Microfluidic tectonics: A comprehensive constructionplatform for microfluidic systemsDavid J. Beebe*†‡, Jeffrey S. Moore*, Qing Yu*, Robin H. Liu*, Mary L. Kraft*, Byung-Ho Jo*, and Chelladurai Devadoss*

*The Beckman Institute for Advanced Science and Technology, University of Illinois at Urbana–Champaign, Urbana, IL 61801; and †Department ofBiomedical Engineering, University of Wisconsin, Madison, WI 53706

Edited by Jiri Jonas, University of Illinois at Urbana–Champaign, Urbana, IL, and approved September 22, 2000 (received for review June 13, 2000)

A microfluidic platform for the construction of microscale compo-nents and autonomous systems is presented. The platform com-bines liquid-phase photopolymerization, lithography, and laminarflow to allow the creation of complex and autonomous microflu-idic systems. The fabrication of channels, actuators, valves, sensors,and systems is demonstrated. Construction times can be as short as10 min, providing ultrarapid prototyping of microfluidic systems.

Construction of microscale systems generally has been ap-proached from two perspectives. Either the components are

fabricated separately and then assembled (as at the macroscale)or lithography-based microfabrication methods are used tocreate the components at their desired locations (e.g., polysiliconsurface micromachining). Assembly of micrometer-sized objectshas proven to be nontrivial because electrostatic and othersurface forces are overwhelming at the microscale, makingmanipulation difficult (1). Through appropriate geometric de-sign, these forces can be harnessed to self-assemble small parts(2). Conventional lithographic approaches show promise, but themany disparate materials and processes hinder the fabrication ofcomplex systems. For example, the processes used to constructone system component (e.g., a sensor) may be incompatible withthose for other components (e.g., pumps and valves). To realizemicroscale systems for many different applications, unconven-tional approaches are needed to overcome these difficulties. Thephysics of scaling (i.e., laminar flow, high surface-to-volumeratio) can lead to significantly improved performance in somemedical and biological applications and also allow for in-channelconstruction. Whitesides and coworkers (3) have demonstratedseveral in-channel fabrication techniques that use laminar flowto create textured walls and to build metal traces within micro-channels. Smela et al. (4) demonstrated conductive microscaleactuators built on flat substrates by patterning conductive poly-mers using lithography. Two-photon polymerization has beenused to create three-dimensional (3D) structures from a polymergel precursor (5, 6).

Previously, we reported the ability to build in-channel auton-omous hydrogel valves by using a photopolymerization process(7). It has been demonstrated that stimuli-responsive hydrogelsare the natural materials for microfluidic systems in terms ofscaling physics because smaller size leads to faster volumechanges for these diffusion-controlled processes. Here, we ex-pand this photopolymerization method to a fabrication platformfor total system construction. This fabrication platform, whichwe refer to as microfluidic tectonics (mFT), utilizes microfluid-ics, photopolymerization, and materials chemistry to createautonomous microfluidic systems controlled by the local f luidicenvironment. mFT allows one to develop a wide variety ofmicrofluidic systems by using one common construction plat-form, providing several key advantages. First, it provides ageneral, integrated platform for the construction of all systemstructures and components. This is a departure from traditionalapproaches, which require a wide range of materials and sens-ingyactuation technologies, making integration costly and diffi-cult. Second, the hydrogels function as both sensor and actuator,providing a stand-alone detectionyreadout component that links

a microscale event (sensing of local f luid environment) to amacroscale response (volume change), thereby eliminating theneed for complicated instrumentation (sensors and electronics).Finally, the platform can be adapted to manufacture differentcomponents for carrying out a variety of system functions(filtering, separation, mixing, etc.) while incorporating localautonomous control of chemistry, f low, and other parameters.

