Stimuli-Responsive Microfibers

Microfibers, characterized by micron-scale diameters and lengths of up to several centimeters, play pivotal roles in natural structures. For example, plant tendrils use their microstructure and responsiveness to the environment to climb and provide support during growth. Their microstructures enable twisting and initiating helical chirality for bundles, providing significant performance at larger scales.¹ This study presents a new approach for preparing microfibers utilizing nanoimprint lithography (NIL) and reactive ion etching (RIE). This method has been successfully employed to fabricate microfibers from polydimethylsiloxane (PDMS) elastomers and poly(acrylic acid)--based copolymer (PAA) hydrogels. The prepared microfibers exhibit exceptional responsiveness to stimuli such as pH, solvent, and electric fields.<br/><br/>The responsive control of microfiber morphologies is dictated by the surface and near-surface chemical and mechanical properties induced by RIE. For PDMS microfibers, their surface tension and near-surface moduli increase after RIE treatment. These modifications provide the driving force for coiling into curved structures with size scales set by the balance of elastic and interfacial interactions when the microfibers are released in a liquid environment. Their curvature can reversibly change when switching between different liquids. Likewise, RIE treatment stiffens PAA microfibers’ surface, resulting in less swelling in water compared to the subsurface region. This difference creates a helical structure when swollen. When placed near the anode, mobile protons in the gel migrate toward the cathode with water molecules due to Coulomb forces. This electro-osmotic flow causes the microfiber to shrink and straighten.^2,3 Upon reversing the electric field’s polarity, the microfibers revert to the swollen helical form. Through the control of the hydrogel’s swollen and shrunken phase transition in electric fields,⁴ the PAA microfibers realize the reversible transformations between coiled and straight configurations. Microfibers with stimuli-responsiveness exhibit the potential in sensors, actuators, electronics, mechanics, fluids, and photonics.⁵<br/><br/>(1) Wang, J. S.; Wang, G.; Feng, X. Q.; Kitamura, T.; Kang, Y. L.; Yu, S. W.; Qin, Q. H. Hierarchical Chirality Transfer in the Growth of Towel Gourd Tendrils. <i>Scientific Reports </i><b>2013</b>, <i>3</i> (1), 1–7.<br/>(2) Yamaue, T.; Mukai, H.; Asaka, K.; Doi, M. Electrostress Diffusion Coupling Model for Polyelectrolyte Gels. <i>Macromolecules</i> <b>2005</b>, <i>38</i> (4), 1349–1356.<br/>(3) Kishi, R.; Hasebe, M.; Hara, M.; Osada, Y. Mechanism and Process of Chemomechanical Contraction of Polyelectrolyte Gels under Electric Field. <i>Polymer Advanced Technologies</i> <b>1990</b>, <i>1</i> (1), 19–25.<br/>(4) Tanaka, T.; Nishio, I.; Sun, S.-T.; Ueno-Nishio, S. Collapse of Gels in an Electric Field. <i>Science (1979)</i> <b>1982</b>, <i>218</i> (4571), 467–469.<br/>(5) Shang, Y.; He, X.; Li, Y.; Zhang, L.; Li, Z.; Ji, C.; Shi, E.; Li, P.; Zhu, K.; Peng, Q.; Wang, C.; Zhang, X.; Wang, R.; Wei, J.; Wang, K.; Zhu, H.; Wu, D.; Cao, A. Super-Stretchable Spring-Like Carbon Nanotube Ropes. <i>Advanced Materials</i> <b>2012</b>, <i>24</i> (21), 2896–2900.