Polyphosphonium‐based bipolar membranes for rectification of ionic currents     

Erik O. Gabrielsson  Magnus Berggren 《Biomicrofluidics》2013,7(6)

Bipolar membranes (BMs) have interesting applications within the field of bioelectronics, as they may be used to create non-linear ionic components (e.g., ion diodes and transistors), thereby extending the functionality of, otherwise linear, electrophoretic drug delivery devices. However, BM based diodes suffer from a number of limitations, such as narrow voltage operation range and/or high hysteresis. In this work, we circumvent these problems by using a novel polyphosphonium-based BM, which is shown to exhibit improved diode characteristics. We believe that this new type of BM diode will be useful for creating complex addressable ionic circuits for delivery of charged biomolecules.Combined electronic and ionic conduction makes organic electronic materials well suited for bioelectronics applications as a technological mean of translating electronic addressing signals into delivery of chemicals and ions.¹ For complex regulation of functions in cells and tissues, a chemical circuit technology is necessary in order to generate complex and dynamic signal gradients with high spatiotemporal resolution. One approach to achieve a chemical circuit technology is to use bipolar membranes (BMs), which can be used to create the ionic equivalents of diodes^{2, 3, 4, 5} and transistors.^{6, 7, 8} A BM consists of a stack of a cation- and an anion-selective membrane, and functions similar to the semiconductor PN-junction, i.e., it offers ionic current rectification^{9, 10} (Figure ​(Figure1a).1a). The high fixed charge concentration in a BM configuration make them more suited in bioelectronic applications as compared to other non-linear ionic devices, such as diodes constructed from surface charged nanopores¹¹ or nanochannels,¹² as the latter devices typically suffers from reduced performance at elevated electrolyte concentration (i.e., at physiological conditions) due to reduced Debye screening length.¹³ However, a unique property of most BMs, as compared to the electronic PN-junction and other ionic diodes, is the electric field enhanced (EFE) water dissociation effect.^{10, 14} This occurs above a threshold reverse bias voltage, typically around −1 V, as the high electric field across the ion-depleted BM interface accelerates the forward reaction rate of the dissociation of water into H⁺ and OH⁻ ions. As these ions migrate out from the BM, there will be an increase in the reverse bias current. The EFE water dissociation is a very interesting effect and is commonly used in industrial electrodialysis applications,¹⁵ where highly efficient water dissociating BMs are being researched.¹⁶ Also, BMs have also been utilized to generate H⁺ and OH⁻ ions in lab-on-a-chip applications.^{2, 17} However, the EFE water dissociation effect diminishes the diode property of BMs when operated outside the ±1 V window, which is unwanted in, for instance, chemical circuits and addressing matrices for delivery of complex gradients of chemical species. The effect can be suppressed by incorporating a neutral electrolyte inside the BM,^{10, 18} for instance, poly(ethylene glycol) (PEG).^{2, 6, 7} However, as previously reported,² the PEG volume will introduce hysteresis when switching from forward to reverse bias, due to its ability to store large amounts of charges. This was circumvented by ensuring that only H⁺ and OH⁻ are present in the diode, which recombines into water within the PEG volume. Such diodes are well suited as integrated components in chemical circuits for pH-regulation, due to the in situ formed H⁺ and OH⁻, but are less attractive if, for instance, other ions, e.g., biomolecules, are to be processed or delivered in and from the circuit. Furthermore, a PEG electrolyte introduces additional patterning layers, making device downscaling more challenging. This is undesired when designing complex, miniaturized, and large-scale ionic circuits. Thus, there is an interest in BM diodes that intrinsically do not exhibit any EFE water dissociation, are easy to miniaturize, and that turn off at relatively high speeds. It has been suggested that tertiary amines in the BM interface are important for efficient EFE water dissociation,^{19, 20, 21} as they function as a weak base and can therefore catalyze dissociation of water by accepting a proton. For example, anion-selective membranes that have undergone complete methylation, converting all tertiary amines to quaternary amines, shows no EFE water dissociation,¹⁹ although this effect was not permanent, as the quaternization was reversed upon application of a current. Similar results were found for anion-selective membranes containing alkali-metal complexing crown ethers as fixed charges.²¹ Also, EFE water dissociation was not observed or reduced in BMs with poor ion selectivity, for example, in BMs with low fixed-charge concentration⁵ or with predominantly secondary and tertiary amines in the anion-selective membrane,²² as the increased co-ion transport reduces the electric field at the BM interface. However, due to decreased ion selectivity, these membranes show reduced rectification. In this work, we present a non-amine based BM diode that avoids EFE water dissociation, enables easy miniaturization, and provides fast turn-off speeds and high rectification.Open in a separate windowFigure 1(a) Ionic current rectification in a BM. In forward bias, mobile ions migrate towards the interface of the BM. The changing ion selectivity causes ion accumulation, resulting in high ion concentration and high conductivity. At high ion concentration, the selectivity of the membranes fails (Donnan exclusion failure), and ions start to pass the BM. In reverse bias, the mobile ions migrate away from the BM, eventually giving a zone with low ion concentration and low conductivity. Reverse bias can cause EFE water dissociation, producing H⁺ and OH^- ions. (b) Chemical structures of PSS, qPVBC, and PVBPPh₃. (c) The device used to characterize the BMs and the BM1A, BM2A, and BM1P designs. The BM interfaces are 50 × 50 μm.An anion-selective phosphonium-based polycation (poly(vinylbenzyl chloride) (PVBC) quaternized by triphenylphospine, PVBPPh₃) was synthesized and compared to the ammonium-based polycation (PVBC quaternized by dimethylbenzylamine, qPVBC) previously used in BM diodes² and transistors,^{7, 8} when included in BM diode structures together with polystyrenesulfonate (PSS) as the cation-selective material (Figure ​(Figure1b).1b). Three types of BM diodes were fabricated using standard photolithography patterning (Figure ​(Figure1c),1c), either with qPVBC (BM1A and BM2A) or PVBPPh₃ (BM1P) as polycation and either with (BM2A) or without PEG (BM1A and BM1P). Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) electrodes covered with aqueous electrolytes were used to convert electronic input signals into ionic currents through the BMs, according to the redox reaction PEDOT⁺:PSS⁻ + M⁺ + e⁻ ↔ PEDOT⁰ + M⁺:PSS⁻.The rectifying behavior of the diodes was evaluated using linear sweep voltammetry (Figure ​(Figure2).2). The BM1A diode exhibited an increase in the reverse bias current for voltages lower than −1 V, a typical signature of EFE water dissociation,^{10, 14} which decreased the current rectification at ±4 V to 6.14. No such deviation in the reverse bias current was observed for BM2A and BM1P, which showed rectification ratios of 751 and 196, respectively. In fact, for BM1P, no evident EFE water dissociation was observed even at −40 V (see inset of Figure ​Figure2).2). Thus, the PVBPPh₃ polycation allows BM diodes to operate at voltages beyond the ±1 V window with maintained high ion current rectification.Open in a separate windowFigure 2Linear sweep voltammetry from −4 to +4 V (25 mV/s) for the BM diodes. The inset shows BM1P scanning from −40 V to +4 V (250 mV/s).The dynamic performance of the diodes was tested by applying a square wave pulse from reverse bias to a forward bias voltage of 4 V with 5–90 s pulse duration (Figure ​(Figure3).3). To access the amount of charge injected and extracted during the forward bias and subsequent turn off, the current through the device was integrated. For BM2A, we observed that the fall time increased with the duration of the forward bias pulse. This hysteresis is due to the efficient storage of ions in the large PEG volume, with no ions crossing the BM due to Donnan exclusion failure.² As a result, during the initial period of the return to reverse bias, a large amount of charge needs to be extracted in order to deplete the BM. After a 90 s pulse, 90.6% of the injected charge during the forward bias was extracted before turn-off. This may be contrasted with BM1P, where the fall time is hardly affected by the pulse duration, and the extracted/injected ratio is only 15.4% for a 90 s pulse. For this type of BM, the interface quickly becomes saturated with ions during forward bias, leading to Donnan exclusion failure and transport of ions across the BM.⁴ Thus, less charge needs to be extracted to deplete the BM, allowing for faster fall times and significantly reduced hysteresis.Open in a separate windowFigure 3Switching characteristics (5, 10, 20, 30, 60, or 90 s pulse) and ion accumulation (at 90 s pulse) of the BM2A and BM1P diodes. BM1A showed similar characteristics as BM1P when switched at ±1V (see supplementary material).²⁴Since the neutral electrolyte is no longer required to obtain high ion current rectification over a wide potential range, the size of the BM can be miniaturized. This translates into higher component density when integrating the BM diode into ionic/chemical circuits. A miniaturized BM1P diode was constructed, where the interface of the BM was shrunk from 50 μm to 10 μm. The 10 μm device showed similar IV and switching characteristics as before (Figure ​(Figure4),4), but with higher ion current rectification ratio (over 800) and decreased rise/fall times (corresponding to 90%/–10% of forward bias steady state) from 10 s/12.5 s to 4 s/4 s. Since the overlap area is smaller, a probable reason for the faster switching times is the reduced amount of ions needed to saturate and deplete the BM interface. In comparison to our previous reported low hysteresis BM diode,² this miniaturized polyphosphonium-based devices shows the same rise and fall times but increased rectification ratio.Open in a separate windowFigure 4(a) Linear sweep voltammetry and (b) switching performance of a BM1P diode with narrow junction.In summary, by using polyphosphonium instead of polyammonium as the polycation in BM ion diodes the EFE water dissociation can be entirely suppressed over a large operational voltage window, supporting the theory that a weak base, such as a tertiary amine, is needed for efficient EFE water dissociation.^{17, 18} As no additional neutral layer at the BM interface is needed, ion diodes that operate outside the usual EFE water dissociation window of ±1 V can be constructed using less active layers, fewer processing steps and with relaxed alignment requirement as compared to polyammonium-based devices. This enables the fabrication of ion rectification devices with an active interface as low as 10 μm. Furthermore, the exclusion of a neutral layer improves the overall dynamic performance of the BM ion diode significantly, as there is less hysteresis due to ion accumulation. Previously, the hysteresis of BM ion diodes has been mitigated by designing the diode so that only H⁺ and OH⁻ enters the BM, which then recombines into water.² Such diodes also show high ion current rectification ratio and switching speed but are more complex to manufacture, and their application in organic bioelectronic systems is limited due to the H⁺/OH⁻ production. By instead using the polyphosphonium-based BM diode, reported here, we foresee ionic, complex, and miniaturized circuits that can include charged biomolecules as the signal carrier to regulate functions and the physiology in cell systems, such as in biomolecule and drug delivery applications, and also in lab-on-a-chip applications.  相似文献   

