CA1329083C - Ultrasonic nebulizer - Google Patents

Abstract

Abstract of the Disclosure An improved ultrasonic nebulizer bowl of the type having a bowl-shaped cavity communicating with a piezoelectric crystal disposed under the cavity for nebu-lizing liquid medication disposed in the bowl. The improved crystal construction can also be used in other applications. The improvement prevents self-destruction of the surface of the crystal and comprises a protective pla-nar cover of a glass-like material disposed over the sur-face of the crystal in close spaced adjacent relationship thereto. An energy-coupling fluid is disposed between the protective planar cover and the surface of the crystal.
Finally, sealing means are disposed about the periphery of the protective planar cover for preventing fluid loss from between the protective planar cover and the surface of the crystal. The protective planar cover can comprise the bot-tom of the cavity, the bottom of a separate bowl insert, or a sheet peripherally attached to the top of the crystal.
Additionally, the piezoelectric crystal, which has a top disk electrode and a bottom disk electrode for making electrical contact therewith on the top and bottom surfaces thereof, is modified by having the bottom electrode of substantially smaller diameter than the diameter of the piezoelectric crystal whereby the electrical contact to the piezoelectric crystal on the bottom thereof and, therefore, the energy concentration is adjacent the center-thereof.

Description

1 3~0~3 658g5-1~5 Thls inventlon relates to medical devlces for the inhalation of medlcation and, more partlcularly, to a novel and lmproved nebulizer whlch is spPclflcally adapted for use in the inhalation of medlcatlon but havlng useful appllcatlon on other devlces employlng a plezoelectrlc crystal for ultrasonlc stlmu-latlon and excltatlon such as emulslflers, cleaners and the like.
Pulmonary drug dellvery systems, commonly referred to medlcally as nebulizers, come ln various forms. These, in turn r`; 10 can be broken down lnto varlous sub-groups. For example, the atomlzer type of devlce uses the venturi prlnclple of air pass-ing across a plpe or orlflce to draw llquld medlcatlon from a storage receptacle and atomlze lt lnto small partlcles. Such devlces can be operated by squeezlng a bulb or with a pressur-ized contalner.
When ultrasonlc energy of the right frequency and power ls applled to a llquld, a very flne partlcle mlst ls released from the sur~ace. At the frequency requlred to con-vert llquids, such as, water to a mlst, the ultrasonic energy can be produced by electrlcally exclting a plezoelectrlc materlal, such as, lead zlrconate tltonate, and mechanlcally coupllng that materlal to the llquld. Of the total energy whlch enters a system of thls type, some ls converted to heat ln the plezoelectrlc materlal, some may be converted to heat ln the llquld, and the remalnder ls consumed at the llquid surface in the process of breaklng away partlcles to form the mlst.

'r;
.`' ~

t 3~qO83 In a medical application, this process is called ~ nebulization and is used to convert medication to a mist :,~ for inhalation in the treatment of respiratory disease. In ;.
~` order to do this most effectively, the medication should be. ~
nebulized into particles or droplets of a particular size range and, as a general rule, the smaller the par~icles the ~etter the penetration of the particles into the lungs and the bronchial passageways.
s~ Earlier versions o~ ultrasonic nebulizers were intended for use primarily in the home or medical facility environment. However, miniaturiæation and the availability rl of small, highly efficient, rechargeable battery packs maXeit highly desirabl~ to provide portable ultrasonic nebuli-~ zers which can be hand-caxried and u~ed as required in the ,:,,:.
treatment. o~ respiratory disease.
i-;.`::
In the past, ultrasonic nebulizers which have employed a piezoelectric material have encountered numerous ., .
~ problems, among which is the tendancy of the material to .~.:
rapidly degrade owing to the generation of heat, cavitation of the li~uid caused by the high acoustic energy level, and chsmical attack of the surface by medications and cleaning agents. Each time that acoustic energy crosses from one . , !,~ matexial to another, some is passed and some i~ reflected.
Any material positioned between the transducer and the x~ liquid or protecting the surEace s~ould possess high ; energy transmission ef-ficiency and low energy reflection back to the transducer. It has been found that this con-dition can be created by providing a thin coating or - plating of approximately 1/100 W, such as, te10n, polyimide or gold, or providing a cover having a thickness of W/2 or a multiple thereof, such as, W, 3W/2, 2W attached ~ 2 --:
~, 1 32~083 , to the transducer surface where "W" i~ the wavelength of - the excitation signal. Glass is a preferred material for ~ such a cover because it presents an easily cleaned and ., .
durable surface to the liquid and can tolerate high tem-peratures. Neverth~less, a coupling agent is required to bridge the air gap between the two surfaces. In U.S.
Letters Patent No. 4,109,863 to Olson et al, it was pro-posed to employ adhesives for this purpose. However, high temperatures tend to weaken the bond of the adhesive and cau~e poor acoustic coupling and increased reflected energy. Olson et al proposed to solve the problem of high temperatures at the transducer surface by circulating a cooling water over the transducer and glass, but this method i5 not feasible for a portable handheld device and has the additional undesirable effect o acoustically damping the back side of the transducer and thus reducing the efficiency of the nebulizer system.
We have found it desirable to employ oil of the correct viscosity and temperature capability as a coupling agent. The oil tends to migrate toward the high energy density center of the transducer/glass inter~ace and occurs even af~er high temperatures have forced some of the oil to the periphery. In order to overcome any tendancy of the oil film to be too thin, causing reduced nebulization, the gap between the protective cover and the transducer surface must be so shaped as to provide an optimum oil film thickness thereacross which will avoid regularly generated reflections. Also it is important to contain the oil so ~;
that gravity and capillary forces do not carry it away from the gap during periods of inactivity. Accordingly it i~

: .
:
~( - 3 -, ~ 329083 . . .
,., important that the oil be confined or sealed in such a way as to assure that it will migrate towards the center of the gap when energy is applied. Moreover, another problem associated with the use of oil as a coupling agent is the presence of sntrapped gas which, when released during operation, may displace the oil and uncouple the glass ` cover. It is therefore desirable to minimize the amount of -~ entrapped gas present in the oil in the process of as~embling the elements of the nebulizer and to make provi- -sion for accumulation of any entrapped gas which may escape from the transducer surface during its life.
~mong various other prior art techniques, U.S.
Letter Patent 31433, 461 ~o Scarpa employs a piezoelectric ~i crystal bonded to a support layer. Both the crystal and ;:! the support layer were chosen to be one-half wavelength in thickness and bonded together with an adhesive to form a ~; composite structure one wavelength in thickness. This structure is supported around its periphery and contains a ,~ thin web between the vibrating center and supported ~$: 20 periphery to prevent support ~tructure loading which is counter productive to efficient high energy vibration.
Other patents of interest are Patent Nos. 4,094,317 to Wasnich and ~o. 3,861,386 to Harris where an acoustic wave form is shaped to perform ultrasonic nebulization of the ~3 liquid and to iæolate the liquid from the piezoelectric ~, . crystal and prevent dry operation of the device.

