PDMS Microfluidic Channel Research Articles

Optimizing Tris(2-Carboxyethyl)phosphine and Mercaptohexanol Concentrations for Thiolated Oligonucleotide Immobilization on Platinum Electrodes in Microfluidic Platforms.

In this study, we propose a strategy to explore the impact of the proportion of tris(2-carboxyethyl)phosphine (TCEP) and 6-mercaptohexanol (MCH) on the efficiency of oligonucleotide functionalization on PDMS microfluidic channels equipped with pairs of homemade microfabricated platinum microelectrodes. We identified an optimal concentration of these compounds that enables the effective orientation and distribution of probes, thereby facilitating subsequent target hybridization. The experiment included optimizing sample injection into microfluidic channels. We used TCEP as a reducing agent to help the DNA probes adhere to the channel electrode better. This stopped the formation of disulfide bonds during the probe immobilization step. We found the optimal TCEP/MCH mixture ratio (5 mM TCEP and 50 mM MCH), which led to a more uniform distribution and orientation of the DNA probes on the platinum electrode. These optimized conditions resulted in a more compact DNA monolayer and enhanced detection capabilities. The biosensor's performance was evaluated by the detection of the hybridization of complementary DNA sequences in the presence of equimolar Fe(CN)63-/Fe(CN)64-. The detection of the synthetic GP8 resistance gene is facilitated by a measurable decrease in the electron transfer rate, which is directly proportional to its concentration. Under the optimized conditions, the DNA biosensor showed excellent sensitivity (with a detection limit of 10-17 M) and high specificity when tested against noncomplementary DNA strands.

Compact lab-on-printed circuit board (PCB) for free-surfactant silver nanomaterial synthesis

This study introduces a compact Lab-on-Printed Circuit Board (PCB) device featuring thermal integrated microfluidic channels designed for nanoparticle synthesis. Commonly, complex heating chambers are employed to expedite reactions and enhance nanomaterial productivity, yet they pose challenges in temperature control and mixing rates within the reaction chamber. To address these issues, we propose a compact device composed of PDMS microfluidic channels and a PCB platform. The PCB heater, constructed to precisely regulate temperature within the microfluidic channels, utilizes copper lines as heating resistors, while an Arduino kit is employed for temperature measurement and control. Leveraging a Proportional–integral–derivative (PID) controller through programming, the device achieves rapid temperature increase and stable control within the microfluidic channels. Experimental validation demonstrates the operational principles and capabilities of this device, showcasing observable changes in nanoparticle size and morphology. Discussions on these observed results are included in this work.

Fabrication of microfluidic pH sensing chip based on sputtered Sb2O3/Sb thin film working electrode and AgIO3/Ag thin film reference electrode for detection of A549 cells

In this study, we present the fabrication and characterization of a microfluidic pH sensing chip designed to detect human lung cancer (A549) cells. The chip utilizes a Sb2O3/Sb thin film as the working electrode and an AgIO3/Ag thin film as the reference electrode, both deposited using the radio frequency (RF) sputtering technique. The PDMS microfluidic channel was prepared using a 3D-printed mold, followed by curing of PDMS within the mold. The fabricated sensing chip demonstrated excellent linear response (R2 ∼ 0.99) and appreciable Nernstian sensitivity (average = -57.02 ± 1.23 mV/pH, n = 4) in a wide pH range of 1.68–10.01. It exhibited good stability over a 48-hour duration with acceptable drift voltage and negligible temperature impact on the sensing performance. Further, in-vitro pH testing of human lung cancer (A549) cells was performed to investigate the real-time pH measurement ability of the developed microfluidic pH sensing chip.

