Anal. Chem. 2004, 76, 1850-1856
Fluid Flow Past an Aperture in a Microfluidic Channel Mark C. Peterman,*,† Jaan Noolandi,‡ Mark S. Blumenkranz,‡ and Harvey A. Fishman‡
Department of Applied Physics, Stanford University, Stanford, California 94305-4090, and Department of Ophthalmology, Stanford University, Stanford, California 94305-5308
Electroosmotically driven flow in neurotransmitter-based retinal prostheses offers a novel approach to interfacing the nervous system. Here, we show that electroosmotically driven flow in a microfluidic channel can be used either to eject or to withdraw fluid through a small aperture in the channel wall. We study this fluid movement numerically using a finite-element method and experimentally using microfabricated channels and apertures. Two devices are used to test the concept of fluid ejection and withdrawal: (1) a single, large channel with four apertures and (2) a prototype neural interface with four individually addressable apertures. We compared experimental and numerical results in microchannels using the observed pH dependence of the fluorescent dye fluorescein, finding good agreement between the results. Because of the simplicity and rapid response of electroosmotic flow, this technique may be useful for neurotransmitter-based neural interfaces. Neurotransmitter-based neural prostheses present a novel approach to interfacing the human nervous system.1,2 The notion of these prostheses is to release a small amount of neurotransmitter at a well-defined location, in effect mimicking the number of molecules that are released during synaptic transmission. Our work has focused on stimulating a biological system with a microfabricated device that acts like an artificial synapse. To mimick synaptic release on a chip, we have fabricated a microfluidic device that releases transmitter through a small aperture connected to a microfluidic channel.3 By driving fluid through the channel and through the aperture, we can eject fluid into a biological system of interest. This concept of neurotransmitter stimulation is not limited to neural prostheses; the ability to release chemicals at an array of well-defined locations to a biological system has applications in drug discovery, tissue engineering, drug delivery, and fundamental biological research in chemotaxis or cell signaling. In the artificial synapse chip (ASC) system that we have developed, electroosmotic (EO) flow is used to drive fluid through †
Department of Applied Physics. Department of Ophthalmology. (1) Peterman, M. C.; Bloom, D. M.; Lee, C.; Bent, S. F.; Marmor, M. F.; Blumenkranz, M. S.; Fishman, H. A. Invest. Ophthalmol. Visual Sci. 2003, 44, 3314-3149. (2) Vastag, B. JAMA 2002, 288, 1833-1834. (3) Peterman, M. C.; Ziebarth, J. M.; Braha, O.; Bayley, H.; Fishman, H. A.; Bloom, D. M. Biomed. Microdevices 2002, 4, 231-236. ‡
1850 Analytical Chemistry, Vol. 76, No. 7, April 1, 2004
small apertures in the side of a microfluidic channel (Figure 1a). Because EO flow is fast, has low power consumption, and is more effective in small channels than applied pressure, it is very popular in MEMS and microfluidic devices.4-7 The design of the ASC consists of a rectangular microfluidic channel, tens of micrometers wide. One side of the channel is a thin silicon nitride membrane,
