Electronics Guide

Understanding Flexible Circuit Technology

Flexible circuits use polyimide or polyester substrates instead of the rigid fiberglass-reinforced epoxy found in conventional PCBs. This flexibility enables circuits to bend, fold, and twist while maintaining electrical connectivity. Key characteristics of flexible circuit technology include:

• Substrate materials: Polyimide (Kapton) offers excellent thermal stability and chemical resistance, suitable for soldering and harsh environments; polyester (PET) provides lower cost for less demanding applications
• Copper adhesion: Adhesive-based and adhesiveless constructions affect flexibility, reliability, and fine-pitch capability; adhesiveless laminates offer superior performance but at higher cost
• Bend radius limits: Minimum bend radius depends on copper thickness, layer count, and whether the circuit undergoes static (one-time) or dynamic (repeated) bending
• Stiffener regions: Rigid sections added to flexible circuits where components mount, providing stable soldering surfaces while maintaining flexibility elsewhere
• Rigid-flex construction: Combines rigid multilayer boards with flexible interconnects, eliminating connectors while providing both mounting stability and mechanical flexibility

Flexible Development Board Options

Several development platforms support prototyping with flexible circuits:

• Adafruit Flex PCB: Pre-made flexible circuit boards with NeoPixel LEDs or sensor arrays, providing starting points for wearable light displays and sensing applications
• SparkFun Flexible Qwiic: Flexible I2C sensor boards compatible with the Qwiic ecosystem, enabling rapid prototyping of conformable sensor systems
• Seeed Studio Flexible PCBA: Rapid prototyping service providing assembled flexible circuits from design files, supporting small-quantity flexible prototypes
• Texas Instruments Flex Modules: Reference designs for flexible battery management, power delivery, and sensing suitable for wearable applications
• Würth Elektronik Flex Design Kits: Educational kits demonstrating flex circuit design principles with sample boards and design guidelines

For custom flexible circuit development, services such as OSH Park Flex, PCBWay Flex, and JLCPCB Flexible PCB provide accessible prototyping with quick turnaround at reasonable cost for small quantities.

Design Considerations for Flex Circuits

Successful flexible circuit design requires attention to factors beyond standard PCB design:

• Trace routing: Route traces perpendicular to bend axes when possible; avoid routing on the neutral bend axis where strain is minimized
• Via placement: Keep vias away from bend regions where they create stress concentrations and potential failure points
• Component placement: Mount components only on stiffened regions; avoid placing components where bending occurs
• Copper weight: Thinner copper (typically half-ounce or one-ounce) improves flexibility; use minimum copper weight that meets current-carrying requirements
• Coverlay openings: Design coverlay (flexible solder mask) openings carefully; incorrect openings can cause delamination during bending
• Strain relief: Incorporate strain relief features at transitions between rigid and flexible regions to prevent conductor fracture

Conductive Thread and Yarn Systems

Conductive threads form the interconnects in e-textile designs, replacing conventional wires with sewable, flexible conductors:

• Stainless steel thread: Fine stainless steel fibers twisted into thread, providing good conductivity (1-10 ohms per foot typical) and durability; can be harsh on skin for direct contact applications
• Silver-coated nylon: Nylon thread with conductive silver coating, offering better hand feel than steel but lower conductivity and potential tarnishing; typical resistance 5-30 ohms per foot
• Copper thread: Copper-based conductive threads providing excellent conductivity but less flexibility and potential for corrosion
• Carbon-loaded thread: Polymer threads containing carbon particles, providing moderate conductivity suitable for resistive sensing applications
• Hybrid constructions: Multi-material threads combining structural fibers with conductive elements for optimized mechanical and electrical properties

Development kits from suppliers like Adafruit, SparkFun, and Loomia include assortments of conductive threads with various properties, enabling experimentation to find optimal materials for specific applications.

Sewable Electronics Components

Specialized components designed for textile integration feature large, fabric-friendly connection points:

• LilyPad Arduino: The pioneering e-textile microcontroller platform featuring a circular form factor with large sew tabs, programmable via Arduino IDE; available in multiple variants including USB, Simple, and Tiny versions
• Adafruit Flora: Arduino-compatible circular e-textile controller with built-in USB and optional Bluetooth; ecosystem includes matching sensors, LEDs, and power components
• Adafruit GEMMA: Tiny e-textile controller for space-constrained applications, programmable via Arduino or CircuitPython
• SparkFun LilyPad components: Extensive range of sewable sensors (light, temperature, accelerometer), LEDs (individual and sequenced), switches, and power management components
• Teknikio: Education-focused e-textile components with simplified connections suitable for classroom use

