A Blumlein-type, nanosecond pulse generator with ... - IEEE Xplore

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S. Romeo et al.: A Blumlein-type, Nanosecond Pulse Generator with Interchangeable Transmission Lines

A Blumlein-type, Nanosecond Pulse Generator with Interchangeable Transmission Lines for Bioelectrical Applications Stefania Romeo, Claudio D’Avino, Olga Zeni National Research Council of Italy Institute for Electromagnetic Sensing of the Environment (IREA) via Diocleziano 328, 80124, Naples, Italy

Luigi Zeni Second University of Naples Department of Industrial and Information Engineering Via Roma 29, 81031, Aversa, Italy and National Research Council of Italy Institute for Electromagnetic Sensing of the Environment (IREA) via Diocleziano 328, 80124, Naples, Italy

ABSTRACT The design and realization of a nanosecond, high-voltage electric pulse generator for bioelectrical applications is reported in this paper. A Blumlein type architecture was adopted, with some modifications, and realized in a microstrip line configuration with meander-shaped conducting strips, and with ultra-fast, high voltage solid state switches. Three microstrip-lines have been realized in such a way that, being interchangeable within the structure, they allow to match different load impedances. A fiber optic-based system was also realized to separate the high voltage side of the system from the switch-control circuit. The system is suitable for applying high voltage nanosecond electric pulses, with variable pulse duration, amplitude, repetition rate and polarity, to liquid media with different electromagnetic characteristics, hosted in electroporation cuvettes with different gap dimensions. Index Terms — high voltage nanosecond pulses, Blumlein pulse forming network, microstrip line, interchangeable transmission lines, bioelectrics

1 INTRODUCTION THE usage of high voltage electric pulses has gained considerable attention over the past decades for their applications in medicine, biotechnology, environment and food processing [1]. In particular, medical and biotechnological applications of intense electric pulses, with durations in the millisecond to microsecond time scale, have been successfully developed such as electrochemotherapy [2], electrogenetherapy [3], nonthermal ablation of cancer tissues [4] and cell-lysis for extraction of intracellular material [5]. All these techniques exploit the electroporation phenomenon, a nonlethal alteration of the plasma membrane permeability induced by the application of intense pulsed electric fields, which allows the entry of genetic material and pharmaceutical agents into living cells [6]. More recently, Manuscript received on 29 September 2012, in final form 23 January 2013.

a new branch in the framework of this research field has been opened by shortening the pulse duration to the submicrosecond and nanosecond time scale. Such nanosecond pulsed electric fields (nsPEFs) have been demonstrated to interact with both the plasma and the intracellular membranes, inducing morphological (membrane permeabilization) and functional (enhanced calcium release and gene expression, apoptosis) effects [7], and showing promising applications for cancer treatment [8]. Moreover, pulsed electric field electro-technologies have also found large acceptance in the field of food processing and preservation [9]. Both the pulse parameters (amplitude, duration, rise/fall time, repetition rate) and the electromagnetic characteristics of the exposed medium, affect the electroporation phenomenon. Therefore, the study of nsPEF effects requires pulse generation systems providing well defined high voltage pulses (in terms of amplitude, duration, number and repetition rate), to be delivered to the biological load in

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IEEE Transactions on Dielectrics and Electrical Insulation

