Options for Medical Oxygen Technology Systems in Low-resource Settings:

A framework for comparison

B.D. Bradley, S. Qu,Y.-L. Cheng Centre for Global Engineering University of Toronto, Canada bev.bradley@mail.utoronto.ca

D. Peel Ashdown Consultants Hartfield, East Sussex, UK

S.R.C. Howie Medical Research Council Unit

The Gambia

Abstract—Four different medical oxygen system configurations are presented and compared across a wide range of criteria relevant to low-resource settings. Two systems make use of power when available to generate and store oxygen for later use; the others use a backup battery bank (grid- or solar-charged) during power interruptions. Some system designs have been realized as prototypes with some field experience, and others are still conceptual. The results of this review and analysis have implications for future work involving the evaluation, development, and prototyping of alternative oxygen supply systems for settings with poor grid power and limited financial and technological resources.

Keywords- oxygen; oxygen concentrator; cylinder; systems; developing countries; alternative energy; battery; solar; pneumonia; The Gambia; sub-Saharan Africa

I. INTRODUCTION & BACKGROUND Medical oxygen is an essential medicine, especially for

treating childhood pneumonia, the leading cause of death in children under five worldwide [1]. Yet, many hospitals and health centres in developing countries struggle to provide a reliable supply of oxygen. Many low-resource settings rely on compressed oxygen cylinders and/or oxygen concentrators; there is ample literature discussing the attributes and limitations of each of these options [2–6]. Cylinders are costly to refill and logistically challenging to transport, particularly to more rural areas with poor access roads. Programs supporting the installation, training, and use of oxygen concentrators have proven to be effective in reducing mortality from childhood pneumonia in Papua New Guinea (PNG) [7] and Malawi [8], where health facilities had suitable grid power or reliable backup generator power. In settings where electricity is not reliable however, oxygen concentrators are less suitable [3].

There is a need to find cost-effective, appropriate, and sustainable oxygen supply options for health centres in low- resource settings to address challenges such as intermittent power, high capital and operating costs, and high cost and skill requirements for maintenance [2], [3]. This will involve developing effective system designs, as well as ways to assess and evaluate these different technology configurations.

A. Oxygen Options for Poor Power: Field Experiences The lack of reliable grid power remains a challenge in

many low-income countries. Many hospitals and health centres

have backup generators, but they are often only used in emergency situations. Although other off-grid energy sources such as solar and wind energy are increasingly being used, to our knowledge, there is limited experience in using such alternative energy sources to operate oxygen concentrators.

In 2006, the Medical Research Council (MRC) Unit in The Gambia implemented an uninterrupted power supply (UPS) system for oxygen concentrators in their 42-bed hospital. The system has since been successful in supplying power to concentrators during short power outages. In other health centres in The Gambia, however, we observed power outages lasting as long as 13.5 hours, with average outage durations for individual health centres ranging from 36 minutes to almost four hours [9]. Electricity was available just over 50% of the time in the health centres studied. UPS can only compensate for short power outages (i.e. less than one hour), thus, locations such as these would be unable to rely entirely on this option.

In 2009, we developed a battery backup power system which operates an oxygen concentrator 24 hours a day with as little as four hours of grid charging time. Results of in-house bench testing at the MRC were previously reported [9], and the system is currently installed at a health centre in The Gambia. Another reported example of the use of batteries (a 45-amp car battery) was for a single concentrator in Nigeria, but sustainability of the system was not described [10].

The only published attempt to use solar panels as an alternative power source for oxygen concentrators was reported from The Gambia in 2001 [11]. This isolated case showed that a solar operated system can be a cost-effective alternative to cylinders where there is sufficient demand for oxygen (i.e. more than six treatment days per month at one litre per minute (LPM), or approximately 100,000 litres per year). However, this conclusion was based solely on a comparison of monthly operating costs. The initial capital investment required for solar technology may be prohibitively expensive in many cases, limiting its widespread use. There are no reports of follow-up to this project. The cost of solar panels has been dropping rapidly; sources estimate price decreases of over 30% in the United States between 1998 and 2008 [12], and over 60% in the last 3 years internationally (e.g. Germany, China, Japan) [13]. We therefore revisit this option as part of this report, as well as in other work in which we present a strictly cost-effectiveness comparison of different oxygen system configurations [14].

