BLE + Sensors + Battery in a plastic. We already done 85% of your product.
SensiTHING Environmental Monitoring Module with Wireless Charging Capability
Application note: How to add AA battery To SensiBLE 1.0 or SensiSUB 1.0
Why lithium batteries do not suit Wearable IoT application…or do they?
This IoT Starter Platform Also Helps with What Comes Next