Radiopaque Medical Devices Containing Bismuth Oxychloride ...

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Bismuth subcarbonate has limited use and is usually compounded with low melting point polymers since it
decomposes at temperatures higher than 225 o C. Its use has decreased due to its thermal instability and poor
dispersibility
Bismuth trioxide has a strong yellow color which may not be desirable if white or other colors are needed.
Furthermore, it makes tubing with rough surfaces.
Bismuth oxychloride powder is white with excellent skin feel. It can be compressed easily and dispersed well in
polymers.
Tungsten has a very high density and can be loaded up to 90% by weight into polymers, which makes it useful
for highly radiopaque devices, such as very thin-wall catheters. However, tungsten has a black color, is
flammable and very abrasive. It can wear out the processing equipment extremely fast.
Table 1: Properties of various fillers
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Barium sulfate 4.5 56 white stable good good
Bismuth oxychloride 7.7 83 white stable up to
600 o C
Bismuth subcarbonate 8.0 83 white stable up to
225 o C
good good
poor to fair fair
Bismuth trioxide 8.9 83 yellow stable fair to good poor to fair
Tungsten 19.35 74 black stable 1
good poor
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Tungsten is flammable. Proper care must be taken when heating tungsten metal powder.
When a special color is required for a device, the desirable fillers are certain grades of bismuth oxychloride
powders and/or barium sulfate since they have white color with relatively good transparency.
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A comparative study of catheter tubing filled with a bismuth oxychloride powder or barium sulfate was
performed and the results are listed in Table 2.
Table 2: Mechanical property data* of catheters filled with bismuth oxychloride or barium sulfate
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Weight (%) 25 25
Tensile, breakload (MPa) 35.4 39.9
Elongation (%) 92.5 81.5
Stiffness (mg) 375 288
*The polymer is polyurethane with a hardness of Shore 84A. Stiffness was measured on a Gurley Stiffness
tester. Tensile strength was measured by an Instron machine.
It appears that the mechanical properties of both catheter samples filled with bismuth oxychloride or barium
sulfate are similar. At equal weight, it is clear that the barium sulfate filled catheter is much less radiopaque
than the bismuth oxychloride filled catheter (Figure 1).
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Figure 1. X-ray image of catheters filled with identical amount of barium sulfate (left) and
bismuth oxychloride (right)
SEM pictures of cross-sections of the tubing samples were taken to compare the dispersibility of bismuth
oxychloride and barium sulfate in the polymer (Figure 2). It is apparent from the SEM pictures that barium
sulfate filled tubing has a very grainy appearance. However, both bismuth oxychloride and barium sulfate are
generally well-wetted.
Figure 2. SEM pictures of cross-section tubing of barium sulfate (left) vs. cross-section tubing
of bismuth oxychloride (right)
It is well known that the loading level (by volume) of filler affects the mechanical properties of a plastic.
Because of the differences in density, the weight percentage of bismuth oxychloride and barium sulfate was
converted to volume percentage and the data are listed in Table 3.
Usually, it is not recommended to fill a polymer with excessive quantity of filler in terms of volume percentage
in order to maintain the polymer’s mechanical properties. In the case of barium sulfate, 20 - 40% by weight is
commonly used to make catheters with reasonable radiopacity and has little effect on the physical properties of
the plastics. At 40% by weight loading level, barium sulfate occupies 14.0% of the total volume of the finished
compound, while bismuth oxychloride powder represents only 8.7% by volume. Furthermore, it has much
higher radiopacity than the same amount of barium sulfate. Therefore, when higher radiopacity is needed,
barium sulfate is apparently not satisfactory since excessive loading will cause the loss of mechanical properties
of the polymer. On the other hand, up to 50% by weight of bismuth oxychloride powders can be used to fill the
polymer and still maintain the required integrity of the polymer.
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Table 3: Volume versus weight percentage of bismuth oxychloride powders and barium sulfate
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Weight % Volume % Volume %
10 2.6 1.6
20 5.8 3.5
30 9.5 5.8
40 14.0 8.7
50 19.6 12.5
60 26.8 17.7
70 36.3 25.0
Nowadays, new generation X-ray machines are operated at higher radiation energy (kVp) and consequently
higher radiopaque materials are essential to provide sufficient attenuation of the radiation energy. Barium
sulfate filled catheters will not be able to exhibit a bright and sharp image due to its lower K-edge energy, which
as a rule of thumb should be comparable with the kVp of a machine. Therefore, the development of new
instrument technology also accounts for the increasing popularity of bismuth oxychloride powders in catheter
applications.
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Bismuth oxychloride is versatile and has been used in various applications, such as cosmetics and personal care
products in addition to medical devices. Its usage in medical devices, particularly in catheters, has increased
steadily due to many of its advantageous attributes.
