Mechanisms of Hydrophobic Recovery of Poly(dimethylsiloxane) Elastomers after Plasma/Corona Treatments: A Minireview

Plasma/corona treatment could alter the wettability of a poly(dimethylsiloxane) (PDMS) surface from being hydrophobic to being hydrophilic, which has attracted many researchers' attention. However, the treated surface will gradually recover its hydrophobicity as it ages. To understand the recovery, many studies have been performed. Although there is still no general consensus on the recovery mechanisms, several models have been proposed that can explain the reported wetting behavior of hydrophobic recovery. In this minireview, we summarized the reported mechanisms underlying the hydrophobicity-recovery of oxidized PDMS surfaces, which are certainly affected by varied factors including temperature, aging time, stored conditions, and treatment conditions. We hope this minireview can give beginners in the field of microfluidics a better understanding on the various mechanisms that contribute to the hydrophobic recovery of PDMS surfaces and thus take appropriate measures to efficiently maintain the surface wettability of oxidized PDMS chips to prolong their performance.

A one-step process for multi-gradient wettability modification on a polymer surface

The surface modification technique is applied in microfluidic devices to modify wettability and achieve different flow velocities. Currently available methods for poly(dimethylsiloxane) (PDMS) surfaces may reliably induce wettability changes, but only one area can be altered at a time. This work introduces the controlled gradient oxygen plasma modification (CGPM) technique, which layers several resin masks with varying porosities on top of the PDMS surface. Selective wettability of the PDMS surface can be achieved by varying the oxygen plasma density above the modified material's surface by manipulation of the porosity value. Through the implementation of the COMSOL plasma module, the impact of the mask's porosity, through-hole size, distribution, and distance from the PDMS surface on wettability was studied. The suggested CGPM approach was characterized by contact angle measurements. During the 25-second CGPM procedure, the PDMS surface's contact angle continually changed from 8.77° to 76.98°. An integrated microfluidic device was created and manufactured to identify D-dimers to illustrate this method. In comparison with standard oxygen plasma treatment, the D-dimer assay was finished in 10 minutes and had a dynamic range of 1-1000 ng mL-1, with a peak fluorescence signal augmentation of 78.3% and an average fluorescence intensity enhancement of 31.1%.

Alignment-free construction of double emulsion droplet generation devices incorporating surface wettability contrast

Although polydimethylsiloxane (PDMS) is a versatile and easy-to-use material for microfluidics, its inherent hydrophobicity often necessitates specific hydrophilic treatment to fabricate microchip architectures for generating double emulsions. These additional processing steps frequently lead to increased complexity, potentially creating barriers to the wider use of promising microfluidic techniques. Here we describe an alignment-free spatial hydrophilic PDMS patterning technique to produce devices for the creation of double emulsions using combinations of PDMS and PDMS/surfactant bilayers. The technique enables us to achieve selective patterning and alignment-free bonding, producing reliable and reproducible water-in-oil-in-water W/O/W droplet emulsions. Our method involves processing devices in a vertical orientation, with the wetting transition contrast being achieved simply by imaging whilst adjusting the PDMS pouring speed (using a mobile phone, for example). We successfully obtain hydrophilic surfaces without distinguishable hydrophobic recovery using a range of surfactant concentrations. Droplet emulsions were produced with low coefficients of variation aligned with those generated with other, more complex, techniques (e.g. 3.8% and 3.1% for the inner and outer diameters, respectively). As a further example, the methods were also demonstrated for liposome production. In future we anticipate that the technique may be applied to other fields, including e.g. reagent delivery, DNA amplification, and encapsulated cell studies.

Preparation of ultra-thin elastomeric films

When polydimethylsiloxane elastomers are produced, in the absence of great care, chains remain that are unbound to the cross-linked matrix. Due to the unbound chains swelling the crosslinked matrix, these materials are gels. We have developed a simple process to prepare well-controlled elastomeric thin films which do not rely on unknown commercial formulations.

