Symposium F.FL01—Bioelectronic Materials for Neural Interfaces—Stimulation, Sensing, Power and Packaging

Digitizing biomarkers analysis by quantifying them at the single-molecule level is the new frontier for advancing the science of precision health. Such an occurrence will enormously enhance their ability of curing diseases by supporting better prognosis and permitting the application of precise treatment methods. The SiMBiT project will develop a bio-electronic smart system leveraging on an existing lab-based proof-of-concept that can perform single-molecule detection of both proteins and DNA bio-markers (pancreatic cancer precursors) in a minimal sample volume, with enhanced sensing capabilities. In its final shape, it comprises a 96-well plate with the electrolyte gated organic transistors (EG-TFTs) sensing array and the bio-functionalized gates module along with addressing and front-end electronics and an electronic plate reader.
Here we present the activity of IIT group on SiMBiT project which is related to the fabrication, with large-area compatible processes such as printing, and optimization of the EG-TFT structure, which will be integrated in a 8x12 matrix. We will show the complete fabrication process and the electrical characterization of the EG-TFTs, together with the the firsts sensing measurements which can show the high sensitivity and specificity of the devices, together with the high reproducibility and stability, which are fundamental for the development of a platform of biosensors.

Peripheral nervous system (PNS) injuries affect millions of people worldwide each year, and peripheral nerve repair remains challenging due to the limited functional recovery and the associated co-morbidities seen in current methods [i]. Modern nerve repair often relies on nerve regeneration, which has been reported to be sensitive to multiple external stimuli, including electromagnetic stimulus and topographical cue. Here we present encouraging results that neurite outgrowth of dorsal root ganglia (DRG) neurons is promoted by applied static magnetic field and, separately, also by topographical guidance from glass-coated magnetic microwires in their demagnetized state. These microwires (diameter ~ 20 microns), comprised of a metallic glass Fe(Co)SiB-based core coated with pyrex glass, possess an extraordinarily responsive magnetic character, with a saturation magnetization around 1.5 T of the metallic core, coercivity in the range of several A/m, and permeability up to around 10⁵ [ii]. We hypothesize that employment of magnetized glass-coated microwires will significantly enhance nerve regeneration with combined applications of magnetic and topographical cues.
Magnetic stimulation was applied to DRG neurons using an apparatus developed with Ansys electromagnetic simulation software. This apparatus employed a pair of axially magnetized neodymium-iron-boron magnets joined by an electrical steel flux path to deliver a nominally uniform in-plane magnetic field μ₀H_app ~500 G (0.05 T) to the cells. Topographical guidance was provided to these cells by a parallel assembly (1:5 mesh density) of glass-coated microwires in the absence of an applied magnetic field. As these magnetic microwires were in a demagnetized state, the cells were not subjected to a magnetic field. All cells were cultured for three consecutive days under controlled conditions in an incubator. Data derived from immunocytochemistry and image analysis with MATLAB revealed that the magnetic field application increased the overall neurite outgrowth volume by up to 152% compared to unstimulated controls. No directional guiding effect was observed from the applied magnetic field. However, the topographical cues from the aligned microwire assembly enhanced the directionality of neurite extension with an increase of 39% in average longest neurite length along the wires, but not in total outgrowth. Future investigations involve growing DRG neurons on magnetized glass-coated microwires meshes, providing combined magnetic and topographical stimuli.

Acknowledgments: This research was conducted under the auspices of Northeastern University, the Fulbright Scholar Program, and ICMM/CSIC, Spain.

Reference:
[i] Koppes, A.N., Zaccor, N.W., Rivet, C.J., Williams, L.A., Piselli, J.M., Gilbert, R.J. and Thompson, D.M., 2014. Neurite outgrowth on electrospun PLLA fibers is enhanced by exogenous electrical stimulation. Journal of neural engineering, 11(4), p.046002.
[ii] Vázquez, M., 2007. Advanced magnetic microwires, in Handbook of Magnetism and Advanced Magnetic Materials, Ed. S. Parkin & H. Kronmuller, John Wiley Vol 4, 2192-2222

Cancer is one of the leading causes of death with approximately 7 million deaths each year. Although knowledge surrounding the prevention and treatment of cancer is increasing, there is a clear need of highly-sensitive techniques for early detection of cancer to increase the survival rate. This presentation describes recent findings of our investigations of multi-functional nanomaterials for Surface-enhanced Raman scattering (SERS) detection of cancer biomarkers. One important focus is the preparation of magnetic and plasmonic nanoparticles as the SERS probes with high sensitivity, high selectivity, and low detection limit. We have developed a duel-functional nanoprobe system for the SERS detection of the biomarkers. Results on the preparation of gold and core-shell nanoparticles with a magnetic core and plasmonic gold shell will be discussed, focusing on the control of size, composition, and surface properties. Tunable bio-conjugation of the nanoprobes with detection/capture antibodies for the detection of the cancer biomarker, e.g., carcinoembryonic antigen, has been demonstrated. Implications of the results for optimizing the magnetic-plasmonic properties and multiplexing detection of cancer biomarkers will also be discussed.

The recent introduction of soft materials to design electrodes that conform and mimic neural tissue drives the development of neural interfaces with improved functionality and biointegration. While being used routinely in academic research, soft neural interfaces have not reached clinical translation, in part because scaling, reproducibility and reliability standards are yet to be established. Materials science and engineering is therefore at the core of translational research.
We propose a translational framework focused on electrode implants and based on systematic steps in design, manufacturing, in vitro biomimetic validation and in vivo functional evaluation to advance the technological readiness of soft neural interfaces. Geometrical scaling is driven by anatomy but also surgical handling and procedure. Systematic control of manufacturing processes guides reproducibility in terms of layout, electrical and electrochemical properties. Validation is first conducted in vitro using a multimodal platform that mimics the biological environment within which the interface is to be implanted, applies mechanical loading similar to those to be withstood in vivo, and enables electrochemical ageing and characterization. The shift from stiff to soft electrode materials requires adaptation of the models and characterization methods to understand and predict electrode performance. Following iterations in characterization and manufacturing improvement, the soft neural interface is next evaluated in large animal models where therapeutic use may be implemented. Transition to medical grade materials and quality-controlled manufacturing are the next technological steps to implement and envision clinical evaluation of the soft neural interface.
This talk will illustrate this translational framework taking the example of the soft neurotechnology named e-dura – surface electrode arrays that mimic the natural dura mater and record or modulate neural activity in the central nervous system.

The brain is a massively-interconnected and constantly-evolving network of specialized circuits, a systematic understanding of which requires an interface that functions at diverse spatial and temporal scales. Implanted electrodes provide a unique approach to decipher brain circuitry by allowing for time-resolved electrical detection of individual neuron activity. However, scalable and stable neural recording that can track and map a large ensemble of neurons across days, weeks and months remains challenging. We recently demonstrated that ultraflexible, cellular-dimensioned neural electrodes afford seamless integration with brain tissue and stable recording of individual neurons for over a year. Building upon this platform, I will also present our recent progress in further decreasing their form factors, and their massive scaling-up of channel count and density in behaving animals. I will finally discuss about our on-going efforts in applying these ultraflexible electrodes in fundamental and translational neurosciences.

As our understanding of the brain’s physiology and pathology progresses, increasingly sophisticated technologies are required to advance discoveries in neuroscience and develop more effective approaches to treating brain disease. There is a tremendous need for advanced materials solutions at the biotic/abiotic interface to improve the spatiotemporal resolution of neuronal recording and stimulation. Organic electronic devices offer a unique approach to these challenges, due to their mixed ionic/electronic conduction, mechanical flexibility, enhanced biocompatibility, and capability for drug delivery. We designed, developed, and characterized conformable organic electronic devices in the form of electrodes, ion gated transistors, conformable batteries and ionic communication units to efficiently interface with the brain and acquire neurophysiological activity not previously accessible with recordings from the brain surface. These devices have facilitated large-scale rodent neurophysiology experiments and uncovered a novel oscillatory interaction. The biocompatibility of the devices allowed intra-operative recording from patients undergoing epilepsy and deep brain stimulation surgeries, highlighting the translational capacity of this class of neural interface devices. In parallel, we are developing the high-speed electronics and embedded acquisition and storage systems required to make high channel count, chronic neurophysiological recording from animals and human subjects possible.This multidisciplinary approach will enable the development of new devices based on organic electronics, with broad applicability to the understanding of physiologic and pathologic network activity, control of brain-machine interfaces, and therapeutic closed-loop devices.