It is therefore an object of the present invention to provide for a novel and improved ultrasonic nebulizer .`~ which is capable o efficient acoustical coupling of energy `.i 30 from a piezoelectric crystal into the medication being :
nebulized without significant attenuation and wherein t~e :.,.

~ 329Q8~ -..
crystal itself is protected from undesired early destruc-~;, tlon.
Another object of the present invention i~ to pro-vide in an ultrasonic nebulizer for a novel and approved piezoelectric transducer and wherein the means for acousti-' cally coupling the ultrasonic energy from the piezoelectric element to a face plate is a liquid; and further wherein ,:
the gap formed between the face plate and piezoelectric element is so formed that the energy transfer is maximized ; 10 and heating minimi~ed so as to avoid the necessity for external means of cooling.
~ r.
A further object of the present invention is to provide in an ultrasonic nebulizer for a novel and improved ; method and means for mounting a piezoelectric crystal and :.
;~ protective cover and face plate which will maintain an .~ .
optimum gap between the elements notwithstanding pressure '; increases created by thermal expansion of the coupling ~,~ fluid; and urther wherein the mounting means will act as a ~.~
~' reservoir for the coupling fluid and retain any entrapped :
gases therein so as to prevent decay of the coupling effi-ciency otherwise resulting from gas build up.
, ., It i~ an additional object of the present inven-~ion to provide for a method of assembly of a piezoelectric i~' crystal in an ultrasonic nebulizer to minimize the amount : .',,'~
of entrapped gases in a coupling fluid between the crystal and face plate and to effectively seal the crystal and the face plate along with the coupling fluid for most efficient ~ r, energy transfer between the crystal and the face plate.

It is still an additional object of ~he present :.
;~ 30 invention to provide for a novel and improved nebulizer which will maximize the isolation and removal of optimum , ~

` 1 3~9083 size particles for inhalation by the user as well as to more effectively collect and renebulize larger particles in $
such a way as to minimize clogging or impaction of the par-ticles.
,.~
~ In accordance with the present invention, the ; foregoing o~jectives have been attained in an assembly having a cavity therein for holding a liquid medication to j~ be nebulized by a piezoelectric crystal disposed in com-;~; munication with the cavity and separated from medication 0 contained in the cavity by a thin cover of a material having characteristics for allowing it to match acoustical -~ impedance~ without significant attenuation, such as, those pos3essed by gla~s, the crystal being disposed adjacent to the cover; a thin film of a enexgy coupling fluid inter-~`~ posed between the cover and the surface of the crystal;
: '.Y
~ and, sealing means for the cover and the surface of the ~,., j ~ii crystal for preventing loss of fluid from between the cover ,:
and the surface of the crystal.
In one embodiment, the cavity has a hole in the bottom thereof communicating with the surface of the crystal; and, the cover comprises a sheet of glass, or the :,, like, di~posed over the curface of the crystal.
In a second emboidment, the cavity has a hole in the bottom thereof communicating with the surface o~ the ,~; srystalg and~ the cover comprises the bottom of a unitary : .,.
.` bowl in~ert of a glass-like meterial inserted into the ~; cavity. In a third emvodiment, the bowl unit is of unitary construction and the cover comprises the bottom of the cavity which is formed therein.
:
A preferred apparatus for the ultrasonic nebuliza-~; tion of fluids wherein a piezoelectric transducer is .~, .
,, s - 6 -: ~
s -i t 3~9~83 .'.:
--~ disposed in communication with a reservoir for nebulizing liquid medication in the reserYoir, the improvement .~ comprising a protective cover member of a predetermined wavelength superimposed on the transducer, the cover member ; and transducer having generally planar confronting surfaces and a common in-terface therebetween, annular sealing means : in outer surrounding relation to the cover member to pre-.- vent the loss of fluid from the interface, and oil coupling means in the interface Eor coupling the ultrasonic energy '~ 10 from the transducer to the cover member. Most desirably ,.
~ the piezoelectric crystal has a top electrode extending ,,.
down the sides and wrapping around the bo-ttom edge and a bottom disc el~ctrode of sub~tantially smaller diameter ~; than the diameter of the piezoelectric crystal whereby the ~-. electrical contact to the piezoelectric crystal on the bot-.~ tom thereof is adjacent to the center thereof.
. Figure 1 is a partially exploded, simplified, ~; cutaway drawing of a prior art ultrasonic nebulizer;
~ Figure 2 is a drawing of the prior art nebulizer `-~ 20 of Figure 1 shown assambled and showing its manner of operation;
`; Figure 3 is an enlarged, detailed, cutaway drawing of the bowl portion of a nebulizer-according to the present ~. invention in a first embodiment thereof;
.~ Figure 4 is a simplifiPd cutaway drawing through a .~ piezoelectric cry~tal as employed in prior art ultrasonic nebulizers showing the manner of vibrational propagation .~ tharethrough;
,~, Figure 5 is a simplified cutaway drawing through a . 30 piezoelectric crystal as employed in the ultrasonic nebu-lizer of the present invention with its protective covering :, :`