Conductive Fabric and Flexible Sensors

Conductive fabrics provide larger-area conductors and enable fabric-based sensors:

• Woven conductive fabric: Fabrics incorporating metal fibers throughout, suitable for shielding, grounding planes, and large-area electrodes
• Pressure-sensitive fabric: Velostat and similar materials change resistance under pressure, enabling fabric pressure sensors and pressure-mapped surfaces
• Stretch sensors: Conductive materials that change resistance when stretched, enabling motion and respiration sensing
• Capacitive fabric: Conductive textiles configured for capacitive touch sensing, creating soft touch interfaces
• Thermochromic fabrics: Materials that change color with temperature, usable as visual temperature indicators when combined with heating elements

E-Textile Development Kits

Comprehensive e-textile development kits provide components, tools, and guidance for beginning e-textile projects:

• Adafruit Flora Wearable Electronics Platform: Complete kit with Flora controller, NeoPixel LEDs, sensors, conductive thread, and needles; includes online tutorials covering project basics through advanced techniques
• SparkFun LilyPad Design Kit: Assortment of LilyPad components with conductive thread and protoboard for learning e-textile techniques
• Loomia Electronic Fabric Development Kit: Industrial-oriented kit featuring flexible circuits designed for textile integration, including heating elements, sensors, and LED arrays with textile-compatible connectors
• KOBAKANT DIY Kit: Educational kit focusing on DIY e-textile techniques, including component making and soft circuit construction

E-Textile Design Challenges

Successful e-textile development requires addressing challenges unique to textile integration:

• Washability: Protecting electronics from water and detergent damage; solutions include encapsulation, removable modules, and inherently water-resistant components
• Connection reliability: Ensuring sewn connections remain conductive through flexing and wear; techniques include multiple connection points, reinforcement stitching, and conductive adhesives
• Short circuit prevention: Avoiding unintended connections when conductive threads cross or fabric layers overlap; insulating layers and careful routing are essential
• Comfort: Electronics and connections should not create uncomfortable pressure points, stiffness, or irritation against skin
• Durability: Components and connections must survive the mechanical stresses of wearing, including stretching, twisting, and abrasion

Photoplethysmography (PPG) Platforms

PPG sensors use light to detect blood volume changes, enabling heart rate and blood oxygen measurement:

• Maxim MAX30102: Integrated pulse oximetry and heart rate sensor with red and infrared LEDs, photodetector, and signal processing; development kits available from Maxim and third parties
• Texas Instruments AFE4404: Analog front-end IC for PPG applications supporting multiple LED wavelengths and configurable timing; evaluation module provides complete reference design
• ams AS7038: Compact PPG sensor optimized for wrist-worn applications, integrating LEDs, photodetector, and analog front-end in small package
• Silicon Labs Si117x: Multi-channel optical sensor family with integrated algorithms for heart rate and SpO2; development kit includes wrist-band reference design
• Analog Devices ADPD4100: High-performance optical analog front-end supporting complex multi-wavelength PPG configurations for research-grade measurements

PPG development considerations include LED wavelength selection (green for heart rate, red and infrared for SpO2), optical path design for consistent skin coupling, motion artifact rejection, and ambient light rejection.

Electrocardiogram (ECG) Development

ECG sensors detect the electrical activity of the heart, providing medical-grade cardiac monitoring capability:

• Analog Devices AD8232: Single-lead ECG analog front-end in small package, widely used in wearable ECG development; multiple evaluation boards and reference designs available
• Texas Instruments ADS1292: Dual-channel 24-bit ADC with integrated ECG front-end, providing high-resolution acquisition for multi-lead applications
• Maxim MAX30003: Clinical-grade single-lead ECG analog front-end with R-R detection and biopotential measurement capability
• SparkFun AD8232 Heart Rate Monitor: Breakout board and development guide for learning ECG signal acquisition and processing
• Protocol OpenECG: Open-source ECG development platform providing hardware designs and software for multi-lead ECG research

ECG development requires careful attention to electrode design and placement, common-mode rejection, high-pass filtering for baseline wander removal, and power line interference rejection.