One of the most important scientific and technological frontiers of our time is the interfacing of electronics with the human brain. This endeavour promises to help understand how the brain works and deliver new tools for diagnosis and treatment of pathologies including epilepsy and Parkinson’s disease. Current solutions, however, are limited by the materials that are brought in contact with the tissue and transduce signals across the biotic/abiotic interface. Recent advances in organic electronics have made available materials with a unique combination of attractive properties, including mechanical flexibility, mixed ionic/electronic conduction, enhanced biocompatibility, and capability for drug delivery. I will present examples of novel devices for recording and stimulation of neurons and show that organic electronic materials offer tremendous opportunities to study the brain and treat its pathologies.

The use of microcoils to magnetically stimulate neurons of the cortex has been shown to have several important advantages over conventional electrode-based stimulation. For example, the spatially asymmetric electric fields arising from microcoils can be used to activate specific populations of neurons [1] while avoiding others. This helps to confine activation to a focal region around the coil, and as a result, a microcoil-based approach may be an attractive platform where focal activation is critical, e.g. for next generation cortical visual prostheses that strive to restore high acuity vision. Initial studies have validated, using both computational and experimental approaches, the ability of a single microcoil to generate small, focal regions of activation [1], and the effects of varying coil shape/design on the both the spatial selectivity and strength of activation [2]. While devices that consist of a single microcoil may be useful in targeting a single, isolated region of the cortex, the ability to focally activate multiple cortical columns, or multiple layers within a single cortical column, either in synchrony or in temporally modulated intervals (interleaved), would be useful where the creation of multiple regions of neural activity is the goal, e.g. to elicit complex spatial patterns with a prosthesis. As a step towards this goal, we designed and fabricated an array of four individually accessible microcoils, each capable of activating a distinct region. Coils and interconnects were fabricated from copper and sandwiched between two layers of SU-8, which functioned as both conformable support substrate and passivation layer; the low young’s modulus of SU-8 helped to minimize mechanical mismatch with brain tissue. Individual coils were V-shaped, a geometry previously shown to be effective in activating pyramidal neurons [2]. Inter-coil spacing was 250um, the approximate separation between adjacent cortical columns. Computational modeling of this design revealed that the magnitude of electric field gradients along the length of pyramidal neurons would be strongest along the edge and at the tip of the “V” , thus increasing the likelihood of focal activation of neuronal processes located close to the tip while minimizing activation further away. Basic functionality of the array was confirmed by the measurement of electrical artifacts resulting from the passing of electrical current through individual coils. Use of a patch-clamp setup helped to prevent saturation of amplifiers and revealed that artifact shape was consistent with theoretical predictions. In-vitro experiments using GCaMP6 mice allowed the fluorescent response from neuronal populations to be measured in response to stimulation from individual coils; the transparency of SU8 allowed clear visualization of the fluorescent signals. As expected, activation from a single coil was largely confined to a focal region around the coil tip, thereby confirming functional performance of the coils. Future efforts will investigate the spatial activation patterns which result from the stimulation of multiple coils in the array.

[1] S. W. Lee, F. Fallegger, B. D. F. Casse, and S. I. Fried, “Implantable microcoils for intracortical magnetic stimulation,” Science Advances, vol. 2, no. 12, p. e1600889, Dec. 2016, doi: 10.1126/sciadv.1600889.
[2] S. W. Lee, K. Thyagarajan, and S. Fried, “Micro-coil design influences the spatial extent of responses to intracortical magnetic stimulation,” IEEE Transactions on Biomedical Engineering, pp. 1–1, 2018, doi: 10.1109/TBME.2018.2877713.

As a leading cause of death worldwide in recent years, cardiac disease needs continuous monitoring of cardiac activity for its prompt diagnosis and treatment. Pacemaker, a conventional technique for diagnosing and treating cardiac diseases such as arrhythmia, has adverse side effects at implantation sites such as infection and damage to blood vessels, and also causes discomfort in daily life due to its rigid and bulky form. Currently, with the emergence of soft bioelectronics that intimates 3D curvilinear bio tissues, various studies have been conducted to monitor the function of cardiac activities by attaching a device onto cardiac surface. Most recent soft electronic devices that detect cardiac functions are based on electrophysiology sensing such as electrocardiograms (ECGs). However, these devices that measure the electrical flow of cardiac surface as their sensing mechanism have limitations in implementing simultaneous electrical stimulation while sensing the electrical signal. This is due to the electrical interdependent between sensing and stimulation electrodes, resulting in unnecessary signal interferences in ECGs. Herein, we demonstrate an implantable and flexible cardiac motion sensor that can simultaneously detect cardiac beating rate and intensity, via pressure measurement of cardiac relaxation, and apply electrical pacing to modulate cardiac contractile function. This cardiac motion sensor consists of active-matrix arrays of pressure-sensitive Si transistors with air dielectric layers, and integrated platinum black electrodes for electrical stimulation. Also, an alginate-based hydrogel patch that is functionalized by multi-crosslinking moieties to enhance tissue adhesiveness and elastic modulus is reported. This hydrogel patch amplifies the pressure sensitivity of the motion sensor while securely fixing the motion sensor onto the epicardium surface without any surgical adjustment. Our results have shown that the flexible cardiac motion sensor is able to accurately detect cardiac mechanical activity such as systole and diastole, which are comparable with ECG trace during in-vivo trials using New Zealand White rabbits. Stimulation of synchronized electrical pulses to the left ventricle by our flexible cardiac motion sensor has modulated the cardiac beating frequency at a low threshold voltage while simultaneously detecting changes in the beating frequency and intensity of the pressure signals without any signal noise. Our motion-sensing diagnostic tool specifically targets those associated with abnormal cardiac activities such as arrhythmia, cardiac hypertrophy, and dilated cardiomyopathy, etc. Thus, we envision that our demonstration will provide a new intuitive platform that enables the diagnosis and treatment of various cardiac diseases.

Human stem cell derived organoids provide cellular models of human diseases without the need for in vivo trials, removing non-genetic pathological factors in disease modelling. Current physical modelling of myocardial infarction and drug cardiotoxicity in cardiac organoid is limited to optical image analysis. This method of analysis ranges from spatial proteomics to fluorescent imaging. However, diagnostic modelling of cardiac organoids requires a higher degree of precision and sensitivity in order to accurately measure beating frequency and intensity. In mechanically energetic cardiac organoids, actual collection and interpretation of raw physical data of organoid beating are important to better understand the contractility implication after drug screening. Herein, we introduce an innovative technology that analyzes the activity of cardiac organoid using a highly sensitive pressure sensor, which is fabricated from Si transistor with an air dielectric component. This device is integrated with electrical stimulating capability that can detect and modulate cardiac organoid beating simultaneously. This sophisticated active-matrix pressure array has a high spatial resolution, providing 2D tactile pressure mapping of cardiac organoid beating. Our novel device is capable of detecting real-time cardiac organoid physical functions, such as beating frequency and intensity, to model various cardiac conditions based directly on the organoids’ mechanical parameter. The periodic beatings of cardiac organoids are converted from physical parameters into electrical signals within the pressure sensor. Our device innovatively replaces the conventional method of image analysis, where the organoid beating measurement is derived by dissecting video frames, for myocardial infarction and drug cardiotoxicity modelling.

Biointerface materials and devices can probe fundamental biological dynamics and improve the lives of human beings. However, the direct application of traditional rigid electronics onto soft tissues or cells can cause signal transduction and biocompatibility issues, partly due to the mechanical mismatch at the biointerfaces. One solution to this mismatch is the use of a soft-hard composite as a biomaterial; the hard components are stable, easy to control and provide the active functionalities, while the soft components promote compliant mechanical adhesion or active biointegration. Additionally, as substrates or encapsulation layers, the soft components can protect the high modulus electronic elements, and can facilitate mechanical operations (e.g., stretching, bending, delivery, removal) of the entire composites. Advanced soft materials can also display dynamically responsive or even living behaviors currently lacking in rigid electronic systems.
In this talk, I will discuss our recent efforts of designing different soft-hard composites for neural interface applications, such as in vitro and in vivo non-genetic neuromodulation. For example, dopant-modulated silicon membranes, when integrated with porous PDMS, can be implanted over the motor and somatosensory cortex regions and are capable of inducing limb motion in animals upon light stimulation. Silicon carbide-based devices can be produced directly with laser ablation from a PDMS substrate and were used for neural and cardiac modulation in flexible configurations. Besides using nonliving composites, our lab has also explored several living composites for neural and cardiac interfaces. These bio-hybrids hold great promise as minimally invasive interfaces to naturally occurring cells and tissues for cell-based therapeutics and control. Finally, we have started the initial effort in creating tissue-like materials for neural interfaces. For example, we recently found that granules-based hydrogel composites can be considered better analogs of biological tissues in terms of both the structure and the static and dynamic mechanical properties. I will end this talk by proposing new soft-hard components for future neural interfaces.

