Abstract:
An injection needle for introducing a product into a human or animal body, the needle including a distal needle section with a needle point and a proximal needle section, both sections formed along the injection needle such that the proximal needle section must penetrate the skin to introduce the product, the distal needle section having greater flexural rigidity than the proximal needle section.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S)  
       [0001]     This application is a continuation of International Application No. PCT/EP2005/000317, filed Jan. 14, 2005, which claims priority to German Application No. 10 2004 002 472.3, filed Jan. 16, 2004, the entire contents of both of which are incorporated herein by reference. 
     
    
     BACKGROUND  
       [0002]     The present invention relates to devices for delivering, dispensing, administering or injecting a substance, and to methods of making and using such devices. More particularly, it relates to an injection needle for introducing or administering a substance or product into organic tissue, to the use of an injection needle in an infusion set, perfusion set or other delivery device, and to an injection needle assembly in which the injection needle is guided axially. The administration can be made into the skin, subcutaneously, into deeper-lying tissue layers, for example into muscle tissue, or intravenously.  
         [0003]     One form of administration of medical or cosmetic products, or of health care products in general, is the subcutaneous administration by a cannula. An example of subcutaneous administration of a product is the administration of insulin in diabetes treatment. For this administration, a cannula tip is positioned subcutaneously. For this purpose, the cannula has to be guided through the skin. Steel cannulas are generally used that have a sufficient flexural rigidity for piercing and penetrating the skin. Because of the required flexural rigidity, however, these injection cannulas are inflexible during the administration and are therefore a source of discomfort, especially if the cannula remains in the body tissue over a considerable period of time. For cannulas remaining at the injection site, infusion sets are known which comprise a catheter head whose inlet end is connected to a catheter that delivers the product to be administered, and whose outlet end comprises the injection cannula. The lack of flexibility of the injection cannula is a disadvantage of these infusion sets in particular.  
         [0004]     To overcome this disadvantage, infusion sets are known that comprise a cannula which remains in the tissue during the administration and which is so flexible that it does not cause discomfort when implanted. Because of their flexibility, however, the flexible cannulas cannot be introduced into the tissue without supporting aids. They would bend or even buckle if an attempt was made to insert them into the skin. To remedy this drawback, steel needles are generally used. The steel needle extends through the flexible cannula and protrudes from the distal or free end thereof. The flexible cannula bears tightly on the steel needle so that it can be pushed together with the steel needle into and through the skin and in this way can be positioned subcutaneously. After the subcutaneous positioning, the steel needle is withdrawn from the cannula, and the cannula remains in the tissue for administration. However, such systems require additional work for sealing after withdrawal of the steel needle and also necessitate careful removal of air. Therefore, their handling is awkward compared to simple injection cannulas made of steel and they are more prone to malfunctioning.  
       SUMMARY  
       [0005]     An object of the present invention to make available an injection needle for introducing a product into organic tissue, which needle is easy to handle when introducing it into the tissue, and flexible within the tissue.  
         [0006]     In one embodiment, the present invention comprises an injection needle for introducing a product into a human or animal body, the needle comprising a distal needle section with a needle point and a proximal needle section, both sections formed along the injection needle such that the proximal needle section must penetrate the skin to introduce the product, the distal needle section having greater flexural rigidity than the proximal needle section.  
         [0007]     In one embodiment, an injection needle according to the present invention can be pushed as such into and through the skin and remains in the tissue with a flexibility that is sufficient to ensure that the injection needle is not a source of discomfort after its introduction into the tissue. The injection needle comprises a distal needle portion (which also may be referred to as the free or front portion) which extends as far as and comprises a tip of the injection needle, and a proximal needle portion (which also may be referred to as the rear portion). These two needle portions are formed along the longitudinal axis of the injection needle in such a way that the proximal needle portion has to be pushed into the skin for the injection needle to reach the desired depth. The injection needle is composed only of the two needle portions.  
         [0008]     According to the invention, the distal needle portion has a greater flexural rigidity than the proximal needle portion. The proximal needle portion forms a flexible site at least locally, but in some embodiments along its entire length. The distal needle portion of greater flexural rigidity lying deeper in the tissue therefore causes no discomfort, or causes much less discomfort than in the case of the known steel cannulas. Of course, the proximal needle portion still has to have sufficient stability to ensure that it does not become completely or largely constricted in the tissue, for example by buckling, such that proper administration of the product is then no longer guaranteed. However, the proximal needle portion can have a flexural rigidity that is as low as the flexural rigidity of the known flexible cannulas.  
