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How to design an insulin pump

Posted: 13 Jun 2013 ?? ?Print Version ?Bookmark and Share

Keywords:Insulin pump? blood glucose level? analogue front-end?

The device includes:
A control unit: A processing module is required to control the position of the piston (shown in Figure 3).
A reservoir: A reservoir or a cartridge is needed to store the insulin that is to be fed to the body. Typical capacity is around 2-3 ml (200 to 300 insulin units). The reservoir needs to be re-filled whenever it is near empty.
A disposable infusion set: The disposable infusion set includes a cannula (a tube with an injection needle) for subcutaneous insertion under the skin (the layer under the skin where the injection needle is inserted) and a tubing system to interface the insulin reservoir to the cannula (similar to the typical syringe with piston and needle).

A related sub-product for controlling/managing diabetes is a continuous blood glucose monitor. This device provides real-time glucose-level monitoring through a subcutaneous sensor. The sensor can be left in place for several days at a time, which reduces the need for the patient to test multiple individual blood samples. Future developments would bring in a closed loop system with the glucose monitor as a feedback sensor to change the dosage rate. The glucose monitor used in the feedback loop can also be designed using a programmable SoC.3 A separate SoC can be used for this or the entire closed loop system can be integrated in a single SoC.

Implementing an insulin monitor using a PSoC
Figure 4 shows a block diagram of the interface between the processor C in this case, an SoC from Cypress' PSoC family C and the reservoir and infusion set.

Figure 4: Block diagram showing the implementation of an insulin pump built using a programmable SoC, in this example the PSoC family from Cypress.

Section 1: Controlling the injection of insulin
As discussed earlier, some logic must be available to control the position of the piston. A DC motor or even a stepper motor may be used to drive a screw, which in turn pushes the piston. Since the rate at which the insulin is injected is extremely small, the piston has to be pushed very slowly with many revolutions of the motor. This is achieved using a gear logic.

Note that this motor control logic cannot be used as an open loop system as it would lead to a change in speed if the load is changed. Thus, some feedback sensors like rotary encoders can be used to monitor the current speed at which the motor is running and also some logic can be used to compare it with the required speed and accordingly modify certain settings in the motor drive control. Link [4] provides a detailed implementation of how to monitor/control the motor speed by the use of rotary encoders using the PSoC3/5.

Since the insulin injected has to be greater during the bolus stage and lessor during the basal stage, the motor has to be driven at a faster speed during the bolus stage and at a slower speed during the basal stage. Thus, the logic must be able to shift between the two modes.

Section 2: RTC and EEPROM
We can use the SoC's internal RTC (real-time clock) block to store the current date and time in internal EEPROM (electrically erasable programmable read-only memory). This allows the date and time to be stored even if the device is powered off. The system can also store the date and time at which the cartridge/ reservoir has to be refilled (easily determined since the system knows the rate at which insulin is pumped into the body) as well as the capacity of the reservoir. The system can also drive an alarm (i.e., a loudspeaker driven by an internal DAC) to indicate when the reservoir becomes empty.

As stated in the previous section, the system needs to know when to switch the motor speed to the one suited for bolus mode and when to the one suited for basal mode. For this, counter blocks available in PSoC 3 and 5 can be used with a very low frequency source clock on the order of 1Hz. This 1Hz clock can be derived from the 32.768 crystal used to drive the RTC.

Section 3: Avoiding blockage through the flowpath
As a safety precaution, it is important to monitor if the insulin is properly being injected into the body or if there is any blockage in the flowpath. If there is a blood clot or a tissue development at the place where the needle is injected that blocks the flow of insulin, for example, the pressure in the cartridge will increase. We can use a pressure sensor (silicon pressure sensors are available)5 surrounding the cannula and feed its output to the processor. Similar to a strain gauge sensor, a pressure sensor converts pressure to a corresponding change in resistance. To detect a change in resistance, the sensor can be placed in a wheatstone's bridge to generate a differential voltage which can then be fed into the SoC for further processing.

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