Z. Li, K. Mathwig, O. A. Arotiba, L. Tshwenya , E. B. Carneiro-Neto, E. C. Pereira, F. Marken, Curr. Opin. Electrochem. 39 (2023) 101280. [link, pdf]

Ionic diodes have emerged repeatedly in the literature for gel interfaces, for nanopores and channels, for nano-/micro-fluidic systems, and for asymmetrically ionomer-covered microholes. Concentration polarisation is likely to be the key to understanding the diode function and the diode time constant τ_diode, i.e. the time for approaching steady state following a potential/polarity switch. For frequencies higher than ω_diode = 2πf = (τ_diode)^-1, the polarization mechanism is too slow for ion current rectification. Below the frequency associated with the diode time constant, irreversibility in ion flow is induced and the diode switches between two resistive states at opposite potentials (“open” and “closed”). The irreversible flow of ions allows energy conversion from electrical to electrochemical. For energy conversion, two coupled ionic diodes are necessary driven by alternating current (AC) electricity to minimise driver electrode electrolysis and energy losses. Opportunities for AC-desalination and for electroosmotic water harvesting with coupled ionic diodes are discussed.

P. de Haan, K. Mathwig, L. Yuan, B. W. Peterson, E. Verpoorte, Organs-on-a-Chip 5 (2023) 100026. [link, pdf]

Microperforated membranes are essential components of various organ-on-a-chip (OOC) barrier models developed to study transport of molecular compounds and cells across cell layers in e.g. the intestine and blood-brain barrier. These OOC membranes have two functions: 1) to support growth of cells on one or both sides, and 2) to act as a filter-like barrier to separate adjacent compartments. Thin, microperforated poly(dimethylsiloxane) (PDMS) membranes can be fabricated by micromolding from silicon molds comprising arrays of micropillars for the formation of micropores. However, these molds are made by deep reactive ion etching (DRIE) and are expensive to fabricate. We describe the micromolding of thin PDMS membranes with easier-to-make SU-8 epoxy photoresist molds. With a multilayer, SU-8, pillar microarray mold, massively parallel arrays of micropores can be formed in a thin layer of PDMS, resulting in a flexible barrier membrane that can be easily incorporated and sealed between other layers making up the OOC device. The membranes we describe here have a 30-μm thickness, with 12-μm-diameter circular pores arranged at a 100-μm pitch in a square array. We show application of these membranes in gut-on-a-chip devices, and expect that the reported fabrication strategy will also be suitable for other membrane dimensions.

V. Khandan, V. Boerkamp, A. Jabermoradi, M. Fontana, J. Hohlbein, E. Verpoorte, R. C. Chiechi, K. Mathwig, arXiv:2212.11503 [physics.flu-dyn]. [link, pdf]

Viscosity is a fundamental property of liquids. It determines transport and diffusion of particles in solution. Nonetheless, it is an open question how a gradient of viscosity – causing a gradient in diffusivity – can lead to viscophoretic transport, i.e., directed transport of particles and molecules in solution. Here, we determine viscophoretic drift experimentally. We generate steep, stable viscosity gradients in a microfluidic device and image transport of suspended nanoparticles in these gradients using high-resolution microscopy. We observe high viscophoretic drift velocities which significantly exceed theoretical predictions. In addition, we demonstrate a new method for trapping and concentrating particles by using the interplay of viscophoresis and diffusiophoresis. We believe that a quantification of viscophoresis will advance the understanding and application of transport processes of gradients of viscosity occurring in very diverse fields such as cell biology, chromatography, and membrane technology.

P. A. Goldsteen, A. M. Sabogal Guaqueta, P. P. M. F. A. Mulder, I. S. T. Bos, M. Eggens, L. Van der Koog, J. T. Soeiro, A. J. Halayko, K. Mathwig, L. E. M. Kistemaker, E. M. J. Verpoorte, A. M. Dolga, R. Gosens, Frontiers Pharmacology 13 (2022) 991072, specialty section Respiratory Pharmacology. [link, pdf]

