The Role of Side Chains and Hydration on Mixed Charge Transport in n-Type Polymer Films

Introducing ethylene glycol (EG) side chains to a conjugated polymer backbone is a well-established synthetic strategy for designing organic mixed ion-electron conductors (OMIECs). However, the impact that film swelling has on mixed conduction properties has yet to be scoped, particularly for electron-transporting (n-type) OMIECs. Here, the authors investigate the effect of the length of branched EG chains on mixed charge transport of n-type OMIECs based on a naphthalene-1,4,5,8-tetracarboxylic-diimide-bithiophene backbone. Atomic force microscopy (AFM), grazing-incidence wide-angle X-ray scattering (GIWAXS), and scanning tunneling microscopy (STM) are used to establish the similarities between the common-backbone films in dry conditions. Electrochemical quartz crystal microbalance with dissipation monitoring (EQCM-D) and in situ GIWAXS measurements reveal stark changes in film swelling properties and microstructure during electrochemical doping, depending on the side chain length. It is found that even in the loss of the crystallite content upon contact with the aqueous electrolyte, the films can effectively transport charges and that it is rather the high water content that harms the electronic interconnectivity within the OMIEC films. These results highlight the importance of controlling water uptake in the films to impede charge transport in n-type electrochemical devices.

Complementary integration of organic electrochemical transistors for front-end amplifier circuits of flexible neural implants

The ability to amplify, translate, and process small ionic potential fluctuations of neural processes directly at the recording site is essential to improve the performance of neural implants. Organic front-end analog electronics are ideal for this application, allowing for minimally invasive amplifiers owing to their tissue-like mechanical properties. Here, we demonstrate fully organic complementary circuits by pairing depletion- and enhancement-mode p- and n-type organic electrochemical transistors (OECTs). With precise geometry tuning and a vertical device architecture, we achieve overlapping output characteristics and integrate them into amplifiers with single neuronal dimensions (20 micrometers). Amplifiers with combined p- and n-OECTs result in voltage-to-voltage amplification with a gain of >30 decibels. We also leverage depletion and enhancement-mode p-OECTs with matching characteristics to demonstrate a differential recording capability with high common mode rejection rate (>60 decibels). Integrating OECT-based front-end amplifiers into a flexible shank form factor enables single-neuron recording in the mouse cortex with on-site filtering and amplification.

A Cross-linked n-Type Conjugated Polymer with Polar Side Chains Enables Ultrafast Pseudocapacitive Energy Storage

Pseudocapacitors bridge the performance gap between batteries and electric double-layer capacitors by storing energy via a combination of fast surface/near-surface Faradaic redox processes and electrical double-layer capacitance. Organic semiconductors are an emerging class of pseudocapacitive materials that benefit from facile synthetic tunability and mixed ionic-electronic conduction. Reported examples are mostly limited to p-type (electron-donating) conjugated polymers, while n-type (electron-accepting) examples remain comparatively underexplored. This work introduces a new cross-linked n-type conjugated polymer, spiro-NDI-N, strategically designed with polar tertiary amine side chains. This molecular design aims to synergistically increase the electroactive surface area and boost ion transport for efficient ionic-electronic coupling. Spiro-NDI-N demonstrates excellent pseudocapacitive energy storage performance in pH-neutral aqueous electrolytes, with specific capacitance values of up to 532 F g-1 at 5 A g-1 and stable cycling over 5000 cycles. Moreover, it maintains a rate capability of 307 F g-1 at 350 A g-1 . The superior pseudocapacitive performance of spiro-NDI-N, compared to strategically designed structural analogues lacking either the cross-linked backbone or polar side chains, validates the essential role of its molecular design elements. More broadly, the design and performance of spiro-NDI-N provide a novel strategy for developing high-performance organic pseudocapacitors.

Flexible switch matrix addressable electrode arrays with organic electrochemical transistor and pn diode technology

Due to their effective ionic-to-electronic signal conversion and mechanical flexibility, organic neural implants hold considerable promise for biocompatible neural interfaces. Current approaches are, however, primarily limited to passive electrodes due to a lack of circuit components to realize complex active circuits at the front-end. Here, we introduce a p-n organic electrochemical diode using complementary p- and n-type conducting polymer films embedded in a 15-μm -diameter vertical stack. Leveraging the efficient motion of encapsulated cations inside this polymer stack and the opposite doping mechanisms of the constituent polymers, we demonstrate high current rectification ratios ([Formula: see text]) and fast switching speeds (230 μs). We integrate p-n organic electrochemical diodes with organic electrochemical transistors in the front-end pixel of a recording array. This configuration facilitates the access of organic electrochemical transistor output currents within a large network operating in the same electrolyte, while minimizing crosstalk from neighboring elements due to minimized reverse-biased leakage. Furthermore, we use these devices to fabricate time-division-multiplexed amplifier arrays. Lastly, we show that, when fabricated in a shank format, this technology enables the multiplexing of amplified local field potentials directly in the active recording pixel (26-μm diameter) in a minimally invasive form factor with shank cross-sectional dimensions of only 50×8 [Formula: see text].