         [0009]     About its outer circumference, the injection needle can form one or more outwardly open channels in which the product is introduced into the tissue. In such designs, the injection needle is not hollow, or it additionally has an inner lumen for the product to pass through. However, in some preferred embodiments, the injection needle is an injection cannula, i.e. a hollow injection needle with at least one, in some cases, exactly one, inner lumen through which the product is introduced into the tissue. Where the following text refers to an injection cannula instead of an injection needle and refers not to a needle but instead to a cannula or cannula portions, the intent is to describe any structure suitable for delivering or adminsistering a substances, e.g., a needle, cannula, catheter, conduit, tube, etc, including non-hollow injection needles which have one or more outwardly open channels on an exterior surface for guiding the product.  
         [0010]     In some preferred embodiments, the distal needle portion is made of a material having a greater modulus of elasticity than the material from which the proximal needle portion is made. The material forming the distal needle portion is therefore harder. The greater the modulus of elasticity of the material, the thinner it is possible to make the cannula tip and/or the sharper the distal edge of the distal needle portion, as a result of which the forces that lead to bending or even to buckling during insertion into and through the skin are reduced.  
         [0011]     The flexural rigidity is the product of modulus of elasticity and geometrical moment of inertia. The greater flexural rigidity can therefore in principle also be achieved through a greater geometrical moment of inertia of the distal needle portion compared to the proximal needle portion. However, with an increased geometrical moment of inertia, the modulus of elasticity of the material forming the distal needle portion should not be less than the modulus of elasticity of the material forming the proximal needle portion.  
         [0012]     In some preferred embodiments, the material forming the distal needle portion is a composite material. When using a composite material, the statements made above concerning the modulus of elasticity may also apply for the composite. It is clear, however, that at least one of the materials forming the composite must as such have a greater modulus of elasticity than the material forming the proximal needle portion, in which case the material forming the proximal needle portion can also be a composite material. Thus, for example, a first material can be a support material for embedded strengthening elements which as such have a greater modulus of elasticity than the material forming the proximal needle portion. If the material forming the proximal needle portion and the support material are the same materials, the composite forming the distal needle portion already has the greater modulus of elasticity on account of the embedded fibers.  
         [0013]     One preferred composite material for the distal needle portion is formed from a support material and a coating with which the support material is coated on its inner circumferential surface or its outer circumferential surface. If appropriate, the support material can be provided with a coating both on the inside and on the outside, or a suitable material can be integrated into the support material. The coating is composed of a material having such a high modulus of elasticity that the composite of support material and coating in each case has a higher modulus of elasticity than the material that forms the proximal needle portion. The coating can be formed by means of a liquid which is dried on the support material and thus hardened. It forms, as it were, a kind of paint. The coating can be homogeneous and, as such, have the sufficiently high modulus of elasticity. However, it too can already be a composite material with embedded strengthening elements, for example fibers, oriented in the longitudinal direction of the injection needle. Instead of fibers, or in addition to fibers, it is also possible for hard grains to be embedded in the coating. For this purpose, a powder or finely particulate grains can be mixed in finely distributed form into a coating liquid.  
         [0014]     In some preferred embodiments, the proximal needle portion is formed by an elastically resilient base needle which, in the distal needle portion, is strengthened to increase the flexural rigidity. This strengthening may be achieved by coating the base needle, as has been described above; the base needle forms the support material.  
         [0015]     The greater flexural rigidity in the distal needle portion can be obtained not only by means of a coating, but in principle also by the thickening of the distal needle portion made as a whole from a material including composite material with a higher modulus of elasticity than the material forming the proximal needle portion. In such cases too, support materials with embedded or integral strengthening elements, fibers and/or hard grains, can be used as composite materials.  
         [0016]     In another embodiment, a base material that can be converted by a chemical reaction, for example a cross-linking reaction of a plastic, into a harder material forms the proximal needle portion, and the converted, harder material forms the distal needle portion. Directly during the forming of the needle or after its forming, the distal needle portion that includes the needle tip goes through a process step effecting the reaction, whereas the more flexible distal needle portion is not converted, to maintain the greater flexibility desired there.  
