J.
McGrath
,
M.
Jimenez
and
H.
Bridle
*
Heriot-Watt University, Microfluidic Biotech Group, Institute of Biological Chemistry, Biophysics and Bioengineering (IB3), Riccarton, Edinburgh, UK. E-mail: H.L.Bridle@hw.ac.uk; Tel: +44 (0)131 4513355
First published on 4th September 2014
Deterministic lateral displacement (DLD), a hydrodynamic, microfluidic technology, was first reported by Huang et al. in 2004 to separate particles on the basis of size in continuous flow with a resolution of down to 10 nm. For 10 years, DLD has been extensively studied, employed and modified by researchers in terms of theory, design, microfabrication and application to develop newer, faster and more efficient tools for separation of millimetre, micrometre and even sub-micrometre sized particles. To extend the range of potential applications, the specific arrangement of geometric features in DLD has also been adapted and/or coupled with external forces (e.g. acoustic, electric, gravitational) to separate particles on the basis of other properties than size such as the shape, deformability and dielectric properties of particles. Furthermore, investigations into DLD performance where inertial and non-Newtonian effects are present have been conducted. However, the evolvement and application of DLD has not yet been reviewed. In this paper, we collate many interesting publications to provide a comprehensive review of the development and diversity of this technology but also provide scope for future direction and detail the fundamentals for those wishing to design such devices for the first time.
The purpose of this review is to focus on one of these passive techniques, the Deterministic Lateral Displacement (DLD). Deterministic lateral displacement was first reported by Huang et al. in 2004 to separate particles on the basis of size in continuous flow with a resolution of down to 10 nm.1 Since invention, this technique has been used to separate millimetre,2 micrometre3–7 and even sub-micrometre1 sized particles and has been applied to diverse purposes, although mostly medical related (separation of trypanosomes,17 white blood cells (WBCs),6 red blood cells (RBCs),9 circulating tumour cells18 (CTCs) or platelets19 from blood for instance). To extend the range of potential applications, the specific arrangement of geometric features in DLD has also been adapted and/or coupled with external forces (e.g. acoustic,8 electric,4,9 gravitational10) to separate particles on the basis of other properties than size such as the shape, deformability and dielectric properties of particles.
Deterministic lateral displacement is a technology which utilises the specific arrangement of posts within a channel to precisely control the trajectory of and facilitate separation of particles larger and smaller than a critical diameter (Dc). Each succeeding row within a constriction is shifted laterally at a set distance from the predecessor, this leads to the creation of separate flow laminae which follow known paths through the device. The separation mechanism of DLD works in that if the centre of a particle is out with the width of the first streamline, it then becomes displaced into the second streamline when negotiating a post. This action continues each time such a particle passes a post, with the particle said to be larger than Dc. Meanwhile, particles that are smaller than Dc remain centred within the first streamline and follow the defined route of this streamline through the device (Fig. 1). Particles smaller and larger than Dc will then be separated from one another along the length of a device.
![]() | ||
Fig. 1 The streamline orientation and basic principle of DLD with and without an external force. (A) The orientation of flow lamina induced as a consequence of lateral row shifting in a device with N = 5. (B) Position of fluid streamlines (P1, P2, P3…) between two pillars. (C) The normal motion of particles in a DLD; particles smaller than Dc (red) remain within the first streamline influenced by drag force (FDrag) and continue through the device in a zigzagged mode according to the path highlighted by the example lamina. Particles that are larger than Dc (green) are continually displaced into the next streamline at each successive pillar, thus facilitating particle separation. As two particles traverse the length of the device, the distance between them becomes larger. (D) When negative dielectrophoresis is induced in polarisable particles nominally smaller than Dc, they move away from the insulating posts due to dielectrophoretic force (FDEP) and act as if they were larger than Dc, thus entering displacement mode. Adapted from ref. 4 with permission from The Royal Society of Chemistry. |
For 10 years, DLD has been extensively studied, employed and modified by researchers in terms of theory, design, microfabrication and application to develop newer, faster and more efficient separation and processing20 tools. However, since invention the evolvement and application of this technology has not been reviewed. Due to the wide ranging applications, the diversity in size of particles and cells being separated, the variation in design features, the prospective future applications of this device and the differences in description throughout literature – for example both DLD and deterministic ratchet are used to describe the same technology – a review is long overdue to synthesis the progress to date and to highlight necessary future work.
Firstly, an introduction to the related theory will be provided before design considerations and several of the many applications are discussed – where a comprehensive list of the uses of DLD and the conditions such uses were applied in is detailed in Table 1. This table will allow readers to quickly understand the operating conditions in the referenced applications and we have generated a toolbox to assist with device design for those who are new to the technology.
![]() | (1) |
Where ρ, υ, p and η refer to fluid density, velocity, pressure and viscosity respectively. The non-linear terms (ν·∇ν) on the left side can be disregarded as inertial effects are negligible,22 thus giving the Stokes equation:
![]() | (2) |
From the Stokes equation, a dimensionless number known as the Reynolds number (Re), which is used to show the ratio of inertial force densities to viscous force densities can be derived22,23
![]() | (3) |
In eqn (3), DH represents the hydraulic diameter, which can be calculated using
DH = 2wh/(w + h), | (4) |
The Peclet number (Pe) gives the ratio of the rates of convection and diffusion of particles, in terms of the time required to move a certain distance by radial diffusion and axial convection and is defined as22
![]() | (5) |
![]() | (6) |
Of the terms in the numerator, k represents the Boltzmann constant and T is the absolute temperature. For the terms in the denominator, a symbolises the hydrodynamic radius of the particle or molecules.
