Adam Lewis

Review on the development of truly portable and in-situ capillary electrophoresis systems

Capillary electrophoresis (CE) is a technique which uses an electric field to separate a mixed sample into its constituents. Portable CE systems enable this powerful analysis technique to be used in the field. Many of the challenges for portable systems are similar to those of autonomous in-situ analysis and therefore portable systems may be considered a stepping stone towards autonomous in-situ analysis. CE is widely used for biological and chemical analysis and example applications include: water quality analysis; drug development and quality control; proteomics and DNA analysis; counter-terrorism (explosive material identification) and corrosion monitoring. The technique is often limited to laboratory use, since it requires large electric fields, sensitive detection systems and fluidic control systems. All of these place restrictions in terms of: size, weight, cost, choice of operating solutions, choice of fabrication materials, electrical power and lifetime. In this review we bring together and critique the work by researchers addressing these issues. We emphasize the importance of a holistic approach for portable and in-situ CE systems and discuss all the aspects of the design. We identify gaps in the literature which require attention for the realization of both truly portable and in-situ CE systems.

Evaluating the Influence of Current on the Wear Processes of Au/Cr-Au/MWCNT Switching Surfaces

Wear processes of the switching contact pair consisting of a gold layer on multiwall carbon nanotubes (Au/MWCNT) and a chromium and gold coated hemispherical ball (Au/Cr) are evaluated over a range of current conditions. The switching experiments were conducted using typical conditions for a MEMS relay application, i.e. 4 V, with static contact force 1 mN with currents level of 20-200mA. The Au coated MWCNT substrate exhibited a transfer process over a large number of switching cycles. On the assumption that the transfer process was a combination of a fine transfer and delamination processes; a previous experimental investigation was considered by dividing the wear processes into four stages of a failure behavior. It is observed that the fine transfer process dominates in the unstable and stable contact resistance stage and then the delamination dominates in the rising stage and a failure process. The increase of contact resistance was used to identify contact failure.

Uncertainty prior to pulmonary rehabilitation in primary care A phenomenological qualitative study in patients with chronic obstructive pulmonary disease

Pulmonary rehabilitation (PR) is recommended for patients functionally restricted by chronic obstructive pulmonary disease (COPD). However, attendance and adherence to PR remains suboptimal. No previous research has explored COPD patient experiences during the key period from referral to initiation of PR in the United Kingdom. This research aimed to explore the lived experience of COPD patients referred to PR programmes prior to participation. COPD participants were recruited from referrals to two community PR programmes. Semi-structured interviews with COPD participants occurred following PR referral, but prior to programme initiation. Data were analyzed using applied interpretive phenomenology. Twenty-five COPD participants aged 42–90 were interviewed. ‘Uncertainty’ affected participants throughout their lived experience of COPD that negatively impacted illness perceptions, PR perceptions and increased participant’s panic and anger. Participants who perceived COPD less as a chronic condition and more as a cyclical process experienced fewer feelings of panic or anger. The experience of uncertainty was disabling for these COPD participants. Recognition of the role that uncertainty plays in patients with COPD is the first step towards developing interventions focused on reducing this uncertainty, thereby reducing the burden of the disease for the individual patient and facilitating PR attendance.

A nano-indentation study of the contact resistance and resistivity of a bi-layered Au/multi-walled carbon nanotube composite

A bi-layer metal-carbon nanotube composite has been developed as a potential low-force electrical contact surface, for application in micro-electromechanical systems switching devices. The samples consist of a vertically aligned forest of multi-walled carbon nanotube (MWCNT), sputter coated with a layer of Au. The effect of varying the components and composition are investigated by means of a modified nano-indenter. By measuring the contact resistance of the composites under various loading conditions, the electrical properties and performance can be evaluated. The composites are shown to have homogenous properties, with each of the layers influencing the total electrical characteristics of the samples. The internal structure of the sample, the MWCNT height and penetration of gold into the forest is shown to directly influence the performance and characteristics of the samples. By analyzing the samples as bulk, the effective resistivities of the composites are also determined to have values from 303 nΩ m down to 54 nΩ m, depending on the composition of the samples.

The Effect on Switching Lifetime of Chromium Adhesion Layers in Gold-Coated Electrical Contacts under Cold and Hot Switching Conditions

Gold is commonly used for electrical contacts due to its many desirable electrical and mechanical properties. Throughout the switch lifetime, the contacts are required to survive a large number of opening and closing cycles and therefore it is important to understand the failure mechanisms. Adhesion layers (e.g. chromium or titanium) can be deposited to increase the adhesion of the gold layer to the contact surface. In this work, the inclusion of a chromium adhesion layer shows an improvement of the switching lifetime of gold-coated electrical contacts under cold and hot switching conditions. These testing conditions further the understanding of the failure mechanisms (e.g. fine transfer, etc.). The mechanism of failure when no chromium adhesion layer was used is attributed to delamination of the gold layer from one contact to the other. This failure mechanism is different in the cases where a chromium adhesion layer is included. We present a model which was developed in line with experimental results. These describe the effect of load current on material transfer between gold contacts and the contact failure.

