
David HolmesSphere Fluidics Ltd. · Physics and Engineering Group
David Holmes
PhD
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96
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Introduction
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Education
October 1998 - June 2003
Publications
Publications (96)
Nanoparticle interactions with micron-scale environments can be induced using dielectrophoresis (DEP). A new DEP signaling and additive noise model is developed from first principles. It is applied to fluorescence microscope CCD video images of fluorescently labeled, 200 nm diameter, latex nanospheres moved by pulsed, positive DEP (pDEP) onto 5 mic...
The prevalence of clinically-relevant bacterial strains resistant to current antibiotic therapies is increasing and has been recognized as a major health threat. For example, multidrug-resistant tuberculosis and methicillin-resistant Staphylococcus aureus are of global concern. Novel methodologies are needed to identify new targets or novel compoun...
© Duellberg et al.The function of microtubules relies on their ability to switch between phases of growth and shrinkage. A nucleotide-dependent stabilising cap at microtubule ends is thought to be lost before this switch can occur; however, the nature and size of this protective cap are unknown. Using a microfluidics-assisted multi-colour TIRF micr...
Fig. S1. pDEP collection rate ratio data as a function of frequency. The data are fitted to a scaled starting with a line of best fit and refined with the scaled , as shown. The fit yields values for the nanoparticle conductivity of 25.8 mS/m and surface conductance of 1.29 nS.
Real-time image signalling and quantification methods are described that allow easy-to-use, fast extraction of the electrical properties of nanoparticles. Positive dielectrophoretic (pDEP) collection rate analysis enables the dielectric properties of very small samples of nanoparticles to be accurately quantified. Advancing earlier work involving d...
Newly developed information processing tools are described that enable easy-to-use, fast extraction of the electrical properties of nanoparticles. Positive dielectrophoretic (pDEP) collection rate analysis enables the dielectric properties of very small samples of nanoparticles to be accurately quantified. We report the latest advances of semiautom...
Determining cell mechanical properties is increasingly recognized as a marker-free way to characterize and separate biological cells. This emerging realization has led to the development of a plethora of appropriate measurement techniques. Here, we use a fairly novel approach, deterministic lateral displacement (DLD), to separate blood cells based...
Although bulk changes in red blood cell concentration between vessels have been well characterised, local distributions are generally overlooked. Red blood cells aggregate, deform and migrate within vessels, forming heterogeneous distributions which have considerable effect on local haemodynamics. The present study reports data on the local distrib...
We show, via three-dimensional
immersed-boundary-finite-element-lattice-Boltzmann simulations, that
deformability-based red blood cell (RBC) separation in deterministic lateral
displacement (DLD) devices is possible. This is due to the
deformability-dependent lateral extension of RBCs and enables us to predict a
priori which RBCs will be displaced...
Image processing methods and techniques for high-throughput quantification of di-electrophoretic (DEP) collections onto planar castellated electrode arrays are devel-oped and evaluated. Fluorescence-based dielectrophoretic spectroscopy is an impor-tant tool for laboratory investigations of AC electrokinetic properties of nanoparticles. This paper d...
Image processing methods and techniques for high-throughput quantification of dielectrophoretic (DEP) collections onto planar castellated electrode arrays are developed and evaluated. Fluorescence-based dielectrophoretic spectroscopy is an important tool for laboratory investigations of AC electrokinetic properties of nanoparticles. This paper deta...
A new dielectrophoretic spectroscopy method and supporting theoretical model is developed to systematically quantify the dielectric properties of nanoparticles using continuously pulsed dielectrophoretic (DEP) collection rates. Initial DEP collection rates, that are dependent on the nanoparticle dielectric properties, are an attractive alternative...
In this work we present data showing deformability-based cell separation in a deterministic lateral displacement (DLD) device. We use cells of defined stiffness (glutaraldehyde cross-linked erythrocytes) to test the performance of the device across a range of cell stiffness and applied shear rates. Optical stretching is used as an independent metho...
The sensitivity of a microfluidic impedance flow cytometer is governed by the dimensions of the sample analysis volume. A small volume gives a high sensitivity, but this can lead to practical problems including fabrication and clogging of the device. We describe a microfluidic impedance cytometer which uses an insulating fluid to hydrodynamically f...
