Innovative Bone-Conduction Device Development

Main requirements:

* Device should sit behind the ear on the mastoid bone area
* Hidden inside hair so almost invisible during normal use
* Very lightweight and comfortable for long use
* Bone conduction vibration only for wearer
* Minimal sound leakage to nearby people
* Bluetooth connectivity with mobile phone
* Rechargeable battery
* Compact charging system (USB-C preferred)
* Smallest possible size

I want two possible designs:

1. Fully self-contained unit behind ear
(battery + Bluetooth + transducer all together)
2. Ultra-small transducer behind ear with separate battery/control module hidden in collar/pocket connected by thin wire

Needed help:

* Component selection
* Circuit design
* PCB design
* Battery optimization
* Small enclosure design
* Comfortable mounting method (skin tape, magnetic clip, hair clip, etc.)
* Prototype development

Preferred features:

* Clear voice audio
* Low latency
* 2–3hour battery backup minimum
* Safe temperature during use
* Android/iPhone compatibility

Please also suggest:

* Best bone conduction transducer size/type
* Best Bluetooth audio module
* Approximate total size possible
* Estimated cost for prototype

I want the design to be extremely discreet and wearable daily.