Polydimethylsiloxane Devices Research Articles

Atmospheric pressure atomic layer deposition for in-channel surface modification of PDMS microfluidic chips

Polydimethylsiloxane (PDMS) is one of the materials of choice for the fabrication of microfluidic chips. However, its broad application is constrained by its incompatibility with common organic solvents and the absence of surface anchoring groups for surface functionalization. Current solutions involving bulk-, ex-situ surface-, and in-situ liquid phase modifications are limited and practically demanding. In this work, we present a simple, novel strategy to deposit a metal oxide nano-layer on the inside of bonded PDMS microfluidic channels using atmospheric pressure atomic layer deposition (AP-ALD). Using three important classes of microfluidic experiments, i.e., (i) the production of micron-sized particles, (ii) the cultivation of biological cells, and (iii) the photocatalytic degradation in continuous flow chemistry, we demonstrate that the metal oxide nano-layer offers a higher resistance against organic solvent swelling, higher hydrophilicity, and a higher degree of further functionalization of the wall. We demonstrate the versatility of the approach by not only depositing SiOx nano-layers, but also TiOx nano-layers, which in the case of the flow chemistry experiment were further functionalized with gold nanoparticles through the use of AP-ALD. This study demonstrates AP-ALD as a tool to broaden the applicability of PDMS devices.

Luteinizing Hormone-Releasing Hormone (LHRH)-Conjugated Cancer Drug Delivery from Magnetite Nanoparticle-Modified Microporous Poly-Di-Methyl-Siloxane (PDMS) Systems for the Targeted Treatment of Triple Negative Breast Cancer Cells.

This study presents LHRH conjugated drug delivery via a magnetite nanoparticle-modified microporous Poly-Di-Methyl-Siloxane (PDMS) system for the targeted suppression of triple-negative breast cancer cells. First, the MNP-modified PDMS devices are fabricated before loading with targeted and untargeted cancer drugs. The release kinetics from the devices are then studied before fitting the results to the Korsmeyer-Peppas model. Cell viability and cytotoxicity assessments are then presented using results from the Alamar blue assay. Apoptosis induction is then elucidated using flow cytometry. The in vitro drug release studies demonstrated a sustained and controlled release of unconjugated drugs (Prodigiosin and paclitaxel) and conjugated drugs [LHRH conjugated paclitaxel (PTX+LHRH) and LHRH-conjugated prodigiosin (PG+LHRH)] from the magnetite nanoparticle modified microporous PDMS devices for 30 days at 37 °C, 41 °C, and 44 °C. At 24, 48, 72, and 96 h, the groups loaded with conjugated drugs (PG+LHRH and PTX+LHRH) had a significantly higher (p < 0.05) percentage cell growth inhibition than the groups loaded with unconjugated drugs (PG and PTX). Additionally, throughout the study, the MNP+PDMS (without drug) group exhibited a steady rise in the percentage of cell growth inhibition. The flow cytometry results revealed a high incidence of early and late-stage apoptosis. The implications of the results are discussed for the development of biomedical devices for the localized and targeted release of cancer drugs that can prevent cancer recurrence following tumor resection.

Surface-segregating zwitterionic copolymers to control poly(dimethylsiloxane) surface chemistry.