         [0017]     Finally, the greater flexural strength can also be achieved by insertion or attachment of a solid sleeve which forms the cannula tip and which is made of a flexurally rigid, hard material, for example steel. The sleeve can be inserted into a base needle already forming the proximal needle portion as such or can be fitted over it.  
         [0018]     In some embodiments, the proximal needle portion should be longer than the distal portion. In one preferred embodiment, it is at least twice as long as the distal needle portion.  
         [0019]     The following are exemplary tolerances or specifications of some embodiments of the present invention:  
         [0020]     the distal needle portion should have a length of at least approximately 0.5 mm, measured from the needle tip. It is at most approximately 8 mm in length. If the injection needle is inserted deeper than is customary for subcutaneous administration, for example for intravenous administration, then the values for the preferred minimum length and maximum length change proportionally according to the total length of the injection needle.  
         [0021]     the modulus of elasticity of the material from which the distal needle portion is made should be at least approximately 1000 MPa-3000 MPa.  
         [0022]     the proximal needle portion should have a length of at least approximately 2 mm.  
         [0023]     the modulus of elasticity of the material from which the proximal needle portion is made is less than approxiamately 3000 MPa-2000 MPa, but should be greater than approximately 500 MPa-1000 MPa.  
         [0024]     A catheter through which the product to be administered is delivered to the injection needle can form the proximal needle portion in one piece. If appropriate, a transition is formed by the catheter thinning from a greater catheter cross section to a smaller cannula cross section.  
         [0025]     In some preferred embodiments, the injection needle is part of an infusion set. The infusion set comprises a housing with an underside which can be positioned on the skin at the injection site. Although, during use, the housing can in principle be held at the injection site by means of the injection needle inserted into the tissue, in some preferred embodiments, the underside of the housing is itself made ready for fixing to the injection site. This can be achieved, for example, by an adhesive pad located on the underside, as in conventional infusion sets. The injection needle is supported by a holding part of the housing and protrudes from an underside of the holding part. The infusion set further comprises a catheter for delivery of the administered product to the housing, and a fluid connection which is formed in or on the housing and which serves to connect the catheter to the injection needle. The fluid connection can be formed with the catheter in one piece. The upstream end of the connecting line can be connected to the catheter by means of a quick-coupling mechanism formed by the housing and is able to be detached again from the catheter. The fluid connection is permanently connected to the injection needle.  
         [0026]     In one preferred infusion set in accordance with the present invention, the housing guides the injection needle in an axial movement and stabilizes it against buckling and bending during the axial movement. In these embodiments, the housing forms a needle guide. The needle guide thus provides lateral support for the injection needle. In a preferred embodiment, the needle guide can be axially shortened to permit insertion of the needle into the skin. In such a configuration, the needle guide can be axially shortened in an elastic or permanent manner. The axial shortening can be achieved by designing the needle guide so that it can collapse or fold up. A collapsible needle guide can be formed as a balloon or as a porous structure. A bellows structure can, for example, form a foldable needle guide.  
         [0027]     In some embodiments, when a pressure is applied to the skin, the housing tensions the skin before injection, and the pressure force needed to insert the needle into the skin is reduced. The guiding and stabilizing may be thought of as separate, distinct functions or features, but they may be applied in combination, i.e. the housing then forms a needle guide that tensions the skin.  
