METHOD AND APPARATUS FOR SUPERFICIAL SKIN HEAT TREATMENT

Abstract: A method and apparatus for depth controlled conductive heat treatment of superficial layers preferably biological tissue such as the epidermal layer is disclosed. The method of the invention is to press a thin radiation absorptive layer attached to a bulk substrate with low thermal difussivity in direct (or indirect via a protective overcoat layer on the absorptive layer) in physical contact with the object surface to heat. The cross section area of the absorption layer is typically 1cm2. The absorptive layers is firmly attached to an optically transparent bulk substrate that is cooled in a controlled manner in order to remove excessive heat from the absorbing layer and bulk controlled manner in order to remove excessive heat from the absorbing layer and bulk substrate. In a preferred embodiment the absorptive layer is heated by the radiation from a modulated or pulsed in-coherent electromagnetic-source preferably a pulsed arc-lamp. Typical pulse duration is 0.5-50 ms with a repetition frequency of 0.5-5hz. The depth of the heat affected zone in the tissue is fine tuned by optimization of the duration time of the electromagnetic radiation pulse and the amount of energy in the pulse from the flash lamp source and the pre-pulse cooling via the waveguide.

Description: METOD AND APPARATUS FOR SUPERFICIAL SKIN HEAT TREATMENT FIELD OF THE INVENTION The present invention relates to an apparatus and method of superficial heat treatment of biological tissue preferably heat treatment and coagulation of the epidermal layers-stratum cornea, stratum lucidum and stratum granolosum. Selective heat treatment of the epidermal layers is made for a number of reasons such as wrinkle removal, non-permanent hair removal, soft facial skin rejuvenation, enhancement of percutaneous drug administration and treatment of psoriasis, among others. BACKGROUND OF THE INVENTION The prior art technology for coagulation, evaporation and ablation of the epidermal layers is based on direct interaction of coherent laser radiation with the tissue. However, only a few types of laser sources are suitable for depth controlled tissue removal. The problem is that the extinction depth of the laser wavelength in the epidermal layers must be in the range of a few ϊms or less in the tissue to achieve a superficial absorption of the radiation. The most commonly used lasers for superficial tissue removal are EπYAG and excimer laser at with emission wavelength of 2940 nm and 193 nm respectively. The corresponding extinction depths are 3-5 im (in water) and 0.1 im (in protein). The CO2 laser with emission wavelength of 10.6 ϊm is also used to some extent for tissue removal but due to the large extinction depth 30-50 im (in water) there are problems with blistering and bleeding in the deeper tissue layers which requires a recovering period of 2-6 months. This is also the problem with the EπYAG laser even though less pronounced. The reason for this may be explained by the fact that stratum corneum (thickness 15-30 im) consists basically of keratin with a quite low water content under normal conditions. The extinction coefficient of e=2940 im in keratin may be considerably larger than 3-5 im in water which may explain these unexpected and undesirable results. The only laser type which can perform precise and predictable removal of epidermal layers is the excimer laser with emission wavelength of 193 nm. Excimer lasers are very expensive and tedious to work with. Excimer lasers at 193 nm are also less suitable for beam delivery through standard fiber optical links due to quite high attenuation in waveguide based on fused silica (quartz) glass. The long term effects of exposing living cells to UV radiation is not fully investigated. There maybe an elevated risk of developing cancer associated with excimer laser treatment. Laser sources in combination with contact probes with radiation absorbing layers are in principle possible to use for superficial tissue removal. The use of a totally radiation absorbing layer covering a part of the surface of the probe will eliminate the problem with the extinction depth since all of the thermal energy will be transferred by conduction to the tissue. The contact probes aimed for lasers normally have a very small effective area typically only a few square millimeters. There are several reasons for making these laser fed contact probes with such small surfaces areas. One important aspect is that the pulse energy from lasers is limited due to the low electro optical efficiency of used lasers. Typically only a few percent of the energy required for laser pumping is converted to laser radiation energy. Therefore laser contact probes with radiation absorbing layers are less suitable for large area treatment. Another important problem related to the use of lasers