On Classification of Water-in-Oil and Oil-in-Water Droplet Generation Regimes in Flow-Focusing Microfluidic Devices

The objective of this research work is to propose a phase diagram that can be used to find a proper operating condition for generating droplets of different types. It is found that the phase diagram of QR versus CaD can effectively classify the droplet generation into three vivid regimes: dripping, jetting and tubing. For the dripping regime, its operating condition is in the range of either CaD < 10−4 and QR < 50 or 10−3 < CaD < 10−4 and QR < 1. For the jetting regime, its operating condition is in the range of either CaD < 1.35 × 10−2 and QR > 100 or CaD > 1.35 × 10−2 and QR > 1. For the tubing regime, its operating condition is in the range of CaD > 1.35 × 10−2 and QR < 1.

Simplified, Shear Induced Generation of Double Emulsions for Robust Compartmentalization during Single Genome Analysis

Drop microfluidics has driven innovations for high throughput, low input analysis techniques such as single-cell RNA-seq. However, the instability of single emulsion (SE) drops occasionally causes significant merging during drop processing, limiting most applications to single-step reactions in drops. Here, we show that double emulsion (DE) drops address this critical limitation and completely prevent drop contents from mixing. DEs show excellent stability during thermal cycling. More importantly, DEs undergo rupture into the continuous phase instead of merging, preventing content mixing and eliminating unstable drops from the downstream analysis. Due to the lack of drop merging, the monodispersity of drops is maintained throughout a workflow, enabling the deterministic manipulation of drops downstream. We also developed a simple, one-layer DE drop maker compatible with simple surface treatment using a plasma cleaner. The device allows for the robust production of single-core DEs at a wide range of flow rates and better control over the shell thickness, both of which have been significant limitations of conventional two-layer devices. This approach makes the fabrication of DE devices much more accessible, facilitating its broader adoption. Finally, we show that DE droplets eliminate content mixing and maintain compartmentalization of single virus genomes during PCR-based amplification and barcoding, while SEs mixed contents due to merging. With their resistance to content mixing, DE drops have key advantages for multistep reactions in drops, which is limited in SEs due to merging and content mixing.

Simple One-Step and Rapid Patterning of PDMS Microfluidic Device Wettability for PDMS Shell Production

Double emulsion (DE) droplets with controlled size and internal structure are a promising platform for biological analysis, chemical synthesis, and drug delivery systems. However, to further “democratize” their application, new methods that enable simple and precise spatial patterning of the surface wettability of droplet-generating microfluidic devices are still needed. Here, by leveraging the increase in hydrophilicity of polydimethylsiloxane (PDMS) due to the plasma-treatment used to permanently bond to glass, we developed a one-step method to selectively pattern the wettability of PDMS microfluidic devices for DE generation. Our results show that both Aquapel-treated and 1H,1H,2H,2H-Perfluorodecyltriethoxysilan (PFDTES)-treated devices are functionally showing the generality of our method. With the resulting microfluidic devices, both water-in-oil-in-water (w/o/w) and oil-in-water-in-oil (o/w/o) DE droplets can be produced. Using a PDMS mixture containing cross-linking agents, we formed PDMS microcapsules by solidifying the shell layer of water-in-PDMS-in-water DE droplets. We also characterize the morphological properties of the generated droplets/microcapsules. We anticipate the method developed in this work could be used in a broad range of applications of DE droplets.

