Wearable Medical FPC CGM/ECG
Posted by SZFRS Engineering Team
Wearable medical devices have grown from a niche segment into a substantial market in the past five years. Continuous glucose monitors (Dexcom G7 and G6, Abbott FreeStyle Libre 2 and 3, Medtronic Guardian) have made diabetes monitoring continuous rather than fingerstick-based for millions of patients. ECG patches (iRhythm Zio XT and Zio AT, BioTel MCT, VitalConnect VitalPatch, Bardy Diagnostics CAM) have moved arrhythmia detection from in-clinic Holter monitors to wearable patches operators wear for 7-14 days. The cable and FPC architecture inside these devices is fundamentally different from traditional medical electronics — moving from rigid PCB to flexible printed circuit, integrating sensors directly with the substrate, and supporting wear life of 1-2 weeks under skin contact. This article walks through what that means in practical terms.
Table of Contents
TL;DR — Quick Answer
Continuous glucose monitors use small FPC inside the sensor body connecting the glucose sensing element (subcutaneous filament) to the BLE radio and battery. Sensor wear life 7-15 days; FPC must survive sweat, body temperature cycling, and skin oil exposure without performance shift. ECG patches use larger FPC integrating ECG electrodes, signal conditioning, BLE radio, battery, and sometimes additional sensors (3-axis accelerometer for activity correlation). 7-14 day wear life. Heart rate and sleep monitor patches follow similar architecture with simpler signal processing. Compliance is FDA Class II for medical-claim devices, with ISO 10993 biocompatibility for skin contact materials. Below covers what’s actually inside.
The Shift From Rigid PCB to FPC
Traditional medical wearables (Holter monitors, ambulatory blood pressure monitors, older fitness trackers) use rigid PCB inside a clamshell enclosure with separate sensor wires running to electrodes. The form factor is necessarily bulky — patient straps a battery box to their belt or body, with sensor wires running to skin electrodes.
Modern wearable medical devices integrate sensor, electronics, battery, and skin adhesive into a single thin patch. This requires:
- Flexible PCB substrate. Polyimide film (Kapton or equivalent) replaces FR4 rigid PCB. The substrate flexes around the body’s contour and accommodates skin movement during normal patient activity. Custom FPC for these applications uses fine-pitch routing to fit components in compact area.
- Integrated electrode contacts. ECG patches and similar devices integrate the patient electrode contacts directly on the FPC substrate. Hydrogel or wet conductive adhesive provides skin contact; the FPC carries signal from electrode contact area to the signal conditioning chip directly on the same FPC.
- Component placement on FPC. SMT components (BLE module, microcontroller, signal amplifier, battery contacts) place directly on the FPC substrate. Component selection skews toward small package types (0201, 01005 SMT passives, WLCSP active components) to minimize FPC area.
- Battery integration. Coin cell or small lithium-polymer battery integrates with the FPC. Some products use printed battery technology directly on the FPC; others use small discrete batteries with contact pads on the FPC for connection.
- Antenna integration. BLE antenna prints directly on the FPC as copper trace, eliminating discrete antenna components. The FPC layout has to acheive the right impedance and clearance for adequate antenna efficiency.
The result is a complete medical sensor system in a thin, flexible patch a few square centimeters in area. Form factor that wasn’t possible 10 years ago is now the standard.
Continuous Glucose Monitors — The Volume Application
CGM (Continuous Glucose Monitor) is the highest-volume wearable medical FPC application. The major products:
- Dexcom G7 and G6. 10-day wear (G7) or 10-day wear (G6) CGM systems. The sensor body contains a subcutaneous glucose sensing filament, signal processing FPC, BLE 5.0 radio, and battery. Compact disc-form factor approximately 24mm diameter and 4.6mm thick on the G7.
- Abbott FreeStyle Libre 2 and 3. 14-day wear CGM systems. Similar form factor to Dexcom — small disc with subcutaneous sensor element. Libre 3 reduced the sensor diameter substantially over Libre 2.
- Medtronic Guardian Connect / Guardian Sensor 4. Used with Medtronic insulin pumps for closed-loop diabetes management. 7-day wear; integrates with Medtronic family of pump and CGM products.
