Design of affordable and ruggedized biomedical devices using virtual instrumentation
1.0 Abstract
This paper presents the designs of four low-cost and ruggedized biomedical devices,
including a blood pressure monitor, thermometer, weighing scale, and spirometer, designed
for the East African context. The design constraints included a mass-production price point
of $10, accuracy and precision comparable to commercial devices, and ruggedness to
function effectively in the harsh environment of East Africa. The blood pressure device,
thermometer and weighing scale were field-tested in Kenya and each recorded data within
6% error of the measurements from commercial devices and withstood the adverse climate
and rough handling. The spirometer functioned according to specifications but a redesign is
needed to improve operability and usability by patients. This article demonstrates the
feasibility of designing and commercializing virtual instrumentation-based biomedical
devices in resource-constrained environments through context-driven design. The next
steps for the devices include designing them such that they can be more easily
manufactured, use standardized materials, are easily calibrated in the field, and have more
user-friendly software programs that can be updated remotely.
Keywords: Biomedical Devices, Telemedicine, Rugged Design, Robustness, Virtual
Instrumentation, Africa
2.0 Introduction
More than half the world’s population lives on less than $2.50/day, with millions of people
lacking access to clean water, sufficient food, and adequate healthcare services [1]. Human
suffering from poverty, and hunger is increasing, and over one billion people worldwide
lack access to quality healthcare [2]. Approximately 70% of the 50 poorest countries of the
world are in sub-Saharan Africa [3]. The inhabitants of this region still suffer from diseases
that are no longer prevalent in developed parts of the world. The maternal mortality rate in
Africa is the world’s highest, estimated at 1 death per 100 successful births compared to
12.7 deaths per 100,000 births in the United States [3]. The lack of adequate funding for
quality medical facilities has led to overcrowded hospitals and insufficient access to
medicine. Furthermore, the fact that a large portion of the healthcare budget is spent on
equipment and devices that are not specifically designed for the East African context is a
major contributing factor to the healthcare crisis [4].
Poorly engineered biomedical devices exacerbate health challenges in Sub-Saharan Africa.
The vast majority of the biomedical equipment is designed and manufactured in western
countries by engineers who are not familiar with the physical, socio-cultural, and economic
environment of these countries [4]. Over 95% of the devices found in public hospitals in
developing countries are imported [5]. Five years after arrival, 96% of this donated
equipment no longer functions [6]. In sub-Saharan Africa, almost 70% of donated medical
equipment is not in use due to lack of spare parts, insufficient means for maintenance, or
simply because local personnel do not know how to use the devices [7]. Biomedical
engineers associated with Engineering World Health (EWH) surveyed 33 hospitals in 10
developing countries to determine why devices typically fail. The group found that of the
975 pieces categorized as broken, 644 (66%) could be repaired. The most common reasons
for failure were problems with the power supply of the device, user error, reasons unknown
to the participant, or lack of spare parts [8]. Reliable access to power in developing regions
is a rare luxury. Even when electricity is available, it is often extremely expensive and
prone to frequent power outages, which renders electro-mechanical devices useless.
Additionally, over 150 incidents of broken devices were attributed to user error by the
operator or health professional.
When the EWH staff provided appropriate training, all the technical misuse problems were
fixed. The EWH team also determined that even though most medical device parts were
available in these developing countries, hospitals chose not to spend scarce finances on
replacing them [5]. For example, a particular hospital reported that a $5 repair to an oxygen
concentrator they owned was not justified when they could instead request a new one from
their European sponsor [5]. In addition, since devices are designed for physical and social
environments, resources, operator education levels, use preferences, and economics of
healthcare systems in developed countries, the current medical device market does not meet
the needs of developing countries. Thus, there is a need for devices designed specifically
for resource-constrained environments. Adopting a context-driven approach can increase
the lifespan, effectiveness and usability of the medical equipment. This would ultimately
lead to multi-dimensional improvements in the healthcare system and reduce the per capita
cost of healthcare.
This paper details four affordable and ruggedized biomedical devices designed and fieldtested over a four-year period. These devices are designed to meet the requirements for the
context of a developing country, and are used in conjuction with the Mashavu
Telemedicine System. The four devices are a weighing scale, a blood pressure device, a
spirometer, and a thermometer. All four devices exemplify six design constraints accuracy, affordability, ruggedness, ease of use, anthropometric requirements, and sociocultural factors. The objective of this project was to create each of these devices under the
above design constraints but with a price point of $10. Measurements from each of the
devices were compared to readings of commercial devices with hopes that the readings of
our devices were within 5 percent error of the commercial data. The data in this article
demonstrates the feasibility of designing and commercializing virtual instrumentationbased biomedical devices in resource-constrained environments. Before delving into the
devices themselves, the Mashavu telemedicine system, previous biomedical devices for
developing countries, design constraints, and the value of virtual instrumentation are
presented in the following sections.
2.1 Mashavu telemedicine system
Mashavu, which means “chubby-cheeked” in Swahili, is a telemedicine system aimed at
connecting patients in rural communities with qualified healthcare professionals. Students
from the Humanitarian Engineering and Social Entrepreneurship (HESE) program at Penn
State began developing Mashavu in 2007 and piloted the telemedicine system in Kenya
from 2009 to 2012. Through the Mashavu system, trained kiosk operators (MKOs) obtain
customer’s vital measurements such as blood pressure and temperature using commercial
medical devices. The MKOs send this and other pertinent health information, such as social
and medical history, over a wireless network to a secure website, where a nurse views the
information and provides feedback. The MKO operating the kiosk informs the patient of
the nurse’s suggestions in-person, by phone or with a text message. Rigorous studies
demonstrate that this system of operations is as reliable as comparable telemedicine
systems in the United States and other developing countries [9]. However, Mashavu
currently relies on expensive commercial devices that are often incompatible with the rural
Kenyan environment. These disparities, combined with the process’s vulnerability to
human error in data collection and entry, could lead to inaccurate recording of patient
records.
