Abstract:
A straight cutting tip for a straight bore subcutaneous implantation instrument is provided. An incising shaft body defines an axial bore extending continuously throughout the incising shaft body&#39;s length. The axial bore is open on both distal and proximal ends of the incising shaft body and has a non-circular cross section of at least five millimeters. A beveled surface is transversely formed beginning on a top surface and ending on a bottom surface of the distal end of the incising shaft body. A straight and sharpened cutting edge with rounded ends on each side curves inwardly towards the proximal end of the incising shaft body. The cutting edge is defined only along a bottom distal edge of the beveled surface. An attachment point is formed on the proximal end of the incising shaft body.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This patent application is a continuation of application Ser. No. 11/345,617, filed Feb. 1, 2006, now U.S. Pat. No. 7,780,625, issued Aug. 24, 2010; which is a continuation of U.S. Ser. No. 11/025,770, filed Dec. 20, 2004, abandoned; which is a continuation of U.S. patent application Ser. No. 10/222,719, filed Aug. 15, 2002, abandoned; which is a continuation of application Ser. No. 09/644,666, filed Aug. 24, 2000, now U.S. Pat. No. 6,436,068, issued Aug. 20, 2002, the priority dates of which are claimed and the disclosures of which are incorporated by reference. 
    
    
     FIELD 
     The present invention relates in general to subcutaneous implantation instruments and methods and, in particular, to straight cutting tip for a straight bore subcutaneous implantation instrument. 
     BACKGROUND 
     A major part of health care assessment involves the review and analysis of physiological measurements collected and recorded by electronic data sensors. In addition to vital signs, physiological measures can include detailed measurements of organ functions, body fluid chemistry and rates, activity levels, and similar measures, both measured directly and derived. 
     The type and quality of physiological measures depends greatly on the type and location of the sensor employed. External sensors, such as thermometers, blood pressure cuffs, heart rate monitors, and the like, constitute the most common, and least invasive, form of sensors. However, these sensors are extremely limited in the kinds of information which they are able to collect and encumber the patient with wearing and maintaining an external sensor. On the other extreme, implantable in situ sensors provide the most accurate and continuous data stream through immediate proximity to organs and tissue of interest. However, implantable sensors are invasive and generally require surgery for implantation. 
     Recent advances in microchip technology have created a new generation of highly integrated, implantable sensors. For instance, PCT Application Nos. PCT/GB99/02389, to Habib et al., filed Jul. 22, 1998, pending, and PCT/GB99/02393, to Habib et al., filed Jul. 22, 1998, pending, respectively describe an implantable sensor chip and treatment regiment, the disclosures of which are incorporated herein by reference. The sensor chip is adapted to receive and rectify incoming electromagnetic signals and to transmit data relating to treatment parameters by wireless telemetry to a receiver external to a body. Similarly, the emerging Bluetooth wireless communication standard, described at http://www.bluetooth.com/developer/specification/specification.asp, proposes a low cost, small form factor solution to short range data communications, potentially suitable for use in implantable sensor technology. 
     Even though implantable sensor technology is trending towards smaller and more specialized microchip sensors, in humans, these sensors must still be implanted via surgical procedure. Minimally invasive implantation using large bore needles is impracticable because sensors, particularly when embodied using microchip technology, favor a prismatic shape with substantially rectangular cross sections. A large bore needle can cause a core of flesh or skin (or hide, when used in domesticated animals) to form in the pointed tip as the needle is inserted. Cylindrical needles also severely limit solid sensor sizes, shapes and dimensions to only those that can be inserted through a circular bore. 
     Although current surgical approaches attempt to minimize the size of incision and decree of invasion, implantation is, at best, costly, time-consuming, traumatic, requires multiple instruments and maneuvers, and potentially risky to the patient. Subcutaneous implantable sensors offer the best compromise between in situ sensors and external sensors and are potentially insertable with a simple injection. These sensors are typically implanted below the dermis in the layer of subcutaneous fat. The subcutaneous implantation of solid materials has been described in the prior art as follows. 
