U.S. patent application number 10/461459 was filed with the patent office on 2004-02-12 for generation of nanoparticles and microparticles of controllable size using acoustic and ultrasonic vibration.
This patent application is currently assigned to Auburn University. Invention is credited to Chattopadhyay, Pratibhash, Gupta, Ram B..
Application Number | 20040026804 10/461459 |
Document ID | / |
Family ID | 25327980 |
Filed Date | 2004-02-12 |
United States Patent
Application |
20040026804 |
Kind Code |
A1 |
Gupta, Ram B. ; et
al. |
February 12, 2004 |
Generation of nanoparticles and microparticles of controllable size
using acoustic and ultrasonic vibration
Abstract
The current invention, Supercritical Antisolvent Precipitation
with Enhanced Mass Transfer (SAS-EM) provides a significantly
improved method for the production of nano and micro-particles with
a narrow size distribution. The processes of the invention utilize
the properties of supercritical fluids and also the principles of
virbrational atomization to provide an efficient technique for the
effective nanonization or micronization of particles. Like the SAS
technique, SAS-EM, also uses a supercritical fluid as the
antisolvent, but in the present invention the dispersion jet is
deflected by a vibrating surface that atomizes the jet into fine
droplets. The vibrating surface also generates a vibrational flow
field within the supercritical phase that enhances mass transfer
through increased mixing. Sizes of the particles obtained by this
technique are easily controlled by changing the vibration intensity
of the deflecting surface, which in turn is controlled by adjusting
the power input to the vibration source. A major advantage of the
SAS-EM technique is that it can be successfully used to obtain
nanoparticles of materials that usually yield fibers or large
crystals in SAS method. Microencapsulation via coprecipitation of
two or more materials can also be achieved using the SAS-EM
technique.
Inventors: |
Gupta, Ram B.; (Auburn,
AL) ; Chattopadhyay, Pratibhash; (North Royalton,
OH) |
Correspondence
Address: |
Michael Sofocleous
Roberts, Mlotkowski & Hobbes, PC
Suite 850
8270 Greensboro Drive
McLean
VA
22102
US
|
Assignee: |
Auburn University
Auburn
AL
|
Family ID: |
25327980 |
Appl. No.: |
10/461459 |
Filed: |
June 16, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10461459 |
Jun 16, 2003 |
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09858301 |
May 16, 2001 |
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6620351 |
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60206644 |
May 24, 2000 |
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Current U.S.
Class: |
264/7 ; 264/407;
264/9; 425/174.4; 425/6 |
Current CPC
Class: |
Y10S 977/773 20130101;
A61K 9/14 20130101; B01J 2/18 20130101; A61K 9/1688 20130101; B01J
2/04 20130101; B01J 13/04 20130101; B82Y 30/00 20130101; Y10S
977/786 20130101; Y10S 977/84 20130101 |
Class at
Publication: |
264/7 ; 264/9;
264/407; 425/6; 425/174.4 |
International
Class: |
B29B 009/00 |
Goverment Interests
[0002] The U.S. Government has a paid-up license in this invention
and the right in limited circumstances to require the patent owner
to license others on reasonable terms as provided for by the terms
of Grant No. CTS-9801067 awarded by the National Science Foundation
Grant and Grant No. 1-R55-RR13398-01 awarded by the National
Institute of Health.
Claims
What is claimed is:
1. A method for generating a solid particle containing a compound
of interest, the method comprising: providing a reservoir
containing a solution of the compound in a solvent; providing an
antisolvent in a contained space in fluid communication with the
solution in the reservoir, wherein the antisolvent is selected such
that the compound is less soluble in the antisolvent than in the
solvent; and applying focused acoustic energy to the reservoir so
as to produce a droplet of the solution in the antisolvent in the
contained space, whereby admixture of the solution droplet and the
antisolvent results in the precipitation of the compound, forming a
solid particle.
2. The method of claim 1, wherein focused acoustic energy is
applied to the reservoir at a plurality of loci so as to eject a
plurality of droplets, whereby a plurality of solid particles is
provided.
3. The method of claim 1, wherein the solvent is aqueous.
4. The method of claim 1, wherein the solvent is non-aqueous.
5. The method of claim 4, wherein the solvent is organic.
6. The method of claim 1, wherein the antisolvent is a
supercritical fluid.
7. The method of claim 4, wherein the antisolvent is a
supercritical fluid.
8. The method of claim 5, wherein the antisolvent is a
supercritical fluid.
9. The method of claim 1, wherein the antisolvent is gaseous.
10. The method of claim 1, wherein the precipitation of the
compound of interest comprises crystallization thereof.
11. The method of claim 1, wherein the solution and the antisolvent
are both present in the reservoir.
12. The method of any one of claims 6, 7, 8, 9, 10 and 11, wherein
the antisolvent is selected from the group consisting of carbon
dioxide, water, ammonia, nitrogen, nitrous oxide, methane, ethane,
ethylene, propane, butane, n-pentane, benzene, methanol, ethanol,
isopropanol, isobutanol, monofluoromethane, trifluoromethane,
chlorotrifluoromethane, monofluoromethane, hexafluoroethane,
1,1-difluoroethylene, 1,2-difluoroethylene, toluene, pyridine,
cyclohexane, m-cresol, decalin, cyclohexanol, 0-xylene, tetralin,
aniline, acetylene, chlorotrifluorosilane, xenon, sulfur
hexafluoride, propane and combinations thereof.
13. The method of claim 12, wherein the antisolvent is carbon
dioxide.
14. The method of claim 1, wherein the solution droplet is ejected
onto a surface of a substrate having the antisolvent thereon.
15. The method of claim 1, wherein the solution is a saturated
solution.
16. The method of claim 1, wherein the compound of interest is
hydrophilic, the solvent is hydrophilic, and the antisolvent is
lipophilic.
17. The method of claim 1, wherein the compound of interest is
hydrophilic, the solvent is aqueous, and the antisolvent is
lipidic.
18. The method of claim 1, wherein the compound of interest is
lipophilic, the solvent is lipophilic, and the antisolvent is
hydrophilic.
19. The method of claim 1, wherein the size of the particle is in
the range of approximately 0.1 nm to about 5 .mu.m.
20. The method of claim 19, wherein the size of the particle is in
the range of approximately 5 nm to about 2.5 .mu.m.
21. The method of claim 1, wherein the compound of interest is
metallic.
22. The method of claim 1, wherein the solution comprises two or
more compounds of interest.
23. The method as claim 1, wherein the solution further comprises a
biodegradable polymer.
24. The method of claim 1, wherein the compound of interest has a
lipophilic group, the solvent is lipophilic, and the antisolvent is
lipophilic.
25. The method of claim 1, wherein the compound of interest has a
hydrophilic group, the solvent is hydrophilic, and the antisolvent
is hydrophilic.
26. The method of claim 1, wherein the compound of interest has a
hydrophilic group, the solvent is hydrophilic and CO.sub.2 philic,
and the antisolvent is CO.sub.2.
27. A method for generating a plurality of particles containing a
compound of interest, the method comprising: providing a plurality
of reservoirs each containing a solution of the compound in a
solvent; providing an antisolvent in a contained space in fluid
communication with the solution in each reservoir, wherein the
antisolvent is selected such that the compound is less soluble in
the antisolvent than in the solvent; and applying focused acoustic
energy to each reservoir so as to produce droplets of the solution
into the antisolvent in the contained space, whereby admixture of
the solution droplets and the antisolvent results in the
precipitation of the compound, forming solid particles.