Single cell microfluidic impedance cytometry has been used to identify cells at high speed, on the basis of their dielectric properties. However, there is no electrical analogue to a fluorescent label, meaning that it is not possible to identify subpopulations of cells. We demonstrate discrimination and enumeration of antigenically defined cell sub...
Recent advances in the bio- and nanotechnologies have led to the development of novel microsystems for bio-particle separation and analysis. Microsystems are already revolutionising the way we do science and have led to the development of a number of ultrasensitive bioanalytical devices capable of analysing complex biological samples. These devices...
A novel microfabricated flow cytometer for simultaneous impedance and optical measurement of single cells and particles is described in this chapter. We discuss the sensitivity of the system with regard to the impedance sensor and describe the optical setup. The relevant parameters related to the experimental setup and sample preparation are discus...
Miniature high speed label-free cell analysis systems have yet to be developed, but have the potential to deliver fast, inexpensive and simple full blood cell analysis systems that could be used routinely in clinical practice. We demonstrate a microfluidic single cell impedance cytometer that performs a white blood cell differential count. The devi...
Microparticles incorporating micrometer-sized diffractive bar codes have been modified with oligonucleotides and immunoglobulin Gs to enable DNA hybridization and immunoassays. The bar codes are manufactured using photolithography of a chemically functional commercial epoxy photoresist (SU-8). When attached by suitable linkers, immobilized probe mo...
Identification and analysis of phytoplankton is important for understanding the environmental parameters that are influenced by the oceans, including pollution and climate change. Phytoplanktons are studied at the single cell level using conventional light-field and fluorescence microscopy, but the technique is labour intensive. Flow cytometry enab...
Impedance spectroscopy is a non-invasive technique which allows analysis of the electrical properties of biological cells and other materials. Typically, the electrical properties of a cell suspension are measured using relatively large volumes (>100 mul). This approach suffers from the significant drawback that the measurement is averaged over sev...
A novel impedance spectroscopy technique has been developed for high speed single biological particle analysis. A microfluidic cytometer is used to measure the impedance of single micrometre sized latex particles at high speed across a range of frequencies. The setup uses a technique based on maximum length sequence (MLS) analysis, where the time-d...
A microfabricated flow cytometer has been developed for the analysis of micron-sized polymer beads onto which fluorescently labelled proteins have been immobilised. Fluorescence measurements were made on the beads as they flowed through the chip. Binding of antibodies to surface-immobilised antigens was quantitatively assayed using the device. Part...
Over the last century a number of techniques have been developed which allow the measurement of the dielectric properties of biological particles in fluid suspension. The majority of these techniques are limited by the fact that they only provide an average value for the dielectric properties of a collection of particles. More recently, with the ad...
Cellular analysis requires a combination of biophysical and biochemical approaches for counting, manipulation and characterization of biological cells. In recent years, considerable attentions have been paid to single cell analysis based on lab-on-a-chip (LOC) technology, which offers the characterization of a large amount of cells one by one.Indiv...
A novel impedance spectroscopy technique has been developed for high speed single biological particle analysis. A microfluidic cytometer is used to measure the impedance of single micrometre sized latex particles at high speed across a range of frequencies. The setup uses a technique based on maximum length sequence (MLS) analysis, where the time-d...
Cellular analysis requires a combination of biophysical and biochemical approaches for counting, manipulation and characterization of biological cells. In recent years, considerable attentions have been paid to single cell analysis based on lab-on-a-chip (LOC) technology, which offers the characterization of a large amount of cells one by one.Indiv...
To date the development of in situ chemical and biological sensors has focused on the production of macro instruments for single point deployment. With the exception of oxygen sensors and biological sensors based on fluorometry (that are now commercially available) chemical and biological sensors are not able to make repeated synoptic measurements...
In this paper, we describe the design and function of a microchip which is used to detect, analyse and count single micron-sized particles at high speed. The device uses multi-frequency electrical impedance together with single particle fluorescence spectroscopy. Impedance is measured using microelectrodes fabricated within a microfluidic channel....