Microfluidic Capture Device for Simple, Cell Surface Marker-Based Quantification of Senescent CD8+ T Cells

Among human CD8+ T cells, senescent cells are marked by the expression of CD57. The frequency of senescent CD57+CD8+ T cells is significantly correlated with aging and age-associated disorders, and it can be measured by multi-color flow cytometry. However, multi-color flow cytometry presents challenges in terms of accessibility and requires significant resource allocation. Therefore, developing a rapid and straightforward method for quantifying CD57+CD8+ T cells remains a key challenge. This study introduces a microfluidic device composed of a PDMS microfluidic channel with a pre-modified glass substrate for anti-CD8 antibody immobilization. This design allows blood samples to flow through, enabling the selective capture of CD8+ T cells while minimizing the required blood sample volume. This technology enables accurate and reliable quantification of CD57+ cells among captured CD8+ T cells through fluorescence image analysis. The ability of the device to easily quantify senescent CD57+CD8+ T cells is anticipated to contribute significantly to both immunological research and clinical applications.

Development of Liquid-Phase Plasmonic Sensor Platforms for Prospective Biomedical Applications.

Localized Surface Plasmon Resonance (LSPR) is an optical method for detecting changes in refractive index by the interaction between incident light and delocalized electrons within specific metal thin films' localized "hot spots". LSPR-based sensors possess advantages, including their compact size, enhanced sensitivity, cost-effectiveness, and suitability for point-of-care applications. This research focuses on the development of LSPR-based nanohole arrays (NHAs) as a platform for monitoring probe/target binding events in real time without labeling, for low-level biomolecular target detection in biomedical diagnostics. To achieve this objective, this study involves creating a liquid-phase setup for capturing target molecules. Finite-difference time-domain simulations revealed that a 75 nm thickness of gold (Au) is ideal for NHA structures, which were visually examined using scanning electron microscopy. To illustrate the functionality of the liquid-phase sensor, a PDMS microfluidic channel was fabricated using a 3D-printed mold with a glass slide base and a top glass cover slip, enabling reflectance-mode measurements from each of four device sectors. This study shows the design, fabrication, and assessment of NHA-based LSPR sensor platforms within a PDMS microfluidic channel, confirming the sensor's functionality and reproducibility in a liquid-phase environment.

Resonance-Based Sensing of Magnetic Nanoparticles Using Microfluidic Devices with Ferromagnetic Antidot Nanostructures.

We demonstrated resonance-based detection of magnetic nanoparticles employing novel designs based upon planar (on-chip) microresonators that may serve as alternatives to conventional magnetoresistive magnetic nanoparticle detectors. We detected 130 nm sized magnetic nanoparticle clusters immobilized on sensor surfaces after flowing through PDMS microfluidic channels molded using a 3D printed mold. Two detection schemes were investigated: (i) indirect detection incorporating ferromagnetic antidot nanostructures within microresonators, and (ii) direct detection of nanoparticles without an antidot lattice. Using scheme (i), magnetic nanoparticles noticeably downshifted the resonance fields of an antidot nanostructure by up to 207 G. In a similar antidot device in which nanoparticles were introduced via droplets rather than a microfluidic channel, the largest shift was only 44 G with a sensitivity of 7.57 G/ng. This indicated that introduction of the nanoparticles via microfluidics results in stronger responses from the ferromagnetic resonances. The results for both devices demonstrated that ferromagnetic antidot nanostructures incorporated within planar microresonators can detect nanoparticles captured from dispersions. Using detection scheme (ii), without the antidot array, we observed a strong resonance within the nanoparticles. The resonance's strength suggests that direct detection is more sensitive to magnetic nanoparticles than indirect detection using a nanostructure, in addition to being much simpler.

Reconfigurable logic gates in biological crossbar neural networks using STDP learning.

McCulloch and Pitts provided a way to describe brain functions in logic gates. The idea of Boolean neurons made a tremendous contribution to the development of artificial intelligence. However, logic gates based on biological neural networks have never been implemented. To investigate where logic gates can be demonstrated using biological neural networks. We constructed biological crossbar neural networks on multi-electrodes arrays (MEA) using PDMS microfluidic channels. The fluorescence optical images revealed that crossbar array was connected by axons. We measured the activation curves using the stimulus-evoked spiking probability, and found that the threshold voltage of activation curves were affected by the spike-timing-dependent-plasticity (STDP) learning. As a result, reconfigurable logic gates of AND and OR could be successfully implemented through STDP learning. Many neuromorphic systems have the crossbar array structure; thus, our study may contribute to the development of neuromorphic research.

