Botanical‐Inspired 4D Printing of Hydrogel at the Microscale

Abstract

Botanical systems have evolved the intriguing ability to respond to diverse stimuli due to long‐term survival competition. Mimicking these dynamic behaviors has greatly advanced the developments in wide fields ranging from soft robotics, precision sensors to drug delivery and biomedical devices. However, realization of stimuli‐responsive components at the microscale with high response speed still remains a significant challenge. Herein, the miniature biomimetic 4D printing of pH‐responsive hydrogel is reported in spatiotemporal domain by femtosecond laser direct writing. The dimension of the printed architectures is at the microscale (<102 µm) and the response speed is reduced down to subsecond level (<500 ms). Shape transformation with multiple degrees of freedom is accomplished by taking advantage of pH‐triggered expansion, contraction, and torsion. Biomimetic complex shape‐morphing is enabled by adopting flexible scanning strategies. In addition, application of this 4D‐printed micro‐architecture in selective micro‐object trapping and releasing is demonstrated, showcasing its possibilities in micromanipulation, single‐cell analysis, and drug delivery.

4D Printed Hydrogels: Fabrication, Materials, and Applications

“…Hu et al reported on the different swelling behaviors of an acrylic acid (AAc)‐based hydrogel in alkaline and acid solutions. [ 92 ] As shown in Figure 7c, when the pH value was greater than 9, the carboxylic groups of AAc were ionized by releasing protons, resulting in stronger electrostatic repulsion forces between the carboxylate moieties such that the hydrogel expanded in volume. On the other hand, at lower pH, the decreasing amount of charged carboxylate moieties led to smaller repulsion forces between the molecules, thus the AAc‐based hydrogel contracted in the acidic environment.…”

4D Printing: A Review on Recent Progresses

“…Hu et al studied acrylic acid (AAc)-based hydrogels, placed them in alkaline and acidic solutions, and analyzed their swelling behavior in both states [ 162 ]. As shown in Figure 15 A, when the pH value is greater than 9, the carboxyl group of AAc releases protons, causing its internal electrostatic repulsion to increase, which in turn causes the hydrogel to expand in volume.…”

“… ( A ) Counterclockwise and clockwise twist of biomimetic micro-structures after contraction triggered by pH. ( B ) Functional microcages are prepared for selective capture and release of micro-objects by controlling the pore size (Reproduced with permission from [ 162 ]). …”

Laser Fabrication of Bioinspired Graphene Surfaces With Superwettability

“…CW lasers can provide continuous output of light, which interacts with graphene or its related materials as a consequence of photothermal effect. While pulsed lasers (e.g., femtosecond and picosecond laser) can interact with graphene-related materials in an ultrafast process, which can reduce the heat-affected zone and thus result in a high resolution (Hu et al, 2019;Lao et al, 2020). Therefore, different laser sources may lead to graphene surfaces with different morphologies and properties, which is beneficial for the production of various functional surfaces.…”