An Implantable Movement
Sensor
MSc Project Report
Author: Jorge Chacon Caldera
Supervisor: Professor Nick Donaldson
21/07/2010
MSc Biomedical Engineering and Medical Imaging
Department of Medical Physics
University College London
Word Count: 8,948
Abstract
Neurological disorders can cause urge incontinence and provoke the lack of urgency-to-void sensation
known as the micturition reflex, this can produce excessive stretching of the bladder, a permanent
increase of its capacity and a decrease of compressive force of the detrusor muscle; incomplete voidance
of the bladder is a common problem under those circumstances. Residual urine can cause renal failure
which is a considerable mortality cause for neurological incontinent patients. The lack of sensation also
produces stress incontinence as a full bladder can leak out urine due to increased vesicular pressure.
An electronic circuit for an externally powered implant has been successfully built. The circuit is able to
measure the size of the bladder using a silicone tube filled with saline as sensor that changes its resistance
when it is stretched. The sensor is meant to be implanted around the bladder so as the bladder expands, it
stretches the sensor. Pulses are generated by the implant at repetition frequencies depending on the
sensor. The pulses are received outside the body by a signal processing unit; implant power and signal
reception from the implant is achieved using electromagnetic coupling.
This report is intended to explain the construction and function of the implant circuit that can permanently
provide information regarding the volume of the bladder.
Acknowledgment
I would like to thank Professor Nick Donaldson for his supervision, guidance and support, Dr. Anne
Vanhoest, and all the people at the implant devices group for making this project possible.
I feel very grateful to Professor Alan Cottenden, Dr. Marin Fry, BEMI lecturers and colleagues for their
contributions to the project.
Finally, I would like to express my gratitude and appreciation to CONACYT for the financial support and
my family for both the economical and emotional unconditional support.
Chapter 1 Introduction 1
1.1 Aims of the project 2
1.2 Motivations 2
1.2.1 Estimation of the bladder size using imaging methods 2
1.2.2 Estimation of the bladder size using implants 2
1.3 Layout of Report 3
Chapter 2 Literature Review 4
2.1 Clinical Review 4
2.1.1 Incontinence 5
2.2 Electronics Review 6
Chapter 3 Passive Electronics circuit
development 7
3.1 Passive Electronics Circuit Theory 7
3.1.1 Two pendulums transfer of energy 7
3.1.2 Two pendulums electrical equivalence 8
3.1.3 Electromagnetic coupling requirements 10
3.2 Passive Electronics Circuit Experiment 10
3.2.1 Two pendulums experiment 10
3.2.2 RLC passive implant experiment 13
3.2.3 Detection of signals outside the body 14
3.2.3.1 Sensitivity improvement by increasing Electromagnetic Field 14
3.2.3.2 Sensitivity improvement by resonance 15
3.2.3.3 RLC signal readout outside the body 16
3.2.3.4 Improving the RLC signal readout outside the body 17
Chapter 4 Active Electronics circuit development
20
4.1 Active Electronics Circuit Theory 20
4.2 Active Electronics Circuit Experiment 21
4.2.1 Design of the active implant 22
4.2.2 Operation of the active implant 22
4.2.3 Design of the external circuit 23
4.2.3.1 Power generation and power transmission to the implant 23
4.2.3.2 Signal detection and signal processing outside the body 25
Chapter 5 Sensor development 27
5.1 Sensor theory 27
5.2 Sensor Experiment 29
5.2.1 Operation of the sensor 29
5.2.2 Location of the sensor in the implant circuit 29
5.2.3 Finding the optimal resistance value 29
5.2.4 Sensor characteristics 31
Chapter 6 Performance Measurements 34
Chapter 7 Discussion 37
Chapter 8 Conclusions and Future Work 38
References 39
Personal Contact Details 41
Chapter 1 Introduction
A silicone rubber tube filled with saline will be designed to be implanted surrounding the bladder so its
length and cross-sectional area can be modified as the bladder expands or contracts.
The saline solution (“a solution of 0.91% w/v of NaCl, about 300 mOsm/L” [1]) contained within the tube has
resistive properties that oppose the electrical current flow due to the resistivity of sodium and chlorine
[2][3]. If inert platinum electrodes are placed on both ends of the tube and it is sealed with silicon, it is
possible to connect the tube to an electronic circuit. The electronic circuit will pass an AC current through
the tube to prevent electrolysis, this will cause the tube to act as a variable resistance whose value will be
driven by the size of the bladder; a simple and reliable sensor that is inert and causes no harm by being
implanted into the human body is created this way. The saline solution will not change its concentration
since it is isotonic with the blood, which means that the concentration of the NaCl of blood and saline
solution is similar. This also means that both the blood and the saline solution have the same resistivity,
depending on the exact concentration but it is generally considered to be 150 ohmCm for normal
concentration at body temperature [4].
The value of the resistance and therefore, the size of the bladder can be sent by the implant circuit to an
external processing circuit outside of the body. The transmission of signals between the implant and the
outside will be achieved using Electro Magnetic Coupling of two coils, one implanted and one outside the
body as shown in the oversimplified diagram in Figure 1.1.
Figure 1.1. The sensor (silicone tube) surrounding the bladder is represented in orange, the information
then transmitted using electromagnetic coupling of the implant and the outside coils. The signal is then
processed to know the size of the bladder.
1
1.1 Aims of the project
The aim of this project is to build a simple implantable sensor that is able to size and detect the changes in
the shape of the bladder to prevent over-distension of the bladder due to lack of filling sensation .This
may be used in combination with a stimulator implant to prevent and/or decrease incontinence problems.
The main specification for the project is simplicity. The implant has to be cost-effective and reliable.
1.2 Motivations

According to the NHS, there are around 3 million people in the United Kingdom that suffer of
incontinence [5].

“It costs an estimated $19.5 billion a year in the United States to manage urinary incontinence”
[6].
One main restrain for solving urinary incontinence caused by neurological disorders is the need of
information regarding the size of the bladder at any time.
There have been several attempts to measure the size of the bladder. Two approaches are then manly
taken: using medical imaging techniques and using implants.
