one example of a penetrating medical device that may be used in embodiments of the invention is a stylet . in one embodiment , as shown in fig4 , an inflatable , elastic membrane 400 is housed in a distal end 402 of a hollow stylet 404 containing a fluid ( e . g ., saline ). as illustrated in fig5 a and 5 b , stylet 404 is held within a needle 500 as both elements are advanced through tissue . a proximal end 406 of the stylet may include a second membrane 408 that is in communication with membrane 400 as a result of the fluid between them . exertion of force on one membrane tends to displace the other membrane . in the embodiment illustrated , membrane 408 is configured and positioned similarly to a plunger at the end of a syringe . such a configuration allows a human operator to exert a force on membrane 408 and to sense the amount of force needed to be applied to membrane 408 to displace membrane 400 . in the embodiment illustrated , membrane 400 is displaced by inflating and the amount of force needed to inflate distal membrane 400 depends on the resistance of the material in which membrane 400 is positioned . the operator detects the penetration of the stylet 404 ( and hence a needle tip 502 ) into certain tissues or tissue compartments by haptically sensing changes in resistance while exerting pressure on distal membrane 408 . in the embodiment illustrated , an operator may sense a change in resistance because inflation of membrane 400 causes a displacement of fluid from the proximal end to the distal end , with a corresponding motion of membrane 408 . for example , as shown in fig5 a and 5 b , the amount of force required to inflate membrane 400 may be used to determine when the tip 502 of the needle exits high resistance matter 504 and enters a tissue compartment , such as an epidural space 506 . when needle tip 502 reaches the epidural space 506 , membrane 400 expands because of the pressure being applied to membrane 408 creating a pressure on membrane 400 greater than that generated by the low resistance of epidural space 506 . an operator may haptically sense the pressure change by maintaining contact with membrane 408 . though any suitable mechanism may be used to sense the change in pressure , including , in some embodiments , a pressure sensor ( not shown ) which may be coupled to the stylet to sense when a sudden pressure drop occurs , and thus indicate that the epidural space has been reached by tip 502 . for purposes herein , membrane 400 is considered to be a member having a structural surface . in a needle having a distal tip with fluid directly exposed through an opening in the distal tip , the fluid is not considered to have a structural surface exposed through the opening , although the fluid is still considered to be a member . any suitable material may be used to form membrane 400 . membrane 400 may be degradable or non - degradable depending on the desired uses of the apparatus and / or the desire characteristics of the apparatus . in some embodiments , membrane 400 and / or membrane 408 may be housed within a needle instead of within a stylet within a needle . also , a penetrating medical device other than a needle may be used . for example , a trocar or an apparatus without a sharp tip may be used in conjunction with one or more membranes or other embodiments described herein . as needle 500 is being advanced , to prevent membrane 400 from being pushed into the barrel of the stylet , stylet tip 402 may include a protective element ( not shown ) between membrane 408 and the interior of the stylet . the protective element may be , for example , a meshwork or a fenestrated surface made from synthetic or natural materials which are degradable or non - degradable materials or combinations thereof . in some embodiments , the apparatus may contain a mechanism that prevents the membrane from remaining inflated during withdrawal of the needle . detection of a dural puncture with prior art epidural needles can be made by observing fluid leakage from the body into the syringe . to permit a similar method of detecting a dural puncture with embodiments disclosed herein , a channel or other longitudinal space may be provided between stylet 404 and interior walls of needle 500 . this channel allows fluid leakage toward the proximal end of the needle so that the operator can observe fluid leakage . currently used epidural placement methods include removing the stylet from the needle and inserting a syringe filled with air or saline to check for loss of resistance to the injection of air or saline . sometimes , multiple exchanges of the stylet and the syringe are required . additionally , in some instances , undesirable amounts of air or saline may be injected into the body . through the use of various embodiments disclosed herein , an operator may detect entry of the needle tip into specific tissue or tissue compartment without the associated complications of injecting air or saline into the body . by not