Integrated microfabricated systems including a purification module and an on-chip nano electrospray ionization interface for biological analysis

Microfluidics in male reproduction: is ex vivo culture of primate testis tissue a future strategy for ART or toxicology research?

The significant rise in male infertility disorders over the years has led to extensive research efforts to recapitulate the process of male gametogenesis in vitro and to identify essential mechanisms involved in spermatogenesis, notably for clinical applications. A promising technology to bridge this research gap is organ-on-chip (OoC) technology, which has gradually transformed the research landscape in ART and offers new opportunities to develop advanced in vitro culture systems. With exquisite control on a cell or tissue microenvironment, customized organ-specific structures can be fabricated in in vitro OoC platforms, which can also simulate the effect of in vivo vascularization. Dynamic cultures using microfluidic devices enable us to create stimulatory effect and non-stimulatory culture conditions. Noteworthy is that recent studies demonstrated the potential of continuous perfusion in OoC systems using ex vivo mouse testis tissues. Here we review the existing literature and potential applications of such OoC systems for male reproduction in combination with novel bio-engineering and analytical tools. We first introduce OoC technology and highlight the opportunities offered in reproductive biology in general. In the subsequent section, we discuss the complex structural and functional organization of the testis and the role of the vasculature-associated testicular niche and fluid dynamics in modulating testis function. Next, we review significant technological breakthroughs in achieving in vitro spermatogenesis in various species and discuss the evidence from microfluidics-based testes culture studies in mouse. Lastly, we discuss a roadmap for the potential applications of the proposed testis-on-chip culture system in the field of primate male infertility, ART and reproductive toxicology.

Electrospray cone-jet mode for weakly viscoelastic liquids

We study theoretically the influence of viscoelasticity on the steady cone-jet mode of electrospray for small stress relaxation times. For this purpose, we numerically integrate the leaky-dielectric model together with the Oldroyd-B constitutive relationship and calculate both the base flow and linear eigenmodes characterizing its stability as a function of the governing parameters. We describe the effect of the polymeric stresses on both the cone-jet mode and the minimum flow rate stability limit. There are considerable differences between the Newtonian and viscoelastic electrospray realizations even for relatively small stress relaxation times due to the intense extensional deformation suffered by the fluid particles in the cone-jet transition region The axial polymeric stress shrinks the liquid meniscus and stabilizes it by pushing the fluid particle in the cone-to-jet transition region.

Microfluidic Chip-LC/MS-based Glycomic Analysis Revealed Distinct N-glycan Profile of Rat Serum

The rat is an important alternative for studying human pathology owing to certain similarities to humans. Glycomic studies on rat serum have revealed that variations in the N-glycans of glycoproteins correlated with disease progression, which is consistent with the findings in human serum. Therefore, we comprehensively characterized the rat serum N-glycome using microfluidic chip-LC-ESI-QTOF MS and MS/MS techniques. In total, 282 N-glycans, including isomers, were identified. This study is the first to present comprehensive profiling of N-glycans containing O-acetylated sialic acid, among which 27 N-glycans are novel. In addition, the co-existence of N-acetylneuraminic acid (NeuAc) and N-glycolylneuraminic acid (NeuGc) in a single N-glycan ('mixed' N-glycan) was detected and represents a new type of N-glycan in rat serum. The existence of O-acetylated sialic acid is the characteristic feature of rat serum that distinguishes it from mouse and human sera. Comparisons between the rat, mouse, and human serum glycomes revealed that the rat glycome is more similar to that of human sera than to that of mouse sera. Our findings highlight the similarities between the glycomic profile of rat and human sera and provided important selection criteria for choosing an appropriate animal model for pathological and pharmacological studies.

SU-8 as a material for lab-on-a-chip-based mass spectrometry

This short review focuses on the application of SU-8 for the microchip-based approach to the miniaturization of mass spectrometry. Chip-based mass spectrometry will make the technology commonplace and bring benefits such as lower costs and autonomy. The chip-based miniaturization of mass spectrometry necessitates the use of new materials which are compatible with top-down fabrication involving both planar and non-planar processes. In this context, SU-8 is a very versatile epoxy-based, negative tone resist which is sensitive to ultraviolet radiation, X-rays and electron beam exposure. It has a very wide thickness range, from nanometres to millimetres, enabling the formation of mechanically rigid, very high aspect ratio, vertical, narrow width structures required to form microfluidic slots and channels for laboratory-on-a-chip design. It is also relatively chemically resistant and biologically compatible in terms of the liquid solutions used for mass spectrometry. This review looks at the impact and potential of SU-8 on the different parts of chip-based mass spectrometry - pre-treatment, ionization processes, and ion sorting and detection.