When Pe is high, the rate of convection greatly exceeds the rate of diffusion and this limits the mixing of fluids. The Peclet number is typically high, from 10–105, in microchannels25 and this coupled with low Reynolds numbers results in long mixing times for fluids, giving greater predictability of fluid flow. If we consider the diffusivities of a small protein (40 μm2 s−1) and a mammalian cell (0.02 μm2 s−1), which are typically 5 nm and 10 μm in size22 and travelling in fluid at 100 μm s−1 in a 100 μm wide channel, then according to eqn (5) the small protein has Pe = 250 whilst it is 500000 for the mammalian cell. This means that the small protein requires 250 channel widths, or a 2.5 cm long channel and 250 s to diffuse across the width of the channel in fluid travelling at 100 μm s−1. Moreover, this means that in 25 s the protein will have diffused a distance of 10 μm across the channel width. Alternatively, the mammalian cell requires 500
000 channel widths or a 50 m microchannel to diffuse across its width in similar conditions. This illustrates how reducing particle size may lead to more prominent, diffusive effects. This parameter is of first importance in DLD since it could strongly alter the separation efficiency of small particles that tend to diffuse.
![]() | (7) |
The pressure difference along the channel is symbolised by Δp. It is apparent that a larger value of R in the denominator would serve to decrease Q.
For rectangular microchannels with high aspect ratio, where either channel width or height (h) is greater than the other and when taking channel length (l) into account, the resistance is typically devised using21
![]() | (8) |
Alternatively, in a rectangular microchannel with a low aspect ratio (w ≈ h), resistance is calculated using21
![]() | (9) |
When the aspect ratio is particularly large in a DLD, for example in the devices used by Davis26 where device depth is at least five times larger than the gap between two pillars, the 3D parabolic profile is dominated by the smaller dimension, which is the gap between pillars. In such devices, rearrangement of eqn (8) allows calculation of the resistance in a single gap as
![]() | (10) |
If we consider a device with a gap between pillars of 10 μm, pillar length of 10 μm and height of 50 μm and then compare this to a device with a gap size of 5 μm – the reduction of the gap size by half, whilst all other parameters remain constant, results in an 8× increase in the resistance according to eqn (10). Although in this specific example it would still be possible to introduce fluid into the system as the pressure requirements are not excessive, the scenario shows how reducing the dimensions of a DLD can cause a marked increase in fluidic resistance, thus affecting possible flow rates and particle sorting times.
According to theory, when two differently sized particles following the same streamline enter the constriction and negotiate a post, assuming that the particles do not alter streamlines and do not interact with one another, a particle smaller than a defined critical diameter (Dc) will remain in the first streamline (Fig. 1C) as its hydrodynamic centre is not out with the width of the first streamline (β). Alternatively, a particle larger than Dc is displaced into the next streamline due to its hydrodynamic centre being out with the boundary of the first streamline – this action continues at every post and is termed displacement mode. Particles larger than Dc are displaced in accordance with the displacement angle (θ) which arises due to lateral row-shifting (Fig. 2). A zigzagged but ultimately straight course through the device ensues for particles smaller than Dc – appropriately termed zigzag mode. Given sufficient time, space and a capable geometry, rigid, spherical particles that are larger or smaller than Dc will be directed to alternate outflows, allowing for collection of separated particles.
The posts contained within one row in a DLD are at a constant centre-to-centre distance from one another, λ, which is the sum of the gap distance, G, and post diameter, Dp. There is a set distance, Δλ, at which each successive row is shifted laterally with reference to its predecessor in a rhombic array (Fig. 2), where rows are perpendicular to the fluid flow with columns tilted. In the titled square array, rows and columns are perpendicular to one another but the array is tilted so that it is not perpendicular to the fluid flow. In the case of the tilted square array, the parameter Δλ does not exist, however all arguments of DLD theory (to be described) are said to hold true if in this instance Δλ is calculated as
λ tan![]() | (11) |
As mentioned, the angle θ develops as a result of lateral row shifting and represents the alignment of each column relative to flow direction. When the posts of row N + 1 are in the same lateral position as the first row, the period is said to be N, which is also related to λ and Δλ:
![]() | (12) |
For Fig. 1, N = 5 and there are N flow lamina or streamlines between two posts, illustrating that the period N dictates the number of streamlines. The inverse of eqn (11) can be used to describe the row shift fraction (ε):
![]() | (13) |
Analytically, the Dc at which a particle will enter displacement mode is approximated using3
DC = 2β = 2α·Gε | (14) |
A unit-less correction factor, α, is used to accommodate non-uniform flow in the DLD and assuming a parabolic flow profile as demonstrated by Beech.23
Davis26 devised an empirical formula for approximation of Dc using over 20 devices with varying gap size and spherical particle size based on a parabolic flow profile. The derived formula is
DC = 1.4Gε0.48. | (15) |
For some rhombic array devices, the gap between the pillars of a column (Dy) is smaller than G rather than equal and a parallelogram-shaped array becomes apparent. In this instance Dc can be calculated using:28
DC = 2α·Gε′ | (16) |
![]() | (17) |
![]() | (18) |
Here, n represents the row number. Meanwhile the gap between the right side wall and posts (GR) can be derived by
![]() | (19) |
Numerical simulations of D'Avino suggest that the use of non-Newtonian fluids can allow tuning of Dc; shear-thinning fluids give rise to lower Dc in DLD constrictions when compared to a Newtonian equivalent.12 The velocity profile is altered due to viscosity thinning, changing flow lane distribution and subsequently reducing Dc. D'Avino derived the following equation to allow calculation of Dc when non-Newtonian fluids are used12
![]() | (20) |
A(f) = 1.86 + 1.08f + 1.38f2. | (21) |
![]() | ||
Fig. 3 Geometry of obstacles within a DLD used to investigate the separation efficiency at moderate Reynolds number. Quadrilateral posts, mirrored quadrilateral posts and round posts are used in designs 1, 2 and 3. Adapted from ref. 33 with permission from Elsevier. |
Determining the shear stress acting upon a particle brings complexity, as a particle alone causes flow perturbation – for example, particles that are much smaller than the gap do not tend to cause significant perturbation but particles slightly smaller than the gap are known to cause large perturbations and if soft may be capable of deformation, which would further influence perturbation.23 Additionally, particle–post interactions may cause particle deformation and flow perturbation.