Methods and ResultsmFT integrates liquid-phase photopolymerization, lithography,and laminar flow to build autonomous microfluidic systems (Fig.1). The construction of a mFT system starts with a universalcartridge. The method then employs photomasks, a light source,and photopolymerizable prepolymer mixtures of both structural(nonfunctional) and functional (stimuli-responsive) materials.Micromolding techniques are used to produce the universalcartridges that include a wide, shallow fluid reservoir serving asblank slates. Polymer components (both functional and struc-tural) are created inside the cartridge via direct photopatterningof a liquid-phase prepolymer mixture. Through the applicationof liquid-phase polymerization, lithography, and laminar flow,all system components including the microchannels are con-structed easily by using mFT.

In a typical procedure, a fluid reservoir ranging from 100 to250 mm deep and 500 to 25,000 mm wide is filled with aprepolymer mixture consisting of monomers and a photoinitia-tor. The mixture is allowed to reach a quiescent state and thenexposed to UV light through a photomask placed on top of thecartridge. Polymerization times typically range from 5 sec to 10min depending on the photoinitiator, monomer mixture, andlight intensity. A convenient light source is the filtered lightsource from a standard fluorescence microscope. When thepolymerization is complete, the channel is f lushed with suitablesolvent (e.g., water, methanol, etc.) to remove the unpolymerizedliquid. Multiple structures can be created simultaneously byusing a photomask with a multistructure pattern (parallel poly-merization) or by refilling the fluid reservoir with anotherprepolymer mixture and repeating the polymerization in asequential fashion (sequential polymerization). Objects that areclose together (,250 mm) cannot be fabricated simultaneouslybecause partial polymerization occurs between the objects. Thepartially polymerized mixture cannot be removed easily byf lushing the channel. Therefore, sequential polymerizationneeds to be used to solve this problem. Objects with a spacing of100 mm have been made. The combination of parallel andsequential polymerization allows polymer components of differ-

This paper was submitted directly (Track II) to the PNAS office.

Abbreviations: 3D, three-dimensional; mFT, microfluidic tectonics; ; EGDMA, ethyleneglycoldimethacrylate; PDMS, polydimethylsiloxane.

‡To whom reprint requests should be addressed. E-mail: dbeebe@engr.wisc.edu.

The publication costs of this article were defrayed in part by page charge payment. Thisarticle must therefore be hereby marked “advertisement” in accordance with 18 U.S.C.§1734 solely to indicate this fact.

Article published online before print: Proc. Natl. Acad. Sci. USA, 10.1073ypnas.250273097.Article and publication date are at www.pnas.orgycgiydoiy10.1073ypnas.250273097

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ent materials, shapes, and sizes (illustrated in Fig. 2) to beintegrated directly into microfluidic systems.

Fig. 2 illustrates the basic procedures and example geometriesof microchannel construction. The structural material normallyconsists of a mixture of isobornyl acrylate (IBA), 2,2-bis[p-(29-hydroxy-39-methacryloxypropoxy)phenylene]propane or tetra-ethyleneglycol dimethacrylate (TeEGDMA), and Irgacure 651as photoinitiator. These monomers have two advantages: lowdegree of shrinkage during polymerization and fast polymeriza-tion kinetics. Typical polymerization times range from 5 to 40sec. After UV exposure, the liquid prepolymer mixture turnsinto a clear, rigid material. By adjusting the monomer compo-sition, microchannels resistant to a variety of common solvents(e.g., water, ethanol, and acetone) can be made.

Resolution is limited by both optical and chemical effects.Typically, light scattering and reflections enlarge the base of theimage in a negative resist in traditional photolithography. Theline widths continue to enlarge with exposure time. The pho-topolymerizable liquid used here can be thought of as a liquid-phase negative resist. The polymerization of relatively deepliquid-phase mixtures introduces other effects, including thedepth of focus of the exposing light, diffusion during polymer-ization, and different optical properties between the liquid phaseand the polymer. Perfect collimation is not achieved in themicroscope and contact aligner used for exposure. Thus, a fewdegrees of divergence exist and are most significant in deep(150–300 mm) channels. The divergence effect appears to bemore prominent in hydrogel polymerization than scattering andreflection. The interplay of optical, material, and diffusioneffects has not been analyzed fully yet. The side wall profilesnormally vary from near-vertical to, at most, 10% variance (topto bottom), as shown by the photograph and confocal images inFig. 3. For many applications, these tolerances are acceptable.

