Biocompatibility of Liquid Silicone Rubber (LSR): Part II

PART II – BIOCOMPATIBILITY TESTS AND IMPROVEMENT OF THE BIOCOMPATIBILITY PROPERTIES

In the United States, the Food and Drug Administration (FDA) is the entity in charge of approving materials that come into contact with human body, based on the material’s biocompatibility and biostability properties. There are a variety of methods that can be used to verify a material’s properties. However not all materials will require testing, the FDA states “if there is sufficient knowledge about the biocompatibility and toxicity of the material, then it is not necessary for further biocompatibility testing”.

There are two types of methodologies used in biocompatibility and biostability testing: In Vivo and In Vitro. In Vivo refers to experiments using a living organism, for example when the tissue reaction is examined after chronic subcutaneous implantation [1]. It is not uncommon with in vivo experiments, to achieve positive results that cannot subsequently be repeated.  In Vitro experiments are techniques used for biocompatibility testing that do not involve living organisms. An example of an in vitro experiment is a neutral red cytotoxicity procedure used to analyze the elicited cellular response [1].

It should be noted that in vitro experiments can also use modelling, examining a material’s behavior under conditions that resemble real ones [2].

The following are the steps used to approve new materials, or for approval of existing materials in new applications:

1. Physical and chemical characterization of the material in compliance with Good Laboratory Practices and ISO 9000 quality management system

1. Characterization of the precursor materials, solvents, catalysts, curing agents, reinforcement agents, crosslinking agents
2. Composition and catalyst ratios
3. Description of the formulation and characterization of the material

• Molecular weight (ASTM D3593)
• Glass transition temperature, melting temperature, curing temperature (ASTM D3418)
• Metal content (ASTM F1372)
• Thermal stability (ASTM D5023, ASTM D5026)
• Extractable (ISO 10993-12)

2. According to ISO 10993-1, Issue 4.0, determining the device category as a function of body contact. The categories are:

1. 1.1 Non-contact
2. 1.2 Surface contacting (skin, mucosal membranes, breach, or compromised body surfaces)
3. 1.3 External communicating
4. 1.4 Implantable devices
5. 2 Limited, prolonged, or permanent contact

3. Several physical and chemical characterizations should be performed

4. Phase 1 (ISO 10993-1)

1. Cytotoxicity (ISO 10993-5) – Cell culture techniques
2. Sensitization (ISO 10993-10) – Potential for contact sensitization
3. Irritation (ISO 10993-1) – Estimation for irritation in skin, eyes and mucosal membranes
4. Intracutaneous reactivity (ISO 10993-10) – Inappropriate reaction of the tissue
5. Systemic toxicity (ISO 10993-11) – Potential harmful effects of implants in the first 24 hours

5. Acute and sub-chronic toxicity (ISO 10993-11) – Potential harmful effects of implants after 24 hours

• Genotoxicity (ISO 10993-3) – Evaluation of gene mutations, changes in chromosome structure, DNA, or gene toxicities
• Mammalian cell transformation assay – C3H/10T1/2 o BALB/c3T3
• Chromosomal aberrations (Test 479)
• Implantation (ISO 10993-6): Evaluation of pathological effects on the tissue
• Hemocompatibility (ISO 10993-4): Evaluation of the material’s effect on the blood

5. Phase 2 – For chronically implanted devices

1. Chronic toxicity (ASTM F1983-14) – Potential harmful effects during the total life span
2. Carcinogenicity (ISO 10993-3, Subsection 5.0) – Only if the genotoxicity test results are positive
3. Reproductive and developmental toxicity (ISO 10993-3) – Only if the device has potential impact on reproductive potential

6. Potential for biodegradation (ISO 10993-3, Subsection 9.0) – Identification and quantification of potential degradation products

7. Material stability in vitro:

1. Oxidation (ASTM D6954-04)
2. Hydrolysis (ASTM F1635-16)
3. Environmental stress cracking (ASTM F561-13)
4. Swelling (ASTM D3616-95)
5. Extraction (ASTM F619-14)
6. Wear (ASTM F732-00)
7. Creep (ASTM D3479/D3479M-12)
8. Mineralization (ASTM F2347-15)

8. Material stability in vivo

9. Human clinical evaluation (Clinical trial)

The ISO 10993-1 standard also provides guidelines regarding factors for consideration in determining if materials need to be reevaluated.

