Overcoming the adhesion paradox and switchability conflict on rough surfaces with shape-memory polymers.

Smart adhesives that can be applied and removed on demand play an important role in modern life and manufacturing. However, current smart adhesives made of elastomers suffer from the long-standing challenges of the adhesion paradox (rapid decrease in adhesion strength on rough surfaces despite adhesive molecular interactions) and the switchability conflict (trade-off between adhesion strength and easy detachment). Here, we report the use of shape-memory polymers (SMPs) to overcome the adhesion paradox and switchability conflict on rough surfaces. Utilizing the rubbery-glassy phase transition in SMPs, we demonstrate, through mechanical testing and mechanics modeling, that the conformal contact in the rubbery state followed by the shape-locking effect in the glassy state results in the so-called rubber-to-glass (R2G) adhesion (defined as making contact in the rubbery state to a certain indentation depth followed by detachment in the glassy state), with extraordinary adhesion strength (>1 MPa) proportional to the true surface area of a rough surface, overcoming the classic adhesion paradox. Furthermore, upon transitioning back to the rubbery state, the SMP adhesives can detach easily due to the shape-memory effect, leading to a simultaneous improvement in adhesion switchability (up to 103, defined as the ratio of the SMP R2G adhesion to its rubbery-state adhesion) as the surface roughness increases. The working principle and the mechanics model of R2G adhesion provide guidelines for developing stronger and more switchable adhesives adaptable to rough surfaces, thereby enhancing the capabilities of smart adhesives, and impacting various fields such as adhesive grippers and climbing robots.

Rapid digital light 3D printing enabled by a soft and deformable hydrogel separation interface.

The low productivity of typical 3D printing is a major hurdle for its utilization in large-scale manufacturing. Innovative techniques have been developed to break the limitation of printing speed, however, sophisticated facilities or costly consumables are required, which still substantially restricts the economic efficiency. Here we report that a common stereolithographic 3D printing facility can achieve a very high printing speed (400 mm/h) using a green and inexpensive hydrogel as a separation interface against the cured part. In sharp contrast to other techniques, the unique separation mechanism relies on the large recoverable deformation along the thickness direction of the hydrogel interface during the layer-wise printing. The hydrogel needs to be extraordinarily soft and unusually thick to remarkably reduce the adhesion force which is a key factor for achieving rapid 3D printing. This technique shows excellent printing stability even for fabricating large continuous solid structures, which is extremely challenging for other rapid 3D printing techniques. The printing process is highly robust for fabricating diversified materials with various functions. With the advantages mentioned above, the presented technique is believed to make a large impact on large-scale manufacturing.

Programmable and scalable transfer printing with high reliability and efficiency for flexible inorganic electronics.

Transfer printing that enables heterogeneous integration of materials in desired layouts offers unprecedented opportunities for developing high-performance unconventional electronic systems. However, large-area integration of ultrathin and delicate functional micro-objects with high yields in a programmable fashion still remains as a great challenge. Here, we present a simple, cost-effective, yet robust transfer printing technique via a shape-conformal stamp with actively actuated surface microstructures for programmable and scalable transfer printing with high reliability and efficiency. The shape-conformal stamp features the polymeric backing and commercially available adhesive layer with embedded expandable microspheres. Upon external thermal stimuli, the embedded microspheres expand to form surface microstructures and yield weak adhesion for reliable release. Systematic experimental and computational studies reveal the fundamental aspects of the extraordinary adhesion switchability of stamp. Demonstrations of this protocol in deterministic assemblies of diverse challenging inorganic micro-objects illustrate its extraordinary capabilities in transfer printing for developing high-performance flexible inorganic electronics.

Universal SMP gripper with massive and selective capabilities for multiscaled, arbitrarily shaped objects.