     An insertion and tunneling tool for a subcutaneous wire patch electrode is described in U.S. Pat. No. 5,300,106, to Dahl et al., issued Apr. 5, 1994. The tunneling tool includes a stylet and a peel-away sheath. The tunneling tool is inserted into an incision and the stylet is withdrawn once the tunneling tool reaches a desired position. An electrode segment is inserted into the subcutaneous tunnel and the peel-away sheath is removed. Although providing a toot for subcutaneous implantation, the Dahl device requires an incision into the subcutaneous fat layer and forms an implantation site larger than the minimum sized required by the electrode segment. Further more, the cylindrical bore precludes the injection of non-conforming solid sensors or materials. 
     An implant system for animal identification that includes a device for implanting an identification pellet in a fat layer beneath the hide or skin of an animal is described in U.S. Pat. No. 4,909,250, to Smith, issued Mar. 20, 1990. The device includes a curved needle-like tube that terminates at a tapered, sharpened point. An elongated, flexible plunger is slidably received within the needle-like tube. The pointed tip is inserted through the hide or skin and the plunger is actuated to drive the identification pellet from the tip into the fat layer. However, the Smith device uses an oversized open bore which can cause coring of the hide or flesh. 
     A trocar for inserting implants is described in PCT Application No. PCT/US99/08353, to Clarke et al., filed Oct. 29, 1999, pending. An implant retention trocar includes a cannula for puncturing the skin of an animal and an obturator for delivering the implant. A spring element received within the cannula prevents an implant from falling out during the implant process. The cannula has a distal tip design which causes a minimum of trauma and tearing of tissue during implant insertion. However, the distal tip design is specifically directed to cannulas having a substantially circular bore and thereby limits the size and shape of implant which can be inserted through the Clarke trocar. 
     An instrument for injecting implants through animal hide is described in U.S. Pat. No. 5,304,119, to Balaban et al., issued Apr. 19, 1994. The instrument includes an injector having a tubular body divided into two adjacent segments with a hollow interior bore. A pair of laterally adjacent tines extend longitudinally from the first segment to the distal end of the tubular body. A plunger rod has an exterior diameter just slightly larger than the interior diameter of the tubular body. With the second segment inserted beneath the animal hide, the push rod is advanced longitudinally through the tubular body, thereby pushing the implant through the bore. As the implant and rod pass through the second segment, the tines are forced radially away from each other, thereby dilating or expanding the incision, and facilitating implant. The instrument is removed from the incision following implantation. Though avoiding the coring of animal hide or flesh, the instrument forms an implantation site larger than the minimum sized required by the implant and causes potentially damaging compaction of the implant against the laterally adjacent times during implant delivery. 
     Therefore, there is need for a non-surgical instrument and method for subcutaneous implantation of sensors and solid materials that preferably does not require an incision preparatory to instrument insertion. 
     There is a further need for a subcutaneous implantation instrument and method capable of implanting sensors and other solid materials that are not readily disposed to implantation through a substantially circular bore. 
     Moreover, there is a further need for a subcutaneous implantation instrument and method which is minimally invasive, preferably creating the smallest needed implantation site, and capable of implantation without exposing the implant to longitudinal stresses. 
     SUMMARY 
     The present invention provides an implantation instrument and method of use for implanting sensors and other solid materials in a subcutaneous or other site. As used herein, “subcutaneous” refers generally to those implantation sites located within a body below the skin. The implantation instrument consists of an incising shaft attached to a syringe body. The syringe body and incising shaft both define a substantially non-circular hollow bore for accommodating the sensor or solid material. The subcutaneous site is formed by a cutting edge on the distal end of the incising shaft. The subcutaneous site can be cleared using a clearing trocar slidably received within the hollow bore. The sensor or solid material is advanced through the hollow bore and delivered into the subcutaneous site. The depth of the subcutaneous site can be limited using a penetration limiting mechanism. 
