A multifunctional shape-morphing elastomer with liquid metal inclusions

Abstract

Natural soft tissue achieves a rich variety of functionality through a hierarchy of molecular, microscale, and mesoscale structures and ordering. Inspired by such architectures, we introduce a soft, multifunctional composite capable of a unique combination of sensing, mechanically robust electronic connectivity, and active shape morphing. The material is composed of a compliant and deformable liquid crystal elastomer (LCE) matrix that can achieve macroscopic shape change through a liquid crystal phase transition. The matrix is dispersed with liquid metal (LM) microparticles that are used to tailor the thermal and electrical conductivity of the LCE without detrimentally altering its mechanical or shape-morphing properties. Demonstrations of this composite for sensing, actuation, circuitry, and soft robot locomotion suggest the potential for versatile, tissue-like multifunctionality.

Keywords: conductive elastomer; liquid crystal elastomer; liquid metal; shape memory; soft actuator.

Fig. 1.

LM–LCE composites and related functional demonstrations. Illustrations of the molecular (A), microscale (B), and mesoscale (C) ordering of LM–LCE composites. (D) Micrographs of unstrained (Top) and strained (Bottom) composites. (Scale bars, 1 mm.) (E) Photographs highlighting the compliance and deformability of an unstretched (Top) and stretched (Middle) electrically conductive 50 vol % LM–LCE powering an LED. Circuit traces form through mechanical damage (Bottom). (Scale bar, 1 cm.) (F) Photographs of a 50 vol % LM–LCE composite lifting a 100-g weight (∼45 kPa). (Scale bar, 3 cm.) (G) Photographs of zero-stress shape change enabled by photoinitiated shape programming. (Scale bar, 1 cm.) (H) Photographs of a multifunctional architecture. LM–LCE composites function as a conductive wire to run current through an LED, as a transducer to sense touch, and as a Joule-heated actuator to lift a weight. An LED turns on when the sensing composite responds to touch, and internal Joule-heated actuation is activated. An example of the process is shown in Movie S1. (Scale bar, 1 cm.)

Fig. 2.

Material properties of LM–LCE composites. (A) Representative stress versus strain for LCEs and LM–LCE composites; (B) strain limit at break and (C) tensile modulus. Error bars represent SD for ≥ 3 measurements. (D) Representative storage modulus versus temperature for filled and unfilled materials. (E) Representative normalized length change versus temperature during cooling for filled and unfilled materials with a 20-kPa applied load. (F) Thermal conductivity versus volume fraction for unstretched materials for n ≥ 300 measurements with at least 3 samples tested for each volume fraction (blue circles), and for stretched unfilled LCE and 50 vol % LM–LCE, deformed to about 60% elongation (n ≥ 100, orange circles). Error bars represent SDs. The dashed traces represent the theoretical predictions using the Bruggeman effective medium theory formulation (blue) and a modified Bruggeman effective medium formulation for elongated inclusions (orange) (21, 23). (G and H) Histograms of thermal conductivity along the direction of strain (at 60% strain) for a 50 vol % LM–LCE composite and an unfilled LCE; (I) IR heat map showing heat dissipation of a 50 vol % LM–LCE composite and an unfilled LCE after initial heating. The temperature scale ranges from 20 °C (blue) to 120 °C (orange). (J) Normalized change in electrical resistance, R, versus strain of a conductive LM–LCE strip with the red line representing the average and the shaded region representing the SD for 3 samples tested 3 times each (n = 9) with a comparison to predictions from Ohm’s law (dashed curve). (K) Photographs of autonomous self-reconfiguring electrical conductivity upon puncturing through the traces.

Fig. 3.

Joule-heated actuation properties of LM–LCE composites. (A) Normalized stroke as a function of cycling frequency and actuation time (i.e., active duty cycle), for the 50 vol % LM–LCE to lift the hanging weight. SDs for n ≥ 4 actuation cycles were less than 8% of the measured values. (B) Specific work as a function of normalized load for a 50 vol % LM–LCE. Error bars represent SDs for ≥3 actuation cycles. (C) Normalized stroke over 15,000 cycles for a 50 vol % LM–LCE actuating with a 14-kPa hanging wait at a frequency of 0.007 Hz.

Fig. 4.

(A) Sequence of damage resilience of a Joule-heated 50 vol % LM-LCE lifting a hanging weight. The sample continues to lift the weight after being punctured. (Scale bar, 10 mm.) (B) Sequence of soft crawler composed of a 50 vol. % LM-LCE. A ruler is used for scale. (C) Damage detection and response of an LM–LCE composite. In the first panel, a conductive trace powers an LED. As the sample is damage (second panel), the LED remains on. After severe damage, Joule heating lifts the weight hanging from the composite (third and fourth panels). (Scale bar, 20 mm.)

Fig. 5.

Influence of inclusion deformability on composite properties. (A) Tensile moduli for unfilled LCE, 50 vol % LM–LCE, 50 vol % Ga–LCE (l), and 50 vol % Ga–LCE (s) where the (l) and (s) designations refer to the physical state of the gallium microparticles (liquid or solid). Measurements were made at room temperature. Error bars represent SDs for ≥3 measurements. (B) Photograph of Ga–LCE (s) with a hanging weight. The rigid inclusions do not permit extension. (C) Photograph of Ga–LCE (l) with a hanging weight. A heat gun is visible, providing heat to melt the gallium inclusions and permitting the composite to extend. (D) Photograph of Ga–LCE (l) after heating above the nematic-to-isotropic transition temperature (T_ni) of the LCE. The composite is able to retract to its original position. (Scale bar, 20 mm.) (E) Photograph of a hanging Ga–LCE composite with specific regions selectively heated to form rigid (solid Ga inclusions) and deformable (liquid Ga inclusions) domains. The deformable domain is mechanically sintered to induce electrical conductivity. (Scale bar, 20 mm.) (F) Photograph of a hanging Ga–LCE bending and buckling during Joule heating due to the rigid domain resisting contraction while the deformable domain contracts. (G) Photograph of a flat Ga–LCE with specific regions selectively heated to form rigid and deformable domains. (H) Photograph of a Ga–LCE bending and buckling during zero-stress actuation induced by Joule heating. (Scale bar, 20 mm.)