         [0028]     In other preferred embodiments, the injection needle is part of a perfusion set. Perfusion sets can be used in particular for diagnostic purposes, for example for determining the glucose content in a body fluid, for example in diabetes therapy. By means of such perfusion sets, body fluid is transported out of the body by means of a flushing liquid, comparable with the flushing in dialysis, and is delivered to a sensor, in this example a glucose sensor. The statements made herein concerning infusion sets apply equally to perfusion sets. A part of such sets that can be positioned on body tissue forms a needle injection unit comprising the injection needle and the needle guide. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0029]      FIG. 1  shows an injection needle according to one illustrative embodiment of the present invention,  
         [0030]      FIG. 2  shows a cross section through a distal needle portion of the injection needle,  
         [0031]      FIG. 3  shows a cross section through a needle portion of an injection needle according to another illustrative embodiment,  
         [0032]      FIG. 4  shows a cross portion through a needle portion of an injection needle according to another embodiment,  
         [0033]      FIG. 5  shows an injection needle according to another illustrative embodiment in a longitudinal section,  
         [0034]      FIG. 6  shows another embodiment of the present invention comprising a catheter head and an injection needle,  
         [0035]      FIG. 7  shows the catheter head at an injection site before the injection needle is inserted into the skin,  
         [0036]      FIG. 8  shows the catheter head before the injection, but with pressure applied,  
         [0037]      FIG. 9  shows the catheter head after the injection needle has been inserted into the skin,  
         [0038]      FIG. 10  shows a catheter head according to another illustrative embodiment,  
         [0039]      FIG. 11  shows the catheter head of  FIG. 10  after the injection needle has been inserted into the skin,  
         [0040]      FIG. 12  shows a catheter head according to another illustrative embodiment with an injection needle,  
         [0041]      FIG. 13  shows the catheter head of the embodiment of  FIG. 12 , and  
         [0042]      FIG. 14  shows the catheter head of embodiment of  FIG. 12  after the injection needle has been inserted into the skin. 
     
    
     DETAILED DESCRIPTION  
       [0043]      FIG. 1  shows an illustrative embodiment of the present invention comprising an injection needle in the form of an injection cannula with two different cannula portions. The cannula portions include a distal cannula portion  1 , which forms a free tip of the injection cannula, and a proximal cannula portion  2  which adjoins the distal cannula portion  1 . A transition between the cannula portions  1  and  2  is substantially linear in a cross-sectional surface perpendicular to a longitudinal axis L of the injection cannula.  
         [0044]     The distal cannula portion  1  has a length L 1  measured along the longitudinal axis L, and the proximal cannula portion  2  has a length L 2 . The sum of the lengths L 1  and L 2  corresponds to the length of conventional injection cannulas for subcutaneous administration of a medicament, for example insulin, and it is for this purpose that the injection cannula according to the invention is also employed. The total length L 1 +L 2  of the injection cannula thus amounts to between 4 and 16 mm. The table below shows approximate lengths for these injection cannulas, the data regarding L 1  being shown with approximate upper and lower limit values, and the difference to L 1 +L 2  being compensated by L 2 :  
                                                   L1 + L2   L1                            4 mm   0.1-2 mm              6 mm   1-3 mm            8 mm   1-4 mm           10 mm   1-5 mm           12 mm   1-6 mm           14 mm   1-7 mm           16 mm   1-8 mm                      
 
         [0045]     In some embodiments, the shape of the injection cannula generally corresponds to that of conventional injection cannulas, i.e. it is circular and has a circular hollow cross section. The distal cannula portion  1  is beveled to form the tip. The proximal cannula portion  2  is made of a plastic material. Possible plastic materials in question are all suitable materials, including those that are used in conventional, flexible infusion cannulas. Polymer acrylate is one example.  
         [0046]     The distal cannula portion  1  is composed of a two-layer composite material. The inner of the two layers is composed of the same material as the proximal cannula portion. The inner layer of the distal cannula portion  1  and the proximal cannula portion  2  form a one-piece base cannula  3 . To obtain greater flexural rigidity in the distal cannula portion  1 , the base cannula  3  in the distal cannula portion  1  is provided with an outer coating  4 . The outer coating  4  is applied uniformly on the outer circumferential face of the base cannula  3 . Its thickness is much smaller than the thickness of the base cannula  3 . The coating  4  is formed as a hard lacquer layer which also covers the tip of the injection cannula. The modulus of elasticity of the coating  4  should be at least twice as great as the modulus of elasticity of the material of the base cannula  3 . The combination of base cannula  3  and coating  4  has a greater overall flexural rigidity than the base cannula  3  and, therefore, than the proximal cannula portion  2 . This is related in each case to the annular cross section per portion  1  and  2 . The coating  4  is formed by application of a liquid that hardens or is hardened after application. The coating  4  can be applied to the inside and outside, for example by immersing the cannula portion  1  in an immersion bath. To obtain the coating  4  with a small layer thickness, the applied liquid has a low viscosity, so that the coating  4  is like a paint.  
