G

S

R

R

KD

a

ARRAA

KRFMFBC

C

1

o[[d

0d

ARTICLE IN PRESSModel

NB-12686; No. of Pages 11

Sensors and Actuators B xxx (2010) xxx–xxx

Contents lists available at ScienceDirect

Sensors and Actuators B: Chemical

journa l homepage: www.e lsev ier .com/ locate /snb

eview

eversible sealing techniques for microdevice applications

halid Anwar, Taeheon Han, Sun Min Kim ∗

epartment of Mechanical Engineering, Inha University, 253 Younghyun-dong, Nam-gu, Incheon 402-751, Republic of Korea

r t i c l e i n f o

rticle history:eceived 12 September 2010eceived in revised form 17 October 2010ccepted 1 November 2010vailable online xxx

a b s t r a c t

Reversible sealing of two different functional layers is an advancing and effective technique for the fabri-cation of microdevices. Reversible sealing enables microdevices to be dismountable and reusable, whichare promising features for the high throughput analysis by means of spatial, temporal, and parallel pro-cess. It is therefore reversible sealing is potentially being used for various research fields such as microand nanodevice fabrication, flow analysis at microscale, biomolecule analysis, cell analysis and other

eywords:eversible sealingabrication techniqueicrodevice coupling

low analysis

related fields. This review reports on the materials and techniques used for reversible sealing and theirapplications in various research areas, and illustrates their advantages and disadvantages. We provideexamples, where necessary and comment on the outlook for reversible sealing applied to microdevices.

© 2010 Elsevier B.V. All rights reserved.

io-molecules analysisell studies

ontents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 002. Material compatibility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 003. Sealing techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

3.1. Sealing by self-adhesion properties of material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 003.2. Vacuum seal by aspiration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 003.3. Sealing by magnetism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

4. Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 004.1. Fabrication of micro and nanodevices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 004.2. Flow analysis in microscale . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 004.3. Bio-molecules analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 004.4. Cell studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

5. Conclusion and outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00Biographies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

. Introduction

Microdevices have great importance to applications in vari-

decreased waste production, reduced reaction time and powerrequirements, increased throughput and portability, integrationwith other miniaturized devices, and potential for parallel opera-

Please cite this article in press as: K. Anwar, et al., Reversible sealing techndoi:10.1016/j.snb.2010.11.002

us biological fields including protein analysis [1], DNA analysis2–7], enzymatic assays [8], immunoassays [9,10], and cell analysis11–13]. Microdevices have many advantages over conventionalevices such as smaller size, less usage of samples and reagents,

∗ Corresponding author. Tel.: +82 32 860 7328; fax: +82 32 860 7328.E-mail address: sunmk@inha.ac.kr (S.M. Kim).

925-4005/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.snb.2010.11.002

tions [14–16]. Great efforts have been made to make microdevicesmore functional, efficient and cost effective, including the findingof new materials, and fabrication techniques [17,18].

iques for microdevice applications, Sens. Actuators B: Chem. (2010),

Developing novel materials and proper sealing methods is akey aspect in the fabrication of microdevices. Material selectionis important for proper sealing and to ensure compatibility for aparticular application. Sealing methods are also important to com-bine layers to make microdevices, and to interconnect or couple

ING

S

2 d Actu

wSvcetasetc

adeisfisshsi[butavppyHatn[

iiiea

2

katadaptc[ctmplrs

ARTICLEModel

NB-12686; No. of Pages 11

K. Anwar et al. / Sensors an

ith different devices to obtain a more robust and efficient system.ealing methods can be generally classified into two types: irre-ersible (permanent seal), and reversible (with weak bonding thatan be disassembled when needed). The important sealing param-ters for microdevices are leakage and the capability to withstandhe working pressure inside the device. Irreversible sealing is usu-lly adopted for the microdevice working with high-pressure fluid,ince it has high sealing strength without leakage problem. Nev-rtheless, reversible sealing has emerged as an alternative sealingechnique for microdevices that are very prone to clogging, espe-ially in molecular analysis [19–25].

Reversible sealing enables the high throughput and multiplenalyses by allowing flexible alignment and facilitating repeatedisassembly and assembly of the device. This sealing method alsonables reversible coupling interconnectivity of different devicesn a line (plug-and-play), which promotes a continuous and con-ecutive operation. A variety of commonly used materials for theabrication of microdevices are compatible with reversible seal-ng. Most polymers can be reversibly sealed with hard materialsuch as glass, metal, and silicon substrate with or without aurface treatment [26–32]. Several reversible sealing techniquesave been demonstrated to improve sealing strength and otherealing properties [26,27,33,34]. A wide range of reversible seal-ng strength levels for leak-proof devices has been investigated34]. The techniques used for reversible sealing are (1) sealingy the self-adhesive property of materials bonding, (2) vac-um seal by aspiration, and (3) sealing by magnetism. Theseechniques are biocompatible and have biological and chemicalpplications. Reversible sealing is useful for coupling microde-ices [35,36], micromixing [37], surface patterning (chemicalatterning, cell or biomolecular patterning, and microelectrodeatterning) [32,38–42], biosensor [24,43–45], biomolecular anal-sis [30,46–48], bio-fuel cells [49], and cell co-culture [50,51].owever, a concern with reversible sealing is the potential for leak-ge of fluids inside the device at high working pressure. Therefore,he working pressures and flow rates of different sealing tech-iques and materials have been investigated for leak-proof devices26,33,34].

Here, we present a study on the materials that are compat-ble for reversible sealing and the different techniques used tomprove reversible sealing quality. Applications of reversible seal-ng in different areas of research, how they can be used for theasy fabrication of flexible and efficient microdevices, and theirdvantages and disadvantages, are discussed in detail.

. Material compatibility

The selection of materials for microdevices depends on severaley factors including biocompatibility, transparency, permeability,nd wettability [29,30,32,52]. In addition, the selection depends onhe specific application of a microdevice [31,39,41]. Initially, siliconnd glass were the most attractive hard materials for microdevicesue to well-developed fabrication technologies for semiconductornd micro-electro-mechanical systems (MEMS) [53,54]. Recently,olymers have become popular for microdevice fabrication dueo their inexpensive manufacturing techniques (replica molding,asting, injection molding and embossing) and low material cost55]. Moreover, polymers have physical, mechanical, and chemi-al properties that meet the demands of microdevice applications:hese polymers include polydimethylsiloxane (PDMS), polymethyl-

Please cite this article in press as: K. Anwar, et al., Reversible sealing techndoi:10.1016/j.snb.2010.11.002

ethacrylate (PMMA), polyethylene diacrylate (PEG-DA), parylene,olyurethane, polycarbonate, polystyrene, polyethyleneterephtha-

ate glycol (PETG), polyvinylchloride, and polyethylene [29,56]. Foreversible sealing, most polymers can be used to seal to smoothurfaces either by conformal contact or using surface pretreat-

PRESSators B xxx (2010) xxx–xxx

ment. However, sealing with different materials allows differentstrengths of adhesion, which in turn determines the limits ofworking pressures and flow rates in microdevices. Moreover, theproperties of the sample solutions are important criteria for theselection of materials, especially polymer materials. Most of themicrodevices for bioassay have been operated in aqueous buffersystems, however, organic synthesis in organic solvents withmicrodevices also has been increasing its applications includingmicroreactors for organic reactions [57–59]. Some polymer mate-rials are not compatible with high concentrations of some organicsolvents [60] and particularly swell in contact with nonpolar sol-vents (e.g., hydrocarbons, toluene, and dichloromethane) [59].

Among polymer materials, PDMS is the most popular due toits properties (including permeability, transparency, flexibility, andbio-compatibility). The surface of native PDMS is hydrophobicdue to the repeating –OSi(CH3)2− units. The surface can be madehydrophilic by the surface modification with plasma, UV/ozone, orcorona discharge [29,61,62]. These surface modification techniquesform a smooth oxide layer of a few nanometer thick over the sur-face, and increase the surface energy. The oxidized surface remainshydrophilic if it stays in contact with water. The surface of oxidizedPDMS can be modified further by treatment with functionalizedsilanes [60].

PDMS can be sealed reversibly to a variety of materials such asglass, hard plastic, silicon, flat metal, photoresist, and native PDMS[26,28–30,60]. PDMS also allows reversible sealing with silicone orcellophane adhesive tapes that facilitate mechanical flexibility andeasy formation by cutting [61]. Parylene is also a promising mate-rial for reversible sealing and is used in medical research due to itsbiocompatibility and flexibility. Parylene can be sealed reversiblyto PDMS, methacrylate, glass, polystyrene, and other flat surfaces.Polyurethane is another material that can be reversibly sealed toitself [62]. PEG-DA can be sealed reversibly to a gold or a siliconsubstrate by UV curing [31]. Thus, a wide range of material com-patibility makes the reversible sealing approach more useful andapplicable to different types of microdevices.

