Conducting Polymer-Reinforced Laser-Irradiated Graphene as a Heterostructured 3D Transducer for Flexible Skin Patch Biosensors.

Flexible skin patch biosensors are promising for the noninvasive determination of physiological parameters in perspiration for fitness and health monitoring. However, various prerequisites need to be met for the development of such biosensors, including the creation of a flexible conductive platform, bending/contact stability, fast electrochemical kinetics, and immobilization of biomolecules. Here, we describe a conducting polymer-reinforced laser-irradiated graphene (LIG) network as a heterostructured three-dimensional (3D) transducer for flexible skin patch biosensors. LIG with a hierarchically interconnected graphene structure is geometrically patterned on polyimide via localized laser irradiation as a flexible conductive platform, which is then reinforced by poly(3,4-ethylenedioxythiophene) (PEDOT) as a conductive binder (PEDOT/LIG) with improved structural/contact stability and electrochemical kinetics. The interconnected pores of the reinforced PEDOT/LIG function as a 3D host matrix for high loading of "artificial" (Prussian blue, PB) and natural enzymes (lactate oxidase, LOx), forming a compact and heterostructured 3D transducer (LOx/PB-PEDOT/LIG) for lactate biosensing with excellent sensitivity (11.83 µA mM-1). We demonstrated the fabrication of flexible skin patch biosensors comprising a custom-built integrated three-electrode system achieve amperometric detection of lactate in artificial sweat over a wide physiological linear range of 0-18 mM. The advantage of this facile and versatile transducer is further illustrated by the development of a folded 3D wristband lactate biosensor and a dual channel biosensors for simultaneous monitoring of lactate and glucose. This innovative design concept of a heterostructured transducer for flexible biosensors combined with a versatile fabrication approach could potentially drive the development of new wearable and skin-mountable biosensors for monitoring various physiological parameters in biofluids for noninvasive fitness and health management.

Precise and rapid solvent-assisted geometric protein self-patterning with submicron spatial resolution for scalable fabrication of microelectronic biosensors.

Precise and high-resolution coupling of functional proteins with micro-transducers is critical for the manufacture of miniaturized bioelectronic devices. Moreover, electrochemistry on microelectrodes has had a major impact on electrochemical analysis and sensor technologies, since the small size of microelectrode affects the radial diffusion flux of the analyte to deliver enhanced mass transport and electrode kinetics. However, a large technology gap has existed between the process technology associated with such microelectronics and the conventional bio-conjugation techniques that are generally used. Here, we report on a high-resolution and rapid geometric protein self-patterning (GPS) method using solvent-assisted protein-micelle adsorption printing to couple biomolecules onto microelectrodes with a minimum feature size of 5 µm and a printing time of about a minute. The GPS method is versatile for micropatterning various biomolecules including enzymes, antibodies and avidin-biotinylated proteins, delivering good geometric alignment and preserving biological functionality. We further demonstrated that enzyme-coupled microelectrodes for glucose detection exhibited good electrochemical performance which benefited from the GPS method to maximize effective signal transduction at the bio-interface. These microelectrode arrays maintained fast convergent analyte diffusion displaying typical steady-state I-V characteristics, fast response times, good linear sensitivity (0.103 nA mm-2 mM-1, R2 = 0.995) and an ultra-wide linear dynamic range (2-100 mM). Our findings provide a new technical solution for the precise and accurate coupling of biomolecules to a microelectronic array with important implications for the scaleup and manufacture of diagnostics, biofuel cells and bioelectronic devices that could not be realized economically by other existing techniques.

Processable and nanofibrous polyaniline:polystyrene-sulphonate (nano-PANI:PSS) for the fabrication of catalyst-free ammonium sensors and enzyme-coupled urea biosensors.

