This study focused on the development of a chemical micropropulsion system suitable for primary propulsion and/or attitude control for a nanosatellite. Due to the limitations and expense of current micropropulsion technologies, few nanosatellites with propulsion have been launched to date; however, the availability of such a propulsion system would allow for new nanosatellite mission concepts, such as deep space exploration, maneuvering in low gravity environments and formation flying. This work describes the design of “dual mode” monopropellant/bipropellant microthruster prototype that employs a novel homogeneous catalysis scheme. Results from prototype testing are reported that validate the concept. The micropropulsion system is designed to be fabricated using a combination of additively-manufactured and commercial off the shelf (COTS) parts along with non-toxic fuels, thus making it a low-cost and environmentally-friendly option for future nanosatellite missions. These satellites are being developed for numerous applications related to research, defense and industry. An important subcategory of nanosats are those built on the CubeSat specification, which allows for cube-shaped satellites, as small as 10 cm per side, with masses as low as 1.33 kg . CubeSatThere are many potential advantages of incorporating propulsion systems on future CubeSat missions. Some of these advantages include extended mission lifetime, maneuvering ability, formation flying, proximity operations, fine attitude control, drag make-up, and de-orbiting . Today, There are numerous types of CubeSat propulsion systems in active development, including those based on solar sails, electrodynamic tethers, ion electrospray, miniaturized Hall thrusters, solid rocket motors, liquid chemical thrusters, cold gas thrusters, and pulsed plasma thrusters. Recent reviews of propulsion technologies and trends for small satellites and cubesats can be found in Refs. –9. WhileAmong the most appealing of these technologies are chemical propulsion systems, owing to their combination of simplicity, reliability, and low power requirements. Liquid propulsion systems are of particular interest both as primary propulsion and attitude control thrusters, as demonstrated by recent efforts such as the University of Vermont Discrete Monopropellant Microthruster  and the In a previous study by the authors , a methoOne potential application for a dual-mode miniaturized propulsion system is that of cubesat-based exploration of near-Earth objects, asteroids and comets –16. As tIn this work, the design overview of a dual-mode propulsion system is presented along with supporting results from preliminary experiments demonstrating the concept. This article describes the selection of the fuel/catalyst combination (in both monopropellant and bipropellant operation), development of the monopropellant attitude control thrusters, and development of the dual-mode primary divert thrusters.2.For a propulsion system based on a hydrogen peroxide monopropellant, the operation is based on the idealized chemical decomposition reaction that proceeds according to the reaction described in n represents a mole fraction of water present in the hydrogen peroxide fuel; typically, the hydrogen peroxide concentrations ranges 85–90% by mass. In this propulsion design, the decomposition is achieved using a homogeneous catalysis approach wherein the catalyst is dissolved in a liquid solvent and reacts with the monopropellant in the liquid phase. The resulting exothermic reaction produces high temperature gaseous products that are subsequently passed through a converging–diverging supersonic nozzle to produce thrust. A high-level representation of this process is shown schematically in Here, auto-decomposition of hydrogen peroxide, resulting in a pressure increase in the storage vessels without the need for an external pressure source. Not only does this approach mitigate the need for a separate pressurization system aboard the cubesat, but it also satisfies current NASA regulations for pressurized elements on cubesats during launch.The detailed operation of this propulsion concept can be broken down into three stages: storage, mixing and reaction, and expulsion. First, in the storage stage, the catalyst solution and hydrogen peroxide are both stored separately in chemically compatible vessels. In this system, the storage vessels are pressurized by the slow, controlled secondary combustion reaction. The secondary combustion reaction releases additional thermal energy into the flow thereby increasing the Isp of the propulsion system; this aspect represents a significant and novel aspect of this design.The second stage, mixing and reaction, occurs when the hydrogen peroxide and catalyst solutions are allowed to flow into the mixing and reaction chamber of the thruster. Here, the catalyst solution is mixed with the hydrogen peroxide resulting in a fast decomposition reaction. This reaction produces steam, oxygen, and heat. The heat released by the decomposition reaction vaporizes the catalyst solvent. Due to the high temperature created by the decomposition reaction, the catalyst solvent vapors react with the oxygen resulting in a After the reactions have occurred in the mixing and reaction stage, the hot product gases enter the third stage of the propulsion system, the converging–diverging nozzle. The decrease in mass density of the gaseous reaction products compared to the density of the liquid reactants results in substantial pressure increase in the mixing and reaction chamber. This pressure drives the reaction products through the converging–diverging nozzle to achieve supersonic conditions and thereby generate the thrust for the device.At the lowest operational ratio of catalyst solution to hydrogen peroxide, the thrust produced will approach the results of a monopropellant thruster; this is the “pseudo-monopropellant” mode of operation. When operating in pseudo-monopropellant mode, the energy released approaches that of a monopropellant hydrogen peroxide system. In this mode, approximately 586 calories of heat per gram of 85% hydrogen peroxide is released and the adiabatic flame temperature during operation is 886 K .In contrast, by increasing the ratio of catalyst solution to hydrogen peroxide to a stoichiometric ratio allows all of the oxygen produced by the hydrogen peroxide decomposition to participate in a subsequent combustion reaction with the catalyst solvent. Coupling the combustion of the catalyst solvent with the decomposition of hydrogen peroxide results in a dramatic increase in thrust. This mode of operation is similar to a traditional bipropellant system, and is thus referred to as the “bipropellant mode” of operation. When the decomposition of hydrogen peroxide is coupled with the combustion of a catalyst solvent, the overall reaction products are carbon dioxide and steam and the total number of gas particles created by the reaction is increased. The addition of a stoichiometric ratio of energy-dense solvent significantly increases the energy evolved resulting in an increased adiabatic flame temperature. In comparison to the pseudo-monopropellant mode, the use of a stoichiometric mixture of hexanol and pure hydrogen peroxide results in an energy density of approximately 1050 calories per gram of mixture with an adiabatic flame temperature of 2670 K.The theoretical differences between the pseudo-monopropellant mode and bipropellant mode are summarized in 3.The CubeSat Design Specification, which creates a standard that ensures that CubeSats and their launch vehicle are compatible, sets strict limiting factors for the characteristics of CubeSat propulsion systems. Along with overall mass limitations for a 1U and 3U CubeSat at 1.33 kg and 4 kg, respectively, the maximum pressure contained in any component is limited to 1.2 atmospheres. Due to the high energy density, low vapor pressure, and nontoxic characteristics of hydrogen peroxide, it is a promising choice for use in a CubeSat propulsion system. In comparison to many other compounds used in propulsion today, such as hydrazine and oxides of nitrogen, high test hydrogen peroxide (HTP) has low toxicity and is considered a “green” monopropellant. The volume specific impulse of 90% HTP is greater than most comparable green propellants currently available. Another advantage of using HTP as a propellant is the benign nature of the products of hydrogen peroxide decomposition, steam and diatomic oxygen; unlike other promising high-energy green monopropellants such as ADN, HAN, and HNF, the decomposition of HTP results in products with relatively low molecular weight. This is an advantage because the operational temperature of the exhaust gases in HTP thrusters can be maintained much lower than that of other “green” propellants resulting in decreased materials and manufacturing costs . Due to 2) nanostructures as an in situ microchannel catalyst showed signs of incomplete decomposition, most likely caused by the inability of the HTP to wet the surface of the catalyst [Fe(III)Cl3) and ferrous chloride (Fe(II)Cl2) [To decompose the HTP, a catalyst is used to accelerate the rate of the decomposition reaction. Previous work using heterogeneous ruthenium-oxide Cl2) . Due to (II)Cl2) .An advantage of using an ionic compound as the catalyst for the reaction is the ability to use a polar solvent in the catalyst solution. Many polar solvents have high energy densities, low vapor pressures, and include many low toxicity compounds. Among such solvent options are water and alcohols. Several different catalyst solvents were tested in this study, each with a varying degree of polarity. The catalyst solvents were chosen based on several criteria: the ability to dissolve the catalyst, the miscibility with hydrogen peroxide, energy density, auto ignition temperature, and vapor pressure. Based on published data or estimates of these characteristics, six solvents of interest were chosen: hexanol, 2-propanol, ethanol, methanol, hexane, and water. Water was chosen as a baseline, non-reactive solvent. The polarity of the catalyst solvent allows the catalyst solution to be miscible with hydrogen peroxide. Although hexane’s dipole moment is significantly lower than the other solvents of interest, it was chosen to determine the degree of polarity necessary to dissolve the catalyst solutes. Nonpolar catalyst solvents tend to be immiscible with hydrogen peroxide and would greatly hinder mixing and hence unacceptably slow reactions in the thruster mixing chamber.The fast rate of decomposition of the hydrogen peroxide allows for exhaust gases to be generated on demand while maintaining high flow rates during steady state operation. Therefore, it is important that the catalyst used for the reaction results in sufficiently fast reaction rates. The use of homogeneous catalysis eliminates catalyst beds completely, resulting in the elimination of problems caused by a contaminated catalyst surfaces . The constant mixing of fresh catalyst in the homogeneous catalysis design means that catalyst degradation over time is not an issue of concern in this design.A key advantage of using the fuel as the catalyst solvent is the ability to vary the reaction kinetics by varying the ratio of hydrogen peroxide to fuel. If the system is run at the full stoichiometric ratio (1:9 fuel-to-oxidizer ratio for 2-propanol), the system operates as a bipropellant thruster with complete combustion of the fuel. By reducing the fuel injected into the reaction chamber, the system will transition to pseduo-monopropellant mode, where the majority of the reaction energy is coming from the exothermic decomposition of the hydrogen peroxide. The result of this control is the ability to use the same thruster, and related hardware, with a number of different operating profiles.When the decomposition of hydrogen peroxide is coupled with the combustion of a catalyst solvent, the overall reaction products are carbon dioxide and steam. The total number of gas particles created through the combustion reaction is increased. This, along with an increase in the enthalpy of reaction results in larger stagnation pressures and temperatures upstream of the nozzle. The bipropellant mode of the microthruster design takes advantage of these properties resulting in an increased thrust and impulse bit.When the desired values of thrust and impulse bit are below the minimum threshold for bipropellant operation, the ratio of catalyst solution to hydrogen peroxide is decreased significantly. This causes the enthalpy of reaction and product stream compositions to shift towards that of a hydrogen peroxide monopropellant thruster. The overall performance shift can be used when small adjustments are necessary, such as attitude control and small delta-V maneuvers.