When irregularly shaped particles flow between pillars in a DLD they tend to become orientated in a manner that makes their smallest dimension the critical dimension.23 Additionally, the mode of transport also influences particle behaviour and consequently the effective size; particles tend to rotate continuously due to asymmetric viscous drag when in displacement mode, meanwhile particles in zigzag mode instead deform, as the effective shear experienced varies between flow lanes.34 The shear rate, deformation and relaxation time of a particle influences which of deformation or rotation influence dominates.34 In order to limit the range of possible orientations of irregularly shaped particles within a DLD, Holm et al. reduced device depth.17 This work demonstrates the use of a very shallow constriction to ensure that RBCs pass posts with their width as the critical dimension, rather than thickness. This effect ensures that the critical dimensions of RBCs and trypanosomes are not similar and facilitates their separation.
The use of streamlined posts was modelled and proposed by Beech as a method of reducing the areas surrounding circular pillars with zero flow velocity, to increase recovery and reduce clogging.5
The use of I-shaped posts is aimed at separating non-spherical and/or deformable particles within a DLD. Zeming et al. developed this particular post shape in order to induce a series of rotations in non-spherical particles which serve to increase hydrodynamic radius whilst passing I-shaped obstacles within the constriction, thus facilitating separation.36
Diamond and airfoil posts were modelled by Al-Fandi et al. with a view to reducing the clogging and deformation issues that soft, deformable particles experience when negotiating circular posts, where the author concluded that airfoil posts were most suitable.37 Airfoil post simulations indicated that flow exerts less variation in velocity gradient, very low forces at the post surface and higher values of tangential forces when compared to circular and diamond posts; leading the author to conclude that this design would inhibit the clogging, sticking or deformation of particles in this constriction. However, there appears to be no experimental data related to the efficiency of airfoil posts, perhaps due to the complexity concerned with manufacturing such a device.
![]() | ||
Fig. 5 DLD device designs with several separation arrays. (A) A multiple array for use where the largest particle diameter is no larger than the gap size of the final array. (B) A chirped array where row shift fraction (ε) is varied to increase separation range and reduce clogging in comparison to the multiple array. As ε increases the displacement angle (θ) also increases; see eqn (13). (C) A cascade array with separate non-clogging outflows to increase the separation range further. Black arrows indicate particle trajectories. Reproduced from ref 26 with permission. |
– Calibration curves are proposed in ref. 3 for circular posts and in ref. 35 for triangular posts.
– Triangular posts allow a larger gap G between the posts than circular ones.
– I-shaped or square posts induce rotation of non-spherical particles to increase their effective diameter.
– Based on Dc, define the gap G and row shift fraction ε.3 See Fig. 6 for the ratio of particle diameter divided by G versus ε to approximate the particle trajectory.
![]() | ||
Fig. 6 Experimental points of the particle diameter divided by the gap, G, versus the row shift fraction, ε. For the work of Inglis et al.3 (in black) and that of Huang et al.1 (in grey), open points represent bump mode and solid points represent zigzag mode. Zigzag mode particles follow the streamlines, while bump mode particles follow the array slope, ε. Adapted from ref. 3 with permission from The Royal Society of Chemistry. |
– Maximum dynamic range in a chirped array 3–5.7
– Typical displacement angles (θ) are 1 to 6°.19
– Refer to ref. 35 for design help for triangular posts which allow a larger gap G for similar Dc and ε.
– Tall posts lead to a higher throughput, but the post aspect ratio is limited by the moulding step. Polydimethylsiloxane (PDMS) posts with an aspect ratio that is more than 2 have an unacceptably high probability of tipping over during assembly. An aspect ratio of 2 for an injection moulded plastic device is at the limit of current manufacturing methods. Extremely large posts, relative to the gap also reduce the critical size, whereas extremely small posts are expected to increase the shear rate.38
Where GL is the width of the gap between the left sidewall and the first pillar of the nth row, within an array with period N.
– Right boundary correction
Where GR is the width of the gap between the last pillar in the nth row and right sidewall.
– Non-clogging outflows of cascade arrays should be designed to ensure that their pressure drop is the same as the next array.
– Divisions of outlet channels should have identical resistance to maintain the profile of flow leaving the constriction.
– Lateral separation is determined by the displacement angle and device length – this calculation will determine outlet positioning.