A multiscale, vertical-flow perfusion system with integrated porous microchambers for upgrading multicellular spheroid culture.

Spheroid formation assisted by microengineered chambers is a versatile approach for morphology-controlled three-dimensional (3D) cell cultivation with physiological relevance to human tissues. However, the limitation in diffusion-based oxygen/nutrient transport has been a critical issue for the densely packed cells in spheroids, preventing maximization of cellular functions and thus limiting their biomedical applications. Here, we have developed a multiscale microfluidic system for the perfusion culture of spheroids, in which porous microchambers, connected with microfluidic channels, were engineered. A newly developed process of centrifugation-assisted replica molding and salt-leaching enabled the formation of single micrometer-sized pores on the chamber surface and in the substrate. The porous configuration generates a vertical flow to directly supply the medium to the spheroids, while avoiding the formation of stagnant flow regions. We created seamlessly integrated, all PDMS/silicone-based microfluidic devices with an array of microchambers. Spheroids of human liver cells (HepG2 cells) were formed and cultured under vertical-flow perfusion, and the proliferation ability and liver cell-specific functions were compared with those of cells cultured in non-porous chambers with a horizontal flow. The presented system realizes both size-controlled formation of spheroids and direct medium supply, making it suitable as a precision cell culture platform for drug development, disease modelling, and regenerative medicine.

Effect of microfluidic channel integration onto gold microelectrode on its redox electrochemistry.

Microfluidic systems have attracted significant interest in recent years as they are extensively employed in lab-on-chip and organ-on-chip research. Their combination with electrochemical platforms offers many advantages, promising a high potential for sensing applications, still the microfluidic-channel integration onto electrodes might induce challenges related to changes in signal-to-noise ratios and mass transport conditions. In this study, we investigated the effect of microfluidic channel integration in redox behavior of thermally deposited gold thin film microelectrodes by voltammetric (CV and SWV) electrochemical measurements. Using different dimensions of PDMS microfluidic channels (i.e. widths of 50, 100, 250, and 500 μm) and a constant electrode dimension (200 μm), we analyzed the relationship between altered electroactive area and electrochemical response against target redox molecules. The increases in electroactive area which were determined by the microfluidic channel sizes were in well-correlation with the obtained CV and SWV redox currents as expected. There was no significant decrease in signal-to-noise ratio in microchannel-integrated electrodes. AFM and SEM characterization demonstrated that thermally deposited thin film electrodes had significantly lower (approximately 25 fold) surface roughness in comparison to commercial screen-printed electrodes. Additionally, we have observed a clear microelectrode-to-macroelectrode transition, from hemispherical to linear (planar) diffusion in other terms, with the increasing channel size.

Comparative Study and Simulation of Capacitive Sensors in Microfluidic Channels for Sensitive Red Blood Cell Detection.

Microfluidics provides an indispensable platform for combining analytical operations such as sample preparation, mixing, separation/enrichment, and detection onto a single compact platform, defined as a lab-on-a-chip (LOC) device with applicability in biomedical and life science applications. Due to its ease of integration, 1D interdigital capacitive (IDC) sensors have been used in microfluidic platforms to detect particles of interest. This paper presents a comparative study on the use of capacitive sensors for microfluidic devices to detect bioparticles, more specifically red blood cells (RBCs). The detection sensitivities of 1D, 2D, and 3D capacitive sensors were determined by simulation using COMSOL Multiphysics® v5.5. A water-filled 25 μm × 25 μm PDMS microfluidic channel was used with different sizes (5–10 μm) of red blood cells passing across the capacitive sensor regions. The conformal mapping was used for translating the 1D IDC sensor dimensions into equivalent 2D/3D parallel plate capacitance (PPC) sensor dimensions, creating similar absolute sensor capacitance. The detection sensitivity of each capacitive sensor is determined, and a new 3D PPC sensor structure was proposed to improve the sensitivity for high-resolution RBC detection in microfluidic channels. Proposed 2D and 3D sensors provide a 3× to 20× improvement in sensitivity compared to the standard 1D IDC structures, achieving a 100 aF capacitance difference when a healthy RBC passes in the structure.