1.2.1 Estimation of the bladder size using imaging methods
It is a huge advantage to use medical imaging since it relies on non-invasive methods to estimate the
size/volume of the bladder. “Ultrasonography is the gold standard technique for measuring bladder
volume and post-void residual urine” [7], X-rays [8] and MRI [9] have also been used; although, there are
important downsides to the imaging approach:

There are no portable devices that can be estimating the size of the bladder permanently. This is
crucial because the project is aiming to help neurological stress incontinent patients that require
knowing the size of the bladder constantly to know when it is necessary to void it.

The size of the bladder needs to be estimated from complicated algorithms that include empirical
found approximation factors and are not sufficiently specific for each patient and therefore are not
very reliable [10-14].
1.2.2 Estimation of the bladder size using implants
Not many implants have been created to know the size of the bladder and the ones that have been created
are temporary implants using batteries and aimed to know the pressure inside the bladder and relate that
pressure to the volume contained [15-18]. Although, in theory it can be a viable method, in practice, the
pressure changes are small because the bladder is an elastic container and it adapts its size to hold more
liquid without a significant increase of the inner pressure, which is also not linear [19]. The pressure
inside the bladder or vesical pressure depends also on abdominal pressure and the detrusor pressure
2
following the relation Pves = Pdet + Pabd [20], the pressure inside the bladder will change with corporal
movement and increased pressure on either the abdomen or the detrusor muscle [21]. They also use
overcomplicated and expensive methods such as a wireless communication between the implant and the
exterior and a pressure sensor inside the bladder driven by a microcontroller, the problems about this type
of implants are the interference over wireless, their cost, the risk of failure of the microprocessor and the
way of measuring the volume of urine stored in the bladder derived from the pressure.
Another sensor to know the size of the bladder has been created by means of surrounding the bladder with
elastic material that contained Polypyrrole as a strain gauge to provide a change in resistance; although
the polypyrrole is biocompatible, the elastic fabric and therefore the sensor is not inert and it will be
degraded with time, changing the output of the sensor. Further, to fully cover the bladder interferes more
to the physiology of the bladder and the sensor does not consider the transmission of the value of the
impedance to valuable information received outside the body [22].
The main motivation for the development of this implant is to help patients with neurological diseases to
maintain or recover safe and appropriate dimensions of their bladder and improve the functioning of their
urological system.
1.3 Layout of Report
This document aims first to help the reader familiarize with the problem that the project is targeted to
decrease or solve; information about incontinence and the bladder anatomy and physiology as well as the
physics and electronic bases are presented in the literature review. Readers familiar with these subjects
might like to skip the literature review.
There is a separation between the sensor and the electronics since they can be developed separately.
Technical information about the work carried out on the circuits begins on chapter 3 and 4, where
electronics are treated more deeply.
Theory, development and experiments applied to the sensor are presented in chapter 5. It is strongly
recommended to the reader not to skip the theory in chapters 3, 4 and 5 so a general overview of the
project can be acquired.
3
Chapter 2 Literature Review
2.1 Clinical Review
The bladder is a vital part of the urologic system. It is an organ located in the pelvic cavity behind the
pubic symphysis formed mainly from an extraordinarily flexible tissue and smooth muscle which is used
for the storage of urine before the liquid is released from the body [23].
When the bladder contains no urine, it has the shape of a tetrahedron with one fixed vertex to the pelvic
floor, such vertex is the neck of the bladder and it is connected to the urethra where the urine is released
during the voiding phase of the bladder [24]. As the bladder receives liquid from the urethers its superior
side expands as a balloon in slightly different shapes that vary from person to person but that can be
validly approximated to a spherical or ovoid shape [25].
Oversimplifying, the bladder is an elastic reservoir which receives the urine from the kidneys through the
urethers, generally holding a volume of around 500 ml in a normal adult person but its capacity can hold
up to one litre [24].
The bladder sends signals through the nerves to let the brain know when it is reaching its volume limit
and it is necessary to be emptied, something that we perceive as urgency to void; this is commonly known
as micturiction reflex and it helps maintaining the volume stored inside the bladder within safe limits.
Holding more urine than safe, at once, becomes almost impossible since the micturiction reflex can
become so intense that it provokes an insufferable sensation and even pain before uncontrolled void
happens to protect the bladder from over-distension. Over-distension nevertheless, is possible by
exploding the viscoelastic properties of the bladder tissue. This means that the volume capacity of the
bladder can be permanently increased over a period of time when excess of storage volume is repeatedly
forced to remain in the bladder [26].
The bladder is innervated by the Lumbar and Sacral region of the spinal cord. If a neurological problem
occurs and the sensory limb of the spinal reflex arc or the sensory components of the ascending spinal
cord tracts are interrupted, the patient will not be aware of bladder filling and not receive any afferent
stimuli from the bladder. The lack of micturiction reflex causes infrequent voiding as the patients do not
know and tend to forget when they have to urinate.
4
Figure 2.1: the bladder is innervated by the Lumbar and Sacral region of the spinal cord. When there is an
injury above those regions, the sensitive and sometimes the motor communication channels between the
bladder and the brain are broken so the patient does not feel when it is necessary to void the bladder.
Infrequent voiding develops large bladder capacity which affects the ability of the detrusor muscle to
contract and empty the bladder, so if the sensory communication channel is broken, there is no way to
know the volume that the bladder contains, this tends to overextend the bladder until a point where plastic
deformation appears and it is no longer possible for the bladder to recover its original size when empty;
this, added to the loss of muscle contraction power causes incomplete voidance of the bladder and
bacteria can be formed inside the bladder causing very severe infections that could lead to death due to
renal failure [27].
The motor control communication channel disruption between the brain and the muscles provokes a loss
of the detrusor and pelvic floor muscles control and they get atrophied, becoming too weak to hold the
stored urine inside the bladder when the vesicular pressure of the bladder increases naturally as the
amount of liquid contained in the bladder also increases or due to an increase of abdominal pressure
known as stress, this causes incontinence. The muscles then can be stimulated to help the bladder hold the
urine, but even in this case, the knowledge of the volume of the bladder is required.
2.1.1 Incontinence
“Incontinence is the unintentional leaking or release of urine” (NHS et al) due to a malfunction of the
bladder mainly but it can involve some deficiencies in the nervous system that receives signals from the
bladder regarding its status (when filled, the signal is perceived as the need to urinate) and also sends
signals that controls the contraction of the bladder in order to release the stored urine when it is necessary.
