Metamaterial adhesives for programmable adhesion through reverse crack propagation.

Adhesives are typically either strong and permanent or reversible with limited strength. However, current strategies to create strong yet reversible adhesives needed for wearable devices, robotics and material disassembly lack independent control of strength and release, require complex fabrication or only work in specific conditions. Here we report metamaterial adhesives that simultaneously achieve strong and releasable adhesion with spatially selectable adhesion strength through programmed cut architectures. Nonlinear cuts uniquely suppress crack propagation by forcing cracks to propagate backwards for 60× enhancement in adhesion, while allowing crack growth in the opposite direction for easy release and reusability. This mechanism functions in numerous adhesives on diverse substrates in wet and dry conditions and enables highly tunable adhesion with independently programmable adhesion strength in two directions simultaneously at any location. We create these multifunctional materials in a maskless, digital fabrication framework to rapidly customize adhesive characteristics with deterministic control for next-generation adhesives.

Tough Bonding of Liquid Metal-Elastomer Composites for Multifunctional Adhesives.

Liquid metal (LM) composites, which consist of LM droplets dispersed in highly deformable elastomers, have recently gained interest as a multifunctional material for soft robotics and electronics. The incorporation of LM into elastic solids allows for unique combinations of material properties such as high stretchability with thermal and electrical conductivity comparable to metals. However, it is currently a challenge to incorporate LM composites into integrated systems consisting of diverse materials and components due to a lack of adhesion control. Here, a chemical anchoring methodology to increase adhesion of LM composites to diverse substrates is presented. The fracture energy increases up to 100× relative to untreated surfaces, with values reaching up to 7800 J m-2 . Furthermore, the fracture energy, tensile modulus, and thermal conductivity can be tuned together by controlling the microstructure of LM composites. Finally, the bonding technique is used to integrate LM composites with functional electronic components without encapsulation or clamping, allowing for extreme deformations while maintaining exceptional thermal and electrical conductivity. These findings can accelerate the adoption of LM composites into complex soft robotic and electronic systems where strong, reliable bonding between diverse materials and components is required.

Deterministic control of adhesive crack propagation through jamming based switchable adhesives.

Controlling delamination across a material interface is a foundation of adhesive science and technology. This ranges from creating permanent, strong adhesives which limit crack propagation to reversible adhesives which initiate cracks for release. Methods which dynamically control cracks can lead to more robust adhesion, however specific control of crack initiation, propagation, and arresting is challenging because time scales of crack propagation are much faster than times scales of mechanisms to arrest cracks. Here we show the deterministic control of crack initiation, propagation, and arresting by integrating a granular jamming layer into adhesive films. This allows for controlled initiation of a propagating crack by reducing rigidity and then rapidly arresting the crack through jamming, with a rise in stiffness and an 11× enhancement in adhesion. This process is highly reversible and programmable, allowing for numerous crack initiation, propagation, and arresting cycles at arbitrary selectable locations in a peeling adhesive. We demonstrate this crack-control approach in single and multiple peel directions under fixed load conditions in response to diverse pressurization input signal profiles (i.e. time varying propagation and arresting scenarios).

Shape morphing mechanical metamaterials through reversible plasticity.

Biological organisms such as the octopus can reconfigure their shape and properties to perform diverse tasks. However, soft machines struggle to achieve complex configurations, morph into shape to support loads, and go between multiple states reversibly. Here, we introduce a multifunctional shape-morphing material with reversible and rapid polymorphic reconfigurability. We couple elastomeric kirigami with an unconventional reversible plasticity mechanism in metal alloys to rapidly (<0.1 seconds) morph flat sheets into complex, load-bearing shapes, with reversibility and self-healing through phase change. This kirigami composite overcomes trade-offs in deformability and load-bearing capacity and eliminates power requirements to sustain reconfigured shapes. We demonstrate this material through integration with onboard control, motors, and power to create a soft robotic morphing drone, which autonomously transforms from a ground to air vehicle and an underwater morphing machine, which can be reversibly deployed to collect cargo.

Cancer cell migration in collagen-hyaluronan composite extracellular matrices.

Hyaluronan (HA) is a key component in the tumor microenvironment (TME) that participates in cancer growth and invasiveness. While the molecular weight (MW) dependent properties of HA can cause tumor-promoting and -repressing effects, the elevated levels of HA in the TME impedes drug delivery. The degradation of HA using hyaluronidases (HYALs), resulting in fragments of HA, is a way to overcome this, but the consequences of changes in HA molecular weight and concentration is currently unknown. Therefore, it is critical to understand the MW-dependent biological effects of HA. Here we examine the influence of HA molecular weight on biophysical properties that regulate cell migration and extracellular matrix (ECM) remodeling. In our study, we used vLMW, LMW and HMW HA at different physiologically relevant concentrations, with a particular interest in correlating the mechanical and structural properties to different cell functions. The elastic modulus, collagen network pore size and collagen fiber diameter increased with increasing HA concentration. Although the collagen network pore size increased, these pores were filled with the bulky HA molecules. Consequently, cell migration decreased with increase in HA concentration due to multiple, long-lived and unproductive protrusions, suggesting the influence of steric factors. Surprisingly, even though elastic modulus increased with HA molecular weight and concentration, gel compaction assays showed an increased degree of ECM compaction among HMW HA gels at high concentrations (2 and 4 mg mL-1 [0.2 and 0.4%]). These results were not seen in collagen gels that lacked HA, but had similar stiffness. HA appears to have the effect of decreasing migration and increasing collagen network contraction, but only at high HA molecular weight. Consequently, changes in HA molecular weight can have relatively large effects on cancer cell behavior. STATEMENT OF SIGNIFICANCE: Hyaluronan (HA) is a critical component of the tumor microenvironment (TME). Overproduction of HA in the TME results in poor prognosis and collapse of blood vessels, inhibiting drug delivery. Hyaluronidases have been used to enhance drug delivery. However, they lead to low molecular weight (MW) HA, altering the mechanical and structural properties of the TME and cancer cell behavior. Understanding how HA degradation affects cancer cell behavior is critical for uncovering detrimental effects of this therapy. Very little is known about how HA MW affects cancer cell behavior in tumor-mimicking collagen-HA composite networks. Here we examine how MW and HA content in collagen-HA networks alter structural and mechanical properties to regulate cell migration and matrix remodeling in 3D TME-mimicking environments.

