Advances in Bioelectrode Design for Developing Electrochemical Biosensors

The critical performance factors such as selectivity, sensitivity, operational and storage stability, and response time of electrochemical biosensors are governed mainly by the function of their key component, the bioelectrode. Suitable design and fabrication strategies of the bioelectrode interface are essential for realizing the requisite performance of the biosensors for their practical utility. A multifaceted attempt to achieve this goal is visible from the vast literature exploring effective strategies for preparing, immobilizing, and stabilizing biorecognition elements on the electrode surface and efficient transduction of biochemical signals into electrical ones (i.e., current, voltage, and impedance) through the bioelectrode interface with the aid of advanced materials and techniques. The commercial success of biosensors in modern society is also increasingly influenced by their size (and hence portability), multiplexing capability, and coupling in the interface of the wireless communication technology, which facilitates quick data transfer and linked decision-making processes in real-time in different areas such as healthcare, agriculture, food, and environmental applications. Therefore, fabrication of the bioelectrode involves careful selection and control of several parameters, including biorecognition elements, electrode materials, shape and size of the electrode, detection principles, and various fabrication strategies, including microscale and printing technologies. This review discusses recent trends in bioelectrode designs and fabrications for developing electrochemical biosensors. The discussions have been delineated into the types of biorecognition elements and their immobilization strategies, signal transduction approaches, commonly used advanced materials for electrode fabrication and techniques for fabricating the bioelectrodes, and device integration with modern electronic communication technology for developing electrochemical biosensors of commercial interest.

Inkjet Printing: A Viable Technology for Biosensor Fabrication

Printing technology promises a viable solution for the low-cost, rapid, flexible, and mass fabrication of biosensors. Among the vast number of printing techniques, screen printing and inkjet printing have been widely adopted for the fabrication of biosensors. Screen printing provides ease of operation and rapid processing; however, it is bound by the effects of viscous inks, high material waste, and the requirement for masks, to name a few. Inkjet printing, on the other hand, is well suited for mass fabrication that takes advantage of computer-aided design software for pattern modifications. Furthermore, being drop-on-demand, it prevents precious material waste and offers high-resolution patterning. To exploit the features of inkjet printing technology, scientists have been keen to use it for the development of biosensors since 1988. A vast number of fully and partially inkjet-printed biosensors have been developed ever since. This study presents a short introduction on the printing technology used for biosensor fabrication in general, and a brief review of the recent reports related to virus, enzymatic, and non-enzymatic biosensor fabrication, via inkjet printing technology in particular.

Micro and Nanoscale Technologies for Diagnosis of Viral Infections

Viral infection is one of the leading causes of mortality worldwide. The growth of globalization significantly increases the risk of virus spreading, making it a global threat to future public health. In particular, the ongoing coronavirus disease 2019 (COVID-19) pandemic outbreak emphasizes the importance of devices and methods for rapid, sensitive, and cost-effective diagnosis of viral infections in the early stages by which their quick and global spread can be controlled. Micro and nanoscale technologies have attracted tremendous attention in recent years for a variety of medical and biological applications, especially in developing diagnostic platforms for rapid and accurate detection of viral diseases. This review addresses advances of microneedles, microchip-based integrated platforms, and nano- and microparticles for sampling, sample processing, enrichment, amplification, and detection of viral particles and antigens related to the diagnosis of viral diseases. Additionally, methods for the fabrication of microchip-based devices and commercially used devices are described. Finally, challenges and prospects on the development of micro and nanotechnologies for the early diagnosis of viral diseases are highlighted.

3D printed nanomaterial-based electronic, biomedical, and bioelectronic devices

The ability to seamlessly integrate functional materials into three-dimensional (3D) constructs has been of significant interest, as it can enable the creation of multifunctional devices. Such integration can be achieved with a multiscale, multi-material 3D printing strategy. This technology has enabled the creation of unique devices such as personalized tissue regenerative scaffolds, biomedical implants, 3D electronic devices, and bionic constructs which are challenging to realize with conventional manufacturing processes. In particular, the incorporation of nanomaterials into 3D printed devices can endow a wide range of constructs with tailorable mechanical, chemical, and electrical functionalities. This review highlights the advances and unique possibilities in the fabrication of novel electronic, biomedical, and bioelectronic devices that are realized by the synergistic integration of nanomaterials with 3D printing technologies.