3. Sealing techniques

Sealing of microdevices can be generally classified intoreversible or irreversible sealing. Irreversible sealing is the pre-ferred option for the microdevices that require high workingpressure. However, reversible sealing has been employed for exper-imental benefits as stated previously. Different techniques havebeen reported to make reversibly sealed microdevices with differ-ent materials (Table 1). Different reversible sealing techniques areexplained as follows.

3.1. Sealing by self-adhesion properties of material

The most common reversible sealing techniques are to usethe inherent adhesive properties of materials, or to use surfacetreatments. For example, PDMS can easily be reversibly sealed toselected materials or to itself by conformal contact after bringingthem in contact one another. Reversible sealing by the conformalcontact can be induced via van der Waals force or enhanced by thegeneration of electrostatic charge over surface using mechanicalfriction. The van der Waals force is caused by the attraction betweenmolecules and surfaces. This force differs from covalent and ionicbonding in that they are caused by correlations in the fluctuat-

iques for microdevice applications, Sens. Actuators B: Chem. (2010),

ing polarizations of nearby particles; a consequence of quantumdynamics [63–66]. The sealing strength from this technique isweak; but, a water tight seal is created. Due to weak bonding,both layers can be peeled apart easily without leaving any stress orchanges in the counterpart layer. Thus, this technique is useful for

ARTICLE IN PRESSG Model

SNB-12686; No. of Pages 11

K. Anwar et al. / Sensors and Actuators B xxx (2010) xxx–xxx 3

Table 1Sealing strength of reversible sealed device respect to different techniques.a

Technique Material Max. pressure (kPa) Max. flow rate

Self-adhesive properties of material due to surface energy PDMS + PDMS/glass/flatsurface/other polymerPEG-DA + gold/silicon

35 [26] ∼10 �l/min

Aspiration PDMS channel + glass/flatsurface

100 [33] ∼Fit for negative pressure flow

MS/gMS/g

brMaiecsuscpcergogTsstctcw

3

aswwiToavorcwfcT1afilscit

Magnetism PDMS ratio (polymer: curing agent) from 5:1 to 20:1 PDMS + PDPlasma bounded irreversible device PDMS + PD

a Error is estimated to be within 2 kPa [34].

iological applications such as cell handling and drug screening thatequire repeatable sealing of channel layers for multiple analysis.oreover, for the surface patterning with proteins or biomolecules

nd microelectrode patterning for electrophoresis where the work-ng pressure is not too high, this technique is very useful due to itsasy handling, and no extra fabrication steps. External measuresan be additionally applied to avoid leakage and enhance sealingtrength such as patch clamps for continuous pressure [27], man-al pressure for a short periods (<30 s) [61], or quick-fix adhesive toeal the PDMS edges [59,66]. This sealing technique by conformalontact is applicable for a wide range of materials including mostolymers to any flat surface (such as glass, silicon, or itself), specifi-ally PDMS is found most promising material for this technique. Thelectrostatic attraction is another technique to create or enhanceeversible seal. Electrostatic attraction was used to seal PEG-DA toold or silicon reversibly by inducing negative charge generationn the surface of a film mold and brought into contact with theold or silicon substrate having a positive charge on its surface [31].his conformal contact creates reversible sealing with no externaltimulus such as temperature or mechanical pressure. Using thisimple technique, reversible sealing can be achieved strong enougho confine fluid within a microfluidic manifold without leaking andan also allow a small negative working pressure by suction upo 35 kPa [26]. Hence, this technique can be useful in biological,hemical research, and in fabricating the nanostructures where theorking pressure is compatible with the sealing strength.

.2. Vacuum seal by aspiration

Vacuum seal is another technique to achieve reversible sealingnd device can be taken apart after release the vacuum. Reversibleealing of two layers by aspiration uses applied vacuum pressureithin the microdevice by adding an additional microchannel net-ork (Fig. 1). The microchannel network for aspiration is fabricated

n the top layer surrounds the working microfluidic manifolds.he bottom layer can be any flat surface, including cover glassr transparent polymeric materials, for optical convenience. Thedhesive force for reversible sealing is generated by a moderateacuum in the microchannel network after placing the top layerver the bottom layer. The device can be easily dismounted afterelease of the vacuum pressure. The negative pressure at the outletan enhance the sealing strength and maintain a flow inside theorking microfluidic manifolds. This technique is more suitable

or sealing elastic polymers to any flat surface. For instance, Espe-ially, PDMS gives better results due to its soft elastomeric property.he aspiration technique allows for a working pressure of up to00 kPa in the functional channel [33,38]. Microchannel networknd working microfluidic manifolds should be separated by suf-cient gap to avoid leakage in-between and uneven surfaces and

Please cite this article in press as: K. Anwar, et al., Reversible sealing techndoi:10.1016/j.snb.2010.11.002

arge debris should be avoided to prevent sealing failure. In thisealing technique, no surface treatment is needed and top layeran be reversibly sealed any time to a preformed substrate includ-ng living cell monolayers and protein [67,68]. Since some surfacereatments can cause changes in the chemical properties of materi-

lass/flat surface 50–145 [34] 100–215 ml/minlass/flat surface 210–345 [19]

als and may also be harmful for biological organisms, this techniquecan be beneficial for chemical and biological applications. How-ever, this technique has some restrictions as it requires a networkof crossing microchannels that adds an additional fabrication step.For adequate sealing of the microdevice the aspiration networkrequires additional space, which is not good for a wide array offunctional microfluidic channel devices.

3.3. Sealing by magnetism

Magnetic force can hold different layers of a device to form areversible seal. This technique could seal any device with a useof material compatible with magnetic field. The device that isreversibly sealed by magnetism can withstand a wide range ofworking pressure from 50 to 145 kPa depending on the magneticfield strength (Fig. 2). For example, Rafat et al. [34] sealed PDMSto glass using a neodymium bar, slab magnets, or fine iron powderwith a magnetic susceptibility, over a stable range of gel elasticity.HeLa cell culture was reported with this technique, which showsbio-compatibility. Magnetic sealing is an inexpensive techniquethat allows for straight forward reconstruction when the clean-ing and replacing of any part is necessary therefore, this techniquecan be usefully applied to bio-sensors, drug delivery, and tissueengineering.

4. Applications

Recently, reversible sealing has become effectively used inmicrodevice-based research including the fabrication of micro andnanodevices, flow analysis at the microscale, biomolecules analysis,and cell studies. Applications of reversible sealing for each appli-cation area and the associated advantages and disadvantages arediscussed as follows.

4.1. Fabrication of micro and nanodevices

The use of microdevices is growing rapidly, and improved meth-ods are needed to fabricate micro and nanostructures and fluidicsystems for the study of biological species (e.g., DNA, proteins,and cells) by transport, sensing, and separation [69–73]. Generaltechniques used in micro and nanochannel fabrication include con-ventional micromachining [74–76], nanoimprinting lithography[77], and simple bonding processes employing irreversible fusionor anodic bonding [78–80]. These nanochannel fabrication tech-niques are used for silicon, glass, PMMA and polyimide based deviceand includes complicated fabrication processes. Micromolding incapillaries (MIMIC) was the first technique, as reported by White-sides and co-workers, to use reversibly bonded microcapillaries[81,82]. To develop nanostructures reversibly bonded nanocapil-

iques for microdevice applications, Sens. Actuators B: Chem. (2010),

laries (RBNs) has been reported [31]. The RBNs were reversiblyattached to the substrate, which was patterned with the nanos-tructures. The patterning material was then filled into the channeland cured, and after removing the RBNs, the nanopattern wasdeveloped onto the substrate (Fig. 3). The main difficulty with

ARTICLE IN PRESSG Model

SNB-12686; No. of Pages 11

4 K. Anwar et al. / Sensors and Actuators B xxx (2010) xxx–xxx

Fig. 1. (A) Schematic diagram of microfluidic device by aspiration. (i) Top layer consisting of microchannel and crossing channel network for aspiration placed over substrate.(ii) Bottom substrate, which has to be pattern. (iii) Application of vacuum pressure by aspiration to reversibly seal both layers. (iv) Filled micofluidic channel with patterningm the pp oresce( rred to

t1wbotytUbcgtttsgn

aterial. (v) After releasing aspiration the microfluidic system can be removed, andatterned surface with different proteins after successive use of microchannel. FluFor interpretation of the references to color in this figure legend, the reader is refe

his technique is to make the resolution of the RBNs less than00 nm because the PDMS has a low elastic modulus (∼3.2 MPa),hich results in deformation or buckling of the channel. A num-

er of studies have been performed to overcome this shortcomingf PDMS by enhancing the PDMS properties and using alterna-ive molding materials [83–86]. However, these attempts did notield the desired result regarding hardness of material necessaryo achieve the required resolution of the nanostructure. Recently, aV-curable PEG-DA mold sheet was used for RBNs [31] that had aetter elastic modulus (>70 MPa) and hardness which can preventsollapse and provides better resolution, and could reversibly seal toold and other substrates for patterning nanostructures. A nanos-ructure was fabricated with PEG-DA RBNs on gold substrate as

Please cite this article in press as: K. Anwar, et al., Reversible sealing techndoi:10.1016/j.snb.2010.11.002

he resolution of 50–500 nm width and 300–1000 nm height. Fur-hermore, microchannel on glass has been depicted by reversiblyealing the PDMS channel to the glass substrate and the followinglass etching process [87]. In this case, PDMS with a defined chan-el size was used as an etch mask. After it was reversibly sealed on

attern developed over surface can be used for further operations. (B) Micrograph ofnce colors of BSA–FITC and BSA–Cy3 proteins are green and red, respectively [33].the web version of the article.)

the glass, etching was performed by the flow of the etching solutionand the depth of the microchannel can be controlled by varying theetching time.