Pico-washing: simultaneous liquid addition and removal for continuous-flow washing of microdroplets

Droplet microfluidics is based on a toolbox of several established unit operations, including droplet generation, incubation, mixing, pico-injection, and sorting. In the last two decades, the development of droplet microfluidic systems, which incorporate these multiple unit operations into a workflow, has demonstrated unique capabilities in fields ranging from single-cell transcriptomic analyses to materials optimization. One unit operation that is sorely underdeveloped in droplet microfluidics is washing, exchange of the fluid in a droplet with a different fluid. Here, we demonstrate what we name the "pico-washer," a unit operation capable of simultaneously adding fluid to and removing fluid from droplets in flow while requiring only a small footprint on a microfluidic chip. We describe the fabrication strategy, device architecture, and process parameters required for stable operation of this technology, which is capable of operating with kHz droplet throughput. Furthermore, we provide an image processing workflow to characterize the washing process with microsecond and micrometer resolution. Finally, we demonstrate the potential for integrated droplet workflows by arranging two of these unit operations in series with a droplet generator, describe a design rule for stable operation of the pico-washer when integrated into a system, and validate this design rule experimentally. We anticipate that this technology will contribute to continued development of the droplet microfluidics toolbox and the realization of novel droplet-based, multistep biological and chemical assays.

Formation of double emulsion micro-droplets in a microfluidic device using a partially hydrophilic-hydrophobic surface

The objective of this paper is to propose a surface modification method for preparing PDMS microfluidic devices with partially hydrophilic-hydrophobic surfaces for generating double emulsion droplets. The device is designed to be easy to use without any complicated preparation process and also to achieve high droplet encapsulation efficiency compared to conventional devices. The key component of this preparation process is the permanent chemical coating for which the Pluronic surfactant is added into the bulk PDMS. The addition of Pluronic surfactant can modify the surface property of PDMS from a fully hydrophobic surface to a partially hydrophilic-hydrophobic surface whose property can be either hydrophilic or hydrophobic depending on the air- or water-treatment condition. In order to control the surface wettability, this microfluidic device with the partially hydrophilic-hydrophobic surface undergoes water treatment by injecting deionized water into the specific microchannels where their surface property changes to hydrophilic. This microfluidic device is tested by generating monodisperse water-in-oil-in-water (w/o/w) double emulsion micro-droplets for which the maximum droplet encapsulation efficiency of 92.4% is achieved with the average outer and inner diameters of 75.0 and 57.7 μm, respectively.

Rapid, Simple, and Inexpensive Spatial Patterning of Wettability in Microfluidic Devices for Double Emulsion Generation

Water-in-oil-in-water (w/o/w) double emulsion (DE) encapsulation has been widely used as a promising platform technology for various applications in the fields of food, cosmetics, pharmacy, chemical engineering, materials science, and synthetic biology. Unfortunately, DEs formed by conventional emulsion generation approaches in most cases are highly polydisperse, making them less desirable for quantitative assays, controlled biomaterial synthesis, and entrapped ingredient release. Microfluidic devices can generate monodisperse DEs with controllable size, morphology, and production rate, but these generally require multistep fabrication processes and use of different solvents or bulky external instrumentation to pattern channel wettability. To overcome these limitations, we propose a rapid, simple, and inexpensive method to spatially pattern wettability in microfluidic devices for the continuous generation of monodisperse DEs. This is achieved by applying corona-plasma treatment to a select zone of the microchannel surface aided by a custom-designed corona resistance microchannel to strictly confine the plasma-treatment zone in a single polydimethylsiloxane (PDMS) microfluidic device. The properties of PDMS channel surfaces and key microchannel regions for DE generation are characterized under different levels of treatment. The size, shell thickness, and number of inner cores of generated DEs are shown to be highly controllable by tuning the phase flow rate ratios. Using DEs as templates, we successfully achieve a one-step generation and collection of gelatin microgels. Additionally, we demonstrate the biological capability of generated DEs by flow cytometric screening of the encapsulation and growth of yeast cells within DEs. We expect that the proposed approach will be widely used to create microfluidic devices with more complex wettability patterns.