         [0047]      FIG. 2  shows the distal cannula portion  1  of the injection cannula from  FIG. 1  in a cross section. The wall thickness of the base cannula  3  can correspond to the wall thickness of conventional flexible infusion cannulas. The thickness of the coating  4  amounts to approximately 10% or less of the thickness of the base cannula  3 .  
         [0048]      FIG. 3  shows an injection cannula in another illustrative embodiment, again in cross section through its distal cannula portion  1 . The proximal cannula portion  2  of the injection cannula in the second illustrative embodiment corresponds to the proximal cannula portion  2  of the first illustrative embodiment. The cannula portion  1  of the second illustrative embodiment differs from that of the first illustrative embodiment only in terms of a modified coating  4 . In contrast to the coating  4  of the first illustrative embodiment, which is composed homogeneously of a hardened coating material, the coating  4  of the second illustrative embodiment has longitudinal fibers  5  embedded or integrated into the same support material, these fibers  5  further increasing the modulus of elasticity of the coating  4  and therefore also the modulus of elasticity of the combination of base cannula  3  and coating  4  relative to the first illustrative embodiment. With the same cross-sectional shape and surface area as in the first illustrative embodiment, the flexural rigidity as a product of the modulus of elasticity and geometrical moment of inertia is correspondingly greater.  
         [0049]     Another illustrative embodiment of an injection cannula is shown in  FIG. 4 . Again, only a cross section through the distal cannula portion  1  of the third illustrative embodiment is shown. The cannula portion  1  of this embodiment is likewise composed of a composite material, which however consists of only one layer of the material of the base cannula  3  of the first and second illustrative embodiment and of longitudinal fibers  5  embedded therein. Unlike in the second illustrative embodiment, the longitudinal fibers  5  are thus not embedded only in a coating, but directly in the whole cross-sectional surface area of the base cannula  3 , which forms the proximal cannula portion  2 , too.  
         [0050]     Instead of or in addition to the longitudinal fibers  5 , granular particles can be embedded in the coating  4  or in the base cannula  3  in the distal cannula portion  1 , these particles also leading to an increase in the modulus of elasticity compared to the material of the base cannula  3 . If appropriate, the distal cannula portion  1  can also be made up of more than two concentric layers. It is also possible to apply a coating which works into the material of the base cannula  3  and there leads to an increased modulus of elasticity across the entire cross-sectional area or at least in an outer part of the cross-sectional area.  
         [0051]      FIG. 5  shows an injection cannula according to a fourth illustrative embodiment of the present invention. A base cannula  3 , which is sufficiently flexible (like the base cannula  3  of the other illustrative embodiments) so as not to cause discomfort in the inserted state, but which on the other hand is sufficiently stable to ensure an adequate cross section of flow for product administration despite the desired resiliency and the associated deformation, extends along almost the entire length L 1 +L 2  of the injection cannula. To obtain the greater flexural rigidity in the distal cannula portion  1 , a thin sleeve  6  is inserted into the base cannula  3 , the length of which sleeve  6  is intended to correspond to the lengths L 1 , as indicated in the first illustrative embodiment, plus an additional length of approximately 5 to 20%. The sleeve  6  is pressed into the base cannula  2  so that the base cannula  3  is tensioned around the sleeve  6 . The hollow cross section of the sleeve  6  generally corresponds to the hollow cross section of the unloaded base cannula  3 . The sleeve  6  protrudes from the base cannula  3  by the stated additional length and forms the cannula tip. This results in an injection cannula being obtained which is somewhat reminiscent of the conventional systems with a flexible cannula and with a steel needle extending through the latter. However, the base cannula  3  and the sleeve  6  are fixedly connected to one another and form one unit. During administration of the product, the sleeve  6  remains in the base cannula  3  and is also discarded together with the latter after use, such that handling is made much simpler compared to the known, two-part systems.  
         [0052]     The sleeve  6  can be a steel sleeve and correspond to a short portion of conventional steel cannulas for subcutaneous administration of products. Instead of an inner sleeve, the injection cannula can also be formed with an outer sleeve.  
         [0053]     FIGS.  6  to  14  show cannula units which are obtained with an injection cannula according to the present invention and which are in the form of catheter heads of infusion sets, for example an infusion set for administration of insulin. Such infusion sets may be used for self-administration, i.e. administration to oneself. The catheter head guides the injection cannula in axial movement and supports it laterally, such that the injection cannula is stabilized against bending and buckling when pressed into and through the skin.  
