Scientists have created a paper-like material derived from pollen that can alter its mechanical characteristics in response to external stimuli. Combined with digital printing, pollen paper may hold promise for the fabrication of a new generation of programmable natural actuators — components in a machine that are responsible for moving and controlling a mechanism.
The team demonstrated the paper's properties by folding it into a flower that “blooms” in the presence of water vapor. They also showed a strip of pollen-based paper that is able to “walk” by adjusting the material's physical properties.
Much progress has been made in developing bioinspired sensors and actuators based on engineered synthetic materials but these materials come with limitations such as issues with environmental sustain-ability and relatively high cost. There remains a critical need to incorporate cost-effective and eco-friendly materials. The pollen paper created from naturally abundant pollen grains responds as an actuator to changes in environmental humidity. This process also renders pollen and the products created from it non-allergenic.
To form the paper, the team first transformed ultra-tough pollen grains from sunflowers into a pliable, gel-like material through a process similar to conventional soap-making. This process includes removing the sticky, oil-based pollen cement that coats the grain's surface before incubating the pollen in alkaline conditions for hours. The resulting gel-like material is then cast into a mold and left to dry, forming a paper-like material. Using scanning electron microscopy, the scientists observed that the pollen-based paper comprises alternating layers of pollen particles, with the top layer significantly rougher than the bottom layer.
The top surface of the pollen paper, which appears frosted to the naked eye, showed remnants of the sunflower pollen grains’ distinct spikes, contributing to its roughness. The bottom surface, which takes on a mirror-like surface finish, was relatively smoother. This structural difference in pollen particle layers means that in the presence of water vapor, the paper starts to bend and under dry conditions, it unbends. Repeated cycles of humid and dry conditions cause the paper to perform a flipping motion over time.
To show that it is possible to customize the water vapor responsiveness of pollen paper, the team adjusted processing parameters — chiefly, the alkaline incubation time of the pollen grains. They joined two pollen paper samples, each prepared under different incubation times (3 hours and 12 hours), to form a strip of bi-material pollen paper with a visible boundary. When the bi-material pollen paper was exposed to a humid-dry cycle, the two pollen paper samples’ different reactions to humidity caused the paper to “walk” like a caterpillar that moves by alternatingly expanding and contracting its soft body.