U.S. patent application number 13/547894 was filed with the patent office on 2012-11-15 for piezoelectric film.
This patent application is currently assigned to ST. JUDE MEDICAL AB. Invention is credited to Tom ERIKSSON, Anna-Karin JOHANSSON, Koroush LASHGARI, Karin LUNGSTROM, Kenth NILSSON, Annika POHL, Gunnar WESTIN.
Application Number | 20120289807 13/547894 |
Document ID | / |
Family ID | 37481895 |
Filed Date | 2012-11-15 |
United States Patent
Application |
20120289807 |
Kind Code |
A1 |
ERIKSSON; Tom ; et
al. |
November 15, 2012 |
PIEZOELECTRIC FILM
Abstract
A method for producing a biocompatible material of the formula
Na.sub.xK.sub.yNbO.sub.3, 0.ltoreq.x.ltoreq.0.8,
0.2.ltoreq.y.ltoreq.1, x+y=1 includes the steps of a) providing a
Na-precursor and a K-precursor for Na.sub.xK.sub.yNbO.sub.3, b)
mixing the precursors in solution wherein said precursors first
react to form a sol and thereafter a gel, c) heat treating the gel
to obtain an oxide of the material Na.sub.xK.sub.yNbO.sub.3,
0.ltoreq.x.ltoreq.0.8, 0.2.ltoreq.y.ltoreq.1, x+y=1. The material
can be produced as a film, and the material or film can be provided
on the exterior surface of a medical implant that will come into
contact with body tissue and/or body fluids upon implantation
thereof.
Inventors: |
ERIKSSON; Tom; (Uppsala,
SE) ; NILSSON; Kenth; (Akersberga, SE) ;
JOHANSSON; Anna-Karin; (Vallentuna, SE) ; LUNGSTROM;
Karin; (Hasselby, SE) ; LASHGARI; Koroush;
(Sundyberg, SE) ; POHL; Annika; (Tyreso, SE)
; WESTIN; Gunnar; (Stockholm, SE) |
Assignee: |
ST. JUDE MEDICAL AB
Jarfalla
SE
|
Family ID: |
37481895 |
Appl. No.: |
13/547894 |
Filed: |
July 12, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11920380 |
Jan 26, 2009 |
8246929 |
|
|
PCT/SE2005/000823 |
May 31, 2005 |
|
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13547894 |
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Current U.S.
Class: |
600/373 ;
607/116 |
Current CPC
Class: |
C01P 2002/88 20130101;
C04B 2235/3251 20130101; C04B 2235/80 20130101; C23C 18/1216
20130101; C23C 18/1225 20130101; C04B 2235/6562 20130101; C01P
2002/82 20130101; C01G 33/00 20130101; C01P 2002/72 20130101; H01L
41/318 20130101; C23C 18/1254 20130101; C04B 2235/3201 20130101;
H01L 41/1873 20130101; A61N 1/056 20130101; C23C 18/1283 20130101;
A61N 1/36542 20130101; A61N 1/36564 20130101; C04B 35/624 20130101;
C04B 35/62675 20130101; C04B 35/62645 20130101; C01P 2004/03
20130101; C04B 35/495 20130101; C04B 2235/3255 20130101; C04B
2235/441 20130101; A61N 1/0565 20130101; C01G 33/006 20130101; C01P
2002/77 20130101; C04B 2235/768 20130101 |
Class at
Publication: |
600/373 ;
607/116 |
International
Class: |
A61B 5/04 20060101
A61B005/04; A61N 1/05 20060101 A61N001/05 |
Claims
1. A medical implant comprising: an implant body having an exterior
surface; and a biocompatible material on the exterior surface
comprising Na.sub.xK.sub.yNbO.sub.3, 0.ltoreq.x.ltoreq.0.8,
0.2.ltoreq.y.ltoreq.1, x+y=1produced by a method comprising; a)
providing a Na-precursor and a K-precursor for
Na.sub.xK.sub.yNbO.sub.3; b) mixing the precursors in solution
wherein the precursors first react to form a sol and thereafter a
gel; and c) heat treating the gel to obtain an oxide of the
material Na.sub.xK.sub.yNbO.sub.3, 0.ltoreq.x.ltoreq.0.8,
0.2.ltoreq.y.ltoreq.1, x+y=1; wherein the precursors are
NaNb(OEt).sub.6 and KNb(OEt).sub.6.
2. The implant of claim 1, wherein the implant is an electrode
and/or sensor with piezoelectric properties for implantation in the
body of a patient for sensing physiological variables in the body
of a patient or for stimulating tissue.
3. The Implant of claim 2, wherein the electrode/sensor is
configured to be located at a distal part of a lead for an
electrical tissue stimulator.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This is a division of U.S. patent application Ser. No.
