Tissaphern Mirfakhrai

Polymer artificial muscles

The various types of natural muscle are incredible material systems that enable the production of large deformations by repetitive molecular motions. Polymer artificial muscle technologies are being developed that produce similar strains and higher stresses using electrostatic forces, electrostriction, ion insertion, and molecular conformational changes. Materials used include elastomers, conducting polymers, ionically conducting polymers, and carbon nanotubes. The mechanisms, performance, and remaining challenges associated with these technologies are described. Initial applications are being developed, but further work by the materials community should help make these technologies applicable in a wide range of devices where muscle-like motion is desirable.

Torsional Carbon Nanotube Artificial Muscles

Carbon nanotube yarns are used to make fast, multirotational torsional actuators. Rotary motors of conventional design can be rather complex and are therefore difficult to miniaturize; previous carbon nanotube artificial muscles provide contraction and bending, but not rotation. We show that an electrolyte-filled twist-spun carbon nanotube yarn, much thinner than a human hair, functions as a torsional artificial muscle in a simple three-electrode electrochemical system, providing a reversible 15,000° rotation and 590 revolutions per minute. A hydrostatic actuation mechanism, as seen in muscular hydrostats in nature, explains the simultaneous occurrence of lengthwise contraction and torsional rotation during the yarn volume increase caused by electrochemical double-layer charge injection. The use of a torsional yarn muscle as a mixer for a fluidic chip is demonstrated.

Electrochemical actuation of carbon nanotube yarns

We report on actuation in high tensile strength yarns of twist-spun multi-wall carbon nanotubes. Actuation in response to voltage ramps and potentiostatic pulses is studied to quantify the dependence of the actuation strain on the applied voltage. Strains of up to 0.5% are obtained in response to applied potentials of 2.5 V. The dependence of strain on applied voltage and charge is found to be quadratic, in agreement with previous results on the actuation of single-wall carbon nanotubes, with the magnitude of strain also being very similar. The specific capacitance reaches 26 F g−1. The modulus of the yarns was found to be independent of applied load and voltage within experimental uncertainty.

A delay prediction approach for teleoperation over the Internet

Based on the notion of wave variables and the idea of wave-integral transmission, a new method is suggested to match the system parameters with changes in the delay. An autoregressive model is used as a predictor to forecast the future values of the delay. The predictions are used with a lookup table to tune the gain with which the wave integrals are to be fed to the system. This gain scheduling and tuning improves the system performance and decreases the mismatch between forces and velocities at the master and slave sides.

Supramolecular Assembly of Carbohydrate‐Functionalized Salphen–Metal Complexes

Metallosalphen complexes with peripheral glucose and galactose substituents were synthesized and characterized. Their self-assembled supramolecular structures were then studied with transmission electron microscopy (TEM), scanning electron microscopy (SEM) and atomic force microscopy (AFM). It was found that all of the complexes displayed aggregation in the solid-state. Zinc-salphen complexes showed a remarkably homogeneous helical nanofibrillar morphology, whereas the other metal complexes only displayed micron-sized clusters.

Mechanoelectrical Force Sensors Using Twisted Yarns of Carbon Nanotubes

Yarns spun by twisting multiwalled carbon nanotubes (MWNTs) have been reported. Here, we report the application of these yarns as mechanical force sensors. When electrochemically charged, the yarns can respond to a change in the applied tension by generating a change in the cell current (up to about 1.2 nA/MPa per centimeter length of the yarn) or the open-circuit potential of the cell (up to 0.013 mV/MPa per centimeter length of the yarn) corresponding to the applied tension force. The MWNT yarns are mechanically strong with tensile strengths reaching 1 GPa. These properties together make them prime candidates for many applications as fast and efficient sensors. Their sensitivity as mechanical strain sensors is about 0.5 V/micorstrain, which is comparable to the sensitivity of metal film strain gauge sensors. The sensitivity is expected to improve by using thicker bundles of yarns.

Rate Limits in Conducting Polymers

Conducting polymer actuators are of interest in applications where low voltage and high work density are beneficial. These actuators are not particularly fast however, with time constants normally being greater than 1 second. Strain in these actuators is proportional to charge, with the rate of charging being found to limit the speed of actuation. This rate of charging is in turn limited by a number of factors, the dominant factor depending on the actuator and cell geometry, the potential range, the composition and the timescale of interest. Mechanisms that slow response can be as simple as the RC charging time arising from the actuator capacitance and the series resistances of the electrolyte and the contacts, or may involve polymer electronic or ionic conductivities, which can in turn be functions of potential. Diffusion can also be a factor. An approach is presented to help estimate the relative magnitudes of these rate limiting factors, thereby enabling actuator designs to evaluated and optimized for a given application. The general approach discussed is also useful for analyzing rate limits in carbon nanotube actuators and other related technologies.

Carbon nanotube yarns as high load actuators and sensors

Carbon nanotubes have attracted extensive attention in the past few years because of their appealing mechanical and electronic properties. Yarns made through spinning multiwall carbon nanotubes (MWNTs) have been reported. Here we study the application of these yarns as electrochemical actuators, and as force sensors. MWNT yarns are mechanically strong with tensile strengths reaching one GPa. When charge is stored in the yarns they change in length. This is thought to be because of a combination of electrostatic and quantum chemical effects. We report strains up to 0.6 %. The charged yarns can also generate current and change in voltage in response to a change in the applied tension. Electrostatic and quantum effects contributing to actuation are introduced along with the effect of the yarn geometry on actuation and other contributing factors.

A model for time-delays for teleoperation over the Internet

The stochastic nature of the communication delay in teleoperation systems makes it hard to predict the behavior of such systems. Although achieving a general-purpose model for the Internet is not easy, if possible at all, stochastic models for specific cases enable us design predictors, which can help in stabilizing the teleoperation systems. In this paper we have developed an initial notion to create an autoregressive model for delays over the Internet. By probing the system behavior at different times of the day for a period of time, the general behavior of the delay can be comprehended and so partially predicted. Introducing a predictor in the teleoperation system can improve the system behavior through for example gain scheduling at the master controller.

Electromechanical actuation of single-walled carbon nanotubes: an ab initio study

The mechanical actuation of a (5, 5) single-walled carbon nanotube as a result of added charge is simulated using first-principles calculations. It is observed that while both positive and negative charging tend to expand the nanotube in the axial direction for most levels of charge, radial actuation is less even and symmetric with respect to charge. The spin distribution of the additional charges is investigated, and it is predicted that in some cases unpaired spin configurations are energetically favourable, significantly affecting actuation strains.