U n i v e r s i t e i t T w e n t e faculteit der elektrotechniek U n i v e r s i t y C o l l e g e L o n d o n Department of Medical Physics and Bioengineering D e s i g n o f a d e - m u l t i p l e x e r I C f o r L A R S I s t i m u l a t o r Master’s thesis report S&S-BME 98-3 A.J. Krabbendam committee Prof. dr. ir. P. Bergveld Dr. N. de N. Donaldson Dr. J. Holsheimer Dr. ir. R.F. Wassenaar Laboratory of Signals and Systems - BME Department of Electrical Engineering University of Twente period January – July 1998 printed London, 28 July 1998.
Transcript
Universiteit Twente
faculteit derelektrotechniek
University College LondonDepartment of Medical Physicsand Bioengineering
Design of a de-multiplexer ICfor LARSI stimulator
Master ’s thesis repor tS&S-BME 98-3
A.J. Krabbendam
committeeProf. dr. ir. P. BergveldDr. N. de N. DonaldsonDr. J. HolsheimerDr. ir. R.F. Wassenaar
Laboratory of Signals and Systems - BMEDepartment of Electrical EngineeringUniversity of Twente
periodJanuary – July 1998
printedLondon,28 July 1998.
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Preface
This report describes the work I did in the Implanted Devices Group of the Department ofMedical Physics and Bioengineering at the University College London, United Kingdom.The work is part of a project that continues until the end of November this year. The partdescribed in this report is meant as a MSc assignment on its own. It is the final assignmentfor the study Electrical Engineering to obtain the master’s degree from the Department ofElectrical Engineering of the University of Twente.
Last year a friend of mine also did his final assignment in the Implanted Devices Group.He was very enthusiastic about his time in London and told me about that. By occasion Ihad already seen a TV-documentary about the work here, just before I left for my PracticalTraining about 2 years ago, which was also in the field of Biomedical Engineering. It fedmy interest for the neurostimulation research more and the possibility to do a IC-designrelated to the my Biomedical interests was the ultimate challenge. I was glad to get incontact with Nick Donaldson of the Implanted Devices Group, who offered me theopportunity to do the work as a MSc assignment.
I would like to thank all the people who helped me with the work in London: Nick for allthe nice discussions about the project, Tim for his encouragement and suggestions forsmall electronic problems, John for his participation in the project, Martin, Lixia andAndreas for their accompany, help and advice. Also thanks to the Dutch members of myMSc graduation committee, Jan and Roel, who supported me via email and finally PietBergveld for his confidence in my change to this assignment.
Arjan Krabbendam.
London, 28 July 1998.
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Abstract
The Implanted Devices Group at the University College London is investigating methodsto restore functions of the leg of paraplegics by using implanted stimulators. The Lumbo-sacral Anterial Root Stimulator Implant is based on a 10-year old design that needs anextention to make the implant useful for control of bladder as well. Also crosstalk effectsbetween muscles that were noticed should be reduced in the to-be-designed de-multiplexerIC.
In an experiment done to investigate the background of the crosstalk effect it was shownthat current distribution over anodes that were connected together is almost uniform. Theseresults are the basis for the specifications of the Grouped Mode of the de-multiplexer IC,where several anodes are joined together.
The design implementation prevents improper use of the IC and has two groups of outputswith different presetable currents. The extra features are the changeable current setting,once the stimulator has been implanted and the feasibility to drive 14 channels with anodeswitching.
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Samenvatting
In de Implanted Devices Group van het University College London wordt onderzoekgedaan naar methoden om (gedeeltelijk) verlamde patiënten met gebruik vanimplanteerbare stimulatoren de beenspier-functies terug te geven. De "LumbosacralAnterial Root Stimulator Implant" stimulator is gebaseerd op een 10-jaar oud ontwerp, dateen uitbreiding behoeft om ook de blaasspier te stimuleren. De 'crosstalk' effecten die zijngeconstateerd tussen spieren onderling moeten worden verminderd in de nog te ontwerpende-multiplexer IC.
In een experiment dat werd gedaan om de achtergrond van bovengenoemde 'crosstalk'effecten te bestuderen kwam naar voren dat de stroomverdeling over meerdere anode's, dieonderling waren verbonden, uniform verdeeld was. Deze resultaten vormen de basis voorde specificaties van de nieuwe de-multiplexer IC wanneer deze in 'Grouped Mode' werkt.In die mode wordt de IC in een configuratie gebruikt waar de anode's onderling zijnverbonden.
Foutieve ingangsignalen worden door het ontwerp afgevangen en leiden niet totongewenste stimulatie. Verder zijn twee extra mogelijkheden in dit ontwerpgeïmplementeerd. In de eerste plaats kan de stroominstelling worden veranderd nadat destimulator is geimplanteerd. Daarnaast kunnen de 14 kanalen afzonderlijk wordenaangestuurd met anodeschakelaars.
4.1 Literature 494.2 Crosstalk experiment results 51
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1. Introduction
The Implanted Devices Group at the University College London is working on systems torestore functions to legs of paraplegics. With Functional Electrical Stimulation (FES)muscle contraction is induced by electrical current pulses that are applied to either thenerve (nerve stimulation) or surface (surface stimulation). The contraction strengthdepends on the current amplitude and duration [11]. Once a nerve has been activated byFES the action potentials propagates in exactly the same way to the muscles, as if theywere originated from the central nervous system. However the number of nerves that canbe stimulated by FES is limited by the stimulator system.
To restore leg functions the Lumbosacral Anterial Root Stimulator Implant (LARSI)project started in 1992. The LARSI project uses the Mark-5 stimulator that was designedin the mid-80’s, while many aspects of the electrode design: cable type, surgical approachand implantation technique are based on the knowledge collected from the Sacral AnterialRoot Stimulator Implant (SARSI) that is used to control the bladder and was firstimplanted in 1976 [18], [2]. At present 2 English patients have received the LARSIimplant. The LARSI project is part of the NEUROS program in which several Europeanuniversities work together to obtain a system for paraplegics to restore functions to theirlegs: particularly standing. Sensors for movement and muscles activity development,stimulation research, as well as control development is part of this program.
Stimulating the nerve roots in the cauda equina instead of previously used methods ofstimulating nerves close to the muscles, has the advantages that the cables are shorter,because the stimulator itself is relatively close to the spinal cord. This reduces the risk offailures. Also the amount of surgery time, which was more than a day, has been reduced toabout eight hours [18]. However the disadvantage is over the muscles less specific control,since there is a non-linear mapping between the stimulation intensities and the resultingcontractions [14].
In a short EMG experiment done in 1996, single channels were switched whose electrodeswere in one 'book' (electrode mount for 3 nerve roots). M-wave and the thresholds wererecorded for the epselateral and contralateral legs. Rather low crosstalk thresholds werefound. This crosstalk effect could be at one of the following locations:
• in the stimulator;• between the electrodes;• between the nerves.
Before investigating the possible changes to the stimulator, it is first interesting tounderstand more about the results obtained from that EMG experiment, so an in vitroinvestigation was performed. The possibility of nerve roots innervating the contralaterallegs is neglected because the nerves lead to different legs.
