Infusion Pump

Description

An infusion pump is a medical device that delivers fluids into a patient in precisely controlled amounts. The pump typically regulates flow from an IV bag through a tube and syringe into a patient's body. There are many types of infusion pumps, including large volume, patient-controlled analgesia (PCA), elastomeric, syringe, enteral, and insulin pumps. These pumps are used in clinical settings such as hospitals and nursing homes, and in ambulatory settings including the home.

The type of pump mechanism used in an infusion pump can vary by application. Common pump mechanisms include peristaltic and syringe. Both types provide precision fluid delivery and both are driven by an electric motor.

Click the "Design Considerations" tab to gain an understanding of the key parameters and circuitry needed to build an infusion pump. Click the "Circuits" or "Block Diagrams" tab to view reference designs and products suggested for use in implementing various design functions.


Design of an infusion must take into account reliability and safety. As more functions are built into the product, attention must be paid to the user interface, both the control interface and the operating interface, to ensure that the pump can be easily programmed and that the therapy is delivered as required.

Many of the components for an infusion are available in module form, for example, motor and gearing mechanism with shaft encoder. The primary focus of an infusion pump designer will be selecting or specifying modular components and sensors and then integrating the components and designing the analog signal chains needed to interface the pump and sensors to the microcontroller(s). The firmware within the microcontroller will establish the ultimate functionality of the design.

An infusion pump design must control a pump to deliver a precise programmable volume of liquid over a certain period of time. The design uses a special pump type (i.e., peristaltic, syringe) along with the control electronics to meet this objective. The design considerations for an infusion pump are discussed by function below.

Pump and Pump Driver


Both peristaltic and syringe pump mechanisms are driven by a motor and the motor is controlled by the system microcontroller. Feedback sensors provide information about rotation count, motor position, and current usage. The type of motor used depends on the application. Stepper motors, brushed DC motors, and BLDC motors are all used. DC and BLDC motors are typically matched with a gearing system and fitted with angular-position sensors or a rotary encoder to provide rotation feedback to the system controller. Stepper motors can directly drive some pump mechanisms.

The type of motor-driving circuitry selected depends on the type of motor used and the application.

Part Selection

Maxim offers innovative brushed DC motor drivers that are perfect for smaller infusion pumps. For larger pumps and those using BLDC motors, Maxim offers MOSFET driver ICs. Some designs will monitor the current in one or more phases of the motor. For this application, the motor current monitoring signal chain consists of a current-sense amplifier connected to a multichannel 12-bit ADC. Refer to the Motor Driver Signal Chain block diagram for recommended Maxim products for these applications.

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Fluid Flow Monitoring


The infusion pump must deliver precise amounts of liquid over a certain time period. The flow rate will be primarily influenced by the motor speed. However, for safety reasons, the fluid flow must also be monitored. The flow is generally monitored by occlusion sensors placed before the pump and after the pump. These sensor values are monitored to make sure they remain within an operating range. If a reading is recorded outside a threshold, an alarm is sounded and the pump is stopped.

Part Selection

The occlusion sensor is usually a form of strain gauge and the unamplified output can be in the millivolts range. The overall pressure range that needs to be measured is very limited (compared to an industrial pressure application), so an accurate 10- or 12-bit analog-to-digital converter (ADC) generally provides enough divisions to effectively monitor these sensors. The typical signal chain for an occlusion sensor is a 10- or 12-bit ADC fronted by an op amp. Refer to the Pressure Sensor/Air-in-Line Signal Chain block diagram to view Maxim products recommended for this application.

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Air-in-Line Sensing


Air-in-line sensors are generally ultrasonic sensors that output a voltage that changes when bubbles are detected flowing through the delivery tube. These sensors generally provide higher output levels than an occlusion/pressure sensor, so they can sometimes be attached directly to an ADC input or fronted by a scaling op amp and fed directly into a microcontroller-based ADC. The exact signal chain required will depend on the sensor selected.

Part Selection

Depending on the sensor selection, this signal chain may consist of either a high voltage output (e.g., 0V to 5V) or millivolt output. Either output type might be fed directly into a microcontroller ADC port through a scaling op amp, if the microcontroller has an ADC. Or these sensors can be fed into a single multichannel external ADC that also handles the occlusion sensor. Refer to the Pressure Sensor/Air-in-Line Signal Chain block diagram to view Maxim products recommended for this application.

