Micromanufacturing Technologies in Dielectrophoresis Applications

Rodrigo Martinez-Duarte

Department of Mechanical Engineering, Clemson University, Clemson, SC
Email: rodrigm@clemson.edu

Dielectrophoresis (DEP) is a powerful technique for particle manipulation that was first described by Pohl in the 1950s [1]. Even though the great potential of this technique to selectively manipulate targeted particles was well realized then, it was not until the establishment of miniaturization techniques in the 1990s that DEP really became prominent as a research field. From other excellent articles included in the AES Dielectrophoresis resource page, it is noted that Dielectrophoresis requires a non-uniform electric field; and that the magnitude of the electric field E is given by the ratio between the applied voltage V and distance between electrodes d (E=V/d). The use of microfabrication techniques enabled the positioning of electrodes very close to each other, by micrometers, and therefore the use of practical voltages, tens of volts, instead of thousands of volts required in those initial experiments by Pohl where electrodes were separated by centimeters.

The 1990s saw an explosion of DEP publications, mainly from the groups of Pethig, Gascoyne, Fuhr and Morgan and Green who used planar metal microelectrodes to sort a wide variety of cells as reviewed a number of times before [2, 3]. The concurrent development of microfluidics also allowed for the creation of better devices for flow management and better understanding of the interaction between hydrodynamic and electrokinetic forces. Starting in the 2000s alternative techniques started to arise to overcome common problems in metal-electrode DEP, such as electrode fouling, and/or to increase the throughput of the system. Insulator-based DEP (4, 5), Glass-beads DEP (6) and light-induced DEP (7,8) are the most significant examples together with Doped Silicon DEP (9). Liquid-electrode DEP (10), contactless DEP (11) and carbon-electrode DEP (12) became active in the later 2000s. Most recently, the use of doped PDMS (polydimethylsiloxane) (13-15) and Polypyrrole (16) as electrode materials was demonstrated. The development of these new techniques and the constant desire of the community to develop practical solutions have led to devices which are more user friendly, less expensive and are capable of high throughput. Below you will find a rather short overview of the different micro manufacturing processes so far developed to fabricate experimental DEP devices. The field is wide and the reader is encouraged to consult a more comprehensive review recently published by this same author: “Microfabrication technologies in Dielectrophoresis applications – a review” published in Electrophoresis, volume 33, pages 3110–3132. doi: 10.1002/elps.201200242.

Important factors to consider when choosing a fabrication technique include 1) cost of the required infrastructure for fabrication, 2) fabrication time and complexity, 3) material cost, 4) capability to resolve small gaps between electrodes and 5) potential to fabricate 3D structures. The sum of the first three parameters will give a total cost per device. Such cost must be low to allow DEP to impact the highest number of application. Besides low cost, the devices must be robust, user-friendly and achieve highly reproducible results. The capability to resolve small gaps between electrodes is important since, as stated above, the electric field gradient is inversely proportional to the distance between electrodes. However, the gap must also be optimized to the size of particles intended to be manipulated. In the case of biomolecules, such as DNA and proteins, very small gaps, in the order of few micrometers or even tens of nanometers, are desired (17, 18). If one intends to manipulate cells, the gap between electrodes must be increased to tens of micrometers (19). The use of 3D electrodes may not be needed in all applications but their use significantly impacts the performance of devices in high throughput applications (12).

A summary of the so far published processes and materials used to fabricate DEP devices is presented in the table below. The cost column is a comparison between the different techniques ($ = less expensive, $$$$= most expensive). The complexity, cost and number of fabrication processes used in DEP techniques such as light-induced, glass-beads, carbon-electrode and contactless; is less than those in conventional techniques such as metal-electrode and traditional insulator-based DEP (iDEP) devices based on glass micro-structures. A basic light-induced DEP device features the simplest fabrication. Fabrication of iDEP devices based on the use of glass beads seems to be the least expensive. Remarks about the most common ways to incorporate a microfluidics network to a DEP-active element are also included. This is to provide the reader with an overview of the cost and complexity of all the processes needed to obtain an experimental device. Please note that the fabrication methods for microfluidics elements are interchangeable and are not exclusive to a given DEP technique. For a detailed explanation of each of these processes the reader is highly encouraged to read “Microfabrication technologies in Dielectrophoresis applications – a review” published in Electrophoresis, volume 33, pages 3110–3132. doi: 10.1002/elps.201200242.