– Significant deformation of gap size has been reported for standard glass–PDMS devices.40
– Flow velocity profile between heterogeneous surfaces in e.g. glass–PDMS devices has been reported as asymmetrical for certain aqueous fluids.40
– PDMS devices deform considerably under pressure.38
Application | Critical size | Post shape | Design parameters | Manufacturing details | Pre-treatment | Containing fluid/buffer | Flow speed/driving pressure/flow rate | External forces (comments) | Recovery rate/purity/resolution/related comments | |
---|---|---|---|---|---|---|---|---|---|---|
Notes: the column headings of Table 1 have been ordered in a chronological manner from the desired application to design considerations, manufacture, experimental details, any external forces applied and details of separation efficiency. Depending on application, the referenced work has been categorised as beads, blood, pathogenic cells, droplets or other and where possible the works contained under each category are listed in ascending order of size. For applications enlisted within the blood category, those using alike cells have been grouped together to improve legibility for the reader. | ||||||||||
BEADS | 300 and 500 nm, 5.0 and 6.6 μm, 6.6 and 7 μm (ref. 8) | Tuneable | None | h = 15 and 45 μm, θ ~ 45° | Polydimethylsiloxane (PDMS) chamber bonded to 0.5 mm thick lithium niobate substrate with 5/250 nm chrome aluminium IDTs arrayed on top | — | Deionized water (DI) with 0.2% polyethylene glycol/none | 4.1 μL min−1 for 5, 6.6 and 7 μm beads 0.45 to 1.8 μL min−1 for 300 and 500 nm beads | Acoustic/electric forces to create virtual pillars | 99.1 ± 0.7% and 99.3 ± 1.3% of 5.0 μm and 6.6 μm successfully separated with DEP. 99.5 ± 0.5% and 97.3 ± 2.7% using SAW. 80–90% separation of 6.6 μm and 7 μm particles. 87% separation of 500 nm particles from 300 nm |
0.40 to 1.03 μm (ref. 1) | 0.64 μm to 1.10 μm | Circular | D post = 6.4 μm, G = 1.6 μm, ε = 0.1 | Silicon device manufactured using deep reactive ion etching (DRIE) | — | Aqueous solution/none | ~40 μm s−1 and ~400 μm s−1 (30 to 300 mbar) | None | Resolution of ~20 nm | |
1.1 to 3.1 μm (ref. 45) | ~1.4 to 1.9 μm | Right isosceles triangular posts | D post = 6 μm, G = 4 μm, θ = 5.71°, h = 10 μm | Silicon by DRIE sealed with a PDMS-coated glass slide | — | —/— | ~250 μm s−1 | None | — | |
1.9 to 3.8 μm (ref. 35) | D c/G from 0.25 to 0.55 in Fig. 2.b, Dc ~ 1.6 to 3.5 μm | Triangular |
θ = 2.86° to 11.46° (Fig. 2.b), ε = 0.05 to 0.2 (tan![]() |
Silicon by DRIE sealed with a PDMS-coated glass slide | — | —/— | ~100 μm s−1 | None | — | |
2 to 10 μm (ref. 4) | 6 to 2 μm | Circular | l = 25 mm, ε = 0.1, G = 12 μm, Dpost = 30 μm, h = 34 μm | PDMS and glass using replica molding/ | — | —/0.5× TBE, 0.1% (w/v) SDS with 2.5% PVP | Between 90 and 260 μm s−1 for 5 μm beads (10 to 100 mbar) | Coupled with DEP | Author comments that D-DLD gives poorer resolution than DLD | |
2.1, 4.2 and 5.7 μm (ref. 43) | Array 1: Dc = 3.1 μm. Array 2: Dc = 4.6 μm measured by electron microscope (were designed to be 3.5 and 5 μm) | Circular | Inlet 1 – 820 μm wide, inlet 2 – 5180 μm wide. 2 arrays: array 1 – 33.7 mm long, G = 10.5 μm, θ = 2.86°. Array 2 – 16.9 mm long, G = 10.5 μm, θ = 5.7°. 3 outlets | PDMS-glass device manufactured using soft lithographic techniques | Flushed with 0.2 μm filtered DI water for 20 min at 5 psi | 2.1, 4.2 and 5.7 μm beads diluted at ratio 2![]() ![]() ![]() ![]() |
Beads introduced at 5 psi | None | 99% recovery of 4.2 μm particles and 96% removal of 2.1 μm and 5.7 μm particles | |
2.3 to 22 μm (ref. 3) | — | Circular | Up to 22 combinations, ε = 0.01 to 0.33, G = 12 to 38 μm, Dpost/G = 0.32 to 1.36, h = 25 μm, l (bump array) = 2 cm | Features in silicon, device in PDMS coated glass cover slips | Devices soaked in a 2 g L−1 solution of Pluronic F108 | —/— | ~500 to 1500 μm s−1 (0.03 to 0.14 bar) | None | — | |
Stainless steel balls (3, 6, 6.4, 7.1 mm in diameter) in glycerol2 | — | Circular | D post = 7.8 mm, G = 16 mm, θ = 13–30° | Lego® | — | —/Glycerol | — | None | — | |
3.4, 4.0, 5.0, or 6.0 μm (ref. 31) | — | Circular | Devices placed in module with entry and exit channels to connect syringes, 2 inlets and outlets of equal width, h = 40 μm, w = 15 mm, l = 15 mm, Dpost = 3.2–8 μm, G = 8–9 μm | Silicon devices manufactured using lithography and DRIE | 0.25 wt% Synperonic PEF108 solution pumped through for 30 min | Bead stock suspensions diluted with MilliQ water to volume concentration of 0.05%/MilliQ water | Buffer and sample fluids introduced at 4 μL h−1. | (Analysis of mixed motion) | — | |
Silica particles 4.32, 10, 15, 20 μm (ref. 10) | — | Circular | h ~ 40 μm, Dpost = 17.5 μm, G = 22.5 μm | SU-8 device spin coated onto microscope slide using standard photolithography procedures | — | Particles suspended in 1 mM KOH solution/— | — | Gravity-driven DLD | At a driving angle of 14° resolution is ~1.35 (see paper for resolution equation) | |
