Final Report
Disposable Mattress Cover That Detects When the Sheets Are Wet
ECE 4884/4007 Senior Design Project
Section L03, D.M.U. Team
Ankit Hemani
Jacob Huang
Yuval Rooz
Heonwoo Song
Sarju Vasavada
Submitted
December 14, 2007
TABLE OF CONTENTS
Executive Summary .......................................................................................................... ii
1. Introduction ..................................................................................................................1
1.1
1.2
1.3
Objective .............................................................................................................1
Motivation ...........................................................................................................1
Background .........................................................................................................2
2. Project Description and Goals ....................................................................................2
3. Technical Specifications ..............................................................................................3
4. Design Approach and Details
4.1 Design Approach ..................................................................................................3
4.2 Codes and Standards .............................................................................................5
4.3 Constraints, Alternatives, and Tradeoffs ..............................................................5
5. Schedule, Tasks, and Milestones.................................................................................7
6. Project Demonstration.................................................................................................8
7. Marketing and Cost Analysis ......................................................................................8
7.1 Marketing Analysis ...............................................................................................8
7.2 Cost Analysis ......................................................................................................10
8. Summary and Conclusions .......................................................................................13
9. References ...................................................................................................................14
EXECUTIVE SUMMARY
A disposable mattress cover is an efficient and sustainable hospital accessory that can be
used to cover hospital beds and to detect discharge of fluids through a patient’s body. Due to the
high cost of hospital bed sheet washing, it is desirable for mattress covers to be replaced after
each use. A cost-effective disposable mattress cover, designed with monitoring functionalities,
will provide an excellent solution to maintain hygienic conditions for patients in hospitals.
The disposable mattress cover, which is made up of biodegradable polyethylene material
and stitched with conductive threads, will detect the presence of fluids in the bed periodically
and immediately transmit an alarm whenever a change of bed sheet is required. An alarm will be
also transmitted for a broken circuit on the mattress cover. This disposable mattress cover will
cost $2.00 and has an excellent growth potential in the market since hospital bed demand in U.S.
will increase by as much as 43% in next 25 years due to demographic shifts [2].
Our project aims to build a working prototype of a disposable mattress sheet cover that
can detect urine and blood on it. After rigorous testing, this prototype can be used to manufacture
a sturdier commercial product that can be implemented in hospitals and private nursing homes
around the world. The targeted manufacturing cost of this product is $2.00 per sheet with the
detection unit being sold separately.
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DISPOSABLE MATTRESS COVER THAT DETECTS WHEN THE SHEETS ARE WET
This report details the development of a mattress cover that detects wetness. It will cover the
technical details, cost analysis, and the future of this product.
1. INTRODUCTION
1.1
Objective
This project aims to design and build a prototype of a mattress cover that is disposable and is
able to detect wetness from urine, blood, or any electrolytic fluid. The product that implements
our design will be mass-produced and replaced after every use. Due to this design’s short
expected use time, the target customers are hospitals, nursing homes, and bed-ridden patients
who will need to change sheets regularly. These customers require frequently replaced products
to have a low cost, so the target price of this mattress cover is less than $2.00.
1.2
Motivation
The primary use of this product is to reduce exposure to moisture of bed-ridden patients. The
CDC reports that there are 1.7 million incidents of nosocomial (hospital-acquired) infections
each year [1], many of which are attributed to wet bed sheets. Bacteria are naturally more prone
to damp areas, so if a mattress cover is left moist for extended periods of time, any exposed
wound will have a greater risk of developing infection. The accompanying circuitry sends out a
signal that, depending on the implementation, may alert any servicing personnel to replace the
mattress cover. Another way this design will reduce nosocomial infections is by eliminating any
contamination from previous patients. To disinfect bed sheets, it must be washed at high
temperatures, a costly procedure. By using disposable mattress covers instead of reusable ones,
the contamination hazard is removed. This constant disposal creates an environmental concern,
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so the design has used only bio-degradable materials. Besides the mattress cover, the mattress
also absorbs fluids and may grow bacteria. To reduce this risk, a bio-degradable polyethylene
1.3
Background
Many products or patent systems include the ability to detect if a material is wet. They are
usually fabricated for toddlers as diapers or for hospital patients as bed sheets. Children products
are used to detect urine for bed wetting training or for a diaper change reminder. The Malem
Bedwetting Alarm by Bedwetting Stores has a detector in the underwear that is connected to an
alarm [2]. This product is not desirable in a hospital since it is invasive. Systems for hospital
patients like [3] are specifically designed to reduce the likelihood of bedsores by detecting long
periods of inactivity and measuring wetness, but there are no commercial products that utilize
that design. Other similar products include a wet sheet detector that uses biological materials to
detect wetness by measuring the change in resistance when the sheet is wet, but are not
affordable for daily replacements. The mattress cover proposed in this report builds upon these
previous designs, but it will be more disposable and inexpensive to accommodate the needs of
the hospital.
