Evaporation from microreservoirs

Microfluidic Evaporation, Pervaporation, and Osmosis: From Passive Pumping to Solute Concentration

Evaporation, pervaporation, and forward osmosis are processes leading to a mass transfer of solvent across an interface: gas/liquid for evaporation and solid/liquid (membrane) for pervaporation and osmosis. This Review provides comprehensive insight into the use of these processes at the microfluidic scales for applications ranging from passive pumping to the screening of phase diagrams and micromaterials engineering. Indeed, for a fixed interface relative to the microfluidic chip, these processes passively induce flows driven only by gradients of chemical potential. As a consequence, these passive-transport phenomena lead to an accumulation of solutes that cannot cross the interface and thus concentrate solutions in the microfluidic chip up to high concentration regimes, possibly up to solidification. The purpose of this Review is to provide a unified description of these processes and associated microfluidic applications to highlight the differences and similarities between these three passive-transport phenomena.

Peristaltic on-chip pump for tunable media circulation and whole blood perfusion in PDMS-free organ-on-chip and Organ-Disc systems

Organ-on-chip (OoC) systems have become a promising tool for personalized medicine and drug development with advantages over conventional animal models and cell assays. However, the utility of OoCs in industrial settings is still limited, as external pumps and tubing for on-chip fluid transport are dependent on error-prone, manual handling. Here, we present an on-chip pump for OoC and Organ-Disc systems, to perfuse media without external pumps or tubing. Peristaltic pumping is implemented through periodic compression of a flexible pump layer. The disc-shaped, microfluidic module contains four independent systems, each lined with endothelial cells cultured under defined, peristaltic perfusion. Both cell viability and functionality were maintained over several days shown by supernatant analysis and immunostaining. Integrated, on-disc perfusion was further used for cytokine-induced cell activation with physiologic cell responses and for whole blood perfusion assays, both demonstrating the versatility of our system for OoC applications.

Rapid Manufacturing of Multilayered Microfluidic Devices for Organ on a Chip Applications

Microfabrication and Polydimethylsiloxane (PDMS) soft-lithography techniques became popular for microfluidic prototyping at the lab, but even after protocol optimization, fabrication is yet a long, laborious process and partly user-dependent. Furthermore, the time and money required for the master fabrication process, necessary at any design upgrade, is still elevated. Digital Manufacturing (DM) and Rapid-Prototyping (RP) for microfluidics applications arise as a solution to this and other limitations of photo and soft-lithography fabrication techniques. Particularly for this paper, we will focus on the use of subtractive DM techniques for Organ-on-a-Chip (OoC) applications. Main available thermoplastics for microfluidics are suggested as material choices for device fabrication. The aim of this review is to explore DM and RP technologies for fabrication of an OoC with an embedded membrane after the evaluation of the main limitations of PDMS soft-lithography strategy. Different material options are also reviewed, as well as various bonding strategies. Finally, a new functional OoC device is showed, defining protocols for its fabrication in Cyclic Olefin Polymer (COP) using two different RP technologies. Different cells are seeded in both sides of the membrane as a proof of concept to test the optical and fluidic properties of the device.

Digital holographic interferometry investigation of liquid hydrocarbon vapor cloud above a circular well

The current study investigates evaporation of liquid hydrocarbons from a circular well cavity of small depth. Gravimetric analysis is performed to measure the evaporation rate and digital holographic interferometry is used for the measurement of normalized mole fraction profile inside the vapor cloud above the well. Phase unwrapping has been implemented to obtain continuous phase distribution in the image plane. The Fourier-Hankel tomographic inversion algorithm is implemented to obtain the refractive index change distribution inside the object plane, i.e., vapor cloud. Four liquid hydrocarbons, i.e., pentane, hexane, cyclohexane, and heptane, are studied. The radius of circular well cavities is varied in the range of 1.5 to 12.5 mm. Results using a quasi-steady, diffusion-controlled model are compared with the experimental evaporation rate. Measured evaporation rates are higher than the diffusion-limited model calculation for all working fluids and well sizes. This difference is attributed to natural convection occurring inside the vapor cloud due to the density difference between the gas-vapor mixture and the surrounding air. Holographic analysis confirms the presence of natural convection by revealing the formation of a flat disk-shaped vapor cloud above the well surface. Experimentally obtained vapor cloud shape is different from the hemispherical vapor cloud obtained using the pure diffusion-limited evaporation model. The gradient of vapor mole fraction at the liquid-vapor interface is higher compared to that of the diffusion-limited model because of the additional transport mechanism due to natural convection. Transient analysis of the vapor cloud reveals time invariant overall shape of the vapor cloud with a reduction in average magnitude of vapor concentration inside the vapor cloud during evaporation. The existing correlation for sessile droplet cannot successfully predict the evaporation rate from a liquid well. A new correlation is proposed for evaporation rate prediction, which can predict the evaporation rate within a root mean square error of 5.6% for a broad size range of well cavity.