Moreover, a reversible assembly technique is used to fabricatethe valve and pump which are important components of inte-grated microdevices in order to control and achieve the desiredflow in the device for bio-chemical analysis using multilayer softlithography [88]. Fabrication of a pneumatically actuated three-way multilayer microvalve has been demonstrated using reversibleand irreversible techniques by Hosokawa and Maeda [89]. The lay-ers can be aligned easily since the PDMS is transparent and canseal reversibly with a weak bonding. Negative pressure was usedto control the valve, which could resist up to 10 kPa positive pres-

iques for microdevice applications, Sens. Actuators B: Chem. (2010),

sure.Recently, the coupling of microdevices has been studied to

improve interconnectivity. Initially, standard High-performanceliquid chromatography (HPLC) PEEK (polyether ether ketone) tub-ing was used to interconnect microchips. Verpoorte et al. [90]

ARTICLE IN PRESSG Model

SNB-12686; No. of Pages 11

K. Anwar et al. / Sensors and Actuators B xxx (2010) xxx–xxx 5

F m). (ii( ) Irono age. ([

sictcactpbccitwdsspvfgcteoi(cub[

4

i

ig. 2. (A) (i) Adhesion of PDMS to glass using bar magnets (15 mm × 5 mm × 3.2 miii) Magnet slab (102 mm × 25.4 mm × 6.4 mm) used to adhere the PDMS to glass. (ivf use of magnetic sealed microchannel for staining HeLa cell. (i) Phase contrast im34].

howed a coupling of polymerase chain reactions (PCR) and cap-llary electrophoresis using a plastic liner and epoxy adhesive toonnect two chips together, and Woolley et al. [91] demonstratedhree-dimensional (3-D) micro-flow by stacking of microfluidicomponents on top of each other. However, leakage and alignmentre basic problems of these systems, thereby requiring compli-ated fabrication processes. Reversible sealing allows the deviceo be used immediately without a complex setup process; i.e., thelug-and-play concept. The reversible sealing method for glass haseen presented using room temperature bonding technique forapillary electrophoresis [92]; however, it still involves a complexleaning step. Thus, a simple and rapid method of chip-to-chipnterconnection was developed [35]. Using reversible interconnec-ion, the injection and separation chip were aligned and sealed,hich supports electroosmotic flow. The devices were coupled toevelop microchip-based electrophoresis used for genetic analy-is [93–95] and separation based immunoassays [96]. A reversibleealed device also has been proposed to couple a microdialysis sam-ling system to microchip electrophoresis through PDMS basedalve [97]. Hence, the device can be used repeatedly and cleanedrequently, since microchip electrophoresis is very prone to clog-ing. This reversibly sealed holding valve was used to integrate aontinuous flow sampling system in microchip electrophoresis, forhe continuous monitoring and analysis of processes that occurither off-chip (such as microdialysis sampling) or on-chip fromther integrated functions [36]. Moreover, this PDMS-based valven a reversibly sealed device was utilized to integrate microdialysisMD), sampling microchip electrophoresis (ME), and electrochemi-al detection (EC) [98]. The integration of MD/ME/EC system can beseful for neurochemistry and pharmacokinetics, which requiresoth electrophoresis separation and electrochemical detection99,100].

Please cite this article in press as: K. Anwar, et al., Reversible sealing techndoi:10.1016/j.snb.2010.11.002

.2. Flow analysis in microscale

Understanding fluid flow behavior in microscale has greatmportance for microfluidic research. The fluid flow behavior in

) Channel fixed by magnets (15 mm × 5 mm × 3.2 mm) embedded in the PDMS gel.filings embedded in PDMS around the channel. Scale bar is 1 cm. (B) Demonstrationii) F-actin and nuclear stain. Scale bar is 25 �m. Arrows indicate the channel edge

presence of mechanical, thermal, optical, or electrical stimuli isimportant based on the applications of microdevices such as mix-ing at microscale for homogenization of sample reagents [16,101],capillary electrophoresis [102,103], and electrokinetic phenomenaof fluid [37,104–106]. The performance of these devices is signif-icantly dependent on the fluid flow behavior. The key features ofthe microdevices are to work with smaller reagent volumes, shorteranalysis times, and to improve efficiency and throughput with thepossibility of parallel operations. Microchip-based electrophoresis(MCE) systems are being considered as an analytical approach forMicro total analysis system (�TAS), which includes pretreatment(concentration, labeling, and detection) and separation of analytes.Although MCE is widely employed for �TAS, it has difficulties suchas precise alignment, reusability, device integration, and suscepti-bility to clogging. Reversible sealing has been utilized to overcomethese difficulties.

To reduce the cost of a disposable device by making it reusable,an electrophoresis microdevice with an integrated electrode usinga reversible seal has been studied [107,108]. The reversible sealedlayer can be dismounted so that it can be cleaned and reused, ortaken apart and to save the electrode material (platinum, gold, orother expensive materials). As the heat treatment process can dam-age gold as an electrode material, the reversible sealing technique,which does not include any a heat treatment process, has beenapplied to duel electrode electrochemical detection systems [103].To increase the capacity of the microchip, a small square of perfo-rated PDMS was used and sealed reversibly around the detectionreservoir. This reversibly sealed device was stable for a long periodof time (as long as four months) and could be easily cleaned, whichdoes not damage the electrode. In addition, reversible sealing wasused to prevent light scattering and allow precise alignment in anoptical detection system [20], and to enable reconstruction of the

iques for microdevice applications, Sens. Actuators B: Chem. (2010),

device very easily [22].Although the number of separation techniques used in microde-

vice is increasing, fabrication of a miniaturized microelectrode thatcan be integrated with other parts of the device in microchip elec-trophoresis is still required. Reversible sealing is an approach to

ARTICLE IN PRESSG Model

SNB-12686; No. of Pages 11

6 K. Anwar et al. / Sensors and Actuators B xxx (2010) xxx–xxx

Fig. 3. (A) Schematic diagram of a reversibly sealed nanocapillary to fabricate a nano structure over a substrate. (i) Arrangement of PET film, PEG-DA, and master mold. (ii)UV curing of PEG-DA over Si-master. (iii) Peeled PEG-DA replica (negatively charged) with 50 �m–thick PET film. (iv) PEG-DA mold that is negatively charged and sealedreversibly onto positively charged gold or silicon by electrostatic attraction after being brought in contact with each other. (v) Channel filled with UV-curable prepolymer( ctureg , (ii) 3b old wit ofiber

sPTtiftptaw

tema[tatsetr

NOA 71) by capillary action. (vi) Peeling off the PEG-DA mold reveals the nanostruold substrate using RBNs for (i) 500 nm lines with an apparent contact angle of 55◦

etween the nanoline mold (70 nm) and gold substrate, (v) 50 nm wells with the mhe nanopillar mold (50 nm) and gold substrate, and (viii) freestanding 300 nm nan

et the dimension of the electrode-over-glass substrate using aDMS based microchannel that is filled with carbon ink and cured.he PDMS device is then removed for molding the microelec-rode onto a glass substrate [102]. The fabricated microelectrodes then integrated with the microfabricated palladium decoupleror microchip capillary electrophoresis and amperometric detec-ion. This approach eliminates the fabrication of a new electrodelate. Using the same approach, a precise and smaller microelec-rode was fabricated to fix the leakage problem that occurred withlarge electrode [42], by precise alignment of the microchannelith respect to the bare palladium decoupler electrode.