IMPROVING BIOCOMPATIBILITY OF THE LIQUID SILICONE RUBBER (LSR)

For parts that will have long-term contact with body tissue, relying on just the inherent inertness of LSR may not be sufficient. For example, when Liquid Silicone Rubber comes in direct contact with blood, the blood will typically coagulate on the device surface, and in the worst case scenario, a thrombus could form. Although capsule contracture is considered an inevitable complication with the use of Liquid Silicone Rubber (LSR) implants, bacteria could also colonize on the device surface leading to infections and even death of the patient [3]. Depending on the application, the surface tension of the Liquid Silicone Rubber should be modified to preserve the characteristics of the material. Also it has been demonstrated that surface topography has a significant impact on the improvement of the biocompatibility; depending on the topography, the characteristic surface tension can be modified [4]. The most common methods for improving the biocompatibility are: ion implantation, sintering, electrochemical deposition, and sol-gel coating.

Ion implantation is easy to perform, inexpensive, and will not affect the mechanical and physicochemical properties of the Liquid Silicone Rubber. When carbon ions are implanted into the silicone, cytocompatibility is improved due to the surface characteristics and morphology [4]. The presence of carbon ions prevents bacteria colonization on the part and on the surface of the mold/die, after 24 hours of implantation [3].

Graft polymerization is used to generate a strong polymeric material. The graft polymerization of water-soluble monomers, normally a hydrogel, reduces protein adsorption and platelet adhesion on the device surface. Water-soluble monomers, as the name implies, have very high water content which inhibits the cell adhesion. However, because of the low mechanical properties of hydrogels, it may not be feasible for some applications [5].

Some metallization processes are employed to modify the surface properties. The advantage of this process is that it does not alter the biocompatibility of silicon rubber [6]. For example, Liquid Silicone Rubber can be coated with polymethyl metacrylate (PMMA) before treatment with laser generating patterns. The surface over which the laser passed is activated and metallized in a platinum bath, and the residue is then removed with a chloroform bath. Another option for surface modification of the silicone rubber is grafting with hydrophilic hydrogels, such as polyacrylamide (PAAm) [7]. The laser radiation described above is also employed with this method. Blending is yet another option. The Liquid Silicone Rubber can be mixed with a cross-linked hydrogel, improving the biocompatibility and the hydrophilic properties.

Biocompatibility improvement of the LSR can also be achieved by slowly releasing Nitric Oxide (NO) during compounding. Nitric Oxide has the ability to attenuate platelet activation, and will also prevent infections due to its antimicrobial properties. Another advantage of using Nitric Oxide, is that the NO formulation can be adjusted as needed for each medical application [8].

The biostability and life of Liquid Silicone Rubber (LSR) in long-term implantation depends on the type of chemical structure used by the elastomer, the application conditions, additives, and the crosslinking density.

Liquid Silicon Rubber (LSR) will behavior differently depending on the application in the human body. Since biostability research is in its early stages, the mechanism involved in the degradation of the LSR after two years should be examined. A combined effort between polymer science, polymer processing, and medicine is required.

References

1. Dolezel, B., Adamírová, L., Vondracek, P., Naprstek, Z. In vivo degradation of polymers. II. Change of mechanical properties and cross-link density in silicone rubber pacemaker lead insulations during long-term implantation in the human body. Biomaterial, 10, 387-392, 1985.
2. Hassler, C., Boretius, T., Stieglitz, T. Polymers for neural implants. Journal of Polymer Science: Part B: Polymer Physics, 49, 18-33, 2011.
3. Zhou, X., Chen, X., Mao, T.-C., Li, X., Shi, X.-H., Fan, D.-L., Zhang, Y.-M. Carbon ion implantation – A good method to enhance the biocompatibility of silicone rubber. Plastic and Reconstructive Surgery, 137, 690-699, 2016.
4. Lei, Z.-Y., Liu, T., Li, W.-J., Shi, X.-H., Fan, D.-L. Biofunctionalization of silicone rubber with microgroove-patterned surface and carbon-ion implantation to enhance biocompatibility and reduce capsule formation. International Journal of Nanomedicine, 11, 5563-5572, 2016.
5. Fujimoto, K., Tadokoro, H., Ueda, Y., Ikada, Y. Polyurethane surface modification by graft polymerization of acrylamide for reduced protein adsorption and platelet adhesion. Biomaterials, 14, 442-448, 1993.
6. Vondracek, P., Dolezel, B. Biostability of medical elastomers: A review. Biomaterials, 5, 209-214, 1984.
7. Fallahi, D., Mirzadeh, H., Khorasani, M.T. Physical, mechanical, and biocompatilibity evaluation of three different types of silicone rubber. Journal of Applied Polymer Science, 88, 2522-2529, 2003.
8. Brisbois, E.J., Major, T.C., Goudie, M.J., Bartlett, R.H., Meyerhoff, M.E., Handa, H. Improved hemocompatibility of silicone rubber extracorporeal tubing via solvent swelling-impregnation of S-nitroso-N-acetylpenicillamine (SNAP) and evaluation in rabbit thrombogenicity model. Acta Biomateralia, 37, 111-119, 2016.