5.7 to 11.9 μm (ref. 9) | Tuneable | None | w = 1.7 mm, l = 2.3 mm, h = 14.4 μm, θ ~ 21° | Electrodes deposited on glass wafer by sputtering. Reactive ion etching used to fabricate spots on electrode arrays. PDMS device produced from SU-8 master using replica moulding and bonded to glass via plasma treatment | — | PBS diluted in DI+ 0.2 w/w Tween 20/— | Buffer 0.2–0.3 μL min−1 sample 0.1 μL min−1 | DEP to create virtual pillars | Depending on electric field applied work demonstrates over 99% separation purity for all PS particles used | |
10 to 16 μm (ref. 44) | 14 to 18 μm | Circular | ε = 0.05, G = 54 μm | PDMS device manufactured using standard rapid prototyping and replica molding techniques | — | 0.001% mass/volume suspension of polystyrene beads/0.1% solution of Pluronics F127 | ~500 μm s−1 | Coupled with mechanical stretching | 100% separation | |
209 to 277 μm and 309 to 532 μm (ref. 33) | For designs 1, 2 – Dc = 400 μm, for design 3 – Dc = 330 μm | Circular/quadrilateral/mirrored quadrilateral | G = 0.56 or 0.6 mm, Dy = 1.13 or 1.80 mm, quadrilateral obstacles – 0.8 × 1.6 mm, Dpost of round obstacles 0.68 mm, ε = 0.25 or 0.17, h = 2.5 mm | Polyether ether ketone (PEEK) device manufactured by milling and placed in stainless steel module with Polymethyl methacrylate (PMMA) lid | Chamber washed initially with demi water + 20% v/v glycerol + 1.5% w/v SDS at 80 mL min−1 | Experiments 1 & 3: demineralised (demi) water + 20% v/v glycerol + 1.5% w/v SDS. Experiment 2: demi water + PEG-400 to give solution with 164 or 220 mPa s−1 viscosity. Note – only one inlet so bead/buffer introduced together as mixture | 20–275 mL min−1. | (Inertial effects in system at elevated Re) | Separation efficiency ratio of 47 (see paper for derivation of ratio) | |
BLOOD | WBCs (5–20 μm) from RBCs (~8 μm × 2 μm) and plasma7 | From 3 μm to 9 μm | Circular | 13 functional regions, Dpost = 12 μm, G = 10 μm, ε = 0.04 to 0.4, h ~ 25 μm | Standard photolithography to construct silicon devices. Bosch silicon etching process used to give near vertical sidewalls. Devices coated in fluorosilane vapour and sealed with glass coverslips coated in PDMS | 2 g per liter solution of the triblock copolymer F108 | Blood/PBS with 1% BSA and 0.09% sodium azide | ~1000 μm s−1 (cell) (blood flow 0.3 nL s−1, pressure −0.1 bar) | None | 99% of RBCs in channel 1. 99% of granulocytes and 99.6% of all WBCs displaced into channels 2 and 3 |
WBCs from RBCs9 | Tuneable | None | w = 1.7 mm, l = 2.3 mm, h = 14.4 μm, θ ~ 21° | Electrodes deposited on glass wafer by sputtering. Reactive ion etching used to fabricate spots on electrode arrays. PDMS device produced from SU-8 master using replica moulding and bonded to glass via plasma treatment | — | Blood diluted 10 times in 0.3 M sucrose buffer with 0.2% EDTA/— | Buffer 0.1 μL min−1 sample 0.01 μL min−1 | DEP to create virtual pillars | Over 99% separation purity of WBCs from RBCs | |
WBCs from RBCs6 | ~8 μm | Circular | h = 20 μm, l = 7 mm, w = 1.8 mm, G = 14 μm, Dpost = 46 μm | PDMS device made using DRIE moulds mounted on glass slides | — | Blood diluted (2, 5 and 10 times) with Ficoll-Paque Plus/— | 0.08 μL min−1 sample 0.4 μL min−1 buffers | None (impact of the blood dilution and freshness) | Initial WBC![]() ![]() ![]() ![]() ![]() ![]() |
|
WBCs, RBSs and platelets from blood42 | 3.8 and 6.1 μm | Circular | w = 1.6 mm, l = 6.8 mm 2 stages stage 1: G = 20 μm, Dpost = 60 μm, ε = 0.025 stage 2: G = 20 μm, Dpost = 60 μm, ε = 0.0625 | PDMS devices made using soft lithography from DRIE silicon moulds | PBS+ heparin at 1 μL min−1 for 10 min | Blood diluted 50 times in PBS + heparin/— | 0.1 μL min−1 sample 1 μL min−1 buffers | None | The ratio of separated RBCs to platelets to WBCs was found about 470![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
|
RBCs from blood36 | 3.33 μm for circular, between 2.5 and 3 μm for I-shaped | Circular/square/I-shaped | 3 inlets, 3 outlets with 40 outlet subchannels, Dpost = 15 μm, G = 10 μm, l ~ 20 mm, w ~ 2 mm, h ~ 15 μm, θ = 2.86° | Silicon-PDMS/photolithography | 1% w/v Pluronic F127 in deionized water for 30 min | Blood diluted 10 times in PBS/PBS | Blood 0.2 μL min−1, buffers 0.5 μL min−1 | None (impact of the pillar shape) | 100% separation of RBCs from blood | |
RBCs depending on their size, morphology, deformability46 | From 3 to 9 μm | Circular | 13 sections, Dpost = 20 μm, G = 12 μm, ε = 0.025 to 0.27, h = 10.84 μm and h = 4.27 μm | PDMS device bound to fluorisilane-coated silicon wafer created using standard lithography, DRIE and sandblasting (entry/exit holes) | 0.2% PLL(20)-g[3.5]-PEG(2) in DI water and left to rest for at least 20 min, then rinsed with autoMACS® | Blood diluted 5 times in autoMACS® with sodium salicylate and TritonX-100/autoMACS® with EDTA and BSA | From 30 μm s−1 to 18 cm s−1 (driving pressure form 5 to 800 mbar) | None (Impact of the depth) | — | |
Enrichment of leukocytes from blood38 | D c1 = 7.3 μm Dc2 = 4.5 μm | Circular | 2 arrays and 6 parallel devices (3 mirroring pairs) – no buffers. Array 1: Dpost = 22 μm, G = 22 μm, ε = 0.05, h = 40 μm, w = 836 μm, l = 26 367 μm array 2: Dpost = 22 μm, G = 13 μm, ε = 0.05, h = 40 μm, w = 840 μm, l = 25![]() |