In order to diminish room for error, bioengineering students at Penn State designed
inexpensive biomedical devices that are rugged, robust, and culturally appropriate.
Mashavu’s drastic cost reductions and increased simplicity are achieved by deviating from
standard commercial devices that embed the primary sensor, signal conditioning, user
display and networking support into a single unit. In contrast, each Mashavu biomedical
device consists of only a sensor and minimal essential hardware, instead relying on virtual
instrumentation to process and display the data. The sensor’s output is digitized by a data
acquisition card (DAQ) and the signal conditioning and user display is done in software on
the computer. All sensors are attached to a 12-bit DAQ device in referenced single-ended
(RSE) mode. The DAQ connects to a netbook running LabVIEW software, which
processes the sensor signals. This offboard computer software is less expensive and easier
to modify than the integrated processors of commercial biomedical devices. Together, this
virtualized approach to device design minimizes manufacturing costs and complexity and
reduces repair difficulty, making Mashavu devices more suitable to the east African
context.
By integrating low-cost biomedical devices into the Mashavu Telemedicine System, the
overall telemedicine system becomes more affordable and effective. Individually, the
devices are designed to cost less than $10 (US) to produce, keeping the overall system
under $200. Each of these devices were developed as part of a are junior design course by
students in Bioengineering 401: Introduction to Bioengineering Research and Design that is
offered at Penn State. Through the semester-long class, student teams use a wide variety of
applicable skills such as computer modeling, programming, bioengineering ethics, and
engineering analysis to design a specific device. Throughout the semester, students
complete a webpage that includes information on their device and subsequently serves as a
blueprint for future design teams and manufacturers. Over the last four years, students have
created a number of devices including a pulse oximeter, blood pressure device,
thermometer, weighing scale, spirometer, ears nose and throat (ENT) scope, and
dermascope. These devices are field-tested in Kenya and based on the findings, they are
refined further.
2.2 Review of low-cost biomedical devices
Engineers recognize the need for low-cost medical devices specifically manufactured for
the developing world. These devices are made for people who live in developing countries,
where modestly expensive tests and even basic infrastructure—i.e., reliable power,
refrigeration, and trained personnel—are often unavailable [10]. For instance, a research
team at the University of Wisconsin is designing low-cost spirometers, pulse oximeters,
and thermometers, which people in some parts of the developing world cannot otherwise
access. These devices are key to combating acute respiratory infection, the most common
cause of death in the developing world among children under five years of age [11].
Similarly, EssentialMed, a 2010 start-up from the Science Park at Ecublens, Switzerland, is
developing essential low-cost devices such as X-ray machines, incubators for newborn
babies, and equipment for anesthetics [12]. The University of Michigan has cataloged
many of these efforts in a website about collaborative sustainable solutions, appropriate
technology, and poverty [13]. Hundreds of different designs are included in this database to
help alleviate the worldwide healthcare crisis, ranging from blood pressure devices to $25
implantable knees.
2.3 Design constraints for low-cost biomedical devices
Six overarching constraints drive the design of the Mashavu biomedical devices. These
overarching factors are accuracy, affordability, ruggedness, ease of use, anthropometric
requirements, and socio-cultural factors. Past teams were required to document and defend
every design decision to ensure that the device successfully met the needs of the specific
context and users. This approach also ensures that the design decisions can be easily
revisited for localizing the device for a different context or demographic.
Accuracy: The most important aspect of device functionality is accuracy, for without it the
devices are of little use. The aim is to achieve a target accuracy within 5% error of
commercial device readings. This threshold accuracy is enough for a doctor to make
informed decisions about the state of a patient’s health. The specific 5% target is consistent
with similar studies that investigate accuracy of primary care medical devices [14,15].
Affordability: Affordability is a key factor in determining whether a device will be
sustainable in the developing world. The target cost is set at $10 in bill of material (BOM)
cost per device when manufactured for a large-scale. This cost is influenced by sensor
choice, surrounding hardware materials, lengths and types of wires required, and necessary
signal processing. Additionally, the target cost for the devices was $10 so that the entire
cost of the equipment and devices for the telemedicine system would be $120 [8]. Final
designs must balance each device’s affordability with both its usability and accuracy.
Ruggedness: The devices are designed to be usable and sustainable in a physical
environment that poses serious challenges to their functionality. Climate demands of the
East African context include high humidity, direct sunlight, high dust levels, rapid
barometric pressure changes, and severe rainstorms. In addition, unpaved roads lead to
difficulty of transportation, and lack of advanced infrastructure compounds the difficulties
of biomedical device usage, transport, and maintenance.
Ease of use: The user interface guides an individual through a software program and
facilitates smooth operation of a device. Devices are designed to be intuitive and easy to
navigate, such that users with varying skill levels could complete the task in one or two
steps. Often, symbols and icons that are well-understood metaphors in the US do not have
corresponding meaning in other contexts. Correct color choice, organization, and units of
measurement are critical to the user interface. All devices are simplified to minimize the
number of steps for the operator.
Anthropometric Requirements: The devices are designed to meet anthropometric
measurements of east Africans. Specific measurements include arm circumference for the
blood pressure device, feet size for the scale, mouth diameter for the spirometer, and
underarm size for the thermometer. Each of these measurements can be found in the
Anthropometric Requirements sections of their respective devices in the sections of this
paper titled “Device Design and Results”.
Socio-cultural Factors: Socio-cultural factors are influences derived from the customs,
traditions, perceptions, and beliefs of the specific context and people. They can be a key
determinant in whether or not a person even agrees to seek care and use the devices. To
increase the usability of the design, devices were designed to fit seamlessly into East
African societal norms by not resembling anything with a negative social connotation, not
disrupting gender roles, and not compromising the user’s privacy.
2.4 Value of virtual instrumentation
Low-cost medical devices become even more powerful when combined with virtual
instrumentation technologies, popularly supported by graphical development environments
like LabVIEW. Virtual instrumentation devices utilize a digital interface on a computer and
LabVIEW software for analysis and display. The rapid advancement and falling prices of
personal computers and smartphones, the development of low-cost, high-performance data
converters, and the emergence of system design software have made virtual
instrumentation systems accessible to a very broad base of users. The ability to connect
low-cost medical devices directly to the virtual instrumentation applications greatly extends
the possibility for growth of telemedicine systems.
