WSN application – proper storage of medicines like insulin
Type 1 diabetics take multiple injections of Insulin every day to control their blood sugar level. Insulin has to be stored properly otherwise it looses its efficacy. High blood sugar level is highly detrimental to health and should be avoided at all costs.
Unopened insulin (in vials or cartridges) should be stored in a refrigerator between 2 to 8 degrees celcius. Once in use, insulin can be used for up to 4 weeks as long as it is stored below 25 degrees celcius and it does not have to be kept inside a refrigerator. Insulin must not be frozen, or exposed to excessive heat or light.
Usually a patient keeps multiple unopened insulin vials or cartridges in the fridge and uses a single vial/cartridge at a time. We need a small form factor detachable wireless node with a temperature sensor and ambient light sensor. This is achievable if we populate a single PCB with a microcontroller, radio and a chip antenna. In fact integrated micro + radio chips are already available. The node can be powered using a very small form factor lithium button/coin cell or a rechargeable NiMH button/coin cell. The obvious trade off is between size and capacity. Another important factor is maximum allowed burst current draw before the battery’s output voltage drops below the minimum required by the micro/radio. All this information will be available in the battery spec.
This node should operate in reduced functionality mode (RFN) to keep energy consumption to a minimum. WiSense nodes use the MSP430G2553 microcontroller which has an on board temperature sensor. It is accurate enough for our purpose which means you do not need an external one which will impact both the cost and size of the PCB. The node needs to wake up periodically and sample the temperature sensor. The periodicity will depend on the battery capacity. If the measured temperature is within the allowed range, it can go back to sleep immediately. If not, it can send an alarm message to the gateway node directly or through one or more intermediate nodes if necessary. The gateway can then send an SMS to the person using the insulin. A light sensor can be used to monitor ambient lighting. The node can send an alarm if the insulin is subjected to excessive light. A small audio buzzer can also be added to aid in locating the insulin vial/cartridge around the house. The node should also monitor the battery voltage and raise an alarm if it falls below a threshold. The MSP430G2553/G2955 micros have an ADC channel dedicated to measuring the micro’s supply voltage. This allows the micro to monitor the battery voltage without any external components assuming that there is no dc-dc converter between the battery and the micro’s supply pins. This voltage value should be sent in every message sent out by the node. In the case that ambient conditions are good for an extended time, the node should still send a few messages to the gateway periodically (in the absence of alarm messages) to show that it is alive. A few messages per day should be enough. If the ability to locate the insulin vial/cartridge is required, then the design becomes more complicated and prone to battery failure. The node needs to wake very frequently to check if the gateway is looking for it.
A network to monitor insulin is highly feasible in a house. A small number of sensor nodes should be enough to cover the whole house. The sensor node will work inside a refrigerator too as long as there is a relay node close to the fridge. We have tested WiSense sensor nodes inside a refrigerator.