2.
PROJECT DESCRIPTION AND GOALS
The design of the mattress cover consists of conductive threads sewn onto a layer of
absorbent paper manufactured covered by a thin layer of biodegradable polyethylene. The
conductive pattern stitched onto the mattress cover will serve as a wetness detection sensor. The
polyethylene plastic layer prevents the leakage of electrolytic fluids, reducing infections. The
conductive threads will form a continuous loop, and each loop will be checked for continuity.
 Sew connective thread to paper using a sewing machine.
 Attach both paper and plastic together efficiently using adhesives.
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 Design an automated assembly process to manufacture the mattress cover
 Detect changes in resistance due to leakage with a differential amplifier circuit
 Use LED to indicate wetness or broken circuit.
3.
TECHNICAL SPECIFICATIONS
Table 1. Technical Design Specifications
Design Aspect
Proposed Specification
Designed Specification
Voltage source
9V batteries
Outlet power
Threshold resistance
TBD
35 kΩ
Output voltage levels
0V and 5V
0V and 5V
Conductive thread
Silver plated nylon
Stainless steel
Adhesive to attach
paper to plastic
3M Super 77 Spray
Adhesive
3M Super 77 Spray
Adhesive
External connection to
circuit
4 conductive traces
attached binder clip
fastened by electric tape
Plastic clip with 4
extruding wires,
connects to box by
RS232 connector
Weight of Mattress
Cover
TBD
150g
Automated Process to
mass produce mattress
covers
TBD
Automated stitching
machine
4.
TECHNICAL DETAILS
4.1
Design Details
Designing a detection circuit for the mattress sheet cover was a challenge because the
circuit had to be safe, robust and at the same time cheap. In order to reduce costs we decided to
implement it with comparator op-amps to compare the resistance of the threads on the mattress
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sheet cover with a known value of resistance. Once we decided to switch our thread from silver
coated nylon to stainless steel, the task was easy because the resistance of the stainless steel
thread is only about 100Ω/ft as compared to 1.2kΩ/ft for the stainless steel thread. Although the
threshold resistances were set by measuring the dry and wet resistances of thread on the
prototype, these values take into account the minor changes in thread resistance due to the
manufacturing process up to a tolerance of 10%.
One of the major decisions while designing the circuit was whether to use DC or AC.
Since we were concerned with safety, we decided to use DC batteries as the power supply since
voltage surges do not occur in a battery. Although DC current can cause ionization and thus
increase resistance of the thread after it has been wet for a while, we tested this theory and found
that the resistance increases up to a certain value and then stabilizes. After taking measurements
of the power consumption, we found that a 9V battery would supply power for only 1-2 days.
After discussing with our advisor, we decided to use an AC-DC wall adapter with an isolation
transformer. The isolation transformer prevents any surges in the AC power to directly transfer
into the DC output. Further, we used a 100 mA fuse to make sure that power surges in the AC
supply do not enter our circuit.
The output of the detection circuit needs to be connected to a computer that can monitor
when the detection circuit sends a signal (for either wetness or tear in the sheet). As there may be
power surges on the computer end, we used an optoisolator to isolate the output signal from the
opamp outputs and thus prevent any surges from the output end getting into our circuitry.
In order to make sure that our circuit uses low current (for safety and cost), we used
resistances in the order of hundreds of kilo ohms. This way, the entire circuit consumes no more
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than a few milliamps and the current passing through the conductive threads at anytime is no
more than 9 micro amps.
Finally, the stitching of the sheet itself was done in such a way that the patient never gets
into contact with the conductive threads.
The prototype was built with an LED to indicate wetness or a tear in the mattress cover
so that there was no need for a voltmeter to show the output signal during the demonstration.