3.0 Device designs and results
This section summarizes the design decisions that informed each of the four devices, fieldtesting results, and a concluding discussion on their future. More than 250 measurements
were taken to test each of the devices while hosting clinics at temporary kiosks in Kenya.
All subjects were residents living in local villages surrounding Nyeri Town in Central
Province. The median age of the patients was 43 years, with the youngest patient being 18
years old, and the oldest being 110 (distribution shown in Figure 1). The most common
occupations, accounting for over 60% of the patients, were businessperson, farmer, and
housewife. When patients arrived at the kiosk, they were first instructed to sit while basic
demographic information was recorded. This allowed time for patients to rest so that travel
to the kiosk would not have any effect on the recorded measurements. Upon entering the
kiosk, measurements were taken and recorded using a single-blind system. Measurements
were first recorded with commercial devices at one table. The four commercial devices
used for comparison were the Veridian Health Manual Blood Pressure Monitor with
Stethoscope, the Precision One Analog Bath Scale, the Microlife Digital Peak Flow Meter,
and MOBI TempTalk Digital Thermometer. Patients were taken to a separate table, and
measurements were taken using the Mashavu devices without knowledge of the
commercial results. The Mashavu and commercial readings were then compared and
analyzed. The following section discusses the design decisions for each device, which are
divided into anthropometric requirements, form factor/materials, signal analysis/user
experience, and socio-cultural factors of the developing context
90
80
70
60
Number of 50
Patients 40
30
20
10
0
18-25
26-35
36-45
46-55
Age
Fig. 1: Age distribution of users
3.1 Blood pressure monitor
56-65
66+
Fig. 2: Blood pressure device
Fig. 3: Blood pressure user interface
3.1.1 Anthropometric requirements
The dimensions of the blood pressure monitor were modeled after the human arm. The
typical arm circumference of a child ranged from 18 to 28 cm, while the circumference for
an adult arm ranged from 23 cm to 33 cm [16]. To account for these size differences, 41 cm
of Velcro was added the blood pressure device so that the cuff could accommodate arm
sizes of people within an arm circumference range of 13 cm to 41 cm.
3.1.2 Form factor/materials
An important aspect of the design was finding the optimum size of the air release valve.
This hole size controlled the flow rate, which affected sampling frequency and duration.
The design of the air release hole was a particular innovation intended to make the cuff
easy to use by making the air release rate automatic. The design strategy had an additional
benefit of making it easier to detect pressure pulses from the pressure decay waveform.
COMSOL Multiphysics was used to simulate the size of the aperture and mean air velocity
such that the pressure decay curve could be predicted. Once this curve was known, the
analysis program was written to detect pressure pulses and calculate blood pressure..
Typical commercial devices record systolic and diastolic measurements in about 20-30
seconds. For our device, the hole size was adjusted to meet the target reading time of 30
seconds.
3.1.3 Signal analysis overview/user experience
The most important components of the computer interface were the green button (see
Figure 3, LabVIEW Interface), the pressure gauge, and the systolic and diastolic
measurement readouts. The operator opened the valve, and clicked the green button to
commence the data collection. The systolic and diastolic pressures appeared in bright bold
numbers on the screen. A pressure gauge, identical in appearance to a physical gauge, was
also displayed on the interface so that the patient and the kiosk operator could see the
pressure decreasing as the measurement was recorded.
The signal analysis of the blood pressure device was centered on a peak detection code that
analyzed the mmHg vs. time response curve, measurements which were obtained using the
integrated piezoelectric pressure sensor. The signal analysis initiated once the user had
pumped the cuff to 180 mmHg. Once the user selected “click once valve is open” (green
button), the software ran and determined the systolic and diastolic measurement based on
the detected pressure peaks.
3.1.4 Socio-cultural factors/physical environment and Kenyan context
Women, men, and children in Kenya tended to dress with many more layers of clothing
than those in Westernized countries. There was no foreseeable way monitoring patients’
blood pressure over clothing. Instead, operators were trained to communicate the
importance of the blood pressure readings relative to their health with hopes that patients
would allow for removal of layers of clothing to take the measurement.
3.1.5 Final price point
The cost analysis for the device considered the cost of replacing Velcro, as Velcro has a
lifespan of about one year, and its ability to close tightly will be compromised by the dusty
environment that characterizes East Africa. A tight cuff closure was essential for accurate
blood pressure measurement and the most cost-effective option was to replace the Velcro
on an annual basis.
A plastic casing was also chosen over more durable options, such as metal. Although a
metal casing would provide a much more sound housing for the sensor, it was not a
desirable choice due to its additional weight and cost. The casing was made from a thick
durable plastic that could be easily replaced in Kenya. The sensor housing helped to
provide strain relief so that the wires were not pulled from the sensor. Strain relief was
provided by soldering the wires to the sensor and covering this connection with heat shrink
tubing. When the housing was closed, a tight seal prevented the sensor from moving and
protected it from damage. The sensor was screwed into the casing providing additional
strain relief. This fixed the sensor’s position within the casing, preventing unnecessary
bouncing against the case.
Table 1: Price point analysis of blood pressure device
*Wholesale Quantity refers to the number or amount of product when purchased in bulk;
Wholesale Cost refers to the cumulative price of the Wholesale Quantity; Cost per Device
refers to the price of the material or product when mass produced using bulk quantities.
