Part 2 | The Anatomical Site
Site Selection: The Anatomical Site Is Part of the Skin-Device Interface
Neither the 2016 nor the 2026 FDA human factors guidance addresses anatomical wear-site selection in detail. However, both documents provide a framework for treating site-related activities as part of the user-device interaction when the intended location affects the user’s ability to identify, prepare, reach, apply, secure, inspect, tolerate, or remove the device safely and effectively.
From a dermatologic perspective, anatomical site should be treated as a design input rather than a placement decision made after the device has already been developed.
The appropriate site is shaped by four interdependent considerations:
The function of the device
The intended user
The use environment
The device user interface
Together, these determine whether a skin-worn product can perform its intended function while remaining tolerable throughout foreseeable use.
Some consumer wearables are not regulated by FDA as medical devices or may fall within FDA’s general-wellness enforcement policy. Although their regulatory obligations differ, the physical reality at the skin does not. The same considerations of device function, intended user, use environment, and user interface remain relevant as sound design practice for the skin-device interface regardless of regulatory status.
1. The Function of the Device
Site selection begins with what the device is intended to measure, monitor, or deliver and where on the body that function can be accomplished accurately and practically.
A sensor may require access to a particular physiologic signal. A therapeutic device may need proximity to a target tissue. An activity monitor may need a location that captures movement reliably. These functional requirements narrow the available anatomical sites, but they do not determine suitability on their own.
A technically desirable site may still be difficult to reach, highly mobile, exposed to pressure or friction, densely haired, prone to sweating, poorly visible, or unsuitable for repeated application. The preferred site must therefore balance signal quality or therapeutic function with the biological and mechanical demands placed on the skin.
2. The Intended User
In FDA human factors guidance, the intended user is the person who operates or handles the device. For skin-worn products, this most commonly includes patients, consumers, caregivers, and healthcare professionals who apply, wear, monitor, adjust, or remove the product.
A dermatologic assessment should also characterize the user’s skin at the intended anatomical site because its physiological and mechanical features directly shape the interaction with the device.
Relevant Site-Specific Characteristics
Skin thickness
Contour
Local mobility through traction and twist
Hair density and caliber
Sweating (Sensible water loss)
Transepidermal water loss (Insensible water loss)
Friction
Pressure
Visibility
Accessibility
Capacity for site rotation
Skin pigment and presence of tattoos
The user’s physical abilities also matter. Dexterity, flexibility, coordination, tactile sensitivity, medical condition, comorbidities, and general health may affect whether the person can reach the site, position the device accurately, inspect the skin, respond to symptoms, and remove the product safely.
A site that is appropriate for clinician application may not be practical for self-application. A location that is easy to reach may be difficult to see. A site that works well for a healthy adult may be less suitable for a child, an older adult, or a person with fragile skin, limited mobility, neuropathy, edema, or another condition that changes skin tolerance or the ability to recognize injury.
3. The Use Environment
In FDA human factors guidance, the use environment is framed primarily as the physical setting and surrounding conditions in which the user interacts with the device and that may influence how the user perceives, accesses, and interacts with the device user interface.
For skin-worn medical and consumer devices, BOHLD extends this concept to include the conditions that affect the microenvironment at the skin-device interface during anticipated use. From a dermatologic perspective, this includes how the intended anatomical site, ambient environmental conditions, user exertion, duration of wear, and clothing worn over or around the device alter that local microenvironment and affect skin tolerance, adhesion, sensor contact, signal quality, data accuracy, device retention, and therapeutic or functional performance.
3.1 Conditions of Wear
Wearables may be used for continuous monitoring, therapy delivery, rehabilitation, diagnosis, activity tracking, or performance monitoring. They may remain on the body during sleep, exercise, work, travel, bathing, swimming, or routine daily activity.
Performance-monitoring wearables and fitness trackers provide a useful example. The same device may be worn during sedentary activity, sleep, and office work, then exposed to vigorous exercise, heavy sweating, repetitive movement, heat, humidity, tight clothing, protective equipment, swimming, or prolonged outdoor use.
