Rapid mixing of sub-microlitre drops by magnetic micro-stirring

Bruyker, Dirk De; Recht, Michael I.; Bhagat, Ali Asgar S.; Torres, Francisco Eduardo; Bell, Alan G.; Bruce, Richard H.

doi:10.1039/c1lc20354a

Cited by 29 publications

(36 citation statements)

References 28 publications

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“…To make a fair comparison, we synthesized large stir bars (ca. Most of the micrometer-sized stir bars reported were in the range of 30-400 mm in width; [7,9,10] they should sediment more quickly than our largest stir bars. 10 mm in length) by coating a thick layer of silica, making them close to but still smaller in size than the typical micro-sized magnetic stir bars.…”

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confidence: 76%

“…Generating such disturbance must involve an external input of energy, for example, transduction of magnetic field energy through the use of commercial magnetic stir bars. [10] Because of gravitational and magnetic attraction, micro-sized stir bars tend to stir only at the bottom of the vessel, [10] leaving most part of the solution unstirred. [2] The main challenge lies in the fabrication of low-cost stir bars that are sufficiently small but still able to transduce external energy for the mixing.…”

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confidence: 99%

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Stirring in Suspension: Nanometer‐Sized Magnetic Stir Bars

et al. 2013

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Rapid mixing is essential for achieving effective chemical and biological reactions. Because passive diffusion is a slow process, it is often necessary to agitate or stir a solution. Generating such disturbance must involve an external input of energy, for example, transduction of magnetic field energy through the use of commercial magnetic stir bars. But such use is impractical for the tiny channels and droplets, which are of great importance for lab-on-chip applications [1] and microliter bioassay. [2] The main challenge lies in the fabrication of low-cost stir bars that are sufficiently small but still able to transduce external energy for the mixing. Moreover, the stir bars should be introduced, operated, and extracted with ease.In microfluidic research, a number of methods have been developed to improve mixing. For a lamellar flow confined in a channel, turbulence can be induced by forcing the solution through a complex winding channel, [1b, 3] or by pulsed injection of additional solution/bubbles from the side channels, [4] thermal gradients can be used to induce convection, [2c, 5] direct physical agitation can be achieved using ultrasound or piezoelectric transducers, [6] and in-channel stirring was achieved using magnetic turbines that were fabricated by lithographic methods. [7] These mixing schemes were often fabricated together with the channel systems, because it is too difficult to introduce them after the fabrication.In static microdroplets, however, transducing external energy for mixing remains a challenge, particularly in arrays of microdroplets. Ultrasonication and violent stirring can break up the droplets, while thermal gradients across tiny droplets are impractical. To date, magnetic stirring is still the most convenient option. Micro-sized stir bars have been reported, for example, linear chains of polymer beads embedded with magnetic nanoparticles (NPs), [8] star-shaped micro-stirrers made by soft lithography, [9] and cobalt-based magnetic bars cut by laser micromachining. [10] Because of gravitational and magnetic attraction, micro-sized stir bars tend to stir only at the bottom of the vessel, [10] leaving most part of the solution unstirred. While facile and scalable synthesis remains a challenge, the main problem of microsized stir bars is still their size: they are too large to remain suspended, but too small to churn up the whole solution.Herein, we report a simple and scalable method for fabricating magnetic stir bars, which are tunable from 75 nm-1.4 mm in width and up to around 17 mm in length (Figure 1). They are straight single-line chains assembled from 40 nm magnetic NPs and then preserved in silica shells of variable thickness. These rigid magnetic chains showed immediate response to a common magnetic stir plate and can be easily recovered. Being small and in large number, they can remain suspended and stir independently in all parts of the solution. These nano stir bars can be readily dispensed for parallel stirring of droplets down to picoliter in volume (4 picoliter). ...

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Stirring in Suspension: Nanometer‐Sized Magnetic Stir Bars

et al. 2013

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“…ACET, AC electrothermal; DEP, dielectrophoresis mixing in droplet-based microfluidic applications, including electrowetting-on-dielectric [6,8], acoustic excitation [9], and magnetic stirring [10]. However, most of the reported strategies perform the mixing process through the movement of the droplet, which inhibits on-the-spot mixing that may otherwise be absolutely essential for performing a desired biochemical or biomedical task (such as medical diagnostics), in situ [7,11].…”

Section: Abbreviationsmentioning

confidence: 99%

“…For a needle-and-planeelectrode configuration of droplet actuation, two kinds of hydrodynamic motions were found for two different regimes of frequencies [20]. At relatively low frequency (10)(11)(12)(13)(14)(15), the flow appears due to droplet oscillations. However, at high frequency (35-256 kHz), strong internal flow was observed.…”

Section: Introductionmentioning

confidence: 99%

Strong rotating flow in stationary droplets in low power budget using wire electrode configuration

2019

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We explore a simple strategy of generating strong rotating flow in a stationary surface‐droplet, using an intricate interplay of local electrical and thermal fields. Wire electrodes are employed to generate on‐spot heating without necessitating any elaborate micro‐fabrication, which causes strong local gradients in electrical properties to induce mobile charges into the droplet. Applying a low voltage (∼10 V), strong rotational velocity of the order of mm/s can be achieved in the system, within the standard operating ranges of operating and geometrical parameters. Further, altering the diameter of the electrode, vortices can be tuned locally or globally in low power budget, without incurring any droplet oscillations. These results may turn out to be of immense consequence in enhancing micromixing in a plethora of droplet based applications ranging from thermal management to medical diagnostics to be potentially employed in resource‐limited settings.

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“…Active mixing requires external energy, such as magnetic, acoustic field force or electric force [2]. Bruyker et al [3] designed an active mixing based on magnetic actuation using micro machined magnetic stir bars last year. Ryu et al [4] designed a micro magnetic stir-bar integrated in parylene.…”

Section: Introductionmentioning

confidence: 99%

Simulation and experimental research on micro-channel for detecting cell status in bio-artificial liver

Wu¹,

Cao²,

Huo

et al. 2015

THC

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Abstract. BACKGROUND: Bioartificial liver support system (BALSS) based on culturing hepatocytes is an important research field for the treatment of acute liver failure. It is necessary to monitor the state of liver cell functions during the treatment of BALSS in order to guide clinical treatment. OBJECTIVE: To design a micro-channel chip to achieve flash mixing for timely detection of liver cell status in bioreactors and improving liver cells growth environment to ensure the efficacy of the bio-artificial liver support system. METHODS: Alanine aminotransferase (ALT) and Urea are chosen as detection indicators to reflect the degree of liver cell injury and the detoxification function. A diamond tandem structure micro-channel is designed and optimized to achieve the efficient mixing of serum and ALT or Urea reagent. RESULTS: The simulation and experimental results show that the diamond tandem structure micro-channel can significantly improve the mixing efficiency and meet the online detecting requirements. CONCLUSION: The easily controllable diamond tandem structure micro-channel combines the advantages of active and passive mixer and can effectively mix the serum and ALT or Urea reagent. It lays the foundation for online monitoring of liver cells and will help to improve the viability of liver cell in the bioreactor.

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Rapid mixing of sub-microlitre drops by magnetic micro-stirring

Cited by 29 publications

References 28 publications

Stirring in Suspension: Nanometer‐Sized Magnetic Stir Bars

Stirring in Suspension: Nanometer‐Sized Magnetic Stir Bars

Strong rotating flow in stationary droplets in low power budget using wire electrode configuration

Simulation and experimental research on micro-channel for detecting cell status in bio-artificial liver

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