Skin treatment device with incoherent pulsed light source

SKIN TREATMENT DEVICE WITH INCOHERENT PULSED LIGHT SOURCE

FIELD OF THE INVENTION

The present invention relates to collection and focusing of non-laser type pulsed light, to optimization of its spectrum and to application of the light to treatment of skin disorders, and more particularly, to treatment of skin irregularities and treatment of blood vessels in the vicinity of the skin using non-laser, incoherent, pulsed light sources.

BACKGROUND OF THE INVENTION

The prior art includes extensive use of lasers to treat irregularities of the skin. For examples of such treatment procedures and their effectiveness, reference is made to the following prior art publications:

1) Is there an optimal Laser treatment for Port Wine Stains? Martin J. C. van Gemert et al, Lasers in Surgery and Medicine Vol 6 p.76-83, 1986;

2) Histologic response of Port Wine Stains treated by Argon, Carbon Dioxide, and Tunable Dye Lasers , Oon Tian Tan et al, Archives of Dermatology, vol 122 , pl016-1022, 1986;

3) The Pulsed dye Laser: its use at 577nm Wavelength Jerome M. Garden et al, J. Derm. Surg. Oncol. Vol 13:2, p 134-138, 1987;

4) Ultarstructure: the Effect of Melanin Pigment on Target Specificity using a pulsed Dye Laser (577nm), Arthur K. F. Tong et al in The Journal of Investigative Dermatology Vol 88 no 6

p.747-752, 1987.

In all the cases described in the prior art, laser radiation was used to treat these skin disorders. In US Patent 4,829,262 to Furumoto. there is described a method of constructing a dye laser that may be appropriate for dermatology applications. US Patent 4,298,005 to Mutzhas describes a continuous ultraviolet lamp with cosmetic, photobiological, and photochemical applications. The lamp is a continuous and not a pulsed lamp. The treatment is based on use of the UV portion of the spectrum and its photochemical interaction with the skin and not on local, pulsed heating or evaporation of the treated area. The power on the skin using the lamp described is quoted as 150W/m2, and this power level does not have any significant effect on the skin temperature.

British patent 2,212,010 to Mendes et al describes a low-power, narrow bandwidth device for providing incoherent light from an LED light source for providing therapy on the skin.

Many types of lasers have been used for dermatological procedures. These include Argon lasers, CO2 lasers, Nd(Yag) lasers. Copper vapor lasers, Ruby lasers and dye lasers, and they are described in the prior art.

There are two main types of skin irregularities which can be treated by laser radiation. The first are external skin irregularities such as local differences in the pigmentation or structure of the skin. The second are vascular disorders lying deeper under the skin, which generate a variety of skin abnormalities such as port wine stains, leg veins and cherry and spider angiomas. The basic mechanism of laser treatment of the above-mentioned skin disorders is by heating of the treated area by absorption of the laser radiation. This heating generates changes In the treated area of the skin that changes or corrects the skin disorder and causes its full or partial disappearance.

In the case of external disorders, the heating of the skin is very fast and the skin reaches a high enough temperature that causes its evaporation. In the treatment of deeper-lying disorders associated with blood vessels, the blood is heated to a high enough temperature that causes its coagulation and the eventual disappearance of the disturbance.

The depth of heat penetration, induced by a light pulse to which the skin is exposed, depends on the light absorption and scattering in the different layers of the skin and the thermal properties of the skin. Another important parameter is the pulse-width. For a pulsed light source the energy of which is absorbed in an infinitesimally thin layer, the depth of heat penetration (d) by thermal conductivity during the pulse can be written as shown in Equation 1:

d = 4 (kAt/CpJ (Eq. 1) where k = heat conductivity of the material being illuminated; /\t = the pulse-width of the light pulse;

C - the heat capacity of the material; p = density of the material.

It is clear from Equation 1 that the depth of heat penetration can be controlled by the pulse-width of the light source. Thus, a variation of pulse-width in the range of -5 sec to 100msec will result in a variation in the thermal penetration by a factor of 100.