11/920,380, filed Jan. 26, 2009, which claims priority from
International Application No. PCT/SE2005/000823, filed May 31,
2005.
FIELD OF THE INVENTION
[0002] The invention refers to a new method for preparing a
biocompatible material with the formula Na.sub.xK.sub.yNbO.sub.3
(sodium potassium niobate), as well as a material and a film
prepared by the method, and a piezoelectric implant comprising the
biocompatible material.
BACKGROUND OF THE INVENTION
[0003] Piezoelectric materials have a widespread use in the medical
field. The materials have for instance found use in electrodes and
sensors for implantation in the human or animal body.
[0004] The piezoelectric materials used in the medical field for
sensing purposes or for mechanical stimulation must meet high
standards in regard of for instance sensitivity and durability. One
consequence of this is that many prior art piezoelectric materials
are less suitable for this purpose.
[0005] A piezoelectric material that is often used is lead
zirconium titanate (PZT). However, this material has some
recognized drawbacks and handling problems. Commonly occurring
problems with PZT are inhomogeneous phases, reactions with the
substrate, impurities of pyrochlore type and PbO formation at the
surface. This is mainly due to the chemistry of lead: it has a low
melting point and is easily reduced. This can lead to formation of
Pb droplets in the material during the synthesis and shortage of Pb
in the active material, which reduces the piezoelectricity. It can
be noted, that an excess of Pb is often used in the synthesis to
obtain the right composition in the PZT material. Further, PZT
deposition on a Pt substrate (commonly used for implantation) is
not recommended, since Pb alloys with Pt.
[0006] NKN (Sodium potassium niobate) does not exhibit these
drawbacks. It does not alloy with Pt, and NKN can be heat treated
at a higher temperature than PZT (NKN: 1000.degree. C.; PZT:
500.degree. C.).
[0007] WO99/54266 discloses a biocompatible ceramic material for
implants comprising Na.sub.xK.sub.yNbO.sub.3,
0.ltoreq.x.ltoreq.0.8, 0.2.ltoreq.y.ltoreq.1, x+y=1. The object of
this invention is to provide a long-term stable material that can
be wholly or selectively polarized in order to obtain piezoelectric
properties for tissue growth promoting purposes. W099/53972
discloses a piezoelectric implant comprising
Na.sub.xK.sub.yNbO.sub.3, 0.ltoreq.x.ltoreq.0.8,
0.2.ltoreq.y.ltoreq.1, x+y=1. The object of this disclosure is to
provide an implant that has a sensitivity and a durability that
meet the high standards required and which further is
biocompatible.
[0008] Thus, NKN is known and has also shown excellent properties
for use in implants. In addition, it is possible to polarize the
material in order to provide it with piezoelectric properties. The
material combines a very high level of biocompatibility, mechanical
and chemical stability that are expected to be at least ten years,
a piezoelectricity constant d33 that can exceed 100 pC/N,
resistivity that can exceed 10.sup.12 .OMEGA.m, and a Curie
temperature >160.degree. C. The material will function as
desired at a working temperature of 36-41.degree. C., and a band
width of 0.3-20 Hz. Thus, NKN is a highly desired piezoelectric
material within this field
[0009] The conventional NKN-preparation methods include:
(1) calcinations and milling together with sintering, where however
milling often brings contaminations from the milling equipment.
Also, sintering may lead to oxygene defects in the material, which
seem to be a result of the choice of sample holder during the
sintering process. Moreover, sintering can e.g. be air-fired, hot
pressed or made by hot isostatic pressure. The NKN-material can
e.g. be made as a bulk material by means of the hot isostatic
pressing methods using sodium carbonate, potassium carbonate and
niobium pentoxide as precursors as defined in the following
articles from American Ceramic Society Bulletin: Egerton-Dillon in
42(1959) pp 438-442, Jaeger-Egerton in 45(1962) pp 209-213 and
Egerton-Bieling in 47(1968) pp 1151-1156. Normally hot pressed
materials give a higher d.sub.33-value (measure of
piezoelectricity) than air-fired; (2) pulsed laser deposition (PLD)
or laser ablation, having the drawback that Na- and K-compounds are
volatile, which may lead to Na- and/or K-deficiency in the material
(Self-assembling ferroelectric Na0.5K0.5NbO3 thin films by pulsed
laser deposition" Choong-Rae Cho, Alex Grishin, Appl. Phys. Lett.
