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Available via license: CC BY-NC 3.0
Content may be subject to copyright.
Electronic cigarettes: product characterisation
and design considerations
Christopher J Brown, James M Cheng
Center for Tobacco Products,
U.S. Food and Drug
Administration, Rockville,
Maryland, USA
Correspondence to
Christopher J Brown, Center
for Tobacco Products, U.S.
Food and Drug Administration,
Office of Science, 9200
Corporate Blvd, Rockville,
MD 20850, USA;
Christopher.Brown@fda.hhs.gov
Received 3 December 2013
Accepted 4 March 2014
To cite: Brown CJ,
Cheng JM. Tob Control
2014;23:
ii4–ii10.
ABSTRACT
Objective To review the available eviden ce regarding
electronic cigarette (e-cigarette) product characterisation
and design features in order to understand their
potential impact on individual users and on public
health.
Methods Systematic literature searches in 10 reference
databases were conducted through October 2013. A
total of 14 articles and documents and 16 patents were
included in this analysis.
Result s Numerous disposable and reusable e-cigarette
product options exist, representing wide variation in
product configuration and component functionality.
Common e-cigarette components include an aerosol
generator, a flow sensor, a battery and a nicotine-
containing solution storage area. e-cigarettes currently
include many interchangeable parts, enabling users to
modify the character of the delivered aerosol and,
therefore, the product’s ‘effectiveness’ as a nicotine
delivery product. Materials in e-cigarettes may include
metals, rubber and ceramics. Some materials may be
aerosolised and have adverse health effects. Several
studies have described significant performance variability
across and within e-cigarette brands. Patent applications
include novel product features designed to influence
aerosol properties and e-cigarette efficiency at delivering
nicotine.
Conclusions Although e-cigarettes share a basic
design, engineering variations and user modifications
result in differences in nicotine delivery and potential
product risks. e-cigarette aerosols may include harmful
and potentially harmful constituents. Battery explosions
and the risks of exposure to the e-liquid (especially for
children) are also concerns. Additional research will
enhance the current understanding of basic e-cigarette
design and operation, aerosol production and
processing, and functionalit y. A standardised e-cigarette
testing regime should be developed to allow product
comparisons.
BACKGROUND
Electronic cigarettes (e-cigarettes) comprise a sub-
category of a broader range of products described
as personal vaporisers (PV), advanced personal
vaporisers (APV) or electronic nicotine delivery
systems (ENDS). These products have a range of
designs. Adequate characterisation of e-cigarette
design features is necessary to evaluate the potential
risks and benefits associated with their use.
METHODS
Systematic literature searches were conducted
through October 2013 to identify research related
to e-cigarettes and electronic nicotine delivery
systems. Ten reference databases (Web of
Knowledge, PubMed, SciFinder, Legacy Tobacco
Documents Library, Embase, EBSCOhost,
Espacenet, Google Scholar, Google Patent and the
US Patent Office) were searched using a set of rele-
vant search terms used singly or in combination.
Search terms included the following: ‘thermal
runaway’ OR ‘battery fire’ OR ‘battery explosion’
OR ‘lithium battery explosion’ OR ‘electronic nico-
tine devices’ OR ‘electronic nicotine delivery
systems’ OR ‘electronic cigarettes’ OR ‘e-cigarette’
OR ‘electronic’ AND ‘cigarette’.
To be considered for inclusion, the article or
patent (granted and applications) had to (1) be
written in English; (2) be publicly available; and (3)
deal partly or exclusively with engineering design
or operation, or lithium battery fires or explosions.
The search yielded a total of 296 e-cigarette articles
or documents that met the inclusion criteria.
Article titles and abstracts (when titles provided
insufficient detail) were then screened for rele-
vance. In addition, thousands of battery and patent
documents were identified; approximately 100
documents related to battery operation and 460
patents were screened for inclusion. Overall, the
search yielded 54 articles and 28 patents for full-
text review, which included a manual search of the
reference lists of selected articles to identify add-
itional relevant publications.
Following the full-text review, 14 articles and
documents and 16 patent documents were deemed
directly relevant for this analysis. The articles and
patent documents were published between 2004
and 2013. The validity and strength of each study
were determined based on a qualitative assessment
of depth and breadth of analysis, uniqueness and
relevance to engineering concerns.