The electrokinetic fluid flow in a microdevice for dielec-rophoresis (DEP), travelling wave dielectrophoresis (TWD) andlectrorotation (ROT) have been investigated and used foranipulation, concentration, separation, collection and the char-

cterization of electrically polarizable bio and non bio-particles109–112]. Recently, a modular analysis platform was reportedhat took advantage of reversible sealing to integrate DEP, ROTnd TWD on a single chip that was easy to fabricate, cost effec-

Please cite this article in press as: K. Anwar, et al., Reversible sealing techndoi:10.1016/j.snb.2010.11.002

ive, and did not require re-wiring or repositioning [66]. Reversibleealing made this integrated device re-configurable, and enablexperimentation with multiple electrode configurations since upo 60 electrodes could be introduced on the same chip. Because ofeversible sealing the fluidic channel can be dismounted to clean

on the gold or silicon substrate. (B) SEM images of fabricated nanostructures on a00 nm lines with one detached freestanding line, (iii) 50 nm lines, (iv) the interfaceth pillar geometry, (vi) large-area fabrication for nanowells, (vii) interface betweens delaminated from the substrate [31].

the channel and the electrode layer and resealed again, or a newfluidic channel can be mounted. This reusability of the electrodelayer significantly reduces the time required to fabricate a newelectrode.

Material selection is an important factor for the microdevicesused for the electrokinetic fluid flow analysis because an electricaland chemical property of materials has great influence on elec-trokinetic phenomena in microdevices. For example, PDMS, whichhas negative polarity, has been characterized for its basic elec-trokinetic phenomena using a reversibly sealed PDMS-to-PDMSand PDMS/glass hybrid channel [113]. The field effect for directcontrol of electro-osmosis in PDMS was also investigated usinga hybrid microchannel fluidic system made by reversible sealingof the PDMS channel onto a silicon wafer in order to miniaturizean electrokinetic capillary instrument as a microchip device [114].Hence, reversible sealing can be advantageous for flow analysis inmicrodevice, particularly electrokinetic flow and separation by pro-viding easy and precise patterning of microelectrode, facilitatingcleaning and making device reusable.

iques for microdevice applications, Sens. Actuators B: Chem. (2010),

4.3. Bio-molecules analysis

Chemical and biomolecular analysis in microscale requiressophisticated and complex methodology to achieve a quick and

ARTICLE IN PRESSG Model

SNB-12686; No. of Pages 11

K. Anwar et al. / Sensors and Actuators B xxx (2010) xxx–xxx 7

Fig. 4. (A) Schematic diagram to fabricate multiphenotype cell arrays over microwell patterned substrates using reversible sealing of microfluidic channel. Initially, a PDMSmicrofluidic array was aligned on an array of microwells. (i) Array of microwells on substrate, (ii) aligned and reversible combined PDMS microfluidic channel on the microwellpatterned substrate, with each well line centrally placed in microchannel and each cell type introduced in an individual channel. (iii) Cell docking in microwells. (iv) Removedmicrochannel after cell docked in microwells. (v) Another microchannel placed orthogonally onto the cell-docked microwell pattern, which makes a multiphenotype cellarray inside each microchannel for multiple tests in parallel. (vi) Different types of cell groups. (B) Experimental result of the formation of multi-phenotype cell arrays withinmicrochannel. The fluorescent images are those of various cell types stained with two membrane dyes, CFSE (green) and SYTO (red) (right to left: ES/AML12/NIH-3T3 cells).( and d( ultiplr

erapmd[

estembhfprsr

acicmptm

i) Reversible sealing of primary channel on microwell pattern, each cell type flowediii) Secondary microchannel placed orthogonally with reversible seal to run the meader is referred to the web version of the article.)

xact result. Extensive research has been performed to analyzeeactions of reagents and to realize rapid and parallel reactions onsingle device. Surface preparation by patterning with chemicals,roteins, and biomolecules, is necessary to provide biocompatibleicroenvironments. Reversible sealing is being used efficiently in

ifferent surface patterning techniques (e.g., microcontact printing115,116] and, MIMIC [47,117–120]).

Dispense a nano liter volume of liquid into array of microw-ll onto microwell patch has been demonstrated using reversiblyealed PDMS device [39]. A silicon oil bath was used to peel offhe microchannel of the PDMS from the patterned surface to avoidvaporation and spilling a nanoliter volume of liquid from theicrowells. Flat surfaces can also be patterned by reversible sealing

y either hard or soft microfluidic networks [121]. Capillary actionas been adopted to reduce regent volume and fill microchannels

or developing bioassays [118]. Wright et al. presented a preciseatterning of biomaterials atop different types of flat surfaces usingeusable, reversible sealed parylene membranes [32]. Reversibleealing is also convenient for direct patterning of protein and cell-esistant polymeric monolayers [122].

The deposition of cells and proteins over a planer surface using3-D microdevice [123] can accommodate extra potentialities for

ell biology, the production of bio-sensors, and tissue engineer-ng [40,124]. 3-D microfluidic device can overcome topographical

Please cite this article in press as: K. Anwar, et al., Reversible sealing techndoi:10.1016/j.snb.2010.11.002

onstraint associated with quasi-two-dimensionality of existingethods (soft lithographic techniques), for example, microcontact

rinting (mCP) and MIMIC, such as 2-D patterning has been limitedo simple, continuous structure and one substrate at a time. 2-D

icrodevice requires multiple steps for patterning with multiple

ocked in microwells. (ii) Stable cell in microchannel after removal of PDMS channel.e tests [27]. (For interpretation of the references to color in this figure legend, the

materials for 3-D patterning. The development of complex, discon-tinuous patterns of multiple biological materials and cells has beenreported by employing a 3-D microdevice sealed reversibly on aflat surface [40]. Hence, reversible sealed device has a potentialto develop precise and complex surface patterning with multiplematerials in a parallel manner.

4.4. Cell studies

Understanding the behavior and physiology of cell and theanalysis of biomaterial is a major concern for biomedical, bio-engineering, tissue engineering and pharmacological research.Reversible sealing has been applied for cell based research, becausethe cells are very sensitive, and reversible sealing left no resid-uals on the cells or channel. In addition, microdevices for cellstudy are generally not subjected to high working pressure so thatthere could not be a leakage problem. In particular, drug screen-ing requires a device that can perform biochemical treatments ofcells in a highly parallel manner and screen drugs containing hun-dreds of chemical compounds. A PDMS device was developed tofabricate arrays of different types of cells by capturing cells in amicrofluidic system (Fig. 4). Using reversible sealing this devicecan be subjected to the sequential biochemical operations with theflow of various fluids in different channels or other cells can be

iques for microdevice applications, Sens. Actuators B: Chem. (2010),

patterned onto specific positions of the substrate [27]. Reversiblesealing allows flexible alignment techniques so that wells can bepositioned at the center of the microchannel. Substrates, after pat-terning with different cells, dismount easily and safely, and thenfurther align and combine with the microchannel. The combination

ARTICLE IN PRESSG Model

SNB-12686; No. of Pages 11

8 K. Anwar et al. / Sensors and Actuators B xxx (2010) xxx–xxx

F -cultus e-c sts es. (iiim on th

oi

asmhT[rfracrpmgciMr

aitc

ig. 5. (A) Schematic diagram of the process used to generate static and dynamic coteps in the formation of dynamic co-cultures using Parylene-c stencils. (i) Paryleneeded with mES cells. (ii) Patterned co-cultures of mES cells and AML12 hepatocytES cells. (iv) After depositing a layer of FN, a third cell type (NIH-3T3) was seeded

f second microchannel with a rotation of 90◦ increases throughputn multiple folds.

In diagnostic research, electrochemical analysis involvesmperometric detection of released materials from cells afterome stimulation. By taking advantage of reversible sealingount/dismount technology, a device for amperometric detection

as been reported that used a reversible sealed electrode plate.he device can be dismounted, cleaned and reuse as necessary125]. Amperometrically monitoring the amount of catecholamineseleased from rat pheochromocytoma (PC 12) cells was per-ormed in microchip-based system using reversible sealing. In thisesearch, cells were immobilized in the channel, peeled further,nd resealed reversibly to the electrode plate in order to detectatecholamines released after stimulation with calcium [126]. Aeversibly sealable microchannel array which is compatible with alate reader is reported for biological assays with normal mouseammary gland epithelial cells (NMuMG) and primary mammary

land epithelial cells (MECs) [127]. Reversibly sealed bio-chip alsoan be used for sensing pathogens, by immobilizing biomolecule,n an arranged fashion, and applied to different sensing reagents.

oreover, cell culture on a chip can also be used to detect allergicesponse [128].

