Smart hydrogels are useful elements in microfluidic systems because they respond to environmental stimuli and are capable of storing reagents. We present here a concept of using hydrogels (poly(N-isopropylacrylamide)) as an interface between continuous and discontinuous microfluidics. Their swelling and shrinking capabilities allow them to act as storage elements for reagents absorbed in the swelling process. When the swollen hydrogel collapses in an oil-filled channel, the incorporated water and molecules are expelled from the hydrogel and form a water reservoir. Water-in-oil droplets can be released from the reservoir generating different sized droplets depending on the flow regime at various oil flow rates (dispensing functionality). Different hydrogel sizes and microfluidic structures are discussed in terms of their storage and droplet formation capabilities. The time behaviour of the hydrogel element is investigated by dynamic swelling experiments and computational fluid dynamics simulations. By precise temperature control, the device acts as an active droplet generator and converts continuous to discontinuous flows.

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 utomation 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 selfstabilizing 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.

Smart hydrogels are useful elements in microfluidic systems because they respond to environmental stimuli and are capable of storing reagents. We present here a concept of using hydrogels (poly(N-isopropylacrylamide)) as an interface between continuous and discontinuous microfluidics. Their swelling and shrinking capabilities allow them to act as storage elements for reagents absorbed in the swelling process. When the swollen hydrogel collapses in an oil-filled channel, the incorporated water and molecules are expelled from the hydrogel and form a water reservoir. Water-in-oil droplets can be released from the reservoir generating different sized droplets depending on the flow regime at various oil flow rates (dispensing functionality). Different hydrogel sizes and microfluidic structures are discussed in terms of their storage and droplet formation capabilities. The time behaviour of the hydrogel element is investigated by dynamic swelling experiments and computational fluid dynamics simulations. By precise temperature control, the device acts as an active droplet generator and converts continuous to discontinuous flows.

Bisensitive interpenetrating polymer network (IPN) hydrogels of temperature sensitive net-poly(Nisopropylacrylamide) and pH sensitive net-poly(acrylic acid-co-acrylamide) for microfluidic applications were prepared via a sequential synthesis using free radical polymerization. The IPN indicated a suitable reversible alteration of swelling in response to the change in pH and temperature. The adequate change of the hydrogel volume is a basic requirement for microfluidic applications. Using the introduced correction factor f, it is possible to determine the cooperative diffusion coefficient (Dcoop) of cylindrical samples at any aspect ratio. The determined cooperative diffusion coefficient allowed the evaluation of varying swelling processes of different network structures. The presence of the second sub-network of the IPN improved the swelling behaviour of the first sub-network compared to the individual networks.

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 hydrogelbased components, including microfluidic one-directional valves, microelectromechanical systems valves, self-regulating, chemomechanical valves, and chemofluidic transistors, is provided.