Electrical nerve stimulation: re-designing, producing and testing a portable stimulator.

Transcription

1 Electrical nerve stimulation: re-designing, producing and testing a portable stimulator. Nimesh Shah Supervisors: Dr. Anne Vanhoest Professor Nick Donaldson The Implanted Devices Group at UCL has necessity for an electrical stimulator, and there is one that exists. This report covers how the old circuit which produced a square pulse, and an exponential pulse was adapted. The circuit was changed so that the square pulse had the capability of being a stepped square pulse too. This was done by introducing a new monostable, and extra logic components. The parameters that were preliminarily defined were not completely adhered to. Therefore, further work still needs to be conducted to refine the stimulator and there are suggestions concerning that.

2 Contents Acknowledgement Introduction What nerve stim specification of the box Old circuit New circuit Additional Circuit Finished Stimulator Typical Setup Parameters Waveforms Periods max. min. Amplitudes Charge Balance. Future Work Bibliography 1

3 1. Introduction 1.1 What is Electrical Stimulation? Electrical stimulation of a nerve is the generation of an action potential by the application of a current waveform (in the instance of this project). Electrical stimulation is used to restore function in neurologically impaired individuals. Electrical stimulation can help with loss of hearing by stimulation of the auditory nerve (Grill and Mortimer 1995), or foot drop in hemiplegic patients. (McNeal and Bowman 1985). It has been shown that the use of different pulse shapes have different physiological effects (Vuckovic, Rijkhoff and Struijk 2004). It is important for the Implanted Devices Group at University College London to have an electrical nerve stimulator, which is able to produce different pulse shapes, and thus differing effects. The use of such a stimulator varies, from as a control when comparing against another stimulator, or the ability to set one waveform and be able to see the impact of changing the electrodes. There is an electrical nerve stimulator which is currently in use, however, the stimulator does not have the capabilities of producing a stepped square pulse. This will be the scope of my project. 2

4 1.2 Specification of a Stimulator In the 19 th Century it was discovered that the relationship between the amount of current needed to activate a nerve depended on the pulse width in a hyperbolic relationship as shown in fig (Donaldson 2009) Figure Further investigation by Geddes and Bourland (1985) saw that the least amount of current is needed when pulse widths are less than chronaxie. In humans chronaxie occurs at about 100µs. The parameters shown in table 1.2.1, that I set out to achieve given agree with these findings. Table Parameter Minimum Maximum Frequency 5Hz 500Hz Pulse Width 50µs 5ms Delay 1µs 100µs 3

5 Figure

6 2. Protoboard Circuit 2.1 Old Circuit This is the original circuit, as seen in fig , that was first designed by Prof. Donaldson and Eleanor Comi (2002). This was continued by Dr. Anne Vanhoest. This circuit can switch between two types of pulses; a square pulse, and one that decays exponentially as shown in figs and Figure Figure The desired waveform can be selected at Switch 2 on the circuit. It is important to note that this is the logic circuit, which produces two voltage waveforms. There is an additional circuit which is responsible for converting the voltage waveform, into a current waveform this will be further discussed in section Clock Loop We can clearly see the clock loop indicated in the circuit diagram in fig The monostables involved, MONO1 and MONO2, produce the time intervals. MONO1 produces the interval 1-2, and MONO2 produces the interval 2-5. These intervals are determined by the RC circuits R1C1 and R2C2, the two resistors are variable and R2 is present on the front panel allowing user control of frequency. We can see though 1-5 is the period, the user only has control over the interval 2-5. The 5

7 feedback into the NOR gate will be discussed in section Square Pulse The formation is more easily described if each stage is considered individually. The pulse is bound by the time intervals 1, 2, 3, 4, and 6. Figure The first stage of the pulse occurs in the interval 1-2. This is generated by MONO1 and is indicated as the clock loop in fig The returning pulse around the loop, instant 6 triggers the monostable on the rising edge. Figure The interval 2-5 is determined by the second monostable which is triggered by the falling edge of instant 2 and contributes to the determination of instant 6. Instant 6 is equivalent to instant 4, (the end of the charge balancing phase of either the square pulse or the exponentially decaying pulse) when instant 4 is greater than 6

