Conducting polymer scaffolds: a new frontier in bioelectronics and bioengineering

Conducting polymer (CP) scaffolds have emerged as a transformative tool in bioelectronics and bioengineering, advancing the ability to interface with biological systems. Their unique combination of electrical conductivity, tailorability, and biocompatibility surpasses the capabilities of traditional nonconducting scaffolds while granting them access to the realm of bioelectronics. This review examines recent developments in CP scaffolds, focusing on material and device advancements, as well as their interplay with biological systems. We highlight applications for monitoring, tissue stimulation, and drug delivery and discuss perspectives and challenges currently faced for their ultimate translation and clinical implementation.

Multi-Sensor Origami Platform: A Customizable System for Obtaining Spatiotemporally Precise Functional Readouts in 3D Models

Bioprinting technology offers unprecedented opportunities to construct in vitro tissue models that recapitulate the 3D morphology and functionality of native tissue. Yet, it remains difficult to obtain adequate functional readouts from such models. In particular, it is challenging to position sensors in desired locations within pre-fabricated 3D bioprinted structures. At the same time, bioprinting tissue directly onto a sensing device is not feasible due to interference with the printer head. As such, a multi-sensing platform inspired by origami that overcomes these challenges by "folding" around a separately fabricated 3D tissue structure is proposed, allowing for the insertion of electrodes into precise locations, which are custom-defined using computer-aided-design software. The multi-sensing origami platform (MSOP) can be connected to a commercial multi-electrode array (MEA) system for data-acquisition and processing. To demonstrate the platform, how integrated 3D MEA electrodes can record neuronal electrical activity in a 3D model of a neurovascular unit is shown. The MSOP also enables a microvascular endothelial network to be cultured separately and integrated with the 3D tissue structure. Accordingly, how impedance-based sensors in the platform can measure endothelial barrier function is shown. It is further demonstrated the device's versatility by using it to measure neuronal activity in brain organoids.

A Conformable Organic Electronic Device for Monitoring Epithelial Integrity at the Air Liquid Interface

Air liquid interfaced (ALI) epithelial barriers are essential for homeostatic functions such as nutrient transport and immunological protection. Dysfunction of such barriers are implicated in a variety of autoimmune and inflammatory disorders and, as such, sensors capable of monitoring barrier health are integral for disease modelling, diagnostics and drug screening applications. To date, gold-standard electrical methods for detecting barrier resistance require rigid electrodes bathed in an electrolyte, which limits compatibility with biological architectures and is non-physiological for ALI. This work presents a flexible all-planar electronic device capable of monitoring barrier formation and perturbations in human respiratory and intestinal cells at ALI. By interrogating patient samples with electrochemical impedance spectroscopy and simple equivalent circuit models, disease-specific and patient-specific signatures are uncovered. Device readouts are validated against commercially available chopstick electrodes and show greater conformability, sensitivity and biocompatibility. The effect of electrode size on sensing efficiency is investigated and a cut-off sensing area is established, which is one order of magnitude smaller than previously reported. This work provides the first steps in creating a physiologically relevant sensor capable of mapping local and real-time changes of epithelial barrier function at ALI, which will have broad applications in toxicology and drug screening applications.

In Vitro Models for Investigating Intestinal Host-Pathogen Interactions

Infectious diseases are increasingly recognized as a major threat worldwide due to the rise of antimicrobial resistance and the emergence of novel pathogens. In vitro models that can adequately mimic in vivo gastrointestinal physiology are in high demand to elucidate mechanisms behind pathogen infectivity, and to aid the design of effective preventive and therapeutic interventions. There exists a trade-off between simple and high throughput models and those that are more complex and physiologically relevant. The complexity of the model used shall be guided by the biological question to be addressed. This review provides an overview of the structure and function of the intestine and the models that are developed to emulate this. Conventional models are discussed in addition to emerging models which employ engineering principles to equip them with necessary advanced monitoring capabilities for intestinal host-pathogen interrogation. Limitations of current models and future perspectives on the field are presented.

