Why Overmolding Works Well For Human-Factors Engineering

By Rob Bodor, VP and GM of the Americas, Proto Labs

Design issues are crucial considerations in the medical industry, especially in cases in which healthcare workers or patients interact with devices and various device-use environments where design flaws may be present.

Just how crucial are these design issues? Numbers tell the story.

Between January 2010 and December 2015, device design issues were the most prevalent cause of medical device recalls, accounting for nearly 35 percent of the (on average) 2,600 annual recalls, according to data provided by FDA’s Center for Devices and Radiological Health (CDRH). In 2014, device recalls reached an all-time high of 2,823.

Human-Factor Design

FDA is placing more emphasis than ever on human factors engineering, an effort intended to support manufacturers in improving device design to minimize use errors and their possible resulting harm, as stated in the agency’s most recent guidance on the subject.

The FDA defines human factors engineering, also known as usability engineering, as the application of human skills and abilities to medical device design — including mechanical and software interfaces, displays, controls, packaging labels, instructions, and training — “to maximize the likelihood that new medical devices will be safe and effective for the intended users, uses and use environments.”

Factors driving the adoption of human factors engineering include market demand for intuitive products, the world’s growing, aging population, mass customization of products, and greater patient engagement. User preference now is almost as important as clinical efficacy, as customer feedback and performance metrics affect how hospitals and other providers are compensated by insurance companies and Medicare.

Soft-touch handles on screwdrivers, kitchen tools, and toothbrushes illustrate the application of human-factors principles to everyday objects. The manufacturing technique and materials that make those consumer products possible — the process known as overmolding, often used in conjunction with thermoplastics and liquid silicone rubber (LSR) — also are ideal for medical device makers.

Further, regulatory and other forces are making human-factor engineering a key element of the future of health care and device development. The new guidance reflects a shift from blaming users for errors to making manufacturers responsible for designing devices to minimize mishaps. Considering how people use a device and how to make it easier and safer to use can apply to all aspects of a product. 

When strength, ergonomics, and aesthetics are among those considerations for a device or component, the manufacturing process of rapid overmolding offers a promising solution.

Rapid overmolding — which involves applying a soft, durable material, such as liquid silicone rubber (LSR), to an underlying plastic or metal part — is responsible for the soft-touch grips on those aforementioned everyday items, such as screwdriver and toothbrush handles. Similarly, overmolding produces surgical instruments with nonslip grips, chemical resistance, and biocompatibility; instrument housings with impact resistance, noise and vibration control and improved aesthetics; and monitors that are impact- and abrasion-resistant and control noise. Overmolded syringes are chemical resistant and have nonslip grips and built-in seals.

For sealed components, overmolding can create seals or gaskets that keep water, air, and dust out of a device.

A Multi-Step Process

Rapid overmolding is a multi-step injection molding process in which a flexible thermoplastic or LSR is overlaid on a rigid “skeleton,” or a substrate of thermoplastic or metal, to cover fully or partially the underlying part.

Overmolding also supports the trend in mass customization, enabling, for example, multiple versions of the same product to have overmolded handles of varying sizes to fit different users’ hands. A two-color design, meanwhile, can help brand a product or, when choosing a neutral color for a product in development, can help elicit unbiased feedback from trial users who don’t associate the unfamiliar color with a particular company's products.

Overmolding also can save on the total cost of manufacturing a finished part by eliminating the assembly steps necessary if a manufacturer is molding two separate parts and affixing them together. The overmolded part, in this case, also is representative of the production part. The tooling may cost more up front, but eliminating labor costs will compensate for the extra expense, especially as volume grows. A “pick-n-place” overmolding process, producing the substrate in its own mold and then manually placing it into an overmold for injection of the second material, is cost-effective for up to 10,000 parts. Rapid overmolded parts, created in a matter of days, can help manufacturers seeking FDA premarket clearance or approval for a device get parts as close to final design as possible, up front, avoiding delays to market and costly resubmissions.

Prototyping is critical to overmolding as validation that the finished part meets both functional and aesthetic goals. The design also has to take into account how the two materials in the finished part will bond (i.e., through a chemical or a mechanical process). Mechanical adhesion, encouraged to prevent the overmolded material from peeling off the substrate, occurs when the overmolded material wraps around the substrate or locks into undercuts. Some resins, if properly overmolded, form a chemical bond, but the materials must be compatible or they will peel apart easily.

Adopting Human-Factor Engineering

The transformation in diabetes glucose monitors over the past 10 years illustrates the influence of human-factors engineering on a medical product likely to generate greater demand as the population ages. While previous incarnations of the devices had tiny buttons and displays that frustrated older users, in particular, today’s simplified monitors feature larger, high-contrast displays and just a couple of thumb-sized buttons. Modern glucose monitors also are smaller than their predecessors.

Product developers and engineers considering applying human-factor engineering to new devices will have to examine how that impacts their development schedule and R&D spending. That cost may be higher early on, but the investment can prevent elevated costs down the road.

About The Author

Rob Bodor is currently VP and GM of the Americas at Proto Labs, a leading online and technology-enabled digital manufacturer of rapid prototypes and on-demand production parts. At Proto Labs, he has also held roles as CTO and director of business development. Prior to joining Proto Labs, Bodor held leadership roles at Honeywell and McKinsey & Company, and has been on the executive team of two early-stage software companies in the Twin Cities. Robert holds B.S., M.S., and Ph.D. degrees in engineering and computer science.