3D-Printed SARS-CoV-2 RNA Genosensing Microfluidic System.

Additive manufacturing technology, referred as 3D printing technology, is a growing research field with broad applications from nanosensors fabrication to 3D printing of buildings. Nowadays, the world is dealing with a pandemic and requires the use of simple sensing systems. Here, the strengths of fast screening by a lab‐on‐a‐chip device through electrochemical detection using 3D printing technology for SARS‐CoV‐2 sensing are combined. This system comprises a PDMS microfluidic channel integrated with an electrochemical cell fully 3D‐printed by a 3D printing pen (3D‐PP). The 3D‐PP genosensor is modified with an ssDNA probe that targeted the N gene sequence of SARS‐CoV‐2. The sensing mechanism relies on the electro‐oxidation of adenines present in ssDNA when in contact with SARS‐CoV‐2 RNA. The hybridization between ssDNA and target RNA takes a place and ssDNA is desorbed from the genosensor surface, causing a decrease of the sensor signal. The developed SARS‐CoV‐2/3D‐PP genosensor shows high sensitivity and fast response.

Modeling and Numerical Studies of Three‐Dimensional Conically Shaped Microwells Using Non‐Uniform Photolithography

AbstractConical microwells have found a wide range of applications such as in cancer diagnostics, cell spheroid formation, and three‐dimensional oncology models. Numerous microengineered fabrication techniques have been developed for the formation of such conically shaped microwells. Among many, non‐uniform photolithography (NUPL) in a PDMS microfluidic channel with a glass substrate, can be used to create polymeric microwells with tapered‐bottom and parabolic curvatures. Here, the parabolic well formation of microwells is numerically investigated using NUPL to better understand its key mechanisms. Temporal and spatial free‐radical diffusion are incorporated into the modeling of photopolymerization. In addition, the 3D‐shape tuning ability of NUPL is modeled for the synthesis of microwells through a variation of UV light intensity induced by the presence of opaque materials. The model and simulation results predict the time‐evolution of microwell formation with parabolic well features. The effects of the conical shape‐tuning parameters are numerically determined, i.e., the aspect ratio of the well diameter to channel height and the non‐uniformity of UV light intensity on the microwell depth and parabolic curvature. Numerical work provides meaningful insights into NUPL to optimize and design polymeric microwells with shape features tuned to their application.

Validation of the spatial resolution of super-resolution ultrasound imaging using vessel mimicking microfluidic channels

Super-resolution ultrasound (SRU) imaging is an emerging technology that can identify microvessels with unprecedented spatial resolution beyond the acoustic diffraction limit. Our group introduced a deconvolution-based SRU imaging technology and successfully applied it on mouse and human kidneys for assessing the renal microvascular changes during the progress of kidney diseases. However, a thorough validation on the most achievable spatial resolution has not yet been systematically performed due to the limitation with in vivo imaging including imperfect co-registration between SRU image and ground truth such as histology. In this study, we evaluated the spatial resolution of our SRU imaging algorithm using a custom-designed PDMS microfluidic channels as the ground truth. In the reconstructed SRU image, the two microchannels separated by wall-to-wall distance of 30 μm were successfully identified, which assured the expected spatial resolution of SRU using 15 MHz transducer. The validation using custom-designed microfluidic channels as the ground truth in the scale of tens of microns was performed for the first time to the best of our knowledge for SRU imaging. Such channels of various diameters and separation distances can be an effective tool for the systematic evaluation of the imaging capabilities of different SRU algorithms.

