In this paper, we describe a method for encapsulation of biomaterials in hydrogel beads using a microfluidic droplet-merging channel. We devised a double T-junction in a microfluidic channel for alternate injection of aqueous fluids inside a droplet unit carried within immiscible oil. With this device, hydrogel beads with diameter\100 lm are produced, and various solutions containing cells, proteins and reagents for gelation could merge with the gel droplets with high efficiency in the broad range of flow rates. Mixing of reagents and reactions inside the hydrogel beads are continuously observed in a microchannel through a microscope. By enabling serial injection of each liquid with the dispersed gel droplets after they are produced from the oil-focusing channel, the device simplifies the sample preparation process, and gelbead fabrication can be coupled with further assay continuously in a single channel. Instantaneous reactions of enzyme inside hydrogel and in-situ formation of cell-containing beads with high viability are demonstrated in this report.

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We investigate stochastic resonance in a globally coupled oscillator system with time delay. The system shows multistability of a desynchronized state and two synchronized states with different collective frequencies, which may be interpreted as multistable perception of ambiguous or reversible figures. Under the influence of a weak periodic external signal, the system exhibits a maximum in the signal-to-noise ratio at an optimum noise level -the characteristic signature of stochastic resonance. We also show stochastic resonance between two limit-cycles in the system. [S0031-9007 (99)08392-1] PACS numbers: 05.40. -a, 07.05.Mh, 42.66.Si, 87.10. + e Synchronization of oscillators has been the topic of much recent investigation [1-3]. Recent experiments [4]showed the synchronized oscillations of neuronal activity in the visual cortex of the cat suggesting that information processing is a cooperative process of neurons. A coupled oscillator model was suggested to understand the temporal and spatial coherence of the oscillations of neuronal activity [2]. The inclusion of time delay into the system is natural in the realistic consideration of finite transmission of interaction. Recently, it was shown that the coupled oscillator system with time delay exhibits multistability of synchronized and desynchronized states [5]. In the synchronized state, the system has also two limit-cycles with different collective frequencies; one is larger than the intrinsic frequency and the other is smaller than the intrinsic frequency. This multistability was interpreted as the perception of ambiguous or reversible figures. Perception of the ambiguous or reversible figures is characterized by noisy multistable dynamics, that is, the different interpretations of the figures are switched with a stochastic time course [6]. Recently, there have been studies on the noisy multistable dynamics in connection with the development of dynamical models of brain function during such switchings in perception [7].There has also been a growing interest in stochastic resonance (SR) associated with noisy nonlinear dynamical systems [8]. SR is characterized by the optimization of the response of the system to an input signal as a function of the input noise strength. The response of the system is measured by a signal-to-noise ratio (SNR) which shows a peak as a function of the input noise strength. This implies that noise may enhance the transmission of information. SR has been demonstrated in numerous physical experiments such as electronic trigger circuits [9], twomode ring lasers [10], and mammalian neuronal networks [11]. The possible importance of SR for the processing of information in neural systems seems evident at all levels of information processing. Indeed, it has long been recognized that noise can improve the performance of certain neural networks [12], and that an optimum noise level can achieve the maximum improvement. SR in low dimensional dynamical systems with attractors is by now a familiar phenomenon, but SR in spatially distributed sys...

For disposable microfluidic devices, easy and inexpensive fabrication is essential. Consequently, replication of microfluidic devices, using injection molding or hot embossing, from a master-mold is widely used. However, the conventional master-mold fabrication technique is unsatisfactory in terms of time and costs. In this regard, direct Ni growth (electroplating) from a back plate is promising when the photoresist is well-defined. Here, we demonstrate the use of SU-8 as a photoresist to define the Ni-growth region. We accomplish this application by focusing on the adhesion, the sidewall profile, and the removal of SU-8: the adhesion is enhanced by controlling the exposure dose, the soft-baking time, and by choosing the adhesion-promoting layer; the sidewall profile is regulated by selecting the intensity of each exposed wavelength, showing an aspect ratio of up to 20.9; and, easy removal is achieved by choosing a proper photoresist-stripper. Using the master-mold fabricated by this method, we test the mechanical stability of the features according to the aspect ratio and length; in the hot embossing process, the features are stable in the aspect ratio of up to 5.8 at a length of 200 microm. In addition, the plastic devices fabricated from this method are applied to the passive stop valves, showing a capillary pressure (-0.2 to -7.2 kPa).

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