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Abstract

Abstract. An inexpensive and simple pumping principle is described that is capable of delivering both small and constant ow rates (10±1,000 nl/min) over a longer period of time (days to weeks). The concept is based on controlled evaporation of a liquid through a membrane into a gas space containing a sorption agent. As long as the sorption agent keeps the vapor pressure in the gas phase below saturation,¯uid evaporated from the membrane is replaced by capillary forces inducing¯ow from a reservoir. In a feasibility study, a total volume of 300 ml of Ringer's solution has been continuously pumped over a period of six days, resulting in an constant average¯ow rate of 35 nl/min (590 pl/s). The maximum liquid volume transported is limited by sorption capacity and amount of the sorption agent. Low fabrication costs, high reliability (no moving parts), the suitability for integration into planar system architectures and the lack of a special external energy source besides an environment of regulated temperature are important features of the concept, in particular with regard to its potential application in continuous patient monitoring. Truly continuous¯ow can be achieved in contrast to many other pump mechanisms leading to discontinuous, pulse-type¯ow. A challenge for a broader range of applications is the inherent temperature dependence of thē ow rate. In its current version, the pump can only be used in a suction-mode.