Impedance-based Biosensors. Pittsburgh, PA 15213, U.S.A.

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1 Impedance-based Biosensos X. Huang, 1 D.W. Geve, 1 I. Nausieda, 1 D. Nguyen, 2 and M.M. Domach 2 1 Depatment of Electical and Compute Engineeing, Canegie Mellon Univesity, Pittsbugh, PA 15213, U.S.A. 2 Depatment of Chemical Engineeing, Canegie Mellon Univesity, Pittsbugh, PA 15213, U.S.A. ABSTRACT Impedance measuements on aays of micoelectodes can povide infomation about the gowth, motility, and physiology of cells gowing on the electodes. In this talk, we epot ecent esults obtained fo the gowth of 3T3 mouse fiboblasts and HCT116 human cance cells on gold electodes appoximately.4 mm 2 in aea. Cells poduce a chaacteistic peak in the impedance change plotted as a function of fequency. With the aid of electical modeling of the cell-electode system, the details of the changes in the measued impedance can be coelated to the cell size, factional electode coveage, and cell-electode gap. In paticula, compaison of impedance measuements of these two cell types show clea diffeences in the gowth ate and the atio of the cell-electode gap to the cell size. In addition to pesenting these expeimental esults illustating the utility of electode impedance measuements, we will outline the issues encounteed when electodes ae scaled to cell size and incopoated into a matix-addessed aay. INTRODUCTION Impedance-based micoelectode senso aays ae potentially useful fo pefoming dug sceening expeiments and also fo studies of cell adhesion and micomotion. The use of impedance sensos to study cell behavio was fist epoted by Giaeve and his cowokes in 1986 who monitoed cell polifeation, mophology, and motility [1]. The impedance senso consists of two metal electodes: one lage common efeence electode and one small woking electode. When cells ae cultued on the electodes the measued AC impedance changes in a way which depends on the fequency of measuement, the cell coveage, and the cell-electode gap. We ae developing aays of cell-sensing electodes which can be used to monito many individual cells o many individual clustes. In the following, we fist biefly outline the impedance changes which esult fom cell gowth and the elation between these impedance changes and the cell coveage and cell-electode gap. We then pesent data on the speading and gowth of nomal mouse fiboblasts and human cance cells. Finally, we descibe the design of an electode aay fabicated using a CMOS technology and discuss some of the design issues. Figue 1 shows the basic senso configuation. A small sensing electode and a lage counteelectode ae immesed in cell gowth medium. Except at vey high fequencies, the measued impedance is dominated by the suface impedance of the smalle electode. That suface impedance depends on tanspot though the electical double laye and monotonically deceases in magnitude with inceasing fequency. Cells gowing on the electode adhee to the electode only at focal adhesion egions which epesent a small (5-1%) faction of thei aea. At low fequencies, cuent can flow fom egions beneath the cell and though the medium in the cell-electode gap and thee is no change in the measued impedance. At modeate fequencies, howeve, cells obstuct the cuent flow and thee is an incease in the measued impedance. Analytic theoy [2] and simulations [3] show that cells lead to a chaacteistic peak

2 in the impedance change as a function of fequency. The magnitude of the impedance change depends on the cell coveage and the cell-electode gap. HP4192A:T1 cell gap HP4192A:T2 gold electode Figue 1. Effect of cell on measued electode impedance (left) small sensing electode and lage counte-electode and (ight) cell on sensing electode showing cuent flow though the gap between the cell and the electode. Figue 2a,b show a lumped equivalent cicuit which epesents the electode with and without cells. Because of the stong fequency dependence of the electode suface impedance, it is convenient to pesent data by plotting the nomalized impedance change = (Z cell -Z no cell )/Z no cell. Figue 2c shows the pedicted fequency dependence of fo a cell coveage of 9% of the electode aea. Z (ω) / A R S (a) 5 4 (c) 3 Z( ω) /( A Acell ) ' R S (b) 2 1 Z (ω) / A cell R gap fequency (Hz) Figue 2. Effect of cells on electode impedance: (a) equivalent cicuit without cells; (b) equivalent cicuit with cells; and (c) nomalized impedance change fo 9% coveage of.13 mm 2 electode ( ).25 µm cell-electode gap and ( ).25 µm cell-electode gap. EXPERIMENTAL Electode aays wee fabicated using a lift-off pocess as peviously epoted [4]. We epot hee on measuements made using two diffeent electode aay design [5].The electodes wee gold with a chomium adhesion laye and wee fabicated on fused silica substates. Paasitic capacitance between the inteconnect lines and the liquid medium was educed by painting the inteconnect about 15 µm away fom the electodes with silicone EP3HT fom Mastebond Inc. (Hackensack, NJ). Aay A had a lage gounded electode in addition to a common electode and fou sensing electodes anging in aea fom.25 mm 2 to.44 mm 2 Aay B had nine identical electodes with exposed aea between.15mm 2 and.2 mm 2 afte painting. A bottomless plastic well cut fom a 24-well cultue dish was bonded onto the

