Working With Hypodermic Tube

When working with hypodermic medical tubing it is especially important to know how the tubing is made, especially if an assembly is being designed where an obturator, stylet, catheter, wire, tube, or rod is being designed to fit into the inner diameter (ID) of the tube.

3 "Joseph-Frederic-Benoit Charriere, a 19th century Parisian maker of surgical instruments, has by virtue of his ingenuity and advanced thinking, continued to have his presence felt in medicine throughout the 20th century. His most significant accomplishment was the development of a uniform, standard gauge specifically designed for use in medical equipment such as catheters and probes. Unlike the gauge system adopted by the British for measurement of needles and intravenous catheters, Charriere's system has uniform increments between gauge sizes (1/3 of a millimeter), is easily calculated in terms of its metric equivalent, and has no arbitrary upper end point. Today, in the United States, this system is commonly referred to as French (Fr) sizing. In addition to the development of the French gauge, Charriere made significant advances in ether administration, urologic, and other surgical instruments, and the development of the modern syringe." Iserson, K.V., J.-F.-B. Charriere: the man behind the French scale, J. Emerg. Med., 5, 545-548, 1987.

Table 5.1 Units Conversion Chart

Gauge Number

Metric (mm)

French Catheter (Fr.) (mm x 3)

Stubs Gauge

American (A.W.G.) or Brown and Sharpe (inch)

6

5.16

15.5

0.203

0.1620

7

4.57

13.7

0.180

0.1442

8

4.19

12.6

0.165

0.1284

9

3.76

11.3

0.148

0.1144

10

3.40

10.2

0.134

0.1018

11

3.05

9.2

0.120

0.0907

12

2.77

8.3

0.109

0.0808

13

2.41

7.2

0.095

0.0719

14

2.11

6.3

0.083

0.0640

15

1.83

5.5

0.072

0.0570

16

1.65

5

0.065

0.0508

17

1.47

4.4

0.058

0.0452

18

1.27

3.8

0.049

0.0403

19

1.07

3.2

0.042

0.0358

20

0.91

2.7

0.035

0.0319

21

0.82

2.4

0.032

0.0284

22

0.72

2.2

0.028

0.0253

23

0.64

1.9

0.025

0.0225

24

0.57

1.7

0.022

0.0201

25

0.51

1.5

0.020

0.0179

26

0.46

1.3

0.018

0.0159

27

0.41

1.2

0.016

0.0141

28

0.36

1

0.014

0.0126

29

0.34

0.013

0.0112

30

0.31

0.012

0.0100

31

0.26

0.010

0.0089

32

0.23

0.009

0.0079

The first consideration is this: tubing is made by reducing the OD of the tube through a die. What this means is that the OD is controllable. The ID of the tube then becomes a function of the OD minus the nominal wall thickness of the tube after forming. This means that the ID is not absolutely controlled. The ID is a theoretical number. This can be seen in the accompanying illustration. This must be taken into account when calculating tolerances between the ID of the tube and whatever you are designing to slide into the tube. (See Figure 5.1.)

Figure 5.1 Typical metal tube drawing methods. (Courtesy of Microgroup, Inc.)

It is possible to draw tubing over a mandrel of a precise size, or hone the inner diameter; however, this is more expensive than using readily available standard-size hypotube.

Also, when designing a part to fit in to the inner lumen of a hypotube, remember that tubes are never perfectly round, nor perfectly straight, nor perfectly smooth on the inside. All of these factors will affect how much tolerance to allow in order to fit a part into the hypotube lumen.

If you are planning to insert a long part into a long hypotube, remember to allow enough tolerance. Even if a part fits easily into a short section of tube, frictions and tolerance stack-ups rapidly accumulate, where a part may fit initially, but becomes jammed as the part is advanced through the full length of the tube.

When measuring tubing with a pin gauge, be sure that the end of the tube is free from burrs. Deburring the end of the tube with a 60 ° cone burr held in a pin vise is a convenient way to clean up a tube before measuring. Also, remember that a gauge pin the exact diameter of the tube will not fit in the tube. For example, a 0.125-inch pin will not fit in a 0.125-inch lumen.

COMMON HYPODERMIC TUBING MATERIALS

The most common is 300 series stainless; 400 series stainless is required for heat treating. Nickel-titanium tubing is also now readily available from

Figure 5.2 Typical hypodermic needle features. (Illustration courtesy of Popper and Sons, Inc.)

vendors such as Memry Corporation (Bethel, CT) and Nitinol Devices Corporation (NDC) (Fremont, CA). Tubing of other alloys such as titanium are available for use in magnetic resonance imaging (MRI) radiology applications.

If you look at a hypodermic needle, you will notice that the end is not ground to a simple bevel. Hypodermic needles are usually ground with a compound bevel, typically called a lancet point, and the angles of these bevels give the needle its characteristics. (See Figure 5.2.) Some needles are designed to pierce veins and arteries, others to penetrate into muscle, and yet others to penetrate tough fascia and joint capsule tissue.

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