Wearable ECG patch sensor

Product description

The device is a single-lead ECG recorder: Holter monitoring, event recording, post-operative and home monitoring, sports heart rate. Single-use versions pair with a hydrogel electrode and are thrown away after use; reusable versions use a strap or medical tape, so the housing has to survive repeated wiping and disinfection. The two routes differ in housing structure, compound and validation list, but share one requirement: the board inside has to survive the full product life.

Full or partial encapsulation is the first decision. Full encapsulation moulds the PCBA, cell and switches inside one silicone body: sweat has no path in, the part does not come apart when dropped, and the outside is clean with no parting line. The price is that it cannot be opened or repaired, and heat and moisture are held inside. Partial encapsulation uses a silicone housing with a rigid or bonded lid, which can be serviced and can take a replaceable cell, at the cost of one more sealing interface. Service strategy and product life decide this, not unit cost.

Temperature and stress are the two gates in molding. Silicone needs heat to cure; cells and some components do not want it. Raising the mould temperature to shorten the cycle shows up months later as capacity spread and consistency loss. A low-temperature curing system (mould at 80-110 degrees C) with a longer hold suits these parts better, and the cavity and cell temperatures are measured with thermocouples — not read off the machine setpoint. On the pressure side, fill velocity drags components: uniform thin walls, radiused transitions, zoned evacuation and low injection speed are done together, which is what brings stress at joints and 0402 parts down.

Permeability is where this part is most often misjudged. Silicone is not a vapour barrier; compared with epoxy and most polyurethanes its moisture vapour transmission is higher. On the skin side that is an advantage — sweat vapour escapes and the patch does not feel clammy — but inside it is a risk: vapour gets in and stays in, condensing on the cold face as the day cools. So "fully encapsulated" does not mean waterproof and moisture-proof: liquid water is kept out, while internal vapour still needs conformal coating, local potting, a dry structure or a breathable membrane. Validate in two separate groups — immersion, and damp heat with thermal cycling — and require both.

On the skin side the material and the edge do the work. Medical-grade platinum-cured compound, no talc, low migration; skin irritation and sensitisation assessed to ISO 10993 for the target market; edges radiused R1 or more so continuous wear does not leave a mark; reusable models checked for appearance and function after the number of alcohol wipes in the customer's cleaning protocol, with that number written into the validation list.

The cell and the switches decide how the patch feels to use. Coin cells (CR2032, CR1632) or thin-film cells both work; the battery well needs room to swell and must not take pressure during molding, and power is taken either by welded nickel tabs or spring contacts depending on the cell. The silicone over a dome switch is commonly 0.4-0.8 mm at Shore A 40-60, with travel measured as a force curve at sampling rather than judged by hand. Because the part cannot be opened, the cell's cycle life is the ceiling on product life.

There are two electrode interfaces. A metal snap (stainless steel or nickel-plated) passes through the housing, sealed by an O-ring at its root plus structural compression, and accepts standard lead wires. A conductive silicone electrode keeps the contact flat and removes metal from the skin, but volume resistance, contact area and ageing in sweat have to be controlled together. The indicator light uses clear silicone overmoulded a second time or a light guide; the window is built into the tool, not cut afterwards.