Conjugated Polyelectrolyte Thin Films for Pseudocapacitive Applications

A subclass of organic semiconductors known as conjugated polyelectrolytes (CPEs) is characterized by a conjugated backbone with ionic pendant groups. The water solubility of CPEs typically hinders applications of thin films in aqueous media. Herein, it is reported that films of an anionic CPE, namely CPE-K, drop cast from water produces single-component solid-state pseudocapacitive electrodes that are insoluble in aqueous electrolyte. That X-ray diffraction experiments reveal a more structurally ordered film, relative to the as-obtained powder from chemical synthesis, and dynamic light scattering measurements show an increase in aggregate particle size with increasing [KCl] indicate that CPE-K films are insoluble because of tight interchain contacts and electrostatic screening by the electrolyte. CPE-K film electrodes can maintain 85% of their original capacitance (84 F g-1 ) at 500 A g-1 and exhibit excellent cycling stability, where a capacitance retention of 93% after 100 000 cycles at a current density of 35 A g-1 . These findings demonstrate that it is possible to use initially water soluble ionic-organic materials in aqueous electrolytes, by increasing the electrolyte concentration. This strategy can be applied to the application of conjugated polyelectrolytes in batteries, organic electrochemical transistors, and electrochemical sensors, where fast electron and ion transport are required.

SpyDirect: A Novel Biofunctionalization Method for High Stability and Longevity of Electronic Biosensors

Electronic immunosensors are indispensable tools for diagnostics, particularly in scenarios demanding immediate results. Conventionally, these sensors rely on the chemical immobilization of antibodies onto electrodes. However, globular proteins tend to adsorb and unfold on these surfaces. Therefore, self-assembled monolayers (SAMs) of thiolated alkyl molecules are commonly used for indirect gold-antibody coupling. Here, a limitation associated with SAMs is revealed, wherein they curtail the longevity of protein sensors, particularly when integrated into the state-of-the-art transducer of organic bioelectronics-the organic electrochemical transistor. The SpyDirect method is introduced, generating an ultrahigh-density array of oriented nanobody receptors stably linked to the gold electrode without any SAMs. It is accomplished by directly coupling cysteine-terminated and orientation-optimized spyTag peptides, onto which nanobody-spyCatcher fusion proteins are autocatalytically attached, yielding a dense and uniform biorecognition layer. The structure-guided design optimizes the conformation and packing of flexibly tethered nanobodies. This biolayer enhances shelf-life and reduces background noise in various complex media. SpyDirect functionalization is faster and easier than SAM-based methods and does not necessitate organic solvents, rendering the sensors eco-friendly, accessible, and amenable to scalability. SpyDirect represents a broadly applicable biofunctionalization method for enhancing the cost-effectiveness, sustainability, and longevity of electronic biosensors, all without compromising sensitivity.

Recyclable Conjugated Polyelectrolyte Hydrogels for Pseudocapacitor Fabrication

In alignment with widespread interest in carbon neutralization and sustainable practices, we disclose that conjugated polyelectrolyte (CPE) hydrogels are a type of recyclable, electrochemically stable, and environmentally friendly pseudocapacitive material for energy storage applications. By leveraging ionic-electronic coupling in a relatively fluid medium, one finds that hydrogels prepared using a fresh batch of an anionic CPE, namely, Pris-CPE-K, exhibit a specific capacitance of 32.6 ± 6.6 F g-1 in 2 M NaCl and are capable of 80% (26.1 ± 6.5 F g-1) capacitance retention after 100,000 galvanostatic charge-discharge (GCD) cycles at a current density (J) of 10 A g-1. We note that equilibration under a constant potential prior to GCD analysis leads to the K+ counterions in the CPE exchanging with Na+ and, thus, the relevant active material Pris-CPE-Na. It is possible to remove the CPE material from the electrochemical cell via extraction with water and to carry out a simple purification through dialysis to produce a recycled material, namely Re-CPE-Na. The recycling workup has no significant detrimental impact on the electrochemical performance. Specifically, Re-CPE-Na hydrogels display an initial specific capacitance of 26.3 ± 1.2 F g-1 (at 10 A g-1) and retain 77% of the capacitance after a subsequent 100,000 GCD cycles. Characterization by NMR, FTIR, and Raman spectroscopies, together with XPS and GPC measurements, revealed no change in the structure of the backbone or side chains. However, rheological measurements gave evidence of a slight loss in G' and G''. Overall, that CPE hydrogels display recyclability argues in favor of considering them as a novel materials platform for energy storage applications within an economically viable circular recycling strategy.