In the case of the coagulation of blood vessels in the skin, the pulse length should be chosen to achieve as uniform heating of the entire thickness of the vessel as possible in order to achieve its efficient coagulation. Typical blood vessels that need to be treated in the skin have thicknesses in the range of 0.5mm. For such a case, the optimal pulse-width, taking into account the thermal properties of blood, would be 100msec. If shorter pulses are used, the heat can still be conducted through the blood to achieve coagulation conditions, however, the instantaneous temperature of part of the blood in the vessel and surrounding tissue will be higher than the temperature required for coagulation and may cause unwanted damage.

In the case of evaporation of external skin disorders, a very short pulse-width would result in very shallow thermal -5 penetration of the skin. For example, a 10 sec pulse will penetrate (by thermal conductivity) a depth of only 55 microns into the skin. Thus, in this case only a thin layer is heated, the instantaneous temperature that can be reached is very high and the external mark of the skin can be evaporated. An optimal source for treatment of skin disorders will have therefore a large variation in the pulse-width built into it. Long pulses will be used to treat blood vessel associated disorders, short pulses will be used to treat external disorders by evaporation.

The absorption and scattering coefficients of the different constituents of the skin are a function of wavelength. In the treatment of external marks the wavelength of the light should be chosen so that the absorption coefficient is maximized. The absorption coefficient of light in the epidermis and dermis is a slowly varying, monotonically decreasing function of wavelength. Lasers are presently used for similar applications such as tattoo removal and removal of birth and age marks.

Lasers are limited, however, since the monochromaticity of the laser, though very effective for one case of skin pigmentation disorder, may be very ineffective in another case where the specific wavelength of the laser is not absorbed efficiently. A wide band source that covers the near UV and the visible pot I.ion of the spectrum is optimal for treatment of external skin disorders by evaporation, since the optimal wavelength can be chosen for the specific pigmentation of the skin disorder that is being treated.

Vascular disorders are characterized by their blood contents. Oxyhemoglobin is the main chromophore which controls the optical properties of blood and has strong absorption bands in the visible. The strongest absorption peak of oxyhemoglobin occurs at 418nm and has a band-width of 60nm. Two additional absorption peaks with lower absorption coefficients occur at 542 and 577nm. The total band-width of these two peaks is on the order of lOOnm.

Light in the wavelength range of 500 to 600nm is optimal for the treatment of blood vessel disorders of the skin since it penetrates well through the skin and is absorbed by the blood. Longer wavelengths up to lOOOnm are also effective since they can penetrate deeper into the skin, heat the surrounding tissue and if the pulse-width is long enough, can also contribute to heating of the blood vessel by thermal conductivity.

As can be seen from the above, skin treatment with light requires a wide range in wavelengths, with the optimal wavelength range chosen according to the specific treatment. The intensity of the light has to be high enough to achieve the required thermal effect by achieving the required temperature. The variation in wavelength of the light source has to be wide enough so that maximum effectiveness is achieved for each specific application. The pulse-width should be variable over a wide enough range so as to achieve the optimal penetration depth for each application.

Therefore, it would be desirable to provide a light source having a wide range of wavelengths, which can be selected according to the required skin treatment, with a controlled pulsewidth and a high enough energy density for application to the affected area.

SUMMARY OF THE INVENTION

Accordingly, it is a principal object of the invention to overcome the above-mentioned disadvantages of the prior art and provide a skin treatment device using non-laser, incoherent, pulsed light sources. The present invention represents a significant simplification in the design and construction of the light source for these applications.

In accordance with a preferred embodiment of the present invention, there is provided a skin treatment device comprising a non-laser, incoherent light source operable to provide a high power, pulsed light output over a skin area.

Further in accordance with a preferred embodiment of the invention, the skin treatment device comprises:

a housing in which said light source is disposed, said pulsed light output exiting through an opening in said housing;

means for controlling the pulse-width of said light output;

focusing means mounted proximate said light source for controlling the power density of said light output; and optical filter means mounted proximate said housing opening for controlling the spectrum of said light output, said controlled density, filtered, pulsed light output being directed via said opening toward said treated skin area.

