Grafting epoxy-modified hydrophilic polymers onto poly(dimethylsiloxane) microfluidic chip to resist nonspecific protein adsorption

Forming Single-Cell-Derived Colon Cancer Organoid Arrays on a Microfluidic Chip for High Throughput Tumor Heterogeneity Analysis

Single-cell-derived tumor organoids (STOs) possess a distinct genetic background, making them valuable tools for demonstrating tumor heterogeneity. In order to fulfill the high throughput demands of STO assays, we have developed a microfluidic chip containing 30 000 microwells, which is dedicated to a single cell culture approach for selective expansion and differential induction of cancer stem cells. The microwells are coated with a hydrophilic copolymer to eliminate cell adhesion, and the cell culture is supported by poly(ethylene glycol) (PEG) to establish a nonadhesive culture environment. By utilizing an input cell density of 7 × 103·mL-1, it is possible to construct a 4000 single cell culture system through stochastic cell occupation. We demonstrate that the addition of 15% PEG10000 in the cell culture medium effectively prevents cell loss while facilitating tumor stem cell expansion. As were demonstrated by HCT116, HT29, and SW480 colon cancer cells, the microfluidic approach achieved a STO formation rate of ∼20%, resulting in over 800 STOs generated from a single culture. Comprehensive analysis through histomorphology, immunohistochemistry, drug response evaluation, assessment of cell invasion, and biomarker detection reveals the heterogeneity among individual STOs. Specifically, the smaller STOs exhibited higher invasion and drug resistance capabilities compared with the larger ones. The developed microfluidic approach effectively facilitates STO formation and offers promising prospects for investigating tumor heterogeneity, as well as conducting personalized therapy-focused drug screening.

Selective expansion of renal cancer stem cells using microfluidic single-cell culture arrays for anticancer drug testing

The suboptimal prognosis associated with drug therapy for renal cancer can be attributed to the presence of stem-cell-like renal cancer cells. However, the limited number of these cells prevents conventional drug screening assays from effectively assessing the response of renal cancer stem cells to anti-cancer agents. To address this issue, the present study employed microfluidic single-cell culture arrays to expand renal cancer stem cells by exploiting the anti-apoptosis and self-renewal properties of tumor stem cells. A microfluidic chip with 18 000 hydrophilic microwells was designed and fabricated to establish the single-cell culture array. Over a 7 day culture, the large-scale single-cell culture yielded a limited quantity of single-cell-derived tumorspheres. The sphere formation rates for Caki-1, 786-O, and ACHN cells were determined to be 8.74 ± 0.53%, 12.02 ± 1.43%, and 4.98 ± 1.68%, respectively. The expanded cells exhibited stemness characteristics, as indicated by immunofluorescence, flow cytometry, serial passaging, and in vitro differentiation assays. Additionally, the comparative transcriptomic analysis showed significant differences in the gene expression patterns of the expanded cells compared to the differentiated renal cancer cells. The drug testing indicated that renal cancer stem cells exhibited reduced sensitivity towards the tyrosine kinase inhibitors sorafenib and sunitinib, compared to differentiated renal cancer cells. This reduced sensitivity can be attributed to the elevated expression levels of tyrosine kinase in renal cancer stem cells. This present study provides evidence that the utilization of microfluidic single-cell culture arrays for selective cell expansion can facilitate drug testing of renal cancer stem cells.

Microfluidic delivery of cutting enzymes for fragmentation of surface-adsorbed DNA molecules

We describe a method for fragmenting, in-situ, surface-adsorbed and immobilized DNAs on polymethylmethacrylate(PMMA)-coated silicon substrates using microfluidic delivery of the cutting enzyme DNase I. Soft lithography is used to produce silicone elastomer (Sylgard 184) gratings which form microfluidic channels for delivery of the enzyme. Bovine serum albumin (BSA) is used to reduce DNase I adsorption to the walls of the microchannels and enable diffusion of the cutting enzyme to a distance of 10mm. Due to the DNAs being immobilized, the fragment order is maintained on the surface. Possible methods of preserving the order for application to sequencing are discussed.

Three-dimensional human neural culture on a chip recapitulating neuroinflammation and neurodegeneration

Neuroinflammation has either beneficial or detrimental effects, depending on risk factors and neuron-glia interactions in neurological disorders. However, studying neuroinflammation has been challenging due to the complexity of cell-cell interactions and lack of physio-pathologically relevant neuroinflammatory models. Here, we describe our three-dimensional microfluidic multicellular human neural culture model, referred to as a 'brain-on-a-chip' (BoC). This elucidates neuron-glia interactions in a controlled manner and recapitulates pathological signatures of the major neurological disorders: dementia, brain tumor and brain edema. This platform includes a chemotaxis module offering a week-long, stable chemo-gradient compared with the few hours in other chemotaxis models. Additionally, compared with conventional brain models cultured with mixed phenotypes of microglia, our BoC can separate the disease-associated microglia out of heterogeneous population and allow selective neuro-glial engagement in three dimensions. This provides benefits of interpreting the neuro-glia interactions while revealing that the prominent activation of innate immune cells is the risk factor leading to synaptic impairment and neuronal loss, validated in our BoC models of disorders. This protocol describes how to fabricate and implement our human BoC, manipulate in real time and perform end-point analyses. It takes 2 d to set up the device and cell preparations, 1-9 weeks to develop brain models under disease conditions and 2-3 d to carry out analyses. This protocol requires at least 1 month training for researchers with basic molecular biology techniques. Taken together, our human BoCs serve as reliable and valuable platforms to investigate pathological mechanisms involving neuroinflammation and to assess therapeutic strategies modulating neuroinflammation in neurological disorders.