The use of microfluidic devices in biomedicine is growing rapidly in applications such as organs-on-chip and separations. Polydimethylsiloxane (PDMS) is the most popular material for microfluidics due to its ability to replicate features down to the nanoscale, flexibility, gas permeability, and low cost. However, the inherent hydrophobicity of PDMS leads to the adsorption of macromolecules and small molecules on device surfaces. This curtails its use in "organs-on-chip" and other applications. Current technologies to improve PDMS surface hydrophilicity and fouling resistance involve added processing steps or do not create surfaces that remain hydrophilic for long periods. This work describes a novel, simple, fast, and scalable method for improving surface hydrophilicity and preventing the nonspecific adsorption of proteins and small molecules on PDMS through the use of a surface-segregating zwitterionic copolymer as an additive that is blended in during manufacture. These highly branched copolymers spontaneously segregate to surfaces and rearrange in contact with aqueous solutions to resist nonspecific adsorption. We report that mixing a minute amount (0.025 wt%) of the zwitterionic copolymer in PDMS considerably reduces hydrophobicity and nonspecific adsorption of proteins (albumin and lysozyme) and small molecules (vitamin B12 and reactive red). PDMS blended with these zwitterionic copolymers retains its mechanical and physical properties for at least six months. Moreover, this approach is fully compatible with existing PDMS device manufacture protocols without additional processing steps and thus provides a low-cost and user-friendly approach to fabricating reliable biomicrofluidics.

Design and fabrication of a polydimethylsiloxane device for evaluating the effect of pillar geometry and configuration in the flow separation using deterministic lateral displacement.

The advancement of microfluidics and the manufacturing of microdevices has led to a strategic change in the biomedical industry. The flow through narrow channels and the pillars are placed strategically, leading to the phenomenon of particle separation through deterministic lateral displacement (DLD). In such a phenomenon, the shape, size, location and orientation of the obstacles play an important role. For the first time, particle separation is achieved with DLD modules having high row shift angles of 25°, 30° and 35°, reducing the number of pillars. The significance of circular and triangular micropillars executing deterministic lateral displacement, oriented at different angles, has been investigated, and it is found that the triangular pillars oriented at 75° resulted in better separation compared to the other configurations. In this report, the fabrication, location, orientation of the micropillars and the selection of appropriate process parameters are detailed. The structures are fabricated on silicon wafers using the standard photolithography process followed by the deep reactive ion etching process. These dies are further used to fabricate the polydimethylsiloxane-based microfluidic chips. These fabricated devices are characterised by their size, structure and quality using 3D microscopy and scanning electron microscopy. Further, blood plasma separation is carried out using the devices fabricated in this work, and the particles at the inlet and outlets are evaluated using microscopy and a novel image processing technique, replacing the use of a hemocytometer. The path traced by the particles at different flow conditions is numerically evaluated and validated with experiments. The novel device is capable of separating blood cells from plasma with a recovery factor varying from 44% to 100%. PDMS-PDMS bonding experiments using oxygen and argon plasma have been carried out to evaluate the maximum bond strength and flow velocity in the devices. It is observed that the oxygen plasma results in a bond strength of 0.404 N mm-1, thus a high throughput of 135.34 μL s-1 is achieved using the fabricated device.

Compound Absorption in Polymer Devices Impairs the Translatability of Preclinical Safety Assessments.

Organotypic and microphysiological systems (MPS) that can emulate the molecular phenotype and function of human tissues, such as liver, are increasingly used in preclinical drug development. However, despite their improved predictivity, drug development success rates have remained low with most compounds failing in clinical phases despite promising preclinical data. Here, it is tested whether absorption of small molecules to polymers commonly used for MPS fabrication can impact preclinical pharmacological and toxicological assessments and contribute to the high clinical failure rates. To this end, identical devices are fabricated from eight different MPS polymers and absorption of prototypic compounds with different physicochemical properties are analyzed. It is found that overall absorption is primarily driven by compound hydrophobicity and the number of rotatable bonds. However, absorption can differ by >1000-fold between polymers with polydimethyl siloxane (PDMS) being most absorptive, whereas polytetrafluoroethylene (PTFE) and thiol-ene epoxy (TEE) absorbed the least. Strikingly, organotypic primary human liver cultures successfully flagged hydrophobic hepatotoxins in lowly absorbing TEE devices at therapeutically relevant concentrations, whereas isogenic cultures in PDMS devices are resistant, resulting in false negative safety signals. Combined, these results can guide the selection of MPS materials and facilitate the development of preclinical assays with improved translatability.