A microfluidic system containing multiple channels and anautonomous valve has been constructed by using the mFTprocess. The system includes a T junction, a straight channel, andan injection channel controlled by an autonomous hydrogelvalve. Fig. 4 illustrates (both schematically and experimentally)the steps required to build a functional microfluidic system byusing mFT. A polydimethylsiloxane (PDMS)yglass cartridge firstis injected with a prepolymer mixture of the constructionmaterial through the inlet channel by using a syringe. Cartridgefilling is facilitated by making the chamber hydrophilic (eithervia chemical or plasma treatment). The prepolymer mixture thenis polymerized through a photomask, which defines the channelpattern as well as a poststructure in the injection channel. Theunexposed region then is f lushed with water to remove theunpolymerized mixture, revealing the desired channel patternand the post (Fig. 4 C and D). Next, a second (functional)prepolymer mixture is injected into the newly formed micro-channels, and the mixture is exposed to a UV source from anOlympus Epi-Fluorescent microscope (Olympus, New HydePark, NY) for 5 min through a patterned photomask (300-mmhole) aligned over the previously formed post. By controlling thepressures on all inputsyoutputs, the functional prepolymer mix-ture can fill specific channels as desired. After removing theunexposed mixture by flushing with water, a pH-sensitive hy-drogel valve jackets the post. As is evident from Fig. 4, the mFTconstruction method allows rapid fabrication with a high degreeof design freedom. The time required to build the system shownis less than 10 min when using the previously prepared cartridgeand photomask. Both sequential and parallel polymerization areused in this demonstration to simplify the fabrication. We

Fig. 1. Overview of mFT. The fabrication of a hydrogel jacket valve in a T channel is illustrated as an example in the schematics. (A) After the reservoir is filledwith the prepolymer mixture of a structural material (blue), it is exposed to UV light through a photomask. (B) After flushing away the unpolymerized mixture,a T channel and a cylindrical post were fabricated in parallel. (C) After the T channel is filled with the prepolymer mixture of a stimuli-responsive hydrogel (green),it is exposed to UV light again through a second photomask. (D) After removing the unpolymerized mixture, a hydrogel jacket valve is fabricated within a Tchannel. Channel fabrication and component fabrication are combined via parallel or sequential polymerization to build a functional complex system.

Fig. 2. Construction of microchannels using mFT. The monomer mixtureconsists of IBA, TeEGDMA (5 wt %), and Irgacure 651 (3 wt %). (a– c) Basicprocedures. (a) Photomask placed on top of the channel. (b) After photopo-lymerization. (c) After removing the unpolymerized monomer mixture withMeOH. (d– f ) A variety of possible channel geometries with the correspondingphotomask shown at a reduced size in the upper right corner of each picture.(Bars 5 500 mm.)

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envision that the process can be simplified further by replacingthe physical photomask with a light array.

To create useful, functionally complex microfluidic systems,f low control is required. Several hydrogel valve designs havebeen demonstrated, ranging from simple two-dimensional de-signs to hybrid (PDMSyhydrogel) 3D designs, to biomimeticdesigns based on two different material beams. In the most basicconfiguration, a single hydrogel cylinder is formed by using thephotopolymerization methods described above. The hydrogelregulates the flow by expanding or contracting to seal or openthe channel. These single-channel valve designs will be useful inmany applications. However, it also would be beneficial to beable to sense the local environment in one channel and regulatethe flow in another channel. We have developed two valveschemes to accomplish this goal. In the first case, a hydrogel plugis polymerized in a gap between two adjacent parallel channels.Through appropriate placement and relative channel dimen-sions, the hydrogel responds to the pH in the sensing channel andcontrols the flow in the regulated channel by either contractingor expanding (Fig. 5 a–c). One of the limitations of the abovehydrogel valve design is the potential for cross-talk betweenchannels because of diffusion of species of interest through thehydrogel itself. To address this problem, we developed a hybridPDMSyhydrogel valve by combining 3D PDMS fabrication (8)and in situ polymerization techniques (Fig. 5 d–h). A PDMSmembrane (ion-impermeable) was created between the sensingand regulated flow streams with a pH-sensitive hydrogel on topof it that deflects the PDMS membrane when swollen. The

