We present a portable, simple-to-operate, point-of-use analyte surface localization device. We take advantage of a set of hydrodynamic design features and components that achieve passive analyte localization by means of a single vacuum input. The vacuum source can be supplied by mechanical or battery-operated vacuum sources that are portable and allow point-of-use operation in the absence of electricity. We discuss the governing hydrodynamic principle and design parameters in detail. In a case study, we demonstrate the applicability of our technology to successfully localize a solution of rhodamine on a polydimethylsiloxane (PDMS) substrate and produce sub-millimetre-sized spots via application of a mild vacuum pressure of less than 10 kPa. In addition, we demonstrate local staining of breast cancer cell blocks and on human breast cancer tissue sections.
Surface biochemistry is critical in several areas pertaining to biomedical diagnostics [1][2][3] , cell biology [4][5][6] , and drug screening [7][8][9] , for example. In conventional biochemistry laboratories, the entire biological sample is exposed to a reagent of interest to produce a certain result or to make a diagnosis (see figure 1A). In recent years, the concept of analyte localization has been introduced to perform, for example, multiplexed surface biochemistry with high spatial resolution 10 . With localization, only confined regions on the sample will be exposed to each reagent, resulting in reduced sample and reagent consumption, multiplexing potential, and enhanced mass transfer. In addition, high-resolution localization presents new possibilities such as single-cell studies (see figure 1B). Micropipettes 11,12 , nanopipettes [13][14][15] , microfluidic probes [16][17][18][19] , and scanning probe microscopy (SPM)-based instruments, such as atomic force microscope (AFM) dip-pen 2,20 , nanofountain probe 21,22 and FluidFM [23][24][25] , are all examples of technologies developed to perform localized micro-and nano-scale processing on substrates.
The localization techniques introduced to-date are technically sophisticated, and there is tremendous merit in their use within modern laboratory settings. However, they simply cannot be used as flexible, handheld, point-of-use tools. Therefore, there remains a gap for the development of a portable, simple-tooperate, point-of-use analyte-localization technology. A major roadblock to achieving such a system is the need for sophisticated bulky motorized stages and flow-control systems, such as syringe pumps and pressure controllers, and electricity. Another obstacle is the complexity of operation and the need for expert skills to calibrate and operate the technologies. Furthermore, widespread use of such localization instruments is hampered by the cost and tedious microfabrication processes involved in their development. An ideal point-of-use localization technology must overcome the above-mentioned obstacles in order to become a simple, flexible and portable tool. The schematics in figure 1D describe and envision such a system. Of course, such a system can potentially also have a reduced resolution and precision compared with full-scale laboratory tools.
We present a hand-held analyte-localization device (hALD) that requires only a single vacuum input supplied through a portable vacuum source to operate. In designing the hALD, we have taken advantage of a set of hydraulic and hydrodynamic principles and features that enable analyte localization. The particular hydrodynamic design of the hALD and its processing apex enable semi-passive localization of an analyte on various surfaces upon contact with its apex, thus allowing flexible hand-held operation of the device without a need for precision motorized stages or other types of sophisticated holders. Furthermore, our devised technology is suited for manual operation on substrates of varying topology without the need for extremely flat or smooth surfaces or extensive alignment and calibration, a capability that has not been present in the previously-invented analyte localization techniques. Vacuum required for hALD operation can be supplied using mechanical or battery-operated sources that are portable, allowing point-of-use operation for field applications with no need for compressed air or electricity.
In this paper, we first introduce the working principle and design rules for our localization device, and then present analytical and numerical models describing the consumption rate and mass transport in hALD (section 2). We perform detailed hydrodynamic analysis of hALD operation, and describe variations in analyte-consumption rate as a function of the applied vacuum, the channel network geometry and the pressure head of the reservoir during operation. Using computational models, we demonstrate effective mass transport from hALD’s apex to the substrate at all vacuum levels applied. In section 3, we validate our hALD technology experimentally: we describe the experimental procedures and showcase a representative application of hALD for surface localization and spotting. Surface spotting is done through localization of a solution containing rhodamine on a polydimethylsiloxane and latex substrates.
A rendered embodiment of our hand-held analyte-localization device is shown in figure 2A andB. As demonstrated in figure 2, the technology is self-contained with an integrated analyte reservoir and a vacuum port for supplying the negative pressure to localize the reagent at a recessed and textured processing apex (figure 2B). Within the hALD, there is a network of microchannels or capillaries (see figure 2C) that helps guide the analyte to the surface (injection line) and confine and remove the injected analyte and the immersion liquid from the surface (vacuum or aspiration line). The channel dimensions and el
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