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Modular Optoelectronic Microfluidic Backplane for Fluid Analysis Systems

Article  in  Journal of Microelectromechanical Systems · March 2013

DOI: 10.1109/JMEMS.2012.2233717

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462 JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 22, NO. 2, APRIL 2013

Modular Optoelectronic Microfluidic Backplanefor Fluid Analysis Systems

Marko Brammer, Christof Megnin, Achim Voigt, Manfred Kohl, and Timo Mappes

Abstract—We report on the development of backplane moduleswith integrated microvalves and optical switches enabling custom-made system design of analysis systems. The backplane modulesare reversibly interconnected by magnetostatic connectors andprovide optical and fluidic coupling to four neighboring backplanemodules and one optical sensor module, which is mounted ontop. This concept allows for selectively guiding fluids and light tothe sensor modules to be operated. We integrated shape memoryalloy (SMA) microvalves in different designs and two kinds ofoptical switches, i.e., one linearly actuated assembly of opticalelements and one based on an electrostatically deflectable mirror.We manufactured the modules in polymers and carried out opticaland fluidic characterization. The functionality of the backplaneis demonstrated by interconnecting two optical sensor moduleswith integrated spectrometer and photodiode color sensor, re-spectively. Herewith, we carried out fluorescence transmissionexperiments. [2012-0141]

Index Terms—Backplane, microfluidics, modular system de-sign, optical switch, optofluidics.

I. INTRODUCTION

MODULAR system design is a promising means of de-veloping a product family based on only one platform.

Certain applications, e.g., in biomedical, environmental, andprocess analytics, request a broad range of similar but indi-vidual configurations and device combinations [1]. A modulardesign approach could reduce development time and costs ofcomplex systems by assembling different functional modulesand using the same connectors and interfaces.

Recently, considerable progress has been made in microflu-idics and optofluidics toward micro total analysis systems forfluid analysis applications [2], [3]. In particular, new sensorshave been developed, enabling the detection of various fluidparameters, such as refractive index, fluorescence, and ab-sorption [4]–[6]. The continuous development of the sensors

Manuscript received May 26, 2012; revised August 26, 2012; acceptedOctober 22, 2012. Date of publication January 10, 2013; date of current versionMarch 29, 2013. This work was supported by Bürkert Technology Center.Subject Editor H. Seidel.

M. Brammer is with Festo AG, 73734 Esslingen, Germany (e-mail:marko.brammer@googlemail.com).

C. Megnin, A. Voigt, and M. Kohl are with the Institute of MicrostructureTechnology, Karlsruhe Institute of Technology, 76128 Karlsruhe, Germany(e-mail: christof.megnin@kit.edu; achim.voigt@kit.edu; manfred.kohl@kit.edu).

T. Mappes is with Corporate Research and Technology, Carl Zeiss AG,07745 Jena, Germany (e-mail: timo.mappes@kit.edu)

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/JMEMS.2012.2233717

Fig. 1. Schematic of the modular system, consisting of three backplane layers,i.e., (blue) microfluidic, (red) optical, and (green) electronic, (white) for theinterconnection of optical sensor modules. Interaction between the interfaces ismarked with arrows.

enables more extensive monitoring and lower detection limitsfor applications. However, for operating analysis systems, theindividual sensor elements need to be interconnected with actu-ators and supply elements, such as microvalves, micropumps,and light sources. One approach is the integration of theseactive elements directly in the same chip as the sensor itself.This has been proposed in several publications of so-calledlaboratory-on-a-chip, inheriting high functional integration andshort distances between elements, therefore enabling fast signalresponse and low analyte consumption [2], [7], [8]. Since allfunctional elements are predetermined by the chosen design,this approach lacks the possibility to be reconfigured and easilyadjusted to different applications. This drawback is consideredto be a reason for the underdevelopment of microfluidics forindustrial implementations [9]. To overcome these difficul-ties, a modular system design may be a promising alterna-tive approach promoting the broad use of optofluidics andmicrofluidics.

Here, we present a modular platform, consisting of backplanemodules that are individually interconnected (see Fig. 1). Eachbackplane module consists of three virtual layers: 1) a microflu-idic backplane layer with fluidic channels and microvalves;2) an optical backplane layer with optical fibers and switches;and 3) an electronic backplane layer with a microcontroller.The backplane modules in our setup have the size of 40×40× 50 mm3, each providing an interface for an optical sensormodule with optical and fluidic connectors. The interfacesbetween the backplane modules are designed to adapt to anyconfiguration required by arbitrary applications.