for large area heat treatment is the inherent formation of speckle and mod patterns due to the coherency of laser radiation. These phenomena's are present even after the passage through an optical waveguide. Laser radiation guided through an circular optical fiber often show an annular ring formed intensity pattern with overlapping speckle spots. Use of waveguides with other types of cross section geometry e.g. rectangular one will not reduce mode pattern problem. Thus, the combination of lasers with contact probes are not suitable for treatment of large surfaces using a large area contact probe surface since uniform heating is very important. Generally, laser light sources are much more expensive in comparison with arc lamp sources. Other common methods used for superficial skin removal are mechanical dermabrasion and chemical peeling which both suffer from lack of depth control and predictability. SUMMARY OF THE INVENTION In general, the invention features a method of selective coagulation and shrinking of the epidermal layers, stratum cornea, stratum lucidum and stratum granolosum, without significantly damaging the underlaying tissue layers. The method of the invention is based on skin surface contact heating probe with a radiation absorbing layer. The absorbing layer is heated in a controlled manner by the concentrated radiation from a pulsed in-coherent arc lamp. The radiation absorbing layer firmly attached to the distal surface of an bulk substrate that is brought in physical contact and gently pressed towards the skin in order to achieve a uniform contact surface. The bulk substrate should be optically transparent over the complete emission spectrum of the radiation source. The radiation absorbing layer is heated rapidly when illuminated by the concentrated incoherent radiation from a pulsed arc lamp. The complete emission wavelength spectrum (typically 0.2-2 im) generated by an arc lamp will contribute to the heating of the absorptive layer. By choosing coating material having an wavelength insensitive and high absorption coefficient matching the emission spectrum of the lamp the major part of the optical energy will be converted to heat in the absorptive layer. Thus, the overall energy efficiency compared to lamp pump lasers is improved considerably. Preferable choice of material for the absorbing layer having a high absorption coefficient in the range of a 0.2-3 im are carbon or a number oxides made from nickel, zirconium etc. The thickness of the absorbing layer should be in the range 1-50 im preferably in the range 2-10 im. If the material composition of the absoφtive layer is not tissue compatible a protective overcoat layer of alumina oxide or silicon oxide would be necessary. The thickness of the protective layer by minimized typically 1-5 im. To further improve the heat conduction from the probe surface to the skin a very thin layer of liquid or gel can be applied onto the skin before treatment with the contact probe. The viscosity of this contact fluid must be sufficiently low to admit excessive paste to be pressed out from the probe surface contact region since the thickness of the contact liquid layer must be less than 5-15 im. Examples of contact fluids are water, saline solution, or ultrasonic contact gel. The fluids may be based on silicone oils or also based on hyaluronic acids. The most preferable choice is a gel mixture compound made of water, denatured alcohol, propylene glycol, glycerin, sodium hydroxide, PEG40 hydrogenated castor oil, panthenol and carbomer and Xylocaine. The Xylocaine is a cutaneous anesthetic gel and will reduce pain during treatment. The depth control of the heat effected (coagulated) tissue is made primarily by regulation of radiation pulse duration time and controlling the power density of the radiation on the absorbing layer. The radiation power density is controlled by the regulation of the discharge current through the arc lamp. The power level is adapted for the specific optical geometry and probe surface area. A smooth heating of the absorbing layer is guaranteed by mixing the light from the lamp in an optical waveguide having a length of at least ten times the diameter of probe cross section which typically is 10 mm. The cross section of the wave guide is preferably round or rectangular having a cross sectional area of 0.5-5 cm 2 preferably 1-2 cm 2. A typical pulse duration time of the lamp is in the range 0.1-500 ms with a repetition rate of typically 0.5-5hz. The heat effected depth of the epidermal layer would correspond to pulse length range of 0.1-500 ms will be 7-500 im according to Figure 2. Used definition of heat effected depth is the distance form the (heated) surface to a point inside the tissue where the increment of temperature is app. 10% of tissue surface temperature increment at the end of the heat pulse. Assuming that the