Microfluidic Networks for Chemical Patterning of Substrates: Design and Application to Bioassays

A Novel Implantable Piezoresistive Microsensor for Intraocular Pressure Measurement

Glaucoma is the world's second-leading irreversible eye disease causing blindness. Although the pathogenesis of glaucoma is not particularly well understood, high intraocular pressure (IOP) is widely recognized as a significant risk factor. In clinical practice, various devices have been used to measure IOP, but most of them cannot provide continuous measurements for a long time. To meet the needs of glaucoma patients who experience frequent fluctuations in the IOP and require constant monitoring, we fabricated an implantable piezoresistive IOP sensor based on microfluidic technology. The sensor has a sensitivity of 0.00257 Ω/mbar and demonstrates excellent linearity, stability, and repeatability. According to the calibration data, the average measurement error is ±0.5 mbar. We implanted it into the vitreous of a rabbit and successfully detected its IOP fluctuations. The sensor is simple in design, easy to fabricate, and can be used for long-term continuous IOP measurements. It presents a new approach for microfluidic-based IOP sensors and offers a novel method for the daily care of patients with glaucoma.

Chip-integrated non-mechanical microfluidic pump driven by electrowetting on dielectrics

A microfluidic pump is presented that generates its pumping action via the EWOD (electrowetting-on-dielectric) effect. The flow is generated by the periodic movement of liquid-vapor interfaces in a large number (≈106) of microcavities resulting in a volume change of approx. 0.5 pl per cavity per pump stroke. The total flow resulting from all microcavities adds up to a few hundred nanolitres per cycle. Passive, topologically optimized, non-mechanical Tesla valves are used to rectify the flow. As a result, the micropump operates without any moving components. The dimensioning, fabrication, and characterization process of the micropump are described. Device fabrication is done using conventional manufacturing processes from microsystems technology, enabling cost-effective mass production on wafer-level without additional assembly steps like piezo chip-level bonding, etc. This allows for direct integration into wafer-based microfluidic or lab-on-a-chip applications. Furthermore, first measurement results obtained with prototypes of the micropump are presented. The voltage- and frequency-dependent pump performance is determined. The measurements show that a continuous flow rate larger than 0.2 ml min-1 can be achieved at a maximum pump pressure larger than 12 mbar.

Drug testing of monodisperse arrays of live microdissected tumors using a valved multiwell microfluidic platform

Cancer drug testing in animals is an extremely poor predictor of the drug's safety and efficacy observed in humans. Hence there is a pressing need for functional testing platforms that better predict traditional and immunotherapy responses in human, live tumor tissue or tissue constructs, and at the same time are compatible with the use of mouse tumor tissue to facilitate building more accurate disease models. Since many cancer drug actions rely on mechanisms that depend on the tumor microenvironment (TME), such platforms should also retain as much of the native TME as possible. Additionally, platforms based on miniaturization technologies are desirable to reduce animal use and sensitivity to human tissue scarcity. Present high-throughput testing platforms that have some of these features, e.g. based on patient-derived tumor organoids, require a growth step that alters the TME. On the other hand, microdissected tumors (μDTs) or "spheroids" that retain an intact TME have shown promising responses to immunomodulators acting on native immune cells. However, difficult tissue handling after microdissection has reduced the throughput of drug testing on μDTs, thereby constraining the inherent advantages of producing numerous TME-preserving units of tissue for drug testing. Here we demonstrate a microfluidic 96-well platform designed for drug treatment of hundreds of similarly-sized, cuboidal μDTs ("cuboids") produced from a single tumor sample. The platform organizes a monodisperse array of four cuboids per well in 384 hydrodynamic traps. The microfluidic device, entirely fabricated in thermoplastics, features 96 microvalves that fluidically isolate each well after the cuboid loading step for straightforward multi-drug testing. Since our platform makes the most of scarce tumor tissue, it can potentially be applied to human biopsies that preserve the human TME while minimizing animal testing.

Membrane-Free Lateral Flow Assay with the Active Control of Fluid Transport for Ultrasensitive Cardiac Biomarker Detection

Membrane-based lateral flow immunoassays (LFAs) have been employed as early point-of-care (POC) testing tools in clinical settings. However, the varying membrane properties, uncontrollable sample transport in LFAs, visual readout, and required large sample volumes have been major limiting factors in realizing needed sensitivity and desirable precise quantification. Addressing these challenges, we designed a membrane-free system in which the desirable three-dimensional (3D) structure of the detection zone is imitated and used a small pump for fluid flow and fluorescence as readout, all the while maintaining a one-step assay protocol. A hydrogel-like protein-polyelectrolyte complex (PPC) within a polyelectrolyte multilayer (PEM) was developed as the test line by complexing polystreptavidin (pSA) with poly(diallyldimethylammonium chloride) (PDDA), which in turn was layered with poly(acrylic acid) (PAA) resulting in a superior 3D streptavidin-rich test line. Since the remainder of the microchannel remains material-free, good flow control is achieved, and with the total volume of 20 μL, 7.5-fold smaller sample volumes can be used in comparison to conventional LFAs. High sensitivity with desirable reproducibility and a 20 min total assay time were achieved for the detection of NT-proBNP in plasma with a dynamic range of 60-9000 pg·mL-1 and a limit of detection of 56 pg·mL-1 using probe antibody-modified fluorescence nanoparticles. While instrument-free visual detection is no longer possible, the developed lateral flow channel platform has the potential to dramatically expand the LFA applicability, as it overcomes the limitations of membrane-based immunoassays, ultimately improving the accuracy and reducing the sample volume so that finger-prick analyses can easily be done in a one-step assay for analytes present at very low concentrations.

