OCT for 2D and 3D velocity measurement in micro-fluidic flows

This project aims to use optical coherence tomography (OCT) for velocity measurement in sub-millimetre flow channels. A major application for flow-measurement in this regime is in the design and assessment of microfluidic systems, which exemplify an increasing trend for miniaturization pervading many aspects of technology. In fluid-flow engineering, miniaturised instruments offer a reduced laboratory footprint, reduced requirements for energy and expensive or hazardous reagents and the ability to acquire data simultaneously from many systems within a single instrument. Microfluidics is a rapidly-growing field with benefits in healthcare (rapid, lab-on-a-chip, techniques for, e.g. immunoassays and biological phase separation), energy technology (membrane fuel cells) and high-throughput, portable systems for chemical analysis.
Currently, the main technique for investigation of micro-fluidic flows is micro particle image velocimetry, which uses high-resolution photography to determine positions of 'seed' particles within a plane illuminated by a thin sheet of laser light. Two images, acquired in rapid succession, allow the distance moved by particles between frames to be calculated, yielding velocity components in the image plane.
In very small ducts, a sufficiently thin light sheet cannot be generated. Illumination of a small volume, through a microscope arrangement, is usual, and the measurement plane thickness is defined using optics that exclude light from regions outside the focal plane. For the smallest channels, seed particles must be smaller than the illumination wavelength (approx. 1 micron). This causes difficulties, because light scattering decreases rapidly as particle diameter drops, and can fall so low that the signal-to-noise (SNR) is inadequate for acceptable images. Fluorescent particles and intensified cameras are then required, with filtering to separate the fluorescent signal from the background.

OCT is used mainly in medical environments, for detailed biological tissue imaging. However, its high spatial resolution, combined with particle-tracking techniques carried over from PIV, offer the possibility of 2- or 3-component velocity measurement in 2-D or 3-D regions, for flow velocities experienced in micro-fluidic systems. Micro-fluidic flow is not well described by classical flow theory, and experimental techniques are needed to validate models in designing micro-fluidic devices such as mixers, heat exchangers and fuel cells. It is important, for example, to eliminate 'dead zones' in the flow, and to understand the fluid motion in curved or bifurcated micro-channels. With appropriate processing, OCT can acquire images in three perpendicular planes with access from only one direction; a big advantage in micro-fluidics, when access is necessarily limited. High-resolution structural imaging of the channels is possible, alongside velocity measurement, which will help in detecting small variations or defects in wall structure that can have a large effect on flow.
OCT offers excellent optical sectioning capability, the image plane thickness being a few micrometres. Strong rejection of scattered light from outside the measurement region eliminates the need for fluorescent particles and eases near-wall measurements. The SNR of OCT is such that signals can be obtained from depths of hundreds of micrometres within turbid biological tissue, which suggests that flow measurements will be possible at higher seeding densities, or greater depths, than for comparable implementations of PIV. Typically, update rates for 2D OCT images are a few tens of Hz. Although micro-fluidic velocities are generally low, the update interval limits measurable velocities to a few mm/s. A shorter interval would be very advantageous in raising this limit. Multiplexing of images acquired from multiple illumination beams is proposed here, to reduce the inter-image interval and allow multiple image planes to be defined simultaneously

This is an applied research proposal, requesting funds to research and construct a new flow-measurement instrument, or instruments. The ultimate goal for such a project is the development of the resulting prototype into a commercial device for the technical instrument market, benefiting both scientific research and the UK economy. At this stage, the aim is to demonstrate that the technology can be successfully implemented. Protection of any IP arising will be a priority (via our Technology Transfer facility, Cranfield Ventures, in partnership with ISIS Enterprise), followed by publication in the open literature. Our department has a strong track record of engagement with industry, including directly-funded research projects and partnerships managed under programmes run by the EU and the Technology Strategy Board, UK. We have worked with small and large UK companies (e.g.Oxford Instruments, Alphasense) and multinationals (e.g. Procter and Gamble, Airbus, EADS) on projects resulting in new or improved commercial products. Industrial engagement, with an eye to follow-on funding opportunities to enable ultimate transfer of the technology, will be a key task throughout the duration of the grant.

The world market for optical coherence tomography (OCT) equipment is robust and growing. Commercial uptake, by one or more existing manufacturers of OCT equipment, is a strong possibility. This is a timely proposal; engineering applications for OCT have received relatively little attention until recently, with the first conference in OCT for non-destructive testing held in Austria in Spring 2013, and the benefits of OCT imaging in horticultural research just beginning to be recognised (e.g.'Inside Food' Symposium, Leuven 2012).

In recent years, OCT has had enormous impact in clinical environments, particularly ophthalmology. Commercial OCT instruments are becoming established in high-street opticians, and are present in large numbers of research hospitals worldwide. Additional tools, extending functionality for in-vivo flow measurement of blood and other bio-fluids within turbid tissue, will interest medical users.
Non-medical markets foreseen for the technology include academic communities in fields listed in the academic beneficiaries section, and commercial organisations in related fields. Micro-fluidics 'lab-on-a-chip' researchers, manufacturers, and end-users, require precise, high-resolution flow measurement tools for and evaluation of products, which are increasingly finding their way into biochemical separation systems, clinical diagnostics and micro-chemical reactors. Efficient, miniaturised heat exchange for electronics cooling is another vigorous research area where full exploitation will depend upon the availability of experimental tools to test and validate flow models.
The proposed OCT-based methodology offers unique benefits in environments where fluids or the surrounding material are turbid, such as adhesives manufacture and plant science, precluding or limiting use of micro-PIV techniques. Micro-fluidics in a wider sense is relevant in many fields where characterisation of small-scale fluid flows is important, e.g. the ability to monitor flow, cure and creep characteristics of an adhesive will lead to better understanding of its bonding properties, with knock-on benefits such as improved designs for resin-bonded composites.

Additional benefits include the training and personal development of the staff and research student (funded by the department), in disciplines with strong commercial applications. All staff and students leaving our department are successful in obtaining suitable employment, many of them in industrial companies. OCT and micro-fluidics are both strong and growing areas commercially, offering good employment prospects for experienced staff. Workers on the project will also gain experience in transferable skills including project management, oral and written reporting and commercial awareness.