Tailoring conducting polymers (CPs) such as polyaniline (PANI) to deliver the appropriate morphology, electrochemical properties and processability is essential for the development of effective polymer-based electrochemical sensors and biosensors. Composite PANI electrodes for the detection of ammonium (NH4+) have been previously reported, but have been limited by their reliance on the electrocatalytic reaction between NH4+ and a metal/nano-catalyst. We report an advanced processable and nanofibrous polyaniline:polystyrene-sulphonate (nano-PANI:PSS) as a functional ink for the fabrication of catalyst-free NH4+ sensors and enzyme-coupled urea biosensors. The PSS provides both a soft-template for nanofibre formation and a poly-anionic charge compensator, enabling the detection of NH4+ based on an intrinsic doping/de-doping mechanism. The nanostructured morphology, chemical characteristics and electrochemical properties of the nano-PANI:PSS were characterised. We fabricated 3D-hierarchical sensor interfaces composed of inter-connected nano-PANI:PSS fibres (diameter of ~50.3 ± 4.8 nm) for the detection of NH4+ with a wide linear range of 0.1-11.5 mM (R2 = 0.996) and high sensitivity of 106 mA M-1 cm-2. We further demonstrated the coupling of the enzyme urease with the nano-PANI:PSS to create a urea biosensor with an innovative biocatalytic product-to-dopant relay mechanism for the detection of urea, with a linear range of 0.2-0.9 mM (R2 = 0.971) and high sensitivity of 41 mA M-1 cm-2. Moreover, the nano-PANI:PSS-based sensors show good selectivity for the detection of NH4+and urea in a urine model containing common interfering molecules. This processable and fibrous nano-PANI:PSS provides new advance on CP-based transducer materials in the emerging field of printed organic sensors and biosensors.

Tunable 3D nanofibrous and bio-functionalised PEDOT network explored as a conducting polymer-based biosensor.

Conducting polymers that possess good electrochemical properties, nanostructured morphology and functionality for bioconjugation are essential to realise the concept of all-polymer-based biosensors that do not depend on traditional nanocatalysts such as carbon materials, metal, metal oxides or dyes. In this research, we demonstrated a facile approach for the simultaneous preparation of a bi-functional PEDOT interface with a tunable 3D nanofibrous network and carboxylic acid groups (i.e. Nano-PEDOT-COOH) via controlled co-polymerisation of EDOT and EDOT-COOH monomers, using tetrabutylammonium perchlorate as a soft-template. By tuning the ratio between EDOT and EDOT-COOH monomer, the nanofibrous structure and carboxylic acid functionalisation of Nano-PEDOT-COOH were varied over a fibre diameter range of 15.6 ± 3.7 to 70.0 ± 9.5 nm and a carboxylic acid group density from 0.03 to 0.18 µmol cm-2. The nanofibres assembled into a three-dimensional network with a high specific surface area, which contributed to low charge transfer resistance and high transduction activity towards the co-enzyme NADH, delivering a wide linear range of 20-960 µM and a high sensitivity of 0.224 µA µM-1 cm-2 at the Nano-PEDOT-COOH50% interface. Furthermore, the carboxylic acid groups provide an anchoring site for the stable immobilisation of an NADH-dependent dehydrogenase (i.e. lactate dehydrogenase), via EDC/S-NHS chemistry, for the fabrication of a Bio-Nano-PEDOT-based biosensor for lactate detection which had a response time of less than 10 s over the range of 0.05-1.8 mM. Our developed bio-Nano-PEDOT interface shows future potential for coupling with multi-biorecognition molecules via carboxylic acid groups for the development of a range of advanced all-polymer biosensors.

Soft and flexible material-based affinity sensors.

Recent advances in biosensors and point-of-care (PoC) devices are poised to change and expand the delivery of diagnostics from conventional lateral-flow assays and test strips that dominate the market currently, to newly emerging wearable and implantable devices that can provide continuous monitoring. Soft and flexible materials are playing a key role in propelling these trends towards real-time and remote health monitoring. Affinity biosensors have the capability to provide for diagnosis and monitoring of cancerous, cardiovascular, infectious and genetic diseases by the detection of biomarkers using affinity interactions. This review tracks the evolution of affinity sensors from conventional lateral-flow test strips to wearable/implantable devices enabled by soft and flexible materials. Initially, we highlight conventional affinity sensors exploiting membrane and paper materials which have been so successfully applied in point-of-care tests, such as lateral-flow immunoassay strips and emerging microfluidic paper-based devices. We then turn our attention to the multifarious polymer designs that provide both the base materials for sensor designs, such as PDMS, and more advanced functionalised materials that are capable of both recognition and transduction, such as conducting and molecularly imprinted polymers. The subsequent content discusses wearable soft and flexible material-based affinity sensors, classified as flexible and skin-mountable, textile materials-based and contact lens-based affinity sensors. In the final sections, we explore the possibilities for implantable/injectable soft and flexible material-based affinity sensors, including hydrogels, microencapsulated sensors and optical fibers. This area is truly a work in progress and we trust that this review will help pull together the many technological streams that are contributing to the field.

