Digital Light Processing of Liquid Crystal Elastomers for Self-Sensing Artificial Muscles
Digital Light Processing of Liquid Crystal Elastomers for Self-Sensing Artificial Muscles
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Artificial muscles based on stimuli-responsive polymers usually exhibit mechanical compliance, versatility, and high power-to-weight ratio, showing great promise to potentially replace conventional rigid motors for next-generation soft robots, wearable electronics, and biomedical devices. In particular, thermomechanical liquid crystal elastomers (LCEs) constitute artificial muscle-like actuators that can be remotely triggered for large stroke, fast response, and highly repeatable actuations. Here, we introduce a digital light processing (DLP)–based additive manufacturing approach that automatically shear aligns mesogenic oligomers, layer-by-layer, to achieve high orientational order in the photocrosslinked structures; this ordering yields high specific work capacity (63 J kg−1) and energy density (0.18 MJ m−3). We demonstrate actuators composed of these DLP printed LCEs’ applications in soft robotics, such as reversible grasping, untethered crawling, and weightlifting. Furthermore, we present an LCE self-sensing system that exploits thermally induced optical transition as an intrinsic option toward feedback control.
Artificial muscles based on stimuli-responsive polymers usually exhibit mechanical compliance, versatility, and high power-to-weight ratio, showing great promise to potentially replace conventional rigid motors for next-generation soft robots, wearable electronics, and biomedical devices. In particular, thermomechanical liquid crystal elastomers (LCEs) constitute artificial muscle-like actuators that can be remotely triggered for large stroke, fast response, and highly repeatable actuations. Here, we introduce a digital light processing (DLP)–based additive manufacturing approach that automatically shear aligns mesogenic oligomers, layer-by-layer, to achieve high orientational order in the photocrosslinked structures; this ordering yields high specific work capacity (63 J kg−1) and energy density (0.18 MJ m−3). We demonstrate actuators composed of these DLP printed LCEs’ applications in soft robotics, such as reversible grasping, untethered crawling, and weightlifting. Furthermore, we present an LCE self-sensing system that exploits thermally induced optical transition as an intrinsic option toward feedback control.
Published 1 paper as the 3rd author on Science Advances
DLP of LCEs. (A) Chemical structures of mesogenic monomers (RM257 and RM82), flexible linker (EDDT), and cross-linker (TATATO) used to synthesize the thermomechanical LCEs. (B) Differential scanning calorimetry curves showing glass transition temperature (Tg) and nematic-to-isotropic transition temperature (TNI) of the cross-linked LCE, as well as melting temperature (Tm) and cold crystallization temperature (Tcc) of the LCE liquid resin. (C) Polymerization speed of the thiol-acrylate/thiol-ene click reaction measured by the gel fraction of LCE exposed to UV irradiation with different duration. (D) Schematic illustration of shear alignment caused by the cyclic rotation of the resin tray. (Inset) From left to right, morphologies of molecular orientation in the sheared aligned resin, aligned cross-linked polymer, and nonsheared resin.
Characterizations of orientational order. (A) POM images of printed LCE showing birefringence caused by shear-induced alignment. (B) Stress-strain curves of LCEs that are parallel shear printed, nonshear printed, and perpendicular shear printed with respect to the direction of the axial stress in tensile testing. SD is demonstrated as the colored shaded areas. (C) Comparison of order parameters calculated from nonshear printed LCE and shear printed LCE with different shearing speeds. (D) Principle of thermal bending (T > TNI) in the printed LCE actuators. (Inset) Schematic showing reversible reconfiguration of cross-linked LCE polymer network in the layered structure.
Thermomechanical shape morphing. (A) Thermal bending of LCE actuators in response to homogeneous heat of different temperatures. (B) Thermal bending of LCE actuators that are printed with different thickness. (C) Thermal bending of LCE actuators that are printed with different fraction of shearing (FS) along their thickness. (D) Response time for actuation and relaxation of LCE actuators that are printed with different fraction of shearing along their thickness. (E) Cyclic actuation and relaxation (10 cycles) of printed LCE actuators in response to intermittent heat stimulation of high (~140°C) and low (~90°C) temperature gusts. Superimposed photos under different experimental conditions are shown in inlets; all scale bars, 1 cm.
Object manipulation and locomotion. (A) A soft robotic gripper (dimensions, 18 mm by 15 mm by 0.2 mm; fraction of shearing FS = 1.0) is used for (a and b) controlled grasping, (c to e) delivering, and (f) releasing of a preheated metal spring. Scale bar, 2 cm. (B) Crawling of a soft maneuvering robot (dimensions, 18 mm by 13 mm by 0.2 mm, FS = 0.7) on a ratchet surface. (a to e, left) Designed heat actuation sequence shows bending of forelimb, torso, and hindlimb of the crawling robot for effective locomotion. (a to e, right) Snapshots of the crawling robot demonstrate the corresponding gaits.
Weightlifting. (A) Image sequence of an LCE actuator (h = 800 μm, m = 56 mg) lifting a 40-g weight in response to applied temperature. (B) Reversible reconfiguration of polymer network of the three cases as shown in (A). (C) Specific work and actuation strain of the actuator when lifting different weights. (D) Uniaxial actuation of the LCE under mechanical load of 40 g.
Optomechanical self-sensing. (A) Left: Schematic of the LCE bending actuator (dimensions, 15 mm by 5 mm by 0.8 mm) and optoelectronic components. Right: Bending deformation of the actuator in two modes [(top) mode 1: thermally induced bending causes the disappearance of Schlieren texture and thus transparency in the actuator; (bottom) mode 2: nonthermally induced (external force) bending does not change the polydomain Schlieren texture, so the actuator remains whitish opaque]. (B) Optical signals detecting and differentiating thermally induced bending and nonthermally induced bending.
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