- Senseonics Eversense. Implanted CGM (180-day wear); different architecture from skin-patch CGMs but similar miniaturization theme.
The FPC inside a CGM connects the subcutaneous sensor filament (which extends below the skin into interstitial fluid) to the electronics on the surface. Signal levels from the glucose sensor are very low (nanoamps); FPC trace routing matters for signal integrity. The FPC also supports the BLE radio that transmits readings to the patient’s smartphone or insulin pump every 1-5 minutes.
ECG Patches — Cardiac Monitoring on the Body
ECG patches replace traditional Holter monitors for ambulatory cardiac monitoring. Patient wears the patch for 7-14 days while continuing normal daily activity; the patch records continuous ECG (and sometimes accelerometer activity for context). At end of wear, the patch is mailed to a service center or uploads data via paired smartphone.
Major ECG patch products and OEMs:
- iRhythm Zio XT and Zio AT. 14-day wear continuous ECG patch. The Zio XT records and stores; Zio AT adds real-time event transmission. Dominant share in the US ambulatory cardiac monitor market.
- BioTel Heart MCT (mobile cardiac telemetry). Real-time monitoring patch with paired transmitter. Used for higher-acuity patients.
- VitalConnect VitalPatch. Multi-parameter patch (ECG, heart rate, respiration, temperature, posture) for hospital and post-discharge monitoring.
- Bardy Diagnostics CAM. P-wave-centric ECG patch optimized for atrial arrhythmia detection.
- Preventice BodyGuardian / BG MINI. Mobile cardiac telemetry patch.
- Philips ePatch. Reusable ECG patch with disposable adhesive interface.
FPC for ECG patches integrates two or more ECG electrode contact pads (typically positioned for Lead I or modified Lead II ECG capture), signal amplification chip directly on FPC, BLE module, microcontroller, flash memory for waveform storage, and battery. The FPC area is larger than a CGM (typically 5-8 cm² versus 2-3 cm² for a CGM) because the ECG electrode separation distance is needed for signal capture.
Heart Rate, Sleep, and Activity Monitors
Heart rate monitor patches and sleep-monitoring patches share architecture with ECG patches but typically simpler signal processing. WHOOP, Oura Ring (different form factor — ring rather than patch), Polar H10 chest strap, and various medical-grade heart rate patches operate in this space. Sleep-monitoring patches focus on heart rate variability, breathing rate, and movement during sleep.
FPC for these products is generally simpler than ECG patches because the signal capture is heart rate (extracted from photoplethysmography or single-lead ECG) rather than full ECG waveform. The FPC includes optical sensor (for PPG), or single ECG electrode pair, microcontroller, BLE radio, and battery.
Compliance and Materials
Medical wearable FPC has specific compliance requirements:
- ISO 10993 biocompatibility. Any material that contacts skin must pass ISO 10993 cytotoxicity, sensitization, and irritation testing. Polyimide FPC substrate is generally compatible; the cover layer (solder mask, conformal coating, or adhesive) needs specific selection. Skin adhesive (typically hydrocolloid or acrylic medical adhesive) requires its own biocompatibility documentation.
- FDA Class II classification. Most medical-claim wearables are FDA Class II — moderate risk, requires 510(k) submission demonstrating substantial equivalence to predicate device. The FPC and cable design becomes part of the 510(k) technical documentation.
- EU MDR 2017/745 for European market. Class IIa or IIb for most medical wearables; technical file includes FPC design and material biocompatibility.
- Sweat and moisture resistance. Skin contact products see sweat for the entire wear period. The FPC, encapsulation, and electronics must function continuously under perspiration exposure. IPX5 or higher water resistance ratings are typical.
- EMI / EMC compliance. BLE radio operating near skin contact requires SAR (Specific Absorption Rate) testing for radio emissions. The FPC layout affects radio efficiency and SAR; iteration during design is normal.
Our medical solutions work covers FPC for wearable medical applications including the documentation flow that medical OEMs need for their submission packages.