Dihydrofolate reductase activity controls neurogenic transitions in the developing neocortex

One-carbon/folate (1C) metabolism supplies methyl groups required for DNA and histone methylation, and is involved in the maintenance of self-renewal in stem cells. Dihydrofolate reductase (DHFR), a key enzyme in 1C metabolism, is highly expressed in human and mouse neural progenitors at the early stages of neocortical development. Here, we have investigated the role of DHFR in the developing neocortex and report that reducing its activity in human neural organoids and mouse embryonic neocortex accelerates indirect neurogenesis, thereby affecting neuronal composition of the neocortex. Furthermore, we show that decreasing DHFR activity in neural progenitors leads to a reduction in one-carbon/folate metabolites and correlates with modifications of H3K4me3 levels. Our findings reveal an unanticipated role for DHFR in controlling specific steps of neocortex development and indicate that variations in 1C metabolic cues impact cell fate transitions.

A single n-type semiconducting polymer-based photo-electrochemical transistor

Conjugated polymer films, which can conduct both ionic and electronic charges, are central to building soft electronic sensors and actuators. Despite the possible interplay between light absorption and the mixed conductivity of these materials in aqueous biological media, no single polymer film has been utilized to create a solar-switchable organic bioelectronic circuit that relies on a fully reversible and redox reaction-free potentiometric photodetection and current modulation. Here we demonstrate that the absorption of light by an electron and cation-transporting polymer film reversibly modulates its electrochemical potential and conductivity in an aqueous electrolyte, which is harnessed to design an n-type photo-electrochemical transistor (n-OPECT). By controlling the intensity of light incident on the n-type polymeric gate electrode, we generate transistor output characteristics that mimic the modulation of the polymeric channel current achieved through gate voltage control. The micron-scale n-OPECT exhibits a high signal-to-noise ratio and an excellent sensitivity to low light intensities. We demonstrate three direct applications of the n-OPECT, i.e., a photoplethysmogram recorder, a light-controlled inverter circuit, and a light-gated artificial synapse, underscoring the suitability of this platform for a myriad of biomedical applications that involve light intensity changes.

Interactions of Catalytic Enzymes with n-Type Polymers for High-Performance Metabolite Sensors

The tight regulation of the glucose concentration in the body is crucial for balanced physiological function. We developed an electrochemical transistor comprising an n-type conjugated polymer film in contact with a catalytic enzyme for sensitive and selective glucose detection in bodily fluids. Despite the promise of these sensors, the property of the polymer that led to such high performance has remained unknown, with charge transport being the only characteristic under focus. Here, we studied the impact of the polymer chemical structure on film surface properties and enzyme adsorption behavior using a combination of physiochemical characterization methods and correlated our findings with the resulting sensor performance. We developed five n-type polymers bearing the same backbone with side chains differing in polarity and charge. We found that the nature of the side chains modulated the film surface properties, dictating the extent of interactions between the enzyme and the polymer film. Quartz crystal microbalance with dissipation monitoring studies showed that hydrophobic surfaces retained more enzymes in a densely packed arrangement, while hydrophilic surfaces captured fewer enzymes in a flattened conformation. X-ray photoelectron spectroscopy analysis of the surfaces revealed strong interactions of the enzyme with the glycolated side chains of the polymers, which improved for linear side chains compared to those for branched ones. We probed the alterations in the enzyme structure upon adsorption using circular dichroism, which suggested protein denaturation on hydrophobic surfaces. Our study concludes that a negatively charged, smooth, and hydrophilic film surface provides the best environment for enzyme adsorption with desired mass and conformation, maximizing the sensor performance. This knowledge will guide synthetic work aiming to establish close interactions between proteins and electronic materials, which is crucial for developing high-performance enzymatic metabolite biosensors and biocatalytic charge-conversion devices.

A guide for the characterization of organic electrochemical transistors and channel materials

The organic electrochemical transistor (OECT) is one of the most versatile devices within the bioelectronics toolbox, with its compatibility with aqueous media and the ability to transduce and amplify ionic and biological signals into an electronic output. The OECT operation relies on the mixed (ionic and electronic charge) conduction properties of the material in its channel. With the increased popularity of OECTs in bioelectronics applications and to benchmark mixed conduction properties of channel materials, the characterization methods have broadened somewhat heterogeneously. We intend this review to be a guide for the characterization methods of the OECT and the channel materials used. Our review is composed of two main sections. First, we review techniques to fabricate the OECT, introduce different form factors and configurations, and describe the device operation principle. We then discuss the OECT performance figures of merit and detail the experimental procedures to obtain these characteristics. In the second section, we shed light on the characterization of mixed transport properties of channel materials and describe how to assess films' interactions with aqueous electrolytes. In particular, we introduce experimental methods to monitor ion motion and diffusion, charge carrier mobility, and water uptake in the films. We also discuss a few theoretical models describing ion-polymer interactions. We hope that the guidelines we bring together in this review will help researchers perform a more comprehensive and consistent comparison of new materials and device designs, and they will be used to identify advances and opportunities to improve the device performance, progressing the field of organic bioelectronics.

