Hot embossed polyethylene through-hole chips for bead-based microfluidic devices

Abstract

Over the past decade, there has been a growth of interest in the translation of microfluidic systems into real-world clinical practice, especially for use in point-of-care or near patient settings. While initial fabrication advances in microfluidics involved mainly the etching of silicon and glass, the economics of scaling of these materials is not amendable for point-of-care usage where single-test applications force cost considerations to be kept low and throughput high. As such, materials base more consistent with point-of-care needs is required. In this manuscript, the fabrication of a hot embossed, through-hole low-density polyethylene ensembles derived from an anisotropically etched silicon wafer is discussed. This semi-opaque polymer that can be easily sterilized and recycled provides low background noise for fluorescence measurements and yields more affordable cost than other thermoplastics commonly used for microfluidic applications such as cyclic olefin copolymer (COC). To fabrication through-hole microchips from this alternative material for microfluidics, a fabrication technique that uses a high-temperature, high-pressure resistant mold is described. This aluminum-based epoxy mold, serving as the positive master mold for embossing, is casted over etched arrays of pyramidal pits in a silicon wafer. Methods of surface treatment of the wafer prior to casting and PDMS casting of the epoxy are discussed to preserve the silicon wafer for future use. Changes in the thickness of polyethylene are observed for varying embossing temperatures. The methodology described herein can quickly fabricate 20 disposable, single use chips in less than 30 min with the ability to scale up 4 times by using multiple molds simultaneously. When coupled as a platform supporting porous bead sensors, as in the recently developed Programmable Bio-Nano-Chip, this bead chip system can achieve limits of detection, for the cardiac biomarker C-reactive protein, of 0.3 ng/mL, thereby demonstrating that the approach is compatible with high performance, real-world clinical measurements in the context of point-of-care testing.

Figure 1

A) The microfluidic card device consists of layers of laminate and plastics. B) Flow trajectory showing fluid delivery to an array of bead sensors.

Figure 2

A) A master containing multiple arrays of etched wells is produced using standard lithography and anisotropic etching of KOH to produce wells to hold agarose beads. B) An aluminum-based epoxy, poured over the silicon wafer, is cured, released, and hard baked. C) The epoxy stamp and thermoplastic is sandwiched between pairs of elastomer rubber, stainless steel sheets, and Kapton film during the embossing process.

Figure 3

A) SEM image of etched silicon microcontainer shows pyramidal square through-hole structure with a top opening of 500μmx500μm and bottom opening of 100μmx100μm used to hold a 290μm bead. B) SEM image of casted PDMS shows very consistent replication of features. C) SEM image of polyethylene, embossed at 160°C, shows good replication of pyramidal pit. D) Variation in thickness of final polyethylene-based microchip as embossing temperature changes.

Figure 4

A) Epifluorescent images showing results from 50ng/mL CRP delivery with 2 calibrator beads (top left), 2 negative control beads (top right), and 8x redundancy CRP specific bead sensors. B) The 7 point dose curve with a limit of detection of 0.3 ng/mL.

Figure 5

A) The economics of scaling up for silicon and plastics. The cost of a silicon chip levels off at ~$10 while the cost of a plastics chip can go down to pennies. B) Breakdown of costs for individual components of agarose bead-based approach for the detection of CRP.