4.2
Codes and Standards
We talked to Piedmont hospital regarding any codes or standards that must be followed
and they said that there were none. Yet we decided to use some components that would increase
the safety our product like the fuse, isolation transformer, opto-isolator and the stitching method
of the mattress cover.
4.3
Constraints, Alternatives, and Tradeoffs
The major constraint for our design was cost. The mattress cover sheet must cost $2 or
less to produce. Since this product is use and throw, it must be replaced several times and hence
it must be cheap. So we designed a circuit that uses 3 ICs, some resistors and some capacitors.
This way, we kept our costs down to the minimum while still keeping our product safe to use,
which was the other constraint we had.
Some of the alternatives we discussed were AC versus DC for the detection circuitry,
different stitching patterns, connector design, power supply source and threshold resistances.
As mentioned earlier, we decided to go with DC instead of AC along with a battery as the
power source in order to make the circuit safer as well as reduce the complexity of the circuit by
eliminating the need for rectifiers. As for the stitching pattern, we tried to maximize the area
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covered by using as less of the thread as possible while making sure that pattern can be easily
stitched by an automated machine in order to make mass production a little easier.
The connector design was a bit of a problem because we needed something that could be
connected quickly to the mattress sheet cover while at the same time has a good connection.
Since there were no connectors that matched our need, we had to design one on our own.
The power supply was another big point of debate. A battery is safe since there are no
voltage surges in it but its life is only about 2 days for the circuit that we designed. Hence, in
order to reduce costs and the effort required to change the battery daily, we decided to use an
AC-DC wall adapter and make the necessary changes to keep the circuit as safe as possible.
Finally, the threshold resistances changed because at first, we used the silver coated
nylon thread and then switched to stainless steel thread. Once we had finalized on the stainless
steel thread, the thresholds were changed again to account for the tolerances due to
manufacturing.
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5.
SCHEDULE, TASKS, AND MILESTONES
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6.
PROJECT DEMONSTRATION
The final prototype for the mattress sheet cover was demonstrated during our final
presentation. As we did not have a full size mattress cover, the pattern was stitched onto an
actual cotton bed sheet. A volunteer from our group connected the wall adapter into a socket and
its output to the detection circuit. Since the detection circuit was not connected to the sheet at this
time, the LED lit up indicating an open circuit which is equivalent to a tear in the conductive
thread pattern. The circuit was then connected to the mattress sheet cover and the LED switched
off indicating no wetness or tear in the thread. A banana plug cable was then used to short out the
conductive pattern by connecting the two loops. When this was done, the LED lit up to indicate a
short circuit and when the connection was removed, the LED switched off. Finally, a glass of
salty water (with a concentration of 1g/L of common salt in water) was used to simulate the
effects of urine or blood on the sheet. The salty water was slowly dropped onto the center of the
sheet. The LED lit up when there was a circular wet area on the sheet about 4 inches in diameter
indicating that it can successfully detect ionized liquids like blood or urine.
7.
MARKETING AND COST ANALYSIS
7.1
Marketing Analysis
Today many hospital infections are caused by body fluids left undetected on the beds.
More than 80,000 deaths happen because of these kinds of infections [4]. By using disposable
mattress covers that can detect these kinds of fluids, it will be possible to reduce the number of
deaths significantly. These lives motivate the development of the product for hospitals. In
addition, if the product succeeds in hospitals, it would be possible to expand to nursing homes
and home usage.
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As of today, there is no other disposable product that has the capability of effectively
detecting body fluids. Some products that are used to detect urine of people at the age of 3-16
already exist, but they cost $70-$100. The disposable mattress cover will cost approximately
$2.00 (detailed cost breakdown will be discussed in more details in the next section). For
instance, the Bedwetting Store offers Malem Bedwetting Alarm, which has a detector in the
underwear that is connected to an alarm [5]. This product is offered for $79.95-$84.95. In
addition, The Bedwetting Store offers alternative products that are used to detect bed wetting by
an external device that is laid under the bed covers. The Wet Call Bed-side Bedwetting Alarm
with Pad is one of these products from the Bedwetting Store. The device is a mattress that can
detect fluids and is connected to an external alarm [6]. This product costs $84.95.