The mechanism used by the bladder is protective because when it is filled, the pressure inside and the
tension in the wall increase. The bladder could break if the pressure is not released.
5
There are different types of incontinence; this project is targeted to the neurologic stress incontinence
caused by detrusor over-activity in certain diseases such as: Parkinson’s disease, diabetes mellitus, spina
bifida, multiple sclerosis, spinal cord injury, cerebrovascular disease, pelvic surgery, trauma, etc [28]; but
it could also be applied to non-neurological stress incontinence.
Stress incontinence refers to the
unintentional leakage of urine due to sudden increases in abdominal pressure or unintentional detrusor
muscle contractions.
2.2 Electronics Review
Saline solution is a mixture of water and salt (NaCl - Sodium chloride). Both elements of the NaCl
compound can conduct electrical current but as any conductor it will have a certain resistance opposing
the free flow of electrons passing through it.
A silicone tube filled with saline, as any conductive or semi-conductive material, used as a wire will have
a resistance opposing the electrical current according to the formula 2.1 [29]:
𝑙
R= ρ𝐴
Formula 2.1. Where ρ = resistivity of the saline solution; 𝑙 = length of the tube;
𝐴 = cross-sectional area of the tube
In case of a deformation, i.e. when the tube is stretched, the length of the tube and the cross-sectional area
will change and will modify the resistance of the tube. If the tube is placed around the bladder, the
expansion or contraction of the bladder will cause deformation of the tube. Therefore, the changes in the
shape and size of the tube will be reflected on the value of its resistance, so the resistance would be a
direct measurement of the size of the bladder, that way the sensor is created.
Nerve could then be stimulated as needed once the information regarding the size of the bladder is
obtained.
6
Chapter 3 Passive Electronics circuit
development
3.1 Passive Electronics Circuit Theory
Once established that the output of the sensor is a resistance value, it is necessary to define how to use
that resistance given by the tube.
Looking for simplicity, initially the implant was thought to be created using only passive electronic
components (resistors, capacitors and coils) powered from the outside using electromagnetic transfer of
energy to reduce complexity and cost. The problem inherent to this approach is the small energy that can
be transmitted from the implant to be read outside the body.
3.1.1 Two pendulums transfer of energy
There is a very popular physics demonstration that involves two pendulums connected by a string. The
two pendulums are initially at rest until there is a stimulus on one of the coils, this energy causes the
stimulated pendulum to oscillate at its maximum amplitude until the energy starts being transferred
through the string to the other coil, there is then a constant energy transfer from one pendulum to another
with a rhythmic pattern causing one pendulum to be oscillating at its maximum amplitude while the other
pendulum is at rest, after a time, depending on the length and tension of the string, the oscillation pattern
is inverted. The energy causing the oscillations is conserved in the system over a long period of time.
In order to solve this energy problem it was thought of creating the electrical equivalence of this physics
experiment so the energy that reaches the implant would not be lost immediately but it would be
transferred from “pendulum to pendulum” and the way of this transfer would be determined by the
resistance between the pendulums.
7
Figure 3.1. The physics experiment consists in two pendulums and a single impulse in any of them. At the
beginning, only one pendulum will be oscillating, and then the energy of the oscillation will be
transferred to the other pendulum entirely, making this other pendulum oscillate at its maximum while the
other stays still.
3.1.2 Two pendulums electrical equivalence
Because it is required to have two “pendulums” to have energy transfer between them, for the electrical
equivalence, it was logical to have two circuits that can oscillate. The oscillation circuits are LC circuits
resonant at the same frequency connected through a resistance; as the resistance will determine the way
that the energy is transferred, the sensor can be placed between the resonant circuits. The proposed circuit
is shown in Figure 3.2, both LC circuits resonate at a selected frequency of 1 MHz; this is achieved by
matching the impedances of the coil and the capacitor at that particular frequency.
?
Figure 3.2. This is the proposed electrical equivalent circuit to the two pendulums demonstration.
8
In the electrical circuit as in the two pendulums demonstration, the expected transfer of energy is shown
in Figure 3.3, the pattern should then be repeated with decreased amplitude following an exponential
decay.
Figure 3.3. This is the transfer of energy between the pendulums and the expected behaviour of the two
resonating circuits.
For the circuit, a couple of coils were built and connected through different values of resistances as shown
in Figure 3.4, and a third coil connected to a pulse generator circuit was used for the stimulation of one of
the coils as shown in Figure 3.5.
Figure 3.4. Complete schematic for the proposed electrical equivalent to the two pendulums experiment.
The left part is the pulse generation for the stimuli; the right part represents the two resonant circuits
coupled by a variable resistance.
9
3.1.3 Electromagnetic coupling requirements
The technical requirements for the circuit to work were the two LC circuits of the implant resonating at
the same frequency and a pulse that excites the circuits. The pulse has to be set to match the frequency of
the resonating LC circuits or the circuits will not resonate; this is achieved by creating a pulse o f a width
or time constant equals to the inverse of the required frequency since the pulse width is measured in time
domain, this relation is expressed as (Horowitz et al):
1
T=𝑓
Where T = time constant and f = frequency.
Since the implant will be placed behind the skin, a variable layer of fat and the anterior-muscle wall, the
other very important requirement is a large communication distance between the implant and the
processing unit coils, at least 5cm. In order to create a big electromagnetic field and have communication
between the inside and outside coils at a wide range of separation distances, and for the coils to be very
selective to resonate at a specific frequency, the coils need to have a big Q or quality factor, the Q of the
coil depends on the frequency (f), the inductance (L) and inversely to the resistance of the coil (R)
according to the formula [30]:
Q=
𝜔𝐿
𝑅
Formula 3.1. Where ω = 2πf, and R = resistance of the coil.
The inductance is given by the formula [31]:
L=
𝜇 𝑜 K 𝑁2 A
𝑙
Formula 3.2. Where 𝜇𝑜 = permeability of free space; K = Nagaoka coefficient; N = number of turns; A =
cross-sectional area of the coil; l = length of the coil.