Vol. 20, No. 4; August 2013

highly controlled conditions. Pulse generators must interface to interchangeable load chambers and offer variability of pulse parameters over wide ranges [10]. Furthermore, they have to be compatible, from the physical and electrical point of view, with the different sample holders chosen on the base of the biological technique to be employed (electroporation cuvettes, microscope slide-based applicators, needle arrays of electrodes or tweezers). Accordingly, they must be able to operate within the correct electrical impedance matching and pulse shape constraints. Differently from milli and microsecond pulse generators, ns pulsers are not available on the market; thus, different types of pulse generators and applicators have been developed for the research activity in order to match the requirements of bioelectric studies. The choice of a pulse generator topology is driven by the load impedance, which depends on the physical form of load (e.g., cuvette or slide), by the desired pulse amplitude, width and rise time, and by the flexibility of exposure conditions. The implementation of nanosecond pulse generators for biotechnological applications is particularly challenging due to the requirements of faster switching elements, wideband and high-voltage components [11]. The Blumlein pulse forming network (PFN) is a well established circuit topology in pulsed power engineering, and is currently one of the main architectures adopted to generate nanosecond, high voltage electric pulses for electroporation [11, 12]. A Blumlein PFN is usually configured with a highvoltage source charging paired transmission lines, seriesconnected to the load, which is required to have an impedance as large as twice the characteristic impedance of each transmission line. The generator operates in two phases, charge and discharge, driven by the activation of a single switch, and provides rectangular pulses with fixed length. This concept has been recently modified into a two- [13] or four-switch [14] configuration, which allow to generate pulses with variable duration, polarity and repetition rate. In particular, in our previous works, it was demonstrated that pulse length and polarity can be changed by means of the synchronization of two switches [13, 15]. However, all previous implementation of Blumlein pulse forming networks for bioelectrics were realized by using coaxial cable-transmission lines, which pose strong constraints on the load impedance. In this work, a double-switch Blumlein pulse generator realized with microstrip transmission lines, is presented. This solution, adopted here for the first time, and previously introduced as a possible, more compact and versatile alternative to coaxial cables [15], has been designed, realized and fully characterized in the present work. In particular, three microstrip-lines have been realized and are interchangeable within the structure, allowing to match different (low, intermediate and high) load impedances according to the electromagnetic properties of the liquid media hosted in electroporation cuvette. Further, a fiber optic link has been also realized to separate the switch driving circuit from the high voltage side of the pulse generator.

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2 DESIGN CONSIDERATIONS 2.1 THE NS PULSE GENERATION SYSTEM A block diagram of the ns pulse generator is sketched in Figure 1. The system is composed by a command station, a fiber-optic link and the Blumlein PFN. The command station comprises a PC and a couple of low voltage, digital pulse generators (USBpulse100, Elan Digital System, Fareham, UK) which provide the control signals for the switches activation. The devices are PC-controlled via USB ports and provide single pulses or trains of pulses with programmable output high voltage level from 1.5 to 5 V, variable length (from 10 ns to 10 s) and repetition rate (up to 100 MHz). These signals are propagated to the switches by means of a fiber-optic link, consisting of two transmitters (TX) interfaced to the USBpulse100 generators, two receivers (RX) interfaced to the switches, and a fiber-optic patch-cord. This solution has been set up to separate the control side of the system from the high voltage side, thus avoiding interferences that could be conducted through common grounding connections and cause undesired commutation of the switches.

Figure 1. Diagram of the high voltage, nanosecond electric pulse generation system. USBPulse100 indicates the digital, PC-controlled generators which provide the low-voltage pulses triggering the highvoltage switches (HTS_1 and HTS_2). TX and RX refer to the transmitter and receiver circuits in the optical link.

The Blumlein PFN consists of a high voltage DC power supply, two microstrip transmission lines, two solid state switches. The presence of two identical switches, that can be independently triggered with a certain delay time among each other, allows to generate pulses with variable duration and polarity: the duration equals this delay time, while the polarity depends on the activation order of the switches [13]. Two solid state switches of the HTS-UF (ultra fast) series by Behlke company (Germany) have been employed, that are made up of a large number of MOSFETs, combined in a compact, low-inductance bank. They can hold a maximum voltage of 8 kV, and have extremely fast turn-on and turn-off rise times, which remain widely constant over a large range of operating voltages and loads. At the input side, a TTL-compatible signal and a 5 V auxiliary voltage are required. The switches have been mounted on a printed circuit board (PCB) following the recommendations provided by the manufacturer to ensure good performance.