Multiple protein-patterned surface plasmon resonance biochip for the detection of human immunoglobulin-G

A portable surface plasmon resonance (SPR) measurement prototype integrated with a multiple protein-patterned SPR biochip is introduced for label-free and selective detection of human immunoglobulin-G (H-IgG). The polyclonal anti-H-IgG antibodies derived from goat, rabbit, and mouse were immobilized through polydimethylsiloxane (PDMS) microchannels to fabricate the patterned SPR biochip. The PDMS surface was functionalized using 3-aminopropyltrimethoxysilane and bonded to carbodiimide-activated gold substrates to construct irreversibly bonded hydrophilic microfluidic chip at room temperature. For SPR measurement, a custom-made system is developed with a high angular scanning accuracy of 0.005° and a wide scanning range of 30°-80° that avoids the conventional requirement of expensive goniometric stages and detector arrays. The SPR biochip immobilized with 750 μg/mL goat anti-H-IgG demonstrated detection of H-IgG with a detection limits of 15 μg/mL, and linear response through a wide concentration range (15-225 μg/mL) of high coefficient of determination (R²  = 0.99661). The selectivity of the sensor was investigated by exposing them to two different non-specific targets (bovine serum albumin and polyvalent antivenom). The results indicate negligible sensor response towards nonspecific targets (0.25° for 30 μg/mL bovine serum albumin (BSA) and 0.25° for 30 μg/mL polyvalent antivenom) in comparison to H-IgG (1.5° for 30 μg/mL).

Handyfuge Microfluidic for On-Site Antibiotic Susceptibility Testing

Low-cost, rapid, and accurate acquisition of minimum inhibitory concentrations (MICs) is key to limiting the development of antimicrobial resistance (AMR). Until now, conventional antibiotic susceptibility testing (AST) methods are typically time-consuming, high-cost, and labor-intensive, making them difficult to accomplish this task. Herein, an electricity-free, portable, and robust handyfuge microfluidic chip was developed for on-site AST, termed handyfuge-AST. With simply handheld centrifugation, the bacterial-antibiotic mixtures with accurate antibiotic concentration gradients could be generated in less than 5 min. The accurate MIC values of single antibiotics (including ampicillin, kanamycin, and chloramphenicol) or their combinations against Escherichia coli could be obtained within 5 h. To further meet the growing demands of point-of-care testing, we upgraded our handyfuge-AST with a pH-based colorimetric strategy, enabling naked eye recognition or intelligent recognition with a homemade mobile app. Through a comparative study of 60 clinical data (10 clinical samples corresponding to six commonly used antibiotics), the accurate MICs by handyfuge-AST with 100% categorical agreements were achieved compared to clinical standard methods (area under curves, AUCs = 1.00). The handyfuge-AST could be used as a low-cost, portable, and robust point-of-care device to rapidly obtain accurate MIC values, which significantly limit the progress of AMR.

Basic Guide to Multilayer Microfluidic Fabrication with Polyimide Tape and Diode Laser

For normal operations, microfluidic devices typically require an external source of pressure to deliver fluid flow through the microchannel. This requirement limits their use for benchtop research activities in a controlled static environment. To exploit the full potential of the miniaturization and portability of microfluidic platforms, passively driven capillary microfluidic devices have been developed to completely remove the need for an external pressure source. Capillary microfluidics can be designed to perform complex tasks by designing individual components of the device. These components, such as the stop valve and trigger valve, operate through changes in microchannel dimensions and aspect ratios. A direct, maskless fabrication protocol that allows the precise fabrication of microchannels and other microfluidic components is introduced here. A diode laser and polyimide tape on a PMMA substrate are the only components needed to start fabrication. By varying the laser power used and the number of laser repetitions, various depths and widths of the microchannel can be quickly created to meet specific needs. As an example of a functional unit, a trigger valve was fabricated and tested, as proof of the validity of the fabrication protocol.

Full-Cut Manufacture of Skin-Interfaced Microfluidic Patch with Copper Electrode for In Situ Admittance Sensing of Sweat Rate

Sweat-rate measurement has received more and more attention, especially for specific groups, such as athletes, soldiers and manual workers, due to their excessive sweat loss under prolonged intense heat stress, which increases the risk of dehydration and electrolyte imbalance. The highly effective manufacture of a sweat-sensing device is essential to its wide range of applications in perspiration-related physiological information detection. In this work, we propose a simple and cost-effective strategy for the manufacture of a microfluidic sweat-rate-sensing patch via laser cutting and transfer printing technology. A copper foil tape is used as the electrode for in situ admittance based sweat-rate-sensing. The detection circuits and measurement conditions are optimized to prevent the negative effect of an electrochemical reaction between a copper electrode and sweat for precise admittance measurement. In vitro and on-body experiments demonstrate that the copper electrode is applicable for admittance-based sweat sensing and is capable of achieving equivalent sensing accuracy as a gold electrode and that the proposed sensor structure can perform consecutive and accurate sweat-rate-sensing and facilitates a significant increase in manufacturing efficiency.

Surface Wettability for Skin-Interfaced Sensors and Devices

The practical applications of skin-interfaced sensors and devices in daily life hinge on the rational design of surface wettability to maintain device integrity and achieve improved sensing performance under complex hydrated conditions. Various bio-inspired strategies have been implemented to engineer desired surface wettability for varying hydrated conditions. Although the bodily fluids can negatively affect the device performance, they also provide a rich reservoir of health-relevant information and sustained energy for next-generation stretchable self-powered devices. As a result, the design and manipulation of the surface wettability are critical to effectively control the liquid behavior on the device surface for enhanced performance. The sensors and devices with engineered surface wettability can collect and analyze health biomarkers while being minimally affected by bodily fluids or ambient humid environments. The energy harvesters also benefit from surface wettability design to achieve enhanced performance for powering on-body electronics. In this review, we first summarize the commonly used approaches to tune the surface wettability for target applications toward stretchable self-powered devices. By considering the existing challenges, we also discuss the opportunities as a small fraction of potential future developments, which can lead to a new class of skin-interfaced devices for use in digital health and personalized medicine.