The main target for the coming years is to establish fabrication processes that allow for inexpensive DEP devices that are user-friendly and feature high throughput and robustness. These considerations are extremely important since often DEP is regarded as a complex, fragile and unreliable technique. Simple and robust plug and play devices are desired to enable their daily use by non-technical staff. The goal, from this author’s point of view, is to accelerate the validation and daily use of DEP in practical settings. Given that the throughput of DEP devices has been steadily increasing in the last 20 years and the cost and complexity of the fabrication techniques are constantly decreasing, this author is confident DEP will become an important technique in settings other than research in the coming years.

Name of DEP Technique Micro manufacturing Processes Employed $ Materials required 3D capability? Cost Remarks on representative microfluidics
Metal-electrode Metal deposition
Polymer mask#
Metal etching&
+
Electroplating, if making 3D structures
Photoresists, glass or plastic substrate, thin film Au, Pt, Al or other metals as electrodes;
Indium Tin Oxide (ITO) commonly used for planar transparent electrodes
No, if using planar electrodes $$$ - Casting of a PDMS part followed by plasma activation and later positioning on top of the electrodes.
- Photolithography of a polymer layer on top of the electrodes. Photoresist deposited by spin coating or dry film lamination. Device closed using glass or polymer substrate and adhesive or glue (i.e. UV, epoxy-based)
Yes, when using electroplating$$$$
Doped Silicon Metal deposition
Polymer mask
Metal etching
Silicon etching (DRIE)
Photoresists, thin film metal, doped silicon, glass Yes $$$$ µm-scale fluidics made together with the silicon electrodes. Network is closed using a glass lid and anodic bonding.
I
N
S
U
L
A
T
O
R
-
B
A
S
E
Insulator matrices Dispensing of dielectric beads
Machining, cutting to pattern macro electrodes
Plastics, dielectric beads (most commonly glass), stainless steel or other metal pieces/foils Yes $ mm-scale fluidics done by machining, molding, etc. Device closed using glass or plastic substrate and adhesive, glue or solvent-assisted
Insulator structures – metal macro electrodes Glass-based devices:
Metal deposition
Polymer mask
Metal etching
Glass wet etching
Photoresists, thin film metal, float glass (quartz has also been used), metal wire Yes $$$$ µm-scale fluidics done together with the glass micro-structures. Network is closed using a glass lid and anodic bonding.
Polymer-based devices: Molding or
Embossing and/or
Machining
Polymer, metal wire, pieces or foil Yes $$ Concurrent fabrication with the insulator structures. Closed by a polymer lid and adhesives, glue or solvent-assisted
Contactless (co-fabrication) Casting
+
Mold fabrication: polymer mask, silicon etching (DRIE)
PDMS, metal wire, glass substrate
+
photoresists, and silicon
Yes $$ µm-scale fluidics to hold sample and conductive liquids fabricated together. PDMS part sealed against a glass or plastic substrate after plasmaactivation
Insulator-structures – metal micro electrodes Metal deposition
Polymer mask
Metal etching
Thick-film photolithography#
Photoresists, Au, ITO, glass or plastic substrate Yes $$$ Photolithography of a spin-coated polymer layer. Device closed using glass or polymer substrate and adhesive, glue or solvent-assisted
Liquid-electrode Metal deposition
Polymer mask
Metal etching
Photolithography
No Network done together with the SU-8 layer enabling the “liquid-electrode” concept. Device closed before each experiment using a PDMS part and mechanical pressure
Light-induced Deposition ITO, amorphous silicon, glass or plastic substrate No $$ Basic fluidic chambers using adhesive or polymer spacers
Carbon-electrode Thick-film photolithography
Pyrolysis
Photoresists, fused silica or silicon substrate Yes $$ Sub mm-scale network cut in double-sided adhesive. Stack of adhesive and drilled plastic sealed around electrode array using rolling press
Liquid metal
(co-fabrication)
Casting
Metal injection
+
Mold fabrication: photolithography
Glass substrate, PDMS, liquid-phase or low melting point metals
+
photoresists
Yes $$ µm-scale fluidics to hold sample and cavities for metal injection fabricated together. PDMS part sealed against a glass or plastic substrate after plasmaactivation
Doped PDMS Casting
Mixing
+
Mold fabrication: photolithography
Conductive particles, PDMS, glass substrate
+
photoresists
Yes $$ µm-scale fluidics fabricated in the pure PDMS part. The PDMS-Doped PDMS part is then sealed against a glass or PDMS substrate after plasmaactivation
Casting
+
Mold fabrication: photolithography
+
Metal ion implantation, metal and ITO patterning,
PDMS, Au, ITO, glass substrate $$$ µm-scale fluidics fabricated in the PDMS part that is later implanted with metal ions. Metal-implanted PDMS part is sealed against the ITO-coated glass substrate after plasmaactivation
Conductive Polymers Metal deposition
Polymer mask#
Metal etching&
Polypyrrole growth
Thin film Au, Cr, Photoresists
Polypyrrole (PPy)
Yes $$$ Sub mm-scale network cut in double-sided adhesive. Stack of adhesive and drilled plastic sealed around electrode array using rolling press

