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This is an Author's Accepted Manuscript of an article published in Assistive Technology: The Official Journal of RESNA, 23(4),
218-224, 2011 [copyright RESNA], available online at: http://www.tandfonline.com/doi/abs/10.1080/10400435.2011.614675
Urinary incontinence: A vibration alert system for detecting
pad overflow
Bosco Fernandes, Patrick Gaydecki1, Felicity Jowitt and Eleanor van den
Heuvel2
1School of Electrical and Electronic Engineering, University of Manchester,
Manchester M13 9PL, United Kingdom
2Brunel Institute for Bioengineering, Brunel University, Uxbridge, UB8 3PH, United
Kingdom
Email: bosco.fernandes@manchester.ac.uk
Abstract. A sensor and electronics system is described that monitors the leakage of urine from
continence pads into surrounding underwear. Urinary incontinence is involuntary loss of urine
and occurs when the bladder muscles contract without warning or sphincter muscle
surrounding the urethra are too weak to prevent leakage. The system comprises a wetness
sensor and electronics unit. The sensor is stitched into the underwear and detects overspills of
urine from the pad. The electronics unit is attached to the underwear and responds by
vibrating, signalling to the wearer that pad has failed. This system has application for
individuals who use continence pads in the community but it could also be used in care homes.
1. Introduction
Urinary incontinence is common in the adult population, but there is a wide range of prevalence
rates reported in the literature. Some studies report levels as low as 11.5% in women and 6.9% in
men, aged over 65 living in the community [1] but another study reported 58% urinary incontinence in
healthy middle aged women [2]. The wide variation in reported prevalence is caused by
methodological differences between studies [3] with many studies not using standardised validated
questionnaires, different time periods over which participants had to report and different definitions of
urinary incontinence. The international continence society changed their definition of urinary
incontinence from “Urinary incontinence is the involuntary loss of urine that is a social or hygienic
problem” [4] to “-any involuntary loss of urine” [5], in an attempt to facilitate comparative
epidemiological studies. Although precise prevalence levels are difficult to specify, it is generally
accepted that prevalence of incontinence increases with age [6] and that more women than men are
affected by urinary incontinence [7]. Surgical, medicinal and physiotherapeutic treatments that are
available help to alleviate incontinence but if these treatments are not tried or to adhered to, while
waiting for treatments to be effective or when treatments fail, continence pads are often chosen as a
management option [8]. The global market for adult continence pads in 2008 was $44 billion and it is
also known that significant numbers of women manage their continence needs with menstrual pads
[9]. It is therefore clear that vast numbers of people are regularly using pads. Whilst pads are useful
for maintaining “social continence” [10], there are a number of problems associated with pad use.
Research on the effect of wearing continence pads has shown that the ability of the pad to hold urine
was the most important feature in both daytime and night time pad use. However, many women were
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concerned that the pad would leak and lived in a constant state of tension [11]. The inspiration for the
device described in this paper came from Cheryle Gartley, the founder and president of The Simon
Foundation for Continence whose professional and personal experience had impressed on her the need
for an early warning system for pad leaks.
The system here described forms part of a large programme of work that is geared towards tackling
the problems associated with continence difficulties in older people. At its core is a specially designed
undergarment that informs the wearer of failure of the continence pad. The new system will give an
early warning of pad leakage that will allow the user to change pads before wetness spreads to outer
clothing and furniture. This will reduce embarrassment and worry for the wearer and the burden of
cleaning wetted clothes and furniture.
The complete work programme encompasses a wide variety of different disciplines. Seven UK
academic organisations are working on the project, in fields such as gerontology, odour chemistry,
design of public facilities in the built environment and the development of pad leakage detection
systems, of which the authors are contributors.
Urination or micturition is the process of disposing of urine from the bladder to the outside of the
body via the urethra. Its operation is managed by the higher brain centres under the voluntary control
of the individual. The urinary system has two distinct phases of operation; storage and voiding. Urine
is stored in the bladder and can be voided by conscious brain activity. The stimuli for the voiding
phase originates from sensory fibres at the bladder whose firing rates trigger the decision making
process. A partially full bladder produces a low sensor firing rate [12]. During this state, the urethra
and sphincter muscle are contracted and the bladder is distended [13]. At high bladder volume, the
firing rate increases, causing a conscious sensation which makes the individual have an urge to void.