     One embodiment provides a straight cutting tip for a straight bore subcutaneous implantation instrument. An incising shaft body defines an axial bore extending continuously throughout the incising shaft body&#39;s length. The axial bore is open on both distal and proximal ends of the incising shaft body and has a non-circular cross section of at least five millimeters. A beveled surface is transversely formed beginning on a top surface and ending on a bottom surface of the distal end of the incising shaft body. A straight and sharpened cutting edge with rounded ends on each side curves inwardly towards the proximal end of the incising shaft body. The cutting edge is defined only along a bottom distal edge of the beveled surface. An attachment point is formed on the proximal end of the incising shaft body. 
     A further embodiment provides a straight cutting tip for a straight bore subcutaneous implantation instrument with penetration limiting mechanism. An incising shaft body defines an axial bore extending continuously throughout the incising shaft body&#39;s length. The axial bore is open on both distal and proximal ends of the incising shaft body and has a non-circular cross section of at least five millimeters. A stopping flange is mounted on a bottom surface of the incising shaft body at an angle of between approximately 30° and 60° bent in a proximal direction. A beveled surface is transversely formed beginning on a top surface and ending on a bottom surface of the distal end of the incising shaft body. A straight and sharpened cutting edge with rounded ends on each side curves inwardly towards the proximal end of the incising shaft body. The cutting edge is defined only along a bottom distal edge of the beveled surface. An attachment point is formed on the proximal end of the incising shaft body. 
     A still further embodiment provides a straight cutting tip for a polygonal straight bore subcutaneous implantation instrument. An incising shaft body defines an axial bore extending continuously throughout the incising shaft body&#39;s length. The axial bore is open on both distal and proximal ends of the incising shaft body and has a cross section in the shape of a simple closed polygon with a height of at least five millimeters. A beveled surface is transversely formed beginning on a top surface and ending on a bottom surface of the distal end of the incising shaft body. A straight and sharpened cutting edge with rounded ends on each side curves inwardly towards the proximal end of the incising shaft body. The cutting edge is defined only along a bottom distal edge of the beveled surface. An attachment point is formed on the proximal end of the incising shaft body. 
     A still further embodiment provides a straight cutting tip for a curved bore subcutaneous implantation instrument. An incising shaft body is formed into a curved arc that defines a curved axial bore extending continuously throughout the incising shaft body&#39;s length. The axial bore is open on both distal and proximal ends of the incising shaft body and has a non-circular cross section of at least five millimeters. A beveled surface is transversely formed beginning on a top surface and ending on a bottom surface of the distal end of the incising shaft body. A straight and sharpened cutting edge with rounded ends on each side curves inwardly towards the proximal end of the incising shaft body. The cutting edge is defined only along a bottom distal edge of the beveled surface. An attachment point is formed on the proximal end of the incising shaft body. 
     A still further embodiment provides a straight cutting tip for a curved bore subcutaneous implantation instrument with penetration limiting mechanism. An incising shaft body is formed into a curved arc that defines a curved axial bore extending continuously throughout the incising shaft body&#39;s length. The axial bore is open on both distal and proximal ends of the incising shaft body and has a non-circular cross section of at least five millimeters. A stopping flange is mounted on a bottom surface of the incising shaft body at an angle of between approximately 30° and 60° bent in a proximal direction. A beveled surface is transversely formed beginning on a top surface and ending on a bottom surface of the distal end of the incising shaft body. A straight and sharpened cutting edge with rounded ends on each side curves inwardly towards the proximal end of the incising shaft body. The cutting edge is defined only along a bottom distal edge of the beveled surface. An attachment point is formed on the proximal end of the incising shaft body. 
