Superstretchable,Self‐Healing Polymeric Elastomers with Tunable Properties

Utilization of self‐healing chemistry to develop synthetic polymer materials that can heal themselves with restored mechanical performance and functionality is of great interest. Self‐healable polymer elastomers with tunable mechanical properties are especially attractive for a variety of applications. Herein, a series of urea functionalized poly(dimethyl siloxane)‐based elastomers (U‐PDMS‐Es) are reported with extremely high stretchability, self‐healing mechanical properties, and recoverable gas‐separation performance. Tailoring the molecular weights of poly(dimethyl siloxane) or weight ratio of elastic cross‐linker offers tunable mechanical properties of the obtained U‐PDMS‐Es, such as ultimate elongation (from 984% to 5600%), Young's modulus, ultimate tensile strength, toughness, and elastic recovery. The U‐PDMS‐Es can serve as excellent acoustic and vibration damping materials over a broad range of temperature (over 100 °C). The strain‐dependent elastic recovery behavior of U‐PDMS‐Es is also studied. After mechanical damage, the U‐PDMS‐Es can be healed in 120 min at ambient temperature or in 20 min at 40 °C with completely restored mechanical performance. The U‐PDMS‐Es are also demonstrated to exhibit recoverable gas‐separation functionality with retained permeability/selectivity after being damaged.     

Temperature‐Induced Switchable Adhesion using Nickel–Titanium–Polydimethylsiloxane Hybrid Surfaces

A switchable dry adhesive based on a nickel–titanium (NiTi) shape‐memory alloy with an adhesive silicone rubber surface has been developed. Although several studies investigate micropatterned, bioinspired adhesive surfaces, very few focus on reversible adhesion. The system here is based on the indentation‐induced two‐way shape‐memory effect in NiTi alloys. NiTi is trained by mechanical deformation through indentation and grinding to elicit a temperature‐induced switchable topography with protrusions at high temperature and a flat surface at low temperature. The trained surfaces are coated with either a smooth or a patterned adhesive polydimethylsiloxane (PDMS) layer, resulting in a temperature‐induced switchable surface, used for dry adhesion. Adhesion tests show that the temperature‐induced topographical change of the NiTi influences the adhesive performance of the hybrid system. For samples with a smooth PDMS layer the transition from flat to structured state reduces adhesion by 56%, and for samples with a micropatterned PDMS layer adhesion is switchable by nearly 100%. Both hybrid systems reveal strong reversibility related to the NiTi martensitic phase transformation, allowing repeated switching between an adhesive and a nonadhesive state. These effects have been discussed in terms of reversible changes in contact area and varying tilt angles of the pillars with respect to the substrate surface.     

Autonomous Self-Healing Elastomers with Unprecedented Adhesion Force

Self-healable elastomers are extremely attractive due to their ability to prolong product lifetime. An additional function that could further expand their applications is strong adhesion force to clean and dusty surfaces. This study reports a series of autonomous self-healable and highly adhesive elastomers (ASHA-Elastomer) that are fabricated via a simple, efficient, and scalable process. The obtained elastomers exhibit outstanding mechanical properties with elongation at break up to 2102% and toughness (modulus of toughness) of 1.73 MJ m^–3. The damaged ASHA-Elastomer can autonomously self-heal with full recovery of functionalities, and the healing process is not affected by the presence of water. The elastomers are found to possess an ultrahigh adhesion force up to 3488 N m⁻¹, greatly outperforming previously reported self-healing adhesive elastomers. Furthermore, the adhesion force of the ASHA-Elastomer is negligibly affected by dust on the surface, in stark contrast with regular adhesive polymers that have adhesion strengths extremely sensitive to dust. The successful development of high-toughness, autonomous self-healable, and ultra-adhesive elastomers will enable a wide range of applications with enhanced longevity and versatility, including their use in sealants, adhesives, and stretchable devices.     

A Stiff yet Rapidly Self-Healable Elastomer in Harsh Aqueous Environments

Rapid underwater self-healing elastomers with high mechanical strength at ambient temperature are highly desirable for dangerous underwater operations. However, current room temperature self-healing materials have shortcomings, such as low healing strength (below megapascal), long healing time (hours), and decay of healing functions in harsh environments (salty, acidic, and basic solutions), limiting their practical applications. Herein, it is introduced water-stable Debye forces and high-density nano-sized physical crosslinking into one network to achieve a stiff yet rapid self-healing elastomer that can work in harsh aqueous environments. The obtained elastomer possesses a high Young's modulus of 48 MPa (24 times than that of natural elastomer), and it can achieve 90% of maximum mechanical strength healing for 10 s at ambient temperature in all types of harsh aqueous conditions, outperforming three orders of magnitudes in healing speed of reported room-temperature self-healing elastomers with Young's modulus over 10 MPa. The new stiff yet rapidly healable elastomers have great potential in emergent repair in urgent and dangerous cases.     