75, 268 (1999)); (3) sputtering (e.g. Rf-magnetron sputtering),
whereby the major drawback of Rf-magnetron is oxygene defects in
the material, sometimes together with a
Na.sub.2Nb.sub.4O.sub.11-contamination. The NKN-material may also
be made in the form of films or layers on substrates by means of
cathode sputtering methods as for instance described in Margolin et
al, "(K, Na)NbO.sub.3 ferroelectric films synthesized by cathode
sputtering", Sov. Phys. Tech. Phys. 33(12), December 1988, or by
other suitable thin film techniques; (4) solid-state reaction
methods (sometimes with a subsequent hot pressing) (see e.g. Ichiki
et al., Journal of the European Ceramic Society, 2004,
24;6:1693-97). By using this method, the synthesis requires a
relatively long time. Also, it is difficult to obtain a homogenous
composition; or (5) chemical vapor deposition (CVD) (Choong-Rae,
Materials Letters, 2002, 57;4:781-786). In this disclosure, a
NKN-film is deposited from precursors that are pre-evaporated at
700-750.degree. C. However, the NKN material that is disclosed
exhibit Nb deficiencies (the composition is estimated to comprise
Na:K:Nb 1.00:1.00:1.47). Further, a mixture of NKN and the
Si-substrate occurs in the interface, which results in a varying
NKN composition.
[0010] With conventional NKN-production methods it is common with
oxygene defects and other material problems. NKN having oxygene
defects are often treated in oxygene in order to fill the defects,
but that results in an additional production step, which makes the
production more expensive. Also, for bulk material it is often
difficult to remove the defects in the entire material.
[0011] Moreover, a common problem when using conventional synthesis
methods for piezoelectric materials and NKN, is that the methods
make it difficult to control the composition. This results in low
phase purity, in a low piezoelectric effect, if any. Further, this
makes it difficult to produce a thin piezoelectric film (which
often is desirable in e.g. sensor applications) having reliable
physical and chemical properties, since the material
characteristics are varying and/or unpredictable.
SUMMARY OF THE INVENTION
[0012] Accordingly, there is a strong need in the art for a novel
way of providing NKN that makes it easier to control the
composition that is synthesized, and further solving other problems
with prior art methods and compositions.
[0013] An object of the present invention is to provide a method
that solves the problems of the prior art, and which method makes
it possible to provide a biocompatible NKN-material having the
desired characteristics.
[0014] The above object is achieved in accordance with the
invention by a method for producing a biocompatible material with
the formula Na.sub.xK.sub.yNbO.sub.3, 0.ltoreq.x.ltoreq.0.8,
0.2.ltoreq.y.ltoreq.1, x+y=1 by using a sol-gel synthesis.
[0015] In accordance with the invention a so-called
"sol-gel"-method is used for producing the NKN-material (Brinker
and Scherer, Sol-Gel Science, Academic Press, 1990). The sol-gel
method makes it easier to control the synthesized composition.
General advantages with the sol-gel method include
(i) simple technique, (ii) low processing temperatures can be used,
(iii) the stoichiometry is easily controlled, (iv) gives highly
homogenous products and (v) large areas can be uniformly
coated.
[0016] Furthermore, NKN prepared by sol-gel also has proved to have
a strong adhesion to the substrate.
[0017] Thus, the present invention provides a method that is
efficient and that results in a NKN-material having a good
homogeneity. Moreover, it is easier to obtain a uniform NKN film
than by using prior art methods.
[0018] The above object also is achieved in accordance with the
invention by a method for producing a film by using a sol-gel
synthesis embodying the biocompatible material of the invention
described above.
[0019] In accordance with the invention a film having a desired
thickness is readily provided. By using the present invention it is
possible to form a film having a thickness with a preferred
thickness of 0.1-10 .mu.m.
[0020] The above object also is achieved in accordance with the
invention by a biocompatible material comprising
Na.sub.xK.sub.yNbO.sub.3, 0.ltoreq.x.ltoreq.0.8,
0.2.ltoreq.y.ltoreq.1, x+y=1, obtained by the method according to
the invention described above.
[0021] The above object also is achieved in accordance with the
invention by a biocompatible film essentially composed of the
biocompatible material of the invention described above.
[0022] The above object also is achieved in accordance with the
invention by an implant that may come into contact with body tissue
and/or body fluids, comprising the biocompatible material of the
invention or the biocompatible film of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] FIG. 1 is a picture of the sol-gel synthesis wherein the
molecules are mixed in a solution and reactions are initiated and
first a sol is formed, i.e. a suspension of small particles in the
solution, and then the gel is annealed to form an oxide.
[0024] FIG. 2 shows the probable structure of NaNb(OEt).sub.6 and
KNb(OEt).sub.6 in solution.
[0025] FIG. 3 shows thermogravimetric studies of gels wherein the
heating rate is 5.degree. C.-min.sup.-1 unless otherwise
stated.
[0026] FIG. 4 shows IR spectra of gels and samples heated to
different temperatures.
[0027] FIG. 5 shows powder XRD diffractograms of gels and samples
heated to different temperatures.