In the preferred embodiment, light from a pulsed, incohoront (non-laser) light source is directly focused on the skin to achieve energy and power densities that will repair irregularities in the human skin. The pulsed, non-laser light can be used to treat skin disorders associated with blood vessel irregularities in the skin or in areas in the immediate vicinity of the skin. The pulsed, non-laser type light can also be used to evaporate, remove or clear irregularities of the skin such as birth marks, age marks, tattoos or vascular lesions of the skin. A typical light source for this application is a flashlamp (sometimes called flash tube or pulsed discharge arc lamp). The light of the flashlamp is focused on the surface of the skin by the usage of proper reflectors in order to achieve the required energy and power densities.

A set of optical filters and of fluorescent materials is incorporated into the device to achieve a spectrum of light that is optimized for the specific skin irregularity that is being treated. The pulse-width of the light source can be controlled over a wide range from a few microseconds to tens of milliseconds. Pulse-width variation is used to control the instantaneous power and the depth of heat penetration into the treated skin; shorter pulses are used to treat irregularities on the surface or very close to the surface of the skin by evaporating them. Longer pulses are used to treat deeper lying skin irregularities associated with blood vessel disorders by heating and coagulating the blood vessels.

Pulsed non-laser type light sources such as linear flashlamps are well suited for this application. The intensity of the emitted light can be made high enough to achieve the required thermal effects. The pulse-width can be varied over a wide range so that control of thermal depth penetration can be accomplished. The typical spectrum covers the visible and ultraviolet range and the optical bands most effective for specific applications can be selected, or enhanced using flourescent materials.

Non-laser type light sources such as flashlamps are much simpler and easier to manufacture than lasers, are significantly less expensive for the same output power and have the potential of being more efficient and more reliable. These light sources have all the physical properties such as energy and power density that can be reached on the exposed area of the skin. They have a wide spectral range that can be optimized for the specific skin treatment application. These sources also have a pulse length that can be varied over a wide range which is critical for the different types of skin treatments.

Other features and advantages of the invention will become apparent from the following drawings and description.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention, reference is made to the accompanying drawings, in which like numerals designate corresponding elements or sections throughout, and in which:

Fig. 1-2 are respectively, cross-sectional and side views of an incoherent, pulsed light source skin treatment device constructed and operated in accordance with the principles of the present invention; and

Fig. 3 is a schematic diagram of a pulse-width forming network for use with the skin treatment device of Figs. 1-2.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

Referring now to Figs. 1-2, there are shown, respectively, cross-sectional and side views of an incoherent, pulsed light source skin treatment device 10 constructed and operated in accordance with the principles of the present invention. The invention is based on the application of pulsed, non-laser type light sources to dermatology. Device 10 comprises a housing 12, in which there is mounted a light source 14. A typical light source that can be used for this application is a gas filled, linear flashlamp 14. The spectrum of light emitted by these sources depends on the current density, the type of glass envelope material and the gas mixture used in the tube of flashlamp 14. For high enough current densities the spectrum is similar to a black body radiation spectrum. Typically, most of the energy is emitted in the 300 to 700nm wavelength range.

The required light density on the skin can be achieved with a focusing arrangement that is shown in Fig. 1. The figure shows a cross-sectional of a reflector 16 that is used to focus the light from flashlamp 14. The cross-section of reflector 16 in a plane that is perpendicular to the axis of flashlamp 14 is an ellipse. The linear flashlamp 14 is located on one focus of the ellipse and reflector 16 is positioned in such a way that the treated surface of the skin is located on the other focus. The focal length f is also shown in Fig. 1.

A 3ide view of flashlamp 14, housing 12 and handle 13 are shown in Fig. 2. This arrangement assures efficient coupling of light from flashlamp 14 to the skin. Similar focusing arrangements are used in flashlamp-excited solid-state lasers.