2012 IEEE Global Humanitarian Technology Conference

978-0-7695-4849-4/12 $26.00 © 2012 IEEE

DOI 10.1109/GHTC.2012.53

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2012 IEEE Global Humanitarian Technology Conference

978-0-7695-4849-4/12 $26.00 © 2012 IEEE

DOI 10.1109/GHTC.2012.53

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Generating and storing oxygen when power is available is an alternative option to storing energy in batteries, however experience with such systems for this context is limited. We are aware of three commercial oxygen generation and storage systems. The Airsep Ultrox (Buffalo, NY) is a transportable unit designed to fill multiple oxygen cylinders simultaneously. It is intended for high volume situations such as disaster relief or temporary medical or military facility installations. It can fill up to two H-size cylinders (2 × 7000L) of oxygen per day. The Invacare Homefill Ambulatory system (Cleveland, OH) is designed for patient home use, and can fill one cylinder at a time, up to size D (425L) in 3.5 hours. Lastly, the Diamedica (Devon, UK) Oxygen Reservoir Filling System can fill either a 20 or 100 litre reservoir to 500 kilopascals giving 100 or 500 litres of usable oxygen, respectively. The estimated costs of such systems range from about $5,000USD (for the Invacare and Diamedica systems) to $20,000USD (for the Ultrox).

B. Existing Models for Comparing Oxygen System Options There has been a lot of work done to compare the cost-

effectiveness of cylinders versus concentrators in low-resource settings [3], [10], [11], [15]. The economic implications of these two different oxygen supply options are quite different (i.e. cylinders require high recurrent expenditures over time, whereas concentrators require a larger initial capital investment and have lower operating costs over time [10]), making comparisons of cost-effectiveness a challenge. Whereas some have compared the cost-effectiveness of these options solely on operating costs (e.g. the solar system presented in [11]), others have proposed that a proper cost comparison should incorporate initial capital costs into estimates of on-going operational costs over time [3], [10]. For example, in a study from a Nigerian neonatal ward capital costs were amortized over time by estimating the lifespan of the equipment needed and converting the capital outlay into a discounted annual charge [10]. They concluded that concentrators were the most cost-effective option (in terms of cost per year) in this neonatal context. In a 2008 study, a computer-based Microsoft Access tool called OxOp was used to compare the cost-effectiveness of three oxygen system options for health centres in The Gambia: concentrators with grid power supply, concentrators with a generator power supply, or cylinders [3]. The tool recommends, at an individual health centre level, one of the three options based on several health centre characteristics (e.g. estimate of yearly oxygen demand, power reliability, number of beds, practicality of cylinder transport, etc.). The tool also estimates the cost of each recommended option (in $USD per 1000L and $USD per year). This tool is the first attempt at a systematic decision-support tool, based on cost-effectiveness, for oxygen supply options in a low-resource setting.

We have recently developed a cost-effectiveness model, which compares oxygen system configurations that encompass more than just cylinders and concentrators to compensate for poor power. Using this tool we have previously presented a cost-effectiveness analysis of two energy storage options (grid- and solar-charged battery backup systems), and two oxygen storage options (commercially available and non-commercial) which generate and store oxygen when power is available [14]. The analysis was based on The Gambia (a widely accepted

model for Sub-Saharan Africa) and accounted for hours of grid power expected per day, health centre child admission rate (and oxygen need), estimated equipment lifespan, and costs of electricity and maintenance. The four system setups presented in [14] provide the premise for the systems compared herein.

Models for comparing other attributes of oxygen systems in addition to cost-effectiveness are also needed. A survey conducted in 2009, which compared commercially available oxygen concentrators, considered criteria relevant for tropical environments (e.g. operating temperature, humidity, altitude ranges) [5]. That survey also compared price and efficiency, and introduced a cost index, defined as the capital cost divided by the maximum flow output. Eleven different concentrator types from seven manufacturers were ranked according to both performance specifications and the cost of replacement and spare parts. Of all the comparison models reviewed herein, this is the only one that considered criteria beyond just cost-effectiveness to assess oxygen technologies, but the study was limited to oxygen concentrators and did not include the entire technological system required to deliver oxygen to a patient.

The goal of this paper is to present alternative oxygen technology systems for settings with unreliable power, as well as a framework for comparison beyond mere cost-effectiveness. We aim to extend our thinking beyond the battery backup system we implemented in The Gambia [9], draw upon new insights related to the cost aspects of alternative oxygen options gained from our recent modeling work [14], and think critically about how these systems would compare against less-quantifiable but relevant subjective criteria specific to the context in which they are intended to operate. At present, there are no revolutionary oxygen technologies for medical use under consideration for ISO standardization; thus the oxygen sources considered here are currently the best available for this context. The aim and scope of this work is not to evaluate new technologies but to evaluate in a more effective way existing technologies in new configurations appropriate for this context.