28. The method of claim 27, wherein the focused acoustic energy is
applied to each reservoir simultaneously using a plurality of
acoustic ejection devices.
29. The method of claim 27, wherein the focused acoustic energy is
applied to each reservoir in succession using a single acoustic
ejection device.
30. A device for making solid particles of a compound of interest,
comprising: a reservoir containing a solution of the compound in a
solvent; an antisolvent in a contained space in fluid communication
with the solution in the reservoir such that droplets ejected from
the solution are directed into the antisolvent, wherein the
antisolvent is selected such that the compound is less soluble in
the antisolvent than in the solvent; an acoustic ejector comprising
an acoustic radiation generator for generating acoustic radiation
and a focusing means for focusing the acoustic radiation at a focal
point within the solution in the reservoir so as to eject a droplet
therefrom; and a means for positioning the ejector in acoustic
coupling relationship to the reservoir.
31. The device of claim 30, comprising a single acoustic
ejector.
32. The device of claim 30, comprising a plurality of acoustic
ejectors positioned to direct focused acoustic energy to a
plurality of loci within the solution so as to eject a plurality of
droplets, whereby a plurality of solid particles is provided.
33. The device of claim 30, further comprising a means for
maintaining the solvent in the reservoir at a constant
temperature.
34. The device of claim 30, wherein the acoustic coupling
relationship between the ejector and the solution in the reservoir
is established by providing an acoustically conductive medium
between the ejector and the reservoir.
35. The device of claim 30, wherein acoustic coupling between the
ejector and the fluid in each reservoir is established at a
predetermined distance between the ejector and each reservoir.
36. The device of claim 30, wherein the solvent is aqueous.
37. The device of claim 30, wherein the solvent is non-aqueous.
38. The device of claim 37, wherein the solvent is organic.
39. The device of claim 30, wherein the antisolvent is a
supercritical fluid.
40. The device of claim 37, wherein the anti solvent is a
supercritical fluid.
41. The device of claim 38, wherein the anti solvent is a
supercritical fluid.
42. The device of claim 30, wherein the antisolvent is gaseous.
43. The device of claim 30, wherein the solution and the
antisolvent are both present in the reservoir.
44. The device of any one of claims 30 through 43, wherein the
antisolvent is selected from the group consisting of carbon
dioxide, water, ammonia, nitrogen, nitrous oxide, methane, ethane,
ethylene, propane, butane, n-pentane, benzene, methanol, ethanol,
isopropanol, isobutanol, monofluoromethane, trifluoromethane,
chlorotrifluoromethane, monofluoromethane, hexafluoroethane,
1,1-difluoroethylene, 1,2-difluoroethylene, toluene, pyridine,
cyclohexane, m-cresol, decalin, cyclohexanol, 0-xylene, tetralin,
aniline, acetylene, chlorotrifluorosilane, xenon, sulfur
hexafluoride, propane and combinations thereof.
45. The device of claim 41, wherein the antisolvent is carbon
dioxide.
46. The device of claim 30, wherein the solution is a saturated
solution.
47. The device of claim 30, wherein the compound of interest is
hydrophilic, the solvent is hydrophilic, and the antisolvent is
lipophilic.
48. The device of claim 30, wherein the compound of interest is
hydrophilic, the solvent is aqueous, and the antisolvent is
lipidic.
49. The device of claim 30, wherein the compound of interest is
lipophilic, the solvent is lipophilic, and the antisolvent is
hydrophilic.
50. The device of claim 30, wherein the compound of interest is
metallic.
51. The device of claim 30, wherein the solution comprises two or
more compounds of interest.
52. The device as claim 30, wherein the solution further comprises
a biodegradable polymer.
53. A device for making solid particles of a compound of interest,
comprising: a plurality of reservoirs each containing a solution of
the compound in a solvent; an antisolvent in a contained space
above and in fluid communication with the solution in each
reservoir such that droplets ejected from the solution are directed
into the antisolvent, wherein the antisolvent is selected such that
the compound is less soluble therein than in the solvent; an
acoustic ejector comprising an acoustic radiation generator for
generating acoustic radiation and a focusing means for focusing the
acoustic radiation at a focal point within the solution in the
reservoir so as to eject a droplet therefrom; and a means for
positioning the ejector in acoustic coupling relationship to the
reservoir.
54. The device of claim 53, comprising a single acoustic
ejector.
55. The device of claim 53, comprising a plurality of acoustic
ejectors positioned to direct focused acoustic energy to each
reservoir so as to eject a plurality of droplets therefrom.
56. The device of claim 53, wherein each of the reservoirs is
removable from the device.
57. The device of claim 53, wherein the reservoirs are individual
wells in a well plate.
58. The device of claim 53, wherein the reservoirs are
substantially acoustically indistinguishable.
59. The device of claim 53, comprising at least about 10,000
reservoirs.
60. The device of claim 59, comprising at least about 100,000
reservoirs.
61. The device of claim 60, comprising in the range of about
100,000 to about 4,000,000 reservoirs.
62. The device of claim 53, wherein each reservoir is adapted to
contain no more than about 100 nanoliters of fluid.
63. The device of claim 53, wherein each reservoir is adapted to
contain no more than about 10 nanoliters of fluid.
64. The device of claim 53, further comprising means for
maintaining the solvent in each reservoir at a constant
temperature.
65. The device of claim 53, wherein the acoustic coupling
relationship between the ejector and the fluid in each reservoir is
established by providing an acoustically conductive medium between
the ejector and each reservoir.
66. The device of claim 53, wherein acoustic coupling between the
ejector and the fluid in each reservoir is established at a
predetermined distance between the ejector and each reservoir.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] The present application is a continuation of application
Ser. No. 09/858,301, filed May 16, 2001, the teachings of which are
incorporated herein by reference. The present application claims
benefit from the Provisional patent application Serial No.
60/206,644, filed May 24, 2000 and entitled METHOD OF FORMING
NANOPARTCLES AND MICROPARTICLES OF CONTROLLABLE SIZE USING
SUPERCRITICAL FLUIDS AND ULTRASOUND, the teachings of which are
incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0003] 1. Field of Invention
[0004] The current invention relates to a method for the production
of micron or nanometer size particles by precipitation, wherein a
dispersion containing the substance of interest is contacted with a
supercritical fluid antisolvent under near or supercritical
conditions in order to maximize micro or nanoparticle formation.
The invention also provides techniques to control the particle
size, particle size distribution and particle morphology. The
invention also includes supercritical fluid coating or composite
material particle formation, wherein encapsulation of one substance
by another substance or coprecipitation of more than one substance
in the form of micro or nanoparticles are achieved in the
supercritical fluid antisolvent.
[0005] 2. Background and Prior Art
[0006] Nanoparticles are of considerable importance in numerous
technological applications. Nanoparticles of materials in fact
exhibit properties significantly different from those of the same
material with larger sizes. Some nanostructured materials with
novel properties include: fullerenes, zeolites, organic crystals,
non-linear optical material, high temperature superconductors,
molecular magnetic materials, starburst dendrimers, piezoelectric
materials, shape changing alloys and pharmaceuticals. The novel
properties of these nanostructured materials can be exploited and
numerous potential applications can be developed by using them in
different industries. One such industry where the need for
nanoparticles is particularly pronounced is the pharmaceutical
industry where nanoparticles of different pharmaceutical materials
are used for designing `drug delivery systems` for controlled
release and targeting.
[0007] Several techniques have been used in the past for the
manufacture of nanoparticles but these techniques suffer from some
inherent limitations. Some of the conventional techniques include:
Spray drying, which is one of the well-known techniques for
particle formation and can be used to produce particles of 5 .mu.m
or less in size. The major disadvantage of this technique is that
it requires high temperature in order to evaporate the solvent in
use, and this makes it unsuitable for treating biological and
pharmaceutical substances. Furthermore, the final product yield may
be low in case of small-scale applications. Milling can be used to
produce particles in the 10-50 .mu.m range, but the particles
produced by this method have a broad size distribution. Fluid
energy grinding can produce particles in the 1-10 .mu.m range but
this process involves the use of high-velocity compressed air,
which leads to electrostatically charged powders. In addition,
particle size reduction by this process tends to be more efficient
for hard and brittle materials such as salt and minerals, but much
less so for soft powders, such as pharmaceuticals and other
biological substances. Lyophilization produces particles in the
desired range, but with a broad distribution. A main disadvantage
of this process is that it employs the use of organic solvents that
may be unsuitable for pharmaceutical substances. In addition,
control of particle size can also be difficult, and a secondary
drying step is required to remove residual solvents. In the case of
precipitation of protein particles, not all proteins can be
lyophilized to stable products, and the process must be tailored to
each protein.
[0008] Thus, none of these methods are entirely satisfactory, and
it is therefore important to explore alternative methods that will
produce particles from 5 .mu.m down to as low as 10 nm.
[0009] Particle Technology Based on Supercritical Fluids
[0010] One of the first uses of supercritical fluids in particle
formation was proposed by Krukonis et al. in 1984 for processing a
wide variety of difficult-to-handle solids. Since then, several
experimental studies have been conducted to develop methods for
particle formation using this technology. The two primary methods
utilizing supercritical fluid technology for particle processing
include Supercritical Antisolvent (SAS) Precipitation technique and
the Rapid Expansion of Supercritical Solutions (RESS) technique.
For many years now, these techniques have been successfully used to
produce microparticles of various compounds including difficult to
handle explosives (Gallagher et al., 1989), lysozyme, trypsin
(Winter et al, 1993), insulin (Yeo et al., 1993; Winter et al,
1993), prednisolone acetate (U.S. Pat. No. 5,803,966), polystyrene
(Dixon et al., 1993), HYAFF-11 polymers (Benedetti et al., 1997),
different steroids (Larson and King, 1985), and numerous other
organic substances. Other areas of application of supercritical
fluids include formation of solvent free, drug loaded polymer
micro-spheres for controlled drug release of therapeutic agents
(Tom et al., 1992; Mueller and Fischer, 1989), production of
ultra-fine and chemically pure ceramic precursors (Matson et al.,
1985 a,b, 1987 a,b; Peterson et al. 1985), formation of intimate
mixtures of ceramic precursors (Matson et al., 1987a), and for
formulation of crystalline powders of labile pharmaceutical drugs.
Dixon and coworkers (1993) used the supercritical CO.sub.2
antisolvent process to make polystyrene particles ranging from 0.1
to 20 .mu.m by spraying polymer/toluene solutions into CO.sub.2 of
varying densities. A major advantage of supercritical fluid
precipitation process is that they can generate particles having a
narrow size distribution unlike other conventional processes that
provide a wide size distribution. Further the particles formed by
supercritical fluid precipitation process are free of organic
solvents and the formation of powdered blends, thin films and
micro-encapsulation of materials is straightforward.
[0011] The Working of the RESS Technique
[0012] In the RESS process, the solid of interest is first
solubilized in supercritical CO.sub.2 and then sprayed through a
nozzle into a low-pressure gaseous medium. Rapid expansion of the
solution on being passed through the nozzle causes a reduction in
CO.sub.2 density and also a reduction in the solvent power of
supercritical CO.sub.2 and this subsequently leads to the
recrystallization of the solid in the form of fine particles.
[0013] RESS provides a useful tool for controlling the size and
morphology of the precipitated powders. The influence of operating
conditions on the process has been studied by several
investigators, sometimes with different and conflicting results
(Larson and King, 1985; Mohamed et al., 1989; Peterson et al.,
1985). When RESS is carried out in the usual mode, solvent free
particles are obtained which makes the technique advantageous for
processing pharmaceutical substances. No surfactants or nucleating
media are required to trigger the nucleation and the solvent is
removed by a simple mechanical separation.
[0014] One of the main constraints in the development of the RESS
process however is supercritical fluid solvent capacity. For
example, carbon dioxide, which is the preferred solvent in many
applications, has a low solubility towards polar substances.
Different supercritical fluids can be chosen in case of such a
problem: a second solvent (cosolvent) can be added to enhance the
CO.sub.2 solvent capacity, but these solvents remains within the
precipitated product as impurities. In general, polymers possess
low solubility in supercritical fluids, including CO.sub.2 (with or
without cosolvents), and for such materials other processing
methods are more suitable.
[0015] The Working of the SAS Process
[0016] In the SAS process, the supercritical fluid is used as the
antisolvent. First the solid of interest is dissolved in a suitable
organic solvent. Then this solution is introduced into the
supercritical fluid using a nozzle. The supercritical fluid
dissolves the solvent, precipitating the solid out as fine
particles.
[0017] The volumetric expansion of the liquid when in contact with
the SCF plays a key role in the process. For example, experiments
conducted by Yeo et al. (1993a,b) for dimethylsulfoxide
(DMSO)-CO.sub.2 system at two temperatures, shows that CO.sub.2
produces a remarkably high volumetric expansion of DMSO (as high as
1000%) near the mixture's critical point. The increase of
antisolvent amount in the mixed solvent and the evaporation of the
organic liquid into the SCF eventually cause the precipitation of
the solute as fine particles.
[0018] Several methods of applying the SAS technique have also been
proposed. In the semibatch mode, the SCF is introduced continuously
at the operating pressure into a stationary bulk liquid phase
(Gallagher et al., 1989; Krukonis, 1988). If the liquid solution
and the SCF are fed continuously to the precipitation tank, a SAS
continuous process takes place (Yeo et al., 1993a,b). When the
solvent used has a high volatility, it is possible to continuously
feed the solution and the supercritical fluid into the
precipitation vessel and, at the same time to discharge the dry
precipitated particles (Randolph et al., 1993). Finally, a full
batch mode is performed where the solution is loaded with the
supercritical solvent from the initial condition at P=1 atm. to the
high pressure (Yeo et al., 1993a,b).
[0019] Note that, in all cases, a cleaning step is necessary after
the precipitation step in order to completely remove the liquid
solvent from the particles. One of the interesting features of SAS
is that the particles may be dried with CO.sub.2, and the CO.sub.2
may be depressurized at supercritical fluid conditions.
Supercritical fluid drying removes the solvent thoroughly, which is
often a major challenge. When liquids are evaporated from a matrix,
the surface tension of the shrinking droplets often causes the
matrix to collapse due to capillary forces. For a supercritical
fluid, there is no surface tension, and the surface forces due to
adsorption are minimal, so that the structure is preserved. Indeed
the world's lightest solids have been formed with critical point
drying (Rangarajan and Lira, 1991).
[0020] Current Limitations of the SAS Process
[0021] The SAS technique can be used to produce particles having a
narrow size distribution in the 1-10 .mu.m size range.
Unfortunately these techniques cannot produce much smaller
particles in the nanometer range. Nanometer size particles are
extremely important for many pharmaceutical applications. New
applications of nanoparticles of other substances can also emerge
if the nanoparticles are manufactured successfully. In any SAS
technology, mass transfer rate of the antisolvent into the droplet
is the key factor in obtaining a high super-saturation rate and a
smaller particle size, and hence mass transfer is the limiting
factor in the SAS process. Techniques that can enhance mass
transfer and provide faster diffusion of CO.sub.2 into the droplets
are thus needed for the formation of smaller particles having a
narrower size distribution. Operating temperature, pressure,
concentration of the injecting solution, and flow rate of the
solution have so far been investigated as size control parameters
but none of these parameters were found to have a significant
effect on the particle size over a wide range.
[0022] In the past few years several modifications (mostly in the
manner of jet break up) in the SAS process have been proposed in
order to overcome some of its limitations. For example in PCT
publication WO 95/01221 the use of a coaxial nozzle for
co-introduction of supercritical fluid and the solution has been
proposed. Such nozzles cause effective breakup or atomization of
the solution jet into tiny droplets. But, again a rigorous size
control process variable is lacking. The use of high frequency
sound waves for atomization has been known for many years for the
atomization of liquid surfaces into tiny droplets. High frequency
sound waves can be generated using various types of transducers
namely piezoelectric, magnetorestrictive, electromagnetic, and
pneumatic devices.
[0023] A specialized ultrasonic nozzle (Sonotek, 120 khz) was
employed by Randolph et al.(1993) in the precipitation of poly
(L-lactic acid) particles using the SAS technique. But they were
unsuccessful in reducing the particle size as a result of the use
of ultrasound. U.S. Pat. Nos. 5,833,891 and 5,874,029 disclose the
use of ultrasound in small particle production. They disclose the
use of a commercial ultrasonic nozzle (Sonomist, Model 600-1) for
the droplet atomization. The sonic waves in this case are created
when an energizing gas passes through a resonator cavity at the
velocity of sound. The frequency of the sonic waves created is not
constant and it is difficult to specify the frequency of the sound
waves generated. Trying to vary the sonic energy might interfere
with other process conditions and as a result it may not be used as
a size control variable.
SUMMARY OF THE INVENTION
[0024] The Supercritical Antisolvent Precipitation with Enhanced
Mass Transfer
[0025] (SAS-EM) Process
[0026] The present invention provides a novel way to produce very
small particles in the nanometer range, having a narrow size
distribution. It also provides techniques to control the particle
size. The processes and methods involved in the invention can be
used for producing nanoparticles of a wide variety of materials
such as polymers, chemicals, pesticides, explosives, coatings,
catalysts and pharmaceuticals. Like the SAS technique, the current
invention also uses a supercritical fluid as the antisolvent, but
in this invention the dispersion jet is deflected by a vibrating
surface that atomizes the jet into micro-droplets. The dispersion
jet once introduced into supercritical fluid and onto the vibrating
surface spreads evenly over the surface forming a thin liquid film.
A set of wavelets then form on the free liquid layer due to the
vibrating surface. The oscillatory vibrations of the liquid surface
causes these wavelets to increase in amplitude until the wavelet
tips break off and the droplets are emitted from the surface into
the supercritical fluid media. Rapid transfer of CO.sub.2 into
these droplets and the solvent out of these droplets causes them to
expand rapidly, leading to a decrease in the droplet's ability to
keep the solute molecules dissolved causing the molecules to
precipitate as fine particles. The vibration field generated by the
vibrating surface inside the supercritical phase helps in enhancing
mass transfer between the solvent and the supercritical fluid due
to increased turbulence and mixing. The reduced mean droplet
diameter coupled with enhanced turbulence within the supercritical
phase cause rapid precipitation of the particles and thus act as
major factors that are responsible for the formation of
nanoparticles.
[0027] The present invention uses high frequency vibrations for
atomization. The atomization process is brought about by
introducing the dispersion on to a surface vibrating at a high
frequency. No specialized nozzles are necessary in this invention.
Any tube made of a standard material (for example: Stainless Steel,
Fused Silica) can be used to spray the dispersion onto the horn
surface. The diameter of the tube can be varied based on the
desired size and desired yield of the micronized particles.
[0028] A schematic representation of the apparatus used for
particles processing using the SAS-EM technique in the batch mode
has been shown in FIG. 1. All the particle precipitation runs can
be carried out using the methods in the invention either in the
batch mode or in the semibatch mode. The first step, as in any
antisolvent precipitation technique involving supercritical fluids,
is filling up the particle production vessel with the antisolvent.
This is done up to the desired operating pressure which is
typically around or above the critical pressure of the antisolvent.
Any antisolvent can be used including carbon dioxide, propane,
butane, isobutene, nitrous oxide, sulfur hexafluoride and
trifluoromethane, but carbon dioxide is the most preferred
antisolvent due to its low cost, environmental friendliness and the
ease of availability. The temperature inside the vessel is
maintained constant at the desired value by placing the vessel in a
temperature controlled zone. The temperature is typically above or
around the critical temperature of the antisolvent. Dispersion
containing desired substance is prepared. The horn inside the
vessel is then turned on to vibrate at the desired amplitude by
adjusting the input power to the vibrating source. The horn in fact
provides the vibrating surface inside the supercritical phase for
both dispersion jet atomization and increased mixing. The
dispersion is then injected inside the precipitation chamber
through a silica capillary tube, onto the vibrating surface. It is
important to note here that tubes having different diameters can be
used to carry out the precipitation process but in our case a 75
.mu.m (internal diameter) capillary tube was used. As the
dispersion jet makes contact with the vibrating horn surface, it is
atomized into tiny droplets and particles are formed due to the
rapid removal of the solvent by supercritical CO.sub.2 from these
droplets. Motion between the particles inside the chamber is
increased due to the vibration field generated by the vibration
surface, which in turn prevents them from agglomerating together
and also increases the mass transfer rate of CO.sub.2 into the
droplet, and the solvent out of the droplet.
[0029] In one preferred embodiment of the current invention the
precipitation or recrystallization process using SAS-EM is carried
out in a continuous manner. In this form of the invention the
supercritical fluid is preheated and pumped into the vessel in a
continuous manner at a desired flow rate. A preheated dispersion,
having at least one solid of interest dissolved in at least one
suitable solvent, is then injected into the vessel and onto the
vibrating surface inside the precipitation vessel in a continuous
manner at a desired flow rate. CO.sub.2 flow rate is kept high
enough to completely dry the particles and remove all solvents.
[0030] A major advantage of the present invention over other forms
of supercritical fluid particle precipitation techniques is that
the sizes of the particles formed by this technique can be easily
controlled by changing the vibration intensity of the deflecting
surface, which in turn can be controlled by adjusting the input
power to the vibrating source. For instance the size control
parameters investigated so far in the SAS process are pressure and
temperature of the antisolvent, concentration of the dispersion and
the flow rate of the dispersion into the supercritical fluid
antisolvent. All these parameters are not robust enough to generate
a pronounced change in particle size. Besides, conflicting results
have been obtained by different researchers about the actual effect
of these parameters on particle size and distribution and no
general trend has been established.
[0031] Another major advantage of the present invention is that it
can be used to produce nanoparticles of compounds that cannot be
obtained using the SAS method. In other words compounds that give
long fibers or large crystals using the SAS technique can be
processed using the SAS-EM technique to form nanoparticles or
microparticles.
[0032] One of the main requirements of such small particles in
several applications is a narrow particle size distribution. SAS-EM
can be used to produce particles with narrow size distributions as
a result of uniform droplet atomization.
[0033] In another preferred embodiment of the current invention
encapsulation of one substance can be achieved using another
substance to form coated nanoparticles. The core particle to be
coated is dispersed in a suitable medium and mixed with a
dispersion containing the desired substance and sprayed on to the
deflecting surface in the vessel to obtain very small particles
coated with the desired substance. Change in vibration intensity is
used to decrease the particle size of such coated or encapsulated
particles. Composite nanoparticles of two or more substances can
also be obtained using the preferred embodiments of the current
invention. In this aspect of the invention, the dispersion for
injection is prepared by dissolving the substances to be
co-precipitated in a suitable solvent or a mixture of solvents.
Surfactants may also be employed for dispersing some substances in
the suitable medium or solvent. The above dispersion is then
sprayed onto the deflecting vibrating surface for atomization and
production of particles. For example the co precipitation can be
used to produce drug loaded polymer nanoparticles or magnetite
encapsulated polymer nanoparticles that can be used for controlled
release and drug targeting.
BRIEF DESCRIPTION OF THE DRAWINGS
[0034] FIG. 1 is a schematic representation of the apparatus
employed for particle production using the SAS-EM technique.
[0035] FIG. 2 is a schematic representation of the particle
production vessel.
[0036] FIG. 3 is a representation of the mechanism for the liquid
film disintegration on the horn surface.
[0037] FIG. 4 is SEM micrographs of the particles obtained from
each experiments at (a) 0 W, (b) 12 W, (c) 30 W, (d) 60 W, (e) 90
W, (f) 120 W power supply to the horn. The volume average of
particles obtained are (a) 2000 nm, (b) 730 nm, (c) 653 nm, (d) 240
nm, (e) 189 nm, (f) 227 nm.
[0038] FIG. 5 is a representation of particle size distribution of
lysozyme particles obtained from experiments conducted at different
horn input powers.
[0039] FIG. 6 is an SEM micrograph of untreated lysozyme sample as
obtained from the manufacturer. The solid is in the form of flakes
a few millimeters in size.
[0040] FIG. 7 is a representation of mean lysozyme particle size
versus power supply to the horn.
[0041] FIG. 8 is a representation of change in the standard
deviation of lysozyme particles with change in the value of the
total power supply to the horn.
[0042] FIG. 9 is the results of the lysozyme assay tests. Lysozyme
supplied by the manufacturer (Top), Lysozyme particles obtained at
96.5 bar, 37.degree. C. and at 60 watt power supply (bottom).
Lysozyme particles obtained by the SAS-EM technique retained about
87% of its activity.
[0043] FIG. 10 is SEM micrographs of tetracycline particles
produced by the SAS-EM process at 96.5 bar, 35.degree. C., at (a1,
a2) 30 W; (b1, b2) at 60 W; (c1, c2) at 90 W and (d1, d2) at 120 W
power supply.
[0044] FIG. 11 is SEM micrographs of tetracycline fibers and
particles produced by the SAS-EM process at 96.5 bar, 35 .degree.
C. with no vibration. Most of the solid is in the form of fibers as
show in (a-c). A few particles were also obtained as shown in
(d).
[0045] FIG. 12 is a representation of the size distribution of
lysozyme particles obtained from experiments conducted at varying
input powers.
[0046] FIG. 13 is a representation of average tetracycline particle
sizes versus power supply to the horn: (a) number average, and (b)
volume average.
[0047] FIG. 14 is a representation of standard deviation in the
size of the lysozyme particles versus power supplied to the
vibrating horn.
[0048] FIG. 15 is the IR spectra of tetracycline as obtained from
the manufacturer and after processing with SAS-EM technique.
[0049] FIG. 16 is SEM micrographs showing the change in the
morphologies of Griseofulvin particles obtained from experiments
conducted at different input power supply to the vibrating source,
using DCM as solvent.
[0050] FIG. 17 is SEM micrographs of spherical shaped Griseofulvin
nanoparticles obtained from experiments conducted at different
input power supply, using DCM solvent.
[0051] FIG. 18 is SEM micrographs showing the change in the
morphologies of Griseofulvin particles obtained from experiments
conducted at different values of input power supply, using THF
solvent.
[0052] FIG. 19 is SEM micrographs of spherical shaped Griseofulvin
nanoparticles obtained from experiments conducted at different
values of input power supply, using THF solvent.
[0053] FIG. 20 is SEM micrographs illustrating the change in the
morphology of the Griseofulvin particles with increasing power
supply to the horn using DCM as solvent.
[0054] FIG. 21 is a representation of volumetric mean size of
spherical shaped Griseofulvin obtained particles versus power
supply.
[0055] FIG. 22 is a representation of volume of long needle shaped
Griseofulvin crystals obtained versus power supply.
[0056] FIG. 23 is SEM micrographs of spherical shaped polymer
encapsulated magnetite nanoparticles obtained from experiments
conducted at different input power, using DCM as solvent.
[0057] FIG. 24 is a TEM micrograph of PLGA encapsulated magnetite
particles. The dark and shady regions are due to magnetite
particles inside PLGA.
[0058] FIG. 25. SEM micrographs of tetracycline particles obtained
using the SAS-EM technique at 96.5 bar, 35.degree. C. and at a
vibration frequency of 20 kHz. The nozzle used in this case was a
760 .mu.m stainless steel tube.
DETAILED DESCRIPTION OF THE INVENTION
[0059] Definitions:
[0060] "Particles" means
[0061] A particle is a relatively small discrete portion of a given
material.
[0062] "Desired substance" means
[0063] The material comprising of one or more substances of
interest.
[0064] "Dispersant" means a fluid that helps in dispersing or
scattering a material in a medium.
[0065] "Dispersion" means
[0066] A homogenous or a heterogeneous mixture of the desired
substance in one or more suitable solvents with or without
dispersants or coreparticles.
[0067] "Solvent" means
[0068] A fluid or a combination of fluids, which can dissolve the
desired substance in order to form a homogenous solution.
[0069] "Surface" means
[0070] The exterior or the boundary of the horn tip excluding any
nozzle surface onto which the dispersion is sprayed.
[0071] "Vibrating the surface" means
[0072] Moving the surface at a rapid rate by means of an external
source.
[0073] "Desired frequency" means a frequency being from 0.5 khz to
1 Mhz, preferably from 10 khz to 200 khz.
[0074] "Antisolvent" means a fluid that does not substantially
dissolve a desired substance.
[0075] "Near or supercritical conditions" means the temperature and
pressure of the fluid being closer to or higher than the critical
temperature and critical pressure of the fluid respectively.
Preferably, the temperature being from 0.7 T.sub.c (K) to 1.5
T.sub.c (K) and pressure being from 0.2 P.sub.c to 10 P.sub.c.
[0076] "Miscible" means two substance being soluble in each
other.
[0077] "Substantially Insoluble in the antisolvent" means the
desired substance has no or very little solubility in the
antisolvent.
[0078] "Droplets" means
[0079] A relatively small drop that can exist independently in the
supercritical fluid medium.
[0080] "Supercritical fluid" means
[0081] A fluid whose temperature and pressure are kept above its
critical temperature and pressure respectively.
[0082] "Size of the particles" means
[0083] The dimensions of the precipitated particles. Typically, it
is the diameter of the particle if it is fairly spherical and the
length and the width if the particle is in the form of a rod or a
needle.
[0084] "Distribution of the particles" means the distribution of
the particle counts for different particle sizes.
[0085] "Intensity of the vibration" means
[0086] The degree of vibration or the extent to which the surface
vibrates. It is directly proportional to the input power to the
vibrating source. Higher the intensity of vibration greater is the
amplitude of vibrating the surface.
[0087] "Piezoelectric" means
[0088] A material capable of generating vibrations when subjected
to applied voltage.
[0089] "Magnetorestrictive" means
[0090] A material capable of generating vibrations when subjected
to a change in its state of magnetization.
[0091] "Agglomeration of the particles" means
[0092] The particles being clustered together to form a larger mesh
or a sphere like structure.
[0093] "Collecting the particles in a continuous manner" means
[0094] Collection of the produced particles in a manner that does
not require stopping the production of particles
[0095] "Coreparticles" means particles that are to be coated or
surrounded by the desired substance.
[0096] "Encapsulated coreparticles" means coreparticles being
surrounded or coated by the desired substance.
[0097] "Medicaments" means substances used in the diagnosis,
treatment, or prevention of disease and for restoring, correcting,
or modifying organic functions.
[0098] "Morphology of theparticle" means external structural
appearance or the form of the particle.
[0099] "close to the vibrating surface" means close to the
vibrating surface so as to get exposed to at least one wavelength
of vibration. Typically one wavelength of vibration with 20 Khz
frequency in the vessel is about 2 cm.
[0100] Description
[0101] The current form of the invention can be practiced either in
a batch mode, or in a continuous manner for particle collection.
FIG. 1 is a schematic of the apparatus used in particle production
using SAS-EM. Pump 1 is used to pump CO.sub.2 at a constant
pressure and at a desired flow rate. Similarly, pump 4 is used to
flow the dispersion at a constant pressure and desired flow rate.
Both streams are pumped through individual temperature controlled
zones to maintain a desired inlet temperature into the particle
production vessel 17. The CO.sub.2 inlets are located close to the
bottom of the vessel and the flow rates in the individual inlets
can be controlled by a control valve 16. The dispersion 3 is
sprayed through a dispersion inlet 14 at an angle between 0 to 90
degrees to the horn surface 13. The horn surface 13 is vibrated
through either piezoelectric or magnetorestrictive means. The
transducer 10 allows to control the intensity or input energy to
the vibration source which in turn controls the amplitudes of
vibration. The vessel is kept in a temperature controlled zone 7
and CO.sub.2 outlet 11 is located at the top of the vessel and
CO.sub.2 is further taken for recycle. The windows 15 are used for
visual inspection and for online particle measurements. Temperature
and pressure sensors are employed accordingly at various locations.
The following steps explain the preferred embodiments of the
practice of the current invention.
[0102] The particle production vessel 17 is filled with the
antisolvent up to the desired operating pressure (near or above the
critical pressure of the antisolvent) and maintained at the desired
operating temperature (near and above the critical temperature of
the antisolvent). The antisolvent from source 2 is pumped through a
temperature controlled zone 6 and let into the vessel 17 in a
continuous manner at a desired flow rate.
[0103] The horn surface 13 inside the vessel 17 is then vibrated at
the desired amplitude by adjusting the input power to the
transducer 10. The frequency of vibration is generally kept at a
constant 20 Khz. Vibration can also be produced with
magnetorestrictive, electromagnetic or pneumatic means. This horn
provides the surface 13 on to which the dispersion jet is injected
for atomization. The change in amplitude results in decreased
droplet size which eventually translates to smaller precipitated
particles.
[0104] The dispersion 3 containing one or more substances of
interest in one or more suitable solvents is pumped through the
temperature controlled zone 6 in order to control the inlet
temperature and sprayed through the dispersion inlet 14. The
distance between the outlet of the dispersion inlet 14 and the horn
surface 13 is kept small and can be varied to prevent clogging of
the dispersion inlet 14 tip.
[0105] As soon as the dispersion 3 jet is in contact with the
vibrating surface, it is atomized into tiny droplets and particles
are formed due to the rapid removal of the solvent/solvents by
supercritical CO.sub.2 from the droplets. The mass transfer rate
between solvent/solvents and supercritical CO.sub.2 is greatly
enhanced due to increased mixing caused by the vibration field
generated by the horn surface 13. Increased mixing also leads to an
increase in particle motion inside the precipitation vessel 17 and
this further prevents agglomeration of the precipitated
particles.
[0106] The vibration field generated by the horn surface 13 causes
vibration streaming inside the particle production vessel 17, which
keeps the particles in constant motion.
[0107] The flow rate of CO.sub.2 is maintained high enough so that
all the solvents in the dispersion 3 are removed to obtain dry
particles.
[0108] Dry particles are collected in a particle barrier 9. This
collection can be made continuous by moving the collection zone
away from the precipitation zone.
[0109] The particle morphology is also controlled by the change in
input power intensity to the vibration source. This changes the
amplitude of vibrations of the horn surface. Change in intensity
also produces narrower particle size distribution.
[0110] Various aspects of the current invention and its salient
features have been demonstrated by the following examples, which
set forth techniques, process parameters, operating conditions and
also a list of the obtained experimental results. Test results to
prove that no structural or biological change in the precipitated
compounds took place as a result of the precipitation process have
also been listed. Examples 1-3 relate to the precipitation of
pharmaceuticals such as lysozyme, tetracycline and Griseofulvin
(GF). Example 4 illustrates the precipitation of fullerene
nanoparticles. Potential applications of these nanoparticles can be
envisioned once their unique physical and chemical properties have
been determined after their manufacture. Example 5 relates to
coating of a coreparticle with one or more desired substances. The
coreparticles are dispersed in the chosen solvent with use of a
surfactant and this mixture is mixed with a solution containing the
desired substance. The resultant dispersion is injected onto the
deflecting surface inside the particle production vessel 17. In the
example, polymer encapsulated magnetite particles have been
produced using the methods of the current invention.
[0111] Formation of Lysozyme Particles
[0112] The SAS-EM technique was applied to the formation of
lysozyme particles of different sizes, using the power supplied to
the horn as the size tuning parameter. The particle production
vessel was kept constant at 96.5 bar and 37.degree. C. and the
frequency of the horn vibrations was maintained at 20 kHz. The
solution containing lysozyme in dimethyl sulfoxide (DMSO)
(concentration 5 mg/ml) was introduced into the vessel at different
horn vibration amplitudes corresponding to 0-120 W input power
supply. As soon as the solution was injected lysozyme particles
were formed inside the vessel which were then collected and taken
for analysis. FIGS. 4a-f show scanning electron (SEM) micrographs
of particles obtained in experiments conducted at the different
vibration amplitudes.
[0113] With no vibration (i.e., when the input power/amplitude is
zero) the volume distribution mean size of particle is around 2
.mu.m with standard deviation of 1 .mu.m. It is important to note
here that the experiment conducted at zero amplitude is the same as
the conventional SAS technique and the nozzle in this case was kept
parallel to the horn surface 13. In SAS-EM experiments, nozzle is
placed at angle to the horn surface 13 (0-90.degree.) to maximize
the solution jet exposure to the horn surface 13. As the horn
amplitude values are increased, there is a considerable decrease in
particle size to as low as 0.26 .mu.m at the amplitude
corresponding to 60 W power supply, as shown in Table 2. FIGS. 5a-f
show a comparison of particle size distribution of lysozyme
particles obtained in each of these experiments. FIG. 6 is an SEM
micrograph of the unprocessed lysozyme sample as obtained from the
manufacturer. Comparison of FIGS. 4a-f and 6 clearly illustrate the
change in morphology and the size of the particles due to SAS-EM
processing. FIG. 7 shows the relationship between average particle
size and the input power corresponding to different vibration
amplitudes. These experiments show that both the volume-average
particle size (S.sub.vol) and the number-average particle size
(S.sub.num) decreases with increasing input power to the vibration
source (A) according to following equations 1-2
S.sub.vol=0.0002A.sup.2-0.0358A+1.7448 (1)
S.sub.num=0.0001A.sup.2-0.0211A+1.1137 (2)
[0114] Hence, one can use the input power/amplitude of vibration to
tune the apparatus that gives desired particle size.
[0115] It is interesting to see that the particle size decreases to
a minimum value for input power of 90 W. Further increase in the
power does not change particle size significantly.
[0116] Apart from a decrease in the particle size there is also a
considerable decrease in the standard deviation with increasing
power as shown in FIG. 8. This is due to the narrow droplet size
distribution obtained in the SAS-EM technique, which leads to the
formation of uniform sized particles.
[0117] The vibration is helping favorably in terms of decreasing
the particle size. But for biological molecules, it is also
important that no other chemical changes are caused that may reduce
the activity of the substance. Hence, experiments were also
conducted to check the biological activity of the protein particles
that were exposed to vibration during their formation.
[0118] A bacterial suspension was prepared by mixing 20 mg of
micrococcus lysodeikticus with 90 ml of phosphate buffer (pH=7) and
10 ml of 1% NaCl solution. Lysozyme solution of concentration 0.04
mg/ml was also prepared in the phosphate buffer (pH=7). Now, 0.25
ml of the protein solution was added to 2.5 ml of the bacterial
suspension and mixed. The biological activity of lysozyme was
determined by measuring the rate of change in ultraviolet (UV)
absorbance at 450 nm using a spectrophotometer (Spectronic
Genesys-2). The results of the experiments have been shown in FIG.
9. The rate of absorbance is linear for 4 minutes and is
proportional to the concentration of the biologically active
lysozyme. Based on these results it can be concluded that lysozyme
particles obtained from the SAS-EM technique at vibration amplitude
corresponding to 60W power supply, retained 87.+-.5% of their
activity. Hence there appears to be no significant loss in the
enzymatic activity of the particles obtained from the SAS-EM
technique.
[0119] (2) Formation of Tetracycline Particles
[0120] The SAS-EM technique was carried out at different amplitude
of vibration of the horn surface 13 to produce tetracycline
particles of different sizes. The particle production vessel was
kept constant at 96.5 bar and 35.degree. C. while the vibration
frequency of the horn was maintained at 20 kHz. The solution
containing tetracycline in tetra hydrofuran (THF, concentration 5
mg/ml) was then introduced into the vessel at different horn
amplitudes corresponding to 0-120 watt input power. FIGS. 100a1-d2
are SEM micrographs of particles obtained from experiments
conducted at the different horn amplitudes. With no vibration i.e.
when the input power was zero tetracycline fibers around 2 .mu.m in
diameter were obtained. A few particles having a mean size of 800
nm were also obtained but most of the solids were in the form of a
fine mesh of fibers having a low mechanical strength as shown in
FIGS. 11a-d. It is important to note here that the experiment
conducted at zero amplitude was similar to the conventional SAS.
The nozzle was placed parallel to the horn surface 13 without
touching the horn for SAS experiments. In SAS-EM experiments,
nozzle is placed at angle to the horn surface 13 (0-90.degree.) to
maximize the solution jet exposure to the horn surface 13. As the
power supply to the horn was increased there was a considerable
decrease in the size of the particles obtained as shown in Table 3.
FIGS. 12a-d show a comparison of particle size distribution of
tetracycline particles obtained from experiments conducted at
different horn vibration amplitudes.
[0121] Vibration Intensity (Input Power Supply) for Controlling
Particle Size
[0122] From the results in Table 3 it is interesting to note that
with an increase in the power supply (i.e., increase in the horn
vibration amplitude), there is a considerable decrease in the
particle size. As low as 100 nm size particles are obtained at 120
W power supply. FIG. 13 showing the relationship between average
particle size and power to the horn, clearly illustrates the trend.
The volume average (S.sub.vol) and number average (S.sub.num)
particle sizes are related to the input power (P) as
S.sub.vol=-0.0016P.sup.3+0.3644P.sup.2-26.461P+795.8 (1)
S.sub.num=-0.0018P.sup.3+0.4298P.sup.2-34.141P+1097.1 (2)
[0123] where, S.sub.vol and S.sub.num are in nm and P is in
Watts.
[0124] Apart from a decrease in the particle size there is also a
considerable decrease in the standard deviation in the particle
size at higher horn vibration amplitudes as shown in FIG. 14. This
is due to the narrow droplet size distribution obtained in the
SAS-EM technique, which leads to the formation of more uniform
sized particles.
[0125] Fourier Transform Infrared Spectroscopy (FTIR) Analysis of
Tetracycline Nanoparticles
[0126] FT-IR analysis was performed to check if there is any
difference in the structures of the original tetracycline (as
supplied by the manufacturer) and that obtained from the
precipitation experiments using the SAS-EM technique at 120 W power
supply. FIG. 15 shows the IR spectra obtained in both the cases.
Comparison of the two spectra show that there is no variation in
the molecular structure of the two tetracyclines. In the case of
tetracycline, the carbonyl region between 1500-1600 cm.sup.-1 and
the amide region between 3000-4000 cm.sup.-1 are of greatest
importance to chemists. These regions seem to be similar in case of
both the original and the SAS-EM precipitated tetracycline samples
confirming that no structural changes took place in the SAS-EM
process.
[0127] (3) Formation of Griseofulvin (GF) Nanoparticles
[0128] The SAS-EM technique was used to produce Griseofulvin
particles of different sizes. The results of the different
precipitation runs have been summarized in Table 4. Precipitation
of GF was carried out using two different solvents, dichloromethane
(DCM) and Tetrahydrofuran (THF). All SAS-EM particle production
experiments were carried out at 96.5 bar and at 35.degree. C. The
vibration frequency of the horn surface 13 was kept constant at 20
KHz while the amplitude of vibration was varied by changing the
input power supply to vibrating source. The concentration of the GF
solution used during the precipitation experiment was 5 mg/ml of
the solvent. FIGS. 16a-f and 18a-e are SEM micrographs of particles
obtained from experiments conducted at the different horn
amplitudes using DCM and THF as solvents respectively.
[0129] When DCM was used as the solvent and when there was no power
supply to the transducer, long needle shaped crystals of several
millimeters in length were obtained (FIG. 16a). It is important to
note here that experiments conducted with no vibration were
basically the SAS process. Results obtained in these cases were
similar to the ones obtained by Reverchon et al. (1999) during
their SAS experiments. In experiments using SAS-EM, nozzle was
placed at angle to the horn surface 13 (0-90.degree.) to maximize
the solution jet exposure to the horn surface 13. As the power
supply to the vibration source was increased, mixtures of long
needle shaped crystals of GF and small spherical shaped GF
nanoparticles were obtained. FIGS. 17a-d are SEM micrographs of the
spherical shaped GF nanoparticles obtained from each of these
experiments corresponding to different values of input power. When
the total power supply was 90 W, narrower and shorter needle shaped
crystals of GF were obtained (FIG. 16c). A low yield of spherical
shaped GF particles were also obtained, but most of the solid was
in the form of long needle shaped crystals 50 .mu.m long and 2.5
.mu.m wide.
[0130] As the power supply to the transducer was increased, a
drastic change in the morphology of the particles was observed.
Relatively a small amount of long needle shaped GF crystals were
obtained when the total power supply to the transducer was 120 W.
The volumetric mean of the spherical GF nanoparticles obtained in
this case was 0.13 .mu.m (FIG. 17b) while the larger needle like GF
crystals were 7.3 .mu.m long and 2.7 .mu.m wide (FIG. 16d).