     A still further embodiment provides a straight cutting tip for a polygonal curved bore subcutaneous implantation instrument. An incising shaft body is formed into a curved arc that defines a curved axial bore extending continuously throughout the incising shaft body&#39;s length. The axial bore is open on both distal and proximal ends of the incising shaft body and has a cross section in the shape of a simple closed polygon with a height of at least five millimeters. A beveled surface is transversely formed beginning on a top surface and ending on a bottom surface of the distal end of the incising shaft body. A straight and sharpened cutting edge with rounded ends on each side curves inwardly towards the proximal end of the incising shaft body. The cutting edge is defined only along a bottom distal edge of the beveled surface. An attachment point is formed on the proximal end of the incising shaft body. 
     Still other embodiments of the present invention will become readily apparent to those skilled in the art from the following detailed description, wherein is described embodiments of the invention by way of illustrating the best mode contemplated for carrying out the invention. As will be realized, the invention is capable of other and different embodiments and its several details are capable of modifications in various obvious respects, all without departing from the spirit and the scope of the present invention. Accordingly, the drawings and detailed description are to be regarded as illustrative in nature and not as restrictive. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of an instrument for implanting sensors or solid materials in a subcutaneous or other tissue location in accordance with the present invention; 
         FIG. 2A  is a longitudinal cross-sectional view of the implantation instrument with a straight incising shaft 
         FIG. 2B  is a longitudinal cross-sectional view of the implantation instrument with a curved incising shaft; 
         FIG. 3  is a diagrammatic view illustrating the implantation of a sensor or solid material into a subcutaneous site; 
         FIG. 4A  is a diagrammatic view illustrating the clearing of a subcutaneous site using the implantation instrument fitted with a clearing trocar in accordance with a further embodiment; 
         FIG. 4B  is a diagrammatic view illustrating the subcutaneous implantation of a sensor using the implantation instrument fitted with a pushing stylet in accordance with a further embodiment; 
         FIGS. 5A-D  are transverse cross-sectional views of the implantation instrument illustrating, by way of example, various bore configurations; 
         FIG. 6  is a segmented side view of a clearing trocar; 
         FIG. 7  is a segmented side view of a pushing stylet; and 
         FIGS. 8A-8B  are section views illustrating penetration limiting mechanisms for use with the implantation instrument; and 
         FIG. 9  is a perspective view of an instrument for implanting sensors or solid materials in a subcutaneous or other tissue location in accordance with a further embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a perspective view of an instrument  10  for implanting sensors or solid materials in a subcutaneous or other tissue location in accordance with the present invention. The implantation instrument  10  consists of two principal groups of components, an incising body consisting of an incising shaft  11  and a syringe body  15 , and a delivery assembly consisting of a plunger assembly  20 . The delivery assembly is received into the syringe body bore by sliding the plunger assembly  20  through proximal bore opening  19 . 
     The incising shaft  11  is formed with a beveled and rounded tip  12  that tapers into a surgically sharp cutting edge  13  formed on a distal edge. The beveled tip  12  includes a distal bore opening  14  through which the sensor or solid material is delivered into the implantation site. 
     In the described embodiment, the sensor or solid material (implant) has approximate dimensions of 5 mm by 10 mm by 20 mm. The critical dimension is the cross-sectional profile, that is, the height and width, of the implant which must conform to passage through the syringe body and incising shaft bores. Other non-linear, prismatic shapes are equally usable provided the implant can fit within the confines of the syringe body and incising shaft bores. The implant could also be folded or compacted to minimize the cross-sectional profile with the implant unfolding or expanding upon implantation. As well, the implant is preferably protected against damage by encasement within, for example, a mannitol pellet in the case of a solid drug delivery system or epoxy in the case of an implantable sensor. 
     An implantable sensor microchip suitable for use in the present invention is described in PCT Application No. PCT/GB99/02389, to Habib et al., filed Jul. 22, 1998, pending, the disclosure of which is incorporated herein by reference. Such a sensor could be used for monitoring and collecting physiological or chemical measures. Similar devices for therapeutic uses, including treating cancer, and for health care giving, including administering solid medication in the form of boluses, are possible. As well, the present invention has equal applicability to implantation of sensors, including location and identification sensors, and solid materials in domesticated animals. The sensor could also constitute or include a data transmitter with which to exchange information and telemetered signals. 