Blocking Nonspecific Interactions Using Y-Shape Poly(ethylene glycol)

Nonspecific interactions play a significant role in physiological activities, surface chemical modification, and artificial adhesives. However, nonspecificity sometimes causes sticky problems, including surface fouling, decreased target specificity, and artifacts in single-molecule measurements. Adjusting the liquid pH, using protein-blocking additives, adding nonionic surfactants, or increasing the salt concentration are common methods to minimize nonspecific binding to achieve high-quality data. Here, we report that grafting heteromorphic polyethylene glycol (Y-shape PEG) with two inert terminates could noticeably decrease nonspecific binding. As a proof-of-concept, we performed single-molecule force spectroscopy and fluorescence staining imaging experiments to verify the feasibility of Y-shape PEG in blocking nonspecific interactions. Our results indicate that Y-shape PEG could serve as a prominent and efficient candidate to minimize nonspecificity for scientific and biomedical applications.

Mechanism of Morphology Development in HDGEBA/PAMS Hybrid Thermosets: Monte Carlo Simulation and LSCM Study

Reactive combinations of aliphatic epoxy resins and functional polysiloxanes form a class of hybrid thermosetting materials with properties that may come from both the organic and the inorganic phases. The two typically immiscible phases form a suspension whose morphology, composition, and thermal properties vary with curing time. The aim of this research was to elucidate the mechanism by which morphology changed with time and to simulate it through Metropolis-Monte Carlo. The selected system was hydrogenated epoxy (HDGEBA) and a synthetic polyaminosiloxane (PAMS). It was studied by DSC, FTnIR, gel point, viscometry, and in-situ laser scanning confocal microscopy. A mechanism for morphology generation was proposed and simulated, exploring a wide range of values of the "a priori" relevant variables. The essential features were captured by simulations with a reasonable agreement with experimental data. However, the complete process was more complex than the geometrical approach of the simulation. The main deviations that were found and qualitatively explained are: (i) the induction period on the rate of coalescence, and (ii) PAMS-rich domain average size increases faster than predictions.

Gel-Free Single-Cell Culture Arrays on a Microfluidic Chip for Highly Efficient Expansion and Recovery of Colon Cancer Stem Cells

The microgel single-cell culture approach we developed to expand tumor stem cells (TSCs) is associated with limited TSC production, which can be attributable to cell viability loss in microgel formation and tumorsphere expansion limitation caused by hydrogel stiffness. In this work, we developed a gel-free single-cell culture array on a microfluidic chip to overcome these issues. The microfluidic chip used in the study has a 16,000 hydrophilic microchamber array, which can capture ∼2000 single cells at a time. After cell capturing, the cell culture chambers were enclosed by forming a chitosan layer through interactions between chitosan and alginate, thus preventing cell loss in the gel-free culture. The hydrophilic coating prevented cell adhesion, so only TSCs with anti-apoptosis and self-renewal properties can survive the harsh culture and form tumorspheres. After a 7 day culture, 19.04% of the HCT116 colon cancer cells formed single-cell-derived tumorspheres with an average size of 46.59 ± 10.58 μm. Compared with the microgel single-cell culture, sphere-forming rate and TSC expansion efficiency were significantly improved by using this gel-free single-cell culture array. After cell culture, the chitosan layer could be destabilized easily, thus allowing recovery of the tumorspheres from the microchip by applying a reverse flow. Approximately 13,600 cells could be obtained in a single culture, which can be used for off-chip cell assays. Flow cytometry analysis indicated high proportions of LGR5(+) and SOX2(+) cells within the single-cell-derived tumorspheres. Moreover, the differentiation experiments confirmed the multi-lineage differentiation potential of single-cell-derived tumorspheres. The gel-free single-cell culture offers a label-free approach to obtain sufficient amounts of TSCs, which is valuable for tumor biology research and the development of TSC-specific therapeutic strategies.

Accurate Evaluation of Hepatocyte Metabolisms on a Noble Oxygen-Permeable Material With Low Sorption Characteristics

In the pharmaceutical industry, primary cultured hepatocytes is a standard tool used to assess hepatic metabolisms and toxicity in vitro. Drawbacks, however, include their functional deterioration upon isolation, mostly due to the lack of a physiological environment. Polydimethylsiloxane (PDMS) has been reported to improve the function of isolated hepatocytes by its high oxygen permeability when used as a material of microphysiological systems (MPS). However, its high chemical sorption property has impeded its practical use in drug development. In this study, we evaluated a new culture material, 4-polymethyl-1-pentene polymer (PMP), in comparison with PDMS and conventional tissue culture polystyrene (TCPS). First, we confirmed the high oxygen permeability and low sorption of PMP, and these properties were comparable with PDMS and TCPS, respectively. Moreover, using primary rat hepatocytes, we demonstrated maintained high levels of liver function at least for 1 week on PMP, with its low chemical sorption and high oxygen permeability being key factors in both revealing the potential of primary cultured hepatocytes and in performing an accurate evaluation of hepatic metabolisms. Taken together, we conclude that PMP is a superior alternative to both PDMS and TCPS, and a promising material for a variety of drug testing systems.