[0028] FIG. 6 shows SEM micrographs showing the cross section of
the NKN-films on Pt/TiO.sub.2/SiO.sub.2 substrate prepared using
solutions with the concentrations 0.6 mol-dm.sup.-3 (a) and 0.3
mol-dm.sup.-3(b).
[0029] FIG. 7 shows an SEM micrograph showing a typical NKN-film
prepared on Pt/TiO.sub.2/SiO.sub.2 substrate, wherein types of
crystals can be seen.
[0030] FIG. 8 shows Gracing Incidence (GI) XRD diffractograms of
films prepared using heating program A and a constant heating rate
20.degree. C.-min.sup.-1 to 700.degree. C.
[0031] FIG. 9 shows SEM micrographs of films prepared using heating
program A.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0032] The present invention involves the use of a sol-gel
synthesis for preparing a biocompatible material.
[0033] Sol-gel is a soft chemistry synthesis method that is
commonly used for producing ceramics, nano-sized particles, or thin
material layers. Mixing metal-alkoxides (M-OR) with water, at the
right pH, forms a gel by hydrolysis. A coating technique is
normally used to apply the gel onto a substrate, e.g. dip-coating,
spray-coating or spin-coating.
[0034] Preparation of materials with complicated compositions from
solutions has many advantages, e.g. high purity, moderate
preparation temperatures and excellent possibilities to control the
composition. Metal-organic sol-gel synthesis is used as an example
in this invention. This is based on reactive metal-organic
compounds that are dissolved and mixed in organic solvents, which
form a solid oxide-based network, i.e. a gel, by addition of water.
The gel is then annealed to obtain the preferred oxide. The
composition mixture of the compounds in the solution can be
maintained in the oxide if the temperature treatment is made in a
controlled way. (FIG. 1).
[0035] In a first aspect the invention provides a method for
producing a biocompatible material with the formula
Na.sub.xK.sub.yNbO.sub.3, 0.ltoreq.x.ltoreq.0.8,
0.2.ltoreq.y.ltoreq.1, x+y=1, comprising the steps of:
(a) providing a Na-precursor and a K-precursor for
Na.sub.xK.sub.yNbO.sub.3; (b) mixing the precursors in solution,
whereby they first react to form a sol and thereafter a gel; (c)
heat treating the gel to obtain an oxide of the material
Na.sub.xK.sub.yNbO.sub.30.ltoreq.x.ltoreq.0.8,
0.2.ltoreq.y.ltoreq.1, x+y=1.
[0036] The NKN material of the invention is with the formula
Na.sub.xK.sub.yNbO.sub.3, 0.ltoreq.x.ltoreq.0.8,
0.2.ltoreq.y.ltoreq.1, x+y=1. In a preferred embodiment, the
material is of the formula Na.sub.0.5K.sub.0.5Nb0.sub.3.
[0037] In a second aspect, the invention refers to a method for
producing a film comprising a biocompatible material of the formula
Na.sub.xK.sub.yNbO.sub.3, 0.ltoreq.x.ltoreq.0.8,
0.2.ltoreq.y.ltoreq.1, x+y=1, comprising the steps of:
(a) providing a Na-precursor and a K-precursor for
Na.sub.xK.sub.yNb0.sub.3; (b) mixing the precursors in solution,
whereby they first react to form a sol and thereafter a gel; (c)
depositing the gel to a substrate in order to obtain a film; (d)
heat treating the gel to obtain an oxide of the material
Na.sub.xK.sub.yNb0.sub.3, 0.ltoreq.x.ltoreq.0.8,
0.2.ltoreq.y.ltoreq.1, x+y=1; (e) if necessary, repeating steps (c)
and (d).
[0038] The production of a thin film with the
Na.sub.xK.sub.yNb0.sub.3 material is performed by the use of a
sol-gel method, where the substrate is covered by the gel, e.g. by
dipping (or spraying, or spinning). The material on the substrate
is heat-treated at an elevated temperature (300-900.degree. C.) to
form the right composition and crystallinity after the coating
procedure. The procedure of dipping, drying and firing can be
repeated a number of times until an even film with the desired
thickness is produced without cracks or holes.
[0039] The morphology is also controlled by using adequate heat
treatment conditions, while the composition in the deposited
material is controlled by manipulating the molar amount of the
starting material. Also, by using sol-gel a substrate having
basically any geometry can be deposited, which is not possible with
other techniques. Moreover, by using sol-gel the composition of
matter can be controlled to a higher degree than with other
techniques.
[0040] The selection of precursor materials is very important in
sol-gel synthesis and there are many possibilities. The group of
alkoxides is based on many considerations the best precursors.
These consist of metal ions bound to alkoxide groups (OR,.
deprotonated alcohols). A series of hydrolysis and condensation
reactions is started by adding water. During hydrolysis alkoxide
groups (OR) are changed to hydroxyl groups, which then react with
other alkoxide molecules in condensation reactions and form M-O-M
bonds (reaction scheme 1; simplified illustration of hydrolysis (a)
and condensation (b-e): M=metal, O=oxygen, H=hydrogen, R=alkyl,
ROH=alcohol. Only the groups taking part in the reactions are
shown).
