IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING, VOL. BME-33, NO. 2, FEBRUARY 1986 Thin-Film Multiple Electrode Probes: Possibilities and Limitations OTTO J. PROHASKA, MEMBER, IEEE, FETHI OLCAYTUG, P. PFUNDNER, AND HEINZ DRAGAUN Abstiwt-Thin-fm multiple electrode probes are produced by means of thin-film techniques. They are successfully employed for potential measurements in brain research. The most advantageous feature of these probes is that the electrodes can be designed and arranged ac- curately and close together. The geometric size of the electrode areas is usually in the range of between 50 and 10 000 zm;. The size limitations of these thin-film probes are mainly determined by the electrode-elec- trolyte interface and insulation layer qualities. Since medical research problems, as well as surgical requests, are stressing these limitations, some estimates of maximum resolution and probe dimensions are pre- sented. INTRODUCTION \ 4[ULTIPLE recordings, simultaneously performed at IVidifferent, closely spaced, well-defined locations in the brain, are of increasing interest in medical research and surgery. Although years of experience enable the interpretation of typical electrical activity patterns in the electroencephalogram (EEG) in terms of pathological changes within the brain, very often only limited expla- nations are possible, as in the case of interactions between the cortical structures and peripheral nerves [1]. It would be very desirable for clinicians to know how neuronal ac- tivities of deeper brain layers influence the electrical sig- nal recordable at the surface of the brain; the localization of pathological tissue areas by che means of the EEG could become very much improved. Another problem results from interest in the study of local neuronal circuits in or- der to correlate electrical activity with anatomical tissue structures within the brain. In these cases, recordings of potential profile changes within small tissue areas are nec- essary, and very delicate probes are required in order to minimize tissue damage. The insertion of stereotactic probes for brain surgery can be done only through X-ray control. Stereotactic probes, which are equipped with potential sensors in order to render a continuous control of the probe insertion pro- Manuscript received March 30, 1985; revised July 3, 1985. This work was supported by the Austrian Fonds zur Foerderung der wissenschaft- lichen Forschung, Project S22/09. 0. Prohaska is with the Department of Biomedical Engineering, Case Western Reserve University, Cleveland, OH 44106. F. Olcaytug and P. Pfundner are with the Institut fur Allgemeine Elek- trotechnik und Elektronik, Technical University of Austria, Vienna, Aus- tria. H. Dragaun is with Technologisches Gewerbemuseum Laboratory fur Kunststofftechnik, Vienna, Austria. IEEE Log Number 8406349. cess by recording the electrical brain activity at different depths at the same time, will, in turn, represent a helpful supplement to the conventionally used control methods. The origin of an electrical activity in the brain and/or its propagation or correlation to another source of electri- cal activity can be determined by means of various math- ematical techniques [2]-[5], as soon as simultaneous re- cordings from well-defined locations within the brain are available. In some research cases, extremely high resolution of the spatial potential distribution within the tissue might be re- quired and electrode sites in the 1 Im2 range might be necessary; in most of the cases, electrode areas in the 100 Atm2 range will be sufficient. The probes themselves should also be as small as possible for research purposes; their length will seldom have to exceed 5 cm. In brain surgery, on the other hand, electrode areas in the range of 5000 Am2 might still be able to provide the required informa- tion; the probe itself, however, might have to be up to 20 cm long. Conventional intracortical multiple electrode probes consist of insulated metal wires or electrolyte-filled glass electrodes, glued together [6]. With increasing electrode numbers, these probes all have the disadvantage of being very destructive during the insertion into the tissue. It is also extremely hard to place a large number of electrodes with well-defined recording sites at distinct points within a small area [7]. Developments in thin-film and integrated circuit tech- niques [8], [9] enable the fabrication of extremely small and precise structures which can be laid out in almost any desired way in order to match the -recording site pattern with the tissue under investigation. A short description of the thin-film multiple electrode probe construction and its electrical qualities will provide the basis for the discussion of the possibilities and limitations of this probe type, since the final accomplishment of this probe type depends on the field of application, either in medical research or in surgery, and whether dc, low frequency (i.e., EEG up to about 100 Hz), or high frequency (i.e., single neuronal signal in the range of kilohertz) measurements are re- quired. PROBE DESIGN Thin-film multiple electrode probes [10] are success- fully in use in neurophysiological research [11], [12]. Var- 0018-9294/86/0200-0223$01.00 © 1986 IEEE 223