Single cell impedance spectroscopy label free analysis of cells

Transcription

1 Single cell impedance spectroscopy label free analysis of cells The Coulter counter Each cell that passes through the aperture reduces the current gives a spike. Cell behaves like an insulating sphere. Measuring the size of the current change is used to size and count the cells. 1

2 Microfabricated devices in silicon electrodes Analysis electronics Koch M et al. Microfabricated electrodes for the construction of a four-electrode impedance based Coulter Counter. J Micromech Microeng (1999). Frequency spectrum of permittivity for cells in suspension (dielectric spectroscopy) Two dispersions: β-dispersion - interfacial polarisation (Maxwell- Wagner polarisation). Characterise cell membrane α-relaxation - counterion polarisation, dominated by cell surface charges (little studied) < 100kHz Frequency / Hz 100kHz to 10MHz 2

3 Flow through devices (micro flow cytometry) Broad-band dielectric spectroscopy Average Signal from > 10 6 cells ~ V(jω) Z mix V( jω) = I( jω) Single cell impedance spectroscopy - microchip with dimensions the size of the cell Single cell analysis in a fluid flow Typical voltage signal (at one frequency) as cell passes electrodes Transit time is sub ms, up to 100 cells/sec 3

4 Discrete SINGLE Frequency Analysis Integrating Optics Multi-colour fluorescence AND Impedance 4

5 Analysis NO PARTICLE (CELL) Cube of conducting liquid, electrodes top and bottom Gap g A C = ε oε r g 1 g R = σ A Electrode area A G = (g/a) is the inverse geometrical cell constant of the system, (units m -1 ). Impedance (or dielectric) spectroscopy of cells Z mix V( jω) 1 = = I( jω) jωε G mix ~ In a system where the field is not uniform, replace G with G f, which takes into account the field divergence, through a modified geometrical cell constant 5

6 Maxwell s Mixture Equation ε mix 1+ 2Φf = εs 1 Φf CM CM ε mix ε p ε m 1+ 2Φ ε p + 2 ε m = εm ε p ε m 1 Φ ε + 2 ε p Φ the volume fraction, ratio of particle volume to total volume This equation is approximately correct up to 20% volume fraction. 10μm diameter cell in a 20μm cube is 7% vol fraction. m ε = ε jσ / ω p The permittivity (capacitance) and conductivity of the cell Shell model and equivalent electrical circuit ε Mem, σ Mem ε m, σ m ε i, σ i R d Cell with membrane Specific Membrane conductance is nearly zero, ignored Specific Membrane capacitance C = ε ε Mem o Mem d 6

7 Circuit analysis & Mixture theory ε ε m i εε 1 3φ o m 2ε + ε m i Electrodes 3 1 Chip Stray σ (1 m 2 φ) Capacitance ε m, σ m C m φ: volume fraction C DL 9φ 4ε 0 RC Mem C Mem R m R i C DL ε 0 + 9φ 2σ m σ i Current around cell (medium) Intracellular current path We measure Z mix Z mix U( jω) 1 = = I( jω) jωε G mix Z After some maths, using mixture theory, the complex permittivity can be written as electrical circuit components: mix = Rm(1 + jωrc i Mem) jωr C + (1 + jωrc )(1 + jωr C ) m Mem i Mem m m The individual components are related to the physical properties of the cells through the following table: 7

8 SUMMARY TABLE of Relationships Size of cell (given a knowledge of G f ) Cell membrane capacitance Typical plot of permittivity vs frequency for interfacial relaxation (β-relaxation) due to cell membrane charging typical data measured in traditional dielectric spectroscopy. 8

9 Complete system includes electronic circuitry Two voltage outputs for each applied frequency Real (X) and Imaginary (Y) impedance amplitude, Frequency spectra for a single CELL taking into account the electronics 18 Low Frequency (<1MHz): cell volume (affected by EDL) Medium Frequency (1MHz-10MHz): membrane capacitance High Frequency (>10MHz): internal properties (cytoplasm/nucleus) 9