The elliptical reflector 16 shown in Fig. 1 is a metallic reflector. Polished aluminum is an easily machinable reflector and has a very high reflectivity in the visible and the UV range of the spectrum. Other bare or coated metals can also be used for this purpose. The optical and neutral density filters 18 are also shown schematically in Fig. 1. Filters 1Θ can be moved into the beam or out of the beam to control the spectrum and intensity of the light. The fluorescent material is deposited on the outer glass tube 15 that is located coaxially with flashlamp 14 and is also shown in Fig. 1.

Other shapes or configurations of flashlamps 14 such as circular, helical, short arc and multiple linear flashlamps can also be used for this application. Other reflector 16 designs such as parabolic or circular reflectors are also applicable. The light source can also be used without a reflector where the required energy and power density are achieved by locating the light source in close proximity to the treated area.

The set of optical filters 1Θ is used to control the spectrum of the light that is coupled to the skin. Typically. 50 to lOOnm bandwidth filters as well as low cutoff filters in the visible and ultraviolet portions of the spectrum are used. In some procedures most of the spectrum can be used and only the UV portion needs to be cut off. In other applications, mainly for deeper penetration, narrower bandwidths can be used. The bandwidth filters and the cutoff filters are readily available commercially.

In the case of coagulation of blood vessels, the energy efficiency of device 10 can be optimized by using a fluorescent glass in association with flashlamp 14. These materials can be chosen to absorb the UV portion of the spectrum of flashlamp 14 and generate light in the 500 to 600nm range that is optimized for absorption in the blood. Similar materials are coated on the inner walls of commercial fluorescent lamps. A typical material used to generate warm white light in fluorescent lamps has a conversion efficiency of 80%, has a peak emission wavelength of 570nm and has a bandwidth of 70nm and is optimized for absorption in blood. The few millisecond decay time of these phosphors is consistent with long pulses that are required for the treatment of blood vessels.

The size of the exposed skin area is controlled by an iris 20 that controls the length (along the flashlamp axis) and the width of the exposed area. The length of flashlamp 14 controls the maximum length that can be exposed. Typically a Bcm long (arc length) tube will be used. Only the central 5cm of the tube will be exposed. This assures a high degree of uniformity of energy density in the exposed skin area.

The iris 20 will enable exposure of skin areas of a maximum length of 5cm and a minimum length of one millimeter. The width of the exposed skin area can be controlled in the range of 1 to 5mm for a 5mm flashlamp. Larger exposed areas can be easily achieved by using longer flash tubes or multiple tubes. The larger area that is simultaneously exposed by device 10 of the present invention compared to lasers is very effective in the coagulation of blood vessels since blood flow interruption over a longer section of the vessel is more effective in coagulating it.

The detector 22 shown in Fig. 1 can be used to monitor the light reflected from the skin. Detector 22 combined with optical filters IB and neutral density filters can be used to achieve a quick estimate of the spectral reflection and absorption coefficients of the skin. This diagnostics of the skin can be carried out at a low energy density level prior to the application of the main treatment pulse. Measurement of the optical properties of the skin prior to the application of the main pulse is important to achieve optimal treatment conditions. The wide spectrum of the light emitted from the non-laser type source enables investigation of the skin over a wide spectral range and choice of optimal treatment wavelength.

Another detector system can be used for real time temperature measurement of the skin during its exposure to the pulsed light source. This is important for skin thermolysis applications with long pulses. In these application light is also absorbed in the epidermis and dermis. When the external portion of the epidermis reaches too high a temperature it may result in permanent scarring of the skin. The temperature of the skin can be measured using the infra-red emission of the heated skin.

A typical detector system would measure the infra-red emission of the skin at two specific wavelengths by using two detectors and filters. The ratio between the signals of the two detectors can be used to estimate the instantenuous skin temperature. The operation of the pulsed light source can be stopped if too high a skin temperature, that can be preset by the physician, is reached. This measurement is relatively easy since the temperature threshold for pulsed heating that may cause skin scarring is of the order of 50 °C or more which is easily measurable using infra-red emission.