Fully Printed μ-Needle Electrode Array from Conductive Polymer Ink for Bioelectronic Applications

Microelectrode arrays (MEAs) are widely used platforms in bioelectronics to study electrogenic cells. In recent years, the processing of conductive polymers for the fabrication of three-dimensional electrode arrays has gained increasing interest for the development of novel sensor designs. Here, additive manufacturing techniques are promising tools for the production of MEAs with three-dimensional electrodes. In this work, a facile additive manufacturing process for the fabrication of MEAs that feature needle-like electrode tips, so-called μ-needles, is presented. To this end, an aerosol-jet compatible PEDOT:PSS and multiwalled carbon nanotube composite ink with a conductivity of 323 ± 75 S m^-1 is developed and used in a combined inkjet and aerosol-jet printing process to produce the μ-needle electrode features. The μ-needles are fabricated with a diameter of 10 ± 2 μm and a height of 33 ± 4 μm. They penetrate an inkjet-printed dielectric layer to a height of 12 ± 3 μm. After successful printing, the electrochemical properties of the devices are assessed via cyclic voltammetry and impedance spectroscopy. The μ-needles show a capacitance of 242 ± 70 nF at a scan rate of 5 mV s^-1 and an impedance of 128 ± 22 kΩ at 1 kHz frequency. The stability of the μ-needle MEAs in aqueous electrolyte is demonstrated and the devices are used to record extracellular signals from cardiomyocyte-like HL-1 cells. This proof-of-principle experiment shows the μ-needle MEAs' cell-culture compatibility and functional integrity to investigate electrophysiological signals from living cells.

Ultrasoft Silicone Gel as a Biomimetic Passivation Layer in Inkjet-Printed 3D MEA Devices

Multielectrode arrays (MEAs) are versatile tools that are used for chronic recording and stimulation of neural cells and tissues. Driven by the recent progress in understanding of how neuronal growth and function respond to scaffold stiffness, development of MEAs with a soft cell-to-device interface has gained importance not only for in vivo but also for in vitro applications. However, the passivation layer, which constitutes the majority of the cell-device interface, is typically prepared with stiff materials. Herein, a fabrication of an MEA device with an ultrasoft passivation layer is described, which takes advantage of inkjet printing and a polydimethylsiloxane (PDMS) gel with a stiffness comparable to that of the brain. The major challenge in using the PDMS gel is that it cannot be patterned to expose the sensing area of the electrode. This issue is resolved by printing 3D micropillars at the electrode tip. Primary cortical neurons are grown on the fabricated device, and effective stimulation of the culture confirms functional cell-device coupling. The 3D MEA device with an ultrasoft interface provides a novel platform for investigating evoked activity and drug responses of living neuronal networks cultured in a biomimetic environment for both fundamental research and pharmaceutical applications.

Inkjet Printing of Sc-Doped TiO2 with Enhanced Photoactivity

Here we report the methodology for nanocomposite fabrication based on the inkjet printing technique. Doped TiO2 nanoparticles with Sc contents up to 10 wt.% were synthesized and adapted towards a facile fabrication of microscale structures and thin film printing. Implementation of the state-of-the-art low-temperature synthesis allowed to us successfully incorporate high concentrations of Sc3+ ions into the TiO2 lattice and improve the light absorption characteristics of the resulting materials. Without affecting the anatase structure substantially, Sc doping gave rise to an intensified absorbance capacity and provided the means for the efficient fabrication of Sc-TiO2 microarchitectures via the inkjet printing technique. The changes in the spectral and structural characteristics of the Sc-TiO2 composites were observed by Energy Dispersive X-Ray spectroscopy (EDX), X-ray diffraction (XRD), and UV-vis methods. The rheological parameters of the colloidal suspension based on the synthesized Sc-TiO2 nanoparticles were adapted for inkjet printing in terms of the optimal viscosity, morphology, and surface tension. The developed individual ink characteristics allowed us to produce a close coherence between the enhanced optical properties of the Sc-TiO2 prepared the sol–gel method and the inkjet-printed films. The introduced methodology features the possibility to inkjet-print doped and pure TiO2 robust films for potential large-scale fabrication.

Three-Dimensional CeO2 Woodpile Nanostructures To Enhance Performance of Enzymatic Glucose Biosensors

Fabrication of detection elements with ultrahigh surface area is essential for improving the sensitivity of analyte detection. Here, we report a direct patterning technique to fabricate three-dimensional CeO₂ nanoelectrode arrays for biosensor application over relatively large areas. The fabrication approach, which employs nanoimprint lithography and a CeO₂ nanoparticle-based ink, enables the direct, high-throughput patterning of nanostructures and is scalable, integrable, and of low cost. With the convenience of sequential imprinting, multilayered woodpile nanostructures with prescribed numbers of layers were achieved in a "stacked-up" architecture and were successfully fabricated over large areas. To demonstrate application as a biosensor, an enzymatic glucose sensor was developed. The sensitivity of glucose sensors can be enhanced simply by increasing the number of layers, which multiplies surface area while maintaining a constant footprint. The four-layer woodpile nanostructure of CeO₂ glucose sensor exhibited enhanced sensitivity (42.8 μA mM^-1 cm^-2) and good selectivity. This direct imprinting strategy for three-dimensional sensing architectures is potentially extendable to other electroactive materials and other sensing applications.