The LARSI system has a control box and transmitter outside the body and a stimulator /receiver implanted. In a very global view the control box generates a stimulation pattern,from information on its inputs or memory, the transmitter sends this data (and power) tothe implant where the data is demodulated, decoded and de-multiplexed. This very last
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stage needs now to be replaced in order to be able to control 14 instead of 12 channels: twochannels more for bladder control. Together with this extension some other improvementswill be made, to avoid crosstalk (as described above) and to prevent improperdemultiplexing, for example when there is an error in the control-box program.
The description and results of the “crosstalk experiment” is written in chapter 4, after ashort introduction to the theory of FES and the background of the LARSI project inchapter 3. First the “ research definition” of this assignment will be given in the nextchapter. In chapter 5 the chipdesign specification is discussed, while in chapter 6 theimplementation is described. Finally in the last 2 chapters a discussion about the completework written in this report is discussed and the conclusions are drawn.
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2. Research definition
In the LARSI project a stimulator is used that was designed about 10 years ago and uses acommercially-available chip to de-multiplex the time-multiplexed signals to 12 channels(anodes and cathodes). There is need for a new chip that meets the following requirements:
• Each nerve should be stimulated by an anode-cathode pair of electrodes that are bothswitched to avoid current to flow to other electrodes;
• Stimulation pulse amplitudes can be independently set for groups of outputs (i.e.Lumbar or Sacral);
• The switching logic should be modified to reduce the risk of improper operation.
While independent switches for every anode and cathode will perform that there is no’current spread’ to adjacent nerve roots, this requires more wires than only cathodeswitching like is done at the moment. To investigate the possibilities to avoid crosstalk anadditional experiment will be done. This so-called “crosstalk experiment” is to measure thecurrent distribution over several electrodes, based on the existing stimulator design. Theresults from this experiment can help in achieving specifications for the chip, but also tofeed ideas about improper operation and grouping of channels with the same amplitude.
Based on the “crosstalk experiment” results and theory, the final specifications for the chipwill be made. Finally the drafts of the chip design are presented, with a description of therequired behaviour of the different parts of the chip.
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3. Theory and background
3.1 Introduction
3.1.1 Neurophysiology
The human nervous system comprises of the Central Nervous System (CNS) and thePeripheral Nervous System (PNS). The CNS consists of the brain and the spinal cord andhas the purpose of control, while the PNS innervates muscles (’motor action’) and collectssensory information for the CNS. The motor neurons (neurons that are responsible formuscle activation) have their cell bodies in the CNS with their axons terminating at themuscle. The information carried by axons is ’encoded’ as a train of electrical pulses (actionpotentials). Action potentials propagate along an axon by movement of charge. Initiallythere is no action potential; the interior resting potential compared with exterior is about70 mV negative. The resting situation can be changed by rising the interial potential to aless negative value of -50 mV. The interion ion concentration will now change quickly,because of the membrane ion permeability is voltage-dependent, and the membranepotential will rise to a slightly positive value. This is called firing or depolarization. If anaxon is seen as a chain of small segments, the positive charge inside at the firing segmentmoves to the adjacent segment, as the positive charge outside moves from the adjacentsegment to the firing segment (see Fig. 1). The charge flow in the adjacent segment willdepolarize this segment and so the action potential propagates.
- - - -
- - - - + +
+ +
- - - -
- - - -
- -+ + + + + + + +
- -+ + + + + + + +
Active segmentCurrent flow
Fig. 1: Charge flow in axon causing depolarisation in adjacent segments.
3.1.2 Neuro-stimulation
Depolarization can also be induced by a current density present close to the nerves, whichchanges the resting potential and launches an action potential. This is called neuro-stimulation and can be achieved by using two electrodes, an anode and a cathode. At the
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cathode site the current is forced to flows out from the axon and causes depolarization. Aconstant current (DC) applied to the membrane does not induce stimulation, because themembrane rest potential accommodates to it. It must however be avoided in stimulationsystems, because the harmful electrolytic effects at the electrodes like acid and chlorinegeneration and metal corrosion.
3.1.3 Strength-duration curve
Plotting the amplitude, which is just sufficient to activate the nerve(fibre) of a rectangularstimulus pulse versus its duration generates the strength-duration curve, which providesinformation about the probability of stimulation. The curve can be described by:
)1(d
crI +⋅=
where I (current amplitude) and d (pulse duration) describe the Lapicque1 curve. The’rheobase’ (r) defines the current amplitude that for long pulses just not evokes an actionpotential (stimulation). The pulse duration where twice the ’rheobase’ value is necessaryfor causing stimulation is defined by the ’chronaxie’ (c) [11]. The product of currentamplitude and pulse duration that causes stimulation is constant.When the energy function, derived from the pulse-duration function, is minimised thelowest energy can be found. The prevent tissue from heating a low energy is preferred. Thelowest energy applied to the tissue can be found to be at the pulse duration equal to’chronaxie’.
3.1.4 Nerve distance to electrod es
Several models have been presented in the literature to describe the electrical behaviour ofnerve membranes and the propagation of action potentials. Also separate models for thecurrent distribution from a unipolar electrode for different geometries can be made.However it is interesting for this project to know what the threshold will be for thestimulation of S3 and S4, the two channels that are going to be extended to the existingstimulator system to handle bladder contractions.The size of axons and the threshold value has been related: smaller neurons have higherthresholds [17], [15]. Another impact on the threshold value seen from the electrode is theamount of tissue that grow between the electrode and the nerve fibre, like shown in Fig. 2.
1 L. Lapicque published about the relationship between amplitude and pulse duration versus stimulationeffects, in the article "Definition experimentale de l’excitabilite", in 1909.
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electrode
distance to small fiber distance to larger fiber
small fiber larger fiber
Silicone base (‘book’ )
alternative locationsmall fiber
Fig. 2: Different distance between electrode and fibre, for small and large fibers.
When the nerve is smaller more tissue can grow, which will reduce the current (density),because tissue has a lower conductivity [13]. So the current that has to be applied to theelectrodes needs to be larger. As Sacral roots are smaller than Lumbar roots, they requirehigher stimulation intensities to be activated.
3.2 LARSI project
3.2.1 Implanted systems
Neuro-stimulation, as described in 3.1.2, is based on a pair of electrodes close to thenerves. It is also possible to stimulate muscles through the skin, by using surfaceelectrodes. The advantage of using surface stimulation is that it does not imply surgery.However it requires a higher current than for neuro-stimulation and care for the skin toprevent allergic reactions and pain. Also the stimulation effects change with motion, as theskin and consequently the electrodes move relative to the underlaying muscles.Implanted electrodes can be divided over 3 groups: percutaneous intramuscular electrodesrequire minimally invasive surgery for implantation. However they are susceptible tobreakage and demand permanent care of the site where they penetrate the skin. Nerve-cuffor other nerve-root electrodes needs a surgical procedure to insert them round peripheralmotor nerves. And the third group is the epimysial electrodes (attached to the musclesurface, near motor point), which needs less dissection, but may involve an extensivesurgery for multi channel systems. The use of implanted cable is necessary, where forlower limb breakdown of the cable is likely [18], [19].
3.2.2 Considerations for LARSI system
Stimulators for bladder control, the so-called SARSI stimulator systems2, have beenimplanted in patients since 1976, the 500th implant was delivered in 1992. The electrodedesign, cable type and surgical approach now used in LARSI were based on the experiencewith it in the SARSI project. The electrode used is slightly different from nerve-cuffelectrodes: tripolar electrodes made from wire are used. They stimulate only from one sideof the nerve (instead of around it). The electrodes are placed intradurally, which requires asurgical approach to do a laminectomy of L2-L4. 2 SARSI stands for Sacral Anterior Root Stimulator Implant and was developed by G.S. Brindley et al.