All-Dielectric Metasurface Refractive Index Sensor with Microfluidics

Magnetic resonance and electrical resonance can be excited simultaneously by the incident light source in high refractive index all-dielectric metasurfaces. They can manipulate light waves to a certain extent and can be used in many fields. Silicon is widely used in all-dielectric metasurfaces due to its high refractive index and low loss in near infrared range. In this paper, we use silicon nanodisk structure as refractive index sensor and PDMS microfluidic channels as detection channels to realize liquid refractive index sensing. The magnetic resonance peak of the nanodisk structure is selected for sensing. The position of the magnetic resonance is shifted in the transmission spectrum with the change of the refractive index of the surrounding environment. When the refractive index increases, the resonance peak moves towards the long-wave direction, resulting in red shift, and vice versa. In this paper, the shift of resonance peak reaches 230 nm RIU−1.

Planar hydrodynamic traps and buried channels for bead and cell trapping and releasing.

We present a novel concept for the controlled trapping and releasing of beads and cells in a PDMS microfluidic channel without obstacles present around the particle or in the channel. The trapping principle relies on a two-level microfluidic configuration: a top main PDMS channel interconnected to a buried glass microchannel using round vias. As the fluidic resistances rule the way the liquid flows inside the channels, particles located in the streamlines passing inside the buried level are immobilized by the round via with a smaller diameter, leaving the object motionless in the upper PDMS channel. The particle is maintained by the difference of pressure established across its interface and acts as an infinite fluidic resistance, virtually cancelling the subsequent buried fluidic path. The pressure is controlled at the outlet of the buried path and three modes of operation of a trap are defined: idle, trapping and releasing. The pressure conditions for each mode are defined based on the hydraulic–electrical circuit equivalence. The trapping of polystyrene beads in a compact array of 522 parallel traps controlled by a single pressure was demonstrated with a trapping efficiency of 94%. Pressure conditions necessary to safely trap cells in holes of different diameters were determined and demonstrated in an array of 25 traps, establishing the design and operation rules for the use of planar hydrodynamic traps for biological assays.

Nanoadhesive layer to prevent protein absorption in a poly(dimethylsiloxane) microfluidic device.

Poly(dimethylsiloxane) (PDMS) is widely used as a microfluidics platform material; however, it absorbs various molecules, perturbing specific chemical concentrations in microfluidic channels. We present a simple solution to prevent adsorption into a PDMS microfluidic device. We used a vapor-phase-deposited nanoadhesive layer to seal PDMS microfluidic channels. Absorption of fluorescent molecules into PDMS was efficiently prevented in the nanolayer-treated PDMS device. Importantly, when cultured in a nanolayer-treated PDMS device, yeast cells exhibited the expected concentration-dependent response to a mating pheromone, including mating-specific morphological and gene expression changes, while yeast cultured in an untreated PDMS device did not properly respond to the pheromone. Our method greatly expands microfluidic applications that require precise control of molecule concentrations.

Rapid Assessment of Surface Markers on Cancer Cells Using Immuno-Magnetic Separation and Multi-frequency Impedance Cytometry for Targeted Therapy

The rapid qualitative assessment of surface markers on cancer cells can allow for point-of-care prediction of patient response to various cancer drugs. Preclinical studies targeting cells with an antibody to “activated” matriptase conjugated to a potent toxin show promise as a selective treatment for a variety of solid tumors. In this paper, we implemented a novel technique for electrical detection of proteins on surfaces of cancer cells using multi-frequency microfluidic impedance cytometry. The biosensor, consists of two gold microelectrodes on a glass substrate embedded in a PDMS microfluidic channel, is used in conjugation with immuno-magnetic separation of cancer cells, and is capable of differentiating between bare magnetic beads, cancer cells and bead-cell aggregates based on their various impedance and frequency responses. We demonstrated proof-of-concept based on detection of “activated” matriptase proteins on the surface of cultured Mantle cells.