Organic Electronic Platform for Real-Time Phenotypic Screening of Extracellular-Vesicle-Driven Breast Cancer Metastasis

Tumor-derived extracellular vesicles (TEVs) induce the epithelial-to-mesenchymal transition (EMT) in nonmalignant cells to promote invasion and cancer metastasis, representing a novel therapeutic target in a field severely lacking in efficacious antimetastasis treatments. However, scalable technologies that allow continuous, multiparametric monitoring for identifying metastasis inhibitors are absent. Here, the development of a functional phenotypic screening platform based on organic electrochemical transistors (OECTs) for real-time, noninvasive monitoring of TEV-induced EMT and screening of antimetastatic drugs is reported. TEVs derived from the triple-negative breast cancer cell line MDA-MB-231 induce EMT in nonmalignant breast epithelial cells (MCF10A) over a nine-day period, recapitulating a model of invasive ductal carcinoma metastasis. Immunoblot analysis and immunofluorescence imaging confirm the EMT status of TEV-treated cells, while dual optical and electrical readouts of cell phenotype are obtained using OECTs. Further, heparin, a competitive inhibitor of cell surface receptors, is identified as an effective blocker of TEV-induced EMT. Together, these results demonstrate the utility of the platform for TEV-targeted drug discovery, allowing for facile modeling of the transient drug response using electrical measurements, and provide proof of concept that inhibitors of TEV function have potential as antimetastatic drug candidates.

3D organic bioelectronics for electrical monitoring of human adult stem cells

Three-dimensional in vitro stem cell models have enabled a fundamental understanding of cues that direct stem cell fate. While sophisticated 3D tissues can be generated, technology that can accurately monitor these complex models in a high-throughput and non-invasive manner is not well adapted. Here we show the development of 3D bioelectronic devices based on the electroactive polymer poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate)-(PEDOT:PSS) and their use for non-invasive, electrical monitoring of stem cell growth. We show that the electrical, mechanical and wetting properties as well as the pore size/architecture of 3D PEDOT:PSS scaffolds can be fine-tuned simply by changing the processing crosslinker additive. We present a comprehensive characterization of both 2D PEDOT:PSS thin films of controlled thicknesses, and 3D porous PEDOT:PSS structures made by the freeze-drying technique. By slicing the bulky scaffolds we generate homogeneous, porous 250 μm thick PEDOT:PSS slices, constituting biocompatible 3D constructs able to support stem cell cultures. These multifunctional slices are attached on indium-tin oxide substrates (ITO) with the help of an electrically active adhesion layer, enabling 3D bioelectronic devices with a characteristic and reproducible, frequency dependent impedance response. This response changes drastically when human adipose derived stem cells (hADSCs) grow within the porous PEDOT:PSS network as revealed by fluorescence microscopy. The increase of cell population within the PEDOT:PSS porous network impedes the charge flow at the interface between PEDOT:PSS and ITO, enabling the interface resistance (R1) to be used as a figure of merit to monitor the proliferation of stem cells. The non-invasive monitoring of stem cell growth allows for the subsequent differentiation 3D stem cell cultures into neuron like cells, as verified by immunofluorescence and RT-qPCR measurements. The strategy of controlling important properties of 3D PEDOT:PSS structures simply by altering processing parameters can be applied for development of a number of stem cell in vitro models as well as stem cell differentiation pathways. We believe the results presented here will advance 3D bioelectronic technology for both fundamental understanding of in vitro stem cell cultures as well as the development of personalized therapies.

In situbiosensing technologies for an organ-on-a-chip

Thein vitrosimulation of organs resolves the accuracy, ethical, and cost challenges accompanyingin vivoexperiments. Organoids and organs-on-chips have been developed to model thein vitro, real-time biological and physiological features of organs. Numerous studies have deployed these systems to assess thein vitro, real-time responses of an organ to external stimuli. Particularly, organs-on-chips can be most efficiently employed in pharmaceutical drug development to predict the responses of organs before approving such drugs. Furthermore, multi-organ-on-a-chip systems facilitate the close representations of thein vivoenvironment. In this review, we discuss the biosensing technology that facilitates thein situ, real-time measurements of organ responses as readouts on organ-on-a-chip systems, including multi-organ models. Notably, a human-on-a-chip system integrated with automated multi-sensing will be established by further advancing the development of chips, as well as their assessment techniques.

