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CUTTING DEVICE FOR SKIN FOR
OBTAINING SMALL BLOOD SAMPLES IN
ALMOST PAIN-FREE MANNER
The present invention addresses a device for cutting skin to obtain small blood samples from human or animal tissue in an almost pain-free manner. It comprises a blade with a cutting edge having a length of less than 10 mm and an oscillator to make the blade oscillate essentially parallel to the cutting edge.
To take small amounts of blood from the finger or the earlobe for diagnostic purposes, one uses lancets which are pricked into the corresponding body part either manually or with the aid of a simple apparatus by the doctor or lab personnel. It is a matter of fact that the lancet has to be sharp and sterile. Otherwise, there are no particularly high demands to be met by the doctors' offices as blood samples are collected from individual patients in greater time intervals and the procedure is carried out by specially trained staff.
The requirements are significantly higher when blood lancet devices are used which are to be operated by the patient. They are necessary to allow patient groups that are at a particular risk to regularly determine certain analytical blood values by home monitoring.
This applies in particular to diabetics who regularly and frequently have to monitor their blood glucose level to keep it within certain levels by adjusting the amount of insulin to be injected which depends on food intake, physical activity, and other factors. This is of utmost importance for the health of these patients to avoid delayed serious damage such as blindness and amputation of body parts.
For this reason, one has developed easy to handle and relatively inexpensive analytical systems which usually comprise blood test strips and a corresponding evaluation instrument. Although the possibility of performing an easy and relatively inexpensive analysis is available to every patient today, the self-monitoring of blood glucose levels has still not reached the desired general acceptance among diabetics. The main reason being the pain that accompanies the pricking procedure when taking the blood sample.
Prior art knows lancet devices where a spring-propelled lancet pricks the tissue at a very high speed. Blood emerges from the so-created wound which can then be used for diagnostic purposes. Devices of this kind are known, for example from U.S. patents U.S. Pat. No. 4,203,446 and U.S. Pat. No. 4,895,147. There is a great variety of such devices with differently ground lancets and different spring mechanisms available on the market. Experiments have shown, however, that the pain caused by this kind of device cannot be lowered under a limit which the user clearly identifies as being unpleasant. What makes it even more difficult in the case of diabetics is that the test must be carried out frequently; the affected skin parts such as the finger tips and earlobes develop cornifications which in turn require deeper penetration levels thus causing more pain during pricking.
Numerous different blood lancet devices have been developed that are suitable to generate the wound necessary to collect the blood specimen in a simple and relatively pain-free manner. Examples are described in patents U.S. Pat. No. 4,442,836, U.S. Pat. No. 4,469,110, U.S. Pat. No. 4,535,769, and U.S. Pat. No. 4,924,897. Blood sampling device for driving a lancet and lancets are made to match one another and are also referred to as blood collection systems. Despite some progress, the pain caused by the cutting procedure when using blood lancet device designed for patient use is still too great.
The pain sensed during cutting could be reduced in blood lancet devices where the needle is guided in a controlled manner (U.S. Pat. No. 4,824,879 and U.S. Pat. No. 5,318, 584). The devices of this kind are known as controlled 5 guidance systems, as opposed to ballistic systems where the lancet, propelled by a spring, contacts the skin surface in a relatively uncontrolled manner.
It was, hence, an object of the present invention to propose a device which causes less pain during blood 10 sampling as is the case with prior arts systems. It was another object of the invention to provide a blood lancet where the penetration depth can be reproduced in a more precise and improved manner than is the case with known devices.
15 The object was accomplished in accordance with the invention in that the skin is penetrated in a manner where there is no pressure wave directed perpendicular to the skin and where a relatively small amount of pain receptors is activated due to precise guidance and a relatively small
20 penetration depth of the lancet/blade. This can be achieved in that the skin is penetrated/opened in a procedure that is carried out essentially parallel to the skin surface. This cutting procedure causes significantly less pain than pricking.
25 In accordance with the invention, the pain sensed when obtaining small amounts of blood is reduced in that a rapid oscillating movement is superimposed on the movement (usually a linear movement) of the blade that is necessary to enter the tissue. The excursion of the blade generated by this
30 oscillating movement can principally be transverse to the tissue or essentially perpendicular to the tissue. With respect to these two embodiments, two different blade types have proven to be expedient so that the embodiments are described separately hereinafter.