II. OXYGEN SYSTEM CONFIGURATIONS PROPOSED The comparative systems are presented as general system

configurations without specific component details. All of the system diagrams below depict a concentrator connected to a flow-splitting assembly with two inputs and five output flowmeters, which can be connected to tubing to supply up to five children simultaneously at 0.1 to 2 LPM. As a backup oxygen supply, a cylinder can be connected to the second input on the flow-splitting assembly and can be switched on by manually turning a knob on a changeover switch.

The following sections provide a description of each system setup, along with a list of their key attributes and limitations. These points of discussion provide the basis for the overall relative rankings with respect to the criteria for comparison summarized in Sections III and IV.

A. Grid-charged battery backup oxygen concentrator system 1) Description: This system consists of a bank of batteries,

a charger, and an inverter (Fig. 1). The charger is plugged into a grid power outlet with the inverter turned on, and the

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concentrator on or off as desired. When grid power is available, the batteries will be charging, whether the concentrator is on or not. In the event of a power outage, the system automatically switches to battery discharge mode [9]. Although more costly, maintenance-free sealed batteries were chosen for the prototype system in The Gambia. When installed at the health centre, the batteries, charger, and inverter were isolated in an adjacent room. Wiring connects new outlets in the ward, which are designated for concentrator use only, to the battery system.

2) Attributes: • Oxygen delivery is direct to patient from concentrator.

• Can be integrated into a health centre such that health workers interface with concentrators and flowmeters only, by wiring sockets directly to the battery system in another room, as demonstrated in The Gambia.

• In the event of a power outage, no user intervention is required and there is no interruption in oxygen supply.

• Components expected to last at least 5 years [9].

• Maintenance-free batteries can be used. Minimal maintenance required for other components.

• Maintenance-free batteries also emit less volatile fumes, which can be a safety concern with other battery types.

• Power from inverter is of better quality than that of the grid, which contributes to extending concentrator life.

• When batteries are fully charged, and grid power is on, charger switches to ‘conservation charging’ mode such that batteries do not overcharge and produce fumes.

• Cost-effective for settings with both poor or good power (four to 10 hours per day) and a wide range of health centre sizes (i.e. between 500 and 6000 under five admissions per year, by Gambian standards); cost is comparable to the solar system (option B) depending on sun vs. grid charging hours available per day [14].

• For the prototype system implemented in The Gambia, all components were readily available.

3) Limitations: • Charger and inverter components tend to be weakest

link with shortest lifespans. They are vulnerable to extreme voltage surges, although built-in surge protection protects against fire risk. Surge protection equipment will add to the overall system cost.

• If batteries, charger, and inverter are isolated in another room, problems may not be immediately apparent.

• Electrical sockets designated for concentrator use only may be subject to misuse in the event of a power outage unless properly monitored.

• The concentrator will run off the batteries at all times; may affect the life of both the batteries and inverter.

• Battery life sensitive to high ambient temperature [16].

Figure 1. Grid-charged battery oxygen delivery system diagram [9].

B. Solar-charged battery backup oxygen concentrator system 1) Description: In this system, the alternative mechanism

to grid power for charging batteries is solar panels (Fig. 2). Concentrators operate from the battery bank when the sun is not available for charging; otherwise solar panels power the concentrators directly as regulated by the charge controller. The mode of operation would be similar to the battery system in that concentrators would operate from the battery bank at all times to avoid user intervention in the event of power outages.

2) Attributes: • Similar attributes to the grid-charged system, plus;

• No grid power required; although the system could be configured to charge from grid power when available.

• Panel life expectancy is quite long (>25 years) [17].

• Operating costs are relatively low [11], [14]. The economic case will continue to improve as solar cell prices drop and electricity rates rise.

• All components would be readily available; at least one system has been assembled in The Gambia [11].

Figure 2. Off-grid solar-powered oxygen system diagram.