Accelerometer-measured daily physical activity related to aerobic fitness in children and adolescents     

Magnus Dencker  Lars B. Andersen 《Journal of sports sciences》2013,31(9):887-895

Abstract

Maximum oxygen uptake ([Vdot]O_2PEAK) is generally considered to be the best single marker for aerobic fitness. While a positive relationship between daily physical activity and aerobic fitness has been established in adults, the relationship appears less clear in children and adolescents. The purpose of this paper is to summarise recently published data on the relationship between daily physical activity, as measured by accelerometers, and [Vdot]O_2PEAK in children and adolescents. A PubMed search was performed on 29 October 2010 to identify relevant articles. Studies were considered relevant if they included measurement of daily physical activity by accelerometry and related to a [Vdot]O_2PEAK either measured directly at a maximal exercise test or estimated from maximal power output. A total of nine studies were identified, with a total number of 6116 children and adolescents investigated. Most studies reported a low-to-moderate relationship (r = 0.10–0.45) between objectively measured daily physical activity and [Vdot]O_2PEAK. No conclusive evidence exists that physical activity of higher intensities are more closely related to [Vdot]O_2PEAK, than lower intensities.  相似文献   

Scaling of upper-body power output to predict time-trial roller skiing performance     

Tomas Carlsson  Magnus Carlsson  Daniel Hammarström  Christer Malm  Michail Tonkonogi 《Journal of sports sciences》2013,31(6):582-588

Abstract

The purpose of the present study was to establish the most appropriate allometric model to predict mean skiing speed during a double-poling roller skiing time-trial using scaling of upper-body power output. Forty-five Swedish junior cross-country skiers (27 men and 18 women) of national and international standard were examined. The skiers, who had a body mass (m) of 69.3 ± 8.0 kg (mean ± s), completed a 120-s double-poling test on a ski ergometer to determine their mean upper-body power output (W). Performance data were subsequently obtained from a 2-km time-trial, using the double-poling technique, to establish mean roller skiing speed. A proportional allometric model was used to predict skiing speed. The optimal model was found to be: Skiing speed = 1.057 · W ^0.556 · m ^?0.315, which explained 58.8% of the variance in mean skiing speed (P < 0.001). The 95% confidence intervals for the scaling factors ranged from 0.391 to 0.721 for W and from ?0.626 to ?0.004 for m. The results in this study suggest that allometric scaling of upper-body power output is preferable for the prediction of performance of junior cross-country skiers rather than absolute expression or simple ratio-standard scaling of upper-body power output.  相似文献   

An improvement index to quantify the evolution of performance in running     

Steve J. Haake  David M. James 《Journal of sports sciences》2013,31(7):610-622

Abstract

Improvements in track and field sports have been attributed to factors such as population increase, drugs and new technologies, but previous research has found it difficult to distinguish the contributions from specific influences. Here it is shown how this is possible by means of a performance improvement index based on useful work done combined with modelling of the annual top 25 performances. The index was set to 100 in 1948 and showed that, by 2012, it had increased in running events to between 110.5 and 146.7 (men’s 100 m and marathon). Underlying global effects accounted for the majority of all improvements (16.2 to 46.7) with smaller influences attributable to an influx of African runners (3.6 to 9.3), and a 4 -year oscillation that arose from staging of the Olympic Games (±0.2 to ±0.6). Performance decreased with the introduction of compulsory random drug testing (?0.9 to ?3.9) the World Anti-Doping Agency (WADA; ?0.5 to ?2.5) and fully automated timing (?0.6 to ?2.5). Changes in elite sporting performance since the 1890s are attributable to societal changes caused by the industrial revolution and globalisation superimposed on millennia of human evolution.  相似文献   