35 When coupling a slow pricking movement with an oscillating movement, it is advantageous that despite an extremely high relative movement of the blade with respect to the skin, a pressure wave affecting deeper layers of the blade is almost completely avoided since the excursion of
40 the blade is very small despite the high speed (preferably ranging around 10-200 fim) and there is virtually no pressure component being generated perpendicular to the tissue surface. The blade "falls" into the skin. The penetration principle is, hence, based on a cutting procedure which can
45 be carried out by an "oscillating" blade without a significant pressure component running perpendicular to the direction of movement. Owing to the small stroke of the blade, the cut is limited to very small dimensions and, hence, easy to control.
50 Coupling a slow blade movement with an oscillating movement leads to cutting procedures which cause less pain than do the methods for obtaining small blood samples known from prior art.
Time-consuming tests have shown that the disadvantages
55 of the blood lancets known in prior art are likely due to the painfullness of the skin penetration process which is essentially caused by the activation of pain receptors and their afferent signals. When the lancet arrives on the tissue a pressure wave is generated that propagates predominantly in
60 the direction of movement. This pressure wave precedes the moving blade and thus activates additional pain receptors which would not be activated during penetration and cutting if the lancet would execute a mere cutting and ripping process. Avoiding a pressure wave when the lancet hits the
65 skin can, hence, significantly reduce the pain sensation. Moreover, the undefined penetration depth and the uncontrolled movements of the lancet are likely to be additional
reasons for unnecessary high pain sensation when using known systems.
The device of the invention can be further improved when the skin opening is kept as small as possible, preferably smaller than 1.5 mm, more preferably smaller than 1 5 mm, and when the cutting movement is carried out in a controlled manner.
It is also important that the site opening be generated in a controlled movement and not in an uncontrolled "rip". Ripping the tissue would lead to a corresponding pressure 10 wave and/or pulling of the tissue which then causes pain. Advantageous movements are those where the blade is guided such that forward and backward movements are uniform. It is also advantageous to control the movement such that the forward movement upon reaching a predefined 15 dead point changes into a backward movement.
The following figures are given to describe the invention in greater detail.
FIG. 1: Principal interaction between penetration depth and cutting width when using pointed lancets 20
FIG. 2: Arrangement of blade and tissue with respect to one another
FIG. 3: Manually operated vampire blood lancet
FIG. 4: Blood lancet with spring as propelling mechanism 25
FIG. 5: Blood lancet with an eccentric disk as a propelling mechanism
FIG. 6: Blade types
FIG. 7: Cross sections of blades
FIG. 8: Principal representation of a Mucke-type blood 30 lancet
FIG. 9: Manually operated blood lancet of the Mucke-type
FIG. 10: System of coordinates for defining directions of movement 35
FIG. 11: Representation of the interaction between projection depth (A), deformation (D) and penetration depth (E)
In order to define the directions of movement of the lancet, FIG. 10 shows a system of coordinates. The tissue surface is located in the XZ-plane. In order to penetrate the 40 tissue, the lancet must carry out a movement with a Y-component, i.e. a movement having a component perpendicular to the tissue surface. In the case shown here, the cutting edge (2) is located on the Z-axis while the blade (1) is located within the XY-plane. The blade can now be guided 45 perpendicularly, i.e. in direction of the negative Y-axis, into the tissue; or it can be moved in a plane E' which is tilted with respect to the XY-plane. The description "essentially perpendicular to the tissue surface" when used with respect to the direction of movement should also include move- 50 ments which are tilted by, e.g. 30°, with respect to the XY-plane. In a first embodiment the blade is made to oscillate transversely to the direction of movement. In the system of coordinates of FIG. 10, this is an oscillating movement of the blade in direction Z. 55
The present invention is further directed to a process for obtaining small volumes of blood by a process where an oscillating blade is being moved in direction towards the tissue to be penetrated, is being lowered into the tissue in a direction primarily vertical to the skin surface and is being 60 retracted out of the tissue. This can be accomplished by an active movement of the blade in direction towards the tissue as well as by an active movement of the tissue. Furthermore it is not necessary that the blade is already in oscillation while contacting the tissue surface. It is also possible to start 65 oscillation of the blade after contact between blade and tissue has been established.