Wholesale
Quantity*
Wholesale Cost*
(US Dollars)
Cost per Device*
(US Dollars)
Commercial Cuff
Pressure Sensor
Integrated Pressure Sensor
(MPX5050GP) from
mouser.com
20
36.00
1.80
100
8.94
0.09
Plastic (Dental Floss) Case
24
28.94
1.21
Valves and Caps
100
7.99
0.08
Heat Shrinking Tubing
30 meters
7.55
0.08
3/4" X 24" Velcro Strips
228.6 meters
1.97
0.01
Small Screws and nuts
Total:
Negligible
Negligible
<0.01
3.28
Material
3.1.6 Results and observations
A total of 217 patients had their systolic and diastolic blood pressure measured with both
the low-cost Mashavu prototype and a commercial monitor in Kenya. The overall percent
error for the systolic measurement was 4.4%, while the percent error for the diastolic
measurement was 5.8%. The average standard deviation for systolic was 3.9 mmHg, and
was 3.6 mmHg for diastolic.
18
16
Percent Error (%)
14
12
10
8
6
4
2
0
Systolic
Diastolic
Fig. 4 Paired data comparison of the STD of the percent error of blood pressure readings
between Mashavu device and commercial device.
Velcro worked well to hold the cuff in place; however, it must be sewn properly or superglued instead of relying on self-adhesion to the cuff due to the glue adhesive on the back
side of the Velcro. The shrink wrap surrounding the wires and sensor provided strain relief
for the sensor-wire connection point. The hard casing around the sensor provided
protection from both dirt and rain. The sensor remained in place when screwed to the edge
of the casing, which eliminated the potential for intermittent or inadequate electrical
contact caused by sensor movement within the case. The plastic casing also provided
protection from dust, rain, and shock from accidental dropping. The device was able to be
packed efficiently, facilitating transport.
3.2 Thermometer
Fig. 5: Thermometer
Fig. 6: Circuit diagram
Fig 7: COMSOL analysis of temperature readings using a glass thermistor
Fig 8: COMSOL analysis of temperature readings using a steel thermistor
3.2.1 Anthropometric requirements
The auxiliary position (under arm as opposed to under the tongue) was identified as the
most comfortable location for measuring temperature, both for the operator and for the
patient. For this reason, the thermometer needed to be small enough to move through layers
of clothes, but large enough to be easily used by the patient or doctor. This led to the
creation of a thin, rigid-tubed thermometer closely resembling the dimensions of an
average ballpoint pen. The tube was 12.5 cm long with a diameter of 0.75 cm. The coneshaped top was approximately 2 cm in length and tapered into a diameter of about 0.5 cm
at the tip where the thermistor was encased. The connection wires were approximately 90
cm long.
3.2.2 Form factor/materials
Since the thermometer read temperature from the auxiliary position, the appropriate model
consisted of a long plastic tube, which tapered into a cone-shape with the thermistor case at
its tip. The length of the tube allowed for the thermometer to maneuver under clothing
while remaining in a position parallel to the ground. The sturdy tube protected the wires
and supported the device as the measurement was taken. The thermistor was surrounded by
steel wool to prevent excess movement and then enclosed within a metal casing. Shrink
wrap was placed around the entire device (leaving the conducting metal exposed), keeping
it intact and thermally connected to the outside.
To determine which material the thermistor should be made out of, a COMSOL analysis
was conducted that compared the time it took for a steel and a silicon thermistor to read an
expected body temperature. Figure 7 shows that the glass covered thermistor takes about
66.5s to reach within 0.05*C of the expected body temperature using a steel cap and air as
the filler. Figure 8 shows that the silicone covered thermistor takes about 103.5s to reach
within 0.05*C of the expected body temperature using a steel cap and air as the filler. A
steel thermistor was included in the final thermometer because it allowed the device to
measure body temperature in a time frame comparable to that of a commercial device .
3.2.3 Signal analysis overview/user experience
The thermistor was located in the tip of the thermometer and was wired into a voltage
divider circuit. The divider circuit consisted of a nominal 5.1 K Ohm resistor and the
thermistor itself. The DAQ device provided the 5 V DC input voltage.
With the value of the resistance of the thermistor known, the Steinhart-Hart equation,
programmed into the LabVIEW software, was used to calculate the temperature from the
resistance. An array of 1/T values was created, [1/T1; 1/T2; 1/T3] , which was then
multiplied by the inverse matrix of [(ln[R1] x ln[R3])^3); (ln[R2] x ln[R2])^3); [(ln[R3] x
ln[R3])^3)] to find the three unknowns necessary for the Steinhart-Hart equation. With
these values known, the temperature reading was displayed in the LabVIEW program’s
user interface.
3.2.4 Socio-cultural factors/physical environment and Kenyan context
In order to obtain an accurate reading from the axillary position, the conductive metal
needed direct contact with the skin under the arm. This procedure posed a few problems
depending on if the patient was male, female, and/or a child. Since there was usually a
large amount of fabric covering the patient’s body, it was difficult to get the thermometer
under the clothes and into the underarm area. Men usually did not have a problem with
taking their garments off, however women felt uncomfortable with clothing being adjusted.
Children often felt uncomfortable in an unfamiliar situation. Therefore, patients were
allowed to control the thermometer themselves if necessary so that they felt more
comfortable with the testing procedures.
3.2.5 Final price point
A negative temperature coefficient (NTC) thermistor was selected for the thermometers,
primarily due to its high response rate. This was ideal because patients often did not have
the patience to sit for an extended period of time. Another deciding factor for the choice
stemmed from the temperature range being measured (expected body temperatures: 33-45
degrees Celsius). Glass thermistors were also considered for their accuracy and fast
performance. However, they were also slightly more expensive and more susceptible to
damage, making them impractical for this design context. The materials used in the
construction of the thermometer included various pen pieces, the thermistor, heat wrap,
electrical wires, conducting paste, hot glue, a 5 K Ohm resistor, solder, and electrical tape.
