Wearable technology requires clean, safe materials for direct or indirect contact with the human body. Fitness trackers that wrap around the wrist, heart monitors that strap across the chest, and smart clothing or jewelry all need biocompatible materials that won’t irritate the skin, stain clothing, or expose the wearer to toxins. Wearable technology that incorporates sensors, cameras, or displays must also handle the heat associated with electronic components while keeping wearers safe and comfortable.
Whether the application is health, medicine, gaming or fashion, wearable designers can choose silicones or thermoplastic elastomers (TPEs) for components such as straps, cases, glasses, or headgear. Silicones that meet FDA and USP Class VI requirements for biocompatibility won’t produce a toxic or immunological response when exposed to the skin, or to bodily fluids such as blood or perspiration. Some TPEs are also biocompatible, so choosing the right compound can seem challenging. Here’s what to consider.
Material Properties and Temperature
Silicones and TPEs have similar properties at room temperatures but exhibit some important differences at elevated temperatures. For example, a TPE wearable that’s subjected to heat and repetitive usage may deform permanently. The body temperatures associated with exercise probably aren’t enough to affect a wristband, but a molded case that houses densely-packed electronics is exposed to higher levels of heat. With their low thermal conductivity, silicones also “resist” the movement of heat towards the wearer.
At higher temperatures, silicones also have better chemical and abrasion resistance. That’s important for straps or headgear that may be subjected to sterilization, or at least to cleaning and scrubbing with soap and hot water. Yet silicones also tend to attract lint and dirt while TPEs can be formulated to repel them. For wearables worn in outdoor environments, there are additional considerations. Silicones naturally resist sunlight but TPEs have only moderate resistance to the sun’s ultraviolet (UV) rays.
Market Acceptance and Performance
Designers also need to consider the look, feel and smell of wearable technology. Silicones and TPEs are similar in appearance and support the use of colorants but can feel different to the touch. Silicones are also odor-free and hypoallergenic while TPEs require additives to achieve these properties. Both types of materials are available in a range of durometers on the Shore A scale, but silicones come in a softer range (3 to 80) with a lower “sweet spot” (50). TPEs have a higher range (20 to 95) and a harder “sweet spot”.
During the design process then, it’s important to determine whether a softer or harder material is required. For wearables that require a super-soft elastomer, gel-like silicones on the Shore 00 Hardness Scale are available. These elastomers have low levels of surface bleed, the release of oil from the surface of cured rubber. Super-soft TPEs are available, too, but these materials have higher levels of surface bleed and could stain clothing.
Overmolding and Cost-Effective Manufacturing
Silicones and TPEs both support overmolding, an injection molding process where one material is molded over another. Because silicones only bond readily to silicones, overmolding is easier with TPEs when there’s a plastic substrate. Plus, the first shots in silicone molding may require preheating if there’s a delay between processes. Co-molding machinery can support a continuous process, but the machines and molds are so expensive that small-volume overmolding is more cost-effective with separate machines.
Before choosing silicones or thermoplastic elastomers (TPEs) for fitness trackers, medical wearables, or other wearable technology, designers need to consider all of their application requirements. Material properties and manufacturing are important, but don’t forget about market acceptability. Extreme Molding can help you to determine whether silicone or TPE is the right choice. Contact us to learn more.