The microfluidic backplane layer provides the control of thefluid flow to the dedicated sensors. Recently, we developedfluidic channel plates made of polymers [10] and shape memory

1057-7157/$31.00 © 2013 IEEE

BRAMMER et al.: MODULAR OPTOELECTRONIC MICROFLUIDIC BACKPLANE FOR FLUID ANALYSIS SYSTEMS 463

Fig. 2. Microfluidic backplane module. (a) Schematic fluidic circuit withfluidic channels and nine microvalves. Fluidic connectors to the neighboringmodules are marked with blue arrows, connectors to the mounted device withblue circles. (b) Photograph of the fluidic channel plate.

alloy (SMA) microvalves in different designs, allowing eithermonostable [11] or bistable [12], [13] switching within thefluidic network. Here, we integrated nine microvalves in eachbackplane module and mounted them on top of a fluidic channelplate either by a magnetostatic plug-in connection or by laser-beam welding.

The optical backplane allows for selectively guiding lightfrom external or internal light sources to the optical sensormodules. In the center of each module, an optical switch isintegrated, coupling light between five optical fibers that areperpendicularly orientated to each other. The optical switchhas been fabricated in two different designs, i.e., one as arobust solution with low coupling losses but slow switchingtime, based on a linearly actuated mirror assembly [14], andanother one with higher losses but faster switching, basedon an electrostatically actuated microelectromechanical-system(MEMS) mirror. The concepts permit the controlled guiding oflight from one neighboring module horizontally to the othersand vertically to a supplied sensor module. Thus, the supply ofmultiple sensor modules with only one light source is possible,or alternatively, light from different sources may be guided toone sensor.

The proposed design is particularly advantageous for fluidflows that are continuously analyzed with respect to variousparameters. The continuous monitoring of fluid parameters,such as pH-value, oxygen content, color, turbidity, or contentscreening for particles, is crucial for several applications inlife sciences, medicine, environmental, and process analysis.In the following, the design, the prototype fabrication, and thecharacterization of the microfluidic backplane (see Section II),the optical backplane (see Section III), and the complete system(see Section IV) are presented.

II. MICROFLUIDIC BACKPLANE

The microfluidic backplane in each module consists of ninemicrovalves integrated into a fluidic network, interconnectingfour fluidic interfaces to the neighboring modules, and twofluidic interfaces to the mounted sensor module (see Fig. 2).Switching the microvalves allows for guiding the fluid betweentwo arbitrary fluidic interfaces of the module.

A. Fluidic Channel Plate

The fluidic channels with a diameter of 0.8 mm were fab-ricated in cyclic-olefin copolymer (COC; TOPAS 6013) bymicromilling. The channels are covered by a plate made ofCOC. To enable laser-beam transmission welding of both parts,this plate is doped with 3-vol.% carbon to absorb the light of thediode laser (λ = 940 nm) [15]. The welding laser was operatedwith a constant power of 12 W and had a beam spot size of0.32 mm × 0.41 mm (full width at half-maximum). It wasdeflected by a mirror and scanned over the sample at a speedof 200 mm/s and line spacing of 0.3 mm to weld the polymer at150 ◦C, as measured by a pyrometer.

B. SMA Microvalves

The integrated microvalves are actuated by a micromachinedcold-rolled TiNi foil of 20 μm thickness. The TiNi microac-tuator exhibits the thermal shape memory effect that is basedon a reversible phase transformation between cold (marten-site) and hot (austenite) states, offering high energy density(107 J/m3) [16]. The TiNi thin film is structured by lithographyand wet chemical etching as a wheel-shaped microbridge [11].Three designs have been implemented, i.e., with monostablenormally open (NO), monostable normally closed (NC), andbistable switching principle. In the case of the NO valve, theactuator is predeflected in the open state of the microvalve bythe applied pressure of the fluid. For the NC valve, a springelement has been added, which provides the force to predeflectthe actuator and close the valve in the off-state. In the caseof the bistable microvalve, the stable states are realized bymagnetostatic latches, which hold a mechanically coupled pairof two counteracting actuators in one of the two states [12].By an electrical current, the TiNi thin film is heated above thereverse transition temperature of 55 ◦C causing the transitionfrom the predeflected shape to its initial planar shape. Electricalconnection of the actuator is achieved by spring contacts thatare integrated in the valve housing.