Tuning the Surface Wettability of Cyclic Olefin Copolymer by Plasma Treatment and Graphene Oxide Deposition and Reduction

Selective altering of surface wettability in microfluidic channels provides a suitable platform for a large range of processes, such as the phase separation of multiphase systems, synthesis of reaction controlled, nanoliter sized droplet reactors, and catalyst impregnation. Herein we study the feasibility to tune the wettability of a flexible cyclic olefin copolymer (COC). Two methods were considered for enhancing the surface hydrophilicity. The first is argon/oxygen plasma treatment, where the effect of treatment duration on water contact angle and COC surface morphology and chemistry were investigated, and the second is coating COC with GO dispersions of different concentrations. For enhancing the hydrophobicity of GO-coated COC surfaces, three reduction methods were considered: chemical reduction by Hydroiodic acid (HI), thermal reduction, and photo reduction by exposure of GO-coated COC to UV light. The results show that as the GO concentration and plasma treatment duration increased, a significant decrease in contact angle was observed, which confirmed the ability to enhance the wettability of the COC surface. The increase in hydrophilicity during plasma treatment was associated with the increase in surface roughness on the treated surfaces, while the increase during GO coating was associated with introducing oxygen-containing groups on the GO-coated COC surfaces. The results also show that the different reduction methods considered can increase the contact angle and improve the hydrophobicity of a GO-coated COC surface. It was found that the significant improvement in hydrophobicity was related to the reduction of oxygen-containing groups on the GO-coated COC modified surface.

Rapid Purification of Immunoglobulin G Using a Protein A-​immobilized Monolithic Spin Column with Hydrophilic Polymers

A rapid purification method was developed for antibody production in Chinese hamster ovary (CHO) cells using a Protein A-immobilized monolithic silica spin column with hydrophilic polymers. Monolithic silica modified with copolymers of 2-hydroxyethylmethacrylate (HEMA) and glycidyl methacrylate (GMA) showed lower non-specific protein absorption than that modified with a silane reagent. The epoxy group of GMA was converted to an amino group, and Protein A was modified by the coupling reagent. The amount of immobilized Protein A was controlled by changing the ratio of GMA to HEMA and the mesopore size of monolith. A modified monolith disk was fixed to a spin column for rapid antibody purification. The linear curves (for the antibody concentrations over 10 - 300 μg/mL) had a correlation coefficient of >0.999. Our column had various analytical advantages over previously reported columns, including a shorter preparation time (<10 min) and smaller sample volumes for purification with Protein A-immobilized agarose.

In situ photografting during direct laser writing in thermoplastic microchannels

A method for in situ photografting during direct laser writing by two-photon polymerization is presented. The technique serves as a powerful approach to the formation of covalent bonds between 3D photoresist structures and thermoplastic surfaces. By leveraging the same laser for both pattern generation and localized surface reactions, crosslinking between the bulk photoresist and thermoplastic surface is achieved during polymerization. When applied to in-channel direct laser writing for microfluidic device fabrication, the process yields exceptionally strong adhesion and robust bond interfaces that can withstand pressure gradients as high as 7 MPa through proper channel design, photoinitiator selection, and processing conditions.

Obtaining an immunoaffinity monolithic material: poly(GMA-co-EDMA) functionalized with an HPV-derived peptide using a thiol-maleimide reaction

Metabolites have great potential for the design of biomarkers, since their presence or absence provides valuable information about a biological system. In this context, polyclonal antibodies are important metabolites for diagnostic procedures, but in some pathologies, it has been found that these metabolites are present at low concentrations, so it could be difficult to detect them. In this investigation, an organic monolithic material of poly(GMA-co-EDMA) was functionalized with a peptide via Michael addition (thiol-maleimide) click chemistry. The peptide, covalently bound to the monolith, contains the SPINNTKPHEAR sequence derived from the human papilloma virus L1 protein. It was determined that the obtained monolithic support allows selectively isolating polyclonal antibodies against the SPINNTKPHEAR sequence, since they are retained on the chemical surface of the material by an immunoaffinity interaction. The monolithic material functionalization protocol reported here could be applied to incorporate any peptide with a terminal cysteine in order to recover a specific analyte. A new method was developed for isolating and pre-concentrating antibodies using monolithic materials, which could contribute to the improvement of disease detection strategies based on immunoaffinity interactions.

Lactoferrin coated or conjugated nanomaterials as an active targeting approach in nanomedicine

A successful drug delivery to a specific site relies on two essential factors including; efficient entrapment of the drug within the carrier and successful delivery of drug- loaded nanocarrier to the target site without opsonisation or drug release in the circulation before reaching the organ of interest. Lactoferrin (LF) is a glycoprotein belonging to the transferrin (TF) family which can bind to TF receptors (TFRs) and LF membrane internalization receptors (LFRs) highly expressed on the cell surface of both highly proliferating cancer cells and blood brain barrier (BBB), which in turn can facilitate its accessibility to the cell nucleus. This merit could be exploited to develop actively targeted drug delivery systems that can easily cross the BBB or internalize into tumor cells. In this review, the most recent advances of utilizing LF as an active targeting ligand for different types of nanocarriers including: inorganic nanoparticles, dendrimers, synthetic biodegradable polymers, lipid nanocarriers, natural polymers, and nanoemulstions will be highlighted. Collectively, LF seems to be a promising targeting ligand in the field of nanomedicine.