AC CONCENTRATOR

5 FLOWMETER SET

SUPPLYSOURCE SWITCH

BATTERY BANK

INVERTER

REGULATOR

CHARGER

BACKUPCYLINDER

CHARGECONTROLLER

AC CONCENTRATOR

5 FLOWMETER SET

SUPPLYSOURCE SWITCH

BATTERY BANK

INVERTER

REGULATOR

BACKUPCYLINDER

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3) Limitations: • Initial capital investment is relatively high (e.g.

estimates range from $10,000USD [14] to $13,000USD [11] in The Gambia).

• Dependent on viable sun charging hours per day (seasonal fluctuations due to rainy or Hamartan (dusty) season), and may require backup cylinders if not properly specified for these low periods of sunlight.

• Panels require regular (daily) washing otherwise charging efficiency is greatly reduced [11].

• Solar panels installed outside the facility (i.e. roof or ground mounted) may be subject to tampering.

C. Commercial oxygen generation and storage system 1) Description: This system would operate continuously

when grid power is available; all oxygen generated would be compressed and stored in either gas cylinders or reservoirs for later use. For this comparison, we have considered two commercial configurations; a high-pressure cylinder filling system and a low-pressure reservoir filling system. If such systems were to be used in a low-resource clinical setting, the setup might be as shown in Figure 3. Cylinders would be filled outside the patient area, with oxygen delivered directly to patients from stored oxygen sources via a flow-splitting device.

2) Attributes: • Can dispense the exact amount of oxygen required by

patients; minimizes oxygen loss because all oxygen generated is stored.

• In the case of the Ultrox, a touch-screen controlled microprocessor offers built-in diagnosis for unattended operation, and will shut down once refill is complete.

• Does not involve complex electrical components.

3) Limitations: • Extra training/personnel may be required to manage

and maintain the cylinder or reservoir refilling process.

• In the case of the Ultrox, capital costs may be prohibitive, and oxygen output over-specified, for a small health centre.

Figure 3. Commercial oxygen generation / storage system diagram. Note: a

cylinder filling plant in the likeness of the Ultrox is shown, adapted from [18].

• Experience with such systems in developing countries (i.e. field experience and/or knowledge of maintenance requirements or life-expectancy) is limited [18].

D. Non-commercial oxygen generation and storage system 1) Description: This “self-assembled” system would also

operate continuously when grid power is available, storing all oxygen generated for later use. A system configuration we have considered in detail previously [14] was a high-pressure compressor system with cylinders (Fig. 4). Exploring possibilities for a low-pressure compressor and reservoir system will be investigated in future work.

2) Attributes: • Can dispense the exact amount of oxygen required by

patients; minimizes oxygen loss because all oxygen generated is stored.

• Choice of components can be scaled to the size and oxygen demand of individual health centres (a limitation of Ultrox); enables lower capital investment.

• Cost-effective option for very small health centres (i.e. < 500 under-5 admissions per year).

3) Limitations: • High technological complexity; high degree of staff

intervention is needed during operation.

• Untested technology; compatibility between concentrator output pressure and compressor inlet pressure need to be considered.

• Compressor maintenance costs are high based on supplier estimates of parts replacement frequency [14].

III. METHODS The systems described above were ranked by an assessment

team comprised of an engineering consultant with 23 years of experience working on oxygen-related projects in ten countries around the world (including Egypt, PNG, Laos, The Gambia), a research clinician with ten years of experience at a medical research centre located in The Gambia, two Canadian engineering students (one with almost one year of on-the- .

Figure 4. Non-commercial oxygen generation / storage system

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ground experience with oxygen projects in The Gambia), and a professor of chemical engineering in Canada. Oxygen projects in The Gambia are also supported by a team of five Gambian biomedical engineering technologists with over 65 years of combined experience working with medical technologies.

A. Criteria for Comparison The comparison criteria considered in this framework are:

feasibility/availability, sustainability, technical complexity, maintainability, usability, adaptability, and cost-effectiveness.

Feasibility/availability includes considerations such as the commercial availability of the system, availability of required components, and the stage of development of the system (e.g. assembled, proof-of-concept prototype, or untested concept). Sustainability refers to the life expectancy of the system in the environment in which it must operate. Technical complexity includes the complexity of all components as well as the complexity of the integration process that would be required to put a system into use. Maintainability refers to the level of day-to-day care required to keep the system in good operating condition. When considering system maintenance we refer to maintenance of those components in addition to the basic components used in each system (concentrators and/or cylinders, flowmeters, regulators, tubing, etc.). Usability refers to the level of health worker intervention required to operate each setup, and/or how different the setup would be from what they would be accustomed to, requiring additional training.