Airway cholinergic nerves play a key role in airway physiology and disease. In asthma and other diseases of the respiratory tract, airway cholinergic neurons undergo plasticity and contribute to airway hyperresponsiveness and mucus secretion. We currently lack human in vitro models for airway cholinergic neurons. Here, we aimed to develop a human in vitro model for peripheral cholinergic neurons using human pluripotent stem cell (hPSC) technology. hPSCs were differentiated towards vagal neural crest precursors and subsequently directed towards functional airway cholinergic neurons using the neurotrophin brain-derived neurotrophic factor (BDNF). Cholinergic neurons were characterized by ChAT and VAChT expression, and responded to chemical stimulation with changes in Ca²⁺ mobilization. To culture these cells, allowing axonal separation from the neuronal cell bodies, a two-compartment PDMS microfluidic chip was subsequently fabricated. The two compartments were connected via microchannels to enable axonal outgrowth. On-chip cell culture did not compromise phenotypical characteristics of the cells compared to standard culture plates. When the hPSC-derived peripheral cholinergic neurons were cultured in the chip, axonal outgrowth was visible, while the somal bodies of the neurons were confined to their compartment. Neurons formed contacts with airway smooth muscle cells cultured in the axonal compartment. The microfluidic chip developed in this study represents a human in vitro platform to model neuro-effector interactions in the airways that may be used for mechanistic studies into neuroplasticity in asthma and other lung diseases.

Z. Li, T. Pang. J. Shen, P. J. Fletcher, K. Mathwig, F. Marken, Micro and Nano Engineering 16 (2022) 100157. [link, pdf]

Microscale ionic rectifier effects are commonly observed in devices based on semipermeable ionomer coated on an array of microholes with potential applications in alternating current (AC) driven desalination and/or electroosmotic pumping. The efficiency of devices is dependent on ionic diode switching speed, the rectification ratio, and the design of materials and the ionic circuit. Here, a new circuit is proposed based on coupling in parallel (i) a cationic diode based on the cation conductor Nafion and (ii) an anionic diode based on the anion conducting Sustainion. With an alternating driving voltage, a net desalination effect is observed without any moving parts and without significant side reactions. Experimentally, a 4-electrode configuration and a 2-electrode configuration are compared. The ionic diode desalination system is shown to work with only two carbon mat driver electrodes, but the performance in particular at higher ionic strengths (>10 mM) still needs to be improved. Based on the experimental prototype, the current/power efficiency are investigated and challenges for future improvements are discussed. 

J. Mestres, F. Leonardi, K. Mathwig, Micromachines 13 (2022) 362. [link, pdf]

Electrochemical sensors are powerful tools for the detection and real-time monitoring of a wide variety of analytes. However, the long-term operation of Faradaic sensors in complex media is challenging due to fouling. The protection of the electrode surface during in vivo operation is a key element for improving the monitoring of analytes. Here, we study different EUDRAGIT^® controlled release acrylate copolymers for protecting electrode surfaces. The dissolution of these polymers—namely EUDRAGIT^® L 30 D-55 and EUDRAGIT^® FS 30 D—is triggered by a change in pH of the environment, and it is electrochemically monitored by detecting electrode access by means of a redox probe. The full dissolution of the polymer is achieved within 30 min and the electrode response indicates a complete recovery of the original electrochemical performance. We demonstrate that amperometric sensing is a practical and straightforward technique for real-time and in situ sensing of EUDRAGIT^® dissolution profiles. It will find future applications in determining the protection of polymer electrode coating in real matrices and in vivo applications. 

M. Fontana, Š. Ivanovaitė, S. Lindhoud, E. van der Wijk, K. Mathwig, W. van den Berg, D. Weijers, J. Hohlbein, Advanced Biology 6 (2022) 2100953. [link, pdf]

Single-molecule fluorescence detection offers powerful ways to study biomolecules and their complex interactions. Here, nanofluidic devices and camera-based, single-molecule Förster resonance energy transfer (smFRET) detection are combined to study the interactions between plant transcription factors of the auxin response factor (ARF) family and DNA oligonucleotides that contain target DNA response elements. In particular, it is shown that the binding of the unlabeled ARF DNA binding domain (ARF-DBD) to donor and acceptor labeled DNA oligonucleotides can be detected by changes in the FRET efficiency and changes in the diffusion coefficient of the DNA. In addition, this data on fluorescently labeled ARF-DBDs suggest that, at nanomolar concentrations, ARF-DBDs are exclusively present as monomers. In general, the fluidic framework of freely diffusing molecules minimizes potential surface-induced artifacts, enables high-throughput measurements, and proved to be instrumental in shedding more light on the interactions between ARF-DBDs monomers and between ARF-DBDs and their DNA response element

