HIGH-SPEED CHEMICAL SIGNAL GENERATION
WITH MULTI-PLUG MODULATORS
Farouk Azizi and Carlos H. Mastrangelo
Case Western Reserve University, USA.
ABSTRACT
We report the implementation and testing of a chemical signal generator chip that
uses high-frequency multi-plug modulators (MPMs). In MPMs analog concentration
signals are generated from an averaged discrete plug stream. The MPM chip output
plug rate was 25 plugs/sec producing high fidelity chemical signals with output
bandwidth of up to ~3 Hz at flow velocities of 12 cm/s. Several test output signals
including fast (2 s) dopamine releases were synthetically generated.
KEYWORDS: Chemical signals, PCM, PDMS, Modulator
INTRODUCTION
Chemical messenger molecules are crucial in the life sciences [1]. The understanding of their role in in-vitro environments requires the ability of generating synthetic messenger signals in microfluidic environments [1-4]. For example, many
messenger proteins such as cytokines, hormones, growth factors and neurotransmitters regulate and control vital aspects of cellular response through their autocrine,
paracrine and endocrine communication modes. Our ability to artificially generate
such signals can be an enabling key to the probing, understanding, control and regulation of such complex biological systems. In this paper we introduce a new microfluidic chip for chemical signal generation using Multi- Plug Modulators (MPM).
CHIP DESIGN AND FABRICATION
Recently, compact microfluidic single-bit pulse-code modulators (PCM) were
developed to reproduce synthetic extracellular releases [4]. Single-bit PCMs such as
those shown in Figures 1a-1b produce output signals by averaging a serial stream of
digitally encoded analyte plugs. The output signal bandwidth is determined by the
plug rate which depends on the PCM valve clock frequency ( ~10 Hz), the plug
residence time (= plug length/flow velocity), and the signal resolution ( 32 levels
). For the single-bit PCM of Fig. 1b, the bandwidth was
~ /2 ~0.15 Hz;
hence precluding the generation of faster signals.
Unlike single-bit PCMs, Multi-Plug Modulators (MPMs) produce higher plug
rates hence faster signals. The MPM scheme shown in Fig. 1c requires two main
steps. In the first step, the output flow is interrupted by closing valve φ, and multiple
parallel plugs, corresponding to a digital input code, are loaded simultaneously into
storage nodes placed along a loading zone along the output flow line. In the second
step, the output flow is resumed thus flushing the plugs from the loading zone and
into the output flow resistor. The output resistor behaves as a low pass filter (LPF)
element that averages the incoming plug stream producing a smooth output concentration signal.
Twelfth International Conference on Miniaturized Systems for Chemistry and Life Sciences
October 12 - 16, 2008, San Diego, California, USA
978-0-9798064-1-4/µTAS2008/$20©2008CBMS
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Solute
φ Solvent
Analyte
MPM‐1
φ
LPF
C(t)
1‐bit MUX
LPF
Cout(t)
C(t)
PCM
signal
(a)
Analyte
Solvent
Signal Average
B0
φ
B1
φ
B0
B1
φ
φ
B1
B1
Solvent
φ
Bn
…
φ
B0
φ
φ
B2
(b)
C(t)
Plug
stream
LPF
φ
Cout(t)
φ
Bn
B2
B2
φ
φ
(c)
B0
φ
B1
B2
φ
B2
B1
B2
Return
B2
φ
B2
φ
B1
φ
B2
Solvent
φ
B2
B2
φ
B0
plug storage nodes B1
Analyte
B1
B2
B2
Return
mp Plugs
B0
B1
B2
φ
B2
φ
B2
Output
φ
Return
B1
φ
B1
Solvent
φ
Analyte
MPM‐2
Figure 1. (a) block diagram of 1-bit PCM, (b)
Implementation of 1-bit fluidic PCM, and (c)
Multi-Plug Modulator (MPM) scheme.