In vitro models to study human gut-microbiota interactions: Applications, advances, and limitations

In vitro models of the human gut help compensate for the limitations of animal models in studying the human gut-microbiota interaction and are indispensable in the clarification the mechanism of microbial action or in the high-throughput screening and functional evaluation of probiotics. The development of these models constitutes a rapidly developing field of research. From 2D¹ to 3D² and from simple to complex, several in vitro cell and tissue models have been developed and continuously improved. In this review, we categorized and summarized these models and described their development, applications, advances, and limitations by using specific examples. We also highlighted the best ways to select an appropriate in vitro model, and we also discussed which variables to consider when imitating microbial and human gut epithelial interactions.

Getting closer to modeling the gut-brain axis using induced pluripotent stem cells

The gut microbiome (GM), the gut barrier, and the blood-brain barrier (BBB) are key elements of the gut-brain axis (GBA). The advances in organ-on-a-chip and induced pluripotent stem cell (iPSCs) technology might enable more physiological gut-brain-axis-on-a-chip models. The ability to mimic complex physiological functions of the GBA is needed in basic mechanistic research as well as disease research of psychiatric, neurodevelopmental, functional, and neurodegenerative diseases, such as Alzheimer’s disease and Parkinson’s disease. These brain disorders have been associated with GM dysbiosis, which may affect the brain via the GBA. Although animal models have paved the way for the breakthroughs and progression in the understanding of the GBA, the fundamental questions of exactly when, how, and why still remain unanswered. The research of the complex GBA have relied on equally complex animal models, but today’s ethical knowledge and responsibilities demand interdisciplinary development of non-animal models to study such systems. In this review we briefly describe the gut barrier and BBB, provide an overview of current cell models, and discuss the use of iPSCs in these GBA elements. We highlight the perspectives of producing GBA chips using iPSCs and the challenges that remain in the field.

Organ-on-a-chip with integrated semitransparent organic electrodes for barrier function monitoring

Organs-on-a-chip (OoC) are cell culture platforms that replicate key functional units of tissues in vitro. Barrier integrity and permeability evaluation are of utmost importance when studying barrier-forming tissues. Impedance spectroscopy is a powerful tool and is widely used to monitor barrier permeability and integrity in real-time. However, data comparison across devices is misleading due to the generation of a non-homogenous field across the tissue barrier, making impedance data normalization very challenging. In this work, we address this issue by integrating PEDOT:PSS electrodes for barrier function monitoring with impedance spectroscopy. The semitransparent PEDOT:PSS electrodes cover the entire cell culture membrane providing a homogenous electric field across the entire membrane making the cell culture area equally accountable to the measured impedance. To the best of our knowledge, PEDOT:PSS has never been used solely to monitor the impedance of cellular barriers while enabling optical inspection in the OoC. The performance of the device is demonstrated by lining the device with intestinal cells where we monitored barrier formation under flow conditions, as well as barrier disruption and recovery under exposure to a permeability enhancer. The barrier tightness and integrity, and the intercellular cleft have been evaluated by analyzing the full impedance spectrum. Furthermore, the device is autoclavable paving the way toward more sustainable OoC options.

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.