−10 °C. Due to the freezing point depression caused by the colligative properties in the catalyst solution, the freezing points are depressed far below the freezing point of hydrogen peroxide for each combination of catalyst and solvent of interest. As a result, no additional heat maintenance is required for the described bipropellant system than a hydrogen peroxide monopropellant thruster.For operation in a deep space environment, it is worth noting the freezing point of 90% propellant grade hydrogen peroxide is The decomposition of hydrogen peroxide is extremely exothermic; when used in monopropellant mode, the energy released is approximated by the energy released through only the decomposition of hydrogen peroxide, 586 calories of heat per gram of 85% HTP decomposed with a resulting adiabatic flame temperature of 886 K . The addThe addition of water to the hydrogen peroxide through the use of aqueous ferric chloride solution decreases the adiabatic flame temperature, as shown in By replacing water solvent with a fuel at a stoichiometric ratio of fuel to hydrogen peroxide, the adiabatic flame temperature of the mixture is increased significantly. If ethanol, methanol, 2-propanol, hexane, or hexanol are used as the catalytic solvent, the resulting adiabatic flame temperatures are significantly higher as shown in 3.1.Results from monopropellant and bipropellant data were divided into three categories based on the observed reaction delay time. The “long delay” category contained solutions with an observed delay of greater than one second, the “short delay” category contained solutions with a delay less than or equal to one second, and the “no delay” category contained solutions with no observed delay before the reaction. The reactions were classified into “fast” or “slow” reactions if the reaction after the delay period was shorter or longer than 0.5 s. Using data from the reaction observations, each solvent was determined to be either fit or unfit for use in this application. If the solution had no delay and a fast reaction, it was put into the fit category; if the solution had a delay longer than 1 s or a reaction longer than 0.5 s after the delay, the solution was unfit. These data are shown in combination with collected solubility data in 3.2.The viable catalyst solutions were chosen by combining the solubility data and the chemical reactivity data. Viable catalyst solutions, which possess characteristics necessary for use in a functioning thruster, were chosen as solutions that were homogeneous and noncolloidal at −10 °C, and have no observed delay followed by a fast decomposition reaction during reactivity testing. Through this process, the original 36 possible catalyst solutions were further reduced to seven viable catalyst solutions.Other characteristics that impact the catalyst solution effectiveness are the energy density of the chemical storage space, the solute concentration, and the relative theoretical maximum specific impulse. Each of these characteristics contribute to the thruster’s ability to provide consistent and reliable performance. The energy density of interest is the total energy per unit volume of the stoichiometric ratio of catalyst solution to hydrogen peroxide. When operating in bipropellant mode, the volumetric ratio of hydrogen peroxide and solvent differs between solvents due to the combustion chemistry of the reaction. When the volumetric ratios of hydrogen peroxide and solvent are taken into consideration, the solvent with the highest combined energy density will provide the thrust system with the highest delta-V per unit of combined hydrogen peroxide and catalyst storage space. This is important because of the tight volume constraints imposed by the CubeSat platform. There is a possibility that the catalyst will recrystallize and form a precipitate layer of catalyst on the walls of the nozzle during thruster operation. Due to this, the concentration of the catalyst in the solvent has the possibility to affect the performance of the thruster over its operational lifetime. This problem is mitigated by using the minimum catalyst concentration in the catalyst solution while maintaining high reaction kinetics in the device. Minimizing the mass of catalyst also decreases the total mass of the system.ISPR), relative chemical storage energy density (UR), and relative solute concentration (ConcR). The expected performance was characterized using an empirical, weighted formula given by :To characterize the expected performance of each of the catalyst solutions, the theoretical relative specific impulse of each was calculated. Theoretical relative specific impulse is the theoretical maximum achievable specific impulse for each solvent normalized by the maximum achievable specific impulse of 85% hydrogen peroxide and water. By determining which solvents have the highest relative specific impulse, the performance of the solvents can be directly compared. The relative expected performance was then determined by normalizing the expected performance with the highest performing solvent, in this case, 2-propanol.Based on this metric, the catalyst solution predicted to provide the thruster system with the highest performance is 15% ferric chloride in 2-propanol. This solution was chosen by taking relative specific impulse, average storage energy density, and catalyst concentration into consideration. This solution showed promising results in preliminary chemical reaction and solubility testing. It was determined that this solution will take advantage of high performance of the combustion of 2-propanol with a relatively low concentration of catalyst, increasing thrust and reliability.4.2O2) and fuel (2-propanol) tanks, the valves and manifold. Specific details of the two different thrusters are provided in the following subsections.With the solvent/oxidizer and fuel identified, a proposed system was developed to offer primary propulsion and attitude control for a 6U CubeSat. The envisioned system, illustrated in 4.1.H2O2 requires no preheating, and the decomposition temperature is significantly below the melting point of stainless steel, so no thermal management is required during continuous operation of the thrusters. Each attitude control thruster is paired with two miniature solenoid valves, which control the HTP and fuel-catalyst injection. Upon entering the reaction chamber, the two fluids mix and the HTP spontaneously decomposes. By maintaining the mixture ratio significantly below the stoichiometric level, these thrusters operate as a monopropellant thruster. The decomposition gases are ejected through a linear converging–diverging micronozzle, with a 30° half-angle. This configuration was selected as it provides a balance between minimizing the volume the nozzles occupy and the expected performance of the miniaturized supersonic nozzle including viscous effects at lower Reynolds numbers [The attitude control thrusters are additively manufactured out of stainless steel using Direct Metal Laser Sintered (DMLS). This manufacturing process allows the plenum, reaction chamber, and converging–diverging nozzle to be printed in a single step, reducing production costs and simplifying production. Decomposition of  numbers .4.2.The primary thruster is additively manufactured out of Inconel using DMLS. Inconel, a nickel-chromium alloy, is selected based on its corrosion resistance and ability to operate in extreme environments of pressure and heat. In the present case, the operating temperature of the primary thruster is significantly above the melting point of stainless steel. Like the attitude control thrusters, the use of DMLS allows the inlet plenum, reaction chamber, and converging–diverging nozzle to be printed in a single piece. Here, owing to the larger Reynolds number of the flow, a conventional conical nozzle with a 15° half-angle is used.The primary thruster uses two miniature solenoid valves to control the fuel and oxidizer. By pulsing the valves, the flow rates can be varied to operate either in bipropellant or pseudo-monopropellant mode. The system has no active thermal management, relying on radiative cooling, which limits continuous firing of the thruster in bipropellant mode to short bursts of under 30 s followed by a 5 min cooling period. In pseudo-monopropellant operation, the system can operate continuously with no thermal concerns.5.Due to the significant difference in thrust levels between the attitude control thrusters and the primary driver thruster, different measurement techniques were required for performance characterization. Three indirect measurement techniques were used to demonstrate the flow characteristics of the 50 mN attitude control thrusters: Schlieren imaging, thermal imaging, and exit plane temperature analysis. The thrust of the 1.25 N primary driver thruster was measured directly on a thrust stand. Specific descriptions of the methods used follow.5.1.To assess the micronozzle performance, Schlieren photography was performed using ambient temperature compressed air. The house air stagnation pressure was measured to be approximately 55 psig. According to the compressible flow relation the ratio of the stagnation pressure to outlet pressure necessary to achieve sonic flow at the throat was 1.57 (using γ = 1.4 for air), resulting in a necessary minimum stagnation pressure of approximately 38 psig when vented directly to the laboratory. Due to this, the pressure drop over the nozzle resulting from the supplied house air was determined to be theoretically large enough for supersonic flow in the divergent section of the nozzle.The Schlieren photography apparatus is shown in For testing, a simplified flow network design was created that eliminated one of the inlets from the nozzle design, leaving one inlet for the house air line. A photograph of the nozzle that was used in the Schlieren imaging is shown in 5.2.Thermal imaging was used to visualize the temperature distribution within the device along with the plume at the nozzle exit during operation. A photograph of the experimental set-up is shown in Thermal imaging of the system was performed using a thermoIMAGER TIM T900 manufactured by Micro-Epsilon . Thermal images were collected and analyzed using Micro-Epsilon’s thermal imaging software, TIM Connect. Along with thermal images, standard video was recorded using a PixeLink PL-B774U Color Camera. The thermal imaging camera was supported above the thruster, while the PixeLink camera was mounted to capture the side view of the thruster. Although the thermal camera was not calibrated to measure precisely the absolute temperatures of the materials, the thermal images nonetheless provide invaluable insight in the flow dynamics