Motion and Activity Sensing

Inertial measurement units (IMUs) enable activity recognition, step counting, gesture detection, and motion tracking:

• Bosch BNO055: Nine-axis absolute orientation sensor with sensor fusion processor, simplifying motion sensing with quaternion and Euler angle outputs
• InvenSense ICM-42688: Six-axis IMU with low power consumption and built-in step counter, suitable for always-on activity monitoring
• STMicroelectronics LSM6DSO: Six-axis IMU with machine learning core for on-sensor activity classification, reducing processor load
• Analog Devices ADXL362: Ultra-low-power accelerometer consuming less than 2 microamps in measurement mode, enabling always-on motion detection
• Xsens MTi: High-accuracy IMU modules with advanced sensor fusion for research-grade motion capture applications

Bioimpedance Measurement

Bioimpedance sensors measure electrical impedance of body tissues, enabling body composition analysis and hydration monitoring:

• Analog Devices AD5940: High-performance bioimpedance analog front-end supporting multiple measurement configurations; evaluation board and reference designs available
• Maxim MAX30009: Bioimpedance analog front-end optimized for wearable applications with low power consumption
• Texas Instruments AFE4300: Multi-frequency bioimpedance measurement IC designed for body composition analysis

Bioimpedance development requires understanding of electrode configurations (two-electrode versus four-electrode), excitation frequency selection, and calibration techniques for accurate measurements.

Galvanic Skin Response (GSR)

GSR sensors measure skin conductance, which varies with emotional arousal and stress levels:

• Grove GSR Sensor: Simple skin conductance measurement module compatible with Arduino and Seeed ecosystem
• Shimmer GSR+: Research-grade wearable GSR sensor with data logging and wireless connectivity for psychophysiology research
• SparkFun GSR Sensor: Breakout board for skin conductance measurement with basic signal conditioning

GSR development considerations include electrode material selection (typically stainless steel or silver-silver chloride), electrode placement, AC versus DC measurement techniques, and environmental artifact rejection.

Integrated Biometric Development Platforms

Several platforms combine multiple biometric sensors into integrated development systems:

• Maxim MAXREFDES104: Wrist-worn health sensor platform integrating PPG, ECG, temperature, and accelerometer with reference firmware
• Silicon Labs Thunderboard Sense: Multi-sensor development kit including motion, environment, and biometric sensing capabilities
• Empatica E4: Research-grade wearable platform measuring PPG, GSR, temperature, and motion with cloud data management
• Polar Verity Sense: Armband optical heart rate sensor with SDK access for development and research applications

E-Paper (Electronic Paper) Displays

E-paper displays consume power only when changing content, making them ideal for applications with infrequently updated information:

• E Ink development kits: Official E Ink reference designs demonstrating various display sizes, resolutions, and color capabilities (black/white, grayscale, and color)
• Good Display modules: Economical e-paper modules in various sizes with SPI interface, supported by Arduino and other microcontroller platforms
• Waveshare e-Paper: Extensive range of e-paper displays with development boards and example code for common microcontrollers
• Plastic Logic flexible e-paper: Flexible e-paper displays enabling curved and conformable display applications

E-paper development considerations include refresh time (typically 1-15 seconds for full refresh), operating temperature range, and ghosting management for partial updates.

Memory-In-Pixel (MIP) and Reflective LCD

MIP displays maintain their image without continuous refresh, dramatically reducing power consumption:

• Sharp Memory LCD: Ultra-low-power displays consuming as little as 15 microwatts while displaying static content; development modules available in various sizes
• Japan Display reflective LCD: Reflective LCD technology providing good visibility in bright ambient light without backlight power consumption
• Qualcomm Mirasol: MEMS-based reflective display technology (discontinued but represents an important technology approach)

MIP displays offer fast refresh compared to e-paper while maintaining low power for static content, making them suitable for applications requiring occasional animation or updates.

OLED and AMOLED Displays

Organic LED displays provide excellent contrast and visibility, though at higher power than reflective technologies:

• Adafruit OLED displays: Small OLED modules (typically 0.96 inch to 1.3 inch) with SPI or I2C interface, suitable for compact wearable displays
• SparkFun Micro OLED: Tiny OLED breakout boards for minimal display requirements
• Midas OLED modules: Industrial OLED display modules with various interface options and development support
• Round and shaped OLEDs: Circular and custom-shaped OLED displays enabling watch-style and other non-rectangular wearable form factors

OLED power consumption depends heavily on displayed content; dark interfaces minimize power while bright, white content maximizes consumption.

