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.

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.

Hydrogel Patterns in Microfluidic Devices by Do-It-Yourself UV-Photolithography Suitable for Very Large-Scale Integration.

The interest in large-scale integrated (LSI) microfluidic systems that perform high-throughput biological and chemical laboratory investigations on a single chip is steadily growing. Such highly integrated Labs-on-a-Chip (LoC) provide fast analysis, high functionality, outstanding reproducibility at low cost per sample, and small demand of reagents. One LoC platform technology capable of LSI relies on specific intrinsically active polymers, the so-called stimuli-responsive hydrogels. Analogous to microelectronics, the active components of the chips can be realized by photolithographic micro-patterning of functional layers. The miniaturization potential and the integration degree of the microfluidic circuits depend on the capability of the photolithographic process to pattern hydrogel layers with high resolution, and they typically require expensive cleanroom equipment. Here, we propose, compare, and discuss a cost-efficient do-it-yourself (DIY) photolithographic set-up suitable to micro-pattern hydrogel-layers with a resolution as needed for very large-scale integrated (VLSI) microfluidics. The achievable structure dimensions are in the lower micrometer scale, down to a feature size of 20 µm with aspect ratios of 1:5 and maximum integration densities of 20,000 hydrogel patterns per cm². Furthermore, we demonstrate the effects of miniaturization on the efficiency of a hydrogel-based microreactor system by increasing the surface area to volume (SA:V) ratio of integrated bioactive hydrogels. We then determine and discuss a correlation between ultraviolet (UV) exposure time, cross-linking density of polymers, and the degree of immobilization of bioactive components.

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

Compartmentalized microfluidic devices with immobilized catalysts are a valuable tool for overcoming the incompatibility challenge in (bio) catalytic cascade reactions and high-throughput screening of multiple reaction parameters. To achieve flow control in microfluidics, stimuli-responsive hydrogel microvalves were previously introduced. However, an application of this valve concept for the control of multistep reactions was not yet shown. To fill this gap, we show the integration of thermoresponsive poly(N-isopropylacrylamide) (PNiPAAm) microvalves (diameter: 500 and 600 µm) into PDMS-on-glass microfluidic devices for the control of parallelized enzyme-catalyzed cascade reactions. As a proof-of-principle, the biocatalysts glucose oxidase (GOx), horseradish peroxidase (HRP) and myoglobin (Myo) were immobilized in photopatterned hydrogel dot arrays (diameter of the dots: 350 µm, amount of enzymes: 0.13-2.3 µg) within three compartments of the device. Switching of the microvalves was achieved within 4 to 6 s and thereby the fluid pathway of the enzyme substrate solution (5 mmol/L) in the device was determined. Consequently, either the enzyme cascade reaction GOx-HRP or GOx-Myo was performed and continuously quantified by ultraviolet-visible (UV-Vis) spectroscopy. The functionality of the microvalves was shown in four hourly switching cycles and visualized by the path-dependent substrate conversion.

Furfuryl- and Maleimido Polysaccharides: Synthetic Strategies Toward Functional Biomaterials.

In context with facile and efficient syntheses of functional polymeric materials, the combination of polysaccharides and functional moieties based on renewable resources is a sustainable and valuable approach. This review presents alternatives to prominent click reactions utilizing biopolymer derivatives with furfuryl and maleimide groups. On the one hand, the cross-linking by Diels-Alder reaction of these polymers enables the synthesis of novel materials in the fields of self-healing polymers, tissue engineering, and drug delivery. On the other hand, thiol-ene click reactions allow their conjugation to complex (bio)molecules. Different synthetic strategies are reviewed and the applicability of functional materials is evaluated.

Synthesis and film formation of furfuryl- and maleimido carbonic acid derivatives of dextran.

Carbonic acid derivatives of dextran possessing furfuryl- and maleimido moieties were synthesized and processed into thin films by spin coating. First, products with different degrees of substitution (DS) of up to 3.0 and substitution patterns were obtained and characterized by NMR- and FTIR spectroscopy, as well as elemental analysis. Thin films possessing maleimide groups were obtained by spin coating of maleimido dextran (furan-protected) and dextran furfuryl carbamate that was converted with bismaleimide. The removal of the protecting group (furan) on the thin film was monitored by QCM-D and compared with gravimetric analysis of the bulk material. Film morphology and wettability were determined by means of AFM and contact angle measurements.