Development of a low-cost graphene-based impedance biosensor

PhD ThesisThe current applicability and accuracy of point-of-care devices is limited, with the need of future technologies to simultaneously target multiple analytes in complex human samples. Graphene’s discovery has provided a valuable opportunity towards the development of high performance biosensors. The quality and surface properties of graphene devices are critical for biosensing applications with a preferred low contact resistance interface between metal and graphene. However, each graphene production method currently results in inconsistent properties, quality and defects thus limiting its application towards mass production. Also, post-production processing, patterning and conventional lithography-based contact deposition negatively impact graphene properties due to chemical contamination. The work of this thesis focuses on the development of fully-functional, label-free graphene-based biosensors and a proof-of-concept was established for the detection of prostate specific antigen (PSA) in aqueous solution using graphene platforms. Extensive work was carried out to characterize different graphene family nanomaterials in order to understand their potential for biosensing applications. Two graphene materials, obtained via a laser reduction process, were selected for further investigations: reduced graphene oxide (rGO) and laser induced graphene from polyimide (LIG). Electrically conductive, porous and chemically active to an extent, these materials offer the advantage of simultaneous production and patterning as capacitive biosensing structures, i.e. interdigitated electrode arrays (IDE). Aiming to enhance the sensitivity of these biosensors, a novel, radio-frequency (RF) detection method was investigated and compared with conventional electrochemical impedance spectroscopy (EIS) on a well-known biocompatible material: gold (standard). It was shown that the RF detection methods require careful design and testing setup, with conventional EIS performing better in the given conditions. The method was further used on rGO and LIG IDE devices for the electrochemical impedance detection of PSA to assess the feasibility of the graphene based materials as biosensors. The graphene-based materials were successfully functionalized via the available carboxylic groups, using the EDC-NHS chemistry. Despite the difficulty of producing reproducible graphene-based electrodes, highly required for biosensor development, extensive testing was carried out to understand their feasibility. The calibration curves obtained via successive PSA addition showed a moderate-to-high ii sensitivity of both rGO and LIG IDE. However, further adsorption and drift testing underlined some major limitations in the case of LIG, due to its complex morphology and large porosity. To enable low contact resistance to these biosensors, the electroless nickel coating process is shown to be compatible with various graphene-based materials. This was demonstrated by tuning the chemical nickel bath and method conditions for pristine graphene and rGO for nickel contacts deposition

Electrical and Electro-Optical Biosensors

Electrical and electro-optical biosensing technologies are critical to the development of innovative POCT devices, which can be used by both professional and untrained personnel for the provision of necessary health information within a short time for medical decisions to be determined, being especially important in an era of global pandemics. This Special Issue includes a few pioneering works concerning biosensors utilizing electrochemical impedance, localized surface plasmon resonance, and the bioelectricity of sensing materials in which the amount of analyte is pertinent to the signal response. The presented results demonstrate the potential of these label-free biosensing approaches in the detection of disease-related small-molecule metabolites, proteins, and whole-cell entities

High-resolution 3D direct-write prototyping for healthcare applications

The healthcare sector has much to benefit from the vast array of novelties erupting from the manufacturing world. 3D printing (additive manufacturing) is amongst the most promising recent inventions with much research concentrated around the various approaches of 3D printing and applying this effectively in the health sector. Amongst these methods, the direct-write assembly approach is a promising candidate for rapid prototyping and manufacturing of miniaturised medical devices/sensors and in particular, miniaturised flexible capacitive pressure sensors. Microstructuring the dielectric medium of capacitive pressure sensors enhances the sensitivity of the capacitive pressure sensor. The structuring has been predominantly achieved with photolithography and similar subtractive approaches. In this project high-resolution 3D direct write printing was used to fabricate structured dielectric mediums for capacitive pressure sensors. This involved the development and rheological characterisation of printability-tuned water soluble polyvinyl pyrrolidone (PVP) based inks (10%-30% polymer content) for stable high-resolution 3D printing. These inks were used to print water soluble micromoulds that were filled and cured with otherwise difficult to structure low G’ materials like PDMS. Our approach essentially decouples ink synthesis from printability at the micrometre scale. The developed micro moulding approach was employed for printing pyramidal micro moulds, that were used as templates for fabricating pyramid structured dielectric mediums for capacitive pressure sensing. The power of the approach was used to alter the microstructures and reap enhanced pressure sensing characteristics for effective miniaturised capacitive pressure sensors. A pressure sensing ring – that could be worn by doctors and surgeons – was prototyped with our approach and employed successfully to monitor in real-time the radial pulse signal of a 29 year old male volunteer. The print resolution of the inks was enhanced by formulating and rheologically characterising a PVP/PVDF polymer blend ink that would wet the printing nozzle less due to the hydrophobicity of the PVDF