The microvalves are mounted on the fluidic channel plateeither by laser-beam welding or by a magnetostatic plug-in con-nector (see Fig. 3). For permanent connection, we used laser-beam welding to fix the housing of the microvalves (transparentCOC) on the fluidic channel plate (nontransparent COC). In thiscase, the valve seat is directly structured into the fluidic chan-nel plate, subsequently covered with a sealing membrane, theactuator, and the valve housing. The sealing around the valvechamber is implemented by a 12-μm-thick membrane made ofpolyimide. The welding process has been carried out with thesame parameters as for welding the fluidic channel plates.

The magnetostatic connector is implemented by soft mag-netic plates (material: steel, 1.4105) screwed onto the fluidicchannel plate and a permanent-magnetic ring (8-mm outerdiameter × 4-mm inner diameter × 3-mm height; material:NdFeB) integrated into the microvalve housing. Thus, bymagnetostatic forces, a 0.23-mm sealing membrane made ofsilicon is compressed. This type of connector allows for easyand fast reversible detachment of the microvalves. Therefore,different types of microvalves, such as monostable or bistable,

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Fig. 3. Schematics of NO-microvalves mounted on the fluidic channel platein cross-sectional view. The sealing membrane is colored in green, andthe actuator in on- and off-state is colored in red and blue, respectively.(a) Valve with magnetostatic plug-in connector in closed state. (b) and (c) Valvewith welded connection in closed and open states, respectively. Fluid flow isillustrated with blue arrows.

Fig. 4. Microfluidic backplane module with three NC-microvalves and sixplace holders with preselected states. The valves are mounted by magnetostaticconnectors on the fluidic channel plate.

Fig. 5. Electronic backplane for control of the microvalves. (a) Simplifiedschematic electronic circuit with microcontroller (μC), I/O logic, analog–digital converter (ADC), pulsewidth–modulated (PWM) signal, field-effecttransistors (FETs), microvalves, LEDs, and multiplexer (MUX). (b) Photographof electronic backplane mounted on the microfluidic backplane.

may be mounted as requested by the application (see Fig. 4).Furthermore, valve place holders may be used with manuallymechanical switching actuators or preselected fixed states.

C. Electronic Control

The electronic backplane for microvalve control consistsof a microcontroller, which generates a pulsewidth-modulatedsignal (f = 40 Hz). By controlling the duty cycle in tenlevels, the mean control current may be adjusted to differenttypes of microvalves. The communication between the ProfiLabprogrammed personal computer (PC) user interface and themicrocontroller is implemented by a RS232 protocol. Fig. 5(a)illustrates the electronic circuit, in which the microcontrollercontrols nine field-effect transistors, one for each microvalve.The current through each actuator is determined from thevoltage drop across single serial measuring resistors after eachvalve. By the analog–digital converter of the microprocessor,the average current is transmitted and displayed in the PC user

Fig. 6. Measured flow rate of water as a function of the applied pressuredifference between inlet and outlet of a backplane module. (a) Results for aflow path through (red) two (blue) or three SMA microvalves mounted on themicrofluidic backplane module. (b) Results for different mean control currents,adjusted by duty cycle of the pulsewidth-modulated signal.

interface. Since the chosen microcontroller (Atmel ATmega88)only provides eight analog–digital converter ports, an analogmultiplexer is used. Light-emitting diodes display the operatingstate of each microvalve.

The electronic backplane has gold contact pads, connectingthe electric spring contacts of the microvalves. It is mounted ontop of the microfluidic backplane layer [see Fig. 5(b)].

D. Fluidic Characterization

We used water to characterize the microfluidic backplanewith respect to flow rates, leakage, and dynamic response. As aliquid flowmeter, we used Bürkert 8708.

Fig. 6(a) depicts the flow rate of water as a function of theapplied pressure difference between the inlet and the outlet ofone backplane module, amounting to 5–7 ml/min for a pressuredifference of 105 Pa. Since the main fluidic resistance originatesfrom the smallest dimension within the valve chambers, theflow rate depends on the number of microvalves the water isguided through.

The flow rate may be controlled by adjusting the duty cycleof the pulsewidth-modulated control current with the user-interface-controlled electronic backplane. Measured results ofthe flow rate characteristics for a voltage of 0.6 V and differentmean currents are illustrated in Fig. 6(b).

The leakage (for mean current equal to 0) is less than 2% ofthe flow rate for the completely open state. Measurements ofthe dynamic behavior of the fluidic backplane modules resultedin switching times of 0.4–0.6 s.