Triggering molecular assembly at the mesoscale for advanced Raman detection of proteins in liquid

An advanced optofluidic system for protein detection based on Raman signal amplification via dewetting and molecular gathering within temporary mesoscale assemblies is presented. The evaporation of a microliter volume of protein solution deposited in a circular microwell precisely follows an outward-receding geometry. Herein the combination of liquid withdrawal with intermolecular interactions induces the formation of self-assembled molecular domains at the solid-liquid interface. Through proper control of the evaporation rate, amplitude of the assemblies and time for spectral collection at the liquid edge are extensively raised, resulting in a local enhancement and refinement of the Raman response, respectively. Further signal amplification is obtained by taking advantage of the intense local electromagnetic fields generated upon adding a plasmonic coating to the microwell. Major advantages of this optofluidic method lie in the obtainment of high-quality, high-sensitivity Raman spectra with detection limit down to sub-micromolar values. Peculiarly, the assembled proteins in the liquid edge region maintain their native-like state without displaying spectral changes usually occurring when dried drop deposits are considered.

A self-driven miniaturized liquid fuel cell

We present an evaporation-driven miniaturized liquid fuel cell with high power density and high portability simultaneously.

A micropillar array for sample concentration via in-plane evaporation

We present a method to perform sample concentration within a lab-on-a-chip using a microfluidic structure which controls the liquid-gas interface through a micropillar array fabricated in polydimethylsiloxane between microfluidic channels. The microstructure confines the liquid flow and a thermal gradient is used to drive evaporation at the liquid-gas-interface. The evaporation occurs in-plane to the microfluidic device, allowing for precise control of the ambient environment. This method is demonstrated with a sample containing 1 μm, 100 nm fluorescent beads and SYTO-9 labelled Escherichia coli bacteria. Over 100 s, the fluorescent beads and bacteria are concentrated by a factor of 10.

Engineers are from PDMS-land, Biologists are from Polystyrenia

As the integration of microfluidics into cell biology research proceeds at an ever-increasing pace, a critical question for those working at the interface of both disciplines is which device material to use for a given application. While PDMS and soft lithography methods offer the engineer rapid prototyping capabilities, PDMS as a material has characteristics that have known adverse effects on cell-based experiments. In contrast, while polystyrene (PS), the most commonly used thermoplastic for laboratory cultureware, has provided decades of grounded and validated research conclusions in cell behavior and function, PS as a material has posed significant challenges in microfabrication. These competing issues have forced microfluidics engineers and biologists to make compromises in how they approach specific research questions, and furthermore, have attenuated the impact of microfluidics on biological research. In this review, we provide a comparison of the attributes of PDMS and PS, and discuss reasons for their popularity in their respective fields. We provide a critical evaluation of the strengths and limitations of PDMS and PS in relation to the advancement and future impact on microfluidic cell-based studies and applications. We believe that engineers have a responsibility to overcome any challenges associated with microfabrication, whether with PS or other materials, and that engineers should provide options and solutions that assist biologists in their experimental design. Our goal is not to advocate for any specific material, but provide guidelines for researchers who desire to choose the most suitable material for their application, and suggest important research directions for engineers working at the interface between microfabrication technology and biological application.

Mapping spatiotemporal molecular distributions using a microfluidic array

The spatial and temporal distributions of an extensive number of diffusible molecules drive a variety of complex functions. These molecular distributions often possess length scales on the order of a millimeter or less; therefore, microfluidic devices have become a powerful tool to study the effects of these molecular distributions in both chemical and biological systems. Although there exist a number of studies utilizing microdevices for the creation of molecular gradients, there are few, if any, studies focusing on the measurement of spatial and temporal distributions of molecular species created within the study system itself. Here we present a microfluidic device capable of sampling multiple chemical messengers in a spatiotemporally resolved manner. This device operates through spatial segregation of nanoliter-sized volumes of liquid from a primary sample reservoir into a series of analysis microchannels, where fluid pumping is accomplished via a system of passive microfluidic pumps. Subsequent chemical analysis within each microchannel, achieved via optical or bioanalytical methods, yields quantitative data on the spatial and temporal information for any analytes of interest existing within the sample reservoir. These techniques provide a simple, cost-effective route to measure the spatiotemporal distributions of molecular analytes, where the system can be tailored to study both chemical and biological systems.

