Features

Silicone materials have been around for more than 70 years and since the 1960s have played an important and evolving part in products designed for the medical field. Since that time, silicone materials have been developed in various medical grades for skin contact, as well as short-term and long-term implantation. These materials can be formed into products using compression molding, transfer molding, and injection molding. This provides engineers, designers, and inventors with a high level of flexibility when selecting a molding process to produce their medical devices. Medical devices such as tubing for medical diagnostic systems to drug delivery devices like tear duct (punctual) plugs have been produced out of silicone.

History

Fig. 1 – Compression molding requires the mold to be clamped under pressure with the silicone material inside for a certain cycle time to create a part.

The first commercially viable silicone product was created by Dr. J. Franklin Hyde of Corning Glass. This product was used to impregnate and coat glass cloth for electrical insulation purposes. In 1943, Corning Glass and The Dow Chemical Company formed the Dow Corning Corporation to pursue the commercialization of silicone materials. In the 1950s, High Temperature Vulcanization (HTV) and Room Temperature Vulcanization (RTV) materials were added to many silicone suppliers’ product lines. While these materials are still used today, they are not as common as Liquid Silicone Rubber (LSR) or High Consistency Rubber (HCR).

HCR, a form of silicone typically referred to as gum stock, was introduced in the late 1950s to early 1960s by various silicone suppliers. HCR was most suitable for transfer molding, which is today, a legacy technology. LSR was introduced in the 1970s and today is the most common material used in molding medical device components. LSR provides medical device manufacturers a hypoallergenic material with a high level of biocompatibility, a high degree of thermal stability, and extreme temperature range performance, while being resistant to bacteria and mold growth. As time has progressed, silicones have been made available that are refined further for medical device use beyond skin contact. These newer cutting edge materials can be used inside the body for short- and long-term use.

Materials

Medical device designers, engineers, and inventors have many silicone material choices available today. Many medical grade silicone materials have been formulated by silicone manufacturers to meet the demands and requirements of various medical devices. Silicone suppliers have created guidance for the use of these materials by creating in-house certifications for bio-contact applications. In order for these materials to become certified, they must pass a series of United States Pharmacopeia (USP) tests. The USP creates product quality guideline standards for regulatory agencies and manufacturers of medicine, food ingredients, and medical devices.

Table 1 – USP Classification of Plastics to determine tests to be conducted for various USP grades of material.
There are a total of six USP test challenges, identified as Level I through Level VI (See Table 1. Classification of Plastics). Materials that meet the requirements of the six tests may be considered the most bio-compatible. Although the silicone supplier approves the material to a certain level, for example Grade 6, this does not mean that additional testing is not required. The medical device will still need to undergo USP testing despite the fact that the silicone material meets a certain USP grade.

The silicone materials currently available to medical device manufacturers include medical non-implantable, medical restricted implantable, and medical unrestricted implantable. The selection of each material is dependent on the medical device’s end use. The higher USP grade of material directly correlates to the purity of the material and level of expected response by the body.

Medical non-implantable materials are widely available today. Many of the silicone suppliers manufacture this grade of material in a variety of durometers so that medical devices of different hardness can be made. Non-implantable medical grade materials are acceptable for skin contact. However, they cannot be used inside the body. Many food grade materials are also in this category. That is the main limitation of this grade of material.

The next step up in medical grade silicones is the restricted grade for implantation. Materials belonging to this group are typically only able to be implanted for 29 days or less. These materials are used for implants that are needed for short-term solutions, often related to surgical procedures. In addition, these materials can be loaded with a low percentage of barium or tungsten to be detected inside the body using X-rays, CT scans, and even magnetic resonance imaging.

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