repeatedly exchanging a syringe and a stylet , a more rapid detection of the epidural space also may be achieved by using embodiments described herein . in addition , opening an inflatable membrane into the epidural space may facilitate passage of a catheter for administration of anesthesia . an apparatus similar to the one illustrated in fig5 a and 5 b may be used to perform central line placement . when performing central line placement , the operator is able to detect entry into a vein or artery based on relative differences in pressure . thus , embodiments disclosed herein may be used to quickly assess entry of the needle into various levels of tissue or tissue compartments . by helping to indicate false arterial cannulization , the apparatus provides the operator with time to reposition the needle prior to insertion of large bore catheters , which is the step that results in most of the complications of cvc discussed above . in some embodiments , the apparatus may be configured to attach to currently existing needles and / or syringes . in the embodiment of fig4 and fig5 a and 5 b , a proximal and a distal membrane are in communication such that resistance encountered at the distal surface is communicated to the proximal membrane for sensing . as described , fluid within a stylet provides communication between the proximal and distal surfaces . however , any suitable mechanism may be used to form those surfaces and to provide communication between those surfaces . according to some embodiments , as illustrated in fig6 a and 6 b , a penetrating medical device such as needle 500 contains a continuous balloon 600 which reaches both the distal tip 502 of the needle and a proximal end 510 of the needle . continuous balloon 600 may be filled with a biocompatible fluid , for example , saline . in use , a pressure is applied to proximal end 612 such that , while needle tip 502 is positioned within a high resistance matter 604 , as shown in fig6 a , a distal end 606 of the balloon remains un - inflated or slightly inflated . however , the pressure on surface 612 is such that when needle tip 502 is advanced into a low resistance matter 608 , as shown in fig6 b , distal end 606 of the balloon inflates . inflation of the distal end 606 can be sensed at the proximal end , such as by a decreased fluid pressure at a compliant diaphragm 610 at a proximal end 612 of balloon 600 . the decrease in fluid pressure at proximal end 612 of the balloon is significant enough for an operator to sense the change in pressure , or significant enough for a pressure sensor to detect the change . alternatively or additionally , inflation of distal end 606 may result in a displacement of fluid sufficient to create a noticeable change in volume at the proximal end or be reflected in any other suitable way at an observable location . initial formation of balloon 600 may occur within needle 502 , or balloon 600 may be manufactured outside of the needle and later inserted into the needle . as shown in fig7 , needle 500 and balloon 600 may include a necked section 700 . the reduction in diameter at necked section 700 restricts the forward movement of balloon 600 to provide a limit on the distance balloon 600 can travel in the distal direction . as shown in fig8 , rounded inside edges 800 of needle tip 502 may be provided to help prevent puncturing of the balloon as it inflates and deflates . the shape of balloon 600 as it inflates out of needle tip 502 need not be symmetric or spherical . in some embodiments , one of which is shown in fig9 , distal end 606 of balloon 600 inflates into a bent shape that takes it out of the direct path of portions of needle tip 502 . in the embodiment illustrated , balloon 600 is shaped so that , as it inflates , it presses against a curved edge and expands away from a sharp edge . it is not necessary that resistance at the tip of a penetrating device be sensed using a proximal membrane . in the embodiment illustrated in fig1 , needle 500 is coupled to a device 1002 which supplies a pressurized fluid 1004 to the needle . needle 500 includes a membrane 1006 at the distal end of the needle . a pressure is applied to the fluid by device 1002 such that membrane 1006 either does not inflate or inflates an insignificant amount when needle tip 502 is positioned within a high resistance matter . when the needle tip is advanced into a low resistance matter with an operator &# 39 ; s insertion force , a measurable pressure decrease may be sensed as membrane 1006 inflates . device 1002 may be a pump box in some embodiments . though , any suitable effect of membrane 1006 encountering a lower resistance may be sensed . such effects may include sensing a flow of fluid as membrane 1006 expands or sensing the pressure required to maintain fluid within needle 500 in equilibrium . in embodiments which include a continuous balloon within needle 500 , device 1002 may be coupled to pressurized fluid within the balloon , and upon reaching low