     The incising shaft  11  is fixably attached to the syringe body  15  through frictional, adhesive, or preformed constructive means, as is known in the art. Both the incising shaft  11  and syringe body  15  define a substantially non-circular hollow bore extending continuously along a shared longitudinal axis, as further described below with reference to  FIGS. 5A-D . 
     The plunger assembly includes a plunger  16 , an interconnecting plunger shaft  17  and a plunger end piece  18 . The plunger  16  is conformably shaped to fit within the syringe body bore. The plunger end piece  18  facilitates deployment of the plunger assembly through the syringe body bore and is preferably shaped to fit a thumb or palm impression. 
     In the described embodiment, the implantation instrument  10  is designed for inexpensive and disposable use utilizing low-cost, sanitizable materials. The incising shaft  11  can be fashioned from surgical grade stainless steel and has the approximate dimensions of approximately 10 mm by 5 mm in cross section. The incising shaft  11  is approximately 50 mm long and the length can be varied to accommodate different implantation depths. The plunger  16  is formed from plastic and rubber and preferably forms a watertight seal within the syringe body bore and has the approximate dimensions of approximately 8 mm by 3 mm in cross section. The plunger shaft  17  and plunger end piece  18  are formed from plastic or similar material. Other materials, as would be recognized by one skilled in the art, could be substituted. 
     In a further embodiment, the syringe body  15  and plunger assembly can be replaced by an automated injection system, such as used with immunization injection guns or similar devices. These devices typically employ compressed air or other inert gases to administer medication in lieu of manual plungers. Other automated variations include spring-loaded and similar mechanical injection systems. The incising shaft  11  is fixably attached to the automated injection system which functions as a delivery mechanism in place of the syringe body  15  and plunger assembly. Thus, the implant would be pushed through the incising shaft bore using the compressed air or gas, or mechanical equivalent. 
       FIG. 2A  is a longitudinal cross-sectional view of the implantation instrument  10  with a straight incising shaft  11 . The hollow bore defined by both the incising shaft  11  and the syringe body  15  runs along a common shared axis. The incising shaft bore  22  is sized to allow the implant to advance smoothly into the implantation site under the forward lateral urging of the plunger assembly  20 . The syringe body bore  23  must be at least as large as the incising shaft bore  22 , but can be slightly larger to accommodate lubricants, anesthetizing agents, or similar coatings, such as mannitol, applied over the sensor or solid material. 
     The syringe body  15  preferably includes a circular collar  21 , pair of winglets, ears, or eyelets, or similar structure, optionally formed on a proximal end of the syringe body  15  to assist a user in depressing the plunger assembly  20 . 
       FIG. 2B  is a longitudinal cross-sectional view of the implantation instrument with a curved incising shaft  24 . The curved incising shaft  24 , as well as the syringe body  15  and related components, are shaped into a substantially continuous curve along the ventral side. The curvature helps regulate the penetration depth of the incising shaft and, in the described embodiment, has an are of approximately 20 degrees. 
       FIG. 3  is a diagrammatic view illustrating the implantation of a sensor  28  or solid material into a subcutaneous site. Prior to delivery, the sensor  28  is fed through the proximal bore opening  19  of the syringe body  15  and then further advanced through the syringe body bore  23 . During operation, the incising shaft  11  is inserted through the dermis  25  and guided into the layer of subcutaneous fat  26 , above the layer of muscle  27 . The sensor  28  is then advanced through the incising shaft bore  22  by the plunger  16  into the subcutaneous site. Note that although the foregoing view illustrates an implant into the subcutaneous fat layer, one skilled in the art would appreciate that subcutaneous implantation locations are not strictly limited to the subcutaneous fat layer and are generally termed as those implantation locations situated within a body under the skin. 