deflection causes the membrane to seal against the orifice belowit, blocking flow. The maximum differential pressures were 50psi for the postjacket valve design (limited by an off-chipconnection failure) and 27 psi for the hybrid valve design whenthey are closed. The valves have been operated for hundreds ofcycles without failure. Force generation in the hybrid valve isestimated via analytical models to be more than 20 mN.

Fig. 3. Confocal images and photograph showing nonvertical side walls. (aand b) Confocal images of the top and the cross-section (along the line) of acylindrical hydrogel object polymerized in a 180-mm-deep microchannel. Afluorescent dye was trapped inside the hydrogel for visualization purposes. breveals a slightly tapered shape with a smaller diameter at the bottom thanthe top. (c and d) Confocal images of the top and the cross-section of an in situfabricated microchannel filled with a fluorescent dye solution. d suggestsnear-vertical side walls with a slightly smaller channel width at the bottomthan the top. (e) The photograph of a hydrogel object that flipped over duringexpansion. The hydrogel showed side walls that deviated from the surfacenormal by '6°. (Bar 5 250 mm.)

Fig. 4. System construction using mFT. (a) A schematic of the steps tofabricate a functional microfluidic system using mFT. (b) The PDMSyglasstectonics cartridge. PDMS substrate with a shallow and wide fluidic chamber(i.e., Tectonics cartridge; 12-mm 318-mm wide, 200-mm deep) was molded ina Petri dish by using a positive relief master. The inletyoutlet connectionchannels were cored through the side of the block to retain complete opticalaccess over the entire cartridge. A microscope cover glass was bonded to thePDMS substrate as the cartridge top after activating the surface with oxygenplasma in an reactive ion etch system. (c and d) The tectonics cartridge aftersimultaneous fabrication of a microchannel and a post using a multipatternphotomask. The prepolymer mixture for this construction material consists ofIBA and TeEGDMA (in a 9:1 weight ratio) and Irgacure 651 (3.0 wt %). The UVsource is a 200-W mercury lamp ('400 mJycm2), and the exposure time is 1 min.(e) The pH-sensitive hydrogel jacket around the post after exposure andflushing. The prepolymer mixture for this pH-sensitive hydrogel consists ofacrylic acid and 2-hydroxyethyl methacrylate (HEMA) (in a 1:4 molar ratio),ethyleneglycol dimethacrylate (EGDMA, 1.0 wt %), and Irgacure 651 (3.0 wt%). [Bars 5 5,000 mm (b and c) and 500 mm (d and e).]

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To further improve the time response of the functionalhydrogel valve, two approaches have been demonstrated. Thefirst is a post-jacket design (7), which improves the time responseby reducing the diffusion distance required for complete volumechanges. In addition to modifying the geometry of the hydrogelobjects, porosity can be introduced to the hydrogel structure tospeed up the diffusion process. The introduction of porosity isachieved by inducing phase separation during polymerization. Itis observed that the step response improves more than 10-fold forboth swelling and shrinking processes upon such modification.