and operation of the system.5.3.The measurement of the temperature of the working fluid at the exit plane of the nozzle can be used to verify that the flow within the nozzle is in the supersonic regime. Supersonic flow through the nozzle is achieved if the pressure drop over the nozzle is high enough to choke the flow at the nozzle throat. As the nozzle diverges, a supersonic flow will accelerate as the flow area increases. The flow through a converging–diverging nozzle can be described using the well-known quasi-one-dimensional isentropic flow relation.M, the ratio of the flow area to the area of the throat, A/A∗, and the specific heat ratio of the gases flowing through the nozzle, γ. From this relationship, the geometry of the nozzle can be used to determine a theoretical Mach number at the nozzle exit plane Me Using the temperature relation for isentropic supersonic flow,T0 to the exit temperature Te can be calculated.at this exit Mach number, the ratio of the stagnation temperature The adiabatic flame temperature of the reactants for each trial is the approximate stagnation temperature in the system. From this, it is possible to determine a theoretical working fluid temperature at the exit plane of the nozzle. These calculations result in the lowest achievable fluid temperature at the exit plane during experimentation due to the isentropic assumptions made in the calculation of the fluid exit temperature. Further, these temperature calculations allow the experimental exit plane temperature to be compared to the theoretical (isentropic) temperature. For these experiments, the catalyst solution used was 15 weight % aqueous ferric chloride. The ferric chloride itself was a laboratory grade anhydrous powder. The catalyst solution was prepared less than an hour prior to testing to eliminate any possible catalyst degradation due to oxygen exposure. Hydrogen peroxide used for the experiment was HTP manufactured by the FMC Corporation ; the concentration was measured as 87.6 % using a Brix refractometer. With this combination of aqueous catalyst and HTP, the maximum adiabatic flame temperature expected during steady-state operation of the thruster was 685 K.The exit plane temperature measurement was performed using an Omega digital thermometer, model HH501AK, and a type K thermocouple. The thermocouple was placed directly at exit plane of the nozzle so that the end of the thermocouple was in the center of nozzle exit. The thermal imaging system was also used to capture thermal images of the plume during the tests. Along with thermal images, standard video was recorded using a PixeLink PL-B774U color camera. The experimental set-up for this experiment was similar to the arrangement used for the thermal imaging of the plume experiment and is shown in 5.4.Measurements of thruster performance were taken on a custom-built torsional thrust stand, with a sensitivity of 1.0 mN and a sampling rate of 20,000 samples/s. The position of the torsion arm is measured by a Digital Voltage Reluctance Transmitter , and captured using a National Instruments Model USB-6001 Digital Acquisition System  . The positional data were converted real-time into raw thrust data and recorded for post-processing. The flow rate of the hydrogen peroxide and catalyst solutions entering the prototype thrusters was controlled via syringe pump. The prototype thruster was attached to the thrust stand and data collection began once the hydrogen peroxide and catalyst solution entered the mixing chamber.6.6.1.An image obtained from Schlieren imaging appears in The results obtained from the Schlieren imaging confirm that the nozzle operates properly with stagnation pressures as low as 55 psig. Due to the rapid decomposition and combustion reaction during operation, it is expected that the chemical propulsion system will have adequate pressure upstream of the nozzle to achieve supersonic flow in the diverging section of the nozzle when operating with the vacuum back-pressure conditions in space.Shown in At high flow rates, the plume developed instabilities which caused the direction of the plume to oscillate between the walls and the center of the nozzle . When th6.2.Using the quasi-1D relations in The correlation observed between the experimental values and those calculated from the isentropic nozzle theory suggests that the nozzle design and mixing scheme were performing as expected during steady-state operation. These data also suggest that, for the nozzle size scale being tested, the Reynolds numbers were sufficiently large that the viscous boundary layers within the nozzle expander do not significantly impact the supersonic flow. This is in agreement with the numerical predictions by Louisos and Hitt for supersonic micronozzles  at the RIn an effort to further evaluate whether the flow within the thruster became supersonic, the ratio of the measured plume temperature to the adiabatic flame temperature was also computed; these data appear in 6.3.Thrust measurements of the device operating in steady-state were collected using the thrust stand apparatus shown in 7.The thruster system outlined in this paper is a novel solution to providing attitude control and primary propulsion for the CubeSat platform at low cost. By leveraging microscale effects, the total mass and volume of the proposed propulsion system is less than similar, commercially available, CubeSat propulsion systems. The chemical propulsion system described in this paper has the ability to operate in both “psuedo-monopropellant” and “bipropellant” modes, which increases its flexibility for mission planning. The psuedo-monopropellant mode operation offers lower impulse bits while the bipropellant mode offers higher thrust. The thrust generated is controlled by adjusting the ratios of catalyst solution to hydrogen peroxide fed to the thrusters, the smallest ratio of catalyst solution to hydrogen peroxide that allows for decomposition of the hydrogen peroxide is used for the psuedo-monopropellant mode and a stoichiometric ratio of catalyst solution to hydrogen peroxide is used for the bipropellant mode. The bipropellant mode further takes advantage of hydrogen peroxide as an oxidizer that oxidizes the catalyst solvent resulting in a combustion reaction.The prototype in this study was designed for operation and testing under ambient conditions. Future design modifications necessary for space operations involve only nozzle performance optimization, as the propellant reaction process is unaffected by ambient conditions. Specifically, the nozzle expansion ratio and chamber pressure will need to be adjusted to provide the target thrust level under vacuum exit conditions.


In this work, the characterisation of reaction kinetics of a methylene diphenyl diisocyanate (MDI)-based fast curing polyurethane resin (PUR) and the mathematical description of its curing process are presented. For the modelling of the reaction process isoconversional methods, which are also called model-free approaches, are used instead of model-based approaches. One of the main challenges is the characterisation of a reactive system with a short pot life, which already starts to crosslink below room temperature. The main focus is the evaluation of the applicability of isoconversional methods for predicting the reaction kinetics of fast curing polyurethane resins. In order to realise this, a repeatable methodology for the determination of time- and temperature-dependent reaction curves using differential scanning calorimetry (DSC) is defined. The cure models defined by this method serve as the basis for process simulations of PUR processing technologies such as resin transfer moulding (RTM) or reactive injection moulding (RIM) and reactive extrusion (REX). The characterisation of the reaction kinetics using DSC measurements is carried out under isothermal and non-isothermal conditions. Within this work isoconversional methods have been applied successfully to experimentally determined DSC data sets. It is shown that the reaction kinetics of fast curing polyurethane resins can be predicted using this methods. Furthermore, it is demonstrated that the time-dependent change of conversion of the considered polyurethane under isothermal curing conditions can also be predicted using isoconversional methods based on non-isothermal DSC measurements. This results in a significant reduction in the experimental effort required to characterise and model the curing process of polyurethanes. The processing of thermoset moulding materials like polyurethanes (PUR) is a very important field of plastics technology. The various properties of this material, especially the wide range of mechanical properties, are apparent in various application areas like the automotive industry, the construction industry as well as in products for everyday domestic needs. The mechanical properties of polyurethanes, depending on their chemical structure, provide the entire spectrum of material behaviour of engineering plastics, from rubber-elastic to hard elastic brittle deformation behaviour.Therefore, it can be assumed that due to the application-specific variable material properties the importance of polyurethanes in all areas of technical applications will increase. In order to meet the requirements of modern development processes, it is necessary to carry out specific research and further developments, especially in the area of numerical simulation of polyurethane manufacturing processes, the calculation of mechanical stresses in polyurethane components and the prediction of process specific component properties.Numerical process and structure simulations are now an essential part of the development processes of plastic components due to the complexity of the material behaviour of plastics. For polyurethane processing, the already published studies on process simulation can be divided into the manufacturing processes reactive injection moulding (RIM) ,2, resinAll process simulations have in common that the characterisation and modelling of the reaction kinetics of the curing process is required for their definition. Regarding the characterisation and modelling of the reaction kinetics, studies on pure PUR systems as well as composite materials ,14 can bIn , FernandThe model accuracy is strongly dependent on the quality of the experimentally determined reaction processes. It is important to ensure that the exothermic curing reactions of the considered resins are detected precisely using thermal analysis methods. Difficult test conditions are especially present if fast curing resins are the subject of the reaction kinetics characterisation. In contrast to conventional resins, which are treated in many published papers, fast curing resins exhibit no significant inhibition time. In many cases, resins of this type already react well below room temperature. This results in particular difficulties during the thermal analysis of the reaction process and a potential negative influence on the quality of the measured data for modelling the curing kinetics. For this reason, a repeatable measurement methodology has to be developed for the characterisation of such resins and their suitability has to be evaluated within the modelling procedure.In this work a commercially available polyurethane resin system of the type NEUKADUR MultiCast 7 made by Altropol Kunststoffe GmbH  was used. The resin is an unfilled fast curing resin. The PUR-System consists of a polyol formulation of branched polyesters and polyethers and 4,4′-methylene diphenyl diisocyanate . The polyol and isocyanate have to be mixed in a ratio of 100:80 (parts by weight). For the thermal characterisation, a batch of 40 g was prepared for each measurement. From this batch, a sample was taken for the thermal