Increase in the power supply beyond 120 W further increased the
yield of spherical shaped GF nanoparticles. The volumetric mean of
the spherical GF particles obtained corresponding to 150 W total
power supply was 0.5 .mu.m (FIG. 17c) while the larger needle like
GF crystals were 3.8 .mu.m long and 1.4 .mu.m wide (FIG. 1 Se). At
180 W power supply the volumetric mean of the spherical shaped GF
nanoparticles was 0.4 .mu.m (FIG. 17d). A low yield of large GF
particles 2.0 .mu.m long and 1.6 .mu.m wide were also obtained
(FIG. 16f).
[0131] When THF was used as the solvent, with no power supply to
the transducer, long fibers of GF were obtained (FIG. 18a). When
the total power supply was increased to 90 W, there was a change in
the morphology of the particles and long needle shaped crystals of
GF 45 .mu.m long and 2.5 .mu.m wide were obtained (FIG. 18b). As
the power supply was further increased to 120 W, there was again a
change in the morphology of the particles and a mixture spherical
and long needle shaped particles of GF were obtained. FIGS. 19a-c
are SEM micrographs of spherical shaped GF nanoparticles obtained
from each of these experiments corresponding to different values of
total power supply. The volumetric mean size of the spherical
shaped nanoparticles was 0.2 .mu.m (FIG. 19a) while the mean size
of the needle shaped GF crystals was 8.0 .mu.m long and 1.0 .mu.m
wide (FIG. 18c). The volumetric mean of the spherical GF particles
when the power supply was 150 W was 0.3 .mu.m (FIG. 19b) while the
mean size of the needle shaped GF crystals was 3.8 .mu.m long and
1.6 .mu.m wide (FIG. 18d). At 180 W power supply, spherical GF
particles having a volumetric mean size of the 0.2 .mu.m (FIG. 19c)
were obtained. Very few larger needle shaped GF particles 2.1 .mu.m
long and 1.7 .mu.m wide were also obtained (FIG. 18e).
[0132] Effect of Vibration Intensity on Size and Morphology of
Griseofulvin Nanoparticles
[0133] From the above results it is interesting to note that, with
an increase in power supply (i.e. increase in horn vibration
amplitude) there is an increase in the yield of small spherical
Griseofulvin nanoparticles. Further, there is also a decrease in
the size and the yield of the larger needle shaped Griseofulvin
crystals obtained. This has been illustrated in FIGS. 16a-f, 18a-e
and 20 where upon visual inspection one can see a change in
morphology of the particles with increased power supply and also a
decrease in the yield of large needle shaped Griseofulvin crystals.
FIG. 21 is a graph showing the relationship between the mean size
of the spherical particles and the input power supply corresponding
to different horn vibration amplitudes. From the figure one can
infer that Griseofulvin nanoparticles having a volumetric mean as
low as 130 nm have been obtained corresponding to 120 W power
supply and when DCM was used as the solvent. FIG. 22 is a graph
showing the relationship between the volume of the large needle
shaped Griseofulvin crystals and input power supply. There is a
considerable decrease in the volume of Griseofulvin crystals with
increasing power supply in case of both the solvents. Based on the
FIGS. 21 and 22, no particular trend can be established about the
effect of the solvent on the size and morphology of Griseofulvin
particles.
[0134] (4) Formation of Fullerene Particles
[0135] In order to demonstrate the effectiveness of the current
invention for processing other materials besides pharmaceutical
substances, the SAS-EM technique was applied for the precipitation
of fullerene C.sub.60 nanoparticles. In this case, the particle
production vessel was kept constant at 96.5 bar and 37.degree. C.
and the frequency of the horn vibrations was maintained at 20 kHz.
A solution of fullerene in toluene (concentration, 0.6 mg/ml) was
used for all the precipitation experiments. The first experiment,
as in all the earlier cases, was performed with no vibration and
was similar to the SAS technique for precipitation of particles.
The particles obtained by this technique were 96 nm in size with
standard deviations of around 43 nm.
[0136] Next, the particle production experiment was performed with
the vibrating horn surface 13 inside the vessel and input power set
at 30 W power. The 75 .mu.m capillary tube in this case was placed
parallel to the horn surface 13 touching it completely. Particles
formed in this case were extremely small having a mean diameter of
30 nm and a standard deviation of 13 nm.
[0137] (5) Formation of Polymer Encapsulated or Coated Magnetite
Particles
[0138] The use of SAS-EM technique was also demonstrated for the
encapsulation or coating of core particles by one or more compounds
to form composite nanoparticles. Similar to earlier examples,
SAS-EM precipitation experiments were carried out at 96.5 bar and
at 35.degree. C. The vibration frequency of the horn surface 13 was
kept constant at 20 KHz while the amplitude of vibration was varied
by changing the total power supply to the vibration source. A
sample of commercial magnetite particles (Ferrofluid) was obtained
that had magnetite particles (10 nm) suspended in a hydrocarbon
mineral oil using a fatty acid surfactant. The solution for
injection into the particle production vessel was prepared by
dissolving the polymer (poly(lactide-co-glycolide)(PLGA), 100 mg)
and the above ferrofluid (49 mg) in 10 ml of dicholoromethane
(DCM).
[0139] When there was no vibration (i.e similar to a SAS
experiments) PLGA encapsulated magnetite particles having a mean
size of 1.7 .mu.m were obtained as shown in FIG. 23a. FIG. 24 is a
TEM micrograph of the obtained composite particles clearly showing
the magnetite particles encapsulated in the polymer matrix. When
the power supply to the vibration source was increased to 60 W,
there was a reduction in mean particle size to 0.7 .mu.m as shown
in FIG. 23b. With increase in the power supply there is a further
reduction in mean particle size to as much as 0.4 .mu.m as shown in
FIG. 23c.
[0140] (6) Formation of Tetracycline Particles Using a Higher
Diameter Nozzle:
[0141] In all the previous experiments a 75 .mu.m silica capillary
tube was used to spray the solution having at least one substance
of interest and in at least one solvent onto or near the horn
surface. In the present experiments we have used a nozzle having a
higher diameter in order to study the effect of increase in nozzle
diameter on the size and the morphology of the particles.
[0142] Like the earlier experiments here the SAS-EM technique was
carried out at different amplitude of vibration of the vibrating
horn surface to produce tetracycline particles of different sizes
and morphologies. The diameter of the stainless steel capillary
used in this case however was a 760 .mu.m. The precipitation cell
in this case was kept constant at 96.5 bar and 35.degree. C. while
the frequency of the titanium horn was maintained at 20 kHz. The
solution jet was then introduced into the cell at different horn
amplitudes corresponding to 0-120W power supplied. FIGS. 25a-f are
SEM micrographs of particles obtained from these experiments. With
no vibration i.e. when the horn amplitude was zero, tetracycline
fibers around 1-2 .mu.m in thick were obtained. Most of the solid
was in the form of this fine mesh of fibers having a low mechanical
strength as shown in FIGS. 25a, b. It is important to note here
that the experiment conducted at zero amplitude was similar to the
conventional SAS. The nozzle was placed parallel to the horn
surface without touching the horn for SAS experiments. As the power
supply to the horn was increased there was a drastic change in the
morphology of the particles. When the power supply was 60 W flaky
crystals of tetracycline about 5.0 .mu.m long and 1.0 .mu.m wide
were obtained as shown in FIGS. 25c, d. Further increase in the
power supply again resulted in a drastic change in morphology of
the particles. Fine nanoparticles of tetracycline having a
volumetric mean diameter of 0.28 .mu.m and a standard deviation of
0.13 .mu.m were obtained when the power supply was 120 W (FIGS.
25e, f).
* * * * *