Bismuth oxychloride is non-toxic and stable under normal compounding and extrusion conditions. It has a high
specific gravity (7.7 g/cm 3 ) and does not melt and only starts to decompose at temperatures above 600 o C.
Bismuth oxychloride renders very smooth skin feel due to its layered crystal structure (Figure 3) and partial
agglomeration (Figure 4), and it offers a smooth, silky finished surface for the plastics. The surfaces of bismuth
oxychloride powder crystals are also somewhat hydrophobic, therefore, they disperse readily in the oily and
hydrophobic systems.
Bismuth oxychloride is compatible with a wide range of polymer resins used in catheter applications, such as
nylon elastomer, polyurethane, polyethylene copolymers, etc.
Figure 3. Schematic drawing of the crystal structure of bismuth oxychloride
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Figure 4. SEM picture of a bismuth oxychloride powder
Bismuth oxychloride not only offers high radiopacity, but also can be used in combination with other colorants
if colors other than white are desired. It can be blended with barium sulfate for a broad range of applications.
It has not been clearly understood why bismuth oxychloride is sensitive to UV light. Under direct sunlight, it
turns gray. However, this darkening effect is partially reversible, i.e. if a darkened bismuth oxychloride is kept
in the dark, it will become white again, but not to the original whiteness prior to the light exposure. Many
efforts have been made to stabilize bismuth oxychloride against the UV light and have yielded commercially
available bismuth oxychloride powders with much improved light stability. Catheters made with more light
stable bismuth oxychloride powders exhibit better light stability than those made with regular bismuth
oxychloride powders. In general, it is recommended to use UV absorbers in clear packaging to prevent
photograying of the device prior to use.
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High purity bismuth oxychloride powders can be manufactured by hydrolysis of bismuth salts (derived from
pharmaceutical grade bismuth metal) in an acidic aqueous system in the presence of excess chloride ions.
Bi 3+ + Cl - +H2O → BiOCl + 2H +
Under different precipitation conditions, such as temperature, pH, concentration, various types of bismuth
oxychloride powders can be obtained. It is possible to produce bismuth oxychloride with extremely little
mechanical or chemical impurities and to meet the specifications for cosmetic and external applied drug
applications according to 21 CFR 73.1162 and 73.2162 (2).
Furthermore, bismuth oxychloride powders can be produced with very low moisture content which is
advantageous for compounding with plastics. Because of the crystalline (non-porous) nature and the smooth
surface of the crystal platelets, the oil absorption of bismuth oxychloride is relatively low, in the range of 20-60
g/100g.
Bismuth oxychloride powders contain mostly irregular platelet-shaped crystals. Typically, they have a diameter
of 2-20 µm and a thickness of less than 1 µm and tend to agglomerate to some extent. Various grades of
bismuth oxychloride powders are commercially available with different opacity, bulk density, and light stability.
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In addition to the radiopacity that bismuth oxychloride renders, it was discovered that bismuth oxychloride also
improved the laser marking property of the plastic in which the bismuth oxychloride was compounded (3).
Photos shown below (Figure 5) are laser marked HDPE chips containing various levels of bismuth oxychloride
powder (1-25% w/w). It appears that when low concentration of bismuth oxychloride is used, the mark is
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visible but blurry. Once the loading level of bismuth oxychloride is increased up to 10%, clear marks with good
resolution are obtained.
1% BiOCl 5% BiOCl 10% BiOCl
15% BiOCl 20% BiOCl 25% BiOCl
Figure 5. HDPE chips filled with various levels of bismuth oxychloride powder
Laser marking property of catheters with 25% loading of bismuth oxychloride or barium sulfate is compared in
Figure 6. It is apparent that the catheter with barium sulfate has no mark while the one with bismuth
oxychloride exhibits very sharp and clear mark.
Figure 6. Laser marked polyurethane filled with 25% barium sulfate (left) or 25% bismuth oxychloride (right)
This advantageous property of bismuth oxychloride can help the manufacturers of medical devices to use less or
no laser marking materials in their products to achieve the required marks for their devices if laser marking is
desired.
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Bismuth oxychloride is an unique filler and renders many advantages over other fillers for radiopaque medical
devices. It provides high radiopacity and sharper contrast image than barium sulfate, allows high loading
without adverse effect on the mechanical properties of the polymers and facilitates more efficient manufacturing
process because of its excellent powder dryness. Furthermore, bismuth oxychloride can improve the laser mark
property of the plastics.
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1. Gachter R. and Muller H., 3ODVWLFV $GGLWLYHV, Third Edition. Carl Hanser Verlag, Munich, Germany
(1990).
2. Code of Federal Regulations, Food and Drugs, 21 Parts 1 to 99 (Revised as of April 1, 2004).
3. Peng, Q, Pending patent application (2005).
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