Fibrillar Elastomeric Micropatterns Create Tunable Adhesion Even to Rough Surfaces

Biologically inspired, fibrillar dry adhesives continue to attract much attention as they are instrumental for emerging applications and technologies. To date, the adhesion of micropatterned gecko‐inspired surfaces has predominantly been tested on stiff, smooth substrates. However, all natural and almost all artificial surfaces have roughnesses on one or more different length scales. In the present approach, micropillar‐patterned PDMS surfaces with superior adhesion to glass substrates with different roughnesses are designed and analyzed. The results reveal for the first time adhesive and nonadhesive states depending on the micropillar geometry relative to the surface roughness profile. The data obtained further demonstrate that, in the adhesive regime, fibrillar gecko‐inspired adhesive structures can be used with advantage on rough surfaces; this finding may open up new applications in the fields of robotics, biomedicine, and space exploration.     

Physically Crosslinked Biocompatible Silk‐Fibroin‐Based Hydrogels with High Mechanical Performance

Developing hydrogel which combines superior mechanical performance and biocompatibility attracts researchers’ attention in recent years. Here, a novel biocompatible hydrogel with excellent mechanical performance, comprised of regenerated silk fibroin (RSF) and hydroxypropyl methyl cellulose (HPMC), is fabricated by simply mixing and heating. It is found that both of compressive modulus and tensile modulus of the optimal RSF/HPMC hydrogel are over 1.0 MPa. Meanwhile, the break energy is up to 3500 J m^?2, which is higher than that of some natural elastomers, such as cartilage, cork, and skin. The investigation of gelation mechanism reveals that more uniformly dispersed crosslinks dominated by smaller β‐sheet structures, which is attributed to the synergistic effects of hydrogen bonding and hydrophobic interaction between HPMC and RSF molecules, contribute to the superior mechanical performance of RSF/HPMC hydrogel. This biocompatible high strength silk protein based hydrogel diversifies the robust hydrogels and holds a great promise as candidates for load‐bearing materials in biomedical field.     

Self-Contained Underwater Adhesion and Informational Labeling Enabled by Arene-Functionalized Polymeric Ionogels

Adhesives and water exhibit a conflicting correlation as indicated by the failure of most synthetic adhesives in submerged and humid environments. Development of instant, strong, reversible, and long-lasting adhesives that can adhere to wet surfaces and function in underwater environments presents a formidable challenge, yet it is of paramount importance in biomedical and engineering applications. Herein, viscoelastic and moldable ionogels are developed based on synergistic engineering of aromatic substituents, fluorinated counterions, ionic building blocks, and 3D cross-linked networks. The molecular design and structural engineering result in a facile synthesis, two bonding methods (glue- and tape-type), and the combined mechanisms of enhanced adhesion and cohesion. The high underwater adhesion strength of over 8.9 MPa is among the best-performing tape-type underwater adhesives reported to date. A combination of excellent durability, reliability, deformation resistance, salt tolerance, water proof, antiswelling, and self-healing properties demonstrates the “self-contained” underwater adhesion. Furthermore, the extended π-conjugation of the aromatic pendant groups confers a new functionality to the ionogels – visible fluorescence, enabling intriguing applications such as underwater labeling, information encryption, and signal transmission. This study shines lights on the fabrication of ionogel-based adhesives and provides their future perspectives in underwater sealing, self-repair, crack diagnosis, and informational labeling.     

Elastic Energy Storage Enabled Magnetically Actuated,Octopus-Inspired Smart Adhesive

Octopus suckers offer remarkable adhesion performance against nonporous surfaces and have inspired extensive research to develop artificial adhesives. However, most of existing octopus-inspired adhesives are either passive without an actuation strategy or active but not energy efficient. Here, a novel design of a magnetically actuated, energy-efficient smart adhesive with rapidly tunable, great switchable, and highly reversible adhesion strength inspired by the elastic energy storage mechanism in octopus suckers is reported. The smart adhesive features two cavities separated by an elastic membrane with the upper cavity filled with magnetic particles while the lower one empty. The deformation of the elastic membrane can be actively controlled by an external magnetic field to change the cavity volume, thus generating a cavity-pressure-induced adhesion. Systematically experimental and theoretical studies reveal the fundamental aspects of design and operation of the smart adhesive and give insights into the underlying adhesion mechanisms. Demonstrations of this smart adhesive in transfer printing and manipulation of various surfaces in both dry and wet environments illustrate the potential for deterministic assembly and industrial or robotic manipulation.     