Performance of PEDOTOH/PEO-based Supercapacitors in Agarose Gel Electrolyte

Poly(3,4-ethylenedioxythiophene) (PEDOT) is a prime example of conducting polymers materials for supercapacitors electrodes that offer ease of processability and sophisticated chemical stability during operation and storage in aqueous environments. Yet, continuous improvement on its electrochemical capacitance and stability upon long cycles remains a major interest in the field, such as the developing PEDOT-based composites. This work evaluates the electrochemical performances of hydroxymethyl PEDOT (PEDOTOH) coupled with hydrogel additives, namely poly(ethylene oxide) (PEO), poly(acrylic acid) (PAA), and polyethyleneimine (PEI), fabricated via a single-step electrochemical polymerization method in an aqueous solution. The PEDOTOH/PEO composite exhibits the highest capacitance (195.2 F g-1) compared to pristine PEDOTOH (153.9 F g-1), PEDOTOH/PAA (129.9 F g-1), and PEDOTOH/PEI (142.3 F g-1) at a scan rate of 10 mV s-1. The PEDOTOH/PEO electrodes were then assembled into a symmetrical supercapacitor in an agarose gel. The type of supporting electrolytes and salt concentrations were further examined to identify the optimal agarose-based gel electrolyte. The supercapacitors comprising 2 M agarose-LiClO4 achieved a specific capacitance of 27.6 F g-1 at a current density of 2 A g-1, a capacitance retention of ~94% after 10,000 charge/discharge cycles at 10.6 A g-1, delivering a maximum energy and power densities of 11.2 Wh kg-1 and 3.45 kW kg-1, respectively. The performance of the proposed supercapacitor outperformed several reported PEDOT-based supercapacitors, including PEDOT/carbon fiber, PEDOT/CNT, and PEDOT/graphene composites. This study provides insights into the effect of incorporated hydrogel in the PEDOTOH network and the optimal conditions of agarose-based gel electrolytes for high-performance PEDOT-based supercapacitor devices.

A role for a DEAD box RNA helicase in natural killer cells

The X chromosome linked RNA helicase DDX3X plays numerous roles in regulation of cellular RNA metabolism and immune responses. Loss of function mutations in this helicase are linked to neurological disease and malignancies, including NK/T-cell lymphoma. To investigate how DDX3X contributes to development of NK cells, we generated mice with a conditional deletion Ddx3x in Ncr1-expressing cells. Female mice with homozygous deletion of NK cells exhibit a complete loss of NK cells. By comparison, the frequency of NK cells was significantly reduced in hemizygous male mice. The reduction in NK cells is associated with increased apoptosis in this lineage early during differentiation in the bone marrow. Our ongoing work is focused on elucidation of the mechanisms by which DDX3X regulates NK cell survival, differentiation, and malignant transformation, as well as on the capacity of DDX3Y to compensate for loss of DDX3X in hemizygous male mice. Our data identify a novel role for DDX3 RNA helicases in the development and survival of NK cells in mice.

Purification of astrocyte subtype based on a double reporter mice approach for downstream transcription profiling

Characterizing the molecular signature of a cell subtype leads to a better understanding of cell diversity, as this molecular data can identify new cellular markers and offer insights about cell function. Here, we describe an efficient protocol to separate a subtype of astrocytes, the Olig2-AS, from other glial cells by using a double reporter mouse approach and to determine the transcriptome profile of the Olig2-AS from the postnatal spinal cord using RNA-sequencing analysis. For complete details on the use and execution of this protocol, please refer to Ohayon et al. (2021).

Organic Bioelectronic Devices for Metabolite Sensing

Electrochemical detection of metabolites is essential for early diagnosis and continuous monitoring of a variety of health conditions. This review focuses on organic electronic material-based metabolite sensors and highlights their potential to tackle critical challenges associated with metabolite detection. We provide an overview of the distinct classes of organic electronic materials and biorecognition units used in metabolite sensors, explain the different detection strategies developed to date, and identify the advantages and drawbacks of each technology. We then benchmark state-of-the-art organic electronic metabolite sensors by categorizing them based on their application area (in vitro, body-interfaced, in vivo, and cell-interfaced). Finally, we share our perspective on using organic bioelectronic materials for metabolite sensing and address the current challenges for the devices and progress to come.

Transcriptome profiling of the Olig2-expressing astrocyte subtype reveals their unique molecular signature

Astrocytes are recognized to be a heterogeneous population of cells that differ morphologically, functionally, and molecularly. Whether this heterogeneity results from generation of distinct astrocyte cell lineages, each functionally specialized to perform specific tasks, remains an open question. In this study, we used RNA sequencing analysis to determine the global transcriptome profile of the Olig2-expressing astrocyte subtype (Olig2-AS), a specific spinal astrocyte subtype that segregates early during development from Olig2 progenitors and differs from other spinal astrocytes by the expression of Olig2. We identified 245 differentially expressed genes. Among them, 135 exhibit higher levels of expression when compared with other populations of spinal astrocytes, indicating that these genes can serve as a “unique” functional signature of Olig2-AS. Among them, we identify two genes, inka2 and kcnip3, as specific molecular markers of the Olig2-AS in the P7 spinal cord. Our work thus reveals that Olig2 progenitors produce a unique spinal astrocyte subtype.