Comparison of PDMS and NOA Microfluidic Chips: Deformation, Roughness, Hydrophilicity and Flow Performance.

Microfluidic devices are frequently manufactured with polydimethylsiloxane (PDMS) due to its affordability, transparency, and simplicity. However, high-pressure flow through PDMS microfluidic channels lead to an increase in channel size due to the compliance of the material. As a result, longer response times are required to reach steady flow rates, which increases the overall time required to complete experiments when using a syringe pump. Due to its excellent optical properties and increased rigidity, Norland Optical Adhesive (NOA) has been proposed as a promising material candidate for microfluidic fabrication. This study compares the compliance and deformation properties of three different characteristic sized (width of parallel channels: 100, 40 and 20 µm) microfluidic devices made of PDMS and NOA. The comparison of the microfluidics devices is made based on the Young's modulus, roughness, contact angle, channel width deformation, flow resistance and compliance. The experimental resistance is estimated through the measurement of the flow at a given pressure and a precision flow meter. The characteristic time of the system is extracted by fitting the two-element resistance-compliance (RC) hydraulic circuit model. The compliance of the microfluidics chips is estimated through the measurement of the characteristic time required for channels to achieve an output flow rate equivalent to that of the input flow rate using a syringe pump and a precision flow meter. The Young modulus was found to be 2 MPa for the PDMS and 1743 MPa for the NOA 63. The surface roughness was found to be higher for the NOA 63 than for the PDMS. The hydrophilicities of materials were found comparable with and without plasma treatment. The results show that NOA devices have lower compliance and deformation than PDMS devices.

Scalable Processing of Cyclic Olefin Copolymer (COC) Microfluidic Biochips.

Microfluidics evolved with the appearance of polydimethylsiloxane (PDMS), an elastomer with a short processing time and the possibility for replication on a micrometric scale. Despite the many advantages of PDMS, there are well-known drawbacks, such as the hydrophobic surface, the absorption of small molecules, the low stiffness, relatively high cost, and the difficulty of scaling up the fabrication process for industrial production, creating a need for alternative materials. One option is the use of stiffer thermoplastics, such as the cyclic olefin copolymer (COC), which can be mass produced, have lower cost and possess excellent properties. In this work, a method to fabricate COC microfluidic structures was developed. The work was divided into process optimization and evaluation of material properties for application in microfluidics. In the processing step, moulding, sealing, and liquid handling aspects were developed and optimized. The resulting COC devices were evaluated from the point of view of molecular diffusion, burst pressure, temperature resistance, and susceptibility to surface treatments and these results were compared to PDMS devices. Lastly, a target DNA hybridization assay was performed showing the potential of the COC-based microfluidic device to be used in biosensing and Lab-on-a-Chip applications.

Penetration Coefficients of Commercial Nanolimes and a Liquid Mineral Precursor for Pore-Imitating Test Systems—Predictability of Infiltration Behavior

Nanolimes have been commercially available for over a decade as a remineralization agent for natural stone to combat deterioration. While they have been applied successfully and studied extensively, their penetration abilities in different materials have not yet been readily quantifiable in situ and in real time. Using two transparent pore-imitating test systems (acrylic glass (PMMA) and polydimethylsiloxane (PDMS)) and light microscopy, the penetration coefficients (PCs) of two nanolimes (CaLoSiL (CLS) and Nanorestore Plus (NRP)), as well as their solvents, were determined experimentally in square channels of about 100 µm diameter. Their PCs and those for a previously published glass-resin-based test system were also predicted based on measurable material parameters or literature values using the Lucas-Washburn equation. Additionally, a liquid mineral precursor (LMP) of calcium carbonate based on complex coacervation (CC) was investigated as an alternative to the solid particle dispersions of nanolime. In general, the dispersions behaved like their pure solvents. Overall, trends could be reasonably well predicted with both literature and experimentally determined properties using the Lucas-Washburn equation. In absolute terms, the prediction of observed infiltration behavior was satisfactory for alcohols and nanolimes but deviated substantially for water and the aqueous LMP. The commercially available PMMA chips and newly designed PDMS devices were mostly superior to the previously published glass-resin-based test system, except for the long-term monitoring of material deposition. Lastly, the transfer of results from these investigated systems to a different, nontransparent mineral, calcite, yielded similar PC values independently of the original data when used as the basis for the conversion (all PC types and all material/liquid combinations except aqueous solutions in PDMS devices). This knowledge can be used to improve the targeted design of tailor-made remineralization treatments for different application cases by guiding solvent choice, and to reduce destructive sampling by providing a micromodel for pretesting, if transferability to real stone samples proves demonstrable in the future.