skin surface temperature just reaches 100°C (the water in the tissue then starts to boil and evaporate) at the end of the heat pulse-pulse length 0.1-500 ms - the corresponding coagulation depth would be 2-127 im, i.e. thickness of epidermis where the temperature is 70-72°C or higher, see Figure 4. The fundamental relationship between temperature distribution inside a bulk body that is surface heated with constant power density (W/m ) is given by the equation: T(z)=T(z=0) * V π * ierfc(z/(2* [(K*e)) : temperature profile ( 1 ) wherein K= e/(p*c) :thermal difussivity (m7s) (2) Equation (1) is valid for one dimensional heat flow which is true if the heat affected depth is much smaller than the cross section dimension of the surface heat source. T(z=0)=2*P/A* l/e*(K*e/TT) 2 Surface temperature vs. pulse time (3) P = power of radiation A = heated cross section area z = distance from heated surface e = heat conductivity of material p = density of material c = heat capacity of material To = T(z-O) s = 2* (K*e)' /2 ....(4) s = mean heat affected distance from surface due to heat diffusion z = distance from surface Eq. (1) gives the following relative temperature increments (see figure 3) for different values of z: z=0.5*s : distance from surface where the temperature has reached 35.4 % of the surface temperature. z=s : distance from surface where the temperature has reached 8.9% of the surface temperature. z=2*s : distance from surface where the temperature has reached 0.2% of the surface temperature. The total energy of the heat pulse per unit area is given by the equation E = P *e (5) Eq (5) with eq (2) and (3) gives and estimation of required pulse energy per unit area for pulse length e: The total energy required for heating both tissue and the bulk substrate on which the absorbing layer is attached gives the following equation : * (e*p*c) ,/2 *e' /2 [(e*p*c) t ' /2 +[(e*p*c) s Vl ] (7) (e*p*c) t 2 = "energy term" related to the thermal properties of tissue (water) (e*p*c) s 2 = "energy term" related to thermal properties of bulk substrate From eq (6) it is obvious that the term (e*p*c) 2 has great influence on required pulse energy for given values of surface temperature T 0 and the duration time of the heat pulse e. The two main choices of bulk substrate material for the absorbing layer are sapphire and quartz glass since both of these materials have very good resistance to thermal chock and are also optically transparent. The term (e*p*c) 2 for tissue (water), quartz glass and sapphire is 1530, 1499, and 6743 (Si-units) correspondingly. With these figures the obvious choice of bulk substrate for the absorbing layer is quartz glass since the required pulse energy in this case is only 37% in comparison with sapphire as bulk substrate. The thermal energy dissipated into the bulk substrate layer during each heat pulse must be drained away before the next pulse is fired. Otherwise the probe will preheat the tissue during the time before the next pulse is fired. Preheating of tissue is undesirable. A preferred embodiment of a heat zinc is to attach the uncoated side of the bulk substrate (quartz) to a sufficiently large body of sapphire which has a high heat conductivity. The sapphire body should continuously be cooled preferably using a peltier element in contact with the sapphire body. The sapphire body is positioned in the beam path. In a preferred embodiment the sapphire body is the distal end of an optical waveguide i.e. the sapphire body is apart of the optical system. If the bulk substrate attachment to the sapphire body is made in an appropriate manner it could be made a disposal part. For example, the surface opposite to the absorbing layer can be prepared with a tin adhesive film so that the bulk substrate easily can be replaced when a new treatment starts. A disposal probe bulk substrate prevents transfer of contagious infections among individuals. The thickness of the bulk substrate which preferably is made of quartz should be in the range 10-1000 im. The optimum thickness maybe calculated for a specific pulse repetition rate and pulse duration time. BRIEF DESCRIPTION OF THE DRAWINGS These and other objects of the present invention and various features and details of the operation and construction thereof are hereinafter more fully set forth with reference to the accompanying drawings, wherein: Fig. 1 is represents a schematic of the device to be used in the practice of the invention. In the above device, reference numeral 1 designates a pulsed electromagnetic source such a laser concentrated by an appropriate optical system 2. Reference numeral 3 represents a waveguide, while 4 is a disposable bulk substrate attached to the exit cross section of the waveguide. Element 5 is a cooling device such as a Peltier unit. Fig. 2 is a graph of the heat affected depth in the epidermis vs duration time of heat pulse. Fig. 3 is a plot of the relative temperature profile versus relative depth inside the bulk body. Fig. 4 is a graph of coagulation depth in epidermis versus duration time of energy pulse. Fig. 5 is a schematic representation of how the device of figure 1 is used to treat the epidermis. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. 1 and FIG. 5, the numeral 1 designates a pulsed electromagnetic source preferably a gas discharge lamp or a laser. The radiation emitted from the light source 1 is concentrated by use of an appropriate optical system 2. In the case where the electromagnetic source comprises a discharge lamp the light going in the backward direction is redirected towards the optical waveguide by the use of a back reflector system e.g. a parabolic or elliptical mirror. Other types back reflectors such as white diffuse ceramics can also be used if the dimensions of the light source is adapted to the size of the entrance geometry of the waveguide 3. When the light source is a laser the concentrating optical system normally comprises a lens array system. All radiation going in the forward direction is then concentrated as much as possible into the entrance surface of the waveguide 3. The typical cross section area of the waveguide entrance surface is 0.3-3 cm . The waveguide 3 is preferably made of crystalline sapphire having a round or rectangular cross section. Other geometry's may also be used such as elliptical ones. The choice of sapphire material in the waveguide is due to the high thermal conductivity of this material. The appropriate length of the waveguide 3 is in the range is 20-60 mm. A large cross section requires a longer waveguide since the main function of the waveguide besides pure radiation guidance is to smooth out the radiation field from the light source 1. A disposable bulk substrate 4 is attached to the exit cross section of the waveguide with thin layer of radiation transparent optical glue 4.6 a combination of optical matching liquid together with means for mechanical fixation of the bulk substrate. The complete bulk substrate should be able to exchanged after medical treatment of an individual. The dimensions of cross section geometry of the bulk substrate 4 should be made a little larger than the corresponding cross section dimensions of the waveguide 3 to avoid leakage of radiation. The bulk substrate 4 comprises of a sheet of quarts glass 4.1 and a stacked layer of coatings comprising an radiation absoφtive layer 4.2, preferably a sputtered layer of carbon or oxide film having a thickness of 1-10 im and a non toxic and tissue compatible layer preferably made of polycrystalline sapphire having a thickness of 0.2-2 im. To improve the heat conductive into the tissue layer a contact brining layer of paste or liquid should be applied on the epidermal layer 4.5 of the tissue 6 before treatment. The contact bringing layer 4.4 comprising of paste or liquid shall have a boiling point of at least 100°C preferably 150 to 250°C. Tissue anaesthetics could be included into the paste or liquid for pain control if necessary. Excessive heat generated from the radiation absoφtion in the layer 4.2 should be removed by attachment of a cooling device to the proximal end of the waveguide 3.2. The cooling device 5 could either be Peltier unit or metal body cooled by a circulating coolant. Although the present invention has been described herein above by way of preferred embodiments thereof, it can be modified, without departing from the spirit and nature of the subject invention as defined in the appended claims.

Claims: CLAIMS What is claimed is: 1. An apparatus and method for conductive heat treatment of superficial tissue layers comprising: (1) means for generation of electromagnetic radiation; (2) means for concentration of said radiation; (3) means for conversion of said electromagnetic radiation energy into heat in a thin absorbing layer; (4) means for effective cooling of redundant heat generated in the absorbing layer and pre-cooling of dermal layers; and (5) means for effective thermal conduction between the absoφtive layer and the object surface to be heated. 2. An apparatus according to claim 1 wherein said means for generation electromagnetic radiation energy is an electrical lamp with a pulse length of at least 0.1 im and maximum 500 ms. 3. The method of claim 2 wherein the electrical lamp is a gas discharge arc lamp. 4. The method of claim 2 wherein the electrical lamp can produce an optical power 2 density on the absorbing layer in the range of 0.1-200 J/cm . 5. The method of claim 1 wherein said means for generation of electromagnetic energy is a laser with pulse length of at least 1 us. 6. The method of claim 5 wherein the laser source is a high power diode laser device. 