         [0054]      FIG. 6  shows, in a first illustrative embodiment, a cannula unit comprising an injection cannula with portions  1  and  2 , a cannula guide  10  for the injection cannula, and a pressure force distributor  7 . The cannula unit serves for subcutaneous administration of a liquid product, a medicament, for example insulin. With its proximal end, the cannula portion  2  forms a securing portion  2   a  which is at an angle, in the illustrative embodiment at a right angle, to the distal part of the cannula portion  2 . The securing portion  2   a  is connected to a catheter  8  for delivery of the product. The pressure force distributor  7  has a planar configuration, in the form of a round plate in the illustrative embodiment.  
         [0055]     The injection cannula and the pressure force distributor  7  are separately produced parts. The injection cannula is held with frictional engagement in the central passage of the pressure force distributor  7  and is secured lying flat on the top face of the pressure force distributor  7 . In a modified design, the injection cannula and the pressure force distributor  7  can also be formed in one piece, or the injection cannula can be embedded with its securing portion  2   a  in the pressure force distributor  7  and cohesively connected to the pressure force distributor  7 .  
         [0056]     The cannula guide  10  is an air-filled balloon with a flexible balloon wall  11 , so that a cannula guide is obtained which has a flexible axial portion  15  between an underside  13  and a top face  14 . The balloon  10  is annular and encloses the injection cannula. The cannula tip is set back a short distance behind an underside  13  of the balloon  10 . The pressure force distributor  7  is secured lying on the top face  14  of the balloon  10 . The balloon  10  bears with its internal pressure uniformly on the injection site. The internal pressure of the balloon  10  is at least as great as the atmospheric pressure, and an overpressure prevails inside the balloon wall  11 .  
         [0057]     Arranged in the balloon  10  there is a support structure  12 , approximately at the axial center of the injection cannula. The support structure  12  is, as the name is intended to suggest, planar and flat in the axial direction, i.e. in the longitudinal direction of the injection cannula. In the illustrative embodiment, the support structure  12  is a thin support plate, a support membrane, which can be deformed into a flat shell. The support structure  12  extends, transversely with respect to the injection cannula  1 , across the entire radial width of the balloon  10 , from its annular outside wall to its annular inside wall and thus forms, in addition to the annular inside wall of the balloon  10 , a local support for the injection cannula.  
         [0058]     The underside  13  of the balloon  10  is provided, for example coated, with an adhesive, so that an outer adhesive surface is obtained which ensures an adhesive connection of the cannula unit  10  to the surface of the body tissue, generally the surface of the skin. The balloon wall  11  is likewise provided with an adhesive across its entire inner surface. Similarly, the support structure  12  is provided with an adhesive on its underside directed toward the underside  13  and on its top face directed toward the top face  14 . In this way, inner adhesive surfaces  16  are obtained which adhere to one another in a collapsed state of the balloon  10 . It would in principle suffice to provide an adhesive only on the underside and top face of the support structure  12  and/or only on the inner surfaces of the balloon wall  11  on the underside  13  and top face  14  of the balloon  10 .  
         [0059]      FIGS. 7, 8  and  9  show the cannula unit of the first illustrative embodiment in use.  
         [0060]     In  FIG. 7 , the cannula unit is placed on the surface of the body tissue  9  and fixed adhesively by means of its underside  13  formed as an outer adhesive surface. No external force is applied to the cannula unit, or at most a light pressure force which is directed axially in the direction of the surface of the body tissue  9  and which is sufficient to establish the adhesive connection. The cannula tip is located a short distance above the surface of the body tissue  9 , i.e. there is still no contact with the body tissue  9 .  
         [0061]      FIG. 8  shows the cannula unit of the first illustrative embodiment in the initial phase of insertion of the injection cannula into the skin. By means of a pressure force F exerted on the pressure force distributor  7  in axial continuation of the injection cannula and directed axially in the direction of the body tissue  9 , the pressure force distributor  7  presses against the balloon  10  via the top face  14  of said balloon  10 , and the latter accordingly deforms under the pressure force F. Because of the pressure force F, the injection cannula moves axially in the direction toward the surface of the body tissue  9 , comes into contact with the surface and initially just presses against the surface, until the surface has reached a critical tension at which the cannula tip pierces the surface and penetrates into the body tissue  9 .  FIG. 8  shows the cannula unit directly before it pierces the surface of the body tissue  9 .  
