Ultratough pollen grains turned into soft particles

Scientists at Nanyang Technological University, Singapore (NTU Singapore) have found a way to turn pollen — one of the hardest materials in the plant kingdom — into a soft and flexible material, using a simple chemical process akin to conventional soap-making. The resulting particles, described in the journal Nature Communications, have the potential to serve as building blocks for the design of new eco-friendly materials.

Pollen, described by scientists as the diamond of the plant world for its indestructible traits, encapsulates and transports a plant’s male genetic material within a wall structure composed of two mechanically distinct layers — a tough outer layer (exine) and a soft and elastic cellulose inner layer (intine).

When released from a flower’s male reproductive part, pollen grains become dehydrated and individual grains fold onto themselves. Conversely, when these grains arrive on the plant’s female reproductive structure, they become hydrated and germinate, with a pollen tube growing out of the grain and towards the female part.

The pollen tube growth process is controlled by enzymes within the pollen wall structure that alter the wall’s elasticity and lead to structural changes. These processes inspired NTU researchers to attempt to remodel the pollen’s entire wall structure and alter its material properties.

Pollen grains from the sunflower plant, with their sticky, oil-based ‘pollen cement’ layer removed, were incubated under alkaline conditions for up to 12 hours. This softened the two parts of the pollen wall, and the pollen grain particles swelled and became more gel-like. The longer the grains were incubated, the more gel-like the resulting material became.

Pollen at different incubation times. As incubation time increased, the pollen developed a greater degree of gel-like characteristics, as indicated by decreased resistance to flow under the influence of gravity.

“Our NTU research team has transformed hard pollen grains beyond their natural performance limits and converted them into soft microgel particles that alter their properties in response to external stimuli,” said study co-author and NTU President Professor Subra Suresh. “This holds promise for a wide range of applications that are environmentally sustainable, economically affordable and practically scalable.”

The researchers suggest that, coupled with advances in 3D and 4D printing, the pollen-based particles might one day be turned into a range of different forms, including polymer gels, sheets of ‘paper’ and sponges. In particular, the particles’ biocompatibility — ie, the fact that they do not cause an immunological, allergic or toxic reaction when exposed to body tissues — make them potentially suitable for applications such as wound dressing, prosthetics and implantable electronics.

The team also conducted computer simulations and found that the elastic properties of the outer and inner wall layers need to fall within a precise range for the pollen-derived material to exhibit the gel-like behaviour. This suggests that for an individual pollen particle, there is a chemical and physical pathway that determines whether hydration leads to its successful germination.

“Both our experimental and computational results give insight into the pollen’s basic biological mechanisms and demonstrate how altering the pollen wall structure can cause the pollen particles to swell — much like the shape transformations that occur during biological processes such as harmomegathy (the folding of the pollen grain to prevent water loss) and germination,” said study co-author Professor Cho Nam-Joon. “The results also show that we can go beyond the performance limits of what nature can accomplish by itself.”

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