So at a given frequency and for a cylindrical coil if a big quality factor is desired, it is necessary to have a
considerable number of turns, cross-sectional area and/or the length of the coil, this is a limitation for the
size of the communication channel.
3.2 Passive Electronics Circuit Experiment
3.2.1 Two pendulums experiment
The coils were built with 10 turns over a cross-sectional area of 19 𝑐𝑚2 to achieve:
Coil 1
Coil 2
Q1 = 89
Q2 = 85
R1 = 6.3 kΩ
R2 = 5.7 kΩ
Z1 = 70.22 Ω
Z2 = 67.5 Ω
L1 = 11.17 uH
L2 = 10.7 uH
Table 3.1. Coils characteristics.
10
In order to create a resonant circuit, a capacitor that matches the impedance of the coil at the selected
frequency is required, so the values of the capacitors were found using the formula [32]:
1
C = 𝜔𝑋𝑐
Formula 3.3. Where Xc = Reactance of the capacitor and is equivalent to the impedance (Z) of the
inductor.
The values for the capacitors were:
C1 = 2.26nF and C2 = 2.35nF
Figure 3.5. This is the proposed circuit in reality. The resonant coils are the ones mounted in green plastic
to achieve more durability and consistence of the values for the coils’ characteristics.
Careful handling of the coils is necessary in order to keep their initial characteristics since the wire tends
to “uncoil” itself and gets lose. A way to maintain the coil in optimal status is to leave them mounted in a
support as shown in Figure 3.5, in this case plastic.
The results obtained from the proposed two pendulums equivalent circuit were not as expected since
energy is lost before a second cycle as shown in Figure 3.6.
11
Figure 3.6. The waveform above represents the resonating circuit that receives the impulse. The
waveform below is the second resonating circuit.
It is important to note the scale at which the image was taken: 50mV for the top waveform and 20mV for
the one on the bottom, this values would be enough if the process unit was measuring directly in the
implant, but as the signal must be detected in the outside of the body, the measurement of these signals is
impossible.
Since the transfer of energy between the resonant circuits did not match the requirements with the simple
design of two resonant circuits coupled by a resistance, an alternative was tried placing a capacitor to
couple the resonant circuits instead of the variable resistance, the results for the waveform of the transfer
of energy were closer to the expected, having a second increase of amplitude in the stimulated resonant
circuit as shown in Figure 3.7. The output was still not high enough and the energy transfer was as
expeted, therefore this approach was also not suitable for being used for the implant circuit.
Figure 3.7. Waveforms for the transfer of energy between the resonant circuits coupled by a capacitor
after a impulse stimuli. The waveform at the bottom is the output from the resonant circuit receiving the
stimuli. The waveform at the top belongs to the resonant circuit that was not stimulated from the outside.
12
At this point it had been proven that the proposed circuit had an output waveform similar to the expected
but also that such approach needed to be modified to have a measurement of the bladder size outside the
body.
3.2.2 RLC passive implant experiment
As the energy was lost almost immediately after the pulse, it was considered to be unnecessary to have
two resonant circuits so one of the resonant circuits was removed and the variable resistance was placed
in parallel with the LC resonant circuit of the implant.
Figure 3.8. RLC resonant circuit with a variable resistance to modify the response of the circuit proposed
for an alternative circuit implant.
The impulse generation circuit designed for the outside of the body remained unchanged.
The results appeared to be useful for the project since the value of the resistance placed in parallel with
the resonant LC circuit modifies the response of the circuit in a very distinctive way. This meant that it
was possible to know the value of the resistance by processing the amplitude and the frequency changes
of the waveform generated by the implant as shown in figure 3.9.
13
a)
b)
c)
d)
Figure 3.9. Changes in response of the RLC resonant circuit placed in parallel with different resistor
values. a) 1kΩ; b) 2kΩ; c) 3kΩ; d) 4kΩ.. The waveforms are measured with the oscilloscope at the nodes
of the implant.
3.2.3 Detection of signals outside the body
The results shown in figure 3.9 were obtained by measuring the energy resonating directly on the nodes
of the implant circuit but it is required to have a signal from the implant reaching the outside the body
detected using an external coil.
3.2.3.1 Sensitivity improvement by increasing Electromagnetic Field
The problem remained the same, the readout of the signal coming out from the implant was impossible to
be detected after a couple of centimetres of separation because the energy is too low. To improve the
sensitivity of the reception, a few modifications were made:

An additional transistor was used to increase the input pulse power on the outside coil to create a
greater electromagnetic field.

Switches were added to isolate the coil from any noise coming from the pulse generator so the
circuit was more sensitive to receive the signal generated by the implant.
The modified circuit is shown in figure 3.10.
14
Figure 3.10. Modified circuit used to increase the current flowing through the outside coil to create a
greater Electromagnetic field in order to increase the range of distances separating the implant from the
outside unit. The switches were added to isolate the receiver coil and increase its sensitivity.
3.2.3.2 Sensitivity improvement by resonance
No useful improvement was found by using the approach presented in section 3.2.3.1 and it was decided
to try forcing the outside coil to resonate at the same frequency as the RLC resonating circuit of the
implant, the result was interesting because the interaction of the circuit produced a signal that was closer
to the pendulums circuit that we were initially looking for. Although this approach was no useful for the
aim of the implant, the pendulums equivalent circuit output was closer to the initially proposed circuit, so
it can be established that a good two pendulums equivalence is achieved by using electromagnetic fields
to couple two resonating RLC circuits.
15
Figure 3.11. This image is produced when both the inside and outside coils resonate at the same
frequency. The waveform at the top represents outside coil that produces the impulse. The waveform at
the bottom is the resonating circuit of the implant.
So the initially expected waveform was obtained but the problem with the separation of the coils was still
present. At this point, 3cm separation between coils was the maximum distance achieved; this was not
enough for the purposes of the implant.
A new problem was introduced with this approach: the amplitude and the frequency were varying with
the separation of the coils, this is not desirable since the only variation source the project needs is from
the bladder via the variable resistance. This turned out to be very inconvenient so the decision was to go
back to the approach that gave out the best results: one resonant LC circuit in parallel with the variable
resistance and a pulse generation as shown in figure 3.8.