Composites based on conductive nanowires embedded in elastomers are popular in a wide range of stretchable electronics applications where the requirements are either a stable (electrodes) or a highly increasing (strainsensors) electrical resistance upon strain. Despite the widespread use of such composites, their production protocol is not based on solid theoretical grounds but rather in empirical observations. There is a lack of a predicitive framework for the piezoresistive quality of samples based on their production parameters. This is due to limitations in the methods for studying nanowire meshes, in particular the lack of knowledge on the spatial distribution of the nanowires and their rearrangement under strain. We overcame this hurdle by collecting 3D reconstructed X-ray tomographies of silver nanowires embedded in polydimethylsiloxane (PDMS) under variable deformations and obtained the missing structural information of the nanomaterial by unsupervised artificialintelligence image analysis¹. This allowed us to reveal the precise assembly mechanisms of nanowire systems and derive a precise analytical formula for the piezoresistive response of the composites based on the nanowire material, the number of wires used,their aspect ratio, and the contact resistance between the wires². We demonstrate a lower bound on the relative increase in resistance of the sample (gauge factor) –in particular, these systems cannot exhibit less than a 3-fold increase in their resistance when strained 100%. Finally to simulate the behavior of arbitrary samples in-silico and propose an exploratory design framework, we made an open-source simulation framework, validated by our experimental observations, which allows to simulate any arbitrary nanowire mesh and its piezoresistive behavior.
1. Ihle, S.J., Unsupervised data to content transformation with histogram-matching cycle-consistent generative adversarial networks.Nat Mach Intell1,461–470 (2019)
2. Forro et. Al (in press). Visualizing and analyzing 3D metal nanowire networks for stretchable electronics, Advanced Theory and Simulations

The ability to modulate cellular electrophysiology is fundamental to the investigation of development, function, and disease. Currently, there is a need for remote, non-genetic, light-induced control of cellular activity in two-dimensional (2D) and three-dimensional (3D) platforms. Here, we report a breakthrough hybrid-nanomaterial for remote, non-genetic photothermal stimulation of 2D and 3D neural cellular systems. We combine one-dimensional (1D) nanowires (NWs) and 2D graphene flakes grown out-of-plane for highly controlled photothermal stimulation at subcellular precision without the need for genetic modification, with laser energies lower than hundred nanojoules, 1-2 orders of magnitude lower than Au-, C- and Si-based nanomaterials. Photothermal stimulation using NW-templated 3D fuzzy graphene (NT-3DFG) is flexible due to its broadband absorption and does not generate cellular stress. Therefore, it serves as a novel powerful toolset for studies of cell signaling within and between tissues and can enable new therapeutic interventions.

Understanding how a network of interconnected neurons receives, stores, and processes information in the human brain is one of the outstanding scientific challenges of our time. The ability to reliably detect neuroelectric activities is essential to addressing this challenge. Optical recording using voltage-sensitive fluorescent probes has provided unprecedented flexibility for choosing regions of interest in recording neuronal activities. However, when recording at a high frame rate such as 500 to 1,000 Hz, fluorescence-based voltage sensors often suffer from photobleaching and phototoxicity, which limit the recording duration. Here, we report an approach called electrochromic optical recording (ECORE) that achieves label-free optical recording of spontaneous neuroelectrical activities. ECORE utilizes the electrochromism of poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) thin films, whose optical absorption can be modulated by an applied voltage. Being based on optical reflection instead of fluorescence, ECORE offers the flexibility of an optical probe without suffering from photobleaching or phototoxicity. Using ECORE, we optically recorded spontaneous action potentials in cardiomyocytes, cultured hippocampal and dorsal root ganglion neurons, and brain slices. With minimal perturbation to cells, ECORE allows long-term optical recording over multiple days

With the aging of global populations, there are emerging retinal disorder cases such as retinitis pigmentosa and macular degeneration. The patients lose their vision due to the damage in their photoreceptors; however, the ganglion cells, which transfer the signals from photoreceptors to the optic nerve, are well preserved in most cases. By stimulating the ganglion cells electrically, the sense of vision can be induced intentionally. Based on these principles, retinal prosthetic devices have been developed by combining photodetectors, such as cameras, and stimulating electrodes. Although there have been many studies of the retinal prosthesis that restore vision through electrical stimulus, several challenges remain to be solved. First, the resolution of the conventional retinal prosthesis is significantly lower than that of the actual vision. Second, the planar structure of stimulation electrodes leads to high impedance and high threshold current, resulting in interference between the surrounding pixels in electrical stimulation. Third, the integration of an optoelectronic array device, which detects the external light, and stimulation electrode for the in-vivo test is rarely reported.
Herein, we introduce highly-integrated, flexible phototransistor arrays based on three-dimensional (3D) stimulation electrodes for vision restoration of live mice. The phototransistor arrays allow the device to electrically stimulate the retina with high-resolution. Since we implemented the 3D structured stimulation electrodes instead of the planar, the device shows the much lower impedance, suggesting less interference between the surrounding pixels in electrical stimulation. These improvements enable the target cells of the retina to be accurately stimulated with the light intensity changes. Furthermore, these 3D stimulation electrodes are made of soft, self-healable material to prevent secondary damage to the retina and enable long-term operation. In-vitro and in-vivo animal trials have confirmed real-time array signals generated by retinal cells in electrical stimulation with the change in light intensity. We completed the verification of the biocompatibility and long-term stability of the device. We envision that retinal implants with the soft 3D stimulating electrode will provide a promising strategy to the biomedical field regarding retinal disorders soon.

We develop and study non-genetic cell opto-stimulation techniques based on artificial light actuators that establish functional abiotic-biotic interfaces able to transduce a light signal into a biological stimulus. This talk reports on the state of our research regarding organic bio interfaces for inducing light sensitivity in cells, both in vitro and in vivo. Light actuators comes in different shapes: planar patches, nanoparticle, intramembrane probes. Their coupling mechanism is still far from being understood and attempts to shed light will be introduced. It seems that membrane modification is a crucial step in the photostimulation. Rescue vision in blind people is one of the most appealing practical outcome of this emerging technology, and recent achievements will be summoned.

Optogenetics and covalent approaches to bio conjugation allow achieving large and effective cell photo-stimulation, yet these are invasive methods that might encounter severe limitations on the way towards clinical applications in photopharmacology.¹ The non-covalent affinity of photoresponsive molecules to biotargets represents an attractive alternative. In this context, we have recently proposed an amphiphilic azobenzene photochromic molecule (ZIAPIN2) whose selective dwelling in the plasma membrane enables modulation of the cell potential, remarkably permitting to evoke light-induced neuronal firing in vitro as well in vivo.^2,3
Here, we show that the affinity of ZIAPIN2 to bacterial cell wall can be exploited to modulate bacterial proliferation. In particular, we demonstrated that while in the dark proliferation is hindered, light illumination in the visible range leads to a total restoration of bacterial growth for both Escherichia coli (Gram (-)) and Micrococcus Luteus (Gram (+)). We preliminary attribute this effect to a light-triggered modification of the bacterial membrane potential brought about by ZIAPIN2 isomerization.

1. Zimmerman, J. F. & Tian, B. ACS Nano 12, 4086–4095 (2018).
2. Paternò, G. M. et al. Adv. Sci. 7, 1903241 (2020).
3. DiFrancesco, M. L. et al. Nat. Nanotechnol. 15, 296–306 (2020).