         [0062]     During the movement toward the surface of the body tissue  9 , during the piercing of the surface and during the penetration into the body tissue  9 , the injection cannula slides along the inside wall of the balloon  10  surrounding it. The support structure  12  stabilizes and guides the injection cannula in the first instance. The balloon  10 , in which the support structure  12  is accommodated, additionally supports and guides the injection cannula throughout the entire injection procedure. The support structure  12  and the balloon  10  thus stabilize the cannula portion  2  particularly against bending or even buckling. The injection cannula protruding freely from the underside of the pressure force distributor  7  can therefore have less flexural rigidity, namely a lower modulus of elasticity and/or a lower geometrical moment of inertia, than injection cannulas which are not laterally supported during the piercing of the tissue surface and their onward penetration into the tissue. The injection cannula is accordingly less “bulky” when it is sitting in the body tissue  9  during the administration of product.  
         [0063]     The balloon  10  is constructed such that it bursts when its internal pressure exceeds a predetermined limit value. This limit value is provided for through a suitable dimensioning of the balloon wall  11 , i.e. through the use of a suitable wall material and through the wall thickness. The balloon wall  11  is configured such that, when the pressure limit value is exceeded, it tears and the balloon  10  suddenly collapses. The design of the balloon  10  is advantageously such that the balloon  10  bursts after the cannula tip is already pressing against the body tissue  9  but when the cannula tip has not yet penetrated the body tissue  9 . The penetration, i.e. piercing of the tissue surface, takes place directly upon collapse of the balloon  10 .  
         [0064]     The balloon  10 , and the cannula guide according to the present invention in general, may also be advantageously configured in such a way that, by manual pressure on the top face  14 , i.e. the application of the pressure force F, the surface of the body tissue  9  is tensioned at the injection site and, in this way, the pressure force required for penetration of the surface is reduced.  
         [0065]      FIG. 9  shows the cannula unit in the implanted state. The injection cannula protrudes with its portions  1  and  2  into the body tissue  9 . The balloon  10  has completely collapsed and forms a flat plaster adhering to the surface of the body tissue  9 , since the outer adhesive surface on the underside  13  of the previous balloon  10  adheres to the body tissue  9  and the inner surfaces  16  adhere to one another. In this state, a medicinal product may be administered through the injection cannula over the course of several days.  
         [0066]      FIG. 10  shows a second illustrative embodiment of a cannula unit consisting of an injection cannula, a pressure force distributor  7  and a cannula guide  17 . The injection cannula and the pressure force distributor  7  are designed as in the first illustrative embodiment. The cannula guide  17  also forms a flexible axial portion  15  which, as before in the first illustrative embodiment, extends from the underside  13  to the top face  14  of the cannula guide  17 . The cannula guide  17  of the second illustrative embodiment is designed as a bellows with pairs of support webs  18  pointing at an angle to one another and to the cannula portions  1  and  2 , and folding joints  19   a  and  19   b  which are in each case formed between two adjacent support webs  18 . The inner folding joints  19   a  are not only joints, but at the same time also form a supporting and guiding position for the injection cannula.  
         [0067]     The support webs  18  are of different lengths, with the length increasing from the underside  13  to the top face  14 . Two support webs  18  of identical or substantially identical length are in each case connected to one another in a foldable manner at the outer folding joints  19   b.  When the unit is placed in position on the surface of the body tissue  9 , the most distal support web  18  points obliquely and radially outward from the most distal inner folding joint  19   a,  such that an open funnel is obtained on the underside  13 . Therefore, as in the first illustrative embodiment, when a pressure force F is exerted, the tissue surface is tensioned at the injection site and, this way, penetration of the tissue surface is made easier.  
         [0068]     The bellows structure forming the cannula guide  17  elastically yields in the axial direction when an axial pressure force F is exerted, up to the point where a limit value is reached for the axial pressure force F, but abruptly collapses when the limit value is exceeded. The cannula guide  17  is designed like the cannula guide  10  of the first illustrative embodiment in terms of its deformation properties, as far as the elastic resiliency and abrupt collapse are concerned.  