Side-by-Side Comparison Table
| Product Category | Wear Duration | FPC Area | Key Sensors | FDA Class |
|---|---|---|---|---|
| CGM (Dexcom G7, FreeStyle Libre 3) | 10-15 days | 2-3 cm² | Glucose sensor + BLE | Class II |
| CGM implanted (Eversense) | 180 days | Smaller, integrated | Glucose + readback radio | Class II |
| ECG patch (Zio XT, BG MINI) | 7-14 days | 5-8 cm² | Dual electrode + BLE | Class II |
| ECG real-time (Zio AT, BioTel MCT) | 3-7 days | 5-8 cm² | Dual electrode + cellular | Class II |
| Multi-parameter (VitalPatch) | 3-5 days | 6-10 cm² | ECG + temp + accel | Class II |
| Heart rate patch (medical) | 3-7 days | 3-5 cm² | PPG or single-lead ECG + BLE | Class II |
| Sleep monitor patch | 1-7 nights | 3-5 cm² | HR variability + accel | Class II or wellness |
| Wellness fitness patch | 1-7 days | 3-5 cm² | HR + activity | Wellness only |
FPC Construction Specifics
- Polyimide substrate, typically 1-2 layer. Single-sided or double-sided FPC dominates wearable medical. Thicker copper for power distribution; thinner copper for fine-pitch signal routing.
- Adhesive cover layer. Polyimide or PET cover layer protects copper traces from sweat exposure. Specialty cover layer adhesives meet ISO 10993 biocompatibility for skin-side surfaces.
- Stiffener regions. Component-bearing areas use rigid stiffener (FR4 or thicker polyimide) to support SMT components mechanically. Flex areas have no stiffener for full bend.
- Skin electrode contact pads. Gold-plated copper pads with hydrogel or wet adhesive interface to skin. Electrode area sized to ECG signal capture geometry; spacing matched to lead vector.
- BLE antenna trace. Printed copper antenna on FPC, typically meandered or PIFA design. Layout requires careful clearance from ground plane and other components.
- Battery contact area. Spring contact, conductive adhesive, or solder pads for battery connection. Battery is typically the thickest component on the patch and constrains overall thickness.
- Conformal coating. Parylene or similar conformal coating over component areas for moisture and abrasion protection. The coating extends FPC service life under sweat exposure.
A Common Mistake — Treating Wearable FPC Like Standard FPC
The most frequent design error we see is treating wearable medical FPC like a standard consumer FPC. Standard FPC is designed for inside-product routing — it doesn’t see skin contact, sweat, or 7-14 day continuous body temperature exposure. Wearable medical FPC needs ISO 10993 cover layer materials, sweat-resistant conformal coating, and skin adhesive integration that consumer FPC programs don’t include. Programs that try to ship wearable medical with standard FPC discover skin irritation issues, biocompatibility failures, or in-wear FPC degradation during validation.
The opposite mistake — over-specifying medical-grade for wellness products — also happens. A consumer fitness patch making no medical claims doesn’t need full FDA Class II compliance documentation. Specifying medical-grade FPC and biocompatibility documentation for a wellness-only product wastes development cost. Match specification to claim and target market regulatory class.
Bottom Line
Wearable medical FPC has reshaped medical electronics from rigid PCB clamshell devices to thin flexible patches integrating sensor, electronics, battery, and adhesive. CGM and ECG patches lead the volume; heart rate and sleep monitoring follow. ISO 10993 biocompatibility and FDA Class II compliance shape the FPC and cover layer material choices. Polyimide substrate, sweat-resistant conformal coating, integrated electrode contact, and miniaturized component placement make the form factor possible. Matching FPC construction to the wear life and compliance class keeps the product design aligned with the regulatory submission requirement.
Related Reading
- Custom FPC — wearable medical FPC manufacturing capabilities.
- Medical Cable Assembly — silicone medical cable for non-wearable medical devices.
- Medical Solutions — full medical and wearable application scope.
- Medical Aesthetics Cable — companion guide on aesthetic device cable.
- Cable Insulation Guide — biocompatible material selection.
Wearable Medical FPC Program?
Send us your wearable application — CGM, ECG, heart rate, sleep monitor, or other. Wear duration, sensor architecture, and target compliance class. We’ll match FPC construction to the application.