A Colorimetric Method for the Rapid Estimation of the Total Cannabinoid Content in Cannabis Samples.

A colorimetric method for the estimation of the total content of cannabinoids in cannabis samples is proposed. The assay is based on the reaction of these compounds with the reagent Fast Blue B (FBB), which has been immobilized into polydimethylsiloxane (PDMS). The reaction and detection conditions have been established according to the results obtained for the individual cannabinoids Δ9-tetrahydrocannabidiol (THC), cannabidiol (CBD), and cannabinol (CBN), as well as for ethanolic extracts obtained from cannabis samples after ultrasonication. In contact with the extract and under basic conditions, the reagent diffuses from the PDMS device, producing a red-brown solution. The absorbances measured at 500 nm after only 1 min of exposure to the FBB/PDMS composites led to responses proportional to the amounts of the cannabinoids in the reaction media. Those absorbances have been then transformed in total cannabinoid content using CBD as a reference compound. The potential utility of the proposed conditions has been tested by analyzing different cannabis samples. The selectivity towards other plants and drugs has been also evaluated. The present method is proposed as a simple and rapid alternative to chromatographic methods for the estimation of the total content of cannabinoids.

Hybrid machining of P20 die steel masters for the development of polymer-based microfluidic devices to study the effect of surface roughness on bacterial activity

Compared to the traditional machining and finishing process, a hybrid technique consisting of mechanical micro-milling and electropolishing is advantageous in maintaining dimensional accuracy and fine surface quality without compromising the form. This is a cost-effective, less tedious technique that can fabricate high-quality masters for microfluidic devices. P20-die steel masters are fabricated to evaluate the method with this technique. The dimensional accuracy and surface quality of the masters are studied. It is found that the peak-to-valley surface roughness (Rt) of the masters is reduced by 50%. These die steel masters are then used to make polydimethylsiloxane (PDMS)-based microfluidic devices that could be used for particle separation. As biofouling and bacterial growth are undesirable in most of the microfluidic devices, the fabricated PDMS devices are tested for bacterial growth and adhesion. The growth of DH5α Escherichia coli bacteria in the devices fabricated using the electropolished masters is evaluated. Bacterial growth and adhesion are monitored for 0, 4 and 12 h, and it is found that the growth and adhesion in these devices are reduced by 15–20% and 40–50%, respectively, compared to the devices produced using non-electropolished masters. Enumeration of the bacterial cells in the samples flown is carried out by evaluating the optical density of the sample fluids by UV-visible spectroscopy and quantifying the bacterial cells using the McFarland 0.5 standard.

Long-Term Culture and Monitoring of Isolated Caenorhabditis elegans on Solid Media in Multi-Well Devices.