All-graphene-based open fluidics for pumpless, small-scale fluid transport via laser-controlled wettability patterning.

Open microfluidics have emerged as a low-cost, pumpless alternative strategy to conventional microfluidics for delivery of fluid for a wide variety of applications including rapid biochemical analysis and medical diagnosis. However, creating open microfluidics by tuning the wettability of surfaces typically requires sophisticated cleanroom processes that are unamenable to scalable manufacturing. Herein, we present a simple approach to develop open microfluidic platforms by manipulating the surface wettability of spin-coated graphene ink films on flexible polyethylene terephthalate via laser-controlled patterning. Wedge-shaped hydrophilic tracks surrounded by superhydrophobic walls are created within the graphene films by scribing micron-sized grooves into the graphene with a CO2 laser. This scribing process is used to make superhydrophobic walls (water contact angle ∼160°) that delineate hydrophilic tracks (created through an oxygen plasma pretreatment) on the graphene for fluid transport. These all-graphene open microfluidic tracks are capable of transporting liquid droplets with a velocity of 20 mm s-1 on a level surface and uphill at elevation angles of 7° as well as transporting fluid in bifurcating cross and tree branches. The all-graphene open microfluidic manufacturing technique is rapid and amenable to scalable manufacturing, and consequently offers an alternative pumpless strategy to conventional microfluidics and creates possibilities for diverse applications in fluid transport.

Tuning surface functionalization and collagen gel thickness to regulate cancer cell migration.

Cancer cells have a tremendous ability to sense and respond to extracellular matrix (ECM) stiffness, modulating invasion. The magnitude of the sensed stiffness can either promote or inhibit the migration of cancer cells out of the primary tumor into surrounding tissue. Work has been done on examining the role of stiffness in tuning cancer cell migration by controlling elastic modulus in the bulk. However, a powerful and complementary approach for controlling stiffness is to leverage interactions between stiff-soft (e.g. glass-hydrogel) interfaces. Unfortunately, most work in this area probes cells in 2D environments. Of the reports that probe 3D environments, none have assessed the role of mechanical linkage to the interface as a potential handle in controlling local stiffness and cell behavior. In this paper, we examine the migration of cancer cells embedded in a collagen fiber network between two flat plates. We examine the role of both surface attachment of the collagen network to the stiff interface as well as thickness (50-540 µm) of the collagen gel in driving collagen organization, cell morphology and cell migration. We find that surface attachment and thickness do not operate overlapping mechanisms, because they elicit different cell responses. While thickness and surface chemistry appear to control morphology, only thickness regulates collagen organization and cell migration speed. This suggests that surface attachment and thickness of the collagen gel control cell behavior through both collagen structure and local stiffness in confined fiber-forming networks.

Tunable Mechanical Metamaterials through Hybrid Kirigami Structures.

Inspired by the art of paper cutting, kirigami provides intriguing tools to create materials with unconventional mechanical and morphological responses. This behavior is appealing in multiple applications such as stretchable electronics and soft robotics and presents a tractable platform to study structure-property relationships in material systems. However, mechanical response is typically controlled through a single or fractal cut type patterned across an entire kirigami sheet, limiting deformation modes and tunability. Here we show how hybrid patterns of major and minor cuts creates new opportunities to introduce boundary conditions and non-prismatic beams to enable highly tunable mechanical responses. This hybrid approach reduces stiffness by a factor of ~30 while increasing ultimate strain by a factor of 2 (up to 750% strain) relative to single incision patterns. We present analytical models and generate general design criteria that is in excellent agreement with experimental data from nanoscopic to macroscopic systems. These hybrid kirigami materials create new opportunities for multifunctional materials and structures, which we demonstrate with stretchable kirigami conductors with nearly constant electrical resistance up to >400% strain and magnetoactive actuators with extremely rapid response (>10,000% strain s-1) and high, repeatable elongation (>300% strain).

Kirigami-Inspired Structures for Smart Adhesion.

Spatially controlled layouts of elasticity can provide enhanced adhesion over homogeneous systems. Emerging techniques in kirigami, where designed cuts in materials impart highly tunable stiffness and geometry, offer an intriguing approach to create well-defined layouts of prescribed elastic regions. Here, we show that kirigami-inspired structures at interfaces provide a new mechanism to spatially control and enhance adhesion strength while providing directional characteristics for smart interfaces. We use kirigami-inspired cuts to define stiff and compliant regions, where above a critical, material-defined length scale, bending rigidity and contact width can be tuned to enhance adhesive force capacity by a factor of ∼100 across a spatially patterned adhesive sheet. The directional nature of these designs also imparts anisotropic responses, where peeling in different directions results in anisotropic adhesion ratios of ∼10. Experimental results are well-supported by theoretical predictions in which the bending rigidity and contact width of kirigami-inspired structures and interconnects control the adhesive capacity. These new interfacial structures and design criteria provide diverse routes for advanced adhesive functionality, including spatially controlled systems, wearable kirigami-inspired electronics, and anisotropic kirigami-inspired bandages that enable strong adhesive capacity while maintaining easy release.