Integrated multifunctional microfluidics for automated proteome analyses

Proteomics is a challenging field for realizing totally integrated microfluidic systems for complete proteome processing due to several considerations, including the sheer number of different protein types that exist within most proteomes, the large dynamic range associated with these various protein types, and the diverse chemical nature of the proteins comprising a typical proteome. For example, the human proteome is estimated to have >10(6) different components with a dynamic range of >10(10). The typical processing pipeline for proteomics involves the following steps: (1) selection and/or extraction of the particular proteins to be analyzed; (2) multidimensional separation; (3) proteolytic digestion of the protein sample; and (4) mass spectral identification of either intact proteins (top-down proteomics) or peptide fragments generated from proteolytic digestions (bottom-up proteomics). Although a number of intriguing microfluidic devices have been designed, fabricated and evaluated for carrying out the individual processing steps listed above, work toward building fully integrated microfluidic systems for protein analysis has yet to be realized. In this chapter, information will be provided on the nature of proteomic analysis in terms of the challenges associated with the sample type and the microfluidic devices that have been tested to carry out individual processing steps. These include devices such as those for multidimensional electrophoretic separations, solid-phase enzymatic digestions, and solid-phase extractions, all of which have used microfluidics as the functional platform for their implementation. This will be followed by an in-depth review of microfluidic systems, which are defined as units possessing two or more devices assembled into autonomous systems for proteome processing. In addition, information will be provided on the challenges involved in integrating processing steps into a functional system and the approaches adopted for device integration. In this chapter, we will focus exclusively on the front-end processing microfluidic devices and systems for proteome processing, and not on the interface technology of these platforms to mass spectrometry due to the extensive reviews that already exist on these types of interfaces.

Microchip technology in mass spectrometry

Microfabrication of analytical devices is currently of growing interest and many microfabricated instruments have also entered the field of mass spectrometry (MS). Various (atmospheric pressure) ion sources as well as mass analyzers have been developed exploiting microfabrication techniques. The most common approach thus far has been the miniaturization of the electrospray ion source and its integration with various separation and sampling units. Other ionization techniques, mainly atmospheric pressure chemical ionization and photoionization, have also been subject to miniaturization, though they have not attracted as much attention. Likewise, all common types of mass analyzers have been realized by microfabrication and, in most cases, successfully applied to MS analysis in conjunction with on-chip ionization. This review summarizes the latest achievements in the field of microfabricated ion sources and mass analyzers. Representative applications are reviewed focusing on the development of fully microfabricated systems where ion sources or analyzers are integrated with microfluidic separation devices or microfabricated pums and detectors, respectively. Also the main microfabrication methods, with their possibilities and constraints, are briefly discussed together with the most commonly used materials.

Integrated liquid chromatography-heated nebulizer microchip for mass spectrometry

A new integrated microchip for liquid chromatography-mass spectrometry (LC-MS) is presented. The chip is made from bonded silicon and glass wafers with structures for a packed LC column channel, a micropillar frit, a channel for optional optical detection, and a heated vaporizer section etched in silicon and platinum heater elements on the glass cover. LC eluent is vaporized and mixed with nebulizer gas in the vaporizer section and the vapor is sprayed out from the chip. Nonpolar and polar analytes can be efficiently ionized in the gas phase by atmospheric pressure photoionization (APPI) as demonstrated with polycyclic aromatic hydrocarbons (PAHs) and selective androgen receptor modulators (SARMs). This is not achievable with present LC-MS chips, since they are based on electrospray ionization, which is not able to ionize nonpolar compounds efficiently. The preliminary quantitative performance of the new chip was evaluated in terms of limit of detection (down to 5 ng mL(-1)), linearity (r>0.999), and repeatability of signal response (RSD=2.6-4.0%) and retention time (RSD=0.3-0.5%) using APPI for ionization and PAHs as standard compounds. Determination of fluorescent compounds is demonstrated by using laser-induced fluorescence (LIF) for detection in the optical detection channel before the vaporizer section.

Microchip electrospray performance during gradient elution with bulk conductivity changes

This work identifies dynamic changes in bulk conductivity during reversed-phase HPLC gradient elution as a major source for spray mode changes and instabilities observed in ESI-MS. A commercial microchip-HPLC/ESI-MS configuration was modified to enable electrospray diagnostics based on frequency analysis of the microchip emitter current combined with spray imaging. This approach facilitated detection of different spray modes together with their onset potentials. Water/acetonitrile mixtures containing formic acid were selected as the electrosprayed solutions to represent typical conditions in reversed-phase HPLC. Experimental data are complemented by computational fluid dynamics simulations, treating the electrosprayed solution as leaky dielectric fluid, to address the influence of bulk conductivity and applied potential difference on the developing cone-jet morphology and stability.

