Electric Field Directed Assembly of High-Density Microbead Arrays by cqe15118


									                            Electric Field Directed Assembly of High-Density Microbead Arrays
                                                      Kristopher D. Barbee, Alexander P. Hsiao, Michael J. Heller & Xiaohua Huang
                                              Department of Bioengineering, Jacobs School of Engineering, University of California, San Diego

                                                     Overview                                                                                                                       Results
     Microbead-based platforms have become a popular technology for many high-throughput biological assays such as           A                                         B                                         C
genotyping, DNA sequencing, and protein detection due to the ease in which they enable multiplexing and
miniaturization. Many of these assays involve the immobilization of microbeads onto solid supports by capturing or
assembling them using evaporation, gravity, centrifugation, and magnetic and electric fields. While all of these methods
have been been successfully utilized, they typically suffer from one or more limitations pertaining to assembly speed,
filling efficiency, scalability and control over microbead number, position and order.
     We present a method for rapid assembly of high-density microbead arrays using an electric field. Our approach
offers many advantages over other microbead assembly strategies in that it is fast, efficient, scalable, automatable,
capable of producing arrays of single microbeads with near perfect order and is compatible with microfluidics, biological
assays and real-time epifluorescence microscopy.
     Photolithography is used to create arrays of wells in an epoxy-based photoresist on gold-coated wafers. Thin gaskets
are used to form microfluidic chambers between the wafers and glass coverslips with thin metal lines or an ITO coating.      SEM images of arrays of streptavidin-conjugated polystyrene microbeads assembled within
Wafer-scale arrays of streptavidin-coated microbeads are assembled and captured within the wells by applying low             photolithographically defined wells via an electric field. A. An array of 1 µm beads at a 2.4 µm pitch. B. An array of
voltage, 1 Hz electrical pulses. Microbead binding is robust and occurs through gold-protein interactions, allowing          1 µm beads at a 1.2 µm pitch. C. An array of 0.5 µm beads at a 1.2 µm pitch.
excess beads to be washed away or recycled. The well and bead sizes are chosen such that each well can capture just
one bead. We have demonstrated that: 1) The assembly process can be completed in as little as 30 seconds; 2) Filling         D                                         E                                         F
efficiencies as high as 99.9% can be achieved; 3) Arrays with densities as high as 69 million beads/cm2 can be
assembled using 0.5 and 1.0 µm beads.
     Potential applications for this technology include the assembly of DNA arrays for genome sequencing and antibody
arrays for proteomic studies. This device may also be used to enhance the concentration-dependent processes of
various assays through the accelerated transport of molecules using electric fields.

                             Device Fabrication and Assembly                                                                                                  24 µm                                   24 µm                                      32 µm

A                                        B                                            C                                      Fluorescent micrographs of arrays of streptavidin- and Neutravidin-conjugated polystyrene microbeads
                                                                                                                             assembled using an electric field. D. An array of 1 µm beads at a 2.4 µm pitch. E. An array of 1 µm beads at a 2.4
                                                                                                                             µm pitch. F. An array of 0.5 µm beads at a 1.6 µm pitch.
      Deposit SiO2 via PECVD                                                                                                 G                                         H                                         I
      on a silicon wafer
                                                                                              Apply low-voltage DC pulses

      Deposit Ti and Au films via                                                                                      V

      sputtering or evaporation                                              d


                                                                                              Remove excess microbeads       SEM images of microbead doublets, size variation and alignment. G. An array with a doublet in one well. H. An
                                                                                                                             array showing the size variation among the 1 µm beads. I. An array of 0.5 µm beads assembled into over-sized wells.
      Spin-coat photoresist and                                                                                              Microbead positioning and filling efficiency may be compromised if the wells are significantly larger than the microbeads.
      pattern array of wells
                                                                                                                             We have demonstrated the ability to use electric fields to direct the rapid assembly of arrays of 0.5 and 1 µm protein-
                                                                                                                             conjugated microbeads on photolithographically defined templates. Standard microfabrication procedures are used to
                                                                                                                             generate wafer-scale arrays of wells on gold in a robust, epoxy-based photoresist. Hundreds of millions of microbeads
Device fabrication and assembly. A. Fabrication of an array of microwells on a silicon wafer. The photolithography is        can be assembled within these wells in 30-45 seconds by applying low-voltage, low-frequency DC electrical pulses.
performed on an i-line stepper system, which allows wafer scale arrays of high-resolution wells to be printed in minutes.    Each well contains only one microbead and filling rates as high as 99.9% are easily achieved with minimal defects.
B. Exploded view of the device. a: ITO-coated glass coverslip; b: silicone gaskets with flow channels; c: silicon wafer      Array assembly takes place within a microfluidic device that is compatible with real-time biright-field and epifluorescence
with a high-density array of wells in photoresist on a thin layer of gold; d: double-coated adhesive gasket; e: microscope   imaging. The methods presented here may be applied to the assembly of arrays of microbeads conjugated to
stage insert with fluidic ports. C. Electric field directed assembly of arrays. The protein-conjugated microbeads are        antibodies and DNA for use in high-throughput assays. In addition, the use of such a platform may provide a means of
directed into the wells and captured onto the gold surface by applying a series of electrical pulses across the chamber.     accelerating diffusion-limited assays by actively concentrating molecules of interest via an electric field.

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