BRAMMER et al.: MODULAR OPTOELECTRONIC MICROFLUIDIC BACKPLANE FOR FLUID ANALYSIS SYSTEMS 465

Fig. 7. Switching positions of the linear actuated optical switch. (a) Verticallight deflection at the pyramid mirror. (b) Light transmission through thetransparent glass cube. (c) and (d) Horizontal light deflection at the prismmirrors.

III. OPTICAL BACKPLANE

In each optical backplane module, five polymer optical fibersare perpendicularly orientated to each other, four of whichare horizontally directed to the neighboring modules and onevertically directed to the sensor module. At the crossing ofthe fibers, an optical switch is situated, enabling the couplingbetween each of the horizontally orientated fibers to one of theothers or vertically to the sensor module. In front of each fiber,a lens is positioned, in order to focus light that is coupled out di-vergently. Thus, light from one fiber is focused by a lens, eitherdeflected or transmitted by the optical switch, focused again byanother lens, and finally coupled into the preselected other fiber.Two optical switches, each one for different applications, havebeen developed.

A. Linearly Actuated Optical Switch

One optical switch is designed as linearly actuated assemblyof optical elements (see Fig. 7). The horizontal deflectionis achieved by 2 mm × 2 mm × 2 mm right-angle prisms(Edmund Optics NT45-385) that are coated with aluminum atthe hypotenuse and glued together to a double-sided mirror.The vertical deflection is achieved by an aluminum-coated2 mm × 2 mm × 1 mm pyramid that has been fabricated bymicromachining on top of a 2 mm × 2 mm × 2 mm trans-parent silica cube. The 150-nm aluminum mirror coating ofthe pyramid surfaces has been deposited by ultrahigh-vacuumevaporation deposition (p = 10−7 Pa). Smooth thin-film de-position has been achieved by cooling the sample to 140 Kduring deposition. The surface roughness has been measuredby an atomic force microscope and amounted to Rq = 4.8 nm(root mean square) before and Rq = 7.4 nm after aluminumcoating.

The mirror assembly is mounted on a holder and actuatedby a brushless micromotor (Faulhaber 0308B, 03AS3) with amounted spindle. The holder has a mating internal screw threadintegrated at the bottom and is glued to a linear bearing (IKOLWL1), thus transforming the rotation motion of the motor intoa linear motion (see Fig. 8).

Light is focused by plano-convex lenses (3-mm diameter ×3-mm focal length, Edmund Optics NT49-167) that are alignedto the fibers by holders. Since the upper fiber is misaligned withrespect to the mirror surface of the pyramid, light is focused bya larger lens (6 mm × 6 mm, Edmund Optics NT47-478).

Fig. 8. Linear actuated optical switch. (a) Computer-aided design (CAD)model of the switching element. Fibers and lenses are marked in yellow, theholder for the optical element assembly in red, the spindle of the micromotorin green, the screw nut in dark blue, and the linear bearing in black. The linearmovement is marked by a blue arrow. (b) Photograph of the optical switch.

Fig. 9. MEMS mirror optical switch. (a) CAD model (sectional view) of the.Lenses are colored in yellow, the MEMS mirror surface in red. Sample raysfor coupling between two horizontally oriented fibers are displayed in yellow.(b) Photograph of the optical backplane module.

The main advantages of this concept are its simple ac-tuation principle and the low positioning accuracy required(+/−0.5 mm), since the extension of the light beam is less than1 mm in diameter, while the area to transmit or deflect it hor-izontally is 2 mm × 2 mm. Since the pyramid mirror surfacesfor each fiber are smaller and not centered with respect to theupper lens, the precise positioning of the pyramid is crucial forthe coupling efficiency. To fulfill this requirement, this positionis set by a mechanical stop at the bottom of the base.

B. MEMS Mirror Optical Switch

The other version of the optical switch is based on an elec-trostatically 2-D deflected MEMS mirror (Sercalo Microtech-nology). The maximum mechanical deflection angle is +/− 5◦

around both axes. Thus, light may be selectively deflectedwithin an angle area of 10◦ around the normal axis. The fibersare positioned such as in the first design. In front of eachhorizontally oriented fiber, a double-convex lens (3 mm ×4.5 mm, Edmund Optics NT49-168) is positioned, however, dueto limited space no lens is positioned in front of the verticallyoriented fiber.