Electrochemical detection of DNA mismatches using a branch-shaped hierarchical SWNT-DNA nano-hybrid bioelectrode.

Common approaches for DNA mutation detection are high cost and have difficult or complex procedure. We propose a fast quantitative method for recognition of DNA mutation based on SWNT/DNA self-assembled nanostructure. Covalent SWNT/DNA hybrid nanostructures are widely used in the fabrication of electrochemical biosensors. Interfacing carbon nanotubes with DNA in particular, is used as a detection method for the analysis of genetic disorders or the detection of mismatches in DNA hybridisation. We have designed a self-assembled, branch-shaped hybrid nanostructure by hybridisation of two sticky oligos that are attached to the ends of SWNTs via a linker oligo. These hybrid nanostructures showed a good conductivity that was greater than free SWNTs. Impedance spectroscopy studies illustrated that the conductivity of these hybrid nanostructures depended on the conformation and structure of the hybridised DNA. We demonstrated that the strategy of using SWNT/DNA self-assembled hybrid nanostructure fabrication yields sensitive and selective tools to discriminate mismatches in DNA. Cyclic voltammetry (CV) and impedance spectroscopy clearly revealed that the conductivity of the branch-shaped and hierarchical hybridised SWNT/DNA nanostructure is higher when matched, than when mismatched in a 1 and 1' hybridised SWNT/DNA nanostructure. Rapid biosensing of match and mismatch nanostructure based on carbon printed electrode showed similar results which can be used for rapid and fast detection of DNA mismatch.

Modulating Electrode Kinetics for Discrimination of Dopamine by a PEDOT:COOH Interface Doped with Negatively Charged Tricarboxylate.

The rapidly developing field of conducting polymers in organic electronics has many implications for bioelectronics. For biosensing applications, tailoring the functionalities of the conducting polymer's surface is an efficient approach to improve both sensitivity and selectivity. Here, we demonstrated a facile and economic approach for the fabrication of a high-density, negatively charged carboxylic-acid-group-functionalized PEDOT (PEDOT:COOH) using an inexpensive ternary carboxylic acid, citrate, as a dopant. The polymerization efficiency was significantly improved by the addition of LiClO4 as a supporting electrolyte yielding a dense PEDOT:COOH sensing interface. The resulting PEDOT:COOH interface had a high surface density of carboxylic acid groups of 0.129 µmol/cm2 as quantified by the toluidine blue O (TBO) staining technique. The dopamine response measured with the PEDOT:COOH sensing interface was characterized by cyclic voltammetry with a significantly reduced ΔEp of 90 mV and a 3-fold increase in the Ipa value compared with those of the nonfunctionalized PEDOT sensing interface. Moreover, the cyclic voltammetry and electrochemical impedance spectroscopy results demonstrated the increased electrode kinetics and highly selective discrimination of dopamine (DA) in the presence of the interferents ascorbic acid (AA) and uric acid (UA), which resulted from the introduction of negatively charged carboxylic acid groups. The negatively charged carboxylic acid groups could favor the transfer, preconcentration, and permeation of positively charged DA to deliver improved sensing performance while repelling the negatively charged AA and UA interferents. The PEDOT:COOH interface facilitated measurement of dopamine over the range of 1-85 µM, with a sensitivity of 0.228 µA µM-1, which is 4.1 times higher than that of a nonfunctionalized PEDOT electrode (0.055 µA µM-1). Our results demonstrate the feasibility of a simple and economic fabrication of a high-density PEDOT:COOH interface for chemical sensing, which also has the potential for coupling with other biorecognition molecules via carboxylic acid moieties for the development of a range of advanced PEDOT-based biosensors.

Artificial Muscles Powered by Glucose.

