Endoscopic instruments having low friction sheath

A low friction sheath for endoscopic instruments is made from an extrusion grade of high density polyethylene (HDPE). The sheath is formed as an extruded polyethylene tube having an inner diameter slightly smaller than the outer diameter of the instrument it will cover. In order to place the sheath over the instrument, a containment sleeve is placed over the sheath and a source of pressurized gas is coupled to one end of the sheath. The endoscopic instrument tube is sealed and inserted into the other end of the sheath. The polyethylene sheath is filled with gas so that it expands against the containment sleeve and the endoscopic instrument tube is pushed into the expanded polyethylene sheath. The gas is released from the polyethylene sheath and it contracts against the outer surface of the endoscopic instrument tube. The sheath is preferably made from a HDPE having a microfractured surface with 10-20 micron fractures.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to endoscopic instruments. More particularly, the 
invention relates to a low friction sheath for an endoscopic instrument 
such as an endoscopic biopsy forceps. 
2. State of the Art 
Endoscopic biopsy forceps generally include a relatively long (several 
feet) hollow flexible member (typically a coil) with one or more flexible 
control members extending therethrough. The proximal ends of the hollow 
member and the control members are coupled to a manual actuation device 
for imparting reciprocal axial movement of the control members relative to 
the hollow member. The distal end of the hollow member is coupled to a 
clevis upon which a pair of biopsy forceps jaws are rotatably mounted. The 
distal ends of the control members are coupled to the jaws so that axial 
movement of the control members relative to the hollow member causes the 
jaws to open or close. The endoscopic biopsy forceps typically are 
delivered to the biopsy site through an endoscope. 
An endoscope has a relatively long flexible tube carrying fiber optics and 
a relatively narrow lumen through which the endoscopic biopsy forceps may 
be inserted. The practitioner guides the distal end of the endoscope to 
the biopsy site and uses the fiber optics to view the site. When the 
distal end of the endoscope is near the biopsy site, the practitioner 
inserts the biopsy forceps jaws into the narrow lumen of the endoscope and 
pushes the long hollow flexible member through the lumen until the jaws 
exit the distal end of the endoscope. Since the lumen of the endoscope is 
narrow and the endoscope typically takes a tortuous path to the biopsy 
site, it is often difficult to push the biopsy forceps through the lumen 
to the biopsy site. Thus, the design of endoscopic biopsy forceps is often 
concerned with reducing friction between the outer surface of the hollow 
flexible member of the forceps and the interior surface of the lumen of 
the endoscope. For example, it is now common to provide the hollow 
flexible member with a shrink wrapped "TEFLON" sheath along its entire 
length. Such a "TEFLON" sheath is also useful for electrically insulating 
the hollow flexible member when the forceps are made "hot" for 
electrocautery. 
Other endoscopic tools are often provided with a similar shrink wrapped 
"TEFLON" sheath for electrical insulation and/or for decreasing friction 
on the outer surface of the tool. "TEFLON" has been considered the ideal 
material for these purposes because of its high lubricity and electrical 
insulating properties. However, "TEFLON" is relatively expensive. Since 
many endoscopic instruments are made to be disposable for safety reasons, 
the cost of providing a "TEFLON" sheath can become significant. 
SUMMARY OF THE INVENTION 
It is therefore an object of the invention to provide a low friction sheath 
for endoscopic instruments. 
It is also an object of the invention to provide a low friction sheath for 
endoscopic instruments which has good electrical insulating properties. 
It is another object of the invention to provide a method for applying a 
low cost lubricious electrically insulating sheath to an endoscopic 
instrument, in particular to an endoscopic biopsy forceps. 
In accord with these objects which will be discussed in detail below, the 
endoscopic instrument of the invention is provided with a low friction 
sheath made from polyethylene. Typically, the polyethylene is an extrusion 
grade high density polyethylene (HDPE), although heat shrinkable or 
blow-molding grade HDPE can be utilized. Likewise, the low friction sheath 
can be made from a mixture of HDPE and low density polyethylene (LDPE). 
Regardless, the sheath is formed as an extruded tube of polyethylene 
having an inner diameter slightly smaller than the outer diameter of the 
tube it will cover. The polyethylene sheath according to the invention is 
preferably more lubricious than an FEP such as "TEFLON". Significantly, 
the low friction polyethylene sheath is only a small fraction of the cost 
of a FEP sheath of the same dimensions. The sheath of the invention is 
electrically resistive making it a good insulator for endoscopic 
instruments incorporating electrocautery functions. While HDPE and 
HDPE/LDPE mixed sheaths do not have the same heat resistance properties as 
FEP, it has been found to be adequate for almost all endoscopic 
applications. In situations where cautery significantly raises the ambient 
temperature at the distal end of the instrument, a short FEP sheath can be 
applied adjacent the distal end of the instrument while maintaining the 
HDPE or HDPE/LDPE mixture sheath on the remaining portions of the 
instrument. 