The nematode Caenorhabditis elegans is among the most common model systems used in aging research owing to its simple and inexpensive culture techniques, rapid reproduction cycle (~3 days), short lifespan (~3 weeks), and numerous available tools for genetic manipulation and molecular analysis. The most common approach for conducting aging studies in C. elegans, including survival analysis, involves culturing populations of tens to hundreds of animals together on solid nematode growth media (NGM) in Petri plates. While this approach gathers data on a population of animals, most protocols do not track individual animals over time. Presented here is an optimized protocol for the long-term culturing of individual animals on microfabricated polydimethylsiloxane (PDMS) devices called WorMotels. Each device allows up to 240 animals to be cultured in small wells containing NGM, with each well isolated by a copper sulfate-containing moat that prevents the animals from fleeing. Building on the original WorMotel description, this paper provides a detailed protocol for molding, preparing, and populating each device, with descriptions of common technical complications and advice for troubleshooting. Within this protocol are techniques for the consistent loading of small-volume NGM, the consistent drying of both the NGM and bacterial food, options for delivering pharmacological interventions, instructions for and practical limitations to reusing PDMS devices, and tips for minimizing desiccation, even in low-humidity environments. This technique allows the longitudinal monitoring of various physiological parameters, including stimulated activity, unstimulated activity, body size, movement geometry, healthspan, and survival, in an environment similar to the standard technique for group culture on solid media in Petri plates. This method is compatible with high-throughput data collection when used in conjunction with automated microscopy and analysis software. Finally, the limitations of this technique are discussed, as well as a comparison of this approach to a recently developed method that uses microtrays to culture isolated nematodes on solid media.

Visualization of Flow-Induced Strain Using Structural Color in Channel-Free Polydimethylsiloxane Devices.

Measuring flow of gases is of fundamental importance yet is typically done with complex equipment. There is, therefore, a longstanding need for a simple and inexpensive means of flow measurement. Here, gas flow is measured using an extremely simple device that consists of an Ar plasma-treated polydimethylsiloxane (PDMS) slab adhered on a glass substrate with a tight seal. This device does not even have a channel, instead, gas can flow between the PDMS and the glass by deforming the PDMS wall, in other words, by making an interstice as a temporary path for the flow. The formation of the temporary path results in a compressive bending stress at the inner wall of the path, which leads to the formation of well-ordered wrinkles, and hence, the emergence of structural color that changes the optical transmittance of the device. Although it is very simple, this setup works sufficiently well to measure arbitrary gases and analyzes their flow rates, densities, and viscosities based on the change in color. It is also demonstrated that this technique is applicable to the flow-induced display of a pattern such as a logo for advanced applications.

A simple and accessible approach for processing photopolymer master molds for the fabrication of microfluidic polydimethylsiloxane devices

The use of three-dimensional (3D) printing for fabrication of master molds for microfluidic devices is very attractive due to its availability and simplicity and replaces the standard methods of soft lithography. However, the commercially available photopolymer resins inhibit the curing of polydimethylsiloxane (PDMS), preventing reliable replication of 3D printed master mold structures. Here, we present a simple and safe method to post-process 3D printed photopolymer master molds for PDMS microfluidic devices. This approach expands the possibilities of prototyping microfluidic PDMS devices for a wider research community without complex post-processing tools currently required for fabrication of 3D photopolymer master molds.

PDMS 마이크로 스텐실이 내재된 세포공동배양 미세유체 장치 제작

In this paper, a microfluidic co-culture system comprising an embedded polydimethylsiloxane (PDMS) microstencil was fabricated. The fabricated co-culture system has two micro-channels separated with a PDMS microstencil membrane. Master molds for microchannels and stencil membranes were fabricated by photolithography, then used for casting of PDMS devices. The stencil membrane was 10 thick, with holes 10-μm large in diameter. The fabricated system co-cultured two types of cells (HepG2, NIH-3T3 Cells) successfully for seven days. The viability and stability of the cells were verified through LIVE/DEAD® staining and analysis. Additionally, albumin secretion of HepG2 cells was measured for seven days, using an HSA ELISA kit. The measured data were analyzed, to compare the activity of HepG2 cells. Results confirmed that cells can be co-cultured in the fabricated microfluidic system.