Micro-LED and Emerging Technologies

Emerging display technologies promise improved performance for wearable applications:

• Micro-LED: Inorganic LED displays offering OLED-like image quality with improved efficiency and durability
• Quantum dot enhancement: Improved color gamut and efficiency for LCD and OLED displays
• Holographic displays: Early-stage technology enabling glasses-free 3D and augmented reality applications

Qi Standard Wireless Charging

The Qi standard provides interoperability for inductive wireless charging:

• Wireless Power Consortium reference designs: Official Qi transmitter and receiver reference designs ensuring standard compliance
• Texas Instruments BQ500215: Qi-compliant wireless power transmitter controller with foreign object detection; evaluation modules available
• STMicroelectronics STWLC04: Qi-compliant wireless power receiver for wearable devices with compact footprint
• IDT (Renesas) P9038: Small Qi receiver designed specifically for wearable and hearable applications
• NXP WLC chips: Wireless charging transmitter and receiver ICs with development kits supporting rapid prototyping

Proprietary Wireless Charging

Custom wireless charging solutions can optimize for specific wearable requirements:

• Powermat development kits: Proprietary magnetic resonance charging technology with higher positioning flexibility
• Energous WattUp: RF-based wireless power enabling charging at distance without precise positioning
• Custom inductive solutions: Application-specific coil designs optimized for particular form factors and power requirements

Wireless Charging Design Considerations

Effective wireless charging integration requires attention to several factors:

• Coil design: Receiver coil size and shape affect coupling efficiency, power capability, and positional tolerance; ferrite shielding prevents interference with other electronics
• Thermal management: Wireless charging generates heat from coil losses and rectification; thermal design must prevent battery damage and user discomfort
• Metal interference: Metal near charging coils causes eddy current heating and efficiency loss; careful placement and shielding are essential
• Foreign object detection: Safety features prevent heating of metal objects between transmitter and receiver
• Charging feedback: Visual or other indication of charging status and completion since the device may be face-down on the charger

System-in-Package (SiP) Modules

SiP modules integrate multiple die and passive components in single packages:

• Nordic nRF5340: Dual-core Bluetooth 5.3 SoC with integrated antenna option, providing complete wireless solution in minimal space
• Qualcomm QCC5100: Audio SoC series designed for compact true wireless earbuds, integrating Bluetooth, audio processing, and power management
• Dialog (Renesas) DA1469x: Bluetooth Low Energy SoC with integrated power management and optional display driver for compact wearables
• Silicon Labs BG22: Compact Bluetooth LE SoC suitable for highly miniaturized wearable applications

Chip-Scale Packaging

Chip-scale packages minimize component footprint for maximum density:

• WLCSP (Wafer-Level Chip-Scale Package): Package footprint equals die size; requires careful PCB design for fine-pitch assembly
• Flip chip: Direct die-to-substrate connection eliminating wire bonds; enables highest density but requires advanced assembly
• Fan-out WLP: Redistributes connections from small die to larger ball pattern, easing assembly while maintaining small size

High-Density Interconnect (HDI) PCB

HDI PCB technologies enable routing complex circuits in minimal area:

• Microvia technology: Laser-drilled vias as small as 50 micrometers enable fine-pitch component placement and dense routing
• Stacked microvias: Multiple microvia layers create direct vertical connections between any layer pair
• Embedded components: Passive components embedded within PCB substrate, freeing surface area for active components
• Any-layer HDI: All layers interconnected with microvias, enabling maximum routing density and design flexibility

HDI development requires specialized design tools and manufacturing processes; prototyping services like Multek and AT&S support HDI development for wearable applications.

Component Selection for Miniaturization

Achieving compact designs requires careful component selection:

• Integrated functions: Select ICs that combine multiple functions (e.g., PMIC with battery charger, DC-DC converters, and LDOs)
• Smallest packages: Use 01005 (0.4 mm x 0.2 mm) or 008004 passives where assembly capability permits
• Passiveless designs: Choose ICs that minimize external component requirements through internal integration
• Stacked components: Package-on-package and stacked memory reduce footprint at the cost of thermal considerations
• Crystal elimination: MEMS oscillators or crystal-less designs reduce component count and footprint

Continuous Glucose Monitoring (CGM) Development

CGM devices provide real-time blood glucose information for diabetes management:

• Sensor technologies: Enzymatic electrochemical sensors, fluorescence-based sensors, and emerging non-invasive optical approaches
• Analog front-end design: Ultra-low-current measurement (picoamps to nanoamps) requiring careful noise management
• Wireless protocols: Bluetooth Low Energy common for smartphone connectivity; proprietary protocols for dedicated receivers
• Development platforms: Analog Devices and Texas Instruments offer evaluation platforms for biosensor signal conditioning

CGM development involves significant regulatory requirements for medical devices; early engagement with regulatory strategy is essential.

