INSTITUTE OF PHYSICS PUBLISHING PLASMA SOURCES SCIENCE AND TECHNOLOGY Plasma Sources Sci. Technol. 15 (2006) 582–589 doi:10.1088/0963-0252/15/3/038 Development of a smart positioning sensor for the plasma needle E P van der Laan 1 , E Stoffels 2 and M Steinbuch 1 1 Department of Mechanical Engineering, Technische Universiteit Eindhoven, PO Box 513, 5600 MB Eindhoven, The Netherlands 2 Department of Biomedical Engineering, Technische Universiteit Eindhoven, PO Box 513, 5600 MB Eindhoven, The Netherlands E-mail: e.p.v.d.laan@tue.nl Received 10 July 2005, in final form 12 May 2006 Published 27 June 2006 Online at stacks.iop.org/PSST/15/582 Abstract In the study of plasma–surface interactions, a well-defined distance from the surface is desired. In this paper, an innovative method is proposed for using the intrinsic properties of a low-power atmospheric plasma to control this distance. It is proposed that the vicinity of a (even poorly) conducting surface imposes a change in the plasma impedance. This effects the reflection of electrical power, which can easily be measured, and provides a non-contact sensor for the distance between the plasma source and the substrate. For this sensor to work, a low-loss and efficient matching network is designed, such that changes in the reflected power are relatively high. With this high-quality matching network at hand, a characterization of the plasma needle is performed in terms of impedance and related parameters. Although characterization is not essential for the development of the sensor, it contributes to fundamental knowledge about the plasma source. Finally, coherence measurements are performed to determine the performance of the above described sensor in different experimental settings. (Some figures in this article are in colour only in the electronic version) 1. Introduction 1.1. The plasma needle This paper deals with a very particular type of plasma source. It was proposed that a non-thermal atmospheric plasma could have a healing effect on a living organism. This idea led to the development of the plasma needle and initiated research on the interactions between non-thermal plasmas and biological tissues [1]. This new type of plasma source is a low temperature non-equilibrium capacitively coupled radio- frequency discharge. The discharge is excited and sustained in helium at atmospheric pressure; however other noble gases have also proved to be effective [2]. The plasma is visible as a spherical glow with an average diameter of about 1 mm. The device consists of a grounded metal housing and a plastic tube through which the helium gas flows, as seen in figure 1(a). A metal wire, placed coaxially in the tube, has a sharpened point and functions as an electrode. Insulation of the wire assures that the plasma is generated at the tip only, as seen in figure 1(b). The gas of the small non-thermal plasma remains at ambient (i.e. room) temperature, so it is not likely that the device causes thermal damage to sensitive biological materials [3]. 1.2. Interactions with biological tissue Studies on the effect of the plasma needle on biological tissues can be found in the literature. Since living organisms are complex, the studies so far are limited to cell cultures. It is found that reactive particles, mainly oxygen-containing radicals, escape from the plasma discharge and induce specific sophisticated cell responses [4]. The dose of these reactive particles is an important parameter in the study of plasma– tissue interactions since it discriminates in the final result. With a proper dose, the plasma treatment results in cell detachment, that is, cell removal without damage and inflammation [5]. A small percentage of cells undergo apoptosis (programmed cell death) after a more intensive plasma treatment [6]. A higher dose leads to necrosis (accidental cell death) which is most 0963-0252/06/030582+08$30.00 © 2006 IOP Publishing Ltd Printed in the UK 582