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New Natural Plant Material Can Self-Fold Into “Intelligent” Green Products

The pollen paper is expected to pave the way for convenient, scalable, and sustainable digital manufacturing of electronic and biomedical structures, and storage containers.

A team of scientists from Nanyang Technological University (NTU), led by the university’s president Professor Subra Suresh and Professor Cho Nam-Joon, has developed a pollen paper that can fold itself into desired 3D shapes in response to environmental humidity by printing specific patterns on it using a laser printer. While the folding process is reversible, it can be “frozen” by applying a layer of chitosan, a natural sugar found in shellfish, onto the structure.

“Building on our earlier studies on the unique properties of pollen, we combined easy-to-process pollen grains and cost-effective digital printing to develop a moisture-sensitive biomaterial-based system that can morph on demand into geometrical configurations that lead to complex shapes depending on the patterns we print,” explained Professor Suresh.

Based on origami fabrication, an important design principle found in nature that describes, for example, how flowers are folded within the bud, the new self-folding paper was inspired by the ability of geranium seeds to coil up and straighten in response to humidity. The material is composed of two layers: a paper-like material made from sunflower pollen and a layer of hydrophobic styrene acrylate copolymer that is found in laser toners and other edible inks. Although the scientists specifically used commercial laser printer toners in the study, previous research has shown that edible inks could also be printed on the pollen layer using standard digital printers.

To create the pollen-based paper, the team had to turn tough pollen grains into microgels through a technique similar to soapmaking. The result is a bilayer material that folds and bends reversibly due to the mismatch in volume change, caused by variations in humidity, between the pollen paper and the parts printed with the hydrophobic toner.

The team demonstrated their novel technique by creating several geometrical configurations with varying levels of complexity – from straws and boxes to 3D paper orchids. Through these trials, the scientists found that the geometric orientation of the printed pattern influenced the direction and degree of coiling. For instance, a 3 cm x 0.5 cm piece of pollen-based material with stripes printed at a 60-degree slant coiled up to form a straw. However, when similar patterns were printed with vertical stripes, it would roll up to form a circle. By varying the printing patterns, the scientists were able to create more complex structures.

Besides orientation, the length of printed patterns was also found to affect the angle at which the material would fold. This allowed the team to create an “intelligent” box and pyramid that could open and close by itself depending on changes in humidity.

“The flexibility of this design and printing strategy to produce box-like structures also illustrates pathways to convenient digital manufacturing of containers (such as boxes, cups, and bowls) made of natural materials that are economical, environmentally friendly and amenable to large-volume production,” noted Prof. Cho.

To ensure that the material remains in a folded or coiled state despite changing humidity levels for applications like food containers or boxes transported in humid weather, chitosan can be coated to “switch off” the material’s reversible and autonomous shape change. Lab studies have shown that chitosan-treated pollen paper will prevent the material from morphing despite changes in humidity, effectively “freezing” it.

In addition, the material also shows potential for use in “origami robots,” flat sheets that can autonomously fold into 3D shapes, for electronic and biomedical applications with special shape requirements, shape-dependent tissue engineering, and stimuli-triggered drug delivery. Since pollen is known to be an allergen, the team conducted studies to determine whether the pollen-based paper can cause allergic reactions before finally concluding that they do not.

In line with the university’s 2025 vision and Sustainability Manifesto, their novel invention is expected to help address some of humanity’s pressing concerns. Currently, the research team plans to increase the sensitivity and speed of the pollen’s material response to moisture and refine its stability and controllability. They have also filed a patent application for the potential commercialisation of this technology.

“In this work, we demonstrated how economical and eco-friendly plant-based materials and readily available printing technologies could be utilised to produce autonomous, on-demand deformation and designing complex shapes. By automating the folding process through controlling environmental humidity, origami-inspired machines can be produced without manual folding, reducing the skill and time necessary for fabrication,” said Prof. Cho. [APBN]

Source: Zhao et al. (2021). Digital printing of shape-morphing natural materials. Proceedings of the National Academy of Sciences, 118(43), e2113715118.