Z. Li, R. Malpass-Evans, N. B. McKeown, M. Carta, K. Mathwig, J. P. Lowe, F. Marken, Electrochemistry Communications 103 (2021) 107110. [link, pdf]

Tertiary-amine-based Polymers of Intrinsic Microporosity (PIMs) provide a class of highly porous molecularly rigid materials for the electrochemical transport of both ionic and neutral species. Here, the transport of water molecules together with chloride anions (i.e. the electroosmotic drag coefficient) is studied for the intrinsically microporous polyamine PIM-EA-TB immersed in aqueous 0.01 M NaCl (i) when protonated for pH < 4 or (ii) when not protonated for pH > 4. Preliminary data suggest that in both cases a high electroosmotic drag coefficient is observed based on direct H₂O transport into a D₂O-filled compartment (quantified by ¹H-NMR). For PIM-EA-TB there is a strong pH dependence with a higher electroosmotic drag coefficient in less acidic solutions (going from approx. 400 H₂O per anion at pH 3 to approx. 4000 H₂O per anion at pH 7), although the underlying absolute rate of water transport at a fixed voltage of −1 V appears to be essentially pH independent. Water transport through the PIM-EA-TB microchannels is rationalised based on the relative populations of chloride anions and of water in the micropores (essentially a ‘piston’ mechanism).

A. K. Thompson, K. Mathwig, P. J. Fletcher, R. Castaing and F. Marken, J. Solid State Electrochem. 25 (2021) 2881. [link, pdf]

Commercial resin microbeads are widely applied in ion exchange and extraction. Here, a single anion-selective and phosphate binding resin microbead (FerrIX™) is mounted into an epoxy membrane and investigated by 4-electrode membrane voltammetry and membrane impedance spectroscopy. Anion transport properties are observed to dominate associated with three distinct potential domains: (I) a low bias ohmic potential domain (dominant at high electrolyte concentration), (II) a concentration polarisation potential domain, and (III) an over-limiting potential domain. Voltammetric responses show transient diffusion-migration features at higher scan rates and quasi-steady state features at lower scan rates. Inherent microbead conductivity is shown to be linked to two resistive elements, electrolyte concentration dependent and independent, in series. The effects of phosphate binding are revealed as transient pattern in impedance spectroscopy data. Preliminary data suggest phosphate concentration-dependent peak features in the imaginary impedance versus frequency plot due to phosphate binding into the microbead.

Z. Li, L. Wang, R. Malpass-Evans, M. Carta, N. B. McKeown, K. Mathwig, P. J. Fletcher and F. Marken, ChemElectroChem 8 (2021) 2044. Invited contribution to the Marcin Opałło Festschrift. [link, pdf]

The polymer of intrinsic microporosity PIM‐EA‐TB provides a molecularly rigid micropore structure containing tertiary amine sites and is shown here to interact with hydrogen bonding guest molecules such as caffeic acid. Voltammetric data with a PIM‐EA‐TB film on glassy carbon electrodes show that in both acidic solution (pH 2; PIM‐EA‐TB is protonated) and in neutral solution (pH 6; PIM‐EA‐TB is not protonated) caffeic acid is slowly accumulated into the microporous host. Binding constants are estimated and suggested to be linked to hydrogen bonding causing accumulation of caffeic acid. When employing PIM‐EA‐TB as an asymmetric membrane coated onto a 5 mm thick Teflon support film with 10 mm diameter microholes (using either a single microhole or a 10 × 10 array of microholes), binding of caffeic acid is shown to cause a modulation of the ionic current without affecting the pH‐dependent ionic diode behaviour. Two complementary types of effects of caffeic acid guests are discussed based on blocking anion diffusion pathways and based on removal of positive charges. The caffeic acid transport mechanism/efficiency is investigated in view of selective molecular pumping.