4D Printing of Hygroscopic Liquid Crystal Elastomer Actuators

Abstract

Liquid crystal elastomers (LCEs) are broadly recognized as programmable actuating materials that are responsive to external stimuli, typically heat or light. Yet, soft LCEs that respond to changes in environmental humidity are not reported, except a few examples based on rigid liquid crystal networks with limited processing. Herein, a new class of highly deformable hygroscopic LCE actuators that can be prepared by versatile processing methods, including surface alignment as well as 3D printing is presented. The dimethylamino‐functionalized LCE is prepared by the aza‐Michael addition reaction between a reactive LC monomer and N,N′‐dimethylethylenediamine as a chain extender, followed by photopolymerization. The humidity‐responsive properties are introduced by activating one of the LCE surfaces with an acidic solution, which generates cations on the surface and provides asymmetric hydrophilicity to the LCE. The resulting humidity‐responsive LCE undergoes programmed and reversible hygroscopic actuation, and its shape transformation can be directed by the cut angle with respect to a nematic director or by localizing activation regions in the LCE. Most importantly, various hygroscopic LCE actuators, including (porous) bilayers, a flower, a concentric square array, and a soft gripper, are successfully fabricated by using LC inks in UV‐assisted direct‐ink‐writing‐based 3D printing.

Intrinsic carbon nanotube liquid crystalline elastomer photoactuators for high-definition biomechanics

“…Intelligent devices integrated with smart materials which can convert environmental stimuli ( e.g. , heat, 1,2 light, 3 electric fields, 4 magnetic fields, 5 and moisture 6 ) into shape deformations or mechanical motions have attracted widespread attention in the fields of motors, 7 sensors, 8–11 soft robots 12 and artificial muscles. 13 Among these various types of actuating schemes, light-driven strategies based on photothermal effects are most fascinating because they can be manipulated using a variety of light sources to enable localized actuation, remote control, etc.…”

Robust and Reprocessable Artificial Muscles Based on Liquid Crystal Elastomers with Dynamic Thiourea Bonds

“…[10][11][12] Moreover, the anisotropic mechanical properties of the LCEs can be harnessed to program their shape changes, enabling complex deformations including bending, coiling, twisting, and helixing. [3,13,14] Recently, significant progress has been achieved in the field of LCEs for materials synthesis, [15][16][17][18][19] fabrication, [16,[20][21][22][23][24] and expansion of LCE applications., [25][26][27][28][29][30][31] which has led to the renaissance of LCEs from both scientific and engineering aspects. However, the preparation of LCEs as actuators typically requires mesogen alignment prior to cross-linking, which leads to complicated fabrication processes unlike those required for fabricating conventional elastomers.…”

Direct Ink Writing of 4D Structural Colors

“…To achieve responsivity to humidity, the pendant amine groups in the ChLCE were protonated using a 0.1 m hydrochloric acid solution, forming a hygroscopic ammonium group in close proximity to the polymer backbone (Figure 3a). [33] Attenuated total reflection Fourier-transform infrared (ATR FT-IR) spectroscopy confirms that during the acid treatment tertiary amine groups (ν̃ ≈ 2750 and 2850 cm −1 ) are converted into tertiary ammonium groups (ν̃ ≈ 2940 cm −1 , see Figure 3b). The broad band formed around ν̃ ≈ 3400 cm −1 can be attributed to H 2 O absorbed from the atmosphere, as the hygroscopic ChLCE readily attracts moisture into its polar interior.…”

“…As demonstrated for DIW 4D-printed soft actuators, this can be done by using a chain extender containing a pendant tertiary amine group such as N,N-dimethylpropane-1,3-diamine (4), which can be positively charged after processing of the ink into an object. [33] The formation of charges on the polymer provides a strong incentive for water to swell the cholesteric elastomer. Table S1 (Supporting Information) provides details on the reactants used for ink synthesis.…”