Ideally, the individual eventually consciously initiates voiding, causing the bladder to contract and the
outlet sphincter to relax. Voiding continues until the bladder empties completely. The bladder then
relaxes and the urethral sphincter contracts, allowing the bladder to be refilled [12].
A number of elements in the bladder control cycle can become less efficient and can lead to urinary
incontinence. Continence problems can also be exacerbated or caused by limited mobility as travelling
to a suitable facility and transferring on to the toilet are an essential part of the continence process. For
the purposes of this development project, a means of managing the continence problem is important
but the cause of urinary incontinence is not relevant. Anyone who uses a pad to manage their
continence and is concerned about potential pad leaks could benefit from the alerting system
described. Unlike most assistive technologies, this device does very little to directly improve function
or safety; the benefits of using the device are mainly in increased confidence for the user which
should, in turn, facilitate greater participation.
2. Current methods for monitoring urinary incontinence
Several researchers and organisations worldwide have devised methods that allow urinary
incontinence to be monitored and controlled. Most of these are designed to be used in care homes and
consist of sensors placed inside continence pads and feature wireless signal transmission to care
workers. A selection of the current methods is described below.
The Simpad is a design that incorporates a reusable sensor into a standard continence pad and uses
a wireless network and central computer to monitor the patients and alert carers in a home [14]. It also
has facilities for recording continence events.
The Stay-Dri design is marketed as a continence management system that links signals from
reusable continence pads to wireless computer systems [15]. This system continuously monitors and
records continence data in care homes. It is also interactive and can be used to provide prompts for
restorative bladder programs. The alerts from this system can be used with pagers.
A device called SenseUrine measures minute changes in body temperature around the bladder area.
The information is designed to be used to determine the urine volume in the bladder and alert the
wearer in advance of an incontinence event occurring. [16].
Researchers have experimented with temperature measurement techniques to determine the onset
of incontinence events [17]. They embedded sensors into continence pads that relied on the sudden
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rise in pad temperature due to urine to trigger an alarm signal. Their results showed that although the
system operated successfully, low urine volume triggering could only be guaranteed by careful sensor
positioning. Furthermore, the system could become unreliable if the temperature difference between
abdominal skin and centre of the pad was low. The same researchers also investigated the use of an
impedance method to detect incontinence. Their method relied on the urine to provide a conductive
pathway between two electrodes on the pad, thus triggering the sensing electronics. They found that
the system was successful in detecting urinary leaks but was also sensitive to pus exuding from
bedsores.
Other systems exist that measure the flow profile of micturition, such as the Pe-Que sensor pad
[18]. This device is used in urodynamics laboratories to investigate disorders in bladder function.
This system is intended for clinical diagnostic investigation and therefore does not incorporate any
alert signalling into the design.
The systems described above are all non-invasive since they do not require insertion into the body
to detect and provide information about incontinence events. In contrast, other work has concentrated
on the development of implantable systems. For example, a patent has been filed for an insertable
sensor array system to monitor urinary incontinence [19]. The array contains pressure, leak detection,
neuromuscular fatigue and electromyogram sensors, with a wireless unit for recording data. This
system is designed to be diagnostic. It is mounted on a catheter that is inserted into the urethra for a
period of 24 hours to several days. At the time of writing this document, such systems have not been
found to be in physical existence.
None of the systems described in this section performs the same function as the new device
described in this paper. While many different systems have been developed for monitoring
incontinence, none was available to monitor the performance of the continence pad until the system
described in this paper was developed.
3. System design
The system developed by the authors is different from the ones described above in two areas. The
first area concerns the pad and undergarment. Continence pads tend to leak as the urine flow rate from
the body often exceeds the speed of absorbance into the pad, allowing urine to flow freely across the
surface of the pad without being absorbed. Furthermore, urine is always deposited in the same small
area of the pad and is required to wick away from this area in order to allow more urine to be
absorbed. Poor fit and incorrect application of the pad are also factors that cause leakage. If overspill
on to the underwear occurs the pad wearer is likely not to be able to detect this immediately, especially
if he or she is seated. Since the pad is next to the skin, there is unlikely to be a discernible temperature
change that could signal overspill. This creates a situation where urine can seep from the wetted
underwear onto outer clothing and thence to the surface of the seat. This sensor monitors the
undergarment covering the pad and is able to detect urine coming into contact with the underwear. It
does not obtain information directly from the continence pad itself.