Please cite this article in press as: K. Anwar, et al., Reversible sealing techndoi:10.1016/j.snb.2010.11.002

Cells in living organisms experience different types of cues suchs biochemical, physical, and cell-to-cell interactions. To mimicn vivo conditions many approaches are used such as surface pat-erning with extracellular matrix (ECM), and furnishing differentues (physical, electrical, and biochemical) and co-culture systems

res. (B) Light micrograph (left) and corresponding fluorescent images (right) of theencil, coated with hyaluronic acid, was reversibly sealed on FN-treated PDMS and) To generate dynamic co-cultures, the stencil was gently peeled away, leaving thee exposed surface [50].

for cell-to-cell interaction. Reversible sealing is very helpful for pat-terning surfaces with appropriate material and investigating cellprotein compatibility [129]. This surface treatment can be used asECM for cell culture, or for creating a cell pattern by trapping cellsonto it [33,51]. Parylene-c has been used for co-culture in static anddynamic systems (Fig. 5). Type 1 cells can be patterned onto thesurface using the reversible sealing properties of Parylene-c, there-after type 2 cells can be patterned over type1 cells after removingthe Parylene-c stencils [32]. This process is called static co-culture.As Parylene-c can be recovered to its original condition after oneexperiment by a plasma cleaning process, it can be reused severaltimes to save time and cost. In a dynamic co-culture system, thereare more than two types of cells to be cultured. Primary type 1cells must be cultured with type 2 cells and subsequently with type3 cells [50]. This dynamic co-culture is useful for many biologicalprocesses, such as stem cell differentiation and to fabricate tissueengineering constructs.

5. Conclusion and outlook

As microdevices rapidly advancing, reversible sealing of devices

iques for microdevice applications, Sens. Actuators B: Chem. (2010),

has emerged as an alternative sealing technique for various appli-cations. This technique is being adopted for the fabrication ofmicro and nano devices, flow analysis in microscale, bio-moleculesanalysis, and cell studies. The reversible seal technique is usefulfor a wide range of material including polymers (For example,

ING

S

d Actu

Ppmotpocsn

brTtdcchcpot

clsmoofsardst

A

G

R

ARTICLEModel

NB-12686; No. of Pages 11

K. Anwar et al. / Sensors an

DMS, PMMA, PEG-DA, parylene, polyurethane, polycarbonate,olystyrene, PETG, polyvinylchloride, and polyethylene), hardaterial (e.g., glass, silicon, gold, and any other flat surface), and

thers. Reversible sealing allows a device to be dismountable sohat device can be cleaned when needed, as microdevices are veryrone to clogging during molecular analysis. Reversible sealing alsoffers reusability, and portability. In addition, reversible and pre-ise interconnectivity of microdevices is possible using reversibleealing techniques because flexible alignment is allowed which isot possible with irreversible sealing.

To create a better reversible sealing different approaches haveeen attempted: (1) sealing by the self-adhesive property of mate-ials; (2) vacuum seal by aspiration; and (3) sealing by magnetism.he main applications of reversible seals are high throughput, mul-iple tests in a parallel manner, subsequent operations, static andynamic cell co-cultures and surface treatments to provide spe-ific cues to cells, which makes the device more functional forell studies. Using reversible sealing micro and nanostructures ofigh resolution can be fabricated over the substrate without anyomplexity. Reversible interconnectivity of devices, known as thelug-and-play concept, is used to couple different analysis systemsnto one platform and can be established without having to gohrough a complex setup process.

While reversible sealing leads to high throughput, ease of fabri-ation, cost effectiveness, and more functional devices, its majorimitation is sealing strength and leakage. Although reversibleealing can be achieved strong enough to confine fluid within aicrofluidic manifold without leaking and can also allow flow

f fluid in microchannel under negative pressure by suction atutlet, this is less than the strength of irreversible sealing. There-ore, a device that experiences high working pressure should beealed irreversibly. However, since reversible sealing technologiesnd capabilities are rapidly advancing, they should play a criticalole in fulfilling the ever increasing requirements for cell studies,rug screening, tissue engineering, molecular and cellular biologytudies, bio-assays, device fabrication, electrophoresis, and minia-urized micro-analysis systems.

cknowledgement

This research is supported by the Inha University Researchrant.

eferences

[1] S.M. Kim, M.A. Burns, E.F. Hasselbrink, Electrokinetic protein preconcen-tration using a simple glass/poly(dimethylsiloxane) microfluidic chip, Anal.Chem. 78 (2006) 4779–4785.

[2] M.D. Adams, J.M. Kelley, J.D. Gocayne, M. Dubnick, M.H. Polymeropoulos, H.Xiao, C.R. Merril, A. Wu, B. Olde, R.F. Moreno, et al., Complementary DNAsequencing: expressed sequence tags and human genome project, Science252 (1991) 1651–1656.

[3] J. Messing, R. Crea, P.H. Seeburg, A system for shotgun DNA sequencing,Nucleic Acids Res. 9 (1981) 309–321.

[4] K.Y. Lien, W.C. Lee, H.Y. Lei, G.B. Lee, Integrated reverse transcription poly-merase chain reaction systems for virus detection, Biosens. Bioelectron. 22(2007) 1739–1748.

[5] S. Li, D.Y. Fozdar, M.F. Ali, H. Li, D. Shao, D.M. Vykoukal, J. Vykoukal, P.N.Floriano, M. Olsen, J.T. McDevitt, P.R. Gascoyne, S. Chen, A continuous-flowpolymerase chain reaction microchip with regional velocity control, J. Micro-electromech. Syst. 15 (2006) 223–236.

[6] Z. Chen, A. Chauhan, DNA separation by EFFF in a microchannel, J. ColloidInterface Sci. 285 (2005) 834–844.

[7] C.H. Lin, J.H. Wang, L.M. Fu, Improving the separation efficiency of DNAbiosamples in capillary electrophoresis microchips using high-voltage pulsed

Please cite this article in press as: K. Anwar, et al., Reversible sealing techndoi:10.1016/j.snb.2010.11.002

DC electric fields, Microfluid. Nanofluid. 5 (2008) 403–410.[8] H.L. Lee, S.C. Chen, Microchip capillary electrophoresis with electrochemical

detector for precolumn enzymatic analysis of glucose, creatinine, uric acidand ascorbic acid in urine and serum, Talanta 64 (2004) 750–757.

[9] A Bernard, B. Michel, E. Delamarche, Micromosaic immunoassays, Anal. Chem.73 (2001) 8–12.

PRESSators B xxx (2010) xxx–xxx 9

[10] K. Hotakainen, P. Tanner, H. Alfthan, C. Haglund, U.H. Stenman, Comparisonof three immunoassays for CA 19-9, Clin. Chim. Acta 400 (2009) 123–127.

[11] F.M. Benes, N. Lange, Two-dimensional versus three-dimensional cell count-ing: a practical perspective, Trends Neurosci. 24 (2001) 11–17.

[12] P. Malatesta, E. Hartfuss, M. Gotz, Isolation of radial glial cells by fluorescent-activated cell sorting reveals a neuronal lineage, Development 127 (2000)5253–5263.

[13] W. Gu, X. Zhu, N. Futai, B.S. Cho, S. Takayama, Computerized microfluidic cellculture using elastomeric channels and Braille displays, Proc. Natl. Acad. Sci.U.S.A. 101 (2004) 15861–15866.

[14] P.A. Auroux, D. Iossifidis, D.R. Reyes, A. Manz, Micro total analysis systems.2. Analytical standard operations and applications, Anal. Chem. 74 (2002)2637–2652.

[15] P.S. Dittrich, K. Tachikawa, A. Manz, Micro total analysis systems. Latestadvancements and trends, Anal. Chem. 78 (2006) 3887–3907.

[16] H.A. Stone, A.D. Stroock, A. Ajdari, Engineering flows in small devices:microfluidics toward a lab-on-a-chip, Annu. Rev. Fluid Mech. 36 (2004)381–411.

[17] A. Clow, P. Gaynor, B. Oback, Coplanar film electrodes facilitate bovine nucleartransfer cloning, Biomed. Microdevices 11 (2009) 851–859.

[18] Y.S. Choi, H.B. Kim, G.S. Kwon, J.K. Park, On-chip testing device for elec-trochemotherapeutic effects on human breast cells, Biomed. Microdevices11 (2009) 151–159.

[19] J.C. McDonald, D.C. Duffy, J.R. Anderson, D.T. Chiu, H. Wu, O.J.A. Schueller, G.M.Whitesides, Fabrication of microfluidic systems in poly(dimethylsiloxane),Electrophoresis 21 (2000) 27–40.

[20] H.F. Li, J.M. Lin, R.G. Su, K. Uchiyama, T. Hobo, A compactly integrated laser-induced fluorescence detector for microchip electrophoresis, Electrophoresis25 (2004) 1907–1915.

[21] C.S. Effenhauser, G.J.M. Bruin, A. Paulus, M. Ehrat, Integrated capillary elec-trophoresis on flexible silicone microdevices: analysis of DNA restrictionfragments and detection of single DNA molecules on microchips, Anal. Chem.69 (1997) 3451–3457.

[22] R.S. Martin, A.J. Gawron, B.A. Fogarty, F.B. Regan, E. Dempsey, S.M. Lunte,Carbon paste-based electrochemical detectors for microchip capillary elec-trophoresis/electrochemistry, Analyst 126 (2001) 277–280.

