This paper reports the fabrication and testing of a helical cell separator that uses insulator-based dielectrophoresis as the driving force of its separation. The helical channel shape’s main advantage is its constant curvature radius which generates a constant electric field gradient. The presented separator was fabricated by extruding a sacrificial ink on rotating spindles using a computer-controlled robot. After being assembled, connected to the reservoir and encapsulated in epoxy resin, the ink was removed to create a helical microchannel. The resulting device was tested by circulating polystyrene microbeads of 4 and 10 μm diameter through its channel using a voltage of 900 VDC. The particles were separated with efficiencies of 94.0% and 92.5%, respectively. However, roughness in some parts of the channel and connections that had larger diameters compared to the channel created local electric field gradients which, doubtless, hindered separation. It is a promising device that could lead the way toward portable and affordable medical devices.
Cell separation is an essential part of the sample preparation prior to medical testing, as in the case of the concentration and separation of erythrocytes from other blood for the detection of malaria [
Dielectrophoresis (DEP) is the physical process by which forces are applied to particles polarized by an inhomogeneous electric field. DEP can efficiently be used to separate particles since the magnitude and orientation of the forces depend on different properties such as the radii of the particles, the permittivities of the particles and medium as well as the electric field gradient. As such, DEP has been used to separate carbon nanotubes by chirality [
The first type of DEP to be discovered [
In insulator-based dielectrophoresis (iDEP), the device is non-conductive and the inhomogeneity of the electric field arises from the shape of the device itself [
Different iDEP-based separator shapes have been proposed such as an insulating hurdle [
This shortcoming could be overcome by using a three-dimensional (3D) spiral equivalent: the helix. In a helical channel, the curvature radius is constant, thus resulting in constant separation forces throughout the channel regardless of its length. Despite being very powerful for the fabrication of 2D shapes, conventional microfabrication methods, such as soft lithography, are incapable of producing complex 3D shapes such as a helical microchannel.
This article reports a new 3D iDEP particle separator relying on helical microchannels fabricated by conformal spindle printing. This method consists of a computer-controlled fugitive ink deposition on rotating spindles creating helices that are assembled and connected to reservoirs before being encapsulated into an epoxy resin. The ink is removed to create the helical channels. Although 3D devices have been reported [
This paper presents the operating principle of the separator, its design, its fabrication process, the particle suspension preparation and the description of the separation experiments, respectively. The fabrication capabilities and separation results are presented next, followed by concluding remarks.
Particles suspended in a medium between two electrodes under tension are polarized by the ensuing electric field. Charges inside the particles accumulate near the particle-medium interfaces and attract charges inside the medium. Charges also accumulate at the interfaces on the medium side. These accumulations of electric charges form what are called electric double layers [
The polarizabilities of both medium and particles determine the relative quantity of charges inside and outside the particles. As negative and positive charges attract, and effectively cancel each other, electric dipoles are created. These relative differences determine the orientation of the dipoles [
A net dipole inside an electric field is subjected to forces resulting from the attraction of opposite charges and the repulsion of identical charges. In the case of a symmetrical electric field generated by electrodes of identical sizes and shapes, for example, the forces cancel each other and no resulting force occurs. If the electric field is inhomogeneous, when electrodes of different sizes are used for example, a resulting force is applied on each particle. This force can be expressed by [
where
where
Particles being acted upon by dielectrophoresis accelerate inside the medium. The acceleration is dictated by the dielectrophoretic force and by the drag. As the velocity of the particles increase, so does the drag. When those two forces become equal, the particles reach their terminal velocity. Usually, the particles reach their terminal velocities inside the medium in a very short time (in the order of 10−4 s for 10 μm particles) [
where
where
The electric field distribution for a voltage of 900 VDC is shown in
After the alignment, the particles entered the second helix which was coiled in the opposite direction and in which the outside wall became the inside wall. The electric field gradient pushed the particles with the same respective velocities toward the outside wall. The difference in velocities separated the particles until they reached
Schematics of the design and operation of the helical separator. (a) Isometric view of the separator. The inlet reservoir contains a mixed suspension which is separated into its two constituents which are present in the outlet reservoirs. (b) Illustration of the electric field intensity in the curved channel for a voltage of 900 VDC and its effect on particles. (c) Slightly inclined side view of the separator showing three stages of particle separation with a 900 VDC voltage. (d) Randomly distributed particles enter the first helix. (e) Particles are aligned after their transit through the first helix. (f) Separated particles exit the second helix and are split toward the two output reservoirs
the end of the second helix. That length was shorter than that of the first helix to insure that the particles did not reach the outside wall but were properly positioned with respect to the bifurcation shown in
The microchannels were fabricated using the direct-write fabrication method [
Fugitive ink was deposited on rotating epoxy 1.2 mm diameter spindles with the direct-write method to create the helices, as shown on
