Silicone rubber is recognized for being inherently biocompatible with human tissues, resistant to bacteria, and it does not degrade in the presence of fluids like blood, saliva, and others. This material is used more and more in Medical Applications, but increasingly strict requirements make it necessary to modify the Silicone in order to use it. Additives are a common option for increasing mechanical and physical properties, but silicone can also be combined with other materials such as thermoplastics, thermosets, thermoplastic elastomers, ceramics, and metals to comply with application requests. In the industrial processing of materials, the challenge is always to create high-performance products in the shortest time with the least amount of the components. There are two options for that: (i) select the best material that fits with all the requirements of the final application; in some cases, this can be almost impossible since it is common that application requests are opposing, for example, rigid in one area but flexible in another area of the part. There is also option (ii): design a single part made with multiple materials that fulfill the application requirements, which is manufactured using co-injection or over-molding, otherwise called two-shot or multi-shot injection molding; this process offers extended functionality, better appearance, and high quality of the product. In this special process, critical variables include mold temperature, shrinkage, and deformation of the second injected material, and they must be analyzed carefully because the right choice will determine the success of the product.

Design Considerations

Before the processing of a multicomponent part, which usually combines hard and soft materials, it is necessary to consider the design of it. In addition to the good adhesion that must exist between the materials that make up the product, it is necessary that there be a mechanical and physical synergy between them. This bond can also be improved by creating physical interlockings called mechanical bonds, where silicone rubber in injected into “runners” or a “mesh” inside of the second material. First, the requirements of the silicone rubber (the soft part of the product) are considered. In this case, medical applications of multicomponent products are being analyzed, so the most important properties are those related to biocompatibility. Sometimes, medical silicone rubber must also be modified to increase the surface tension (and avoid adhesives, which would mean additional process steps) [1]. Second, it is necessary to know the compatibility of the silicone rubber; in medical applications, the combination of silicone rubber-thermoplastics and silicone rubber-metal are the most used and the focus will be on these two groups. In addition to that, the other material(s) that are part of the product must be compatible with the human tissues/fluids, or in another case, not be in direct contact with them. The biocompatibility of thermoplastics can be measured by the ISO 10993-1 standard [2] where several tests are included to measure the cytotoxicity, sensitization, irritation, genotoxicity, hemocompatibility, cancerogenicety, and biodegradation of the materials at short, protracted, and continuous use of materials in contact with the body surface, in contact with the interior of body, from the outside, and implantable medical products [3]. As a result of this analysis, Table 1 summarizes the use of thermoplastics in medical applications using the classification of the Health Industry Manufacturers Association. Related to metals, the metals and alloys employed must not be susceptible to corrosion due to the terrible effects that this could have in the human body. For that reason, stainless steel such as 316L, cobalt-chromium alloys, titanium alloys, and Nitinol (Nickel-Titanium) alloys are used. Despite corroding like all metals, they have an inherent passivation of the oxide layer. However, the major disadvantage of metals is in long-term applications; for example, implants in which, at some point, metal ions are released as the oxide layer diffuses can then accumulate in the tissue, creating a layer of fibrous tissue that in some cases is dangerous for the health. Table 2 summarizes the use of metals in medical applications using the same classification of the previous table.

Table 1. Use of thermoplastics in medical applications [3]

Table 2. Use of metals in medical applications [4]

Now the question that arises is whether there is compatibility between these materials and silicone rubber. It can be said that a multicomponent part is a bulk blend between two or more materials, so the same criteria analysis for blends or alloys can be applied to determine the compatibility (adhesion and heat compatibility). It should be considered that during co-injection or multi-shot injection molding (MSM), the second material must be injected immediately after the first material to increase the adhesion between them.