Recent advances in droplet microfluidics for enzyme and cell factory engineering

Enzymes and cell factories play essential roles in industrial biotechnology for the production of chemicals and fuels. The properties of natural enzymes and cells often cannot meet the requirements of different industrial processes in terms of cost-effectiveness and high durability. To rapidly improve their properties and performances, laboratory evolution equipped with high-throughput screening methods and facilities is commonly used to tailor the desired properties of enzymes and cell factories, addressing the challenges of achieving high titer and the yield of the target products at high/low temperatures or extreme pH, in unnatural environments or in the presence of unconventional media. Droplet microfluidic screening (DMFS) systems have demonstrated great potential for exploring vast genetic diversity in a high-throughput manner (>10⁶/h) for laboratory evolution and have been increasingly used in recent years, contributing to the identification of extraordinary mutants. This review highlights the recent advances in concepts and methods of DMFS for library screening, including the key factors in droplet generation and manipulation, signal sources for sensitive detection and sorting, and a comprehensive summary of success stories of DMFS implementation for engineering enzymes and cell factories during the past decade.

An ultrahigh-throughput screening platform based on flow cytometric droplet sorting for mining novel enzymes from metagenomic libraries

Uncultivable microbial communities provide enormous reservoirs of enzymes, but their experimental identification by functional metagenomics is challenging, mainly due to the difficulty of screening enormous metagenomic libraries. Here, we propose a reliable and convenient ultrahigh-throughput screening platform based on flow cytometric droplet sorting (FCDS). The FCDS platform employs water-in-oil-in-water double emulsion droplets serving as single-cell enzymatic micro-reactors and a commercially available flow cytometer, and it can efficiently isolate novel biocatalysts from metagenomic libraries by processing single cells as many as 10⁸ per day. We demonstrated the power of this platform by screening a metagenomic library constructed from domestic running water samples. The FCDS assay screened 30 million micro-reactors in only 1 h, yielding a collection of esterase genes. Among these positive hits, Est WY was identified as a novel esterase with high catalytic efficiency and distinct evolutionary origin from other lipolytic enzymes. Our study manifests that the FCDS platform is a robust tool for functional metagenomics, with the potential to significantly improve the efficiency of exploring novel enzymes from nature.

Polylactic is a Sustainable, Low Absorption, Low Autofluorescence Alternative to Other Plastics for Microfluidic and Organ-on-Chip Applications

Organ-on-chip (OOC) devices are miniaturized devices replacing animal models in drug discovery and toxicology studies. The majority of OOC devices are made from polydimethylsiloxane (PDMS), an elastomer widely used in microfluidic prototyping, but posing a number of challenges to experimentalists, including leaching of uncured oligomers and uncontrolled absorption of small compounds. Here we assess the suitability of polylactic acid (PLA) as a replacement material to PDMS for microfluidic cell culture and OOC applications. We changed the wettability of PLA substrates and demonstrated the functionalization method to be stable over a time period of at least 9 months. We successfully cultured human cells on PLA substrates and devices, without coating. We demonstrated that PLA does not absorb small molecules, is transparent (92% transparency), and has low autofluorescence. As a proof of concept of its manufacturability, biocompatibility, and transparency, we performed a cell tracking experiment of prostate cancer cells in a PLA device for advanced cell culture.

Wettability patterning in microfluidic devices using thermally-enhanced hydrophobic recovery of PDMS

Spatial control of wettability is key to many applications of microfluidic devices, ranging from double emulsion generation to localized cell adhesion. A number of techniques, often based on masking, have been developed to produce spatially-resolved wettability patterns at the surface of poly(dimethylsiloxane) (PDMS) elastomers. A major impediment they face is the natural hydrophobic recovery of PDMS: hydrophilized PDMS surfaces tend to return to hydrophobicity with time, mainly because of diffusion of low molecular weight silicone species to the surface. Instead of trying to avoid this phenomenon, we propose in this work to take advantage of hydrophobic recovery to modulate spatially the surface wettability of PDMS. Because temperature speeds up the rate of hydrophobic recovery, we show that space-resolved hydrophobic patterns can be produced by locally heating a plasma-hydrophilized PDMS surface with microresistors. Importantly, local wettability is quantified in microchannels using a fluorescent probe. This "thermo-patterning" technique provides a simple route to in situ wettability patterning in closed PDMS chips, without requiring further surface chemistry.

