Hydrogels have been developed to respond to a wide variety of stimuli, but their use in macroscopic systems has been hindered by slow response times (diffusion being the rate-limiting factor governing the swelling process). However, there are many natural examples of chemically driven actuation that rely on short diffusion paths to produce a rapid response. It is therefore expected that scaling down hydrogel objects to the micrometre scale should greatly improve response times. At these scales, stimuli-responsive hydrogels could enhance the capabilities of microfluidic systems by allowing self-regulated flow control. Here we report the fabrication of active hydrogel components inside microchannels via direct photopatterning of a liquid phase. Our approach greatly simplifies system construction and assembly as the functional components are fabricated in situ, and the stimuli-responsive hydrogel components perform both sensing and actuation functions. We demonstrate significantly improved response times (less than 10 seconds) in hydrogel valves capable of autonomous control of local flow.

A microfluidic platform for the construction of microscale components and autonomous systems is presented. The platform combines liquid-phase photopolymerization, lithography, and laminar flow to allow the creation of complex and autonomous microfluidic systems. The fabrication of channels, actuators, valves, sensors, and systems is demonstrated. Construction times can be as short as 10 min, providing ultrarapid prototyping of microfluidic systems. C onstruction of microscale systems generally has been approached from two perspectives. Either the components are fabricated separately and then assembled (as at the macroscale) or lithography-based microfabrication methods are used to create the components at their desired locations (e.g., polysilicon surface micromachining). Assembly of micrometer-sized objects has proven to be nontrivial because electrostatic and other surface forces are overwhelming at the microscale, making manipulation difficult (1). Through appropriate geometric design, these forces can be harnessed to self-assemble small parts (2). Conventional lithographic approaches show promise, but the many disparate materials and processes hinder the fabrication of complex systems. For example, the processes used to construct one system component (e.g., a sensor) may be incompatible with those for other components (e.g., pumps and valves). To realize microscale systems for many different applications, unconventional approaches are needed to overcome these difficulties. The physics of scaling (i.e., laminar flow, high surface-to-volume ratio) can lead to significantly improved performance in some medical and biological applications and also allow for in-channel construction. Whitesides and coworkers (3) have demonstrated several in-channel fabrication techniques that use laminar flow to create textured walls and to build metal traces within microchannels. Smela et al. (4) demonstrated conductive microscale actuators built on flat substrates by patterning conductive polymers using lithography. Two-photon polymerization has been used to create three-dimensional (3D) structures from a polymer gel precursor (5, 6).Previously, we reported the ability to build in-channel autonomous hydrogel valves by using a photopolymerization process (7). It has been demonstrated that stimuli-responsive hydrogels are the natural materials for microfluidic systems in terms of scaling physics because smaller size leads to faster volume changes for these diffusion-controlled processes. Here, we expand this photopolymerization method to a fabrication platform for total system construction. This fabrication platform, which we refer to as microfluidic tectonics (FT), utilizes microfluidics, photopolymerization, and materials chemistry to create autonomous microfluidic systems controlled by the local fluidic environment. FT allows one to develop a wide variety of microfluidic systems by using one common construction platform, providing several key advantages. First, it provides a general, integrated platform for the construction of...

A biomimetic hydrogel valve capable of directional flow control in response to the local fluid environment in microfluidic channels is described. The valve consists of a pair of photopolymerized hydrogel bistrips with different pH sensitivities, and becomes activated or deactivated reversibly based on local pH as a result of volume and shape changes. When activated at pH 8, the valve mimics anatomic venous valves both functionally and structurally, allowing fluid flow in one direction under forward pressure while restricting flow in the opposite direction. At an applied pressure of 0.71 psi, the forward flow rate was 140 μL/min, compared to a backward leakage rate of 2.3 μL/min.

Several microvalves utilizing stimuli-responsive hydrogel materials have been developed. The hydrogel components are fabricated inside microchannels using a liquid phase polymerization process. In-channel processing greatly simplifies device construction, assembly, and operation since the functional components are fabricated in situ and can perform both sensing and actuation functions. Two in situ photopolymerization techniques, "laminar stream mode" and "mask mode," have been explored. Three two-dimensional (2-D) valves were fabricated and tested (response time, pressure drop, maximum differential pressure). In addition, a hydrogel/PDMS three-dimensional (3-D) hybrid valve that physically separates the sensing and regulated streams was demonstrated. Analytical modeling was performed on the 3-D valve. Hydrogel-based microvalves have a number of advantages over conventional microvalves, including relatively simple fabrication, no external power requirement, no integrated electronics, large displacement (185 m), and large force generation (22 mN).[597]