Is this a Bluetooth, or a Bluetooth Low Energy device? Is it yours (do you own the firmware, and know what it does)? Or is it a device from a third party that you would like to use?
The easiest type of device to support is BLE.
Have you looked here: https://developer.apple.com/bluetooth/ ?
If you've never done this before, start with the Heart Rate Monitor app and use an off-the-shelf heart rate monitor. The Heart Rate Monitor profile is a published standard profile. If you're comfortable with it, you can also use a low-cost BLE development kit to make your own BLE device, with any profile you like, using chips from TI, Maxim IC, Silicon Labs, Nordic, Dialog etc