1 32~083 and showing the manner of vibrational propagation ^~ therethrough;
.~ Figu.re 6 is an enlarged, detailed, cutaway drawing of the bowl portion of a nebulizer according to the present ; invention in a second embodiment thereof;
.
. Figure 7 is an enlarged, detailed, cutaway drawing of the bowl portion of a nebulizer according to the present :,: invention in a third embodiment thereof;
'f'i'~ Figure 8 is a simplified cutaway drawing through a prior art vibrational surface specially shaped to direct the energy waves toward the center thereof;
; Figure 9 is a simplified cutaway drawing through a '. prîor art camposite structure for a piezoelectric crystal ~,~ wherein the crystal is adhesively attached to a vibrating ~ support structure and the virbating portion is decoupled 't~`'` from the actual support area;
,~, ~,. Figure 10 is a simplified cutaway through a prior art structure wherein the liquid to be nebulized is con-tained in a separate container;
. :
Figure 11 is a ~ectional view of a preferred form ~` of portable nebuIizer unit in accordance with the present ~, invention;
Figure 12 is a detailed view in section of the piezoelectric transducer assembly; and Figure 13 is a bottom view of the preferred form of crystal employed in accordance with the present inven ,: .
~, tlon.
-~ As a setting for the present invention, prior art ~ nebulizers, such as, that generally indicated at 10 in .~,.
~, 30 Figure 1 in exploded view, comprise a power pack 12 con-taining batteries, not shown, if it is a portable unit, or 't.
~ 8 ;~ ' :"

` 1 32~083 , are adapted to be plugged into a wall outlet in the cas~ of non-portable units. The power pack 12 is connected by power cable 14 to the nebuliæing unit 16. The nebulizing unit 16, in turn, comprises a bowl portion 18 over which a cover 20 is adapted to be positioned. The cover 20 has a mouthpiece mounting tube 22 through the sidewalls thereof into which mouthpiece 24 can be press fit. Yor ease of manufacture and assembly, the bowl portion 18, cover 20, tube 22 and mouthpiece 24 are usually cylindrical in cross ~i 10 section~ The same components are also typically made of high impact plastic for light weight, ease of cleaning, and j ., non~contamination. The bowl portion 18 contains a bowl-. shaped cavity 26 in the -top thereof into which the liquid ~ , ~
. medication 28 ~see Figure 2) is poured. ~ piezoelectric crystal 30 is positioned under the cavity 26 and the bottom oE the cavity 26 has a circular hole 32 therein com-municating with the top of the piezoelectric crystal 30.
To activate the unit, the button 34 on the -top of the power ~ pack 12 is depressed, causing the piezoelectric crystal 30 `~ 20 to have power applied thereto. That, in turn, causes the crystal 30 to vibrate at ultrasonic frequency and nebulize a por-tion of the liquid medication disposed within the hole 32 and on top of the surface of the piezoelectric crystal ~ 30. The atomized droplets 36 produced are inhaled through '~:' s~ the mouthpiece 24 in combination with air, indicated by the ~` arrows 38, which is drawn in through the entry pipe 40 in .~ the top of the cover 20 provided for that purpose.
.~ Such prior art devices made and opera-ting according to the foregoing description work adequately for their intended purpose with a major shortcoming that the piezoelectric crystal is rapidly destroyed in the process.

- g_ ~ 32~Q83 This, of course, requires frequent and costly repair or replacement of the bowl portion 18. One problem with using ;~ a bare crystal is that the plated electrode thereof is ~ attacked by some of the cleaning solutions (e.g., vinegar~. ;
Another problem arises because of the wide difference bet-ween the acoustical impedances of a water based liquid (i.e., the medication to be nebulized) and air. If the acoustical impedances are properly matched, then the acoustical energy is radiated; if not, a majority of the energy is reflected back into the crystal. When the energy ~` is reflected back, it is in a very small region and produ~
~ ces high localized temperatures. These high temperatures ~
tend to cause the crystal material, along with its plated electrode t to rapidly degrade. When there is medicine pre-sent, a good acoustical impedance match exists and there is , ~ j good nebulization. When the medicine is not present or exists in only a thin layer, a poor acoustical impedance match exists~ causing very destructive conditions for the crystal. When the bare crystal fails due to the acoustical r,~, 20 impedance mismatch, the high energy densities locate in the nterface between the plated electrode and the piezo crystal itself. This develops thermal gradients which, in ~ turn, cause the plating to develop pin holes therethrough.
;~; Once the pin holes appear, the plating quickly degrades as the medicine seeps in. Additionally, cavita-tion effects accelerate the destruction once the holes appear.
":~
~ Attempts at coating the piezoelectric crystal have ; ~":~
met with little or no success to date. Either the ultraso-~. nic vibration of the crystal is not coupled into the :, covering, or poorly coupled r such that nebulization is inefficient or non-existent; or, the coating is simply .

.

, ~ 32~83 .:
destroyed in the same manner, followed shortly thereafter by the crystal itself. For example, with a coating, such as r Teflon or a high temperature polyimide as previously employed in the art, the same impedance problem is encoun-~.:
tered; except, it is complicated by the coating having its own characteristic acoustic impedance as well. Now, the energy is concentrated in the interface between the .:, ~- electrode and the coating. Subsequently, the coating sees ~ the high energy densities and associated thermal gradients.
;~ 10 Failure is quite similar to the bare crystal. Small bubbles first begin to appear in the center of the assembly, i.e~ the most active portion. Once a bubble 'i- i -, appears, the assembly quickly degrades in the manner ;~ described above.
'~ In the prior art structure of Figure 9, the . :, piezoelectric crystal 88 is bonded to a support layer 90 which actually contacts the liquid. Both the crystal 88 ;i,,;
and the support layer 90 were chosen to b~ one-half wave-length in thickness and then bonded together with an adhe-sive 92 to form a composite structure 86 one wavelength in thickness. The composite structure 86 is supported about its periphery and contains a thin web 94 between the :~, ~ vibrating center portion and the supported periphery 96 to ,................... .
prevent support structure loading thereof which is coun-terproductive to efficient high energy vibration.
..
As depicted in Figure 8, a vibrating surface is ~, shown on which the liquid to be nebulized is in contact in order to focus the energy waves towards the center of the liguid in a concentrated energy area. Thus, for example, the vibrating surface 98 in Figure 8 is in the form of a Fresnel lens, which aims or directs the vibrational energy, '`
, 1 ~29Q~3 . indicated by the dotted lines 100~ towards a plane passing ,~ through the center.
. A second technique is shown in simplified form in Figure 10 and comprises placing the liquid to be nebulized - 102 in a small container 104 having a thin, vibratable bot-.
.~ tom surface 106 and placing the small container 104 in a . large container 108 filled with liquid 110 (such as water) ~ and having the pieæoelectric crystal 30 at the bottom .~ thereof. In this manner, the crystal 30 is always liquid-~,'. 10 covered. When the crystal 30 is vibrated, the energy waves travel through the liquid 110 in the large container 108 ;~ and strike the bottom surface 106 of the small container ;.~
~ 104, causing it to vibrate an amount sufficient to nebulize ~.:
. the liquid 102 and cause the droplets 112.
.: The improved bowl portion of an ultrasonic nebu-lizer similar to the prior art nebulizer of Figures 1 and 2 ~. and modif.ied according to the present invention is shown in '~ detail in Figure 3 and labelled therein as 18'. It i9 ~3:
~, designed to be used in conjunction with a power pac.k 12 and cover 20, as previously described with respect to Figures 1 ~: and 2, and in the interest of simplicity and the avoiding of redundancy in the description and drawings, those por-tions will not be shown or described hereinafter.
~,~ The improvement to a piezoelectric crystal ~` constructed assembl~ of the present invention as used in a nebulizer, for example, includes a protective covering for the piezoelectric crystal and a revised construction of the ' ~t' crystal itself. As best seen in the detailed cutaway view ~ o~ Figure 3, the piezoelectric crystal 30' of the present A~ 30 invention in the first embodiment thereof has a protective ,~ pyrex glass covering 42 disposed over the surface thereof i - 12 -,...
, .
., ,. .
::