Surface patterning techniques for proteins on nano- and micro-systems: a modulated aspect in hierarchical structures

The surface patterning of protein using fabrication or the external functionalization of structures demonstrates various applications in the biomedical field for bioengineering, biosensing and antifouling. This review article offers an outline of the existing advances in protein patterning technology with a special emphasis on the current physical and physicochemical methods, including stencil patterning, trap- and droplet-based microfluidics, and chemical modification of surfaces via photolithography, microcontact printing and scanning probe nanolithography. Different approaches are applied for the biological studies of recent trends for single-protein patterning technology, such as robotic printing, stencil printing and colloidal lithography, wherein the concepts of physical confinement, electrostatic and capillary forces, as well as dielectrophoretics, are summarised to understand the design approaches. Photochemical alterations with diazirine, nitrobenzyl and aryl azide functional groups for the implication of modified substrates, such as self-assembled monolayers functionalized with amino silanes, organosilanes and alkanethiols on gold surfaces, as well as topographical effects of patterning techniques for protein functionalization and orientation, are discussed. Analytical methods for the evaluation of protein functionality are also mentioned. Regarding their selectivity, protein pattering methods will be readily used to fabricate modified surfaces and target-specific delivery systems for the transportation of macromolecules such as streptavidin, and albumin. Future applications of patterning techniques include high-throughput screening, the evaluation of intracellular interactions, accurate screening and personalized treatments.

Inhibitory effect of nucleotides on acetylcholine esterase activity and its microflow-based actuation in blood plasma

The inhibitory effect of nucleotides on the catalytic activity of acetylcholine esterase (AChE) was rationalized and similar inhibition trend was observed when analyzing the macroscopic fluid flow generated by surface...

Biopatterning: The Art of Patterning Biomolecules on Surfaces

Patterning biomolecules on surfaces provides numerous opportunities for miniaturizing biological assays; biosensing; studying proteins, cells, and tissue sections; and engineering surfaces that include biological components. In this Feature Article, we summarize the themes presented in our recent Langmuir Lecture on patterning biomolecules on surfaces, miniaturizing surface assays, and interacting with biointerfaces using three key technologies: microcontact printing, microfluidic networks, and microfluidic probes.

A flux-adaptable pump-free microfluidics-based self-contained platform for multiplex cancer biomarker detection

Microfluidics drives technological advancement in point-of-care (POC) bioanalytical diagnostics towards portability, fast response and low cost. In most microfluidic bioanalytical applications, flowing antigen/antibody reacts with immobilized antibody/antigen at a constant flux; it is difficult to reach a compromise to simultaneously realize sufficient time for the antigen-antibody interaction and short time for the entire assay. Here, we present a pump-free microfluidic chip, in which flow is self-initialized by capillary pumping and continued by imbibition of a filter paper. Microfluidic units in teardrop shape ensure that flow passes through the reaction areas at a reduced flux to facilitate the association between antigen and antibody and speeds up after the reaction areas. By spotting different antibodies into the reaction area, four types of biomarkers can be measured simultaneously in one microfluidic chip. Moreover, a small-sized instrument was developed for chemiluminescence detection and signal analysis. The system was validated by testing four biomarkers of colorectal cancer using plasma samples from patients. The assay took about 20 minutes. The limit of detection is 0.89 ng mL^-1, 1.72 ng mL^-1, 3.62 U mL^-1 and 1.05 U mL^-1 for the assays of carcinoembryonic antigen, alpha-fetoprotein, carbohydrate antigen 125 and carbohydrate antigen 19-9, respectively. This flux-adaptable and self-contained microfluidic platform is expected to be useful in various POC disease-monitoring applications.

New flow control systems in capillarics: off valves

Capillary systems are a promising technology for point-of-care microfluidics, since they are pre-programmable and self-powered. This work introduces "off valves" as a key building block for capillaric circuits, providing easy-to-use, multi-purpose valving functionality and autonomous flow control. To this end we present a set of switching valve designs that use trigger channels and liquid input alone to close or open connections between channels in a highly controllable fashion. The key element of all these valve designs is a new off trigger valve, which is characterised in detail here and holds the potential for transistor-like switching and resistance tuning. As an example for the potential applications of switching valves, we demonstrate how they can be used for flow resistance control in a complex microfluidic circuit and for sequential chemical loading into a reaction chamber. Use of the switching valves for the latter in particular allowed for the tuning of incubation times and volumetric measurement, thus confirming applicability of the valves for automated and self-powered immunoassays in point-of-care environments.

Microdissected "cuboids" for microfluidic drug testing of intact tissues

As preclinical animal tests often do not accurately predict drug effects later observed in humans, most drugs under development fail to reach the market. Thus there is a critical need for functional drug testing platforms that use human, intact tissues to complement animal studies. To enable future multiplexed delivery of many drugs to one small biopsy, we have developed a multi-well microfluidic platform that selectively treats cuboidal-shaped microdissected tissues or "cuboids" with well-preserved tissue microenvironments. We create large numbers of uniformly-sized cuboids by semi-automated sectioning of tissue with a commercially available tissue chopper. Here we demonstrate the microdissection method on normal mouse liver, which we characterize with quantitative 3D imaging, and on human glioma xenograft tumors, which we evaluate after time in culture for viability and preservation of the microenvironment. The benefits of size uniformity include lower heterogeneity in future biological assays as well as facilitation of their physical manipulation by automation. Our prototype platform consists of a microfluidic circuit whose hydrodynamic traps immobilize the live cuboids in arrays at the bottom of a multi-well plate. Fluid dynamics simulations enabled the rapid evaluation of design alternatives and operational parameters. We demonstrate the proof-of-concept application of model soluble compounds such as dyes (CellTracker, Hoechst) and the cancer drug cisplatin. Upscaling of the microfluidic platform and microdissection method to larger arrays and numbers of cuboids could lead to direct testing of human tissues at high throughput, and thus could have a significant impact on drug discovery and personalized medicine.