Experimental testing for metrological traceability and accuracy of liquid microflows and microfluidics

Micro-flows are growing more important and useful for a wide variety of scientific and engineering fields such as bioanalysis, drug development and administration, Organ-on-a-Chip, etc, but accurate and reliable measurements traceable to the International System of Units can be challenging in micro-to-femto flow operating ranges. In this paper a gravimetric method has been used to quantify the measurement error of the volumetric flow rate of water from 1000 μL/h to 1 μL/h delivered by a syringe pump in different experimental conditions that replicate the current microfluidics setups. Several sources of error have been determined such as the mass measurement, the fluid evaporation dependent on the gravimetric methodology implemented and the repeatability, believed to be closely related to the operating mode of the stepper motor and drive screw pitch of the syringe pump. The elastic deformability of the medical grade 1 mL polypropylene syringes commonly used in micro-flows has greatly affected the volumetric flow rate, unlike the glass syringes tested. Finally, testing the gravimetric method while adding PDMS microfluidic channels to the flow circuit served to demonstrate that material, dimensions and shape of the microchannels did not show significant influence on the volumetric flow rate error for the flow rates imposed, important to establish well defined methodologies for microfluidics.

Surface Modification with Gallium Coating as Nonwetting Surfaces for Gallium-Based Liquid Metal Droplet Manipulation

We report gallium (Ga) coating as a simple approach to convert most common microfluidic substrates to nonwetting surfaces against surface-oxidized gallium-based liquid metal alloys. These alloys are readily oxidized in ambient air and adhere to almost all surfaces, which imposes significant challenges in mobilizing liquid metal droplets without leaving residue. Various flat substrates (e.g., PDMS, Si, SiO2, SU-8, glass, and parylene-C coated PDMS) were coated with thin film (75-200 nm in thickness) of gallium by evaporation and the coated gallium formed nanoscale uneven and rough surface through Ostwald ripening with its surface covered with oxide shell. Static and dynamic contact angles of thegallium-coated surfaces were found to be greater than 160°, while dynamic contact angle measurements showed contact angle hysteresis in the range of 6.5-24.4°. Surface-oxidized gallium-based liquid metal alloy droplets were shown to bounce off and roll on the gallium-coated surfaces without leaving any residue which confirms the nonwettability of the gallium-coated flat surfaces. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) showed the gallium-coated flat substrates consist of nanoscale hemispherical structures with average surface roughness of 33.8 nm. Pneumatic actuation of surface-oxidized liquid metal droplets in PDMS microfluidic channels coated with gallium was conducted to confirm the feasibility of utilizing gallium coating as an effective surface modification for surface-oxidized gallium-based liquid metal droplet manipulation.

Aptamer surface functionalization of microfluidic devices using dendrimers as multi-handled templates and its application in sensitive detections of foodborne pathogenic bacteria

A microfluidic system that incorporates both dendrimers and aptamers to detect E. coli O157:H7 is developed. To achieve this, generation 7-polyamidoamine dendrimers were immobilized onto the detection surfaces of PDMS microfluidic channels; subsequently aptamers against E. coli O157:H7 were conjugated onto the microchannel surfaces via the immobilized dendrimers as templates. Surface modifications were characterized by FTIR, XPS, water contact angles, fluorescence microscopy and AFM to confirm the success of each surface modification steps. The efficacy of this simple microchannel in detection was investigated using E. coli O157:H7 spiked samples. Our results showed that this interesting approach significantly increased the amount of aptamers available on the microfluidic channel surfaces to capture E. coli O157:H7 cells to allow sensitive detection, which in turn resulted in detections of E. coli O157:H7 cells at a low limit of detection of 102 cells mL−1. The results also demonstrated that in comparison with the generation 4-polyamidoamine dendrimers (G4) modified microchannels, those modified with G7 showed enhanced detection signals, improved target capturing efficiencies, and at higher throughput. This simple whole cell detection design has not been reported in the literature and it is an interesting and effective approach to developing a sensitive and rapid detection platform for foodborne pathogenic bacteria.