Collecting peptide release from the brain using porous polymer monolith-based solid phase extraction capillaries

Porous polymer monolithic (PPM) columns are employed to collect and concentrate neuronal release from invertebrate and vertebrate model systems, prior to their characterization with mass spectrometry. The monoliths are fabricated in fused-silica capillaries from lauryl methacrylate (LMA) and ethylene glycol dimethacrylate (EDMA). The binding capacities for fluorescein and for fluorescently labeled peptides are on the order of nanomoles per millimeter of length of monolith material for a capillary with an inner diameter of 200 microm. To evaluate this strategy for collecting peptides from physiological solutions, angiotensin I and insulin in artificial seawater are loaded onto, and then released from, the monoliths after a desalination rinse, resulting in femtomole limits of detection via matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. Positioned in the extracellular media near Aplysia californica bag cell neurons, upon electrical stimulation, these LMA-EDMA monoliths are also used to collect and concentrate peptide release, with egg-laying hormones and acidic peptide detected. In addition, the collection of several known peptides secreted from chemically stimulated mouse brain slices demonstrates their ability to collect releasates from a variety of neuronal tissues. When compared to collection approaches using individual beads placed on brain slices, the PPM capillaries offer greater binding capacity. Moreover, they maintain higher spatial resolution, compared to the larger-volume, solid-phase extraction collection strategies.

Structure-transport analysis for particulate packings in trapezoidal microchip separation channels

This article investigates the efficiency of particulate beds confined in quadrilateral microchannels by analyzing the three-dimensional fluid flow velocity field and accompanying hydrodynamic dispersion with quantitative numerical simulation methods. Random-close packings of uniform, solid (impermeable), spherical particles of diameter d(p) were generated by a modified Jodrey-Tory algorithm in eighteen different conduits with quadratic, rectangular, or trapezoidal cross-section at an average bed porosity (interparticle void fraction) of epsilon = 0.48. Velocity fields were calculated by the lattice Boltzmann method, and axial hydrodynamic dispersion of an inert tracer was simulated at Péclet numbers Pe = u(av)d(p)/D(m) (where u(av) is the average fluid flow velocity through a packing and D(m) the bulk molecular diffusion coefficient) from Pe = 5 to Pe = 30 by a Lagrangian particle-tracking method. All conduits had a cross-sectional area of 100d(p)(2) and a length of 1200d(p), translating to around 10(5) particles per packing. We present lateral porosity distribution functions and analyze fluid flow profiles and velocity distribution functions with respect to the base angle and the aspect ratio of the lateral dimensions of the different conduits. We demonstrate significant differences between the top and bottom parts of trapezoidal packings in their lateral porosity and velocity distribution functions, and show that these differences increase with decreasing base angle and increasing base-aspect ratio of a trapezoidal conduit, i.e., with increasing deviation from regular rectangular geometry. Efficiencies are investigated in terms of the axial hydrodynamic dispersion coefficients as a function of the base angle and base-aspect ratio of the conduits. The presented data support the conclusion that the efficiency of particulate beds in trapezoidal microchannels strongly depends on the lateral dimensions of the conduit and that cross-sectional designs based on large side-aspect-ratio rectangles with limited deviations from orthogonality are favorable.

A decade of microfluidic analysis coupled with electrospray mass spectrometry: an overview

This review presents a thorough overview covering the period 1997-2006 of microfluidic chips coupled to mass spectrometry through an electrospray interface. The different types of fabrication processes and materials used to fabricate these chips throughout this period are discussed. Three 'eras' of interfaces are clearly distinguished. The earliest approach involves spraying from the edge of a chip, while later devices either incorporate a standard fused-silica emitter inserted into the device or fully integrated emitters formed during chip fabrication. A summary of microfluidic-electrospray devices for performing separations and sample pretreatment steps before sample introduction into the mass spectrometer is also presented.