In almost all parts of its body, the human being possesses epithelial skin where blood sampling with lancets is not possible at all or very difficult as this type of skin, owing to its high retraction properties, will close immediately after pricking or cutting so that the blood emerging from the blood vessels cannot reach the surface of the skin or is released into subcutaneous tissue parts causing microhematomas. Skin parts that are suitable for obtaining blood samples include the tips of fingers and toes and the earlobes. On his finger and toe tips the human being possesses ridged skin. The top layer of this skin is the epidermis with a thickness of 0.1 to 0.5 mm. Below the epidermis is the corium consisting of the stratum papillare and the stratum reticulate. Below the corium is the subcutis. The upper part of the corium and the stratum papillare are of particular importance for obtaining small amounts of blood in almost pain-free manner. The stratum papillare features a finely woven net of blood vessels and pain receptors located closely together. In this part of the tissue the blood vessels can be divided into capillaries and microvessels. Capillaries are shaped like hair needles reflected in the papillae of the skin. Arterial blood rises from the inside of the tissue to the curvature in the hair needle to release oxygen to the surrounding tissue; it is then via the venous system conducted back into deeper layers of tissue in the form of venous blood. Within the present invention it has been found that due to anatomic properties, it suffices to penetrate the epidermis and cut open a few hair needle-like blood vessels of the papillary layer and/or cut open a few microvessels located in this region of the tissue to obtain an amount of blood that is sufficient to satisfy the diagnostic purpose (appr. 5 fA-30 fil).
Prior art lancets, however, penetrate the corium much deeper and in an uncontrolled manner, i.e. with heavy movements perpendicular to the propelling direction. The reason for this can be understood from FIG. 1. In order to generate a wound of width d on the surface of the corium, the tip of the lancet must enter the corium at a depth h. With the technology known from prior art, this depth h can only be reduced when a lancet with a flatter tip is selected, i.e. reducing the h/d-ratio. More blunt lancets, however, cause more pain during pricking as was shown in experiments.
The First Embodiment (Vampire)
In accordance with the invention, the pricking pain is reduced in that the blade is made to oscillate a high rate while the oscillating blade is lowered into the tissue are relatively slow speed, preferably slower than 2 km/h. In a first embodiment of the invention, the blade is oscillated essentially parallel to the cutting edge.
FIG. 2 shows the principal arrangement of blade and tissue. The blade (1) shown has a cutting edge (2) which executes an oscillating movement in direction toward the double arrow shown in the figure. When the blade, while oscillating, approaches the epidermis, it is capable of penetrating it without causing significant pain. Due to the oscillating movement, the epidermis is then cut open; as compared to currently available pricking lancets, it is now possible to lower the blade into epidermis and corium at a much slower speed. From FIG. 2 it can be seen that this embodiment uses a blade type where a sufficiently wide piece d of the tissue is cut open while the penetration depth of the corium remains very small. Owing to the arrangement of the hair-needle-like blood vessels and other microvessels found in this region within the corium, said vessels are effectively cut open by the oscillating movement of the blade. A cutting width d of 300 to 900 fim is already sufficient in order to obtain a sufficient amount of blood. For
obtaining larger amounts of blood it is in accordance with the invention of course also possible to use correspondingly longer blades.
The Oscillator 5
In order to allow the blade to enter the tissue in an almost pain-free manner, experience has shown it to be important that the blade oscillate essentially parallel to the cutting edge at frequency above 500 Hz. The cutting device in accordance with the invention could further be significantly 10 improved by raising the oscillation frequency to above 700 Hz. Particularly efficient cutting is achieved with frequencies in the kilo-Herz range or above 900 Hz. Pain sensation during cutting is significantly influenced by the oscillation amplitude. Experiments have shown that the amplitude 15 should be below 500 fim. Apreferred amplitude is one below 300 fim, particularly preferred below 250 fim. Oscillators to implement the described requirements include in particular piezo elements, ultrasonic ceramics, electrodynamical transformers as well as mechanical transformers. Piezo elements 20 have proven particularly well as oscillators which execute changes in length upon application of an electrical voltage. As these changes usually amount to only one or a few fim, it is necessary to have a mechanical transformation to achieve larger amplitudes. This can be accomplished by a 25 piezo element coupled to one end of a bar and a blade coupled to the other end. The bar should have a low weight in order to minimize a damping of the oscillation. The bar is preferably made of a rigid material in order to achieve a proper energy transfer from the oscillator onto the blade. Bar 30 and piezo element are properly coupled to a mass. It is, however, also possible to advantageously use a stack of piezo elements where the changes in length of the individual elements are added up.