       FIG. 4A  is a diagrammatic view illustrating the clearing of a subcutaneous site using the implantation instrument  10  fitted with a clearing trocar  29  in accordance with a further embodiment. The clearing trocar  29 , as further described below with reference to  FIG. 6 , is mounted to its own handle or plunger assembly and has a sharp cutting tip  30  for optionally clearing a subcutaneous site prior to delivery of the implant. 
     Prior to implantation, the clearing trocar  29  is slidably received into the syringe body  15  and is advanced until the cutting tip  30  is even with the proximal bore opening  19  of the incising shaft  11 . During operation, the incising shaft  11  and clearing trocar  29  are inserted through the dermis  25  and guided into the layer of subcutaneous fat  26 , above the layer of muscle  27 . 
     The cutting edge  13  of the beveled tip  12  makes an entry incision through the dermis  25  and is laterally pushed into the subcutaneous fat  26  until the cutting edge  13  is adjacent to the subcutaneous site. The clearing trocar  29  is then urged through the subcutaneous fat  26  by advancement of its handle or plunger assembly to prepare the implantation site for delivery of the sensor  28  or solid material. The clearing trocar  29  is then withdrawn from the subcutaneous site and out of the implantation instrument  10 . 
       FIG. 4B  is a diagrammatic view illustrating the subcutaneous implantation of a sensor  28  using the implantation instrument  10  fitted with a pushing stylet  31  in accordance with a further embodiment. The pushing stylet  31 , as further described below with reference to  FIG. 7 , has a blunt tip  32  for advancing the sensor  28  (or solid material) through the syringe body bore  23  and incising shaft bore  22  and into the subcutaneous site. The cross section of the pushing stylet  31  closely conforms to the incising shaft bore  22  while the plunger  16  closely conforms to the syringe body bore  23 . The pushing stylet  31  thus extends the reach of the plunger assembly  20  and allows the syringe body bore  23  to have a different cross-section than the incising shaft bore  22 . 
     The pushing stylet  31  is used while the incising shaft  11  is in situ in the subcutaneous layer  26 . Prior to delivery, the sensor  28  is fed through the proximal bore opening  19  of the syringe body  15  and further advanced within the syringe body bore  23  by contact with the plunger  16 . The pushing stylet  31  is slidably received into the syringe body  15  and is advanced until the blunt tip  32  contacts the sensor  28 . During operation, the sensor  28  is urged through the incising shaft bore  22  by the pushing stylet  31  and into the subcutaneous site by advancement of the plunger assembly. Upon delivery of the sensor  28  into the subcutaneous site, the incising shaft  11  and pushing stylet  31  are withdrawn. 
     Although operation of the implantation instrument  10  is described with reference to the implantation of sensors or solid materials into a subcutaneous site situated within the layer of subcutaneous fat  26 , implantations could also be effected in other subcutaneous, intramuscular, intraperitoneal, intrathoracic, intracranial, intrajoint, as well as other organ or non-subcutaneous sites, as would be recognized by one skilled in the art. In addition, the foregoing procedure could be modified to forego the use of the clearing trocar  29  for small sensors  28  or solid materials. The clearing effect of the clearing trocar  29  can be approximated by use of the incising shaft  11  alone whereby the incising shaft  11  is inserted into the subcutaneous site and then withdrawn by reverse deployment, thereby forming a slightly overwide implantation site. 
     The operations of subcutaneous implantation can be carried out over a plurality of sites and with the same or different sensors  28  and solid materials. Similarly, several sensors  28  and solid materials could be implanted at the same subcutaneous site during a single implantation operation. 
       FIGS. 5A-D  are transverse cross-sectional views of the implantation instrument  10  illustrating, by way of example, various bore configurations.  FIG. 5A  illustrates an incising shaft  35  with a substantially rectangular bore  36 .  FIG. 5B  illustrates an incising shaft  37  with a substantially square bore  38 .  FIG. 5C  illustrates an incising shaft  39  with a substantially oval bore  40 . And  FIG. 5D  illustrates an incising shaft  41  with a substantially hexagonal bore  42 . Note the circumferential shape of the incising shaft need not follow the internal shape of the incising shaft bore. Other bore configurations, including variations on oval, rectangular, square, pentagonal, hexagonal, heptagonal, octagonal, and similar equilateral or non-equilateral shapes, are feasible. 
