Biocompatible silicones

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

Patient safety is paramount in medical technology. Silicones that come into direct or indirect contact with the human body must meet strict 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 any harmful reactions in living organisms. For medical devices, this is not only a question of quality, but also a regulatory requirement. The European Medical Device Regulation (MDR 2017/745) and the US FDA require proven biocompatible materials for all products that come into contact with the body.

For manufacturers, this means that without documented biocompatibility testing, there will be no market approval. In Switzerland, Swissmedic checks compliance with the relevant standards as part of the approval process. The choice of materials is also critical in terms of liability law—incidents caused by unsuitable materials can result in substantial claims for damages and reputational losses.

Silicones naturally offer many advantages for medical applications: they are chemically inert, temperature-resistant, stable over time, and have low surface energy. However, not every silicone is automatically biocompatible. The quality of the raw materials, additives, catalyst systems, and processing are crucial factors.

ISO 10993: The series of standards 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: Grundlegende Bewertung und Testauswahl basierend auf Kontaktart (Haut, Schleimhaut, Blut) und Kontaktdauer (kurzzeitig <24h, längerfristig 24h-30d, dauerhaft >30d)
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 that come into contact with the skin
ISO 10993-11: Tests for systemic toxicity – for implants and products with longer-term body contact

A common misunderstanding: 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.

Note device master record

Biocompatibility testing always refers to the defined end product as specified in the Device Master Record (DMR). Changes to materials, processing, or sterilization require a reassessment of biocompatibility. Carefully document all material specifications and supplier changes.

USP Class VI: The US standard

The United States Pharmacopeia (USP) Class VI is an older standard for biocompatible materials that is still widely used in the US. It comprises three main tests:

Systemic Injection Test: Extracts of the material are injected into mice and monitored 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 strict 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 usually also meets many of the requirements of ISO 10993—but the reverse is not necessarily true.

Which standard applies when? ISO 10993 is the relevant standard for the European market. US customers and the FDA often require USP Class VI as well. Many manufacturers of biocompatible silicones have both tests carried out to ensure global market acceptance.

ISO 10993 vs. USP Class VI: Direct comparison

criterion	ISO 10993	USP Class VI
origin	International (ISO), preferred in Europe	USA (United States Pharmacopeia)
scope of testing	Modular, risk-based according to contact type and duration	Three standard tests for all materials
flexibility	High – Tests are selected according to application	Low – always the same three tests
acceptance	Worldwide, especially EU, Switzerland, Asia	USA, increasingly recognized internationally
test duration	Depending on the parts, 2–12 weeks	Typically 4–6 weeks
expenses	Variable, depending on the tests selected (CHF 5,000–25,000)	Fixed, approx. $8,000–12,000
Regulatory validity	MDR/IVDR compliant, FDA accepted	FDA compliant, not always sufficient for the EU

Material types: Which silicones are biocompatible?

Not all types of silicone are suitable for medical applications. The decisive factors are purity, cross-linking system, and additives used:

RTV-2 Addition-curing silicones

Two-component silicones that vulcanize at room temperature with a platinum catalyst. They cure without releasing by-products and achieve high degrees of purity. Bluesil RTV 141 and similar products are available in biocompatible versions. Typical applications: prototypes, seals, impression compounds for prostheses.

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 150–200°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, textile coatings. Mostly peroxide-cross-linked, therefore thorough post-cross-linking 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 come into contact with large areas of skin.

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 the supplier.

Processing instructions for biocompatible silicones

Even the best biocompatible silicone can lose its properties if it is not processed correctly. Key requirements:

cleanroom environment

Medical devices should be processed in at least ISO Class 8 (cleanroom class 100,000) conditions. 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 for biocompatible materials only. Silicones easily absorb plasticizers from PVC tubing or residues from release agents. These migrants can negatively affect biocompatibility tests.

Post-cure (post-curing)

After cross-linking, volatile components (VOCs) often remain in the material. Thermal post-treatment (typically 4 hours at 200°C or 24 hours at 150°C) reduces residual monomers and low-molecular 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, 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. These applications place the highest demands on long-term stability and tissue compatibility. Typically LSR with 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, liners for leg prostheses. Silicone allows for a skin-like feel and translucency. Biocompatibility must be guaranteed over many years of wear.

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 when sweating and moving.

diagnostic equipment

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

Sterilization processes and their effect on silicones

Medical devices must be sterile when placed on the market. The choice of sterilization method influences the material properties:

Procedure	Temperature/Method	Suitability for silicones	effects
autoclaving	121–134°C, saturated steam, 15–30 min	✓ Very suitable	No damage, possibly slight discoloration on light-colored materials
gamma radiation	25–50 kGy ionizing radiation	✓ Suitable, but testing required	Can increase degree of cross-linking (hardening) or split chains (softening), depending on dose and formulation
Ethylene oxide (EtO)	37–63°C, EtO gas, several hours	✓ Very suitable	No mechanical changes, but sufficient degassing is necessary (EtO residues are toxic).
Plasma (H₂O₂)	40–50°C, hydrogen peroxide plasma	✓ Suitable	Very gentle, no residue, but slow process

Recommendation: Check the material properties again 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)

No. Food safety (FDA CFR 21, EU 10/2011) means that a material is approved for contact with food—it tests for the migration of substances into food. Biocompatibility tests biological reactions (cytotoxicity, sensitization, systemic toxicity). A food-safe silicone is not automatically biocompatible and vice versa. Only biocompatibility is relevant for medical devices.

The duration depends on the standard chosen. USP Class VI typically takes 4–6 weeks, as implantation tests require several weeks of observation. ISO 10993 tests vary: cytotoxicity (Part 5) is completed in 1–2 weeks, while systemic toxicity or implantation tests (Parts 6, 11) can take 8–12 weeks. Allow 3–4 months for a complete test, including sample preparation and report generation.

High-quality, fully cross-linked and post-cured silicones are very stable over the long term. They are more resistant to UV radiation, oxidation, and hydrolysis than most other polymers. However, mechanical stress (stretching, compression), aggressive media (strong acids/alkalis), or extreme temperatures can lead to changes in properties over time. Long-term studies over 10+ years are required for implants. Accelerated aging tests (e.g., storage at 70°C) simulate aging and help to predict service life.

The costs vary greatly depending on the scope. A basic cytotoxicity test (ISO 10993-5) costs approximately CHF 2,000–3,000. A complete ISO 10993 test series for an implant (parts 5, 6, 10, 11, possibly 3, 4, 18) can cost CHF 20,000–40,000. USP Class VI costs CHF 8,000–12,000. Added to this are costs for sample production, shipping, toxicological evaluation (Biological Evaluation Report), and any follow-up tests. Use existing data from the material supplier to reduce costs—many manufacturers offer master files.

Ja, aber mit Einschränkungen. Farbpigmente müssen selbst biokompatibel sein. Anorganische Pigmente (Eisenoxide, Titandioxid) sind meist unkritisch. Organische Farbstoffe können problematisch sein, wenn sie migrieren oder zytotoxisch wirken. Jede Farbzugabe verändert die Materialzusammensetzung und erfordert theoretisch eine Neubewertung der Biokompatibilität. In der Praxis akzeptieren Behörden oft Daten für das ungefärbte Basismaterial, wenn die Pigmentkonzentration gering ist (<1%) und der Pigmenthersteller Biokompatibilitäts-Dokumentation liefert. Für hautfarbene Epithesen sind speziell zertifizierte Pigmentsysteme erhältlich.