Creating entrepreneurial networks: academic entrepreneurship, mobility and collaboration during PhD education     

Dzamila Bienkowska  Magnus Klofsten 《Higher Education》2012,64(2):207-222

Network-building activities of PhD students are an important area of study in furthering our understanding of academic entrepreneurship. This paper focuses on PhD students’ participation in network-building activities defined as mobility and collaboration, as well as own interest in and perceived grade of support for commercialisation from various levels of the university hierarchy. The results of a large-scale survey (of 1,126 PhD students at Link?ping University, Sweden, 41% response rate) presented here show that the majority of PhD students are engaged in collaborations with external organisations, though quite few (one quarter) have spent a part of their PhD education outside their home university. PhD students from all faculties are on average interested in commercialisation and in favour of it. However, PhD students from the faculty of Health Sciences state that it is difficult for them to combine research and commercialisation. Furthermore, interest in commercialisation of research results is relatively lowest amongst those PhD students who are undertaking mobility placements at other universities, thus pointing to an experienced incompatibility of research and academic entrepreneurship.  相似文献   

Some problems encountered by disabled students at the University of Oslo – whose responsibility?     

Solveig Magnus Reindal 《欧洲特需教育杂志》2013,28(3):227-241

ABSTRACT

A survey was undertaken among students at the University of Oslo in order to include their comments in a recommendation that was being prepared to elucidate the disabled students' situation in the future. Students were requested to write shorter or longer comments concerning their day as a disabled student, in order to expose barriers, weaknesses in the organization of services and other concerns. They were also requested to make recommendations as to improvements. The students’ answers indicated that they encountered several practical problems in their education, including a lack of understanding and cooperation from administrators, faculty staff and lecturers, a lack of adaptive aids and other resources and inaccessibility of grounds and buildings. Students made recommendations to improve the delivery of services. On the background of the information given by the students, the paper reflects on the issue of higher education's responsibility in relation to adjustments. The Reform‐94 within upper secondary education makes the issue of responsibility even more current.  相似文献   

From Identity to Facility—The New Buildings for the Faculty of Humanities at the University of Copenhagen     

Magnus Gabrielsen 《Scandinavian Journal of Educational Research》2013,57(5):531-546

This analysis seeks to unite both an architectural and an educational perspective on the transformation of the university into a postmodern institution. Our point of departure is an analysis of the new buildings for the Faculty of Humanities University of Copenhagen which are located on the island area of Amager, to the south of Copenhagen's centre. To put the transformation into perspective we will also consider the respective older buildings that house the universities of Copenhagen and Oslo. The analysis will illustrate how the university as an institution has changed in terms of characteristics, position in society and its understanding of itself. Previously, the university was a clearly defined institution, which had both the process of Bildung (liberal education) and the production of objective knowledge as its goal. Today's university has become a institution, which seeks to develop competence rather than Bildung and to facilitate knowledge in the Network Society. These changes can be explicated by the French philosopher Lyotard's analysis of the position of knowledge within the postmodern sphere.  相似文献   

Older adults’ experiences of a reablement process. “To be treated like an adult,and ask for what I want and how I want it”     

Gunnel Östlund  Viktoria Zander  Magnus Lars Elfström  Lena-Karin Gustavsson  Els-Marie Anbäcken 《Educational gerontology》2013,39(8):519-529

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Parental governmentality: involving ‘immigrant parents’ in Swedish schools     

Magnus Dahlstedt 《British Journal of Sociology of Education》2009,30(2):193-205

In Sweden, calls for partnership between state institutions and local communities punctuate discussions of a number of areas of public policy. In this article, the discourse of partnership is analyzed in recent developments in Swedish educational policy, and particularly the involvement of ‘immigrant parents’ as partners collaborating with the school. In the article it is argued that, in partnerships between the school and ‘immigrant parents’, the ‘rules of the game’ are most often dictated by one of the partners (i.e. the Swedish school). Here, ‘immigrant parents’ are by various techniques being ‘measured’ and exhorted to adapt to an imagined ‘Swedish normality’, in order to become a ‘responsible’ parent and equal partner.  相似文献   

Doing Leadership in Higher Education: The gendering process of leader identity development     

Haake  Ulrika 《Tertiary Education and Management》2009,15(4):291-304

Tertiary Education and Management - How leadership is done in higher education is analysed through a longitudinal interview study among the heads of department at a Swedish university. The focus is...  相似文献