Application of perfluoropolyether elastomers in microfluidic drug metabolism assays

Recently, increasing attention has been paid to liver-on-a-chip models for both pharmacokinetics and toxicity (ADMET) screenings. Although polydimethylsiloxane (PDMS) is the most popular material for the fabrication of microfluidic devices, its extensive sorption of hydrophobic drugs limits its applications. Therefore, we investigated a chemically repellent material, perfluoropolyether (PFPE) elastomer, as an alternative to PDMS. Primary rat hepatocytes cultured in the PFPE microfluidic device were polygonal or cuboidal in shape and had one or two prominent nuclei, as when cultured in 96-well plates. When hepatocytes were cultured in the PFPE microfluidic device and exposed to dynamic flow, the production of albumin and urea increased 3.94- and 1.72-fold, respectively, compared with no dynamic flow. Exposure to dynamic flow did not result in obvious changes in the expression of cytochrome P450, but increased the metabolic activity of hepatocytes compared to under static conditions. PFPE devices did not absorb midazolam, which was extensively absorbed by PDMS devices. However, the sorption of bufuralol could not be avoided even with PFPE devices. Solvent swelling experiments highlighted much better chemical repellency with PFPE than with PDMS. Hansen solubility parameters and sphere radius were estimated from the solvent swelling experiments. The relative energy distance (RED) of bufuralol to PFPE was much smaller than that of other three drugs tested, reasonably explaining the high sorption of bufuralol to PFPE. Although sorption into PFPE cannot be completely avoided, PFPE microfluidic devices may provide a better performance in ADMET evaluation than PDMS.

Microfluidic Processing of Piezoelectric and Magnetic Responsive Electroactive Microspheres.

Poly(vinylidene fluoride) (PVDF) combined with cobalt ferrite (CFO) particles is one of the most common and effective polymeric magnetoelectric composites. Processing PVDF into its electroactive phase is a mandatory condition for featuring electroactive behavior and specific (post)processing may be needed to achieve this state, although electroactive phase crystallization is favored at processing temperatures below 60 °C. Different techniques are used to process PVDF-CFO nanocomposite structures into microspheres with high CFO dispersion, with microfluidics adding the advantages of high reproducibility, size tunability, and time and resource efficiency. In this work, magnetoelectric microspheres are produced in a one-step approach. We describe the production of high content electroactive phase PVDF and PVDF-CFO microspheres using microfluidic technology. A flow-focusing polydimethylsiloxane device is fabricated based on a 3D printed polylactic acid master, which enables the production of spherical microspheres with mean diameters ranging from 80 to 330 μm. The microspheres feature internal and external cavernous structures and good CFO distribution with an encapsulation efficacy of 80% and prove to be in the electroactive γ-phase with a mean content of 75%. The microspheres produced using this approach show suitable characteristics as active materials for tissue regeneration strategies and other piezoelectric polymer applications.

Abstract 6031: Microfluidics: Simulating the gut microbiome for early cancer detection

