Part 1 | The Skin-Device Interface

The Skin Is Not a Passive Surface

FDA’s human factors framework considers three interacting elements: the user, the use environment, and the device user interface. During device use, these elements come together and may result in either safe and effective use or unsafe or ineffective use (Figure 1).

Figure 1. FDA human factors framework for medical devices. The interaction among the user, use environment, and device user interface provides a useful model for understanding skin safety, wear conditions, and product performance in skin-worn devices. Reproduced from the U.S. Food and Drug Administration, Applying Human Factors and Usability Engineering to Medical Devices, 2016.

Drawing on BOHLD’s dermatologic expertise and our view of the skin as living, responsive tissue that actively interacts with the device, we apply this framework with particular attention to the skin as the biological part of the user that directly contacts the product.

The User

The user is the individual using the device and the characteristics of their skin that directly contacts the device.

The ideal skin-device interaction maintains a microenvironment beneath the device that remains as close as reasonably possible to the skin’s normal condition. When heat, moisture, pressure, friction, occlusion, or chemical exposure move that environment too far from baseline, the skin may respond with discomfort, itching, redness, barrier disruption, or other signs of irritation.

Skin is not an inert covering. It is a complex organ that acts as a physical barrier, regulates heat and water loss, helps protect against infection, and continuously interacts with the outside world.

If device materials contain irritants, sensitizers, toxic substances, or harmful levels of radiation, the skin may be injured. For this reason, medical devices are evaluated for biological safety using the ISO 10993 framework (e.g. Biocompatibility). Consumer wellness products may not be subject to the same regulatory requirements, but selecting appropriately evaluated materials is still an important risk-reduction practice.

Biocompatibility, however, does not answer every skin-safety question. It is best viewed as a starting point for skin safety and user comfort, not the end of the analysis.

A material can be biologically acceptable and still perform poorly against the skin because of its physical properties. It may trap heat and moisture, create friction, concentrate pressure, resist movement, or make removal more traumatic.

A simple comparison is the choice between wearing clothing made from breathable cotton versus wetsuit material on a hot, humid day. Both materials may be safe to touch, but they create very different microenvironments against the skin. Cotton allows greater airflow and moisture movement. Wetsuit material is insulating and moisture-retaining.

The same question applies to wearable devices. Silicone-style watch bands are often marketed for exercise because they are soft, flexible, durable, and relatively inert. But silicone is also non-breathable and can retain heat and moisture against the skin. For prolonged athletic use, a more porous, moisture-wicking material will likely create a more favorable skin environment.

The lesson is not that silicone is unsafe. It is that biological safety and suitability for a specific use environment are not the same thing.

Design teams should begin with the understanding that skin is living tissue. The device should support skin comfort, barrier function, and physiologic balance throughout the intended period of use.

The Use Environment

The use environment includes the real-world conditions the skin and device experience together. A product may be worn during sleep, exercise, bathing, work, travel, or ordinary daily activity.

Heat, humidity, sweat, water, friction, pressure, movement, clothing, and occlusion can all change the microenvironment beneath the product. These effects may increase with prolonged wear or repeated use at the same or nearby sites.

Real-world use will also test the limits of the original design assumptions. Users may wear the device longer, expose it to more sweat, water, friction, pressure, or movement than anticipated, or continue using it under conditions that were not fully represented during testing. Foreseeable workarounds, such as reinforcing a lifting edge, tightening a strap, covering the device, or delaying removal, may further change the skin-device interface.

The use environment therefore includes not only where the device is worn, but also how long it is worn, how people actually use it, and what the skin must tolerate during that time.

The Device User Interface

The device user interface includes the physical features that contact or affect the skin and the information that guides the user’s actions.

·  The Physical Interface includes the adhesive, strap, housing, geometry, edge profile, sensors, applicator, and removal features. These elements influence pressure, friction, heat, moisture, movement, shear, traction, adhesion, and removal forces.

·  The Informational Interface includes instructions for skin preparation, application, wear, monitoring, removal, aftercare, and warnings, as well as any prompts or feedback provided by the device. These elements help the user decide how to apply the product, what changes to watch for, and when adjustment or removal may be necessary.

Unlike regulated medical devices, many consumer wearables rely heavily on app-based onboarding, on screen prompts, or online user manuals rather than providing complete printed instructions with the product. Although this digital approach makes information available, it also creates the foreseeable possibility that users will begin wearing the product without reviewing detailed guidance on fit, skin monitoring, cleaning, wear duration, or when the device should be adjusted or removed.

The skin outcome is therefore shaped by more than the material alone. It reflects the interaction among the user’s skin, the conditions of wear, the physical design of the product, and how relevant guidance is delivered, whether actively through prompts, onboarding, or alerts, or passively through manuals, websites, or other reference materials that the user may never review.

From the Skin-Device Interface to Site Selection

For skin-worn products, safe and effective use depends on more than selecting biocompatible materials. The user’s skin, the real-world conditions of wear, and the physical and informational features of the device, and how guidance is delivered and used  all shape the interaction.

Viewing the skin as living, responsive tissue broadens the design question. It reinforces that the skin-contact system is part of the human factors interface, not only a material boundary. As dermatology consultants, BOHLD brings real-world clinical dermatology experience and expertise in both medical and consumer devices to help companies better understand the interface between skin and technology.

Stay tuned for Part 2 of the Human Factors Series, which will examine how anatomical site influences skin response and how device design can be optimized to meet the specific demands of the intended wear location.