characterisation using a micropipette and placed in a DSC pan.The characterisation of reactive moulding materials such as polyurethane as well as epoxy or polyester resins is usually carried out by means of differential scanning calorimetry (DSC). The exothermic polymerisation reaction is reflected in the thermograms of a DSC measurement curve so that its time and temperature-dependent progression as well as the reaction enthalpy ΔH  and convFor isothermal measurements, the total enthalpy Based on these, specific characteristics of the reaction can be derived such as the conversion rate ersion α  which arIn this study, a differential scanning calorimeter of the type DSC8500 with an intracooler from PerkinElmer  was used. Curing processes under both isothermal and non-isothermal conditions were considered during characterisation. The characterisation was performed for non-isothermal measurements in the temperature range of −50–150 °C. Heating rates of 3, 4, 5 and 7.5 °C/min were considered. In isothermal DSC measurements, curing temperatures in the range of 30–80 °C with a step of 10 °C were investigated. The curing temperature was applied using a heating rate of 700 °C/min to take into account as much of the heat of reaction as possible during the characterisation and subsequent modelling. After the sample preparation, the DSC pans were placed in the measurement cell, which had already been pre-cooled to a temperature of −50 °C. After a short isothermal holding phase for temperature homogenisation, the measurements were initiated. Due to the fact that the resin system starts to react at temperatures well below room temperature, the isothermal DSC measurements were also started at a temperature of −50 °C, unlike to other works where the cell is pre-tempered to the corresponding curing temperature. The sample weights lie in the range between 6–10 mg.The conversion rateIn this case, Different approaches for describing the reaction processes of thermoset materials can be found in the literature. Most of them can be divided into model-fitting and model-free approaches ,25. The Many reactions as well as the curing processes of thermosets show a complex and multi-step behaviour. Often different reaction mechanisms can be observed due to parallel reactions and their mutual influence. Therefore, a constant activation energy can no longer be assumed in reactions like this. A mathematical description using established reaction models as illustrated by some examples in Isoconversional or model-free methods are based on the isoconversional principle, which eliminates the requirement of using an already existing reaction model or deriving a new model. In addition, these methods take into account the dependence of the activation energy For The isoconversional principle indicates that the rate of conversion of a reaction for a defined conversion is only a function of temperature. The conversion-dependent activation energy Isoconversional methods are subdivided into differential and integral methods. The differential method according to Friedman  can be dshown in  EquationBesides differential methods, there are also integral approaches. Based on Equation (2) and using the integral form of the reaction model For non-isothermal applications under constant temperature changes, this relationship is defined as follows:by Doyle  and usedby Doyle  and Ozawby Doyle :(10)lnln(β Starink : (12)ln and the previously determined conversion-dependent activation energy To predict the reaction process When using isoconversional methods to describe the curing process of thermosets and to determine kinetic triplets, it should be considered that these have to be applied consistently, since the calculated parameters of integral and differential methods are not exchangeable. Although in  it is shFor more detailed information on isoconversional methods of thermally activated processes, reference is made to Vyazovkin . VyazovkThe results of the DSC measurements under isothermal and non-isothermal curing conditions are shown in The results indicate that the PUR system already starts to react at approximately −25 °C. For the thermal characterisation, this results in several challenges during preparation and handling of the sample. If possible, the curing reaction should be stopped until the measurement is started by pre-cooling the DSC measuring cell. However, the raw materials must also be mixed within an ice water bath or by using alternative cooling methods, as already described in some other studies. In any case, the individual measurements should always be carried out in the same way. The procedure described above for the preparation of the DSC samples and the calorimeter enables to determine thermograms that are suitable for the modelling of the reaction kinetics described below.During the interpretation of curing processes, the effect of vitrification must be taken into account. With increasing degree of crosslinking while curing, the glass transition temperature increases. If the glass transition temperature reaches the level of the curing temperature, this leads to a rapid reduction in molecular chain mobility and, as a result, a decrease of the curing rate. The reaction becomes diffusion-controlled. Within a DSC thermogram this effect is expressed as a decreasing heat flow, which can be misinterpreted as a slowing down of the curing reaction. Therefore, vitrification occurs when the temperature of an isothermal curing process is below the glass transition temperature of a fully cured resin system In chnology  can be uFor the further considerations of this work, the average total enthalpy As shown in Furthermore, the reaction processes determined under isothermal curing conditions are not considered at this point, because non-isothermal measurements are sufficient using isoconversional methods to define a complete model and predict both isothermal and non-isothermal reaction processes.The conversion rate r time t . The conAccording to the isoconversional principle, the conversion-dependent activation energy The corresponding conversion-dependent activation energy can be determined from the slopes of these lines. It can be seen that the curves of the integral isoconversional methods and the activation energy determined by Vyazovkin’s method are almost identical and differ only in their level. The activation energy curve determined by Friedman shows a higher slope at the beginning of the reaction process, which becomes closer to that of the integral methods as it progresses further.Using isoconversional methods also the pre-exponential factor and the reaction model of a curing reaction can be determined, even though they are not necessary for the prediction of the reaction kinetics for states outside the conditions considered in the experiment. For the computation of the pre-exponential factor and the reaction model only the calculated activation energy according to Vyazovkin is considered in the following. Applying the kinetic models 1–13 from The compensation effect factors The calculation of a generic reaction model It can be seen that the reaction model of the considered PUR system has a characteristic that lies between that of the D2 and D3 model. These models represent the two- and three-dimensional mathematical description of chemical reactions coupled with diffusion processes. Therefore, for the modelling of curing kinetic using a model-based approach, the D3 model would be used, as this kinetic model represents the investigated curing reaction most closely.In The modelling of isothermal curing exhibits a very good agreement with the experimentally determined progressions of the reaction conversion. However, the model does not accurately depict the courses of conversion for curing conditions In the scope of this work, the characterisation of the reaction kinetics of fast curing MDI-based PUR resins using DSC was presented. The investigated PUR system has no significant inhibition time and reacts already well below room temperature. In this context, a repeatable methodology for determining the reaction process for isothermal and non-isothermal curing conditions was prepared and successfully applied. The experimental data obtained were used for the mathematical description and modelling of curing processes using isoconversional methods. It could be shown that the crosslinking reaction of fast curing PUR resins can also be modelled with high accuracy using this method for curing conditions


We additionally assessed whether nudging effects would remain robust when a disclosure message was included. The field experiment was conducted over a seven-week period. The measured outcome was the sales of the targeted healthy food products.The accessibility nudge significantly increased the sales of the fresh fruits. The impact of the salience nudge was limited presumably due to existing preferences or habits that typically facilitate bread purchases. As the sales of the yoghurt shakes remained consistently low over the seven-week period the impact of the social proof nudge remained unexamined. Critically, disclosing the purpose of the nudges did not interfere with effects.Current findings suggest nudging as an effective strategy for healthy food promotion, and offer implications for topical debate regarding the ethics of nudges. There is an urgent need to counter unhealthy eating on a societal level as the growing prevalence of overweight and obesity contributes majorly to the rise of non-communicable diseases  that not only pose increasing financial strain on healthcare systems , but evenudging, defined as “any aspect of the choice architecture that alters people’s behaviour in a predictable way without forbidding any options or significantly changing their economic incentives” [Research has consistently shown that consumers make food choices in a mindless manner with minimal deliberation, with many consumption behaviours occurring outside of awareness often as the result of environmental influences –7. In reentives” , as an aNudging interventions are based on the theoretical rationale derived from dual-process models of behaviour. Contemporary dual-processing models posit that behaviours result from the interaction of two modes of processing: an unconscious, fast, and automatic mode (System I) on one hand, and a slow, conscious, and deliberative mode (System II) on the other hand , 10. SysIn accordance with this theory, while many consumers intend to eat healthily and express weight concerns , much ofThe use of nudging interventions has increasingly attracted interest from several governments , 23, nonThe accessibility to food on the basis of physical proximity influences people’s consumption of that food, such that people tend to consume a greater amount of food that is closer in proximity compared to food that is further away . The assPeople have a natural tendency to approach objects that they find rewarding , and theThe food choices of others often have a strong influence on people’s own consumption decisions, and the operation of social norms has been proposed as a mechanism underlying such influence . The socThe goal of nudging in healthy food promotion is to redirect an automatic and mindless choice towards a healthier outcome by changing the environment in such a way that the healthy choice becomes a more convenient, attractive, or normal choice, all in the interest of consumers . CorrespNonetheless, there is scarce research systematically evaluating whether nudging strategies are indeed only effective in covert conditions where consumers are unaware of being nudged. Put differently, it remains an open question whether effects of nudging would still be observed when their purpose is disclosed, potentially stimulating consumers to be