Wearable PCB Design for Medical Devices: Flex and Power

A wearable medical device has to be small, comfortable, electrically safe against the skin and able to run for days on a battery the size of a fingernail. Those requirements pull in different directions, and the board is where the compromise is settled. This guide covers the construction choices, the safety constraints and the design habits that make a wearable succeed.

What Makes a Wearable PCB Different

The board is part of the product rather than a component inside it. It flexes with the body, sits close to skin, and often carries the sensor, the radio and the battery on the same thin substrate. Mechanical compliance is therefore a first order requirement, not a packaging detail.

Electrical requirements follow: very low quiescent current, stable performance when the board is bent or stretched, and isolation that keeps any fault current away from the user. Each of those shapes both the circuit and the construction.

<img src="https://www.gopcba.com/wp-content/uploads/2026/05/smart-energy-PCBA-1.jpg" alt="Wearable PCB flexible circuit for a medical monitoring patch” />

Rigid, Flexible and Rigid-Flex Construction

A wearable pcb is usually a flexible circuit, either on its own or combined with a small rigid section that carries the fine pitch devices. The flexible part conforms to the body, and the rigid part provides the dimensional stability that fine pitch assembly requires.

Layer count is kept low to maximise bendability, and the stackup is arranged symmetrically around the neutral axis so that bending produces tension and compression that balance each other. The rules for via and stack selection apply with the added requirement that no via sits in a region that will flex.

Materials and Skin Contact

Skin contact drives the material selection. The substrate, the coverlay and any adhesive that could reach the surface have to be assessed for biocompatibility, and the standard applied depends on whether contact is brief, prolonged or permanent.

A coating or an overmould is usually the barrier. A conformal coating or a thin polymer encapsulation protects the circuit from sweat and moisture, and it also separates the user from the conductors, which is why its integrity under repeated flexing has to be qualified.

Electrical Safety Against the Body

Applied part requirements limit the leakage current that may flow through a patient, and the limits are tighter than those for ordinary equipment. Isolation, insulation thickness and creepage distance all have to be designed for the worst case rather than for normal operation.

Even a battery powered device has to be considered. A single fault that connects the supply to an electrode has to be prevented by construction, which usually means two independent barriers, adequate spacing and a design that fails safe rather than failing closed.

Low Power Design

Power is the constraint that governs everything else, because battery capacity is limited by what can be worn comfortably. Duty cycling, low quiescent current regulators, efficient conversion and careful choice of sensors all reduce the average load before any energy harvesting is considered.

The layout contributes by keeping the high current loops small and the ground returns short, which reduces both loss and noise. Even a low power design has switching nodes, and a current calculation on the battery and charging paths prevents an avoidable voltage drop when the device transmits.

Sensors, Electrodes and Signal Quality

The signal from a body worn sensor is small and the environment is noisy. Electrodes present a high impedance and pick up movement artefacts, so the front end needs short connections, guarding where appropriate and a reference that is genuinely quiet.

Filtering belongs as close to the source as the circuit allows. A long trace between an electrode and its amplifier converts the body into an antenna, and the resulting interference is often misinterpreted as a sensor fault during development.

Radio and Antenna Integration

Wireless links need an antenna, and in a wearable the antenna competes for the same small area as everything else. Ground plane size, the proximity of the body and the presence of a battery all affect performance, and the matching network has to be tuned in the final mechanical configuration.

Keepout regions around the antenna are usually required, and components near it should be avoided even when they are electrically unrelated, because a nearby metal part detunes the structure and reduces the range of the link.

Mechanical Durability and Testing

The board will be bent, twisted, washed and worn against a moving body. Flex testing, repeated bending to a defined radius, thermal cycling and moisture exposure are the tests that expose the weakness, and the failure usually appears as a cracked trace or a delaminated coverlay at the transition from flexible to rigid.

Designing for those tests means keeping conductors perpendicular to the bend line, avoiding abrupt width changes in flexing areas and adding stiffeners where a component must be mounted on a flexible section.

Manufacturing and Assembly Notes

Flexible circuits are processed on a carrier, and the panel is designed so that the flexible material is supported through assembly. Component placement on a flexible area is minimised, and any part that must sit there is small, light and anchored with adhesive.

Inspection follows the same priorities as the design. Continuity under flex, coating integrity, cleanliness and dimensional conformance at the rigid to flex transition matter more than the cosmetic appearance of a part that will never be seen.

Comfort and Form Factor

A device that is uncomfortable will not be worn, and a device that is not worn does not collect data. Thickness, stiffness and the position of any rigid island against the skin all influence whether the user tolerates the product through a working day.

The board layout therefore has to be developed alongside the enclosure and the strap or patch that holds it. Moving a rigid section away from a bony area, or splitting one stiff region into two smaller ones, often improves comfort more than reducing the overall size.

Working With the Supply Chain

Flexible and rigid-flex fabrication is a narrower set of capabilities than rigid board work, and the assembly house has to be able to handle the carrier and the flexible material. Confirming both before the design is frozen avoids a redesign triggered by a process limitation.

Medical devices also bring regulatory obligations. Material declarations, process traceability and change control are part of the deliverable rather than an administrative extra, and a supplier that already works in that environment will have the documentation structure in place.

The same thinking applies to the battery. A cell that is accessible for replacement simplifies the mechanical design but adds a connector and a seam; a sealed cell removes those parts and makes the charging path part of the circuit, which shifts the definition of low power design from a preference to a requirement.

FAQ

Does a wearable always need a flexible circuit? Not always, but a rigid board has to be small enough or shaped so that body movement does not load the solder joints, which is difficult in a device that is worn continuously.

How is a wearable board protected from sweat? Usually by a conformal coating or a polymer encapsulation, sometimes combined with an overmould. The barrier has to be qualified for flexing, not just for moisture resistance.

What limits battery life most in these devices? Quiescent current. Standby consumption in the regulator, the radio and the sensor interface usually dominates the average load, ahead of the current drawn during active measurement.

Can a rigid board be used in a wearable? It can where the enclosure absorbs movement and the board is small. As soon as the electronics must follow the body, a flexible or rigid-flex construction becomes the practical choice.