Miniature Fluidic Devices for Rapid Biological Detection

During the last 20 years, the use of nanopore membranes to separate molecules depending on their size, charge or other characteristics, have increased in interest. These more ordered and defined nanopores have several advantages compared to traditional ultrafiltration membranes and provide possibilities to combine with, e.g., both electrical and optical sensing schemes. In this chapter, we discuss some of the more common nanopore membranes and how they can be used both for separation and sensing of analytes.

Surface plasmons (SP) are depicted in the classical picture as a fundamental electromagnetic mode of an interface between a metal (or a semi-conductor) and a dielectric medium and involving surface collective electronic oscilSurface plasmonslations Dror and William (Modern introduction to surface plasmons. Cambridge University Press, Cambridge, UK, [1]).

Among other factors, the performance of an affinity-based biosensor is dependent on the rate at which analyte is transported to, and captured by, its active sensing surface. The efficiency of analyte delivery can be increased via the use of microfluidics, albeit not without detraction, as microfluidic biosensors are often subjected to severe diffusion limitations when used for the detection of biologically relevant analytes. Such conditions lead to the formation of a boundary layer, void of analyte, which acts to resist the rate at which analyte is captured. It is often proposed to mix the fluid in the sensing chamber, where the exchange of depleted solution with fresh analyte can potentially increase sensor performance. The nature of analyte transport in a mixed channel is complex, however, and simply mixing the contents of a microchannel does not guarantee success. In this chapter, we review developments in the characterization (and prediction of) analyte transport in both mixed and unmixed channels. Our discussion focuses on the conditions under which mixing will (and will not) be beneficial and furthermore, the magnitude of performance increase that can be expected. Special attention is given to flow in the staggered herringbone mixer (SHM): a passive chaotic micromixer often used to enhance the performance of a biosensor. We review relevant experimental works on the topic and compare the results from several studies with the behavior expected from theory. Finally, we note several challenging aspects regarding the detection of circulating tumor cells which, due to their large size, are subject to additional transport mechanisms with respect to smaller analytes.

Analytical sensors using varying detection strategies have been widely and successfully employed for advances in areas such as drug discovery, disease diagnosis and study of biological systems. Many of these sensors utilize plasmonic metallic nanostructures which can concentrate electromagnetic fields in nanoscale regions leading to many fold enhancement in optical signal obtained from the molecules. They employ techniques including fluorescence, surface plasmon resonance (SPR)-based refractive index sensing, surface-enhanced Raman spectroscopy (SERS) and other forms of vibrational spectroscopy for molecular characterization. However, the performance of these devices relies on effective transport of the target molecules to these nanoscale detection sites. Guided transport is extremely important for fast detection in cases where the concentration of molecules is really low and for accurate measurements of protein–protein binding kinetics. In this chapter, we discuss nanostructured biosensing substrates which can spontaneously direct the flow of molecules in solution towards the sensing hotspots. These devices demonstrate improved detection sensitivity, while minimizing the limitations and complexity imposed upon the system. Additionally, they can trap biological particles such as organelles and liposomes on the sensor surface, facilitating on-chip analysis of single particles. This chapter discusses a few methods which have been utilized for concentration of molecules on plasmonic sensing surfaces, without the application of external power sources.

Performance of surface-based plasmonic sensors is often plagued by diffusion-limited transport, which complicates detection from low-concentration analytes. By harnessing gradient forces available from the sharp metallic edges, tips, or gaps that are often found in the plasmonic sensors, it is possible to combine a dielectrophoretic concentration approach to overcome mass transport limitations. A transparent electrode is combined with the plasmonic substrates that allow dielectrophoresis without interfering with the optical detection. Detection from pM-level protein solution is expedited by more than 1000 times as compared to the case of diffusion. Also, enhanced Raman spectroscopic detection is demonstrated using carbon nanotubes and biological particles. Finally, to improve the performance of dielectrophoresis, the gap between the electrodes is reduced to sub-10 nm and ultralow voltage trapping experiments are shown. The ultralow power electronic operation combined with plasmonic detection can enable high-density on-chip integration and portable biosensing.

Digital Holographic Microscopy (DHM) is a technique that uses optical interference patterns to record a three-dimensional optical field for imaging, sensing, and microscopy applications. “Lensless” in-line DHM is the simplest arrangement, requiring no lenses, no mirrors, and typically only a light source, sample, and a digital imager chip such as a CCD or CMOS pixel array. Despite this simplicity, lensless in-line DHM is capable of producing high-resolution images over a wide field of view and allows researchers to record the amplitude and phase of a light field, and to digitally reconstruct the shape, thickness, 3D position, velocity, refractive index, and other parameters of cells or small particles. There are therefore many potential opportunities for combining in-line DHM with microfluidics, optical flow velocimetry, low-cost imaging, point-of-care diagnostics, single cell tracking, cell cytometry, counting, sorting, and lab-on-a-chip technologies.

In recent years, the tendency to adopt digital microfluidics (DMF) for lab-on-a-chip (LOC) applications has increased extensively.

Microfluidics is becoming a technology of growing interest to build miniature culturing systems, capable of mimicking tissue functions and multi-tissue interactions in so-called “body-on-a-chip” applications while featuring integrated readout functionalities. This chapter presents a highly versatile, modular and scalable analytical platform technology, which combines microfluidic hanging-drop networks with multi-analyte biosensors for in situ monitoring of the metabolism of 3D microtissues. The microfluidic platform is based on the hanging-drop network technology, which has been designed for formation, cultivation, and analysis of fluidically interconnected organotypic spherical 3D microtissues that can be obtained from various different cell types. The sensor modules were designed as small glass plug-ins, which allow for convenient functionalization and calibration of the sensors and do not interfere with the microfluidic functions. They were placed in the ceiling substrate, from which the hanging drops that host the spheroid cultures were suspended. The detection of secreted lactate of single microtissue spheroids will be presented. Further, we will demonstrate that it is possible to monitor microtissue lactate secretion and glucose consumption in parallel.