Pressure Sensitive Adhesion of an Elastomeric Protein Complex Extracted From Squid Ring Teeth

The pressure sensitive adhesion characteristic of a protein complex extracted from squid ring teeth (SRT), which exhibits an unusual and reversible transition from a solid to a melt, is studied. The native SRT is an elastomeric protein complex that has standard amino acids, and it does not function as adhesives in nature. The SRT can be thermally shaped into any 3D geometry (e.g., thin films, ribbons, colloids), and it has a glass transition temperature of 32 °C in water. Underwater adhesion strength of the protein film is approximately 1.5–2.5 MPa. The thermoplastic protein film could potentially be used in an array of fields, including dental resins, bandages for wound healing, and surgical sutures in the body.     

Wet‐Responsive,Reconfigurable, and Biocompatible Hydrogel Adhesive Films for Transfer Printing of Nanomembranes

This study presents a wet‐responsive and biocompatible smart hydrogel adhesive that exhibits switchable and controllable adhesions on demand for the simple and efficient transfer printing of nanomembranes. The prepared hydrogel adhesives show adhesion strength as high as ≈191 kPa with the aid of nano‐ or microstructure arrays on the surface in the dry state. When in contact with water, the nano/microscopic and macroscopic shape reconfigurations of the hydrogel adhesive occur, which turns off the adhesion (≈0.30 kPa) with an extremely high adhesion switching ratio (>640). The superior adhesion behaviors of the hydrogels are maintained over repeating cycles of hydration and dehydration, indicating their ability to be used repeatedly. The adhesives are made of a biocompatible hydrogel and their adhesion on/off can be controlled with water, making the adhesives compatible with various materials and surfaces, including biological substrates. Based on these smart adhesion capabilities, diverse metallic and semiconducting nanomembranes can be transferred from donor substrates to either rigid or flexible surfaces including biological tissues in a reproducible and robust fashion. Transfer printing of a nanoscale crack sensor onto a bovine eye further demonstrates the potential of the reconfigurable hydrogel adhesive for use as a stimuli‐responsive, smart, and versatile functional adhesive for nanotransfer printing.     

Engineered Bicomponent Adhesives with Instantaneous and Superior Adhesion Performance for Wound Sealing and Healing Applications

Surgical adhesives are playing an important role in wound repair and emergency hemostasis in clinical treatment. However, the development of strong bioglue with rapid in situ adhesion, durable adhesiveness, and flexibility in dynamic and moist physiological environments is still challenging. Herein, a new type of biosynthetic protein bioadhesives with superior adhesion performance is reported by developing a protein aldimine condensation strategy. Lysine-rich recombinant proteins are designed and massively biosynthesized to instantaneously react with aldehyde cross-linkers to realize in situ strong adhesion. The obtained bioadhesives show an ultra-high adhesion strength of ≈101.6 kPa on porcine skin, outperforming extant clinical bioglues. In addition, they possess super biocompatibility, flexibility, biodegradability, and compliance with the tissues. Owing to the strong and instantaneous adhesion properties, the bioadhesives are qualified for dynamic wound closure, facilitating wound repair, and noncompressible hemorrhage. Importantly, they can be industrially encapsulated into custom-made cartridge delivery tubes at low cost for clinical use. Therefore, biosynthetic bioadhesives have great potential for biological applications and are capable of scaling up to the industrial level for clinical transformation, which will be a successful paradigm for reforming existing clinical products.     

In Situ Observation Reveals Local Detachment Mechanisms and Suction Effects in Micropatterned Adhesives

Fibrillar adhesion pads of insects and geckoes have inspired the design of high‐performance adhesives enabling a new generation of handling devices. Despite much progress over the last decade, the current understanding of these adhesives is limited to single contact pillars and the behavior of whole arrays is largely unexplored. In the study reported here, a novel approach is taken to gain insight into the detachment mechanisms of whole micropatterned arrays. Individual contacts are imaged by frustrated total internal reflection, allowing in situ observation of contact formation and separation during adhesion tests. The detachment of arrays is found to be governed by the distributed adhesion strength of individual pillars, but no collaborative effect mediated by elastic interactions can be detected. At the maximal force, about 30% of the mushroom structures are already detached. The adhesive forces decrease with reduced air pressure by 20% for the smooth and by 6% for the rough specimen. These contributions are attributed to a suction effect, whose strength depends critically on interfacial defects controlling the sealing quality of the contact. This dominates the detachment process and the resulting adhesion strength.     

