Mechanical properties of conducting polymers are an essential consideration in the design of flexible and stretchable electronics, but the guidelines for the material design having both high mechanical and electrical properties remain limited. Here we provide an important guideline for the design of mechanically robust, electroactive polymer thin films in terms of the molecular weight of the polymers. These studies based on a highly efficient, representative n-type conjugated polymer (P­(NDI2OD-T2)) revealed a marked enhancement in mechanical properties across a narrow molecular weight range, highlighting the existence of a critical molecular weight that can be exploited to engineer films that balance processability and mechanical and electronic properties. We found the thin films formed from high molecular weight polymers (i.e., number-average molecular weight (M n) ∼ 163 kg mol–1) to exhibit superior mechanical compliance and robustness, with a 114-fold enhanced strain at fracture and a 2820-fold enhanced toughness, as compared to those of low molecular weight polymer films (M n = 15 kg mol–1). In particular, we observed a jump in the mechanical properties between the M n = 48 and 103 kg mol ‑1, yielding a 26-fold enhanced strain at fracture and a 160-fold enhanced toughness. The significant improvement of tensile properties indicates the presence of a critical molecular weight at which entangled polymer networks start to form, as supported by the analysis of the thermal and crystalline properties, specific viscosity, and microstructure. Our work provides useful guidelines for the design of conjugated polymers with recommendations for the best combinations of mechanical robustness and electrical performance for flexible and stretchable electronics.

We present systematic measurements of the surface rheology of monolayers of liquid-condensed (LC) dipalmitoylphosphatidylcholine (DPPC) at the air-water interface. Using microfabricated, ferromagnetic 'microbuttons' as new microrheological probes, we measure the linear viscoelastic moduli of LC DPPC monolayers as both surface pressure and frequency are varied. Visualization of this interface reveals that the interlocked liquid crystalline domains that comprise an LC-DPPC monolayer give rise to a viscoelastic solid response. Two distinct behaviors arise as surface pressure is increased: for low surface pressures (8 mN m À1 # P # 12-14 mN m À1 ), the monolayer behaves like a two-dimensional emulsion, with a surface elastic modulus G 0 s that is relatively constant, as would be expected from a line tension-mediated elasticity. The surface viscosity increases exponentially with P, as would be expected for a condensed liquid monolayer. Above 12-14 mN m À1 , however, both moduli increase exponentially with P, albeit with a weaker slope-a response that would not be expected from line-tension-mediated elasticity. This transition would be consistent with a second-order phase transition between the LC and solid-condensed phase, as has been observed in other phospholipid monolayers. Finally, we employ a controlled-stress (creep) mode to find a stress-dependent viscosity bifurcation, and thus the yield stress of this monolayer.

Two-dimensional films of surface-active agents—from phospholipids and proteins to nanoparticles and colloids—stabilize fluid interfaces, which are essential to the science, technology and engineering of everyday life. The 2D nature of interfaces present unique challenges and opportunities: coupling between the 2D films and the bulk fluids complicates the measurement of surface dynamic properties, but allows the interfacial microstructure to be directly visualized during deformation. Here we present a novel technique that combines active microrheology with fluorescence microscopy to visualize fluid interfaces as they deform under applied stress, allowing structure and rheology to be correlated on the micron-scale in monolayer films. We show that even simple, single-component lipid monolayers can exhibit viscoelasticity, history dependence, a yield stress and hours-long time scales for elastic recoil and aging. Simultaneous visualization of the monolayer under stress shows that the rich dynamical response results from the cooperative dynamics and deformation of liquid-crystalline domains and their boundaries.

With the increasing interest and demand for epidermal electronics, a strong interface between a sensor and a biological surface is essential, yet achieving such interface is still a challenge. Here, a calcium (Ca)-modified biocompatible silk fibroin as a strong adhesive for epidermal electronics is proposed and the physical principles behind its interfacial and adhesive properties are reported. A strong adhesive characteristic (>800 N m −1 ) is observed because of the increase in both viscoelastic property and mechanical interlocking through the incorporation of Ca ions. Furthermore, additional key characteristics of the Ca-modified silk: reusability, stretchability, biocompatibility, and conductivity, are reported. These characteristics enable a wide range of applications as demonstrated in four epidermal electronic systems: capacitive touch sensor, resistive strain sensor, hydrogel-based drug delivery, and electrocardiogram monitoring sensor. As a reusable, biocompatible, conductive, and strong adhesive with water-degradability, the Ca-modified silk adhesive is a promising candidate for the next-generation adhesive for epidermal biomedical sensors.

High internal phase emulsions have been widely used as templates for various porous materials, but special strategies are required to form, in particular, particle-covered ones that have been more difficult to obtain. Here, we report a versatile strategy to produce a stable high internal phase Pickering emulsion by exploiting a depletion interaction between an emulsion droplet and a particle using water-soluble polymers as a depletant. This attractive interaction facilitating the adsorption of particles onto the droplet interface and simultaneously suppressing desorption once adsorbed. This technique can be universally applied to nearly any kind of particle to stabilize an interface with the help of various non- or weakly adsorbing polymers as a depletant, which can be solidified to provide porous materials for many applications.