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Efficient method to isolate Olig2-AS from other spinal glial cells

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Provide astrocyte subtype transcriptome from the post-natal spinal cord

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Identification of two specific markers of the Olig2-AS

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Bioinformatics identifies functional specificity of Olig2-AS in synapse regulation

Microfluidic Integrated Organic Electrochemical Transistor with a Nanoporous Membrane for Amyloid-β Detection

Alzheimer's disease (AD) is a neurodegenerative disorder associated with a severe loss in thinking, learning, and memory functions of the brain. To date, no specific treatment has been proven to cure AD, with the early diagnosis being vital for mitigating symptoms. A common pathological change found in AD-affected brains is the accumulation of a protein named amyloid-β (Aβ) into plaques. In this work, we developed a micron-scale organic electrochemical transistor (OECT) integrated with a microfluidic platform for the label-free detection of Aβ aggregates in human serum. The OECT channel-electrolyte interface was covered with a nanoporous membrane functionalized with Congo red (CR) molecules showing a strong affinity for Aβ aggregates. Each aggregate binding to the CR-membrane modulated the vertical ion flow toward the channel, changing the transistor characteristics. Thus, the device performance was not limited by the solution ionic strength nor did it rely on Faradaic reactions or conformational changes of bioreceptors. The high transconductance of the OECT, the precise porosity of the membrane, and the compactness endowed by the microfluidic enabled the Aβ aggregate detection over eight orders of magnitude wide concentration range (femtomolar-nanomolar) in 1 μL of human serum samples. We expanded the operation modes of our transistors using different channel materials and found that the accumulation-mode OECTs displayed the lowest power consumption and highest sensitivities. Ultimately, these robust, low-power, sensitive, and miniaturized microfluidic sensors helped to develop point-of-care tools for the early diagnosis of AD.

Polaron Delocalization in Donor-Acceptor Polymers and its Impact on Organic Electrochemical Transistor Performance

Donor-acceptor (D-A) polymers are promising materials for organic electrochemical transistors (OECTs), as they minimize detrimental faradaic side-reactions during OECT operation, yet their steady-state OECT performance still lags far behind their all-donor counterparts. We report three D-A polymers based on the diketopyrrolopyrrole unit that afford OECT performances similar to those of all-donor polymers, hence representing a significant improvement to the previously developed D-A copolymers. In addition to improved OECT performance, DFT simulations of the polymers and their respective hole polarons also reveal a positive correlation between hole polaron delocalization and steady-state OECT performance, providing new insights into the design of OECT materials. Importantly, we demonstrate how polaron delocalization can be tuned directly at the molecular level by selection of the building blocks comprising the polymers' conjugated backbone, thus paving the way for the development of even higher performing OECT polymers.

Influence of Side Chains on the n-Type Organic Electrochemical Transistor Performance

n-Type (electron transporting) polymers can make suitable interfaces to transduce biological events that involve the generation of electrons. However, n-type polymers that are stable when electrochemically doped in aqueous media are relatively scarce, and the performance of the existing ones lags behind their p-type (hole conducting) counterparts. Here, we report a new family of donor-acceptor-type polymers based on a naphthalene-1,4,5,8-tetracarboxylic-diimide-bi-thiophene (NDI-T2) backbone where the NDI unit always bears an ethylene glycol (EG) side chain. We study how small variations in the side chains tethered to the acceptor as well as the donor unit affect the performance of the polymer films in the state-of-the-art bioelectronic device, the organic electrochemical transistor (OECT). First, we find that substitution of the T2 core with an electron-withdrawing group (i.e., methoxy) or an EG side chain leads to ambipolar charge transport properties and causes significant changes in film microstructure, which overall impairs the n-type OECT performance. We thus show that the best n-type OECT performer is the polymer that has no substitution on the T2 unit. Next, we evaluate the distance of the oxygen from the NDI unit as a design parameter by varying the length of the carbon spacer placed between the EG unit and the backbone. We find that the distance of the EG from the backbone affects the film order and crystallinity, and thus, the electron mobility. Consequently, our work reports the best-performing NDI-T2-based n-type OECT material to date, i.e., the polymer without the T2 substitution and bearing a six-carbon spacer between the EG and the NDI units. Our work provides new guidelines for the side-chain engineering of n-type polymers for OECTs and insights on the structure-performance relationships for mixed ionic-electronic conductors, crucial for devices where the film operates at the aqueous electrolyte interface.