Construction of other devices and components can be realizedby using our approach. For example, by combining two strips ofhydrogel material with different pH sensitivity, a shape-changingstructure that bends in response to changes in local pH has beendemonstrated. In a typical procedure, after polymerizing a stripof pH-sensitive hydrogel, the channel is f lushed and filled witha non-pH-sensitive monomer mixture. A strip of non-pH-sensitive hydrogel is then polymerized at an adjacent location,slightly overlapping with the previous hydrogel strip. The non-pH-sensitive hydrogel strip also has an anchor that fixes one endof the bistrip hydrogel to the channel top and bottom at thedesired location. When exposed to a basic solution, the pH-sensitive strip swells whereas the other strip remains the samevolume, causing the bistrip gel to curve toward the non-pH-sensitive strip. A biomimetic check valve based on such a bistriphydrogel is demonstrated in Fig. 6. When triggered by high pH,the pair of bistrip hydrogels extends and bends to close thechannel. In this way, it operates as the passive valves found inveins, allowing the fluid flow in only one direction. The pH-sensitive strip serves as a spring to provide a restoring force forthe valve. When contracted in acidic solution, the valve becomesdeactivated and remains permanently open (Fig. 6f ). In thisexample, the valve responds to the local chemical environmentin addition to the local f luid flow characteristics.

Finally, laminar flow (without photomasks) can be used tocontrol geometry during fabrication as illustrated in Fig. 7. Inthis case, three fluid streams are pumped through a microchan-

Fig. 5. Hydrogel valve designs. (a– c) A two-dimensional hydrogel valvedesign. (a) A hydrogel plug is polymerized in the gap between two adjacentparallel channels. The hydrogel composition is the same as that described inFig. 4e, with a pH volume transition at pH 5 5. The sensing channel (600-mmwide) was constructed wider than the regulated channel (300-mm wide) tokeep the sensing channel open when the regulated channel is completely shutoff. The microchannel is made of EPON (Nano XP SU-8 50; Microchem, Newton,MA). Transparent adhesive tape (regular packaging tape) was used to seal the200-mm-deep EPON microchannels. (b) A pH 5.7 buffer is pumped through thesensing channel while water is pumped through the regulated channel. Thehydrogel valve expands to seal off the flow in the regulated channel. Thearrows denote the direction of the fluid flow. (c) The valve reopens theregulated channel when a pH 3.8 solution was pumped through the sensingchannel. (d– h) A 3D PDMSyhydrogel hybrid valve design. (d) A schematic ofthe PDMSyhydrogel hybrid valve. (e and f ) The top view and the side view ofthe hydrogel structure when it expands and deforms a membrane, blockingthe flow in an adjacent channel. The fluid in the blocked channel has beendyed for visualization purposes. (g and h) The hydrogel valve contracts, andthe membrane returns to a position that allows flow in the adjacent channel.[Bars 5 400 mm (a– c) and 250 mm (e– h).] Note in the side views that the edgeof the hydrogel actuator has been outlined for clarity.

Fig. 6. A biomimetic valve based on bistrip hydrogel. (a) After simultaneouspolymerization of the pH-sensitive strips. The hydrogel composition is thesame as that described in Fig. 4e. (b) After polymerization of the non-pH-sensitive strips to form the bistrip hydrogel valve with anchor. The prepolymermixture for the non-pH-sensitive hydrogel consists of HEMA, EGDMA (1.0 wt%), and Irgacure 651 (3.0 wt %). (c) When exposed to basic solution, the bistriphydrogel expands and curves to form a normally closed valve. The bistrip valvecan be pushed open (d) to allow flow in one direction (from left to right) whilerestricting flow in the opposite direction (e). ( f) When exposed to acidicsolutions, the valve is deactivated, returning to the permanently open state.(Bars 5 500 mm.)

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nel. The two outer streams contain functional prepolymermixtures, and the middle stream consists of an inert f luid. Aftersteady laminar flows are established by using syringe pumps, theflow rates are gradually reduced to zero followed by immediateUV irradiation at the desired area to initiate polymerization.The result of polymerization is the formation of pH-sensitivehydrogels along the edges of the channel. These walls subse-quently can be used to regulate flow based on the pH value ofthe fluid. By incorporating laminar flow, it may be possible torealize 3D fabrication by using mFT. In addition, componentsmade of different materials that are otherwise incompatible canbe fabricated simultaneously by using this method because theinert stream eliminates the necessity of direct contact betweenthe two outer streams.