Temperature-triggered attachment and detachment of general human bio-foulants on zwitterionic polydimethylsiloxane

Biofouling has long been a problem for biomaterials, so being able to control the fouling on the surface of a biomaterial would be ideal. In this study a copolymer system was designed comprising three moieties: an epoxy containing group, glycidyl methacrylate (GMA); a thermoresponsive segment, N-isopropylacrylamide (NIPAAm); and an antifouling zwitterionic unit, sulfobetaine methacrylate (SBMA). The copolymers (pGSN), synthesized via free radical polymerization with these 3 moieties, were then grafted onto polydimethylsiloxane (PDMS). The presence of a critical temperature for both the copolymers and the coated PDMS was evidenced by particle size and contact angle measurements. The coated PDMS exhibited controllable temperature-dependent antifouling behaviors and stimuli-responsive phase characteristics in the presence of salts. The interactions of the coated PDMS with biomolecules were tested via attachment of fibrinogen protein, platelets, human whole blood, and tumor cells (HT1080). The attachment and detachment of these biomolecules were studied at different temperatures. Exposed hydrophobic domains of thermoresponsive NIPAAm-rich pGSN containing NIPAAm at 56 mol% generally allows molecular and cellular attachment on the PDMS surface at 37 °C. On the other hand, the coated PDMS with a relatively high content of SBMA (>41 mol%) in the copolymer started to exhibit fouling resistance and lower the thermoresponsive properties. Interestingly, the incorporation of zwitterionic SBMA units into the copolymers was found to accelerate the hydration of the PDMS surfaces and resulted in biomolecular and cellular detachment at 25 °C, which is comparable to the detachment at 4 °C. This modified surface behavior is found to be consistent through all biofouling tests.

Robust Formation of an Epithelial Layer of Human Intestinal Organoids in a Polydimethylsiloxane-Based Gut-on-a-Chip Microdevice

Polydimethylsiloxane (PDMS) is a silicone polymer that has been predominantly used in a human organ-on-a-chip microphysiological system. The hydrophobic surface of a microfluidic channel made of PDMS often results in poor adhesion of the extracellular matrix (ECM) as well as cell attachment. The surface modification by plasma or UV/ozone treatment in a PDMS-based device produces a hydrophilic surface that allows robust ECM coating and the reproducible attachment of human intestinal immortalized cell lines. However, these surface-activating methods have not been successful in forming a monolayer of the biopsy-derived primary organoid epithelium. Several existing protocols to grow human intestinal organoid cells in a PDMS microchannel are not always reproducibly operative due to the limited information. Here, we report an optimized methodology that enables robust and reproducible attachment of the intestinal organoid epithelium in a PDMS-based gut-on-a-chip. Among several reported protocols, we optimized a method by performing polyethyleneimine-based surface functionalization followed by the glutaraldehyde cross linking to activate the PDMS surface. Moreover, we discovered that the post-functionalization step contributes to provide uniform ECM deposition that allows to produce a robust attachment of the dissociated intestinal organoid epithelium in a PDMS-based microdevice. We envision that our optimized protocol may disseminate an enabling methodology to advance the integration of human organotypic cultures in a human organ-on-a-chip for patient-specific disease modeling.

Complete inclusion of bioactive molecules and particles in polydimethylsiloxane: a straightforward process under mild conditions

By applying a slow curing process, we show that biomolecules can be incorporated via a simple process as liquid stable phases inside a polydimethylsiloxane (PDMS) matrix. The process is carried out under mild conditions with regards to temperature, pH and relative humidity, and is thus suitable for application to biological entities. Fluorescence and enzymatic activity measurements show that the biochemical properties of the proteins and enzyme tested are preserved, without loss due to adsorption at the liquid-polymer interface. Protected from external stimuli by the PDMS matrix, these soft liquid composite materials are new tools of interest for robotics, microfluidics, diagnostics and chemical microreactors.

In situ probing of switchable nanomechanical properties of responsive high-density polymer brushes on poly(dimethylsiloxane): An AFM nanoindentation approach

Nanomechanical characteristics of end grafted polymer brushes were studied by AFM based, colloidal probe nanoindentation measurements. A high-density polymer brush of poly(2-hydroxyethyl methacrylate) (PHEMA) was precisely prepared on the surface of a flexible poly(dimethylsiloxane) (PDMS) substrate oxidized in ultraviolet/ozone (UVO). Exposure times less than 10min resulted in laterally homogeneous oxidized surfaces, characterized by a SiO_x thickness ∼35nm and an increased modulus up to 9MPa, as shown by AFM nanoindentation measurements. We have demonstrated that a high surface density of up to ∼0.63chains/nm² of the well-defined PHEMA brushes can be grown from the surface of oxidized PDMS by surface-initiated atom transfer radical polymerization (SI-ATRP) from trimethoxysilane derivatives mixed-SAM. The reversible nanomechanical changes of PHEMA layer between extended (hydrated state) and collapsed (dehydrated state) chain upon immersing in selective and non-selective solvents were investigated by in situ AFM nanoindentation analysis in liquid environments. The elastic modulus derived from force-indentation curves obtained for swollen PHEMA grafted chains in water was estimated to be equal 2.7±0.2MPa, which is almost two orders of magnitude smaller than the modulus of dry PHEMA brush. Additionally, under cyclohexane immersion, the modulus of the PHEMA layer decreased by one order of magnitude, indicating a more compact chain packing at the PDMS surface.

