Laser-driven noncontact bubble transfer printing via a hydrogel composite stamp

Transfer printing based on tunable and reversible adhesive that enables the heterogeneous integration of materials is essential for developing envisioned electronic systems. An adhesive with both adhesion enhancement and reduction capabilities in a rapid and selective manner is challenging. Here, we report a laser-induced adhesive, featuring a geometrically simple shape memory polymer layer on a glass backing, with excellent adhesion modulation capability for programmable pickup and noncontact printing of microchips. Selective and rapid laser heating substantially enhances the adhesive's adhesion strength from kilopascal to megapascal within 10 ms due to the shape fixing effect, allowing for precise and programmable pickup. Conversely, the enhanced adhesion can be quickly reduced and eliminated within 3 ms through the shape recovery effect, enabling noncontact printing. Demonstrations of transfer printing microlight-emitting diodes (LEDs) and mini-LEDs onto various low-adhesive flat, rough, and curved surfaces highlight the unusual capabilities of this adhesive for deterministic assembly.

A theoretical model of reversible adhesion in shape memory surface relief structures and its application in transfer printing

Transfer printing techniques based on tunable adhesives that enable heterogeneous integration of materials in desired layouts are essential for developing existing and envisioned systems such as flexible electronics and micro‐LED (µ‐LED) displays. Here, a novel thermal controlled tunable adhesive, which not only has the ability of eliminating the interfacial adhesion for printing but also provides a new strategy for enhancing the interfacial adhesion for pick‐up is reported. The tunable adhesive features cavities filled with air on the surface. This simple construct offers thermal controlled suction and thrust with their amplitudes on the order of a few tens of kPa within 100 °C temperature change, which enables a reliable damage‐free transfer printing. Systematically theoretical and experimental studies reveal the underlying thermal induced pressure change mechanism and provide insights into the design and operation of the thermal controlled tunable adhesive. Demonstrations of this smart adhesive in manipulation of various surfaces and transfer printing of micro‐scale Si inks and µ‐LEDs illustrate its unusual capabilities for deterministic assembly by transfer printing.

Microscale elastomeric relief structures with “pedestal” shapes provide enhanced operation in stamps designed for deterministic materials assembly via transfer printing. Experimental measurements of adhesion and finite element analysis both show that for certain geometries, exceptionally large enhancements in adhesion strength (over 15×) can be achieved. Transfer printing of microscale platelets of silicon and ultrathin gallium nitride light emitting diodes onto a silicon substrate without adhesive coatings demonstrates some capabilities in assembly that result from this type of stamp, of interest in diverse applications, including those that involve heterogeneous materials integration.

Transfer printing is an emerging technique for materials assembly and micro-/nano-fabrication. An important emerging variant of this process involves laser-induced impulsive heating to initiate separation at the interface between a soft, elastomeric stamp and hard micro-/nano-materials (i.e. inks) on its surface, due to a large mismatch in coefficients of thermal expansion. The result is the active ejection of the inks from the stamp to a spatially separated receiving substrate, thereby representing the printing step. In the following, a thermo-mechanical model is established to analytically obtain the temperature field, and the energy release rate for delamination at the interface between the stamp and ink in the form of a rigid plate. The normalized critical laser pulse time for interfacial delamination depends only on the normalized total heat flux at the interface and the normalized width of the ink structure. This scaling law has been verified by experiments and the finite element method.

Transfer printing of patterned metal films using parylene C coated surfaces

The preparation of well-defined molecular monolayers and their patterning on the microscale and nanoscale are key aspects of surface science and chemical nanotechnology. In this article, we describe the modification of amine-functionalized surfaces using a new type of contact printing based on chemically patterned, flat PDMS stamps. The stamps have discrete areas with surface-bond tetrafluorophenol (TFP) groups, which allow the attachment of carboxylic acids in the presence of coupling agents such as diisopropylcarbodiimide (DIC). The generated active esters can be reacted by placing the stamps in contact with amine-functionalized surfaces. The process leads to the transfer of acyl residues from the stamp to the substrate and therefore to a covalent attachment. Patterning occurs because of the fact that reaction and transfer take place only in areas with TFP groups present on the stamp surface. Different types of amine-decorated surfaces were successfully modified, and the transfer was visualized by fluorescence microscopy. To the best of our knowledge, the covalent transfer printing (CTP) of an immobilized molecular monolayer from one surface to another surface is unprecedented.

The adhesion-based transfer printing process allows the devices to be fabricated through low temperature process, which is important for realizing flexible electronics with high mobility. Therefore, comprehensive understanding about adhesion between the solid object and the surface of elastomeric stamp (generally, polydimethylsiloxane is used) and the optimal strategy to control adhesion can support the effective and high yield transfer. In this paper, a novel adhesive force measurement system is developed to measure dynamic adhesive forces from the surface on the flexible stamp. The thermodynamic work of adhesion at the moment of debonding of a tip of sensor from the flexible stamp is modeled and used for developing the adhesion control strategy. The measurement results show that the work of adhesion is strongly dependent on the peel-off velocity of tip, while the indentation force has only minor effects on it. The effect of Young’s modulus of elastomeric stamp is also investigated using the model of work of adhesion based on the relation between the adhesive force and peel-off velocity. The elastomeric stamp with low Young’s modulus is preferred for efficient transfer printing.