Development of a MEMS test platform for investigating the use of multi-walled CNT composites electric contacts

The use of gold-coated multi-walled carbon nanotube (Au/MWCNT) composites have been shown to extend the life of electrical contacts, in previous work. Due to the long lifetimes (which are of the order of 106 up to 108 cycles) the lifetime testing tends to be highly time consuming. In this work we discuss the design and development of an electrostatically actuated MEMS cantilever beam which enables testing at higher frequencies than our previous experimental rig. Following calculations using fundamental cantilever beam equations, a computational model of the designed beam was developed to accurately predict the characteristics of the beam, including the resonant frequency, pull-in voltage and contact force. Where possible the values from the model have been compared with the fabricated MEMS cantilever beam. A MEMS-based electrostatically actuated cantilever beam has been fabricated and incorporated with Au/MWCNT composite surfaces to form a MEMS switch test platform. Initial results show the improved performance over a PZT based test platform.

Evolution of Voltage Transients During the Switching of a MEMS Relay With Au/MWCNT Contacts

Gold is commonly used for microelectromechanical electrical contacts due to its desirable electrical and mechanical properties; however, the lifetime of gold contacts is limited, particularly in the case of hot switching. To improve the lifetime of electrical contacts, we have developed a gold-coated multiwalled carbon nanotube bilayer composite. Experiments with these composites have shown that the switching dynamics vary over the lifetime of the switch. The change in potential across the switch contacts during the contact-break process, referred to as the transient opening voltage, has been monitored at a number of intervals throughout the switch life. The transient opening voltage shows behavior indicative of the molten metal bridge (MMB) phenomenon. While stable for most of the contact lifetime, the duration of this behavior increases sharply as the contacts approach failure. Throughout the switch lifetime, the contacts are required to survive a large number of opening and closing cycles and, therefore, it is important to understand the switching dynamics. A contact pair was investigated to failure where the experimental conditions were: load current and voltage of 50 mA and 4 V, respectively. Failure occurred after 28 million hot-switched cycles. The average energy of the MMB process was evaluated as 1.54 μJ per opening event during the stable region, this rose to 13.2 μJ shortly before failure. An experiment run under similar conditions but with a lower load current of 10 mA, which was switched for over 500 million cycles, showed a stable contact resistance and an average MMB energy of 5 Ω and 57 nJ, respectively.

Influence of the height of Carbon Nanotubes on hot switching of Au/Cr-Au/MWCNT contact pairs

Gold coated Multi-Walled Carbon Nanotube (MWCNT) composites have been used for electrical contact surfaces in previous studies. It was shown that the composite could significantly improve switch lifetime, and exhibited potential as a useful contact material for MEMS switches. The reason is attributed to the contribution of the compressibility of the MWCNT forest, which creates a compliant layer under the gold films. In this study, experiments were performed for samples with different heights of MWCNT forests, namely 30 μm, 50 μm and 80 μm. The influence of MWCNT height on the switching behavior and lifetime were studied. An outcome of the work was that the sample with 80 μm height forests showed the longest lifetime, however the composites with 30 μm and 50 μm forest heights showed lower contact resistances, a parameter of high importance for MEMS switch applications.

Composite material hollow core fibers - functionalisation with silicon and 2D materials

Hollow Core Anti-resonant fibers allow for guidance of mid-infrared light at low attenuation and can be used for a variety of applications, such as high power laser transmission and gas sensing. Recent work has seen the integration of silicon into such fibers with linear losses potentially as low as 0.1dB/m. Due to the change in refractive index difference of silicon via for example the free carrier plasma dispersion effect, the prospect of an all optical modulator using such a fiber has been proposed. Here, further work has been undertaken on the integration of functional materials inside hollow core fibers via the deposition of the TMD semiconductor material MoS2, in its few-layered form. Through the use of a liquid precursor, a high quality MoS2 film can be deposited over 30cm length of fiber, as confirmed via Raman spectroscopy. The transmission spectra of these novel composite material hollow core fibers has also been analysed, showing additional loss of around 5dB/m, despite being only around 2nm in thickness. This implies that the refractive index of the integrated material is potentially able to modify the guidance properties of the fiber sample. We will present a comparison of the composite material hollow core fibers we have fabricated to date and discuss the prospects for using these novel waveguides in the active manipulation of light, including optical switching, sensing and frequency generation.

Time-course of changes to intrathoracic pressure induced by CPAP in normal subjects

Introduction: An understanding of the changes in intra-thoracic pressure in response to application of CPAP (Continuous Positive Airway Pressure) is important in the study of airway and ventilator mechanics. It is unclear how quickly intrathoracic pressure measured directly with balloon catheters responds to change in CPAP. We have studied the time course of stabilisation of pressures in normal subjects.. Methods: Mouth pressure (Pmo) was measured directly at the facemask with a NIPPY 3 CPAP; oesophageal (Poes) and gastric(Pga) pressures were measured with balloon catheters in 5 normal subjects, aged 32-65, seated at rest, with 10 minutes no CPAP, then 20 minutes at CPAP 5cmH 2 O, then 10cm H 2 O and then 10 minutes no CPAP. Results: Mouth pressure (Pmo) at the facemask was lower than prescribed CPAP; CPAP 5cmH 2 O=Pmo 4.3cm H 2 O;CPAP 10cmH 2 O=8.14cmH 2 O. Change in prescribed CPAP pressure was followed by change in Pmo and Poes within 60 seconds for all pressure changes, but the corresponding change in Pga was more variable; up to 470 seconds to stabilise. Conclusions: Mouth pressure with CPAP is less than prescribed through leakage. Gastric pressure change in response to change in CPAP takes longer and varies more than for Pmo and Poes. This may reflect variation in diaphragm tonicity as noted before (Am J Respir Crit Care Med 1999;160:513-22).It may have methodological implications for studying the impact of pressure change with CPAP.