Antibody Printing Technologies

Antibody microarrays are routinely employed in the lab and in the clinic for studying protein expression, protein-protein, and protein-drug interactions. The microarray format reduces the size scale at which biological and biochemical interactions occur, leading to large reductions in reagent consumption and handling times while increasing overall experimental throughput. Specifically, antibody microarrays, as a platform, offer a number of different advantages over traditional techniques in the areas of drug discovery and diagnostics. While a number of different techniques and approaches have been developed for creating micro and nanoscale antibody arrays, issues relating to sensitivity, cost, and reproducibility persist. The aim of this review is to highlight current state-of the-art techniques and approaches for creating antibody arrays by providing latest accounts of the field while discussing potential future directions.

Polymer Microchannel and Micromold Surface Polishing for Rapid, Low-Quantity Polydimethylsiloxane and Thermoplastic Microfluidic Device Fabrication

Polymer-based micromolding has been proposed as an alternative to SU-8 micromolding for microfluidic chip fabrication. However, surface defects such as milling marks may result in rough microchannels and micromolds, limiting microfluidic device performance. Therefore, we use chemical and mechanical methods for polishing polymer microchannels and micromolds. In addition, we evaluated their performance in terms of removing the machining (milling) marks on polymer microchannel and micromold surfaces. For chemical polishing, we use solvent evaporation to polish the sample surfaces. For mechanical polishing, wool felt polishing bits with an abrasive agent were employed to polish the sample surfaces. Chemical polishing reduced surface roughness from 0.38 μm (0 min, after milling) to 0.13 μm after 6 min of evaporation time. Mechanical polishing reduced surface roughness from 0.38 to 0.165 μm (optimal pressing length: 0.3 mm). As polishing causes abrasion, we evaluated sample geometry loss after polishing. Mechanically and chemically polished micromolds had optimal micromold distortion percentages of 1.01% ± 0.76% and 1.10% ± 0.80%, respectively. Compared to chemical polishing, mechanical polishing could better maintain the geometric integrity since it is locally polished by computer numerical control (CNC) miller. Using these surface polishing methods with optimized parameters, polymer micromolds and microchannels can be rapidly produced for polydimethylsiloxane (PDMS) casting and thermoplastic hot embossing. In addition, low-quantity (15 times) polymer microchannel replication is demonstrated in this paper.

Switchable Microvalves Employing a Conducting Polymer and Their Automatic Operation in Conjunction with Micropumps with a Superabsorbent Polymer

Automated microfluidic devices integrated with microvalves and micropumps were developed. To realize an efficient and automatic control of solution transport, we newly developed microvalves comprising a polypyrrole (PPy) film electropolymerized on patterned platinum electrodes and doped with a surfactant. The surface of the doped PPy film exhibits a nearly hydrophobic state or a hydrophilic state when oxidized or reduced under the application of an appropriate potential, enabling the control of the solution transport via capillary action. The simple structure and fabrication of the microvalves facilitated the integration of many valves in various flow channel structures. To improve the performance, simple suction and injection micropumps with freeze-dried discs made of a superabsorbent polymer (SAP) were additionally incorporated along with the microvalves. The former withdraws the solution by directly absorbing it onto the SAP, whereas the latter applies a pressure to the solution through an elastic diaphragm by absorbing a priming solution into the SAP. The significant volume changes of the SAP discs enabled an efficient transport of the solutions. Repeated injection and withdrawal of the solutions in and out of a reaction chamber were demonstrated using four injection and suction pumps and eight valves.

Surface-directed spinodal decomposition on chemically patterned substrates

Surface-directed spinodal decomposition (SDSD) is the kinetic interplay of phase separation and wetting at a surface. This process is of great scientific and technological importance. In this paper, we report results from a numerical study of SDSD on a chemically patterned substrate. We consider simple surface patterns for our simulations, but most of the results apply for arbitrary patterns. In layers near the surface, we observe a dynamical crossover from a surface-registry regime to a phase-separation regime. We study this crossover using layerwise correlation functions and structure factors and domain length scales.

Study on Functionality and Surface Modification of a Stair-Step Liquid-Triggered Valve for On-Chip Flow Control

Distinctive from other forms of microfluidic system, capillary microfluidics is of great interest in autonomous micro-systems due to its well-engineered fluidic control based on capillary force. As an essential component of fluidic control in capillaric circuits, micro-valves enable sequential fluidic operations by performing actions such as stopping and triggering. In this paper, we present a stair-step liquid-triggered valve; the functionality of the valve and its dependencies on geometry and surface modification are studied. The surface contact angle of the microfabricated valves that are coated by polyethylene glycol (PEG) or (3-Aminopropyl) triethoxysilane (APTES) is evaluated experimentally, and the corresponding reliability of the valve structure is discussed. Moreover, the variation in the surface contact angle over time is investigated, indicating the shelf time of the device. We further discuss the overall fluidic behavior in such capillary valves, which benefits the capillaric circuit designs at the initial stage.

Digital Manufacturing for Microfluidics

The microfluidics field is at a critical crossroads. The vast majority of microfluidic devices are presently manufactured using micromolding processes that work very well for a reduced set of biocompatible materials, but the time, cost, and design constraints of micromolding hinder the commercialization of many devices. As a result, the dissemination of microfluidic technology-and its impact on society-is in jeopardy. Digital manufacturing (DM) refers to a family of computer-centered processes that integrate digital three-dimensional (3D) designs, automated (additive or subtractive) fabrication, and device testing in order to increase fabrication efficiency. Importantly, DM enables the inexpensive realization of 3D designs that are impossible or very difficult to mold. The adoption of DM by microfluidic engineers has been slow, likely due to concerns over the resolution of the printers and the biocompatibility of the resins. In this article, we review and discuss the various printer types, resolution, biocompatibility issues, DM microfluidic designs, and the bright future ahead for this promising, fertile field.

