Philipp Jan Mehner

Stimuli‐sensitive hydrogels have an outstanding potential for miniaturized, integrated sensor, and actuator systems and especially for lab‐on‐chip technology, but the application is still in its infancy. One major reason may be that design and realization of hydrogel‐based systems are exceptionally complex and demanding. Here, the design parameters of a key component, the hydrogel‐based valve, are discussed in their entirety. Key developments in the fields of stimuli‐sensitive hydrogels are highlighted and the necessary know‐how in material behavior, microstructuring technologies, modeling and name five essential design guidelines as well as scaling laws for hydrogel‐based components, including microfluidic one‐directional valves, microelectromechanical systems valves, self‐regulating, chemomechanical valves, and chemofluidic transistors, is provided.

Despite great progress of lab‐on‐a‐chip (LoC) technology platforms in the last 30 years, there is a lack of standardized microfluidic components, real on‐chip automation and progressive functional scalability of the fluidic circuits. Hydrogel‐based microfluidic circuits have a high scaling potential and provide on‐chip automation, but are complex in system design. An advanced circuit concept for planar microfluidic chip architectures, originating from the early era of the semiconductor‐based resistor‐transistor‐logic (RTL) is presented and the hydrogel‐based chemical volume phase transition transistor (CVPT) for logic gate operations is implemented. The circuit concept (CVPT‐RTL) is robust and simple in design, feasible with common materials and manufacturing techniques of the LoC technology. Thereby, three major challenges are solved: contamination issues, maintaining the signal compliance for cascadability, and chemical signal inversion. As a central element, a CVPT cascode is introduced. The functionality of the concept is verified by a 24 h test of the NAND gate operation and a self‐automated chemofluidic analog‐to‐digital converter, utilized as interface between bioreactors and extended microfluidic logic circuits. Moreover, the CVPT‐RTL cascode demonstrates the expected self‐stabilizing performance of the NAND gate. Accompanying simulations of the component behavior based on a network description implemented in Matlab Simscape match with the experimental results.

Background: Endoscopic and laparoscopic electrosurgical devices (ED) are of great importance in modern medicine but can cause adverse events such as tissue injuries and burns from residual heat. While laparoscopic tools are well investigated, detailed insights about the temperature profile of endoscopic knives are lacking. Our aim is to investigate the temperature and the residual heat of laparoscopic and endoscopic monopolar instruments to increase the safety in handling ED. Methods: An infrared camera was used to measure the temperature of laparoscopic and endoscopic instruments during energy application and to determine the cooling time to below 50 °C at a porcine stomach. Different power levels and cutting intervals were studied to investigate their impact on the temperature profile. Results: During activation, the laparoscopic hook exceeded 120 °C regularly for an up to 10 mm shaft length. With regards to endoknives, only the Dual Tip Knife showed a shaft temperature of above 50 °C. The residual heat of the laparoscopic hook remained above 50 °C for at least 15 s after activation. Endoknives cooled to below 50 °C in 4 s. A higher power level and longer cutting duration significantly increased the shaft temperature and prolonged the cooling time (p < 0.001). Conclusion: Residual heat and maximum temperature during energy application depend strongly on the chosen effect and cutting duration. To avoid potential injuries, the user should not touch any tissue with the laparoscopic hook for at least 15 s and with the endoknives for at least 4 s after energy application. As the shaft also heats up to over 120 °C, the user should be careful to avoid tissue contact during activation with the shaft. These results should be strongly considered for safety reasons when handling monopolar ED.

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.

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.

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.

We present a technique for the simultaneous photostructuring of hydrogels on the μm scale with different compositions on one substrate. The existing methods (e.g. spin-coating of the hydrogel precursor and application of simple moulds or spacers) have less potential to integrate spatially separated functional hydrogel dots as needed for the fabrication of complex polydimethylsiloxane (PDMS) on-glass microfluidic chips. Here, we report a method for the fast production of precisely placed hydrogel structures with adjustable height (≤80 μm) and width (≤500 μm) on glass substrates utilizing black polyoxymethylene moulds. The integration of several chambers for different hydrogel precursor solutions in one mould allows the simultaneous photopolymerization of hydrogels with different compositions. To visualize the difference in composition of the hydrogel dots and to demonstrate the precision in structuring, various quantum dots were added to the hydrogel precursor solutions and physically entrapped during the photopolymerization. To show the high potential of our photostructuring method, a microfluidic double-chamber reactor was designed and the variability in the performance of tri-enzymatic cascade reactions with optionally spatially separated or non-separated enzymes β-galactosidase, glucose oxidase, and horseradish peroxidase was visualized.

We propose a planar hydrogel-based micro-valve design which is modeled as a library element for Matlab Simulink. For this test case, a pressure pump (voltage source) in series with a micro-valve model (variable fluidic resistance) is built up. The micro-valve subsystem is separated in four main parts. Based on the applied temperature stimulus, the equilibrium length is determined according to an experimentally verified fit function. Furthermore, the equilibrium length considers a static hysteresis effect which is modeled in analogy to the saturation of magnetization in electric transformers. In a second step, the transient behavior follows a first order differential equation, but the cooperate diffusion coefficient is size dependent affecting the rise time of the system. This causes a faster swelling than deswelling of the hydrogel. In the third section, the stiffness property is implemented to calculate the maximum sealing pressure and the resulting gap between the hydrogel and the wall. The fluidic resistance of the micro-valve considers a three-dimensional geometry and is calculated based on a look-up table, extracted from a fluid-structure-interaction (FSI) model generated from a finite element structure. The proposed model allows a full description of the fluidic hydrogel-based micro-valve and is part of an upcoming microfluidic toolbox for Matlab Simulink containing passive elements and optional chemical reactions like mixing fluids and enzyme reactions for future applications.

We propose an improved micro reactor design for a scalable microfluidic device, in which enzymes are immobilized in a hydrogel matrix. Furthermore, fluid flow is controlled by means of hydrogel-based micro-valves. In this work, computational flow simulations will be compared to experimental results to highlight new design ideas and to improve wetting and concentration distribution through the entire chamber volume, even for high aspect ratios. Additionally, modelling concepts will be introduced to efficiently describe multi-domain problems like enzyme reactions. With the help of a computer-aided design process which is capable to simulate hydrogel-based microfluidic systems it is possible to better understand, predict and visualize the behavior of micro-reactors and support the development of highly integrated hydrogel-based microfluidic circuits.

We propose a reduced order flow chart model of a hydrogel-based micro-valve, which focuses on the implementation of pressure and flow regimes as well as swelling kinetics of hydro-gels within a micro-valve seat. As material behavior, the transient swelling and the quasi-static phase-change are demonstrated. This phase-change is triggered by a concentration level which is determined by the flow behavior of the microfluidic network. Furthermore, a library model is created and the simulation of 100 valves connected in a microfluidic network is performed simultaneously. It is aimed to create a user-friendly toolbox for hydrogel-based microfluidic circuits in Matlab Simulink which is capable of accurately predicting flow and hydrogel behaviors and improve the rapid prototyping design process for highly complex microfluidic systems.