다중혈관 관상동맥 환자에서 y-문합을 이용하여 양쪽 내흉동맥만을 사용한 우회술의 조기 성적

[신간의 별자리x] 우리/미술, 그리고 ‘슬픔의 박물관’

Hydrogel Adhesion: A Supramolecular Synergy of Chemistry, Topology, and Mechanics

Two-dimensional (2D) materials have generated great interest in the past few years as a new toolbox for electronics. This family of materials includes, among others, metallic graphene, semiconducting transition metal dichalcogenides (such as MoS2), and insulating boron nitride. These materials and their heterostructures offer excellent mechanical flexibility, optical transparency, and favorable transport properties for realizing electronic, sensing, and optical systems on arbitrary surfaces. In this paper, we demonstrate a novel technology for constructing large-scale electronic systems based on graphene/molybdenum disulfide (MoS2) heterostructures grown by chemical vapor deposition. We have fabricated high-performance devices and circuits based on this heterostructure, where MoS2 is used as the transistor channel and graphene as contact electrodes and circuit interconnects. We provide a systematic comparison of the graphene/MoS2 heterojunction contact to more traditional MoS2-metal junctions, as well as a theoretical investigation, using density functional theory, of the origin of the Schottky barrier height. The tunability of the graphene work function with electrostatic doping significantly improves the ohmic contact to MoS2. These high-performance large-scale devices and circuits based on this 2D heterostructure pave the way for practical flexible transparent electronics.

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Graphene-MoS2 Hybrid Technology for Large-Scale Two-Dimensional Electronics

The Physics of Rubber Elasticity (J. Meixner)

In materials of all types, hysteresis and toughness are usually correlated. For example, a highly stretchable elastomer or hydrogel of a single polymer network has low hysteresis and low toughness. The single network is commonly toughened by introducing sacrificial bonds, but breaking and possibly reforming the sacrificial bonds causes pronounced hysteresis. In this paper, we describe a principle of stretchable materials that disrupt the toughness-hysteresis correlation, achieving both high toughness and low hysteresis. We demonstrate the principle by fabricating a composite of two constituents: a matrix of low elastic modulus, and fibers of high elastic modulus, with strong adhesion between the matrix and the fibers, but with no sacrificial bonds. Both constituents have low hysteresis (5%) and low toughness (300 J/m2), whereas the composite retains the low hysteresis but achieves high toughness (10,000 J/m2). Both constituents are prone to fatigue fracture, whereas the composite is highly fatigue resistant. We conduct experiment and computation to ascertain that the large modulus contrast alleviates stress concentration at the crack front, and that strong adhesion binds the fibers and the matrix and suppresses sliding between them. Stretchable materials of high toughness and low hysteresis provide opportunities to the creation of high-cycle and low-dissipation soft robots and soft human-machine interfaces.

https://www.pnas.org/content/pnas/116/13/5967.full.pdf

Stretchable materials of high toughness and low hysteresis.

Flaw sensitivity of highly stretchable materials

Stretchable and fatigue-resistant materials

A rubber band is an elastic dissipater. It has narrow hysteresis on load and unload, but dissipates all its elastic energy on snap. We hypothesize that the dissipation of elastic energy can be a potent toughening mechanism. A polyacrylamide hydrogel has narrow hysteresis, but appreciable toughness, providing an ideal candidate to test the hypothesis of elastic dissipater. We peel the polyacrylamide hydrogel of various thicknesses, and record the steady peel forces. A transition thickness exists, below which the steady peel force increases linearly with thickness, and above which the steady peel force is independent of thickness. The transition thickness is comparable to the fractocohesive length, defined by the ratio of toughness over the work of fracture, and is ∼1 mm for the polyacrylamide hydrogel. The linear slope is comparable to the work of fracture. The linear extrapolation of the steady peel force to vanishing thickness defines the peel threshold, and the thickness-independent steady peel force defines the toughness. The former is much below the latter. We interpret these experimental findings in terms of elastic dissipater. A crack not only cuts a layer of polymer chains, but also breaks some polymer chains in a zone of thickness comparable to the fractocohesive length. Even though only a small fraction of the chains in the zone rupture, all the elastic energy stored in this zone dissipates and contributes to toughness. The entire zone acts as an elastic dissipater, not just a layer of individual polymer chains. We discuss the importance of the elastic dissipater in creating materials of high toughness, high threshold, and low hysteresis.

Zhengjin Wang

Papers

Stretchable materials of high toughness and low hysteresis.

Flaw sensitivity of highly stretchable materials

Stretchable and fatigue-resistant materials

Polyacrylamide hydrogels. II. elastic dissipater

Compressive crushing of novel aluminum hexagonal honeycombs with perforations: Experimental and numerical investigations