Holographic Micro Flow Meter for Biological Sensing

Lead Research Organisation: University of Glasgow
Department Name: School of Engineering

Abstract

Ever been caught by a speed camera? Most work by taking two images in quick succession the speed of your car being calculated from the separation of its images. We have invented a similar technique for measuring the speed of micron sized objects moving within a fluid. Our technique relies on optical tweezers, tightly focussed beam of light to trap and hold micro-sized test particles in the fluid. Pulsing the tweezers repeated releases the objects into the fluid flow and the speed camera records their velocity, before the particles are re trapped. The result of many measurements is averaged to give the velocity of the fluid flow to an accuracy of one micron per second. One area where we will apply our technique is micro-fluidics, micron-sized networks of fluid channels allowing the transfer and mixing of chemical reagents, probes or bio-objects. Such systems are the basis of 'lab on chip' technology, cheaper, more compact and faster than their full sized lab equivalents. The small scale of lab chips means that fluid behaviour is non-intuitive, water flow being more like concrete, hence the ability to measure the fluid is both elegant and instructive. Beyond gaining an understanding of micro-fluidic flow, our aim is to apply the technique to various bio-applications. Fluid flow around a cell creates mechanical stress, modifying the cell response with a parameter that is not understood; motile cells can swim in a fluid, but again the interplay between the creation and sensing of fluid flow is poorly understood. We believe that our ability to measure the fluid flow with both spatial and temporal resolution combined with our proven expertise in other imaging and sensing technologies will allow us to address these and many other interesting issues.

Technical Summary

The aim of the proposal is to develop a new sensitive sensor technology that will, for the first time provide a method to monitor real-time changes of fluid flow in and around biological and bio-analytical microstructures, with extremely high temporal and spatial resolution. Our new technique is based on modulated holographic optical tweezers with which we deliberately release the particle from the optical trap and map its velocity and direction within the fluid. The velocity field is calculated from successive images (c.f. a speed camera) several millisec apart. Then re-establish the trap and the particle is drawn back into the trap by the gradient force of the tweezers. This process is repeated between 40 and 50 times a second. The velocity, as measured from many such cycles, can be averaged to give a precise value. This value is determined whilst the probe particle is in free flow (when the tweezers beam is off) and is therefore totally independent of any dynamic change in the tweezers performance. Our 'tweezers-speed-camera' technique hence combines the accuracy of PIV with the non-perturbing performance of a trapped particle tweezers. The methods will be implemented on a microscope as an imaging platform and will therefore be ideally suited to making measurements in a range of biological and sensory systems including diagnostic lab-on-a-chip devices, biosensors, isolated cells and their organelles. We are particularly interested in using the flow sensors, in combination with microfluidic systems in order to study two specific biological questions, namely: (i) the effects of shear-stress on single cells; and, in a closely related study (ii) the function of cilia and flagella as sensory organelles as well as actuators of fluid flow. Co-funded by EPSRC Life Science Interface Programme.

Publications

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Leach J (2009) Comparison of Faxén's correction for a microsphere translating or rotating near a surface. in Physical review. E, Statistical, nonlinear, and soft matter physics

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Preece D (2011) Optical tweezers: wideband microrheology in Journal of Optics

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Tassieri M (2010) Measuring storage and loss moduli using optical tweezers: broadband microrheology. in Physical review. E, Statistical, nonlinear, and soft matter physics

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Yao A (2009) Microrheology with optical tweezers. in Lab on a chip

 
Description The resources generated include the following three products or processes:
Methodologies: The development of techniques to control microparticles in microfluidic channels. One immediate benefit of this enabled us to work with Parasitologists, such as Professor Mike Barrett, studying the protozoan flagellate Trypansoma.
We performed initial work using optical tweezers to study the activity of the flagella (as an objective). The development of
this technique had led to the successful application to the Gates Foundation ($100,000 awarded) using a different kind of optical tweezer, called an optoelectronic tweezers, which can be used both to enrich blood containing parasites, and to
sense them. The technique can also be used to study the way in which drugs interact with the protozoa.
Analalytical Methods: We have developed a new label free technique to probe physiological changes in single cells.
Attaching a bead to the cell, we are able to probe changes in the viscoelastic properties of the membrane, as a
consequence of changes in osmolarity within the cell, and more recently on addition of drugs such as blebbistatin.
Software: New software, developed in this project, and used for particle tracking of probes or bead actuators in the
microfluidic channels has become the subject of interest from Malvern Instruments (contact Frazer McNeil-Watson, Strategic Development Group Manager). Malvern Instruments are now concluding a developmental propgramme agreement with Padgett-Cooper to use this software with their products. They will also fund two further CASE awards on viscoelastic and microrheological measurements, spanning the Cooper-Padgett laboratories.