Sleep Monitoring Systems

Sleep monitoring combines multiple sensor modalities to assess sleep quality:

• Motion sensing: Accelerometer-based detection of sleep stages from movement patterns
• Heart rate variability: PPG-based HRV analysis provides indicators of sleep stage and sleep quality
• Respiratory monitoring: Chest movement sensing or airflow detection for sleep apnea screening
• Blood oxygen: SpO2 monitoring detects hypoxia events associated with sleep apnea
• Development platforms: Maxim Integrated (now Analog Devices) reference designs combine multiple sensors for comprehensive sleep monitoring

Cardiac Monitoring Wearables

Wearable cardiac monitors enable extended ECG recording and arrhythmia detection:

• Patch-based monitors: Adhesive patches worn on the chest for multi-day continuous ECG recording
• Wrist-based ECG: Single-lead ECG from wrist contact electrodes, as implemented in Apple Watch and similar devices
• Event recorders: Patient-activated recording triggered by symptom occurrence
• Development platforms: AD8232-based development kits provide starting points; Maxim MAX30003 evaluation kit offers clinical-grade capability

Cardiac monitoring devices face regulatory oversight as medical devices; understanding classification and compliance requirements is essential for commercial development.

Fitness and Activity Tracking

Consumer fitness tracking forms the largest market segment for wearable health devices:

• Step counting: Accelerometer-based algorithms detect walking and running patterns
• Activity classification: Machine learning identifies specific activities (walking, running, cycling, swimming)
• Calorie estimation: Combines activity data with user demographics for energy expenditure calculation
• VO2 max estimation: Heart rate and activity data enable aerobic capacity estimation
• Development platforms: Nordic nRF52 DK and similar Bluetooth LE platforms provide complete solutions for fitness tracker development

Stress and Mental Health Monitoring

Emerging wearable applications address mental health through physiological indicators:

• Heart rate variability: HRV metrics correlate with stress levels and autonomic nervous system balance
• Electrodermal activity: Skin conductance variations indicate sympathetic nervous system arousal
• Skin temperature: Temperature variations can indicate stress responses
• Multi-modal fusion: Combining multiple physiological signals improves stress detection accuracy
• Development platforms: Empatica E4 and similar research platforms provide validated sensor configurations for psychophysiology research

Wearable Operating Systems

Specialized operating systems address wearable device requirements:

• Zephyr RTOS: Open-source RTOS with excellent Bluetooth support and low-power management, widely used in wearable development
• FreeRTOS: Popular RTOS with tickless idle mode for power-sensitive applications; extensive ecosystem and cloud connectivity options
• Mbed OS: ARM-backed RTOS with connectivity focus and power management features
• RIOT OS: Open-source IoT operating system with small footprint suitable for constrained wearable devices
• Wear OS: Google's smartwatch platform for feature-rich wearable applications
• watchOS: Apple's wearable platform providing development framework for Apple Watch applications

Sensor Fusion Libraries

Sensor fusion algorithms combine multiple sensor inputs for improved measurements:

• Bosch SensorTec libraries: Proprietary sensor fusion for Bosch IMUs, providing orientation and motion detection
• InvenSense MotionApps: Embedded motion processing for InvenSense IMUs
• Open-source implementations: Madgwick and Mahony filters provide orientation estimation from IMU data
• Machine learning frameworks: TensorFlow Lite Micro and Edge Impulse enable on-device activity classification

Power Optimization Tools

Tools for analyzing and optimizing wearable device power consumption:

• Nordic Power Profiler: Real-time power measurement with correlation to firmware execution
• Silicon Labs Energy Profiler: Integrated power analysis in Simplicity Studio development environment
• Otii by Qoitech: Programmable power supply with precise current measurement and software correlation
• Joulescope: Precision DC energy analyzer for battery life optimization

Assembly Considerations

• Component accessibility: Design for automated assembly while maintaining compact form factor
• Test point access: Include test points for production testing while minimizing their impact on size
• Enclosure integration: Design PCBs to integrate with enclosure features, mounting points, and mechanical constraints
• Waterproofing: Consider sealing methods, gasket integration, and cable routing for water resistance

Regulatory Compliance

• Wireless certification: FCC, CE, and other regional certifications required for devices with wireless capability
• Medical device regulations: FDA 510(k), CE marking, and other medical device requirements for health-related applications
• Safety standards: IEC 62368-1 for consumer electronics, IEC 60601-1 for medical devices
• Biocompatibility: ISO 10993 testing for materials contacting skin for extended periods