M-OR+H.sub.2O.fwdarw.M-OH+ROH (a)
M-OH+M-OR.fwdarw.M-0-M+ROH (b)
M-OH+M-OH.fwdarw.M-O-M+H.sub.2O (c)
M-OH+M-O(H)R.fwdarw.M-0-M+ROH (d)
M-OH+M-0H.sub.2.fwdarw.M-0-M+H.sub.2O (e)
[0041] Alkoxides are very reactive and forms therefore very pure
gels where the organic groups are almost entirely removed by
hydrolysis and evaporated. The high reactivity of metal alkoxides
often makes the moisture in the air sufficient to obtain fast
hydrolysis and condensation. This has been utilized in this
invention. Another benefit from using alkoxides is the possibility
to prepare heterometallic-alkoxide molecules that comprise two or
more different metal ions in the same molecule, which allows a
simultaneous hydrolysis of the different alkoxides and thereby is
the atomic composition from the solution preserved in the gel.
[0042] Hydrolysis and condensation reactions thus form M-O-M bonds
and larger units are successively created. Long polymers or large
clusters are formed (i.e. a sol) through further condensation in
the solution and a network, i.e. a gel, is formed. The gel normally
contains hydroxo and some alkoxo groups besides the M-0-M network
and the encapsulated solvent. During drying of the gel (solvents
and water are evaporated), it often cracks and collapses to a fine
powder and this xerogel powder (xero=dry) is thereafter annealed to
form an oxide.
[0043] Possible precursor compounds for sol-gel preparation of NKN
include alkoxides, alcohols, carboxylates, nitrates and citrates.
Citrates and nitrates have the advantage that they are
water-soluble. The R-group in alkoxides, alcohols and carboxylates
can be any alkyl group. Examples of alkoxides include ethoxides,
metoxides, isopropoxide isobutoxide and metoxyetoxides. Examples of
carboxylates include acetate (ethanoate), propionate and oxalates.
It should be bared in mind that the carboxylate, nitrate. and
citrate salts will react in a different way compared to alkoxides.
Other salts can also be used e.g. acetylacetonate. Mixtures of
different type of precursors can also be used; two examples are
Nb-alkoxide+acetate and Nb-alcohols+acetate. It should be noted
that (i) nitrates and citrates have to be used in water or water
mixtures with polar solvents and (ii) oxalates may be difficult to
dissolve. Furthermore, chlorides are commonly used for sol-gel
synthesis but are not good precursor candidates for NKN since NaCl
will most likely form, which will be difficult to remove.
[0044] In a preferred embodiment according to the first and second
aspect, the precursors are NaNb(OEt).sub.6 and KNb(OEt).sub.6.
[0045] All dry organic solvents are possible as candidates in a
sol-gel synthesis of NKN. Examples are alcohols, toluene, hexane,
pure ethanol, isopropanol, metoxyetanol, etoxymetanol and ethers. A
mixture between a polar and a non-polar solvent are commonly used.
The solvents used are to a large extent determined by the
precursors used. Certain precursors rewire water based solutions.
Halogenated solvents are not suitable, since these will react with
the ethoxides.
[0046] Maximum temperature is determined by the choice of substrate
and evaporation of Na- and K-oxides. Thermogravimetry shows that
temperatures up to 1200.degree. C. can be used. Heating rates from
1-100.degree. C. min.sup.-1 can be used and rates of 10 or
20.degree. C. min.sup.-1 are preferred. However, even higher
heating rates up to 2000.degree. C. min.sup.-1 may be used should
it for instance be desired to shorten the process time. The heating
program used can have one or several annealing steps at
temperatures up to 1200.degree. C. for any amount of time.
[0047] In a preferred embodiment, the heat treatment includes an
annealing step or constant temperature plateau in the interval
200-400.degree. C. for 30-120 minutes, most preferably at
300.degree. C.
[0048] In another preferred embodiment, the heat treatment includes
a temperature gradient from about 300 to about 700.degree. C. at
10-30.degree. C./min.
[0049] In yet another preferred embodiment, the heat treatment
includes a constant temperature plateau in the interval from
700.degree. C. to 1200.degree. C. for 30-90 minutes, most
preferably at 700.degree. C.
[0050] In yet another aspect, the invention refers to a
biocompatible material comprising Na.sub.xK.sub.yNbO.sub.3,
0.ltoreq.x.ltoreq.0.8, 0.2.ltoreq.y.ltoreq.1, x+y=1, characterized
by that it is obtained by a method according to the invention.