Grayscale surface patterning using electrophoretic motion through a heterogeneous hydrogel material

Chemical surface patterning can be an incredibly powerful tool in a variety of applications, as it enables precise spatial control over surface properties. But the equipment required to create functional surface patterns-especially "grayscale" patterns where independent control over species placement and density are needed-is often expensive and inaccessible. In this work, we leveraged equipment and methods readily available to many research labs, namely 3D printing and electroblotting, to generate controlled grayscale surface patterns. Three-dimensional-printed molds were used to cast polyacrylamide hydrogels with regions of variable polymer density; regions of low polymer density within the hydrogels served as reservoirs for proteins that were later driven onto a target surface using electrophoresis. This mechanism was used to deposit grayscale patterns of fluorescently labeled proteins, and the fluorescent intensity of these patterns was measured and compared to a theoretical analysis of the deposition mechanism.

Multiplexed drug testing of tumor slices using a microfluidic platform

Current methods to assess the drug response of individual human cancers are often inaccurate, costly, or slow. Functional approaches that rapidly and directly assess the response of patient cancer tissue to drugs or small molecules offer a promising way to improve drug testing, and have the potential to identify the best therapy for individual patients. We developed a digitally manufactured microfluidic platform for multiplexed drug testing of intact cancer slice cultures, and demonstrate the use of this platform to evaluate drug responses in slice cultures from human glioma xenografts and patient tumor biopsies. This approach retains much of the tissue microenvironment and can provide results rapidly enough, within days of surgery, to guide the choice of effective initial therapies. Our results establish a useful preclinical platform for cancer drug testing and development with the potential to improve cancer personalized medicine.

A microfluidic platform for functional testing of cancer drugs on intact tumor slices

Present approaches to assess cancer treatments are often inaccurate, costly, and/or cumbersome. Functional testing platforms that use live tumor cells are a promising tool both for drug development and for identifying the optimal therapy for a given patient, i.e. precision oncology. However, current methods that utilize patient-derived cells from dissociated tissue typically lack the microenvironment of the tumor tissue and/or cannot inform on a timescale rapid enough to guide decisions for patient-specific therapy. We have developed a microfluidic platform that allows for multiplexed drug testing of intact tumor slices cultured on a porous membrane. The device is digitally-manufactured in a biocompatible thermoplastic by laser-cutting and solvent bonding. Here we describe the fabrication process in detail, we characterize the fluidic performance of the device, and demonstrate on-device drug-response testing with tumor slices from xenografts and from a patient colorectal tumor.

Fully Dried Two-Dimensional Paper Network for Enzymatically Enhanced Detection of Nucleic Acid Amplicons

Two-dimensional paper networks (2DPNs) have enabled the use of paper-based platforms to perform multistep immunoassays for detection of pathogenic diseases at the point-of-care. To date, however, detection has required the user to provide multiple signal enhancement solutions and been limited to protein targets. We solve these challenges by using mathematical equations to guide the device design of a novel 2DPN, which leverages multiple fluidic inputs to apply fully dried solutions of hydrogen peroxide, diaminobenzidine, and horseradish peroxidase signal enhancement reagents to enhance the limit-of-detection of numerous nucleic acid products. Upon rehydration in our unique 2DPN design, the dried signal enhancement solution reduces the limit-of-detection (LOD) of the device to 5 × 10¹¹ nucleic acid copies/mL without increasing false positive detection. Our easy-to-use device retains activity after 28 days of dry storage and produces reliable signal enhancement 40 min after sample application. The fully integrated device demonstrated versatility in its ability to detect double-stranded and single-stranded DNA samples, as well as peptide nucleic acids.

Microfluidics for interrogating live intact tissues

The intricate microarchitecture of tissues - the "tissue microenvironment" - is a strong determinant of tissue function. Microfluidics offers an invaluable tool to precisely stimulate, manipulate, and analyze the tissue microenvironment in live tissues and engineer mass transport around and into small tissue volumes. Such control is critical in clinical studies, especially where tissue samples are scarce, in analytical sensors, where testing smaller amounts of analytes results in faster, more portable sensors, and in biological experiments, where accurate control of the cellular microenvironment is needed. Microfluidics also provides inexpensive multiplexing strategies to address the pressing need to test large quantities of drugs and reagents on a single biopsy specimen, increasing testing accuracy, relevance, and speed while reducing overall diagnostic cost. Here, we review the use of microfluidics to study the physiology and pathophysiology of intact live tissues at sub-millimeter scales. We categorize uses as either in vitro studies - where a piece of an organism must be excised and introduced into the microfluidic device - or in vivo studies - where whole organisms are small enough to be introduced into microchannels or where a microfluidic device is interfaced with a live tissue surface (e.g. the skin or inside an internal organ or tumor) that forms part of an animal larger than the device. These microfluidic systems promise to deliver functional measurements obtained directly on intact tissue - such as the response of tissue to drugs or the analysis of tissue secretions - that cannot be obtained otherwise.