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3.3 Electrode representation
Electrical stimulation of spinal roots using implanted electrodes involves electro-chemicalbehaviour. Stimulation effects with metal, placed in the Cerebrospinal Fluid (CSF) may bemore easily understood by using a model to describe the interaction between the electrodeand CSF. In the representation that is given in Fig. 3, the nonlinear behaviour due toelectrolysis is neglected, on the assumption that excessive charge is not passed through theelectrodes.
Warburg impedance
Ra
C
Cd Rd
Rleak
Electrolyte (‘access’ ) resistivity
Fig. 3: Electrode representation, with the gas generation effects omitted.
This is only the representation for one electrode, in a real system Ra is shared with otherelectrode(s). In the figure above, Ra denotes the ’access resistance’, which is due to theresistivity of the electrolyte. The capacitance C is caused by the ’ion layer’ positioned closeto the electrode surface. Rleak is the leaky resistance in parallel to this electrodecapacitance, however it has been connected in series with the so-called ’Warburgimpedance’. This impedance was introduced to the network to represent the influence ofthe diffusion rate of charged ions attracted to the electrode surface.In an actual situation of two (plate) electrodes opposite to each other there could also bedirect capacitance behaviour, as occurs between two well conducting surfaces.
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4. Crosstalk effects between pseudo-tr ipolar electrodes
4.1 Introduction
Each root is stimulated via two or more electrodes. Considered as a circuit diagram, it issupposed that there is one anode and one cathode per channel to stimulate each nerve root.The way the electrodes are connected to the stimulating current source differs for differentapplications: it depends on the number and type of electrodes used and therefore thenumber of cables required to connect those electrodes. Also the available space in thespinal canal to lead the cables from the stimulator implant to the electrodes is a limitation.Further considerations are stimulation patterns (e.g. ’simple’ rectangular, or with anodeblocking, and so on) applied to the electrodes and the area in the implant where the cablescan be mounted for termination.Basically there are two options for connecting when only one current source is used for allchannels, as shown in Fig. 4. One possibility is to connect only the cathodes separately tothe stimulator and join the anodes together, which needs in total n+1 wires (with nchannels). An alternative is to connect each anode and each cathode separately to thestimulator, which requires 2n wires.
Fig. 4: Left: ’common anode’ configuration. Right: ’isolated channel’ configuration. The grey lines representthe cables from the electrodes to the stimulator.
For the Lumbosacral Anterial Root Stimulation Implant (LARSI) system the ’commonanode’ configuration was used. In a short experiment done in 1996, in which singlechannels were stimulated, the threshold of muscle activity and the muscle M-wave activitywas recorded. The location of each root nerve within the electrode mount (’books’) hadbeen noted during surgery so it was possible to compare the threshold for muscles on thesame side as the stimulated root on the contralateral side. Assuming that neurons do notcross the midline of the body, this crosstalk must be due to ’current spread’ from the activeelectrode to nerve fibres near to, but not in, the slot of the active electrode. The adjacentnerve to the one that was stimulated indeed led to right leg muscles. Therefor the amount
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of crosstalk that was measured in muscle contraction could directly be related to acrosstalk effect between the two channels in effect.Because the nerve roots lay in the spinal cord fluid (CSF) there is a good conductingmedium between the electrodes of the channels. Crosstalk based on this Ohmic path couldoccur. This arises the question: what is the actual current distribution over the anodes andcould this cause stimulation and so muscle contraction. To answer this question alaboratory experiment was done, using saline solution and the same kind of electrodes thatare used in the LARSI implant. The results of these experiments are reported in thischapter.
4.2 Electrode-books
The stimulation system used in the LARSI project employs ’books’ as illustrated in Fig. 5.The books are made of silicon rubber and have a base of 9 by 9 mm and a height of 3 mm.Each contains of three slots, 2 mm wide. The electrodes in each slot (two anodes and onecathode) are made of PtIr (with a diameter of 0.1 mm). The de-insulated electrode wire issewn through the silicon rubberbase, with the bottom of the book coated by silicone rubberadhesive ("Silastic 734 RTV"). In the slot, a length of about 3 mm of electrode wire isexposed. Since this sewing is done by hand under a microscope, there is a substantialspread in the surface area of the electrodes, due to variation in the length of exposedelectrode wire. Under a light microscope a variation up to a factor 4 has been observed.
Fig. 5: Schematic view book, with three slots, each containing two anodes that are connected together and acathode in the middle.
4.3 Electrode 'access resistance’
Assuming a cylindrical electrical field close to any electrode in the books described in theprevious paragraph, the ’access resistance’ (the real part of the impedance) can be describedby:
)ln(0r
r
lR
πρ ⋅=
When this ’access resistance’ close to the electrode is relatively high compared to therestivity in the electrolyte elsewhere, it will determine the current distribution. Since the’access resistance’ is linear related to the surface area of the electrode, as seen in the aboveformula, the variation in area will determine the current spread.An electrode representation has been given in the previous chapter, which contains acapacitator in series with the ’access resistance’ and a ’Warburg impedance’ in parallel. The
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use of relative high frequency pulses in experiments to obtain the ’access resistance’ only,will neglect the influence of these capacitance [6].An experiment was performed to investigate the relative differences between electrodesurface, where the current through the individual electrodes to a large reference electrodewas measured. The reference electrode was made of 50 cm 80/20-PtIr wire with diameterof 0.1 mm. A square wave signal with a frequency of 100 kHz was applied to either apotentiometer or the ’electrode-reference electrode’ configuration (see Fig. 6). Thepotentiometer was adjusted to a value that gave an equal voltage drop (current change)measured through the small resistor for the ’electrode-reference electrode’ circuit. Hencethe drop was measured, the electrode capacitance was eliminated, because the transientcurrent through an impedance initially only meets the resistance, the capacitor acts as ashort. So only the required ’access resistance’ was obtained by measuring the potentiometerresistance value with an Ohm-meter. The frequency used was low enough not to beinfluenced by the ‘direct capacitance’ between the electrodes.
10 Ω
Fig. 6: Access resistance measurement set-up to measure the ’access resistance’ of an electrode, by using acomparison with a potentiometer to eliminate capacitor influence.
To compare the measured 'access resistance' of an electrode with respect to others and thusestimate relative surface area, all resistances are converted into conductance. Theconductance of each electrode relative to the sum of conductances of all electrodes inparallel is visualised in Fig. 7.
Fig. 7: Relative ’access conductance’ for the electrodes that are used in the crosstalk experiment.
The observed difference in electrode area, by a factor 4 can also can be deduced from theseresults, because the minimum conductivity is 2% of the total and the maximum is 9%; arange of 4.5:1.