Grippers are widely used for the gripping, manipulation, and assembly of objects with a wide range of scales, shapes, and quantities in research, industry, and our daily lives. A simple yet universal solution is very challenging. Here, we manage to address this challenge utilizing a simple shape memory polymer (SMP) block. The embedding of objects into the SMP enables the gripping while the shape recovery upon stimulation facilitates the releasing. Systematic studies show that friction, suction, and interlocking effects dominate the grip force individually or collectively. This universal SMP gripper design provides a versatile solution to grip and manipulate multiscaled (from centimeter scale down to 10-µm scale) 3D objects with arbitrary shapes, in individual, deterministic, or massive, selective ways. These extraordinary capabilities are demonstrated by the gripping and manipulation of macroscaled objects, mesoscaled steel sphere arrays and microparticles, and the selective and patterned transfer printing of micro light-emitting diodes.

Laser-driven programmable non-contact transfer printing of objects onto arbitrary receivers via an active elastomeric microstructured stamp.

Transfer printing, as an important assembly technique, has attracted much attention due to its valuable merits to develop novel forms of electronics such as stretchable inorganic electronics requiring the heterogeneous integration of inorganic materials with soft elastomers. Here, we report on a laser-driven programmable non-contact transfer printing technique via a simple yet robust design of active elastomeric microstructured stamp that features cavities filled with air and embedded under the contacting surface, a micro-patterned surface membrane that encapsulates the air cavities and a metal layer on the inner-cavity surfaces serving as the laser-absorbing layer. The micro-patterned surface membrane can be inflated dynamically to control the interfacial adhesion, which can be switched from strong state to weak state by more than three orders of magnitude by local laser heating of the air in the cavity with a temperature increase below 100°C. Theoretical and experimental studies reveal the fundamental aspects of the design and fabrication of the active elastomeric microstructured stamp and the operation of non-contact transfer printing. Demonstrations in the programmable transfer printing of micro-scale silicon platelets and micro-scale LED chips onto various challenging receivers illustrate the extraordinary capabilities for deterministic assembly that are difficult to address by existing printing schemes, thereby creating engineering opportunities in areas requiring the heterogeneous integration of diverse materials such as curvilinear electronics and MicroLED displays.

Fast Digital Patterning of Surface Topography toward Three-Dimensional Shape-Changing Structures.

Exiting strategies for 3D shape-changing structures are constrained by either the complicated fabrication process or the harsh demands of active materials. Facile preparation of 3D shape-changing structures with an extremely simple approach based on the elastomeric polymer still remains a challenging topic. Here, we report a fast digital patterning of surface topography of a single-layer elastomeric polymer toward 3D shape-changing structures. The surface topography features digitally engraved grooves by a laser engraver on a poly(dimethylsiloxane) (PDMS) sheet, which is surface oxidized by the UV-ozone treatment. The resulting engraved PDMS sheets exhibit programmable shape-changing behaviors to form various 3D structures under the action of organic solvent. Experimental and numerical studies reveal the fundamental aspects of surface topography-guided 3D shape-changing structures. Demonstrations of this concept in developing various complex 3D shape-changing structures illustrate the simplicity and effectiveness of our approach, thereby creating engineering opportunities in a wide range of applications such as actuators and soft robots.

Rapidly tunable and highly reversible bio-inspired dry adhesion for transfer printing in air and a vacuum.

Tunable and reversible dry adhesion has attracted much attention in academia and industry due to its wide applications ranging from releasable joints to stamps for transfer printing. Here, a simple yet robust magnetically actuated, aphid-inspired design of an elastomeric surface that provides rapidly tunable and highly reversible adhesion strength is reported. The magnetically actuated adhesive features open reservoirs filled with magnetic particles and encapsulated by a thin surface membrane, which can be deformed in a controlled manner via the magnetic field, thus, to tune the adhesion. The combination of the rate dependent effect and magnetic actuation of the thin surface membrane offers continuously tunable adhesion with a great switchability and a quick response. Experimental and theoretical studies reveal the underlying physics and provide design guidelines to optimize geometries for the broad control of adhesion. Demonstrations of this concept in stamps for transfer printing of silicon wafers in air and in a vacuum with a selective and programmable mode illustrate the capabilities for deterministic assembly and the potential in the semiconductor industry.