Ewen O. Blair

Enhanced Electroanalysis in Lithium Potassium Eutectic (LKE) Using Microfabricated Square Microelectrodes

Molten salts (MSs) are an attractive medium for chemical and electrochemical processing and as a result there is demand for MS-compatible analysis technologies. However, MSs containing redox species present a challenging environment in which to perform analytical measurements because of their corrosive nature, significant thermal convection and the high temperatures involved. This paper outlines the fabrication and characterization of microfabricated square microelectrodes (MSMs) designed for electrochemical analysis in MS systems. Their design enables precise control over electrode dimension, the minimization of stress because of differential thermal expansion through design for high temperature operation, and the minimization of corrosive attack through effective insulation. The exemplar MS system used for characterization was lithium chloride/potassium chloride eutectic (LKE), which has potential applications in pyrochemical nuclear fuel reprocessing, metal refining, molten salt batteries and electric power cells. The observed responses for a range of redox ions between 400 and 500 °C (673 and 773 K) were quantitative and typical of microelectrodes. MSMs also showed the reduced iR drop, steady-state diffusion-limited response, and reduced sensitivity to convection seen for microelectrodes under ambient conditions and expected for these electrodes in comparison to macroelectrodes. Diffusion coefficients were obtained in close agreement with literature values, more readily and at greater precision and accuracy than both macroelectrode and previous microelectrode measurements. The feasibility of extracting individual physical parameters from mixtures of redox species (as required in reprocessing) and of the prolonged measurement required for online monitoring was also demonstrated. Together, this demonstrates that MSMs provide enhanced electrode devices widely applicable to the characterization of redox species in a range of MS systems.

Development and Optimization of Durable Microelectrodes for Quantitative Electroanalysis in Molten Salt

Microfabricated square electrodes with finely controlled highly reproducible dimensions have been developed for electrochemical analysis of high-temperature molten salt (MS). These microelectrodes have been fabricated using photolithographic techniques on silicon wafers and have been designed for operation in lithium chloride/potassium chloride eutectic salt at and ~500 °C. The electrodes are constructed from a series of patterned layers, and their development has involved a systematic study and optimization of a number of different material combinations. This has resulted in a process for making electrodes that represents a step change in capability, delivering the first robust microelectrode device capable of quantitative electroanalysis in a MS system at 500 °C.

Test structures for the characterisation of sensor packaging technology

This paper presents three test structures targeted at characterising sensor packaging materials for liquid environments. The test structures enable the evaluation of: 1) the successful removal of packaging material on sensing areas, 2) the permeability of the packaging material to its environment, 3) electrical continuity through the packaging process, and 4) the ingress of the liquid environment between the packaging material and the chip surface. The paper presents an example of the evaluation of a UV curable resin as packaging process for a biomedical sensor.

Implantable Microsystems for Personalised Anticancer Therapy

The Implantable Microsystems for Personalised Anti-Cancer Therapy (IMPACT) project aims to produce an implantable wireless sensor device for monitoring tumour physiology. Real-time measurements will be used to improve radiotherapy by allowing treatment to be responsively delivered at the most effective time and location. We are developing miniaturised microfabricated sensors for measuring local oxygen concentration and pH within the tumour, using technologies that are amenable to integration on CMOS. In addition, we have established proof of concept for a range of electrochemical biosensors that can respond to enzyme biomarkers. Together these sensors will allow comprehensive monitoring of tissue physiology before and after radiotherapy treatment. For clinical use, the complete system will be equipped with circuits for wireless power and communications and packaged in biocompatible materials. This is a very challenging application for sensors integrated on CMOS. Here we provide a brief background to medical aspects of the work and describe our progress towards solving the engineering challenges it has presented.

Test structures to support the development and process verification of microelectrodes for high temperature operation in molten salts

This paper reports the design and application of test structures used for the development and characterisation of microelectrodes for operation in the harsh, caustic environment of molten salts operating at 450°C. These structures have been employed to evaluate the effect of electrode area and the dielectric integrity of insulating layers in the molten salt. This has been useful in identifying failures mechanisms, which has facilitated the optimisation of both the design and fabrication of the microelectrodes while at the same time also providing valuable information for process verification.

Test structures for optimizing polymer electrolyte performance in a microfabricated electrochemical oxygen sensor

Test structures were produced for optimizing the design and fabrication of a patterned solid polymer electrolyte in an electrochemical oxygen sensor. Measurements showed that choice of photoresist developer and the underlying insulator material affected durability of the polymer structures. Test electrodes covered by the polymer were effective at supporting electrochemical oxygen detection.

Improving the Yield and Lifetime of Microfabricated Sensors for Harsh Environments

This paper details improvements in the design and fabrication of electrodes intended to function in the high temperature, corrosive environment of a molten salt. Previously reported devices have displayed low yield and lifetimes and this paper presents two strategies to improve these aspects of their performance. The first one involves reducing the critical area, which increased both the electrode yield and lifetimes. The second element utilized test structures, targeted at identifying failure mechanisms, which helped facilitate the materials/design modifications required to make the devices more robust.

Biocompatibility of common implantable sensor materials in a tumor xenograft model: Biocompatibility of common implantable sensor material

Abstract Real‐time monitoring of tumor microenvironment parameters using an implanted biosensor could provide valuable information on the dynamic nature of a tumors biology and its response to treatment. However, following implantation biosensors may lose functionality due to biofouling caused by the foreign body response (FBR). This study developed a novel tumor xenograft model to evaluate the potential of six biomaterials (silicon dioxide, silicon nitride, Parylene‐C, Nafion, biocompatible EPOTEK epoxy resin, and platinum) to trigger a FBR when implanted into a solid tumor. Biomaterials were chosen based on their use in the construction of a novel biosensor, designed to measure spatial and temporal changes in intra‐tumoral O2, and pH. None of the biomaterials had any detrimental effect on tumor growth or body weight of the murine host. Immunohistochemistry showed no significant changes in tumor necrosis, hypoxic cell number, proliferation, apoptosis, immune cell infiltration, or collagen deposition. The absence of biofouling supports the use of these materials in biosensors; future investigations in preclinical cancer models are required, with a view to eventual applications in humans. To our knowledge this is the first documented investigation of the effects of modern biomaterials, used in the production of implantable sensors, on tumor tissue after implantation.

A low cost patternable packaging technology for biosensors

This paper demonstrates a simple and low cost technology to reliably and accurately package integrated chips. Microchannels and cavities of minimum feature size of 500 μm can be reliably reproduced. In addition, the curing depth in relation to the exposure time was investigated. A simple microfluidic device, consisting of a 500 μm channel and 2 mm ports, was manufactured to demonstrate the possibilities of this technology. Extensive electrochemical experiments showed that the packaging material is a good insulator and leaves no residue on the chip.

Test Structures for the Characterisation of Conductive Carbon Produced from Photoresist

Conductive carbon films are highly attractive for use as electrodes in electrochemistry and biosensing applications. Patterned photoresist films can be transformed into carbon electrodes using standard photolithographic techniques followed by pyrolysation of the photoresist in a furnace under a reducing atmosphere. Previous studies have been made of the electrical properties of blanket carbon films created using this method of fabrication. However, there is a need to investigate pattern dependent effects, particularly the extent to which the dimensions of the patterned films shrink during the high temperature processing. This study applies microfabricated test structures to the process characterisation of conductive carbon produced from standard positive photoresists.