Yonago Acta medica 1999;42:185–188

Membrane Humidifier That Does Not Require Addition of Water
Naoto Burioka, Hiroki Chikumi, Hisashi Suyama, Takanori Sako, Hidemi
Teramoto, Yukio Matsumoto and Kazukiyo Takano*
Third Department of Internal Medicine, Faculty of Medicine, Tottori University, Yonago
683-0826 and *Sanyo Electric Industries, Okayama 703-8221, JapanWe developed a new device called “a membrane humidifier” which does not require an
external water supply. Fifteen patients inhaled humidified-oxygen from the membrane
humidifier and were asked about their subjective impression. The relative humidity ofroom air and that of humidified-oxygen from the membrane humidifier or a conventional
bubble water humidifier were measured with a digital hygrometer. The relative humidity of the oxygen humidified by humidifiers was measured after the gas was flowed intoa partially opened 500-mL container for 30 min. None of the patients experienced
dryness of the nose or throat. All patients answered that there was no difference intheir subjective impression between breathing oxygen from the membrane humidifier
and from the conventional bubble water humidifier. A significant regression was observed between the relative humidity of room air and that of the oxygen humidified
from the membrane humidifier. The membrane humidifier was able to produce humidification very well. This new compact device can be used not only in hospitals, but canalso be incorporated in home oxygen concentrators. This new device also saves theprocedure of changing water.
Key words: humidifier; humidity; membrane humidifier

Bubble water humidifiers have conventionally been used to humidify dry gas (Hayes andRobinson, 1970; Mercke, 1975; Chalon, 1980).
It is troublesome to clean the humidifier container and change the water. To reduce the riskof infection, the water reservoir must be cleaned
periodically and the water must be changed frequently. We have developed a new device
called “a membrane humidifier” whose function does not require the addition of external
water for humidification. This new system obtains moisture from room air. It can be used not
only in hospitals, but can also be incorporated
in home oxygen concentrators (Burioka et al.,1997a,1997b). This technical study evaluatedthe membrane humidifier to determine its efficiency in humidifying dry oxygen.

Subjects and Methods

Structure of the membrane humidifier
The membrane humidifier is composed of acompressor and a steel cylinder containing several hundred hollow fibers made from polyimide resin (Fig. 1). The different permeation
rate of gases is used to separate water moleculesfrom the air (Mulder, 1996). Water vapor canpermeate the polyimide membrane of a hollow
fiber (UBE membrane, UBE Industries, Ltd.,Tokyo, Japan) hundreds of times more readily
than either nitrogen or oxygen (Nishimura,1986). Compressed air with water vapor ispassed outside the hollow fibers. As the room
air is passed under high pressure through thespace around the hollow fibers (outside passage), the water molecules in the air permeate
the membrane of the hollow fibers. Dry oxygenfrom the hospital’s oxygen supply is passed

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through the hollow fibers(inside passage) within
the membrane humidifier,and is humidified withwater vapor (Fig. 2). One
unit of hollow fibers ishighly durable. The polyimide membrane of a hollow fiber can be sterilizedwith alcohol or disinfecting gases. The pressure of
the compressed air was
196 kPa (2 kgf/cm 2 ) inthis study although it was
98 kPa (1 kgf/cm2) in ourprevious study (Buriokaet al., 1997a, 1997b).

Fig. 1. Membrane humidifier. The membrane humidifier is composed of acompressor and a steel cylinder containing several hundred hollow fibers.

Patients

Measurement

We examined 15 patients (8 males and 7 females, mean age; 67.3 years old) who were hospitalized for chronic pulmonary disease (emphysema: 5 patients, interstitial pneumonia
with collagen disease: 2 patients, sequelae oftuberculosis: 4 patients, idiopathic interstitial
pneumonia: 2 patients, diffuse panbronchiolitis:
2 patients). Their clinical condition was stable.
They were receiving oxygen therapy in the hospital. The study was explained and informed
consent was obtained from patients before theirparticipation.

After inhaling humidified-oxygen from boththe membrane humidifier and conventional
bubble water humidifier (Koike Medical Co.,Tokyo) for 5 h, all patients were asked to fill out
a questionnaire. We asked the patients whether
their throat or nose was dried by the oxygen
inhalation from the 2 humidifiers, and we also
asked whether they felt some difference in theirimpression of inhalation between breathingoxygen from the new device and breathingoxygen from the conventional bubble water
humidifier.
We measured the relative humidity of theroom air, that of the dry oxygen from the hos-Compressed air with vapor

Air-outlet

H2O (outside passage)
Humidified
oxygenH2O
Hollow fibers(inside passage)

Dry oxygen from
hosptal's supply

CylinderFig. 2. Principle of humidification in one unit of hollow fibers. Compressed air with water vapor passes
through the outside passage. Water vapor only permeates the hollow fibers. Dry oxygen from the hospital’s
supply passes inside the hollow fibers, and is humidified with water vapor.

