A bioinspired snap-through metastructure for manipulating micro-objects

A hyperelastic torque-reversal mechanism for soft joints with compression-responsive transient bistability

Snap-through, a rapid transition of a system from an equilibrium state to a nonadjacent equilibrium state, is a valuable design element of soft devices for converting a monolithic stimulus into systematic responses with impulsive motions. A common way to benefit from snap-through is to embody it within structures and materials, such as bistable structures. Torque-reversal mechanisms discovered in nature, which harness snap-through instability via muscular forces, may have comparative advantages. However, the current intricacy of artificial torque-reversal mechanisms, which require sophisticated kinematics/kinetics, constrains design possibilities for soft joints and devices. Here, we harnessed hyperelasticity to implement a torque-reversal mechanism in a soft joint, generating repetitive cilia-like beating motions through an embedded tendon. The developed hyperelastic torque-reversal mechanism (HeTRM) exhibits transient bistability under a specific compressive displacement/force threshold, with snap-through occurring at the point where the transience ends. To validate the effectiveness of this design principle, we explored the functionalities of HeTRM in energy storage and release, dual modes for impulsive and continuous motion, mechanical fuse, and rapid three-dimensional motions, through proof-of-concept soft machines. We expect that this design principle provides insight into incorporating snap-through behavior in soft machines and may aid in understanding the relationship between torque-reversal mechanisms and bistability.

On-demand Reprogrammable Mechanical Metamaterial Driven by Structure Performance Relations

The physical reprogrammability of metamaterials provides unprecedented opportunities for tailoring changeable mechanical behaviors. It is envisioned that metamaterials can actively, precisely, and rapidly reprogram their performances through digital interfaces toward varying demands. However, on-demand reprogramming by integration of physical and digital merits still remains less explored. Here, a real-time reprogrammable mechanical metamaterial is reported that is guided by its own structure-performance relations. The metamaterial consists of periodically tessellated bistable building blocks with built-in soft actuators for state switching, exhibiting rich spatial heterogeneity. Guided by the pre-established relations between state sequences and stress-strain curves, the metamaterial can accurately match a target curve by digitally tuning its state within 4 s. The metamaterial can be elastically tensioned and compressed under a strain of 4%, and its modulus tuning ratio reaches >30. Moreover, it also shows highly tunable shearing and bending performances. This work provides a new thought for the physical performance reprogrammability of artificial intelligent systems.

Self-locking and stiffening deployable tubular structures

Deployable tubular structures, designed for functional expansion, serve a wide range of applications, from flexible pipes to stiff structural elements. These structures, which transform from compact states, are crucial for creating adaptive solutions across engineering and scientific fields. A significant barrier to advancing their performance is balancing expandability with stiffness. Using compliant materials, these structures achieve more flexible transformations than those possible with rigid mechanisms. However, they typically exhibit reduced stiffness when subjected to external pressures (e.g., tube wall loading). Here, we utilize origami-inspired techniques and internal stiffeners to meet conflicting performance requirements. A self-locking mechanism is proposed, which combines the folding behavior observed in curved-crease origami and elastic shell buckling. This mechanism employs simple shell components, including internal diaphragms that undergo pseudofolding in a confined boundary condition to enable a snap-through transition. We reveal that the deployed tube is self-locked through geometrical interference, creating a braced tubular arrangement. This arrangement gives a direction-dependent structural performance, ranging from elastic response to crushing, thereby offering the potential for programmable structures. We demonstrate that our approach can advance existing deployment mechanisms (e.g., coiled and inflatable systems) and create diverse structural designs (e.g., metamaterials, adaptive structures, cantilevers, and lightweight panels).Weanticipate our design to be a starting point to drive technological advancement in real-world deployable tubular structures.

Fibrillar adhesives with unprecedented adhesion strength, switchability and scalability

Bio-inspired fibrillar adhesives have received worldwide attention but their potentials have been limited by a trade-off between adhesion strength and adhesion switchability, and a size scale effect that restricts the fibrils to micro/nanoscales. Here, we report a class of adhesive fibrils that achieve unprecedented adhesion strength (∼2 MPa), switchability (∼2000), and scalability (up to millimeter-scale at the single fibril level), by leveraging the rubber-to-glass (R2G) transition in shape memory polymers (SMPs). Moreover, R2G SMP fibrillar adhesive arrays exhibit a switchability of >1000 (with the aid of controlled buckling) and an adhesion efficiency of 57.8%, with apparent contact area scalable to 1000 mm2, outperforming existing fibrillar adhesives. We further demonstrate that the SMP fibrillar adhesives can be used as soft grippers and reusable superglue devices that are capable of holding and releasing heavy objects >2000 times of their own weight. These findings represent significant advances in smart fibrillar adhesives for numerous applications, especially those involving high-payload scenarios.