Torrent Frog‐Inspired Adhesives: Attachment to Flooded Surfaces

Anatomic differences on the toe pad epithelial cells of torrent and tree frogs (elongated versus regular geometry) are believed to account for superior ability of torrent frogs to attach to surfaces in the presence of running water. Here, the friction properties of artificial hexagonal arrays of polydimethylsiloxane (PDMS) pillars (elongated and regular) in the presence of water are compared. Elongated pillar patterns show significantly higher friction in a direction perpendicular to the long axis. A low bending stiffness of the pillars and a high edge density of the pattern in the sliding direction are the key design criteria for the enhanced friction. The elongated patterns also favor orientation‐dependent friction. These findings have important implications for the development of new reversible adhesives for wet conditions.     

A Self‐Healing Poly(Dimethyl Siloxane) Elastomer

Self‐healing functionality is imparted to a poly(dimethyl siloxane) (PDMS) elastomer. This new material is produced by the incorporation of a microencapsulated PDMS resin and a microencapsulated crosslinker into the PDMS matrix. A protocol based on the recovery of tear strength is introduced to assess the healing efficiency for these compliant polymers. While most PDMS elastomers possess some ability to re‐mend through surface cohesion, the mechanism is generally insufficient to produce significant recovery of initial material strength under ambient conditions. Self‐healing PDMS specimens, however, routinely recover between 70–100 % of the original tear strength. Moreover, the addition of microcapsules increases the tear strength of the PDMS. The effect of microcapsule concentration on healing efficiency is also investigated.     

Ultrafast UV-Curable Adhesives for Optical Pick-Ups

This paper describes novel ultraviolet (UV)-curable adhesives with an ultrafast curing rate which are fully cured within 8 s for optical pick-up (OPU) applications. Two kinds of oligomers (novolac epoxy acrylate and urethane acrylate), additives, and inorganic fillers were prepared for the formulation of the adhesives. In addition, three kinds of photo-initiator [2,2-dimethoxy-2-phenylacetophenone and 2-hydroxy-2-methylpropiophenone for surface curing and (2,4,6-trimethylbenzoyl) diphenyl phosphine oxide (TMDPO) for deep curing] were mixed to increase the curing rate. Photo-differential scanning calorimetry (photo-DSC) analyses showed that the newly formulated UV adhesives had faster curing rate than conventional UV adhesives. The UV adhesives were applied to OPUs for DVD/CD-RW, and five kinds of reliability tests, i.e., thermal shock, low-temperature storage, high-temperature storage, high temperature/high humidity, and nonoperation shock tests, were conducted to evaluate the adhesive reliability. According to the results of reliability tests and thermal stress simulations, the UV adhesives with lower storage modulus (E′) showed better thermal shock reliability due to lower thermal stresses. In addition, OPUs assembled using the UV adhesives passed all reliability tests. Consequently, the UV adhesives were successfully applied to OPUs in OPU production lines, contributing to mass production.     

Noise performance at cryogenic temperatures at AlGaAs/InGaAs HEMT'swith 0.15-μm T-shaped WSix gates

T-shaped 0.15-μm WSi_x gate HEMTs have been fabricated on AlGaAs/InGaAs MBE wafers. Their S-parameters, output noise spectral density P_no, and noise temperatures T _e at cryogenic temperatures, were measured. The current gain cutoff frequency f_T increases from 61 GHz at 295 K to 87 GHz at 90 K. P_no and T_e measurements indicate that the hot-electron effect is noticeable at low temperatures at high drain current. At 30 GHz, the noise temperature is 19±3 K with an associated gain of 10.4 dB at the physical temperature of 20 K. The results demonstrate the great potential of AlGaAs/InGaAs HEMTs for low-temperature applications     

Fabrication of Stable Metallic Patterns Embedded in Poly(dimethylsiloxane) and Model Applications in Non‐Planar Electronic and Lab‐on‐a‐Chip Device Patterning