Biosensing is another area of interest in microfluidics. Byusing the mFT method, we have modified hydrogels with lipid orfatty acid coatings that will be the foundation of a new class ofbiosensor devices (9). In an ideal design, this approach willconsist of a pH-sensitive hydrogel matrix modified on its surfacewith a lipid bilayer that contains channel proteins or receptors.As such, we call this type of component a ‘‘cell gel.’’ Because thelipid coating is impermeable to ions, a different pH can bemaintained within the hydrogel interior, allowing the hydrogel toremain contracted while bathed in a pH environment thatnormally would cause volume expansion. Upon exposure to amembrane-disrupting agent, the lipid bilayer will leak, causingthe hydrogel to swell and, thereby, amplifying the original signal.

We have completed the preliminary steps of this approach.First, a pH-sensitive hydrogel containing a pH indicator (phe-nolphthalein) is polymerized in a microchannel by using theabove technique. The hydrogel then is bathed in benzene andmodified by covalently linking fatty acids to the surface. Finally,the modified hydrogel is exposed to a basic solution (whichnormally would cause the hydrogel to swell) and the hydrogeldiameter is measured at timed increments to test the efficiencyof the fatty acid layer as an ion barrier. It was found thathydrogels modified in this manner remain stable for a severaldays without visible change in color or size, demonstrating theability to maintain chemical gradients. In contrast, unmodified

hydrogels that contain the pH indicator (phenolphthalein) be-come fully expanded within 40 min along with a color changefrom clear to pink. The modified hydrogels are capable ofswelling and changing color once the fatty acid layer is physicallydisrupted, as illustrated in Fig. 8. When exposed to a buffersolution with a pH of 12, a modified hydrogel shows no changeuntil the fatty acid layer was physically disrupted by piercing thegel with a micropipette tip, causing the gel to rapidly swell andchange color.

DiscussionmFT offers an alternative approach to microfluidic systemfabrication. The combination of liquid-phase polymerization,lithography, and laminar flow provides a universal constructionplatform that allows for continued growth and expansion in thedesign parameter space. The construction platform, compo-nents, and systems described in this paper demonstrate theversatility of the approach.

A variety of materials can be used within the fabricationprocesses, including both nonfunctional construction materialsand stimuli-responsive hydrogels. Known functionality includesresponse to stimuli such as pH, temperature, electric fields, light,carbohydrates, and antigens. Thus, functionally complex systemscan be fabricated by using a single construction platform. Theperformance of the hydrogel components in terms of displace-ment and force generation are excellent, and the observedtemporal response is one to two orders of magnitude better thantypical macroscopic hydrogel structures because of scaling.

Fig. 7. Geometry control during fabrication by using laminar flows. (a) Aschematic of the fabrication method using laminar flows to control geometry.(b) Steady laminar flows are obtained before UV irradiation. The outerstreams of prepolymer mixture contain acrylic acid, EGDMA, water, and aphotoinitiator. The middle stream consists of glycerin. The channel fabricationis the same as described in Fig. 5a. (c) After UV irradiation for 2 min, pH-sensitive hydrogel walls are formed while the center of the channel remainsopen. (d) When exposed to a basic solution, the pH-sensitive hydrogels expandto seal off the channel. (Bars 5 500 mm.)

Fig. 8. A pH-sensitive hydrogel modified with a fatty acid surface layer. Acylindrical hydrogel with a diameter of 400 mm is polymerized in a microchan-nel. The hydrogel material is the same as described in Fig. 4e, which containshydroxyl termini. Fatty acid segments then are covalently linked to the hy-droxyl groups on the outer surface of the hydrogel via in-channel treatment(a). When a pH 5 12 buffer is flowed through the channel, the modifiedhydrogel (r) remains stable for hours without obvious change (b), whereas anunmodified hydrogel of the same composition and size (■) starts to swellinstantly. After piercing the hydrogel with a micropipette tip to physicallydisrupt the surface layer (c), the modified hydrogel starts to swell at thepierced location (d and e) and eventually becomes fully swollen accompaniedby a color change ( f). (Bars 5 100 mm.)

