Bio-compatibility Interactions
Nowadays, we apply metals, ceramics, and polymers materials in medical devices to improve human health and life quality. The typical biological materials are polymers, natural rubbers, polyurethanes, and silicone rubbers. Therefore, we can formulate these materials for specific biomedical applications with unique characteristics.
Liquid Silicone Rubber has excellent mechanical and chemical properties. Furthermore, it is also nontoxic, non-irritant and low cost. These desirable characteristics make it a perfect bio-material. Bio-material in definition is a synthetic or natural material in biomedical devices. Meanwhile, it can replace living system or contact with living body for intended methods and period.
Difference between Bio-compatibility and Bio-stability
Bio-compatibility is the ability to contact with a living system without adverse effect. Meanwhile, Bio-stability is the ability to resist the degrading effect of hosts. Once the material loses bio-stability, it will be no longer bio-compatible.
Normally, bio-compatibility depends on two main factors: bulk properties and surface properties. Firstly, bulk properties are related to mechanical function between devices and surrounding tissues, such as weight, shape, flexibility. Secondly, surface properties are related to chemical and biological interaction between material and tissue. All in all, any implanted devices are designed with minimal impact, the body response is depended on the chemical composition of the surface.
Key properties of liquid silicone rubber in bio-medical applications.
| Biological | Physicochemical |
| Inertness Non-toxic Moderate Biocompatibility Protein absorption Blood-compatibility Microbial adhesion | Thermal stability Low density Excellent dielectric behavior Low curing temperature Resistance to UV, oxygen, ozone and sunlight Resistance to aging and bio-degradation Chemical stability in hydrocarbons, oils and solvents Flexibility&elastic, easy mold and shape Hydrophobic character High gas permeability Good optical transparency |
Sterilization
Liquid Silicone Rubber in bio-medical applications must be sterilized without any side effect. Normally, the sterilization process is in dry or steam atmosphere with temperature between 110℃ and 190℃. Furthermore, there are new sterilization method in process, such as:
- Firstly, ethylene oxide gas (EOG) is another method of sterilization, there is little EOG residue on silicone rubber surface, in reason of carbon dioxide (CO2) can displace any absorbed EOG.
- Secondly, ClO2 is also used for sterilizing, but it will change LSR part crystallization and plasticization level.
- Thirdly, gamma irradiation is becoming more and more popular sterilization method, it never involves any toxic materials. However, this irradiation will result in molecular fracture of silicone rubber, and impact mechanical properties and cross-linking.
Biomedical Applications
Medical grade Liquid Silicone Rubber in biomedical applications is easy fabrication. In addition, it is bio-stale in contact with living organism tissues. However, its inherent hydrophobic characteristic also give rise to some issues in medical devices.
Medical devices are divided into four categories by Health Industry Manufacturers Association, liquid silicone rubber is applied in all four categories. Implant applications of Liquid Silicone Rubber:
| Medical Treatment | Applications |
| Reconstructive surgery | Reconstruction of nose, chin, ear, armature Breast reconstructionConstruction of hearing assist devices. |
| Ophthalmology | Prosthetic eye Detached retina correction |
| Orthopedic surgery | Reconstruction of fingers, wrists, thumbs Maxilla-facial prosthesis |
| Cardiovascular surgery | Ball-and-cage heart valve Coating on pacemaker and lead-wiresConstruction of artificial hearts |
Liquid Silicone Rubber is widely applied in clinical implants in reason of its inherent bio-inert property. We apply silicone rubber for facial reconstruction in facial and oral surgery, in reason of its elastic, odorless and bio-compatible properties. In addition, it can replace polythene as artificial heart valves, such as aortic, mitral and tricuspid cardiac valve. Furthermore, silicone rubber is also used in dental liners, impression bands and materials.
RTV and LSR Biomedical Applications
In bio-medical industry, we can apply both Room Temperature Vulcanization(RTV) silicone rubber and Liquid Silicone Rubber. But these materials should meet the following conditions:
- No by-products after vulcanization reactions, these by-products will contact and migrate with body fluids, result in infection or encapsulation. Addition silicone rubber is the best option due to its no by-products in reactions.
- Any silicone rubber components cannot contain any carcinogenic, toxic, allergic, thrombogenic or inflammatory compounds.
RTV silicone rubber is based on poly-dimethyl-siloxanes in two-part materials. It is vulcanized with platinum, organ-platinum, stannous octoate, or irradiation in the curing process. Meanwhile, the C-H and C-Si bonds fracture, to generate the cross-linking in RTV silicone rubber. However, LSR applies peroxides, condensation, metal salt, and vinyl addition in curing systems.
Normally, the filler materials in LSR compounding is inorganic, such as fumed silica, TiO2, carbon black. However, these fillers are not suitable for bio-medical applications, because fillers cannot bond to silicone backbones, and are hazardous to the human body.
Interaction between Silicone Rubber and Cells
The cell-silicone interaction is depending on the surface physical and chemical properties. Such as: surface charge, surface wet, surface free energy, rigidity, micro-structure, and hydro-phobicity. Furthermore, silicone in body is very stable in contact with physiological environments, LSR silicone implants compatibly with body tissues.
However, silicone rubber has huge difference in physical and mechanical behavior for long-term applications, this effect is greater especially in dynamic applications. Duration of two years is considered as the maximum limit of implant silicone rubber devices. In reason of slow degradation of implanted products by lack of light, excessive heat, and environment stress cracking.
In addition, silicone rubber can undergo another degradation in human body. Hydrolysis degradation is generated by enzyme, tissue PH and physiologic ions. It will change the silicone rubber structure and dynamic properties. Furthermore, mineralization will also occur once body deposit calcium salts on silicone rubber devices.
An efficient way to determine silicone rubber degradation is its mechanical properties’ alternation, such as, tensile stress decreases 10%, the elongation at break will increase at similar value. In addition, shape lose, yellowish color, swelling, cracking, plasticizing effect is also found in degradation.
As the time of implantation, silicone rubber will absorb corporal fluids. Although this absorption won’t result in functional failure, we also need to improve and optimize manufacturing process conditions, especially in curing and post-curing.
Conclusion
In conclusion, Miwo provides leading precision, high volume LSR molding of medical, food and other grades in China. We have professional LSR team and advanced technology for custom LSR, two-shot and over-mold LSR production. Billions of our products are applied in medical, healthcare, infant care, electronics, automotive and industrial market.
Contact us for more information about our LSR molding capability.