Transfer printing, a foundational manufacturing route for integrating heterogeneous materials onto diverse substrates, holds great promise for applications in flexible electronics, emerging displays, and soft robotics. As applications grow more complex, the quality of ultrathin films integrated onto applicable substrates, initially fabricated on rigid substrates by spin-coating, physical vapor deposition, or chemical vapor deposition [e.g., photoresists, metals, two-dimensional (2D) materials] with thicknesses below 10 μm, becomes critical to the performance of advanced devices, including flexible/curved electronics and 2D films-based electronics. However, achieving damage-free transfer printing of ultrathin films requires resolving the trade-off in traditional methods between interfacial fracture and in-plane film damage. Herein, this review elucidates the principles underlying damage-free transfer printing of ultrathin films, highlights recent innovations that enable on-demand control of interfacial adhesion during the process, and summarizes typical applications based on transferred damage-free films. Finally, we provide perspectives on the remaining challenges and future developments needed to enable industrial-scale manufacturing and inspire continued innovation.

We report the effect of epoxy adhesion layers with different mechanical or physical property on a magnetoelectric (ME) composite laminate composed of FeBSi alloy (Metglas)/single-crystal Pb(Mg1/3Nb2/3)O3-Pb(Zr,Ti)O3/Metglas to achieve an improved ME conversion performance. Through theoretical simulation, it was revealed that the Young's modulus and the thickness of interfacial adhesives were major parameters that influence the conversion efficiency in ME composites. In the experimental evaluation, we utilized three epoxy materials with a distinct Young's modulus and adjusted the average thickness of the adhesion layers to optimize the ME conversion. The experimental results show that a thin epoxy layer with a high Young's modulus provided the best performance in the inorganic-based ME conversion process. By tailoring the interfacial adhesion property, the ME laminate generated a high conversion coefficient of 328.8 V/(cm Oe), with a mechanical quality factor of 132.0 at the resonance mode. Moreover, we demonstrated a highly sensitive alternating current magnetic field sensor that had a detection resolution below 10 pT. The optimization of the epoxy layers in the ME laminate composite provided significant enhancement of the ME response in a simple manner.

Improvement of interfacial adhesion in vertical GaN-based LEDs by introducing O 2 plasma cleaning and intermediate layers

Damage evolution behavior of bi-adhesive repaired composites under bending load by acoustic emission and micro-CT

Biomodification of eroded and abraded dentin with epigallocatechin-3-gallate (EGCG)

The agile locomotion of adhesive animals is mainly attributed to their sophisticated hierarchical feet and reversible adhesion motility. Their structure–function relationship is an urgent issue to be solved to understand biologic adhesive systems and the design of bionic applications. In this study, the reversible adhesion/release behavior and structural properties of gecko toes were investigated, and a hierarchical adhesive bionic toe (bio-toe) consisting of an upper elastic actuator as the supporting/driving layer and lower bionic lamellae (bio-lamellae) as the adhesive layer was designed, which can adhere to and release from targets reversibly when driven by bi-directional pressure. A mathematical model of the nonlinear deformation and a finite element model of the adhesive contact of the bio-toe were developed. Meanwhile, combined with experimental tests, the effects of the structure and actuation on the adhesive behavior and mechanical properties of the bio-toe were investigated. The research found that (1) the bending curvature of the bio-toe, which is approximately linear with pressure, enables the bio-toe to adapt to a wide range of objects controllably; (2) the tabular bio-lamella could achieve a contact rate of 60% with a low squeeze contact of less than 0.5 N despite a ±10° tilt in contact posture; (3) the upward bending of the bio-toe under negative pressure provided sufficient rebounding force for a 100% success rate of release; (4) the ratio of shear adhesion force to preload of the bio-toe with tabular bio-lamellae reaches approximately 12, which is higher than that of most existing adhesion units and frictional gripping units. The bio-toe shows good adaptability, load capacity, and reversibility of adhesion when applied as the basic adhesive unit in a robot gripper and wall-climbing robot. Finally, the proposed reversible adhesive bio-toe with a hierarchical structure has great potential for application in space, defense, industry, and daily life.

The poor adhesion performance of typical gels still remains a challenge to find a simple method to achieve strong and reversible adhesion with the existence of water. Here, a poly(acryloyloxyethyl trimethyl ammonium chloride-co-2-vinyl-4-6-diamino-1,3,5-triazine) (P(DAC-co-VDT)) gel with high and adjustable interfacial adhesion is fabricated by combining cation-triazine π interaction and multiple hydrogen bonding and through a one-pot route. Characterization of the gels reveals that the two types of interactions are introduced into the gel network and that the gel-gel and gel-glass interfacial adhesion can be readily adjusted in a wide range from 15.98 to 123.60kPa. This approach enables the creation of high-strength composites using P(DAC-co-VDT) gel as matrix, anionic monomer sodium p-styrene sulfonate as ion concentration adjustor, and discrete quartz sands as filler with easy and repeated moldability and self-healing capability.