Microfluidic chips for point-of-care immunodiagnostics

We might be at the turning point where research in microfluidics undertaken in academia and industrial research laboratories, and substantially sponsored by public grants, may provide a range of portable and networked diagnostic devices. In this Progress Report, an overview on microfluidic devices that may become the next generation of point-of-care (POC) diagnostics is provided. First, we describe gaps and opportunities in medical diagnostics and how microfluidics can address these gaps using the example of immunodiagnostics. Next, we conceptualize how different technologies are converging into working microfluidic POC diagnostics devices. Technologies are explained from the perspective of sample interaction with components of a device. Specifically, we detail materials, surface treatment, sample processing, microfluidic elements (such as valves, pumps, and mixers), receptors, and analytes in the light of various biosensing concepts. Finally, we discuss the integration of components into accurate and reliable devices.     

Addressable Microfluidic Polymer Chip for DNA‐Directed Immobilization of Oligonucleotide‐Tagged Compounds

A microfluidic polymer chip for the self‐assembly of DNA conjugates through DNA‐directed immobilization is developed. The chip is fabricated from two parts, one of which contains a microfluidic channel produced from poly(dimethylsiloxane) (PDMS) by replica‐casting technique using a mold prepared by photolithographic techniques. The microfluidic part is sealed by covalent bonding with a chemically activated glass slide containing a DNA oligonucleotide microarray. The dimension of the PDMS–glass microfluidic chip is equivalent to standard microscope slides (76 × 26 mm²). The DNA microarray surface inside the microfluidic channels is configured through conventional spotting, and the resulting DNA patches can be conveniently addressed with compounds containing complementary DNA tags. To demonstrate the utility of the addressable surface within the microfluidic channel, DNA‐directed immobilization (DDI) of DNA‐modified gold nanoparticles (AuNPs) and DNA‐conjugates of the enzymes glucose oxidase (GOx) and horseradish peroxidase (HRP) are carried out. DDI of AuNPs is used to demonstrate site selectivity and reversibility of the surface‐modification process. In the case of the DNA–enzyme conjugates, the patterned assembly of the two enzymes allows the establishment and investigation of the coupled reaction of GOx and HRP, with particular emphasis on surface coverage and lateral flow rates. The results demonstrate that this addressable chip is well suited for the generation of fluidically coupled multi‐enzyme microreactors.     

Microfluidic Blood Cell Sorting: Now and Beyond

Blood plays an important role in homeostatic regulation with each of its cellular components having important therapeutic and diagnostic uses. Therefore, separation and sorting of blood cells hasa been of a great interest to clinicians and researchers. However, while conventional methods of processing blood have been successful in generating relatively pure fractions, they are time consuming, labor intensive, and are not optimal for processing small volume blood samples. In recent years, microfluidics has garnered great interest from clinicians and researchers as a powerful technology for separating blood into different cell fractions. As microfluidics involves fluid manipulation at the microscale level, it has the potential for achieving high‐resolution separation and sorting of blood cells down to a single‐cell level, with an added benefit of integrating physical and biological methods for blood cell separation and analysis on the same single chip platform. This paper will first review the conventional methods of processing and sorting blood cells, followed by a discussion on how microfluidics is emerging as an efficient tool to rapidly change the field of blood cell sorting for blood‐based therapeutic and diagnostic applications.     

Stochastic Particle Barcoding for Single‐Cell Tracking and Multiparametric Analysis

This study presents stochastic particle barcoding (SPB), a method for tracking cell identity across bioanalytical platforms. In this approach, single cells or small collections of cells are co‐encapsulated within an enzymatically‐degradable hydrogel block along with a random collection of fluorescent beads, whose number, color, and position encode the identity of the cell, enabling samples to be transferred in bulk between single‐cell assay platforms without losing the identity of individual cells. The application of SPB is demonstrated for transferring cells from a subnanoliter protein secretion/phenotyping array platform into a microtiter plate, with re‐identification accuracies in the plate assay of 96±2%. Encapsulated cells are recovered by digesting the hydrogel, allowing subsequent genotyping and phenotyping of cell lysates. Finally, a model scaling is developed to illustrate how different parameters affect the accuracy of SPB and to motivate scaling of the method to thousands of unique blocks.     

Nanoporous elements in microfluidics for multiscale manipulation of bioparticles

Solid materials, such as silicon, glass, and polymers, dominate as structural elements in microsystems including microfluidics. Porous elements have been limited to membranes sandwiched between microchannel layers or polymer monoliths. This paper reports the use of micropatterned carbon-nanotube forests confined inside microfluidic channels for mechanically and/or chemically capturing particles ranging over three orders of magnitude in size. Nanoparticles below the internanotube spacing (80 nm) of the forest can penetrate inside the forest and interact with the large surface area created by individual nanotubes. For larger particles (>80 nm), the ultrahigh porosity of the nanotube elements reduces the fluid boundary layer and enhances particle-structure interactions on the outer surface of the patterned nanoporous elements. Specific biomolecular recognition is demonstrated using cells (≈10 μm), bacteria (≈1 μm), and viral-sized particles (≈40 nm) using both effects. This technology can provide unprecedented control of bioseparation processes to access bioparticles of interest, opening new pathways for both research and point-of-care diagnostics.     

