806 IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING, VOL. 48, NO. 7, JULY 2001 High-Speed Solution Switching Using Piezo-Based Micropositioning Stages Shane Stilson, Annette McClellan, and Santosh Devasia*, Member, IEEE Abstract—Motion-induced vibration is a critical limita- tion in high-speed micropositioning stages used to achieve solution switching. Controlled rapid solution switching is used to study the fast activation and deactivation kinetics of ligand-gated ion-channel populations isolated in excised membrane patches—such studies are needed to understand fundamental mechanisms that mediate synaptic excitation and inhibition in the central nervous system. However, as the so- lution-switching speed is increased, vibration induced in the piezo-based positioning stages can result in undesired, repeated, ligand application to the excised patch. The article describes a method to use knowledge of the piezo-stage’s vibrational dynamics to compensate for and reduce these unwanted vibrations. The method was experimentally verified using an open-electrode tech- nique, and fast solution switching (100 s range) was achieved. Index Terms—Micropositioning, piezo-actuator, receptor kinetics, solution switching. I. INTRODUCTION F UNDAMENTAL mechanisms that mediate synaptic excitation and inhibition in the central nervous system can be understood by investigating high-speed channel gating caused by neurotransmitter binding. Studies of channel gating can be done using solution-switching techniques [1]–[4], which are used to experimentally simulate brief synaptic transmitter conditions and physiologically relevant conditions. For example, solution-switching techniques have been used to apply neurotransmitters to isolated receptor population (excised patch) and obtain direct measurement of kinetic transitions. Such studies have been used to understand alterations in neurotransmitter receptor function in the presence of therapeu- tically relevant drugs. For example, understanding the kinetic properties of GABA ( -aminobutyric acid type A) receptors, which are the primary inhibitory neurotransmitter receptors in the mammalian brain, allows the determination of mechanisms by which anticonvulsant drugs increase the receptor function to control seizures [5], and allows the study of functional differences between subtypes of receptors [6], [7]. However, current solution-switching systems are relatively slow—which Manuscript received July 23, 1999; revised March 21, 2001. This work was supported by the National Science Foundation (NSF) under Grant CMS 9813080. Asterisk indicates corresponding author. S. Stilson is with the Mechanical Engineering Department, University of Utah, Salt Lake City, UT 84112 USA. A. McClellan was with the Pharmacology Department, University of Utah, Salt Lake City, UT 84119 USA. She is now with BioMedical Engineering So- lutions, Inc., Salt Lake City, UT 84112 USA. *S. Devasia is with the Mechanical Engineering Department, Univer- sity of Washington, Box 352600, Seattle, WA 98195-2600 USA (e-mail: devasia@u.washington.edu). Publisher Item Identifier S 0018-9294(01)05139-4. limits the speed of neurotransmitter application to the excised patch of receptors. Development of new techniques to allow rapid ligand exposure of receptors is needed so that the rate limiting determinants reflect the channel properties rather than the ligand application (i.e., solution switching) parameters [8]. The slow application of neurotransmitter also makes it impos- sible to characterize the activation kinetics of relatively fast neurotransmitter receptors. Therefore, there is a need to develop high-speed solution-switching systems—this article studies the reduction of unwanted motion-induced vibrations and to achieve high-speed, precision micropositioning that is critical to the development of fast solution-switching techniques [8]. While the focus of this article is on solution switching, we note that the technique presented here is also applicable to other precision positioning systems where high-speed is desired. A. Patch-Clamp Techniques Patch-clamp techniques have long been used to study elec- trophysiological properties of channels and ligand-receptor interaction [1]. Briefly, membrane patches containing one or more receptor-channel complexes can be excised from the cell surface and exposed to various experimental solutions. It is noted that activation of ligand-gated receptor channels requires binding of ligands (i.e., neurotransmitters) to the receptor, which cause conformational changes in the channel protein—mechanisms that facilitate these structural changes open integral protein pores and permit ion flux across the membrane. The ensuing changes in ion flux associated with opening and closing of the channel protein can then be mea- sured as changes in current, when the membrane voltage is clamped, to study the kinetics of opening (activation) and closing (deactivation) for ion channel populations [2]. The central idea is to change the solution that is flowing across an excised membrane patch—rapid solution switching can be accomplished by moving a double lumen theta tube with adjacent streams of control and test solutions (see Fig. 1). B. Induced Vibrations During High-Speed Solution Switching The theta tube (see Fig. 1) can be moved using a piezo-based micropositioning stage (here after referred to as the “piezo-stage”) to change the solution that is flowing across the excised membrane patch on the recording pipette. Such piezo-based solution switchers have been used to study rapid kinetic transition of ligand gated ion channels [3]–[5]. The solution-switcher must satisfy two requirements. First, solution switching must occur faster than the kinetic transition under investigation, for example, activation transitions in some inhibitory ligand gated ion channels (for example, in 0018–9294/01$10.00 © 2001 IEEE