The two epoxy spindles were glued upright on a glass slide. The inlet and outlet channels were deposited and connected to the lower part of the helices as shown on
Fabrication steps of the helical separator. (a) Deposition of the fugitive ink on a rotating spindle. (b) Deposition of the reservoirs and filaments linking the upright helices to the reservoirs. (c) Encapsulation of the reservoirs and filaments in liquid epoxy resin. (d) After curing, the top part of the epoxy is cut to expose the filaments that are connected with another fugitive ink filament
The particle suspension was prepared by mixing 4 and 10 µm polystyrene microbeads (Fluka Analytical, Buchs, Switzerland) with an aqueous solution of 100 ppm of sodium chloride. The solution was also used without particles to fill the channel and the outlet reservoirs. The final suspension had a conductivity measured of 210 µS/cm and contained approximately 5 × 106 particles per mL of each type for a total of 107 particles per mL. The conductivities of the particles were 10 µS/cm and 4 µS/cm for the 4 µm and 10 µm particles, respectively (these conductivities were calculated using the surface conductance recommended by Ermolina and Morgan [
The outlet reservoirs and channel were filled with the NaCl solution using a syringe. The inlet reservoir was then filled with approximately 30 µL of particle suspension using a pipette. The gold electrodes were immersed inside the reservoirs. The voltage was supplied by a 1000 VDC electrophoresis power supply (FB1000Q, Cole- Parmer, Montréal, Canada). The inlet reservoir received the positive potential and the outlet reservoirs were grounded. The reservoirs were carefully filled to the same level to avoid the interference of a pressure induced flow.
Particle motion was captured using an Evolution VF Camera (MediaCybernetics, Rockville, MD, USA) and an SZX12 stereomicroscope (Olympus, Richmond Hill, Ontario, Canada). Close-up fluorescent optical images were taken using an Evolution VF camera and a BX61 microscope (Olympus). Pictures were treated with Image Pro 7 (MediaCybernetics). Close-up images were assembled from several images with different focus heights to extend the depth of field.
The overall dimensions and shape of the designed separator were respected. The curvature radius was relatively constant in the device and the channel dimensions had variations of less than 7%. Despite the overall smoothness, one part of the output channel had some visible roughness that induced local electric field gradients and could potentially have hindered the separation. The difference between the total length and the effective length did not contribute to the separation but reduced the available voltage in the helices. The connections between the different parts of the channel had diameters that were nearly twice that as large as the average diameter of the helices, which also created local electric field distortions. Finally, the fabrication process is complex and heavily reliant on manual operations.
(a) Optical image of the fabricated separator. (b) Fluorescent side view image of the two helices. (c) Fluorescent top view image of the bifurcation. (d) Fluorescent slightly inclined bottom view of the two helices
Separation experiments have been performed with the method described in Section 2.5. Voltages between 800 and 1000 VDC were tested. The highest separation efficiency was obtained using a voltage of 900 VDC.
Difficulties were encountered during the separation experiments. The results were difficult to reproduce because valid tests were challenging to obtain. The fine adjustment of the level of solution in each reservoir was especially difficult to make. An uneven level of solution led to pressure-induced flow which deviated the particles and prevented efficient separation. The settling of the particles during the short initial period between the introduction of the particle in the input reservoir and the beginning of the separation experiments was also a problem observed. Since the 10 µm particles settled more rapidly than the 4 µm particles, the smaller particles were more numerous altogether during the experiments even though the same number of each type of particle was introduced in the input reservoir.
Reported 2D separators have efficiencies varying between 90% and 98% [
The fabrication of a functional dual helix dielectrophoretic separator was presented in this paper. Separation
Separation results of the 3D separator using a voltage of 900 VDC. (a) Optical image of the 4 µm and 10 µm particles before separation. (b) Bifurcation of the channel during a separation at 900 VDC. (c) Particles collected from the top reservoir after separation. (d) Proportion of the particles reaching the expected reservoir (e) Particles collected from the bottom reservoir after separation
tests at 900 VDC obtained separation efficiencies of 94.0% and 92.5% for 4 µm and 10 µm particles respectively. These results are similar to those obtained with 2D DEP-based separator that were reported in the literature. In order to prove the applicability of this device to the biomedical field and to better compare it with other existing devices, further separation experiments will have to be performed using living cells. As polystyrene separation is very common in the literature as a proving ground for new separation devices, these results constitute a definite and encouraging proof of concept for the helical DEP separator because even with some geometrical flaws, the prototype was able to obtain separation efficiencies comparable to 2D devices. A spiral-shaped separator has two intertwined spirals, thus binding the length of the two spirals. Having two independent helices, the dual helix shape also lends a flexibility in the number of coils that a 2D separator does not have. This allows a precise tailoring of the number of coils for each helix to optimize the separation efficiency. The helical separator also has a smaller planar footprint than 2D separators. The efficient length of the helices is not limited because of its constant curvature radius. Helical DEP separators thus have the potential to outperform 2D separators while being more flexible and occupying less space.