Untethered actuation is important for robotic devices to achieve autonomous motion, which is typically enabled by using batteries. Using enzymes to provide the required electrical charge is particularly interesting as it will enable direct harvesting of fuel components from a surrounding fluid. Here, a soft artificial muscle is presented, which uses the biofuel glucose in the presence of oxygen. Glucose oxidase and laccase enzymes integrated in the actuator catalytically convert glucose and oxygen into electrical power that in turn is converted into movement by the electroactive polymer polypyrrole causing the actuator to bend. The integrated bioelectrode pair shows a maximum open-circuit voltage of 0.70 ± 0.04 V at room temperature and a maximum power density of 0.27 µW cm-2 at 0.50 V, sufficient to drive an external polypyrrole-based trilayer artificial muscle. Next, the enzymes are fully integrated into the artificial muscle, resulting in an autonomously powered actuator that can bend reversibly in both directions driven by glucose and O2 only. This autonomously powered artificial muscle can be of great interest for soft (micro-)robotics and implantable or ingestible medical devices manoeuvring throughout the body, for devices in regenerative medicine, wearables, and environmental monitoring devices operating autonomously in aqueous environments.

A repertoire of biomedical applications of noble metal nanoparticles.

Noble metals comprise any of several metallic chemical elements that are outstandingly resistant to corrosion and oxidation, even at elevated temperatures. This group is not strictly defined, but the tentative list includes ruthenium, rhodium, palladium, silver, osmium, iridium, platinum and gold, in order of atomic number. The emerging properties of noble metal nanoparticles are attracting huge interest from the translational scientific community and have led to an unprecedented expansion of research and exploration of applications in biotechnology and biomedicine. Noble metal nanomaterials can be synthesised both by top-down and bottom up approaches, as well as via organism-assisted routes, and subsequently modified appropriately for the field of use. Nanoscale analogues of gold, silver, platinum, and palladium in particular, have gained primary importance owing to their excellent intrinsic properties and diversity of applications; they offer unique functional attributes, which are quite unlike the bulk material. Modulation of noble metal nanoparticles in terms of size, shape and surface functionalisation has endowed them with unusual capabilities and manipulation at the chemical level, which can lead to changes in their electrical, chemical, optical, spectral and other intrinsic properties. Such flexibility in multi-functionalisation delivers 'Ockham's razor' to applied biomedical science. In this feature article, we highlight recent advances in the adaptation of noble metal nanomaterials and their biomedical applications in therapeutics, diagnostics and sensing.

Integrated Printed Microfluidic Biosensors.

Integrated printed microfluidic biosensors are one of the most recent point-of-care (POC) sensor developments. Fast turnaround time for production and ease of customization, enabled by the integration of recognition elements and transducers, are key for on-site biosensing for both healthcare and industry and for speeding up translation to real-life applications. Here, we provide an overview of recent progress in printed microfluidics, from the 2D to the 4D level, accompanied by novel sensing element integration. We also explore the latest trends in integrated printed microfluidics for healthcare, especially POC diagnostics, and food safety applications.

Generic Neutravidin Biosensor for Simultaneous Multiplex Detection of MicroRNAs via Electrochemically Encoded Responsive Nanolabels.

Current electrochemical biosensors for multiple miRNAs require tedious immobilization of various nucleic acid probes. Here, we demonstrate an innovative approach using a generic neutravidin biosensor combined with electrochemically encoded responsive nanolabels for facile and simultaneous multiplexed detection of miRNA-21 and miRNA-141. The selectivity of the biosensor arises from the intrinsic properties of the electrochemically encoded responsive nanolabels, comprising biotinylated molecular beacons (biotin-MB) and metal nanoparticles (metal-NPs). The procedure is a simple one-pot assay, where the targeted miRNA causes the opening of biotin-MB followed by capturing of the biotin-MB-metal-NPs by the neutravidin biosensor and simultaneous detection of the captured metal-NPs by stripping square-wave voltammetry (SSWV). The multiplexed detection of miRNA-21 and miRNA-141 is achieved by differentiation of the electrochemical signature (i.e., the peak current) for the different metal-NP labels. The biosensor delivers simultaneous detection of miRNAs with a linear range of 0.5-1000 pM for miRNA-21 and a limit of detection of 0.3 pM (3σ/sensitivity,  n = 3), and a range of 50-1000 pM for miRNA-141, with a limit of detection of 10 pM. Furthermore, we demonstrate multiplexed detection of miRNA-21 and miRNA-141 in a spiked serum sample.

Integrating an ex-vivo skin biointerface with electrochemical DNA biosensor for direct measurement of the protective effect of UV blocking agents.