Multilayer SU-8 and PDMS device manufactured using standard lithography and plasma bonding | De-ionized water + 0.1% (v/v) Tween-20 for 5 min for beads. AutoMACS® buffer for 5 min for blood | Whole or diluted blood in AutoMACS®/none | Whole undiluted blood ~115 μL min−1 atm−1 (0.2 atm) | None | Capture of 98% and approximately ten-fold enrichment of leukocytes in whole blood | |
CD4+ T helper lymphocytes from other WBC42 | 23 μm | Circular | G = 47 μm, ε = 0.15, Dpost = 13 μm | PDMS devices made using soft lithography from DRIE silicon moulds | PBS+ heparin at 1 μL min−1 for 10 min | WBCs in PBS (1 × 106 cells mL−1) with antibodies coated beads (25 μm)/— | 0.2 μL min−1 sample 1.2 μL min−1 buffers | (Attached antibodies to WBC for a subtype separation) | 100% separation of 25 μm beads with 91% recovery of T lymphocytes | |
Platelets (~3.2–3.6 μm in diameter, ~0.9–1.1 μm thick), from blood19 | D c = 2.3–5.3 μm in array 2 | Circular | Array 1: G = 17 μm array 2: 11 steps, Dpost = 20 μm, G = 6–17 μm, ε = 0.01–0.125, h = 18 μm | PDMS-glass device | Deionized water +2 g L−1 Pluronic F108 and placed under vacuum for 2 h | Blood in anticoagulant citrate dextrose with PE-conjugated antihuman CD41/Auto MACS® buffer | Blood 0.8 nL min−1, 14 kPa | None | — | |
Undiluted blood plasma from whole blood7 | From 4 to 1 μm | Circular | 3 functional regions 1: l = 17.6 mm, w = 1.1 mm, Dpost = 10 μm, θ = 2.8°, G = 20 μm 2![]() ![]() |
Standard photolithography to construct silicon devices combined with Bosch silicon etching. Devices coated in fluorosilane vapour and sealed with PDMS-coated glass coverslips | 2 g per liter solution of the triblock copolymer F108 | Blood/PBS with 1% BSA and 0.09% sodium azide | (Blood flow 0.4 μL min−1, pressure 0.3 bar) | None | 100% removal of all components greater than 1 μm from blood plasma | |
Circulating tumor cells (15–30 μm) from blood (other cells 2–15 μm)18 | 7 μm | Triangular | 1 input, 2 outputs, mirrored array. w = 2.5 mm, l = 25 mm, Dpost = 58 μm, G = 42 μm, θ = 2.86°, h = 60 μm | Silicon wafer sealed with a PDMS-coated glass cover slide – standard lithography | 1× PBS, 1% BSA, and 1 mM EDTA | Cell suspension in buffer or diluted blood (between 5 and 20 times) with buffer/none (3.75 × 106 cells mL−1) | 1.5 to 150 cm s−1 (0.1 to 10 mL min−1) 4 atm at 10 mL min−1 | None | Captured over 85% of CTCs from blood | |
Circulating tumour cells (15–25 μm, 1 in 109 cells) from diluted peripheral blood (other cells 5–15 μm)41 | ~5–6 μm | Circular/triangular | One inlet, three outlets, mirrored design. l = 35 mm, w = 3.5 mm, h = 30 μm, circular posts: Dpost = 50 μm, G = 25 μm, θ = 3.2° triangular posts: Dpost = 25 μm, G = 25 μm, θ = 3.8° | Standard photolithography and soft lithography – PDMS bonded to glass sides | — | Cultured cells in PBS (105 cells mL−1) or cultured cells in 10 times diluted blood with physiological saline (~104 cells mL−1)/none | 0.01 to 2 mL min−1 | None (tests with 5 human cancer cell lines- comparison between triangular and circular posts) | 90% capture yield and more than 50% capture purity | |
PATHOGENIC CELLS | E. coli (0.5 μm in diameter, 2 μm long) in DI36 | 1.12 μm for the circular shape | Circular/I- shaped | D post = 6 μm, G = 4 μm, l ~ 20 mm, w ~ 500 μm, h ~ 8 μm, θ = 2.00° | Silicon device manufactured by standard lithography and DRIE, PDMS cover layer bound to device by plasma treatment | 1% w/v Pluronic F127 for 30 min | Cell culture in DI/DI (8 × 107 cells mL−1) | 0.3 μL min−1 for the wider flow stream, 0.08 μL min−1 for the narrow stream, 0.05 μL min−1 for the sample stream | None | Overall efficiency not given but author states that bacteria tend to stick to posts in ESI |
Trypanosomes (~2.5 μm × 30 μm) from human blood17 | From 3 to 9 μm | Circular | 13 sections, ε = 0.025 to 0.27, Dpost = 20 μm, G = 12 μm, h = 4, 11 and 33 μm | PDMS device generated using replica molding from SU-8 master, patterned PDMS bound to PDMS cover using plasma treatment | 0.2% PLL(20)-g[3.5]-PEG(2) in DI water for at least 20 min before rinsing with DI water for another 20 min | Parasites and blood diluted 20 times in autoMACS® (without blood serum for experiments with blood and parasites)/autoMACS® | ~600 μm s−1 (~1 nL s−1) | None (Impact of the depth) | 99.5% sorting efficiency (fraction of trypanosomes captured with lateral displacement such that 99% of the RBCs are rejected by the device) | |
Trypanosomes (3.7–5.8 μm in effective diameter)37 | 2.7 μm | Circular | D post = 20 μm, G = 10 μm, θ = 2.86° | Not detailed | — | — | — | None | — | |
Mature and immature spores of Aspergillus brasiliensis (0–10 μm).43 | Array 1: Dc = 3.1 μm. Array 2: Dc = 4.6 μm measured by electron microscope (were designed to be 3.5 and 5 μm) | Circular | Inlet 1 – 820 μm wide, inlet 2 – 5180 μm wide. 2 arrays: array 1 – 33.7 mm long, G = 10.5 μm, θ = 2.86°. Array 2 – 16.9 mm long, G = 10.5 μm, θ = 5.7°. 3 outlets | PDMS-glass device manufactured using soft lithographic techniques | Flushed with 0.2 μm PBS for 15 min at 10 psi | 4.4 × 106 spores ml−1 PBS with 0.1% Tween-20/0.2 μm filtered DI water | 10 psi/total volumetric flow through of 40 μL min−1 | None | Two- to three-fold increase in purity of spores | |