Abstract Background: Cancer is the second leading cause of death globally and ~39.5% of people will be diagnosed with cancer at some point during their lifetimes. Thus, there is an unmet need to identify novel strategies for early cancer detection and prevention. The emerging evidence suggests that the gut microbiome has a role in promoting cancer. This microbiome including bacteria plays a vital role in maintaining homeostasis in the body. An imbalance in bacterial composition may cause diseases including cancer. Here we developed a microfluidic chip that can accurately simulate the gut microbiome to test the effects of bacteria and therapies on cancer cells. Methods and Results: To test the causal effect of bacteria on cancer, we developed a new high-throughput microfluidic device for simulating the environment of the gut. Initially, we used the photolithography technique where we designed the chip in AutoCAD and fabricated using photoresist resins and Polydimethylsiloxane (PDMS). Next, we tested the effect of bacteria on the growth of colorectal cancer cells. For this, we cultured colorectal cancer cells (HCT-116) with lipopolysaccharide (LPS), which is found in the outer membrane of bacteria, as well as the Bacillus bacteria in our microfluidics. Our data show that both LPS and Bacillus significantly accelerate the growth of cancer cells 2.02 times (p value = 0.012) and 1.58 times (p value = 0.011), respectively, over a 4 day culture period. These results show that the increased presence of certain bacteria can promote cancer cell growth and that our chip can be used to test the specific correlation between bacteria and cancer cell growth. The previously described method was inefficient and time-consuming. To overcome this limitation, we designed a new chip that allows running 16 samples at once with improved efficiency and accuracy. The template of the device that had 16 microfluidic channels was printed by a 3D printer and used for PDMS replica molding. The PDMS device was attached to the modified multiwell plate to feed media to and collect waste from each channel in a high-throughput manner. In the initial design, the bacteria grew faster than cancer cells taking over the chips. Our new design has dual layered chambers to keep bacteria and cancer cells separated by a membrane, allowing only bacterial secretions to pass through the membrane to cancer cells, mimicking the human gut. The new design also allowed the chip to maintain continuous microfluidic flow and a hypoxic environment. Conclusion: Our research demonstrates that the new microfluidic device has broader implications including simulating other body organs such as the lung and liver, and testing the impact of viruses such as influenza and COVID-19 on human cells. This device can be used to test both the effect of bacteria and new treatment on clinical samples for the identification of personalized therapy, thus reducing the need for mouse model testing, which is a lengthy and expensive process. Citation Format: Ekansh Mittal, Young Bock Kang. Microfluidics: Simulating the gut microbiome for early cancer detection [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 6031.

New In Vitro Model to Study Multicellular and Flow Control of Blood‐Brain Barrier

Brain endothelial cells of the blood‐brain barrier (BBB) have been shown to be regulated by supportive cells, such as pericytes and astrocytes, and shear stress exposure. However, studies investigating the impact of pericytes and astrocytes on brain endothelial cell function have identified both beneficial and detrimental results. Additionally, most studies investigating the relationship between shear stress and brain endothelial cell function lack physiological relevance via the use of sub‐physiological shear stress magnitudes and/or via the absence of pericytes and astrocytes. In this study, we developed a millifluidic device compatible with standard transwell inserts to investigate BBB function. In contrast to standard polydimethylsiloxane (PDMS) microfluidic devices, this model allows for easy, reproducible shear stress exposure without common limitations of PDMS devices such as inadequate nutrient diffusion and air bubble formation. In no‐flow conditions, we first used the device to examine the impact of primary human pericytes and astrocytes on human brain microvascular endothelial cell (HBMEC) barrier integrity. We found that astrocytes, pericytes, and a 1:1 ratio of both cell types increased HBMEC barrier integrity via reduced permeability to 40 kDa fluorescent dextran, which was associated with increased expression of tight junction protein, claudin‐5. Interestingly, we also observed a significantly lower permeability to 3 kDa dextran in HBMEC‐pericyte co‐cultures compared to HBMEC‐astrocyte and HBMEC‐astrocyte‐pericyte co‐cultures. Based on these findings, we hypothesize pericytes may be providing increased barrier support to the BBB model compared to astrocytes although they both function as permeability reducers. After using the device to generate 24‐hour flow at 12 dynes/cm2, we observed that shear stress exposure significantly reduced dextran permeability in HBMEC monolayers, but not in tri‐culture models consisting of HBMECs, pericytes, and astrocytes. These results indicate that co‐cultures may demonstrate a more pronounced impact on overall BBB permeability than flow exposure. In both cases, flow exposure was interestingly associated with reduced expression of both claudin‐5 and occludin. However, ZO‐1 expression, and localization at cell‐cell junctions increased in the tri‐culture but exhibited no apparent change in the HBMEC monolayer. Under flow conditions, we also observed alignment of HBMECs in the tri‐culture while no such phenomenon was observed in HBMEC monolayers, indicating supportive cells and flow are both essential to observe brain endothelial cell alignment in vitro. Collectively, these results support the necessity of physiologically relevant, multicellular BBB models when investigating brain endothelial cell function in relation to the BBB. Additionally, our findings provide clues on the role of shear stress and supportive cells within the BBB, a critical step to elucidating the physiology of the neurovascular unit.