Skin cancer is the most frequent kind of cancer in white people in many parts of the world. UV-induced DNA damage and genetic mutation can subsequently lead to skin cancer. Therefore development of new biosensing strategies for detection of UV-induced DNA damage is of great importance. Here we demonstrate a novel combination of an ex-vivo skin biointerface and an electrochemical DNA sensor for the direct detection of UV induced DNA damage and investigation the protective effect of various UV blockers (Zinc-oxide (ZnO), titanium-dioxide (TiO2) nanoparticles (NPs) and sunscreens) against DNA damage. A diazonium modified screen-printed carbon electrode immobilized with a DNA sequence related to the p53 tumour suppressor gene, the most commonly affected gene in human UV-induced skin cancer, was applied as an electrochemical DNA sensor. Electrochemical impedance spectroscopy (EIS) was employed for the detection of DNA damage induced by UV-A radiation by following the changes in charge transfer resistance (Rct). The protective effects of UV blockers applied onto a pig skin surface (a suitable model representing human skin) were successfully detected by the DNA sensor. We observed that the naked skin has little UV protection showing an 18.2% decreases in ∆R/R values compared to the control, while applying both NPs and NP-formulated sunscreens could significantly reduce DNA damage, resulting in a decrease in ∆R/R values of 67.1% (ZnO NPs), 77.2% (TiO2 NPs), 77.1% (sunscreen 1) and 92.4% (sunscreen 2), respectively. Moreover, doping moisturising cream with NPs could provide a similar DNA protective effect. This new method is a biologically relevant alternative to animal testing and offers advantages such as fast, easy and inexpensive processing, in addition to its miniaturised dimension, and could be used for a range of applications in other sources of DNA damage and the protective effect of different UV blocking agents and other topical formulations.

Positively-charged hierarchical PEDOT interface with enhanced electrode kinetics for NADH-based biosensors.

Poly(ethylenedioxythiophene) (PEDOT) has attracted considerable attention as an advanced electrode material for electrochemical sensors and biosensors, due to its unique electrical and physicochemical properties. Here, we demonstrate the facile preparation of a positively-charged and hierarchical micro-structured PEDOT electrochemical interface with enhanced electrode kinetics for the electrooxidation of NADH. Processable PEDOT colloidal microparticles (PEDOT CMs) were synthesised by template-assisted polymerisation and were then utilised as building blocks for the fabrication of hierarchically-structured electrodes with a larger accessible electroactive surface (2.8 times larger than that of the benchmark PEDOT:PSS) and inter-particle space, thus improving electrode kinetics. The intrinsic positive charge of the PEDOT CMs further facilitated the detection of negatively-charged molecules by electrostatic accumulation. Due to the synergistic effect, these hierarchically-structured PEDOT CMs electrodes exhibited improved NADH electrooxidation at lower potentials and enhanced electrocatalytic activity compared to the compact structure of conventional PEDOT:PSS electrodes. The PEDOT CMs electrodes detected NADH over the range of 20-240 µM, with a sensitivity of 0.0156 µA/µM and a limit of detection of 5.3 µM. Moreover, the PEDOT CMs electrode exhibited a larger peak separation from the interferent ascorbic acid, and improved stability. This enhanced analytical performance for NADH provides a sound basis for further work coupling to a range of NAD-dependent dehydrogenases for applications in biosensing, bio-fuel cells and biocatalysis.

Type I Collagen-Derived Injectable Conductive Hydrogel Scaffolds as Glucose Sensors.

The advent of home blood glucose monitoring revolutionized diabetes management, and the recent introduction of both wearable devices and closed-loop continuous systems has enormously impacted the lives of people with diabetes. We describe the first fully injectable soft electrochemical glucose sensor for in situ monitoring. Collagen, the main component of a native extracellular matrix in humans and animals, was used to fabricate an in situ gellable self-supporting electroconductive hydrogel that can be injected onto an electrode surface or into porcine meat to detect glucose amperometrically. The study provides a proof-of-principle of an injectable electrochemical sensor suitable for monitoring tissue glucose levels that may, with further development, prove clinically useful in the future.

An electrochemical immunosensor for cardiac Troponin I using electrospun carboxylated multi-walled carbon nanotube-whiskered nanofibres.