In summary:
The energy carried by light is fundamental to life on our planet. But as well as energy, light beams carry a momentum. So if I shine a laser pointer at you, in addition to making you slightly hotter you'd feel a small force pushing you away. This force is roughly equal to the weight of a single biological cell and is used within microscopes to move single cells without touching them - a technique called optical tweezers. All we have to do is shine the laser into a microscope, and when we move the laser, the light can produce a force and make the the cells follow the laser beam.

When viewed under a microscope, tiny objects like cells or even inert spheres of glass are seen to be constantly wobbling
around. This wobble is called "Brownian Motion" and is caused by the constant, yet random, bombardment by individual molecules of surrounding air or water. When the optical tweezers is switched on, this Brownian motion is suppressed but can never be eliminated. No matter how powerful the laser and hence the strength of its trap, the object always wobbles slightly around the trap centre. But rather than being a nuisance, these wobbles tell us about the properties surrounding fluid. Indeed the properties of these fluids are very important, not only in industrial biotechnology and the food industry (e.g. how well mayonnaise is extruded from a nozzle), but also in biology. They tell us about the stiffness of the cell, and therefore give us an indication of its wellness.
Traditionally, scientist and engineer have used sophisticated detection systems to measure the properties of these fluids.
In Glasgow we have adopted a different approach. As people in the high-street already know, camera technologies and
resulting performance are advancing very quickly. These technologies mean that low cost cameras are available that can record thousands of images every second in very low light conditions. We use this technology to measure precisely the
frequency and extent of the wobble. It transpires that our accuracy of measurement, using a high-street camera provides us with a measurement which reaches the fundamental limits set by physics, which no system can beat. Furthermore the use of a camera means that we can monitor many objects at the same time. This technique is already of considerable interest to instrument makers who make machines that measure the elastic properties of fluids in the food industry.
So what does this wobble tell us and why might this be useful? In most situations of interest to us the particles are
suspended in a surrounding fluid (think or cells or bacteria in blood). By monitoring the motion of tiny glass beads we can
measure the fluid flow. On this scale the fluid flow is strange, even water feels like flowing concrete, two opposing steams
never mix. Measuring this fluid flow will help us to design tiny laboratories on a single chip, potentially allowing medical
screen on devices embedded in a credit card.
However, we can measure more than just fluid flow. The force exerted by a moving fluid depends upon its viscosity. On a micro-biological scale these visco-elastic properties are the norm and understanding them is central to our understanding of various biological processing ranging from synchronization of swimming cells to the folding and unfolding of protein structures.
One unlikely outcome of us trying to understand the swimming of single cells, has led to us trying to understand the way that single micro-organisms, that cause infectious disease behave (which account for ~60% of all deaths of children aged 5-14). Amongst these diseases are the so called "neglected diseases" such as sleeping sickness, which have a devastating impact on rural communities, in sub-Sahara Africa. The micro-organisms that cause sleeping sickness are now being studied with our new technique to help find new drugs and develop new diagnostic sensors.
Exploitation Route See above
Sectors Healthcare,Manufacturing, including Industrial Biotechology,Pharmaceuticals and Medical Biotechnology

 
Description Unilever have shown interest in funding the work on viscoelastic measurements of polymers and we anticipate some level of support in the near future. We are in a discussion with Malvern instruments about developing new methods to measure the viscoelastic properties of biological membranes. They are also interested in using the new techniques we developed on measuring cell membrane properties with optical tweezers and want to apply this more generally to the area of microrheology. Finally, Malvern Instruments are committing to fund work, using the particle tracking software developed in this programme (see descriptions elsewhere).
First Year Of Impact 2011
Sector Healthcare,Manufacturing, including Industrial Biotechology,Pharmaceuticals and Medical Biotechnology
Impact Types Economic

 
Description Engagement associated with BBE0222431 
Form Of Engagement Activity A broadcast e.g. TV/radio/film/podcast (other than news/press)
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Public/other audiences
Results and Impact We have engaged in various forms of outreach activity that have incorporated both the studies detailed in this grant, as well
as our broader research base. For example, outreach activities motivated by this grant include, but are not restricted to:
- Launch of two free to download educational apps of the apple iStore for iPhone or iPad, illustrating a touch screen
interface to an optical tweezers;
- Public lecture at Cafe Scientifique (www.cafescientifique.org) on swimming on a micron scale;
- You-tube video for optical tweezers (the most downloaded Tweezers video in the World >20,000 viewings);
- Workshop for Scottish Science teachers, at the ASE annual meeting;
- Multiple work experience placements, within Research Group of 16-18 school pupils prior to University Application
Procedure;
- Created YouTube Channel as outlet for our research videos;
- Used micro-scale swimming in low Reynolds number systems as an example for our schools lectures (recruitment to
undergraduate Biomedical Engineering)
Year(s) Of Engagement Activity 2012,2013,2014,2015