4.4 Measurement Board Design
The measurement board used in this experiment was designed to simulate the Mark-5implant3 circuit. The same component values were used and the stimulating signal had apulsewidth of 100µs, with an interval of 20ms, which both is typical. The supply voltagewas set to 15 V and this gave a current of about 5mA (depends on electrode impedance).To be able to measure the current through all the electrodes separately, a differentialamplifier was employed to amplify the voltage across the series resistor in each anodebranch, as well as in the two cathode branches. The common voltage compared to thedifferential voltage fed in the differential amplifier was high; consequently the common-mode-rejection-ratio (CMRR) needed to be high. The preferred value for the CMRR is100 dB or more (at 10 kHz). The used amplifier (matched pair transistor design) performeda CMRR of 105 dB, with a gain of 31. This was sufficient for the measurements.
The set-up for this crosstalk experiment is shown in Fig. 8. From the 15 V power rail thecurrent flows first through a 2
!#"$#% & ' and the potentiometers that represents the cable resistance towards the anodes. During'stimulation' of one of the two cathodes the corresponding transistor is switched on. The
3 The Mark-5 stimulator designed by N. Donaldson is currently used in the LARSI system
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impedance between an anode and a cathode through the electrolyte is several hundreds ofohms, is less than the connected ‘discharge’ resistors of 2.5 ( )*,+-/.01 2(3546728982 3;:1 <passing 5 mA is large enough, compared with the stimulation pulse of 100 µs, to act as ashort.
2 kΩ
2.5 kΩ
15V
0V
3.3µF
2.5 kΩ
3.3µF
10Ω10Ω
10Ω
18 anodes
-
+
Fig. 8: Crosstalk experiment system set-up: 18 anodes connected via cable resistance to the power supplyand two anodes switched to ground via a cable resistance and a blocking capacitor. The 2.5 = >?A@B @CEDF>G@IH>?used for discharging the electrodes after stimulation.
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4.5 Exper imental Set-up
A typical value for CSF’s conductivity in man is 15 mS/cm, but tissue has a conductivityof 1.6 mS/cm [8], [13]. For the experiment physiological saline (0.9 mass percent) waschosen, which has conductivity close to that of CSF. To investigate the influence of growntissue around the books placed in the spinal cord, saline with a 10 times lowerconcentration was used in another experiment The conductivity of 0.9 and 0.09% Salinewas measured with a conductivity meter4 to be 15.0 and 1.5 mS/cm respectively. Tosimulate the conditions in the spinal cord, 3 books were placed in a tank filled with 3.5litre of saline, at body temperature of 37JKMLWith three books you can achieve all the current distribution patterns that can happen inthe spinal cord using similar books. The books have three slots, with anodes, like thoseused in the LARSI system (same size, same dimensions, described in section 4.2). Onebook has also cathodes: in the left and middle slot. With the arrangements that are summedin Table 1, the different distributions for stimulating one of the 12 channels in the LARSIsystem, which should be simulated can be found. The book with the two cathodes is calledthe active book. The distance between the books was from 5 up to 29 mm. In the actualsituation, seen on a photo made during the operation, the books are placed about one booklength apart (the length of a book equals 9 mm).
Order books Stimulating cathodeActive book middle of row Left slot of active bookActive book middle of row Middle slot of active bookActive book end of row Left slot of active bookActive book end of row Middle slot of active book
Table 1: experiment stimulation arrangements.
4.6 Results
The results of one of the 4 arrangements (Table 1) are shown in Fig. 9. The bars representthe relative current through each anode and give an indication about the distribution of thecurrent. The active cathode in this figure was placed in the middle book, first slot. Thebooks with their slots are drawn under the bars to illustrate the arrangement. The grey linesindicate the electrodes (anodes), 18 in total, and the active cathode. Possible currents thatflows from two anodes to the cathode has been drawn as a dashed arrow.
4 Used conductivity meter: Jencons 4070 with probe PCM101. Total accuracy of 0.01mS.
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anodes
activecathode
I (µA)
iA11
iC11
A B C
500
i A11
i C11
250
0
Fig. 9: Typical distribution of anode currents.
The results show an almost uniform current distribution for all anodes. When averagingthe results of all experiments, the currents through the anodes in the slots adjacent to theslot with the active cathode are higher than those of the other anodes. In the experimentswith 0.9% Saline the relative currents are in average 5.2% for the ’far-away’ slots and 7.5%of the total stimulation current for the currents in the adjacent slots. These numbers are 4.7and 8.0% respectively for the experiments with 0.09% Saline. The current distribution didnot correlate with the variation in ‘access resistance’ (conductivity) as was measured in theexperiment described in section 4.3 when all anodes are taken in account. The correlationcoefficients in all 8 experiments were between -0.5 and 0.5. However when only thecurrent measurements of the anodes in the non-active books (A and C) are compared withthe ‘access resistance’ (conductance values) for those anodes, a correlation coefficient of0.80 is found, for detailed results see appendix 9.2. It was noticed that the non-activecathode during stimulation acted as an anode, which is not surprising in retrospect, whenthe potentials in the circuit diagram are calculated.
4.7 Discussion and conclusions
In a ‘common anode’ configuration, as used in the Mark-5 stimulator arrangement in theLARSI project, the current that flows to the switched (‘active’ ) cathode comes from all theanodes with an almost uniform distribution. The distribution of the current that flowsthrough the anodes not adjacent to the slot with the active cathode is well correlated to the'access conductance', the reciprocal of the measured 'access resistance'. The correlationcoefficient is slightly less in the experiments with 0.09% Saline, which indicates that the
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’access resistance’ is less dominating for the current. The increased restitivity of theelectrolyte is an explanation for this. The variation in electrode surface area and thus’access resistance’ will determine the distribution less.In the slot with the active cathode and its adjacent slot(s) the current distribution is notdetermined by the variation in ’access resistance’, since the correlation coefficient is lessthan 0.5 (absolute). This leads to the conclusion that the current distribution close to thecathode is defined differently. This can be the local path distance between cathode andanodes and so the local field. Unfortunately it is not possible to measure that difference indistance accurately to verify this assumption.The fact that the unused cathode acts as an anode should be considered when differentcurrent settings are in use and the discharge resistor is not switched.
4.8 Impact on chip design
Based on the results obtained from this crosstalk experiment some ideas about the chipdesign can be launched. The use of a ‘common –switched- anode’ configuration impliesdirectly that the cathode current comes from all the common switched anodes. This is not aproblem when the cathodes are all set to the same current source, since the currentamplitude at the anodes will be averaged over the number of anodes and so will not causeunintended stimulation.
However, when a higher current setting is needed for some channels (anode/cathode pairs),there is more risk of crosstalk and the anodes should not be joined together. Especiallywhen one current setting is much higher than the other, the channels with the lower currentsetting may be stimulated. This is because a fraction of the currents required for nerveroots with higher thresholds could flow along nearby nerves with a lower threshold andactivate this one.
Because local field effects that may cause problems are still not well understood, in thenew design care should be taken how to avoid crosstalk when nerves with a differentthreshold are placed in adjacent slots, i.e. the same book. At least in that case completeanode switching should be used.
Finally since the new design will have channels meant for nerves with a differentthreshold, two Sacral nerve roots will be added for bladder control, the discharge systemshould be reconsidered. This is because in a design with fixed passive discharge anyunused cathode starts acting as an anode, so you may call the cathodes pseudo-anodes,which have the same crosstalk issues as normal anodes.