Table 2: Price point analysis of thermometer
Material
Wholesale Quantity
1000
304.8 meters
100
50
Wholesale Cost
(US Dollars)
20.00
250.00
26.63
7.20
Cost per Device
(US Dollars)
0.02
0.25
0.26
0.14
5kOhm Resistor
Wires (1.8 meters)
Circuit Board
Thermistor Sensor
5 KΩ negative
temperature
coefficient (NTC)
thermistor from
Images Scientific
Instruments
Plastic Pen Body
Steel Pen Cap
100
1000
10.00
20.00
0.10
0.02
Heat Shrinking
Tubing
Total:
30 meters
7.55
0.08
0.87
3.2.6 Thermometer results and observations
Temperature was measured by placing the Mashavu thermometer or its commercial
counterpart under the arm of 172 patients. The average temperature measured using the
commercial device was 35.7 ° C, while the average temperature using the Mashavu device
was measured at 34.4 C. The average difference between measurements averaged at 1.3 C.
This led to a percent error of 4.5% with a standard deviation of 1.14 C.
16
14
Percent Error (%)
12
10
8
6
4
2
0
Fig. 9 Paired data comparison of the STD of the percent error of temperature between
Mashavu thermometer and commercial thermometer.
The pen cap casing effectively protected the tip of the thermometer. The device was
dropped to the ground several times without affecting performance. The strain relief
provided by interweaving the wires was effective in preventing thermistor detachment, and
the heat shrink was effective in providing additional support during the pilot testing. High
stress points were identified for further reinforcement of the heat shrink and/or inclusion of
stronger wires, specifically at the base of the pen. The thermometer must also be held in the
horizontal position due to the reading abilities of the sensor. When calibrating and taking
measurements, the temperature measurement was observed to be affected if the
thermometer was not held in the same position consistently.
3.3 Scale
Fig. 10: Scale
Fig. 11: Scale user interface
Fig. 12: Diagram of the weighing scale circuit
Fig. 13: 3-D Strain Approximation on the Top Plate for 10 kg Weight
Fig. 14: 3-D Strain Approximation on the Top Plate for 180 kg Weight
3.3.1 Anthropometric requirements
The average foot length for males and females was 28 cm and 23 cm respectively. Based
on these numbers, the device was constructed as a square with each side equal to 30 cm.
Furthermore, the average hip diameter for males and females were 29 cm and 30 cm,
respectively [17]. A scale with 30 cm width, which matched hip size, was slightly larger
than the maximum foot size expected and was intended to increase stability of the scale
when someone stood on it.
3.3.2 Form factor/materials
The basic geometry of the scale consisted of a 30.5 cm x 30.5 cm x 1.9 cm board supported
by four legs (one in each corner), with two load cells placed directly underneath the
locations intended for the patients' feet. The square geometry provided the greatest possible
surface area for patients to place their feet on the scale. The circuit board was attached
directly to the bottom of the scale. This ensured a strong connection to the wires that extend
to the load cells. The load cells were protected by a wooden shell covered in cling wrap.
This permitted easy access to the cell while protecting it from water and dust.
COMSOL modeling was conducted that was aimed to determine the magnitude and the
location of the strain. A 3-D model (30.48cm x 30.48cm x 1.9cm) of the prototype with
four fixed points representing the four scale legs and 2 point loads in the center of the
scale. These models further informed the placement of the load cells and the overall strain
that the top plate would be subjected to under typical weighing. As demonstrated in figures
13 and 14, weights were chosen of 10 kg and 180 kg to represent a minimum and
maximum weight of our expected patient sample size. These models demonstrated that the
maximum strain in the plywood occurred in the center for both weights so sensors were
placed on the sale to measure that strain. Additionally, the plywood both withstood the
strain of maximum strains of both the minimum and maximum weights while still
accurately allowing the sensor to pick up a deformation in the wood thus validating the
selection of plywood.
3.3.3 Signal analysis overview/user experience
The user interface displayed a command button that allows the user to select which of the
prototype scale to use. A second button allowed the operator to zero the scale and weigh a
patient, making operation simple. The LabVIEW user interface of the scale has two
phases. The first phase of the program involved taring the scale and was achieved by the
user selecting the "Zero" button of the user interface. This set the weight to zero by
subtracting the acquired baseline voltage from the two load cells located underneath the
scale. An asymptotic value was acquired by measuring the percentage difference between
consecutive averages. If the averages were within 0.01% of each other, the common value
was considered the baseline and used to tare the scale.
The second phase of the weighing process begun after the patient stepped onto the scale
and a deformation occurred in the strain gauges. Upon clicking the “Weigh” button, the DC
signal entered a low-pass filter and a running average produced an asymptotic voltage
value. This value was subtracted from the baseline voltage attained in phase one. The
resultant voltage was put through the calibration equation to produce a final weight that
was displayed on the user interface.
3.3.4 Socio-cultural factors/physical environment and Kenyan context
To protect the scale from the elements, the plywood board was stained with a polyurethane
coat to prevent water damage. The circuitry components were contained within a housing
to protect them from the dusty Kenyan environment. The load cells were the only
components that are prone to alteration from the climate. The extreme heat of Kenya
caused thermal expansion of the cells, altering the measurements. This was not a factor
during field-testing, as temperatures were relatively mild. If high temperatures were to
occur, a protocol would need to be set in place to recalibrate the scale so that accurate
measurements may be taken. Dust could also get in and disrupt the operation of the strain
gauges.
3.3.5 Final price
The scale employed two strain gauges placed in a specific manner ideal for weight
measurements. The load cells were approximately $3.90 each when purchased in bulk.
Other options were explored, but the load cell of choice gave optimal results and durability
for the best price. These cells required a custom-designed circuit to be used in conjunction
with the LabVIEW program. The circuit utilized a Wheatstone bridge along with a
differential amplifier to relay the signal from the load cell to the DAQ device.
