Here is a great design checklist for wearable devices. When we do a design we check all of these things and more. Often we need to use our engineering judgment on the tradeoff between size, cost, testability, and manufacturability of the board. Power What size battery? What is the appropriate battery lifetime? How often do various parts of it need to be on? What is the maximum current draw (also determines battery size)? How will the battery be charged: wirelessly, plugged in, or replaceable batteries? Wireless What is the appropriate type of wireless transmission? Is a gateway needed or will it connect directly to the Internet? What is the impact on the battery, size and lifetime? What type and size of antenna is needed? How often does it need to be on? Sensors What is the appropriate tradeoff between cost and accuracy? What accuracy and sensitivity is needed? Is calibration needed? How will the sensors be tested and calibrated? If there are biological sensors, how will the signals be picked up from the body, contact or non-contact, invasive or non-invasive? Flex Circuits What is the bend radius? Software Where does the data analysis take place, on the device or elsewhere? How smart does the device have to be (smarter generally means more power and larger)? What is the protocol for communication, standard or proprietary? How will software updates be done, over the air, by plugging in, or not at all? Will there be wired communication? During charging? Packaging How small is the space for electronics? How will everything fit? Where will the antenna be located (away from metal and not too close to the body)? Does it need to be sealed from dust and moisture? To what water depth is it sealed?