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In particular, the 5 V auxiliary voltage has been buffered directly to the switch with a 47 μF capacitance, which has also filtering purposes. Moreover, a 5.6 kΩ, 2 W antiinductive resistance has been added to limit the drain current, and has been dimensioned by considering a high voltage value in excess of 2 kV on the maximum voltage that the switches can hold, and low switching frequencies up to about 100 Hz. Finally, the circuit has been tested for its voltage strength by slowly increasing the operating voltage until it reached its rated value, so that the switching performance could be verified. The load consists of a commercial electroporation cuvette filled with the medium to be exposed to nsPEFs. Electroporation cuvettes are widely used to study the biological in vitro effects of nsPEFs, and allow the exposure of relatively large volume samples, suitable for post-pulse biological investigations (e.g. flow-cytometry). They consist of two planar, stainless-steel electrodes and are available on the market in three main configurations which differ for the gap distance between the electrodes and for the electrode’s surface. In particular, the following characteristics can be found: 0.1 cm gap, 1 cm2 surface; 0.2 cm gap, 2 cm2 surface; 0.4 cm gap, 2 cm2 surface [12]. The equivalent electrical impedance, ZL, of the cuvette containing the liquid medium can be estimated by the second Ohm’s Law: (1)

electroporation cuvettes. In this work, microstrip transmission lines have been chosen since they can be easily designed in such a way to exhibit a particular characteristic impedance, and can be realized with a lowcost and fast procedure, as described in the following section. 2.2 MICROSTRIP TRANSMISSION LINES A microstrip line is a type of planar transmission line consisting of a conducting strip of width w placed on a dielectric substrate of thickness h and located on a ground plane. At low frequencies, typically below a few gigahertz, the fundamental propagation mode is a quasi-TEM mode (i.e. the propagating wave can be approximated as having electric and magnetic fields transverse to the direction of propagation) [17]. When designing the transmission lines for a Blumlein PFN, two are the main design parameters to be considered for each transmission line: 1) the characteristic impedance, and 2) the electrical time delay τ/2, where τ, in the doubleswitch Blumlein, represents the longest possible pulse duration. Treating the propagation mode as quasi-TEM yields approximate results for the characteristic impedance and propagation constant of the microstrip-line. A closed-form, approximate expression for the quasi-static characteristic impedance (Z0) of a microstrip line is [18]:

ln

where d (cm) is the gap distance between the electrodes, A (cm2) is the electrode area covered by the liquid, and ρ (Ωcm) its resistivity, which is defined as: (2) σ, ε0 and εr being the electrical conductivity (S/m) of the medium, the free-space permittivity and the relative permittivity of the solution, respectively. For frequencies up to several tens of MHz, the medium can be considered as mainly conductive, and the equivalent impedance of the load mainly resistive [16]. The resistivity of buffer solutions usually employed for cell cultures (PBS, HBSS) is assumed equal to 100 Ωcm, thus giving equivalent impedances, according to the (1), of 10 Ω for the 0.1 and 0.2 cm gap-electroporation cuvettes (the surfaces being 1 cm2 and 2 cm2, respectively), and of 20 Ω for the 0.4 cm gap-electroporation cuvette [12]. Pulsed electric fields are also applied to non-physiological buffers (sucrose) and to liquid foods, which present lower conductivity values [9], and give rise to much higher load resistance of the filled cuvette (hundreds of Ohms). The Blumlein PFN concept requires the load impedance to be as large as twice the characteristic impedance of each transmission line. Coaxial cables, which have a characteristic impedance of 50 or 75 Ω, are most frequently adopted to set up Blumlein-type pulse generators, but are not flexible enough to guarantee this matching condition for all the type of liquid media to be exposed to nsPEFs in

.

1 .

1

.