Conducting Polymer-ECM Scaffolds for Human Neuronal Cell Differentiation

3D cell culture formats more closely resemble tissue architecture complexity than 2D systems, which are lacking most of the cell-cell and cell-microenvironment interactions of the in vivo milieu. Scaffold-based systems integrating natural biomaterials are extensively employed in tissue engineering to improve cell survival and outgrowth, by providing the chemical and physical cues of the natural extracellular matrix (ECM). Using the freeze-drying technique, porous 3D composite scaffolds consisting of poly(3,4-ethylene-dioxythiophene) doped with polystyrene sulfonate (PEDOT:PSS), containing ECM components (i.e., collagen, hyaluronic acid, and laminin) are engineered for hosting neuronal cells. The resulting scaffolds exhibit a highly porous microstructure and good conductivity, determined by scanning electron microscopy and electrochemical impedance spectroscopy, respectively. These supports boast excellent mechanical stability and water uptake capacity, making them ideal candidates for cell infiltration. SH-SY5Y human neuroblastoma cells show enhanced cell survival and proliferation in the presence of ECM compared to PEDOT:PSS alone. Whole-cell patch-clamp recordings acquired from differentiated SHSY5Y cells in the scaffolds demonstrate that ECM constituents promote neuronal differentiation in situ. These findings reinforce the usability of 3D conducting supports as engineered highly biomimetic and functional in vitro tissue-like platforms for drug or disease modeling.

Organic electronic transmembrane device for hosting and monitoring 3D cell cultures

3D cell models have made strides in the past decades in response to failures of 2D cultures to translate targets during the drug discovery process. Here, we report on a novel multiwell plate bioelectronic platform, namely, the e-transmembrane, capable of supporting and monitoring complex 3D cell architectures. Scaffolds made of PEDOT:PSS [poly(3,4-ethylenedioxythiophene):polystyrene sulfonate] are microengineered to function as separating membranes for compartmentalized cell cultures, as well as electronic components for real-time in situ recordings of cell growth and function. Owing to the high surface area-to-volume ratio, the e-transmembrane allows generation of deep, stratified tissues within the porous bulk and cell polarization at the apico-basal domains. Impedance spectroscopy measurements carried out throughout the tissue growth identified signatures from different cellular systems and allowed extraction of critical functional parameters. This platform has the potential to become a universal tool for biologists for the next generation of high-throughput drug screening assays.

Recent advances in biofabricated gut models to understand the gut-brain axis in neurological diseases

Increasing evidence has accumulated that gut microbiome dysbiosis could be linked to neurological diseases, including both neurodegenerative and psychiatric diseases. With the high prevalence of neurological diseases, there is an urgent need to elucidate the underlying mechanisms between the microbiome, gut, and brain. However, the standardized aniikmal models for these studies have critical disadvantages for their translation into clinical application, such as limited physiological relevance due to interspecies differences and difficulty interpreting causality from complex systemic interactions. Therefore, alternative in vitro gut–brain axis models are highly required to understand their related pathophysiology and set novel therapeutic strategies. In this review, we outline state-of-the-art biofabrication technologies for modeling in vitro human intestines. Existing 3D gut models are categorized according to their topographical and anatomical similarities to the native gut. In addition, we deliberate future research directions to develop more functional in vitro intestinal models to study the gut–brain axis in neurological diseases rather than simply recreating the morphology.

Investigation of Host-Microbe-Parasite Interactions in an In Vitro 3D Model of the Vertebrate Gut

In vitro models of the gut-microbiome axis are in high demand. Conventionally, intestinal monolayers grown on Transwell setups are used to test the effects of commensals/pathogens on the barrier integrity, both under homeostatic and pathophysiological conditions. While such models remain valuable for deepening the understanding of host-microbe interactions, often, they lack key biological components that mediate this intricate crosstalk. Here, a 3D in vitro model of the vertebrate intestinal epithelium, interfaced with immune cells surviving in culture for over 3 weeks, is developed and applied to proof-of-concept studies of host-microbe interactions. More specifically, the establishment of stable host-microbe cocultures is described and functional and morphological changes in the intestinal barrier induced by the presence of commensal bacteria are shown. Finally, evidence is provided that the 3D vertebrate gut models can be used as platforms to test host-microbe-parasite interactions. Exposure of gut-immune-bacteria cocultures to helminth "excretory/secretory products" induces in vivo-like up-/down-regulation of certain cytokines. These findings support the robustness of the modular in vitro cell systems for investigating the dynamics of host-microbe crosstalk and pave the way toward new approaches for systems biology studies of pathogens that cannot be maintained in vitro, including parasitic helminths.