The second area concerns the alert method used. This device is designed to notify the wearer that
the continence pad has failed and that appropriate action therefore needs to be taken. The device is
designed to assist pad wearers by alerting them to pad leaks before urine spreads, creating a potentially
embarrassing situation.
The significant difference about this system is that it alerts the wearer directly, rather than sending
information to a carer. In this regard, it is a portable system that can be used by otherwise healthy
individuals who suffer involuntary loss of urine whilst carrying out their daily tasks. Around 20% of
patients suffering from neurogenic bladder associated with spinal injury use a continence pad as part
of their management strategy [20]. These individuals would only be able to benefit from the current
system if the vibration unit were placed where they have normal sensation. However the authors have
also developed a system which alerts the wearer or their carer of pad leakage by short messaging
service (SMS). This version of the system might be more useful for people with reduced sensation.
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The sensors are integrated into the design of the underwear by embroidering two parallel
conductive pathways on the underside of the underwear, closest to the leg holes. These are the
locations where spillage from the pad is most likely to occur. A plain piece of cotton cloth is stitched
above this layer to eliminate the risk of the pathways making false contact due to material creasing.
The ends of the pathways terminate in press studs, to which the signalling unit is attached.
The signalling unit consists of a single round printed circuit board on which is mounted a
microcontroller, a vibration unit, a coin cell and associated drivers. The device is normally on and in
deep sleep mode, consuming minimal power. It is awoken when urine from a pad spills over and
bridges the gap between the conductive pathways. The pathways effectively act as a switch which
turns on in the presence of urine. When awake, the software within the microcontroller begins to
execute and energises the vibration unit in a particular sequence. Being attached to the underwear, the
user feels the vibration and can respond to the warning. The current flowing in the conductive
pathways during active mode is 10A and the signalling unit can be configured to provide single or
repeated warnings, as necessary. It then returns to deep sleep mode.
Figure 1. Urinary incontinence detection and signaling unit. The sensor is integrated into the
underwear and the electronics unit is attached to it with press studs.
Figure 1 shows the PCB, enclosure and underwear. The PVC enclosure is custom designed to be
smooth edged and of low profile. Standard press studs are used to attach the unit to the undergarment.
The signalling unit is designed to be reusable.
4. Experimentation and results
Preliminary experiments to establish system design were conducted by fixing two conductors to a
piece of cloth 10mm apart and wetting the cloth with tap water, salty water and urine samples. At
each test, the electrical resistance between the two electrodes was measured and recorded. Readings
were taken after the liquid had absorbed into the material for one minute. The duration of time
allowed the resistance value to settle to a constant, measurable value. The results are shown in Table
1.
Table 1. Results of resistance measurement tests on a piece of cloth with electrodes placed 10mm
apart.
Test description
Resistance ()
R of dry cloth
1M
R of cloth soaked in tap water
120k
R of cloth soaked in saltwater (salt concentration of 0.1mol.l-1)
80k
R of cloth soaked in urine
60k
It was found that when the cloth was dry, the resistance was in the megaohm region. With tap
water, the resistance dropped to 120k and with salty water it was 80k . Fresh urine had a
resistance of 60k under the same conditions. These results provided the initial information that
would allow the system to trigger with urine.
Following these tests, the first sensor pad system was designed and tested. It consisted of a folded
piece of fabric on which two symmetrical patterns were embroidered with electrically conductive yarn.
The pattern consisted of parallel lines terminating at a single connection area. Several pads were made
5
with different numbers of lines. The patterns were separated from each other by a sheet of 1mm thick
absorbent fabric. The design is shown in Figure 2.
Figure 2. Initial sensor pad design showing the embroidered pattern, separator and assembled layout.
This system was used to examine its triggering capability in the presence of urine. An arrangement
consisting of the sensor, amplifier, processor and PC was used to record the sensor response to being
wetted by 10ml of salt water solution at a concentration of 0.3mol.l-1. It is shown in Figure 3.
