“Hydration Solids: Unveiling a Revolutionary Class of Matter in Scientific Exploration”

For ages, the absorption of ambient moisture by biological materials has been a well-known fact. However, groundbreaking research conducted at Columbia University has revealed that water plays a far more crucial role in shaping the characteristics of natural substances like pine cones, fungi, plants, and trees than previously understood.

A recent study proposes the existence of a newly identified category of matter known as “hydration solids,” encompassing materials such as wood, bacteria, and fungi. Traditionally, the fields of physics and chemistry have attributed the properties of solid materials to the atoms and molecules they comprise. For instance, the crystalline structure of salt is attributed to the ionic bond formed between sodium and chloride ions. Similarly, the strength of metals like iron and copper stems from the metallic bonds between their constituent atoms, while the elasticity of rubber arises from flexible polymer bonds. This principle has also been applied to substances such as fungi, bacteria, and wood.

However, a paradigm-shifting paper recently published in Nature challenges this notion, arguing that the character of many biological materials is primarily shaped by the water that permeates them. It suggests that water endows these materials with solidity while retaining its liquid characteristics, ultimately defining their properties. The authors of the study introduce the term “hydration solids” to classify these materials, emphasizing that their structural rigidity, the defining feature of the solid state, is derived from the fluid present in their pores. This new understanding holds the potential to answer long-standing scientific inquiries.

Professor Ozgur Sahin, one of the paper’s authors and an expert in Biological Sciences and Physics, expressed his enthusiasm, calling this a significant moment in science. He stated, “It’s unifying something incredibly diverse and complex with a simple explanation. It’s a big surprise, an intellectual delight.”

The research originated from Professor Sahin’s ongoing investigation into the peculiar behavior of spores, which are dormant bacterial cells. Steven G. Harrellson, who recently completed his doctoral studies in Columbia’s physics department and contributed to the study, likened the team’s findings to a building metaphor. He explained that just as the steel frames support a skyscraper, water situated between the molecular building blocks acts as the air inside those frames. The team discovered that certain structures are not sustained by their steel frames but by the air within them.

Although initially difficult to believe, this idea resolves mysteries and enables the prediction of fascinating phenomena in materials, according to Sahin.

When water exists in its liquid form, its molecules maintain a delicate balance between order and disorder. However, when water combines with the molecules constituting biological materials, the balance tilts toward order. Water, seeking to return to its original state, pushes the molecules of the biological matter apart. This force, known as the hydration force, was identified in the 1970s but was previously thought to have limited effects on biological matter. The paper challenges this perception by suggesting that the hydration force almost entirely determines the character of biological matter, including its hardness or softness.

While it has long been known that biological materials absorb ambient moisture, this research reveals that ambient water is far more integral to the character of wood, fungi, plants, and other natural substances than previously realized.

By prioritizing the role of water, the team was able to describe the properties displayed by familiar organic materials using simple mathematical equations. Unlike previous models requiring complex computer simulations, these newly discovered formulas can predict material properties, indicating a significant breakthrough.

For example, the team found that the equation E=Al/λ elegantly explains how a material’s elasticity changes based on factors like humidity, temperature, and molecule size. In this equation, E represents the material’s elasticity, A is a factor influenced by environmental temperature and humidity, l approximates the size of biological molecules, and λ represents the distance over which hydration forces weaken.

Harrellson remarked that as the project progressed, the answers became simpler, a rarity in scientific research. He described the feeling of relief when they realized that by solely considering hydration forces, they could discard previously unsatisfactory formulas. According to Harrellson, “When only hydration forces were left, it felt like our feet finally hit the ground. It was amazing, and a huge relief; things made sense.”

The culmination of their experiments led to the publication of this paper, authored by Steven G. Harrellson, Michael S. DeLay, Xi Chen, Ahmet-Hamdi Cavusoglu, Jonathan Dworkin, Howard A. Stone, and Ozgur Sahin. Unfortunately, Adam Driks from Loyola University Chicago, another contributor to the research, passed away before the work’s completion.

The findings of this paper apply to a significant portion of the world around us. Hygroscopic biological materials, which allow the passage of water, potentially constitute anywhere from 50% to 90% of the living world, encompassing familiar elements such as wood, bamboo, cotton, pine cones, wool, hair, fingernails, pollen grains, animal skin, and bacterial and fungal spores vital for the survival and reproduction of these organisms.

The term “hydration solids,” coined in this paper, encompasses any natural material responsive to ambient humidity. By utilizing the identified equations, the team and other researchers can now predict the mechanical properties of such materials based on fundamental principles of physics. Previously, this level of predictability was primarily limited to gases due to the well-established general gas equation formulated in the 19th century.

Professor Sahin concluded by suggesting a shift in perspective, stating that when we venture into the woods, we should consider the trees and plants as towering structures of water that securely hold sugars and proteins in place. He declared, “It’s really water’s world.”

Reference: “Hydration solids” by Steven G. Harrellson, Michael S. DeLay, Xi Chen, Ahmet-Hamdi Cavusoglu, Jonathan Dworkin, Howard A. Stone, and Ozgur Sahin, published in Nature on 7 June 2023.
DOI: 10.1038/s41586-023-06144-y

The study received funding from the U.S. Department of Energy, the Office of Naval Research, the National Institutes of Health, and the David and Lucile Packard Foundation.

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More about hydration solids

• Nature: “Hydration solids”