Mechanical transformers which lead to an oscillation of the blade are so called spring-massoscillators, which can be accomplished with a tuning fork, for example, at the one arm of which there is attached the blade or a device to hold the blade. It has shown to be particularly useful to operate the 4Q oscillator at a resonance frequency since the amplitude of the blade is high under such conditions.
Blades with a cutting edge of less than 10 mm have 45 proven to be suitable for cutting devices designed to obtained small blood samples. Cutting edges with a length between 0.2 to 2 mm, preferably 0.4 to 1.5 mm have proven to be suitable for obtaining blood samples ranging around 100 (A. Cutting edges of 0.5 to 1.0 mm in length are 50 particularly suitable.
Materials such as steel that are commonly used in prior art lancets haven proven well for the blades. Generally, it is also possible to use metals, glasses, ceramic materials and plastics. Particularly well suited are plastics which can be 55 processed by injection molding and which are rigid enough after cooling down. When using suitable injection molding forms a separate process step for sharpening the blade can be avoided. Experiments have shown it to be advantageous to coat the blades with substances which reduce friction 60 between blade and tissue. By reducing the friction between blade and tissue, it is possible to avoid the transfer of mechanical energy into the tissue and, hence, pain caused by heat. In a particularly preferred embodiment bar and blade are a mechanical unit which can be exchanged. In the 65 medical field it is desirable due to hygienic reasons to have a disposable unit which can be exchanged after each cutting
process. Bar and blade can be made from different materials, however, it is preferred that both are made from the same material.
The Manually Operated Blood Lancet
FIG. 3 shows a blood lancet in accordance with the invention. Said lancet comprises a housing containing the blade and the oscillator. The housing has an outer cover (10) which serves to handle the device. Inside the cover there is a bar (12) to which the blade (13) is attached. Apiezo crystal (14) connected to bar (12) is activated via an electronic oscillation generator (15) such that blade (13) executes oscillating movements essentially parallel to the cutting edge (13a). In the example shown here, another cover (11) is located inside the outer cover (10) where it can be moved such that the portion of the blade which projects out of the outer cover (10) can be adjusted. An axis (18) connects bar (12) with cover (11) such that this end of the bar is attached to a mass. At its bottom end, cover (10) is provided with a contact surface (16) which is manually placed onto the tissue. The portion by which the blade projects beyond the contact surface, hence, defines the depth of the cut in the tissue. The invention proposes that the cutting devices feature an adjusting unit to define the maximum and minimum distances by which the blade can project beyond the contact surface. The adjusting range of this interval is preferably selected such that its lower limit is longer than 200 fim while its upper limit is smaller than 2500 fim This adjusting range is preferably set between 0.5 and 2.0 mm or, even more preferred, between 0.7 and 1.3 mm.
For proper performance of the device it is important that the bar (12) be made of a material that exhibits a sufficiently high modulus of elasticity to transfer the energy of the piezo oscillation to the blade (13). Suitable materials for the bar are, for example, glass, spring steel, plastics and ceramics. Moreover, it is also important that the weight of the oscillating components (bar and blade) is small in comparison to the weight on the opposite site of the piezo element. In FIG. 3 the piezo element is coupled via a connecting piece (11a) to the inner cover (11). It is particularly advantageous to operate the apparatus such that bar and blade oscillate at resonance frequency.
The Spring-Operated Blood Lancet
FIG. 4 shows examples of embodiments where the manual movement which is necessary with the apparatus shown in FIG. 3, is replaced by the effect of a spring element (20). Spring (20) is connected to guide rail (21) which has a recess for a lever (22) to engage. Lever (22) is provided with a push-button located outside the housing. When pressing the push-button, the lock is released and the inner cover (11) is moved relative to the outer cover. Blade (13) is thus moved outside the outer cover. By varying the distance X between the edge of the inner cover (11) and the inside of the contact surface (16) in the initial position, it is possible to adjust the cutting depth by which the blade penetrates the tissue. This can be accomplished, for example, by varying the length of the outer cover, e.g. with the aid of a screw. Additional options to vary the penetration depth are known from U.S. Pat. No. 4,895,147 and U.S. Pat. No. 5,318,584. The arrangement can also be selected such that the blade is fixed in its position while the skin contact surface is provided on the front side of a cover which is mounted to an adjustable abutment in a spring-supported manner.
The unit for adjusting the penetration depth is preferably configured such that it can be set at given levels. At least in