Notes:
# Polymer mask refer to the patterning of a polymer layer to be used as mask for metal etching or lift-off, or silicon etching; the process includes photolithography of a photoresist, usually a positive-tone one, and polymer stripping. In this table, polymer mask is different from photolithography in that the latter is usually done to fabricate structures, most commonly in negative photoresists, that will remain on the experimental device.
& Metal etching is not necessary if lift-off techniques are used. Alternatively, metal patterning can be done directly by using laser ablation or ion-milling instead of the combination of a polymer mask and wet or dry metal etching.


References
  1. [1] Pohl, H. A., J Appl Phys 1951, 22, 869.
  2. [2] Pethig, R., Biomicrofluidics 2010, 4, 022811.
  3. [3] Gascoyne, P. R. C., Vykoukal, J. V., P IEEE 2004, 92, 22-42.
  4. [4] Cummings, E. B., Singh, A. K., Anal Chem 2003, 75, 4724-4731.
  5. [5] Srivastava, S. K., Gencoglu, A., Minerick, A. R., Anal Bioanal Chem 2010, 399, 301-321.
  6. [6] Hayward, R. C., Saville, D. A., Aksay, I. A., Nature 2000, 404, 56-59.
  7. [7] Chiou, P. Y., Ohta, A. T., Wu, M. C., Nature 2005, 436, 370-372.
  8. [8] Suehiro, J., Zhou, G., Imamura, M., Hara, M., IEEE T Ind Appl 2003, 39, 1514-1521.
  9. [9] Iliescu, C., Xu, G. L., Samper, V., Tay, F. E. H., J Micromech Microeng 2005, 15, 494-500.
  10. [10] Demierre, N., Braschler, T., Linderholm, P., Seger, U., van Lintel, H., Renaud, P., Lab Chip 2007, 7, 355.
  11. [11] Shafiee, H., Caldwell, J. L., Sano, M. B., Davalos, R. V., Biomed Microdevices 2009, 11, 997-1006.
  12. [12] Martinez-Duarte, R., Renaud, P., Madou, M. J., Electrophoresis 2011, 32, 2385-2392.
  13. [13] Niu, X. Z., Peng, S. L., Liu, L. Y., Wen, W. J., Sheng, P., Adv Mater 2007, 19, 2682-2686.
  14. [14] Lewpiriyawong, N., Yang, C., Lam, Y. C., Electrophoresis 2010, 31, 2622-2631.
  15. [15] Choi, J.-W., Rosset, S., Niklaus, M., Adleman, J. R., Shea, H., Psaltis, D., Lab Chip 2010, 10, 783.
  16. [16] Perez-Gonzalez, V. H., Ho, V., Kulinsky, L., Madou, M. J. and Martinez, S. O., Lab Chip 2013, DOI: 10.1039/C3LC50893E.
  17. [17] Holzel, R., IET Bionanotechnol 2009, 3, 28-45.
  18. [18] Nakano, A. and Ros, A, Electrophoresis 2013, 34, 1085-1096.
  19. [19] Gagnon, Z. R., Electrophoresis 2011, 32, 2466-2487.