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Further improvements through chemical modification also havebeen achieved, which will lead to rapid sensing and actuating inmany microfluidic systems.

New materials can be developed to add new functionality. Asdescribed above, we recently have demonstrated a hydrogelyfatty acid layer hybrid component that will serve as the basis ofa class of biosensor components. The goal of this class ofbiosensor components is to capture nature’s exquisite ability todetect signals with high sensitivity and specificity without thelimitations of cell-based biosensors. By incorporating molecularrecognition structures (e.g., transmembrane proteins) within thelipid bilayer, we can retain natural biodetection mechanisms andcouple them to an artificial output (e.g., the hydrogel). Thisapproach may provide a robust biosensor that does not requireprecise environmental control to maintain its functionality. Thefatty acid surface layer does not affect the physical properties ofthe interior hydrogel matrix, and the layer remains stable forseveral days until physically disrupted. By extending the physicaldisruption to other types of interactions, these devices can beused as biosensors. The release of a signal such as dye entrappedin the hydrogel would create a visually observable signal alongwith the volume change that would not require special instru-ments to detect molecular-level interactions.

The use of lithography and laminar flow to define componentlocation and geometry allows 3D control. For example, one candefine dimensions in the plane perpendicular to flow by usinglaminar flow (e.g., each quadrant contains a different chemis-try), and lithography can define the dimensions in the orthogonalplanes. In addition, one can choose between sequential andparallel construction. For example, one can easily adapt thetechnique to construct a large array of posts (made of a singlematerial) in parallel or a large array of posts (each made of adifferent material) sequentially. A major advantage of liquid-phase polymerization is that the unpolymerized mixture is easilyremoved by flushing the channel with solvent. In addition, thelow viscosity of the mixture allows it to fill channels easily viacapillary force. Although diffusion in the liquid phase canbecome significant during photopolymerization, lowering theresolution compared with traditional lithography, the overallresolution of mFT is adequate for many practical applications.

Finally, mFT points toward a more organic, more biomimeticapproach to microsystem development. Traditionally integratedcircuit-derived approaches to miniaturization focus on dimen-sional precision and resolution. Silicon-based micromachining(i.e., surface and bulk) allows for high resolution (i.e., on theorder of a micrometer) structures, but the process developmenttime can be lengthy. Traditional stereolithography prototypingmethods allow for the rapid creation of prototypes, but to movethose prototypes into production requires a different process.mFT allows rapid prototyping of functional devices, and the sameprocess then can be used for production. In natural systems, it isfunction that is important, and natural systems are constructedto perform required functions. For example, if you were toexamine the hearts of 10 mice, one would find that each heart isslightly different (in some cases significantly different) from theothers. All of the hearts, however, perform the same function–topump blood throughout the circulatory system. mFT has thissame characteristic in that each valve may not be exactly the samein shape and size, but each valve performs the same function. Inaddition, by fabricating the systems within prefabricated chan-nelsyreservoirs, mFT minimizes the use of clean rooms and thelarge amount of energy and chemicals consumed during theclean room fabrication procedures, making it a more environ-mental-friendly construction method.

ConclusionsWe believe that the mFT platform will facilitate the developmentand the application of microfluidic systems in general. Theapproach provides ultrafast construction, allowing multiple de-sign interactions to occur in a single day. In addition, theplatform has been designed and built specifically with and formicrofluidics. It provides a single platform that can be extendedand expanded by the addition of new materials and a wide varietyof geometric designs without modifying the basic fabricationprotocol.

This work has been supported under grants from the Defense AdvancedResearch Projects Agency-Microsystems Technology Office (F33615-98-1-2853 and F30602-00-1-0570; Dr. Abraham Lee, Program Manager).We thank Joseph M. Bauer for his help in this work.

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