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Power and Battery


Infusion pumps are generally portable instruments typically found residing on a desktop or mounted on a mobile IV rack. They generally use line power, but can also be battery powered depending on the application. Typically the line-powered infusion pumps provide battery backup for reliable operation during power outages or glitches.

The power system generally consists of a line-powered AC-to-DC wall transformer, the output of which is typically a single DC rail of 12V to 24V depending on the system design. This voltage rail is input to the battery charging circuit and also sent to the system DC-DC converters usually through a power source switch. The power source switch will select the DC line power if it is active or automatically switch to battery power if not.

The IC chosen for the battery charging circuit must match the battery chemistry and cell count. Battery types used include sealed lead acid, Li+, and LiPo. Multicell lithium-based batteries need to have each cell charged in balance with the others to provide optimal service life and to prevent explosion. The charging circuit used should be selected with input from the battery manufacturer. Maxim Reference Circuit 3241: Charging Batteries Using USB Power provides a good introduction to battery charging.

In addition to a battery charger IC, it is very important to provide the user an indicator of the charge remaining in the battery. For this application a battery fuel gauge IC is required.

Part Selection

Maxim manufactures many different types of battery charger ICs. The charging IC selected for an application depend on the type of battery selected, and the charging circuit used should be selected with input from the battery manufacturer.

Refer to the main block diagram for recommended battery charging and fuel gauge ICs for this application.

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System Power


The main battery or DC line voltage rail will generally power the motor. The system ICs will have power provided to them through one or more switching regulators in combination with a low dropout linear voltage regulator for some rails. A higher voltage may be needed to power the LCD display, depending on display design, though many display modules require only one 3.3V or 5V rail and perform voltage conversions to power the display within the module.

Part Selection

The number of voltage rails needed in this application will depend on the processor used. A step-up switching regulator might be needed, for example, to provide power for a display. In addition, low dropout voltage regulators (LDOs) are sometimes used to provide cleaner power for certain microcontroller rails. Maxim manufactures many step-down and step-up switching regulators and LDOs that can provide the necessarily voltage rails. Refer to the main block diagram for products typically recommended for this application.

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User Interface and Alarms


The user interface for most infusion pumps consists of a display screen and a set of operating buttons. The display provides operating instructions and the buttons offer operating input. Displays can range from simple character-based LCDs to graphic color TFT LCD screens. Complete display modules are generally used. These modules usually require only one power rail and provide an I2C data interface.

Infusion pumps require audible and visible alarms to alert users to faults or potentially dangerous conditions. Bicolor or tricolor (red/orange/green) LEDs are typically used as visual indicators. Audible alarms vary from simple beepers driven by the microcontroller's pulse-width modulation (PWM) output to more sophisticated alarms, including voice synthesis, created with an audio DAC. Even simple audio beepers should include a self-test feature. This function can be implemented either indirectly by monitoring for a speaker impedance within range or directly by incorporating a microphone near the speaker to register the audio output and confirm that it is at the proper level.

Maxim circuits that can be incorporated into the user interface function of an infusion pump design are shown in the User Interface and Audio block diagrams.

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Electrostatic Discharge


All infusion pumps must pass IEC 61000-4-2 electrostatic discharge (ESD) requirements by either using electronics with built-in protection or by adding ESD line protectors to exposed traces. Maxim offers many interface parts with this high ESD protection built-in, as well as stand-alone ESD diode arrays.

For more design information, review the application notes specified to the right, view the circuits listed under the Circuits tab, and view the block diagrams listed under the Block Diagrams tab.

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Sensors Signal Chain Motor Driver Signal Chain User Interface Communications Audio Signal Chain Multivoltage Supervisor Temperature RTC SD Card Reader Battery Charger Fuel Gauge Battery Isolation Step-Down DC-DC Step-Up DC-DC LDO

Air-in-Line, Occlusion Sensor, Pressure Sensor Signal Chain


Amplifier VREF ADC

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Communications


Isolated Power USB Protection Isolation RS-232 Transceiver NFC

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Motor Driver Signal Chain


DC Motor Driver Half-Bridge Driver CS Amp ADC

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User Interface


LCD Power and Control LCD Backlight Drivers Key Scanner GPIO Indicator Drivers

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Audio


Audio Codec Speaker Driver Piezo Driver

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Type Part Number Title
Evaluation Board MAX9918EVKIT Evaluation Kit for the MAX9918
Evaluation Board MAX9921EVKIT Evaluation Kit for the MAX9921
Evaluation Board DS89C450-KIT Evaluation Kit for the DS89C450