Additional documents considered for review
included conference presentations/posters, reports
not published in peer-reviewed journals, national
and international standards, and government
reports. Three documents from either online
sources or conference proceedings are cited. Two
websites that provide e-cigarette design and oper-
ation information are also cited. Although not peer-
reviewed, these websites and documents provide
valuable insight on product design and operation.
SCIENTIFIC REVIEW
Basic design and operation
e-cigarettes are generally designed to resemble trad-
itional cigarettes in dimensions and, to some
extent, graphic design. The common components
for most e-cigarettes include an aerosol generator, a
flow sensor, a battery and a solution (or e-liquid)
storage area (see figure 1).
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ii4 Brown CJ, et al. Tob Control 2014;23:ii4–ii10. doi:10.1136/tobaccocontrol-2013-051476
Original article
e-cigarettes currently are classified as either disposable or
reusable. Disposable units do not have rechargeable batteries
and are usually not refillable. They may have a light-emitting
diode (LED). The e-liquid container or cartridge may be separ-
ate from the aerosol generator or atomizer; a combined atom-
izer and cartridge is called a cartomizer. Currently marketed
e-cigarettes typically have an aerosol generator with a metal or
ceramic heating element coiled around a wick bundle.
A wide variety of materials may be used in an e-cigarette.
They include metals, ceramics, plastics, rubber, fibres and
foams.
1–5
Some materials may be aerosolised, possibly contrib-
uting to adverse health effects.
Although e-cigarettes range in complexity, the following
describes the basic operation of a first-generation e-cigarette:
1. The user draws upon the e-cigarette, which activates an
airflow sensor.
2. The airfl ow sensor detects pressure changes and prompts the
flow of power to an LED and a heating element.
3. The e-liquid saturates a wick via capillary action and is then
aerosolised by the heating element.
6
4. The aerosolised droplets of e-liquid subsequently flow into
the user’s mouth and lungs.
7
Detailed operation and components
For some advanced e-cigarettes, prior to puffing, the consumer
can select feature adjustments that determine heating element
temperature, air flow rate or other functions. Figure 2 outlines a
more detailed, but typical, e-cigarette operations cycle. The
cycle is initiated by single or multiple sensor responses and/or
the use of a button.
8
The initiating sensor(s) may be an acoustic,
pressure, touch, capacitive, optical, Hall Effect or electromag-
netic field type.
910
The sensor(s) and/or button initiates power
flow to pumps, heating elements, LEDs and other elements.
1
Anecdotal evidence suggests sensors or buttons may provide the
ability to extend puff duration.
11
The cartridge (or cartomizer) and sometimes the battery
holder have air holes to help facilitate the flow of air required
for puffing while also controlling for pressure drop. However,
air holes may serve multiple purposes. For instance, the
European and International Organization for Standardization
Figure 1 Typical e-cigarette configuration. This shows a wick/heater as aerosol generator, gauze saturated with e-liquid, a microprocessor
(optional) to control operations and an LED (optional) to imitate a burning coal.
Figure 2 Basic e-cigarette operation. This flowchart outlines basic actions and functions to transform and deliver e-liquid-based aerosol.
Brown CJ, et al. Tob Control 2014;23:ii4–ii10. doi:10.1136/tobaccocontrol-2013-051476 ii5
Original article
(ISO) have ink pen lid standards that mandate air holes to
prevent a child-choking hazard associated with pen lids.
12
Aerosol production generally involves three stages: preproces-
sing, aerosol generation and postprocessing. The first stage
involves the transport of the e-liquid to the aerosol generator.
Capillary action through a wick is the primary means used by
first-generation and possibly the majority of current e-cigarettes
to control the delivery of e-liquid to the aerosolising element.
Other possible transport mechanisms include programmed or
mechanically controlled pumps, nozzles and diaphragms. The
pump may be peristaltic, plunger, eccentric or screw, and
powered by electrostatic, piezoelectric, magnetorestrictive,
thermal contractive or thermal bubble processes.
113
Additionally, fluid jet, micromesh, microetched screen or elec-
tropermeable membrane methods of transport are available.
19
Another method of e-liquid delivery to the aerosol generator
involves micro-pumps on microelectromechanical systems
(MEMS). Miniaturised pumps and/or nozzles/jets deliver specif-
ically programmed quantities and combinations of e-liquids to
an aerosol generator.