[0051] In preferred embodiments, the biocompatible material has a
piezoelectric constant d.sub.33 of at least 20 pC/N, preferably of
at least 50 pC/N, more preferably of at least 100 pC/N, even more
preferably of at least 150 pC/N, and most preferably of at least
160 pC/N. Further, the material has preferably a relative density
that exceeds 97% and a pore size that is less than 4 .mu.m.
[0052] In still another aspect, the invention refers to a
biocompatible film essentially composed of the biocompatible
material of the invention.
[0053] In still another aspect, the invention refers to an implant
that may come into contact with body tissue and/or body fluids,
comprising the biocompatible material of the invention or the
biocompatible film of the invention.
[0054] In a preferred embodiment, said implant is in the form of a
piezoelectric electrode and/or sensor for implantation in the body
of a patient for sensing physiological variables. For instance, the
material may be used as a layer covering the conducting tip on an
electrode for sensing/stimulating cardiac tissue, this layer being
in direct contact with conducting liquid in blood/tissue. The
conducting tip then will function as one plate of a capacitor and
the liquid as the other plate, the piezo material being the
dielectric. The layer thus will react to a charge applied onto the
plates by generating a mechanical force. The layer will also
generate a charge if a mechanical force is applied. By these means
the cardiac tissue may be stimulated mechanically as well as
electrically when a stimulating pulse is applied to the electrode.
Conversely a pressure variation in the blood or a myocardial
contraction will generate a charge that can be utilized for sensing
purposes. Also, said implant may be used for monitoring motion
exerted by the patient.
[0055] As used herein, "substrate" means any solid surface on which
the material or film of the invention is deposited. This material
can be made of any electronically conducting material. Examples of
material types include metals, alloys, ceramics, composites and
conducting polymers.
[0056] Preferably, the substrate material is chosen from Pt or
Pt/TiO.sub.2/SiO.sub.2/Si.
[0057] For further substrate variants, see W099/53972 and
W099/54266, which hereby is incorporated as a reference.
[0058] Air, moisturized air, oxygen gas, moisturized oxygen gas,
moisturized nitrogen gas and vacuum up to ultra high vacuum are
alternative atmospheres that can be used in the sol-gel method of
the invention.
[0059] Any method of deposition of NKN precursors on the substrate
can be used in the process. For example, dip, spin and spray
coating. Dip or spin coatings are preferred.
[0060] Spin coating, for example, can be used to prepare films with
sol-gel technique. The alkoxide solution is then deposited on a
substrate that rotates with high velocity. A gel is formed as a
consequence of fast solvent evaporation and rapid reactions with
water from the surrounding air as a drop of alkoxide solution hits
the rotating substrate and the excess of alkoxide solution is
removed simultaneously. It is important to control the structure
and composition of the gel to prevent formation of cracks in the
film during drying and heat treatment. This is controlled by
precursor selection, solution concentration, rotation speed, air
moisture, solvent selection and thermal treatment and
atmosphere.
[0061] For a specific application, the preferred film thickness may
e.g. be 0.1-10 .mu.m. Single or multilayer deposition can be used.
Multilayer deposition can give 20-300 nm NKN per deposited layer.
The number of layers will determine the total film thickness.
[0062] The material of the invention can be used in several ways in
the medical field. Due to its strength and durability the material
can be used as a rigid implant material in either bulk form or in
the form of films or layers covering other materials.
[0063] The material according to the invention is easily polarized
as is well known in the art in order to make it piezoelectric and
thus can be used for several different implant applications, such
as pressure sensors, accelerometers measuring patient motion or
motion of a cardiac wall (force measurement of heart strokes)
[0064] The invention will now be further illustrated by way of
examples. These examples are only intended to exemplify the
invention, and should therefore not be regarded as limitations of
the scope of the invention.
EXAMPLE
Sol-Gel Preparation of Na.sub.0.5K.sub.0.5Nb0.sub.3 Films
EXAMPLE 1
Sol-Gel-Synthesis of Na.sub.0.5K.sub.0.5Nb0.sub.3
[0065] Two synthesis paths have been used: preparation of the
binary alkoxides separately and then mixing them or preparation of
one solution with the precursors mixed directly. The later way was
used for film preparation, while the first way was used for the
temperature study of the phase transitions as described below). The
phase evolution from gel to oxide has been studied on air
hydrolyzed powders and the films have been manufactured using
spin-coating.
(A) Preparation of Precursors
[0066] Alkoxides like NaNb(OEt).sub.6 and KNb(OEt).sub.6 are very
sensitive to moisture and also to some extent to oxygen.