Table 3: Price point analysis of scale
Material
Wires (1.8 meters)
20 Ω Trimpots
Wholesale Quantity
1000
100
Wholesale Cost
(US Dollars)
20
25
Cost per Device
(US Dollars)
0.02
0.25
TLC2252CP Op Amp
Op Amp Socket
4.7 Ω Resistors
560 Ω Resistors
1 MΩ Resistors
2-5/16' x 1-15/16'
Proto Board
2-5/16' x 1-15/16'
Proto Board
100
100
300
200
200
60
8
12
8
8
0.6
0.08
0.04
0.04
0.04
100
69
0.69
100
69
0.69
1' x 1' x 3/4" Plywood
2" x 2" Pine
Large-Diameter Flat
100
30.48 m
200
63
100
16
0.63
1
0.08
Washers
Wood Screws #12 x
2''
400
Steel Machine Screw
Hex Nut
400
Load Cells
2 compressive load
cell (16375097) strain
gauges
from
technoweighindia.com 200
52.8
0.132
24
0.06
780
3.9
Plastic Circuit Cover
Polyurethane
Total:
88
30
0.88
0.3
9.432
100
1150 oz
3.3.6 Scale results and observations:
The scale measured weight in kilograms when a patient stood on the scale. Throughout the
course of the trial, 93 patients were weighed, and the data between the commercial scale
and the Mashavu scale was within 5.9% error. The standard deviation for the measurements
was 2.67 kg.
18
16
Percent Error (%)
14
12
10
8
6
4
2
0
Fig. 15 Paired data comparison of the STD of the percent error of weight between a
Mashavu scale and commercial scale.
Problems arose when using the scale due to the immobility of the device and the need for
excessive calibration. Calibration took upwards of ten minutes, and a new calibration
equation was needed every time the scale was moved or displaced. This proved that it
would be worth the investment to purchase a third strain gauge to improve the portability
and calibration time. Using a high-grade wood that was stained to seal and protected from
any water or mud provided a durable and robust design. The wires were protected on the
underside of the scale, and were wrapped around a washer to provide strain relief from and
tugging or pulling on the wires.
3.4 Spirometer
Fig. 16: Spirometer without mouthpiece
Fig. 17: Mouthpiece in SolidWorks
Fig. 18: COMSOL model depicting the pressure differences across the spirometer design at
Time = 0 of the measurement
Fig. 19: COMSOL model depicting the pressure differences across the spirometer design at
Time = 2 second illustrating the pressure change during measurement
3.4.1 Anthropometric requirements
The main physical dimension affecting the use of the spirometer was the size of a person's
mouth. An average adult male has a mouth diameter of 4.3 cm (1.7 inches) while females
have a mouth diameter of 4.1 cm (1.6 inches) [18]. When an individual used the
spirometer, they held the device while blowing into it. The average adult hand length was
19 cm (7.4 inches) for males and 17 cm (6.8 inches) for females. In terms of hand size
across the palm, the average for males was 8.4 cm (3.3 inches) and 7.5 cm (2.9 inches) for
females [19]. Both the length and outer diameter of the spirometer correlated with these
dimensions.
3.4.2 Form factor/materials
The basic geometry of the spirometer consisted of two cylinders with a cone connector,
providing a sleek transition from the inlet to outlet tube as shown in Figure 12. The
mouthpiece was shaped in an elliptical design. This shape was chosen due to ease of
handling for the patient; however, the fundamentals of how the pressure sensor works also
made this geometry most suitable. Based on average mouth sizes, the diameter of the
mouthpiece was designed to be approximately 1 inch. The sensor chosen was a differential
pressure sensor. In order for it to function, one port of the sensor measures the pressure
near the base of the large outlet cylinder, while the other port measures near the base of the
inlet smaller cylinder (model of small cylinder can be seen in Figure 12).
COMSOL was used to model the air flow through the body of the spirometer to validate
both the choice of sensor and the shape of the device. The walls of the spirometer are
modeled as no slip boundary conditions, which is desired. However, because a pressure
will be present at the inlet and outlet, two values were made; the inlet pressure was found
to be around 14.5 kPa to represent the pressure of the average human breath. The outlet
pressure was set to 0 kPa to represent atmospheric pressure. The figures below show the
COMSOL 3-D representation of the spirometer with inlet of 14.5 kPa and outlet of 0
kPa. Figure 18 shows the pressure at Time = 0 seconds while figure 19 shows the pressure
at Time = 2 seconds. From the readings, it can be seen that a pressure drop occurs along
the narrow cylinder, after the eccentric cone that can be easily detected by the sensor.
3.4.3 Signal analysis overview/user experience
The user interface for the spirometer was designed to properly engage both the operator
and the patient using the device. This was done through a graph and fill bar as shown in
figure 19. The graph of flow rate helped the operator in evaluating spirometer read-out. If
the operator saw that the flow rate was non-continuous or that there was severe signal
noise, he or she could better determine the cause for error so that they could re-explain or
demonstrate how to properly hold the device. Additionally, the software displayed the
forced vital capacity (FVC), FEV1/FVC ratio, and maximum flow rate. These three
numbers could be seen on the user interface, just to the right of the graph of flow rate vs.
time.
The pressure sensor of the spirometer was designed to take pressure measurements that are
then provided to the DAQ device as a voltage signal. Since every pressure sensor was
unique, each one was calibrated to derive a relationship between pressure and voltage.
Using the calibration feature in LabVIEW, known pressure differences were delivered to
the sensor and the corresponding voltages were recorded. From that data, a correlation was
made and an equation was derived that related pressure to voltage for each sensor. Using
Bernoulli's equation, flow rate and pressure were related based upon the geometry of the
spirometer body and the density of air. Once the signal had been transformed into a flow
rate, an additional function ensured that negative flow rate values were zeroed. This step
was important because it ensured that the baseline reading was always at zero. Once
calibrated, patients used the device and the flow rate was presented as a real-time graph of
flow rate vs. time. The peak flow rate was also an output. To obtain the forced vital
capacity of the patient, the flow rate was integrated over the time of the patient's use. A
similar method was used to calculate the forced expiratory volume in the first second of
exhalation (FEV1). This time, however, the flow rate was only integrated over the first
second that the patient exhaled into the spirometer.
3.4.4 Socio-cultural factors/physical environment and Kenyan context
One of the most important concerns with the spirometer was the education that
accompanied the use of the device in Kenya. The spirometer was not a commonly-used
device, and many people, specifically in East Africa, did not understand exactly what it
measured and why. Therefore, the educational information was designed to be extremely
clear, detailed, and easy to understand. Users also had to trust that the device was sanitary.
