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.

Perylene-terminated monodendrons 1−7 and phenyl-terminated reference monodendrons 8−14 have been synthesized, and the intramolecular energy transfer has been studied using steady-state as well as time-resolved fluorescence spectroscopy. In the series 2−7, the light-harvesting ability of these compounds increases with increasing generation due to the increase in molar extinction coefficient. However, the efficiency of the energy transfer decreases with increasing generation in this series. With increasing generation, the photoluminescence intensity from the perylene core still increases and the expected level-off in the photoluminescence intensity has not been reached in this series of compounds. Dendrimer 1 is unique in that the energy transfer in this molecule occurs at a very fast rate. The rate constant for energy transfer in 1 is at least 2 orders of magnitude larger than in 2−7. In contrast to monodendrons 2−7, 1 possesses a variable monomer type at each generation that creates an energy funnel. The ultrafast energy transfer in this system is best explained by the presence of this energy gradient.

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...

We have studied the dynamics of directed, multistep energy transport in a class of fractal-like dendrimeric molecules. For particular forms of these highly branched phenylacetylene dendrimers, both theory and experiment put the lowest excitation energy at the center (locus) of the supermolecule. This results in a structurally symmetric and ordered exciton funnel, with a well-directed energy gradient. We have designed and synthesized a derivative of these dendrimers with a perylene moiety at the locus, which acts as an energy trap for the directed exciton funnel. Spectroscopic evidence indicates transfer efficiency of 98% from the photoabsorbing dendrimer backbone to the perylenic trap.

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Communications separating the PPV-rich parts and the pyridine-rich segments. This will be explained in more detail elsewhere.The significant red emission at high voltages is probably associated with the higher efficiency for light emission in pyridine-rich segments. As demonstrated by OnodaI7] the EL efficiency for light emission from PPyV is rather high (7 N 0.5 YO ) and it is further enhanced due to the presence of the junctionsr7 in PPV/PPyV. At the present time, we cannot exclude alternative explanations associated with the formation of exciton or impurity bands in the x-x* forbidden gap of PPV upon substitution of the pyridine moieties in the copolymer. ExperimentalThe 2,6-pyridyl-dimethylene-bis-(tetramethylene sulfonium chloride) monomer was prepared by adding tetrahydrothiophene (THT) (15 ml) to 2,6bis(chloromethy1)-pyridine (5 g) in methanol (150 ml). The reaction mixture was kept at 50°C for 4 days and was then concentrated to gelation. The hissulfonium salt was precipitated by addition of ice cold acetone (250 ml) and was isolated. The 1,4-phenylenedimethylene-bis-(tetramethylene sulfonium chloride) was prepared according to the literature procedure. The copolymerization was typically carried out as follows. A mixture of 20 YO (molar) of 2,6pyridyl-dimethylene-bis-(tetramethylene sulfonium chloride) (1.49 g) and 1,4phenylene dimethylene-bis-(tetramethylene sulfonium chloride) (5.96 g) in water (200 ml) was prepared. Polymerization was initiated under nitrogen at 0-2°C by addition of a stoichiometric quantity of 1 N NaOH in water (21.2 ml) under constant vigorous stirring for 1 h. The mixture was neutralized with 1 N HCl(9.4 ml) corresponding to 54.2 % conversion of the monomers to the copolyelectrolyte. The copolyelectrolyte solution was homogenized and dialyzed (Mw cut-off 12000) at 4°C for ten days.Films for the various measurements were prepared by casting the dialyzed copolyelectrolyte on relevant substrates by spin coating. The polyelectrolyte was then converted to the final copolymer, co(2,6PyV-PV) by heating in vacuum, 10-6torr, at 280°C for 12 h. Film thicknesses were determined by X-ray reflectivity at grazing angle using a 12 kW Rigaku rotating anode and found to be approximately 1000 A. Absorption spectra were measured on a HP 8452 diode array spectrometer. PL spectra were recorded on Perkin Elmer Luminescence Spectrometer LS 50. EL spectra were recorded using an Oriel monochromator and the light intensity was quantified with a photo-multiplier (products from Research Inc. model no. R955).

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