~ 1 3~083 and under the hole 32 in the bottom of the cavity 26. As an indication of the sizes involved in the apparatus being ~- described, in a tested embodiment of this embodiment which ' is to be commercially manufac~ured and sold by the assignee of this application, the piezoelectric crystal 30' is .77 s inches in diameter and the glass covering 42 is .61 inches ~` in diameter. The glass covering 42 is one-half wavelength ;-~ oE the crystal's fre~uency in thickness, which was found to give preferr~d coupling to the medication 28 shown in ~` 10 Figure 2. Since the frequency being employed is about 1.65 ~- MHz, the thickness of the glass covering 42 is approxima-tel~ .064 inches. It should be noted at this point that while the kested example being described here employs a protective pyrex glas~ covering, other materials known to ~ those skilled in the art could, of course, be employed.
!,' For example, certain plastics and ceramic materials would ,~ undoubtedly make good substitutes. Likewise, while a cir-cular disc is shown and preferred for ease of manufacture, ~; ~ other shapes could, of course, be used and the term "disc"
"~ 20 is not to be construed as a limitation, but rather, a term ~ ,.
of convenience only.
Employing a glass covering over the piezoelectric crystal bonded by adhesive would not improve greatly over the prior art approaches mentioned above and would suffer from the same problems. Such approach was initially tried ,;
by the applicants herein with the expected results, or lack thereof. Primarily, there is a major loss of acoustic coupling of the ultrasonic energy into the glass cover as - the adhesive bond deteriorates. One of the possible solu-tions to the non-coupling problem which was tried was the ~..
use of a thin film of commonly available fluids, such as, ., ~` - 13 -.
, ~, ; `:
,~:
",~
. . .

~ ~ 32qo~,~
water as a coupling fluid between the surface of the i crystal 30' and the glass 42. While, initially, these fluids would cause coupling of the energy into the glass cover 42, these common coupling fluids were quickly destroyed in operation, resulting in failure of the unit by loss of acoustic coupling of the energy. Extensive testing ~ finally lead to the discovary of certain fluids which, ,~ under influence of the ultrasonic vibration, acted exactly contrary to the other, normall~ thought of, coupling fluids, which had disintegrated in use; that is, while the ~; other, non-useful fluids had migrated to the edges of the ~; glass 42 and crystal 30' and away from the central active zone of vibrational transfer, the workable coupling fluids discovered by the applicants herein tended to migrate to ~; the center under vibrational stimulation and, thereby, ~',f, effect ma~imum energy trans~er from the crystal 30' into ~,~ the glass cover 42. ~gain, it is worthy of note at this point that while fluids of a particular type and exhibiting ~` particular coupling characteristics are described ,; 20 hereinafter by way of example, those skilled in the art ~ ~ will recognize and appreciake that there are other :
,r,~ materials, which could be easily overlooked in the broad i classification of "fluids", which could be substituted as -~ the coupling fluid. Where the term "fluid" is used in the .1 descriptions herein and in the claims appended hereto, it is the applicants' intent that it be considered in its ~- broadest sense as including other coupling fluids possessing the necessary gualities and characteriestics as set forth in detail hereinafter. As to some of the fluids i~` 30 which were tested and found to be unsuitable, some were ; quickly destroyed while others were not. When destruction '!, 14 -.....
~s~
.: ., ., :-.-:-, t 32qO83 ., was immediate, i.e., in less than ten uses, no amount of time was enough to recover the ability to effectively pass the ultrasonic energy into the medicine bowl. Such fluids included silicone as well as Teflon-based oils and greases.
Other fluids failed because of migration out from under the glass. This resulted in excessively long ~Iwarm up'l times to produce nebulization, which allowed the fluid to go to the center of activity and for electrical conduction through the film of oil between the brass connector ring and the wrap-around electrode. The 'loill' fluids ultimately employed by the applicants as a film between the crystal and its closely adjacent, spaced, protective cover have the :; ~
`~ property of being ~Iself protecting and self healing'l; that is, when destructive conditions appear, the oil film will move out of the most active (and destructive) areas. After the detrimental conditions have lleft; i.e., there is medi-, ~
cine in the bowl and a good impedance match, the oil "pumps" itself back into the necessary areas to allow good . ~ ~
acoustic coupling.
~ ~0 The critical properties of acoustic coupling x fluids which are appropriate include --viscosity, viscosity breakdown, temperature stability, acoustic impedance, che-mical stability, and molecular composition. These proper-ties will now be addressed individually.
Viscosity: Through empirical analysis, the applicants herein were able to determine the workable limits of viscosity. Bounding the low end is the ability ~ to manufacture the assembly. If the fluid is too thin -~ (under 1 centistoke at 100 C for oil), it is very dif--~ 30 ficult to intimately join the crystal and the glass without ~s excessive air entrapping itself (enough to signiEicantly . . .
`,,':

, ,, ,........................................................... .

