Pusan National University Scientists 4D Print Novel Humidity-Responsive Polymer Materials

BUSAN, South Korea, Sept. 3, 2021 /PRNewswire/ — Technological advancement has led to the creation of sensors, weather-responsive architecture, smart textiles, soft robots, biomedical tools, energy harvesting devices, and much more. These are often based on materials that can change shape in response to external stimuli (actuate), like temperature, light, and humidity.

One material with actuating ability is the liquid crystal (LC)-based lightly crosslinked polymer, called LC elastomer (LCE). However, so far, no LCEs have been reported that respond to changes in environmental humidity. The only few reports on such materials are on heavily crosslinked rigid LC networks (LCNs) that comprise of carboxylic acid groups. But the densely crosslinked chain networks in LCNs make them mechanically brittle, which is not desirable. And methods of fabricating LCNs have so far focused on surface alignment, allowing only for the preparation of thin films.

To overcome these challenges and increase applicability, scientists have been attempting to develop soft and deformable materials with multiple geometries. Now, scientists from Pusan National University, Republic of Korea, led by Prof. Suk-kyun Ahn, have become successful! They have developed a new class of smart LCEs, which can be fabricated in various shapes via 3D printing, are highly deformable, and are responsive to environmental humidity.

Explaining the significance of their study, published in Small, Prof. Ahn states, "Our LCE leads the way in designing humidity-responsive complex structures that can be applied to 4D printing technology. This can be one of the key materials technologies that innovates our futures for the industrial revolution, Industry 4.0." 4D printing refers to the mechanism through which 3D printed objects undergo external stimuli-induced transformation in their shape, color, or function over time.

To develop their LCEs, the scientists first prepared dimethylamino group functionalized LC oligomers, as LC ink. They then printed these in multiple geometries—films, porous bilayers, concentric square arrays, flowers, and grippers—using UV-assisted direct-ink-writing based 3D printing technology. Finally, they activated one of the 3D-printed LCE surfaces using an acidic solution, which generates cations on the surface, thus providing asymmetric hydrophilicity to the LCE.

The resulting humidity-responsive LCEs can undergo programmed hygroscopic (upon absorbing moisture from air) actuation in a variety of ways, which can be directed by localizing the activation regions in the LCE or via tuning the cut angle of the LCE film. For example, LCE films can undergo bending or twisting, and even form the letters ‘P,’ ‘N,’ and ‘U’ in high humidity. Furthermore, an LCE flower successfully mimics the hygroscopic responses of morning glory flowers; and a soft LCE gripper could grab, lift, and release a ball!

Excited about the future possibilities of 4D printing and this LCE material, Prof. Ahn surmises, "The day is not far when we may see smart textiles or shoes that can spontaneously change their breathability in response to weather conditions, or soft actuators that can operate driven by moisture, without an electric motor."

Indeed, their findings point to a "smart" future where materials may grow a mind of their own!

Authors: Keumbee Kim, Yuanhang Guo, Jaehee Bae, Subi Choi, Hyeong Yong Song,
Sungmin Park, Kyu Hyun, and Suk-Kyun Ahn
Title of original paper: 4D Printing of Hygroscopic Liquid Crystal Elastomer Actuators
Journal: Small
DOI: https://doi.org/10.1002/smll.202100910 

Corresponding author:
Suk-Kyun Ahn
Lab: http://ahn.pusan.ac.kr 
ORCID: https://orcid.org/0000-0002-6841-4213

About Pusan National University

Website: https://www.pusan.ac.kr/eng/Main.do

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SOURCE Pusan National University