Advanced Materials and Sensors for Microphysiological Systems: Focus on Electronic and Electrooptical Interfaces

Advanced in vitro cell culture systems or microphysiological systems (MPSs), including microfluidic organ-on-a-chip (OoC), are breakthrough technologies in biomedicine. These systems recapitulate features of human tissues outside of the body. They are increasingly being used to study the functionality of different organs for applications such as drug evolutions, disease modeling, and precision medicine. Currently, developers and endpoint users of these in vitro models promote how they can replace animal models or even be a better ethically neutral and humanized alternative to study pathology, physiology, and pharmacology. Although reported models show a remarkable physiological structure and function compared to the conventional 2D cell culture, they are almost exclusively based on standard passive polymers or glass with none or minimal real-time stimuli and readout capacity. The next technology leap in reproducing in vivo-like functionality and real-time monitoring of tissue function could be realized with advanced functional materials and devices. This review describes the currently reported electronic and optical advanced materials for sensing and stimulation of MPS models. In addition, an overview of multi-sensing for Body-on-Chip platforms is given. Finally, one gives the perspective on how advanced functional materials could be integrated into in vitro systems to precisely mimic human physiology.

Vertical Organic Electrochemical Transistors and Electronics for Low Amplitude Micro-Organ Signals

Electrical signals are fundamental to key biological events such as brain activity, heartbeat, or vital hormone secretion. Their capture and analysis provide insight into cell or organ physiology and a number of bioelectronic medical devices aim to improve signal acquisition. Organic electrochemical transistors (OECT) have proven their capacity to capture neuronal and cardiac signals with high fidelity and amplification. Vertical PEDOT:PSS-based OECTs (vOECTs) further enhance signal amplification and device density but have not been characterized in biological applications. An electronic board with individually tuneable transistor biases overcomes fabrication induced heterogeneity in device metrics and allows quantitative biological experiments. Careful exploration of vOECT electric parameters defines voltage biases compatible with reliable transistor function in biological experiments and provides useful maximal transconductance values without influencing cellular signal generation or propagation. This permits successful application in monitoring micro-organs of prime importance in diabetes, the endocrine pancreatic islets, which are known for their far smaller signal amplitudes as compared to neurons or heart cells. Moreover, vOECTs capture their single-cell action potentials and multicellular slow potentials reflecting micro-organ organizations as well as their modulation by the physiological stimulator glucose. This opens the possibility to use OECTs in new biomedical fields well beyond their classical applications.

Engineering Hydrogels for the Development of Three-Dimensional In Vitro Models

The superiority of in vitro 3D cultures over conventional 2D cell cultures is well recognized by the scientific community for its relevance in mimicking the native tissue architecture and functionality. The recent paradigm shift in the field of tissue engineering toward the development of 3D in vitro models can be realized with its myriad of applications, including drug screening, developing alternative diagnostics, and regenerative medicine. Hydrogels are considered the most suitable biomaterial for developing an in vitro model owing to their similarity in features to the extracellular microenvironment of native tissue. In this review article, recent progress in the use of hydrogel-based biomaterial for the development of 3D in vitro biomimetic tissue models is highlighted. Discussions of hydrogel sources and the latest hybrid system with different combinations of biopolymers are also presented. The hydrogel crosslinking mechanism and design consideration are summarized, followed by different types of available hydrogel module systems along with recent microfabrication technologies. We also present the latest developments in engineering hydrogel-based 3D in vitro models targeting specific tissues. Finally, we discuss the challenges surrounding current in vitro platforms and 3D models in the light of future perspectives for an improved biomimetic in vitro organ system.

High-Current-Density Organic Electrochemical Diodes Enabled by Asymmetric Active Layer Design

Owing to their outstanding electrical/electrochemical performance, operational stability, mechanical flexibility, and decent biocompatibility, organic mixed ionic-electronic conductors have shown great potential as implantable electrodes for neural recording/stimulation and as active channels for signal switching/amplifying transistors. Nonetheless, no studies exist on a general design rule for high-performance electrochemical diodes, which are essential for highly functional circuit architectures. In this work, generalizable electrochemical diodes with a very high current density over 30 kA cm^-2 are designed by introducing an asymmetric active layer based on organic mixed ionic-electronic conductors. The underlying mechanism on polarity-sensitive balanced ionic doping/dedoping is elucidated by numerical device analysis and in operando spectroelectrochemical potential mapping, while the general material requirements for electrochemical diode operation are deduced using various types of conjugated polymers. In parallel, analog signal rectification and digital logic processing circuits are successfully demonstrated to show the broad impact of circuits incorporating organic electrochemical diodes. It is expected that organic electrochemical diodes will play vital roles in realizing multifunctional soft bioelectronic circuitry in combination with organic electrochemical transistors.