         [0069]      FIG. 11  shows the cannula unit of the second illustrative embodiment in the implanted state of the injection cannula, in which the latter&#39;s penetrating portion  3  has penetrated completely into the body tissue  9 . In this state, the cannula guide  17  of the second illustrative embodiment likewise forms a flat plaster, because the support webs  18  are folded in pairs on top of one another. To stabilize the cannula guide  17  in the folded state, the support webs  18  are also provided with inner adhesive surfaces  16 . Moreover, those support webs  18  with undersides pointing toward the body tissue  9  are provided with outer adhesive surfaces  13 a on these undersides, such that the support webs  18  on the one hand adhere to one another via their outer surfaces and, because the support web lengths increase from distal to proximal, they also adhere directly on the surface of the body tissue.  
         [0070]      FIG. 12  shows a cannula unit of a third illustrative embodiment. The cannula unit differs from the cannula units of the other illustrative embodiments in terms of its cannula guide  20 , which in the third illustrative embodiment is designed as an umbrella structure, i.e. as a structure which can be deployed, opened or spread open in the manner of an umbrella and can thereby be shorted in the length direction of the injection cannula.  
         [0071]      FIG. 13  shows the cannula unit of the third illustrative embodiment in a state in which it is placed on the body tissue  9  before insertion of the injection cannula into the skin. As can be seen from  FIG. 13 , the cannula guide  20  comprises several spreadable struts  21  which are each attached in an articulated manner to an underside of the force distributor  7  directed toward the body tissue  9 . The articulated attachment is such that the inherently axially stiff spreadable struts  21  can be pivoted toward the underside of the force distributor  7  at their respective articulation. In relation to the injection cannula, the spreadable struts  21  point radially outward from their articulations. They are arranged in uniform distribution around the injection cannula. The spreadable struts  21  are each supported on the injection cannula via several support struts  22 . The support struts  22  are each attached in an articulated manner to the spreadable struts  21  and form an axial slide guide for the injection cannula, which axial guide laterally supports the injection cannula and axially guides it in a linear movement. The articulated attachments of the support struts  22  to the spreadable struts  21  are designated by  23 , and the slide guides at the respective other end of the support struts  22  are designated by  24 . Along the spreadable struts  21 , the articulated attachments  23  are each at a distance from the articulated attachments of the spreadable struts  21  on the force distributor  7  which corresponds to the length of the respective support strut  22 . Thus, for example, the support struts  22  which have the greatest distance a from the articulated attachments of the spreadable struts  21  on the force distributor  7  each have a length a corresponding to the distance. The support struts  22  arranged closer to the force distributor  7  each have lengths corresponding to their distances measured along the spreadable struts  21 . With uniform distribution, as shown in the illustrative embodiment, lengths ⅔ a and ⅓ a are obtained for the further support struts  22 .  
         [0072]      FIG. 14  shows the cannula unit of the third illustrative embodiment with the injection cannula inserted into the body tissue  9 . The spreadable struts  21  are pivoted, about their articulated attachments on the force distributor  7 , toward the force distributor  7  and are thus spread open. The support struts  22  are pivoted about their articulated attachments  23  toward their respective spreadable strut  21  and come to lie one on the other, so that overall a flat structure is obtained in the spread or compressed state, which flat structure at the same time also serves as a plaster for attachment to the tissue surface.  
         [0073]     As is indicated in  FIG. 12  and can be seen in  FIG. 14 , the cannula unit of the third illustrative embodiment comprises a plaster  25  which, in accordance with the spreading mechanism, can be designated as an umbrella-type plaster. The plaster  25  is similar to the cover of an umbrella. It is secured on the spreadable struts  21 . In the non-inserted state, i.e. before being spread open, it hangs loosely like the cover of an umbrella between the spreadable struts  21 , whereas in the inserted state it is stretched out and adheres with its underside on the tissue surface.  
         [0074]     Embodiments of the present invention, including preferred embodiments, have been presented for the purpose of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms and steps disclosed. Obvious modifications or variations are possible in light of the above teachings. The embodiments were chosen and described to provide the best illustration of the principles of the invention and the practical application thereof, and to enable one of ordinary skill in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. All such modifications and variations are within the scope of the invention as determined by the appended claims when interpreted in accordance with the breadth they are fairly, legally, and equitably entitled.