Geometric Determinants of In-Situ Direct Laser Writing

Direct laser writing (DLW) is a three-dimensional (3D) manufacturing technology that offers significant geometric versatility at submicron length scales. Although these characteristics hold promise for fields including organ modeling and microfluidic processing, difficulties associated with facilitating the macro-to-micro interfaces required for fluid delivery have limited the utility of DLW for such applications. To overcome this issue, here we report an in-situ DLW (isDLW) strategy for creating 3D nanostructured features directly inside of—and notably, fully sealed to—sol-gel-coated elastomeric microchannels. In particular, we investigate the role of microchannel geometry (e.g., cross-sectional shape and size) in the sealing performance of isDLW-printed structures. Experiments revealed that increasing the outward tapering of microchannel sidewalls improved fluidic sealing integrity for channel heights ranging from 10 μm to 100 μm, which suggests that conventional microchannel fabrication approaches are poorly suited for isDLW. As a demonstrative example, we employed isDLW to 3D print a microfluidic helical coil spring diode and observed improved flow rectification performance at higher pressures—an indication of effective structure-to-channel sealing. We envision that the ability to readily integrate 3D nanostructured fluidic motifs with the entire luminal surface of elastomeric channels will open new avenues for emerging applications in areas such as soft microrobotics and biofluidic microsystems.

Superhydrophilic Smart Coating for Self-Cleaning Application on Glass Substrate

In general, superhydrophilic coating on glass substrate possesses water contact angle (WCA) below 10° and contains high self-cleaning properties in outdoor environment as compared to noncoated glass substrate panels. In this study, the superhydrophilic coating behavior on glass substrate has been developed. The micro- and nanosized titanium dioxide (TiO2) particles have been utilized to improve the surface roughness, and the polypropylene glycol (PPG) has been utilized to increase the surface energy of glass substrates. The wettability of coating surface shows the coating possess water contact angle (WCA) as low as 5° and suddenly reduce to 0° after 10 s. Superhydrophilic coated glass clearly shows excellent dirt repellent against dilute ketchup solution due to the absence of dirt streak on the glass surface. Meanwhile, the dirt streak is present on the bare glass surface indicating its weak self-cleaning property. The developed superhydrophilic coating on glass substrate was also found to have great antifog property compared to the bare glass substrate. Superhydrophilic surfaces have showed free tiny droplet even at 130°C of hot boiling bath for 10 min and completely dry after 1 min. The superhydrophilic coating surfaces have demonstrated free water streak after impacting with harsh water spraying for 5 min confirming that the superhydrophilic coating on glass substrate is antiwater streak.

Microfluidic channels with renewable and switchable biological functionalities based on host–guest interactions

Microfluidic channels with renewable and switchable biological functionalities were prepared using host–guest interactions.

Orange is the new white: rapid curing of an ethylene-glycidyl methacrylate copolymer with a Ti-bisphenolate type catalyst

The here established crosslinking chemistry opens up a by-product free method for rapid curing of epoxy-functionalised polyethylenes.

Recent advances in nonbiofouling PDMS surface modification strategies applicable to microfluidic technology

In the last decade microfabrication processes including rapid prototyping techniques have advanced rapidly and achieved a fairly mature stage. These advances have encouraged and enabled the use of microfluidic devices by a wider range of users with applications in biological separations and cell and organoid cultures. Accordingly, a significant current challenge in the field is controlling biomolecular interactions at interfaces and the development of novel biomaterials to satisfy the unique needs of the biomedical applications. Poly(dimethylsiloxane) (PDMS) is one of the most widely used materials in the fabrication of microfluidic devices. The popularity of this material is the result of its low cost, simple fabrication allowing rapid prototyping, high optical transparency, and gas permeability. However, a major drawback of PDMS is its hydrophobicity and fast hydrophobic recovery after surface hydrophilization. This results in significant nonspecific adsorption of proteins as well as small hydrophobic molecules such as therapeutic drugs limiting the utility of PDMS in biomedical microfluidic circuitry. Accordingly, here, we focus on recent advances in surface molecular treatments to prevent fouling of PDMS surfaces towards improving its utility and expanding its use cases in biomedical applications.

Advances and Challenges in Recapitulating Human Pulmonary Systems: At the Cusp of Biology and Materials

The aim of this review is to provide an overview of physiologically relevant microengineered lung-on-a-chip (LoC) platforms for a variety of different biomedical applications with emphasis on drug screening. First, a brief outline of lung anatomy and physiology is presented followed by discussion of the lung parenchyma and its extracellular matrix. Next, we point out the technical challenges in recapitulating the complexity of lung in conventional static two-dimensional microenvironments and the need for alternate lung platforms. The importance of scaling laws is also emphasized in designing these in vitro microengineered lung platforms. The review then discusses current LoC platforms that have been used for testing the efficacy of drugs or as model systems for investigating disorders of the lung parenchyma. Finally, the design parameters in developing an ideal physiologically relevant LoC platform are presented. As this emerging field of organ-on-a-chip can serve an alternative platform for animal testing of drugs or modeling human diseases in vitro, it has significant potential to impact the future of pharmaceutical research.