Light coupling out of a horizontal fiber is deflected onto theMEMS mirror by a pyramid mirror, which is positioned upsidedown in the crossing of the fibers, serving as fitting for thevertically oriented fiber (see Fig. 9).

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Fig. 10. Schemes of the positions of the optical switches with simulated geo-metrical parameters. x and y represent distances between the optical elements(simulated parameters illustrated in Fig. 11). Arrows and crosses representpositional tolerances perpendicular to the optical path (simulated parametersillustrated in Fig. 12). (a) Horizontal light deflection at the prism in the linearlyactuated optical switch. (b) Vertical light deflection at the pyramid in thelinearly actuated optical switch. (c) Coupling between two horizontally orientedfibers in the MEMS mirror optical switch. (d) Coupling between a horizontallyand a vertically oriented fiber in the MEMS mirror optical switch.

The pyramid has been fabricated by micromachining andsubsequently coated with aluminum, similarly to the pyramid inthe assembly of the linearly actuated optical switch. The surfaceroughness amounted to Rq = 4.6 nm before and Rq = 6.7 nmafter Al coating. Depending on the mechanical deflection angleof the MEMS mirror, the light is deflected either vertically tothe fiber leading to the sensor module or to one of the pyramidsurfaces, from where it is deflected a horizontally orientated tothe fibers.

C. Optical Simulation

Fig. 10 depicts the geometrical parameters used for opticalray-tracing simulations carried out with ZEMAX. Fig. 11 illus-trates the simulated coupling efficiency as a function of theseparameters shown for the two different optical switches and theswitching states.

For the linearly actuated optical switch, horizontal deflectionat the prism and vertical deflection at the pyramid are pre-sented. The simulation of the horizontal transmission throughthe transparent cube (not illustrated) has shown 6% highercoupling efficiency than the deflection at the prism. First, lightis absorbed at the mirror surface of the prism, and second,the mirror surface is deviated from the ideal diagonal due tothe thickness of the mirror layer (150 μm). The parametersfor vertical light deflection at the pyramid have not been op-timally chosen, since this would reduce the horizontal couplingefficiency.

The results show lower coupling efficiency for the MEMSmirror optical switch than for the linearly actuated opticalswitch. This is caused by higher losses due to the longer opticalpaths between the optical fibers and the additional reflectionsper coupling.

Fig. 11. Ray-tracing simulations of the coupling efficiency of the opticalswitches (λ = 655 nm). Fraction of rays coupling into the second fiber N/N0

in colored scale as a function of the geometrical parameters for differentswitching positions. The distances chosen for the prototype are marked bywhite lines. (a)–(d) Results for the different coupling cases as illustratedschematically in Fig. 10.

Fig. 12. Ray-tracing simulations of the coupling efficiency of the opticalswitches (λ = 655 nm). Normalized efficiency as a function of the positionaltolerances perpendicular to the optical path. (a)–(d) Results for the differentcoupling cases as illustrated schematically in Fig. 10.

Fig. 12 depicts the coupling efficiency as a function ofpositional tolerances of the individual optical elements withinthe optical path between two fibers. The examined positionaltolerances are perpendicular to the optical path and depictedin the inserts with corresponding colors. The graphs show thatcertain positional deviation may lead to higher efficiency forthe associated switching position. Since the switch is bidi-rectionally used, the deviation would lead to lower efficiencyfor other switching positions. The coupling efficiency of theMEMS mirror switch depends stronger from tolerances, due tolonger optical path lengths.

BRAMMER et al.: MODULAR OPTOELECTRONIC MICROFLUIDIC BACKPLANE FOR FLUID ANALYSIS SYSTEMS 467

Fig. 13. Measured coupling efficiency of the optical fiber connector (seeSection IV-A) and the optical switches for the different switching positions.(a) Coupling efficiency of the optical fiber connector at the interconnectioninterface of two modules. (b) Horizontal light transmission through the trans-parent cube. (c) Horizontal light deflection at the prism in the linearly actuatedoptical switch. (d) Vertical light deflection at the pyramid in the linearlyactuated optical switch. (e) Coupling between two horizontally oriented fibersin the MEMS mirror optical switch. (f) Coupling between horizontally andvertically oriented fibers in the MEMS mirror optical switch.

D. Optical Characterization

The coupling efficiency of the optical backplane prototypehas been measured by a silicon optical power head connectedto an optical multimeter (OMH-6703B and OMM-6810B, ILXLightwave). A red laserdiode (λ = 655 nm, Popt = 5 mW,Laser Components ADL65055TL) coupled by a lens to theinput fiber was used as source. As reference, the intensity oflight coupling out of the input fiber was measured. The resultsare illustrated in Fig. 13, including mean values and standarddeviation for different samples.

