Self-Healable and Recyclable Dual-Shape Memory Liquid Metal–Elastomer Composites

Abstract

Liquid metal (LM)–polymer composites that combine the thermal and electrical conductivity of LMs with the shape-morphing capability of polymers are attracting a great deal of attention in the fields of reconfigurable electronics and soft robotics. However, investigation of the synergetic effect between the shape-changing properties of LMs and polymer matrices is lacking. Herein, a self-healable and recyclable dual-shape memory composite, comprising an LM (gallium) and a Diels–Alder (DA) crosslinked crystalline polyurethane (PU) elastomer, is reported. The composite exhibits a bilayer structure and achieves excellent shape programming abilities, due to the phase transitions of the LM and the crystalline PU elastomers. To demonstrate these shape-morphing abilities, a heat-triggered soft gripper, which can grasp and release objects according to the environmental temperature, is designed and built. Similarly, combining the electrical conductivity and the dual-shape memory effect of the composite, a light-controlled reconfigurable switch for a circuit is produced. In addition, due to the reversible nature of DA bonds, the composite is self-healable and recyclable. Both the LM and PU elastomer are recyclable, demonstrating the extremely high recycling efficiency (up to 96.7%) of the LM, as well as similar mechanical properties between the reprocessed elastomers and the pristine ones.

Recent Developments in Shape Memory Elastomers for Biotechnology Applications

“…Range from highly crosslinked to no crosslinks Slightly crosslinked Range from being flexible to rigid Flexible and elastic Wide range of glass transition temperatures Typically low (sub-zero) glass transition temperature 2022, 14, x FOR PEER REVIEW 2 of 21 permanent shape from the temporary deformations upon simultaneous disclosure to various (even multiple) stimuli without additional mechanical effort. Typically, SMPs exhibit at least two phases: the first is a stable form that stabilises the SMP and is responsible for the retention of the original shape and the second that responds to the external trigger [15][16][17][18][19][20][21][22][23][24][25]. SMPs can be categorized into one of four types depending on their chemical structure: thermosets, rubbers, thermoplastics, and block copolymers.…”

“…A category of smart materials, SMEs react to external stimuli such as temperature, pressure, electromagnetic fields, chemicals (and pH), water, light, and so on. The responsive physical properties could be stiffness, shape, and damping, among others [1,9,14,24,[40][41][42]. A recent SME was developed by Rim et al (2022) [43].…”

“…The transformation temperature is typically 15-20 • C higher than the T g , though it may be in the vicinity of or even below that of the T g . When passing the T g from the glassy to the rubbery state, it is known that the network deformability goes through a maximum [24]. A sample model of an SME is presented in Figure 1.…”

Biphasic liquid metal mixtures in stretchable and flexible applications

“…The LM-PU (polyurethane) composite exhibits a bilayer structure and achieves excellent shape-morphing abilities due to the phase transitions of LM and crystalline PU elastomers. 20 Biphasic liquid metal mixtures and their applications in flexible and stretchable electronics are highlighted in this article. Additionally, the properties and tuning methods of LM alloys (biphasic LM mixture with solid metal particles) and composites (biphasic LM mixture with elastomers) with organic materials and patterning techniques for circuit fabrication are summarized.…”

Photothermal Thin Films with Highly Efficient NIR Conversion for Miniaturized Liquid-Crystal Elastomer Actuators

“…In general, LCEs require external heating elements to facilitate shape-change actuation. Previous research reported LCE composites that were electrically actuated by Joule heating using conductive paths such as flexible metal wires [ 13 , 14 , 15 , 16 ], silver ink/paste [ 17 ], carbon coatings [ 18 ], and encapsulated liquid metal [ 19 , 20 ]. These approaches have proven to be effective for repeatable actuation.…”