Microfluidic device delivers molecules for nano-patterning

In recent years, dip-pen nanolithography (DPN) has become a powerful tool for making surface nanostructures.¹ Based on scanning-probe or atomic-force microscopes (AFMs), DPN uses a coated scanning probe tip to deposit molecules onto a substrate in much the same way a quill pen transfers ink to paper. The small size of the tip helps create micro- and nano-patterns with high resolution and multiplexed registration.

One application that can be enhanced by DPN technology is high-throughput deposition of biomolecular arrays. For instance, the industrial biotechnology standards for genomic and proteomic assays require parallel delivery and testing of 48, 96, 384, or an even larger number of unique chemical species. Recently, our group optimized the DPN process by the developing a commercial microfluidic device called Inkwells™.^2,3 The device delivered a maximum of 10 unique chemical species for DPN to individual pens in an array for biotechnology applications in genomics and proteomics.

To maximize the number of chemical species that can be patterned simultaneously by DPN, we developed a microfluidic ink delivery device called Centiwell.⁴ The device conforms to the industrial standard for DNA and RNA micro-arrays and can simultaneously nano-pattern 96 (or more) chemical species. The Centiwell device consists of an array of 96 microwells fabricated by photolithography and wet-etching processes on a silicon substrate. Because each microwell has a volume of only about 1pl, this design enables significant cost savings for the nanolithography of expensive biological samples and reagents. A thermoelectric Peltier cooler is attached to the back side of the silicon substrate for thermal control. Microbeads of polyethylene glycol (PEG) are dispensed into the microwells, forming an array (see Figure 1). The beads, which will become the ‘ink’ of the DPN process, can be placed only in the wells, thus maintaining the cleanliness of the rest of the substrate. This is especially important for avoiding cross-contamination between the different chemical species.

In a typical experiment, the device is placed on the AFM stage. When power is connected to the Peltier cooler, the temperature on the microwell array decreases to below the dew point, permitting drops of water to condense onto the substrate. The water dissolves the PEG microbeads, forming solutions that can be used for writing. When the power to the Peltier cooler is cut, the condensed water outside the microwell array evaporates immediately. The hygroscopic property of PEG, however, captures the water inside the microwells and prevents its sudden evaporation. This allows sufficient time to coat the scanning probe for subsequent DPN patterning.

As a proof of concept, we demonstrated the delivery of a single ink (PEG) to an AFM tip using the experimental apparatus shown in Figure 2. After coating the AFM tip with PEG solutions, the Centiwell was replaced with a mica substrate on the AFM stage. As the probe moved, the PEG on the probe was transferred to the substrate. Figure 3 shows a lateral force image of three lines of PEG patterned by the DPN process.

So far, we have demonstrated that the microfluidic device can deliver an ink to a scanning probe successfully. However, the Centiwell device was been designed to optimize the DPN process by delivering multiple species (up to 96) to an array of passive multiple pens for parallel-write applications. Such performance would meet the current industrial standards for fluid handling in the biotechnology applications of DPN in genomics and proteomics assays. We could also expand the Centiwell device to enable simultaneous nano-patterning of hundreds or even thousands of different inks. Thus, this platform provides the capability for exponential growth in nanofabrication of bio-molecular species.