resistance matter , the expanding balloon results in a measurable pressure change . the ability to communicate an indication of a resistance at the tip of a device to a more proximal location may be used in ways other than to provide an indication of the location of the tip relative to low resistance spaces . the ability to communicate a force may be used to also control a driving force coupled to the penetrating device so that a driving force is coupled to the penetrating device as it encounters high resistance materials but the driving force is removed when the tip is positioned in a target space , such as in a lumen , such as a vein or artery a spinal column or other low resistance tissue or structure . in the embodiment illustrated in fig1 a and 11 b , bellows 1100 are attached directly to needle 500 . needle 502 and bellows 1100 contain a fluid , which may be a biocompatible fluid such as saline . when needle tip 502 is positioned within high resistance matter 604 , bellows 1100 remain pressurized because membrane 1006 inflates insignificantly or not at all . even as bellows 1100 are pushed by the operator to advance the needle , the bellows remain pressurized because less force is needed to advance the needle than to inflate membrane 1006 when advancing the needle tip through the high resistance matter . once needle tip 502 reaches low resistance matter 608 , however , the force needed to advance the needle is higher than the force that results in inflating membrane 1006 . as a result , further pushing the bellows results in compression of the bellows and inflation of the membrane with the biocompatible fluid . in some embodiments , resistance encountered at the tip of a device may be used to control a clutch mechanism to create an apparatus that provides a driving force to a penetrating medical device when the apparatus tip encounters material of high resistance , and when the apparatus tip encounters a low resistance material , no further driving force is applied to the apparatus . such an apparatus may be used to stop advancing the tip of a device upon reaching a desired low resistance area , regardless of whether the operator continues to apply force to certain components . in the embodiment illustrated in fig1 a and 12 b , a catheter core 1202 is positioned within a curved needle 1204 . a distal tip 1206 of needle 1202 is partially inserted into a high resistance matter 604 by pushing on the needle . after the needle has been partially inserted , continued advancement of the needle is provided by pushing on catheter core 1202 . catheter core 1202 does not exit through needle tip 1206 because the high resistance matter present at needle tip 1206 prevents a tip 1208 of the catheter core ( which may be a blunt tip ) from advancing into the matter . at a certain level of force , catheter core 1202 buckles , and catheter core 1202 contacts the interior of curved walls 1210 of needle 1204 . in the embodiment illustrated , the curved walls act like a capstan to wedge the catheter core in place . with the catheter core wedged , further forces applied to the catheter core are transferred to the needle wall , and the needle advances further through the high resistance matter . when the needle is advanced into a region of low resistance matter , the force resisting the catheter core tip decreases and the catheter core moves through the needle and into the low resistance matter . allowed to move into the low resistance matter , the catheter core no longer buckles , no longer transfers significant force to the needle wall , and thus advancement of the needle no longer occurs . in this manner , the catheter inside the curved needle acts like a linear force clutch . for purposes herein , a catheter core , or any other suitable element used in a similar manner , is considered to be a member with a structural surface . in some embodiments , needle 1204 may be formed of 17 gauge thin - walled stainless steel tubing ( 0 . 058 ″ o . d . and 0 . 048 ″ i . d . ), but may be formed of any suitable material and have any suitable size . needle 1204 may include any suitable bend radii , but in some embodiments , bend radii of greater than 0 . 5 ″ are used . catheter core 1202 may be any suitable material that is sufficiently pliable to buckle at forces above those encountered as it passes through a material deemed “ low resistance ,” sufficiently rigid to transmit a force when buckled and sufficiently springy to disengage from the walls of needle 1204 . examples of suitable materials includes ptfe , peek , nylon , nitinol , or any metal or other material conventionally used to make catheter guide wires . in some embodiments , catheter core 1202 may have a diameter of less than 0 . 