A customizable microfluidic platform for medium-throughput modeling of neuromuscular circuits

Neuromuscular circuits (NMCs) are vital for voluntary movement, and effective models of NMCs are needed to understand the pathogenesis of, as well as to identify effective treatments for, multiple diseases, including Duchenne’s muscular dystrophy and amyotrophic lateral sclerosis. Microfluidics are ideal for recapitulating the central and peripheral compartments of NMCs, but myotubes often detach before functional NMCs are formed. In addition, microfluidic systems are often limited to a single experimental unit, which significantly limits their application in disease modeling and drug discovery. Here, we developed a microfluidic platform (MFP) containing over 100 experimental units, making it suitable for medium-throughput applications. To overcome detachment, we incorporated a reactive polymer surface allowing customization of the environment to culture different cell types. Using this approach, we identified conditions that enable long-term co-culture of human motor neurons and myotubes differentiated from human induced pluripotent stem cells inside our MFP. Optogenetics demonstrated the formation of functional NMCs. Furthermore, we developed a novel application of the rabies tracing assay to efficiently identify NMCs in our MFP. Therefore, our MFP enables large-scale generation and quantification of functional NMCs for disease modeling and pharmacological drug targeting.

Laser printing-enabled direct creation of cellular heterogeneity in lab-on-a-chip devices

Lab-on-a-chip devices, capable of culturing living cells in continuously perfused, micrometer-sized channels, have been intensively investigated to model physiological microenvironments for cell-related testing and evaluation applications. Various chemical, physical, and/or biological culture cues are usually expected in a designed chip to mimic the in vivo environment with defined spatial heterogeneity of cells and biomaterials. To create such heterogeneity within a given chip, typical methods rely heavily on sophisticated fabrication and cell seeding processes, and chips fabricated with these methods are difficult to readily adapt for other applications. In this study, laser-induced forward transfer (LIFT)-based printing has been implemented to create heterogeneous cellular patterns in a lab-on-a-chip device to achieve the efficiency in creating heterogeneous cellular patterns as well as the flexibility in adapting different evaluation configurations in lab-on-a-chip devices. Two applications, parallel evaluation of cellular behavior and targeted drug delivery to cancer cells, have been implemented as proof-of-concept demonstrations of the proposed fabrication method. For the first application, the morphology of cells in different extracellular matrix (ECM) materials cultured under varying conditions has been investigated. It is found that less stiff ECM and dynamic culturing are preferred for spreading of fibroblasts. For the second application, different drug carriers have been utilized for targeted delivery of anticancer drugs to breast cancer cells. It is found that targeted drug delivery is important to realize effective chemotherapy and drug release rate from drug carriers affects the chemotherapy effect. Consequently, the proposed laser printing-based method enables direct creation of heterogeneous cellular patterns within lab-on-a-chip devices which improves the efficiency and versatility of cell-related sensing and evaluation using lab-on-a-chip devices.

A Fast and Simple Contact Printing Approach to Generate 2D Protein Nanopatterns

Protein micropatterning has become an important tool for many biomedical applications as well as in academic research. Current techniques that allow to reduce the feature size of patterns below 1 μm are, however, often costly and require sophisticated equipment. We present here a straightforward and convenient method to generate highly condensed nanopatterns of proteins without the need for clean room facilities or expensive equipment. Our approach is based on nanocontact printing and allows for the fabrication of protein patterns with feature sizes of 80 nm and periodicities down to 140 nm. This was made possible by the use of the material X-poly(dimethylsiloxane) (X-PDMS) in a two-layer stamp layout for protein printing. In a proof of principle, different proteins at various scales were printed and the pattern quality was evaluated by atomic force microscopy (AFM) and super-resolution fluorescence microscopy.

Capillary microfluidics in microchannels: from microfluidic networks to capillaric circuits

Microfluidics offer economy of reagents, rapid liquid delivery, and potential for automation of many reactions, but often require peripheral equipment for flow control. Capillary microfluidics can deliver liquids in a pre-programmed manner without peripheral equipment by exploiting surface tension effects encoded by the geometry and surface chemistry of a microchannel. Here, we review the history and progress of microchannel-based capillary microfluidics spanning over three decades. To both reflect recent experimental and conceptual progress, and distinguish from paper-based capillary microfluidics, we adopt the more recent terminology of capillaric circuits (CCs). We identify three distinct waves of development driven by microfabrication technologies starting with early implementations in industry using machining and lamination, followed by development in the context of micro total analysis systems (μTAS) and lab-on-a-chip devices using cleanroom microfabrication, and finally a third wave that arose with advances in rapid prototyping technologies. We discuss the basic physical laws governing capillary flow, deconstruct CCs into basic circuit elements including capillary pumps, stop valves, trigger valves, retention valves, and so on, and describe their operating principle and limitations. We discuss applications of CCs starting with the most common usage in automating liquid delivery steps for immunoassays, and highlight emerging applications such as DNA analysis. Finally, we highlight recent developments in rapid prototyping of CCs and the benefits offered including speed, low cost, and greater degrees of freedom in CC design. The combination of better analytical models and lower entry barriers (thanks to advances in rapid manufacturing) make CCs both a fertile research area and an increasingly capable technology for user-friendly and high-performance laboratory and diagnostic tests.

Power-efficient self-cleaning hydrophilic condenser surface for portable exhaled breath condensate (EBC) metabolomic sampling

In this work, we present a hydrophilic self-cleaning condenser surface for the collection of biological and environmental aerosol samples. The condenser is installed in a battery-operated hand-held breath sampling device. The device performance is characterized by the collection and analysis of exhaled breath samples from a group of volunteers. The exhaled breath condensate is collected on a subcooled condenser surface, transferred into a storage vial, and its chemical content is analyzed using mass spectrometric methods. The engineered surface supports upon it a continuous condensation cycle, and this allows the collection of liquid samples exceeding the saturation mass/area limit of a plain hydrophilic surface. The condenser surface employs two constituent parameters: a low surface energy barrier to enhance nucleation and condensation efficiency, and a network of surface microstructures to create a self-cleaning mechanism for fluid aggregation into a reservoir. Removal of the liquid condensate from the condenser surface prevents the formation of a thick liquid layer, and thus maintains a continuous condensation cycle with a minimum decrease in heat transfer efficiency as condensation occurs on the surface. The self-cleaning condenser surfaces may have a number of applications in the collection of biological, chemical, or environmental aerosol samples. Sample phase conversion to liquid can facilitate sample manipulation and chemical analysis of matrices with low concentrations. Here, we demonstrate the use of a self-cleaning microcondenser for the collection of exhaled breath condensate with a hand-held portable device. All breath collections with the two devices were performed with the same group of volunteers under UC Davis IRB protocol 63701-3.