Polymer-silicone multi-shot

Normally, thermoplastic polymers are used as a substrate. The softening temperature and melting temperature of the thermoplastic are critical during processing because silicone rubber is processed at high temperatures. For that reason, it is necessary to use thermoplastics that can resist temperatures above 300°F (150°C), and the only ones that fulfill this requirement are engineering thermoplastics; these must be non-polar to avoid the use of primers and adhesives. The other option is the modification of the silicone to decrease its curing temperature and expand the type of thermoplastic polymers that can be used. For example, polypropylene (PP) is co-extruded with silicone rubber to produce tubes; PP is used as the inner layer, taking advantage of the inherent hydrophobic properties and low gas permeability which means that there is no interaction between the medium and the circulating fluid. Silicone rubber as the external layer improves the pumping properties and high resilience which prevents the PP from failing catastrophically in case of “high pressure” flows, which occurs in extracorporeal machine (hemodialysis, for example). The PC-silicone combination is one of the most popular in medical applications; the advantage here is that co-injection does not need an intermediate surface treatment for the PC to improve the adhesion. It is used in respirators, masks, IV valves, and grips where silicone rubber is used as seal, damper, and/or soft touch surface and the PC as the main body of the product. Other combinations that are being studied and used more in medical applications are: silicone rubber with glass-filler, polyethylenimine (PEI), polyetheretherketone (PEEK), polyphenylene oxide (PPO), polyphenylene sulfide (PPS), polyurethane (PUR), and polybutadiene terephthalate (PBT), which are used in implants, electrodes, pipes, tubes, catheters, stents and medical equipment that is in contact with human tissues.

A new combination that is being used in medical applications is thermoplastic elastomers based on polyurethane (TPU) and silicone rubber. It is used preferably in external electronic devices that sense vital signs in patients, where abrasion resistance and toughness of the TPU is combined with UV and chemical resistance of silicone rubber [5]. A special case of multi-shot polymer silicone is silicone-silicone. The critical variable here is the cure systems used in the silicones involved: they must match. If the first shot is cured using a platinum catalyst, the second must also have the same system; otherwise, there would be a high probability that the curing of the second shot would be inhibited. Also, post-curing is done once the final product is finished.

Injection with metal insert

In the case of injection with a metal insert, the thermal stability of the substrate is not a problem, but the adhesion between the silicone rubber and the metal can be an issue. Here, mechanical interlocking or using a primer that is activated during the curing of the silicone rubber are the most common methods. Again, considering the application, it is necessary that the primer does not contain substances that are toxic to the human body, much less that it has the capability to migrate to the outside (away from the part) and thus avoid accumulations that can form fibrous tissues.

A new application of a silicone rubber-metal combination is a contact lens made with optical silicone and a sensor inside. The sensor detects changes in the eye’s volume, and the data is sent via wi-fi. This information is used to prevent optic nerve damage, which is one of consequences of glaucoma [6]. The stainless steel-silicone combination has applications from the simplest, such as a medical ID bracelet, to the most complicated, such as shoulder implants where silicone rubber is used as a replacement for tendons. Combinations with stainless steel have changed during the last years to using UHMWPE (ultra-high molecular weight polyethylene), and the first combination was employed in hip implants and replacements.

Numerous combinations of materials with silicone rubber are currently used in virtually all fields of medicine. The combinations are tailored to the application, and the compatibility of the components with the human body, tissues and fluids as well as the mechanical properties needed in them. This concept makes possible new combinations that can be designed to fulfill requirements that at present are unthinkable for a polymeric material like silicone rubber.

References

1. https://www.designnews.com/materials-assembly/multi-shot-liquid-silicone-rubber-and-thermoplastic-injection-molding/24401101942833/page/0/1
2. International Organization for Standardization. ISO 10993-1:2009. Biological evaluation of medical devices – Part 1: Evaluation and testing within a risk management process. October 2009.
3. An overview of the plastic material selection process for medical devices. February 2013.
4. Gotman, I. Characteristics of metals used in implants. Journal of Endourology, 11, 6, 383-389, 1997.
5. https://www.dowcorning.com
6. https://www.medicalplasticsnews.com/news/contact-lens-aids-optic-nerve-treatment/