Output Stage of a Current-Steering Multipolar and Multisite Deep Brain Stimulator Virgilio Valente and Andreas Demosthenous Department of Electrical and Electronic Engineering University College London London, UK WC1E 7JE Email: v.valente@ucl.ac.uk Richard Bayford Department of Natural Sciences Middlesex University London, UK NW4 4BT Abstract— Clinical deep brain stimulation (DBS) is based on the use of cylindrical electrodes driven in monopolar or bipolar configurations. The simulation field spreads symmetri- cally around the electrode modulating both targeted and non- targeted neural structures. Recent advances have focused on novel stimulation techniques based on the use of high-density segmented electrodes, which allow current-steering and field- shaping capability. This paper presents the architecture of a multi-channel current-steering stimulator output stage that allows for monopolar, bipolar, tripolar and quadripolar multi- site stimulation. The core of the output stage comprises N independent high-compliance current drivers (HCCDs), capable of delivering up to 1.5 mA complementary currents in 10 different current ranges. Each of the N HCCDs can drive up to 8 adjacent electrode contacts thanks to a 2-32 multiplexer controlled by a 5-32 decoder. The HCCD was designed in a HV 0.18m CMOS process. The circuits were simulated in Cadence Spectre and simulated results are presented in the paper. I. I NTRODUCTION Clinical deep brain stimulation (DBS) consists of the ap- plication of high frequency voltage pulse to deep regions of the brain, known as basal ganglia, via a 4-contact cylindrical electrode. The structure and geometry of clinical electrodes poses a considerable constraint on the control of the shape and location of the stimulation field. In response to monopolar stimulation, the resulting potential distributes symmetrically around the electrode contact, and the area of stimulation is affected solely by the stimulus configuration [1]. DBS targets are adjacent to neural structures that, if stimulated, may cause considerable motor, sensory and cognitive side effects [2]. Recent technological advances have been aiming at accurate targeting of the stimulation by enabling field-shaping of the electric field within the brain with alternative stimulation configuration [1], [3], novel design of current-steering DBS stimulators [4] and novel lead designs [5]. In addition, multi- site targeting is currently under investigation to improve the efficacy of DBS [6]. Segmented electrode leads can bring substantial benefit to DBS efficacy by achieving longitudinal and radial field shaping and accurate targeting [7]. The use of high-density electrode leads, however, introduces several additional challenges in the design of suitable stimulators, in terms of minimization of chip area, power consumption and complexity. The design of multi-channel stimulators is usually based on independent current source cells to achieve high flexibility and parallelism of stimulation [8] or the use of mul- tiplexers to reduce the number of independent current sources [9]. Boston Scientific (MA, USA) have recently marketed an 8- channel DBS system with 8 independent current sources 1 . This approach, however, is sub-optimal in terms of the overall area of the system and power consumption, and leaves the clinician with an impractical range of configurations to select, in order to optimise the clinical efficacy. This paper presents the architecture of a current-steering multipolar DBS stimulator output stage, with particular emphasis on the design of a high-compliance and high-output impedance versatile current source. The system is suitable for a 32-segment electrode lead with a structure equivalent to [5]. II. MULTIPOLAR CURRENT- STEERING DBS SYSTEM ARCHITECTURE The architecture of a current-steering multipolar DBS sys- tem is shown in Figure 1. The system consists of a 5-bit R2R DAC,  high-compliance current drivers (HCCDs) and a set of selection switches. This paper will focus on the description of the HCCD. The function of each HCCD is the generation of two complementary, biphasic currents, I and I(1-). In order to achieve high-compliance, each output stage is equipped with two voltage-to-current converter (VIC) structures [10], where an amplifier and a transistor, biased in the triode region, are used to set the output current. The inputs of the VICs are generated by 5-bit R2R DAC, which allows to cover voltage ranges between 0 and 320mV in steps of 10mV. A 1-32 multiplexer, , is used to connect the R2R DAC to the ℎ HCCD. Two capacitors, C  and C  , are used to store the voltage inputs,   and   , for the low side and the high side VICs respectively. This way one DAC can serve all  HCCDs. Each HCCD is also equipped with a 2-to-32 multiplexing stage controlled by a 5-to-32 decoder, which selects which and how many electrodes are driven by the same HCCD. Given the impractical number of electrode configurations resulting from the use of 32-contact electrodes, we limit the design of the multiplexing stage to drive groups a maximum of 8 adjacent contacts. The switch array S1 allows to select the return path for the stimulation current. 1 Source: Boston Scientific 978-1-4799-1471-5/13/$31.00 ©2013 IEEE 85