040 inches . in one specific example of a method of using such an apparatus , a needle is partially inserted into the ligamentum flavum by pushing on the needle , and then further advancement is controlled by pushing on the catheter core . while the needle tip is positioned in the ligamentum flavum , pushing on the catheter core buckles the catheter , thereby pushing the needle further into the ligamentum flavum . once the needle advances into the epidural space , the catheter core no longer buckles , and no further advancement of the needle occurs . while the curved needle embodiment described above is described as including a catheter core , other suitable force - providing elements may be used . for example , in some embodiments , a flexible wire may be used within the needle to advance the needle through tissue . in still other embodiments , a force - providing element may be implemented on the outside of the needle . as illustrated in fig1 c , instead of a one - piece needle , one embodiment of which is shown in fig1 a and 12 b , a curved section , such as an s - shaped section 1204 ′ for example , and a straight section 1204 ″ may be separable from one another . a luer - lok ® connector 1214 or other suitable connector may used to permit selective connection of s - shaped section 1204 ′ and straight section 1204 ″. in some embodiments , s - shaped section 1204 ′ and straight section 1204 ″ may be configured to be separable without the use of a connector . straight section 1204 ″ may be a standard needle in some embodiments . when straight section 1204 ″ advances to the low resistance material , s - shaped section 1204 ′ may be separated from straight section 1204 ″ and catheter core 1202 may be removed . a catheter ( not shown ) then may be inserted through straight section 1204 ″. in some embodiments , catheter core 1202 may be left in place when s - shaped section 1204 ′ is removed , and a catheter may be slid over catheter core 1202 and into the low resistance material . as shown in fig1 c , a hand piece 1212 may be included on the penetrating medical device , for example at the proximal end , to provide a grip area for the operator . in some embodiments , hand piece 1212 is a tube in another embodiment , a straight needle is used with a force - providing element to provide a linear force clutch . as illustrated in fig1 a , when needle tip 502 is positioned within high resistance matter 604 , a force - providing element , such as flexible wire 1302 , buckles and contacts the interior of the needle walls . friction with the needle walls transfers the force on the flexible wire to the needle . when needle tip 502 enters low resistance matter 608 , as shown in fig1 b , flexible wire 1302 no longer buckles and it is released from substantial contact with the needle . the flexible wire is then free to advance without pushing the needle further into the low resistance matter 608 . in some embodiments , one example of which is illustrated in fig1 c , the medical device may include serpentine portions in both the needle and the wire . as illustrated in fig1 c , a removable portion of the penetrating medical device may include both a straight section 1306 and an s - shaped section 1308 . flexible wire 1302 ( or other suitable catheter core ) may buckle within straight section 1306 , and thereby contact the interior walls of straight section 1306 . additionally , the curved walls of s - shaped section 1308 act like a capstan to wedge the flexible wire in place . in some embodiments , the flexible wire ( or other suitable catheter core ) also may buckle within portions of s - shaped section 1308 . for embodiments of the penetrating medical device which include both a straight section and a curved section ( e . g ., an s - shaped section ), a mathematical model has been created to estimate the output force of the catheter core ( e . g ., a flexible wire ) as a function of the axial input force on the catheter core . equation ( 1 ) presented below was developed by modeling the amount of force absorbed by the curved and straight sections of the device , and then subtracting the estimated forces from the input force . by conducting test measurements , a correction factor has been incorporated into the equation . the force for the straight section uses the helical buckling equation while the force for the curved section is for the unbuckled state . for the model , it is assumed that the catheter core is always helically buckling in the straight section while not buckled in the curved section . the force output by the catheter core is modeled to be : f out predicted = f in −( μ l curved f in / r + 10μ d core f in 2 r /( ei )) ( 1 ) where π is the coefficient of friction between the catheter core and the interior wall , l curved is the length of the interior of the curved section ( m ) that the catheter core is touching , f in is the input force ( n ) applied at the hand piece , d core is the diameter of the catheter core ( m ), r is the average radius of the curves in the curved section ( m ), r is the radial gap spacing between the catheter core and the interior wall ( r ), e is the modulus of elasticity ( n / m 2 ) of the catheter core , and i is the moment of inertia of