(3)

with: 1 2

1 2

1 1

4

12

where w is the width of the strips, h is the thickness of the dielectric substrate, and εr is the relative permittivity of the dielectric substrate. Thus, the parameters which must be considered in order to obtain a specific Z0, are the width of the track, the material and the thickness of the dielectric layer. The design formula is [18]: 8

2

2

1

ln 2

1

1

2

2

where: 1 60

2

1 0.23 1

0.11

6

5


377 2 √ ln

1


7 0.61

0.39

8

To set the length (L) of each line, the following design formulas have been applied: 9

2

2

√

2

10

where vf is the phase velocity, c is the speed of light in vacuum (3x108 m/s) and εeff, given by the (4), can be approximated to the εr of the dielectric material since, as shown in the following, the ratio h/w is very small. The parameter τ/2 is the electrical delay introduced by each line. The microstrip-lines have been realized by employing commercial PCBs, which are commonly available with different thicknesses of the dielectric layer: 1.6 mm, 0.8 mm or 0.4 mm. The dielectric layer is usually made by Teflon or FR4, the former having a relative permittivity of 2.2, the latter of 4.8 (as specified by the manufacturer, CIF, France). Thus, two out of three parameters (h and εr) are already set, and the width of the tracks, w, can be adjusted to fix the characteristic impedance of the line. Accordingly, three microstrip-lines have been realized, to be matched to 1) a low impedance load (10 Ω) made by a biological medium (σ = 1 S/m) placed inside a 0.1 or 0.2 cm electroporation cuvette; 2) an intermediate impedance load (50 Ω) [16]; 3) a high impedance load (133 Ω) made by a low conductivity medium (σ = 0.15 S/m) placed inside a 0.4 cm electroporation cuvette. Assuming to use FR4-based PCBs, and by applying the (5), (6), (7) and (8), to set the strips and the dielectric dimensions, and the (10) for the strip length, the design parameters have been set as in Table 1. Table 1. Design parameters for the microstrip (MS)-lines. Indices 1, 2 and 3 refer to the low, intermediate and high-impedance-matched lines, respectively. h and w are the thickness of the dielectric layer and the width of the strips, respectively. τ is the maximum pulse length achievable with the MS-line. MS-line

h [mm]

w [mm]

τ [ns]

1

0.4

11.4

40

2

1.6

11.4

40

3

1.6

1.75

100

To keep the dimensions of the whole system as much compact as possible, meander shaped conducting strips have been considered.

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The design of the microstrip lines has been performed by means of the CST Microwave Studio (Darmstadt, Germany) software, which is based on the Finite Integration technique. The structure has been modeled by designing a ground plane made by PEC (Perfect Electric Conductor), a dielectric layer h mm thick modeled as FR4 (εr = 4.8), and two meander-shaped conducting strips. The switches have been simulated by using two discrete voltage ports connecting each strip conductor to the ground plane, and two activation signals have been defined as trapezoidal pulses with the desired delay time between the first and the second signal [15]. The load has been modeled as a lumped element which is placed at the structure center, series connected to the metallic strips. 2.3 FIBER OPTIC LINK The fiber-optic link has been realized by employing: 1) two high-speed, fiber optic transmitters (TX) (HFBR 1414Z, Agilent Technologies) to be interfaced to the USBpulse100 low voltage generators; 2) a 2 m long optical fiber patch-cord (62.5/125 μm); 3) two fiber optic receivers (RX) (HFBR 2402Z, Agilent Technologies), which provide TTL compatible output voltage levels to the HTS-UF switches. The circuit configuration suggested by the manufacturer was considered to set up the optical link. The transmission and reception circuits have been designed by means of a dedicated PCB design tool, and then realized on standard PCBs by means of a circuit board plotter.