more reflective and cognizant in the situation. To our knowledge there is only one field study assessing the effects of the disclosure of nudging specifically targeted at healthy eating promotion, and even so this study has only examined disclosure applied to one type of nudge  . In the The first objective of the current research was to conduct a field experiment to test the effectiveness of an accessibility nudge, a salience nudge, and a social proof nudge to encourage more purchases of fresh fruits, healthy bread rolls, and yoghurt shakes respectively at a take-way food vendor. Considering that in previous research similar nudges have successfully promoted the purchases of healthy products in student cafeterias , 30, 41,The study took place during February, March, and April 2016, at a take-away food vendor that sold a variety of hot and cold beverages , small meal items , and snacks  at a large academic hospital in The Netherlands. Participants consisted of all customers  who made purchases at the take-away food vendor during the seven-week period that the field experiment took place.baseline week where no nudges were implemented; 2) Week 2 was an experimental nudge week where all three nudges were simultaneously implemented  to promote healthy food choices; 3) Week 3, 4, 5, and 6 were washout weeks where all three nudges were simultaneously removed to eliminate carryover effects from the previous nudge week; and 4) Week 7 was an experimental nudge and disclosure week where all three nudges were re-implemented with an additional disclosure slogan conveying the purpose of the nudge.The current experiment employed three different nudges to promote the sales of fruits, healthy bread rolls, and a yoghurt shake. The field experiment was designed over a seven-week course such that: 1) Week 1 was a All purchases were recorded electronically on a weekly basis. The field experiment has been approved by the faculty’s institutional review board.nudge week, nudge and disclosure week) the three different nudges including the accessibility nudge, salience nudge, and social proof nudge (see Nudges below) were set up simultaneously at 7.30 a.m. on Monday morning when the take-away food vendor opened, and removed at 5.00 p.m. on Friday when the vendor closed. During the nudge and disclosure week, an additional sign of the disclosure (see Nudge Disclosure below) informing the purpose of the nudge was displayed adjacent to each nudge. The nudges were not implemented during the baseline or washout weeks. After the study had completed its course, the manager of the take-way food vendor provided the electronically recorded weekly sales data of the seven-week period to the researchers.In both the two experimental weeks  were placed flat on the counter. Customers would not have noticed the labels unless they approached the counter see Fig. a. The sonudge and disclosure week to disclose the intention of the nudges in place.A small sign with the simple one-sentence message, “We help you make healthy choices”, was displayed accompanying each individual nudge during the total daily sales of each food product rather than individual sales transactions, means and standard deviations could not be calculated, and hence statistical analyses could not be carried out for significance testing to examine the differences in sales between the baseline, nudge, and nudge and disclosure week. Nonetheless, in addition to providing descriptives, we conducted chi-square analyses to test the effectiveness of the respective nudges. Specifically, the chi-square compared the sales of the targeted healthy product to the sales of a comparable unhealthy product between the baseline vs. nudge vs. the nudge and disclosure week.In order to test the effectiveness of the accessibility nudge, the salience nudge, and the social proof nudge respectively, we first present the sales data of the targeted healthy products  collected over the seven-week period . We acknowledge that the reported increase or decrease in sales of the targeted healthy products compared between the baseline vs. nudge vs. nudge and disclosure week is only descriptive. Due to the nature of the weekly sales data, which recorded the Additionally, exploratory analyses investigated whether potential spill over nudging effects existed, such that the hypothesized increase of sales for targeted healthy food products would extend from the nudge week to the subsequent washout weeks when the nudges were removed.Table During the baseline week a total amount of 90 pieces of fruit were sold. The total amount of fruit sales increased to 156 during the nudge week, which is equivalent to a 73.3% increase. Furthermore, a total amount of 164 pieces of fruit were sold during the nudge and disclosure week. This was a 82.2% increase compared to the baseline week, and a slight increase of 5.1% compared to the nudge week.Chi-square tests were conducted to test the nudge effectiveness hypothesis. Results are presented in Table Specific pairwise comparisons to examine the sales data of fruits vs. confectionary between the baseline vs. nudge week, baseline vs. nudge and disclosure week, and also nudge vs. nudge and disclosure week. Results from the follow-up pairwise comparisons revealed that the proportion of fruit sales to confectionary sales in the nudge week  was significantly different to the proportion in the baseline week . The proportion of fruit sales to confectionary sales in the nudge and disclosure week  was also significantly different to the proportion in the baseline week. Finally, the proportion of fruit sales to confectionary sales in the nudge week was not significantly different to the proportion in the nudge and disclosure week. Complementing the descriptives, the results from the chi-square analyses demonstrate that the accessibility nudge was effective in promoting fresh fruits .During the baseline week a total of 291 healthy bread rolls were sold. The total amount of healthy bread rolls increased to 318 during the nudge week, which is equivalent to a 9.3% increase. During the nudge and disclosure week a total of 327 healthy bread rolls were sold, which was a 12.4% increase compared to the baseline week, and a 2.8% increase relative to the nudge week.We conducted a chi-square test to compare the sales of healthy bread rolls to the sales of croissants to examine the impact of the salience nudge. The croissants were selected as a comparison group because they were the competitive ‘unhealthy’ alternatives in the same product category. Results from the chi-square indicated that the overall differences in proportion of sales in healthy bread rolls compared to croissants was not significantly different between the baseline week , the nudge week , and nudge and disclosure week . While descriptives suggest the sales of healthy bread rolls were higher in the weeks where the salience nudge was implemented, we did not obtain evidence from the chi-square analysis that the proportion of healthy bread rolls compared to croissants was statistically significant different across the three different weeks.During the baseline week, a total of 7 yoghurt shakes were sold. Comparatively, during the nudge week a total of 6 yoghurt shakes were sold, and, a total of 8 yoghurt shakes were sold during the nudge and disclosure week. Considering the descriptives, it was apparent that the sales of the yoghurt shake across the entire seven-week period remained consistently low and would not have warranted sufficient statistical power for analysis. For this reason, statistical analyses were not conducted to examine the effect of the social proof nudge.In light of the finding that the accessibility nudge significantly increased the sales of fresh fruit in the nudge week compared the baseline week, we explored whether this increase in sales ‘spilled over’ or was sustained in the washout weeks subsequent to the nudge week. Similar to the chi-square analysis used previously, we compared the sales of fresh fruits to the sales of confectionaries between the baseline week vs. nudge week vs. washout week (averaged between the 4 weeks) vs. nudge and disclosure week. The results of the chi-square indicated that there was a significant difference in the proportion of sales between the weeks. Specific follow-up comparisons revealed that the proportion of fresh fruit sales to confectionary sales during the washout week  was significantly different to the proportion in the baseline week . On the other hand, the proportion of fruit sales to confectionary sales during the washout week  was not significantly different to the proportion in the nudge week ; nor to the nudge and disclosure week . These results demonstrate that the increase of sales of fruits on the nudge week from the baseline week sustained during the washout-weeks after the accessibility nudge has been removed, thereby suggesting a potential spillover effect of the accessibility nudge.The automatic basis on which many food choices are made without much deliberation offers a window of opportunity for using choice architectures to gently nudge consumers towards healthy food choices. Specifically we employed an accessibility nudge to increase the convenience for picking healthier fresh fruits, a salience nudge to enhance the visibility and attractiveness of healthy bread rolls, as well as a social proof nudge to promote the popularity of yoghurt shakes at a take-away food vendor located at a university hospital. Considering the sales data, it was evident that the accessibility nudge was a particularly effective nudge in this study. Consistent with previous research findings , 28, simThe salience nudge was intended to enhance the visibility and visual attractiveness of the healthy bread rolls. Compared to the baseline week it was apparent that in terms of absolute sales of healthy bread, there was a relatively greater proportion of healthy bread rolls sold relative to the croissants in the nudge week, as well as in the nudge and disclosure week. The observed increase was however not significant in statistical terms across the 3 weeks. Nonetheless, these findings do not necessarily dismiss the effectiveness of a salience nudge in general. It may be the case that the influence of the salience nudge was overpowered by consumers’ existing preferences and habits for bread purchases. In an in-store experiment, an accessibility nudge to improve the convenience for purchasing whole-wheat bread did not influence sales . SimilarIn current research, we tested the effectiveness of three nudges  in a real-life setting as opposed to a more controlled environment in the lab. We observed that the accessibility nudge appeared to have worked particularly well in promoting a healthy food product in spite of the distractions that were taking place in the setting. On the other hand, due to practical reasons all three nudges were implemented simultaneously in the current study. Future studies could potentially examine whether presenting multiple nudges together would cause interference between the nudges, or whether they could complement each other and have additive effects.intended purpose of the nudge, and not the actual presence of the nudge  in the disclosure message. While our findings suggest that nudging effects remain robust when consumers are made aware of the nudge’s intended purpose through a simple disclosure message, future research should further scrutinize whether this effect still holds when consumers are made aware specifically of the nudge’s presence . Nonetheless, our current finding may be relevant in consideration of the topical debates surrounding the ethics of implementing interventions  that may be influencing individuals at large without their awareness. For example, the 2011 House of Lords Behaviour Change report [As a second objective, the current study also examined the impact of disclosure. Our findings indicated that disclosing the intended purpose of the nudge did not interfere with its effects , which corroborates recent work . We alsoe report  publishechoice does not automatically mean food consumption, which needs to be taken into account when considering implications from this study.The current study has a number of strengths as well as limitation. A first strength of the study is that the different nudges were tested in the field, therefore allowing for observations in a real-life food choice setting. A second strength pertains to the inclusion of disclosure in this study, as transparency of nudging is heavily debated, but empirical evidence is scarce. Of course, a field study also has limitations. For example, although allowing for real-life food choices, the study did not allow for a controlled setting. Therefore, we are not able to control for other contextual influences on food choice during the experiment. A final note of caution relates to the notion that food Our current research has demonstrated nudging to be a low-cost and easy-to-implement strategy to promote healthy food choices. It appears that the fast, non-conscious, and automatic processes are not destined towards unhealthy choices, but could rather be gently nudged by the choice architecture into more optimal, healthy outcomes. These nudging effects are perhaps even immune to conditions when consumers are made aware of being guided toward healthier choices. In closing, with the increasing trend of people eating outside their homes, public eating environments have been identified as strategic places for health promotion , and nud