This article describes the fabrication of durable metallic patterns that are embedded in poly(dimethylsiloxane) (PDMS) and demonstrates their use in several representative applications. The method involves the transfer and subsequent embedding of micrometer‐scale gold (and other thin‐film material) patterns into PDMS via adhesion chemistries mediated by silane coupling agents. We demonstrate the process as a suitable method for patterning stable functional metallization structures on PDMS, ones with limiting feature sizes less than 5 μm, and their subsequent utilization as structures suitable for use in applications ranging from soft‐lithographic patterning, non‐planar electronics, and microfluidic (lab‐on‐a‐chip, LOC) analytical systems. We demonstrate specifically that metal patterns embedded in both planar and spherically curved PDMS substrates can be used as compliant contact photomasks for conventional photolithographic processes. The non‐planar photomask fabricated with this technique has the same surface shape as the substrate, and thus facilitates the registration of structures in multilevel devices. This quality was specifically tested in a model demonstration in which an array of one hundred metal oxide semiconductor field‐effect transistor (MOSFET) devices was fabricated on a spherically curved Si single‐crystalline lens. The most significant opportunities for the processes reported here, however, appear to reside in applications in analytical chemistry that exploit devices fabricated using the methods of soft lithography. Toward this end, we demonstrate durably bonded metal patterns on PDMS that are appropriate for use in microfluidic, microanalytical, and microelectromechanical systems. We describe a multilayer metal‐electrode fabrication scheme (multilaminate metal–insulator–metal (MIM) structures that substantially enhance performance and stability) and use it to enable the construction of PDMS LOC devices using electrochemical detection. A polymer‐based microelectrochemical analytical system, one incorporating an electrode array for cyclic voltammetry and a microfluidic system for the electrophoretic separation of dopamine and catechol with amperometric detection, is demonstrated.     

A Highly-Adhesive and Self-Healing Elastomer for Bio-Interfacial Electrode

Stretchable electrodes are playing important roles in the measurement of bio-electrical signals especially in wearable electronic devices. These electrodes usually adopt commercial elastomers such as polydimethylsiloxane or polystyrene-ethylene-butylene-styrene as substrates, which result in poor stability and reliability due to weak interfacial adhesion between electrodes and human skin. Here, dopamine is introduced into the hydrogen bonding based elastomer as pendent groups. The elastomer shows both mechanical strength and adhesion strength at the same time. It exhibits high stress at break (1.9 MPa) and high fracture strain (5100%). Significantly, it exhibits a high adhesive strength (≈62 kPa) and underwater adhesive strength (≈16 kPa) with epithelial tissue. Thus, a stretchable bio-interfacial electrode is fabricated by spray-coating silver nanowires on the elastic substrate, which is stretchable, self-healable, and highly adhesive and suitable for electromyogram measurement.     

Influence of Joining Conditions on Bonding Strength of Joints: Efficacy of Low-Temperature Bonding Using Cu Nanoparticle Paste

We investigated a new low-temperature bonding process utilizing Cu nanoparticle paste without addition of sintering promoter. Joint bonding strengths above 30 MPa were achieved even at a low bonding temperature of 250ºC. We attribute the higher bonding strengths of joints fabricated using the vacuum preheating process to the rapid progression of Cu nanoparticle sintering due to the activated nanoparticle surface at lower temperatures. The increase in bonding strength depended on the applied pressure, in addition to the bonding temperature. The formation of a dimple-like morphology was confirmed in the ductile fracture area. This indicated that the joint bonded strongly with the bonding layer, in agreement with the results of bonding tests carried out on strongly bonded joints. The bonding ability of the joints obtained using Cu nanoparticle paste could be improved by controlling the joint fabrication conditions.     

Silicon-to-steel bonding using rapid thermal annealing

This paper presents the rapid, low-temperature bonding between silicon and steel using the rapid thermal annealing process. Three different thin-film adhesion layer systems including silver, gold, and nickel were utilized as the intermediate bonding material to assist the eutectic Pb/Sn bonding between silicon and steel. The bonding temperature was set at 220/spl deg/C for 20 s, with a 20-s ramp-up time. Five experiments were conducted to determine the strength of the bond, including static tensile and compressive four-point bend tests, axial extension tests, tensile bending fatigue tests, and corrosion resistance tests. The test results have shown that the gold adhesion layer is the most robust, demonstrating minimal creep during fatigue tests, no delamination during the tensile or compressive four-point bend tests, and acceptable strength during the axial extension tests. Additionally, all adhesion layers have withstood four months of submersion in various high-temperature solutions and lubricants without failure. Simulations of the axial stresses and strains that developed during the four-point bend and axial extension tests were performed and showed that the presence of the silicon die provides a local reinforcement of the bond as observed in the experimental tests.