8 instant 5. Or, instant 6 is equal to instant 5 when instant 4 is less than instant 5. In this case the frequency is as set by the user. This is a condition governed by the feedback from the circuit, which in itself is dependent on charge balance into the flip-flop and the NOR gate. This is shown in fig Figure The pulse exiting from MONO1 not only triggers MONO2, but also MONO3 as well. This creates the interval between 2 and 3. With the interval 2-3, and interval 2-4 being output from the flip flop, all the time intervals needed for the square pulse can be created. The highlighted pulse on fig is created by the connection of switch 5 to a V+ source through a resistor, during the interval 2-3. Figure Interval 3-4 is not a pulse that is output by a monostable, we can however make it by processing pulses with interval 2-4 and interval 2-3. The resulting pulse of interval 3-4 causes a connection to a V- source at switch 7 via a resistor. This in turn causes the highlighted part of the pulse, shown in fig

9 Figure The time interval 4-2 is grounded, this is maintained by switch 6. The combination of these pulses, and the resulting switch connections contribute to the production of the square waveform. The front panel houses the variable resistors controlling the durations of the pulse width (interval 2-3), and the period (interval 2-5). Both of these durations are user controlled Exponentially Decaying Pulse This pulse is selected by switching at switch 2 from the square pulse circuit to the exponentially decaying pulse circuit. This circuit as designed by Prof. Donaldson and E. Comi, previously mentioned, remains unchanged. It is important to have the option of an exponentially decaying pulse, as it may be preferential to avoid a break excitation. (Frankhaueser and Widen 1956) This circuit works mainly in the same way as the square pulse, that is, using a combination of switches to connect to V+, ground and V- at different given intervals. The obvious difference is in the form of the pulse. The exponential decay is achieved with the introduction of a capacitor (C6), and a variable resistor in parallel with switch 2, which allows user control of the decay. 8

10 Figure Once again, the interval 4-2 is kept at ground by S3. This is shown in the highlighted part of the pulse in fig Figure The interval 2-4 is governed by S1 and S2. During the interval 2-4 S4 is connected allowing these switches to have an impact during this time period. This is shown in the highlighted part of the pulse in fig Figure The inverted form of interval 1-3 connects S1 to a high voltage via R5. Although S1 9

11 connects to a high voltage the waveform is limited by S2, which is connected from intervals 2-4. Therefore the high voltage is shown in fig by the square pulse at interval 2. Figure At time interval 3 S1 becomes disconnected, and S2 connects allowing the discharge of the capacitor (C6) across variable resistor R4. We then get the abrupt return to ground from the disconnection of S4 at interval 4. The adjustment of R4 is done by the user, and the controls are situated on the front panel. This controls the decay of the exponential pulse. This is shown in the highlighted part of the pulse in fig The combination of these pulses, and the resulting switch connections contribute to the production of the exponential waveform Integrator, Comparator and Feedback Figure These two components are responsible for charge balancing. The integrator measures the areas of the curves in both the square pulse and the exponential pulse (In both cases it is shown in fig , with the positive being the lighter shade, and the negative being the darker shade). The comparator sums the 2 areas involved in the positive and negative phase of the waveforms and produces instant 4 when the sum of the integrals equal 0. 10

12 Interval 4 is created by the comparator; the flip flop assembles this with interval 2, which is outputted from MONO2. This produces a pulse interval 2-4 from the non Q output. This is then fed back into the NOR gate of the clock loop. The NOR gate within the clock look sets the time interval 6. This NOR gate sets T6 as either T4 or T5, working on the condition: T6 = T4 if T4 > T5 T6 = T5 if T5 > T4 Therefore, the instant 6 is either set by the user by way of adjusting the period (T6=T5) or, is extended until the condition of charge balance is achieved (T6=T4). 2.2 New Circuit The existing circuit, as previously described, produces a square pulse, and an exponentially decaying pulse. Changing the square pulse to a stepped pulse has been shown to allow the use of up to 30% lower charge (Vuckovic, Rijkhoff and Struijk 2004). It is then obvious that this would be a nice option to have. The change in waveform that is necessary is shown below in figs and Figure Figure

13 Legend for Figure Resistors Table Resistor Number Value (Ω) R1 2 k R2 120 k R3 200 k R4 50 k R5 200 k R6 47 R7 47 R8 47 R9 200 k R R11 - R12 8 k R13 10 k R14 1 k Capacitors Table Capacitor Number Value (nf) C1 10 C2 100 C3 47 C4 22 C5 50 C6 47 C7 1 12