Photoresist with low fluorescence for bioanalytical applications

The negative photoresist SU-8 has found widespread use as a material in the fabrication of microelectrical-mechanical systems (MEMS). Although SU-8 has been utilized as a structural material for biological MEMS, a number of SU-8 properties limit its application in these bioanalytical devices. These attributes include its brittleness, nonspecific adsorption of biomolecules, and high fluorescence in the visible wavelengths. In addition, native SU-8 is a poor substrate for cellular adhesion. Photoresists composed of resins with epoxide side groups and photoacids were screened for their ability to serve as a low-fluorescence photoresist with sufficient resolution to generate microstructures with dimensions of 5-10 microm. The fluorescence of structures formed from 1002F photoresist (1002F resin combined with triarylsulfonium hexafluoroantimonate salts) was as much as 10 times less fluorescent than similar SU-8 microstructures. The absorbance of 1002F in the visible wavelengths was also substantially lower than that of SU-8. Microstructures or pallets with an aspect ratio as high as 4:1 could be formed permitting 1002F to be used as a structural material in the fabrication of arrays of pallets for sorting adherent cells. Several different cell types were able to adhere to native 1002F surfaces, and the viability of these cells was excellent. As with SU-8, 1002F has a weak adhesion to glass, a favorable attribute when the pallet arrays are used to sort adherent cells. A threshold, laser pulse energy of 3.5 microJ was required to release individual 50 microm, 1002F pallets from an array. Relative to SU-8, 1002F photoresist offers substantial improvements as a substrate in bioanalytical devices and is likely to find widespread use in BioMEMS.

Microdevices in mass spectrometry

Miniaturization of laboratory instrumentation is becoming critical in achieving the speed and throughput required by the current revolutionary progress in biology. This mini review critically summarizes the present status of microfluidic devices designed for use in mass spectrometry.

Microfluidics for multiplexed MS analysis

Recent advances of microfluidics systems suitable for multiplexed MS analysis are reviewed with respect to fabrication technologies and applications.

On-chip temperature gradient interaction chromatography

This paper reports the first integrated microelectromechanical system (MEMS) HPLC chip that consists of a parylene high-pressure LC column, an electrochemical sensor, a resistive heater and a thermal-isolation structure for on-chip temperature gradient interaction chromatography application. The separation column was 8 mm long, 100 microm wide, 25 microm high and was packed with 5 microm sized, C18-coated beads using conventional slurry-packing technique. A novel parylene-enhanced, air-gap thermal isolation technology was used to reduce heater power consumption by 58% and to reduce temperature rise in the off-column area by 67%. The fabricated chip consumed 400 mW when operated at 100 degrees C. To test the chromatography performance of the fabricated system, a mixture of derivatized amino acids was chosen for separation. A temporal temperature gradient scanning from 25 to 65 degrees C with a ramping rate of 3.6 degrees C/min was applied to the column during separation. Successful chromatographic separation of derivatized amino acids was carried out using our chip. Compared with conventional temperature gradient HPLC system which incorporates "macro oven" to generate temporal temperature gradient on the column, our chip's thermal performance, i.e., power consumption and thermal response, is greatly improved without sacrificing chromatography quality.

Simple photografting method to chemically modify and micropattern the surface of SU-8 photoresist

SU-8 has gained widespread acceptance as a negative photoresist. It is also finding increasing use as a structural material in microanalytical devices. Consequently, methods to tailor the surface properties of SU-8 as well as to micropattern coatings on the surface of SU-8 are needed. The SU-8 photoresist consists of EPON SU-8 resin mixed with the photoacid generator triarylsulfonium hexafluoroantimonate. This photoacid generator can also serve as a photoinitiator generating free radicals when illuminated with UV light. Under the appropriate conditions, sufficient triarylsulfonium hexafluoroantimonate remains within cured SU-8 to act as a source of free radicals and initiate UV-mediated grafting of polymers onto the surface of the SU-8. UV-mediated grafting was used to coat SU-8 surfaces with poly(acrylic acid) and other water-soluble monomers. The SU-8 surface was chemically micropatterned by placing a mask between the UV light and SU-8. The X-Y spatial resolution of micropatterned poly(acrylic acid) on the SU-8 surface was 2 mum. Three applications of these chemically modified SU-8 surfaces were demonstrated. In the first, poly(ethylene glycol) was used to protect the SU-8 from interactions with proteins, yielding a surface resistant to biofouling. In the second demonstration, the SU-8 surface was micropatterned with a cell-resistant layer to guide cellular attachment and growth. In the final application, SU-8 micropallets were encoded with polymer lines. The bar codes were read by either absorbance or fluorescence measurements. Thus, UV-mediated graft polymerization is an efficient and effective method to micropattern coatings onto the surface of SU-8.

Miniaturization: Chip-based liquid chromatography and proteomics

Proteomic research is linked with significant technological challenges such as the high dynamic range and low abundance of biologically interesting proteins. Moreover, there is an increasing need for high-throughput and robustness of routinely performed analyses. Solving these difficulties requires refinements in the capability to fractionate and prepare biological samples as well as improvements in speed, automation, separation power and overall analytical sensitivity.Recent innovations in microfluidic devices with integrated on-chip sample enrichment, liquid chromatography and electrospray emitters and their applicability for specific proteomic applications are presented in this review.: