CA2257582A1 - Apparatus and procedure for depositing materials - Google Patents

Abstract

Material is deposited onto a surface, such as a tooth or a bone, to repair or restore the surface. The material is delivered in powdered form and melted by application of a laser. The molten material is entrained in a high velocity fluid stream, typically a gas such as air, and directed to the surface to be treated. The molten material solidifies. The materials include hydroxyapatite, alumina and silicon dioxide.

Description

CA 022~7~82 1998-12-04 APPAR~TUs AND PROCEDuRE FOR DEPOSITlNG MATERIALS

The present invention relates to deposition of materials.
It is well known to apply coatings to protect or repair selected areas of an 5 object. The coatings typically have a physical characteristic that protects or enhances the object being coated.
Such coatings have been applied by thermal spraying techniques including DC
plasma spraying, E~F plasma spraying and flame spraying techniques. The principle of plasma spray deposition is to feed powder materials into a high pressure plasma where 10 it is both heated and accelerated by the high temperature, high speed gas stream. The deposition of the coatings therefore involves the use of relatively high temperature gas streams rendering it unsuitable for temperature critical applications.
A particularly temperature-sensitive application involves in vivo application ofmaterial to selected areas of the body. It is well known to treat voids or cavities in 15 teeth by applying a filling material which solidifies and inhibits further decay.
Conventionally, such materials have been a mercury-based dental amalgam but morerecently resin-based materials have been used in place of the metal arnalgams. The use of resin-based materials is preferred but there are some indications that they may not be sufficiently long-lasting to effect a permanent repair.

2 0 It has previously been proposed to enhance the biocompatibility of a dental implant or internal prosthesis by applying a coating of hydroxyapatite (HAp) to the implant. This is conventionally done by a plasma spraying technique. HAp is recognized to be one of the best biocompatible materials and is a main inorganicconstituent of hard tissues such as bone or teeth. The deposition of such materials in 2 5 vivo using conventional techniques would however cause a relatively high temperature plasma to impinge upon the tooth leading to severe discomfort for the patient.
It is an object of the present invention to obviate or mitigate the above disadvantages.

3 0 In general terms, the present invention provides a method of depositing a material onto a surface comprising the steps of delivering the material in powder forrn to a heating zone, melting said material at the hating zone and entraining the said material in a molten state in a fluid stream to deposit said material onto said surface SUBSTITUTE S~lEET (RULE 26) CA 022~7~82 1998-12-04 and allowing said material to solidify on said surface In a preferred embodiment for in vivo treatment of teeth, it is preferred to utilize a laser to melt the restorative material for plasma spraying.
Alternatively, a die may be prepared having a surface identical to that of the 5 body part and restorative material plasma-sprayed onto the surface to form a body of filling material that conforms to the cavity. The material may then be removed and inserted into the prepared tooth.
In a further embodiment, a ceramics material may be sprayed onto a bone to repair pathologic defects in the bone.
In the dental environment, this proposed process offers distinct advantages over conventional restorative materials and methods, such as mercury cont~ininE~amalgam alloys, composite resins and laboratory fabricated indirect restorations such as full or partial crowns and coated implants. By lltili7ing the proposed process in the dental environment, a number of advantages may be realized, namely:
(i) Ceramic, tooth coloured or metal restorations could be used in place of mercury-based dental ~m~lg;lm;
(ii) Superior physical properties may be achieved compared to alternative materials, specifically composite resins;
(iii) The need for laboratory processed dental restorations may be elimin~te~l; and (iv) Time and cost savings in fabricating restorations may be obtained.

Embodiments of the invention will now be described by way of example only, with reference to the accompanying drawings, in which 2 5 Figure 1 is a sectional schematic view of an apparatus for the application of plasma-sprayed restorative m~teri~l;
Figure 2 is a schematic reF sentation of an apparatus for the preparation of a restorative insert;
Figure 3 is a schematic representation of a further embodiment of the 3 0 a~al~us of Figure 1;
Figure 4 is a still further embodiment of the apparatus of Figure 1; and Figure S is a yet further embodiment of the ~p~lus of Figure 1.
SUBSTITUTE S~IEET (~ULE 26) CA 022~7~82 1998-12-04 Referring therefore to Figure l, an intra-oral apparatus for repairing cavities in teeth generally indicated at 10 includes a housing 12 having an elongate body 14 and a conical head 16. The body 14 encloses a power supply assembly 18 and a laser 20 driven by the power supply assembly 18.
Interior walls 26 of the conical head 16 defme a no7zle directed towards a prepared cavity 28 of a tooth 30. The walls 32 of the head 16 have internal ducts 32 which can direct cooling air from a remote source (not shown) to impinge upon the cavity 28.
Restorative materials, such as a}uminum oxide or hydroxyapatite ~Ap, in 1 0 powder forrn, are conveyed to the interior of the body 12 through a powder passage 34 formed in the body 14 and which terminates in a delivery tube 36. The powder is transported through the passage 34 by entrainment in a stream of air so that air in the passage 34 induces the flow of powder from the delivery tube 36 and carries it through the head 16.
1 5 The laser 20 produces a beam of coherent radiation that impinges upon the stream of powder being delivered by the delivery tube 36 so that it is melted by the incident laser radiation. A mirror 40 is provided to reflect the beam and provide a dynamic crucible in which the restorative material is melted.
The device 10 is operated such that the restorative material is preferably still2 0 liquid upon impingement upon the tooth, adheres to the tooth and solidifies on the tooth 30 so that a stable solid layer may be built up within the cavity.
Intermittent cooling can be provided by the application of air through the jets 32 onto the tooth so that heat transferred to the tooth 30 is minimi7e~1 Moreover, since the laser beam does not impinge on the tooth but only on the material applied to 2 5 the tooth, direct heating of the tooth is avoided.
The laser 20 may be of a convenient power rating and wavelength to melt the dental restorative materials such as carbon dioxide lasing devices or YAG lasingdevices. The restorative material may be, for example, a porcelain, hydroxyapatite, alumina, silicon dioxide, ~m~ m, ceramic, gold, silver, palladium, chromium cobalt 3 0 or titanium.
To facilitate the bonding of the restorative material to the cavity 28, the surface of the tooth 30 may be acid etched or acid etched and primed with a bonded SUBSTITUTE S~ IEET (I~ULE 26) CA 022~7~X2 1998-12-04 resin prior to plasma spraying.
To facilitate melting, the restorative materials and the laser wavelength shouldbe chosen to utilize the optical absorption property of the m~teri~l. As shown in Table l below, absorption of incident radiation varies significantly with wavelength.

Table 1 Absorption property of four dental materials Absorption (%) -Material (~ m~10 ,um Hydroxyapatite (HAp) 5 92 . 3 Al203(50 ~m) 17 97 . 4 SiO2(80 ,um) 6 9 8 7 amalgam 4 9 31 For hydroxapatite, a GO2 laser with a wavelength in the order of l 0.6 llm would be optimum provided it can be delivered in a practical manner. The absorption characteristics of HAp also provide a local peak at about 3.0 ~,lm so that Er:YAG or Er:YSAG lasers are also suitable. In addition, Holmium (Ho) YAG lasersare also possible for use in the present invention.
In feasibility investigations, a powder was projected from a spray gun onto a glass substrate from a distance of 5 mm. A laser beam was positioned parallel to the surface of the substrate to impinge the powder. The bearn was spaced less than l mm from the surface of the substrate.
Three lasers having the characteristics set out in Table 2 below were tested.
Table 2 Laser system parameters CW Nd:YAG laser Pulsed Nd:YAG laser CO2 Laser Optical Power2W 2 J/pulse~ l 0 Hz, 20 W av., 20 W
200 MW pk SUBSTITUTE~ S~lEEt (RULE 26) CA 022~7~82 1998-12-04 Wavelength 1.064 ~lm 1.064 llm 10.6 ~m Focal Area ~ 0.5 mm ~ 0.5 mm ~ 0.5 mm The results obtained are shown in Table 3 for a spray time of 5 sec., a spray gas of air and a gas speed of < 5 m/sec.

- Table 3 Dental material deposits on glass substrate Laser SiO2 Al2O3 Hap Am~lg~m CW Nd:YAG ! E
Pulsed E E E
Nd:YAG
Focused CO2 E E E
Unfocused 10 ! = not molten; E = a little molten; - = no deposit.
It will be observed that melting of the silicon dioxide, aluminum oxide and HAp was obtained. A relatively low particle velocity resulted in a poor binding of the particles. However, when the air velocity was increased, homogeneous micro crystals were obtained using the pulsed Nd:YAG laser. With a particle size of 50 llm, air1~ velocities greater than 50 m/sec are desirable and homogeneous microcrystal structures were obtained with an air stream having a velocity profile of between 50 m/sec and 149 m/sec with an average velocity of 100 m/sec. Air velocities up to 300 m/sec may be used with velocities in the range of 200 m/sec to 300 m/sec preferred.
While the device has been shown as a self-contained hand-operated unit, it is 2 0 feasible that certain of the components, for example the power supply assembly 18 and the laser 20, might be located remotely to reduce the bulk of the device. In that case, the laser radiation may be delivered through an optic fibre or light pipe.

SUBYI~U~E S~l~L~ UL~ 26) CA 022~7~82 1998-12-04 Although the above embodiments are described with respect to dental restoration, it will be appreciated that similar techniques may be utilized to deposit Hap, for example, directly onto bone in vivo for orthopaedic purposes, to repairpathologic defects and possibly fuse fractured bones and elimin~te the need for casts, 5 splints or metal plates or rods to hold the fractured bones in place while healing occurs. In this application, a suitably configured device 10 is held adjacent the area to be repaired or reconstituted and biocompatible material is deposited onto the existing bone structure. Such a method could result in immediate loading of the bone with the supporting ~Ap deposit eventually being absorbed and remodelled.
Other biologically active materials such as polymers for dermatologic purposes might be deposited in a similar manner permitting a cut or abrasion to be sprayed with a suitable polymer, with antibiotics and other healing inducing pharmacologic chemicals added, to effect an artificial patch or scab which will slough off as the underlying epithelium heals.
An alternative embodiment is shown in Figure 2 in which like reference numerals will denote like components, with the suffix ~a' added for clarity. In the embodiment of Figure 2, a tooth die 30a is located within an evacuated chamber 42 which has an opening 44. The die 30a has a cavity 28a which has been modelled from a cavity prepared in the tooth to be repaired. An electric arc plasma-spraying unit 42, 2 0 including nozzle 26a and a powder delivery duct 36a, is connected at the opening 44 and plasma sprays the restorative material at the prepared cavity 28a. The restorative material delivered as powder through duct 36a is melted by an electric arc applied at the opening of the vacuum chamber so that melted liquified spray of restorative material is directed at the cavity 28a and plasma sprayed onto the die 30a.
2 5 Accordingly, the cavity 28a is filled with an insert of restorative material which, upon solidification, may be removed, fini.~h.od and inserted into the prepared cavity in the tooth where it is bonded in position.
One advantage to this process is that, in a vacuum chamber, the insert would have less porosity and be easier to finish after insertion in the tooth. Moreover, some 3 0 materials, such as metals, are difficult to melt with lasers and must be heated with electric arc plasma spraying units. This technique therefore permits the metal restorative insert to be ~ ~ed without exposing the patient to the high temperature ~UB~TITlJT~ s~ r~ ULE 26) CA 022~7~82 1998-12-04 plasma.
As noted above, deposition is enhanced with the velocity at which the melted restorative material may be deposited on the substrate. ~n embodiment of the apparatus to facilitate deposition is shown in Figure 3 where like reference numerals - 5 will be used to denote like parts from Figure 1, with a suffix 'b' added for clarity.
In the embodiment of Figure 3, powdered restorative material is fed from a reservoir 45 by means of an air feed 46 under relatively low velocity. The air powder mixture is fed into a powder passage 34b which termin~te~ at a delivery tube 36b.
The passage 34b is aligned with a laser 20b and has a mirror 40b at the intersection of 1 0 the supply passage 34b and delivery tube 36b. The beam of coherent radiationemitted by the laser 20b passes through an optical window 48 provided at the end of the powder passage 34b.
A high velocity fluid air stream, typically air or other suitable gas, is introduced through a passage 50 into the delivery tube 36b. The high velocity air 1 5 delivered through the passage 50 entrains the powder carried in the passage 34b and projects it onto the substrate 30b with sufficient velocity to cause splattering and a coherent homogeneous micro crystal structure.
The ~lignment of the laser beam with the powder passage 34b permits the laser to impinge upon the powder over a relatively prolonged period, thereby permitting 2 0 sufficient time for energy transfer and melting of the restorative material as it is delivered to the delivery tube 36b. The high pressure air therefore impinges upon the material in a molten state for delivery to the substrate 30b.
If preferred, the laser beam may be delivered to the powder passage 34b by a light tube or optic fibre if the wavelength permits so that the laser 20b may be located 2 5 remotely from the device 1 Ob.
The separation of the high pressure air from the material supply permits the required high velocity of deposition while at the same time permitting adequate time in the delivery of the restorative material for heat transfer to the material to be facilitated. Moreover, by al,prop.iate selection of the carrier gas, no heat is transferred 3 0 to the carrier gas so that thermal transfer to the substrate 30b is minimi7e(1 To minimi7~ heat transfer, the carrier gas should exhibit a low absorption to the incident radiation and be of relatively low thermal conductivity, e.g. argon rather than air.

SUBYITUTE SHE~T (RULE 26) .

CA 022~7~82 1998-12-04 A similar arrangement is shown in Figure 4 in which the laser 20 is replaced with an incandescent power source. Referring therefore to Figure 4 in which likecomponents will be identified with like reference numerals with a suffix 'c' added for clarity, restorative material is transferred from a reservoir 44c by means of a feed 5 supply 46c. The material is supplied to the powder passage 34c located within housing 12c. A high pressure air supply duct 50c delivers high velocity air to the delivery tube 36c for deposition on the substrate 30c.
Located within the body 12c is an incandescent tungsten bulb 52 which is located within an elliptical cavity 54 formed in the housing 12c. The supply passage 1 0 34c is transparent to the radiation emitted from the tungsten bulb 52 and is located at the focal point of the elliptical cavity 54. The radiation from the bulb 52 is thus directed to the powder passage 34c to melt the restorative material within the passage 34b. As it is delivered to the delivery tube 36c, it is impinged by the high pressure air 50c for deposition on the substrate 30c.
1 5 As shown in Figure 5, in which like elements will be identified with like numerals and a suffix 'd' added for clarity, an alternative configuration of powder passage 34d is adopted. In this arrangement, the powder passage 34d is wound about the tungsten bulb 52d to increase the time during which the restorative material is exposed to the bulb 52d. The powder passage 34d adopts a linear configuration 2 0 adjacent the delivery tube 36d to minimi7P the amount of melted material sticking to the walls of the powder passage 34d. Again, the high pressure air stream 50d impinges upon the molten material delivered to the delivery tube 36d for deposition on the substrate 30d.
Obviously the geometry of the delivery system will depend upon the nature of 2 5 the material, the source of radiation, and the effective heat transfer to the material.
Such heat transfer can be effected as noted in the examples above, even using the direct impingement of the beam on the powdered material in the embodiment as it is delivered to the delivery tube 36 in the embodiment shown in Figure 1.
If the speed of the filling material particles being deposited onto the substrate 3 0 needs to be greater than v = 100 m/s to ensure enough impact (spl~ttering) upon the substrate and to form a well-developed deposit and a focused laser source of 0.5 mm meter focal spot is used to obtain high energy density, the actual time that particles SU~STITUTE StlEET (~UL~ 26) CA 022.,7.,82 1998 - 12 - 04 are under laser radiation in Figure I is less than:

t = (focal spot diameter)/v = 0.5 mm/100 (m/s) = 5 lls The volume of an HAp particle is:

V = ~ d3/6 = 7~ x (50 ~lm)3 /6 = 65449.8 ~lm3 = 6.545 x 10-8 cm3 The cross-section area of an HAp particle is:

S = 7~r2 = ~ x (25 llm)2 = 1.96 x 103 }lm2 = 1.96 x 10-5 cm2 For the HAp particle to be molten during this time period, the minimum energy needed is:
E = (Tm - Troom) x C x V x D/w where Tm is the melting point, Troom is the room temperature, C is the heat capacity, D is the density, and w is the molecular weight of ~IAp which is equal to:
w = (40.1 xlO) + (31x6) + (16x26) + (lx2) = 1005 (g/mol) Therefore, the energy needed to melt the restorative material is:

2 5 E = (1614~C - 25~C) x 184.7 cal/(~C-mol) x 6.545x108 x 3.16 g/cm3 . 1005 g/mol =
6 x 10-5 cal = 2.4 x 10~ J (note: 1 cal = 4.186 J) The optical power needed to melt the restorative material is:

P=E/t=2.5x lO~J/5 ~1s=50W

for a particle diameter of 50 llm (a 500 ~lm particle would require 100 times SUBSTITUTE ~EE~ ~Rl:ll ~ 26~

, CA 022~7~82 1998-12-04 this power).

The power density should be:

I = P / S = 50 W / (1.96x105 cm2) = 2.55 MW/cm2.

On the other hand, the spot size of the pulsed Nd:YAG laser is:

SYAG = ~ x (0.25 mm)2-= 1.96 x 10-' mm2 = 1.96 x 10-3 cm2 and the fluence rate while pulsing is:

FYAG = s J / SYAG = 1-02 kJ/cm The energy deposited on a single particle is:

EHAP = FYAG x S = 1.02 kJ/cm x 1.96 x 10 cm = 2 x 10 J

Since the absorption of HAp is about 5% at this wavelength, the energy 2 0 absorbed by the particle is:

A"Ap=5%x2x 10-2J= 1 x 10-3J

Because AHAp is larger than E calculated above (1 x 10-3 versus 2.4 x 10~), it is 2 5 practical for HAp particles to be melted with the pulsed Nd:YAG laser.
It should be noted that the energy-temperature equation is based upon complete thermal isolation of the m~ten~l. In real situations, heat loss is present due to the thermal conductivity of the carrier gas. Accordingly, a carrier gas of low thermal conductivity, e.g. argon rather than air, may be preferred. Also, it is assumed 3 0 that there is no phase changes (such as melting or evaporation, which consumes energy).
It will be seen therefore that the al ,u~lus shown in Figures 1-5 provides a SUBSTlTUTE StlEET (PlULE 26) CA 022~7~82 1998-12-04 practical means of depositing restorative material directly onto the body part in vivo without excessive thermal transfer to the body part.
Similar apparatus may be used to deposit coatings on other objects, such as electronic components or other industrial items where the object is temperature 5 critical. The coating material will be selected to match the re~uirements of the object and will be delivered in powder form to the furnace. The carrier gas will thus entrain the melted powder and deliver it to the surface to be coated.
In a further embodiment, the method of the invention can be used for plasma spraying coatings onto parts used for aircraft. By avoiding an increase in 10 temperatures of the substrate as with the present invention, it is possible to reduce the fatigue of materials resulting from being heated with known plasma spraying methods.

SU~STITUTE ~ ET (~717 26) ,

Claims (18)

We Claim:

1. A method of in-situ deposition of hydroxyapatite into a dental cavity comprising the steps of delivering said hydroxyapatite in power form to a heating zone, melting said hydroxyapatite at said heating zone by imparting energy to said hydroxyapatite from a laser beam, entraining said hydroxyapatite in a molten state in a fluid stream, depositing said entrained hydroxyapatite into said dental cavity and allowing said hydroxyapatite to solidify in said dental cavity.

2. The method according to claim 1 wherein said laser beam is directed transverse to said fluid stream.

3. The method according to claim 1 wherein the laser is selected from the group comprising CO2, Er:YAG and Er: YSAG lasers.

4. The method according to claim 1 wherein said fluid stream is delivered to said melting zone separately from said hydroxyapatite to inhibit heat transfer to said fluid stream.

5. The method according to claim 1 wherein said fluid stream entrains said hydroxyapatite at a velocity greater than 50 m/sec.

6. The method according to claim 5 wherein said fluid stream entrains said hydroxyapatite at a velocity greater than 300 m/sec.

7. An apparatus for depositing a material onto a surface comprising a housing, a cavity within said housing, a material supply duct to deliver material to said cavity, a laser directing a laser beam along a portion of the material supply duct and across said cavity to elevate the temperature of material in said material supply duct and said cavity and the cause said material to melt a conduit to deliver a fluid stream to said cavity, said fluid stream entraining molten material in said cavity and carrying said material through an outlet in said cavity toward said surface.

8. The apparatus according to claim 7 wherein said fluid stream in transverse to said laser beam.

9. The apparatus according to claim 7 wherein said material is delivered through said material supply duct as a power.

10. The apparatus according to claim 9 wherein said power is fluidized in said material supply duct.

11. The apparatus according to claim 7 wherein the laser is selected from the group comprising CO2, Er:YAG and Er:YSAG lasers.

12. An apparatus for depositing a material onto a surface comprising a bousing, a cavity within said housing, defined by at least one wall having internal ducts therein to direct cooling air from a remote source to impinge on said surface, a material supply duct to deliver material to said cavity, an energy source to elevate the temperature of material in said cavity and to cause said material to melt a conduit to deliver a fluid stream to said cavity, said fluid stream entraining molten material in said cavity and carrying said material through an outlet in said cavity toward the surface.

13. An apparatus according to claim 12 wherein said energy source is a laser directing a laser beam across said cavity.

14. An apparatus according to claim 13 wherein said laser beam extends along a portion of said material supply duct.

15. An apparatus according to claim 13 wherein said fluid stream is transverse to said laser beam.

16. An apparatus according to claim 13 wherein said material is delivered through said material supply duct as a powder.

17. An apparatus according to claim 16 wherein said powder is fluidized in said material supply duct.

18. The apparatus according to claim 12 wherein the laser is selected from the group comprising CO2, Er:YAG and Er:YSAG lasters.