Finding an inexpensive and scalable method for the mass production of memristors will be one of the key aspects for their implementation in end-user computing applications. Herein, we report pioneering research on the fabrication of laser-lithographed graphene oxide memristors. The devices have been surface-fabricated through a graphene oxide coating on a polyethylene terephthalate substrate followed by a localized laser-assisted photo-thermal partial reduction. When the laser fluence is appropriately tuned during the fabrication process, the devices present a characteristic pinched closed-loop in the current-voltage relation revealing the unique fingerprint of the memristive hysteresis. Combined structural and electrical experiments have been conducted to characterize the raw material and the devices that aim to establish a path for optimization. Electrical measurements have demonstrated a clear distinction between the resistive states, as well as stable memory performance, indicating the potential of laser-fabricated graphene oxide memristors in resistive switching applications. After the cycling of the device, there is neither a significant degradation in the performance nor an overlap between the HRS and LRS. The devices can safely be cycled further if the current compliance is set to 20 µA, limiting the power dissipated and preventing irreversible resistive switching. As shown in The stability of the resistive states has been assessed by continuous device cycling. Results represented in TM, there are no metallic agents in the composite .3 domains to conducting sp2 bonds (oxygen vacancies) based on the detachment of oxygen groups under the action of the electric field [The other model to explain the resistive switching in GO films proposes that the conductive path is caused by transforming insulating spic field ,52. Seveic field ,54 have ic field , the autic field , which e12 eV, consistent enough to trigger a significant percentage of the migration and recombination of oxygen ions and functional groups across the basal plane [3 C–O bonds can be roughly estimated in the order of ~1013 C atoms .This second model is likely to explain the switch of the resistance of laser-fabricated graphene oxide memristors. Thus, the memristance relates to a bulk phenomenon involving the drift of oxygen ions and oxygen-containing groups inducing local changes in the level of reduction of the GO turning it into a more conductive state ,56,57. Tal plane , since ttructure  and 60% 2 high-conductive domains are interrupted by low-conductive ones (sp3) at a nanoscale level. When a current is flowing, the electrostatic potential is mostly applied on those non-conductive regions resulting in large local electric fields. sp2 based conductive paths are set up under the action of the current flow leading to an LRS state. The hypothesis of the formation of interconnected high-conductivity domains does not mean that the high conductivity state is related to a homogeneously distributed conduction medium, but rather the LRS is correlated with a confined effect associated to the formation of a percolation path (or paths) of highly reduced GO by the energy accumulated in the device. Under reversed polarity, a partial recombination of the oxygen vacancies and mobile ions is possibly resulting in the recovery of the HRS. According to [rding to , this re3 domains are tuned into sp2 domains (oxygen vacancies). The sp3 domains can be recovered by reverse polarity and these mechanisms are likely to be triggered by dissipative effects and assisted by the electric field.This work addressed the fabrication of organic memristors based on the laser-assisted reduction of graphene oxide. This technique has been demonstrated as a fast and reliable approach for fabricating the devices with a clear advantage in its simplicity and intrinsic lithography. The results have proved that after tuning the photothermal intensity of the ablation process, the devices feature a prominent and stable memristance. Therefore, this fabrication technique may constitute a new high throughput approach for the advent of memristive circuits. According to our experiments and existing theories, the resistive switching of the devices is attributed to oxygen-containing functional groups drift modifying the local stoichiometry of the reduced-graphene oxide layer. Low-resistance conductive paths are formed in the bulk of the material when sp


Our main interest is to identify how humans maintain upright while walking. Balance during standing and walking is different, primarily due to a gait cycle which the nervous system must contend with a variety of body configurations and frequent perturbations . We have identified three mechanisms that healthy young adults use to respond to a visually perceived fall to the side. The lateral ankle mechanism and the foot placement mechanism are used to shift the center of pressure in the direction of the perceived fall, and the center of mass away from the perceived fall. The push-off mechanism, a systematic change in ankle plantarflexion angle in the trailing leg, results in fine adjustments to medial-lateral balance near the end of double stance. The focus here is to understand how the three basic balance mechanisms are coordinated to produce an overall balance response. The results indicate that lateral ankle and foot placement mechanisms are inversely related. Larger lateral ankle responses lead to smaller foot placement changes. Correlations involving the push-off mechanism, while significant, were weak. However, the consistency of the correlations across stimulus conditions suggest the push-off mechanism has the role of small adjustments to medial-lateral movement near the end of the balance response. This verifies that a fundamental feature of human bipedal gait is a highly flexible balance system that recruits and coordinates multiple mechanisms to maintain upright balance during walking to accommodate extreme changes in body configuration and frequent perturbations. Investigations of human balance control have focused primarily on standing, either during unperturbed quiet stance , 2 or inWe have identified three basic mechanisms following visual perturbations in the medial-lateral direction that shift the CoP in the desired direction, the lateral ankle mechanism, the foot placement mechanism, and the push-off mechanism . We provThe lateral ankle mechanism refers to the generation of ankle inversion/eversion torque during the single stance phase. Rolling the ankle while the foot is on the ground can shift the CoP under the foot. The goal is to shift CoP in the direction of the perceived fall under the stance foot. The lateral ankle mechanism is able to act on the CoP at virtually any time during the gait cycle, and is the main means of modulating the CoP under the stance foot during sustained locomotion. However, the magnitude of CoP modulation that the lateral ankle mechanism can produce is constrained to the width of the foot or shoe.The foot placement mechanism is another mechanism that can shift the CoP. The swing foot is shifted in the direction of the perceived fall, and on heel-strike, shifts the CoP. Foot placement is the most commonly reported measure of dynamic stability , 15. MucThe push-off mechanism refers to the modulation of ankle plantarflexion angle during double stance. Very few studies have recognized the push-off mechanism’s role in balance. Ankle plantarflexion torque has been shown to change as a result of bipolar, binaural galvanic vestibular stimulation  and anteThe three identified balance mechanisms are the main available methods to make fine motor adjustments to the CoP and CoM in order to maintain balance while walking. Given that multiple mechanisms are available to the CNS to maintain balance, we analyzed whether these mechanisms are functionally related. For example, modeling results suggest that without any CoP modulation during the stance phase following a sensory perturbation, a 60% larger foot placement is needed to maintain balance in the medial-lateral direction . Here wePortions of this data and detailed data collection methods have already been published , where wAfter explaining the experiment, obtaining consent and placing markers and EMG sensors, subjects first walked for 15 min on the self-paced treadmill in the virtual environment to adapt to this experimental setup. We then stopped the treadmill and told the subjects that we would now perturb their sense of balance by modifying the virtual scene, and asked them to cope with this perturbation “normally” and keep walking forward. Data collection blocks consisted of two alternating phases for metronome and stimulus. During metronome phases, lasting 30 s, subjects were provided an auditory metronome at 90 bpm and asked to use this as an “approximate guideline” for their footsteps, both during metronome and stimulus phases. During stimulus phases, lasting 120 s, the metronome was turned off, and subjects received visual fall stimuli as described above. Data were collected during stimulus phases. Each subject performed four blocks of walking, each block consisting of five metronome and five perturbed phases, always starting with metronome phases, for a total of 12.5 min per block. After each block, the treadmill was turned off and subjects were offered a break. This protocol was implemented in a custom Labview program that sent the head position, treadmill speed and rotation angle to the computer controlling the virtual environment implemented in Unity3d  via User Datagram Protocol communication. The visual rotation angle and treadmill speed were saved in Labview at 100 Hz.