The variation in pulse-width is achieved by feeding flashlamp 14 with a set of pulse forming networks (PFN's). The concept is shown in Fig. 3. The light pulse full width at half maximum (FWHM) of a flashlamp driven by a single element PFN with capacitance C and inductance L is approximately equal to:

£\t ⁵⁵ 21LC1 (Eq. 2)

The flashlamp 14 light source can be driven, in the case shown in Fig. 3 by three different PFN'b. The relay contacts R1¹ , R2’ and R3' are used to select the capacitors that are charged by the high voltage power supply. The relays R1, R2 and R3 are used to select the PFN that will be connected to flashlamp

14. The high voltage switches SI, S2 and S3 are used to discharge the energy stored in the capacitor of the PFN into flashlamp 14.

A simmer power supply (not shown in Fig. 3) may be used to keep the flashlamp in a low current conducting mode. Other configurations can be used to achieve pulse-width variation, such asthe use of a single PFN and a crowbar switch.

In addition to the possibility of firing each PFN separately which generates the basic variability in pulse-width, additional variation can be achieved by firing PFN's sequentially. If, for example, two PFN's having pulse-width /\tl and /\t2 are fired, so that the second PFN is fired after the first pulse has decayed to half of its amplitude, then an effective light pulse-width of this operation of the system will be given by the relation: /\t * /_\tl ⁺ /\t2

The charging power supply typically has a voltage range of 500V to 5kV. The relays should therefore be high voltage relays that can isolate these voltages reliably. The switches S are capable of carrying the current of flashlamp 14 and to isolate the reverse high voltage generated if sequential firing of the PFN's is used. Solid-state switches, vacuum switches or gas switches can be used for this purpose.

Typically, for operation of flashlamp 14 with an electrical pulse-width of 1 tolOmsec, a linear electrical energy density input of 100 to 300J/cm can be used. An energy density of 30 to 100J/cm2 can be achieved on the skin for a typical flashlamp bore diameter of 5mm. The use of a 500 to 600nm bandwidth transmits 2035 of the incident energy. Thus, energy densities on the skin of 6 to 20J/cm2 are achieved. The incorporation of the fluorescent material will further extend the output radiation in the desired range, enabling the same exposure of the skin with a lower energy input into flashlamp 14.

The experience with pulsed laser skin treatment shows that energy densities in the range of 0.5 to 5J/cm2 with pulsewidths in the range of 0.5msec are effective for treating vascular related skin disorders (background references 1 to 4). Longer pulses are not used with lasers since they are much more difficult to achieve at the appropriate wavelength and power density (the main subject of background reference 4 is a way to extend the pulse-width of a dye laser to the millisecond range required for optimal treatment of blood vessels). This range of parameters falls very conveniently in the range of operation of pulsed non-laser type light sources such as the linear flashlamp. A few steps of neutral density glass filters 18 can also be used to control the energy density on the skin.

In the case of treatment of external disorders a typical pulse-width of 5 microsec can be used. A 20J/cm electrical energy density input into a 5mm bore flashlamp results in an energy density on the skin of 10J/cm2. Cutting off the hard UV portion of the spectrum results in 90¾ energy transmission, or skin exposure to an energy density of close to 10 J/cm2. This energy density is high enough to evaporate external marks on the skin.

Device 10 can be provided as two units: a lightweight, unit held by the physician using handle 13, with the hand-held unit containing flashlamp 14, filters 18 and iris 20 that together control the spectrum and the size of the exposed area and the detectors that measure the reflectivity and the instantaneous skin temperature. The power supply, the PFN's and the electrical controls are contained in a separate box that is connected to the hand-held unit via a flexible cable. This enables ease of operation and easy access to the areas of the skin that need to be treated.

Having described the invention with regard to certain specific embodiments thereof, it is to be understood that the description is not meant as a limitation, since further modifications may now suggest themselves to those skilled in the art and it is intended to cover such modifications as fall within the scope of the appended claims.