A sandwich-type nanostructured immunosensor based on carboxylated multi-walled carbon nanotube (CMWCNT)-embedded whiskered nanofibres (WNFs) was developed for detection of cardiac Troponin I (cTnI). WNFs were directly fabricated on glassy carbon electrodes (GCE) by removing the sacrificial component (polyethylene glycol, PEG) after electrospinning of polystyrene/CMWCNT/PEG nanocomposite nanofibres, and utilised as a transducer layer for enzyme-labeled amperometric immunoassay of cTnI. The whiskered segments of CMWCNTs were activated and utilised to immobilise anti-cTnT antibodies. It was observed that the anchored CMWCNTs within the nanofibres were suitably stabilised with excellent electrochemical repeatability. A sandwich-type immuno-complex was formed between cTnI and horseradish peroxidase-conjugated anti-cTnI (HRP-anti-cTnI). The amperometric responses of the immunosensor were studied using cyclic voltammetry (CV) through an enzymatic reaction between hydrogen peroxide and HRP conjugated to the secondary antibody. The nanostructured immunosensor delivered a wide detection range for cTnI from the clinical borderline for a normal person (0.5-2ngmL-1) to the concentration present in myocardial infarction patients (> 20ngmL-1), with a detection limit of ~ 0.04ngmL-1. It also showed good reproducibility and repeatability for three different cTnI concentration (1, 10 and 25ngmL-1) with satisfactory relative standard deviations (RSD). Hence, the proposed nanostructured immunosensor shows potential for point-of-care testing.

Processable enzyme-hybrid conductive polymer composites for electrochemical biosensing.

A new approach for the facile fabrication of electrochemical biosensors using a biohybrid conducting polymer was demonstrated using glucose oxidase (GOx) and poly (3, 4-ethylenedioxythiophene) (PEDOT) as a model. The biohybrid conducting polymer was prepared based on a template-assisted chemical polymerisation leading to the formation of PEDOT microspheres (PEDOT-MSs), followed by in-situ deposition of platinum nanoparticles (PtNPs) and electrostatic immobilisation of glucose oxidase (GOx) to form water processable GOx-PtNPs-PEDOT-MSs. The morphology, chemical composition and electrochemical performance of the GOx-PtNPs-PEDOT-MS-based glucose biosensor were characterised using scanning electron microscopy (SEM), energy-dispersive X-ray spectrometry (EDS), Fourier transform infrared (FTIR) spectroscopy, zeta potential and electrochemical measurements, respectively. The biosensor delivered a linear response for glucose over the range 0.1-10mM (R2 = 0.9855) with a sensitivity of 116.25µAmM-1cm-2, and limit of detection of 1.55µM (3×SD/sensitivity). The sensitivity of the developed PEDOT-MS based biosensor is significantly higher (2.7 times) than the best reported PEDOT-based glucose biosensor in the literature. The apparent Michaelis-Menten constant (Kmapp) of the GOx-PtNPs-PEDOT-MS-based biosensors was calculated as 7.3mM. Moreover, the biosensor exhibited good storage stability, retaining 97% of its sensitivity after 12 days storage. This new bio-hybrid conducting polymer combines the advantages of micro-structured morphology, compatibility with large-scale manufacturing processes, and intrinsic biocatalytic activity and conductivity, thus demonstrating its potential as a convenient material for printed bioelectronics and sensors.

Printable Heterostructured Bioelectronic Interfaces with Enhanced Electrode Reaction Kinetics by Intermicroparticle Network.

Printable organic bioelectronics provide a fast and cost-effective approach for the fabrication of novel biodevices, while the general challenge is to achieve optimized reaction kinetics at multiphase boundaries between biomolecules and electrodes. Here, we present an entirely new concept based on a modular approach for the construction of heterostructured bioelectronic interfaces by using tailored functional "biological microparticles" combined with "transducer microparticles" as modular building blocks. This approach offers high versatility for the design and fabrication of bioelectrodes with a variety of forms of interparticle spatial organization, from layered-structures to more advance bulk heterostructured architectures. The heterostructured biocatalytic electrodes delivered twice the reaction rate and a six-fold increase in the effective diffusion kinetics in response to a catalytic model using glucose as the substrate, together with the advantage of shortened diffusion paths for reactants between multiple interparticle junctions and large active particle surface. The consequent benefits of this improved performance combined with the simple means of mass production are of major significance for the emerging printed electronics industry.