[23] C. Dalton, K. Kaler, A cost effective, re-configurable electrokinetic microfluidicchip platform, Sens. Actuator B: Chem. 123 (2007) 628–635.

[24] N.V. Zaytseva, V.N. Goral, R.A. Montagna, A.J. Baeumner, Development of amicrofluidic biosensor module for pathogen detection, Lab Chip 5 (2005)805–811.

[25] D.C. Duffy, H.L. Gillis, J. Lin, N.F. Sheppard, G.J. Kellogg, Microfabricatedcentrifugal microfluidic systems: characterization and multiple enzymaticassays, Anal. Chem. 71 (1999) 4669–4678.

[26] J.C. McDonald, G.M. Whitesides, Poly(dimethylsiloxane) as a material for fab-ricating microfluidic devices, Acc. Chem. Res. 35 (2002) 491–499.

[27] A. Khademhosseini, J. Yeh, G. Eng, J. Karp, H. Kaji, J. Borenstein, O.C. Farokhzad,R. Langer, Cell docking inside microwells within reversibly sealed microflu-idic channels for fabricating multiphenotype cell arrays, Lab Chip 5 (2005)1380–1386.

[28] D. Erickson, D. Li, Integrated microfluidic devices, Anal. Chim. Acta 507 (2004)11–26.

[29] J.M. Ng, I. Gitlin, A.D. Stroock, G.M. Whitesides, Components for integratedpoly(dimethylsiloxane) microfluidic systems, Electrophoresis 23 (2002)3461–3473.

[30] E. Delamarche, D. Juncker, H. Schmid, Microfluidics for processingsurfaces and miniaturizing biological assays, Adv. Mater. 17 (2005)2911–2933.

[31] P. Kim, K.Y. Suh, Rigiflex, spontaneously wettable polymeric moldfor forming reversibly bonded nanocapillaries, Langmuir 23 (2007)4549–4553.

[32] D Wright, B. Rajalingam, J.M. Karp, S. Selvarasah, Y.B. Ling, J. Yeh, R. Langer,M.R. Dokmeci, A. Khademhosseini, Reusable, reversibly sealable parylenemembranes for cell and protein patterning, J. Biomed. Mater. Res. A 85A(2008) 530–538.

[33] M. Le Berre, C. Crozatier, G.V. Casquillas, Y. Chen, Reversible assembling ofmicrofluidic devices by aspiration, Microelectron. Eng. 83 (2006) 1284–1287.

[34] M. Rafat, D.R. Raad, A.C. Rowat, D.T. Auguste, Fabrication of reversibly adhe-sive fluidic devices using magnetism, Lab Chip 9 (2009) 3016–3019.

[35] M. Arudell, E. Igata, H. Morgan, J.M. Cooper, Interconnected reversible lab ona chip technology, Lab Chip 2 (2002) 65–69.

[36] M.W. Li, R.S. Martin, Integration of continuous-flow sampling with microchipelectrophoresis using poly(dimethylsiloxane)-based valves in a sealed devicereversibly sealed device, Electrophoresis 28 (2007) 2478–2488.

[37] E. Biddiss, D. Erickson, D.Q. Li, Heterogeneous surface charge enhancedmicromixing for electrokinetic flows, Anal. Chem. 76 (2004) 3208–3213.

[38] C. Crozatier, M. Le Berre, Y. Chen, Multi-colour micro-contact printing basedon microfluidic network inking, Microelectron. Eng. 83 (2006) 910–913.

[39] X. Zhou, L. Lau, W.W.L. Lam, S.W.N. Au, B. Zheng, Nanoliter dispensing method

iques for microdevice applications, Sens. Actuators B: Chem. (2010),

by degassed poly(dimethylsiloxane) microchannels and its application in pro-tein crystallization, Anal. Chem. 79 (2007) 4924–4930.

[40] D.T. Chiu, N.L. Jeon, S. Huang, R.S. Kane, C.J. Wargo, I.S. Choi, D.E. Ingber, G.M.Whitesides, Patterned deposition of cells and proteins onto surfaces by usingthree-dimensional microfluidic systems, Proc. Natl. Acad. Sci. U.S.A. 97 (2000)2408–2413.

ING

S

1 d Actu

ARTICLEModel

NB-12686; No. of Pages 11

0 K. Anwar et al. / Sensors an

[41] X.X. Chen, A. Murawski, G. Kuang, D.J. Sexton, W. Galbraith, Sample prepa-ration for MALDI mass spectrometry using an elastomeric device reversiblysealed on the MALDI target, Anal. Chem. 78 (2006) 6160–6168.

[42] L.C. Mecker, R.S. Martin, Use of micromolded carbon dual electrodes with apalladium decoupler for amperometric detection in microchip electrophore-sis, Electrophoresis 27 (2006) 5032–5042.

[43] N.V. Zaytseva, R.A. Montagna, A.J. Baeumner, Microfluidic biosensor for theserotype-specific detection of Dengue virus RNA, Anal. Chem. 77 (2005)7520–7527.

[44] B.D. DeBusschere, G.T. Kovacs, Portable cell-based biosensor system usingintegrated CMOS cell-cartridges, Biosens. Bioelectron. 16 (2001) 543–556.

[45] B.R. Schudel, C.J. Choi, B.T. Cunningham, P.J.A. Kenis, Microfluidic chip forcombinatorial mixing and screening of assays, Lab Chip 9 (2009) 1676–1680.

[46] E. Eteshola, D. Leckband, Development and characterization of an ELISA assayin PDMS microfluidic channels, Sens. Actuators B: Chem. 72 (2001) 129–133.

[47] E. Delamarche, A. Bernard, H. Schmid, B. Michel, H. Biebuyck, Patterned deliv-ery of immunoglobulins to surfaces using microfluidic networks, Science 276(1997) 779–781.

[48] C. Situma, Y. Wang, M. Hupert, F. Barany, R.L. McCarley, S.A. Soper, Fabricationof DNA microarrays onto poly(methyl methacrylate) with ultraviolet pattern-ing and microfluidics for the detection of low-abundant point mutations, Anal.Biochem. 340 (2005) 123–135.

[49] C.M. Moore, S.D. Minteer, R.S. Martin, Microchip-based ethanol/oxygen bio-fuel cell, Lab Chip 5 (2005) 218–225.

[50] D. Wright, B. Rajalingam, S. Selvarasah, M.R. Dokmeci, A. Khademhosseini,Generation of static and dynamic patterned co-cultures using microfabricatedparylene-C stencils, Lab Chip 7 (2007) 1272–1279.

[51] A. Khademhosseini, G. Eng, J. Yeh, P.A. Kucharczyk, R. Langer, G. Vunjak-Novakovic, M. Radisic, Microfluidic patterning for fabrication of contractilecardiac organoids, Biomed. Microdevices 9 (2007) 149–157.

[52] S. Bhattacharya, A. Datta, J.M. Berg, S. Gangopadhyay, Studies on surface wet-tability of poly(dimethyl) siloxane (PDMS) and glass under oxygen-plasmatreatment and correlation with bond strength, J. Microelectromech. Syst. 14(2005) 590–597.

[53] D.J. Harrison, A. Manz, Z. Fan, H. Luedi, H.M. Widmer, Capillary electrophoresisand sample injection systems integrated on a planar glass chip, Anal. Chem.64 (2002) 1926–1932.

[54] D.J. Harrison, K. Fluri, K. Seiler, Z. Fan, C.S. Effenhauser, A. Manz, Micromachin-ing a miniaturized capillary electrophoresis-based chemical analysis systemon a chip, Science 261 (1993) 895–897.

[55] H. Becker, L.E. Locascio, Polymer microfluidic devices, Talanta 56 (2002)267–287.

[56] T.J. Johnson, D. Ross, L.E. Locascio, Rapid microfluidic mixing, Anal. Chem. 74(2002) 45–51.

[57] H. SalimiMoosavi, T. Tang, D.J. Harrison, Electroosmotic pumping of organicsolvents and reagents in microfabricated reactor chips, J. Am. Chem. Soc. 119(1997) 8716–8717.

[58] M. Kakuta, F.G. Bessoth, A. Manz, Microfabricated devices for fluid mixing andtheir application for chemical synthesis, Chem. Rec. 1 (2001) 395–405.

[59] J.N. Lee, C. Park, G.M. Whitesides, Solvent compatibility ofpoly(dimethylsiloxane)-based microfluidic devices, Anal. Chem. 75 (2003)6544–6554.

[60] S.K. Sia, G.M. Whitesides, Microfluidic devices fabricated inpoly(dimethylsiloxane) for biological studies, Electrophoresis 24 (2003)3563–3576.