These transitions can push the skin-device interface well beyond the relatively stable conditions of sedentary wear. A design that remains comfortable and functional at rest may behave differently when moisture, friction, pressure, traction, twist, and repeated movement increase.
The interface may also be exposed to:
Ambient and environmental conditions
Heat and humidity
Ultraviolet exposure
Water exposure
Bathing or immersion
Chlorine
Salt water
Repeated wet-dry cycles
External mechanical influences
Bedding
Sportswear
Compression garments
Protective equipment
Activity-related conditions
Exertion
Sweating
Repetitive movement
The effect of these conditions depends on the anatomical site, duration and frequency of wear, intensity of activity, and whether the device is adhesive, strapped, compressed, inserted through the skin, or repeatedly removed and reapplied.
These conditions can increase temperature, skin hydration, moisture retention, friction, pressure, occlusion, and mechanical stress. They may also affect adhesion, sensor contact, signal quality, device retention, and removal forces.
A device that performs well during brief or controlled wear may behave differently after hours or days of sweating, movement, water exposure, or repeated loading.
3.2 When Real-World Use Stretches Design Boundaries
For skin-worn devices, foreseeable real-world conditions may extend beyond ideal or controlled use. FDA’s HFE/UE framework focuses on use-related hazards rather than hazards arising solely from device or component failure. These may occur when the device itself operates as intended but the use environment affects its safe or effective use in ways the user does not recognize or understand, or when the device is used in foreseeable but inappropriate ways.
Activity and environmental exposure
Continuing use during intense exercise, with increasing heat, sweat, friction, and repetitive movement
Swimming with a device not designed for immersion
Wearing the device beneath compression clothing, sportswear, or protective equipment
User modification or compensation
Reinforcing a lifting adhesive edge
Tightening a strap to improve sensor contact
Repositioning or adjusting the device in ways not anticipated by the design
Delayed removal or prolonged wear
Delaying removal despite pain, burning, itching, or visible skin changes
Extending wear beyond the intended duration
Reapplying the device repeatedly to the same anatomical area without sufficient recovery time
The relevant design question is not simply where the device will be used, but also what physical conditions the skin-device system will encounter and how far real-world use may stretch the intended design boundaries.
4. The Device User Interface
For skin-worn devices, the device user interface includes both the physical features that contact or affect the skin at the intended anatomical site and the information that guides the user in selecting, preparing, applying, monitoring, and removing the product from that site.
4.1 The Physical Interface
The physical interface includes the adhesive, strap, housing, geometry, edge profile, sensors, applicator, and removal features. Their design should reflect both the biological and mechanical demands of the intended site and the functional requirements of the device, including the need to maintain reliable sensor contact, signal quality, or therapeutic delivery.
The same device may behave very differently depending on where it is placed.
A relatively flat, low-motion area may tolerate a broader or more rigid footprint than a curved or highly mobile site. Near joints, the shoulder, abdomen, breast, axilla, neck, or wrist, the skin may bend, fold, stretch, shift, and twist during ordinary movement. These changes can increase traction, friction, pressure, edge lift, and localized stress beneath the device.
4.1a The Concept of Device Footprint
The device footprint is the total area of skin occupied or affected by the device, including adhesives, straps, housings, sensors, electrodes, charging contacts, and other skin-facing components. It should be considered a biological and mechanical design variable rather than simply the physical dimensions of the product.
A larger footprint covers more skin and must accommodate greater variation in contour, movement, sweating, and transepidermal water loss across the contact area.
The footprint should not be treated as a uniform zone. The center of the device may behave differently from its edges. Raised modules, seams, electrodes, charging contacts, rigid housings, adhesive borders, or strap transitions may create localized pressure, friction, occlusion, or mechanical mismatch.