Sulf2a controls Shh-dependent neural fate specification in the developing spinal cord

Sulf2a belongs to the Sulf family of extracellular sulfatases which selectively remove 6-O-sulfate groups from heparan sulfates, a critical regulation level for their role in modulating the activity of signalling molecules. Data presented here define Sulf2a as a novel player in the control of Sonic Hedgehog (Shh)-mediated cell type specification during spinal cord development. We show that Sulf2a depletion in zebrafish results in overproduction of V3 interneurons at the expense of motor neurons and also impedes generation of oligodendrocyte precursor cells (OPCs), three cell types that depend on Shh for their generation. We provide evidence that Sulf2a, expressed in a spatially restricted progenitor domain, acts by maintaining the correct patterning and specification of ventral progenitors. More specifically, Sulf2a prevents Olig2 progenitors to activate high-threshold Shh response and, thereby, to adopt a V3 interneuron fate, thus ensuring proper production of motor neurons and OPCs. We propose a model in which Sulf2a reduces Shh signalling levels in responding cells by decreasing their sensitivity to the morphogen factor. More generally, our work, revealing that, in contrast to its paralog Sulf1, Sulf2a regulates neural fate specification in Shh target cells, provides direct evidence of non-redundant functions of Sulfs in the developing spinal cord.

Organic Bioelectronics: From Functional Materials to Next-Generation Devices and Power Sources

Conjugated polymers (CPs) possess a unique set of features setting them apart from other materials. These properties make them ideal when interfacing the biological world electronically. Their mixed electronic and ionic conductivity can be used to detect weak biological signals, deliver charged bioactive molecules, and mechanically or electrically stimulate tissues. CPs can be functionalized with various (bio)chemical moieties and blend with other functional materials, with the aim of modulating biological responses or endow specificity toward analytes of interest. They can absorb photons and generate electronic charges that are then used to stimulate cells or produce fuels. These polymers also have catalytic properties allowing them to harvest ambient energy and, along with their high capacitances, are promising materials for next-generation power sources integrated with bioelectronic devices. In this perspective, an overview of the key properties of CPs and examination of operational mechanism of electronic devices that leverage these properties for specific applications in bioelectronics is provided. In addition to discussing the chemical structure-functionality relationships of CPs applied at the biological interface, the development of new chemistries and form factors that would bring forth next-generation sensors, actuators, and their power sources, and, hence, advances in the field of organic bioelectronics is described.

Biofuel powered glucose detection in bodily fluids with an n-type conjugated polymer

A promising class of materials for applications that rely on electron transfer for signal generation are the n-type semiconducting polymers. Here we demonstrate the integration of an n-type conjugated polymer with a redox enzyme for the autonomous detection of glucose and power generation from bodily fluids. The reversible, mediator-free, miniaturized glucose sensor is an enzyme-coupled organic electrochemical transistor with a detection range of six orders of magnitude. This n-type polymer is also used as an anode and paired with a polymeric cathode in an enzymatic fuel cell to convert the chemical energy of glucose and oxygen into electrical power. The all-polymer biofuel cell shows a performance that scales with the glucose content in the solution and a stability that exceeds 30 days. Moreover, at physiologically relevant glucose concentrations and from fluids such as human saliva, it generates enough power to operate an organic electrochemical transistor, thus contributes to the technological advancement of self-powered micrometre-scale sensors and actuators that run on metabolites produced in the body.

The Key Role of Side Chain Linkage in Structure Formation and Mixed Conduction of Ethylene Glycol Substituted Polythiophenes

Functionalizing conjugated polymers with polar ethylene glycol side chains enables enhanced swelling and facilitates ion transport in addition to electronic transport in such systems. Here, we investigate three polythiophene homopolymers (P3MEET, P3MEEMT, and P3MEEET) having differently linked (without spacer and with methyl and ethyl spacer, respectively) diethylene glycol side chains. All the polymers were tested in organic electrochemical transistors (OECTs). They show drastic differences in the device performance. The highest μ_OECT C* product of 11.5 F/cm·V·s was obtained for ethyl-spaced P3MEEET. How the injection and transport of ions is influenced by the side-chain linkage was studied with electrochemical impedance spectroscopy, which shows a dramatic increase in volumetric capacitance from 80 ± 9 up to 242 ± 17 F/cm³ on going from P3MEET to P3MEEET. Thus, ethyl-spaced P3MEEET exhibits one of the highest reported volumetric capacitance values among p-type polymers. Moreover, P3MEEET exhibits in dry thin films an organic field-effect transistor (OFET) hole mobility of 0.005 cm²/V·s, highest among the three, which is one order of magnitude higher than that for P3MEEMT. The extracted hole mobility from OECT (oxidized swollen state) and the hole mobility in solid-state thin films (OFET) show contradictory trends for P3MEEMT and P3MEEET. In order to understand exactly the properties in the hydrated and dry states, the crystal structure of the polymers was investigated with wide-angle X-ray scattering (WAXS) and grazing incidence WAXS, and the water uptake under applied potential was monitored using electrochemical quartz crystal microbalance with dissipation monitoring (E-QCMD). These measurements reveal an amorphous state for P3MEET and a semicrystalline state for P3MEEMT and P3MEEEET. On the other hand, E-QCMD confirms that P3MEEET swells 10 times more than P3MEEMT in the oxidized state. Thus, the importance of the ethyl spacer toward crystallinity and mixed-conduction properties was clearly demonstrated, emphasizing the impact of side chain linkage of diethylene glycol. This detailed study offers a better understanding of how to design high-performance organic mixed conductors.

Sulfatase 2 promotes generation of a spinal cord astrocyte subtype that stands out through the expression of Olig2

Generation of glial cell diversity in the developing spinal cord is known to depend on spatio-temporal patterning programs. In particular, expression of the transcription factor Olig2 in neural progenitors of the pMN domain is recognized as critical to their fate choice decision to form oligodendrocyte precursor cells (OPCs) instead of astrocyte precursors (APs). However, generating some confusion, lineage-tracing studies of Olig2 progenitors in the spinal cord provided evidence that these progenitors also generate some astrocytes. Here, we addressed the role of the heparan sulfate-editing enzyme Sulf2 in the control of gliogenesis and found an unanticipated function for this enzyme. At initiation of gliogenesis in mouse, Sulf2 is expressed in ventral neural progenitors of the embryonic spinal cord, including in Olig2-expressing cells of the pMN domain. We found that sulf2 deletion, while not affecting OPC production, impairs generation of a previously unknown Olig2-expressing pMN-derived cell subtype that, in contrast to OPCs, does not upregulate Sox10, PDGFRα or Olig1. Instead, these cells activate expression of AP identity genes, including aldh1L1 and fgfr3 and, of note, retain Olig2 expression as they populate the spinal parenchyma at embryonic stages but also as they differentiate into mature astrocytes at postnatal stages. Thus, our study, by revealing the existence of Olig2-expressing APs that segregate early from pMN cells under the influence of Sulf2, supports the existence of a common source of APs and OPCs in the ventral spinal cord and highlights divergent regulatory mechanism for the development of pMN-derived OPCs and APs.

Direct metabolite detection with an n-type accumulation mode organic electrochemical transistor

The inherent specificity and electrochemical reversibility of enzymes poise them as the biorecognition element of choice for a wide range of metabolites. To use enzymes efficiently in biosensors, the redox centers of the protein should have good electrical communication with the transducing electrode, which requires either the use of mediators or tedious biofunctionalization approaches. We report an all-polymer micrometer-scale transistor platform for the detection of lactate, a significant metabolite in cellular metabolic pathways associated with critical health care conditions. The device embodies a new concept in metabolite sensing where we take advantage of the ion-to-electron transducing qualities of an electron-transporting (n-type) organic semiconductor and the inherent amplification properties of an ion-to-electron converting device, the organic electrochemical transistor. The n-type polymer incorporates hydrophilic side chains to enhance ion transport/injection, as well as to facilitate enzyme conjugation. The material is capable of accepting electrons of the enzymatic reaction and acts as a series of redox centers capable of switching between the neutral and reduced state. The result is a fast, selective, and sensitive metabolite sensor. The advantage of this device compared to traditional amperometric sensors is the amplification of the input signal endowed by the electrochemical transistor circuit and the design simplicity obviating the need for a reference electrode. The combination of redox enzymes and electron-transporting polymers will open up an avenue not only for the field of biosensors but also for the development of enzyme-based electrocatalytic energy generation/storage devices.

Onset of Spinal Cord Astrocyte Precursor Emigration from the Ventricular Zone Involves the Zeb1 Transcription Factor

During spinal cord development, astrocyte precursors arise from neuroepithelial progenitors, delaminate from the ventricular zone, and migrate toward their final locations where they differentiate. Although the mechanisms underlying their early specification and late differentiation are being deciphered, less is known about the temporal control of their migration. Here, we show that the epithelial-mesenchymal transition regulator Zeb1 is expressed in glial precursors and report that loss of Zeb1 function specifically delays the onset of astrocyte precursor delamination from the ventricular zone, correlating with transient deregulation of the adhesion protein Cadherin-1. Consequently, astrocyte precursor invasion into the Zeb1^-/- mutant white matter is delayed, and induction of their differentiation is postponed. These findings illustrate how fine regulation of adhesive properties influences the onset of neural precursor migration and further support the notion that duration of exposure of migrating astrocyte precursors to environmental cues and/or their correct positioning influence the timing of their differentiation.