[61] J.C. McDonald, M.L. Chabinyc, S.J. Metallo, J.R. Anderson, A.D. Stroock, G.M.Whitesides, Prototyping of microfluidic devices in poly(dimethylsiloxane)using solid-object printing, Anal. Chem. 74 (2002) 1537–1545.

[62] E. Piccin, W.K.T. Coltro, J.A.F. da Silva, S.C. Neto, L.H. Mazo, E. Carrilho,Polyurethane from biosource as a new material for fabrication of microfluidicdevices by rapid prototyping, J. Chromatogr. A 1173 (2007) 151–158.

[63] G.T. Roman, R.T. Kennedy, Fully integrated microfluidic separations systemsfor biochemical analysis, J. Chromatogr. A 1168 (2007) 170–188.

[64] H. Lee, B.P. Lee, P.B. Messersmith, A reversible wet/dry adhesive inspired bymussels and geckos, Nature 448 (2007) 338–344.

[65] A. Galliano, S. Bistac, J. Schultz, Adhesion and friction of PDMS net-works: molecular weight effects, J. Colloid Interface Sci. 265 (2003) 372–379.

[66] C. Dalton, K. Kaler, A cost effective, re-configurable electrokinetic microfluidicchip platform, Sens. Actuators B 123 (2007) 628–635.

[67] U.Y. Schaff, M.M. Xing, K.K. Lin, N. Pan, N.L. Jeon, S.I. Simon, Vascular mimeticsbased on microfluidics for imaging the leukocyte-endothelial inflammatoryresponse, Lab Chip 7 (2007) 448–456.

[68] B.G. Chung, J.W. Park, J. Hu, C. Huang, E. Monuki, N.L. Jeon, A hybridmicrofluidic-vacuum device for direct interfacing with conventional cell cul-ture methods, BMC Biotechnol. 7 (2007) 60.

[69] H. Craighead, Future lab-on-a-chip technologies for interrogating individualmolecules, Nature 442 (2006) 387–393.

[70] J. Han, S.W. Turner, H.G. Craighead, Entropic trapping and escape of longDNA molecules at submicron size constriction, Phys. Rev. Lett. 83 (1999)

Please cite this article in press as: K. Anwar, et al., Reversible sealing techndoi:10.1016/j.snb.2010.11.002

1688–1961.[71] J. Rzayev, M.A. Hillmyer, Nanochannel array plastics with tailored surface

chemistry, J. Am. Chem. Soc. 127 (2005) 13373–13379.[72] R. Karnik, K. Castelino, R. Fan, P. Yang, A. Majumdar, Effects of biological reac-

tions and modifications on conductance of nanofluidic channels, Nano Lett. 5(2005) 1638–1642.

PRESSators B xxx (2010) xxx–xxx

[73] D. Mijatovic, J.C.T. Eijkel, A.v.d Berg, Technologies for nanofluidic systems:top-down vs. bottom-up-a review, Lab Chip 5 (2005) 492–500.

[74] N.R. Tas, J.W. Berenschot, P. Mela, H.V. Jansen, M. Elwenspoek, A. van den Berg,2D-confined nanochannels fabricated by conventional micromachining, NanoLett. 2 (2002) 1031–1032.

[75] C. Lee, E.H. Yang, N.V. Myung, T. George, A nanochannel fabrication techniquewithout nanolithography, Nano Lett. 3 (2003) 1339–1340.

[76] J.C.T. Eijkel, J. Bomer, N.R. Tas, A. van den Berg, 1-D nanochannels fabricatedin polyimide, Lab Chip 4 (2004) 161–163.

[77] L.J. Guo, X. Cheng, C.F. Chou, Fabrication of size-controllable nanofluidic chan-nels by nanoimprinting and its application for DNA stretching, Nano Lett. 4(2004) 69–73.

[78] A. Berthold, L. Nicola, P.M. Sarro, M.J. Vellekoop, Glass-to-glass anodic bondingwith standard IC technology thin films as intermediate layers, Sens. ActuatorsA: Phys. 82 (2000) 224–228.

[79] C. Iglesias, M. Seyama, T. Horiuchi, T. Miura, T. Haga, Y. Iwasaki, Fabrication ofa nanofluidic channel for SPR sensing application using glass-to-glass anodicbonding, Electrochemistry 74 (2006) 169–171.

[80] G.J. Cheng, D. Pirzada, P. Dutta, Design and fabrication of a hybrid nanofluidicchannel, J. Micro-Nanolithogr. MEMS MOEMS 4 (2005) 013009.

[81] E. Kim, Y.N. Xia, G.M. Whitesides, Polymer microstructures formed by moldingin capillaries, Nature 376 (1995) 581–584.

[82] E. Kim, G.M. Whitesides, Imbibition and flow of wetting liquids in noncircularcapillaries, J. Phys. Chem. B 101 (1997) 855–863.

[83] H. Schmid, B. Michel, Siloxane polymers for high-resolution, high-accuracysoft lithography, Macromolecules 33 (2000) 3042–3049.

[84] T.W Odom, J.C. Love, D.B. Wolfe, K.E. Paul, G.M. Whitesides, Improved pat-tern transfer in soft lithography using composite stamps, Langmuir 18 (2002)5314–5320.

[85] K.M. Choi, J.A. Rogers, A photocurable poly(dimethylsiloxane) chemistrydesigned for soft lithographic molding and printing in the nanometer regime,J. Am. Chem. Soc. 125 (2003) 4060–4061.

[86] K.Y. Suh, R. Langer, J. Lahann, Fabrication of elastomeric stamps withpolymer-reinforced sidewalls via chemically selective vapor deposition poly-merization of poly(p-xylylene), Appl. Phys. Lett. 83 (2003) 4250–4252.

[87] I. Rodriguez, P. Spicar-Mihalic, C.L. Kuyper, G.S. Fiorini, D.T. Chiu, Rapid pro-totyping of glass microchannels, Anal. Chim. Acta 496 (2003) 205–215.

[88] M.A. Unger, H.P. Chou, T. Thorsen, A. Scherer, S.R. Quake, Monolithic microfab-ricated valves and pumps by multilayer soft lithography, Science 288 (2000)113–116.

[89] K. Hosokawa, R. Maeda, A pneumatically-actuated three-way microvalve fab-ricated with polydimethylsiloxane using the membrane transfer technique,J. Micromech. Microeng. 10 (2000) 415–420.

[90] E.M. Verpoorte, B.H. Vanderschoot, S. Jeanneret, A. Manz, H.M. Widmer,N.F. Derooij, 3-dimensional micro-flow manifolds for miniaturized chemical-analysis systems, J. Micromech. Microeng. 4 (1994) 246–256.

[91] A.T. Woolley, D. Hadley, P. Landre, A.J. deMello, R.A. Mathies, M.A. Northrup,Functional integration of PCR amplification and capillary electrophoresis in amicrofabricated DNA analysis device, Anal. Chem. 68 (1996) 4081–4086.

[92] N. Chiem, L. Lockyear-Shultz, P. Andersson, C. Skinner, D.J. Harrison, Roomtemperature bonding of micromachined glass devices for capillary elec-trophoresis, Sens. Actuators B: Chem. 63 (2000) 147–152.

[93] L.A. Legendre, J.M. Bienvenue, M.G. Roper, J.P. Ferrance, J.P. Landers, A sim-ple, valveless microfluidic sample preparation device for extraction andamplification of DNA from nanoliter-volume samples, Anal. Chem. 78 (2006)1444–1451.

[94] C.N. Liu, N.M. Toriello, R.A. Mathies, Multichannel PCR-CE microdevice forgenetic analysis, Anal. Chem. 78 (2006) 5474–5479.

[95] C.A. Emrich, H. Tian, I.L. Medintz, R.A. Mathies, Microfabricated 384-lanecapillary array electrophoresis bioanalyzer for ultrahigh-throughput geneticanalysis, Anal. Chem. 74 (2002) 5076–5083.

[96] S.B. Cheng, C.D. Skinner, J. Taylor, S. Attiya, W.E. Lee, G. Picelli, D.J. Harri-son, Development of a multichannel microfluidic analysis system employingaffinity capillary electrophoresis for immunoassay, Anal. Chem. 73 (2001)1472–1479.

[97] L.C. Mecker, R.S. Martin, Coupling microdialysis sampling to microchip elec-trophoresis in a reversibly sealed device, JALA Charlottesv Va 12 (2007)296–302.

[98] L.C. Mecker, R.S. Martin, Integration of microdialysis sampling and microchipelectrophoresis with electrochemical detection, Anal. Chem. 80 (2008)9257–9264.

[99] M.I. Davies, J.D. Cooper, S.S. Desmond, C.E. Lunte, S.M. Lunte, Analyticalconsiderations for microdialysis sampling, Adv. Drug Deliv. Rev. 45 (2000)169–188.