3.2.3.3 RLC signal readout outside the body
The output shown in Figure 3.12 corresponds to the implant circuit response in blue and the
transmitter/receiver coil in the outside circuit. It can be noticed the big impulse in the orange waveform
followed by a decay due to the inductance of the coil and controlled by the diode, during this period, the
coil is “blind” to any signal produced by the implant circuit, this can be noticed at the figure looking at
the first 3μs being ignored.
After the fist 3μs the waveform of the outside circuit, represented in orange, follows the shape of the
implant circuit response, represented in blue.
From the output of the coils, it can be further noticed the high frequency oscillations on the outside coil
caused by the capacitance of the circuit causing resonance at the frequency determined by the inherent
characteristics of the coil.
16
Figure 3.12. Output waveforms when using one resonant circuit for the implant and one
transmitter/receiver coil. The response from the outside coil is plotted in orange and the resonance
measured at the implant RLC circuit is shown in blue.
3.2.3.4 Improving the RLC signal readout outside the body
There are two possible solutions to improve the readout of the out-coming signal:
1. Create a low-pass filter that eliminates the high frequency oscillations from the receiver coil.
2. Create separate coils for transmitting pulse to the implant circuit and receiving the signal coming
out of it.
Both solutions were suitable but difficult to implement. However, the second was chosen since the
receiver coil is able to sense any incoming signal even when the transmission of the pulse is taking place.
For the first solution, even if the high frequency oscillations could be eliminated there would be a loss in
data for approximately 3μs, which contain the biggest amplitude of the signal from the implant circuit.
The requirement for this idea to work is to have the two coils one right next to the other without causing
any type of coupling between them. This is achieved by changing the shape of the coils to change the
paths that the current follows in respect to each other to create different electromagnetic fields for each
coil and reduce the coupling coefficient between them. The required shape and the current flows are
shown in Figure 3.13 [33].
17
Figure 3.13. Shape and positioning of coils to reduce the coupling coefficient between them: coil 1 is
shown in red with its respective current flow path; coil 2 is represented in black with its respective current
flow.
In reality, the arrangement of coils had 3 layers, from the surface to the centre:
1. Copper wire forming the coils.
2. PVC plastic tube for support of the coils, one piece per coil.
3. Glass fibre for the PVC to be mounted on, this permits each of the PVC parts to rotate to find the
angle between coils that can reduce the coupling to the maximum.
Besides having inductive coupling, the coils have capacitive coupling due to the wires, this problem is
reduced by using an aluminium ring between the coils connected to earth as shown in figure 3.14, so
capacitive coupling can be “captured” and taken away.
18
Figure 3.14. Two coils were used. To reduce the coupling inductance between them, the coils were built
in the shape shown in figure 13 and figure 14, and an aluminium ring was added between the coils to
reduce the capacitive coupling.
The coupling coefficient between the coils could not be eliminated so the pulses from the transmitting
coil produced resonance on the receiving coil; this can be considered noise and could not be eliminated,
just reduced. The reduction was not enough for reading the output signals from the implant.
Figure 3.15. The input pulse of the transmitting coil in the outside is the wave on the top. On the bottom
the noise that the reading coil receives only from the transmission coil can be observed, the noise is too
big for the signal we are trying to receive from the implant.
The main problem that appears when using passive electronics is that the energy of the signal is not
enough to travel the desired distance to the implant and back from it.
At that point, it had been proved that the use of passive electronics was not suitable for the type of
implant that the project required.
19
Chapter 4 Active Electronics circuit development
4.1 Active Electronics Circuit Theory
An alternative approach using active electronics powered by the outside circuit using electromagnetic
coupling was then applied.
The implant circuit is passive the majority of the time, receiving energy to power the electronics in order
to generate pulses at different pulse repetition frequencies, these pulses will short-circuit the coil,
modifying the electromagnetic coupling of the coils and giving an amplification on the outside coil
waveform. The period or repetition frequency of the pulse is generated by an oscillator circuit in which
the value of the resistance is the silicone tube sensor and a RC filter. The repetition frequency of the pulse
will depend on the size of the bladder.
Figure 4.1. Schematic of the new approach for the project using active electronics in the implant circuit,
powered by electromagnetic coupling. The repetition frequency will vary as the circumference of the
bladder varies.
In order to power the implant, it is necessary to have a signal generator that will pass current through the
coil in a very specific manner so an electromagnetic link can be created between the coils of the circuit
outside the body and the implant circuit.
20
4.2 Active Electronics Circuit Experiment
Figure 4.2. The implant circuit on the middle, the power generator and signal processing circuit on the
bottom. The variable resistance made of a silicone tube filled with saline is at the top.
21
4.2.1 Design of the active implant
For the final design of the implant, the active electronics used include one integrated circuit hex inverter
that contains 6 inverter gates, one voltage regulator and one power transistor; the rest of the circuit is
composed by passive electronics: resistances, diodes, capacitors and one coil.
Figure 4.3. Final design of the implant circuit. AC voltage is rectified by D2 and C3 and controlled by the
Vreg that powers the inverters. The square wave source is generated by the two inverters in the left with
R1, Rvar and C1, this circuit controls also the period or pulse repetition frequency. The pulse width is
controlled by R2 and C4. The short circuit is managed by transistor M1.
4.2.2 Operation of the active implant
The function of this circuit is to produce pulses of variable repetition frequency controlled by the sensor
and it includes 2 main phases:
1. Power generation from the outside signals.
2. Pulse generation
For the first phase, a sinusoidal power signal at the selected resonating frequency is received and
produces an AC power signal. The AC power signal is rectified with the use of a diode and a capacitor so
it is converted into a variable DC voltage source that is controlled by the voltage regulator so it cannot
exceed the maximum voltage supported by the inverter.
For the second phase an AC must be used to go through the sensor, an AC circuit must then be
implemented to prevent electrolysis in the cells and tissues that are in contact with the variable resistor,
this is achieved by two inverters connected with two resistances and one capacitor act as a square wave
generator, this square wave is used to control the period of the pulses with a frequency f = 1 / RC [34]
using a capacitor, a fixed 1M resistance and the variable resistance, the signal is the passed through an RC
filter to eliminate noise and create only clean positive impulses that go from 0v to a maximum voltage
determined by the output voltage of the voltage regulator. The pulses will drive the transistor to short
circuit the capacitor and most importantly, the coil; this is what changes the coupling between the coils
and gives useful data to analyze.
