A class of optimal state-delay control problems

We consider a general nonlinear time-delay system with state-delays as control variables. The problem of determining optimal values for the state-delays to minimize overall system cost is a non-standard optimal control problem – called an optimal state-delay control problem – that cannot be solved using existing optimal control techniques. We show that this optimal control problem can be formulated as a nonlinear programming problem in which the cost function is an implicit function of the decision variables. We then develop an efficient numerical method for determining the cost function’s gradient. This method, which involves integrating an auxiliary impulsive system backwards in time, can be combined with any standard gradient-based optimization method to solve the optimal state-delay control problem effectively. We conclude the paper by discussing applications of our approach to parameter identification and delayed feedback control

Direct detection and measurement of wall shear stress using a filamentous bio-nanoparticle

The wall shear stress (WSS) that a moving fluid exerts on a surface affects many processes including those relating to vascular function. WSS plays an important role in normal physiology (e.g. angiogenesis) and affects the microvasculature's primary function of molecular transport. Points of fluctuating WSS show abnormalities in a number of diseases; however, there is no established technique for measuring WSS directly in physiological systems. All current methods rely on estimates obtained from measured velocity gradients in bulk flow data. In this work, we report a nanosensor that can directly measure WSS in microfluidic chambers with sub-micron spatial resolution by using a specific type of virus, the bacteriophage M13, which has been fluorescently labeled and anchored to a surface. It is demonstrated that the nanosensor can be calibrated and adapted for biological tissue, revealing WSS in micro-domains of cells that cannot be calculated accurately from bulk flow measurements. This method lends itself to a platform applicable to many applications in biology and microfluidics

Reversible Integration of Microfluidic Devices with Microelectrode Arrays for Neurobiological Applications

The majority of current state-of-the-art microfluidic devices are fabricated via replica molding of the fluidic channels into PDMS elastomer and then permanently bonding it to a Pyrex surface using plasma oxidation. This method presents a number of problems associated with the bond strengths, versatility, applicability to alternative substrates, and practicality. Thus, the aim of this study was to investigate a more practical method of integrating microfluidics which is superior in terms of bond strengths, reversible, and applicable to a larger variety of substrates, including microfabricated devices. To achieve the above aims, a modular microfluidic system, capable of reversible microfluidic device integration, simultaneous surface patterning and multichannel fluidic perfusion, was built. To demonstrate the system’s potential, the ability to control the distribution of A549 cells inside a microfluidic channel was tested. Then, the system was integrated with a chemically patterned microelectrode array, and used it to culture primary, rat embryo spinal cord neurons in a dynamic fluidic environment. The results of this study showed that this system has the potential to be a cost effective and importantly, a practical means of integrating microfluidics. The system’s robustness and the ability to withstand extensive manual handling have the additional benefit of reducing the workload. It also has the potential to be easily integrated with alternative substrates such as stainless steel or gold without extensive chemical modifications. The results of this study are of significant relevance to research involving neurobiological applications, where primary cell cultures on microelectrode arrays require this type of flexible integrated solution

Phylogenomics and the rise of the angiosperms

Angiosperms are the cornerstone of most terrestrial ecosystems and human livelihoods1,2. A robust understanding of angiosperm evolution is required to explain their rise to ecological dominance. So far, the angiosperm tree of life has been determined primarily by means of analyses of the plastid genome3,4. Many studies have drawn on this foundational work, such as classification and first insights into angiosperm diversification since their Mesozoic origins5,6,7. However, the limited and biased sampling of both taxa and genomes undermines confidence in the tree and its implications. Here, we build the tree of life for almost 8,000 (about 60%) angiosperm genera using a standardized set of 353 nuclear genes8. This 15-fold increase in genus-level sampling relative to comparable nuclear studies9 provides a critical test of earlier results and brings notable change to key groups, especially in rosids, while substantiating many previously predicted relationships. Scaling this tree to time using 200 fossils, we discovered that early angiosperm evolution was characterized by high gene tree conflict and explosive diversification, giving rise to more than 80% of extant angiosperm orders. Steady diversification ensued through the remaining Mesozoic Era until rates resurged in the Cenozoic Era, concurrent with decreasing global temperatures and tightly linked with gene tree conflict. Taken together, our extensive sampling combined with advanced phylogenomic methods shows the deep history and full complexity in the evolution of a megadiverse clade

Natural commutation technique to reduce voltage surges and switching losses in a cycloconverter-type high frequency-link inverter

This paper presents an isolated cycloconverter-type high-frequency (HF) link inverter that consists of three arms of bidirectional switches at the transformer secondary. This topology has the advantages of light weight and reduced switch count, when compared to other types of HF link inverters. However, like all HF link inverter topologies, this topology suffers from a high-voltage surge problem and power device switching loss. In order to overcome these problems, the natural commutation method that enables soft-switching operation is applied. To verify the viability of the scheme, a low-power laboratory prototype based on the Siemens 80C167 fixed-point microcontroller is constructed. In addition, the simulation of the inverter is also carried out. The prototype inverter is able to supply output voltage at 120 Vrms, with a total harmonics distortion of less than 1%. The total power conversion efficiency reaches 86% when the output power is over 300 W

An isolated bidirectional inverter using high frequency center-tapped transformer

This paper proposes a bidirectional high-frequency link inverter using a high-frequency center-tapped transformer. The main feature of the inverter is fewer number of power switches used. On the secondary side of the transformer, the active rectifier employs only two switches, thus reducing the switching losses. Furthermore, a modified sinusoidal pulse width modulation (PWM) has been applied to increase the efficiency of the inverter. A 1 kW prototype has been constructed and experiments were carried out to verify the viability of the proposed inverter

Preparation of silica or alumina pillared crystalline titanates

Layered crystalline titanates (CT) [Anthony and Dosch, US Patent 5 177 045 (1993)] are pillared with tetraethyl orthosilicate, 3-aminopropyltrimethoxysilane, and aluminum acetylacetonate to prepare porous and high surface area supports for sulfided NiMo catalyst. Tetra-ethyl orthosilicate or aluminum acetylacetonate intercalated CT are prepared by stepwise intercalation. First, the basal distance is increased by n-alkylammonium ions prior to intercalation with inorganic compounds. However, an aqueous solution of 3-aminopropyltrimethoxysilane could directly pillar CT without first swelling the titanate with n-alkylamine. The catalytic activities for hydrogenation of pyrene of sulfided NiMo supported silica or alumina pillared CT were higher than those of commercial catalysts (Shell324 and Amocat1C). The silicon and aluminum contents of the pillared CT, used as supports, have a considerable effect on the catalytic activities and physical properties of the supports