[100] M. Heien, P.E.M. Phillips, G.D. Stuber, A.T. Seipel, R.M. Wightman, Overoxi-dation of carbon-fiber microelectrodes enhances dopamine adsorption andincreases sensitivity, Analyst 128 (2003) 1413–1419.

[101] N.T. Nguyen, Z.G. Wu, Micromixers—a review, J. Micromech. Microeng. 15(2005) R1–R16.

[102] M.L. Kovarik, M.W. Li, R.S. Martin, Integration of a carbon microelec-

iques for microdevice applications, Sens. Actuators B: Chem. (2010),

trode with a microfabricated palladium decoupler for use in microchipcapillary electrophoresis/electrochemistry, Electrophoresis 26 (2005)202–210.

[103] R.S. Martin, A.J. Gawron, S.M. Lunte, C.S. Henry, Dual-electrode electrochemi-cal detection for poly(dimethylsiloxane)-fabricated capillary electrophoresismicrochips, Anal. Chem. 72 (2000) 3196–3202.

ING

S

d Actu

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

Michigan, Ann Arbor in 2005 and 2006, respectively. In 2007, he joined the faculty of

ARTICLEModel

NB-12686; No. of Pages 11

K. Anwar et al. / Sensors an

104] G. Ocvirk, M. Munroe, T. Tang, R. Oleschuk, K. Westra, D.J. Harrison, Elec-trokinetic control of fluid flow in native poly(dimethylsiloxane) capillaryelectrophoresis devices, Electrophoresis 21 (2000) 107–115.

105] M.Z. Bazant, T.M. Squires, Induced-charge electrokinetic phenomena, Curr.Opin. Colloid Interface Sci. 15 (2010) 203–213.

106] N.G. Green, A. Ramos, H. Morgan, Ac electrokinetics: a survey ofsub-micrometre particle dynamics, J. Phys. D: Appl. Phys. 33 (2000)632–641.

107] C.C. Liu, D.F. Cui, Design and fabrication of poly(dimethylsiloxane) elec-trophoresis microchip with integrated electrodes, Microsyst. Technol. 11(2005) 1262–1266.

108] J.C. Sanders, M.C. Breadmore, P.S. Mitchell, J.P. Landers, A simple PDMS-basedelectro-fluidic interface for microchip electrophoretic separations, Analyst127 (2002) 1558–1563.

109] R. Pethig, M.S. Talary, R.S. Lee, Enhancing traveling-wave dielectrophoresiswith signal superposition, IEEE Eng. Med. Biol. Mag. 22 (2003) 43–50.

110] E.G. Cen, C. Dalton, Y. Li, S. Adamia, L.M. Pilarski, K.V.I.S. Kaler, A com-bined dielectrophoresis, traveling wave dielectrophoresis and electrorotationmicrochip for the manipulation and characterization of human malignantcells, J. Microbiol. Methods 58 (2004) 387–401.

111] J. Voldman, R.A. Braff, M. Toner, M.L. Gray, M.A. Schmidt, Holding forces ofsingle-particle dielectrophoretic traps, Biophys. J. 80 (2001) 531–542.

112] F.F. Becker, X.B. Wang, Y. Huang, R. Pethig, J. Vykoukal, P.R. Gascoyne, Separa-tion of human breast cancer cells from blood by differential dielectric affinity,Proc. Natl. Acad. Sci. U.S.A. 92 (1995) 860–864.

113] A.M. Spehar, S. Koster, V. Linder, S. Kulmala, N.F. de Rooij, E. Ver-poorte, H. Sigrist, W. Thormann, Electrokinetic characterization ofpoly(dimethylsiloxane) microchannels, Electrophoresis 24 (2003)3674–3678.

114] J.S. Buch, P.C. Wang, D.L. DeVoe, C.S. Lee, Field-effect flow control in apolydimethylsiloxane-based microfluidic system, Electrophoresis 22 (2001)3902–3907.

115] C.S. Chen, M. Mrksich, S. Huang, G.M. Whitesides, D.E. Ingber, Geometric con-trol of cell life and death, Science 276 (1997) 1425–1428.

116] A. Bernard, E. Delamarche, H. Schmid, B. Michel, H.R. Bosshard, H. Biebuyck,Printing patterns of proteins, Langmuir 14 (1998) 2225–2229.

117] N.L. Jeon, I.S. Choi, B. Xu, G.M. Whitesides, Large-area patterning by vacuum-assisted micromolding, Adv. Mater. 11 (1999) 946–949.

118] E. Delamarche, A. Bernard, H. Schmid, A. Bietsch, B. Michel, H. Biebuyck,Microfluidic networks for chemical patterning of substrate: design and appli-cation to bioassays, J. Am. Chem. Soc. 120 (1998) 500–508.

119] A. Folch, A. Ayon, O. Hurtado, M.A. Schmidt, M. Toner, Molding of deeppolydimethylsiloxane microstructures for microfluidics and biological appli-

Please cite this article in press as: K. Anwar, et al., Reversible sealing techndoi:10.1016/j.snb.2010.11.002

cations, J. Biomech. Eng.: Trans. ASME 121 (1999) 28–34.120] A. Folch, M. Toner, Cellular micropatterns on biocompatible materials,

Biotechnol. Prog. 14 (1998) 388–392.121] D. Juncker, H. Schmid, A. Bernard, I. Caelen, B. Michel, N.d. Rooij, E.

Delamarche, Soft and rigid two-level microfluidic networks for patterningsurfaces, J. Micromech. Microeng. 11 (2001) 532–541.

PRESSators B xxx (2010) xxx–xxx 11

[122] A. Khademhosseini, S. Jon, K.Y. Suh, T.N.T. Tran, G. Eng, J. Yeh, J. Seong, R.Langer, Direct patterning of protein- and cell-resistant polymeric monolayersand microstructures, Adv. Mater. 15 (2003) 1995–2000.

[123] J.R. Anderson, D.T. Chiu, R.J. Jackman, O. Cherniavskaya, J.C. McDonald, H. Wu,S.H. Whitesides, G.M. Whitesides, Fabrication of topologically complex three-dimensional microfluidic systems in PDMS by rapid prototyping, Anal. Chem.72 (2000) 3158–3164.

[124] R.S. Kane, S. Takayama, E. Ostuni, D.E. Ingber, G.M. Whitesides, Pattern-ing proteins and cells using soft lithography, Biomaterials 20 (1999) 2363–2376.

[125] D.M. Spence, N.J. Torrence, M.L. Kovarik, R.S. Martin, Amperometric deter-mination of nitric oxide derived from pulmonary artery endothelial cellsimmobilized in a microchip channel, Analyst 129 (2004) 995–1000.

[126] M.W. Li, D.M. Spence, R.S. Martin, A microchip-based system for immobilizingPC 12 cells and amperometrically detecting catecholamines released afterstimulation with calcium, Electroanalysis 17 (2005) 1171–1180.

[127] H.M. Yu, C.M. Alexander, D.J. Beebe, A plate reader-compatible microchannelarray for cell biology assays, Lab Chip 7 (2007) 388–391.

[128] Y. Matsubara, Y. Murakami, M. Kobayashi, Y. Morita, E. Tamiya, Application ofon-chip cell cultures for the detection of allergic response, Biosens. Bioelec-tron. 19 (2004) 741–747.

[129] T.Y. Chang, V.G. Yadav, S. De Leo, A. Mohedas, B. Rajalingam, C.L. Chen, S.Selvarasah, M.R. Dokmeci, A. Khademhosseini, Cell and protein compatibilityof parylene-C surfaces, Langmuir 23 (2007) 11718–11725.

Biographies

Khalid Anwar received BE degree in Mechanical Engineering from Aligarh Mus-lim University, Aligarh, India in 2008. He successfully completed Masters Degreein Thermodynamics and Fluid Mechanics from Inha University, Incheon, Republicof Korea in 2010. His research interests are Micro Total Analysis Systems (�TAS),Bio-MEMS, Micromixer, and Electrokinetic protein preconcentration.

Taeheon Han received his BS (2003) and MS (2008) degrees in mechanical engineer-ing from Inha University, Incheon, Republic of Korea. Now he is a Ph.D. candidate inthe same institute and his research interests are microfluidic system for biochemicalanalysis and cell based biosensors.

Sun Min Kim received his BS (1997) and MS (1999) degrees in mechanical engi-neering from the Seoul National University, Korea, and an MS degree in biomedicalengineering and his PhD degree in mechanical engineering from the University of

iques for microdevice applications, Sens. Actuators B: Chem. (2010),

the Department of Mechanical Engineering, Inha University, following post-doctoralwork at the Brigham and Women’s Hospital, Harvard Medical School, MA. He isinterested in the fundamental understanding and development of micro/nanofluidicsystems for biochemical sample analysis, cell-based biosensor, and cellanalysis.