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5. Design specification
5.1 Introduction
The design is made to only replace a small part of the stimulator system so there arerestrictions to keep it compatible with the currently used commercial UCN5832multiplexer IC. To give a better understanding of the given specifications later on, therefollows first a description of the working of the multiplexer chip in the LARSI stimulator.This stimulator has 12 channels: 4 books, with each 3 slots that contain two anodes andone cathode. Those channels are stimulated in sequence (see Fig. 10). The stimulation ofone channel is done by biasing the BJT that connects instantly the cathode -via a capacitor-to ground. The AC behaviour is not important here, but the way the digital part operates is.
The so called multiplexer can actually best be seen as a (serial) shift register (SR) with anoutput_enable and clock signal connected to all 12 cells with the data output of one cellbeing the input for the next cell. The first SR-cell clocks in the data signal, that is highduring rising clock edge (see timing in Fig. 10) and would make the output of thisparticular cell high, however this is controlled by the output_enable signal that thereforecontrols the duration of the output pulse (which controls the BJT) and so the stimulationduration. At the next rising clock edge, the data line for the first cell is again low, but nowis high for the second cell that reads in the data from the first cell. Again theoutput_enable signal controls the stimulation duration, but now for the stimulation in thesecond channel and so on. So the shift-register design makes the system stimulate insequence, which is demultiplexing the output_enable signal to the correspondingchannels.
The three input signals are generated by another IC, from the received data signaltransmitted to the implant. This IC is not going to be replaced and it is also preferable notto make extraordinary changes in the program that runs in the control box. This impliesthat the new design should be able to operate from the data on the three data lines in asimilar way to the existing commercial chip.
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Data
Enable
Clock
2 31
12 cathodes
ClockData
Enable
15V
0V
12 anodegroups
1 2
Output ch.1
Output ch.2
Fig. 10: Mark-5 Stimulator: a shiftregister acting as a de-multiplexer for the Enable signal to theappropriate channel.
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5.2 New design
5.2.1 Extension to 14 channels
In the Mark-5 stimulator all 12 channels have anodes that are connected together and arefed by the same 5 mA current. The new design should be able to stimulate 14 channels intotal, because also the bladder will be controlled by stimulating the S3 and S4 nerve roots.These roots have a higher threshold, so a different current amplitude setting is necessaryfor these 2 channels. Actually also S2 (left and right) needs a higher current setting basedon the experience with the two LARSI patients at the moment. From now on the roots L2till S1R will called “Lumbar” and S2, S3 and S4 “Sacral” to distinguish the 2 groups withdifferent current settings. From the results of the crosstalk experiment, it is now knownthat the cathode current is approximately uniformly spread over the commonly-connectedanodes. So if all the anodes were joined together, and one of the two new channels isstimulated, this cathode current that has higher amplitude may cause anode stimulation inchannels that were not meant to stimulate. To avoid this risk, anode switching would be asolution, however any extra anode switch in the chip has a penalty that an extra wire fromthe stimulator to the electrodes is needed. Also there is little space available on thesubstrate to terminate the cables. It is desirable not to use more cables than are necessary toavoid crosstalk. This will further be discussed in section 5.2.3.
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L3L L2L L2R
L3R L4R L5R
S1L L5L L4L
S2R S1R
S2L S3B S4B
A
B
C
D
E
Fig. 11: Proposed new Book arrangement
5.2.2 Preventing improper use
In the system with the commercial multiplexer chip, there is no way the hardware canprevent stimulation of two channels at the same time. At present, the software must nothave a fault that would allow this. This also implies that it is necessary to wait 12 clock(stimulation) cycles before starting stimulating the first channel again. As the risk ofimproper use is important, the prevention of 2 channel stimulation is one of the aims of thenew design.
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5.2.3 Grouped anode switching
From the crosstalk experiments it is now known that when all anodes are interconnected,there is a uniform current spread over these anodes. This has the advantage that thelikelihood of anode stimulation is less than was thought. Together with the fact that foranode stimulation you need a higher current(density) than at the cathode side, it is nowunlikely to get anode stimulation due to crosstalk effects. However the positions of theroots outside the slots are not defined and thus the possibility that there will be inadvertentstimulation of roots remains. Especially when you have two channels with different currentsettings in the same book (fig Fig. 5 in chapter 4) there is a danger of anode stimulation.Based on a preference for maximal 4-wire cables, and the minimisation of the number ofcables in total, the connection of nerve roots L2-S4 to the stimulator can be drawn as inFig. 11 and Fig. 12.
L2R L2L S1L S1R S2R S2L S3B S3B
Fig. 12: Grouped anode switching design.
To eliminate the possibility of anode stimulation in book D, as discussed above, the 2anodes in this book are separately switched. The anodes of books A to C, with theassociated anodes for L2 to S1L, can be joined together since their associated cathodespass the same current. S1R and S2L are both separate anode switched and S2R, S3B andS4B are again anode joined together ("B" stands for both ’left’ and ’right’ nerve in sameslot). Another advantage of joined-anodes, for book A to C, is also the redundancy of theanode wires: if a wire breaks you have still the other anodes [3].
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5.2.4 Overview
Some extra features can be included as well. The first one to think about is a softwarecurrent setting instead of, or in addition to, a hardware (fixed) current setting.
The following major advantages and thus specifications for the new design can beproduced:
• extension with two channels, from 12 to 14, for stimulation of S3 and S4 (both left andright); 2 different current settings for 2 groups of channels;
• hardware prevention for improper input on data signal line; allowing stimulation ofonly the first x channels (1NPO NRQ!STVU;WXGYZ[\^]_[``aQ!Scbd[WWZ`Xe[`;fg[hPXi
• isolate channels by anode switching to prevent crosstalk effects.
5.3 Technologies choice
5.3.1 Voltage considerations
When the BJT’s (i.e. analogue) and the digital part of the design are integrated on the samechip, which is rather preferable with 14 channels, it is necessary to find a process that canhave both a high voltage part (analogue) and a ‘normal’ digital part. High voltage in thiscase means a voltage that exceeds the voltage that can be used in CMOS. For the Sacralroot stimulation, a current of about 32 mA is led through a electrode impedance of severalhundred of ohms, which means you need a voltage of more than 20. However, the chipshould be able to run with a power supply of 15 volts.
So the specification for the process technology will at least be that there should beanalogue layers suitable for high voltage, as well as a digital part –with libraries- to makethe shift register and some other logic.
Based on the specification several technologies are available that can be used. Howeverthe amount of funding is limited, so the costs need to be considered carefully. Fortunatelythe University College London is member of Europractice5 which makes it cheaper to useone of the technologies that are available in that program. Because of the high voltagesinvolved in the specifications, only two technologies were left on the Europractice list,from which the cheapest one actually met the specifications very well. It was not necessaryto use a commercial process, since there was no penalty for using the chosen technology,while it was definitely cheaper than any available commercial technology.
5 EUROPRACTICE (Promoting Access to Components, subsystems and microsystems Technologies forIndustrial Competitiveness in Europe) aims to stimulate the wider application of state-of-the-artmicroelectronics technologies by European Industry in order to enhance European competitiveness in theglobal market-place and is financially supported by the Europerian Commission.