Organic Bioelectronics for In Vitro Systems

Bioelectronics have made strides in improving clinical diagnostics and precision medicine. The potential of bioelectronics for bidirectional interfacing with biology through continuous, label-free monitoring on one side and precise control of biological activity on the other has extended their application scope to in vitro systems. The advent of microfluidics and the considerable advances in reliability and complexity of in vitro models promise to eventually significantly reduce or replace animal studies, currently the gold standard in drug discovery and toxicology testing. Bioelectronics are anticipated to play a major role in this transition offering a much needed technology to push forward the drug discovery paradigm. Organic electronic materials, notably conjugated polymers, having demonstrated technological maturity in fields such as solar cells and light emitting diodes given their outstanding characteristics and versatility in processing, are the obvious route forward for bioelectronics due to their biomimetic nature, among other merits. This review highlights the advances in conjugated polymers for interfacing with biological tissue in vitro, aiming ultimately to develop next generation in vitro systems. We showcase in vitro interfacing across multiple length scales, involving biological models of varying complexity, from cell components to complex 3D cell cultures. The state of the art, the possibilities, and the challenges of conjugated polymers toward clinical translation of in vitro systems are also discussed throughout.

Advances in modelling the human microbiome-gut-brain axis in vitro

The human gut microbiome has emerged as a key player in the bidirectional communication of the gut–brain axis, affecting various aspects of homeostasis and pathophysiology. Until recently, the majority of studies that seek to explore the mechanisms underlying the microbiome–gut–brain axis cross-talk, relied almost exclusively on animal models, and particularly gnotobiotic mice. Despite the great progress made with these models, various limitations, including ethical considerations and interspecies differences that limit the translatability of data to human systems, pushed researchers to seek for alternatives. Over the past decades, the field of in vitro modelling of tissues has experienced tremendous growth, thanks to advances in 3D cell biology, materials, science and bioengineering, pushing further the borders of our ability to more faithfully emulate the in vivo situation. The discovery of stem cells has offered a new source of cells, while their use in generating gastrointestinal and brain organoids, among other tissues, has enabled the development of novel 3D tissues that better mimic the native tissue structure and function, compared with traditional assays. In parallel, organs-on-chips technology and bioengineered tissues have emerged as highly promising alternatives to animal models for a wide range of applications. Here, we discuss how recent advances and trends in this area can be applied in host–microbe and host–pathogen interaction studies. In addition, we highlight paradigm shifts in engineering more robust human microbiome-gut-brain axis models and their potential to expand our understanding of this complex system and hence explore novel, microbiome-based therapeutic approaches.

Electrolyte-gated transistors for enhanced performance bioelectronics

Electrolyte-gated transistors (EGTs), capable of transducing biological and biochemical inputs into amplified electronic signals and stably operating in aqueous environments, have emerged as fundamental building blocks in bioelectronics. In this Primer, the different EGT architectures are described with the fundamental mechanisms underpinning their functional operation, providing insight into key experiments including necessary data analysis and validation. Several organic and inorganic materials used in the EGT structures and the different fabrication approaches for an optimal experimental design are presented and compared. The functional bio-layers and/or biosystems integrated into or interfaced to EGTs, including self-organization and self-assembly strategies, are reviewed. Relevant and promising applications are discussed, including two-dimensional and three-dimensional cell monitoring, ultra-sensitive biosensors, electrophysiology, synaptic and neuromorphic bio-interfaces, prosthetics and robotics. Advantages, limitations and possible optimizations are also surveyed. Finally, current issues and future directions for further developments and applications are discussed.