22
Figure 4.4. Pulse generated by the implant.
4.2.3 Design of the external circuit
The external circuit is more complicated than the implant’s, it includes a signal generation stage to
produce an AC power for the implant using a coil and a pair of capacitors and a signal processing stage
including rectification, signal’s DC component removal and a comparator.
Figure 4.5. The complete power transmission and signal processing unit. The impulse generation is the
part on the left of the coil L1. The signal processing begins in the node connecting C1 and C2. The
rectification of the signal is made by the diodes D1 and D2, resistance R1 and capacitor C3. Capacitor C4
eliminates the DC component of the signal. The OPAMP functions as a voltage comparator of the signal
versus a reference voltage that can be modified to change the sensitivity and range of the electromagnetic
coupling.
4.2.3.1 Power generation and power transmission to the implant
The power signal is generated using a square wave source oscillating at a frequency equal to 1 MHz, this
signal is connected to a hex inverter and a pair transistors, one P-type and one N-type to increase the
23
current and drive the coil. The LC resonant circuit consumes a very high current, up to 300mA are
consumed by the whole power circuit when resonance is achieved by setting the right frequency at the
signal generator source; this current consumption can be observed in figure 4.6 by looking at the peak
valleys (1, 2) in what would be a square wave if the LC circuit was not present or resonating, the current
drawn affects the transistors and the shape and the size of the valleys depend on the power delivered by
the pair of transistors, the top peak (1) is controlled by the P-type and the bottom valley (2) depends on
the N-type transistor. Ideally, the valleys on the peaks should be as small as possible, more than one
transistor can be used to increase the power output, they have to be the same type and be connected in
parallel. The depressions are hints of the resonance of the coils, if the frequency is the correct one, the
two peaks on the sides of the depression (3 & 4, 5 & 6) will be the same size, if one is significantly
bigger than the other, and the frequency has to be corrected.
Figure 4.6. Square wave with depressions on the peaks due to current being drawn by the LC resonant
circuit.
The coil is connected to a couple of capacitors for a double purpose: to force the coil to resonate and to
create a voltage divider.
A peak voltage of 7 volts enters the coil, by the time the current exits the coil, the initial voltage is
multiplied by the quality factor (Q) of the coil producing approximately 700 volts. This voltage is
excessive for further analysis, so it is necessary to decrease the voltage by using the voltage divider. The
coil remains resonating since two capacitors in series have the almost the same capacitance of the smaller
one of them; the small capacitor is the one producing the resonance.
A sinusoidal waveform is created by the effect of the coil and the capacitors driven by the input square
wave from the signal generator. This sinusoidal is the pulse that will power implant circuit.
24
Figure 4.7. Sinusoidal waveform created by the LC resonant circuit that is used to power up the implant.
4.2.3.2 Signal detection and signal processing outside the body
As long as the implant circuit is receiving power passively, the electromagnetic coupling of the coils will
remain unchanged and the sinusoidal will be a stable input to the signal processing part of the project.
When the implant circuit modifies the coupling, the sinusoidal will be altered producing a change in
amplitude.
Figure 4.8. Coupling between coils being modified by a pulse that short-circuits the implant coil. The
bottom waveform shows the pulse that causes the short circuit of the LC implant circuit. The top
waveform is obtained at the voltage divider formed by the capacitors and it is the input to the signal
processing stage.
When the valuable information appears as a change of amplitude in a signal it is commonly known as an
amplitude modulated signal and it is processed by a rectification phase, followed by the removal of the
DC component and finally a voltage comparator, all this is made so by the end of the process, a neat zero
voltage line with a chosen voltage peak is obtained; the peaks appear when the coupling between the coils
25
changes. The frequency at which each coupling change occurs is linearly proportional to the size and
shape of the sensor, which is controlled by the size of the bladder.
Figure 4.9. Changes in the frequencies of the pulses as the value of the tube is stretched provoking
changes in the values of the sensor’s resistance.
A frequency to voltage can be added to be compared with a selected threshold to let the patient know that
it is necessary for him to empty his bladder. A LED or beeping is a simple and reliable solution.
26
Chapter 5 Sensor development
5.1 Sensor theory
The bladder is a flexible reservoir that expands or contracts changing its volume and its circumference
depending on the amount of urine it contains. In the storage phase, the bladder will be expanded as more
urine is passed to it from the urethers changing in size, as represented in Figure 5.1 section a) the size of
the bladder will increase, stretching the silicone tube and therefore increasing the length and reducing the
cross-sectional area of the tube, this modifies the value of the electrical resistance of the tube represented
in section b).
Figure 5.1. a) The bladder expands with time due to the volume of liquid contained inside; b) the shape of
the silicone tube sensor will change in Length and Area changing the value of the resistance.
The bladder can take different shapes but it can be considered as a spherical shape so, as the volume of
urine contained within the bladder increases, the circumference of the bladder will increase, if we implant
the tube around the bladder, the tube will have approximately the same value as the circumference of the
bladder, this relation will be maintained in different proportions that can be calibrated for each patient.
The spherical shape is represented in figure 5.2, where the change in volume stored in the bladder is
plotted against the length of the tube.
27
Bladder Volume vs Tube Length
1200
1000
800
Bladder
Volume (cm^3)
600
400
200
0
15
20
25
30
35
40
Tube Length (cm)
Figure 5.2. Change of the length of the tube as the volume of the bladder increases. Bladder is considered
as a sphere.
The Formula 2.1 expresses the relationship between electrical resistance and the shape of a partially
conductive material. For the type of sensor that will be used for this project, the length and the area will
be determined by the silicone tube that adapts the better to the human anatomy without interfering with
the normal physiology of the body and also on the size of the bladder, the resistivity of the material
indicates how much resistance the material will put against the flow of current. The Formula 2.1 can be
used to calculate the expected values from the changes in the Area and Length of the tube as the bladder
volume changes and therefore the resistance value in the implant circuit. The expected results are
displayed in Table 5.1.