Restraining non-specific adsorption of protein using Parylene C-caulked polydimethylsiloxane

Non-specific adsorption (NSA) of proteins on surface is a critical issue in polydimethylsiloxane (PDMS)-based microfluidics, which may either considerably decrease the efficiency of a continuous flow reaction or cause a large background noise in a heterogeneous sensing. This work introduced a new method to restrain NSA of protein by caulking PDMS with Parylene C, i.e., forming a Parylene C-caulked PDMS (pcPDMS) surface. The caulking depth of Parylene C inside PDMS matrix was characterized by laser scanning confocal microscopy based on a detectable autofluorescence intensity difference between Parylene C and PDMS after being annealed at 270 °C for 2 h in nitrogen. NSA of bovine serum albumin (BSA) on the inner surfaces of PDMS and pcPDMS microchannels was experimentally compared. The results indicated that the adsorbed BSA on the pcPDMS surface were 35.2% of that on the pristine PDMS surface after the BSA solution flowing through the microchannels at a flow rate of 2000 nL/min, a typical scenario of the continuous flow reaction. In a case mimicking the heterogeneous sensing, after a 60 min washing of phosphate buffered saline flow on a pre-saturated BSA adsorbed surface, the residual BSA on the pcPDMS surface was only 4.5% of that on the pristine PDMS surface. Adsorption/desorption coefficients of BSA on the PDMS and the pcPDMS surfaces were extracted from the experimental results based on the first-order Langmuir model, which indicated that the pcPDMS has a lower adsorption coefficient (Ka ) and a higher desorption coefficient (Kd ), compared to those of the pristine PDMS. A preliminary experiment also indicated that Taq polymerase kept 93.0% activity after flowing through a pcPDMS microchannel, while only 28.9% activity was left after passing a pristine PDMS microchannel under the same operation condition.

Fabrication of universal serial bus flash disk type microfluidic chip electrophoresis and application for protein analysis under ultra low voltage

A simple and effective universal serial bus (USB) flash disk type microfluidic chip electrophoresis (MCE) was developed by using poly(dimethylsiloxane) based soft lithography and dry film based printed circuit board etching techniques in this paper. The MCE had a microchannel diameter of 375 μm and an effective length of 25 mm. Equipped with a conventional online electrochemical detector, the device enabled effectively separation of bovine serum albumin, lysozyme, and cytochrome c in 80 s under the ultra low voltage from a computer USB interface. Compared with traditional capillary electrophoresis, the USB flash disk type MCE is not only portable and inexpensive but also fast with high separation efficiency.

Simple and Rapid Immobilization of Coating Polymers on Poly(dimethyl siloxane)-glass Hybrid Microchips by a Vacuum-drying Method

A simple and rapid vacuum-drying modification method was applied to several neutral and charged polymers to obtain coating layers for controlling electroosmotic flow (EOF) and suppressing sample adsorption on poly(dimethyl siloxane) (PDMS)-glass hybrid microchips. In the vacuum-dried poly(vinylpyrrolidone) coating, the electroosmotic mobility (μeo) was suppressed from +2.1 to +0.88 × 10(-4) cm(2)/V·s, and the relative standard deviation (RSD) of μeo was improved from 10.2 to 2.5% relative to the bare microchannel. Among several neutral polymers, poly(vinylalcohol) (PVA) and poly(dimethylacrylamide) coatings gave more suppressed and repeatable EOF with RSDs of less than 2.3%. The vacuum-drying method was also applicable to polyanions and polycations to provide accelerated and inversed EOF, respectively, with acceptable RSDs of less than 4.9%. In the microchip electrophoresis (MCE) analysis of bovine serum albumin (BSA) in the vacuum-dried and thermally-treated PVA coating channel, an almost symmetric peak of BSA was obtained, while in the native microchannel a significantly skewed peak was observed. The results demonstrated that the vacuum-dried polymer coatings were effective to control the EOF, and reduced the surface adsorption of proteins in MCE.

Microfluidics using a thiol-acrylate resin for fluorescence-based pathogen detection assays

We demonstrate thiol-acrylate microfluidics prepared via soft lithography for single-step protein immobilization and fluorescence-based pathogen detection. Such microfluidics are formed via room temperature curing, and bonded without oxygen plasma. The background fluorescence of the resin was found to be similar to PDMS for several filter sets. We also show that thiol-acrylate devices are able to bond to gold-coated surfaces, which allows for integration with microfabricated sensors.

Droplet confinement and leakage: Causes, underlying effects, and amelioration strategies

The applicability of droplet-based microfluidic systems to many research fields stems from the fact that droplets are generally considered individual and self-contained reaction vessels. This study demonstrates that, more often than not, the integrity of droplets is not complete, and depends on a range of factors including surfactant type and concentration, the micro-channel surface, droplet storage conditions, and the flow rates used to form and process droplets. Herein, a model microfluidic device is used for droplet generation and storage to allow the comparative study of forty-four different oil/surfactant conditions. Assessment of droplet stability under these conditions suggests a diversity of different droplet failure modes. These failure modes have been classified into families depending on the underlying effect, with both numerical and qualitative models being used to describe the causative effect and to provide practical solutions for droplet failure amelioration in microfluidic systems.

Preparation of anti-fouling silicone elastomers by covalent immobilization of carboxybetaine

The surface-grafted carboxybetaine (CB) layer could enhance the biocompatibility of polydimethylsiloxane (PDMS) and reduce the adsorption of protein and adhesion of bacteria efficiently.

Promoting the selection and maintenance of fetal liver stem/progenitor cell colonies by layer-by-layer polypeptide tethered supported lipid bilayer

In this study, we designed and constructed a series of layer-by-layer polypeptide adsorbed supported lipid bilayer (SLB) films as a novel and label-free platform for the isolation and maintenance of rare populated stem cells. In particular, four alternative layers of anionic poly-l-glutamic acid and cationic poly-l-lysine were sequentially deposited on an anionic SLB. We found that the fetal liver stem/progenitor cells from the primary culture were selected and formed colonies on all layer-by-layer polypeptide adsorbed SLB surfaces, regardless of the number of alternative layers and the net charges on those layers. Interestingly, these isolated stem/progenitor cells formed colonies which were maintained for an 8 day observation period. Quartz crystal microbalance with dissipation measurements showed that all SLB-polypeptide films were protein resistant with serum levels significantly lower than those on the polypeptide multilayer films without an underlying SLB. We suggest the fluidic SLB promotes selective binding while minimizing the cell-surface interaction due to its nonfouling nature, thus limiting stem cell colonies from spreading.