3 substate, enclosing the sensos and foming a chambe fo cell cultue. Aay B had nine identical electodes with exposed aea between.15mm 2 and.2 mm 2.Duing the measuements, the well was placed in a tempeatue-contolled chambe mounted on an electonic pobe station, with the chambe tempeatue contolled at 37 ± 1 C. The impedance magnitude was measued as a function of fequency ove the ange fom 1 Hz to 1 MHz using an HP 4192A impedance analyze. CELL ATTACHMENT AND GROWTH Figue 3 shows the impedance as a function of time afte deposition of 3T3 mouse fiboblasts. Ove this time peiod, we obseve the settling, adhesion, and speading of the cells. As expected, the maximum nomalized impedance change inceases duing this peiod. At the end of 5 hous the cell coveage obseved by optical micoscopy was about 95% max min fequency (Hz) Time(minutes) Figue 3. Plot of nomalized impedance change () ove 5 hous afte 3T3 fiboblasts wee deposited: ( ) 15 min, ( ).5 h, ( ) 2 h, ( ) 3 h, ( ) 4 h, and ( ) 5 h afte deposition. 9% to 95% cell coveage on substate was obseved by optical micoscope. The electode aea was.13 mm 2. (b). Nomalized impedance peak magnitude ( max ) and magnitude at 3 Hz ( min ) as a function of time afte cell deposition. Afte nealy complete coveage is achieved, continues to change in a way that depends on the cell type. Figue 4 shows the changes that occu fo mouse fiboblasts ove a longe time peiod. Attachment and speading of the cells takes place ove a ~ 6 hou time peiod as befoe. We then obseve a dop in indicating a decease in total aea coveed, due to cell motion and elief fom the suface. Duing the next 18 hous, migation induced by gaps between cells and some polifeation occu. Thee is then a monotonic incease in ove a peiod of 48 hous as the cells gow to cove most of the emaining open aea esulting in the fomation of a compact monolaye.

4 Figue 4. Plot of maximum (nomalized impedance change) as a function of time fo fiboblast cells: (.17 mm 2 ;.37 mm 2 ;.9 mm 2.4 mm 2 ). In contast, diffeent behavio is obseved fo human cance cells. Figue 5 shows the nomalized impedance change as a function of time fo HCT116 human cance cells. Attachment and speading occus ove a longe peiod of about 24 hous. Measuements ove a longe time peiod show a nea step-change in the impedance at about 3 hous afte cells wee intoduced (not shown). We also obseve a consistently lowe peak fequency and a smalle value of max afte the initial attachment peiod. This can be explained by the smalle size of cance cells and/ o a smalle value fo the cell-electode sepaation Fequency (Hz) Figue 5. Plot of maximum (nomalized impedance change) as a function of time fo HCT116 human cance cells. (--) 45 min; ( ) 2 h; ( -) 4 h; ( ) 6 h; ( ) 1 h; ( ) 14 h; ( ) 16 h; ( ) 24 h; and ( ) 48 h.

5 The obseved diffeences between nomal and cance cells indicate the potential fo cell impedance measuements to povide useful diagnostic o pognostic infomation. Pevious wok has suggested that electode impedance measuements can povide infomation about the micomotion of cells [2]. The micomotion is diffeent fo canceous and nomal cells [6,7] so it is possible that measuements of micomotion can be used diagnostically o pognostically. Impedance changes due to micomotion can be expected to patly aveage out when the electode size is lage enough to contain many cells. Consequently we seek to develop electode aays consisting of a lage numbe of small electodes. Small electodes will exhibit lage changes in impedance due to micomotion and the availability of a lage numbe of electodes will make it possible to collect statistically significant infomation. In the following section, we discuss a possible design fo an electode aay. DESIGN OF ELECTRODE ARRAYS Electode aays fo cell impedance measuements have been peviously poposed by Giaeve and Keese [8]. This patent poposed aanging the electodes in a matix and then measuing the impedance between paticula ow and column lines to select the electode at the cosspoint. This is a passive matix aay and suffes fom sneak paths fo lage aays. In ode to fabicate a lage aay, we ae using an active matix design (Fig. 6a) in which a field effect tansisto is used to select a paticula electode. Fo the electode sizes of inteest (3-1 µm squae to measue a single cell o a small cluste of cells), an aay of hundeds of electodes can be fabicated on a chip of a few mm 2 in size. Figue 6b shows a possible design fo one site in the aay. The aay is designed in the MOSIS AMIS 1.5 micon pocess. sensing electodes efeence electode i meas A column select v pobe sin( ωt) Figue 6. Active matix senso aay: (left) cicuit diagam of the aay and (ight) possible layout and coss section. In this design, the electodes ae fomed by using a glass cut to expose Metal2. As shown, the electodes ae 5 µm 5 µm and can be designed with 5 µm spacing. Column lines ae polysilicon and the ow lines ae Metal1. Note that postpocessing of the chip is equied to coat the electodes with gold and to passivate the bond pads and bond wies while exposing the sensing electodes. Details of these aspects of the pocessing will be epoted elsewhee. Hee we focus on the design of the electode aay.