Opto-stimulation of semiconductor-biointerfaces provides efficient pathways towards eliciting neural activity through selective spectral excitation. In visual prosthesis, tri-colour excitation capability, is the key to restoring full-colour vision. Among organic semiconductor materials, donor–acceptor molecules have the most promising properties due to excellent light absorption, high stability and good solubility in organic solvents. Varying the donor and acceptor functionalities, allows fine tuning of the optical properties. These carbon based molecules bear intrinsic affinity to biological systems, and also demonstrate excellent photoresponses, electronic and ionic conductivity, and allow intimate interface with liquid bioenvironment.
We demonstrate tri-colour optoelectronic devices based on solution-processable conjugated donor-acceptor molecules with absorption in red, green and blue spectral regions, mimicking the absorption of human retinal cones. Photo-response is studied via interfacing with biological electrolyte solution and using long-pulse, narrow-band excitations, where both photo-voltage and photo-current responses show clear signatures of capacitive charging at the electrolyte/device interface.
We further explore ink-jet printing to reproduce individual photo-pixels with red, green and blue absorption characteristics, with small diameters of 35 to 45 microns.
We discuss the suitability of organic semiconductors for potential medical applications as retinal bio-engineered prosthesis for the restoration of human vision, and evaluate prospects of wider, colour-selective excitations of bio-interfaces.

Implantable neurostimulation devices play a key role in treating many serious injuries and diseases by providing a direct therapeutic link to the nervous system. This enables brain stimulation for treatment of Parkinson’s disease and epilepsy, nerve guidance and regeneration to remedy spinal cord injury, and retinal prosthetic devices that could cure blindness.^[1] To address such issues, new bioelectronic systems that can deliver electrical stimulation to nerve cells are required. Although silicon microelectronics and metal electrodes have been the historic gold standard for bioelectronic interfaces, their use in clinical practice is limited. The main obstacles to further translation of these devices include a low biocompatibility that reduces in vivo lifetimes,^[2] a mechanical rigidity that is poorly matched with soft tissue, causing inflammation and ineffective electrical contact, and a requirement for costly external power supplies to deliver current.^[3] These issues result in indiscriminate tissue activation, with a consequent lack of spatial selectivity.^[4]

In this work, we report a strategy to simultaneously address these issues by combining soft carbon-based organic conductors and nanoscale science to build new bioelectrodes that allow optical neurostimulation without external power. Our approach creates bioelectronic interfaces from organic conductors that can be formed into customized nanoparticles with established solution-based chemistry methodologies. This approach enables the stimulating electrodes to be combined with targeted pharmaceutical factors in the fabrication procedure, which subsequently optimise connections to the neural network. We tune the optoelectronic properties of the organic conductors to cover standard red, green, and blue spectral regions, allowing spectrally selective platforms for neurostimulation. These conductors are turned into electroactive inks, and subsequently fabricated into pixelated arrays using inkjet printing. This approach establishes a new low-cost manufacturing methodology that is generally applicable to other organic materials and can be used for a variety of bioelectronic devices, creating a new manufacturing paradigm for biomedical technology.

We demonstrate both the anatomical and functional biocompatibility of neural tissue with our organic bioelectronic systems using immunolabelling with neuronal marker MAP2 and visualisation with epifluorescence microscopy to detect neurons cultured on the organic conductors. We demonstrate the controlled release of drugs from the organic conductive nanoparticles, aiding in precise spatial delivery of pharmaceutical factors. Finally, we employ whole-cell patch clamp electrophysiology recordings to demonstrate an exciting result; purely optical neurostimultation of dorsal root ganglion nerve cells. We demonstrate that the organic conductors can trigger changes in the nerve cell membrane potentials via a capacitive coupling mechanism, the efficacy of which can be improved by judicious selection of the device architecture. Importantly, capacitive neurostimulation does not require injection of current into biotic tissue to trigger changes in the membrane potentials. This means very confined stimulation is possible and therefore spatial selectivity is no longer limited by current spread and undesirable electrochemical reactivity is minimised.

References:
[1] B. Zhu, S. C. Luo, H. Zhao, H. A. Lin, J. Sekine, A. Nakao, C. Chen, Y. Yamashita, H. H. Yu, Nat. Commun., 2014, 5, 4523.
[2] T. Someya, Z. Bao, G. G. Malliaras, Nature, 2016, 540, 379.
[3] C. Liao, M. Zhang, M. Y. Yao, T. Hua, L. Li, F. Yan, Adv. Mater., 2015, 27, 7493.
[4] C. de Balthasar, S. Patel, A. Roy, R. Freda, S. Greenwald, A. Horsager, M. Mahadevappa, D. Yanai, M. J. McMahon, M. S. Humayun, R. J. Greenberg, J. D. Weiland, I. Fine, Investig. Opthalmology Vis. Sci., 2008, 49, 2303.

Plasmonic semiconductor nanocrystals are more sensitive to electron transfer events than traditional plasmonic materials such as Au and Ag due to their lower free electron concentrations. Because the plasmon peak position and intensity depend on the free electron concentration, we can quantify electrons transferred to and from the semiconductor nanocrystals in real-time through simple spectroscopic measurements. Further, the surface of doped semiconductor nanocrystals can be functionalized with various ligands post-synthetically to customize solvent compatibility, ensure colloidal stability, and tune the interactions with other species in solution. Leveraging our ability to quantify the change in optical response with the aqueous biocompatibility of colloidal nanocrystal dispersions, we aimed to quantify redox activity from cell metabolism. Specifically, we quantified extracellular electron transfer (EET) rates from Shewanella oneidensis. EET is process by which electron flux from carbon metabolism is directed to metallic substrates in anaerobic environments. The utility of EET as a power source for microbial fuel cells, bioelectronics, and material synthesis has led to significant interest in quantification methods for microbial electron flux.

When dispersed together, plasmonic semiconductor nanocrystals serve as electron accepting substrates for S. oneidensis. We dispersed tin-doped indium oxide (ITO) nanocrystals with S. oneidensis in an aqueous buffer by stripping the nanocrystals of their native, hydrophobic ligands and wrapping them in a hydrophilic, random co-polymer. We used in-situ spectroscopy to track the plasmonic response of ITO in real-time and quantified kinetic EET rate constants from S. oneidensis on a per cell basis. Using this method, we distinguished EET rates of healthy cells from those starved of a carbon source. The consistent measurement of a steady-state electron transfer regime indicated that cellular respiration was primarily directed toward energy production via EET as opposed to growth. Lastly, we quantitatively differentiated electron flux from S. oneidensis strains that were genetically engineered to tune EET activity. The generated current ranged from 0.1 to 6.9 fA per cell, depending on expression levels of a key EET gene, mtrC. Overall, our results indicate that colloidal plasmonic semiconductor nanocrystals are a reliable sensing platform for probing metabolic activity in native biological environments. Looking forward, we aim to assess EET gene turn-on in real-time in response to environmental stimuli, enabling understanding of fundamental gene expression dynamics.

The coupling of electronic charges and ions, in bioelectronic devices based on conjugated polymers, is unique and offers us a great opportunity to explore novel device concepts and also new science. The fundamental principles of charge accumulation and ion exchange in conjugated polymers, often referred to as doping, will be discussed. The dynamic control over doping can be achieved via electrochemical addressing and serves as the principle of operation in an array of organic electrochemical devices. Examples of such devices are the organic electronic sensors and actuators, devices that rely on iontronic electrode, transistor, conductor or diode configurations. Such devices and systems have then been applied to several biological and medical settings, targeting new biotech/med-tech tools, therapy and monitoring of physiology and neuronal functions.

Being the primary energy source of mammalian cells, glucose is involved in numerous biological events. In this work, I will present an electronic platform based on an n-type conjugated polymer for detecting glucose as well as extraction of power from bodily fluids. We employ the n-type mixed (ionic and electronic) conductor in an accumulation mode organic electrochemical transistor (OECT). The polymer is engineered to have specific interactions with catalytic enzymes and requires no electron mediators to transduce enzymatic reactions. This micron-scale device detects glucose alongside lactate and shows excellent sensitivity and selectivity over five orders of magnitude wide detection range (1,2). When assembled as an enzymatic biofuel cell, the same polymer serves as the anode, converting the glucose metabolism in an aqueous medium into power (2). These devices can be fabricated on flexible substrates, paving the way for implantable self-powered metabolite sensing. While introducing these bioelectronic device concepts, I will touch upon approaches to further improve the performance of n-type mixed conductors.