A common problem with high throughput combinatorial chemistry is the inability to easily analyse and track the multitude of molecules produced. This bottleneck is a major problem for the big pharmaceutical companies and other researchers working in the fields of drug screening, DNA analysis, etc. To this end we have developed a microfluidic device...
A micro flow cytometer has been fabricated that detects and counts fluorescent particles flowing through a microchannel at high speed based upon their fluorescence emission intensity. Dielectrophoresis is used to continuously focus particles within the flowing fluid stream into the centre of the device, which is 40 microm high and 250 microm wide....
A microchip has been developed that measures the electrical impedance and optical properties of single particles, such as cells, at the same time. Individual cells flow in a high-speed fluid stream through a detection zone, where the device analyzes thousands of cells per minute. This happens in an enclosed microfluidic environment. Many applicatio...
Micron-sized polymer particles are commonly used in chemistry as substrates for attaching proteins and DNA. The analysis of such particles is important for a wide variety of biochemical assays [1]. One area of particular interest is the development of encoded or barcoded particles. The use of such particles allows for the realization of a number of...
A new design of particle sorting chip is presented. The device employs a dielectrophoretic Gate that deflects particles into one of two microfluidic channels at high speed. The device operates by focussing particles into the central streamline of the main flow channel using dielectrophoretic focussing,. At the sorting junction (T- or Y-junction) tw...
Microfluidic analysis devices, often referred to as Micro Total Analysis Systems or the Lab-on-a-chip, are often based on the manipulation of small volumes of fluid. These devices require the design and fabrication of components for fluid handling, control and measurement, such as micropumps, micromixers and flow sensors. The fabrication of miniatu...
In this work we report on the development of biochips for the rapid analysis of single cells and other particles. We have developed a device that can simultaneously measure the optical and electrical properties of single cells or other micron-scale particles. The micro flow-cytometer chip consists of a planar electrode array onto which a micro-flui...
In this work we report on the development of biochips for the rapid analysis of single cells and other particles. We have developed a device that can simultaneously measure the optical and electrical properties of single cells or other micron-scale particles. The micro flow-cytometer chip consists of a planar electrode array onto which a micro-flui...
A polymer microfluidic device for the formation of artificial bilayer lipid membranes (BLMs) on-chip is described. The device is fabricated from thin, transparent films of poly(methyl methacrylate), allowing for optical monitoring of the BLM. In addition, detection of single fluorescently-labeled lipid molecules using conventional epifluorescence m...
We have developed a new method for the rapid detection of single bio-particles flowing in a micro-channel. Confocal optical detection has been combined with multi-frequency impedance based detection to allow measurements of single bio-particles. The optical detection setup is based around a high numerical aperture microscope objective with free spa...
In this work we report on the development of biochips for the rapid analysis of single cells and other particles. We have developed a device that can simultaneously measure the optical and electrical properties of single cells or other micron-scale particles. The micro flow-cytometer chip consists of a planar electrode array onto which a micro-flui...
Microfabricated barcodes formed in SU8 photopolymer are presented as a method for identifying individual particles based on their diffraction patterns. Two designs of barcoded particles are considered; a single layer SU8 process and a two layer SU8 process. Theoretical data shows that for ideal barcoded particles it should be possible to obtain sev...
A polymer microfluidic device for the formation of artificial bilayer lipid membranes (BLMs) on-chip is described. The device is fabricated from thin, transparent films of poly(methyl methacrylate), allowing for optical monitoring of the BLM. In addition, detection of single fluorescently-labeled lipid molecules using conventional epifluorescence m...
In this work we report on the development of biochips for the rapid analysis of single cells and other particules. We have developed a device that can simultaneously measure the optical and electrical properties of single cells or other micron-scale particules. The micro flow-cytometer chip consists of a planar electrode array onto which a micro-fl...
In this paper we present a cell separation device that uses a combination of dielectrophoresis (DEP) and pressure driven fluid flow to separate cells of different types along the length of an interdigitated micro-electrode array. As the cells enter the device they are focussed into a thin sheet, mid-way between the upper and lower walls of the flow...
We present a high-speed particle sorting and deflection system which is an integral part of a micro flow-cytometer chip capable of high speed detection and sorting of micron-sized particles. The device sorts particles using a combination of DEP and hydrodynamic forces. DEP focusing of particles is used to axially centre particles in a channel. Nega...