Technical complexity, maintainability, and usability have implications for the costs of training and on-going maintenance. Training for both health care workers and health facility technical staff can be a large portion of a project’s overall budget [6], [7]. For an oxygen program in PNG, training costs and follow-up maintenance visits, including cost of spare parts and repairs, comprised over 22% of the total project budget [7]. A recent project in Laos allocated over 30% of its project budget to consultants and training.1 Funds need to be allocated for a comprehensive approach to training and maintenance, if such systems are to be introduced [7].

Adaptability is the system’s ability to meet highly variable or fluctuating oxygen demand, and cost-effectiveness is a measure of the system’s capital and operating costs (i.e. cost per year or cost per 1000 litres of oxygen delivered).

B. Ranking Process We propose to use a relative ranking approach, similar to

that used by Peel and Howie [5], but extended beyond quantifiable characteristics to derive a subjective assessment. Each oxygen system was ranked from one to four for each of the above criteria, one being the best. The first six criteria are applicable to any typical health centre in a low-resource location. Cost-effectiveness is the only criterion directly dependent on availability of power and health centre size.

In our earlier modeling work, cost-effectiveness is explored in detail for the configurations proposed herein [14]. In that analysis, the model scenario considered is a typical Gambian

1 Personal experience of contributing author.

health centre with at least four to ten hours of grid power available per day, and 500 to 6000 under-five child admissions per year (less than 4000 is considered small to medium and up to 6000 is large). We assume approximately 6% of child admissions are given oxygen [19], with treatments specified at 0.5 LPM for 3.6 days [20]. This leads to oxygen consumptions of about 78,000 to 935,000 litres per year. Based on the outcomes of this work, general rankings for cost-effectiveness are provided separately for three general categories; a health centre with at least four hours of power per day (any size), and either a small to medium or a large health centre with at least ten hours of power. The sum of the ranks for these three categories determined the overall rank for this criterion.

IV. RESULTS & DISCUSSION The relative rankings of all systems considered are

summarized in Table 1. In terms of feasibility/availability, the off-the-shelf commercial storage options were ranked higher than the energy storage options (A & B). All components for the grid-charged system would be readily available, but not as a complete system. For the Gambian system, components were procured through an alternative energy company [9]. The next highest ranked system is the solar system; there is at least one such system assembled in The Gambia [11].

For sustainability, the systems with compressors (C & D) ranked lower than the systems with purely electrical components (A & B). Long-term maintenance of compressors involves regular parts replacement due to wear. The grid- and solar-charged systems tied because overall these systems are very similar; the addition of panels, in our view, had little impact on sustainability because of their long expected lifespan (approximately 25 years for some manufacturers [17]). We are however, unaware of any documented evidence of panel durability in the field for this context. Solar equipment warranties are also subject to a company’s longevity in an industry that has grown 51% annually since 2000 [21]. The non-commercial option is the least sustainable, as manufacturer support would be required for all components individually.

In terms of technical complexity and maintenance, due to the overall systemic changes required for integration into a health centre setting, we felt the oxygen storage options were more complex than just introducing an alternative power source for a concentrator. In both the grid- and solar-charged systems, maintenance-free batteries can be used. For the solar system, other maintenance is labor intensive (e.g. washing of panels) and requires commitment, but is not technically difficult. The oxygen storage options introduce issues of frequent parts replacement, but day-to-day maintenance, especially for the low-pressure system, is expected to be low.

In terms of usability, both the grid- and solar-charged battery systems were ranked equally high; oxygen is provided directly to patients from an oxygen concentrator via the flow-splitting device and no user intervention is required in the event of a power outage. For the storage systems, additional training and/or personnel would be required to manage and maintain the cylinder or reservoir refilling process. The low-pressure system would be more user-friendly than the Ultrox due to its smaller size and portability.

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TABLE I. SUMMARY OF OXYGEN SYSTEM COMPARISON RANKINGS

The commercial Ultrox system is the most adaptable option due to its high capacity output, allowing it to cater to a wide range in oxygen demand. The low-pressure Diamedica and non-commercial options are also highly adaptable with options for excess storage capacity to meet varying needs. Between the grid and solar system, the solar system can be configured to match a specific target oxygen demand more closely; however once components are purchased, adaptability is then limited.