1 32qos3 i decrease life or couple effectively). On the high end (above 100 centistoke at 100 C for oil), the fluid still passes the acoustical energy; but~ attenuation becomes significant. The higher viscosity fluids tested produced lower output as compared to the fluid viscosities. It is desirable for the fluid to have a viscosity curve which is as flat as possible. This provides uniformity as the tem-perature of the system changes. The preferred viscosity ~- range is between 4 and 10 centistoke at 100 C for an oil coupling. In this range, there were no d~tectable dif-ferences in either coupling or life.
~.:
Viscosity Breakdown: Because of the high ~;~ energy densities of the reflected acoustic wave when impe-~.i;
dances are mismatched (i.e., air only, no medicine) ,~ corresponding high temperatures are found in the interface ,;, .
between the crystal and the glass containing the fluid. To maintain the desired viscosity range and reasonable life, the fluid should not undergo permanent changes in viscosity when exposed -to the high temperature and energy densities involved.
Temperature Stability: As previously men-tioned, the fluid is subject to very high temperature ~ra-dients. Therefore, the fluid needs to have high operating temperature limits as well as a reasonably flat viscosity .~ curve. Temperature requirements are met if the flash point is above 200 C and there is no viscosity breakdown after repeated thermal cycling.
Acoustic Impedance: The fluid must also have an acoustic impedance which is acceptable to the assembly system at the desired frequency without significant atte--; nuation. Ideal impedance matching between the crystal and ~ - 16 -1 3~9~83 , .
cover is difficult to attain; however, through empirical ,~
testing, the applicants were able to determine that the ; window of acceptability was fairly large in a given family ~; of fluids.
;~
Chemical Stability: Some of the fluids tested failed to pass the acoustic energy after a limited number of uses. It is hypothesized that the high energy densities and thermal gradients caused the fluids to change chemical composition. Therefore, the fluid needs to main-tain chemical composition when exposed to high -temperature and energy densities. Synthetic oils appear to have the ability to take the harsh conditions and maintain their initial properties.
Molecular Composition: Some of the fluids which met the above criteria still failed to pass enough acoustic eneryy to be eEfective. The applicants' best guess in this regard is that the molecular composition is such that these fluids' molecular composition outweighed the characteristics of the above properties.
Preferred Fluid: The fluid preferred by applicants, as a group, is synthetic oil based. The pre-ferred embodiment as presently employed by the applicants in the commercial embodiments of the present invention is an 8 centistoke (at 100 C) Polyalphaolefin base oil.
While the discovery of a workable class of fluids to effect coupling was a major breakthrough in the develop-ment of a long-lasting ultrasonic nebulizer, it, in turn, created new problems to be solved in the overall design of . .~ .
i~ the bowl portion 18'. Because of the close spacing .....
employed between the glass cover and the crystal, one of '"t the problem phenomena was capillary action of the fluid : ~, ~ - 17 -~' , 1 ~290g3 during periods of non-use; that is, the fluid simply moved to the outer edges of the glass and was lost due to capillary ac-tion. There are also transition periods when the unit is turning on and off. During these times, the assembly is attempting to stabilize to the new conditions that the electrical excitement has produced, i.e., mechani-cal movement. When initially excited 9 the assembly attempts to pump fluid into the center of activity to correctly match acoustical impedances. The mechanism of this movement is not fully understood but -the applicants hypothesize tha-t a slow moving~ travelling wave is created ,~ between the crystal surface and the protective glass cover similar to peristaltic pumping. This wave slowly brings in the necessary amount of fluid to complete coupling and i bet-ter match the acoustic impedance of the crystal to the A glass and the medication, i.e~, the preferred fluids .
;; actually ma~e the assembly self-adjusting for optimum per-formance. Once the acoutic impedances are matched, the glass cover attains similar vibrational characteristics to the crystal, resulting in a stable standing wave with the fluid ~t the center of the assembly. When the unit is turned off, the hydrodynamic forces, material forces, and geometry of the crystal and the cover tend to remove the ;.
~luid that is contained in the valley sections o~ the pre-viously formed standing wave. Along ~ith the capillary . . . ~
action previously mentioned, the fluid forced out toward the perimeter of the assembly. The net result was a loss .~.
I~ of enough fluid to disable the mechanism pumping the fluid . .
~` into the center of activity, thus ending efficient acoustic coupli~g.

. :, ~ - 18 -s' ~,, ~ 3~Q83 ~ Initially, the diameter of the glass cover 42 and - crystal 30' were made the same and -the two were merely pressed into the groove under the hole 32 originally pro-vided for the crystal alone. To solve the problem, after ; much testing, the sealing configuration of Figure 3 was finally developed. As can be seen, the diameter of the protective glass cover 42 was made smaller than the , ` diameter of the crystal 30'. After putting the coupling c~ fluid 44 between the glass cover 42 and the surface of the crystal 30', the circumferencial edge was sealed with an ,~ adhesive 46~ The preferred adhesive 46 sealing material is . ; a two part epoxy which, when fully cured, provides a hard, .~ rigid, attachment as well as sealing in the coupling fluid.
.i; Like the surface of the crystal 30', however, if exposed to :.: the medication 28 at ultrasonic vibrational frequencies, . the epoxy adhesive 46 would be quickly destroyed. To eli-~: minate that undesired side effect, a second seal was pro-;~,; .
~rl ~ vided to isolate the edge seal of the epoxy adhesive 4Ç
`.: ;
~` from the medication 28. That second seal is provided by ~: 20 positioning the edge of the outer periphery of the protec-tive glass 42 and the adbesive 46 outside of the 0-ring 48 and which is disposed in a circular groove 50 provided ~. .
~ therefor below and surrounding the hole 32. 0-ring 48 is r ~ of a diameter less than the diameter of the glass cover 42 and greater than the diameter of the hole 32.

The crystal 301, protective glass cover 42 and 0-~,r ring 48 are held in position for operation under the bowl-~` shaped cavity 26 by the same mounting apparatus as in the .~ prior art ultrasonic nebulizers previously discussed 30 broadly under Backgrcund of the Invention~ In particular, ~`~ a metal cylindrical contact 52 connected to one wire 54 :

;' - 19 -~`, , t 32~3 ., .
within power cable 14 is supported against the bottom of the crystal 30' by cylindrical plate 56 as a result of the screws 58 threaded into the bottom 60 of the bowl portion 18'. To provide protection against overheating of the crystal 30', a thermal switch 62, connected to the other wire 64 within power cable 14, is pushed up against the bottom of the crystal 30' by insulating 0-ring 66 and plate 56 in a manner common in the prior art.
Turning now to Figures 4 and 5 with particularity, ::.
~ lO the changes to prior art piezoelectric crystals to optimize .....
the particular objectives of the present invention will now be described in detail. As shown in Figure 4, a ~,s piezoelectric crystal 30 as employed in prior art ultraso-nic nebulizers, emulsifiers, cleaners, and the like, ,..:
comprises a circular disc 68 of piezoelectric crystal material. Over the disc 68 two metal electrical contacts . ,. ~.
70, 72 are bonded or plated~ The upper contact 70 covers the top and sides and extends just around the bottom edge - ~;
~; o~ the disc 68. The bottom contact 72 is a disc that :,.,. :i ;~ 20 covers the bottom of the disc 6~ except for a small circum-'-~ ferencial spacing from the edge of the contact 70 to pro-: .s.
vide electrical isolation therefrom. Electrical contact is provided totally on the bottom surface with the contacts 70, 72 via the cylindrical contact 52 and thermal switch -,.,::
64, respectively. The result is a vibrational pattern such ~` as that indicated by the vibrational waves 74 of Figure 4 which effects the total diam~ter of the piezoelectric ,~;
crystal 30. A similar pattern is obtained where the con-tacts comprise plating of the upper and lower surfaces of the crystal.
`'''' , .
i~ - 20 -`',"' "
", . .

1 3~9~8~

To better accomplish the objectives of the presen-t invention, it was hypothesized that if ~he energy of the crystal 30l could be concentrated in the center where maxi-mum coupling to the glass cover 42 takes place, efficiency would be greatly enhanced due to concentrating the available energy into a smaller area, this smaller area conforming to the mounting scheme used to fixture the piezoelectric crystal. To accomplish this without the use . of specially formed vibrating surfaces as taught in the prior art, the revised contact structure of Figure 5 was developed. While the upper contact 70 remains the same, the lower contact 72' is of greatly reduced diameter resul-ting in what is believed to be a vibrational wave pat-tern 74' which is wedge-shaped ancl centrally located, as ~` depicted.
Turning to Figure 6 a nebulizer bowl portion accor~ing to a second embodiment of the present invention is indicated therein as 18 " . For ease of comparison, like parts are descri~ed by like numbers. In the bowl protion `~ 20 18'', the bowl 26' itself has no hole in the bottom as in the previously described embodiment. Rather, the bottom of ~b~: ~ the bowl 26' comprises a thin planar cover 76 of the same i ~ material as the rest of the bowl portion 18''. This makes ~ ~ the inside surface of the bowl 26' smooth and without ~ j .
cracks or joints. Such a design is sanitary and easy to clean. This embodiment would lend itself particularly well to being formed of one of the modern ceramic materials or high temperature plastics developed from space technology.

As can be seen from the drawing, the piezoelectric crystal 68 is closely spaced and coupled direc-tly to the planar cover 76 by the coupling fluid 44. A peripheral ~eal 78, >

1 3 ~ 3 such as, an O-ring, adhesive, or the like is disposed bet~
ween the crystal 68 and the bottom 80 o~ the planar cover 76 to prevent the loss of the Eluid 44, as previously :,,.
described.
, Turning now to Figure 7 a nebulizer bowl portion ~;~ according to a third embodiment of the present invention is indicated therein as 18 " '. For ease of comparison, like parts are again designated by like numbers. In the bowl portion 18''', the bowl 26~ itself has hole 32' in the bot-tom as in the previously described embodiment of Figure 3O
' To provide an inside surface for receipt of the medication 7;~ which is smooth and without cracks or joints as in the ~ ;
second embodiment, a unitary bowl insert 82 is disposed 7~
~` within the bowl 26'' and over the hole 32'. The bowl inisert can be of pyrex qlacs, plastic, ceramic, or the ...:
~ like, as desired. This design too is sanitary and easy to clean. As can be seen from the drawing, the piezoelectric ~ crystal 68 is closely spaced and coupled directly to -the :,. ~
bottom 84 of the bowl insert 82 by the coupling fluid 44.
~:~ 20 A peripheral seal 78', such as, an 0-ring, adhesive or the like ls disposed between the crystal 68 and the bottom 84 of the bowl insart 82 to prevent the loss of the ~luid 44, %:
as previously described.
A novel and improved form of portable nebulizer unit is illustrated in Figures 11 to 14 and comprises a generally cylindrical housing 120 which is divided into an ~- upper chamber area 121 and a lower bowl section 122. The :~.
~'7~ lower bowl section 122 defines a cavity 123 with a hole 124 ~v in the bottom of the cavity directly above a piezoelectric crystal 126. The crystal 126 is covered by a one-half wave glass cover plate 127, the plate 127 being resiliently .:
,"~
. , .

urged against an upper surface of the crystal by an elasto-;~
- meric boot 128. As best seen from Figure 12, boot 128 has an outer wall 130 which encircles the perimeter of the ; crystal 126 and plate 127 with a reservoir 129 in com-~; munication with the external edge of interface 125 between the crystal 126 and cover plat~ 127. The boot 128 has upper and lower annular re-turn flanges 131 and 132, respec-~- tively, and is dimensioned such that it must be stretched ~; over the crystal 126 and cover plate 127 so that the 0 crystal 126 and cover plate 127 are pressed together in close-fitting sealed relation to the boot 128 without the necessity of an adhesive. An energy coupling oil film 133 ~ is placed in the interface or gap 130.
;~ A conventional thermoswitch 134 is supported beneath the crystal 126 by an annular plastic support member 136, and a cable retainer has a power pack connec-'J tion 139 for a power pack unit, not shown.
A leaf spring contact 142 is interposed between ~` the center or positive electrode 143 on the undersurface of the crystal 126 and i5 connected to a positive electrical strap 144 extending downwardly into the power pack connec-- tion 139. In turn, a negative electrical strap or lead 145 establishes conkact wi-th the outer, negative electrode 146 of the crystal and extends into the power pack connection 138.
' The upper chamber area 121 is made up of inner and outer concentric tubes 150 and 151, respectively, which are arranged coaxially and in direct communication with respect to the bowl unit 122. The inner concentric tube 150 ter-~`~30 minates at its upper end in an enlarged annular seat por-tion 152 which rPceives an on-off valve assembly 154, the ~ - 23 -.~
:

` t 32~3 valve assembly provided with flap valve members 156. The ~' valve member 156 is a thin resilient membrane of generally ,, circular configuration which is secured to a fixed hub 158 ~: and radiates outwardly across openings 160. The valve membrane is normally biased in a direction closing the valve but when a negative pressure is created in the . interior of the tube 150 it will cause the circular membrane to open and admit outside air in the direction of the arrows through the inner tube 150. The necessary nega-$ ~ lo tive pressure or suction is created by means of a mouth- . .
e piece 162 in communication with the concentric space 163 r~' between the tubes 150 and 151 so as to cause air to be : drawn downwardly through the tube and into the cavity of :~ the bowl unit 122 so as to pick up particles of liquid ~:
s;~ medication generated by the piezoelectric crystal and which particles tend to erupt into a somewhat conical-shaped tur-" bulent fountain within the cavity area formed by the bowl.
.-; unit. The particles are entrained in the airstream and ;.;~. ~ flow upwardly along a spixal flow path created by a ~. 20 spirally extending baffle 164 on the external surface of .:
s~: the inner tube 150 benaath the mouthpiece 162.
x:, In devising a functional nebulizer it i5 important to match emitted particle size distribution to that ~r~
.: required physiologically for efEective respiratory treat-.~. ment. Ultrasonic nebuli~ers typically produce a range of particle sizes, a significant number oE which are larger .$~ than those clinically useful. These larger particles do not reach the respiratory tissues targeted for treatment ~ and therefore represented wasted medication. In order to ri prevent particles larger than approxi mately 4 microns from . exiting through the mouthpiece and qoing to the patient, ,.
.,; - 24 -;

~' 1 3~9~
the upper section of the nebulizer is provided wi-th a par~
~ ticle impaction baffle lÇ4 as described. The baffle will `~ accelerate the par-ticles through a circular path on their way to the mouthpiece and only the larger particles will tend to accumulate on the sidewalls and flow back down-,'',: `
`~ wardly into the bowl unit to be renebulized. As a xesult, essentially all liquid medication is converted to a mist of clinically useful particles to insure accurate and economi-cal treatment.
1~ Having considered the overall construction and arrangement of the form of nebulizer illustrated in Figures ,:
~ 11 to 14, a better understanding can be gained of the .~
detailed construction and arrangement of the piezoelectric crystal 126 and its cover plate 127. As hereinbefore described, the crystal 126 has a center electrode 143 and .~ .
'r ~ outer spaced concentric negative electrode 146 surrounding ?`, i;~ the top surface and outer perim~ter of the crystal.
. .
~ Preferably, the electrodes are composed of a thin coating ~i .~,, or plating, such as, nickel, silver or goldO It is impor-~`; 20 tant that the coating selected be such that it will not : tend to separate from the surface oE the crystal and cause acoustic energy coupling losses and resultant reduction in nebulization. The cover plate 127 is preferably a glass material having a thickness oE one-half wave length or a multiple thereof, such as, W, 3W/2, 2W and which is . .
assembled to the transducer or crystal surface. Again, glass is a convenient material for such a cover since it presents an easily cleaned and durable surface to the , liquid and can tolerate high temperatures. The liquid `~ 30 coupling film 133 serves to bridge the gap formed between the two confron~ing surfaces of the crystal 126 and cover :~, , .
;

pla-te 127. Preferably, the gap formed is a generally cir-cular recess defined by a planar sur~ace 126' and concave surface 127' on the crystal 126 and cover plate 127, respectively. This imposes a certain optimum oil film i ~ thickness while permitting the oil to migrate toward the ,,:
high energy density center of the crys-tal glass interface;
and further, the gap or recess avoids problems associated with a uniform recess depth which were found to lead to destruction of the transducer surface caused by concentric standing wave activity in the oil layer. The success of the oil coupling system suggests that the oil be contained SO that gravity and capillary forces do not carry the coupling oil out of the gap during periods of inactivity.
Nevertheless, the reservoir 129 formed in the boot permits storage of excess oil at the periphery of the gap and allows migration of the oil 133 toward the center when ,r; ~
~ energy is applied to the transducer. Thus, the oil reser-,,?;'. voir 129 permits the oil to expand with increased tem-t5,5,"; perature rather than to separate the transducer from the ~ glass. In addition, the boot 128 reduces damping by ,,i. , ;~ acoustically isolating the vibrating transducer and glass :s ~; assembly from the outer housing~
.,":
Another problem associated with piezoelectric crystals in ultrasonic nebulizers is the tendency on the part of the porous surface of the crystal to hold entrapped gas which, when released during operation, can displace the ;;~ oil and uncouple the glass cover so as to retard nebuliza-: tion. In order to control this degenerative effect, it was . found that a preassembly oil saturation treatment is extre-mely helpful and is carried out by placing the crys~als in an oil bath surrounded by very low pressure environment.

..
,:

` 1 3290~3 ..
The peripheral oil reservoir 129 in the boot 128 serves to ~- accumulate any remaining entrapped gas which may escape from the transducer or crystal surface during its life.
.~ Accordingly, selective design of the piezoelectric assembly - in the manner described has been found to result in superior energy transmission efficiency, increased power level tolerance while expanding the thermal range of the . crystal to a point that a coolant is not required.
It is therefore to be understood that various ... ~.
-: 10 modifications and changes may be made in the construction ~, and a.rrangement of elements comprising preferred and alter-nate forms of apparatus as well as the particular sequence .. ..
~: of steps described in connection with the method without , ,,1, departing from the spirit and scope thereof as d~fined by . the appended claims.
.. .
,...
~,...
~, i.! :
~''``

,~

~' r.
r~.

~r~

.

, ,.
: ,,',.
. S"
il ,f i - 27 -:~

,, ,:

Claims (26)

1. In an ultrasonic nebulizer bowl unit having a bowl-shaped cavity therein communicating through a hole in the bottom thereof with a piezoelectric crystal disposed under the hole, and energizing means are provided to energize said crystal for nebulizing liquid medication disposed in the bowl and on the surface of the crystal, the improvement for preventing self-destruction of the surface of the crystal comprising:
(a) protective sheet material means having characteristics for allowing it to match acoustical impe-dances without significant attenuation of acoustic energy, said sheet material means disposed over one surface of said crystal;
(b) fluid means for energy coupling disposed in a gap between said protective sheet material means and one surface of the crystal; and (c) sealing means disposed about the periphery of said crystal and said sheet material means for preventing loss of said fluid means from the gap between said sheet material means and one surface of the crystal.

2. The improvement to an ultrasonic nebulizer bowl unit according to claim 1 wherein:
(a) said protective sheet material means is peripherally smaller than the surface of said piezoelectric crystal; and (b) said sealing means comprises a bead of epoxy adhesive disposed about the periphery of said sheet material means in contact with one surface of said crystal.

