Sensors andActuators B 81 -2001) 49±54 Micromachining of non-fouling coatings for bio-MEMS applications Yael Haneina,*, Y. Vickie Panb,1, Buddy D. Ratnerb,c, Denice D. Dentona, Karl F. BoÈhringera aDepartment of Electrical Engineering, University of Washington, Seattle, WA 98195, USA bDepartment of Bioengineering, University of Washington, Seattle, WA 98195, USA cDepartment of Chemical Engineering, University of Washington, Seattle, WA 98195, USA Received2 April 2001; receivedin revisedform 12 August 2001; accepted18 August 2001 Abstract Standard photolithography is used to pattern a poly -ethylene glycol) -PEG)-like polymer onto silicon substrates. The coating has excellent non-fouling properties and good adhesion to various substrate materials, such as silicon, oxide, nitride, gold, and platinum. This methodallows precise control of the shape, size andalignment of the polymer, thus providinga reliable tool to pattern protein sheets as well as cell cultures. This method also enables the incorporation of patterned cell cultures with various prede®ned elements such as electrodes, channels, and sensors. To demonstrate the properties of our technique, we apply it to build cell cultures and to protect metallic electrodes from protein and cell adhesion. We show that the thin coatings provide excellent protection without compromising the conductivity of the electrodes. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Bio-MEMS; Bio-fouling; Proteins; Cell cultures 1. Introduction example, reducing its life span or increasing its power consumption [6]. The rapidgrowth of micro electro mechanical systems A convenient approach to control protein adsorption is by -MEMS) research in the last few decades has given rise to a surface modi®cation [7]. This approach is particularly con- wide variety of novel applications. Among the most notable venient for bio-MEMS development since it allows the use are small, portable systems for medical and biological of commonly usedsubstrates andmaterials -as long as they applications -bio-MEMS). These devices are currently are non-toxic), andrequires only a ®nal coating procedure. developed with the vision of building miniaturized, fast, Many common non-fouling coating techniques are based and portable analytical instrumentation for drug delivery, on poly -ethylene glycol) -PEG) [8,9]. PEG is a hydrated electrochemical monitoring [1] andDNA analysis [2]. These [10], non-toxic coating andFDA-approvedfor use in bio- devices would speed the analysis time for researchers in technology andconsumer applications. In recent years, the lab, for medical staff at hospitals or might be used as various methods have been adopted to coat surfaces with carry-on devices by chronically ill patients. PEG, including silicon surfaces [11]. In our study, we use The interaction of a device with a biological environment glow discharge plasma polymerization of tetraglyme leads to various challenges that have to be taken into account -CH3ÀOÀ-CH2ÀCH2ÀO)4ÀCH3) to obtain a PEG-like in order to allow its proper operation. A major issue is bio- material. Plasma polymerization is a gas-phase, thin-®lm fouling, the strong tendency of proteins and organisms to deposition process. This process is capable of producing thin physically adsorb to synthetic surfaces [3]. The adsorbed coatings -of the order of 10 nm) with superb surface con- protein layer tends to create undesired perturbations to the formality [10] which is an extremely important factor in operation of devices such as pH [4] and glucose [5] sensors. determining the success of the PEG coating [12]. The excel- In addition, the protein layer can mediate various biological lent surface coverage of the plasma polymerization process responses such as cell attachment andactivation, which may also contributes to its excellent non-fouling properties. interfere with the optimal operation of the device by, for In previous studies, it was demonstrated that plasma polymerizedtetraglyme -pp4G) coatedsurfaces show good resistance to protein adsorption [10]. The focus of our * Corresponding author. E-mail address: [email protected] -Y. Hanein). current study is to examine the compatibility between the 1 Present address: Cellomics, Pittsburgh PA, USA. pp4G coatings andphotolithography andmicro-machining 0925-4005/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0925-4005-01)00925-X 50 Y. Hanein et al. / Sensors and Actuators B 81 +2001) 49±54 processes in order to allow pp4G patterning as a standard for our experiments because it has better corrosion resis- MEMS process. In this paper, we focus on devices with a tance in aqueous environments than oxide [14]. To achieve topography suitable for photolithography. Such conditions the pp4G patterning, photoresist -Clariant AZ 1512, 1.5 mm) are common for many Bio-MEMS devices which consist of was deposited, and then exposed and developed in order to two bonded elements with at least one suitable for photo- create openings in the resist for the later pp4G lift-off. After lithography. Patterning the pp4G directly on high aspect a plasma oxygen descum etch of 15 s in an RIE chamber ratio structures is limitedby the ability to achieve uniform -Trion Technology), the wafers were introduced to the photoresist coatings on these structures. However, such plasma reactor for the pp4G deposition. The substrates were coatings may be achievedby using very thick photo-resist brie¯y cleanedin an argon plasma, the tetraglyme vapor was or a special application procedure [16]. introduced to the chamber and the samples were coated. The We describe below the integration of pp4G coatings into details of the monomer preparation and the deposition microfabrication processes for bio-MEMS applications. We process will be discussed elsewhere [15]. Following the characterizedthe coatings using environmental scanning plasma deposition process the wafers were soaked in acetone electron microscopy -ESEM) which we usedto verify the an ultrasonic bath for approximately 20 s. Finally, the wafers pattern de®nition. Electron spectroscopy for chemical ana- were rinsedwith methanol anddriedwithnitrogen. The lysis -ESCA), a surface sensitive analytical technique, was microfabrication processes were done at the Washington also utilizedto analyze the coating surface chemistry after Technology Center -WTC) andat the Electrical Engineering the photoresist lift-off. We foundthat the patterning process Micro-fabrication Labs at the University of Washington. The did not affect the pp4G coating. pp4G deposition was done at the University of Washington Finally, we discuss experiments which demonstrate the EngineeredBiomaterials -UWEB) labs. capabilities of our technique: A protein adsorption experi- Integrating the lift-off andthe plasma deposition pro- ment demonstrates the spatial control of these interactions cesses was rather straightforward. However, special atten- on patternedpp4G surfaces. We use the patternedpp4G tion hadto be directedto the location of the wafer in the coatings to con®ne cell cultures to speci®c regions on the plasma chamber. We placedthe wafers in low monomer substrate. The advantages over contemporary methods that condensation regions to prevent tetraglyme vapor from consist of soft lithography [13] are discussed. The pp4G was dissolving the photoresist. After the plasma deposition, also testedas a protection layer for electrodesin electro- the wafers were rinsedin acetone -in an ultrasonic bath) chemical sensing. to remove resist leftovers. 2.1. Lift-off results 2. Fabrication To discuss the quality of our results we begin with a visual We usedsilicon wafers as substrates for the pp4G coat- examination of the pp4G coating after lift-off. In Fig. 1a and ings. The wafers were sputteredwith silicon nitrideor b, we show ESEM images of patternedpp4G coatings on oxidized for electrical passivation. Nitride is better suited thermally oxidized substrates. Here, arrays of Au squares Fig. 1. ESEM images of pp4G features after lift-off. The scale bar is 200 mm for both images. -a) The pp4G structure -left) alignedto predefinedCrAu structure. Lift-off without ultrasonic power. -b) ExposedSiO 2 substrate -bright square) with pp4G background. Lift-off is done in an ultrasonic bath. Similar results were obtainedfor structures as small as 10 mm. While current results were produced with low-cost printed masks -features greater than 10 mm), minimum feature size andalignment precision for these protein-resistant coatings is limitedonly by the lithographic resolution. We recently achi evedsimilar results with 2 mm resolution. Y. Hanein et al. / Sensors and Actuators B 81 +2001) 49±54 51 Fig. 2. A fluorescence microscope image of a patternedprotein-resistant pp4G pad-1500 mm  1500 mm) with Au pads -200 mm  200 mm) on SiO2 substrate after incubation in a solution with fluorescently labeled proteins. The pp4G pad appears dark as the adsorbed proteins cover exposed SiO2 andAu areas while pp4G coatedareas resist protein adsorption. were ®rst patternedusing the lift-off technique. Then the the patterning process does not affect the chemistry of the pp4G was deposited and patterned using lift-off. Using a pp4G coating andthat there are no organic residues left on stylus stepper -Alpha-Step), the pp4G coating was measured the exposed or coated surfaces. The ESCA data are dis- to be approximately 20 nm. In Fig. 1a the pp4G padis shown cussedin detailelsewhere [15]. with a prede®ned chrome±gold -CrAu) feature. To perform To assess the