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Membrane humidifierpital’s oxygen supply and that of the oxygen
humidified by the membrane humidifier or thebubble water humidifier. The relative humidity
was measured with a digital hygrometer (TRHCA, Shin-ei Co., Tokyo, Japan), after the gases(1 L/min) had flowed into a partially opened
container (500 mL) for 30 min. The pressure ofthe compressed air in the membrane humidifier
was 196 kPa (2 kgf/cm2). The temperatures in
the laboratory room were maintained from 20 to22°C during the study.

Data analysis
Data are reported as mean ± SD. The significance of the results of multiple comparisons
was calculated with a one-way analysis of variance by Turkey’s test. Correlation between therelative humidity of room air and that of theoxygen flow humidified by the membrane humidifier was calculated using a regression curve

(%)
100

***
***

*Results
None of the patients experienced dryness of
their nose or throat while breathing oxygenfrom the new device or the conventional bubble
water humidifier. All subjects also answeredthat there was no difference in their subjective
impression between while breathing oxygenfrom the new machine and while breathingoxygen from the conventional bubble water
humidifier.
The relative humidity of the air in the laboratory room was 43.8 ± 14.9%. The relative
humidity of the dry oxygen flow delivered byhospital’s oxygen supply was 7.0 ± 1.0% after it
had flowed into a partially opened 500-mL
container for 30 min. A significant difference
was observed between the relative humidity ofthe oxygen from the membrane humidifier(81.4 ± 8.2 %) and that from the conventional
bubbling humidifier (88.9 ± 2.6%) (P < 0.05)(Fig. 3). A significant regression was observed
between the relative humidity of the room air
and that of the oxygen humidified by themembrane device (Fig. 4).

60(%)
100

Relative humidityof oxygen from new device

Relative humidity

80

with the software StatView-J 4.11 (Abacus
Concepts Inc, Berkeley, CA). A level of P < 0.05
was considered statistically significant.

40200Roomair

9080706050

Membranehumidifier

10Bubblehumidifier

20304050

Relative humidity of room air

Fig. 3. Relative humidity of room air, oxygen humidified by the membrane humidifier, and oxygen
humidified by the bubble water humidifier. ***P
< 0.001, *P < 0.05 (Turkey’s test).

60(%)

Fig. 4. There is a strong positive regression between
the relative humidity of room air and that of the oxygen humidified by the membrane humidifier (Y =14.1X – 3.96X2 + 0.0005X3 (r2 = 0.93).

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Discussion
Dry oxygen is generally humidified by sterile
distilled water when a supplement of oxygen isprovided by nasal cannula in Japan. However,sterile distilled water is expensive, and it istroublesome to change the water in the reservoir. We developed a new humidifier that canhumidify dry oxygen without the use of water.
A strong positive regression was observed
between the relative humidity of room air andthat of the oxygen humidified by the membrane
humidifier. A significant difference was observed between the relative humidity of theoxygen humidified by the membrane humidifier and that humidified by the conventional
bubble water humidifier (Fig. 3), because therelative humidity of the oxygen humidified by
the new device was low when the relative humidity of room air was low (about 14–22%).
Since this system obtains its water vapor fromroom air, it appears that its efficiency is diminished when the relative humidity of room air islow (Burioka et al., 1997a,1997b). However,even if the relative humidity of room air isabout 14%, the membrane humidifier could humidify the dry oxygen to 55%. When the relative humidity of room was above 30 %, the relative humidity obtained from the membrane humidifier was almost the same as that of the conventional bubble water humidifier.
None of the patients experienced dryness of
their nose or throat while breathing oxygenfrom the new device or the conventional bubble
water humidifier. The membrane humidifier
supplied well-moistured nasal oxygen without
the need of water. This new device can be also
incorporated in a PSA type oxygen concentrator(Burioka et al., 1997a,1997b) used in homeoxygen therapy. The membrane humidifier will
be beneficial in both hospitals and the homebecause it eliminates the laborious cleaning of
the reservoir and changing of the water.
The polyimide membrane using hollow
fibers has a dense layer pinhole-free surface. Itspore size is less than 10–3 µm (Nishimura, 1986;Mulder, 1996). Water vapor in the air dissolves
and diffuses into a membrane wall. Since bacteria and mycetes can’t permeate the membrane, this new device may be more hygienic
than conventional humidifiers that use water. A
more detailed study will be needed to investigate whether bacterial contamination can becompletely preventable.

Acknowledgments: This study was supported by agrant (No. 10670544) from the Ministry of Education, Science, Sports and Culture, Japan.
References
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(Received August 4, Accepted August 24, 1999)

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