the catheter core ( m 4 ). the embodiments of fig1 a , 12 b , 12 c , 13 a , 13 b , and 13 c provide examples of apparatus including a force translating member that translates an axial force , generated by resistance encountered at the tip of a medical device , to a transverse force , such as a radial force . such force translation allows a driving member , such as a catheter core , to selectively engage a needle based on resistance encountered . other suitable elements may be used to create a “ clutch ” to selectively drive a penetrating medical device . another type of suitable element is illustrated in fig1 a and 14 b . a material or device having a high poisson ratio ( hereinafter , “ poisson element ”) may be used as part of an apparatus which provides a linear force clutch for controlling advancement of a penetrating medical device . in some embodiments , the poisson element may be a balloon or an elastomer . in the embodiment illustrated in fig1 a and 14 b , an element 1402 including high poisson ratio material acting as a poisson element is coupled to a force - providing core 1404 . when the core is pushed with needle tip 502 positioned in high resistance matter 604 such that a distal end 1406 of core 1404 cannot substantially exit needle tip 502 , poisson element 1402 is compressed and expands radially . once sufficiently expanded , the poisson element contacts the interior walls of needle 500 , and the operator &# 39 ; s compressive force on core 1404 is transferred to the needle walls , thereby advancing the needle . once the needle tip advances into low resistance matter 608 , distal end 1406 of core 1404 advances through the low resistance matter , the compressive force on the poisson element decreases , and the poisson element no longer significantly contacts the needle walls . in this manner , further pushing on core 1404 does not further advance needle 500 . as an example of another embodiment of a poisson element , a mechanical expansion spring may be used to selectively transmit force from a core to the needle walls . as shown in fig1 , a mechanical expansion spring 1502 includes two flexible spring members 1504 which are displaced radially when a compressive axial force is applied via core 1404 . with sufficient axial force , flexible spring members 1504 contact the interior needle walls and the friction transfers the force of the core 1404 to needle 500 . the mechanical expansion spring is configured to expand sufficiently radially ( direction b ) when the resistance of expected high resistance matter is encountered by the distal end of core 1404 as needle 500 is being advanced through the high resistance matter . when the needle tip advances into expected low resistance matter , the core is free to extend out of the needle , allowing the expansion spring to elongate in the axial direction ( direction a ) and retract in the radial direction , thereby ending its contact with the needle walls . accordingly , further advancement of the needle does not occur within the low resistance matter . while the embodiment illustrated in fig1 includes two flexible spring members , three , four , or more than four flexible spring members may be included in some embodiments . a sensitivity amplifier may be employed in some embodiments to amplify the effect of a change in resistance encountered at the tip of a penetrating medical device . such amplification may improve the reliability with which a clutch engages or help an operator observe changes in the material through which the needle tip is advancing . for example , as illustrated in fig1 a and 16 b , a needle 1600 may have a distal section 1602 with a first diameter and a proximal section 1604 with a second , smaller diameter . as a piston 1606 is pushed into proximal section 1604 , a release of biocompatible fluid from the distal section that is equal to the volume of fluid occupying a longitudinal length of δout will permit piston 1606 to move a longer , longitudinal length of δin because the cross - sectional area of proximal portion 1604 is less than the cross - sectional area of distal section 1602 . this embodiment of a sensitivity amplifier may be used with a needle having no membrane at needle tip 502 , or it may be used with a needle that has a membrane at the needle tip ( similar to the embodiment illustrated in fig1 a and 11 b ). a sensitivity amplifier may be used with other embodiments described herein , and other suitable sensitivity amplifiers may be used to aid with the sensing of needle advancement . an operator may prefer to retain an open channel within the penetrating medical apparatus . for example , in some cases , the operator may desire to observe fluid flowing from the body to help determine the location of the needle tip . in some cases , the operator may wish to inject fluid into the region in which the needle tip is placed . to accommodate fluid flow through the needle , various embodiments of needle and / or core arrangements may