Breakups of an encapsulated surfactant-laden aqueous droplet under a DC electric field

We investigate the breakups of an encapsulated conducting aqueous droplet under a direct-current electric field via extensive experiments and theoretical analysis. The encapsulating shell phase and the ambient phase consist of leaky dielectric liquids. We change the surface tension by using an aqueous core with different surfactant (Tween 80) concentrations. Moreover, we vary the core size under different electric-field conditions and observe the core dynamics. We present three different breakup modes of the encapsulated droplet. In the first mode, the encapsulated core forms asymmetric Janus shapes after breakup. In the second and third breakup modes, stable and unstable ternary droplets are formed, respectively. We show that the surfactant molecules significantly alter the dynamics of core stretching. According to the theoretical analysis, we identify the critical conditions of instability leading to breakup. We plot the breakup modes in the form of a phase diagram in the electric capillary number (Ca₂₃ = ε₃r_sE_o²/γ₂₃; ratio of interfacial electric to capillary stresses) vs. radius ratio of the core to the shell (β = r_c/r_s) parametric space at different nondimensional surfactant concentrations (C* = C_Tween 80/C_CMC, where C_CMC represents the critical micellar concentration). The study provides essential physical insight into encapsulated emulsions and is useful for their application in various areas of science and technology.

In-Channel Responsive Surface Wettability for Reversible and Multiform Emulsion Droplet Preparation and Applications

We report on a simple approach for in-channel functionalization of a polydimethylsiloxane (PDMS) surface to obtain a switchable and reversible wettability change between hydrophilic and hydrophobic states. The thermally responsive polymer, poly( N-Isopropylacrylamide) (PNIPAAm), was grafted on the surface of PDMS channels by UV-induced surface grafting. PNIPAAm-grafted PDMS (PNIPAAm-g-PDMS) surface wettability can be thermally tuned to obtain water contact angles varying in the range of 24.3 to 106.1° by varying temperature at 25-38 °C. By selectively modifying the functionalized area in the microfluidic channels, multiform emulsion droplets of oil-in-water (O/W), water-in-oil (W/O), oil-in-water-in-oil (O/W/O), and water-in-oil-in-water (W/O/W) could be created on-demand. Combining solid surface wettability and liquid-liquid interfacial properties, tunable generation of O/W and W/O droplet and stratified flows were enabled in the same microfluidic device with either different or the same two-phase fluidic systems, by properly heating/cooling thermal-responsive microfluidic channels and choosing suitable surfactants. Controllable creation of O/W/O and W/O/W droplets was also achieved in the same microfluidic device, by locally heating or cooling the droplet generation areas with integrated electric heaters to achieve opposite surface wettability. Hollow microcapsules were prepared using double emulsion droplets as templates in the microfluidic device with sequential hydrophobic and hydrophilic channel segments, demonstrating the strength of the proposed approach in practical applications.

Droplet control technologies for microfluidic high throughput screening (μHTS)

The transition from micro well plate and robotics based high throughput screening (HTS) to chip based screening has already started. This transition promises reduced droplet volumes thereby decreasing the amount of fluids used in these studies. Moreover, it significantly boosts throughput allowing screening to keep pace with the overwhelming number of molecular targets being discovered. In this review, we analyse state-of-the-art droplet control technologies that exhibit potential to be used in this new generation of screening devices. Since these systems are enclosed and usually planar, even some of the straightforward methods used in traditional HTS such as pipetting and reading can prove challenging to replicate in microfluidic high throughput screening (μHTS). We critically review the technologies developed for this purpose in depth, describing the underlying physics and discussing the future outlooks.

Marker patterning: a spatially resolved method for tuning the wettability of PDMS

This article presents a marker patterning method where a permanent ink is used as a masking layer. During plasma oxidation, the PDMS surfaces are protected leading to a simple and easy wettability patterning.