The intended site should guide decisions about:
Geometry and fit
Size and shape
Conformability
Edge profile
Mechanical behavior
Flexibility and stretchability
Pressure distribution
Accommodation of traction, twist, and repetitive movement
Moisture and microenvironment management
Breathability
Moisture management
Adhesive properties
Occlusion
Thermal and functional considerations
Thermal management
Sensor contact
Stability of signal or therapeutic delivery
Visibility, accessibility, and site management
Ability to position the device accurately
Ability to inspect the skin and device during wear
Ability to adjust or remove the device safely
Availability of sufficient area for site rotation
Need for skin preparation, applicators, or placement aids
Materials that work well on one anatomical site or in one intended population may perform poorly in another. A rigid component may be tolerated on a relatively stable surface but create stress when placed over mobile or contoured skin. A highly occlusive material may remain comfortable during sedentary wear but become less suitable at a site exposed to heavy sweating, sportswear, or repetitive movement.
Similarly, a skin-adhered device initially designed and evaluated in an older, relatively sedentary population may not perform or be tolerated in the same way in a younger, more active population, where greater movement, sweating, friction, clothing interaction, and repetitive loading can alter adhesion, edge behavior, skin hydration, and mechanical stress.
A footprint that is biologically and mechanically suitable may still be impractical if the intended user cannot reliably reach, see, inspect, or rotate the site. Locations such as the back, posterior arm, or scalp may therefore require different applicators, placement aids, geometries, skin-preparation instructions, or removal features. Dense hair, skin folds, prominent bones, and limited available surface area may further constrain placement and rotation.
Device footprint is therefore both site dependent and population dependent. Its size, geometry, materials, functional elements, and distribution of mechanical, thermal, and moisture-related effects should be designed in relation to the biology, contour, movement, visibility, accessibility, and available rotation area of the intended anatomical location, as well as the age, activity profile, skin characteristics, and physical capabilities of the intended users.
4.2 The Informational Interface
The informational interface includes the instructions and guidance that help users select an appropriate anatomical site and manage that site throughout wear.
4.2a Site-Selection Guidance
Instructions should help users distinguish an appropriate site from one that may be less suitable because of skin folds, high mobility, bony prominences, irritated or damaged skin, dense hair, repeated pressure, or interference from clothing or equipment.
4.2b Application Position and Orientation
Posture, skin tension, and device orientation can matter because skin shifts with sitting, standing, bending, reaching, breathing, sleeping, and exercise. Placement instructions should account for these predictable changes where they affect device performance or skin loading.
4.2c Monitoring During Wear
Users should be given practical guidance about findings that may warrant adjustment or removal, such as persistent redness, itching, burning, pain, swelling, maceration, pressure marks, blistering, or localized edge irritation.
4.2d Site Rotation and Recovery
When repeated use is expected, the availability of alternative placement sites and sufficient recovery time should be considered during device and instruction design rather than left entirely to the user.
Conclusion | Site Selection is the Foundation for Use-Related Risk Analysis
From a dermatologic perspective, BOHLD approaches anatomical site selection through four interdependent considerations informed by the FDA HFE/UE process:
1. The function of the device
2. The intended user
3. The use environment
4. The device user interface
Once the intended users, use environments, user interface, and functional requirements of the device are sufficiently defined, the HFE/UE process can proceed to identifying and evaluating use-related hazards (Figure 2). Appropriate risk-control measures can then be developed and tested to reduce the likelihood of use error and resulting harm.
FDA’s Center for Devices and Radiological Health (CDRH), the FDA center primarily responsible for medical-device regulation, considers human factors evaluation and testing an important component of medical-device product development. BOHLD can help guide this process from a dermatological perspective. For skin-worn devices, defining the intended anatomical site and understanding its dermatologic, biological, and mechanical characteristics helps establish the context needed for that subsequent risk analysis.
Future installments in this series will examine these next steps, including how use-related hazards at the skin-device interface can be identified, mitigated through device and interface design, evaluated under realistic wear conditions, and further refined by postmarket experience.
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