Taking A Closer Look At Optical Grade Silicone

An already emerging technology in the consumer marketplace, manufacturing with optical grade silicone is starting to awaken the medical device industry to new possibilities. Still in its infancy within the healthcare market, the benefits of optically clear silicones offer some intriguing opportunities, and it is gaining interest for applications such as wearable technologies, endoscopes, optical sensors and instruments, medical lasers, diagnostics, light guides, and other medical device-related applications.

Every successful product has strict material demands, and it is no secret that the medical device industry is particularly rigorous. When a device does not perform as it is designed to, the consequences can be grave and costly. While the stringent regulations in the healthcare market can prevent these issues, they can also inadvertently create design challenges that many Original Equipment Manufacturers (OEMs) do not have the capacity to work through.

Liquid silicone rubber (LSR) has been instrumental in breaking down these potential barriers, and it is now a standard material choice in the healthcare market. The biggest obstacles facing medical devices include sterilization, contact with living tissue, functionality, and cosmetic quality, and LSR is the ideal material to minimize these issues. Almost every grade of LSR is biocompatible, it can withstand high and low levels of heat, and there are grades specifically developed for implants and similar medical devices.

Benefits and Attributes

Optical grade silicone exhibits the many benefits associated with silicone rubber, but offer the added benefit of optical clarity needed for specific applications. Particularly in the medical device market, quality and functionality are absolutely crucial, and molders are starting to see the positive attributes and characteristics of optically clear silicone within this arena. A similar pattern is developing for the use of this material in the medical device market associated with the benefits already realized with liquid injection molding in general: companies face challenges that require a specific design option and are limited with traditional injection molding methods and materials, so they begin to look elsewhere for solutions. With applications that are based around highly precise features, factors such as viscosity and molding parameters can make or break an entire program, so it is natural that OEMs are searching for alternatives.

When making the decision to work with optical grade silicone, one needs to make the distinction between the current grades available. Because molding this material within the medical device industry is an emerging technology, most material suppliers currently do not offer a standard medical grade optically clear silicone. Silicone material suppliers such as Dow Corning, Shin-Etsu, Nusil, Momentive and Wacker have all developed new high performance optical silicone materials, and industry members can expect medical grade optically clear silicones to follow as more applications require this. Most silicone material suppliers offer medical grades of liquid silicone rubber, but not optically clear silicone, and because of the stringent requirements mentioned above, it is recommended that this be considered while exploring silicones for healthcare-related applications. (See Figure 1)

OEMs and molders have been heading in the optical grade silicone direction because of the physical benefits silicone offers, including bacterial resistance, UV resistance, biocompatibility, low viscosity for molding precise and difficult features, temperature flexibility from -180ºF to 600ºF, chemical resistance, and fatigue and compression set resistance. It is also significantly lighter than most plastics and glass, and silicones in general are lighter in weight than traditional optical materials. Because the material is a thermoset, it does not have to be molded in stresses like thermoplastic resins do.

One of the most exciting aspects of using optical grade silicone is the design flexibility and subsequent reduction of costs. Due to the elastomeric nature of this material, it can reduce the need for additional parts and secondary operations, lowering tooling and non-recurring engineering (NRE) costs. (See Figure 2) For example, in a traditional lighting application where optical silicone is not being used, a lens and a seal would be two separate parts. By molding the lens with an optical grade silicone, the seal can be incorporated into the lens, ultimately eliminating an entirely different part and assembly step. The flexibility of optical grade silicone also allows for designs with undercuts, negative draft angles, and fine features, and assembly can be simplified by adding gasketing, sealing, and mounting features directly onto the part without compromising optical characteristics. This decreasing of the manufacturing process reduces inventory, bill of materials, time-to-market, and overall project cost significantly. (See Figure 3)

Beyond the design and manufacturing benefits, there is a push towards optical grade silicones purely for cosmetic purposes. In the medical device world, quality is held above all else, and anything that appears to be flawed or dirty can be interpreted as a defect. Even if it has no effect on the function of the device, a part that is yellowing or discolored will be misinterpreted by doctors and patients. Regardless of how old it is or where it has been, optical grade silicone does not lose transparency or discolor with age, or with exposure to UV light, moisture, or heat. It requires no polishing after molding and is resistant to scratches, cracks, and breaks. Optical grade silicone is essentially unaffected by environmental factors and remains attractive throughout the life of the device, never worrying a doctor or patient if something is unsafe.

Applications and Considerations

Despite its current categorization as a niche capability, there are many opportunities in the marketplace today that allow for optical grade silicone integration. The most common application that optically clear silicone is being used for currently is to offer alternatives to polycarbonate, glass, or polymethyl methacrylate (PMMA) in situations where traditional materials are limiting the design or function of the device. The options do not stop here, however. Most applications using optical grade silicone center on highly precise geometries that are almost impossible to fabricate with current materials and methods, and the low viscosity before cure makes molding optical grade silicone into complex shapes easier than with either glass or organic polymers.

There are some design considerations that OEMs need to factor in when working with optical silicone. The key benefits for injection molding this material lie within design freedom and flexibility. Because of the low viscosity of the material, liquid injection molding of optical grade silicone allows for the molding of geometries that would not otherwise be possible with other polymers and glass. More precise features, varying wall thickness, reduced likelihood of sink allowing for thicker walls, small undercuts, negative draft, and difficult parts to fill are all possible with this material, allowing designers to really test their product’s limits.

What to Watch For

With testing product limits comes challenges, however, and designers need to be aware of potential hurdles. For example, it is important that designers know the refractive index of the material when designing parts for index matching, as optical grade silicone is quoted with the value of the refractive index at specified wavelengths. To achieve precise index matching, the refractive index measurement should be carried out at the operating wavelength. Be aware that the majority of optical grade silicones will undergo refractive index reductions due to the temperature at a rate of 1x10-4 to -5x10-4 per degree Celsius. This rate of change of refractive index corresponding to any given temperature is known as the thermo-optic coefficient (or dn/dt), and the operating temperature must be known in order to maximize the refractive index at operating temperature. It is critical that molders understand the processing parameters of this material and have experienced tool designers to produce a part that meets the optical requirements for the application needed. Depending on the type of device that is being manufactured, molders will need to work with the customer to create necessary solutions.

Using optical grade silicone in the healthcare industry is an exciting, albeit challenging opportunity for molders and OEMs. The design options with this material are endless, and due to the flexible nature of optically clear silicone, there is unlimited potential for new innovations and devices. We are starting to see a new wave of product development, silicone tooling, and Design for Manufacturing (DFM) work being introduced to the marketplace with no intention of slowing down, and when it comes to designing and manufacturing with optical grade silicone, we have literally not scratched the surface.

This article was written by Rebecca Murphy, Inside Sales and Marketing Coordinator, GW Plastics, Bethel, VT. For more information, Click Here .

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