Integrated sensitive on-chip ion field effect transistors based on wrinkled InGaAs nanomembranes

Self-organized wrinkling of pre-strained nanomembranes into nanochannels is used to fabricate a fully integrated nanofluidic device for the development of ion field effect transistors (IFETs). Constrained by the structure and shape of the membrane, the deterministic wrinkling process leads to a versatile variation of channel types such as straight two-way channels, three-way branched channels, or even four-way intersection channels. The fabrication of straight channels is well controllable and offers the opportunity to integrate multiple IFET devices into a single chip. Thus, several IFETs are fabricated on a single chip using a III-V semiconductor substrate to control the ion separation and to measure the ion current of a diluted potassium chloride electrolyte solution.

A simple method for the evaluation of microfluidic architecture using flow quantitation via a multiplexed fluidic resistance measurement

Quality control of microdevices adds significant costs, in time and money, to any fabrication process. A simple, rapid quantitative method for the post-fabrication characterization of microchannel architecture using the measurement of flow with volumes relevant to microfluidics is presented. By measuring the mass of a dye solution passed through the device, it circumvents traditional gravimetric and interface-tracking methods that suffer from variable evaporation rates and the increased error associated with smaller volumes. The multiplexed fluidic resistance (MFR) measurement method measures flow via stable visible-wavelength dyes, a standard spectrophotometer and common laboratory glassware. Individual dyes are used as molecular markers of flow for individual channels, and in channel architectures where multiple channels terminate at a common reservoir, spectral deconvolution reveals the individual flow contributions. On-chip, this method was found to maintain accurate flow measurement at lower flow rates than the gravimetric approach. Multiple dyes are shown to allow for independent measurement of multiple flows on the same device simultaneously. We demonstrate that this technique is applicable for measuring the fluidic resistance, which is dependent on channel dimensions, in four fluidically connected channels simultaneously, ultimately determining that one chip was partially collapsed and, therefore, unusable for its intended purpose. This method is thus shown to be widely useful in troubleshooting microfluidic flow characteristics.

Self-digitization of sample volumes

This paper describes a very simple and robust microfluidic device for digitizing samples into an array of discrete volumes. The device is based on an inherent fluidic phenomenon, where an incoming aqueous sample divides itself into an array of chambers that have been primed with an immiscible phase. Self-digitization of sample volumes results from the interplay between fluidic forces, interfacial tension, channel geometry, and the final stability of the digitized samples in the chambers. Here, we describe experiments and simulations that were used to characterize these parameters and the conditions under which the self-digitization process occurred. Unlike existing methods used to partition samples into an array, our method is able to digitize 100% of a sample into a localized array without any loss of sample volume. The final volume of the discretized sample at each location is defined by the geometry and size of each chamber. Thus, we can form an array of samples with varying but predefined volumes. We exploited this feature to separate the crystal growth of otherwise concomitant polymorphs from a single solution. Additionally, we demonstrated the removal of the digitized samples from the chambers for downstream analysis, as well as the addition of reagents to the digitized samples. We believe this simple method will be useful in a broad range of applications where a large array of discretized samples is required, including digital PCR, single-cell analysis, and cell-based drug screening.

Passive microfluidic pumping using coupled capillary/evaporation effects

Controlled pumping of fluids through microfluidic networks is a critical unit operation ubiquitous to lab-on-a-chip applications. Although there have been a number of studies involving the creation of passive flows within lab-on-a-chip devices, none has shown the ability to create temporally stable flows for periods longer than several minutes. Here a passive pumping approach is presented in which a large pressure differential arising from a small, curved meniscus situated along the bottom corners of an outlet reservoir serves to drive fluid through a microfluidic network. The system quickly reaches steady-state and is able to provide precise volumetric flow rates for periods lasting over an hour. A two-step mathematical model provides accurate predictions of fluid and mass transport dynamics in these devices, as validated by particle tracking in laboratory systems. Precise flow rates spanning an order of magnitude are accomplished via control of the microchannel and outlet reservoir dimensions. This flow mechanism has the potential to be applied to many micro-total analytical system devices that utilize pressure-driven flow; as an illustrative example, the pumping technique is applied for the passive generation of temporally stable chemical gradients.