1. Pappa et al Sci. Adv. 4, 6, eaat0911 (2018)
2. Ohayon et al Nat. Mater. 19, 456–463 (2020)

Microelectrode arrays (MEAs) are widely used tools for investigating neural activity. To ensure the best sensitivity of the electronic devices to ionic signals and the lowest information loss, their electrochemical interface must be optimized by lowering their surface impedance, with materials that ensure the highest compatibility with the cells at the same time.
Here, we show that by the electropolymerization of thiophene-derivatives, functionalized for higher cell biocompatibility and higher electrochemical performances, one can lower the microelectrodes’ surface impedance by the control of the polymer morphology. The microelectrode structuring with bottom-up grown conducting polymers was monitored in-situ by voltage-ramped impedance spectroscopy upon electropolymerization to track its circuit-elements modification.
Iterative impedance modeling over the growth confirmed the material’s electrochemical dynamic to be controlled by the gradual modifications of specific discrete circuit elements at different frequency ranges, thanks to the surface electrodes microstructuring. More particularly, we systematically evidenced a monotonic change of the electrode charging from ideal capacitor to constant phase element dominated modes, due to the bulk charging of the conducting polymer. The evolution of the materials morphology screened by atomic force microscopy and electron microscopy has been confronted to the modification of the materials circuit element, and confirmed distinctive charging modes for the electrodes that are governed by their different texturing. In addition to the surface morphology, chemical tuning of the electrodeposited polymer has been performed and showed that a fine tuning of the polymer’s glycolation promotes the decrease of the electrodes’ electrochemical impedance down to -15% compared to the unglycolated polymers thanks to a right balance between ionic permeability and electronic performances.
Overall, lower impedance values than commercial MEAs have been systematically reached with performances comparable to spin-coated polymer electrodes’, and with low performance dispersion over the whole population of electrodes in the MEAs.
With the presented preliminary biocompatibility and stability tests, this study aims is to demonstrate that unusual microfabrication techniques derived from electrochemistry can provide unique features at the material level to match properties of future emerging bioelectronics technologies to the strong requirements of sensing involving biological materials with rich material chemistry and morphology.

Conductive polymers (CPs) are in the particular interest of bioelectronics as they combine the mechanical flexibility with the electrical and ionic conductivity. Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) has been widely studied as the most promising CP due to its conductivity, processability, and commercial availability.¹ However, the biocompatibility of this benchmark material for long-term applications is still under debate,² even though a recent study has demonstrated its stability in cell culture for up to four months.³ Therefore, for interfacing electronics with biology, there is still a high demand for biocompatible devices and materials showing mixed electronic/ionic conductivity. Recently, PEDOT has been incorporated with biomolecule dopants, e.g., deoxyribonucleic acid (DNA), hyaluronic acid, dextran sulfonate, heparin, pectin, guar gum to modify the properties.⁴ Among those, PEDOT:DNA has the main advantage of higher ionic conductivity with respect to PEDOT:PSS.⁵

In our work, we prepared PEDOT:DNA and polypyyrole:DNA aqueous dispersions by oxidative chemical polymerization. DNA was engaged as a stabilizer and dopant during the polymerization process of EDOT and pyrrole monomers. Different surfactants and secondary dopants were added into the dispersed solutions to support the thin-film processing using the spin-coating technique as well as to avoid the delamination of the films. Subsequently, we investigated the electrical, optical, and morphological properties of the biocomposite films. We fabricated organic electrochemical transistors (OECTs) involving conducting polymer biocomposites as the organic active layer and compared the transistor characteristics to PEDOT:PSS based OECTs.⁶ Additionally, biocompatibility tests on the conductive films were carried out. The results showed that DNA-based conductive polymers can be considered as novel biocomposites for iontronics such as OECTs and biosensors.


1. Wen, Y.; Xu, J. Scientific Importance of Water-Processable PEDOT:PSS and Preparation, Challenge and New Application in Sensors of Its Film Electrode: A review. J. Polym. Sci. A, 55, 1121–1150 (2017).
2. Wei, B.; Liu, J.; Ouyang, L.: Kuo, C.-C.; Martin, D.C. Significant Enhancement of PEDOT Thin Film Adhesion to Inorganic Solid Substrates with EDOT-Acid. ACS Appl. Mater. Interface, 7, 15388-15394 (2015).
3. Dijk, G; Rutz, A. L.; Malliaras, G. G.; Stability of PEDOT:PSS coated gold electrodes in cell culture conditions, Adv. Mater. Technol., 1900662, (2019).
4. Mantione, D.; Del Agua, I.; Sanchez-Sanchez, A.; Mecerreyes, D. Poly(3,4-etheylenedioxythiophene) (PEDOT) Derivatives: Innovative Conductive Polymers for Bioelectronics. Polymers, 9, 354 (2017).
5. Ner, Y.; Invernale, M.A.; Grote, J.G.; Stuart, J.A.; Sotzing, A.G. Facile Chemical Synthesis of DNA-doped PEDOT. Synthetic Metals, Vol.160, Issue 5-6, 351-353 (2010).
6. Tekoglu, S.; Wielend, D.; Scharber, M. C.; Sariciftci, N. S.; Yumusak, Cigdem; Conductive Polymer-Based Nanocomposites Using Deoxyribonucleic Acid (DNA) as Counterion, Adv. Mater. Technol., 1900699, (2019).

The emerging need for bioelectronic devices that interface with biological substances is accompanied by a growing need for biocompatible, biodegradable, conducting materials. Biomaterials, which are inherently biocompatible, biodegradable, are commonly considered to be "bad conductors". In order to overcome this challenge, our group is developing bio-inspired materials based on de-novo design of peptides that capture many of the features of proteins on one hand, and are optimized for electronic and/ or protonic conduction applications on the other hand. In this talk, I will demonstrate the use of b-sheet forming peptides for the development of conductive materials. Specifically, I will show that the sequence of the peptide can greatly influence the conductivity with sensitivity to even single sequence mutation. These effects are shown to originate both from the differences in the intrinsic ability of a specific amino acid to participate in the charge transport process, and also from their effects on the assembly of the structure, which in return affect the conductivity. Design concepts for improving either electron^1,2 or proton³ conductivity, or both,⁴ will be presented. The structure function relationships we reveal can be used for the design of novel biocompatible organic bio-electronic devices.

References
1. M. Amit, G. Cheng, I. W. Hamley, and N. Ashkenasy, "Conductance of amyloid β based peptide filaments: Structure-function relations". Soft Matter, 8, 8690-8696, (2012).
2. D. Ivnitski, M. Amit, O. Silberbush, Y. Atsmon-Raz, J. Nanda, R. Cohen-Luria, Y. Miller, G. Ashkenasy, and N. Ashkenasy, "Structure Polymorphism Strongly influences the conductivity of peptide fibrils" Angewandte Chemie Int. Ed., 55, 9988 (2016).
3. O. Silberbush, M. Amit, S. Roy and N. Ashkenasy "Significant enhancement of proton transport in bioinspired peptide fibrils by single acidic or basic amino acid mutation" Adv. Func. Mat. 27, 1616-3028 (2017).
4. M. Amit, S. Appel, R. Cohen, G. Cheng, I. W. Hamley, and N. Ashkenasy, "Hybrid Proton and Electron Transport in Peptide Fibrils", Adv. Func. Mat. 24, 5873-5880 (2014)

Implanted devices such as pacemakers and brain stimulators are critical treatment modalities for certain patients. Establishing communication with implanted devices wirelessly remains a difficult task because biologic tissue absorbs most of the electromagnetic waves. Instead, we hypothesized that ions in biological tissue could serve as communicating carriers. The key parameters to achieve megahertz-bandwidth ionic communication with controllable propagation radius is unknown. Here, we describe the working principles of ionic communication (IC) and the effect of material and geometrical properties such as electrode area, material composition, ion type/concentration, and operating frequency on this process. For each of these properties, we performed electrochemical impedance spectroscopy (EIS) and a gain-bandwidth test. Using this approach, we were able to optimize the electrode design for different applications towards such as higher bandwidth or longer-range transmission. We then designed implantable transdermal IC electrodes with several MHz bandwidth. We used these devices to acquire high quality in vivo electrophysiological data from freely moving rats. The proposed working theory of IC and the resulting devices set the foundation for advanced biomedical implantable systems, such as brain-computer interfaces, that require a low consumption, high bandwidth wireless communication system.

