Flexible, absorbent and versatile, hydrogels are found in everyday products like diapers and contact lenses, and have shown promise in repairing electronics, killing drug-resistant bacteria, stemming bleeding and keeping ship hulls barnacle-free. But as useful as they are, manufacturing them can be costly and hard to scale up. Now, scientists at Stanford have developed new hydrogels made from common and inexpensive natural materials, which are easily adjusted to suit a range of applications.
Two relatively basic substances are at the core of the new hydrogel: a cellulose polymer, made from organic materials like wood chips and other agricultural waste products, and a colloidal silica derived from sand. When combined, the ingredients form a gel that's not only simple to manufacture, but can be tweaked to make it useful for a whole range of products.
"When we mix the cellulose and silica together, we get a stable gel," says lead author, Eric Appel. "By altering the formulations, we can tune across an enormous range of mechanical properties. It's not a knife-edge scenario – not a 'gel' or 'no gel' situation. Instead, you can get a whole continuum of gel states that can be useful for different applications."
To demonstrate some of those applications, the team tested the new gels in two different scenarios. First, a special formulation of the hydrogel was used in a winery to clear pipes of any leftover wine, a process that usually requires water and results in about two percent of the winery's saleable product being wasted. The hydrogel not only cleaned the pipes and reduced the amount of water used for cleaning, but it allowed the facility to reclaim much of that wasted wine, since the gel doesn't mix with the liquid.
"And because the hydrogel is composed of materials that are food grade – cellulose and colloidal silica are both used in the food industry – there were no odor or taste issues," says Appel.
The team's hydrogels were also used to make for a more efficient fire retardant in combating wildfires. Chemicals currently used in aerial drops can contaminate the groundwater, and subsequent water bombing tends to wash them away. Mixing the retardant with the team's hydrogel solved both of those issues, and improved the overall usefulness of the compound.
"These hydrogels can be dropped from a greater range of heights with less drift and evaporation," says Appel. "And because it's non-toxic there are no contamination issues with streams or aquifers."
The researchers are currently testing their gels on larger scales, and working to develop new mixtures that may find use in cosmetics and lubricants for drilling operations.