Biocompatible silicones: ISO 10993 and USP Class VI at a glance

In medical technology, patient safety is paramount. Silicones that come into direct or indirect contact with the human body must meet stringent biocompatibility requirements. This guide explains the two most important standards – ISO 10993 and USP Class VI – and shows what developers need to consider when selecting materials and processing methods.

Why biocompatibility is crucial for silicones

Biocompatibility means that a material does not cause harmful reactions in a living organism. For medical devices, this is not only a matter of quality but also a regulatory requirement. The European Medical Device Regulation (MDR 2017/745) and the US FDA mandate demonstrably biocompatible materials for all products that come into contact with the body.

For manufacturers, this means: no market approval without documented biocompatibility testing. In Switzerland, Swissmedic verifies conformity with the relevant standards as part of the approval process. The choice of materials is also critical from a liability perspective – incidents caused by unsuitable materials can lead to substantial claims for damages and reputational harm.

Silicones inherently offer many advantages for medical applications: they are chemically inert, temperature-resistant, age-stable, and have a low surface energy. However, not every silicone is automatically biocompatible. Raw material quality, additives, catalyst systems, and processing are crucial.

ISO 10993: The standard series explained

ISO 10993 is an internationally recognized series of standards for the biological evaluation of medical devices. It comprises over 20 parts covering various aspects of biocompatibility. The following are particularly relevant for silicones:

• ISO 10993-1: Basic assessment and test selection based on contact type (skin, mucous membrane, blood) and contact duration (short-term
• ISO 10993-5 : Cytotoxicity (cell toxicity) tests – the basic test for almost all materials
• ISO 10993-10: Tests for irritation and skin toxicity – important for products that come into contact with skin
• ISO 10993-11 : Systemic toxicity tests – for implants and products with long-term body contact

A common misconception: ISO 10993 is not a "certificate for a material," but rather a testing concept for the finished medical device in its final configuration. A silicone tube may be considered biocompatible in one device but not in another – depending on the sterilization method, contact duration, and other components.

USP Class VI: The US Standard

The United States Pharmacopeia (USP) Class VI is an older, but still very common standard for biocompatible materials in the USA. It comprises three main tests:

• Systemic Injection Test: Extracts of the material are injected into mice, monitoring for toxic reactions.
• Intracutaneous test: Injection under the skin of rabbits to test for local irritation
• Implantation Test: Material samples are implanted subcutaneously and examined histologically after several weeks.

USP Class VI is considered a very stringent test, but it is less differentiated than ISO 10993. It evaluates the material overall, without distinguishing between different contact types. A material that passes USP Class VI usually also meets many of the requirements of ISO 10993 – the reverse is not necessarily true.

When to use which standard? For the European market, ISO 10993 is the standard. US customers and the FDA often additionally require USP Class VI. Many manufacturers of biocompatible silicones have both tests performed to ensure global market acceptance.

ISO 10993 vs. USP Class VI: Direct Comparison

Material types: Which silicones are biocompatible?

Not all types of silicone are suitable for medical applications. Purity, cross-linking system, and additives used are crucial

RTV-2 Addition-curing silicones

Two-component, room-temperature vulcanizing silicones with a platinum catalyst. They cure without the release of byproducts and achieve high purity levels. Bluesil RTV 141 and similar products are available in biocompatible versions. Typical applications: prototypes, seals, and impression materials for prostheses.

LSR (Liquid Silicone Rubber)

Liquid silicones for injection molding, also platinum-cured. Highest purity and reproducibility, ideal for high-volume production. Standard in medical technology for catheters, valves, membranes, and baby products. Process temperatures of 150–200°C enable fast cycle times.

HTV (High Temperature Vulcanizing)

High-temperature cross-linking solid silicones. Cure at 150–200°C, available in biocompatible grades. Used for hoses, molded parts, and textile coatings. Mostly peroxide-cured, therefore thorough post-curing is critical.

silicone gels

Very soft silicones (Shore 00), not fully cross-linked. Used in scar patches, prosthetic cushions, and wearable sensors. Biocompatible gels must be particularly pure, as they often have large areas of skin contact.

Important: Biocompatibility is not an inherent material property, but depends on the raw material batch, manufacturing process, and post-treatment. Request material data sheets and biocompatibility documentation from your supplier.

Processing instructions for biocompatible silicones

Even the best biocompatible silicone can lose its properties if the processing is not correct. Key requirements:

Cleanroom environment

Medical devices should be processed in at least ISO Class 8 (cleanroom class 100,000). Particles, fibers, and microbial contamination must be avoided. Wear gloves – skin contact leaves behind fats and proteins.

Contamination prevention

Use separate tools and mixing containers only for biocompatible materials. Silicones readily absorb plasticizers from PVC tubing or residues of release agents. These migrants can negatively affect biocompatibility tests.

Post-cure (post-curing)

After crosslinking, volatile organic compounds (VOCs) often remain in the material. A thermal post-treatment (typically 4 hours at 200°C or 24 hours at 150°C) reduces residual monomers and low-molecular-weight silicones. This improves not only the mechanical properties but also the biocompatibility.

extraction

Some manufacturers also perform solvent extraction (e.g., with hexane or ethanol) to remove extractable substances. This is particularly important for implants. However, note that aggressive cleaning can also affect the material structure.

Applications of biocompatible silicones in medical technology

Biocompatible silicones have become established in numerous medical fields:

Implants

Breast implants, joint replacement components, cochlear implants, hydrocephalus shunts. These products place the highest demands on long-term stability and tissue compatibility. Typical LSR products undergo complete ISO 10993 testing.

Catheters and tubes

Urinary catheters, venous catheters, feeding tubes, drainage tubes. The smooth surface of silicone reduces the risk of thrombosis and biofilm formation. Platinum cross-linking prevents cytotoxic residues.

Prostheses and orthoses

Epitheses (finger, nose, and ear prostheses), insoles, and liners for leg prostheses. Silicone allows for a skin-like feel and translucency. Biocompatibility over many years of wear must be guaranteed.

Wearable Medical Devices

Housings for insulin pumps, sensor patches, smartwatch wristbands for vital sign monitoring. Silicone gels or soft RTV-2 for skin contact, even with sweating and movement.

Diagnostic devices

Seals in blood analysis devices, membranes in lab-on-a-chip systems, tubing in dialysis machines. No direct patient exposure, but contact with bodily fluids requires biocompatibility.

Sterilization methods and their effect on silicones

Medical devices must be sterile before being placed on the market. The choice of sterilization method affects the material properties:

Recommendation: Re-check the material properties after sterilization. Tensile strength, elongation, and Shore hardness may change. Document the validated sterilization method in the Device Master Record and do not deviate from it.

Frequently Asked Questions (FAQ)