In accord with the preferred method of the invention, the polyethylene 
sheath is placed in a containment sleeve, and a source of pressurized gas 
is coupled to one end of the tube. The tube or coil of the endoscopic 
instrument is filled with a stiffening wire and is inserted into the other 
end of the polyethylene sheath. The polyethylene sheath is filled with gas 
so that it expands against the containment sleeve and the tube of the 
endoscopic instrument with the wire contained therein is pushed into the 
expanded polyethylene sheath. The gas is then released from the 
polyethylene sheath (i.e. the pressure is reduced) and the polyethylene 
sheath contracts against the outer surface of the tube of the endoscopic 
instrument. A silicone based lubricant may then be applied to the outer 
surface of the sheath. Alternatively, a silicon based lubricant can be 
mixed with the polyethylene prior to extrusion of the polyethylene sheath. 
The lubricant adheres to the fractured surface of the polyethylene in 
contrast to the smooth surface of TEFLON, which repels lubricant. 
Additional objects and advantages of the invention will become apparent to 
those skilled in the art upon reference to the detailed description taken 
in conjunction with the provided figures.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows an endoscopic biopsy forceps 10 having a proximal handle 12, a 
pair of distal jaws 14, and a long flexible coil 16 connecting the jaws to 
the handle. The handle 12 includes a slotted shaft 18 having a thumb ring 
20 and a displaceable spool 22. A pair of pull wires 24 coupled to the 
spool 22 extend through the coil 16 and are coupled to the jaws 14. 
Relative movement of the spool and the thumb ring causes opening and 
closing of the jaws. According to the invention, a low friction 
polyethylene sheath 26 is provided on the outer surface of the coil 16 
along substantially all of its length. As mentioned above, if the biopsy 
forceps 10 are provided with cautery capability, a short length of TEFLON 
sheath 28 is preferably provided at the distal end of the coil 16, 
proximal of the jaws 14 so that the polyethylene sheath 26 is not exposed 
to the high temperatures generated by cautery at the jaws 14. 
The lubricious, low friction polyethylene sheath of the invention can also 
be provided on other types of endoscopic tools where a "TEFLON" sheath is 
presently used. For example, the endoscopic surgical instrument 30 shown 
in FIGS. 2 and 2a generally includes a hollow tube 32 having a manual 
actuator 34 coupled to its proximal end and a pair of end effectors 36 
mounted at its distal end. A push rod 38 extends through the tube 32 and 
is coupled at its proximal end to the actuator 34 and at its distal end to 
the end effectors 36. Manipulation of the actuator 34 causes reciprocal 
movement of the push rod 38 through the tube 32 to open and close the end 
effectors 36. According to the invention the outer surface of the tube 32 
is provided with a low friction polyethylene sheath 40 along substantially 
its entire length. As with the biopsy forceps described above, if cautery 
capability is provided in the instrument 30, the distal portion of the 
tube 32 is preferably covered with a short length of TEFLON sheath. 
Turning now to FIGS. 3-7, the polyethylene sheath according to the 
invention is preferably made from an extrusion grade HDPE (such as 
FINA-7740), although a blow-molding grade can be utilized, particularly if 
gels in the blow-molding grade HDPE are filtered out. In some 
applications, however, it may be desirable to blend 75% HDPE with 25% LDPE 
which gives the resulting mixture a bit more elasticity without severely 
compromising its lubricity. If desired, additional lubricity can be gained 
by adding lubricant directly to the HDPE or HDPE/LDPE mixture prior to 
extrusion. Regardless, the HDPE or HDPE/LDPE mixture is extruded to form a 
tube or sheath 26 as shown in FIG. 3. The polyethylene sheath 26 is 
preferably long enough to cover the component to which the sheath will be 
applied, for example biopsy forceps coil 16 as shown in FIG. 4. Moreover, 
the internal diameter of the polyethylene sheath 26 is preferably slightly 
smaller than the external diameter of the coil 16. 
According to the method of the invention, the polyethylene sheath (tube) 26 
is inserted into a containment sleeve 50. The containment sleeve 50 has an 
internal diameter which is slightly larger than the external diameter of 
the polyethylene sheath 26 and is preferably approximately slightly larger 
than the sum of the external diameters of the polyethylene sheath 26 and 
the coil 16 less the internal diameter of the polyethylene sheath 26. The 
containment sleeve 50 is preferably long enough to contain the entire 
polyethylene sheath 26. With the polyethylene sheath 26 inside the 
containment sleeve 50, a source of pressurized gas 52 is coupled to one 
end of the polyethylene sheath through a valve 53 and a fitting 54. A 
rigid member 56 is inserted into the coil 16 to stiffen the coil and to 
substantially seal its hollow interior. An end of the coil 16 is then 
inserted into the other end of the polyethylene tube 26 as shown in FIG. 
5. Those skilled in the art will appreciate that since the external 
diameter of the coil is slightly larger than the internal diameter of the 
polyethylene tube, the coil cannot be fully inserted into the polyethylene 
tube. However, the elasticity of the polyethylene tube will allow enough 
diametrical expansion at the end of the polyethylene tube so that the end 
of the coil can "plug" the end of the polyethylene tube. After the coil 
has been inserted in this manner, the valve 53 is opened and pressurized 
gas enters the interior of the polyethylene tube through the fitting 54. 