−2 for 600 ms around the anterior-posterior axis of the midline of the floor, inducing a feeling of falling to the side. The perturbations were provided randomly on a right or left heel-strike (stimulus trigger), and rotation direction were randomized rotate clockwise or counter-clockwise. Perturbations in which the environment rotates clockwise corresponds to the visual sensation of a fall in the opposite direction, counter-clockwise or to the left. Similarly, a counter-clockwise rotation of the environment corresponds to the sensation of a clockwise fall, or to the right. We will refer to these as fall stimulus to the right and fall stimulus to the left. Furthermore, we will assume body symmetry and pool data for cases where the direction of the perceived fall is towards the triggering leg, i.e. right heel-strike triggering fall stimulus to the right and left heel-strike triggering fall to the left, and cases where the direction of the perceived fall is away from the triggering leg, i.e. right heel-strike triggering fall stimulus to the left and left heel-strike triggering fall stimulus to the right. The resulting rotation of 10.8° was then held constant for 2,000 ms, before being reset to neutral rotation with uniform speed over 1,000 ms. After resetting to neutral rotation, a randomized interval of 10-13 steps elapsed before the next stimulus was triggered. Heel strikes were identified as downward threshold crossings of the vertical heel-marker position. The threshold was set to the vertical heel-marker position of each foot during quiet standing, plus 3 mm.Subjects walked on a split-belt, instrumented treadmill within a virtual environment projected onto a dome surrounding the treadmill by ∼180 degrees . The treadmill was self-paced, using a nonlinear PD-controller in Labview  to keep the middle of the posterior superior iliac spine markers on the mid-line of the treadmill. Perturbations consisted of a rotating virtual environment scene with an acceleration of 60° sReflective markers were placed bilaterally on the feet, lower legs, thighs, pelvis, torso, head, upper arms, forearms, and hands of the subject, using the Plug-in Gait marker set  with sixKinematic data were low pass filtered with a 4th order Butterworth filter at a cut-off frequency of 10Hz. Small gaps in the marker data of up to 100ms length from occlusions were filled using cubic splines. Time points with remaining marker occlusions were excluded from further analysis. From the marker data, we calculated joint angle data based on a geometric model with 15 segments  and 38 degrees of freedom (DoF). We estimated the hip joint centers based pelvis landmarks , 27, and−1 after each heel strike. We visually inspected the result of this automatic identification and applied manual corrections in the rare cases where events were misidentified.We identified heel strike events for each foot by finding negative peaks in the vertical positions of the heel markers with minimal inter-peak distances of 250 ms and peak prominence greater than 2 cm, and push-off events as the first peak in the vertical velocity of the 2nd metatarsal marker with a prominence greater than 0.35 mscontrol mean from all data, including control, for every subject. The control steps are defined as the two steps preceding the heel-strike that triggers the visual perturbation. All data between heel-strikes were normalized to 100 time steps. All of the data here are represented as a change in response from the average of the control steps.All data between heel-strikes were normalized to 100 time steps. We subtract the control steps. This leaves three conditions to analyze, stimulus towards trigger foot, stimulus away from trigger foot, and control. After processing, we were left with 1947 steps for Stimulus Towards, 1930 steps for Stimulus Away, and 3854 steps for Control.The experimental design had four distinct stimulus conditions, where each heel strike could trigger a stimulus to the left or right. Here we assume anatomical symmetry and group the conditions that are anatomically similar . Previous analysis of the data set indicate asymmetries between feet are not present . We alsoComparing the use of the balance mechanisms on every step requires that the mechanisms are summarized by a single value. We use the following definitions to quantify the use of each balance mechanism on every step:control steps of the medial-lateral distance between CoP and CoM during single stance. The integration is performed during single stance to avoid the contribution of the foot placement mechanism on the CoP. Throughout the text we refer to this variable as Ankle.The lateral ankle mechanism is defined as the integrated response from We define the foot placement mechanism by the medial-lateral response of the swing leg heel position at heel strike, relative to the initial medial-lateral position of the trigger foot heel strike. Throughout the text we refer to this variable as Foot.control steps of the trigger foot ankle dorsiflexion/plantarflexion angle during the second double stance phase following stimulus trigger. Throughout the text we refer to this variable as Push.We define the push-off mechanism as the integrated response from We confirmed the assumptions of normality and homoscedasticity by visual inspection of the residual plots for each combination of variables. Linear mixed models were used for each pair of mechanisms to assess interdependence. For each pair of mechanisms, we fitted a linear mixed model and performed an ANOVA to analyze the interdependence of the mechanisms and interaction of stimulus direction, using Satterthwaite’s method  implemenR2 values were calculated for each balance mechanism pair to provide an indication of the strength of the relationship.For visual purposes, we perform a least-squares fit for all pairs of mechanisms. Pearson Correlation R2 values two standard deviations lower than the mean for Ankle-Foot interdependence. Including the subjects resulted in non-converging of the mixed-models in R.All figures show data averaged from eighteen out of twenty subjects. We removed two subjects from this analysis. These two subjects expressed Stimulus Towards and Stimulus Away deviate away from each other. The activation of the lateral ankle musculature leads to a CoP shift towards the direction of the perceived fall, for both Stimulus Towards and Stimulus Away. Stimulus Towards and a dorsiflexion response for Stimulus Away.The balance response to a perceived fall laterally (towards and away from trigger foot) consists of a combined response of the lateral ankle mechanism, the foot placement mechanism and the push-off mechanism, as shown in Stimulus Towards, Stimulus Away and Control, demonstrating a negative interdependent relationship for all conditions  We observe a systematic modulation of ankle plantarflexion angle following a medial-lateral (ML) balance perturbation. It seems unlikely that a response to an ML perturbation would serve only propulsion in the sagittal plane. 2) Propulsion is dependent on the stimulus direction . If, forR2 values between push-off and other mechanisms were very small. We can provide three possible reasons for such small but consistent relationships: 1) The push-off mechanism may provide more of an indirect balance adjustment through modulation of other gait parameters, which we are currently analyzing and plan to address in a future publication; 2) The relationship between the lateral ankle and foot placement mechanisms is so strong that it masks their relationship with the push-off mechanism; 3) There are other mechanisms contributing to the maintenance of balance during walking. As discussed in the previous paragraph, we are confident the push-off is a separate balance mechanism, despite the weak correlations, in that it is activated separately from the lateral ankle and foot placement mechanisms. The small correlations between push-off and the other two balance mechanisms is actually evidence that they are separately activated balance mechanisms. For example, if the lateral ankle and push-off were activated through the same neural pathway, we would expect a strong positive relationship between the two. We actually observe a weak negative correlation between the lateral ankle and push-off mechanisms. The weak correlations may also be a product of the flexibility of the healthy young nervous system. There are so many degrees of freedom available to act on the balance related forces that a moderate amount of error at any point of the gait cycle is acceptable.Despite the systematic nature of the push-off, the Stimulus Towards, Stimulus Away, Control). Thus, interdependence of the balance mechanisms is present in all conditions, but the overall balance response is shifted in the direction of the perceived fall. This emphasizes that a sensory perturbation does not alter the fundamental nature of the balance response observed during unperturbed walking, but in the presence of a perceived threat to balance, the overall balance response shifts in the direction of the perceived fall (i.e. blue shifts towards the trigger foot and yellow shifts away from trigger foot in An interesting finding was that these balance responses are observed on steps both with and without perturbations. This is consistent with the view that heel strike is essentially a perturbation that is inherent to the gait cycle, requiring a control action to maintain upright balance on every step , 38. ThiThe sequential onset of the balance mechanisms in the presence of the perceived fall  in combiPrevious work has identified different strategies that can be recruited to aid in the overall goal of maintaining stability.  has showWe also assume all of the modulation of the CoP with respect to the CoM in the first single stance following the trigger perturbation is a result of the lateral ankle mechanism, but it is certainly possible other mechanisms could be used to assist in this modulation.  also fouWe note a change in ankle plantarflexion in response to the medial-lateral balance perturbation; however, we do not have concrete evidence that the change in ankle plantarflexion angle is due to an increased ankle plantarflexion torque and propulsive ground reaction force. Due to the inherent design of our experiment, calculation of joint torques and ground reaction forces from a single limb are impossible to discern because of scenarios involving one foot on two force plates or two feet on one force plate. We asked people to treat the split-belt treadmill as a sidewalk to create an environment that allows for the most natural gait. Adding medial-lateral balance perturbations increases the number of instances where a single foot may not land squarely on a single force plate. This scenario makes it difficult to provide concrete evidence that the ankle plantarflexion response is producing a “push-off”. Future work will investigate in detail how the systematic change in ankle plantarflexion angle produces medial-lateral balance shifts, but for now we believe there is enough support to assume it does produce a CoM shift in the medial-lateral direction.In conclusion, an interdependent relationship has been uncovered between three basic mechanisms for the maintenance of upright balance during walking. Most evidently, our analysis illustrates that when the lateral ankle mechanism overshoots or undershoots the foot placement will provide a correction later in the gait cycle. The push-off mechanism is not as strongly related to the other two balance mechanisms described, suggesting the push-off is only producing a subtle balance related adjustment. Moreover, we highlight that these mechanisms are used on every step, regardless of whether there is a perturbation. The perturbations result in a shift of the overall balance response in the direction of the perceived fall. This is confirmation that a fundamental feature of human bipedal gait is a highly flexible balance system that recruits multiple mechanisms to maintain upright balance during walking despite extreme changes in body configuration and frequent perturbations.