1
Alternately, a consumer may directly drip
e-liquid onto a heating element before puffing.
11 14
The second stage of aerosol processing involves aerosol gener-
ation, which involves (1) heating, in which the e-liquid comes in
contact with the heating element as described above; and/or (2)
mechanical processing, in which an ultrasonic vibration generator
or other mechanical device produces an aerosol by mechanical
dispersion.
13 15
At least one e-cigarette (cigar) introduced to the overseas
market uses ultrasonic vibration to produce an unheated aerosol
of 0.5–1.5 μm particle size, and another described in patent
documents combines heating and vibrating elements.
16 2
In add-
ition to heating and ultrasonic vibration, e-cigarettes may
incorporate MEMS that use pumps and nozzles ( jets) or ultra-
sonic piezoelectric elements for aerosol generation.
Possible heating element arrangements include straight line,
multiple spiral, cluster, nozzle, laser or element combinations.
Coil arrangements may incorporate wicks and coil covers
(bridged) or no covers (debridged for ‘dripping’). The heating
element’s resistance, material and the voltage across it determine
the current flow and element temperature. The heating element
temperature and temperature duration influence the aerosol
properties. Element degradation, fouling and other factors also
influence the heating element temperature.
The final stage of aerosol processing occurs as the aerosol
travels through the central air passage to the consumer. Unless the
aerosol is heated, its temperature decreases as it flows and conden-
sation occurs. Larger droplets that condense on the inside of the
central air passage may be removed from the passage and subse-
quently discarded or reprocessed into an aerosol.
17
To further
simulate traditional smoking, the appearance of sidestream smoke
is engineered into one e-cigarette design using pumps.
13
Microprocessors, programmable logic units, integrated cir-
cuits and other electronic components may be incorporated into
some e-cigarette products. The electronic components may be
used to power components and in conjunction with a liquid
crystal screen to display and/or record operating state para-
meters, such as battery life, use frequency per day, average use
cycle and safety warnings. However, the microprocessor may
also have additional functions such as the ability to control inte-
grated MEMS (eg, pumps and/or motors) that deliver speci fic-
ally programmed product quantities or concentrations. One
patent describes Bluetooth communication protocol integration
and multiple smoking software programs based on fluid type
and user preference.
13
The presence of microprocessors and memory chips in e-
cigarettes raises concerns about the collection and use of per-
sonal privacy information. A microprocessor may facilitate con-
sumer data collection for dissemination to a third party, either
wirelessly by Bluetooth or through the universal serial bus
(USB) interface when the battery is recharged.
9
Data may
include the smoker’s personal information (eg, gender, age,
address), smoking topography and possibly health-related
data.
918
Of further concern is the ability of the software and
microprocessor to directly or covertly manipulate nicotine deliv-
ery, and software viruses.
Some e-cigarettes may be used while connected by USB cord
to a power source. Additionally, some e-cigarettes may be
powered by permanent rechargeable battery (a manufacturer-
supplied sealed unit), a non-rechargeable battery or a user-
replaceable battery (rechargeable or non-rechargeable). Portable
chargeable carrying cases are available for remote e-cigarette
charging for some brands. Nickel-cadmium (NiCad), nickel
metal-hydride (NiMh), lithium ion (Li-ion), alkaline and lithium
polymer (Li-poly), and lithium manganese (LiMn) batteries may
be used to power e-cigarettes.
19
Performance considerations
Several studies have described significant performance variability
among e-cigarette brands and within the same brand/product
and compared the performance of e-cigarettes with that of con-
ventional cigarettes. The performance parameters investigated
included pressure drop, airflow, aerosol products and puff
count.
Trtchounian et al
20
compared eight brands of traditional
cigarettes to four brands of e-cigarettes. In a comparison of the
first 10 puffs of each brand, three of the four e-cigarette brands
required significantly higher average vacuums to produce an
aerosol density comparable to traditional cigarettes. In the same
study, when five brands (Trtchounian et al added additional
brand) of e-cigarettes were tested, the vacuum required to
produce an aerosol of standardised density increased for each
brand as it was smoked. The puff count at which the increase
would be required varied from a low of 24±12 for one brand
to 121±26, and 114±71 for two brands at the high end. Puff
count to entirely exhaust a cartridge varied for the e-cigarettes
tested from low of 30±43 to a high of 313±115 for different
brands.