The products that are on the market are for home use and are meant to be used more than
one time. Since it is assumed that people with bed wetting problem are going to experience
repetitive bed wetting, the products offer a device that is reusable. The product proposed, on the
other hand, is meant for one time use only making it attractive for hospital usage. Once body
fluids are detected, the mattress needs to be replaced and a new sheet is to be put on the bed.
The advantage is that the product being developed will cost approximately 1/30 of the products
that are currently on the market. This will be particularly useful in hospitals, where there is a
high need for cleanliness and where bed sheets are changed frequently.
Now that the product is finished, there are few additional steps needed to take in order to
complete the marketing process. Since some changes had to be done before the final
presentation, the product was not packaged, therefore, it not in a physical stage that can be
marketed. In addition, the product needs to communicate with the nurses’ station either via
wireless communication or landline. Whichever is the method of communication, the box will
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need to be adjusted to talk with the local communication. Other than these modifications, the
product is ready to be marketed directly to hospitals. Saying that, it is still necessary to sign
mutual agreements with producers of sheets to work together to make the sheet at a price that
meet the price target ($2.00) while giving revenue to all parties that are working together to
produces and sell the product.
7.2
Cost Analysis
This cost analysis assumes an income of $52,200 per year for Electrical Engineering
students from Georgia Institute of Technology [7]. This yields $25 per hour of work.
Determination of Selling Price:
This estimate is based on twelve billion units sold in the first five years assuming 10%
market share [8]. Assembly labor is very low since the sheets are going to be produced by an
automated process. Each sheet will be tested by visual inspection only. Prices for parts are
taken from company quotes. Additional cost that was not taken into account originally was the
customization of each product to the local hospital.
Development Costs:
Fringe Benefits
Item
Health Insurance
Fuel & Other Transportation Costs
Cell Phones
401k
Total
Parts
10 Covers + shipment
Conductive Thread
20 LN741CN Op Amps + shipment
Other electric parts: AC adapter, board, box,
fuse and other electrical parts
Total
Labor
Yuval Rooz
Meetings
D.M.U. (ECE4007L03)
Percentage
5%
5%
1%
1%
12%
Price
$21.70
$15.00 [9]
$19.80 [10]
$80.00
$175.70
Hours Price
161
$4,025.00
64
$1,600.00
10
Classes
Writing Reports
Product Development
Web Development
32
25
30
10
$800.00
$625.00
$750.00
$250.00
Jacob Huang
Meetings
Classes
Writing Reports
Product Development
156
64
32
30
30
$3,900.00
$1,600.00
$800.00
$750.00
$750.00
Gopinath Jayaprakash
Meetings
Classes
Writing Reports
Product Development
Group discussion page maintenance
156
64
32
25
30
5
$3,900.00
$1,600.00
$800.00
$625.00
$750.00
$125.00
Ankit Hemani
Meetings
Classes
Writing Reports
Product Development
151
64
32
25
30
$3,775.00
$1,600.00
$800.00
$625.00
$750.00
Heonwoo Song
Meetings
Classes
Writing Reports
Product Development
151
64
32
25
30
$3,775.00
$1,600.00
$800.00
$625.00
$750.00
Serju Vasavada
Meetings
Classes
Writing Reports
Product Development
151
64
32
25
30
$3,775.00
$1,600.00
$800.00
$625.00
$750.00
Total
926
$23,150.00
Cost and Price Calculations
Fringe Benefits
Overhead
Sales & Marketing Expense
Warranty & Support Expense
D.M.U. (ECE4007L03)
12% of labor
of materials,
20% labor & fringe
15% of selling price
5% of selling price
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Development Cost (Non-recurring Cost)
What it costs the company to develop the product
Parts
Labor
Fringe Benefits, % of Labor
Subtotal
Overhead, % of Matl, Labor &
Fringe
Total
$175.70
$23,150.00
$2,778.00
$26,083.45
$5,216.69
$31,300.14
Determination of Selling Price
What the customer pays the company for the finished product
Based on:
Parts Cost
Assembly Labor
Testing Labor
Total Labor
Fringe Benefits, % of Labor
Subtotal
Overhead, % of Matl, Labor &
Fringe
Subtotal, Input Costs
Sales & Marketing Expense
Warranty & Support Expense
Amortized Development Costs
Subtotal, All Costs
Profit
Selling Price
Total Revenue
Total Profit
12,000,000,000 units
$1.00
$0.05
$0.10
$0.15
$0.02
$1.17
$0.23
$1.40
$0.30
$0.10
$0.00
$1.80
$0.20 9.9%
$2.00
$24,000,000,000
$2,380,768,700
It is crucial to note here that these costs were only for the initial development. Once the
product is ready (packaging and communication), mass production will reduce the costs of the
product significantly. It is important to note that the sheet is the only cost that matters since the
detection unit is purchased only once at initial installation. Since the sheets are for disposable
usage, they will be purchased in bulk.