14 13 Figure 2.2.3

15 There has been an introduction of a time interval, the interphase delay. The new circuit is shown on the previous page in fig The greyed area labelled Square Pulse, and the fourth monostable are the changes which I have completed, they are shown in fig We previously saw in section 2.1 how the original wave was generated. Bibliography 1. Different Pulse Shapes to Obtain Small Fiber Selective Activation by Anodal Blocking - A Simulation Study. Vuckovic, A, Rijkhoff, N and Struijk, J. 5, 2004, IEEE, Vol. 51, pp Figure

16 The pulse with time interval 2-3 is generated from MONO3, this triggers the pulse, with time interval 3-4 from the new MONO4. The period in which Sw5 is connected to a high voltage is not affected by the new circuit, as the controlling pulse is coming from MONO3. The durations in which Sw6 and Sw7 are connected are altered. Sw6 was previously connected from intervals 5-2, providing the ground state outside the positive and negative phases. Sw6 is now connected to ground between the newly formed interval 3-4 as well as seen in the pulse entering Sw6 in figure. The variable resistor on MONO4 allows an adjustment of the duration of the delay. As I introduced the new delay, we also had to change how S7 was connected, as interval 3-4 is being used in the delay, we needed to create a consecutive interval 4-5. This is done by using the pulses from MONO3, MONO4 and the feedback (due to charge balance) As a late addition (not shown on the circuit diagram) the RC circuits on monostables 3, and 4 were connected via a fixed resistor (R3F = 2kΩ, R4F = 430Ω) to high voltage. This ensures the resistance never falls to 0. The combination of these pulses, and the resulting switch connections contribute to the production of the stepped square waveform as shown in fig Figure

17 As we have introduced a new time interval the conditions of when interval 7 change to: T7 = T5 if T5 > T6 T7 = T6 if T6 > T5 2.3 Additional Circuits 16

18 The circuit I have been working on is the logic circuit which is produces a voltage waveform. The circuit shown in fig 2.3.1is responsible for converting this voltage waveform into a current waveform. The voltage waveform is initially split into two proportional waveforms, there is then a voltage to current conversion stage, which results in the current waveforms for the anodes. Figure shows the circuit responsible for this conversion, also in the circuit are stages that deal with voltage conversion, discharge of the blocking capacitor, isolation of the signal and saturation. However this is something that I have not worked on, and I will not focus on it. It is however, important to acknowledge it as it is an important part of the stimulator. Figure Finished Stimulator 17

19 3.1 Typical Setup Power Generator Oscilloscope Circuit Figure Figure3.1.1illustrates the typical setup of the protoboard, and the instruments. 18

20 3.2 Parameters As mentioned in the previous sections the parameters that we set out to achieve are shown in table Table Parameter Minimum Maximum Frequency 5Hz 500Hz Pulse Width 50µs 5ms Delay (square pulse) 1µs 100µs The parameters that the stimulator actually produces are shown in tables and Stepped Square Pulse Table Parameter Minimum Maximum Period 215µs 10ms Frequency 4600Hz 100Hz Pulse width 50µs 9ms Delay 16µs 1.1ms Exponentially Decaying Pulse Table Parameter Minimum Maximum Period 200µs 41.5ms Frequency 5000Hz 24Hz Pulse Width 100µs 9.5ms Decay 0s 41ms The implications of these results are discussed in section 4. 19

21 3.3 Waveforms To demonstrate the capabilities of the stimulator I have set various values and explained them below Stepped Square Pulse Frequency Figure The adjustment of the variable resistor in the RC circuit of monostable 2 allows the control of the time interval 2-6 i.e. a control of frequency. We have previously seen the condition of: T7 = T5 if T5 > T6 T7 = T6 if T6 > T5 20

22 Altering the time interval 2-6 clearly will then alter the frequency of the waveform. Fig shows a high frequency, in this particular case the period of the waveform is 650µs, which corresponds to a frequency of approximately 1.5 khz. Figure Alternatively, we are also able to adjust the resistor so that the time interval 2-5 is very large, reducing the frequency markedly. In this instance, the period of the waveform shown in fig is 4ms, corresponding to a frequency of 250Hz 21

