Features

Healthcare-related tubing is used for fluid management and drainage as well as with anesthesiology and respiratory equipment, IVs, catheters, peristaltic pumps, and biopharmaceutical laboratory equipment. It is essential that the materials employed meet the highest quality requirements.

Breathing tube. (Credit: Wacker Chemie AG)

Silicone elastomers are the material of choice for use in healthcare tubing or other extruded profiles that require compliance to medical device regulatory standards. Once silicone rubber is converted to cured silicone tubing or extruded profiles, it contains no plasticizers that can leach or be extracted, nor does it retain by-products of the curing reaction. A post-cure process is not required, although it is highly recommended to stabilize physical dimensions and mechanical properties.

Processing Characteristics

Table 1. Comparison of silicone, TPE, and PVC.
Small diameter medical tubing. (Credit: Wacker Chemie AG)

Silicone elastomers outperform conventional thermoplastics such as polyvinyl chloride (PVC), polyethylene (PE), polyurethane (PU), ethylene vinyl acetate (EVA), and poly-isoprene in extrusion applications, bringing overall value in processing and performance. Table 1 shows a comparison of silicone, PTE, and PVC. Silicone elastomers possess excellent dimensional stability, and their inherent strength and flexibility hold very thin-wall tubing profiles. Furthermore, they can be extruded as solid profiles with significant cross sections. Production space can be minimized by vertical oven positioning and short-curing tunnels that require fewer heating elements. Throughput of up to 50 ft/min is achievable with the correct equipment. Silicone elastomers are available as a one- or two-component rubber system.

Material Characteristics

Silicone elastomers are available in a wide range of durometer ranges and offer excellent heat resistance and good chemical resistance. Silicone elastomer tubing is resistant to many environmental factors such as temperature, chemicals, UV radiation, and x-rays. Their use in medical applications is driven in part by the fact they resist adhering to body tissue and do not support microbial growth. Silicone elastomer tubing can be sterilized with a variety of methods with minimal performance degradation or changes in mechanical properties. This reflects the molecular stability of silicone. Once made into a finished part, silicone rubber is generally sterilized by steam, ethylene oxide, e-beam, gamma, and beta radiation prior to its end use.

One of the singular traits of silicone elastomers is the silicone-oxygen (Si-O) bond. Energy and molecular stability of silicone is the strongest contributor to biodurability, or the ability to have skin contact and resist body fluids. Silicone's many hardness options remain flexible without additives — something that is very important in biocompatibility. Silicone, by nature, has a high level of translucency due to the compatible refractive indexes of the filler and polymer that are used, and it provides distinct colorability. There are different pigment options; however, silicone elastomers can be highly transparent for fluid and flow visualization.

Another silicone trait is vapor permeability, which is why silicones are used in wound care and prosthetics and have high biocompatibility for skin contact. Silicone is easy to process and is used in most types of rubber processing, e.g., molding using compression or transfer, in addition to extrusion.

In general, silicone rubber has very good resistance to weathering, UV radiation, and x-rays. These products can be made radiopaque by adding barium sulfate, so they are visible in x-rays, especially for short-term implantable parts. The general hydrophobicity of silicone rubber helps inhibit the growth of microbes.

Silicones for Healthcare

A variety of silicones are available, so it is critical to select the right one for the application (see the sidebar, “Selecting the Appropriate Healthcare-Grade Silicone Elastomer.” Wacker, for example, offers several families of extrusion-grade elastomers for healthcare applications. These materials comply with USP Class VI, ISO 10993, FDA 21 CFR 177.2600, German BfR, and European Pharmacopoeia 3.1.9.

These standards ensure that silicones are biocompatible and non-cytotoxic, and that they can be used for short-term implantation of less than 30 days. Device Master Files are with the FDA for the SILPURAN® silicones, which are designed for the healthcare sector, and they are available for select ELASTOSIL® products. SILPURAN® products are cleanroom packaged specifically for healthcare applications. ELASTOSIL® products are general-purpose products that also comply with the healthcare standards. SILPURAN® 8030 and ELASTOSIL® 4305, for example, are families of platinum-cured extrusion bases that work with a catalyst masterbatch at a 100:1.5 ratio. These are offered in several different durometers.

Conclusion

Because of their characteristics, silicone elastomers are the material of choice for medical, pharmaceutical, and biotech applications. Biocompatibility and ease of use lead the list of benefits gained from using silicone elastomers. With such great versatility, they offer an attractive alternative and cost/benefit ratio to thermoplastics and thermoplastic elastomers in extrusion products for healthcare applications.

This article was written by Scott Richardson, Technical Manager Healthcare, Wacker Chemical Corporation, Allentown, PA. For more information, visit here.


Selecting the Appropriate Healthcare-Grade Silicone Elastomer

  1. Look for compliance to the following healthcare standards:
    1. FDA 21 CFR 177.2600 for food additives.
    2. US Pharmacopeia Class VI for biocompatibility.
    3. ISO 10993 for biocompatibility and cytotoxicity.
  2. Know your preferred process for manufacturing the silicone parts:
    1. Extrusion; for various size tubing and solid profiles.
    2. Compression or transfer molding; for manual or semiautomatic processing of medium to large parts.
    3. Liquid injection molding; mostly unattended processing of large numbers of small to medium sized parts.
  3. Know your preferred silicone curative:
    1. Peroxide; a less-expensive base is needed but will require a post-cure heat aging of parts to eliminate the by-products of the curing process. These are offered in various grades depending on which manufacturing process you use.
    2. Platinum; a clean cure without by-products, although a post-cure is still recommended to tighten physical properties. There are also different platinum catalysts available depending on which process is used.
  4. Know which physical properties are required:
    1. Durometer hardness; families of products may have multiple choices.
    2. Tensile and elongation; stress/strain values may vary by durometer.
    3. Tear strength; higher numbers are better for mold release and toughness.
    4. Compression set; low numbers mean less likely to have a permanent deformation in application.
  5. Other questions should be asked of the technical contacts at your material supplier.