the proposed principles and for facilitating various applications.”

Read the abstract in ACS Macro Letters.