23 Pulse Width This is specifically the duration of interval 2-3. As explained previously the integrator and comparator produce instant 5 when the condition of charge balance has been met. Figure Fig shows how the pulse width can be adjusted. This is done by changing the value of the variable resistor in the RC circuit of monostable 3. In this instance we can see the pulse width is at 100µs, the delay is 150µs and the duration until the next waveform is 200µs. The period of the waveform is 550µs. 22

24 Figure The pulse width approaching instant 7 can be seen in fig , which in this case is instant 6. In this instance the pulse width is 200µs, and the delay is 150µs. the period of this wave is also 550µs. 23

25 Figure Fig demonstrates the conditional control over instant 7. Increasing the pulse width, increases the duration of the negative pulse and forces the integrator and comparator to delay instant 5. This increase in pulse width, results then, in a decrease in frequency. In this specific example the pulse width shown is 500µs, and the delay is still 150µs. The period of this waveform is 1150µs, perfectly showing us how the frequency decreases on increase of pulse width. 24

26 Delay Figure Fig and fig show us the limits of the delay. The duration of the delay can be altered by changing the value of the variable resistor in the RC circuit of monostable 4. Fig is showing us how long the duration of the delay can be. This picture shows us that the maximum delay is 1.1ms. 25

27 Figure The scale on the oscilloscope has been adjusted in fig so we can clearly see the duration of the delay. On setting the delay to as low as possible the waveform appears to be a square wave with no step, however, further investigation shows us that a delay is still present, with a duration of 16µs. 26

28 Amplitude Figure As a late addition (which is not shown on the circuit diagram) I added a variable resistor between the low voltage and the switch. This allows the negative phase to be shallower. This feature is also demonstrated on the front panel design shown in section 4.2 as both the positive and negative voltage can be set by the user. Fig shows how the charge balance is maintained when the low voltage is changed. 27

29 Figure The response of the circuit to a variation of the positive voltage amplitude is shown in fig , the maximum amplitude previously was 5V relative to ground. Here the amplitude is 10V, there is charge balance though the low voltage is still -5V, which explains the lack of symmetry. 28

30 Figure Fig shows the breakdown of the waveform at 3.50V when the positive voltage was reduced. However, this is due to the fact that some components were not receiving enough power at 3.50V to produce the pulses. The voltage supply was the same to both the components and the V+ that determines the amplitude of the waveform. 29

31 3.3.2 Exponential Pulse Figure Though nothing has been altered in the exponential pulse, I made the circuit just to check the durations of the time intervals. A typical exponentially decaying waveform can be seen in fig

32 Pulse Width Figure As with the square pulse we are also able to reduce the duration of the pulse width in the exponential pulse. The pulse width was reduced to 150µs as shown in fig

33 Figure Fig shows the effect of increasing the duration of the pulse width to 1ms. 32

34 Exponential Decay Figure By altering the variable resistor across which C5 discharges, we can alter the time taken for the decay. In this situation the capacitor discharges so fast it almost takes on the appearance of a square pulse, this is shown in fig

35 Figure Conversely, we can change the value of the resistor so the time taken for the capacitor to discharge is longer. Here it takes 2.6ms for the charge to be balanced and a return to ground it is shown in fig

36 4. Future Work 4.1 Parameters We can see from the protoboard circuit that the circuit is functional, and is producing charge balanced waveforms. As seen before in section 3.2 there is not perfect agreement in the parameters that I set to achieve, and the ones that were then consequently attained. As we have seen the time intervals are set by the RC circuits attributed to the various monostables. Future work will consist of altering the values of the resistors and capacitors, to achieve the parameters defined. Monostables 1 and 2 follow equation(see Appendix B): Two = K.Rt.Ct (1) Where: Two = Output pulse width (s) Rt = External timing resistor (Ω) Ct = External timing capacitor (F) K = 0.42 for VDD = 5v Monostable 2 is responsible for the interval 2-5, the main interval responsible for period, and therefore frequency. The periods necessary for the frequencies are 2ms 0.2s. The R2C2 circuit can be adapted to fit these values better. Working from equation (1) I could recommend a variable resistor value of 100kΩ, and a capacitor value of 47µF. This gives you a variable period of 0.198ms to 0.198s. The pulse width conforms quite well, however the upper range may need to be tapered. The delay, doesn t fit perfectly either, the R4C4 circuit could also be better optimised. This monostable has equation: 35