The coil 16 with the rigid member 56 substantially prevents gas from 
escaping the interior of the polyethylene tube. The elasticity of the 
polyethylene tube yields to the pressure of the gas and the polyethylene 
tube expands diametrically until it is restrained by the interior of the 
containment sleeve 50. The coil 16 with rigid member 56 is then pushed 
freely into the polyethylene tube as shown in FIG. 6. When the coil is 
substantially covered by the polyethylene tube, the valve 53 is closed and 
the fitting 54 is removed from the polyethylene tube. The polyethylene 
tube contracts diametrically until it engages the exterior surface of the 
coil 16. The polyethylene tube-wrapped coil is removed from the retaining 
sleeve and the rigid member is removed from the coil. The flexibility of 
the coil is not significantly reduced by the polyethylene sheath 26 which 
now embraces the coil 16 as shown in FIG. 7. Moreover, as mentioned above, 
the sheath 26 has a highly lubricious outer surface and is electrically 
non-conductive. 
Those skilled in the art will appreciate that the above-described method 
can be used to apply a sheath to any substantially cylindrical member such 
as the coil 16 or the tube 32 shown in FIGS. 2 and 2a. It will be 
appreciated that when the cylindrical member is substantially rigid such 
as the tube 32, it is not necessary to insert a rigid member 56 into the 
tube 32 before inserting the tube 32 into the polyethylene sheath. Rather, 
it is only necessary to plug the interior of the tube 32 to inhibit gas 
from escaping while the polyethylene sheath is being expanded. In 
addition, while no heating of the polyethylene sheath is required, the 
method can be carried out at elevated temperatures if desirable, for 
example at temperatures between 110.degree. F. and 160.degree. F. The 
heating of the polyethylene sheath prior to and/or during expansion, while 
not preferred, does aid in helping the polyethylene sheath expand as 
required by the method invention. 
It is believed that the lubricity of the polyethylene tube of the invention 
is provided by the surface structure of the polyethylene which includes 
longitudinal microfractures of a desired width. The microfractured surface 
of the polyethylene sheath of the invention is believed to be more 
lubricious than the smooth FEP ("TEFLON") surface because the fractures 
reduce the outer surface contact area, thus decreasing surface tension. In 
addition, lubrication applied to the fractured surface of the polyethylene 
sheath tends to adhere to the surface because of the microfractures. Since 
it is believed that it is the microfractured surface of the polyethylene 
sheath which makes it so suitable for the uses described herein, it should 
be appreciated that materials other than polyethylene which exhibit a 
similarly microfractured surface may also be similarly useful for 
providing an endoscopic instrument sheath. 
For excellent results, the width of the microfractures in the surface of 
the polyethylene tube are preferably between ten and twenty microns. 
However, polyethylene tubes with either smaller and larger characteristic 
microfractures can still provide surfaces which are more lubricious than 
FEP. It should be noted that for purposes herein, the term "lubricity" or 
"lubricious" relates to the lubricity of the outer surface of an 
endoscopic instrument vis-a-vis an endoscope through which the endoscopic 
instrument will pass. Typically, the endoscope surface past which the 
endoscopic instrument will travel is made from polytetrafluoroethylene 
(PTFE). 
While the preferred embodiment of the invention utilizes extrusion grade 
HDPE which is fit over the coil or tube of an endoscopic instrument as 
discussed above with reference to FIGS. 3-7, according to another 
embodiment of the invention, a heat shrinkable grade HDPE can be utilized. 
If a heat shrinkable grade HDPE is utilized, the method of application is 
similar to where a FEP (e.g., "TEFLON") shrink-tubing is utilized; i.e., 
the tubing is placed over the instrument and heated until it shrinks and 
grabs the instrument. 
There have been described and illustrated herein several embodiments of 
endoscopic instruments having low friction (lubricious) sheaths. While 
particular embodiments of the invention have been described, it is not 
intended that the invention be limited thereto, as it is intended that the 
invention be as broad in scope as the art will allow and that the 
specification be read likewise. Thus, while particular grades and 
densities of polyethylene have been disclosed, it will be appreciated that 
other grades and densities could be utilized. Also, while two exemplary 
endoscopic instruments have been shown, it will be recognized that other 
types of endoscopic instruments could be provided with the inventive 
sheath. Moreover, while particular configurations have been disclosed in 
reference to the apparatus used to perform the method of the invention, it 
will be appreciated that other configurations could be used as well. 
Furthermore, while the sheath has been disclosed as being made from 
polyethylene, it will be understood that different materials having the 
properties of the polyethylene described herein may achieve the same or 
similar function as disclosed herein. It will therefore be appreciated by 
those skilled in the art that yet other modifications could be made to the 
provided invention without deviating from its spirit and scope as so 
claimed.