3 RESULTS AND DISCUSSION 3.1 MICROSTRIP LINE The results of the simulation allow to analyze the charging and discharging dynamics of the Blumlein network, to check for the load-matching conditions, and to adjust the dimensions of the conducting strips towards the desired performance. The minimum distance between parallel conductors in the same meander line was set in such a way that it never exceeded the width of the line itself. This choice allows to maximize the electric length of the line (τ), and, at the same time, to avoid air breakdown problems or crosstalk at the interface. Figure 2 shows the obtained results where the model of the conducting strips onto the dielectric layer (Figure 2a) and the electric field distribution in the microstrip-line, during the discharging phase of the line (Figure 2b), are presented. To realize the microstrip transmission lines, a cheap, homemade procedure has been employed. Briefly, the design of the conducting tracks has been transferred onto one of the copper planes of a double-side PCB, and both the conductive strips and the ground plane, have been covered by a protective tape. Then, the PCB has been completely immersed, for about 1 hour, in a solution containing ferric chloride (500 g/L), in order to achieve the etching. Then, after drying of the boards, the protective tape has been removed. Figure 3 reports the obtained microstrip-lines realized to match low (3a), intermediate (3b) and high (3c) impedance biological loads.

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Figure 2. Blumlein microstrip-line. (a) Perspective view of the top layer with the meander conductive strips and the load series connected in the middle. (b) Electric field distribution in the microstrip-line during the discharging phase of the network. The right side line corresponds to the first activated switch, while the discharging phase of the other line results delayed.

Figure 3. Microstrip-lines realized to match a (a) 10 Ω, (b) 50 Ω and (c) 133 Ω load. The maximum pulse length achievable is 40 ns with (a) and (b), and 100 ns with (c).

Figure 4. Microstrip line-based, double-switch Blumlein, high voltage, nanosecond electric pulse generation system



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3.2 PULSE GENERATOR ASSEMBLY The complete pulse generation system has been assembled as shown in Figure 4, where the command station, the double-channel optical link and the microstrip line Blumlein network are shown. The microstrip-lines are interchangeable within the structure to match the load to be exposed. Output pulses across the load have been measured by means of a Tektronix DPO 7104 1 GHz oscilloscope, in conjunction with a Tek P5100 high voltage probe. Representative ns pulses are reported in Figure 5, that have been achieved with the assembled pulse generator by using the intermediate (5a), high (5b) and 1ow (5c) impedance microstrip line, respectively. The charging voltage applied was of about 1 kV, 1.8 kV and 600 V, for signals in Figure (5a), (5b) and (5c), respectively. The capability of the system to generate pulses with variable pulse duration, polarity and amplitude is pointed out. As a matter of fact, a positive 10 ns, a positive 20 ns and a negative 60 ns pulse are reported from top downwards (pulse durations are considered as full width at half maximum, FWHM). It has to be pointed out that the system is able to provide pulses as short as 10 ns, which is particularly interesting towards the possibility of exploring the biological effects of shorter and shorter, intense electric pulses. Overall, the obtained results are satisfactory, although the output pulses across the load suffer from some residual oscillations, which can likely be ascribed to a non-perfect matching between the load and the transmission lines. As a matter of fact, the matching is affected, on one hand, by the tolerances in the homemade fabrications of the meander striplines (imperfect symmetry between the two lines, approximations in the analytical estimation of the characteristic impedance), and, on the other hand, by the uncertainties on the determination of the load impedance, which consists of electroporation cuvettes filled with liquid media with specific resistivity values. Moreover, the interface connections to the microstrip-lines are particularly critical and must be handled with care in order to reduce pulse distortions and efficiency losses.

4 CONCLUSIONS The design and realization of a high voltage, nanosecond pulse generator based on a double-switch Blumlein architecture, has been presented in this work. The system exhibits high flexibility in terms of exposure conditions (variable pulse duration, polarity, amplitude and repetition rate). Moreover, by using microstrip transmission lines, that can be realized with the desired characteristics through a lowcost procedure, different types of biological loads, consisting of liquid media with different electromagnetic properties (cell cultures, liquid foods, non-physiological buffers) can be exposed to ns pulses to carry out biological investigations. In conclusion, it represents an useful tool to perform in vitro experiments, in the framework of mechanistic studies and applications development of nsPEFs.