According to our cost-effectiveness modeling [14], the solar system turns out to be the most cost-effective option when power availability is poor. For the grid-charged battery system to have more favorable economics, the hours of battery charging from the grid must be substantially greater than what it could have attained from the sun. This assessment is based on The Gambia, where average daily irradiation time ranges from 4.96 to 7.35 hours per day for the rainiest and driest months, respectively [22]. Solar irradiation is dependent on geography and affects the power storage necessary to compensate for when batteries are not being charged. We plan to conduct further analyses using our cost-effectiveness modeling tool to study the economics of off-grid solar configurations for differing amounts of solar irradiation per day.

The grid-charged system is ranked first when power availability is good (i.e. > 10 hours) and health centre size is large; the investment in solar panels is not likely to pay off in this case. The low-pressure Diamedica system is ranked second due to similar initial capital investment and oxygen output to the grid-charged system. The Ultrox is suitable for large hospitals given its high capacity output, but the large capital investment still limits its overall cost-effectiveness. Cost-wise the only case when we observed the non-commercial option as a viable competitor was for very small health centres with low patient admissions and power at least half of the day. For most other criteria, it was not an appropriate option.

Overall, this assessment indicates that grid- and solar-charged battery backup systems, as well as the recently developed low-pressure reservoir filling system from Diamedica, are promising oxygen supply options for this context. Although readily available and highly adaptable, the Ultrox system is not as cost-effective as the smaller-scale, more customizable, configurations for the context considered. It still may be suitable for health facilities with very high patient loads.

We recognize several limitations of this analysis. We have not provided rankings for safety, as some of the systems

a. < 4000 under-five admissions per year; b. up to 6000 under-five admissions per year [14] considered are still conceptual at this stage, however safety is also a very important criterion to consider. Any oxygen technology setup will involve inherent risk, either due to the oxygen system as a whole or due to individual components. Oxygen increases the likelihood of ignition and intensity of fire, and cylinders themselves are pressure vessels. The use of oxygen thus requires training on the risks and mitigation of oxygen fires. Cylinders have not been included as an option in this comparison. There is ample literature justifying the use of concentrators over cylinders [3], [10], [15], primarily due to cost, but also feasibility, usability, and safety. Cylinders would not have ranked highly using this framework. We also recognize that the criteria used for comparison could only be assessed subjectively, based on experiences in the field and/or expert opinion. By ranking the options relative to one another, we have attempted to represent this assessment in a quantitative manner. Also, our framework assumes equal weighting of all criteria, which may not be the case in certain decision-making scenarios. A weighting which reflects different priorities could easily be applied to the ranking system proposed.

The results of this review and analysis have implications for future work involving the evaluation, development, and prototyping of alternative oxygen supply systems. The design process requires resources, expertise, and time, a discussion of which was beyond the scope of this work. The aim of this analysis was to highlight those options which are within the realm of possibility and likely to be successful in this context, as well as areas where innovation and development are needed to improve the current state-of-the-art in oxygen technologies. This approach to assessment and the criteria used can easily be applied to new systems which may be developed.

V. CONCLUSION Several types of medical oxygen systems have been

outlined and compared across a wide range of criteria relevant to low-resource settings. The assessment indicates that systems using battery backup are promising options to pursue for this context, as is a recently developed oxygen storage system developed by Diamedica. The results of this review and analysis have implications for future work involving the evaluation, development, and prototyping of alternative oxygen supply systems for settings, both within and beyond Sub-Saharan Africa, experiencing challenges including poor grid power and limited financial and technological resources.

Oxygen System Configuration

Criteria for Comparison

Overall Rank Feasibility/

availability Sustain-ability

Technical Complexity

Maintain-ability Usability Adapt-

ability

Cost-effectiveness

~ 4 hours of power

> 10 hours of power Small/ Meda Largeb

A) Grid-charged 3 1 1 1 1 4 2 2 1 (12) = 1

B) Solar-charged 4 1 2 3 1 5 1 1 3 (18) = 3 C) Comercial storage i) High pressure (Ultrox) ii) Low pressure (Diamedica)

1 1

3 4

4 3

4 2

4 3

1 2

4 3

4 2

4 2

(21) = 4 (17) = 2

D) Non-commercial storage 5 5 5 5 5 2 5 5 5 (32) = 5

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ACKNOWLEDGMENT The authors gratefully Airsep Corp. and Diamedica for their

willingness to field questions about commercial oxygen storage systems. Ebrima Nyassi, a senior biomedical engineering technologist in the MRC Biomedical Engineering Department is acknowledged for his technical assistance and invaluable insight into the realities on the ground.

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