To estimate the costs for the total chip, first the number of bond pads is important. This isone of the most important factors in calculating the surface. For a 40 pins chip, you needan area of approximately five square millimetres (5 mm2), which should be enough for themultiplexer. Whether this is really the case will be reconsidered later.
switchescontrol
14 cathode switches
14 discharge switches
14 anode switches
current sourcescontrol
currentsource A
currentsource B
clockdata1data2grouped_mode
output to 14 anodes 1)
output to 14 cathodes
current changecontrol
change_mode
initial current setting
set IA
set IB
10 4
14
14
1) in grouped mode only 4 outputs used
14 channel nerve root stimulator stage
clockdata1
Fig. 13: Block diagram: design overview.
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5.4 Explanation of the design overview
5.4.1 Switches control
The input signal lines are used in the same way they have been connected to the UCN5832 multiplexer chip in the existing design. So it should be possible to replace this chipwith the new designed chip and obtain the same performance as well as some extrafeatures.
Input signals:
Clock clock rise means stimulation of next channel in sequenceData_1 (data) a specific ’one’ on this line initiates the stimulation of the first
channel in the sequence, and the cancellation of any stimulationsequence that is running.
Data_2 (enable) the length of the high period on this dataline determinate the lengthof a particular stimulation pulse.
Grouped_mode this dataline is pulled high when the chip is mounted and indicatesthat it should work in ungrouped mode, while no wirebonding to thisinput means a grouped mode operation. Grouped Mode means thatanodes are connected as shown in Fig. 12, while in UngroupedMode every anode is connected to the demultiplexer-IC separately.
Output signals (internal):The output signals are used to control the anode, cathode and discharge switches.
5.4.2 Current change control
Clock clock rise means shift data through.Data_1 (data) a special pattern of ’one’s has the meaning of changing one of the
two current settings, this will be explained in detail in the designimplementation sections.
Change_mode this dataline is pulled high when the chip is mounted and indicatesthat it should work in change mode. This means that it reacts tospecial words on the data_1 line so as to change one of the twocurrent settings. If not wire bonded this input will keep the chip infixed-current mode.
5.4.3 Initial current setting
Two resistors connected to the chip determinate the initial value of the two currentsources; while no connected resistor has the meaning of the lowest possible current settingis used (i.e. 2 mA).
5.4.4 Current sources control
From the input signals that give the information about the initial current setting and thepossible change of current setting, the control box sets both current sources by two sets ofbinary (5 bit) output lines.
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5.4.5 Current source A or B
The digital input lines (5 bit) set the current sources to 2, 4, 8, 16 or 32 mA, with anaccuracy of 10%.
5.4.6 Discharge switches
To remove the charge that is build up between the electrodes and in the blockingcapacitors (in series with the cathodes) a discharge resistance is necessary. The resistanceof the switch (or switch plus resistor) should be around 500 jklmknnloqpsrmtnujtvnwpsrdissipate 4 mW. This will remove 99% of charge in 50 ms (the time between stimulationpulses for one channel).
5.4.7 Anode switches
30 Microseconds before the cathode switch of a channel is turned on, the anode switch ofthe channel will be turned on. The switches control will take care of the differencebetween Grouping Mode and Ungrouped Mode. The specifications of the anode switchesare the speed of turning on, in the order of microseconds; the maximum voltage, equal tosupply voltage and the voltage drop, which should be kept low.
5.4.8 Cathode switches
The switches will connect one cathode at a time to the associated current source, duringstimulation, as determined by the switches control, with a speed to obtain 90% of currentsource amplitude in 1 xzy| ~#yx;ysPI;~!~||!y5yqx ~$yshandle is 32 mA and the voltage drop should be kept low. Consideration of the final chiparea is not made here. Design is an interactive process during design in which simulationresults that indicate performance provides information about realistic chip area.
36
37
6. Design implementation
6.1 Introduction
Each process in IC-technology usually comes with documentation on paper about allprocess details and the contents of the libraries. For all the types of transistors available inthe process, standard cells with specific design optimisations (size, current) are given inthe libraries. Based on SPICE models, the I-V characteristics of the transistors are given.This helps start the design. For the transistors however, the design parameters need to beworked out, based on the requirements of the specification.This chapter is divided in two sections. First the high-voltage, analogue part of the chip isdescribed. In this section the anode switches, cathode switches, discharge unit and currentsource design will be given. In the digital section the control logic of these units isdescribed.
6.2 Analogue par t
6.2.1 overview
Before designing the switches that are involved in stimulation, first the potentials in thechannel are considered. Because one of the specifications is that is should be able to runwith a supply voltage of (only) 15 V, it is important to watch the voltage drop over theelectrodes, switches and current source. The several voltage drops are given in Fig. 14.
VswA
VswD
VDR
VswC
VCS
Ve
Fig. 14: Voltage drop over several parts in a stimulation channel.
In the ’Design specification’ the cathode switches and current sources are put in a separatebox, this is not essential: the cathode switches could be designed as switched currentsources. In theory the following option are available, where ’-’ means that the switch actsas an current source:
Cathode switch Current source CommentBJT BJT Switching characteristics are non-linear, in current
source base current effectsBJT FET Switching characteristics are non-linear, simple
current source implementationBJT - Base current effects and control lines for current
setting to all channelsFET BJT Switching characteristics are linear, in current
source base current effectsFET FET Switching characteristics are linear, simple current
source implementationFET - Control lines for current setting to all channels
Based on the comments written in the table, the FET-FET option would probably bechosen, however it should be noticed that there is a size-difference between an MOSFETand a bipolar transistor for high currents and high supply voltages. This difference is about10 times for a current of 32 mA and a saturation voltage of 0.5 V in this process6. Tocompare the real sizes: a bipolar transistor (cell HNHP350A from library) needs52.10-3 mm2, the FET (FND W=10mm) 723.10-3 mm2 and finally the round FET type(FNDR W=10mm) needs 417.10-3 mm2. This disadvantage of the MOSFET’s leads us tothe solution of a BJT as switch, because the total area occupied by them is 14 times less!For the current source three options are still available.
The choice for current source design is now an optimisation on:
• total chip area• minimal voltage supply to operate (less than 1 V in total)• other characteristics (like speed, accuracy)
6 In the previous chapter the process 'Alcatel-Mietec 2µ HBIMOS HV' has been chosen. In this chapter allcalculations apply to this process.
39
For one channel, based on the cells that are available in this process library, the choices aregiven in Fig. 15 (option A), Fig. 16 (option B) and Fig. 17 (option C).
2mA
1x 2x 4x 8x 16x
out
I=32mAI=16mAI=8mAI=4mAI=2mA
stimulation
Fig. 15: One stimulation channel with the use of an adjustable bipolar current-mirror as current source usedfor all channels and a bipolar transistor switching the cathode during stimulation.
Fig. 16: One stimulation channel with the use of an adjustable FET-based current-mirror as current sourcefor all channels and a bipolar transistor switching the cathode during stimulating..
2mA
2x 4x 8x 16x
out
I=32mAI=16mAI=8mAI=4mAI=2mA
stimulation
1x
40
Fig. 17: One stimulation channel with the use of an adjustable bipolar current-mirror as current source foreach channel that also is switched during stimulation.
6.2.3 Anode and discharge swit ches
The same size considerations as for the cathode switches apply to the anode switches. Thismeans that a bipolar transistor is preferable to an FET. Since a bipolar transistor is a three-terminal device, which requires a base current to turn it on, the use of a PNP version hasthe advantage over the NPN version, that the current through the collector, that isdetermined by the current source in the system, will not be affected by any base current.The use of an NPN would it make harder for the current source to keep the current on therequired value, since the base current is added to the emitter current. The size of one PNPtransistor in this process is 97.10-3 mm2, a little larger than a NPN transistor with the samecurrent flow.