Graphene/P(VDF-TrFE) Heterojunction Based Wearable Sensors with Integrated Piezoelectric Energy Harvester

Graphene, with its outstanding material properties, including high carrier mobility, atomically thin nature, and ability to tolerate mechanical deformation related strain up to 20% before breaking, make it very attractive for developing highly sensitive and conformable strain/pressure sensor for wearable electronics. Unfortunately, graphene by itself is not piezoresistive, so developing a strain sensor utilizing just graphene is challenging. Fortunately, graphene synthesized on Cu foil can be transferred to arbitrary substrates (preserving its high quality), including flexible polymer substrates, which will allow the overall flexibility and conformability of the sensing element, to be maintained. Furthermore, a graphene/polymer based sensor devices can be easily patterned into an array over dimensions reaching several feet, taking advantage of large area synthesis of graphene, which will make the ultimate sensor very inexpensive. If a piezo-electric polymer, such as P(VDF-TrFE), is chosen to form a heterojunction with graphene, it will strongly affect the carrier density in graphene, due to the fixed charge developing on its surface under strain or pressure. Taking advantage of the high carrier mobility in graphene, such a charge change can result in very high sensitivity to pressure and strain. Hence, these features, coupled with the flexible nature of the device and ease of fabrication, make it a very attractive candidate for use in the growing wearable technology market, especially biomedical applications and smart health monitoring system as well as virtual reality sensors. In this dissertation, various unique properties of graphene and P(VDF-TrFE), and their current applications and trends are discussed in chapter 1. Additionally, synthesis of graphene and P(VDF-TrFE) and their characterizations by various techniques are investigated in chapter 2. Based on piezoelectric property of P(VDF-TrFE), a highly flexible energy harvesters on PDMS as well as PET substrates have been developed and demonstrated their performances in chapter 3. As follow-up research, graphene/P(VDF-TrFE) heterojunction based wearable sensors with integrated piezoelectric energy harvester on flexible substrates have also been fabricated and demonstrated for practical wearable application in chapter 4. Finally, major findings and future directions of the project are discussed in chapter 5

Driving Monitoring System Application With Stretchable Conductive Inks: A Review

Nowadays the automotive industry is moving towards developing system connected vehicle parameters which can monitor the driver’s behaviour before driving. Most drivers lose focus and are emotionally distracted while driving owing to fatigue, drowsiness and alcohol consumption, that can result in a traffic accidents. The device or equipment used to detect the driver’s health before driving has always posed a problem in terms of the efficiency of the system especially concerning the cable connecting the equipment. Stretchable conductive ink (SCI) via electronic devices have been widely applied in various industries such as fabric, health, automotive, communications, etc. The flexibility allows a circuit to be placed on an uneven or constantly changing surface. However, till to-date, the effective use of the stretchable conductive ink has yet to be proven in the automotive industry. The current driver monitoring system cannot integrate with many of the driver's health level tracking features at one time. A combination of the driver’s monitoring system methods with stretchable conductive ink (SCI) sensors layout design can be used to prevent road accidents as a result of a driver’s behavior and will make the driving monitoring system more effective with soft substrates technology that has the advantage of geometric deformation based on appropriate shapes