Williams and Talbot followed up their collaboration with
Trtchounian and tested four different brands of e-cigarettes that
included duplicates of two brands.
21
An analysis of the first 10
puffs showed that pressure drop, flow rate and aerosol density
remained relatively constant for a given e-cigarette, but varied
among brands. The investigators analysed the ventilation or air
flow for each brand; they concluded that there was a good cor-
relation between air hole area and pressure drop for half the
e-cigarettes tested. The investigators concluded that “the airflow
rate required to produce an aerosol varied significantly among
e-cigarette brands and was usually higher than the airflow rate
required to produce smoke from tobacco-containing cigar-
ettes.”
21
Additionally, they concluded that standard testing pro-
tocols typically used with traditional cigarettes are not
appropriate for e-cigarettes, stating, “E-cigarette laboratory
testing will require its own standard procedure, which is yet to
be developed”.
21
Test protocols and standardisation are a concern when com-
paring results from the two studies. In each study, puff counts
are correlated with both puff volume and duration. The investi-
gators produced aerosols of a particular density. This
ii6 Brown CJ, et al. Tob Control 2014;23:ii4–ii10. doi:10.1136/tobaccocontrol-2013-051476
Original article
presupposes that density will drive the puff volume for all e-
cigarettes. However, the satiating effect may be the driving
factor; thus, standardisation of nicotine delivered/absorbed may
be an alternate means of determining the appropriate test proto-
col. The effect of smoking topography on the studies’ results
and conclusions is unclear and merits further investigation.
Design and aerosol production considerations
Anecdotal evidence suggests that larger capacity tanks, higher
coil voltage and dripping configurations appear to be consumer
innovations adopted by manufacturers. The e-cigarette forums
are a potential source of information concerning emerging
trends regarding e-cigarette design, use and maintenance.
Currently marketed e-cigarettes may have thousands of inter-
changeable parts that modify the character of the delivered
aerosol; connection adapters are available to further enable
interchangeability. Currently advertised features of some
e-cigarettes include
▸ advanced power-on activation (multiple-button click-on
feature)
▸ auto shutoff (safety feature)
▸ short circuit and over current protection (safety feature)
▸ variable voltage ranges (eg, 3–6 V in 0.1 V increments).
It appears that the variable voltage units introduced the
ability to increase heating element temperature. Increasing
heating element temperature subsequently increases the tem-
perature of the air containing the aerosol and increases the
aerosol generation rate. The warmer air can hold more e-liquid
mass per unit of air volume. Additionally, aerosol particle size
may be altered by the temperature of the heating coil.
5
Referencing Trtchounian et al
20
, Zhang et al
22
noted, “Vaping
technique would be especially important if vapers can generate
a different range of particle sizes, or if particle sizes change over
time.” Additional research is required to determine whether the
increased coil temperature significantly increases the e-liquid
mass available and/or alters aerosol particle size.
According to Etter et al
23
, particle size affects absorption and
can directly affect aerosol toxicity. Two studies measured e-
cigarette aerosol particle size. Ingebrethsen et al
24
found that
undiluted e-cigarette aerosols had particle diameters in the 250–
450 nm range and particle density concentration of approxi-
mately 109 particles/cm
3
. However, according to the same
report, these e-cigarette numbers differ somewhat from the 50–
200 nm particle diameter modes reported in another study by
Schripp et al.
25
Zhang et al
22
constructed an apparatus to deter-
mine in vitro particle size distribution and concluded through
testing that “e-cigs and conventional reference cigarettes
produce aerosols having generally similar particle sizes in the
range of 100–600 nm.”
In 2012, Pellegrino et al compared particulate matter (PM)
from aerosols of Italian brand e-cigarettes with the PM of con-
ventional cigarettes. Data showed that concentration of fine and
ultrafine PM was approximately 6–18 times higher for the con-
ventional cigarettes than the e-cigarettes tested.
26
A limited number of patents (granted and applications) were
reviewed to determine what new features have been incorpo-
rated into products currently on the commercial market or
potentially available to the market. One patent described ports
designed to facilitate reprocessing of accumulated condensate.
