Available online at www.sciencedirect.com Sensors and Actuators B 132 (2008) 431–438 Microfluidic valves based on superhydrophobic nanostructures and switchable thermosensitive surface for lab-on-a-chip (LOC) systems G. Londe a , A. Chunder b,c , A. Wesser d , L. Zhai b,c , H.J. Cho d,∗ a Department of Electrical Engineering, University of Central Florida, Orlando, FL 32816-2450, USA b Nanoscience Technology Center, University of Central Florida, Orlando, FL 32826-3271, USA c Department of Chemistry, University of Central Florida, Orlando, FL 32826-2366, USA d Department of Mechanical, Materials and Aerospace Engineering, University of Central Florida, Orlando, FL 32816-2450, USA Available online 4 November 2007 Abstract This paper describes a novel valve concept using a nanostructured functional polymer surface. To prove the concept, two fully integrated microfluidic valves, one with a superhydrophobic polymer surface and the other with a switchable, thermosensitive polymer surface have been fabricated and tested. The passive valve with the superhydrophobic polymer surface selectively inhibits the flow of water-based reagents and passes aqueous solutions containing surfactants. In case of the thermosensitive valve, the switchable polymer surface becomes hydrophobic when heated to temperatures exceeding 65 ◦ C, thus inhibiting the flow of water and becomes hydrophilic at room temperature, thus allowing the flow of water. The microchannels are fabricated by standard photolithography and wet etching techniques. The polymer surface for both the valves is fabricated using the layer-by-layer (LBL) deposition technique, in which multiple layers of polyelectrolytes are coated on a channel wall followed by silica nanoparticle treatment. For the thermosensitive valve, the polymer surface is further coated with the thermosensitive polymer poly(N- isopropylacrylamide) (PNIPAAm). The fabricated microfluidic valve was tested with liquids flowing in the microchannels under capillary action. It is shown that the valve selectively regulates the flow of test samples. © 2007 Elsevier B.V. All rights reserved. Keywords: Passive valve; Layer-by-layer (LBL) deposition; Nanoparticles; Superhydrophobic; Thermosensitive polymer 1. Introduction Surfaces that can tune their properties when subjected to external stimuli have potential applications in a variety of fields including drug delivery [1], bioanalysis [2], pro- tein separation [3] and microfluidics [4]. The development of micropumps and microvalves has made it possible to realize a fully integrated microfluidic system for biochemical analy- sis [5,6]. Nano/microliter fluid handling on LOC by passive fluidic manipulation often employs a hydrophobic surface as a valve in the microchannel network and an external pneumatic control coupled with capillary action to discretely manipulate ∗ Corresponding author at: Department of Mechanical, Materials & Aerospace Engineering, 4000 Central Florida Boulevard, University of Central Florida, Orlando, FL 32816-2450, USA. Tel.: +1 407 823 5014; fax: +1 407 823 0208. E-mail address: joecho@mail.ucf.edu (H.J. Cho). URL: http://www.mmae.ucf.edu/∼joecho (G. Londe). the fluids. Capillary manipulation of fluids by modifying the solid–liquid surface tension, using surfaces with wettability gra- dients have been shown to be well adapted for microfluidic systems [7]. Wettability of a solid surface is an important property of a material as it controls its interaction with the liquid. It depends on various factors, among which surface roughness and surface energy are the dominant ones. For the fabrication of superhy- drophobic surface, a combination of optimum surface roughness and low surface energy is required. Wenzel’s model proposed that the roughness increases the surface area which geometri- cally increases the hydrophobicity [8]. In fact, surfaces with a water contact angle of more than 150 ◦ were fabricated by incor- porating appropriate roughness on materials having low surface energies. By extending these studies, we propose novel microfluidic valves, in which the microchannel network is selectively coated with functionalized polymers, using the layer-by-layer (LBL) deposition technique. Two different microfluidic valves based 0925-4005/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2007.10.052