37 tw = K.REXT.CEXT (2) Where: tw = output pulse width in ns; REXT = external resistor in kω; CEXT = external capacitor in pf; K = constant = 0.55 for VCC = 5.0 V The duration of the delay that we would like is 1µs to 100µs. the values of R4, R4F, and C4, according the formula allow the delay to go as low as 5µs. This however, is not reflected in the measurement, this could be due to the tolerance in manufacturing of the resistors. The delay can be lowered to 1µs, with a decrease in R4F. Currently, all the parameters are adjusted using variable resistors. In the future, it might be worth considering the use of variable capacitors, in cases where a variable resistor is impractical. 4.2 Box Design The next step of this project is to make the box. the design of the old box was not intuitive, and I have therefore redesigned the front panel with approximate dimensions as shown in figure I wanted the new front panel to be intuitive, that someone would be able to work the stimulator who had a knowledge of electrical stimulation, and not necessarily experience in working with this specific stimulator. This new front panel is designed in vertical panels. The top left box, indicates the initial control of the box where the power, triggering, and trigger coupling is set. Below that is where the leads for the power are inserted; they are sufficiently far away to not interfere with the operation of the box. To the control panels right is where the user is able to set the amplitude of the 36

38 wave. This is show in fig To the right of the amplitude control are the waveform parameters, where the user is able to set pulse width, period, delay, and decay durations. Furthest right is the adjustment for the BNC sockets. The BNC sockets go to an oscilloscope, they are separated from the circuit by isolation amplifiers that are powered by batteries. The top control knob selects which of the BNC sockets, outputs a signal. The LEDs associated let the user know when there is a current pulse, and when there is saturation. The bottom control knob allows user control of the ratio of I1/I2 from the 4mm sockets going to the electrodes. 250mm 150mm Figure

39 The designing of the box runs in parallel with the making of the PCB, which also has yet to be done. Once the PCB is made, populated and tested. The box can be finally assembled. 38

40 Appendix A Logic Gates Some of the time intervals are created by the processing of other time intervals through logic gates. The truth tables for the logic gates that I have included in the circuit are drawn below. NOR p q p q NAND p q p q EXNOR p q Inverter p p

41 Appendix B Monostables There are 4 monostables that have been used in this circuit, they are triggered on either a rising, or falling edge depending on how the chip has been wired. On this edge the monostable produces a pulse of standard time, which is determined by the RC circuit. Monostables 1 and 2 are on chip HEF4528B. They follow equation: Two = K.Rt.Ct (1) Where: Two = Output pulse width (s) Rt = External timing resistor (Ω) Ct = External timing capacitor (F) K = 0.42 for VDD = 5v The datasheet for chip HEF4528B can be found on: Monostables 3 and 4 are on chip 74HC/HTC123. They follow equation (when values for CEXT exceed 1nF): tw = K.REXT.CEXT (2) Where: tw = output pulse width in ns; REXT = external resistor in kω; CEXT = external capacitor in pf; K = constant = 0.55 for VCC = 5.0 V The datasheet for 74HC/HTC123 can be found on: 40

42 Bibliography Donaldson, Nick. "Stimulation Basics." Lecture 18 MPHY3013, Medical Electronics Frankhaueser, Bernhard, and Lennart Widen. "ANODE BREAK EXCITATION IN DESHEATHED FROG NERVE." Journal of Physiology, 1956: Geddes, and Bourland. "The Strength Duration Curve." IEEE transactions on bio-medical engineering, 1985: 485. Grill, Warren M, and J.Thomas Mortimer. "Stimulus Waveforms of Selective Neural Stimulation." IEEE Engineering in Medicine and Biology, 1995: McNeal, D R, and B. R. Bowman. "Selective activation of muscles using peripheral nerve electrodes." Medicinal & Biological Engineering & Computing, 1985: Vuckovic, Aleksandra, Nico Rijkhoff, and Johannes Struijk. "Different Pulse Shapes to Obtain Small Fiber Selective Activation by Anodal Blocking - A Simulation Study." IEEE, 2004: