It has been over 20 years since the pioneering work of Arthur Ashkin, and in the intervening years, the field of optical tweezers has grown tremendously. Optical tweezers are now being used in the investigation of an increasing number of biochemical and biophysical processes, from the basic mechanical properties of biological polymers to the multitude of molecular machines that drive the internal dynamics of the cell. Innovation, however, continues in all areas of instrumentation and technique, with much of this work focusing on the refinement of established methods and on the integration of this tool with other forms of single-molecule manipulation or detection. Although technical in nature, these developments have important implications for the expanded use of optical tweezers in biochemical research and thus should be of general interest. In this review, we address these recent advances and speculate on possible future developments.

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Recent Advances in Optical Tweezers

A polymer composition includes a conductive polymer and at least one stretchability and electrical conductivity (STEC) enhancer, wherein a content of the STEC enhancer in the composition is at least about 1 wt.% of the composition.

Highly stretchable, transparent, and conductive polymer

Cells both actively generate and sensitively react to forces through their mechanical framework, the cytoskeleton, which is a nonequilibrium composite material including polymers and motor proteins. We measured the dynamics and mechanical properties of a simple three-component model system consisting of myosin II, actin filaments, and cross-linkers. In this system, stresses arising from motor activity controlled the cytoskeletal network mechanics, increasing stiffness by a factor of nearly 100 and qualitatively changing the viscoelastic response of the network in an adenosine triphosphate–dependent manner. We present a quantitative theoretical model connecting the large-scale properties of this active gel to molecular force generation.

https://science.sciencemag.org/content/sci/315/5810/370.full.pdf

Nonequilibrium mechanics of active cytoskeletal networks.

The rapid development of novel organic technologies has led to significant applications of the organic electronic devices such as light-emitting diodes, solar cells, and field-effect transistors. There is a great need for conducting polymers with high conductivity and transparency to act as the charge transport layer or electrical interconnect in organic devices. Poly(3,4-ethylenedioxythiophene): poly(styrenesulfonic acid) (PEDOT:PSS), well-known as the most remarkable conducting polymer, has this role owing to its good film-forming properties, high transparency, tunable conductivity, and excellent thermal stability. In this Review, various of interesting physical and chemical approaches that can effectively improve the electrical conductivity of PEDOT:PSS are summarized, focusing especially on the mechanism of the conductivity enhancement as well as applications of PEDOT:PSS films. Prospects for future research efforts are also provided. It is expected that PEDOT:PSS films with high conductivity and transparency could be the focus of future organic electronic materials breakthroughs.

Effective Approaches to Improve the Electrical Conductivity of PEDOT:PSS: A Review

Polymers are lightweight, flexible, solution-processable materials that are promising for low-cost printed electronics as well as for mass-produced and large-area applications. Previous studies dem ...

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Semi-metallic polymers

Highly conductive poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) films are obtained using ionic liquids as additives. Upon adding 1-ethyl-3-methylimidazolium tetracyanoborate (EMIM TCB) to the conducting polymer, the conductivity increases to 2084 S cm−1; this is attributed to the phase separation of PSS leading to a structural change in the film. A comparative study with 1-butyl-3-methyl imidazolium tetrafluoroborate (BMIM BF4) shows that EMIM TCB gives higher conductivity and transmittance and can be regarded as one of the most promising additives for the preparation of indium tin oxide (ITO)-free organic devices using PEDOT:PSS/EMIM TCB as electrodes.

Highly Conductive Poly(3,4-ethylenedioxythiophene):Poly (styrenesulfonate) Films Using 1-Ethyl-3-methylimidazolium Tetracyanoborate Ionic Liquid

Hyperuniformity characterizes a state of matter for which (scaled) density fluctuations diminish towards zero at the largest length scales. However, the task of determining whether or not an image of an experimental system is hyperuniform is experimentally challenging due to finite-resolution, noise, and sample-size effects that influence characterization measurements. Here we explore these issues, employing video optical microscopy to study hyperuniformity phenomena in disordered two-dimensional jammed packings of soft spheres. Using a combination of experiment and simulation we characterize the possible adverse effects of particle polydispersity, image noise, and finite-size effects on the assignment of hyperuniformity, and we develop a methodology that permits improved diagnosis of hyperuniformity from real-space measurements. The key to this improvement is a simple packing reconstruction algorithm that incorporates particle polydispersity to minimize the free volume. In addition, simulations show that hyperuniformity in finite-sized samples can be ascertained more accurately in direct space than in reciprocal space. Finally, our experimental colloidal packings of soft polymeric spheres are shown to be effectively hyperuniform. DOI: 10.1103/PhysRevE.91.012302

Diagnosing hyperuniformity in two-dimensional, disordered, jammed packings of soft spheres.

We report a study of the correlated motions of two hydrodynamically coupled colloidal particles, each of which is trapped in a quadratic potential well defined by optical tweezers (optical traps). By setting one of the trapped particles into forced oscillation using oscillating optical tweezers, we measure the displacement and phase shift of each of the particles over a wide frequency range. From the in-phase and out-of-phase motions of both of the particles in the traps, we determine the correlated motions of the coupled mechanical system as a function of frequency. A theoretical model is developed to calculate the response tensor of the coupled mechanical system. The experimental results are in agreement with the prediction of the theoretical model. This method may be extended to more general applications, such as the investigation of the micromechanical properties of viscoelastic and/or heterogeneous media.

Correlated motions of two hydrodynamically coupled particles confined in separate quadratic potential wells.

An aqueous fluid composition useful for the recovery of hydrocarbons from a subterranean formation, including a mixture of water, a water soluble polymer, an inorganic salt and at least one zwitterionic surfactant and methods of using same.

Addition of zwitterionic surfactant to water soluble polymer to increase the stability of the polymers in aqueous solutions containing salt and/or surfactants

We present a rheology study of associating polymers. The associating polymers are telechelic, composed of a water-soluble backbone (polyethylene oxide) terminated by hydrophobic moieties (C16H33). In aqueous solutions, these polymers self-assemble to form micellar structures. Above a critical concentration, approximately 1 wt % of polymer, bridging between the micelles forms a transient network. Traditionally, the viscoelastic response of these polymeric solutions has been described using the Maxwell model. In this work we measure the viscoelastic properties over an extended frequency range (0.01-6000 Hz) using microrheology, and show that at high frequencies the rheology behaves as the square root of the oscillation frequency. To fit the data, we use a combination of the Maxwell model and the Rouse model. The Maxwell model accounts for the hydrophobic associations between the polymeric micelles, and the Rouse model accounts for the microscopic dynamics of the individual micelles.

Lawrence Alan Hough

Papers

Highly Conductive Poly(3,4-ethylenedioxythiophene):Poly (styrenesulfonate) Films Using 1-Ethyl-3-methylimidazolium Tetracyanoborate Ionic Liquid

Diagnosing hyperuniformity in two-dimensional, disordered, jammed packings of soft spheres.

Correlated motions of two hydrodynamically coupled particles confined in separate quadratic potential wells.

Addition of zwitterionic surfactant to water soluble polymer to increase the stability of the polymers in aqueous solutions containing salt and/or surfactants

Viscoelasticity of aqueous telechelic poly(ethylene oxide) solutions: Relaxation and structure