15
In their patent application, Tucker et al
19
describe the following
novel features: diffusers (figure 3), which enhance mixing;
airflow diverters, which apparently allow the heating element to
maintain a desired temperature, thus optimising nicotine deliv-
ery with puff intensity; multiple e-liquid tanks, each with its
own wick heater; aroma strips to add fragrance to the outside
of the e-cigarette (or into the aerosol); and a movable screen
with openings to vary the airflow and thus the pressure drop.
Liu describes multiple heating elements arranged in a series or
in parallel, where aerosol streams converge or remain separ-
ate.
27
In another patent application, Tucker et al
28
describe an
advanced dripping configuration. Alarcon and Healy describe
extensive communications and data collection potential in one
of their patent applications (table 4).
9
Conley et al
29
describe
breath, saliva, sweat, and tissue or cell analysis and conveyance
of the information to a healthcare professional as well as bio-
metrics to couple a device to a particular user. Li et al
30
describe
a feature designed to shift aerosol particle size distribution
towards a range of smaller particles through impacting the
aerosol on a surface.
Safety considerations
Two safety issues are posed by contaminants in the e-liquid or
‘e-juice’ and the handling hazards associated with inadvertent
skin contact. Due to a lack of manufacturing standards and con-
trols, e-liquid purity often cannot be assured, and testing of
some products has revealed the presence of hazardous sub-
stances.
7
The nicotine in e-liquid can be hazardous if mis-
handled and can be toxic to infants and children at the levels
present in e-liquid.
31
Child-safe or child-resistant packaging,
child safety locks (such as those present on cigarette lighters)
and proper instruction on the safe handling of e-liquid can help
mitigate some of these risks. One patent discussed the use of
biometrics and sensors to identify consumers by age; this tech-
nology could possibly be used to prevent some child usage by
using age screening.
29
The use of e-cigarettes with illegal substances is a concern.
One patent states that, “With slight modification of the solution
storage container, the device and connecting structures of the
present invention can be filled with conventional drug for pul-
monary administration apparatus.”
2
Another patent notes that
the unit may be used with narcotics, steroids, the marijuana con-
stituent tetrahydrocannabinol (THC) and other substances.
29
Part interchangeability is of particular concern since the per-
formance, risks and safety associated with a particular brand’s
configuration might change significantly when that configuration
is modified using third-party products. One study showed some
variability in cartomizer unit performance when batteries were
exchanged with those of the same brand.
21
In the same study,
one e-cigarette stopped producing aerosol when parts were
exchanged with another (same model) unit. Furthermore,
brand-specific e-liquid cartridges performed ‘significantly better’
at producing aerosols than third-party cartridges provided by a
vendor.
Figure 3 Diffuser. This diagram shows multiple angled openings that
increase aerosol dispersion and buccal cavity contact. The numbers
reference descriptive text in the patent.
519
Brown CJ, et al. Tob Control 2014;23:ii4–ii10. doi:10.1136/tobaccocontrol-2013-051476 ii7
Original article
Other concerns involve materials used in the aerosol gener-
ation process, aging and fouling. e-cigarettes that use a heating
mechanism to create a nicotine vapour emit metallic particles
and even nanoparticles of heating coil components in the
aerosol, such as tin, iron, nickel and chromium.
3
Lead, nickel
and chromium appear on the US Food and Drug
Administration’s (FDA) ‘harmful and potentially harmful consti-
tuents (HPHC) list’.
32
The safety of the inhalation of these
metallic particles and nanoparticles has not been studied and
could be a cause for concern. The use of alternative aerosol gen-
eration mechanisms may mitigate some of these safety questions,
although it is uncertain whether these alternatives may also gen-
erate particle and nanoparticle emissions. Moreover, long-term
e-cigarette performance and the associated generation of
HPHCs have not been studied. As e-cigarettes age and become
fouled, the products they generate may change. Automatic or
manual heating element cleaning should be design
considerations.
Many e-cigarettes use lithium batteries due to their ability to
store large amounts of energy in a compact amount of space.
However, the inherent characteristics of lithium batteries can
pose a risk of fire and explosion. Poor design, use of low-quality
materials, manufacturing flaws and defects, and improper use
and handling can all contribute to a condition known as
‘thermal runaway’, whereby the internal battery temperature
can increase to the point of causing a battery fire or even an
explosion.
33
The use of overcharging protection circuits,
thermal power cut-offs and internal overpressure relief mechan-
isms can help prevent and mitigate thermal runaway.