The discharge switch with resistor can be implemented by using a FET. Alternativelyresitivity sheets are available in this process, but their value can not be designedaccurately. To remove for example 99% of the charge from the capacitor (and electrodecapacitance) through the electrode impedance and discharge resistor (FET) the later shouldhave a value of 600 ª «,¬®¯'°± ²;¯°®V³´c³¯°³zµ¶¯·¬°¹¸Pº,«»¶µ½¼¾ªG¿ À -3 mm2.
2mA
1x 2x 4x 8x 16x
out
I=32mAI=16mAI=8mAI=4mAI=2mA
stimulation
41
6.3 Digital par t
6.3.1 switches control
The working of the shift register (SR) that act as a de-multiplexer for the signal data2 withthe use of data1 to indicate the start with stimulation of channel 1, has already beendescribed in the Introduction of the ’Design specification’ (par. 5.1). To prevent the systemfrom improper use (the second objective in the research definition), between the SR-cells’AND-gates’ are placed. As soon as data1 becomes high the inverter makes the input forall ’AND-gates’ low and all SR-cells will "clock in" a low, except the first SR-cell.Therefore it will not occur that more than one channel is being stimulated. Actually, in thereal design, the ’AND-gates’ are replaced by ’NOR-gates’, since the later are standarddigital cells. This adjustment in design will be done with finally no effect in the operationof the de-multiplexer, as described above.
clock
data1
data2
Q
Q
D Q
Q
D Q
Q
D Q
Q
D
swA
ch1swB
ch1swA
ch2swB
ch2swA
ch3swB
ch3
Fig. 18: de-multiplexer logic to control anode, cathode and discharge switches and to turn on the currentsources.
Each cell of the shift register in Fig. 18 has two outputs. The first output (A) becomes highas soon as a channel is going to be stimulated. This output can be used for example toclose already the anode switch, disable the discharge unit and activate the current source.The second output (B) will become high at least 30 µs after output A and extent exactlythe intended stimulation pulse duration (see Fig. 19). This signal can be used to close thecathode switch.
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Data1
Data2
Clock
2 31
SWB ch1
SWB ch2
SWA ch1
SWA ch2
Fig. 19: Input and output lines of de-multiplexer logic, example for two channels, during normal operation.
6.3.2 Current change control
In the ’change mode’ of the chip, that is when the input signal change_mode has beenpulled high, the current setting can be changed by a special input sequence. Generally amaximum of 14 channels is stimulated and then "channel one" is used again by the data1input signal becoming high. If for example a virtual 15th channel would be stimulatedbefore starting with channel one again, there is a gab of 15 cells between the ’one’ in thefirst SR-cell and the ’one’ in the 16th SR-cell. (There is no obstruction of an ’AND-gate’ forthe SR-cells 15 till 34.) This gap can be detected as soon as the ’one’ has propagated to the31th SR-cell, see Fig. 23. At that moment the other ’one’ has arrived in the 15th SR-cell.Internal signal B becomes high.
43
1 2 15 31 32 33 34clockdata1
data2
A B C D E
14
Fig. 20: Extended shiftregister to detect special word for change current setting. Virtually stimulating morethan 14 channels will be detected as a special meaning.
Also a gap of 16, 17 or 18 can be generated, which will be detected and respectivelyoutputs C, D, and E become high. To avoid current change by any mistake in the programor hardware failure, the following special sequence should be used:
B-B-E reset both current source settings to initial value. Initial value is set external bya resistor
C-C-E change the current setting of current source A to the next higher levelD-D-E change the current setting of current source B to the next higher level
The detection of these internal signal sequence can be carried out by the design showed inFig. 21. The unit has an input signal (B, C, or D) corresponding to the purpose of it: reset,change CSA or change CSB. The input signal will be ’clocked in’ into a three cell long shiftregister, as soon as an ’one’ arives in the 15th SR-cell.
1 2 3clockB, C, or D
A
E
Reset,change CSA,or change CSB
Change_mode
Fig. 21: Detection of special sequence of internal signals for changing the current setting.
Data1
A
1 15 30 45clock
Fig. 22: Normal operation, signal A becomes high every 15 cycles. Other internal data lines remain low.
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Data1
A
B
16 16 19
15 16 16 19
16
19E
Fig. 23: Timingsdiagram for internal data lines A, B and E. It can be seen that B becomes 2 times high,before E becomes high and both signals can be ’clocked in’ by signal A.
6.3.3 current sources control
The current sources are controlled by five switches, for the corresponding 2, 4, 8, 16 and32 mA, so only one switch will be closed at a time. The logic in this section will controlthose switches. The initial current setting will be set by reading the internal data lines thatcome from the box ’initial current setting’. Basically this device is transparent for theseinternal data signals, when no information is provided by the ’change current detect’segment indicates to change the current.
S/L clk
Q4
Q2
Q3
Q0
Q1
D4
D2
D3
D0
D1
Serial in
Fig. 24: Circulating shiftregister to control current source. The output controls the switches that sets thecurrent source to the corresponding binary value. Parallel input for initial current setting and serial loadingfor changing the current setting to the next higher level.
Fig. 24 gives one of the two basic components that this segment contains. It acts as anordinary shift register that latches the input to the output when S/L_ is low and CLKremains low. These input signals come from the ’change current detect’ segment and theoutput lines lead to the current sources. The input line S/L_ signal is based on the chip’sinput change_mode and the initial resetline in the ’change current detect’ segment. Thismeans that when this signal is made high it is possible to change the current setting. Theshift register will than operate in serial mode and the contents of the five SR-cells will shiftup. This means that the ’one’ that is present in one of the five SR-cells will move to thenext output and will therefore operate the switch in the current source that sets the nexthigher current (or goes back to 2 mA, after it reached 32 mA). Reset to the initial currentvalue is done be loading (latching) the input.
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7. Discussion
7.1 Introduction
A global block diagram of the chip was already showed in the ’design specification’chapter, where also the several segments and their function were described. That chaptercovered the internal datalines too. In the previous chapter the design of the de-multiplexerIC was given. Based on the given specifications a possible implementation for eachsegment has been presented. A possible implementation that was not elaborated in the’design implementation’ chapter is discussed below. The results of the crosstalkexperiments as far as they concern the chip design are discussed in the followingparagraph.
7.2 Crosstalk exper iment
The local distribution of current will be determined by the mutual distance betweenelectrodes, at least there is no correlation found between the ’access resistance’ forelectrodes that are close to the current sink and their leading current. The correlation that isfound between the other electrodes and their ’access resistance’ is conform the expectationsthat the dominating local restitivity determines the current distribution.The explanation for the current distribution for the anodes close to the stimulating cathodecan be found in the electrical field close to the active cathode. The measured ’accessresistance’ is not valid for this situation anymore, since the distance between the electrodesis too small.In the experiments with 0.09% Saline, that represents the grow of tissue, an increasementof discrepancy between the current that flows through the electrodes in the same book asthe active cathode has observed. This means that after a while the crosstalk will increase,because more current will now flow from adjacent slots, instead of the anodes in the otherbooks. This can be explained by the decreased electrolyte conductivity and thereforeincreased dependency on distance from the cathode to the anodes.