Capillarity Guided Patterning of Microliquids

Soft lithography and other techniques have been developed to investigate biological and chemical phenomena as an alternative to photolithography‐based patterning methods that have compatibility problems. Here, a simple approach for nonlithographic patterning of liquids and gels inside microchannels is described. Using a design that incorporates strategically placed microstructures inside the channel, microliquids or gels can be spontaneously trapped and patterned when the channel is drained. The ability to form microscale patterns inside microfluidic channels using simple fluid drain motion offers many advantages. This method is geometrically analyzed based on hydrodynamics and verified with simulation and experiments. Various materials (i.e., water, hydrogels, and other liquids) are successfully patterned with complex shapes that are isolated from each other. Multiple cell types are patterned within the gels. Capillarity guided patterning (CGP) is fast, simple, and robust. It is not limited by pattern shape, size, cell type, and material. In a simple three‐step process, a 3D cancer model that mimics cell–cell and cell–extracellular matrix interactions is engineered. The simplicity and robustness of the CGP will be attractive for developing novel in vitro models of organ‐on‐a‐chip and other biological experimental platforms amenable to long‐term observation of dynamic events using advanced imaging and analytical techniques.     

Formation of bubbles and droplets in parallel, coupled flow-focusing geometries

This paper describes the mechanism of formation of bubbles of nitrogen in water containing Tween 20 as a surfactant, and of droplets of water in hexadecane containing Span 80 as a surfactant. The study of these microfluidic systems compares two or four flow-focusing generators coupled through shared inlets, supplying the continuous phase, and through a common outlet channel. The processes that form bubbles in neighboring generators interact for a wide range of flow parameters; the formation of bubbles alternates in time and space, and the bubbles assemble into complex patterns in the outlet channel. The dynamics of formation of bubbles in these systems are stable for long time (at least 10 min). For a certain range of flow parameters, the coupled flow-focusing generators exhibit two stable modes of operation for a single set of flow parameters. The dynamics of formation of droplets of water in hexadecane by the coupled flow-focusing generators are simpler--the adjacent generators produce only monodisperse droplets over the entire range of flow parameters that are explored. These observations suggest that the mechanism of interaction between coupled flow-focusing generators relies on the compressibility of the dispersed phase (e.g., the gas or liquid), and on variations in pressure at the flow-focusing orifices induced by the breakup of bubbles or droplets.     

Microfluidic Surface‐Enhanced Raman Scattering Sensors Based on Nanopillar Forests Realized by an Oxygen‐Plasma‐Stripping‐of‐Photoresist Technique

A novel surface‐enhanced Raman scattering (SERS) sensor is developed for real‐time and highly repeatable detection of trace chemical and biological indicators. The sensor consists of a polydimethylsiloxane (PDMS) microchannel cap and a nanopillar forest‐based open SERS‐active substrate. The nanopillar forests are fabricated based on a new oxygen‐plasma‐stripping‐of‐photoresist technique. The enhancement factor (EF) of the SERS‐active substrate reaches 6.06 × 10⁶, and the EF of the SERS sensor is about 4 times lower due to the influence of the PDMS cap. However, the sensor shows much higher measurement repeatability than the open substrate, and it reduces the sample preparation time from several hours to a few minutes, which makes the device more reliable and facile for trace chemical and biological analysis.     

Miniaturized Lab-on-a-Disc (miniLOAD)

A miniaturized centrifugal microfluidic platform for lab-on-a-chip applications is presented. Unlike its macroscopic Lab-on-a-CD counterpart, the miniature Lab-on-a-Disc (miniLOAD) device does not require moving parts to drive rotation of the disc, is inexpensive, disposable, and significantly smaller, comprising a 10-mm-diameter SU-8 disc fabricated through two-step photolithography. The disc is driven to rotate using surface acoustic wave irradiation incident upon a fluid coupling layer from a pair of offset, opposing single-phase unidirectional transducers patterned on a lithium niobate substrate. The irradiation causes azimuthally oriented acoustic streaming with sufficient intensity to rotate the disc at several thousand revolutions per minute. In this first proof-of-concept, the capability of the miniLOAD platform to drive capillary-based valving and mixing in microfluidic structures on a disc similar to much larger Lab-on-a-CD devices is shown. In addition, the ability to concentrate aqueous particle suspensions at radial positions in a channel in the disc dependent on the particles' size is demonstrated. To the best of our knowledge, the miniLOAD concept is the first centrifugal microfluidic platform small enough to be self-contained in a handheld device.