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8.
SUMMARY AND CONCLUSION
The design of this project is complete. Each component, sheet, connector, and detection
unit, all function as desired, though there were a few changes from the original design. The sheet
was designed nearly as proposed. The only change was using stainless steel thread instead of
silver thread for the conductive loop. This was due to the reduced resistance to allow for a more
accurate measurement. The connector was also similar to the one proposed. An RS-232 jack
was included to allow for an easy interface between the connector wires and the detection unit
box. The unit was modified for increased safety.
What remains to do is preparation of the design for mass-production. For the mattress
cover, a full-size paper bed sheet needs to be stitched and tested. The connector currently uses
an external binder clip, but the finished product will need a clip built in. The detection unit is
furthest along for production. The finalized unit just needs to be enclosed. Also, depending on
the implementation, an output signal may need to be installed.
As of now, the prototype responds well to moderate levels of moisture. The saline
solution used to simulate urine or blood triggered the detection unit and resulted in the desired
effect. When the two loops were connected by dabbing the bedsheet with a moistened cloth, the
output was triggered successfully.
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9.
REFERENCES
[1]
R. Klevens, J. Edwards, C. Richards, T. Horan, R. Gaynes, D. Pollock, D. Cardo,
“Estimating Health Care-Associated Infections and Deaths in U.S. Hospitals, 2002,”
Public Health Reports, Center of Disease Control, vol. 122, March-April 2007.
[2]
Bedwetting Store, “Malem bedwetting alarm,” [online document], 2007 Sep 05, [cited
2007 Sep 16], Available HTTP:
http://www.bedwettingstore.com/Bedwetting_Alarms/malem_bedwetting_alarm.htm.
[3]
S. E. Kanor, and R. C. Hirsc, “Personal Care Monitoring System,” U.S. patent no.
6384728, May 7, 2002.
[4]
B. Starfield, “Is US health really the best in the world,” The Journal of the American Medical
Association, Vol. 284, no. 4, pp.483-485, 2000.
[5]
Bedwetting Store, “Malem bedwetting alarm,” [online document], 2007 Sep 05, [cited
2007 Sep 16], Available HTTP:
http://www.bedwettingstore.com/Bedwetting_Alarms/malem_bedwetting_alarm.htm.
[6]
Bedwetting Store, “Wet Call Bed-side Bedwetting Alarm with Pad,” [online document],
2007 Jun 20, [cited 2007 Sep 16], Available HTTP:
http://www.bedwettingstore.com/Bedwetting_Alarms/wetcall_bedwetting_alarm.htm.
[7]
Career Services – Georgia Institute of Technology, “Salary survey,” [online document], 2007
Sep 16, [cited 2007 Sep 16], Available HTTP: http://career.gatech.edu/students/bachelor.pdf.
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[8]
MSEC, “Hospital bed sheets,” [online document], 2007 Sep 16, [cited 2007 Sep 16],
Available HTTP: http://home-health-care.medical-supplies-equipmentcompany.com/PPF/page_ID/230/article.asp.
[9]
The Lame Lifesaver, “Stainless steel conductive thread,” [online document], 2006 Feb 04,
[cited 2007 Dec 11], Available HTTP: http://members.shaw.ca/ubik/thread/thread.html.
[10]
RadioShack, “LM741CN Operational amplifier (8-Pin Dip),” [online document], 2007 Sep
16, [cited 2007 Sep 16], Available HTTP:
https://www.google.com/accounts/ServiceLogin?service=mail&passive=true&rm=false&con
tinue=http%3A%2F%2Fmail.google.com%2Fmail%3Fui%3Dhtml%26zy%3Dl&ltmpl=defa
ult&ltmplcache=2&hl=en.
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