Precautions must therefore be taken during handling and preparation
of these compounds. The preparation must be done in inert
atmosphere and all glassware, solvents and starting materials were
dried prior to usage. The synthesizes were done in an Ar(g)
glove-box. The solvents ethanol and toluene were distillated over
CaH.sub.2 in nitrogen atmosphere to remove water. It can be noted,
that simpler equipment and drying processes were used during
process development. The binary alkoxides were prepared from
Na-metal, K-metal and Nb(OEt).sub.5. The weighing of blank Na and K
were also made in the glove-box. Furthermore, the concentration of
Nb(OEt).sub.5 was determined before usage. This was done by
gravimetric analysis of Nb.sub.20.sub.5 after hydrolysis and
heating to 1000.degree. C. for 1 h of an exact volume of the
Nb(OEt).sub.5 solution.
[0067] Synthesis of NaNb(OEt).sub.6 and KNb(OEt).sub.6 were made as
described below:
NaNb(OEt).sub.6: 0.500 g (0.0218 mol) Na was dissolved in 20 ml
toluene:ethanol 1:1 during hydrogen gas evolution according to:
Na+EtOH.fwdarw.Na(OEt)+0.5 H
6.915 g (0.0218 mol) Nb(OEt).sub.5 was then added to the solution
and it was stirred over night. The solvents were evaporated under
reduced pressure after that and white crystals of NaNb(OEt).sub.6
were obtained. KNb(OEt).sub.6: 1.000 g K (0.02558 mol) was
dissolved in 20 ml toluene:ethanol 1:1 during hydrogen gas
evolution according to:
K+EtOH.fwdarw.K(OEt)+0.5 H.sub.2
8.132 g (0.02558 mol) Nb(OEt).sub.5 was then added and the solution
was stirred over night. The solvents were evaporated under reduced
pressure after that and white crystals of KNb(OEt).sub.6 were
obtained.
[0068] For the preparation of Na.sub.0.5K.sub.0.5NbO.sub.3 a
solution of (Na.sub.0.5K.sub.0.5)Nb(OEt).sub.6 in toluene:ethanol
1:1 has been used. This is easily prepared by dissolving NaNb(OEt)6
and KNb(OEt).sub.6 crystals, or directly, without preceding
crystallization, by dissolving equal moles of Na and K together
with toluene:ethanol and then adding Nb(OEt).sub.5.
(B) From Gel to Oxide--Phase Development Studies of Powder
(i) Gel
[0069] The gels are light yellow and amorphous according to X-ray
diffraction (XRD). IR spectroscopy studies show that the gels do
not contain any organic groups, i.e. all alkoxide groups have
reacted during the hydrolysis reactions and the solvents have
evaporated. IR spectroscopy (4000-450 cm.sup.-1) shows that the
gels contain water and carbonate groups (FIG. 2). The later
originates from C0.sub.2 adsorption from air. There are absorption
bands from M-O stretching modes, mainly from Nb--O, in the range
around 1000 cm.sup.-1.
(ii) Heat Treatment
[0070] To study the phase transitions during heat treatment it is
more effective to study powder than thin films, since TGA can be
used to determine at which temperatures reactions take place and
also indicate at what temperature pure oxide is formed. XRD of
powder can also more easily be done at high resolution compared to
thin films and IR studies can also be used to describe the chemical
content.
[0071] Heat treatment to transform gel powder to oxide has been
studied with thermogravimetry (TG) to 1000.degree. C. in air and
oxygen, using different heating rates. Both new and aged gels were
studied. The gels were prepared using hydrolysis in air: the
alkoxide solution was deposited in a thin layer on a glass
substrate and allowed to react with moisture in the air. Some gels
were also treated with water vapor. TG studies of air hydrolyzed
binary K--Nb and Na--Nb gels are compared in FIG. 3.
[0072] The TG investigations show that the weight loss for
Na--K--Nb gels occur in three steps: (i) the gel loses 10-12% of it
weight in the first step from room temperature to ca. 170.degree.
C., (ii) a smaller weight loss, approximately 2-4% takes place from
ca. 170.degree. C. to ca. 500.degree. C., and (iii) the last steep
slope occurs between 500 and 550 or 580.degree. C., and the sample
loses <0.5% of its initial weight from 600 to 1000.degree. C.
This pattern is the same regardless of hydrolysis path, heating
rate and atmosphere used. On the contrary, water vapor treatment of
the gel affects the size of the weight loss in the last step: air
hydrolyzed gels then lose ca. 7% between 500 and 550.degree. C.,
while gels treated with water vapor lose only ca 4% from 500 to
580.degree. C.
[0073] The air hydrolyzed K--Nb gel shows the same behavior as
Na--K--Nb gels up to 400.degree. C., followed by a weight loss of
ca. 4% in two steps (2% in each step) taking place from 400 to
550.degree. C. and from 620 to 700.degree. C., respectively, and
thereafter the curve flattens out. The Na--Nb gel shows the same
behavior as the Na--K--Nb gels up to ca 320.degree. C., and then
losses ca 7% in three steps from 320 to 600.degree. C., followed by
a further weight loss of 1% between 600 and 1000.degree. C.