7. The method of claim 1, wherein said means for conversion of said electromagnetic energy into heat comprises at least one radiation absorbing layer having a thickness of 0.1-1000 im. 8. The method of claim 7 wherein said radiation absorbing layer comprises an oxide or carbon layer with thickness of 0.5-50 im. 9. The arrangement of claim 8, wherein said oxide material layer is an oxide made of at least one of the elements: zirconium, aluminum, nickel, zinc, indium, strontium, barium, silicium. 10. The arrangement of claim 7 wherein said plurality of layers comprises at least one optical transparent bulk substrate layer with low thermal diffusivity that is brought in physical contact with one side of said radiation absorbing layer and wherein the thickness of the bulk substrate is 1-1000 im. 11. The arrangement of claim 10 wherein said bulk substrate is made of quartz glass. 12. The arrangement of claim 10 wherein said bulk substrate is a disposable part attached with removable glue to an optical waveguide having an identical cross section geometry as the bulk substrate. 13. The arrangement of claim 7 wherein said plurality of layers comprises an overcoat layer attached to the tissue side of the absorbing layer in case of non tissue compatible composition of the absorbing layer. 14. The arrangement of claim 14 wherein said overcoat layer is made of a tissue compatible material. 15. The arrangement of claim 14 wherein said layer is made of crystalline (sapphire) or polycrystalline aluminum oxide (alumina) having a thickness of 0.1-10 im, preferably 0.5-2 im. 16. The arrangement of claim 7 wherein the geometry of said absorbing layer is round or rectangular having an area of 0.1-5 cm " . 17. The arrangement of claim 1 wherein said means for concentration of said electromagnetic energy comprises optical elements of which at least one is a refractive/ diffractive optical element and/or at least one reflective optical element. 18. An apparatus according to claim 1 wherein said means for concentration of said electromagnetic energy into heat include at least one optical waveguide transferring said electromagnetic energy to a remote absorbing layer. 19. The arrangement of claim 18 wherein said optical waveguide is an dielectric waveguide based on at least one of the following waveguide materials comprising a combination of doped and/or undoped: quartz glass, crystalline sapphire, flint glass, crown glass. 20. The arrangement of claim 18 wherein said optical waveguide is at least partly coated with a highly reflective metallic layer such as gold, aluminum, or silver. 21. The arrangement of claim 18 wherein the distal portion of said optical waveguide system is made of an optical transparent material having a high thermal conductivity and diffusivity and one side of said waveguide is brought in physical contact with said bulk substrate. 22. The arrangement of claim 21 wherein said distal portion of optical waveguide is made of crystalline sapphire glass. 23. The arrangement of claim 21 wherein said distal portion of optical waveguide is made of crystalline sapphire glass. 24. An apparatus according to claim 1 wherein said means for effective conduction of generated heat in the absoφtive layer onto the surface of biological tissue comprises a gel or liquid with appropriate viscosity and boiling point. 25. The arrangement of claim 24 wherein said paste or liquid is based on silicone oil. 26. The arrangement of claim 24 wherein said paste or liquid is based on hyaluronic acid. 27. The arrangement of claim 24 wherein said paste or liquid includes drugs for transdermal deposition. 28. The arrangement of claim 24 wherein said paste or liquid is based on water or physiological saline solution. 29. The arrangement of claim 24 wherein said paste or liquid include tissue anesthetic. 30. The arrangement of claim 24 wherein boiling point of said gel or liquids is 75-400°C, preferably range is 150-300 °C. 31. An apparatus according to claim 1 wherein said means for effective cooling of redundant heat generated in bulk substrate from the attached absorbing layer comprises a heat sink in physical contact with said bulk substrate with attached absoφtive layer on opposite side. 32. The arrangement of claim 31 wherein said heat sink comprises a Pelier cooling device. 33. The arrangement of claim 31 wherein said heat sink comprises a body with internal channels allowing circulation of a cooling liquid. 34. The arrangement of claim 31 wherein said heat zinc cool down said bulk substrate layer to a temperature in the range of -20 to +20 °C. 5. The arrangement of claim 34 wherein said cooled substrate layer decrease the temperature of the outside tissue layers before heat pulse.

Inventors: TORSTENSSON PER-ARNE, TORSTENSSON, PER-ARNE
Applicants: GUSTAFSSON MORGAN, TORSTENSSON PER ARNE, GUSTAFSSON, MORGAN, TORSTENSSON, PER-ARNE
Application year: 2000
Main classification: A61B18
IPC codes: A45D26, A61B18, A61F7, A61M37