At low mole fractions, cholesterol segregates into 10-to 100-nmdiameter nanodomains dispersed throughout primarily dipalmitoylphosphatidylcholine (DPPC) domains in mixed DPPC:cholesterol monolayers. The nanodomains consist of 6:1 DPPC:cholesterol "complexes" that decorate and lengthen DPPC domain boundaries, consistent with a reduced line tension, λ. The surface viscosity of the monolayer, η s , decreases exponentially with the area fraction of the nanodomains at fixed surface pressure over the 0.1-to 10-Hz range of frequencies common to respiration. At fixed cholesterol fraction, the surface viscosity increases exponentially with surface pressure in similar ways for all cholesterol fractions. This increase can be explained with a free-area model that relates η s to the pure DPPC monolayer compressibility and collapse pressure. The elastic modulus, G′, initially decreases with cholesterol fraction, consistent with the decrease in λ expected from the line-active nanodomains, in analogy to 3D emulsions. However, increasing cholesterol further causes a sharp increase in G′ between 4 and 5 mol% cholesterol owing to an evolution in the domain morphology, so that the monolayer is elastic rather than viscous over 0.1-10 Hz. Understanding the effects of small mole fractions of cholesterol should help resolve the controversial role cholesterol plays in human lung surfactants and may give clues as to how cholesterol influences raft formation in cell membranes.surface rheology | isotherms | free-volume model | AFM M inute fractions of cholesterol lead to dramatic changes in dipalmitoylphosphatidylcholine (DPPC) monolayer morphology (Figs. 1-3) (1, 2) and have equally dramatic effects on monolayer dynamic properties. One weight percent cholesterol reduces the surface viscosity, η s , of DPPC monolayers by an order of magnitude, and 2 wt% reduces η s by two orders of magnitude . Atomic force microscopy (AFM) images and microrheological data show that the cholesterol is segregated to lineactive, locally disordered nanodomains that are dispersed in and separate ordered, primarily DPPC domains. As a result, the monolayer retains many of the features of pure DPPC monolayers including a high collapse pressure, high compressibility, and so on, while having significantly lower surface viscosity. This surface viscosity effect suggests a role for cholesterol in lung surfactant (LS), a lipid-protein monolayer necessary to reduce the surface tension in the lung alveoli during respiration (Fig. S1) (3, 4). At present, even the existence of cholesterol in native LS is questioned, because the lung lavage required to harvest LS inevitably causes blood and cell debris to be coextracted, potentially contaminating LS with cholesterol (5). This lack of consensus over the role of cholesterol is reflected in the composition of replacement lung surfactants for neonatal respiratory distress syndrome (NRDS), which occurs in 20,000-30,000 premature births each year (6). Survanta and Curosurf, two clinically approved animalextract replacement surfact...

Counting of transcripts at each DNA template suggested a stochastic initiation mechanism in the experimental system. We found a prototypical activator (human Sp1) regulates transcription by enhancing PIC assembly (presumably by recruiting TFIID). Real-time TFIID binding to DNA was monitored and coupled to transcription detection at the same DNA template for the first time. We also developed methods to detect the production of RNA transcripts in real-time and couple that to the kinetic measurements of RNA polymerase binding at the single-molecule level. using multiple fluorescently labeled General Transcription Factors (GTFs, namely TFIIB TFIID, TFIIE, TFIIF and TFIIH) and Pol II, we are currently investigating the structure of PIC, pathways of its assembly, and the mechanism of transcription modulation by sequence-specific activators and the core promoter DNA elements.

The development of electronic devices with both excellent electrical conductivity and outstanding flexibility is an active and vibrant research field due to the inherent fragile nature of silicon-based electronics and the large demand for portable electronics, such as portable display, communication, computational, and identification products. [1][2][3][4][5][6][7] Flexible electronic devices (FEDs) consist of various functional organic and/or inorganic features on flexible substrates. In particular, electric circuits on flexible substrates are essential parts in the operation of FEDs. Flexible electric circuits can be generally fabricated by depositing the patterned conductive materials on a flexible substrate via conventional vacuum deposition or printing processes. However, flexible substrates applicable to the fabrication of highly conductive patterns by vacuum deposition and printing processes are mostly limited to impermeable thin-film plastic substrates because forming continuous patterns without disconnections is difficult on light fibrous substrates, such as fabric and paper, by a vacuum deposition or printing process. Moreover, the cost is other major obstacle when preparing flexible electric circuits using the vacuum deposition and printing processes owing to the requirement of expensive equipment and high operating outlays.Fibrous materials, such as textile and paper, have excellent flexibility and foldability, and they can be easily cut and attached to flexible substrates simply by gluing or stitching. Furthermore, fibrous materials are easily accessible at an affordable price, and the processes for preparing patterns consisting of fibrous materials on a substrate are very simple and inexpensive. Once the fibrous materials has a high electrical conductivity and good mechanical endurance against external deformation, they become attractive for use in electronic fields, especially for the simple and low-cost preparation of electric circuits for flexible and wearable electronics.Conductive fibrous materials are fabricated by the electroless silver (Ag) or nickel (Ni) plating onto the surfaces of chemical and/or natural fibers. [8] Although the electroless plating process is a useful tool for the deposition of metallic films onto the surfaces of nonconductive materials, it has a high cost when used for the uniform deposition of metallic film on fibrous materials owing to expensive processes and the price of the Ag and Ni. Furthermore, poor electrical conductivity and mechanical endurance against external deformation can occur in conductive fibrous materials which undergo electroless plating. Another well-established method is polymer coating, in which individual fibers are uniformly coated with a smooth and coherent layer of conductive polymers. [9][10][11] However, the electrochemical stability of polymer-coated conductive fibrous materials decays with age. [12] Recently, conductive carbons, such as carbon nanotube (CNT) and graphene, as well as activated carbon have been used in the production of condu...