Hydrophobic Recovery of PDMS Surfaces in Contact with Hydrophilic Entities: Relevance to Biomedical Devices.

Polydimethylsiloxane (PDMS), a silicone elastomer, is increasingly being used in health and biomedical fields due to its excellent optical and mechanical properties. Its biocompatibility and resistance to biodegradation led to various applications (e.g., lung on a chip replicating blood flow, medical interventions, and diagnostics). The many advantages of PDMS are, however, partially offset by its inherent hydrophobicity, which makes it unsuitable for applications needing wetting, thus requiring the hydrophilization of its surface by exposure to UV or O2 plasma. Yet, the elastomeric state of PDMS translates in a slow, hours to days, process of reducing its surface hydrophilicity—a process denominated as hydrophobic recovery. Using Fourier transform infrared spectroscopy (FTIR) and atomic force microscopy (AFM), the present study details the dynamics of hydrophobic recovery of PDMS, on flat bare surfaces and on surfaces embedded with hydrophilic beads. It was found that a thin, stiff, hydrophilic, silica film formed on top of the PDMS material, following its hydrophilization by UV radiation. The hydrophobic recovery of bare PDMS material is the result of an overlap of various nano-mechanical, and diffusional processes, each with its own dynamics rate, which were analyzed in parallel. The hydrophobic recovery presents a hysteresis, with surface hydrophobicity recovering only partially due to a thin, but resilient top silica layer. The monitoring of hydrophobic recovery of PDMS embedded with hydrophilic beads revealed that this is delayed, and then totally stalled in the few-micrometer vicinity of the embedded hydrophilic beads. This region where the hydrophobic recovery stalls can be used as a good approximation of the depth of the resilient, moderately hydrophilic top layer on the PDMS material. The complex processes of hydrophilization and subsequent hydrophobic recovery impact the design, fabrication, and operation of PDMS materials and devices used for diagnostics and medical procedures. Consequently, especially considering the emergence of new surgical procedures using elastomers, the impact of hydrophobic recovery on the surface of PDMS warrants more comprehensive studies.

Projection micro-stereolithography versus master–slave approach to manufacture a micro-optofluidic device for slug flow detection

In this work, the use of projection micro-stereolithography (PμSL) to 3D print a micro-optofluidic device for slug flow detection is presented. For comparison purposes, a poly-dimethylsiloxane (PDMS)–based device was also manufactured by a novel master–slave 3D printing approach. The micro-optofluidic device has a microfluidic T-junction with a micro-optical section that consists of two optical fiber insertions used for slug flow detection. The design of the device also includes two micro-channels for the optical fiber’s insertion, needed to acquire a light signal, which give a direct information about the microfluidic channel inner flow by exploiting the absorption phenomenon. The working principle in the detection is based on a different light transmission correlated to the fluid interfering with the laser beam in a micro-channel section. The two materials used for the two manufacturing approaches were fully characterized in terms of their surface properties via both Atomic Force Microscopy (AFM) and angle of contact measure.The process within the two micro-channels was monitored optically and a signal correlated to the slug passage was analyzed for the flow tracking. A wide experimental campaign was done for the device manufactured through the PμSL technique in different operative conditions. Thus, the optimal one was identified through the Analysis of Variance (ANOVA). Then, a detailed comparison between the slug process detected inside the HTL resin device and the PDMS device was carried out to evaluate the pros and cons of using different materials and fabrication techniques. The analysis run on the two devices revealed that the HTL resin device can be used for slug flow detection, but future research is still needed to obtain a resin allowing to outperform the PDMS device.