In the case of the linearly actuated optical switch, the cou-pling efficiency is highest for coupling through the transparentcube. In accordance with the simulation, the coupling efficiencyis lower for the deflection at the prism. For vertical deflectionat the pyramid, the efficiency is the lowest, and the standarddeviation is the highest, which is due to the smaller size of thepyramid surface compared with the prism mirror and the largerdistance between the pyramid and the upper lens compared withthe distance between prism mirror and lens. Furthermore, themirror surface of the pyramid is deviated with respect to thecenterline of the upper lens.

In accordance with the simulations, the MEMS mirror op-tical switch showed lower coupling efficiencies, which resultfrom higher reflection losses due to additional reflections andstronger dependence on positional tolerances due to largerdistances between input and output fibers.

Measurements with up to three interconnected modulesshowed that the coupling efficiency of each module staysconstant within the tolerances represented by the standard devi-ation. Thus, the necessary optical power of the light source maybe calculated from the necessary light intensity at the sensor forthe dedicated application and the coupling efficiencies of theinvolved modules.

The coupling losses depend on the integrated optical switchand the active switching position. The overall losses per modulemay be calculated by summarizing the losses at the opticalswitch and the losses at the fiber connector at the moduleinterface (see Section IV-A).

Fig. 14. Photographs of the fluidic and optical connectors. (a) Proto-type of fluidic connector with reversible detachable magnetostatic connector.(b) Prototype of optical fiber connector.

IV. OPERATIONAL BACKPLANE SYSTEM

The entire backplane modules consist of the three backplanelayers that are stacked on top of each other. The modules areinterconnected by modular connectors to form the completenetworking platform. On top of each backplane module, onefunctional module, such as a sensor or a control element, ismounted.

A. Module Connectors

The backplane modules are mechanically interconnected bymagnets (see Fig. 14). This method allows for configurabilityand reassembly of the system. Furthermore, it respects the needfor a clearance fit, in order not to statically overconstrain thesystem. Fluidic sealing is assured by nitrile rubber O-rings withinner diameter and cross section of 1 mm × 1 mm, which areintegrated in each interface around the channel inlets. Leak testsshowed that the fluidic contacts are leak proof for fluid pressuredifferences of up to 24 · 105 Pa.

The optical connector assures the alignment of two opticalfibers with respect to each other, by symmetric oblique surfacesin yoke shape. One optical fiber connector is inserted in eachmodule interface, thus mechanically self-aligning the fibersat module assembly. The radial displacement of the fibersmeasured by an optotactile sensor amounted to +/−30 μm.Losses due to rough fiber surfaces or axial displacement may bereduced by introducing an index-matching gel, which is sealedby an O-ring directly situated around the fiber. The couplingefficiency has been measured by the same setup used forcharacterizing the optical switches and amounted to −0.33 dB.

B. Sensor Modules

The functionality of the backplane as a networking plat-form has been demonstrated with different sensor and controlmodules. Optical sensor modules require the optical input, aswell as the fluidic input and output, whereas sensor modulesbased on chemical principles solely use the fluidic ports. Forthe optical analysis of the fluids, we equipped one sensor mod-ule with a miniature spectrometer (Hamamatsu C10988MA)[see Fig. 15(a) and (c)] and another one with a photodiodecolor sensor (MAZeT MTCS C2, Jencolor Colorimeter 2) [seeFig. 15(b)]. Both sensors allow for detecting light transmittedthrough the fluidic channel element, inserted into the sensormodule. The lateral dimensions of the electronic board onwhich the color sensor is mounted are too large for horizontalpositioning inside the module. Therefore, we used a right-angle

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Fig. 15. Schemes and CAD model of the optical sensor modules. Light pathis illustrated with yellow arrows and fluid flow with blue arrows. (a) Cross-sectional view of the module with integrated spectrometer. (b) Cross-sectionalview of the module with integrated color sensor. (c) CAD model of the modulewith integrated spectrometer.

Fig. 16. Measurement setup with two backplane modules interconnectinga miniature spectrometer and a combined conductivity/temperature sensormodule. The sensor modules are operated with a supply module with integratedlaser diodes, light emitting diodes, a micropump, and sample fluids withdifferent dyes.

prism (5 mm × 5 mm, Edmund Optics NT32-328) to guide thelight beam by the total internal reflection to the sensor surface.

