Hydrogel Microvalves as Control Elements for Parallelized Enzymatic Cascade Reactions in Microfluidics

Boosting Microfluidic Enzymatic Cascade Reactions with pH-Responsive Polymersomes by Spatio-Chemical Activity Control

Microfluidic flow reactors permit the implementation of sensitive biocatalysts in polymeric environments (e.g., hydrogel dots), mimicking nature through the use of diverse microstructures within defined confinements. However, establishing complex hybrid structures to mimic biological processes and functions under continuous flow with optimal utilization of all components involved in the reaction process represents a significant scientific challenge. To achieve spatial, chemical, and temporal control for any microfluidic application, compartmentalization is required, as well as the unification of different sensitive compartments in the reaction chamber for the microfluidic flow design. This study presents a self-regulating microfluidic system fabricated by a sequential photostructuring process with an intermediate chemical process step to realize pH-sensitive hybrid structures for the fabrication of a microfluidic double chamber reactor for controlled enzymatic cascade reaction (ECR). The key point is the adaptation and retention of the function of pH-responsive horseradish peroxidase-loaded polymersomes in a microfluidic chip under continuous flow. ECR is successfully triggered and controlled by an interplay between glucose oxidase-converted glucose, the membrane state of pH-responsive polymersomes, and other parameters (e.g., flow rate and fluid composition). This study establishes a promising noninvasive regulatory platform for extended spatio-chemical control of current and future ECR and other cascade reaction systems.

Microfluidic design in single-cell sequencing and application to cancer precision medicine

Single-cell sequencing (SCS) is a crucial tool to reveal the genetic and functional heterogeneity of tumors, providing unique insights into the clonal evolution, microenvironment, drug resistance, and metastatic progression of cancers. Microfluidics is a critical component of many SCS technologies and workflows, conferring advantages in throughput, economy, and automation. Here, we review the current landscape of microfluidic architectures and sequencing techniques for single-cell omics analysis and highlight how these have enabled recent applications in oncology research. We also discuss the challenges and the promise of microfluidics-based single-cell analysis in the future of precision oncology.

Reversible Molecular Capture and Release in Microfluidics by Host-Guest Interactions in Hydrogel Microdots

The integration of microscopic hydrogels with high specific surface area and physically reactive groups into microfluidic systems for selective molecular interactions is attracting increasing attention. Herein, the reversible capture and release of molecules through host-guest interactions of hydrogel dots in a microfluidic device is reported, which translates the supramolecular chemistry to the microscale conditions under continuous flow. Polyacrylamide (PAAm) hydrogel arrays with grafted β-cyclodextrin (β-CD)  modified poly(2-methyl-2-oxazoline) (CD-PMOXA) chains are fabricated by photopolymerization and integrated into a polydimethylsiloxane (PDMS)-on-glass chip. The β-CD/adamantane (β-CD/Ada) host-guest complex is confirmed by two dimensional Nuclear Overhauser Effect Spectroscopy NMR (2D NOESY NMR) prior to transfer to microfluidics. Ada-modified molecules are successfully captured by host-guest interaction formed between the CD-PMOXA grafted chains in the hydrogel network and the guest molecule in the solution. Furthermore, the captured molecules are released by perfusing free β-CD with higher binding affinity than those grafted in the hydrogel array. A small guest molecule adamantane-fluorescein-isothiocyanate (Ada-FITC) and a macromolecular guest molecule (Ada-PMOXA-Cyanine 5 (Cy5)) are separately captured and released for three times with a release ratio up to 46% and 92%, respectively. The reproducible capture and release of functional molecules with different sizes demonstrates the stability of this hydrogel system in microfluidics and will provide an opportunity for future applications.

Fabrication of Chemofluidic Integrated Circuits by Multi-Material Printing

Photolithographic patterning of components and integrated circuits based on active polymers for microfluidics is challenging and not always efficient on a laboratory scale using the traditional mask-based fabrication procedures. Here, we present an alternative manufacturing process based on multi-material 3D printing that can be used to print various active polymers in microfluidic structures that act as microvalves on large-area substrates efficiently in terms of processing time and consumption of active materials with a single machine. Based on the examples of two chemofluidic valve types, hydrogel-based closing valves and PEG-based opening valves, the respective printing procedures, essential influencing variables and special features are discussed, and the components are characterized with regard to their properties and tolerances. The functionality of the concept is demonstrated by a specific chemofluidic chip which automates an analysis procedure typical of clinical chemistry and laboratory medicine. Multi-material 3D printing allows active-material devices to be produced on chip substrates with tolerances comparable to photolithography but is faster and very flexible for small quantities of up to about 50 chips.

Digital manufacturing for accelerating organ-on-a-chip dissemination and electrochemical biosensing integration

Organ on-a-chip (OoC) is a promising technology that aims to recapitulate human body pathophysiology in a more precise way to advance in drug development and complex disease understanding. However, the presence of OoC in biological laboratories is still limited and mainly restricted to laboratories with access to cleanroom facilities. Besides, the current analytical methods employed to extract information from the organ models are endpoint and post facto assays which makes it difficult to ensure that during the biological experiment the cell microenvironment, cellular functionality and behaviour are controlled. Hence, the integration of real-time biosensors is highly needed and requested by the OoC end-user community to provide insight into organ function and responses to stimuli. In this context, electrochemical sensors stand out due to their advantageous features like miniaturization capabilities, ease of use, automatization and high sensitivity and selectivity. Electrochemical sensors have been already successfully miniaturized and employed in other fields such as wearables and point-of-care devices. We have identified that the explanation for this issue may be, to a large extent, the accessibility to microfabrication technologies. These fields employ preferably digital manufacturing (DM), which is a more accessible microfabrication approach regardless of funding and facilities. Therefore, we envision that a paradigm shift in microfabrication that adopts DM instead of the dominating soft lithography for the in-lab microfabrication of OoC devices will contribute to the dissemination of the field and integration of the promising real-time sensing.