10 Frequency spectrum for latex beads Solid object, no membrane Impedance dominated by size (and permittivity at hf) h.f.). Data obtained at two frequencies simultaneously, reference frequency of 500kHz, other frequency swept. Scatter plots (TWO frequencies) Opacity 5.8μm 7.18μm Size Opacity: ratio of HF impedance to LF impedance. 10.0μm Factors out size variations. Sensitive to changes in membrane properties. The properties of the beads do not vary much with frequency, hence opacity stays constant. There is almost a linear relationship between size and l.f. impedance 10

11 Human Blood Erythrocytes red blood cells ~ RBC 5,000,000 RBC per μl 4,000 11,000 Leukocytes per μl only Monocytes per μl 21 Blood cells capacitance and size T-lymphocytes B-lymphocytes data from Yang J., et al Biophys J, Measured using ROT and DEP Monocytes Granulocytes 11

12 Full Blood Count (FBC) A generic blood test, performed to diagnose disease (virus, cancer) rule out a clinical problem monitor therapy establish a prognosis screen for disease determine effective drug dosage/prevent toxicity. FBC is used to measure properties of blood cells. Normally done by light scattering: Bigger cells scatter more light at small angles Granular cells scatter more light than agranular at large angles Frequency sweeps for white blood cells Data obtained at two frequencies simultaneously, reference frequency of 500kHz, other frequency swept. 12

13 Scatter plots for WBCs. Each sub-population purified, then re-mixed Cell membrane capacita ance Size Single cell impedance Light scatter (FACS) Whole human blood Erythrocytes removed by lysis Cell membrane capacitan nce Size 13

14 Smaller particles limit is set by noise in the system To measure smaller particles, requires smaller electrode and channel dimensions, increasing Φ Platelets Beads RBC Platelets (1μm) and RBCs (5μm) 14

15 Can we increase the sensitivity of the system to detect changes in cell phenotype, i.e. using CD antibody expression Currently performed using fluorescently labelled antibodies Can this be done electrically? Can we replace the fluorescence label with an impedance label Conjugation of beads to cells beads are impedance labels CD4 conjugated beads + CD4+ T-Lymphocyte All other cells (CD4-) 15

16 Simulation showing change in impedance with increasing binding of beads Application in medicine - Counting T lymphocyte sub-population to diagnose HIV AIDS A small number of the lymphocyte population express an antigen called CD4+. A CD4 count of less than 200 cells/μl of whole blood establishes diagnosis of AIDS i.e. the CD4+ cells need to be counted to diagnose HIV AIDS Currently performed using expensive flow cytometry (FACS) and fluorescent antibodies Can it be done with impedance labels? 16

17 White blood cell prep. BEFORE ADDING BEADS AFTER ADDING BEADS Whole blood 17

18 Comparison with commercial FACS machine Cell type FACS (% of total) IS (% of total) Typical Numbers % [Wintrobe 1999] T-cells with CD4 beads 18.0 (11243/62312) 15.6 (1215/7789) Lymphocytes (non CD4+) Total Lymphocytes 12.7 (7914/62312)) (1005/7789) Monocytes with CD4 beads 0.6 (328/62312) 0.8 (66/7789) N/A Monocytes (no beads) Total Monocytes 4.3 (2695/62312) (421/7789) Granulocytes 64.4 (40132/62312) 65.4 (5095/7789) The electrical double layer Electrode C DL R C DL Electrode R DL R DL C double layer bulk electrolyte double layer The capacitance of the Double layer can be approximated to C DL = ε ο ε r κ. κ In PBS, this gives... For a small electrode the D.L. Capacitance leads to a significant series reactance, reducing the signal. Answer: Increase electrode area, either absolute leads to reduced sensitivity or relative, porous electrodes 18

19 Smooth electrodes in PBS 0.5 cm diameter electrodes Nanostructured Pt electrodes in PBS 0.5 cm diameter electrodes 19

20 Conclusions Impedance spectroscopy measures the dielectric properties of single cells the effective polarisability given by Maxwell Wagner relaxation. This data can be used to characterise cells impedance labelling of cells provides an electrical impedance labelling of cells provides an electrical analogue of fluorescent labelling for enhanced cell identification 20