In addition to the sensor modules, we built a supply modulewith integrated laser diodes, light-emitting diodes (LEDs) anda micropump. Fig. 16 illustrates an exemplary assembly ofthree modules interconnected by the modular backplane to ananalysis system.

We used a green laser pointer (λ = 532 nm, Popt = 1 mW)coupled into the input fiber of one module as light sourcefor transmission measurements carried out with the miniaturespectrometer integrated in the modular analysis system. Theresults show a fluorescence signal when analyzing in ethanoldissolved dye Disperse Red 1 [see Fig. 17(a)].

For measurements with the photodiode color sensor, we used awhite LED (Vishay Semiconductors VLCW5100). The referencetransmission spectrum of the white LED through water is embed-ded into the software. Measurement results showed (49500,44530, 45650) in the XYZ tristimulus color scale [see Fig. 17(b)],corresponding to (255, 198, 190) in the RGB color scale.

A variety of sensor modules may be chosen as required by thededicated application and interconnected via the standardizedinterfaces. Particularly promising are sensors based on optical

Fig. 17. Transmission fluorescence measurements carried out with a greenlaser diode (λ = 532 nm) as light source and a miniature spectrometer asdetector. (a) Normalized intensity measured in the spectrometer module as afunction of wavelength for water or Disperse Red 1 dissolved in ethanol asanalyte. (b) Color values measured with the photodiode color sensor for water(blue mark) or Disperse Red 1 (red mark). Color chart with x = X/(X + Y +Z), y = Y/(X + Y + Z), and z = 1− x− y.

Fig. 18. Scheme of an exemplary optofluidic analysis system with twobackplane modules interconnecting one mixing (A) and one sensor module (B).Light path is illustrated with yellow arrows, analyte flow with blue, and mixedfluid with violet arrows.

principles. In this respect, several sensor designs may be ap-plied, thus paving the way to numerous applications.

In addition to the transmission measurements demonstratedin this paper, light scattering or fluorescence emission is de-tectable at right angles to the excitation beam. Furthermore, thelight absorption of the fluid may be detected, either at specificwavelengths or as a function of the broad spectrum of a white-light source.

Another concept would be the integration of photonic cavi-ties as optical transducer elements into the interaction zone ofthe sensor. This would offer the possibility of interconnectingsensors modules based on optofluidic principles, i.e., the controlof the light signal by manipulating the fluid [2]. Fig. 18 illus-trates a general concept on how to use the modular backplane asa platform for an optofluidic sensor. In this example, an analyteis mixed with a reagent in a first module A and guided to a sec-ond module B in which it is analyzed. Light is coupled into theinteraction zone of the sensor module B by a transducer element(e.g., a photonic cavity such as a ringresonator or a photoniccrystal) and detected, for example, by photodiodes. The be-havior of the optofluidic sensor may be tuned by manipulatingthe fluid. This manipulation is achieved by adding differentreagents to the analyte. The fluidic backplane allows for guiding

BRAMMER et al.: MODULAR OPTOELECTRONIC MICROFLUIDIC BACKPLANE FOR FLUID ANALYSIS SYSTEMS 469

different fluids (e.g., analyte, reagent, and cleaning solution)selectively to the mixer and to the optofluidic sensor module.The optical backplane allows for selecting light from dedicatedlight sources and thus analyzing different properties of the fluid.

V. CONCLUSION

In this paper, we have presented the concept for a mod-ular backplane mainly made out of polymers enabling theinterconnection of functional modules, such as optical sensors,fluid control elements, and light sources. The modules may beassembled in a user-defined number, functionality, and config-uration, such that arbitrary applications may be addressed.

We have demonstrated the functionality of fluidic channelplates with mounted SMA microvalves allowing for guidingfluids selectively to the dedicated devices. The microvalves havebeen individually actuated by a microprocessor-driven elec-tronic backplane, which is stacked on top of the microfluidicbackplane layer and connected by electrical spring contacts.

Furthermore, we have presented two functional opticalswitches, one based on a linearly actuated assembly of opticalelements and another one based on a biaxial deflectable MEMSmirror. In each backplane module, one optical switch is inte-grated and allows for guiding light from a chosen source to thededicated sensor module.

