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Silicone tubing has increased its market share due to its performance |
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Some of the most useful properties of silicone include:
• Good electrical insulation making it suitable for a wide range of electrical applications
• Thermal stability (constancy of properties over a wide operating range of 100-250 �C)
• Though not a hydrophobe, it has the ability to repel water and form watertight seals
• Excellent resistance to oxygen, ozone and UV light leading to widespread use in the construction industry (coatings, fire protection, glazing seals), and automotive industry (external gaskets, external trim)
• Non-stick
• Low chemical reactivity
• Low toxicity, but does not support microbiological growth
• High gas permeability: at room temperature (25 �C) the permeability of silicone rubber for gases like oxygen is approximately 400 times that of butyl rubber, making silicone useful for medical applications
Silicone tubing has become more popular in the medical device market over the last five years. Silicone is gradually replacing tubing from other polymers such as Teflon or PU in surgical instrumentation to implantable drug-delivery systems. Silicone tubing offers biocompatibility and ease of manufacturing and also offers good choice in design without sacrificing quality or durability. Good biocompatibility compared to other thermoplastic has finally given silicone tubing a boost. Silicone tubing, mainly due to the following characteristics offers advantages over thermoplastic tubing:
• Low toxicity
• Physiological inertness
• Low occurrence of leachable and extractable
• Hypoallergenicity
Silicone has been widely used in both implantable applications such as defibrillator and pacemaker lead covers and stents and non implantable applications such as wound drainage. The material has also found various applications in biological sensors, prosthetics, and drug-delivery systems. Silicone tubing is available in a wide durometer range (20�90 Shore A hardness), and its color characteristics can include clear, translucent, or opaque versions. In addition, radiopaque tubing in either a striped or solid form can be produced from silicone. The material can be continuously extruded to encompass a broad spectrum of sizes. For example, silicone can be extruded for very small diameters and tolerances (0.01-in. outer diameter, �0.0005 in.). Conversely, the material can be extruded to sizes exceeding 3 in.
Designers sometimes desire a specific shape along the tube length. Silicone is a heat-cured material; partially cured or uncured extruded profiles can be placed into forming trays or other specialized molds to help set a specified shape without altering physical properties or crucial dimensions.
Silicone tubing carries a reputation for not being able to withstand extreme pressures as well as comparably sized thermoplastic tubing. However, a designer requiring a softer or more flexible tube that still has resistance to pressure can specify a reinforced version. Silicone extrusions can be embedded or reinforced by braiding or spiral winding with a variety of materials. Polyester and stainless steel are common supplemental materials.
Braiding occurs as a secondary operation and provides kink resistance as well as an increased working pressure range. The reinforced silicone tube�s inner layer is extruded (either hollow or over a core), braided or spiral wrapped, then re extruded with an additional layer of silicone, encapsulating all the components or layers. The process can be repeated for multiple layers.
The developments in precision braiding equipment, very small stranded materials and steel wire or ribbon can impart significant pressure resistance, kink resistance, and rigidity to small extruded shapes. Braiding requires a minimum inner tubing wall thickness and a minimum encapsulation thickness. Dimensional tolerances are typically larger due to stack-ups, which is a term used to describe the addition of different tolerances that must be considered in the production and functionality of each layer of the part. Core removal may also need to be considered to limit piece length.
Another characteristic of silicone tubing is its ability to separate multilumen tubing for multifunctional applications. This strippable tubing is achieved by including a thin web (0.007 � 0.010 in.) between discrete tubes. Multilumen tubing, with certain design restrictions, can transition to fewer lumens or change lumen size and location within the same tube. This has multiple uses in the wound-drainage market,.
Inertness and ease of manufacture provides advantages to silicone tubing. Silicone tubing has some limitations, including lower tear strength when compared with PU tubing. Low burst pressure of silicone tubing without reinforcement compared to the other thermoplastic tubing is another limitation. Silicone tubing also has limitations in chemical resistance to fluids like concentrated acids or bases, organic solvents, or oils.
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