DROPLETS | Water drops (3.6 to 11.7 mm) in oil47 | — | Circular | D post = 7.8 mm, G = 16 mm θ = 0–45° | Lego® | — | Water/oil | — | Gravity | — |
Droplets (11–30 μm in diameter) with S. cerevisiae encapsulated in oil40 | 24 μm | Circular | D post = 60 μm, ε = 0.1, G = 60 μm, h = 30 μm | PDMS device manufactured by standard lithography from SU-8 mould, plasma treatment used to bond device to glass slide | Treated with a coating agent (Aquapel, PPG industries) and flushed with air | (PBS or S. cerevisiae in YPD medium (2 × 106 cells mL−1)/oil for droplet generation)/oil | 10 μL h−1 PBS, 500 μL h−1 oil (30 μm droplets), 30 μL h−1 PBS, 600 μL h−1 oil (10 μm droplets) and 5 mL h−1 buffer | None | Outlet 6 contained 99.9% large droplets, whereas at the central outlet has >97% small droplets | |
OTHER | Bacterial artificial chromosomes (61 and 158 kb)1 | 1.39 μm based on Davis corr. | Circular | G = 3 μm, Dpost = 5 μm, ε = 0.1 | Fused silica device | — | —/— | ~20 μm s−1 | Electric fields | Resolution of 12% |
H1975 epithelial cell fractionation (10–40 μm)/H1975 epithelial cell line and the 3T3 fibroblast cell line (13.7 ± 3.0 μm)39 | 15 μm | Circular | 3 inlets, 6 outlets G = 37.5 μm, ε = 0.1, Dpost = 50 μm | PDMS device manufactured using standard soft lithography techniques | Devices flushed with ethanol, then rinsed with PBS followed by an injection of 1% BSA. The 1% BSA was allowed to adsorb in the device for 30 min before rinsing with PBS. | Cell in PBS (5 × 105 cells mL−1)/PBS note: clogging issues for higher cell concentration (1 × 106 cells mL−1)/ | 200 μL min−1 sample, 500 μL min−1 buffers | None | ~90% recovery rate of H1975 epithelial cells and 97% separation efficiency of recovered cells = 87.3% of total cells separated |
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Fig. 7 Trajectory of platelets, red blood cells and white blood cells through two stages of a whole blood separation DLD device with heparin used as an anti-coagulant. Sample and buffer flow rates were 0.1 μL min−1 and 1 μL min−1 respectively. (A) Separation of platelets from red blood cells and white blood cells through stage one of the device. (B) Separation of platelets, red blood cells and white blood cells in stage two of the device. Reproduced with permission from ref. 42 © 2007 IEEE. |
Term | Meaning | Unit/value |
---|---|---|
ρ | Density | kg m−3 |
ν | Velocity | m s−1 |
p | Pressure | Pa |
η | Viscosity | Pa s |
D H | Hydraulic diameter | — |
w | Width | m |
h | Height | m |
l | Length | m |
Note: in Fig. 8l refers to post centre–centre distance to satisfy notation of ref. 17 | ||
D | Diffusion coefficient | cm2 s−1 |
k | Boltzmann constant | 1.38 × 10−23 |
T | Absolute temperature | K |
α | Hydrodynamic radius | m |
Q | Flow rate | m3 s−1 |
R | Fluidic resistance | N s m−5 |
Δp | Pressure difference | Pa |
D c | Critical diameter | m |
D c min | Minimum critical diameter | m |
β | Width of first streamline | m |
θ | Displacement angle | Degrees |
Note: in Fig. 8θ represents driving angle | ||
n | Periodicity of array | — |
n | Row number | — |
S n | Streamline number | — |
G | Gap size | m |
D p | Post diameter | m |
D y | Distance between posts in one Row and those in another | m |
λ | Centre-to-centre post spacing | m |
Δλ | Distance that each successive row is shifted laterally | m |
ε | Row shift fraction | — |
ε′ | Row shift fraction in devices with Dy < G | — |
G L | Gap between left sidewall and posts | m |
G R | Gap between right sidewall and posts | m |
f | Degree of fluid shear thinning | — |
F Drag | Drag force | N |
F DEP | Dielectrophoretic force | — |
b c | Critical angle | Degrees |
Re | Reynolds number | — |
Pe | Peclet number | — |
psi | Pounds per square inch | lbf in−1 |
DLD | Deterministic lateral displacement | — |
IMS | Immuno-magnetic separation | — |
FACS | Fluorescence-activated cell sorting | — |
PFF | Pinched flow fractionation | — |
HDF | Hydrodynamic filtration | — |
WBC | White blood cell | — |
RBC | Red blood cell | — |
CTC | Circulating tumour cell | — |
PDMS | Polydimethylsiloxane | — |
DI | Deionized | — |
PEG | Polyethylene glycol | — |
SDS | Sodium dodecyl sulfate | — |
BSA | Bovine serum albumin | — |
PBS | Phosphate buffered saline | — |
AC | Alternating current | — |
IDT | Interdigital transducer | — |
DRIE | Deep reactive ion etching | — |
DEP | Dielectrophoresis | — |
TBE | Tris/borate/EDTA buffer | — |
PVP | Polyvinylpyrrolidone | — |
KOH | Potassium hydroxide | — |
Demi | Demineralized | — |
PEEK | Polyether ether ketone | — |
PMMA | Polymethyl methacrylate | — |
EDTA | Ethylenediaminetetraacetic acid | — |
YPD | Yeast extract peptone dextrose | — |
Beech et al. used pillars manufactured of an insulator material placed between electrodes to modulate an electric field throughout the whole constriction.4 By tuning the applied, low frequency AC electric field which ran perpendicular to the fluid flow direction, it was possible to continuously deflect polystyrene beads smaller than Dc into displacement mode when experiencing negative dielectrophoresis (Fig. 1D). In this instance beads are effectively repelled from the pillars, causing their displacement out of the existing streamline.