However, with the evolution of the freeze-all strategy, a new problemis arising for evaluating the performance of an embryological team. The aim of the study was to present anew representative index, combining fresh and frozen embryo transfer success rates. In this opinion article,apart from the effectiveness of managing fresh gametes and embryos, we wish to evaluate the efficacy ofthe processes of both freezing and thawing of the produced embryos. The reporting of pregnancy rates ofan IVF unit in the past was primarily laying in the fresh embryo transfer (ET) pregnancy rates. Now withthe most frequent utilization of freeze-all strategy, it does not seem logical to report only on poor prognosispatients as all the good cases are postponed for thawed cycles. Ongoing implementation of the freeze-allstrategy has indicated the need to establish a new representative index that may combine the success ofboth fresh and frozen cycles performed in the same woman; an index that may not be biased by the policyof an IVF center towards or against the freeze-all strategy. This newly proposed index, which is referred toas COMFFETI , describes the optimal way toreport final reproductive outcomes in the present opinion article. The efficacy of  However,there are so many confounding factors that might alterthe achievement of a pregnancy. These factors are eitherpresently unmanageable by medicine, such as the geneticquality of the produced eggs, sperms and the endometrialreceptivity capacity of each individual, or manageableby current technology, such as gonadotrophin capacity,equipment capacity, culture media capacity, air quality,scientist’s expertise and so on , putting the probability of pregnancy with fresh embryo at risk in these poor prognosis patients.Previously, the reports on pregnancy rates of an IVF unit were primarily based on the cases with fresh ET. Now with the frequent utilization of the freeze-all strategy there is a risks of reporting only poor prognosis patients, as all the good cases are postponed for thawed cycles.The rationale for the freeze-all strategy and eventually segmentation of the IVF cycle has developed over the recent years and is based on two pathophysiological facts. The main one is the endometrial receptivity, which is definitely violated during ovarian stimulation due to the supra-physiological levels of the steroid hormones. Devroey et al.  have cleOngoing implementation of the freeze-all strategy has indicated the need to establish a new representative index that may combine the success of both fresh and frozen cycles administered in the same woman, an index that may not be biased by the policy of an IVF center in favor of or against the freeze-all strategy , 9. ThisThe proposed COMFFETI index could be defined as a binomial variable [yes (1) or no (0)] reflecting the achievement of a pregnancy or not per individual (couple) at the end of each stimulated cycle; including fresh ET plus the thawed ET obtained by a single multifollicular ovarian stimulation cycle. This is a radically different index from the widely used index of pregnancy/delivery rate per transfer.The basic difference lies in the fact that by the old way of reporting clinical or ongoing pregnancy per fresh ET reflects the potential of pregnancy achievement following this specific fresh ET , withoutThe value of this new index could be demonstrated in case we examine two theoretical examples of different IVF centers, of which the first is in favor of fresh ET, while the second is practicing the freeze-all technique. Table 1 presents the final outcomes of 10 patients undergoing IVF treatment in an IVF center performing fresh ET. In case there were three deliveries achieved by the first ET with fresh embryos, the traditional index would have been 30%. Table 2 presents the same 10 patients undergoing IVF treatment and their final outcomes in a center taking the freeze-all approach. In case an IVF Unit has as mainstream policy to freeze embryos, the traditional delivery index would be only 20%. For instance case number 5 (due to high response) and case number 10 (due to high follicular progesterone) withheld the fresh transfer and took the freeze-all approach. Therefore, a superficial outcome of the two centers would indicate that centers favoring the fresh ET policy are more successful in achieving clinical pregnancies/deliveries.Our own data support what was mentioned in the examples above about the COMFETI index. Based on the latest 50 cases of fresh embryo transfer cycles, the live birth rate has been 46% for the first embryo transfer, while the COMFFETI index has been 74% after two embryo transfers. Relatively, for the last 50 cases of the freeze-all strategy, the live birth rate was 58% for the first embryo transfer per case while the COMFFETI index was 82%. These results indicate the importance of using an index, which reflects the cumulative results of consecutive embryo transfers, especially in the freeze-all-friendly centers.However, the consideration of pregnancies achieved “at the end of the day” according to COMFFETI index would radically change the situation. COMFFETI index would only have a slight increase from 20 to 30% in the first center, favoring fresh ETs, while in the second example, both cases 5 and 10 might have achieved a delivery with frozen embryos, and thus COMFFETI pregnancy rate could rise up to 60%, representing a totally different clinical outcome with regards to traditional index.The additional great advantage that COMFFETI pregnancy rate provides is that it incorporates the implantation potential of all embryos produced from a single stimulated cycle. On the other hand, a drawback might be that COMFFETI rate is significantly related to the efficacy of cryopreservation techniques and the assumed increased success rates reported from the frozen cycles . TherefoThe basic endpoint of a successful IVF treatment cycle is giving a healthy baby to the mother . The intThe COMFFETI index in reproductive medical practice may be used to give the infertile patients a more subjective view about the realistic possibilities to have a successful IVF cycle from the beginning of the treatment. Moreover, reporting to organizations like European Society of Human Reproduction and Embryology (ESHRE) and Center for Disease Control would become increastingly objective by using the above index as a higher number of centers are moving towards the freeze-all policy, and therefore only poor cases or modest responders would be selected for fresh embryo transfers , 14.


Microcirculation plays an important role in supplying nutrients and oxygen to cells in the tissues. Thus, insufficient supplies of oxygen in microcirculation may lead to tissue hypoxia. Tissue hypoxia is considered to be one of the factors of serious diseases, such as cystic fibrosis and chronic bronchitis.Many imaging techniques for ,,To visualize tissue microcirculation, orthogonal polarization spectral (OPS) imaging was developed as a real-time and noninvasive imaging technique of handheld vital microscopes (HVMs).,in vivo experiments. Second, since we calculate the AECs from the SDF images of the turbid phantom experiments, they are not optimized for in vivo SDF images. Third, heat from the light-emitting diodes (LEDs) used for the SDF imaging might affect hemodynamics in microcirculation since the SDF probe is in direct contact with the tissue. We need to investigate the possible effect and that is the focus of this paper.Based on SDF imaging, we have developed a trial SDF probe and spectral image-based oximetry method, which we called SDF oximetry.in vivo experiment, we recalculated AECs from the literature values. Then, we examined the heat influence of the LEDs. Next, we conducted the experiment with rats where we observed the time-lapsed changes in In this study, we conducted hypoxic stimulation experiments with rats by changing fraction of inspired oxygen  for The Hence, the AECs of We calculated these AECs from the spectral distribution of the extinction coefficients of 2.3,As the first experiment, to ensure the time-lapsed changes in 33.1The hypoxic stimulation experiment, described in Sec. in vivo experiment, we measured the time-lapsed changes in the temperature of the surface of SDF probe. We used a thermometer  with a flexible internal temperature sensor  for temperature measurement. As shown in in vivo experiment to evaluate the stability of SDF oximetry during an 11-min period. Prior to performing the 3.2The hypoxic stimulation experiment in this section was repeated using two rats. We describe results for rat 2 first. As shown in Further, we focused on the difference in response to hypoxia between two blood vessels, which are marked by the red and blue arrows in In order to ensure the reproducibility of the hypoxic stimulation experiment, we repeated it with the other rat (rat 3). The experimental conditions were the same as the first time. Our results showed that SDF oximetry could estimate 4in vivo experiments and observed the time-lapsed SO2 changes in dermis microcirculation of rats under hypoxic stimulation. Before performing the in vivo experiment, we calculated the AECs from the literature values because the AECs used in our previous study were optimized only for turbid phantom experiments. Next, to confirm the time-lapsed stability of SDF oximetry, we obtained SDF images under the normoxia during an 11-min period. The results suggested that hemodynamics in microcirculation were not affected by the heat from the LEDs. Then, we conducted the in vivo experiment with rats to evaluate the sensitivity of SDF oximetry under hypoxic stimulation. This is the first in vivo experiment that observed the changes in In this paper, we have applied our SDF oximetry to We confirmed that the estimated in vivo experiments with a sepsis rat model by following the same procedure as described in our previous work.To raise the reliability of