Figure 2. Schematic of a high-speed, 3-bit,
7-plug, dual MPM continuous flow signal
generator.
The MPM scheme multiplies the PCM plug rate thus increasing the output band/2 , where
is the number of simultaneously loaded
width to
~ ·
plugs. In a single MPM the flow is interrupted and flushed; hence two interleaved
MPMs are connected to produce a continuous output flow. Figure 2 shows the
schematic of a 3-bit, 7-plug, dual MPM continuous flow signal generator chip. In
operation, when the MPM-1 is loading plugs with corresponding input digital code,
the MPM-2 is flushing the loading zone and vice versa.
Solvent
0
1
2
3
4
5
6
7
MPM‐1
B2
B0
B1
Analyte
MPM‐2
Output
LPF
Solvent
5 mm
Figure 3. Photograph of 3-bit, dual-MPM
signal generator implemented with twolevel PDMS technology.
Figure 4. Photographs of plug loading for
different codes (0-7).
The dual MPM chip was fabricated using a two-level PDMS stamp process [1,5].
Figure 3 shows a photograph of the dual MPM signal generator. The flow channel
dimensions were 175x16 μm2. The chip was tested using disodium fluorescein (0.1
mg/ml) in H2O as an indicator of analyte concentration. The time dependent intensity
of the fluorescence at different locations downstream from the PCM outputs was recorded with an Olympus MVX10 fluorescence microscope and Hamamatsu EMCCD intensified camera. Figure 4 shows pictures of the loading step for 0-7 plugs.
Twelfth International Conference on Miniaturized Systems for Chemistry and Life Sciences
October 12 - 16, 2008, San Diego, California, USA
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EXPERIMENTAL RESULTS
Figure 5a shows the static transfer characteristic (output intensity versus bit code)
for
42 (6 cycles/code). Using this chip, we have generated several fast signals
shown in Figure 5b and 5c. Figure 5c shows the generation dopamine release synthetically [6]. The valve clock frequency was 3.7 Hz with an overall plug rate of 25
plugs/s and the output flow velocity was ~12 cm/s. At total of 1500 digital clock
pulses were used to synthesize a single 60-second trace in Figure 5b. The bandwidth
in this chip was limited by the multi-plug residence time (~100 ms on the plug loading region), yielding a maximum output signal bandwidth of ~3 Hz.
(a)
(b)
(c)
Figure 5: (a) Concentration versus code plot with fourty-three distinct levels using a
six-cycle scheme. (b) Synthetic saw-tooth, ramp and sinusoidal waveforms and (c)
synthetic dopamine release.
CONCLUSIONS
We demonstrated a new microfluidic signal generator using Multi-Plug Modulators. Since the analyte plugs are loaded in parallel all at once onto the exit channel,
much faster chemical signals can be generated. We demonstrated that the chip could
generate high fidelity chemical signals with bandwidths of several Hertz.
REFERENCES
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chemical waveform synthesizer”, PNAS, 102, pp. 8097-8102, (2005).
[3] L. Chen, F. Azizi and C. H. Mastrangelo, “Generation of Dynamic Chemical
Signals with Microfluidic C-DACs”, Lab Chip, 7, pp. 850-855, (2007).
[4] F. Azizi, C. H. Mastrangelo, “Generation of Dynamic Chemical Signals With
Pulse Code Modulators”, Lab Chip,8 , pp. 907-912, (2008).
[5] M. A. Unger, H. P. Chou, T. Thorsen, A. Acherer, S. R. Quake, “Monolithic
Microfabricated Valves and Pumps by Multilayer Soft Lithography”, Science,
288, pp. 113-116, (2000).
[6] D. L. Robinson, et al., “Detecting Subsecond Dopamine Release with Fast-Scan
Cyclic Voltammetry in Vivo”, Clinical Chemistry, Oct 2003; 49: 1763–1773.
Twelfth International Conference on Miniaturized Systems for Chemistry and Life Sciences
October 12 - 16, 2008, San Diego, California, USA
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