Asymmetric toughening in the lap shear of metamaterial structural adhesives

Metamaterial structural adhesives (MSAs), whose properties primarily rely on structural design, offer promising advantages over traditional adhesives, including asymmetric, switchable, and programmable adhesion. However, the effects of thick backing structures on the adhesion properties remain largely underexplored. Herein, we investigate a series of MSAs featuring a thin adhesive layer and an asymmetric thick beam structure terminated with a film. We conduct lap shear tests on the MSAs with varying terminated film thickness (t) and beam tilting angle (θ) while maintaining an identical adhesive layer. For MSAs with a thick terminated film (t = 2 mm), the effective adhesion energy is double that of solid samples without compromising shear strength, consistent with the theoretical predictions based on the crack trapping mechanism. Conversely, for MSAs with a thin terminated film (t = 0.5 mm), the maximum shear strength and effective adhesion energy are ∼2.8 times and ∼18.6 times those of solid samples, respectively, deviating significantly from the theoretical predictions due to new crack initiations. We further explore adhesion asymmetry by tuning the beam tilting angle (θ). For MSAs with highly tilted beams (θ = 70.3°), we achieve a maximum adhesion strength asymmetry factor of τ2/τ1 ∼ 2.2 for a thick terminated film (t = 2 mm), and a maximum adhesion energy asymmetry factor of Γ1/Γ2 ∼ 5.3 for a thin terminated film (t = 0.5 mm). Our work provides useful insights for designing metamaterial structural adhesives suitable for robotic grippers, wall-climbing robots, and wearable devices, particularly those requiring asymmetric, switchable, and stimuli-responsive adhesion, and adhesives on rough surfaces or in underwater conditions.

A review of bioinspired dry adhesives: from achieving strong adhesion to realizing switchable adhesion

In the early twenty-first century, extensive research has been conducted on geckos' ability to climb vertical walls with the advancement of microscopy technology. Unprecedented studies and developments have focused on the adhesion mechanism, structural design, preparation methods, and applications of bioinspired dry adhesives. Notably, strong adhesion that adheres to both the principles of contact splitting and stress uniform distribution has been discovered and proposed. The increasing popularity of flexible electronic skins, soft crawling robots, and smart assembly systems has made switchable adhesion properties essential for smart adhesives. These adhesives are designed to be programmable and switchable in response to external stimuli such as magnetic fields, thermal changes, electrical signals, light exposure as well as mechanical processes. This paper provides a comprehensive review of the development history of bioinspired dry adhesives from achieving strong adhesion to realizing switchable adhesion.

Switchable Adhesive Based on Shape Memory Polymer with Micropillars of Different Heights for Laser-Driven Noncontact Transfer Printing

Switchable adhesive is essential to develop transfer printing, which is an advanced heterogeneous material integration technique for developing electronic systems. Designing a switchable adhesive with strong adhesion strength that can also be easily eliminated to enable noncontact transfer printing still remains a challenge. Here, we report a simple yet robust design of switchable adhesive based on a thermally responsive shape memory polymer with micropillars of different heights. The adhesive takes advantage of the shape-fixing property of shape memory polymer to provide strong adhesion for a reliable pick-up and the various levels of shape recovery of micropillars under laser heating to eliminate the adhesion for robust printing in a noncontact way. Systematic experimental and numerical studies reveal the adhesion switch mechanism and provide insights into the design of switchable adhesives. This switchable adhesive design provides a good solution to develop laser-driven noncontact transfer printing with the capability of eliminating the influence of receivers on the performance of transfer printing. Demonstrations of transfer printing of silicon wafers, microscale Si platelets, and micro light emitting diode (μ-LED) chips onto various challenging nonadhesive receivers (e.g., sandpaper, stainless steel bead, leaf, or glass) to form desired two-dimensional or three-dimensional layouts illustrate its great potential in deterministic assembly.

In Situ Activation of Snap-Through Instability in Multi-Response Metamaterials through Multistable Topological Transformation

Snap-through instability has been widely leveraged in metamaterials to attain non-monotonic responses for a specific subset of applications where conventional monotonic materials fail to perform. In the remaining more plentiful set of ordinary applications, snap-through instability is harmful, and current snapping metamaterials become inadequate because their capacity to snap cannot be suppressed post-fabrication. Here, a class of topology-transformable metamaterials is introduced to enable in situ activation and deactivation of the snapping capacity, providing a remarkable level of versatility in switching between responses from monotonic to monostable and bistable snap-through. Theoretical analysis, numerical simulations, and experiments are combined to unveil the role played by contact in the topological transformation capable of increasing the geometry incompatibility and confinement stiffness of selected architectural members. The strategy here presented for post-fabrication reprogrammability of matter and on-the-fly response switching paves the way to multifunctionality for application in multiple sectors from mechanical logic gates, and adjustable energy dissipators, to in situ adaptable sport equipment.