A (cm2)
L (cm)
Bladder Volume (cm^3)
Resistance (Ohms)
0.0145
38.97
1000
400,951
0.0151
37.63
900
373,754
0.0157
36.18
800
345,529
0.0164
34.60
700
316,099
0.0172
32.87
600
285,228
0.0183
30.93
500
252,583
0.0197
28.71
400
217,669
0.0217
26.09
300
179,682
0.0249
22.79
200
137,123
0.0314
18.09
100
86,382
Table 3.1. Expected values for the bladder and the sensor as the size of the bladder changes.
28
Therefore: the value of the resistance will vary according to the size of the bladder; this is the aim of the
sensor.
5.2 Sensor Experiment
5.2.1 Operation of the sensor
The variable resistance controls the frequency at which the pulses are sent to drive the transistor to alter
the electromagnetic coupling between the implant and the outside circuit. The time constant is determined
by the product of the values of the resistance and the capacitor according to the relation [35]:
T = RC
Where: T = time constant, R = resistance value and C = capacitance value
5.2.2 Location of the sensor in the implant circuit
The time constant can control the duration of the pulse if it is placed on the RC pulse generation section
of the implant but the time constant is in the order of ms and the changes of the resistance over the pulse
width are very small compared with the changes that the variable resistance can produce on the period,
the pulse repetition frequency. If the resistance is placed on the oscillation signal generation, the
resistance will control the time at which the repetition pulses will appear; the time elapsed between one
pulse and the next is the inverse of the frequency. After the signal generation, we still have a pulse
generation controlled by a constant RC circuit that filters the signal from any undesired random noise.
5.2.3 Finding the optimal resistance value
The circuit was tested using different values of resistances and it was found that the expected values were
inconveniently big because the low repetition frequency allowed the appearance of a source of noise at a
frequency higher than the frequency of the repetition of the pulses (Figure 5.3). In order for the circuit to
discriminate the noise, the sensitivity of the system could have been reduced, reducing the
communication range between coils as well but, if the value of the resistance is decreased, the pulse
repetition frequency will be higher than the noise frequency and it will be overridden by the legitimate
pulses.
29
Figure 5.3. In the image at the left, an unidentified source of noise appears at low frequencies. The
problem is solved when the frequency is increased by decreasing the resistance values of the sensor.
The decrease of resistance for the silicone tube filled with saline, according to Formula 2.1, can be
achieved by increasing the cross-sectional area, reducing the length of the tube or reducing the resistivity
of the material. The length of the tube cannot be changed because it depends on the circumference of the
bladder; the cross-sectional area of the tube needs to remain small since it will be implanted around the
bladder and it must be comfortable and interfere as little as possible with the normal body functioning.
The way that the resistance value of the sensor was chosen was by modifying the saline solution; the
electrical current travels through the tube using the metals of the salt, if the amount of salt is increased,
the resistivity is decreased and the resistance of the sensor is then decreased proportionally.
The principle of the resistance change depending on the stretch remains unchanged so the length and
cross-sectional area modifications will still be proportional to the resistance after the value of the
resistance of the tube is decreased.
The initial saline solution was prepared using 9grms of salt per Litre of water; using careful
experimentation, the salinity was increased using an exponential function beginning with steps of 1gr of
salt each and finishing with steps of 20grs aiming at a resistance with a value of approximately 5kΩ. The
final solution contained approximately 120grs of table salt per Litre of water.
It is very difficult to create a mix of salt and water with the exact resistivity value, so it was decided that
7.01kΩ can be considered good enough for the initial value of the resistance.
30
Figure 5.4. The measurement of the length, resistance and impedance values of the sensor. The
impedance and resistance values are the same as expected from a resistance with no capacitive or
inductive properties.
5.2.4 Sensor characteristics
The resistance created has the following initial characteristics:

Length = 21 cm

Radius = 1 mm or .1cm

Area = 𝜋𝑟 2 = 0.031415927 cm

Volume = A*L = 0.659734457
The volume of saline solution will remain constant within the tube regardless of the changes caused by
the bladder; this, added to the obtained resistance of the unchanged tube permits the calculation of the
resistivity of the saline solution, calculate the bladder volume contents and estimate values for the
changes of length and area of the sensor when it is stretched. The length of the tube is considered to be
equal to the bladder circumference. The results are shown in table 5.2.
31
Tube Area (cm^2)
0.031
0.030
0.029
0.027
0.026
0.025
0.024
0.024
0.023
0.022
Tube Length (cm)
21.000
22.000
23.000
24.000
25.000
26.000
27.000
28.000
29.000
30.000
Bladder Volume (cm^3)
156.389
179.811
205.462
233.444
263.857
296.804
332.384
370.700
411.854
455.945
Expected
Resistance (kΩ)
7.01
7.70
8.42
9.16
9.94
10.76
11.60
12.48
13.38
14.32
Measured
Resistance (kΩ)
7.01
7.85
8.55
9.35
10.16
11.01
11.87
12.79
13.71
14.67
Table 5.2. Changes in the characteristics of the sensor as it is stretched. Expected bladder volume is also
considered.
The most relevant characteristics changes from table 5.2 are plotted in Figure 5.5 and 5.6 where it can be
noticed that the expected values are similar enough to the measured one.
Resistance vs Tube Length
15
14
13
12
Resistance
11
Value (kΩ)
10
9
8
7
Expected Resistance
(kΩ)
Measured Resistance
(kΩ)
20
25
30
Tube Lenght = Bladder Circumference (cm)
Figure 5.5. The change in the expected and the measured resistance values of the sensor is plotted in
respect of the change of the tube length.
32
Resistance vs Bladder Volume
15
14
13
12
Resistance
11
Value (kΩ)
10
9
8
7
Expected Resistance
(kΩ)
Measured Resistance
(kΩ)
150 200 250 300 350 400 450 500
Bladder Volume (cm^3)
Graph 5.6. The change in the expected and the measured resistance values of the sensor is plotted in
respect of the change of the expected bladder volume.
The graphs show very helpful information because it proves the theory that there is a linear relation
between the tube, the resistance changes and more importantly the expected bladder size. The sensor
works properly.