be put to use . for example , as shown in fig1 a , core 1702 may have a diameter that contacts or almost contacts an interior surface 1704 of the needle walls around most of the perimeter of the core . in one or more areas , however , a space 1706 may be formed between core 1702 and interior surface 1704 with a section 1708 of the core that has a smaller radius . the space 1706 forms a longitudinal channel through which fluid may flow . a hollow core 1712 may be used in some embodiments to provide a channel , as illustrated in fig1 b . the core 1712 reaches or nearly reaches the interior surface of the needle around the entire perimeter of the core , but a channel 1714 is provided through the interior of the core . in another embodiment , as illustrated in fig1 c , a substantially solid needle 1718 may be used as a penetrating device , and a portion 1720 of the solid material may have a reduced radius . a needle wall 1722 forms a flow space 1724 in conjunction with reduced radius portion 1720 of the solid needle . in still a further embodiment of a channel within a penetrating medical device , a flow channel may be formed within the area between the needle wall and the outside of a core , and also within a hollow core ( or cores ). for example , as illustrated in fig1 d , two hollow cores 1730 and 1732 may form two channels 1734 and 1736 . additional channels 1738 and 1740 may be formed between the exterior of the cores 1730 , 1732 and the interior surface 1704 of needle 500 . in this manner , multiple channels may be formed for fluid feedback , fluid injection , or any other suitable purpose . a multi - channel device may be configured to allow fluid flow in one or more channels , and the presence of a core and / or poisson element in another channel . in this manner , separated channels may be used for different purposes . in some embodiments , a first channel may be used for holding pressurized fluid to be used as part of a sensing arrangement , and a second channel may be used to deliver fluid to the body . the penetrating medical devices described herein may further employ an advancing arrangement which limits the advancement of a core such as a catheter core , a flexible wire , etc ., even when low resistance matter is encountered . for example , as illustrated in fig1 , a device 1800 with a trigger handle 1802 may be used to push on core 1404 . each complete pull of trigger handle 1802 advances core 1404 out of device 1800 by a certain distance . when core 1404 is wedged in curved needle 1204 , needle 1204 advances by the same distance . once the needle tip reaches matter with a low resistance , the core advances in a step - wise manner into the low resistance matter , such that each discrete motion of the operator advances the core at most by the distance provided by one complete pull of the handle . in this manner , unintentional advancement of core 1404 into low resistance matter can be limited . device 1800 is shown by way of example only , and other suitable devices and methods may be used to limit the advancement of core 1404 upon encountering a low resistance material , such as by limiting advancement to a discrete distance . another embodiment of an advancing arrangement for use with penetrating medical devices described herein is illustrated in fig1 a . a grip tube 1900 holds flexible wire 1302 at a grip location 1902 . grip tube 1900 has a diameter that is slightly larger than straight needle 500 such that grip tube 1900 may be translated axially relative to needle 500 . grip tube 1900 provides off axis stability to one or both of the needle and the flexible wire . in an alternative embodiment , as illustrated in fig1 b , a rigid rod 1904 may be used to apply force to flexible wire 1302 . as with various embodiments disclosed herein , when needle tip 502 is positioned within high resistance matter , a force - providing element , such as flexible wire 1302 , buckles and contacts the interior of the needle walls . friction with the needle walls transfers the force on the flexible wire to the needle . when needle tip 502 enters low resistance matter , flexible wire 1302 no longer buckles and it is released from substantial contact with the needle . the flexible wire is then free to advance without pushing the needle further into the low resistance matter . either of the advancing arrangements illustrated in fig1 a and 19 b may be used as a stand - alone device , or may be used in combination with other components , such as a step - wise advancing device similar to the embodiment illustrated in fig1 . in some embodiments , grip tube 1900 may be sized and configured to translate axially along the interior of needle 500 . when using various embodiments disclosed herein , sufficient resistance may not exist between the penetrating medical device and the surrounding material to stop advancement of the penetrating medical device when it reaches low resistance material . in various embodiments disclosed herein , the needle may include one or more features that increase the resistance between the needle and the surrounding material . for example , as shown in the side view of fig2 a , an outer surface of needle 500 may include an area of surface roughness 2002 that is higher than a standard needle . as illustrated in the cross - sectional view of fig2 b , needle 500 may have a first section 2004 with a first outer diameter and a second section 2006 having a larger outer diameter than first section 2004 . the change in diameter creates an increased resistance to advancement . the cross - sectional view of fig2 c shows needle 500 having first section 2004 with a first outer diameter and second section 2006 having a larger outer diameter than first section 2004 , and additionally includes a transition section 2008 between first section 2004 and second section 2006 . in an alternative embodiment , needle 500 may include a very short or non - existent first section 2004 , such that transition section 2008 is positioned close to or at the distal tip of needle 500 . fig2 d shows a cross - sectional view of a needle 500 having a curved bump 2010 that extends around the circumference of the needle . in an alternative embodiment , a toroidal - shaped component may be positioned around the circumference of the needle . in another embodiment , a semi - spherical bump may be included at one or more distinct positions on the needle . the particular arrangements and sizes of any features that increase resistance with surrounding material may be based on the type of material being penetrated and the type of space being targeted . one embodiment of a catheter core , such as flexible wire 1302 for example , is shown in a cross - sectional view in fig2 . in this embodiment , flexible wire 1302 has a lumen 2102 and one or more passages 2104 that lead to lumen 2102 through the wall of flexible wire 1302 . lumen 1302 may be used for insufflation , drug delivery , or drainage of excess bodily fluids in some embodiments . lumen may be used to inject fluid when a target region is reached by the penetrating medical device . the injection of fluid may be provided by using bellows . in some embodiments , a balloon may be provided within the lumen of flexible wire 1302 , and the balloon may be filled with a fluid to provide pressure feedback to the operator . in various embodiments described above , a membrane is provided at a needle tip . in some embodiments , needle tip may be left open and optionally may have a restricted diameter opening . in such embodiments , fluid at the opening may not have a structural surface , but high resistance matter nonetheless may prevent biocompatible fluid from exiting the needle tip , while low resistance matter may allow the fluid to flow from the needle tip . similar to the inflation of a membrane in low resistance matter , the initiation of flow from the needle tip decreases the pressure sensed by the operator , indicating advancement into low resistance matter . in embodiments which include a membrane or balloon at the needle tip , in addition to providing an indication of reaching low resistance matter , inflation of the membrane or balloon may serve to prevent or reduce further advancement of the needle tip into low resistance matter . for example , an inflated membrane or balloon may block the sharp edge of the needle and / or present a large surface area which cannot be advanced through the low resistance matter . also , an embodiment was described in which a catheter core was used as a driving element . there is no requirement that the driving element be at the center of a catheter . fig1 a - 17 d illustrate various multi - lumen devices and the driving element may be configured in any of suitable lumen or otherwise constrained in a desired position . having thus described several aspects of at least one embodiment of this invention , it is to be appreciated various alterations , modifications , and improvements will readily occur to those skilled in the art . such alterations , modifications , and improvements are intended to be part of this disclosure , and are intended to be within the spirit and scope of the invention . accordingly , the foregoing description and drawings are by way of example only .
an apparatus provides feedback regarding the material in which tip of the apparatus is located as the tip is advanced into matter of varying resistances . the apparatus responds to a change in pressure , force , or other parameter such that when the tip reaches matter of a certain resistance , a change in the parameter is sensed . the apparatus provides a driving force to a penetrating medical device , such as a needle , when the apparatus tip encounters material of high resistance . when the apparatus tip encounters a low resistance material , no further driving force is applied to the apparatus . an inner core may be advanced into the low resistance material for deployment of a gas or a liquid as desired .