Chang and Cho developed a device with electrode pillar arrays to create a tuneable, negative dielectrophoretic effect within a DLD device, where a voltage was applied to the electrode pillars via an electrode backbone.9 Tuning of the voltage enabled the separation of 6, 8, 10 and 12 μm particles, with the larger particles being forced into displacement mode and smaller particles flowing through the device in zigzag mode. Furthermore, the 99% separation of WBCs from RBCs was exhibited using this device.
Devendra and Drazer used gravity to induce particle movement through a DLD constriction by simply tilting the microfluidic device at a set force angle for size-fractionation of mixed particle populations.10 Smaller particles have a smaller critical angle (bc) than large particles, and therefore migration can be controlled by controlling the offset angle as is outlined in Fig. 8. Particles move with an average migration angle of α = 0° with bc = l sin(θc), where l is the post centre-to-centre distance and θc the transition angle. When bc < l sin(θc), particles no longer migrate with α = 0°, thus facilitating size-based separation. The device is tuneable in that changing the offset angle renders particles within a different range susceptible to the separation but the highest resolution of ~1.35 was given at a driving angle of 14°.
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Fig. 8 Schematic trajectories of 4.32 μm (green, left) and 15 μm (red, right) particles colliding with two consecutive cylindrical posts (black) of 20 μm, separated centre-to-centre by distance l. The driving angles are θ = 5° (a), θ = 10° (b) and θ = 20° (c). The dotted circles show the trajectories of the particles in the absence of obstacles. The two particles have two different values of the impact parameter, bc. Initially, both particles move with α = 0°. Each particle then transitions out of this locking direction when bc < l sin(θc). Transitions occur at different θ. The middle cartoon is representative of a separative case. Adapted with permission of Devendra and Drazer.10 Copyright 2012 American Chemical Society. |
Collins et al. used a virtual DLD system with interdigital transducers (IDT's), which produce surface acoustic waves at an angle to flow direction, rather than pillars to enable continuous size-based separation of particles in the micrometer and sub-micrometer range.8 Principally, this system works by trapping particles larger than Dc in the force field produced by the IDT's, which is at 45° to the flow direction. Smaller particles are not sufficiently affected by the force field and consequently separation ensues. The device is tuneable in that the applied voltage can be selectively controlled – the >97% separation of 5 μm from 6.6 μm particles and then ~87% separation of 500 nm from 300 nm particles in the same device demonstrates this.
To date, this technique has been used for the separation of a wide range of particles, from white blood cells to droplets, and from nanometre-sized to millimetre-sized particles. By relying only on hydrodynamics, flow rates as high as 10 mL min−1 have been reported in the literature for the separation of cancer cells from blood corresponding to one of the highest flow rates reported for this purpose using microfluidics. However, fluid volumes processed by DLD's are typically very small (0–1 μL min−1), therefore we expect that in the future work detailing the stacking or running of devices in parallel will be published, in order to increase the capability of this separation technique and its suitability to biomedical applications, for example. On the subject of suitability of devices to specific applications, researchers should detail the recovery rates and purity of target particles from the tested devices, as this information is missing from most publications.
Specific care is required when dealing with DLD since clogging by means of particle–particle or particle–surface interactions can occur but also high resistances can limit its practical implementation. Some of the above limitations can be overcome by adding external forces to the process such as dielectrophoresis or acoustic forces for creating a virtual DLD and avoiding the presence of physical posts in the device.
Clearly though, design considerations are thus crucial for this technique and the most significant devices designed were presented in this review. However, and without compromising its interesting potential for particle separation, there is not yet a “one fits all” solution and one should refer to the most related literature to adapt DLD to the targeted application. By gathering studies related to DLD in a single review, this process will hopefully be simplified, potentially enhancing new applications since there is still much to explore. Additionally, in this paper, a toolbox was proposed to summarize the main design parameters requiring of consideration and to serve as a design aid to those unfamiliar with the technique.
Strong efforts have been reported during the last decade to adapt this technique to the separation of non-spherical biological matters resulting in the consideration of new posts shapes or new designs for the channel, depending on the particles to be separated. In terms of future work, it is expected that work will commence to further characterise device performance where inertial or non-Newtonian effects are present and where target particles are irregularly-shaped and/or deformable as this will enable more appropriate design of a wider range of applications.
The large majority of publications to date refer to the use of DLD alone on chip; however it is conceivable that more devices will be designed and integrated with upstream and/or downstream processes. For example, Liu et al.41 demonstrate particle separation using DLD, before target cells are captured downstream. Perhaps the next stage for developers of DLD is to show that this technology is truly capable of integrated lab-on-chip applications.
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