Progress in Integrative Biomaterial Systems to Approach Three-Dimensional Cell Mechanotransduction

Mechanotransduction between cells and the extracellular matrix regulates major cellular functions in physiological and pathological situations. The effect of mechanical cues on biochemical signaling triggered by cell-matrix and cell-cell interactions on model biomimetic surfaces has been extensively investigated by a combination of fabrication, biophysical, and biological methods. To simulate the in vivo physiological microenvironment in vitro, three dimensional (3D) microstructures with tailored bio-functionality have been fabricated on substrates of various materials. However, less attention has been paid to the design of 3D biomaterial systems with geometric variances, such as the possession of precise micro-features and/or bio-sensing elements for probing the mechanical responses of cells to the external microenvironment. Such precisely engineered 3D model experimental platforms pave the way for studying the mechanotransduction of multicellular aggregates under controlled geometric and mechanical parameters. Concurrently with the progress in 3D biomaterial fabrication, cell traction force microscopy (CTFM) developed in the field of cell biophysics has emerged as a highly sensitive technique for probing the mechanical stresses exerted by cells onto the opposing deformable surface. In the current work, we first review the recent advances in the fabrication of 3D micropatterned biomaterials which enable the seamless integration with experimental cell mechanics in a controlled 3D microenvironment. Then, we discuss the role of collective cell-cell interactions in the mechanotransduction of engineered tissue equivalents determined by such integrative biomaterial systems under simulated physiological conditions.

Strategically Designing a Pumpless Microfluidic Device on an "Inert" Polypropylene Substrate with Potential Application in Biosensing and Diagnostics

This study is an attempt to make a step forward to implement the very immature concept of pumpless transportation of liquid into a real miniaturized device or lab-on-chip (LOC) on a plastic substrate. "Inert" plastic materials such as polypropylene (PP) are used in a variety of biomedical applications but their surface engineering is very challenging. Here, it was demonstrated that with a facile innovative wettability patterning route using fluorosilanized UV-independent TiO₂ nanoparticle coating it is possible to create wedge-shaped open microfluidic tracks on inert solid surfaces for low-cost biomedical devices (lab-on-plastic). For the future miniaturization and integration of the tracks into a device, a variety of characterization techniques were used to not only systematically study the surface patterning chemistry and topography but also to have a clear knowledge of its biological interactions and performance. The effect of such surface architecture on the biological performance was studied in terms of static/dynamic protein (bovine serum albumin) adsorption, bacterial (Staphylococcus aureus and Staphylococcus epidermidis) adhesion, cell viability (using HeLa and MCF-7 cancer cell lines as well as noncancerous human fibroblast cells), and cell patterning (Murine embryonic fibroblasts). Strategies are discussed for incorporating such a confined track into a diagnostic device in which its sensing portion is based on protein, microorganism, or cells. Finally, for the proof-of-principle of biosensing application, the well-known high-affinity molecular couple of BSA-antiBSA as a biological model was employed.

Cell Microarray Technologies for High-Throughput Cell-Based Biosensors

Due to the recent demand for high-throughput cellular assays, a lot of efforts have been made on miniaturization of cell-based biosensors by preparing cell microarrays. Various microfabrication technologies have been used to generate cell microarrays, where cells of different phenotypes are immobilized either on a flat substrate (positional array) or on particles (solution or suspension array) to achieve multiplexed and high-throughput cell-based biosensing. After introducing the fabrication methods for preparation of the positional and suspension cell microarrays, this review discusses the applications of the cell microarray including toxicology, drug discovery and detection of toxic agents.

Spatially selective binding of green fluorescent protein on designed organosilane nanopatterns prepared with particle lithography

A practical approach for preparing protein nanopatterns has been to design surface templates of nanopatterns of alkanethiols or organosilanes that will selectively bind and localize the placement of biomolecules. Particle lithography provides a way to prepare millions of protein nanopatterns with a few basic steps. For our nanopatterning strategy, organosilanes with methoxy and sulfhydryl groups were chosen as a surface template. Green fluorescent protein (GFP) was selected as a model for patterning. Areas of 2-[methoxy (polyethyleneoxy)6-9propyl]trichlorosilane (MPT-silane) are effective as a matrix for resisting the attachment of proteins, whereas nanopatterns with sulfur groups provide reactive sites for binding linker groups to connect proteins. A protocol with particle lithography was designed to make a surface template of nanopatterns of (3-mercaptopropyl)trimethoxysilane (MPTMS) surrounded by a methoxy terminated matrix. The sulfhydryl groups of the MPTMS nanopatterns were activated with a sulfosuccinimidyl-4-(N-maleimidomethyl)cyclohexane-1-carboxylate linker. The activated regions of MPTMS furnished sites for binding GFP. Samples were characterized with atomic force microscopy after successive steps of the patterning protocol to evaluate the selectivity of protein binding. Direct views of the protein bound selectively to designated sites of MPTMS are presented, as evidence of robust and reproducible patterning. Nanoscale patterns of proteins can be used for surfaces of biochips and biosensors, and also for immunochemistry test platforms.