Organs-on-a-chip: a new tool for drug discovery

INTRODUCTION

The development of emerging in vitro tissue culture platforms can be useful for predicting human response to new compounds, which has been traditionally challenging in the field of drug discovery. Recently, several in vitro tissue-like microsystems, also known as 'organs-on-a-chip', have emerged to provide new tools for better evaluating the effects of various chemicals on human tissue.

AREAS COVERED

The aim of this article is to provide an overview of the organs-on-a-chip systems that have been recently developed. First, the authors introduce single-organ platforms, focusing on the most studied organs such as liver, heart, blood vessels and lung. Later, the authors briefly describe tumor-on-a-chip platforms and highlight their application for testing anti-cancer drugs. Finally, the article reports a few examples of other organs integrated in microfluidic chips along with preliminary multiple-organs-on-a-chip examples. The article also highlights key fabrication points as well as the main application areas of these devices.

EXPERT OPINION

This field is still at an early stage and major challenges need to be addressed prior to the embracement of these technologies by the pharmaceutical industry. To produce predictive drug screening platforms, several organs have to be integrated into a single microfluidic system representative of a humanoid. The routine production of metabolic biomarkers of the organ constructs, as well as their physical environment, have to be monitored prior to and during the delivery of compounds of interest to be able to translate the findings into useful discoveries.

Preparation and characterizations of poly(2-methyl-2-oxazoline) based antifouling coating by thermally induced immobilization

Poly[(2-methyl-2-oxazoline)-random-glycidylmethacrylate] was immobilized on a silicon/glass surface via a simple annealing procedure to obtain a covalent and cross-linked antifouling coating.

Highly reproducible chronoamperometric analysis in microdroplets

Here we report a method for highly reproducible chronoamperometric analysis of the contents of microdroplets. Aqueous microdroplets having volumes on the order of 1 nL and separated by a fluorocarbon solvent are generated within a microfluidic device using a T-shaped junction. The key finding is that stable and reproducible quasi-steady-state currents are observed if the electrochemical measurements are made in a narrowed segment of a microchannel. Under these conditions, the microdroplets are stretched, here by a factor of 10, leading to desirable intradroplet mass transfer characteristics. Microdroplet frequencies up to 0.67 s(-1) are accessible using this method. The quasi-steady-state currents resulting from chronoamperometric analysis of microdroplets containing 1.0 mM Ru(NH3)6(3+) have relative standard deviations of just 1.8% and 2.8% at flow rates of 30 nL min(-1) and 60 nL min(-1), respectively. Importantly, the design of the microelectrochemical device ensures direct contact between intradroplet redox molecules and the electrode surface. That is, the fluorocarbon between microdroplets does interfere with inner-sphere electrocatalytic processes such as the oxygen reduction reaction. Finite-element simulations are presented that are in accord with the experimental findings.

A negative-pressure-driven microfluidic chip for the rapid detection of a bladder cancer biomarker in urine using bead-based enzyme-linked immunosorbent assay

This paper describes an integrated microfluidic chip that is capable of rapidly and quantitatively measuring the concentration of a bladder cancer biomarker, apolipoprotein A1, in urine samples. All of the microfluidic components, including the fluid transport system, the micro-valve, and the micro-mixer, were driven by negative pressure, which simplifies the use of the chip and facilitates commercialization. Magnetic beads were used as a solid support for the primary antibody, which captured apolipoprotein A1 in patients' urine. Because of the three-dimensional structure of the magnetic beads, the concentration range of the target that could be detected was as high as 2000 ng ml(-1). Because this concentration is 100 times higher than that quantifiable using a 96-well plate with the same enzyme-linked immunosorbent assay (ELISA) kit, the dilution of the patient's urine can be avoided or greatly reduced. The limit of detection was determined to be approximately 10 ng ml(-1), which is lower than the cutoff value for diagnosing bladder cancer (11.16 ng ml(-1)). When the values measured using the microfluidic chip were compared with those measured using conventional ELISA using a 96-well plate for five patients, the deviations were 0.9%, 6.8%, 9.4%, 1.8%, and 5.8%. The entire measurement time is 6-fold faster than that of conventional ELISA. This microfluidic device shows significant potential for point-of-care applications.

Nonfouling hydrophilic poly(ethylene glycol) engraftment strategy for PDMS/SU-8 heterogeneous microfluidic devices

We report a novel nonfouling passivation method using poly(ethylene glycol) (PEG) engraftment on the surfaces of poly(dimethylsiloxane) (PDMS) microfluidic devices sealed with SU-8. To achieve bonding between the PDMS and SU-8 surfaces, the PDMS surface was first functionalized with amines by treatment with 3-aminopropyltrimethoxysilane (APTMS) for subsequent reaction with epoxide functional groups on SU-8 surfaces. To modify the heterogeneous surfaces of the resulting PDMS/SU-8 microfluidic device further, the remaining SU-8 surfaces were amino functionalized using ethylene diamine (EDA), followed by treating both amino-functionalized PDMS and SU-8 surfaces with mPEG-NHS (N-hydroxysuccinimide) through an amine-NHS reaction for facile PEG immobilizations, thus simultaneously modifying both PDMS and SU-8 surfaces in one reaction. Detailed surface analyses such as the water contact angle, X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM) were conducted to confirm the chemical reactions and characterize the resulting surface properties. To test the efficacy of this surface-modification strategy, we conducted nonspecific protein and particle binding tests using microfluidic devices with and without modifications. The PEG-modified PDMS/SU-8 device surfaces showed a 64.5% reduction in nonspecific bovine serum albumin (BSA) adsorption in comparison to that of the unmodified surfaces and 92.0 and 95.8% reductions in microbead adhesion under both stagnant and flowing conditions, respectively.