[0074] This shows that the K-Nb gels decomposition occurs faster
together with the Na--Nb gel, indicating good homogeneity, since a
bad mixing of the two gels would give raise to the step at
700.degree. C. that only occurs in K--Nb gel.
[0075] The air hydrolyzed Na--K--Nb gels have been heated to 170,
300, 500, 550, 600, 700 and 1000.degree. C., respectively, to study
the phase evolution during heating. These samples were then
analyzed by IR spectroscopy and XRD.
[0076] The IR studies (FIG. 4) together with the TG results show
that the two first steps up to 500.degree. C. originate from water
disappearance (by a combination of evaporation and condensation
reactions) and that the third step derives from decomposition of
carbonates to oxide and carbon dioxide. In some samples small
amounts of carbonates were still detected at 600.degree. C.,
however, these were completely removed at 700.degree. C.
[0077] The XRD results showed that the samples were amorphous up to
300.degree. C., and that crystallization had started at 500.degree.
C.; the diffractogram shows peaks from perovskite (similar to the
preferred NKN phase), however, unidentified peaks from one or more
other phases were also detected. The peaks from these additional
phase/s decrease and the peaks from perovskite increase with
temperature. Small peaks that do not correspond to perovskite are
discernible in the diffraction pattern from the samples heat
treated at 550 and 600.degree. C. These peaks are not detected and
pure perovskite is obtained at 700.degree. C. The perovskite peaks
are better defined after heating to 1000.degree. C., most likely
due to improved crystallization. No trace of additional phase/s was
observed for the sample treated at 1000.degree. C. (FIG. 5).
(C) Production of Films
[0078] Design of the temperature programs for film preparation was
based on the results from the powder studies. The films on flat
Pt--Pt/TiO2/SiO2/Si-substrate have been prepared by spin coating in
air (spin rate 3200-3300 rpm, time 40 s). Toluene:ethanol 1:1
solutions of (Na.sub.0.5K.sub.0.5)Nb(OEt).sub.6 with the
concentrations 0.3 and 0.6 moldm.sup.-3 were used.
[0079] A gel film is formed as a result of solvent evaporation and
reactions with water in the air during spin coating. From the phase
development studies "using powders, it can be concluded that the
gel needs to be heated to at least 700.degree. C. to form pure
perovskite. The gel was therefore transformed to oxide by heating
in air to 700.degree. C. Several different heating rates and
programs were tested.
[0080] The oxide films were investigated using Scanning Electron
Microscopy (SEM) and X-ray Diffraction (XRD). SEM shows that the
film thicknesses were 150-160 nm and 60-70 nm, respectively for the
films prepared from a solution with the concentration 0.6
mol.dm.sup.-3 and 0.3 mol-dm.sup.-3, respectively (FIG. 6). SEM
also shows that the films consist of two types of crystals (FIG.
7), indicating that two different phases have been formed. The
formation of two different phases in the films is confirmed by XRD
(FIG. 8). The perovskite phase was identified together with a
varying amount of (Na,K).sub.4Nb.sub.6O.sub.17 in all films.
[0081] The heating program (A) that gave a high phase purity was
used to prepared thick films by multi layer deposition of 0.5
mol-dm-3 on thin Pt substrates. Heating program A consists of the
following steps: heating from room temperature to 300.degree. C. at
10.degree. C.-min.sup.-1, annealing at 300.degree. C. for 50 min,
heating from 300.degree. C. to 700.degree. C. at 20.degree.
C.-min.sup.-1, annealing at 700.degree. C. for 60 min and free
cooling to room temperature. The program has a plateau at
300.degree. C. to improve the condensation reactions without
crystallization of the gel. This was followed by fast heating to
700.degree. C., passing the critical 500.degree. C. range rapidly.
The phase evolution studies showed that the
(Na,K).sub.4Nb.sub.60.sub.17 phase was found to be formed in the
500.degree. C. range and that a pure and carbonate free oxide was
formed at 700.degree. C. To achieve a complete decomposition of all
carbonates and a higher density the films were sintered at
700.degree. C. for 60 min. The heating program A gave significantly
smaller amounts of the (Na,K).sub.4Nb.sub.60.sub.17 phase, see
FIGS. 8 and 9.
[0082] It can be noted, that tests using 90 or 30 min annealing at
300.degree. C. gave inferior result compared to program A.
Furthermore, results indicate that a longer annealing time (2-10 h)
at 700.degree. C. improves the phase purity further.
[0083] Although modifications and changes may be suggested by those
skilled in the art, it is the intention of the inventors to embody
within the patent warranted hereon all changes and modifications as
reasonably and properly come within the scope of their contribution
to the art.
* * * * *