Dynamic focussing of particles can be used to centre particles within a flow-through channel. This ensures that all particles pass through the same detection volume. In this paper we describe a method for focussing nano-particles within a microfluidic channel using AC electrokinetics. The method differs from other proposed focussing methods in that...
A microfluidic chip for performing impedance measurements on micron-sized particles is presented. The device consists of micro-electrodes patterned onto pyrex glass wafers and a microfluidic channel formed in PDMS, cast using a dry-etched silicon wafer as a master. The PDMS channel was positioned over the electrodes to form a channel to contain the...
Dynamic focusing of particles can be used to centre particles in a fluid stream, ensuring the passage of the particles through a specified detection volume. This paper describes a method for focusing nanoparticles using dielectrophoresis. The method differs from other focusing methods in that it manipulates the particles and not the fluid. Experime...
A novel dielectrophoretic cell-separation technique has been developed for the separation of white blood cells. A microdevice was constructed from two consecutive arrays of microelectrodes, one to focus the particles into a sheet and the second to separate them using a combination of positive and negative dielectrophoresis. At a flow rate of 1 ml/h...
This paper presents results of the focussing and separation of particles in microchannels using specially designed and integrated microelectrodes. The behaviour of a range of particle sizes, down to-5Onm, were numerically simulated in microchannels containing the electrodes. The results of the simulations were then compared with the experimental re...
We present a cell separation device that uses a combination of dielectrophoresis (DEP) and pressure driven fluid flow to separate differing cells types along the length of an electrode array within the device. Cells are initially focussed into a thin sheet mid-way between the upper and lower walls of the flow channel using negative-DEP. As they flo...
INTRODUCTION: In this paper we describe a
micro-fabricated cell separation and positioning
system based on the AC electrokinetic technique,
dielectrophoresis (DEP). The system uses nonuniform
electric fields produced by arrays of
microelectrodes to manipulate cells held in
suspension. Depending upon the mode of operation
one can either concentrate...
A linear travelling wave dielectrophoretic (twDEP) microchip was fabricated and used to investigate both the levitation and the twDEP motion of latex beads as a function of applied potential and frequency, suspending medium conductivity, bead size, and sur, face characteristics. The surface conductance of the latex beads was characterised by measur...
Separation of rare or unwanted bio-particles from heterogeneous mixtures is of great interest both for science and medicine. The ability to purify blood products of virus, isolate rare cell types for diagnostic purposes, obtain pure cell samples for further study are all important problems. We have developed a microfluidic device capable of address...
The isolation of fetal cells from maternal peripheral blood has long been proposed as a method for prenatal diagnosis but with only limited success. Current methods using antibody based separation strategies have proven unreliable. Here we describe a microfluidic system which can isolate specific cell types from a heterogeneous population, dependan...
Non-invasive methods for isolating fetal cells from the maternal circulation have been used for prenatal genetic diagnostic purposes. Unfortunately, most of the currently available separation methods are antibody based, and limited by the lack of specific fetal markers. Our aim is to develop a novel technique for the isolation of trophoblast from t...
This thesis describes experimental and theoretical investigations into new particle handling and separation methods and techniques. It makes a major contribution to the rapidly expanding field of cell separation technology. A novel dielectrophoretic cell separation system has been developed, which is capable of processing large sample volumes (~50m...
A microfluidic chip for performing impedance measurements on micron-sized particles is presented. The device consists of micro-electrodes patterned onto pyrex glass wafers and a microfluidic channel formed in PDMS, cast using a dry-etched silicon wafer as a master. The PDMS channel was positioned over the electrodes to form a channel to contain the...
Microfabricated barcodes formed in SU8 photopolymer are presented as a method for identifying individual particles based on their diffraction patterns. Two designs of barcoded particles are considered; a single layer SU8 process and a two layer SU8 process. Theoretical data shows that for ideal barcoded particules it should be possible to obtain se...
We present a novel device for performing differential impedance spectroscopic analysis of single cells hydrodynamically captured in an array of trapping sites. The method combines the advantages offered by impedimetric analysis used for flow cytometry with the ability to capture multiple single cells and perform long-term transient and steady state...