Simple surface treatment using amphiphilic phospholipid polymers to obtain wetting and lubricity on polydimethylsiloxane-based substrates

Simple surface treatment of polydimethylsiloxane (PDMS) substrates was performed using an aqueous-ethanolic solution of amphiphilic phospholipid polymers to reduce the hydrophobic and high friction characteristics of PDMS. The phospholipid polymers, poly(2-methacryloyloxyethyl phosphorylcholine (MPC)-co-2-ethylhexyl methacrylate (EHMA)-co-2-(N,N-dimethylamino)ethyl methacrylate) (PMED) and poly(MPC-co-EHMA) (PMEH) were synthesized, and the effects of the electric charge of the polymer chain on the stability of the attachment to the PDMS surface was investigated. The polymers were dissolved in a mixed solvent of ethanol and water, and the PDMS samples were treated by a simple dipping method using the polymer solution. Pure ethanol as the solvent was ineffective for the attachment of the polymers to the PDMS surface. It was considered that the hydrophobic interactions and electrostatic attraction forces between the polymer chains and the PDMS surface were too weak for efficient interaction in this solvent. On the other hand, the surface wettability and lubricity of PDMS could be improved by treatment with an aqueous-ethanolic solution of PMED. The static contact angle was decreased from 90° to 20° by this treatment, and the dynamic friction coefficient against a Co-Cr ball was decreased by nearly 80% compared with that of the untreated PDMS. The hydrophobic interactions and electrostatic attraction forces generated by PMED were both essential for the stable adsorption of the polymer layer on PDMS. Furthermore, the solubilized state of the polymers affected the adsorption of the polymer. We concluded that the surface of PDMS could be stably modified using aqueous-ethanolic solutions of PMED without the need for pretreatments.

Polyurethane-based microfluidic devices for blood contacting applications

Protein adsorption on PDMS surfaces poses a significant challenge in microfluidic devices that come into contact with biofluids such as blood. Polyurethane (PU) is often used for the construction of medical devices, but despite having several attractive properties for biointerfacing, it has not been widely used in microfluidic devices. In this work we developed two new fabrication processes for making thin, transparent and flexible PU-based microfluidic devices. Methods for the fabrication and bonding of microchannels, the integration of fluidic interconnections and surface modification with hydrophilic polyethylene oxide (PEO) to reduce protein adsorption are detailed. Using these processes, microchannels were produced having high transparency (96% that of glass in visible light), high bond strength (326.4 kPa) and low protein adsorption (80% reduction in fibrinogen adsorption vs. unmodified PDMS), which is critical for prevention of fouling. Our findings indicate that PEO modified PU could serve as an effective alternative to PDMS in blood contacting microfluidic applications.

Stable low-fouling plasma polymer coatings on polydimethylsiloxane

Polydimethylsiloxane (DMS) is a popular material for microfluidics, but it is hydrophobic and is prone to non-specific protein adsorption. In this study, we explore methods for producing stable, protein resistant, tetraglyme plasma polymer coatings on PDMS by combining extended baking processes with multiple plasma polymer coating steps. We demonstrate that by using this approach, it is possible to produce a plasma polymer coatings that resist protein adsorption (<10 ng/cm(2)) and are stable to storage over at least 100 days. This methodology can translate to any plasma polymer system, enabling the introduction of a wide range of surface functionalities on PDMS surfaces.

Physisorbed surface coatings for poly(dimethylsiloxane) and quartz microfluidic devices

Surface modifications of microfluidic devices are of essential importance for successful bioanalytical applications. Here, we investigate three different coatings for quartz and poly(dimethylsiloxane) (PDMS) surfaces. We employed a triblock copolymer with trade name F(108), poly(L-lysine)-g-poly(ethylene glycol) (PLL-PEG), as well as the hybrid coating n-dodecyl-β-D-maltoside and methyl cellulose (DDM/MC). The impact of these coatings was characterized by measuring the electroosmotic flow (EOF), contact angle, and prevention of protein adsorption. Furthermore, we investigated the influence of static coatings, i.e., the incubation with the coating agent prior to measurements, and dynamic coatings, where the coating agent was present during the measurement. We found that all coatings on PDMS as well as quartz reduced EOF, increased reproducibility of EOF, reduced protein adsorption, and improved the wettability of the surfaces. Among the coating strategies tested, the dynamic coatings with DDM/MC and F(108) demonstrated maximal reduction of EOF and protein adsorption and simultaneously best long-term stability concerning EOF. For PLL-PEG, a reversal in the EOF direction was observed. Interestingly, the static surface coating strategy with F(108) proved to be as effective to prevent protein adsorption as dynamic coating with this block copolymer. These findings will allow optimized parameter choices for coating strategies on PDMS and quartz microfluidic devices in which control of EOF and reduced biofouling are indispensable.