Microfluidic Chip to Produce Temperature Jumps for Electrophysiology Thomas Pennell, † Thomas Suchyna, ‡ Jianbin Wang, † Jinseok Heo, † James D. Felske, † Frederick Sachs, ‡ and Susan Z. Hua* ,†,‡ Department of Mechanical and Aerospace Engineering, State University of New YorksBuffalo, Buffalo, New York 14260, and Department of Physiology and Biophysics, State University of New YorksBuffalo, Buffalo, New York 14214 We developed a microfluidic chip that provides rapid temperature changes and accurate temperature control of the perfusing solution to facilitate patch-clamp studies. The device consists of a fluid channel connected to an accessible reservoir for cell culture and patch-clamp measurements. A thin-film platinum heater was placed in the flow channel to generate rapid temperature change, and the temperature was monitored using a thin-film resistor. We constructed the thermal chip using SU-8 on a glass wafer to minimize the heat loss. The chip is capable of increasing the solution temperature from bath temper- ature (20 °C) to 80 °C at an optimum heating rate of 0.5 °C/ms. To demonstrate the ability of the thermal chip, we have conducted on-chip patch-clamp recordings of tem- perature-sensitive ion channels (TRPV1) transfected HEK293 cells. The heat-stimulated currents were ob- served using whole-cell and cell-attached patch configura- tions. The results demonstrated that the chip can provide rapid temperature jumps at the resolution of single-ion channels. Temperature has strong effects on cell function and cellular processes. In addition to the general thermal effects on ion channel kinetics, the discovery of heat-gated ion channels in neurons 1 has focused interest on thermosensing pathways. 2,3 Neurons detect and react to heat via thermal-sensitive ion channels. The quantita- tive investigation of channel activity requires controlled thermal stimuli. 4,5 Commercially available instruments do not provide sufficient speed to resolve the kinetic response to steps in temperature. Most existing heater devices, such as the ITO (indium tin oxide)-coated glass heater or focused laser apparatus can provide fast rise time of the temperature; however, the fall time is limited by thermal diffusion due to the size of the source, so only small volumes can cool fast. A dual-channel flow can produce faster temperature change; however, this requires moving parts and the actual temperature at the cell is not under tight control. 8 Here we present a microfluidic thermal chip that enables rapid temperature change with accurate temperature control. Microfluidic devices offer the advantage of precise thermal input and, because of their small volume, rapid temperature changes. The resistive heater responds rapidly, 6,7 requires simple fabrication, and offers minimum interference with picoamp- nanoamp patch-clamp electrical signals. Several microheater devices have been built on silicon chips using MEMS (micro- electromechanical systems) technology for various microfluidic applications. 9 Since the thermal conductivity of silicon is relatively large and results in high substrate heat losses, significant effort has been focused on isolating the heating element from the substrate. Typically, a thin silicon nitride membrane with an air gap has been used to isolate the heater from the silicon substrate. With the use of this approach, a fluidic chamber with an integrated platinum heater has been demonstrated for precise temperature control with low power consumption. 7 Rapid temperature cycling has also been achieved by controlling the heat flow via various thermal designs of the heating chamber. 10-13 On the other hand, the high thermal conductivity of silicon has been utilized by constructing arrays of micromachined silicon posts which enabled efficient heat dissipation into a fluidic channel through the microfinned surfaces of the channel. 14 Although the performance of silicon-based heaters has been demonstrated, their complicated fabrication process has limited their applications. Recently, glass substrate has been used to construct polymerase chain reaction (PCR) reactors showing faster thermal cycling. 15,16 * Corresponding author. Phone: 716-645-2593 ext. 2358. Fax: 716-645-3875. 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