Figure 3. Experimental arrangement to test sensor response time.
The results obtained are shown in Figure 4. They indicate that the pads responded within 5 seconds
of being wetted. It was also proven that changing the number of conductive lines did not have a
significant effect on the final voltage output of the system.
A similar experiment to measure the conductivity of water with different concentrations of salt was
also carried out. Salt solutions of 0.1mol.l-1 to 0.6mol.l-1 were prepared and tested using a similar data
acquisition arrangement to the one shown in Figure 2. The results showed an exponential resistance
drop from 80. to 20.with increases in salt concentration. In comparing these results with those
from Table 1 above, it is noted that the resistance measurements taken across a wet piece e of fabric
were different from those taken directly in the liquid, by 3 orders of magnitude. These results
provided vital information to facilitate the design of a robust triggering circuit.
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Figure 4. Results of an experiment to measure sensor response times to pad wetness.
Following the above experimentation, it was determined that the sensor would be fixed to the
inside of an undergarment so that it could be used to detect urine overspill from a continence pad
placed on it. The sensor would consist of four layers of sheeting, two of which would be used as
separators to isolate the conductive layers from each other and the continence pad. The conductive
layers would consist of patterns embroidered into the fabric using electrically conductive yarn. The
initial layout was a comb pattern as shown in Figure 1and could pick up urine from any part of the
underside of the continence pad. When, evaluated experimentally two problems were exposed.
Firstly, it was found that the thickness of the sensor made the underwear uncomfortable to wear.
Secondly, it was noted that urine overspill only occurred at the edges of the pad, rendering the
majority of the embroidered patterned area useless.
As a result of the undergarment tests, an improved sensor, shown in Figure 5 was developed. In
this version, the sensor thickness was reduced by sewing the conductive yarn as single tracks on the
same layer of fabric. The two tracks were separated from each other by 10mm and were positioned
along the area corresponding to the sides of a continence pad, so that they would be ideally located to
conduct upon urine overspill.
Figure 5. New sensor design.
This second design worked correctly in principle, but suffered from false triggering due to short
circuits caused by fraying of the conductive yarn. The fraying had resulted from the use of
multifilament conductive yarn in the sewing process causing it to break. A solution was found by
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using higher quality conductive yarn that remained intact after sewing. This allowed an efficient
sensor design to be realised.
Another aspect of the system that required experimental evaluation was the design of the enclosure
that was to house the signalling unit. The result of several focus group investigations made clear that
the unit was to be small, light in weight, round and of low profile, so that it could be used discreetly.
The final design obtained after a few iterations is shown in the photograph of Figure 1.
The information obtained from the experiments contributed to the design of a sensor that would be
compatible with the signalling unit. At the time of writing, the signalling unit has been tested and
awarded a CE mark. Pilot tests have also been carried out by volunteers and have yielded very
promising results. Plans are now underway for the system to undergo more extensive clinical
evaluation.
5. Conclusion
The system described above builds upon the already existing body of work that has been carried out to
develop a robust method for the detection and warning of continence pad leakage during urinary
incontinence events. The system provides assistance and reassurance for individuals who are
managing a stigmatising condition. The integration of the sensor into specially designed underwear
will allow the system to be used with many different types of incontinence pad. The underwear is
designed to be used by a single individual to eliminate the risk of cross infection.
Although a functional system currently exists, there are several areas which need to be further
investigated before it can become commercially acceptable. Possible false triggering due to sweat is
currently being investigated and will provide information that will enable the sensing electronics and
software to be reconfigured as necessary. The system will be optimised to be able to make an
embedded decision about the type of trigger that was detected, before energizing the alert mechanism.
6. Further work
A fully working system opens up the possibility for a number of new sensing mechanisms to be
incorporated into the already existing framework. A first step in this direction is to undertake a clinical
evaluation of the current system; this is now being planned. The results will prove the system’s
repeatability and reliability so that it can be developed further.
Acknowledgment
The authors wish to thank the following organisations for their collaboration towards this work:
Bristol Urology Institute, Helen Hamlyn centre, Royal College of Art, Sheffield Institute for Studies
on Ageing, Sheffield Hallam University, University of Manchester School of materials, and the
University of West of England.
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