Figure 5. High voltage, nanosecond electric pulses with variable duration, amplitude and polarity generated by means of a microstrip line-based, double switch Blumlein PFN, with interchangeable transmission lines for low (a), intermediate (b) and (c) high impedance loads. Pulse rise time is about 6 ns, 8 ns and 25 ns, respectively

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ACKNOWLEDGMENT The authors are thankful to Professor Rita Massa, from the National Institute of Nuclear Physics (INFN) Section of Naples, for her suggestions and comments to the work, and to Dr. Gianfranco Palmese and Dr. Luca Ciofaniello from CORISTA (Consortium for Research on Advanced Remote Sensing Systems, Naples, Italy) for their support in the design and realization of the PCB circuits.

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Stefania Romeo received the Laurea degree (summa cum laude) in Biomedical Engineering from the University of Naples Federico II, and the PhD in Electronic Engineering from the Second University of Naples in 2008 and 2012, respectively. Since June 2012, she has been with IREA-CNR (Napoli) as Research Fellow. She was visiting student at the University of Southern California, Department of Electrical Engineering and Electrophysics, from September 2010 to March 2011. The research activity of Dr Romeo is in the framework of Bioelectromagnetics, spanning from the design and realization of high voltage, nanosecond pulse generators for in vitro biological applications, to the employment of numerical and experimental dosimetry techniques for in vitro exposures to RF electromagnetic fields.

Claudio D'Avino was born in Naples, Italy, in 1989. He received the Master Degree in Biomedical Engineering (summa cum laude) from the University of Naples Federico II in 2012. Since 2012, he is a scholarship holder within the training project for Embedded Software Engineers, IESWECAN in cooperation with the University of Naples Federico II and Fiat Group Automobiles SpA. His main research interests are related to in vitro dosimetry and high voltage nanosecond pulse generation for biological applications.

Olga Zeni received the Laurea Degree in Biology from the University of Naples, and the PhD in Zootechnical Science from the University of Bologna, in 1990 and 1996, respectively. Since March 2001 she has been with CNR-IREA, Naples, Italy, as Research Scientist. The research activity of Dr Olga Zeni, in the framework of the study on biological effects induced by low and high-frequency electromagnetic fields, mainly deals with the evaluation of cellular parameters related to carcinogenesis (cell viability, proliferation and cell cycle, apoptosis, oxidative stress, DNA molecule integrity) in mammalian cell cultures following electromagnetic field exposures to and co-exposures with environmental pollutants. More recently, she has also been involved with the evaluation of the biological effects of high voltage, nanosecond electric pulses on mammalian cells, and with the evaluation of the cytotoxicity induced by multiwalled carbonnanotubes (buckypaper).

Luigi Zeni took his degree in Electronic Engineering, summa cum laude, from University of Naples in 1988 and his Ph.D. in Electronics and Computer Science, from Italian Ministry of University in 1992. He has been research assistant at the Department of Electronic Engineering of University of Naples "Federico II" and, from 1998 to 2006, he has been associate professor of Electronics at the Second University of Naples. Currently, he is full professor of electronics at the Second University of Naples and president of the Research Consortium on Advanced Remote Sensing Systems – CO.RI.S.T.A. He has been, from 2001 to 2012, vicedirector of the Department of Information Engineering. He worked at DIMES (Delft Institute of Microelectronics and Submicrontechnology) of Technical University of DELFT (The Netherlands) as a visiting scientist. The research interests of Prof. Zeni include the design and realization of optical fiber sensors for distributed measurements of deformation and temperature, the design and characterization of optoelectronic devices with particular emphasis on silicon optoelectronics, optoelectronic integrated sensors, biosensors, and optofluidics, the design and realization of pulse forming networks for bioelectric applications.