Depends what you're trying to measure. You really only need an Arduino/Raspberry Pi and some input device (sensor). For example, if you want a "brain wave wearable", simply connect an EEG to the Pi via RS-232 and process the data. It's really just a 4 step process:

1. Acquire MCU (Pi, Arduino, etc.) 2. Acquire sensor (Heartrate, EEG, etc.) 3. Program processor/UI (Python/C++) 4. 3D print enclosure

Make a dope marketing video and $$. Send me a PM if you want help, I've launched a wearable (it failed but there were lessons learned).