6 Refeing to Fig. 3, we ae inteested in impedance measuements ove the fequency ange fom appoximately 1 3 Hz to 1 MHz. Ove this ange, the impedance of a 5 µm 5 µm squae electode vaies fom appoximately 41 kω to 6.4 kω ohms [4]. We equie the ON esistance of the access tansistos to be substantially less than this value. Fo a maximum eo of 2%, this yields the condition 1 R = < kω W 2k n ( VGS VT ) L The AMIS 1.5 micon pocess has k n 36 µa/v 2 so at V GS -V T = 4.5 V this condition can be satisfied by a tansisto with W/L = 24. A second condition aises because of capacitive coupling between the Metal1 ow lines and the cell gowth medium. This capacitance is appoximately given by C stay ε i A = t i whee A is the total aea of the Metal1 ow line and ε i and t i ae the effective pemittivity and thickness of the intelevel dielectic plus the passivation insulatos, espectively. Fo a Metal1 line 3 µm in width and 3 µm long (coesponding to 3 sensing sites) we estimate 1 C stay <.31 pf. We equie that the impedance magnitude of the stay capacitance be substantially geate than that of the impedance of the sensing electode. This condition is easily satisfied fo an aay with 3 sensing sites along a column line. This analysis suggests that a chip with 9 sensing sites could be fabicated in this pocess with acceptable paasitics. Such a chip would have a sensing aay 3 mm 3 mm and might have a total aea of appoximately 16 mm 2 to allow fo demultiplexing cicuity and bonding pads. Postpocessing and testing of a chip with about 12 sensing sites is pesently unde way. SUMMARY We have pesented measuements of the impedance of cell-coveed electodes which show that thee ae easily obsevable changes which can be elated to the attachment and polifeation of the cells. These obseved impedance changes depend on the cell type. We have also pesented a possible design fo an active-matix aay of a lage numbe of sensing sites. ACKNOWLEDGEMENTS This mateial is based upon wok suppoted by the National Science Foundation unde Gant No. ECS Any opinions, findings, and conclusions o ecommendations expessed in this mateial ae those of the authos and do not necessaily eflect the views of the National Science Foundation. 1 Assuming an insulato thickness at least as geat as 1 µm and the same dielectic constant as SiO 2.

7 REFERENCES 1. I. Giaeve and C.R. Keese, IEEE Tansactions on Biomedical Engineeing BME-33, (1986). 2. I. Giaeve and C.R. Keese, Poc. Natl. Acad. Sci., 88, (1991). 3. X. Huang, D. Nguyen, D.W. Geve, and M.M. Domach, (to be published in IEEE Sensos). 4. X. Huang, D.W. Geve, D. Nguyen, and M.M. Domach, in Poc. IEEE Int. Conf. Sensos 23, pp (IEEE, Piscataway, NJ, 24). 5. D.D. Nguyen, X. Huang, D.W. Geve, and M. M. Domach, (Biotechnology and Bioengineeing, in pess). 6. A.W. Patin, J.T. Isaacs, B. Teige and D.S. Coffey, Cance Reseach (1988). 7. J.J. Latime, in Developing Technologies fo Ealy Detection of Beast Cance, L. Newman, ed., summay of the wokshop Committee on the Ealy Detection of Beast Cance, pp , National Cance Policy Boad, Institute of Medicine, (Washington, DC, 2). 8. I. Giaeve and C.R. Keese, "Cell substate electical impedance senso with multiple electode aay," US patent 5,187,96 (1993).