     In the described embodiment, the rectangular bore  36  has the dimensions of approximately 10 mm by 5 mm. The syringe body bore  23  has a length of approximately 5 cm. 
       FIG. 6  is a segmented side view of a clearing trocar  45 . The clearing trocar  45  consists of a beveled tip  47  on the distal end of the clearing trocar  45  and a clearing trocar shaft  46  affixed, either fixably or removably, to the distal end of a plunger  16 . 
     During a clearing operation, the clearing trocar  45  is fully extended from the distal bore opening  14  of the incising shaft  11 . The clearing trocar shaft  46  is only long enough to clear out the subcutaneous site. The plunger  16  acts as a stop that limits the extent of penetration of the clearing trocar  45 , thereby preventing the clearing trocar  29  from incising too deeply into the subcutaneous fat  29 . In addition, the clearing trocar  29  is sized to approximate the girth of the incising shaft bore  22  and will clear a subcutaneous site only as wide as minimally necessary to facilitate implantation of the sensor or solid material. In the described embodiment, the clearing trocar  45  has a length of approximately 2 cm beyond the tip of the syringe body  15 . 
       FIG. 7  is a segmented side view of a pushing stylet  50 . The pushing stylet  50  consists of a blunt tip  52  on the distal end of the pushing stylet  50  and a pushing stylet shaft  51  affixed, either fixably or removably, to the distal end of a plunger  16 . 
     During a delivery operation, the pushing stylet  50  is extended from the distal bore opening  14  of the incising shaft  11 . The pushing stylet shaft  51  is only long enough to clear the distal bore opening  14 . The plunger  16  acts as a stop that limits the lateral travel of the pushing stylet  50 . In the described embodiment, the pushing stylet  50  has an additional length of approximately 2 cm beyond the tip of the syringe body  15 . 
       FIGS. 8A-8B  are section views illustrating penetration limiting mechanisms for use with the implantation instrument  10 . The penetration limiting mechanisms limit the depth of penetration of the incising shaft  11  and help prevent excessive penetration.  FIG. 8A  shows a fixed penetration limiting mechanism consisting of a stopping flange  55  attached to the incising shaft  11 . The position of the stopping flange  55  along the incising shaft  11  can be adjusted by loosening a hold-down screw  58  and sliding the stopping flange  55  into the desired location. The lower edge of the stopping flange  55  has a bend  57  with an angle τ, preferably between approximately 30° and 60°, thereby forming an elbow  56  which stops lateral travel upon contact with the skin. 
       FIG. 8B  shows an adjustable penetration limiting mechanism consisting of a stopping flange  60  attached a frictional collar  64 . The stopping flange  60  and frictional collar  64  are slidably attached to the incising shaft  11 . An adjustable collar  64 , preferably in threaded communication  65  with the frictional collar  64 , manually stops deployment of the penetration limiting mechanism by tightening the frictional collar  64  against the incising shaft  11 . The lower edge of the stopping flange  60  has a bend  62  with an angle ν, preferably between approximately 30° and 60°, thereby forming an elbow  61  which stops lateral travel upon contact with the skin. 
       FIG. 9  is a perspective view of an instrument for implanting sensors or solid materials in a subcutaneous or other tissue location in accordance with a further embodiment of the present invention. The instrument is equipped with the stopping flange  55  shown in  FIG. 8A . Other forms of penetration limiting mechanisms, both fixed and adjustable, could be used, as would be readily apparent to one skilled in the art. 
     While the invention has been particularly shown and described as referenced to the embodiments thereof, those skilled in the art will understand that the foregoing and other changes in form and detail may be made therein without departing from the spirit and scope of the invention.