The safest way to assure sanitation was to have new mouthpieces for each person.
However, this was unrealistic and expensive for the context. It was decided to use several
removable mouthpieces which could go through a cleaning rotation while different pieces
were being used. After each use, the mouthpiece was cleaned in rubbing alcohol and then
rinsed in clean water to remove the alcohol taste.
3.4.5 Final price point
The spirometer was primarily made of rapid prototype printing material. This material was
durable plastic and inherently had a long lifetime. The mouthpieces had a greater chance of
breaking because they are handled more often. If the body or mouthpiece happened to
break, they could not be replaced in Kenya due to the lack of a 3-D printer. The case
protecting the sensor was made of metal and did not need to be replaced. The mouthpieces
will be made by injection molding in the long-term.
Table 4: Price point analysis of spirometer
Materials
Pressure Sensor
differential pressure
sensor MPXC2011DT1
Rapid Prototyping
PVC tee connector
Small fittings
Black Rubber Tubing
Wiring (<3 feet
device) 1.8 meters
Total:
Wholesale
Quantity
Wholesale Cost
(US Dollars)
Cost per Device
(US Dollars)
100
100
100
100 (2 per one
device)
12 yards (100
feet)
(1foot/spirometer)
300
2300
67
3
23
0.67
74
0.74
14.16
0.1416
1000
20
0.02
27.57
per
3.5.6 Spirometer results and observations:
FEV1 and flow rate were both measured using the spirometer. After testing on 172
patients, there was a 28.5% error in the FEV1, and a 32.8% error in the flow rate. The
standard deviations were approximately 98 L for FEV1, and 90 L/min for flow rate. Errors
were large because many patients did not understand how to use the device, and the
English to Kiswahili translation was not very effective. Since it was not an intuitive device,
inconsistent and incorrect use posed problems for recording accurate measurements. It
should be noted that the price for rapid prototyping increased the cost of the spirometer by
almost three times of the desired price point. Although very durable and effective, future
considerations will be made to develop the spirometer using cheaper materials such as PVC
piping that is available in Kenya.
120
Percent Error (%)
100
80
60
40
20
0
FEV1
Flow Rate
Fig. 20: Paired data comparison of the STD of the percent error of FEV1 between Mashavu
spirometer and commercial spirometer.
Using 3D printing improved the robustness and durability of the device but greatly
increased its overall cost. The use of lower gauge wires and the decision to braid rather
than tape the wires together prevented them from breaking or pulling out of the device. The
use of foam instead of PVC to hold the sensor in place provided additional cushioning for
the sensor.
4.0 Final discussions and observations
Three of the four devices performed within the stipulated margin of error despite the harsh
climate. As shown in Table 5, the blood pressure device, thermometer, and weighing scale
all gave readings about 5% different from the commercial readings obtained.
Measurements were supervised by community healthcare workers and followed necessary
protocols. Data was recorded using a single blind method such that users of the Mashavu
devices were not aware of the recordings of the commercial devices during measurement.
The spirometer recorded percent errors of 28.5% for FEV1 and 32.8% for flow rate.
Although these percent errors were very high and not ideal, this can be attributed to the
language barrier between the locals, students, and community healthcare workers who
operated the devices. Since the spirometer was not a well-known device, it was often
difficult to explain exactly how to use either the commercial or Mashavu devices and
record quality measurements. Overall, the results were promising. There were a few
common themes amongst all instruments that made them suitable for the developing
country context. A carrying case (either box or bag) was necessary to transport all devices
efficiently. Bubble wrap worked well to protect the DAQ devices and the four biomedical
devices during long journeys on poor roads. The wrapping could be easily reused, making
it very cost-effective. When considering strain relief, tying knots in wires, braiding wires
together, and heat shrunk connections all proved durable and effective. The devices were
powered through a small notebook computer that was running from either a battery or
plugged-in main power. Due to the limited time spent in Kenya, analysis of power
consumption was not conducted. This element of the devices will be addressed at a later
date so proper power considerations can be made to better enhance the functionality of our
devices and adjust to the varying quality of power supplies that are found in developing
countries.
Table 5: Final summary of device performances
Device
Number of Tests
Blood pressure device 217
Thermometer
Weighing Scale
Spirometer
172
93
172
Percent Error
5.0% systolic, 6.8%
diastolic
4.5%
5.9%
28.5% FEV1, 32.8%
Flow Rate
Standard Deviations
3.9mmHg systolic,
3.6 mmHg diastolic
1.14o
2.67 kg
98 L FEV1, 90
L/min flow rate
These devices were designed by hand as prototypes. Accuracy was proven to be within
an acceptable range, and the materials used were chosen with climate and environmental
factors in mind. As is, it was determined that these devices would not survive long term in
a developing world climate due to their make-shift manufacturing methods. However, the
intent is that when mass-manufactured using regular manufacturing techniques the
durability will drastically improve resulting in an anticipated life-span of 5 years. As
products of design classes with time and resource constraints, bioengineering students
developed the devices to as advanced a stage as possible. Moving forward, a medical
device company with access to industrialized design equipment and manufacturing
processes would be better suited to bring the designs and devices through the regulatory
process and ultimately to market. While the lifespan cannot be tested until a final product is
field tested and used regularly within the developing market, the 5 year estimate can be
approximated based on the lifespan of the chosen materials.
After the completion of field testing, it is believed that the devices would not suffer from
the same problems as the devices that they are intended to replace. This is because the
devices have been designed specifically with the customers and context in mind. The
current devices are powered from a USB port since computers are becoming increasingly
popular. If the device is durable and can operate on a rechargeable battery, electricity and
power problems will not be an issue. Additionally, one-button programs such as those that
are utilized with virtual instruments and field-tested with Mashavu Kiosk Operators allow
for a better user experience and eliminate the need for operator expertise.
Leapfrogging solutions such as the emergence of smart phones, can potentially eliminate
the need for robust medical devices or a typical device at all.
