Biocompatible Silicones: ISO 10993 and USP Class VI at a Glance

In medical technology, patient safety is of utmost importance. 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 highlights 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 obligation. The European Medical Device Regulation (MDR 2017/745) and the US FDA require demonstrably biocompatible materials for all products that come into contact with the body.

For manufacturers, this means: without documented biocompatibility testing, no market approval. In Switzerland, Swissmedic verifies conformity with the relevant standards as part of the approval process. From a liability perspective, material selection is also critical; incidents caused by unsuitable materials can lead to substantial compensation claims and loss of reputation.

Silicones naturally offer many advantages for medical applications: they are chemically inert, temperature-resistant, stable against aging, and exhibit low surface energy. However, not every silicone is automatically biocompatible. The decisive factors are raw material quality, additives, catalyst systems, and processing.

ISO 10993: The Standard Series Explained

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

• ISO 10993-1: Basic evaluation and test selection based on contact type (skin, mucous membrane, blood) and contact duration (short-term
• ISO 10993-5: Tests for cytotoxicity (cell toxicity), the basic test for almost all materials
• ISO 10993-10: Tests for irritation and skin toxicity: important for products with skin contact
• ISO 10993-11: Tests for systemic toxicity: for implants and products with long-term body contact

A common misunderstanding: ISO 10993 is not a "certificate for a material", but rather a test 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 widely used standard in the USA for biocompatible materials. 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 as a whole, without distinguishing between different types of contact. A material that passes USP Class VI generally also meets many requirements of ISO 10993, but the reverse is not necessarily the case.

Which standard when? For the European market, ISO 10993 is decisive. 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 silicone types are suitable for medical applications. Purity, crosslinking system, and the additives used are decisive:

RTV-2 Addition-Curing Silicones

Two-component, room-temperature vulcanizing silicones with platinum catalyst. They cure without the release of by-products and achieve high purity levels. Bluesil RTV 141 and similar products are available in biocompatible grades. Typical applications: prototypes, seals, molding compounds for epitheses.

LSR (Liquid Silicone Rubber)

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

HTV (High Temperature Vulcanizing)

High-temperature curing solid silicones. Cure at 150 to 200°C, available in biocompatible grades. Used for hoses, molded parts, 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, cushion pads for prostheses, wearable sensors. Biocompatible gels must be particularly pure, as they often have extensive skin contact.

Important: Biocompatibility is not an inherent material property, but depends on raw material batch, manufacturing process, and post-treatment. Request Material Data Sheets and biocompatibility documentation from the 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 at least in ISO Class 8 (cleanroom class 100'000). Particles, fibers, and microbial contamination must be avoided. Wear gloves; skin contact leaves fats and proteins.

Contamination Prevention

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

Post-Cure (Post-Curing)

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

Extraction

Some manufacturers additionally perform solvent extraction (e.g., with hexane or ethanol) to remove extractable substances. This is particularly important for implants. However, please 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. Here, the highest requirements are placed on long-term stability and tissue compatibility. Typically LSR with complete ISO 10993 testing.

Catheters and Tubing

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

Prostheses and Orthoses

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

Wearable Medical Devices

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

Diagnostic devices

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

Sterilization Methods and Their Impact on Silicones

Medical devices must be placed on the market sterile. The choice of sterilization method influences 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)