Fabrication of Thermo-Responsive Controllable Shape-Changing Hydrogel

Temperature response double network (DN) hydrogels comprising a network formed by polymerization of methacrylic acid (MA) modified PVA, N,N’-methylene bis(acrylamide), N-isopropylacryl amide (NIPAM), and one formed from crystalline polyvinyl alcohol (PVA) are prepared in a 3D printed tailor-made mold. The (PVA-MA)-g-PNIPAAm thermoset intermediate is formed in water by a radical, photo-initiated process, and in the presence of dissolved PVA polymers. A subsequent freezing-thawing sequence induces the crystallization of the PVA network, which forms a second network inside the thermoset NIPAM polymer. The prepared hydrogel is thermoresponsive by the phase transition of PNIPAAm segments (T ≈ 32 °C) and has good mechanical properties (tensile strength 1.23 MPa, compressive strength 1.47 MPa). Thermal cycling between room temperature at 40 or 50 °C shows the product converses from a virgin-state to a steady-state, which most likely involves the reorganization of PVA crystals. The swelling-deswelling cycles remain clear at a length change of about 13%.

Reversible Protein Capture and Release by Redox-Responsive Hydrogel in Microfluidics

Stimuli-responsive hydrogels have a wide range of potential applications in microfluidics, which has drawn great attention. Double cross-linked hydrogels are very well suited for this application as they offer both stability and the required responsive behavior. Here, we report the integration of poly(N-isopropylacrylamide) (PNiPAAm) hydrogel with a permanent cross-linker (N,N'-methylenebisacrylamide, BIS) and a redox responsive reversible cross-linker (N,N'-bis(acryloyl)cystamine, BAC) into a microfluidic device through photopolymerization. Cleavage and re-formation of disulfide bonds introduced by BAC changed the cross-linking densities of the hydrogel dots, making them swell or shrink. Rheological measurements allowed for selecting hydrogels that withstand long-term shear forces present in microfluidic devices under continuous flow. Once implemented, the thiol-disulfide exchange allowed the hydrogel dots to successfully capture and release the protein bovine serum albumin (BSA). BSA was labeled with rhodamine B and functionalized with 2-(2-pyridyldithio)-ethylamine (PDA) to introduce disulfide bonds. The reversible capture and release of the protein reached an efficiency of 83.6% in release rate and could be repeated over 3 cycles within the microfluidic device. These results demonstrate that our redox-responsive hydrogel dots enable the dynamic capture and release of various different functionalized (macro)molecules (e.g., proteins and drugs) and have a great potential to be integrated into a lab-on-a-chip device for detection and/or delivery.

Enzymatic Synthesis of Sialic Acids in Microfluidics to Overcome Cross-Inhibitions and Substrate Supply Limitations

Multienzymatic cascade reactions are a powerful strategy for straightforward and highly specific synthesis of complex materials, such as active substances in drugs. Cross-inhibitions and incompatible reaction steps, however, often limit enzymatic activity and thus the conversion. Such limitations occur, e.g., in the enzymatic synthesis of the biologically active sialic acid cytidine monophosphate N-acetylneuraminic acid (CMP-Neu5Ac). We addressed this challenge by developing a confinement and compartmentalization concept of hydrogel-immobilized enzymes for improving the efficiency of the enzyme cascade reaction. The three enzymes required for the synthesis of CMP-Neu5Ac, namely, N-acyl-d-glucosamine 2-epimerase (AGE), N-acetylneuraminate lyase (NAL), and CMP-sialic acid synthetase (CSS), were immobilized into bulk hydrogels and microstructured hydrogel-enzyme-dot arrays, which were then integrated into microfluidic devices. To overcome the cytidine triphosphate (CTP) cross-inhibition of AGE and NAL, only a low CTP concentration was applied and continuously conveyed through the device. In a second approach, the enzymes were compartmentalized in separate reaction chambers of the microfluidic device to completely avoid cross-inhibitions and enable the use of higher substrate concentrations. Immobilization efficiencies of up to 25% and pronounced long-term activity of the immobilized enzymes for several weeks were realized. Moreover, immobilized enzymes were less sensitive to inhibition and the substrate-channeling effect between immobilized enzymes promoted the overall conversion in the trienzymatic cascade reaction. Based on this, CMP-Neu5Ac was successfully synthesized by immobilized enzymes in noncompartmentalized and compartmentalized microfluidic devices. This study demonstrates the high potential of immobilizing enzymes in (compartmentalized) microfluidic devices to perform multienzymatic cascade reactions despite cross-inhibitions under continuous flow conditions. Due to the ease of enzyme immobilization in hydrogels, this concept is likely applicable for many cascade reactions with or without cross-inhibition characteristics.