There are already
applications to monitor wound care, measure heart rate, and guide nutrition. An application
to measure vital signs directly from the phone would eliminate most devices, further
streamlining telemedicine. Nevertheless, several financial and technological barriers exist
to smart phone based devices and telemedicine systems. Despite increased cell phone use,
smart phones are still rare in rural developing contexts. However, in recent years many
companies like Huawei, Nokia and Tecno are striving to make affordable smart phones for
emerging markets in Africa and South Asia. Mozilla is launching Firefox Mobile OS
phones in South American markets in 2013 [20]. Sales of less expensive smart phones are
already growing faster that high-end models in Western Europe, and will be crucial in the
emerging markets [21]. Medical device donors as well as hospitals in the developing world
can benefit from the availability of ruggedized low-cost devices by way of lowered capital
and operational costs. The largest potential for impact is in the rural areas where
community health workers and social ventures are at the forefront of improving access to
pre-primary healthcare for the most marginalized populations.
Conclusions:
Overall, the blood pressure device, scale and thermometer worked effectively in the
Kenyan context. Measurements from the three devices were recorded within a clinical
range and with reasonable precision and accuracy. The spirometer did not record data
similar to the commercial device used during trials. This was caused by improper data
collection techniques as the language barrier between the operator and patient did not allow
an accurate explanation of how to use the device. Design constraints were addressed so that
the devices could be used successfully and long-term in the developing world context. This
article
demonstrated
the
feasibility of
designing
and
commercializing
virtual
instrumentation-based biomedical devices for use in resource-constrained environments.
Before these devices are ready for large-scale commercialization, they need to undergo
industrial design for better usability and be optimized for mass-manufacturability.
Bibliography
(1) United Nations Development Program, 2007 Human Development Report (HDR),
November 27, 2007, p.25
(2) Carr, Dana. “Improving the Health of the World’s Poorest People,” Health Bulletin1
(Washington, DC: Population Reference Bureau, 2004).
(3) Chudi, Ibekwe. "Healthcare Problems in Developing Countries." Medical Practice and
Reviews Apr. 2010,
Volume 1 ed.: 9-11. Print.
(4) Dzombak, Rachel, Khanjan Mehta, and Peter Butler. "Design by Example: A Webbased Tool for Context-Driven Biomedical Device Design." NCIIA Open (2012): Print.
(5) Malkin, Robert A. “Design of Health Care Technologies for the Developing World.”
Annual Review
(6) Peña-Mohr, Jorge. "Distributing and Transferring Medical Technology." International
Journal of Technology Assessment in Health Care 3.02 (1987): 281. Print.
(7) "Kenya." WHO. N.p., n.d. Web. 12 Oct. 2012. <http://www.who.int/countries/ken/en/>.
(8) Fleishman, Aaron., Wittig, Julia., Milnes, Jason., Baxter, Andrew., Moreau, Jennifer.,
and Mehta, Khanjan., “Validation Process for a Social Entrepreneurial Venture in
Tanzania: A Case Study”, International Journal for Service Learning in Engineering:
Humanitarian Engineering and Social Entrepreneurship, Vol 5, No 1, 2010
(9) Qin, R.; Dzombak, R.; Amin, R.; Mehta, K. Reliability of a Telemedicine System
Designed for Rural Kenya. Journal of Primary Care and Community Health. 2012
(10) Martinez, Andres W., Scott T. Phillips, George M. Whitesides, and Emanuel
Carrilho. "Diagnostics for the Developing World: Microfluidic Paper-Based Analytical
Devices." Analytical Chemistry 82.1 (2010): 3-10. Print.
(11) Nimunkar, Amit, Jonathan Baran, David Van Sickle, and John G. Webster. "Lowcost Medical Devices for Developing Countries." IEEEMBS (2009): 1-4. Print.
(12) "Designing Medical Devices for Developing Countries." Mediacom. N.p., 5 Oct.
2011. Web. 12 Nov. 2012.
<www.essentialmed.org/pdf/PR_Mediacom_052011_EN.pdf>.
(13) "Appropedia." N.p., n.d. Web. 17 Oct. 2012.
<http://www.appropedia.org/Welcome_to_Appropedia>.
(14) Heinermann, Megan, Ken Sellick, Claire Rickard, Pam Reynolds, and Matthew
McGrail. "Automated versus Manual Blood Pressure Measurement: A Randomized
Crossover Trial." (n.d.): n. pag. Web.
(15) "An Investigation into the Accuracy of Different Types of Thermometers." Nursing
Times. N.p., n.d. Web. 04 Dec. 2012.
(16) “The Importance of Large Blood Pressure Cuffs (IMLBP)" Blood Pressure Cuff Size.
N.p., n.d. Web. 21 Oct. 2012. <http://www.plus-size-pregnancy.org/lgbpcuffs.htm>.
(17) Msamati, B. C., and P. S. Igbigb. "Anthropometric Profile of Urban Adult Black
Malawians." East African Medical Journal 77.7 (2000): 364-65. Web.
http://www.ajol.info/index.php/eamj/article/viewFile/46663/33056.
(18) Gray, Henry. 1918. Anatomy of the Human Body." Gray, Henry. 1918. Anatomy of
the Human Body. N.p., n.d. Web. 17 Oct. 2012. <http://www.bartleby.com/107/>.
(19) Agnihotri, A K., B Purwar, N Jeebun, and S Agnihotri. "Determination of Sex by
Hand." The Internet Journal of Forensic Science 1.2 (2006).
(20) Kiezzer, G. "Mozilla Promises Firefox OS Phones in Early 2013." Computerworld. N.p., 3
July 2012. Web. 30 Nov. 2012.
<http://www.computerworld.com/s/article/9228770/Mozilla_promises_Firefox_OS_phones_in
_early_2013>.
(21) Ricknas, M. "Nokia Readies Linux OS for Low-end Smartphones." IT News, Technology
Analysis and How-to Resources. N.p., 30 Sept. 2011. Web. 12 Nov. 2012.
<http://www.itworld.com/hardware/208803/nokia-readies-linux-os-low-end-smartphones>.