Formulation of functional materials for inkjet printing: A pathway towards fully 3D printed electronics

Inkjet printing offers a facile route for manufacturing the next generation of electronic devices, by combining the design freedom of additive manufacturing technologies with tuneable properties of functional materials and opportunities for their integration into heterostructures. However, to fully realise this potential, the library of functional materials available for additive manufacturing technologies needs to be expanded. In this review, we summarise current developments in ink formulation strategies, approaches for tailoring the functional properties of inks, and multi-material processing. Material – process – property relationships are reviewed for emerging functional materials, such as polymers, nanomaterials, and composites, with examples of current state-of-the-art devices. The flexibility of combining inkjet deposition with other existing technologies and a variety of substrates is also discussed reviewing current trends in electronics and optoelectronics, including wearable electronics, sensing, and energy applications. The review offers a comprehensive and systematic overview of ink formulations for inkjet deposition of electronic devices, summarising the challenges and perspectives in the advancement of 3D and multi-functional electronic devices and smart electronics

The role of printed electronics and related technologies in the development of smart connected products

The emergence of novel materials with flexible and stretchable characteristics, and the use of new processing technologies, have allowed for the development of new connected devices and applications. Using printed electronics, traditional electronic elements are being combined with flexible components and allowing for the development of new smart connected products. As a result, devices that are capable of sensing, actuating, and communicating remotely while being low-cost, lightweight, conformable, and easily customizable are already being developed. Combined with the expansion of the Internet of Things, artificial intelligence, and encryption algorithms, the overall attractiveness of these technologies has prompted new applications to appear in almost every sector. The exponential technological development is currently allowing for the ‘smartification’ of cities, manufacturing, healthcare, agriculture, logistics, among others. In this review article, the steps towards this transition are approached, starting from the conceptualization of smart connected products and their main markets. The manufacturing technologies are then presented, with focus on printing-based ones, compatible with organic materials. Finally, each one of the printable components is presented and some applications are discussed.This work has been supported by NORTE-06-3559- FSE-000018, integrated in the invitation NORTE59-2018-41, aiming the Hiring of Highly Qualified Human Resources, co-financed by the Regional Operational Programme of the North 2020, thematic area of Competitiveness and Employment, through the European Social Fund (ESF), and by the scope of projects with references UIDB/05256/2020 and UIDP/05256/2020, financed by FCT—Fundação para a Ciência e Tecnologia, Portugal

Analyse of strain and stress on different stretchable conductive ink materials by numerical method

This study determines the optimal stretchability performance of different materials on a conductive pattern by using maximum principal elastic strain and Von Mises stress analysis. It was performed by using finite element analysis (FEA) modelling approaches. The FEA modelling was initiated from previous studies of comparative difference in strain and stress caused by stretching the screen printed straight-line pattern (baseline) and curving wave pattern using graphene conductive ink as material. The research is using a sine wave pattern because it has the best results from the previous studies compared to other patterns. Five different FEA modelling conductive materials were developed, which are copper as the baseline, graphene, carbon nanotube (CNT), carbon black, and silver. The maximum principal elastic strain and equal stress (Von Mises stress) obtained by FEA modelling can be used to approximate which material has better elasticity. After 20% elongation, the maximum principal elastic strain of carbon-based conductive ink carbon black and graphene, 14.521 x 10-3 and 14.578 x 10-3, respectively, produced the best results, with percentage difference values of 2.63% and 2.24% from copper (baseline). As compared to the copper (1761.7MPa) conductive ink, the Von Mises stress value for carbon black (241.76 MPa) and graphene (257.34 MPa) is about 7 and 6 times lower stress respectively. There are no significant differences in strain and stress values between graphene and carbon black conductive inks. The findings show that carbon black can be an alternative to graphene as a good conductive ink. Furthermore, this research demonstrates that the FEA method can be used to investigate the stretchability of conductive ink