3. The improvement to an ultrasonic nebulizer bowl unit according to claim 2 and additionally comprising:
second sealing means disposed between said sheet material means and the bottom of said nebulizer bowl and between the hole and said epoxy adhesive for blocking medication in said bowl from contacting said epoxy adhe-sive.

4. The improvement to an ultrasonic nebulizer bowl unit according to claim 1 and additionally comprising:
said protective sheet material means defined by a glass cover plate; and means for causing said piezoelectrical crystal to concentrate its maximum vibrational forces adja-cent to the center thereof.

5. The improvement to an ultrasonic nebulizer bowl unit according to claim 4 wherein said piezoelectric crystal has a top electrode for making electrical contact therewith on the top surface and extending outward to the sides thereof and a bottom disk electrode for making electrical contact therewith on the bottom surface thereof and wherein said means for causing said piezoelectric crystal to concentrate its maximum vibrational forces adja-cent the center thereof comprises:
the bottom electrode being of substantially smaller diameter than the diameter of the piezoelectric crystal whereby the electrical contact to the piezoelectric crystal on the bottom thereof is adjacent the center thereof.

6. The improvement to an ultrasonic nebulizer bowl unit according to claim 1 wherein:
said fluid means is synthetic oil based.

7. The improvement to an ultrasonic nebulizer bowl unit according to claim 1 wherein:
said fluid means has a viscosity of between 4 and 10 centistoke at 100°C.

8. The improvement to an ultrasonic nebulizer bowl according to claim 1 wherein:
said fluid means is a Polyalphaolefin base oil having a viscosity of 8 centistok at 100°C.

9. In an ultrasonic nebulizer bowl unit having a bowl-shaped cavity therein for holding a liquid medication to be nebulized by a piezoelectric crystal disposed under the cavity, the improvement for protecting the surface of the crystal from destruction comprising:
(a) said piezoelectric crystal being disposed under the bottom of the cavity and cover means for separating said crystal from medication contained in the cavity, said cover means having characteristics for allowing it to match acoustical impedances without signifi-cant attenuation, such as, those possessed by glass, said crystal being disposed in adjacent relation to said cover means;
(b) fluid means for energy coupling disposed between said cover means and one surface of the crystal;
and (c) sealing means disposed between said cover means and the surface of the crystal about the periphery of said fluid for preventing said fluid from flowing out from between said cover means and the one sur-face of said crystal.

10. The improvement to an ultrasonic nebulizer bowl unit according to claim 9 wherein:
(a) the cavity has a hole in the bottom thereof communicating with a surface of said cover means;
and (b) said cover means comprises a planar sheet of glass like material, and sealing means for adhesi-vely attaching said cover means peripherally to the one surface of said crystal.

11. The improvement to an ultrasonic nebulizer bowl unit according to claim 9, wherein:
(a) the cavity has a hole in the bottom thereof communicating with a surface of said cover means;
and (b) said cover means comprises the bottom of a unitary bowl insert of a glass-like material inserted into said cavity.

12. The improvement to an ultrasonic nebulizer bowl unit according to claim 9, wherein:
the bowl unit is of unitary construction and said cover means comprises the bottom of the cavity which is formed therein.

13. The improvement to an ultrasonic nebulizer bowl unit according to claim 9 wherein:
said fluid means is synthetic oil based.

14. The improvement to an ultrasonic nebulizer bowl unit according to claim 9, wherein:
said fluid means has a viscosity of between 4 and 10 centistoke at 100°C.

15. The improvement to an ultrasonic nebulizer bowl unit according to claim 9, wherein:
said fluid means is a Polyalphaolefin base oil having a viscosity of 8 centistokes at 100°C.

16. The improvement to an ultrasonic nebulizer bowl according to claim 9 wherein the piezoelectric crystal has a top electrode for making electrical contact therewith on the top surface and extending to the sides thereof and a bottom disk electrode for making electrical contact therewith on the bottom surface thereof, and means for causing the piezoelectrical crystal to concentrate its maximum vibrational forces adjacent the center thereof.

17. In apparatus for the ultrasonic nebulization of fluids for respiratory inhalation in which a piezoelectric transducer is disposed in communication with a reservoir for nebulizing liquid medication in the reservoir, the improvement comprising:
a protective cover member of a predetermined wavelength superimposed on said piezoelectric transducer, said cover member and transducer having generally planar confronting surfaces and a common interface therebetween;
annular sealing means in outer surrounding relation to said cover member for preventing the loss of fluid from said interface; and oil coupling means in said interface for coupling the ultrasonic energy from said transducer to said cover member.

18. In apparatus according to claim 17, said cover member being in the form of a glass cover plate.

19. In apparatus according to claim 18, said interface between said confronting surfaces of said transducer and said cover plate including a gap tapering radially and out-wardly from a central portion of said interface.

20. In apparatus according to claim 19, said confronting surfaces being of mutually opposed generally concave configuration.

21. In apparatus according to claim 17, said annular sealing means being defined by an elastomeric boot resi-liently urging said cover member and said transducer together, said boot provided with an oil reservoir in surrounding relation to said interface.

22. In apparatus according to claim 21, said elasto-meric boot having an outer wall surrounding the outer periphery of said cover member and said transducer which acts as a pressure limiting diaphragm so that the pressure created by thermal expansion of said oil coupling medium does not alter the gap between said cover member and said transducer.

23. In apparatus according to claim 17, including an air inlet portion in communication with said fluid reser-voir, means for inducing the flow of air into said fluid reservoir via said air inlet whereby to pick up nebulized liquid in particle form from said reservoir and direct mixed air and particles to an outlet, and baffle means in the path of flow of the mixed air and particles from said fluid reservoir toward said outlet to prevent larger sized particles from passing through said outlet.

24. In apparatus according to claim 23, wherein said baffle means is in the form of a generally spirally extending plate in the path of flow of mixed air and par-ticles from said fluid reservoir toward said outlet.

25. In apparatus according to claim 23, said air inlet defined by a generally tubular portion coaxially aligned with said liquid reservoir, and a spirally extending baffle member on an external surface of said tubular portion.

26. In apparatus according to claim 25, said outlet including an outer tubular portion in outer spaced con-centric relation to said air inlet and a mouthpiece in com-munication with the concentric space between said air inlet and said outer cylindrical portion, said baffle means causing said larger particles to drain by gravity back into said reservoir for renebulization.