7.3 Design implementation
All three current sources as presented in the ’design implementation’ chapter have theiradvantages and disadvantages. As mentioned, the optimisation will be on the chip areathey eventually will occupy and their electrical characteristics. The option with the leastarea is the one with bipolar current sources for each channel (see Fig. 17 in the previouschapter). However the use of bipolar transistors in the presetable current source loseaccuracy, because, in the current mirror, base current to drive the output transistors isdeducted from the reference branch collector current. Especially with different emitter areaas proposed in that design, different base currents effect the base-emitter voltage and so thebiasing. Better results can be obtained by changing the current mirror to a cascode, orWilson source, or by adding a third transistor to provide the base current. Theimprovements in accuracy and supply voltage will be shown by simulation.
46
The same considerations apply to the current source design with separate cathode switches(see Fig. 16 in the previous chapter). However this design actually already acts as acascode design, but still has the base current disadvantages. Also this design occupies onlya small chip area, less than 1 mm2 in total.Finally to discuss the use of FETs for the current sources. Despite the fact that FETsoccupy more chip area (this design takes 1.6 mm2 in total), their use avoids the problem ofbase current, simply because a gate does not need any current, assuming leakage can beneglected. Only the output resistance of a FET-based current source, caused by leakagebetween source and drain will influence the performance when the electrode impedance inthe same order of this output resistance.In the digital part of the chip, the control part, the input signals grouped_mode andchange_mode are included intentionally. This allows the new chip to operate in the sameway, with fixed stimulation currents, as the existing de-multiplexer operates, whileexternal wirebonding will activate the extra features. Also it is based on the idea ofinherent safety. When no external wirebonding is done, the de-multiplexer will operate inthe simple mode.
The timing for the cathode and anode switches as well as the discharge unit has not beendefined yet. Certainly, during stimulation, the anode and cathode switches should beclosed and the discharge switch opened. Internally, as can be found in the ’switchescontrol’ segment paragraph, there is a signal available prior to the stimulation signal thatcould be used. The only disadvantage of switching all the switches at the same time (at thestart and end of stimulation pulse) is that the speed will be determined by the slowestswitch. The penalty for switching prior to the stimulation, and using only the cathodeswitch to define the stimulation duration is the waste of current, so energy, in the implant.
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8. Conclusions and recommendations
At present there is a design for a de-multiplexer IC that meets the objectives as pointed outin the ’Research definition’ chapter. It is possible to stimulate each of the 14 channels byseparate cathode and anode switches: this is called ’ungrouped mode’. However to reducethe number of cables, there is also a ’grouped mode’ operation, where several anodes areswitched together. The arrangement of anodes that can be joined together and the ones thatare still separately switched is based on the results that were obtained from the crosstalkexperiment. The current distribution is quite uniform over the connected anodes, with alittle favour for the anodes in the slots in the same book as the stimulating cathode. Thismeans that when two nerves with different threshold have been mounted in the same bookand the one with a higher threshold would have been stimulated and the anodes are joinedtogether, stimulation in the nerve root with this lower threshold could occur. To avoid thisproblem and avoid any risk with stimulating nerves of different threshold, anode switchingis been provided for four groups of channels.
The amplitude can be set for a group of outputs; the first 10 outputs are set to one currentsetting and the next 4 to another. The current setting for these two groups can be setinitially external to the chip, but when the chip operates in ’change current’ mode, it is alsopossible to change the current setting by sending a special stimulation pattern.
Another feature is that the stimulator has been protected from an improper use. Wheneverthe input signal becomes high, to start a stimulation sequence, beginning with the firstchannel, stimulation of all the other channels is cancelled. This means that there will neveroccur stimulation of more than one channel (improper use) and it is possible to stimulateless than 14 channels (extra feature).
The design implementation does not give a definitive and detailed description of allsegments of the chip. Especially the proposed current source options will be simulatedwith a circuit simulator to find a design that best matches the specifications.Notwithstanding the finished design for the digital segments, it needs to be transferred intoVLSI design to produce a layout from which an IC can be manufactured. A test patternwill be needed to check if the logical (digital) behaviour of the control logic is appropriate.
To produce an accurate description of the current distribution with the use of bookelectrodes, as used in LARSI, an additional crosstalk experiment is necessary. More usefuldata can be obtained with experiments where more positions of the cathode relative to theanodes are subject to interest. Also the measurement of the distance between the cathodeand the anodes in the same slot and in the adjacent slots would probably provide moreinformation to verify the assumption that the distance is an important factor in the currentdistribution. Finally I suggest recollection of the data and analysing it with severalmathematical tools.
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49
9. Appendices
9.1 Literature
[1] Berne, R.M., Levy, M.N., Physiology, Mosby Year Book, St. Louis, 1993
[2] Brindley, G.S. “The first 500 patients with sacral anterior root stimualtor implants:general desciption” , in Paraplegia, Vol. 32, 1994, pp. 795-805
[3] Brindley, G.S., “The first 500 sacral anterior root stimulators: implant failures andtheir repair” , in Paraplegia, Vol. 33, 1995, pp. 5-9
[4] Craggs, M.D., Donaldson, N. de N., Donaldson, P.E.K., “Performace of platinumstimulating electrodes, mapped on the limit–voltage plane” (Part 1 and 2), in Medicaland Biological Engineering & Computing, Vol. 24, 1986, pp. 424-438
[5] Donaldson, N. de N., The electrical design of an implantable stimulator to restoremotor control to the paralysed, PhD. Thesis, University of London, 1990
[6] Donaldson, N. de N., Donaldson, P.E.K., “When are actively balanced diphasic(‘Lilly’ ) stimulating pulses necessary in a neurological prosthesis?” (Part 1 and 2), inMedical and Biological Engineering & Computing, Vol. 24, 1986, pp. 41-56
[7] Donaldson, P.E.K., Donaldson, N. de N., Brindley, G.S., “Life of Pt and Pt-Irstimulating electrodes in neurological prostheses” , in Medical and BiologicalEngineering & Computing, Vol. 23, 1985, pp. 84-86
[9] Ganong, W.F., Review of Medical Physiology, Prentice Hall, London, 1997
[10] Geddes, L.A., Bourland, J.D., "The Strength-Duration Curve", in: IEEETransactions on Biomedical Engineering, Vol. 32-6, 1985, pp. 458-459
[11] Geddes, L.A., Bourland, J.D., “The Strength-Duration Curve” , in IEEE Transactionson Biomedical Engineering, Vol. 32-6, 1985, pp. 458-459
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Remarks:temperature: 37 deg.C temperature: 36 deg.C temperature: 37 deg.C temperature: 36 deg.Cconductivity: 19.1 mS conductivity: 19.0 mS conductivity: 19.1 mS conductivity: 19.0 mSdistance between books: 5 and 12 mm distance between books: 23 and 29 mm
Table 2: Results crosstalk experiment for 0.9% Saline.
Remarks:temperature: 37 deg.C temperature: 36 deg.C temperature: 35 deg.C temperature: 34 deg.Cconductivity: 1.8 mS conductivity: 1.8 mS conductivity: 1.6 mS conductivity: 1.6 mSdistance between books: 5 and 12 mm distance between books: 23 and 29 mm
Table 3: Results crosstalk experiment for 0.09% Saline.
