Nontoxic Method Inspired by Nest-Building Creates Cellulose Gels

Researchers from North Carolina State University have devised a nontoxic method for creating cellulose gels, drawing inspiration from the nest-building process of small birds.

The process, based on freeze-thaw cycles, is straightforward, cost-effective, and yields cellulose gels suitable for various applications, including the development of tunable gels for controlled drug delivery. This approach is also compatible with bamboo and potentially other plant fibers that contain lignin.

Cellulose is an exceptional material for producing hydrogels, which find application in a wide range of uses, including contact lenses, wound care, and drug delivery. However, crafting hydrogels from cellulose can be a challenging task, and frequently, the methods employed in creating these hydrogels involve toxic components or processes.

Normally, you have to first dissolve the cellulose and then induce it to crosslink or form the structure of interest, which often requires the use of difficult to handle, unstable, or toxic solvents.”

Lucian Lucia, Co-Corresponding Author and Professor, Forest Biomaterials and Chemistry, North Carolina State University

The Swift family of birds are small avian creatures that employ their saliva to secure twigs in position when constructing their nests.

My then PhD student Zhen Zhang noted that when birds do this, the saliva acts like a natural resin that holds the nest together and encourages the fibers within the nest to interconnect or crosslink. Which is exactly what we want the dissolved cellulose to do when making hydrogels. So we asked ourselves, ‘what if we mimic the birds?’”

Lucian Lucia, Co-Corresponding Author and Professor, Forest Biomaterials and Chemistry, North Carolina State University

Zhang, who is presently a postdoctoral researcher at Texas Tech University, serves as a co-corresponding author of the study.

The researchers followed a method involving the addition of water-soluble cellulose, known as carboxymethyl cellulose (CMC), to an acidic solution, dissolving the CMC. Subsequently, they introduced powdered cellulose fiber into the solution and subjected it to four cycles of freezing and thawing. This process yielded a cellulose gel.

Think of it as adding a thickener to water, like you would a pie filling. By changing the pH of the CMC, the water essentially becomes sticky. Freezing and thawing the solution causes the cellulose to compact and interweave itself into the sticky network, giving you a more organized structure, just as Swifts do when they create their nests. Only we don’t have to use beaks and saliva to do it.”

Lucian Lucia, Co-Corresponding Author and Professor, Forest Biomaterials and Chemistry, North Carolina State University

Freeze-drying the gels led to the formation of cellulose foam. The researchers also replicated the procedure with bamboo fibers, indicating that it has the potential for application with various other plant fibers containing both lignin and cellulose.

“The cellulose gels are robust, stable at room temperature and can be tuned to degrade on a schedule, so would be useful in drug delivery applications, among others,” Lucia notes. “This opens a promising new window for using biomimicry to process these insoluble cellulosic materials in a greener way.”