34
Critical information/tool gap s
Additional scientific studies are needed to evaluate the safety
and effectiveness of e-cigarettes. Topics for future research
include the following:
1. Understanding of the designs and functions of products cur-
rently on the US market is incomplete.
2. Variable and increased voltage e-cigarettes appear to intro-
duce the ability to deliver increased nicotine concentra-
tions.
35
Higher voltages and other features may introduce
the ability to manipulate particle size and increase aerosol
mass. Little research is available concerning these functions.
3. Knowledge of all the materials involved in aerosol produc-
tion is lacking.
Figure 4 Communications features. This diagram shows bidirectional data transfer between the consumer, computer, pack, social networks and
stakeholders. The numbers reference descriptive text in the patent.
9
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Original article
4. Hazards associated with the use of batteries require further
study. Failure mechanisms and the frequency of burn, shock
and explosion hazards are unknown.
5. The possible presence, function and capabilities of the soft-
ware, sensors and microprocessors incorporated into
e-cigarettes are unknown. It is not known what health or
topographical data are being collected or how the data may
influence or affect regulation and health policies. Software
vulnerabilities are also unknown.
6. The absence of standardised testing protocols compromises
comparisons across studies. Standardised test protocols that
allow for meaningful testing, categorisation and comparison
of e-cigarette test results would be a valuable research tool.
7. Knowledge of product lifecycles, degradation over time,
third-party component performance and misuse is needed.
CONC LUSIONS
Although e-cigarettes share a basic design, engineering variations
and user modifications result in differences in nicotine delivery
and potential product risks. Performance appears to greatly vary
among commercial products. e-cigarette aerosols may include
HPHCs defined by the FDA. Battery explosions and risk of
exposure to the e-liquid (especially for children) are also poten-
tial concerns. A number of safety features that could enhance
consumer safety do not appear to be widely used.
Current e-cigarette features have the potential to increase
nicotine delivery though advanced aerosol production methods.
Patents show that novel features are available that will likely
further maximise aerosol properties and e-cigarette efficiency
for delivering nicotine. e-cigarette forums and websites provide
information on current e-cigarette use, misuse, innovations and
concerns that may influence the commercial market.
Additional research will improve the current understanding of
basic e-cigarette design and operation, aerosol production and
processing, data collection capability, sensor and software/micro-
processor functionality and vulnerability, performance variation,
lifecycle degradation, unintended use and user safety. In add-
ition, a standardised e-cigarette testing protocol should be devel-
oped to allow product comparisons. Although of significant
importance, specific guidance for development of a standardised
test regime is beyond the scope of this paper. e-cigarettes with
unique designs may require specialised testing protocols.
What this paper adds
▸ To our knowledge, this is the first comprehensive review of
the literature related to e-cigarette product design features
and their potential health consequences.
▸ e-cigarettes are highly engineered products representing a
wide variation in product configuration, components and
safety features; furthermore, flexibility in many e-cigarette
designs allows user modifications. This results in
cross-product and within-product differences in aerosol
production, nicotine delivery and potential product use risks,
making it difficult to evaluate the impact of e-cigarettes on
individual users and the public health.
▸ Additional research is required to ascertain the health
consequences of e-cigarette use; a standardised e-cigarette
testing regime should be developed in order to facilitate
cross-product and within-product comparisons.
Acknowledgements The authors thank Elizabeth L. Durmowicz, M.D., Deborah
Neveleff, M.B.A., Paul Aguilar, M.P.H., Thomas Eads, Ph.D., M.P.A., and R. Philip
Yeager, Ph.D., DABT, for their support and assistance.
Contributors CJB performed the broad literature search on electronic cigarettes,
and JMC performed the literature search on battery-related items. CJB and JMC
cowrote the article; CJB, the lead author, focused on design, operation and
performance, while JMC focused on batteries and exposure.
Competing interests None.
Provenance and peer review Not commissioned; externally peer reviewed.
Open Access This is an Open Access article distributed in accordance with the
Creative Commons Attribution Non Commercial (CC BY-NC 3.0) license, which
permits others to distribute, remix, adapt, build upon this work non-commercially,
and license their derivative works on different terms, provided the original work is
properly cited and the use is non-commercial. See: http://creativecommons.org/
licenses/by-nc/3.0/
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