Continuously-worn ECG patch

A miniature rechargeable chest patch with two dry/gel electrodes for single-lead ECG in continuous-wear mode for up to 30 days without recharging. Data is sent over BLE to a single dedicated app. Target volume — from 10,000 units.

Architecture

flowchart LR
  subgraph FE["Analog front-end"]
    E1((Electrode 1))
    E2((Electrode 2))
    AFE["MAX30003
ECG AFE
+ R-peak HW detect"]
  end
  MCU["nRF52810/811
Cortex-M4 + BLE 5.x"]
  subgraph SENS["Sensors & memory"]
    ACC["LIS2DW12
Accelerometer"]
    FLASH["W25Q* SPI NOR
4-8 MB buffer
for BLE dropout"]
  end
  subgraph PWR["Power"]
    BAT[("LiPo
100-150 mAh")]
    PMIC["MAX77658
charger + LDO
+ fuel gauge"]
    DOCK["USB-C or
pogo-pin dock"]
  end
  PHONE["📱 heartlab.app"]
  E1 --> AFE
  E2 --> AFE
  AFE -->|"SPI + IRQ"| MCU
  MCU <-->|"I²C"| ACC
  MCU <-->|"SPI"| FLASH
  MCU <-->|"BLE 5.x"| PHONE
  BAT --> PMIC
  DOCK -.-> PMIC
  PMIC -->|"power"| AFE
  PMIC -->|"power"| MCU
  PMIC -->|"power"| ACC
  PMIC -->|"power"| FLASH
    

Key component choices

— MAX30003 — the de-facto irreplaceable chip today: a specialized single-channel AFE for wearable ECG. Biopotential input, ADC, filtering, R-peak detection and RR-interval computation, lead-off detection — all implemented in silicon at a power level unattainable if the same tasks were solved with discrete electronics and an MCU.

— nRF52810/811 — Nordic MCU, optimized for miniature BLE devices.

— LIS2DW12 — accelerometer chip, strongly recommended for motion-artifact rejection.

— SPI flash buffer — data-buffering chip for when there is no phone connection for more than a few minutes.

Power budget

30 days (720 h), data transfer no more than every 30 s, no real-time streaming

Required battery 100–130 mAh, LiPo 5×20×3 mm, ~3 g.

✓ Goal of a month without recharging — looks achievable

Cost estimate

10k+ volume, FOB Shenzhen

The battery can be replaced with primary cells — this cuts cost and removes the charging circuit, but creates enclosure-sealing problems.

✓ Goal of $15–20 cost of goods — looks achievable

✗ Goal of a $15–20 final consumer price — unachievable

Other considerations and risks

• If the device must not merely report HRV but detect dangerous conditions with diagnoses/advice, the project moves into the medical domain and requires expensive, lengthy certification (up to $300k, up to 18 months).
• Prolonged wear may cause skin irritation, electrode detachment, contact degradation and user complaints. Adhesive biocompatibility for >7 days on skin requires a medical-adhesive supplier (3M, Vancive, Scapa Healthcare).
• ECG quality on dry electrodes in real wear requires field trials; the design may need to change or add a 3rd electrode (which sharply improves measurement quality). Overall, the mechanics of electrodes and patch may matter more than the electronics if the electrode lifts when the body rotates or sweat disrupts contact. The prototype should test: standard Ag/AgCl electrodes; dry electrodes; flexible printed electrodes; conductive gel; hydrogel patches; a combined reusable enclosure + disposable electrode sticker.
• The chest-worn patch antenna radiates upward and sideways; with the phone in a back pocket, BLE through the body loses up to −30 dB, and wet synthetic clothing adds another −5 dB. Effective range drops to 1–3 m. Bench tests needed. An SPI flash buffer is required, otherwise data is lost. Alternatively — inform users that the device does not work when the phone is not on them.

Key baseline questions

• Calculations assume accumulation of processed data with BLE transfer once every 30 seconds (burst transfer). If a raw real-time ECG stream is needed, with indication of each heartbeat, battery life may drop to 5 days. We propose providing a raw-ECG mode, but only for rare, short test and diagnostic procedures.
• Depth of protection against the device working with other apps? Consumer-grade is done via a handshake; serious industrial-grade protection against hackers requires an MCU with secure boot and factory provisioning.

Proposed stages, optimistic cost and timeline

Development requires a coordinated team of 2–3 qualified engineers experienced in building devices of this level (available).