33
Chapter 6 Performance Measurements
Typical Performance Characteristics
Frequency Response vs Tube Length
6
5.5
5
4.5
Frequency
4
(kHz)
3.5
3
2.5
2
20
21
22
23
24
25
26
27
28
29
30
Length of Sensor (Cm)
Figure 6.1. Frequency dependence on tube length at room temperature. The value of the resistance was
measured by stretching the sensor linearly and measuring its resistance using an impedance analyzer at
10kHz; the values of the resistance were substituted on the actual circuit to obtain the frequency response
measured using an oscilloscope.
Frequency Response vs Resistance
25
20
15
Frequency
(kHz)
10
5
0
0
1
2
3
4
5
6
7
8
9
10
Resistance
KΩ
Figure 6.2. Frequency dependence on resistance value changes at room temperature. A variable resistance
box was used to input different resistance values to the actual circuit; the frequency response was
obtained from the circuit was measured using an oscilloscope.
34
Frequency Response vs Temperature
11
10
9
Frequency
(kHz)
8
7
6
5
20
25
30
35
40
45
50
55
60
Temperature (°C)
Figure 6.3. Frequency dependence on temperature. The resistance value was heated using a boiler and
measured using an impedance analyzer at 10kHz; the resistance value was placed actual implant circuit
and the frequency was measured using an oscilloscope.
Frequency Response vs Coil Spacing
8
7
6
5
Frequency
4
(kHz)
3
2
1
0
2
3
4
5
6
7
8
9
10
Spacing (cm)
Figure 6.4. Frequency dependence on coil-coil spacing (coupling coefficient) with a resistance with a
value of 5 kΩ to avoid changes in response due to temperature or position of the real sensor;
35
Resistance Changes per Day
6.4
6.38
6.36
6.34
Resistance
6.32
KΩ
6.3
6.28
6.26
6.24
0
1
2
3
4
5
6
Day
Figure 6.5. Resistance value change per day stored in isotonic saline measured using an impedance
analyzer at 10kHz at room temperature. The position of the resistance could not be the same for all
measurements so values of the resistance vary with slight changes of positions of the sensor and
temperature of the room.
Electrical Characteristics
Parameter
Min.
Typ.
Max.
Unit
Supply Voltage
6
7
9
V
Current Consumption (External Unit)
180
220
410
mA
Current Consumption (Implant)
.1
5.3
8.3
mA
Output Frequency
781.3
7.57k
23.15k
Hz
Sensor Resistance
1
5 – 10
60
kΩ
Frequency dependence on power supply voltage
No change within recommended limits
36
Chapter 7 Discussion
One of the main problems when trying to solve or decrease neurological stress incontinence in patients is
the lack of constant real-time information regarding the size of the bladder.
Several attempts have been made to estimate the size of the bladder using non-invasive methods but the
results are not accurate enough. Some implants have been successful at giving approximations for the
bladder size but they are overcomplicated and expensive.
The relative short range of electromagnetic coupling between coils is enough for the type of implant
required and it is very reliable since it is not sensitive to interference from other radio-frequency sources
providing a simple and non-expensive way of transmitting signals between two separate circuits.
The frequency response of the implant to different values of resistance determined by the length of the
sensor decays for increasing lengths in a linear manner (fig. 6.1) in which, the values of frequency can be
validly correlated to the size of the bladder. Figure 6.2 shows that the frequency varies exponentially with
a linear variation of the resistance, this can be useful in further calculations.
The response in frequency is steady for a range of coil-to-coil separation distances from 2 to 8.5 cm (fig.
6.4), this is necessary because the distance may vary with changes of position of the patient, respiration
and body weight changes and it will not affect the output.
The temperature changes show the major effect in response of the implant (fig. 6.3), in the worst case,
more than 1 kHz change occurs per 5 degree Celsius modification, this is equivalent to an error of
approximately 50cm^3 in the final bladder volume reading. The resistance value changes considerably
from day to day stored in isotonic saline (fig. 6.5) showing a random behaviour with a maximum change
of up to 0.13kΩ; if the resistance is stored in isotonic saline, the concentration of the solution giving the
resistance will not change and therefore the changes from day to day can be attributed to room
temperature changes and the position at which the measurement was performed. It is important to notice
that the internal body temperature remains very stable and once the sensor is placed around the bladder
there will be no small displacements that modify its output so the value of the resistance is expected to be
very stable and accurate once implanted.
Reliability, simplicity and cost-effectiveness are three characteristics that the present biomedical
challenges require.
A silicone tube filled with saline has no harmful effect on the body and when added to externally powered
electronics, it creates an implant that can remain safely inside the body for a prolonged period of time.
37
Chapter 8 Conclusions and Future Work
A working implant prototype aiming to monitor the size of the bladder has been created. It uses a silicone
tube filled with saline surrounding the bladder so it can be stretched as the bladder expands during the
storage of urine.
The impedance of the tube is directly related to the circumference of the bladder, when it changes, it
produces frequency variable pulses that are sent via an electromagnetic coupled pair of coils to the
outside of the body for signal processing stage based on amplitude-modulation techniques.
An electrical analogy to the two pendulums physics demonstration was found when trying to create an
implant using only passive electronic components, it was further found that this is only possible for short
range applications and it was not suitable for the project.
For the future work, the circuits will be optimized, and some new features can be added:
A frequency to voltage converter can be added to have a voltage comparison and set a volume threshold.
The circuits can be decreased in size considerably by using a purpose built PCB with surface-mounting
ICs and smaller resistances. The coil can also be reduced in size but the voltage would have to be
increased to achieve the same distance reach for the Electromagnetic field.
Once there is information regarding the current size of the bladder at any time, it is possible to add nerve
stimulation to the implant so the filling sensation and the voiding sensation can be recovered.
Other applications for the implant can include muscular stimulation to solve general stress incontinence
problems by stimulating the bladder neck muscles according to the volume contained within the bladder
so it can retain the urine stored without leaking.
38
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Personal Contact Details
Jorge Chacon Caldera
E-mail:
Jorge.caldera.09@ucl.ac.uk
Permanent address:
Rosario Castellanos #6119.
Colonia Lomas Universidad.
Zapopan, Jalisco.
45016. México.
Mobile number:
+44 (0) 7935992528
Home number:
+53 (33) 31106222
41