Controlled Growth of CH3 NH3 PbI3 Using a Dynamically Dispensed Spin-Coating Method: Improving Efficiency with a Reproducible PbI2 Blocking Layer

It is commonly believed that excess PbI₂ has beneficial effects for perovskite solar cells owing to the modification of charge-transport behavior at interfaces, by surface passivation and by blocking electron-hole recombination. Here, we introduce a dynamically dispensed spin-coating technique in a two-step deposition to form a perovskite layer with controllable quantities of crystalline PbI₂ . Using this technique, the concentration of CH₃ NH₃ I solution is kept constant at the reaction interface, ensuring smooth growth of films. By changing the spinning rate during the reaction, the PbI₂ conversion ratio and perovskite cuboid size can be optimized, resulting in a power conversion efficiency improvement over control devices. This dynamically dispensed technique represents a repeatable method for compositional control in perovskite solar cells and improves our understanding of how a PbI₂ blocking layer improves the performance of perovskite solar cells.

Motor skill learning requires active central myelination

Myelin-forming oligodendrocytes (OLs) are formed continuously in the healthy adult brain. In this work, we study the function of these late-forming cells and the myelin they produce. Learning a new motor skill (such as juggling) alters the structure of the brain's white matter, which contains many OLs, suggesting that late-born OLs might contribute to motor learning. Consistent with this idea, we show that production of newly formed OLs is briefly accelerated in mice that learn a new skill (running on a "complex wheel" with irregularly spaced rungs). By genetically manipulating the transcription factor myelin regulatory factor in OL precursors, we blocked production of new OLs during adulthood without affecting preexisting OLs or myelin. This prevented the mice from mastering the complex wheel. Thus, generation of new OLs and myelin is important for learning motor skills.

A mesenchymal-like ZEB1(+) niche harbors dorsal radial glial fibrillary acidic protein-positive stem cells in the spinal cord

In humans and rodents the adult spinal cord harbors neural stem cells located around the central canal. Their identity, precise location, and specific signaling are still ill-defined and controversial. We report here on a detailed analysis of this niche. Using microdissection and glial fibrillary acidic protein (GFAP)-green fluorescent protein (GFP) transgenic mice, we demonstrate that neural stem cells are mostly dorsally located GFAP(+) cells lying ependymally and subependymally that extend radial processes toward the pial surface. The niche also harbors doublecortin protein (Dcx)(+) Nkx6.1(+) neurons sending processes into the lumen. Cervical and lumbar spinal cord neural stem cells maintain expression of specific rostro-caudal Hox gene combinations and the niche shows high levels of signaling proteins (CD15, Jagged1, Hes1, differential screening-selected gene aberrative in neuroblastoma [DAN]). More surprisingly, the niche displays mesenchymal traits such as expression of epithelial-mesenchymal-transition zinc finger E-box-binding protein 1 (ZEB1) transcription factor and smooth muscle actin. We found ZEB1 to be essential for neural stem cell survival in vitro. Proliferation within the niche progressively ceases around 13 weeks when the spinal cord reaches its final size, suggesting an active role in postnatal development. In addition to hippocampus and subventricular zone niches, adult spinal cord constitutes a third central nervous system stem cell niche with specific signaling, cellular, and structural characteristics that could possibly be manipulated to alleviate spinal cord traumatic and degenerative diseases.

Zfh1 promotes survival of a peripheral glia subtype by antagonizing a Jun N-terminal kinase-dependent apoptotic pathway

In Drosophila subperineurial glia (SPG) ensheath and insulate the nerve. SPG is under strict cell cycle and survival control because cell division or death of such a cell type would compromise the integrity of the blood-nerve barrier. The mechanisms underlying the survival of SPG remain unknown. Here, we show that the embryonic peripheral glia expresses the Zfh1 transcription factor, and in zfh1 mutants a particular SPG subtype, ePG10, undergoes apoptosis. Our findings show that in ePG10, Zfh1 represses the pro-apoptotic RHG-motif gene reaper in a cell-autonomous manner. Zfh1 also blocks the activation of the Jun N-terminal kinase (JNK) pathway, and reducing or enhancing JNK signalling in zfh1 mutants prevents or promotes ePG10 apoptosis. Our study shows a novel function for Zfh1 as an anti-apoptotic molecule and uncovers a cryptic JNK-dependent apoptotic programme in ePG10, which is normally blocked by Zfh1. We propose that, in cells such as SPG that do not undergo self-renewal and survive long periods, transcriptional control of RHG-motif gene expression together with fine tuning of JNK signalling is crucial for cell survival.

The failure of traditionally used desert plants to act against cutaneous leishmaniasis in experimental animals

Several desert plants that are traditionally used by the Bedouin community as folkloristic treatment for skin diseases were examined for their efficacy against cutaneous leishmaniasis (CL) in BALB/c mice. Water and chloroform extracts made from these plants were incorporated into cetomacrogol and soft white paraffin respectively and some were supplemented with DMSO. These preparations were applied twice daily for up to 30 days to CL lesions caused by Leishmania major. None of the extracts tested showed any leishmanicidal effect.