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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?

Also, the insulin to be fed into the body must be maintained within a suitable temperature range to prevent denaturing. This can be achieved by monitoring the temperature of the insulin in the reservoir or the cannula through a sensor like a thermistor that can then be interfaced to SoC.

Finally, two analogue inputs C one from pressure sensor and the other from the temperature sensor C are fed to the SoC to monitor their current status via an integrated ADC. PSoC 3 and 5, for example, have high precision analogue front-ends to do such operations with resolution up to 20 bits and the ability to multiplex the signals through the same ADC. The resultant digital values can be compared with the stored threshold values to detect if there is a blockage (i.e., when the pressure sensor reading exceeds its threshold) or if the insulin has denatured (i.e., when the thermistor reading exceeds its threshold). The SoC can then sound an alarm or flash an LED if a blockage has occurred. This alarm can also be used to sound when the battery is almost drained.

Section 4: Power management in this portable device
The alkaline batteries (i.e., non-rechargeable) used in portable medical devices typically provide up to 1.5 V. An internal boost regulator in the SoC can boost this to the appropriate voltage to operate the SoC, 1.8 V in the case of PSoC 3/5. This boost regulator can boost even voltages as low as 0.5 V to be 1.8 V. Lithium batteries are recommended when using rechargeable batteries.

Since this a handheld portable device, power consumption plays a major role in efficient operation given that the battery cannot be recharged or replaced often. Thus, the SoC needs to support multiple low power modes, including sleep/hibernate when the device is not in use, to aid in conserving battery power.

PSoC 3 and 5 offer an additional low power mode known as alternate active mode where the CPU is turned off but certain digital and analogue blocks can still be operated. This enables an architecture where the insulin pump can operate the majority of time without the CPU. This means CPU operation (say an interrupt) is only needed when we are switching between bolus and basal dosages.

Section 5: Display + I/O interface
If the duration of basal and bolus stage has to be changed or if the concentration has to be changed with the duration fixed, there is no need for re-designing the entire system. The user only has adjust the system by a means such as pressing buttons. The touch sensing solutions, which Cypress Semiconductor provides as an in-built feature in PSoC, can replace the traditional mechanical buttons which are being used as of today. With the ability to also directly control a high-quality Graphics or segment LCD, the same display used to show information such as the current status and when the next re-fill will be required can also serve as the user interface by implementing a resistive touch screen on top of the LCD display screen.

Section 6: USB features
By implementing a communications port such as USB so that the device can talk to a PC, the insulin pump can log important data such as the time instances of insulin injection and dosage duration. USB also enables the option of charging the device's battery through a PC.

Other considerations
From the above description, it is observed that:

1. Each of the blockage sensing methods (pressure sensor, temperature sensor, etc.) requires a high-precision analogue front-end (AFE). With a traditional MCU, discrete components would be required to perform the necessary input measurements, increasing system size and cost compared to a SoC with an integrated analogue front-end that allows many different sensors to be interfaced to the CPU.

2. Insulin pumps are battery-powered devices so active power consumption and sleep current are important considerations. Also there is a need for boosting the voltage as the traditional MCUs operate at higher voltage range when the battery input provides a lower voltage than is required to power the MCU. These issues are eliminated by the high level of integration in SoC architectures.

3. Insulin pumps use a display to show the current status. A processor module supporting LCD direct drive or LCD control simplifies system design.

4. An insulin pump requires memory for storing the dosage history as well as certain thresholds for later comparison. EEPROM or another permanent memory storage technology needs to therefore be available, preferably integrated on the processor or SoC.

5. A serial communications interface such as USB allows data to be easily logged to PC periodically.

6. A touch screen user interface allows for a more intuitive and simplified interface. In addition, it eliminates the need for mechanical buttons which can wear.

7. Proper circuitry is needed for controlling the motor, which in turn pushes the piston to inject insulin into the body.

8. Certain insulin pumps also require the current glucose status within the body to adjust the flow rate. Traditional MCUs aren't appropriate for such a closed loop system as they would require additional external analogue ICs to implemented this. In contrast, an SoC has the elements required to create a closed loop system in a cost-effective manner.

References
[1] Importance of Electronics in Medical ApplicationsPart I. http://www.eetimes.com/design/medical-design/4407326/Electronics-in-medical-apps-Part-IFertility-monitor-design-?Ecosystem=medical-design
[2] FDA rules in the design of medical equipments. http://www.fda.gov/MedicalDevices/DeviceRegulationandGuidance/Standards/default.htm
[3] Blood Glucose Monitor using PSoC. http://www.cypress.com/?rID=43661&source=header
[4] Design of motor control system using SoCs. http://www.embedded.com/design/mcus-processors-and-socs/4405205/1/Trade-offs-between-programmable-SoCs-vsdedicated-MCUs-in-motor-control
[5] Silicon pressure sensors. http://www.mouser.com/catalog/645/usd/2049.pdf

About the author
Asha Ganesan earned her Bachelor's degree in electronics and communication at College of Engineering Guindy. She is currently working as an applications engineer at Cypress Semiconductor. She is gaining experience in PSoC 3 and PSoC 5 products and she assists customers with their PSoC 3/5 projects.

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