High-performance PCB-based capillary pumps for affordable point-of-care diagnostics

Capillary pumps are integral components of passive microfluidic devices. They can displace precise volumes of liquid, avoiding the need for external active components, providing a solution for sample preparation modules in Point-of-Care (PoC) diagnostic platforms. In this work, we describe a variety of high-performance capillary pump designs, suitable for the Lab-on-Printed-Circuit-Board technology (LoPCB). Pumps are fabricated entirely on Printed Circuit Board (PCB) substrates via commercially available manufacturing processes. We demonstrate the concept of LoPCB technology and detail the fabrication method of different architectures of PCB-based capillary pumps. The capillary pumps are combined with microfluidic channels of various hydraulic resistances and characterised experimentally for different micropillar shapes and minimum feature size. Their performance in terms of flow rate is reported. Due to the superhydrophilic properties of oxygen plasma treated FR-4 PCB substrate, the capillary pump flow rates are much higher (138 μL/min, for devices comprising micropillar arrays without preceding microchannel) than comparable devices based on glass, silicon or polymers. Finally, we comment on the technology's prospects, such as incorporating more complicated microfluidic networks that can be tailored for assays.

Microwave Chemical Sensor Using Substrate-Integrated-Waveguide Cavity [corrected]

This research proposes a substrate-integrated waveguide (SIW) cavity sensor to detect several chemicals using the millimeter-wave frequency range. The frequency response of the presented SIW sensor is switched by filling a very small quantity of chemical inside of the fluidic channel, which also causes a difference in the effective permittivity. The fluidic channel on this structure is either empty or filled with a chemical; when it is empty the structure resonates at 17.08 GHz. There is always a different resonant frequency when any chemical is injected into the fluidic channel. The maximum amount of chemical after injection is held in the center of the SIW structure, which has the maximum magnitude of the electric field distribution. Thus, the objective of sensing chemicals in this research is achieved by perturbing the electric fields of the SIW structure.

A Fluidically Tunable Metasurface Absorber for Flexible Large-Scale Wireless Ethanol Sensor Applications

In this paper, a novel flexible tunable metasurface absorber is proposed for large-scale remote ethanol sensor applications. The proposed metasurface absorber consists of periodic split-ring-cross resonators (SRCRs) and microfluidic channels. The SRCR patterns are inkjet-printed on paper using silver nanoparticle inks. The microfluidic channels are laser-etched on polydimethylsiloxane (PDMS) material. The proposed absorber can detect changes in the effective permittivity for different liquids. Therefore, the absorber can be used for a remote chemical sensor by detecting changes in the resonant frequencies. The performance of the proposed absorber is demonstrated with full-wave simulation and measurement results. The experimental results show the resonant frequency increases from 8.9 GHz to 10.04 GHz when the concentration of ethanol is changed from 0% to 100%. In addition, the proposed absorber shows linear frequency shift from 20% to 80% of the different concentrations of ethanol.

Introducing natural thermoplastic shellac to microfluidics: A green fabrication method for point-of-care devices

We present a sustainable fabrication method for cheap point-of-care microfluidic systems, employing hot embossing of natural shellac as a key feature of an energy-efficient fabrication method that exclusively uses renewable materials as consumables. Shellac is a low-cost renewable biomaterial that features medium hydrophilicity (e.g., a water contact angle of ca. 73°) and a high chemical stability with respect to common solvents such as cyclohexane or toluene, rendering it an interesting candidate for low-cost microfluidics and a competitor to well-known systems such as paper-based or polydimethylsiloxane-based microfluidics. Moreover, its high replication accuracy for small features down to 30 μm lateral feature size and its ability to form smooth surfaces (surface roughness Ra  = 29 nm) at low embossing temperatures (glass transition temperature Tg  = 42.2 °C) enable energy-efficient hot embossing of microfluidic structures. Proof-of-concept for the implementation of shellac hot embossing as a green fabrication method for microfluidic systems is demonstrated through the successful fabrication of a microfluidic test setup and the assessment of its resource consumption.

Interfacial Tensions of Aged Organic Aerosol Particle Mimics Using a Biphasic Microfluidic Platform

Secondary organic aerosol (SOA) particles are a major component of atmospheric particulate matter, yet their formation processes and ambient properties are not well understood. These complex particles often contain multiple interfaces due to internal aqueous- and organic-phase partitioning. Aerosol interfaces can profoundly affect the fate of condensable organic compounds emitted into the atmosphere by altering the way in which ambient organic vapors interact with suspended particles. To accurately predict the evolution of SOA in the atmosphere, we must improve our understanding of aerosol interfaces. In this work, biphasic microscale flows are used to measure interfacial tension of reacting methylglyoxal, formaldehyde, and ammonium sulfate aqueous mixtures with a surrounding oil phase. Our experiments show a suppression of interfacial tension as a function of organic content that remains constant with reaction time for methylglyoxal-ammonium sulfate systems. We also reveal an unexpected time dependence of interfacial tension over a period of 48 h for ternary solutions of both methylglyoxal and formaldehyde in aqueous ammonium sulfate, indicating a more complicated behavior of surface activity where there is competition among dissolved organics. From these interfacial tension measurements, the morphology of aged atmospheric aerosols with internal liquid-liquid phase separation is inferred.

Capillary Flow Resistors: Local and Global Resistors

The use of capillary systems in space and biotechnology applications requires the regulation of the capillary flow velocity. It has been observed that constricted sections act as flow resistors. In this work, we also show that enlarged sections temporarily reduce the velocity of the flow. In this work, the theory of the dynamics of capillary flows passing through a constricted or an enlarged channel section is presented. It is demonstrated that the physics of a capillary flow in a channel with a constriction or an enlargement is different and that a constriction acts as a global flow resistor and an enlargement as a local flow resistor. The theoretical results are checked against experimental approaches.

Microfluidic-based photocatalytic microreactor for environmental application: a review of fabrication substrates and techniques, and operating parameters

This article gives an overview of photocatalytic microreactors with an application in environmental science, in particular, the degradation of different toxic dyes within microchannels.