PEDOT:PSS (Poly(3,4 ethylenedioxythiophene):polystyrene sulfonate) coated electrodes have been widely used to evaluate the integrity of cellular barriers. Nevertheless, an experimental and systematic study of the correlation between tissue integrity and impedance of the sensing device has not yet been conducted. In this work, with the use of impedance spectroscopy, we investigate the way that impedance ratio of the biological tissue to the recording device affects the recording ability of the latter. Various sized PEDOT:PSS coated electrodes are employed and the effect of their dimensions to their sensing ability is determined. A simple equivalent circuit is proposed to model the biotic/abiotic ensemble and an analytical expression of the total impedance as a function of frequency is extracted. The study results in a critical impedance ratio of the biological tissue to the sensor, which allows for efficient sensing of the tissue integrity. This work opens new pathways for the realization of improved impedance based biosensors with optimized sensitivity.

Organic electrochemical transistors (OECTs) have been widely used as transducers in electrophysiology and other biosensing applications. Their identifying characteristic is a transconductance that increases with channel thickness, and this provides a facile mechanism to achieve high signal amplification. However, little is known about their noise behaviour. Here, we investigate noise and extract metrics for signal-to-noise ratio and limit of detection in OECTs with different channel thickness. These metrics are shown to improve as channel thickness increases, but the improvement is not proportional and tends to saturate for thicker channels. The results show that OECTs can be easily optimised to show not only high amplification, but also low noise.

Organic electrochemical transistors (OECTs) have gained popularity for biological sensing, for a variety of applications, ranging from neural signal recording to ion detection, due to their amplification capabilities. OECTs are three terminal devices where the organic semiconducting channel’s bulk conductance is modulated via ionic/electronic interactions when operated in electrolyte. The main differences between OECTs and OFETs is the prevalence of bulk (as opposed to interfacial) charging and transport. As such, OECTs can take on smaller form factors, have high transconductance, and do not require the pristine interfaces that OFETs require which allows for more relaxed fabrication constraints. The same advantageous bulk charging, however, can result in parasitic capacitance when the mixed conductor/contact overlap in devices are large. In micro-OECT devices patterned photolithographically, these overlaps (which aid in layer-to-layer alignment) can account for >50% of the total measured device capacitance, slowing device response time.

In this work we simplify pattering and eliminate parasitic capacitance by fabricating OECTs with laser-cut channels and a self-aligning insulation. The self-aligning insulation is made of a hydrophobic coating that allows the semiconducting material to be “dragged and dropped” into the laser cut channel between the source and drain contacts, eliminating the need for photolithography, dry etching and a sacrificial insulation layer to pattern the organic material in the channel. The self-aligned OECTs showed one hundred percent yield and electronic properties (mobility and threshold voltage) similar to those fabricated with traditional methods. These self-aligned OECTs were also fabricated onto flexible substrates to show the applicability for biological applications.

This method of manufacturing was extended for use with vertical OECTs (vOECTs), by stacking another layer of gold contact to create a vertical channel between the source and drain contacts, which eliminates parasitic capacitance once again. Finally, we used these vOECTs to create simple circuits that allow for the possibility of simultaneous on-site amplification and signal processing, for applications such as differential and biochemical sensing, without increasing overall device footprint.

The recent progress in the extracellular microelectrode arrays (MEAs) have greatly improved our ability to probe cellular electrophysiological activities. Nevertheless, passive MEAs are subject to small signal-to-noise ratio and small potential detection.
Recently, organic electrochemical transistors (OECTs) have been identified as a promising device architecture to improve extracellular potentials recording in electroactive cells culture both in-vitro and in-vivo. In addition to unique properties of interest for electrophysiology such as biocompatibility, transparency and flexibility, OECTs operating principle is based on the transduction of ionic currents in the biological medium into electronic currents in the organic semiconductor (e.g. PEDOT:PSS) via electrochemical coupling. The transconductance represents an important figure of merit of OECTs and depends on geometrical and material parameters that rules largely OECTs performances for sensing electrophysiological signals. However, as an organic electronic technology, larger device-property distributions are often encountered with respect to the one of metal- or inorganic-based technologies, inherent to the very nature of the soft organic materials involved in the OECTs transduction process.

Here, we explore the possibility to tune post-fabrication material and geometrical parameters of OECTs with electropolymerization of EDOT. We show that this strategy can be used to simultaneously improve OECT transconductance and its geometrical capacitance. The addressed OECT chips were micro-fabricated on a glass substrates with spin coated PEDOT:PSS. Electropolymerization of EDOT on top of spin-coated PEDOT:PSS was carried on with both fix voltage and ramp voltage techniques. A detail impedance analysis was performed during OECTs functionalization. DC electrical characterizations was used to correlate the transconductance and capacitance tuning due to electropolymerization and to assess device performances improvements. Scanning Electron Microscopy (SEM) was used to correlate morphological changes due to electropolymerization with the enhancement in the transconductance and capacitance of the OECTs. Finally, we performed bio-compatibility assessment between primary neural cells culture and the different possible monomers used for electropolymerization to evaluate the possibility to improve affinity between cultured neurons and electropolymerized materials.
The key novelty of this material engineering technique is to propose a promising method for tunable OECTs sensors development. For instance, this back-end-of-line tuning technique can reduce chip variability in terms of performance yield and bring OECTs technology to the next maturity level. Furthermore, such flexibility can enable matching the electrochemical impedance of the device to the one of the cells, and in the future promote exploratory sensing missions, merging brain-inspired information processing with neuro-sensing.

Aptamer-based target biomolecule sensing is attracting much interest due to their excellent reception with high selectivity and specificity. The 3D structure coupled with the short strand length of aptamers enables detection within the Debye limit of ionic solutions [1]. Field effect transistors (FETs) offer an excellent platform to transduce and amplify the effective change in the charged aptamer morphology on target binding into electrical signals. However, these FETs need to be ultralow power and portable on flexible substrates for seamless integration with micro-probes or wearable devices. Amorphous oxide semiconductors such as InGaZnO (IGZO) demonstrate high mobility, low process complexity, good uniformity, stability in threshold voltage, and reduced leakage due to the wide bandgap [2]. Moreover, their low thermal budgets will allow them to be integrated heterogeneously on Si CMOS fabrication in Back End of Line (BEOL) processes. However, IGZO TFTs have been mostly studied at near threshold operations that require high bias voltages. Interestingly, recent reports have investigated Schottky barrier (SB) source and drain regions where the presence of two reverse biased Schottky diodes facilitate deep subthreshold operation due to gate-bias induced modulation of current injection or extraction at these terminals through the presence of a depletion region that impinges on the channel [3]. The diodes also enable early saturation of the transistor output curves and shield it from deleterious contact effects. Prior work has demonstrated SB formation in IGZO TFT with Ohmic metals such as Ti/Pt/Mo by modulating the vacancy concentration in IGZO via deposition under higher oxygen partial pressure [3], [4]. We report deep subthreshold SB IGZO TFTs by metal-IGZO contact engineering. Experimental verification of the deep subthreshold enabled ultra-low power operation with below unity switching voltages has been completed and almost infinite output resistance (~10¹² Ohms) and subsequent high intrinsic gain (>100) have been derived. Stability tests in this regime have demonstrated extremely low drifts in device behavior. A model is under development for the behavior of the device in this subthreshold region, influenced by a reverse biased Schottky and a diffusion/drift behavior. The low-voltage operation is important for biosensors, where this would allow sensing within the electrochemical window (< 1V) of the analyte solution, prevent local work function change of the electrodes, and enable stable bias stress operation to sample the analyte at regular intervals. We propose a platform of aptamer functionalized low power operation TFTs that can selectively and specifically detect physiological neurotransmitter (NT) concentrations in solutions to be used for diagnosis of mental disorders. Dopamine [2-(3,4-dihydroxyphenyl) ethylamine] is an important catecholamine NT that plays a crucial role in the central nervous, hormonal, renal and cardiovascular system and helps in the control of cognitive/motor and neuroendocrine functions. Imbalances in dopamine secretions can lead to neurological disorders such as post-traumatic stress disorder (PTSD), Parkinson’s, Schizophrenia and Alzheimer’s disease [1]. Therefore, preliminary tests have been conducted to evaluate TFTs for dopamine detection through a novel ‘click chemistry’ functionalization technique. This process incorporates the catalytic adhesion of an alkyne terminated molecule with an azide anchored aptamer which precludes the need for an intermediate molecular adhesion step. Hence, this would lead to a shorter overall bond length as well as lower steric hindrance. The aptamer was specifically synthesized for selectivity towards dopamine target and in vitro initial experiments with a lower limit of 10nM have been performed.
1. Science 2018 Oct 19;362(6412):319-324.
2. Appl. Phys. Lett. 92, 033502 (2008).
3. PNAS March 12, 2019 116 (11) 4843-4848.
4. Science 2016 Oct 21;354(6310):302-304.

Present characterization techniques for the tight junction modulation are either invasive, such as fluorescence marking^[1], or have low ion sensitivity, like trans-epithelial electrical resistance^[2]. The organic electrochemical transistor (OECT) is able to operate in aqueous solutions at low voltages, so it is a biocompatible, label-free and low-cost ion-to-electron converter.^[3] Therefore, OECTs are suited biosensors for drug delivery and related processes as cell barrier integrity.^[4] The passage of nutrients and drugs into the blood is regulated by the barrier functionality of a cell layer. Hence, modulating the barrier functionality by external chemical agents like poly-L-lysine (PLL) is crucial for drug delivery.^[5] The ability of a cell layer to impede the passage of ions through it and therefore to act as a barrier, can be assessed electrically by measuring the resistance across the cell layer. Here, we used an OECT in a current-driven configuration for the evaluation of reversible modulation of tight junctions in the epithelial colon carcinoma (Caco-2) cell line,^[6] found in the small intestine, over time. Operating the OECT in current-driven configuration enhances the sensitivity,^[7] required to study reversible ion permeability through tight junctions of Caco-2 cells. The reversible opening and closing of tight junctions was detected using the PLL at RT and under physiological conditions. Concentration and time dependent experiments resumed that the exposure to low and medium concentrations of PLL initiates reversible modulation, whereas a too high concentration induces an irreversible barrier disruption due to non-functional tight junction proteins. To support electrical measurements occluding-staining has been performed using immunofluorescence imaging. The results demonstrate the suitability of OECTs to in-situ monitor temporal barrier modulation and recovery, which can offer valuable information for drug delivery applications.

[1] M. Ramuz, A. Hama, M. Huerta, J. Rivnay, P. Leleux, R. M. Owens, Adv. Mater. 2014, 7083.
[2] L. H. Jimison, S. A. Tria, D. Khodagholy, Gurfinkel M., E. Lanzarini, A. Hama, G. G. Malliaras, R. M. Owens, Adv. Mater. 2012, 5919.
[3] J. Rivnay, P. Leleux, M. Sessolo, D. Khodagholy, T. Hervé, M. Fiocchi, G. G. Malliaras, Advanced materials (Deerfield Beach, Fla.) 2013, 25, 7010.
[4] N. Y. Shim, D. A. Bernards, D. J. Macaya, J. A. DeFranco, M. Nikolou, R. M. Owens, G. G. Malliaras, Sensors 2009, 9896.
[5] G.T.A. McEwan, M. A. Jepson, B. H. Hirst, N. L. Simmons, Biochemica et Biophysica Acta 1993, 1148, 51.
[6] M. S. Balda, K. Matter, Seminars in cell & developmental biology 2000, 11, 281.
[7] a) M. Ghittorelli, L. Lingstedt, P. Romele, N. I. Crăciun, Z.M. Kovács-Vajna, P.W.M. Blom, F. Torricelli, Nature Communications 2018, 1441; b) L. V. Lingstedt, M. Ghittorelli, M. Brückner, J. Reinholz, N. I. Crăciun, F. Torricelli, V. Mailänder, P. Gkoupidenis, P. W. M. Blom, Advanced healthcare materials 2019, 8, e1900128.

Organic Electrochemical Transistors (OECTs) have exhibited promising performance as transducers and amplifiers of low potentials due to their exceptional transconductance, enabled by the volumetric charging of Organic Mixed Ionic/Electronic Conductors (OMIECs) employed as the channel material. OECT performance in aqueous electrolytes as well as the OMIECs’ redox activity has spurred a myriad of studies employing OECTs as chemical transducers. However, the OECT’s large (potentiometrically derived) transconductance is not fully leveraged in common approaches that directly conduct chemical reactions amperometrically within the OECT electrolyte with direct charge transfer between the analyte and the OMIEC, which results in sub–unity transduction of gate to drain current. Hence, amperometric OECTs do not truly display current gains in the traditional sense, falling short of the expected transistor performance. We have demonstrated in our previous work an alternative device architecture that separates chemical transduction and amplification processes on two distinct electrochemical cells (1). This approach fully utilizes the OECT’s large transconductance to achieve current gains of 10³ and current modulations of four orders of magnitude. Furthermore, harvesting energy from the chemical reaction on the reaction cell renders the gate circuit self–powered. This simple yet overlooked transduction mechanism represents a general approach enabling high–gain chemical OECT transducers.

In this current work, we investigate the device physics behind the different architectures and operation modes of the OECT as redox chemical sensors. Analyses reveal that in the amperometric operation of the OECT, approaches to optimize both chemical transduction and OECT operation are challenging at best, and even contradictory in some cases. Furthermore, conducting faradaic reactions at non-equilibrium conditions within the OECT result in complex effects arising from voltage-dependent redox reaction rates, spatially asymmetric voltage variations across the channel, and mass transport dependencies. In contrast, the operation of the RC-OECT at equilibrium conditions allows for a simple model to describe its operation, enabling straightforward optimization of its performance by materials selection and device design. Furthermore, the separation of chemical transduction and electronic amplification on two separate compartments allow for more feasible optimization of the individual components.

This study sheds light on the challenges faced by the conventional approach of conducting redox sensing reactions within the electrolyte of the OECT. Additionally, it suggests a promising alternative approach to simultaneously achieve chemical transduction and reap the full amplification capabilities of the OECT, including a simple model for understanding device operation. The RC-OECT’s superior performance and predictability of device performance from fundamental materials properties will aid the future development of high-performance chemical sensors for early disease diagnosis.

1. S. T. M. Tan, A. Giovannitti, A. Melianas, M. Moser, B. L. Cotts, D. Singh, I. McCulloch, A. Salleo, Chemical to Electrical Transduction using Fuel Cell Powered Organic Electrochemical Transistors (2020), doi:10.26434/CHEMRXIV.12071136.V1.

Fluidity, mobility, and resistivity are key characteristics of native membranes in our cells where cation channels regulate numerous molecular mechanisms (e.g., the excitation of neurons). Interfacing with biomimetic membranes via electronic devices promises to expand our understanding of the function of these membranes in disease states. The challenge is to form supported lipid bilayer (SLBs) on (semi)conducting films while sustaining high sensitivity and operational stability. In this talk, I will show the traits of n-type organic semiconductors for such biological interfaces. I will show the very first ion channel sensor based on an n-type, accumulation mode, microscale organic electrochemical transistor (OECT), which interfaces an SLB embedded with a pore-forming protein. The polymer is particularly sensitive to divalent cations, endowing the system with biomimetic properties. The side-chain engineering of semiconducting polymers prompts the development of next-generation bioelectronic hybrids where electron-transporting polymers establish bilateral electronic communication with living systems.

A great demand exists for minimally-invasive neuromodulation technologies to enable next-generation bioelectronic medicine. We report on our developments of ultrathin organic (opto)electronic devices for neurostimulation. All of these devices rely on far red/near infrared irradiation in the tissue transparency window to actuate nanoscale organic semiconductor components. Our flagship technology is the organic electrolytic photocapacitor (OEPC) – a device that mimics biphasic current-pulse neurostimulation and thus transduces an optical signal into directly-evoked action potentials in neurons. These devices are not only wireless, but also 100-1000 times thinner than most existing technologies. We will discuss chronic implants capable of stimulating peripheral nerves (sciatic and vagus) when actuated from outside of the body using diode lasers. We have observed stable operation in rodent models for at least 100 days. As a different approach, we will report on favorable harnessing of DC electrochemistry effects, normally considered spurious and undesired. Specifically, we show how electrochemical generation of low doses of hydrogen peroxide can be used to reversibly stimulate M-type potassium channels, providing a novel method of controlling neural excitability.