Each backplane module consists of a microfluidic, an optical,and an electronic backplane layer stacked on top of each other.This stack provides one slot for a functional module, whichis interconnected by defined fluidic and optical connectors.Similar connectors are integrated in each interface to the neigh-boring backplane modules. The mechanical interconnection isassured by reversibly detachable magnetostatic connectors, thusenabling reconfigurable system design. We have demonstratedthe functionality of the system by interconnecting three mod-ules, i.e., one with integrated light sources and micropumps,one with an integrated spectrometer, and one with an integratedphotodiode color sensor.

The backplane modules presented here are compatible withmultiple applications in microfluidics and optofluidics, and maybe considered as a promising approach toward custom-mademicro total analysis systems.

ACKNOWLEDGMENT

This work was carried out at the Institute of MicrostructureTechnology, Karlsruhe Institute of Technology, Karlsruhe, Ger-many. The authors would like to thank C. Schmuck and theworkshop at Bürkert Fluid Control Systems for manufactur-ing the polymeric prototypes; H. Besser from the Institute ofApplied Material Research, Karlsruhe Institute of Technology(KIT), for the laser-beam welding of the prototypes; M. Sieg-farth from the Institute of Microstructure Technology (IMT),KIT, for helping with the module connectors; S. Zimmermannfrom the IMT, KIT, for helping with the electronics circuit;A. Hofmann from the Institute for Applied Computer Science,KIT, for sharing his knowledge on micromotors and bearings;D. G. Rabus from the University of California, Santa Cruz, forfruitful discussions; and F. Hübler and M. Wolf from the Insti-tute of Nanotechnology, KIT, for the evaporation deposition ofthe aluminum on the pyramids.

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Marko Brammer received the Diploma degreein electrical engineering (microelectromechanicalsystems and microelectronics) from the TechnicalUniversity of Hamburg, Hamburg, Germany, in2009 and the Ph.D. degree on the development of amodular optoelectronic microfluidic backplane fromKarlsruhe Institute of Technology, Karlsruhe,Germany, in 2012.

He is currently working for advanced developmentfor process automation with Festo AG, Esslingen,Germany.

Christof Megnin received the Diploma degree inmechanical engineering in 2008 from the Universityof Karlsruhe [now Karlsruhe Institute of Technology(KIT)], Karlsruhe, Germany, where he is currentlyworking toward the Ph.D. degree at the Institute ofMicrostructure Technology and within the BürkertTechnology Center on the development of microflu-idic devices for integration in fluidic control systems.

470 JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 22, NO. 2, APRIL 2013

Achim Voigt received the Diploma degree from theFachhochschule Rüsselsheim, Germany, in 1991.

Since 2006, he has been a Physics Engineerwith the Institute of Microstructure Technology,Karlsruhe Institute of Technology, Karlsruhe,Germany, working on the development of micro-fluidic control electronics and high-frequencyacoustic sensor systems (SAW).

Manfred Kohl received the Ph.D. degree in physicsfrom the University of Stuttgart, Stuttgart, Germany,in 1989.

From 1990 to 1991, he was an IBM Postdoc-toral Fellow with the T. J. Watson Research Center,Yorktown Heights, NY. He is currently a Senior Sci-entist with the Institute of Microstructure Technol-ogy, Karlsruhe Institute of Technology, Karlsruhe,Germany, working on smart materials and their im-plementation in micro- and nanosystems. He haspublished more than 200 publications in conference

proceedings, journals, patents, and books.Dr. Kohl is member of the German Physical Society (DPG) and the Society

of German Engineers (VDI/VDE) on Microelectronics, Micro-, and PrecisionTechnology (GMM).

Timo Mappes received the Ph.D. degree in mechan-ical engineering from the University of Karlsruhe[now Karlsruhe Institute of Technology (KIT)],Karlsruhe, Germany, in 2006.

Since 2007, he has been heading an interdisci-plinary group at KIT. In 2010, for half a year, he wasa Visiting Professor at the Danish Technical Univer-sity, Denmark. In 2011, he was a Visiting Professorwith the Université de Franche-Comté, Besançon,France. He is currently with Corporate Researchand Technology, Carl Zeiss AG, Jena, Germany. His

research focuses on realizing and process engineering highly integrated opticallaboratory-on-a-chip systems out of polymers for biophotonic applications. Aspecialty of his work is the on-chip integration of tunable miniaturized lasers,i.e., DFB lasers based on both solid-state and liquid core (optofluidics), andmicrocavity dye lasers based on a novel process developed in his group. He hascoauthored more than 40 publications in ISI journals and additionally publishedover 100 contributions to conferences, patents, and books.

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