Bioelectronic devices must have high speed and high performance to interact with the often rapid, low amplitude signals generated by neural tissue. They should also be capable of forming complementary logic and analog circuits to allow acquisition, processing and manipulation of a biological environment. However, the lack of readily available p-type polymers for use in enhancement mode transistors and their complicated synthesis significantly limits the investigation and application of such devices in bioelectronics. Here, we develop an enhancement mode ion-gated transistor (e-IGT) that functions based on a reversible redox reaction and hydrated ion reservoirs within the conducting polymer channel to enable long-term stable operation and shortened ion transit time. We combined a highly conductive poly(3, 4-ethylenedioxythiophene) doped with polystyrene sulfonate (PEDOT:PSS) and a reducing agent polyethylenimine (PEI) to enable doping of the polymer in an aqueous environment. The transistor operates as following: (1) the protonated PEI bonds with the PSS- and de-dopes the PEDOT:PSS chain resulting in an initial off-state of the transistor, (2) a subsequent application of negative gate voltage dopes the PEDOT:PSS switching the channel on. To determine an optimal transistor configuration and material composition, we microfabricated transistor arrays of varying geometrical parameters. The high-transconductance and high-speed of the e-IGTs resulted in a gain bandwidth product that is several orders of magnitude above other ion-based transistors. We used these transistors to acquire a wide range of electrophysiological signals, including in vivo recording of neural action potentials. Furthermore, we combined E-IGTs with d-IGTs to create a real-time non-linear rectifier to accurately detect epileptic discharges in vivo. In conclusion, E-IGTs offer a safe, reliable, and high performance building block for chronically implanted bioelectronics, with spatiotemporal resolution to the scale of individual neurons.

One of the neuromorphic engineering aims is using nanoelectronics’ materials and devices to reproduce key features that are used by the brain for computing. Currently, neuromorphic engineering has explored standard silicon-based technologies (i.e. such as complementary metal–oxide–semiconductor ) or more emerging material and devices (iono-electronic materials and resistive memory devices, for example). Most of these technologies are still bounded to a top down approach. However, brain computing largely rely on bottom-up processes. For instance, interconnectivity between cells and formation of communication pathway in neural networks result principaly from bottom-up organization. Here, we show how dendritic growth of organic conductive polymers (PEDOT) can be used to mimic structural branching observed in neural network. Conducting-polymer based dendritic structures with different morphology are synthesized in a two-electrode setup by pulsed voltage-driven electropolymerization derived from state-of-the-art bipolar AC-electrochemical synthetic methods. We show how various AC signals can lead to a large variety of dendritic structures and PEDOT morphologies. In a second part, such dendritic structures are used to implement functionnal OECTs. More importantly, we focus on the transconductance and memory effects that can be obtained in such dendritic OECTs such as short tem plasticity. We report on the relationship between dendrites morphologies and STP time constant.
This work paves the way to new approaches for neuromorphic engineering, such as structural plasticity and neural network topology exploration.

We focus on developing a new class of nanoscale materials and novel strategies for the investigation of biological entities at multiple length scales, from the molecular level to complex cellular networks. Our highly flexible bottom-up nanomaterials synthesis capabilities allow us to form unique hybrid-nanomaterials that can be used in various input/output bioelectrical interfaces. For example, we have developed several bioelectrical platforms based on graphene, a two-dimensional (2D) atomically thin carbon allotrope. We have demonstrated recording of the electrical activity of excitable cells with graphene-based ultra-microelectrodes as small as the size as an axon ca. 2µm in size. Using graphene-based hybrid-nanomaterials, we have formed remote, non-genetic bioelectrical interfaces with excitable cells and modulated cellular and network activity with high precision and low needed power. We have also developed a breakthrough bioelectrical interface, a 3D self-rolled biosensor arrays (3D-SR-BAs) of either active field effect transistors or passive microelectrodes to measure both cardiac and neural spheroids electrophysiology in 3D. Our approach enables electrophysiological investigation and monitoring of the complex signal transduction in 3D cellular assemblies toward an organ-on-an-electronic-chip (organ-on-e-chip) platform for tissue maturation investigations and development of drugs for disease treatment. In summary, the exceptional synthetic control and flexible assembly of nanomaterials provide powerful tools for fundamental studies and applications in life science and open up the potential to seamlessly merge either nanomaterials-based platforms or unique nanosensor geometries and topologies with cells, fusing nonliving and living systems together.

The rapid rise of bioelectronics has enabled a number of innovative approaches to treat neurological disorders, restore and repair lost functions, and modulate neural circuitry to control mood and behavior. The vast majority of bioelectronic interfaces, however, still rely on traditional noble metal and silicon materials, which are expensive to source and process and are intrinsically inadequate to address the mechanical, chemical, and electrical properties of excitable circuits in the body. To establish bioelectronic technologies as the clinical standard, significant innovations in materials and fabrication strategies are still required to achieve safety, softness, biocompatibility, and long-term stability. Nanostructured materials are uniquely positioned to address these challenges because they combine high electrical conductivity with intrinsically high mass-specific surface area and mechanical flexibility. Furthermore, they can interact with biological systems on a molecular scale and can be easily integrated within scalable solution-based processing, thus allowing easy modulation of their electronic and optical properties.
In this talk I will present the ongoing work in my lab to leverage the unique properties of nanostructured materials and engineer novel bioelectronic interfaces for sensing and stimulating neural and neuromuscular circuits. Specifically, I will describe the fundamental electronic, electrochemical, and mechanical properties of 2D transition metal carbides nanomaterials (a.k.a. MXenes) and how these translate into significant impedance and noise reduction when MXenes are integrated bioelectronic interfaces. Then, I will present ad hoc, scalable, rapid manufacturing processes developed in my lab to translate the exceptional material properties at the molecular scale into high-resolution, low impedance bioelectronic interfaces that are also compatible with clinical neuroimaging modalities, such a magnetic resonance imaging (MRI) and computerized tomography (CT). Finally, to illustrate the potential of MXene-based bioelectronics, I will present different example of applications in both implantable and wearable interfaces.

Organic electrochemical transistors (OECTs) have recently attracted tremendous attention in the scientific community due to their unique set of features. Especially, the lack of an insulating layer that separates gate from channel, like in conventional field effect transistors, and their feature to support electrolyte gating open new pathways in the field of bioelectronics. In particular, they render the device ideal for measurements in the aqueous environments that dominate the biological world. Additionally, they provide extra possibilities in terms of the device architecture as the gating electrode can be patterned on the same plane as the conducting channel. In a set up like that, what is of extreme interest is the effect of the gate position with respect to the channel. In this work, the spatiotemporal response of a poly(3,4 ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) OECT is examined and its device physics is studied. This work opens new routes for exploiting the physics of multi terminal OECT devices in bioelectronics, sensing and neuromorphic electronics.

Understanding how a network of interconnected neurons receives, stores, and processes information in the human brain is one of the outstanding scientific challenges of our time. The ability to reliably detect neuroelectric activities is essential to addressing this challenge. Optical recording using voltage-sensitive fluorescent probes has provided unprecedented flexibility for choosing regions of interest in recording neuronal activities. However, when recording at a high frame rate such as 500 to 1,000 Hz, fluorescence-based voltage sensors often suffer from photobleaching and phototoxicity, which limit the recording duration. Here, we report an approach called electrochromic optical recording (ECORE) that achieves label-free optical recording of spontaneous neuroelectrical activities. ECORE utilizes the electrochromism of poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) thin films, whose optical absorption can be modulated by an applied voltage. Being based on optical reflection instead of fluorescence, ECORE offers the flexibility of an optical probe without suffering from photobleaching or phototoxicity. Using ECORE, we optically recorded spontaneous action potentials in cardiomyocytes, cultured hippocampal and dorsal root ganglion neurons, and brain slices. With minimal perturbation to cells, ECORE allows long-term optical recording over multiple days

One of the most important scientific and technological frontiers of our time is the interfacing of electronics with the human brain. This endeavour promises to help understand how the brain works and deliver new tools for diagnosis and treatment of pathologies including epilepsy and Parkinson’s disease. Current solutions, however, are limited by the materials that are brought in contact with the tissue and transduce signals across the biotic/abiotic interface. Recent advances in organic electronics have made available materials with a unique combination of attractive properties, including mechanical flexibility, mixed ionic/electronic conduction, enhanced biocompatibility, and capability for drug delivery. I will present examples of novel devices for recording and stimulation of neurons and show that organic electronic materials offer tremendous opportunities to study the brain and treat its pathologies.